sensors Article
Design of Diaphragm and Coil for Stable Performance of an Eddy Current Type Pressure Sensor Hyo Ryeol Lee 1 , Gil Seung Lee 2 , Hwa Young Kim 3 and Jung Hwan Ahn 1, * 1 2 3
*
School of Mechanical Engineering, Pusan National University, 2, Busandaehak-ro 63 beon gil, Geumjeong-gu, Busan 46241, Korea; [email protected] NOVASEN Co., LTD, 25, Bansong-ro 525 beon gil, Haeundae-gu, Busan 48002, Korea; [email protected] Research Institute of Mechanical Technology, Pusan National University, 2, Busandaehak-ro 63 beon gil, Geumjeong-gu, Busan 46241, Korea; [email protected] Correspondence: [email protected]; Tel.: +82-51-510-3087
Academic Editor: Vittorio M. N. Passaro Received: 7 April 2016; Accepted: 22 June 2016; Published: 1 July 2016
Abstract: The aim of this work was to develop an eddy current type pressure sensor and investigate its fundamental characteristics affected by the mechanical and electrical design parameters of sensor. The sensor has two key components, i.e., diaphragm and coil. On the condition that the outer diameter of sensor is 10 mm, two key parts should be designed so as to keep a good linearity and sensitivity. Experiments showed that aluminum is the best target material for eddy current detection. A round-grooved diaphragm is suggested in order to measure more precisely its deflection caused by applied pressures. The design parameters of a round-grooved diaphragm can be selected depending on the measuring requirements. A developed pressure sensor with diaphragm of t = 0.2 mm and w = 1.05 mm was verified to measure pressure up to 10 MPa with very good linearity and errors of less than 0.16%. Keywords: eddy current; pressure sensor; round-groove; impedance change
1. Introduction Pressure sensors have been widely used in such areas as chemical plants, automobile/ship, aviation, air conditioning, biomedical, and hydraulics industries, etc., where pressure is an important monitored parameter. In chemical plants, pressure sensors are used to monitor partial pressures of gases in industrial units so that large chemical reactions take place under precisely controlled environmental conditions. In automobile and ship engines where it forms an integral part of the engine and its safety, the dynamic cylinder pressure in the combustion chamber is monitored to regulate the power that the engine must deliver to achieve suitable speeds. Engine combustion pressure analysis is a fundamental measurement technique applied universally in the research and development of reciprocating combustion engines [1,2]. The three types of pressure sensors—metal thin-film, ceramic thick-film and piezoresistive—have been most commonly used so far. Among them, silicon piezoresistive MEMS pressure sensors as micromachined devices, are widely used in various systems because of their high accuracy, high sensitivity, and excellent linearity over a wide range of pressures in addition to their low cost and small size [3–7]. Pressure sensors are often exposed to harsh environments with contaminants—such as dust, dirt, and oil—and severe temperature changes, which cause the silicon based sensors to have poorer accuracy, sensitivity, and linearity. In an effort to develop a robust pressure sensor that can endure such harsh environments, an eddy current type pressure sensor is proposed in this research, due to the high resolution, high frequency response, and robustness against contaminants of an eddy current sensor (ECS). ECS are known to Sensors 2016, 16, 1025; doi:10.3390/s16071025
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known to be very competitive for displacement measurement harsh environments under dynamic be very competitive for displacement measurement in harshin environments under dynamic and static and staticFurthermore, pressures. Furthermore, active compensation temperature compensation reduce theeffect temperature pressures. active temperature to reduce the to temperature on ECS has effectproved on ECSpossible has beenthrough proved previous possible through been researchprevious [8,9]. research [8,9]. An eddy current type pressure sensor should be composed two transducer An eddy current type pressure sensor should be composed of twoof transducer parts—aparts—a diaphragm to convert pressure into diaphragm and displacement and andetector eddy current detector to todiaphragm convert pressure into diaphragm displacement an eddy current to convert diaphragm convert diaphragm displacement into eddy current voltage. In this paper, particularly, it is displacement into eddy current voltage. In this paper, particularly, it is described how the design described how the design parameters of these two key components affect the pressure sensor parameters of these two key components affect the pressure sensor performance—i.e., pressure range performance—i.e., pressure range and sensitivity—and what type of diaphragm measure the eddy and sensitivity—and what type of diaphragm measure the eddy current sensor displacement most current sensor displacement most stably and precisely. Finally, the characteristic of an assembled stably and precisely. Finally, the characteristic of an assembled sensor of diaphragm and eddy current sensor of diaphragm and eddy current probe was tested in the real pressure chamber and the probe was tested in the real pressure chamber and the feasibility for practical use was checked. feasibility for practical use was checked.
2. Principle and Structure of Eddy Current Type Pressure Sensor 2. Principle and Structure of Eddy Current Type Pressure Sensor 2.1. Working Principle of a Typical Pressure Sensor 2.1. Working Principle of a Typical Pressure Sensor A typical—generally a force collector type—pressure sensor is composed of a diaphragm A typical—generally a force collector type—pressure sensor is composed of a diaphragm and a and a probe, where a pressurized gas from a high pressure chamber causes the diaphragm to be probe, where a pressurized gas from a high pressure chamber causes the diaphragm to be elastically elastically deflected. And the probe then detects the diaphragm deflection. According to detection deflected. And the probe then detects the diaphragm deflection. According to detection principles, principles, there are several kinds of pressure sensors such as strain gauge type, piezoresistive there are several kinds of pressure sensors such as strain gauge type, piezoresistive type, type, piezoelectric capacitive electromagnetic optical In strain gauge, piezoelectric type, type, capacitive type,type, electromagnetic type, type, and and optical type.type. In strain gauge, piezoresistive, types,the theunit unitthat thatdetects detects the diaphragm deflection is connected piezoresistive,and and piezoelectric piezoelectric types, the diaphragm deflection is connected to to ororbuilt into the diaphragm as a thin layer-bonded, and sputtered etc., while the others are separately built into the diaphragm as a thin layer-bonded, and sputtered etc., while the others are located nearlocated the diaphragm detect its deflection in deflection non-contact Piezoresistive sensors—based separately near the to diaphragm to detect its in mode. non-contact mode. Piezoresistive on measurementon of measurement the diaphragm a good feature aingood that feature the resistance sensors—based of displacement—have the diaphragm displacement—have in that change the for a given amount of pressure is much greater than in metal strain gauges. resistance change for a given amount of pressure is much greater than in metal strain gauges. The type pressure pressuresensor sensorwhich whichisistotobebe developed research is one of the Theeddy eddy current current type developed in in thisthis research is one of the electromagnetic type type with the diaphragm byby a sensor coil. electromagnetic diaphragmdeflection deflectionbeing beingdetected detected a sensor coil. 2.2. PressureSensor Sensor 2.2.Structure Structure of of Eddy Eddy Current Type Pressure Thestructure structure of an pressure sensor to be in this is shown in The aneddy eddycurrent currenttype type pressure sensor todeveloped be developed instudy this study is shown is is composed of of housing, diaphragm, insulator, target plate andand sensor coil coil inFigure Figure1a. 1a.The Thesensor sensor composed housing, diaphragm, insulator, target plate sensor (eddycurrent currentprobe). probe). The diaphragm C22 to to prevent sensors from being rusted (eddy diaphragmisismade madeofofHastelloy Hastelloy C22 prevent sensors from being rusted even under harsh conditions. When pressurized gas from a high pressure chamber is applied to the even under harsh conditions. When pressurized gas from a high pressure chamber is applied to the sensor,as asshown shown in in Figure Figure 1b, moves thethe insulator andand the the target sensor, 1b, the the diaphragm diaphragmisisdeflected, deflected,which which moves insulator target plate; the resulting displacement of the target plate is then detected by the coil. plate; the resulting displacement of the target plate is then detected by the coil.
(a)
(b)
Figure 1. 1. Eddy structure; (b)(b) on on sitesite application. Figure Eddy current currenttype typepressure pressuresensor: sensor:(a)(a) structure; application.
Therefore, the two key parts of an eddy current type pressure sensor—diaphragm and Therefore, the two key electrically parts of an currenttotype pressure sensor—diaphragm coil—should be well-matched andeddy mechanically enhance its sensitivity and linearity inand coil—should well-matched mechanically to enhance its sensitivity andislinearity the aspect ofbe tiny displacementelectrically acquisitionand on the condition that the diaphragm deflection within in the aspect of tiny displacement acquisition on the condition that the diaphragm deflection is within
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the elastic limit. For compatible use with other types of commercialized pressure sensors, the the elastic limit. For compatible use with other types of commercialized pressure sensors, the diameter the sensor is fixed at 10with mm.other types of commercialized pressure sensors, the diameter the elastic of limit. For compatible use diameter of the sensor is fixed at 10 mm. of the sensor is fixed at 10 mm. 3. Design of Sensor Coil 3. 3. Design Designof ofSensor SensorCoil Coil 3.1. Design Parameters 3.1. 3.1. Design Design Parameters Parameters Figure 2 shows a schematic diagram of the sensor coil unit—eddy current probe. Figure schematic diagram diagram of of the the sensor sensor coil coil unit—eddy unit—eddy current current probe. probe. Figure 22 shows shows aa schematic
Figure 2. 2. Geometry Geometry of of coil coil against against target target plate. plate. Figure Figure 2. Geometry of coil against target plate.
Many design parameters are geometric ones, such as: outer diameter (a); inner diameter (b); Many design parameters parameters are geometric ones, such as: outer outer diameter diameter (a); (a); inner inner diameter diameter (b); (b); and height (c) of the coil unit; coil diameter and gap (d) between coil and target plate. Other design and height (c) of the coil unit; coil diameter and gap (d) between coil and target plate. Other design parameters include magneto-electrical ones such as conductivity, temperature resistance and parameters includemagneto-electrical magneto-electrical ones as conductivity, temperature resistance and parameters include ones suchsuch as conductivity, temperature resistance and magnetic magnetic permeability. It is known that self-inductance and mutual-inductance of the sensor coil are magnetic permeability. is known that self-inductance and mutual-inductance of coil the sensor coil are permeability. It is knownIt that self-inductance and mutual-inductance of the sensor are dependent dependent only on geometrical parameters [10]. Copper is used as coil material to get sensitivity and dependent only on geometrical Copper is used as coil to getand sensitivity and only on geometrical parameters parameters [10]. Copper[10]. is used as coil material to material get sensitivity linearity of linearity of detection as high as possible because of its high conductivity and low temperature resistance. linearity as high asbecause possibleofbecause its high conductivity low temperature resistance. detectionofasdetection high as possible its highofconductivity and low and temperature resistance. From the viewpoint of sensor performance referring to the past works, the inner and outer From the viewpoint of sensor performance referring referring to the past works, the inner and outer diameter should be minimized and maximized, respectively, and the height should be minimized as diameter should height should bebe minimized as should be beminimized minimizedand andmaximized, maximized,respectively, respectively,and andthe the height should minimized much as geometrically possible [10,11]. Since the coil should detect the diaphragm deflection much as as geometrically possible as much geometrically possible[10,11]. [10,11].Since Sincethe thecoil coilshould shoulddetect detect the the diaphragm diaphragm deflection appropriately, it is desirable that the diameter of the probe is limited to a maximum of 5 mm. The appropriately, itit isis desirable of of thethe probe is limited to atomaximum of 5 of mm. The desirablethat thatthe thediameter diameter probe is limited a maximum 5 mm. inner diameter and height should be at least 2 mm and 1 mm respectively for convenience of coil inner diameter and height should be at be least mm2and mm 1respectively for convenience of coil The inner diameter and height should at 2least mm1 and mm respectively for convenience manufacturing. The copper wire diameter is at most 0.1 mm, as anything larger reduces the manufacturing. The copper wire wire diameter is atismost 0.1 0.1 mm, asasanything of coil manufacturing. The copper diameter at most mm, anythinglarger larger reduces reduces the sensitivity and the anything smaller is too sensitive to disturbance. The gap is fixed at 0.3 mm to sensitivity and andthe theanything anythingsmaller smaller sensitive to disturbance. is fixed 0.3tomm to is is tootoo sensitive to disturbance. TheThe gap gap is fixed at 0.3at mm avoid avoid the effect of parasitic capacitance largely generated in ranges less than 0.2 mm. avoid the of effect of parasitic capacitance generated in ranges less0.2 than 0.2 mm. the effect parasitic capacitance largelylargely generated in ranges less than mm. 3.2. Signal Processing for Eddy Current Detection 3.2. Signal Processing for Eddy Current Detection The principle of eddy current detection is to detect a variation of coil impedance caused by a The principle of eddy current detection is to detect a variation of coil impedance caused by a change of the gap between coil and target plate. Figure 3 shows a signal processing block for between coilcoil andand target plate.plate. Figure 3 shows a signalaprocessing block for block detecting change of ofthe thegap gap between target Figure 3 shows signal processing for detecting a change of coil impedance and removing disturbances from the acquired R and L impedances. a change of coil impedance and removing disturbances from the acquired R and L impedances. detecting a change of coil impedance and removing disturbances from the acquired R and L impedances.
Figure Figure 3. 3. Functional Functional block block diagram diagram for for signal signal processing processing of of eddy eddy current current acquisition. acquisition. Figure 3. Functional block diagram for signal processing of eddy current acquisition.
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Given kHz) Given the the optimal optimal frequency frequency (250 (250 kHz) kHz) and and amplitude amplitude of of the the coil coil excitation excitation current current (25 (25 mA) mA) by by Given the optimal frequency (250 and amplitude of the coil excitation current (25 mA) by DSP (Digital Signal Processor), DDS (Direct Digital Synthesizer) circuit transmits corresponding DSP (Digital (Digital Signal Signal Processor), Processor),aaaDDS DDS(Direct (DirectDigital DigitalSynthesizer) Synthesizer)circuit circuittransmits transmits aaacorresponding corresponding DSP voltage signal to current pump which supplies corresponding current to the coil. Eddy current voltage signal signal to to aaa current current pump pumpwhich whichsupplies supplies aaacorresponding corresponding current current to tothe thecoil. coil. Eddy Eddy current current voltage electromagnetically induced in the target plate placed close to the sensor coil causes a change of electromagneticallyinduced inducedinin target plate placed the sensor coil causes a of change of electromagnetically thethe target plate placed closeclose to thetosensor coil causes a change voltage voltage in sensor coil, which is by detector is to of the voltage in the thecoil, sensor coil,is which is detected detected by aa voltage voltage detector and is converted converted to aa change change ofcoil the in the sensor which detected by a voltage detector and isand converted to a change of the coil aa demodulator circuit. coil impedance impedance by demodulator circuit. impedance by aby demodulator circuit. 3.3. 3.3. Experiment Experiment Evaluation Evaluation of of Sensor Sensor Coil Coil The characteristics of the the sensor coilare areexamined examinedalong alongwith withthe thegap gapprecisely preciselycontrolled controlledon onaa The characteristics characteristics of sensor coil coil are examined along with the gap precisely controlled on measurement a measurementsystem, system,as shownin Figure4, whichis is composed composed of measurement system, asasshown shown ininFigure Figure 4,4,which which is composed of aa precision precision positioning positioning unit unit the gap and an LCR (inductance (L), capacitance (C), and resistance (R)) meter controlling the gap and an LCR (inductance (L), capacitance (C), and resistance (R)) meter measuring controlling the gap and an LCR (inductance (L), capacitance (C), and resistance (R)) meter measuring coil coil impedance. measuring coil impedance. impedance.
Figure with gap controlled. Figure 4. 4. Coil Coil impedance impedance measurement measurement system system with with gap gapcontrolled. controlled. Figure 4. Coil impedance measurement system
In In order order to to find find out out the the best best target target material material in in the the magneto magneto electrical electrical aspect, aspect, five five materials—copper, materials—copper, In order to find out the best target material in the magneto electrical aspect, five materials—copper, aluminum, brass, bronze and stainless steel—are considered as alternatives. aluminum, brass, bronze and stainless steel—are considered as alternatives. The The results results of of aluminum, brass, bronze and stainless steel—are considered as alternatives. The results of impedance impedance impedance measurements measurements for for the the five five target target materials materials are are displayed displayed in in the the R-L R-L impedance impedance plane, plane, measurements for the five target materials are displayed in the R-L impedance plane, which is specially which which is is specially specially adopted adopted to to improve improve the the typical typical measurement measurement method method based based on on amplitude amplitude data data adopted to improve the typical measurement method based on amplitude data acquired from an LCR acquired from an LCR meter, as shown in Figure 5. acquired from an LCR meter, as shown in Figure 5. meter, as shown in Figure 5.
Figure Figure 5. 5. Coil Coil impedance impedance measurement measurement system system with with gap gapcontrolled. controlled.
Both impedance changes against gap—ΔL/Δd, Both impedance impedancechanges changesagainst againstgap—∆L/∆d, gap—ΔL/Δd, ΔR/Δd—are ΔR/Δd—are almost almost linear linear in range less less than than Both ∆R/∆d—are in aa range range less than 500 μm, although they become nonlinear as the gap increases beyond that. As R is actually affected 500 μm, although they become nonlinear as the gap increases beyond that. As R is actually affected 500 µm, although they become nonlinear as the gap increases beyond that. As R is actually affected more aluminum is over the other materials as more than than LL by by temperature temperature change, copper oror aluminum is preferred preferred over thethe other materials as the the more than temperaturechange, change,copper copperor aluminum is preferred over other materials as target material, because of higher conductivity and lower temperature sensitivity in addition to target material, because of higher conductivity and lower temperature sensitivity in addition to the target material, because of higher conductivity and lower temperature sensitivity in to nonmagnetic nonmagnetic metals metals property. property. Furthermore, Furthermore, in in terms terms of of corrosion-resistance, corrosion-resistance, aluminum aluminum is is much much better better than than copper. copper. Therefore, Therefore, Al6061 Al6061 is is chosen chosen as as the the target target material. material.
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property. Furthermore, in terms of corrosion-resistance, aluminum is much 5 of better 11 than copper. Therefore, Al6061 is chosen as the target material.
4. Design of Round-Grooved Diaphragm 4. Design of Round-Grooved Diaphragm 4.1. Shape of Diaphragm for Stably Measurable Deflection 4.1. Shape of Diaphragm for Stably Measurable Deflection The diaphragm plays an important role in the elastic deflection in response to an applied The diaphragm plays an important role in the elastic deflection in response to an applied pressure. pressure. The shape and geometrical dimensions of a diaphragm should be well-chosen so that its The shape and geometrical dimensions of a diaphragm should be well-chosen so that its deflection deflection matches the performance of the sensor coil designed in Section 3. A small sensor diameter matches the performance of the sensor coil designed in Section 3. A small sensor diameter of 10 mm of 10 mm limits the diaphragm diameter up to 8.6 mm at most. Furthermore, its deflection being limits the diaphragm diameter up to 8.6 mm at most. Furthermore, its deflection being measured more measured more accurately by the sensor coil, the diaphragm should be designed so that some area of accurately by the sensor coil, the diaphragm should be designed so that some area of its central part its central part goes up and down near flatly as the acting pressure goes up or down. goes up and down near flatly as the acting pressure goes up or down. What shape of the diaphragm is needed to meet such restricted requirements? Wang et al. What shape of the diaphragm is needed to meet such restricted requirements? Wang et al. revealed that the center-embossed diaphragm is superior in optical characteristics to the flat one revealed that the center-embossed diaphragm is superior in optical characteristics to the flat one which which is seriously deteriorated by the central area being wrapped under high hydrostatic pressure [12]. is seriously deteriorated by the central area being wrapped under high hydrostatic pressure [12]. In this study, a round-grooved, square-grooved diaphragm—like a center-embossed one—shown in In this study, a round-grooved, square-grooved diaphragm—like a center-embossed one—shown in Figure 6a,b, are considered for rather flatter deflection of the central part, which helps the eddy Figure 6a,b, are considered for rather flatter deflection of the central part, which helps the eddy current current probe measure a gap more stably and precisely. The design parameters, as depicted in probe measure a gap more stably and precisely. The design parameters, as depicted in Figure 6a,b, Figure 6a,b, are the width and thickness of the groove while the thickness of the diaphragm and the are the width and thickness of the groove while the thickness of the diaphragm and the distance of the distance of the groove from the wall are set as 1 mm and 1.5 mm, respectively. For deflection groove from the wall are set as 1 mm and 1.5 mm, respectively. For deflection comparison a flat type, comparison a flat type, shown in Figure 6c, is used as a reference. shown in Figure 6c, is used as a reference.
(a)
(b)
(c) Figure 6. Shape and and design parameter of three types of diaphragm: (a) round-grooved; (b) square Figure 6. Shape design parameter of three types of diaphragm: (a) round-grooved; (b) square grooved; (c) flat. grooved; (c) flat.
4.2. 4.2. Analysis of Diaphragm Deflection Analysis of Diaphragm Deflection Structural analyses were carried out out to investigate the the effects of design parameters on on the the Structural analyses were carried to investigate effects of design parameters maximum deflection andand stress of the diaphragm by increasing pressure in 0.25 MPa increments maximum deflection stress of the diaphragm by increasing pressure in 0.25 MPa increments until yielding occurs using ANSYS Workbench V15. The material properties of Hastelloy C22 are are until yielding occurs using ANSYS Workbench V15. The material properties of Hastelloy C22 elastic modulus 206 GPa, yield strength 438 MPa, and Poisson ratio 0.3. elastic modulus 206 GPa, yield strength 438 MPa, and Poisson ratio 0.3. With thickness (t) 0.2 mmmm andand pressure 2.5 2.5 MPa, Figure 7 shows a typical deflection behavior With thickness (t) 0.2 pressure MPa, Figure 7 shows a typical deflection behavior along the radial axis of the round-grooved, square-grooved, and flat diaphragms, respectively. along the radial axis of the round-grooved, square-grooved, and flat diaphragms, respectively. TheThe point 0 indicates the diaphragm. diaphragm.The Thedeflection deflection whole diaphragm point 0 indicatesthe thecenter center of of the of of thethe whole diaphragm surface surface under 2.5 MPa is displayed on the axis of −4.3~4.3 mm. While a much larger deflection with under 2.5 MPa is displayed on the axis of ´4.3~4.3 mm. While a much larger deflection with no flat no flat central is seen the flat the type, the grooved type deflects much with quite a flat in central part part is seen in theinflat type, grooved type deflects much less withless quite a flat deflection deflection in the center part compared to the flat type, which makes the eddy current probe measure the center part compared to the flat type, which makes the eddy current probe measure the diaphragm the deflection diaphragmwith deflection with more and precision. more stability andstability precision. Figure 8 shows comparisons of the simulated results in terms of maximum deflection andand Figure 8 shows comparisons of the simulated results in terms of maximum deflection maximum stress at each pressure between flat,flat, square-grooved, andand round-grooved types with maximum stress at each pressure between square-grooved, round-grooved types with increasing pressure at the same thickness. increasing pressure at the same thickness.
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(c) (c) (c) Figure 7. 7. Deflection Deflection behavior behavior along along the the diaphragm diaphragm axis axis with with tt == 0.2 0.2 mm, mm, w w == 1.05 1.05 mm, mm, P P = 2.5 2.5 MPa: MPa: Figure Figure 7. Deflection behavior along thethe diaphragm axisaxis withwith t = 0.2 w =w 1.05 mm,mm, P = 2.5 Figure 7. Deflection behavior along diaphragm t = mm, 0.2 mm, = 1.05 P ==MPa: 2.5 MPa: (a) round-grooved; (b) square-grooved; (c) flat. (a) round-grooved; (b) square-grooved; (c)flat. flat. (a)(a) round-grooved; (b)(b) square-grooved; (c) (c) flat. round-grooved; square-grooved;
(a) (a) (a)
(b) (b) (b)
Figure 8.Comparison Comparison ofmaximum maximum deflection/stress against pressure among flat, square-grooved, Figure deflection/stress against pressure among flat, flat, and Figure 8. 8. Comparison ofofmaximum deflection/stress against pressure among flat, square-grooved, Figure 8. Comparison of maximum deflection/stress against pressure among square-grooved, and round-grooved types with w = 1.05 mm: (a) maximum deflection, (b) maximum stress. round-grooved types with w = 1.05 mm: (a) maximum deflection; (b) maximum stress. and round-grooved types with w = 1.05 mm: (a) maximum deflection, (b) maximum stress. and round-grooved types with w = 1.05 mm: (a) maximum deflection, (b) maximum stress.
(b) (b) (b) (a) (a) (a) Figure 9. Analysis results at t at = 0.2= mm, w0.2 =w 1.05 mm, P==1.05 12.5 MPa: (a) stress distribution; yield points. Figure 9. Analysis Analysis results 0.2 mm, 1.05 mm, P == 12.5 12.5 MPa: (a) stress distribution; (b)distribution; yield points. Figure Analysis results t = wmm, mm,MPa: P = 12.5 MPa: (a) (b) stress Figure 9. results at tt =at 0.2 mm, wmm, == 1.05 P (a) stress distribution; (b) yield points. (b) yield points.
ForFor t = 0.20.2 mm, thethe flatflat type hashas a yielding pressure around at 3.55 MPa, which is much lower mm, type yielding pressure around at 3.55 3.55 MPa, which is much much lower For tt == 0.2 mm, the flat type has aa yielding pressure around at MPa, which is lower than the round-grooved type at 13.75 MPa or the square-grooved type at 5.87 MPa. For t = 0.3 mm, than the round-grooved type at 13.75 MPa or the square-grooved type at 5.87 MPa. For t = 0.3 mm, than the round-grooved type at 13.75 MPa or the square-grooved type at 5.87 MPa. For t = 0.3 mm,
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the flat type has a yielding pressure around 8.15 MPa, which is much lower than the round-grooved For20.88 t = 0.2MPa mm,orthe flat type has a yielding around at 3.55 MPa, which is much lower than type at square-grooved type at pressure 12.87 MPa. Figure 9 shows the numerical analytical the round-grooved type at 13.75 MPa or the square-grooved type at 5.87 MPa. For t = 0.3 mm, the flat results of how the stress is distributed on the round-grooved diaphragm and how the yielding stress type has a yielding around MPa,ofwhich is much lower than type at occurs at points 1, 2,pressure 3, and 4 on both 8.15 surfaces the round-groove with t =the 0.2round-grooved mm at 12.5 MPa. 20.88The MPa or square-grooved type at 12.87 MPa. Figure shows measurable the numerical analytical results of round-grooved type has a wider pressure range 9linearly prior to yielding than how the stress is distributed on the round-grooved diaphragm and how the yielding stress occurs at the square-grooved and the flat types. Thus, only the round-grooved type is considered for further points 1, 2, 3, of and on bothofsurfaces of the round-groove with t = 0.2 mm at 12.5 MPa. investigation the4 effects the design parameters. The round-grooved type has a wider pressure range to yielding Table 1 summarizes the 16 analysis cases with the twolinearly designmeasurable parameters prior properly selectedthan for the round-grooved square-grooved diaphragm and the flat and types. Thus, only theanalytical round-grooved is considered for further the their numerical resultstype of maximum pressure and investigation of the effects of the design parameters. deflection. In addition, the sensitivity is calculated as maximum deflection/maximum pressure. Table 1 summarizes the 16 analysis cases with the two design parameters properly selected for the Table 1. 16 diaphragm cases of properly selected design parameters withofanalytical of maximum round-grooved and their numerical analytical results maximumresults pressure and deflection. pressure,the maximum deflection and sensitivity. In addition, sensitivity is calculated as maximum deflection/maximum pressure. Max. Pressurewith Max. Deflection Sensitivity Table 1. 16 Thichness cases of properlyWidth selected design parameters analytical results of maximum pressure, (mm) (mm) (MPa) (μm) (μm/MPa) maximum deflection and sensitivity. Case 1 0.20 1.05 13.75 6.65 0.48 Case 2 0.20Thichness 1.30 Width 9.68Pressure 7.15 0.74 Max. Max. Deflection Sensitivity Case 3 0.20 (mm) 1.55 (mm) 7.38 8.38 1.14 (MPa) (µm) (µm/MPa) Case 4 Case 1 0.20 0.20 1.80 1.05 6.38 10.74 13.75 6.65 0.48 1.68 Case 5 Case 2 0.25 0.20 1.05 1.30 16.88 7.14 9.68 7.15 0.74 0.42 7.38 8.38 1.14 0.58 Case 6 Case 3 0.25 0.20 1.30 1.55 12.88 7.49 Case 4 0.20 1.80 6.38 10.74 1.68 Case 7 0.25 1.55 10.25 8.41 0.82 Case 5 0.25 1.05 16.88 7.14 0.42 Case 8 Case 6 0.25 0.25 1.80 1.30 8.88 10.11 12.88 7.49 0.58 1.14 Case 9 Case 7 0.30 0.25 1.05 1.55 20.88 8.03 10.25 8.41 0.82 0.38 8.88 10.11 1.14 0.49 Case 10 Case 8 0.30 0.25 1.30 1.80 16.38 8.06 20.88 8.03 0.38 0.65 Case 11 Case 9 0.30 0.30 1.55 1.05 12.88 8.35 Case 10 0.30 1.30 16.38 8.06 0.49 Case 12 Case 11 0.30 0.30 1.80 1.55 11.88 10.13 0.82 12.88 8.35 0.65 Case 13 Case 12 0.35 0.30 1.05 1.80 25.00 9.05 11.88 10.13 0.82 0.36 25.00 9.05 0.36 0.44 Case 14 Case 13 0.35 0.35 1.30 1.05 18.88 8.29 18.88 8.29 0.44 0.55 Case 15 Case 14 0.35 0.35 1.55 1.30 15.88 8.69 Case 15 0.35 1.55 15.88 8.69 0.55 Case 16 0.35 1.80 14.38 9.90 0.69 Case 16
0.35
1.80
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14.38
9.90
0.69
(b)
Figure 10. Maximum pressurepressure (a) and maximum deflection (b)deflection with design of round-groove Figure 10. Maximum (a) and maximum (b)parameters with design parameterstype. of round-groove type.
Figure 10 shows the maximum pressure and deflection at varied thickness and width. Figure 10a indicates that as the round-groove thickness increases and width decreases, the maximum pressure Figure 10 shows the maximum pressure and deflection at varied thickness and width. Figure 10a prior to yielding becomes larger. The change rates affected by both parameters are almost equal. indicates that as the round-groove thickness increases and width decreases, the maximum pressure Figure 10b shows that the round-groove width affects the deflection much more than the prior to yielding becomes larger. The change rates affected by both parameters are almost equal. round-groove thickness but does not show a consistent trend when thickness increased. So it would Figure 10b shows that the round-groove width affects the deflection much more than the round-groove be better to use the sensitivity calculated in Table 1 as an indicator to compare how sensitive the thickness but does not show a consistent trend when thickness increased. So it would be better to diaphragm is to pressure. The thinner and wider one is more sensitive to pressure change. The larger use the sensitivity calculated in Table 1 as an indicator to compare how sensitive the diaphragm is to
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pressure. The thinner and wider one is more sensitive to pressure change. The larger width increases width increases the sensitivity much more than the thickness but lowers the measurable maximum the sensitivity much more than the thickness but lowers the measurable maximum pressure too much, pressure too much, as shown in Figure 10a. as shown in Figure 10a. Based on the measurement requirements of pressure and sensitivity, the design parameters of Based on the measurement requirements of pressure and sensitivity, the design parameters of diaphragm can be selected. In this study the round-groove diaphragm with a groove 0.2 mm thick diaphragm can be selected. In this study the round-groove diaphragm with a groove 0.2 mm thick and and 1.05 mm wide is chosen to investigate its performances throughout the experiment. 1.05 mm wide is chosen to investigate its performances throughout the experiment.
5. Sensor 5. SensorPerformance PerformanceEvaluation Evaluationthrough throughPressure PressureMeasurement MeasurementTest Test Applied Pressure Pressure 5.1. Measurements of Diaphragm Deflection According to Applied It is necessary to examine how well the diaphragm deflection deflection relates relates to the applied applied pressure. pressure. Figure 11 shows a setup of an apparatus composed of a pressure chamber, dead weight tester and Figure 11 shows a setup of an apparatus composed of a pressure chamber, dead weight tester and half half bridge transformer (HBT) forexperiment. the experiment. bridge transformer (HBT) for the
Figure equipment with with pressure pressure varied varied by by dead dead weights. weights. Figure 11. 11. Diaphragm Diaphragm deflection deflection measurement measurement equipment
The deflection is measured by a HBT with pressure increased by 2.5 MPa increments up to 20 The deflection is measured by a HBT with pressure increased by 2.5 MPa increments up to MPa. To evaluate the repeatability of diaphragm deflection in response to changes in pressure, the 20 MPa. To evaluate the repeatability of diaphragm deflection in response to changes in pressure, measurement experiments were performed six times with a single specimen. The measurement the measurement experiments were performed six times with a single specimen. The measurement results are listed in Table 2 and displayed in Figure 12, which shows almost the same behavior. results are listed in Table 2 and displayed in Figure 12, which shows almost the same behavior. Table 2. 6 times measured results of diaphragm deflection according to varied pressure Table 2. 6 times measured results of diaphragm deflection according to varied pressure
Pressure Test 1 Test 2 Test 3 Test 4 1 Test 2(μm) Test 3 (μm) Test 4 (MPa) Pressure (μm) Test(μm) (MPa)
0.0
0.00
(µm)
0.26
12.5
0.0 0.5 0.57 2.5 2.85 5.0 7.5 5.70 10.0 12.58.60 15.011.55 17.5 20.014.27
0.00 0.570.82 2.85 3.14 5.70 8.606.05 11.55 14.278.90 16.14 11.81 17.69 18.9414.4
15.0
16.14
16.22
0.5 2.5 5.0 7.5 10.0
(µm)
0.26 0.82 3.14 6.05 8.90 11.81 14.4 16.22 17.71 18.94
(µm)
0.00
0.00 0.56 0.56 2.82 2.82 5.72 5.72 8.59 11.47 8.5914.15 11.4716.01 17.50 14.1518.80
16.01
(µm)
0.09
0.09 0.68 0.68 3.00 3.00 5.94 8.82 5.94 11.7 8.82 14.3 16.11 11.7 17.58 14.3 18.8
16.11
Test 5 Test 5 (μm) (µm)
0.00
Test 6
Standard
Average Test(μm) 6 Standard Deviation Average (µm) Deviation 0.02
0.00 0.59 0.59 2.85 2.85 5.70 8.58 5.70 11.48 8.58 14.16 16.00 11.48 17.47 14.16 18.76
0.02 0.61 0.61 2.93 2.93 5.85 8.72 5.85 11.63 14.218.72 16.0211.63 17.50 18.7614.21
16.00
16.02
0.06 0.64 2.93 5.83 8.70 11.61 14.25 16.08 17.57 18.83
0.06
0.10 0.64 0.10 0.12 2.93 0.15 5.83 0.14 0.13 8.70 0.10 11.61 0.09 0.11 14.25 0.09
16.08
0.10 0.10 0.12 0.15 0.14 0.13 0.10 0.09
17.5 17.69 17.71 curve 17.50 17.58 up to17.47 17.50 17.57 0.11 The deflection-pressure appears linear about 12.5 MPa lower than the simulated
yielding MPa and to remain even up18.83 to 20 MPa, 0.09 although 20.0 pressure 18.94of 13.75 18.94 18.80the elasticity 18.8 seems 18.76 18.76 the deflection rate against pressure becomes lower. This would be due to work hardening of the round-groove of the diaphragm. Through regression analysis linear relationship between The deflection-pressure curve appears linear up to about 12.5the MPa lower than the simulated
yielding pressure of 13.75 MPa and the elasticity seems to remain even up to 20 MPa, although the deflection rate against pressure becomes lower. This would be due to work hardening of the round-groove of the diaphragm. Through regression analysis the linear relationship between
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diaphragm deflection (δ) and pressure (P), δ (μm) = 1.142P (MPa) + 0.087, is of such good fitness that (δ) andof pressure δ (μm) = 1.142P (MPa) + 0.087, is of such good fitness that adiaphragm coefficient deflection of determination 0.999 is(P), obtained. diaphragm deflection (δ) and pressure δ (µm) = 1.142P (MPa) + 0.087, is of such good fitness that a a coefficient of determination of 0.999(P), is obtained. coefficient of determination of 0.999 is obtained.
Figure 12. Plot of measured diaphragm deflection with pressure. Figure 12. Plot of measured diaphragm deflection with pressure. Figure 12. Plot of measured diaphragm deflection with pressure.
5.2. Assembled Pressure Sensor 5.2. 5.2.Assembled AssembledPressure PressureSensor Sensor Figure 13 shows a cross section view of an assembled pressure sensor developed for this research. Figure of an an assembled assembledpressure pressuresensor sensordeveloped developedfor forthis thisresearch. research. Figure13 13shows showsaa cross cross section section view view of
Figure 13. 13. Cross Cross section section view view of of an an assembled assembled pressure pressure sensor. Figure sensor. Figure 13. Cross section view of an assembled pressure sensor.
Its round-groove a round-grooved diaphragm was welded on round-groovemachined machinedininboth bothfaces facesbybyturning, turning, a round-grooved diaphragm was welded Itsone round-groove machined in both faces by turning, a round-grooved diaphragm was welded the one end of housing. An An insulator waswas placed between diaphragm and and target plateplate to prevent heat on the end of housing. insulator placed between diaphragm target to prevent on the one end housing. An insulator was between diaphragm and target plate to prevent transfer from theofdiaphragm since the coil impedance is affected by temperature. heat transfer from the diaphragm since the coilplaced impedance is affected by temperature. heat transfer from the diaphragm since the coil impedance is affected by temperature. 5.3. Performance Evaluation Evaluation of of an an Assembled 5.3. Performance Assembled Pressure Pressure Sensor Sensor According According to to Applied Applied Pressure Pressure 5.3. Performance Evaluation of an Assembled Pressure Sensor According to Applied Pressure The performance of assembled sensor same equipment equipment as in Figure Figure 11 11 with with The performance of an an assembled sensor was was tested tested on on the the same as in aa HBT probe replaced by a multi-meter to measure the pressure output (voltage) depicted in Figure 3. performance of aanmulti-meter assembled to sensor was the tested on theoutput same equipment as in Figure 11 with HBTThe probe replaced by measure pressure (voltage) depicted in Figure 3. The measurement results are summarized in Table 33the andpressure the output is displayed displayed in Figure Figure 14a. a HBT probe replaced byare a multi-meter toin measure output (voltage) depicted in Figure 3. The measurement results summarized Table and the sensor sensor output is in 14a. The sensor output is linearly well related with input pressure up to 10 MPa and has maximum linearity The sensor measurement are summarized Tableinput 3 andpressure the sensor is displayed in maximum Figure 14a. The output results is linearly well relatedinwith upoutput to 10 MPa and has errors lesserrors than 0.16%, seen0.16%, in well Figure 14b, is much smaller than commercialized sensors. The sensor output linearly related with input pressure up is toother 10 MPa and hasthan maximum linearity lessisas than as seenwhich in Figure 14b, which much smaller other linearity errors less than 0.16%, as seen in Figure 14b, which is much smaller than other commercialized sensors. commercialized sensors.
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(a)
(b)
Figure 14. 14. Pressure sensor error(b) (b) along with applied pressure Figure Pressure sensoroutput output(a) (a) and and linearity linearity error along with applied pressure Table 3. Measurement sensoraccording according to varied pressure. Table 3. Measurementresults resultsof ofthe the assembled assembled sensor to varied pressure.
Pressure Pressure (MPa) (MPa) Impedance Change Impedance Change (mΩ) (mΩ) 0 1 2 3 4 5 6 7 8 9 10 6. Conclusions
Sensor Output (V) Sensor Output (V) Error (%)
0.000.00 0 0.00 0.00 1 11.66 0.98 0.98 11.66 2 23.45 1.99 23.45 1.99 3 35.29 2.99 35.29 4 47.14 3.99 2.99 5 58.96 4.99 3.99 47.14 6 70.75 5.99 58.96 4.99 7 82.52 6.99 70.75 8 94.30 7.99 5.99 9 106.14 8.99 6.99 82.52 10 118.10 10.00 94.30 7.99 * Error (%) = (Sensor output (V)—Expected value(V))/(10V) ˆ 100. 106.14 8.99 118.10 10.00
0.00 0.13 0.15 0.12 0.09 0.08 0.10 0.13 0.16 0.13 0.00
Error (%) 0.00 0.13 0.15 0.12 0.09 0.08 0.10 0.13 0.16 0.13 0.00
* Error (%) = (Sensor output (V)—Expected value(V))/(10V) × 100. An eddy current type pressure sensor with an outer diameter of 10 mm was developed and its characteristic was investigated so as to get appropriate values of the design parameters of the sensor’s 6. Conclusions two key components, i.e., diaphragm and coil. Some important findings are summarized as follows.
An eddy current type pressure sensor with an outer diameter of 10 mm was developed and its
(1) The coil designed in this research hasget a good linear relationship between the impedance change characteristic was investigated so as to appropriate values of the design parameters of the and the to targeti.e., platediaphragm up to 500 µm. sensor’s two keydistance components, and coil. Some important findings are summarized (2) A round-grooved diaphragm is much better for stable measurement of deflection even with much as follows. less deflection compared to a flat type one. (1) (3) The Depending coil designed in this research has aand good linear relationship between theparameters impedanceofchange on the required pressure sensitivity measurement, the design a and round-grooved the distance todiaphragm target plate up to 500 μm. can be selected. (2) (4) A round-grooved betterlinearity for stable measurement deflection even The developed diaphragm sensor showsisamuch very good up to 10 MPa withof linearity errors less with much less deflection compared to a flat type one. than 0.16%.
(3) Depending on the required pressure and sensitivity measurement, the design parameters of a Acknowledgments: This work was supported by the Human Resource Training Program for Regional round-grooved diaphragm can be selected. Innovation and Creativity through the Ministry of Education and National Research Foundation of (4) Korea The developed sensor shows a very good linearity up to 10 MPa with linearity errors less than (2013H1B8A2032233). 0.16%. Author Contributions: H.R. Lee and G.S. Lee designed and performed the experiments; H.R. Lee and J.H. Ahn analyzed the data and wrote the paper; G.S. Lee and H.Y. Kim built up the research project.
Acknowledgments: This was supported byofthe Human Resource Training Program for Regional Conflicts of Interest: Thework authors declare no conflict interest. Innovation and Creativity through the Ministry of Education and National Research Foundation of Korea (2013H1B8A2032233). Author Contributions: H.R. Lee and G.S. Lee designed and performed the experiments; H.R. Lee and J.H. Ahn analyzed the data and wrote the paper; G.S. Lee and H.Y. Kim built up the research project. Conflicts of Interest: The authors declare no conflict of interest.
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David, R.R. Engine Combustion: Pressure Measurement and Analysis; SAE International: Warrendale, PA, USA, 2010. André, V.B.; José, A.V.; Luiz, F.M. Chapter 2: Internal Combustion Engine Indicating Measurements. In Applied Measurement Systems; Zahurul, H., Ed.; InTech: Rijeka, Croatia, 2012; pp. 23–44. Perraud, E. Theoretical model of performance of a silicon piezoresistive pressure sensor. Sens. Actuator. A Phys. 1996, 57, 245–252. [CrossRef] Barlian, A.A.; Park, W.T.; Mallon, J.R.; Rastegar, A.J.; Pruitt, B.L. Review: Semiconductor piezoresistance for microsystems. IEEE Proc. 2009, 97, 513–552. [CrossRef] [PubMed] Kim, J.C.; Lee, J.C.; Choi, B.K. Fabrication and characterization of strain gauge integrated polymeric diaphragm pressure sensors. Int. J. Precis. Eng. Manuf. 2013, 14, 2003–2008. [CrossRef] Berg, J.V.; Ziermann, R.; Reichert, W.; Obermeier, E.; Eickhoff, M.; Krotz, G. Measurement of the cylinder pressure in combustion engines with a piezoresistive β-SiC-on-SOI pressure sensor. In Proceeding of the 1998 High Temperature Electronics Conference, Albuquerque, NM, USA, 14–18 June 1998; pp. 245–249. Otmani, R.; Benmoussa, N.; Benyoucef, B. The thermal drift characteristics of piezoresistive pressure sensor. Phys. Proced. 2011, 21, 47–52. [CrossRef] Li, Q; Ding, F. Novel displacement eddy current sensor with temperature compensation for electro hydraulic valves. Sens. Actuator. A Phys. 2005, 122, 83–87. [CrossRef] Wang, H.; Feng, Z. Ultrastable and highly sensitive eddy current displacement sensor using self-temperature compensation. Sens. Actuator. A Phys. 2013, 203, 362–368. [CrossRef] Kim, T.O.; Lee, G.S.; Kim, H.Y.; Ahn, J.H. Modeling of eddy current sensor using geometric and electromagnetic data. J. Mech. Sci. Technol. 2007, 21, 488–498. [CrossRef] Nabavi, M.R.; Nihtianov, S.N. Design strategies for eddy-current displacement sensor systems: Review and recommendations. IEEE Sens. J. 2012, 12, 3346–3355. [CrossRef] Wang, F.; Shao, Z.; Xie, J.; Hu, Z.; Luo, H.; Hu, Y. Extrinsic fabry-perot underwater acoustic sensor based on micromachined center-embossed diaphragm. J. Lightwave Technol. 2014, 32, 4628–4636. [CrossRef] © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
Design of Diaphragm and Coil for Stable Performance of an Eddy Current Type Pressure Sensor Hyo Ryeol Lee 1 , Gil Seung Lee 2 , Hwa Young Kim 3 and Jung Hwan Ahn 1, * 1 2 3
*
School of Mechanical Engineering, Pusan National University, 2, Busandaehak-ro 63 beon gil, Geumjeong-gu, Busan 46241, Korea; [email protected] NOVASEN Co., LTD, 25, Bansong-ro 525 beon gil, Haeundae-gu, Busan 48002, Korea; [email protected] Research Institute of Mechanical Technology, Pusan National University, 2, Busandaehak-ro 63 beon gil, Geumjeong-gu, Busan 46241, Korea; [email protected] Correspondence: [email protected]; Tel.: +82-51-510-3087
Academic Editor: Vittorio M. N. Passaro Received: 7 April 2016; Accepted: 22 June 2016; Published: 1 July 2016
Abstract: The aim of this work was to develop an eddy current type pressure sensor and investigate its fundamental characteristics affected by the mechanical and electrical design parameters of sensor. The sensor has two key components, i.e., diaphragm and coil. On the condition that the outer diameter of sensor is 10 mm, two key parts should be designed so as to keep a good linearity and sensitivity. Experiments showed that aluminum is the best target material for eddy current detection. A round-grooved diaphragm is suggested in order to measure more precisely its deflection caused by applied pressures. The design parameters of a round-grooved diaphragm can be selected depending on the measuring requirements. A developed pressure sensor with diaphragm of t = 0.2 mm and w = 1.05 mm was verified to measure pressure up to 10 MPa with very good linearity and errors of less than 0.16%. Keywords: eddy current; pressure sensor; round-groove; impedance change
1. Introduction Pressure sensors have been widely used in such areas as chemical plants, automobile/ship, aviation, air conditioning, biomedical, and hydraulics industries, etc., where pressure is an important monitored parameter. In chemical plants, pressure sensors are used to monitor partial pressures of gases in industrial units so that large chemical reactions take place under precisely controlled environmental conditions. In automobile and ship engines where it forms an integral part of the engine and its safety, the dynamic cylinder pressure in the combustion chamber is monitored to regulate the power that the engine must deliver to achieve suitable speeds. Engine combustion pressure analysis is a fundamental measurement technique applied universally in the research and development of reciprocating combustion engines [1,2]. The three types of pressure sensors—metal thin-film, ceramic thick-film and piezoresistive—have been most commonly used so far. Among them, silicon piezoresistive MEMS pressure sensors as micromachined devices, are widely used in various systems because of their high accuracy, high sensitivity, and excellent linearity over a wide range of pressures in addition to their low cost and small size [3–7]. Pressure sensors are often exposed to harsh environments with contaminants—such as dust, dirt, and oil—and severe temperature changes, which cause the silicon based sensors to have poorer accuracy, sensitivity, and linearity. In an effort to develop a robust pressure sensor that can endure such harsh environments, an eddy current type pressure sensor is proposed in this research, due to the high resolution, high frequency response, and robustness against contaminants of an eddy current sensor (ECS). ECS are known to Sensors 2016, 16, 1025; doi:10.3390/s16071025
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known to be very competitive for displacement measurement harsh environments under dynamic be very competitive for displacement measurement in harshin environments under dynamic and static and staticFurthermore, pressures. Furthermore, active compensation temperature compensation reduce theeffect temperature pressures. active temperature to reduce the to temperature on ECS has effectproved on ECSpossible has beenthrough proved previous possible through been researchprevious [8,9]. research [8,9]. An eddy current type pressure sensor should be composed two transducer An eddy current type pressure sensor should be composed of twoof transducer parts—aparts—a diaphragm to convert pressure into diaphragm and displacement and andetector eddy current detector to todiaphragm convert pressure into diaphragm displacement an eddy current to convert diaphragm convert diaphragm displacement into eddy current voltage. In this paper, particularly, it is displacement into eddy current voltage. In this paper, particularly, it is described how the design described how the design parameters of these two key components affect the pressure sensor parameters of these two key components affect the pressure sensor performance—i.e., pressure range performance—i.e., pressure range and sensitivity—and what type of diaphragm measure the eddy and sensitivity—and what type of diaphragm measure the eddy current sensor displacement most current sensor displacement most stably and precisely. Finally, the characteristic of an assembled stably and precisely. Finally, the characteristic of an assembled sensor of diaphragm and eddy current sensor of diaphragm and eddy current probe was tested in the real pressure chamber and the probe was tested in the real pressure chamber and the feasibility for practical use was checked. feasibility for practical use was checked.
2. Principle and Structure of Eddy Current Type Pressure Sensor 2. Principle and Structure of Eddy Current Type Pressure Sensor 2.1. Working Principle of a Typical Pressure Sensor 2.1. Working Principle of a Typical Pressure Sensor A typical—generally a force collector type—pressure sensor is composed of a diaphragm A typical—generally a force collector type—pressure sensor is composed of a diaphragm and a and a probe, where a pressurized gas from a high pressure chamber causes the diaphragm to be probe, where a pressurized gas from a high pressure chamber causes the diaphragm to be elastically elastically deflected. And the probe then detects the diaphragm deflection. According to detection deflected. And the probe then detects the diaphragm deflection. According to detection principles, principles, there are several kinds of pressure sensors such as strain gauge type, piezoresistive there are several kinds of pressure sensors such as strain gauge type, piezoresistive type, type, piezoelectric capacitive electromagnetic optical In strain gauge, piezoelectric type, type, capacitive type,type, electromagnetic type, type, and and optical type.type. In strain gauge, piezoresistive, types,the theunit unitthat thatdetects detects the diaphragm deflection is connected piezoresistive,and and piezoelectric piezoelectric types, the diaphragm deflection is connected to to ororbuilt into the diaphragm as a thin layer-bonded, and sputtered etc., while the others are separately built into the diaphragm as a thin layer-bonded, and sputtered etc., while the others are located nearlocated the diaphragm detect its deflection in deflection non-contact Piezoresistive sensors—based separately near the to diaphragm to detect its in mode. non-contact mode. Piezoresistive on measurementon of measurement the diaphragm a good feature aingood that feature the resistance sensors—based of displacement—have the diaphragm displacement—have in that change the for a given amount of pressure is much greater than in metal strain gauges. resistance change for a given amount of pressure is much greater than in metal strain gauges. The type pressure pressuresensor sensorwhich whichisistotobebe developed research is one of the Theeddy eddy current current type developed in in thisthis research is one of the electromagnetic type type with the diaphragm byby a sensor coil. electromagnetic diaphragmdeflection deflectionbeing beingdetected detected a sensor coil. 2.2. PressureSensor Sensor 2.2.Structure Structure of of Eddy Eddy Current Type Pressure Thestructure structure of an pressure sensor to be in this is shown in The aneddy eddycurrent currenttype type pressure sensor todeveloped be developed instudy this study is shown is is composed of of housing, diaphragm, insulator, target plate andand sensor coil coil inFigure Figure1a. 1a.The Thesensor sensor composed housing, diaphragm, insulator, target plate sensor (eddycurrent currentprobe). probe). The diaphragm C22 to to prevent sensors from being rusted (eddy diaphragmisismade madeofofHastelloy Hastelloy C22 prevent sensors from being rusted even under harsh conditions. When pressurized gas from a high pressure chamber is applied to the even under harsh conditions. When pressurized gas from a high pressure chamber is applied to the sensor,as asshown shown in in Figure Figure 1b, moves thethe insulator andand the the target sensor, 1b, the the diaphragm diaphragmisisdeflected, deflected,which which moves insulator target plate; the resulting displacement of the target plate is then detected by the coil. plate; the resulting displacement of the target plate is then detected by the coil.
(a)
(b)
Figure 1. 1. Eddy structure; (b)(b) on on sitesite application. Figure Eddy current currenttype typepressure pressuresensor: sensor:(a)(a) structure; application.
Therefore, the two key parts of an eddy current type pressure sensor—diaphragm and Therefore, the two key electrically parts of an currenttotype pressure sensor—diaphragm coil—should be well-matched andeddy mechanically enhance its sensitivity and linearity inand coil—should well-matched mechanically to enhance its sensitivity andislinearity the aspect ofbe tiny displacementelectrically acquisitionand on the condition that the diaphragm deflection within in the aspect of tiny displacement acquisition on the condition that the diaphragm deflection is within
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the elastic limit. For compatible use with other types of commercialized pressure sensors, the the elastic limit. For compatible use with other types of commercialized pressure sensors, the diameter the sensor is fixed at 10with mm.other types of commercialized pressure sensors, the diameter the elastic of limit. For compatible use diameter of the sensor is fixed at 10 mm. of the sensor is fixed at 10 mm. 3. Design of Sensor Coil 3. 3. Design Designof ofSensor SensorCoil Coil 3.1. Design Parameters 3.1. 3.1. Design Design Parameters Parameters Figure 2 shows a schematic diagram of the sensor coil unit—eddy current probe. Figure schematic diagram diagram of of the the sensor sensor coil coil unit—eddy unit—eddy current current probe. probe. Figure 22 shows shows aa schematic
Figure 2. 2. Geometry Geometry of of coil coil against against target target plate. plate. Figure Figure 2. Geometry of coil against target plate.
Many design parameters are geometric ones, such as: outer diameter (a); inner diameter (b); Many design parameters parameters are geometric ones, such as: outer outer diameter diameter (a); (a); inner inner diameter diameter (b); (b); and height (c) of the coil unit; coil diameter and gap (d) between coil and target plate. Other design and height (c) of the coil unit; coil diameter and gap (d) between coil and target plate. Other design parameters include magneto-electrical ones such as conductivity, temperature resistance and parameters includemagneto-electrical magneto-electrical ones as conductivity, temperature resistance and parameters include ones suchsuch as conductivity, temperature resistance and magnetic magnetic permeability. It is known that self-inductance and mutual-inductance of the sensor coil are magnetic permeability. is known that self-inductance and mutual-inductance of coil the sensor coil are permeability. It is knownIt that self-inductance and mutual-inductance of the sensor are dependent dependent only on geometrical parameters [10]. Copper is used as coil material to get sensitivity and dependent only on geometrical Copper is used as coil to getand sensitivity and only on geometrical parameters parameters [10]. Copper[10]. is used as coil material to material get sensitivity linearity of linearity of detection as high as possible because of its high conductivity and low temperature resistance. linearity as high asbecause possibleofbecause its high conductivity low temperature resistance. detectionofasdetection high as possible its highofconductivity and low and temperature resistance. From the viewpoint of sensor performance referring to the past works, the inner and outer From the viewpoint of sensor performance referring referring to the past works, the inner and outer diameter should be minimized and maximized, respectively, and the height should be minimized as diameter should height should bebe minimized as should be beminimized minimizedand andmaximized, maximized,respectively, respectively,and andthe the height should minimized much as geometrically possible [10,11]. Since the coil should detect the diaphragm deflection much as as geometrically possible as much geometrically possible[10,11]. [10,11].Since Sincethe thecoil coilshould shoulddetect detect the the diaphragm diaphragm deflection appropriately, it is desirable that the diameter of the probe is limited to a maximum of 5 mm. The appropriately, itit isis desirable of of thethe probe is limited to atomaximum of 5 of mm. The desirablethat thatthe thediameter diameter probe is limited a maximum 5 mm. inner diameter and height should be at least 2 mm and 1 mm respectively for convenience of coil inner diameter and height should be at be least mm2and mm 1respectively for convenience of coil The inner diameter and height should at 2least mm1 and mm respectively for convenience manufacturing. The copper wire diameter is at most 0.1 mm, as anything larger reduces the manufacturing. The copper wire wire diameter is atismost 0.1 0.1 mm, asasanything of coil manufacturing. The copper diameter at most mm, anythinglarger larger reduces reduces the sensitivity and the anything smaller is too sensitive to disturbance. The gap is fixed at 0.3 mm to sensitivity and andthe theanything anythingsmaller smaller sensitive to disturbance. is fixed 0.3tomm to is is tootoo sensitive to disturbance. TheThe gap gap is fixed at 0.3at mm avoid avoid the effect of parasitic capacitance largely generated in ranges less than 0.2 mm. avoid the of effect of parasitic capacitance generated in ranges less0.2 than 0.2 mm. the effect parasitic capacitance largelylargely generated in ranges less than mm. 3.2. Signal Processing for Eddy Current Detection 3.2. Signal Processing for Eddy Current Detection The principle of eddy current detection is to detect a variation of coil impedance caused by a The principle of eddy current detection is to detect a variation of coil impedance caused by a change of the gap between coil and target plate. Figure 3 shows a signal processing block for between coilcoil andand target plate.plate. Figure 3 shows a signalaprocessing block for block detecting change of ofthe thegap gap between target Figure 3 shows signal processing for detecting a change of coil impedance and removing disturbances from the acquired R and L impedances. a change of coil impedance and removing disturbances from the acquired R and L impedances. detecting a change of coil impedance and removing disturbances from the acquired R and L impedances.
Figure Figure 3. 3. Functional Functional block block diagram diagram for for signal signal processing processing of of eddy eddy current current acquisition. acquisition. Figure 3. Functional block diagram for signal processing of eddy current acquisition.
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Given kHz) Given the the optimal optimal frequency frequency (250 (250 kHz) kHz) and and amplitude amplitude of of the the coil coil excitation excitation current current (25 (25 mA) mA) by by Given the optimal frequency (250 and amplitude of the coil excitation current (25 mA) by DSP (Digital Signal Processor), DDS (Direct Digital Synthesizer) circuit transmits corresponding DSP (Digital (Digital Signal Signal Processor), Processor),aaaDDS DDS(Direct (DirectDigital DigitalSynthesizer) Synthesizer)circuit circuittransmits transmits aaacorresponding corresponding DSP voltage signal to current pump which supplies corresponding current to the coil. Eddy current voltage signal signal to to aaa current current pump pumpwhich whichsupplies supplies aaacorresponding corresponding current current to tothe thecoil. coil. Eddy Eddy current current voltage electromagnetically induced in the target plate placed close to the sensor coil causes a change of electromagneticallyinduced inducedinin target plate placed the sensor coil causes a of change of electromagnetically thethe target plate placed closeclose to thetosensor coil causes a change voltage voltage in sensor coil, which is by detector is to of the voltage in the thecoil, sensor coil,is which is detected detected by aa voltage voltage detector and is converted converted to aa change change ofcoil the in the sensor which detected by a voltage detector and isand converted to a change of the coil aa demodulator circuit. coil impedance impedance by demodulator circuit. impedance by aby demodulator circuit. 3.3. 3.3. Experiment Experiment Evaluation Evaluation of of Sensor Sensor Coil Coil The characteristics of the the sensor coilare areexamined examinedalong alongwith withthe thegap gapprecisely preciselycontrolled controlledon onaa The characteristics characteristics of sensor coil coil are examined along with the gap precisely controlled on measurement a measurementsystem, system,as shownin Figure4, whichis is composed composed of measurement system, asasshown shown ininFigure Figure 4,4,which which is composed of aa precision precision positioning positioning unit unit the gap and an LCR (inductance (L), capacitance (C), and resistance (R)) meter controlling the gap and an LCR (inductance (L), capacitance (C), and resistance (R)) meter measuring controlling the gap and an LCR (inductance (L), capacitance (C), and resistance (R)) meter measuring coil coil impedance. measuring coil impedance. impedance.
Figure with gap controlled. Figure 4. 4. Coil Coil impedance impedance measurement measurement system system with with gap gapcontrolled. controlled. Figure 4. Coil impedance measurement system
In In order order to to find find out out the the best best target target material material in in the the magneto magneto electrical electrical aspect, aspect, five five materials—copper, materials—copper, In order to find out the best target material in the magneto electrical aspect, five materials—copper, aluminum, brass, bronze and stainless steel—are considered as alternatives. aluminum, brass, bronze and stainless steel—are considered as alternatives. The The results results of of aluminum, brass, bronze and stainless steel—are considered as alternatives. The results of impedance impedance impedance measurements measurements for for the the five five target target materials materials are are displayed displayed in in the the R-L R-L impedance impedance plane, plane, measurements for the five target materials are displayed in the R-L impedance plane, which is specially which which is is specially specially adopted adopted to to improve improve the the typical typical measurement measurement method method based based on on amplitude amplitude data data adopted to improve the typical measurement method based on amplitude data acquired from an LCR acquired from an LCR meter, as shown in Figure 5. acquired from an LCR meter, as shown in Figure 5. meter, as shown in Figure 5.
Figure Figure 5. 5. Coil Coil impedance impedance measurement measurement system system with with gap gapcontrolled. controlled.
Both impedance changes against gap—ΔL/Δd, Both impedance impedancechanges changesagainst againstgap—∆L/∆d, gap—ΔL/Δd, ΔR/Δd—are ΔR/Δd—are almost almost linear linear in range less less than than Both ∆R/∆d—are in aa range range less than 500 μm, although they become nonlinear as the gap increases beyond that. As R is actually affected 500 μm, although they become nonlinear as the gap increases beyond that. As R is actually affected 500 µm, although they become nonlinear as the gap increases beyond that. As R is actually affected more aluminum is over the other materials as more than than LL by by temperature temperature change, copper oror aluminum is preferred preferred over thethe other materials as the the more than temperaturechange, change,copper copperor aluminum is preferred over other materials as target material, because of higher conductivity and lower temperature sensitivity in addition to target material, because of higher conductivity and lower temperature sensitivity in addition to the target material, because of higher conductivity and lower temperature sensitivity in to nonmagnetic nonmagnetic metals metals property. property. Furthermore, Furthermore, in in terms terms of of corrosion-resistance, corrosion-resistance, aluminum aluminum is is much much better better than than copper. copper. Therefore, Therefore, Al6061 Al6061 is is chosen chosen as as the the target target material. material.
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property. Furthermore, in terms of corrosion-resistance, aluminum is much 5 of better 11 than copper. Therefore, Al6061 is chosen as the target material.
4. Design of Round-Grooved Diaphragm 4. Design of Round-Grooved Diaphragm 4.1. Shape of Diaphragm for Stably Measurable Deflection 4.1. Shape of Diaphragm for Stably Measurable Deflection The diaphragm plays an important role in the elastic deflection in response to an applied The diaphragm plays an important role in the elastic deflection in response to an applied pressure. pressure. The shape and geometrical dimensions of a diaphragm should be well-chosen so that its The shape and geometrical dimensions of a diaphragm should be well-chosen so that its deflection deflection matches the performance of the sensor coil designed in Section 3. A small sensor diameter matches the performance of the sensor coil designed in Section 3. A small sensor diameter of 10 mm of 10 mm limits the diaphragm diameter up to 8.6 mm at most. Furthermore, its deflection being limits the diaphragm diameter up to 8.6 mm at most. Furthermore, its deflection being measured more measured more accurately by the sensor coil, the diaphragm should be designed so that some area of accurately by the sensor coil, the diaphragm should be designed so that some area of its central part its central part goes up and down near flatly as the acting pressure goes up or down. goes up and down near flatly as the acting pressure goes up or down. What shape of the diaphragm is needed to meet such restricted requirements? Wang et al. What shape of the diaphragm is needed to meet such restricted requirements? Wang et al. revealed that the center-embossed diaphragm is superior in optical characteristics to the flat one revealed that the center-embossed diaphragm is superior in optical characteristics to the flat one which which is seriously deteriorated by the central area being wrapped under high hydrostatic pressure [12]. is seriously deteriorated by the central area being wrapped under high hydrostatic pressure [12]. In this study, a round-grooved, square-grooved diaphragm—like a center-embossed one—shown in In this study, a round-grooved, square-grooved diaphragm—like a center-embossed one—shown in Figure 6a,b, are considered for rather flatter deflection of the central part, which helps the eddy Figure 6a,b, are considered for rather flatter deflection of the central part, which helps the eddy current current probe measure a gap more stably and precisely. The design parameters, as depicted in probe measure a gap more stably and precisely. The design parameters, as depicted in Figure 6a,b, Figure 6a,b, are the width and thickness of the groove while the thickness of the diaphragm and the are the width and thickness of the groove while the thickness of the diaphragm and the distance of the distance of the groove from the wall are set as 1 mm and 1.5 mm, respectively. For deflection groove from the wall are set as 1 mm and 1.5 mm, respectively. For deflection comparison a flat type, comparison a flat type, shown in Figure 6c, is used as a reference. shown in Figure 6c, is used as a reference.
(a)
(b)
(c) Figure 6. Shape and and design parameter of three types of diaphragm: (a) round-grooved; (b) square Figure 6. Shape design parameter of three types of diaphragm: (a) round-grooved; (b) square grooved; (c) flat. grooved; (c) flat.
4.2. 4.2. Analysis of Diaphragm Deflection Analysis of Diaphragm Deflection Structural analyses were carried out out to investigate the the effects of design parameters on on the the Structural analyses were carried to investigate effects of design parameters maximum deflection andand stress of the diaphragm by increasing pressure in 0.25 MPa increments maximum deflection stress of the diaphragm by increasing pressure in 0.25 MPa increments until yielding occurs using ANSYS Workbench V15. The material properties of Hastelloy C22 are are until yielding occurs using ANSYS Workbench V15. The material properties of Hastelloy C22 elastic modulus 206 GPa, yield strength 438 MPa, and Poisson ratio 0.3. elastic modulus 206 GPa, yield strength 438 MPa, and Poisson ratio 0.3. With thickness (t) 0.2 mmmm andand pressure 2.5 2.5 MPa, Figure 7 shows a typical deflection behavior With thickness (t) 0.2 pressure MPa, Figure 7 shows a typical deflection behavior along the radial axis of the round-grooved, square-grooved, and flat diaphragms, respectively. along the radial axis of the round-grooved, square-grooved, and flat diaphragms, respectively. TheThe point 0 indicates the diaphragm. diaphragm.The Thedeflection deflection whole diaphragm point 0 indicatesthe thecenter center of of the of of thethe whole diaphragm surface surface under 2.5 MPa is displayed on the axis of −4.3~4.3 mm. While a much larger deflection with under 2.5 MPa is displayed on the axis of ´4.3~4.3 mm. While a much larger deflection with no flat no flat central is seen the flat the type, the grooved type deflects much with quite a flat in central part part is seen in theinflat type, grooved type deflects much less withless quite a flat deflection deflection in the center part compared to the flat type, which makes the eddy current probe measure the center part compared to the flat type, which makes the eddy current probe measure the diaphragm the deflection diaphragmwith deflection with more and precision. more stability andstability precision. Figure 8 shows comparisons of the simulated results in terms of maximum deflection andand Figure 8 shows comparisons of the simulated results in terms of maximum deflection maximum stress at each pressure between flat,flat, square-grooved, andand round-grooved types with maximum stress at each pressure between square-grooved, round-grooved types with increasing pressure at the same thickness. increasing pressure at the same thickness.
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(a) (a) (a)
(b) (b) (b)
(c) (c) (c) Figure 7. 7. Deflection Deflection behavior behavior along along the the diaphragm diaphragm axis axis with with tt == 0.2 0.2 mm, mm, w w == 1.05 1.05 mm, mm, P P = 2.5 2.5 MPa: MPa: Figure Figure 7. Deflection behavior along thethe diaphragm axisaxis withwith t = 0.2 w =w 1.05 mm,mm, P = 2.5 Figure 7. Deflection behavior along diaphragm t = mm, 0.2 mm, = 1.05 P ==MPa: 2.5 MPa: (a) round-grooved; (b) square-grooved; (c) flat. (a) round-grooved; (b) square-grooved; (c)flat. flat. (a)(a) round-grooved; (b)(b) square-grooved; (c) (c) flat. round-grooved; square-grooved;
(a) (a) (a)
(b) (b) (b)
Figure 8.Comparison Comparison ofmaximum maximum deflection/stress against pressure among flat, square-grooved, Figure deflection/stress against pressure among flat, flat, and Figure 8. 8. Comparison ofofmaximum deflection/stress against pressure among flat, square-grooved, Figure 8. Comparison of maximum deflection/stress against pressure among square-grooved, and round-grooved types with w = 1.05 mm: (a) maximum deflection, (b) maximum stress. round-grooved types with w = 1.05 mm: (a) maximum deflection; (b) maximum stress. and round-grooved types with w = 1.05 mm: (a) maximum deflection, (b) maximum stress. and round-grooved types with w = 1.05 mm: (a) maximum deflection, (b) maximum stress.
(b) (b) (b) (a) (a) (a) Figure 9. Analysis results at t at = 0.2= mm, w0.2 =w 1.05 mm, P==1.05 12.5 MPa: (a) stress distribution; yield points. Figure 9. Analysis Analysis results 0.2 mm, 1.05 mm, P == 12.5 12.5 MPa: (a) stress distribution; (b)distribution; yield points. Figure Analysis results t = wmm, mm,MPa: P = 12.5 MPa: (a) (b) stress Figure 9. results at tt =at 0.2 mm, wmm, == 1.05 P (a) stress distribution; (b) yield points. (b) yield points.
ForFor t = 0.20.2 mm, thethe flatflat type hashas a yielding pressure around at 3.55 MPa, which is much lower mm, type yielding pressure around at 3.55 3.55 MPa, which is much much lower For tt == 0.2 mm, the flat type has aa yielding pressure around at MPa, which is lower than the round-grooved type at 13.75 MPa or the square-grooved type at 5.87 MPa. For t = 0.3 mm, than the round-grooved type at 13.75 MPa or the square-grooved type at 5.87 MPa. For t = 0.3 mm, than the round-grooved type at 13.75 MPa or the square-grooved type at 5.87 MPa. For t = 0.3 mm,
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the flat type has a yielding pressure around 8.15 MPa, which is much lower than the round-grooved For20.88 t = 0.2MPa mm,orthe flat type has a yielding around at 3.55 MPa, which is much lower than type at square-grooved type at pressure 12.87 MPa. Figure 9 shows the numerical analytical the round-grooved type at 13.75 MPa or the square-grooved type at 5.87 MPa. For t = 0.3 mm, the flat results of how the stress is distributed on the round-grooved diaphragm and how the yielding stress type has a yielding around MPa,ofwhich is much lower than type at occurs at points 1, 2,pressure 3, and 4 on both 8.15 surfaces the round-groove with t =the 0.2round-grooved mm at 12.5 MPa. 20.88The MPa or square-grooved type at 12.87 MPa. Figure shows measurable the numerical analytical results of round-grooved type has a wider pressure range 9linearly prior to yielding than how the stress is distributed on the round-grooved diaphragm and how the yielding stress occurs at the square-grooved and the flat types. Thus, only the round-grooved type is considered for further points 1, 2, 3, of and on bothofsurfaces of the round-groove with t = 0.2 mm at 12.5 MPa. investigation the4 effects the design parameters. The round-grooved type has a wider pressure range to yielding Table 1 summarizes the 16 analysis cases with the twolinearly designmeasurable parameters prior properly selectedthan for the round-grooved square-grooved diaphragm and the flat and types. Thus, only theanalytical round-grooved is considered for further the their numerical resultstype of maximum pressure and investigation of the effects of the design parameters. deflection. In addition, the sensitivity is calculated as maximum deflection/maximum pressure. Table 1 summarizes the 16 analysis cases with the two design parameters properly selected for the Table 1. 16 diaphragm cases of properly selected design parameters withofanalytical of maximum round-grooved and their numerical analytical results maximumresults pressure and deflection. pressure,the maximum deflection and sensitivity. In addition, sensitivity is calculated as maximum deflection/maximum pressure. Max. Pressurewith Max. Deflection Sensitivity Table 1. 16 Thichness cases of properlyWidth selected design parameters analytical results of maximum pressure, (mm) (mm) (MPa) (μm) (μm/MPa) maximum deflection and sensitivity. Case 1 0.20 1.05 13.75 6.65 0.48 Case 2 0.20Thichness 1.30 Width 9.68Pressure 7.15 0.74 Max. Max. Deflection Sensitivity Case 3 0.20 (mm) 1.55 (mm) 7.38 8.38 1.14 (MPa) (µm) (µm/MPa) Case 4 Case 1 0.20 0.20 1.80 1.05 6.38 10.74 13.75 6.65 0.48 1.68 Case 5 Case 2 0.25 0.20 1.05 1.30 16.88 7.14 9.68 7.15 0.74 0.42 7.38 8.38 1.14 0.58 Case 6 Case 3 0.25 0.20 1.30 1.55 12.88 7.49 Case 4 0.20 1.80 6.38 10.74 1.68 Case 7 0.25 1.55 10.25 8.41 0.82 Case 5 0.25 1.05 16.88 7.14 0.42 Case 8 Case 6 0.25 0.25 1.80 1.30 8.88 10.11 12.88 7.49 0.58 1.14 Case 9 Case 7 0.30 0.25 1.05 1.55 20.88 8.03 10.25 8.41 0.82 0.38 8.88 10.11 1.14 0.49 Case 10 Case 8 0.30 0.25 1.30 1.80 16.38 8.06 20.88 8.03 0.38 0.65 Case 11 Case 9 0.30 0.30 1.55 1.05 12.88 8.35 Case 10 0.30 1.30 16.38 8.06 0.49 Case 12 Case 11 0.30 0.30 1.80 1.55 11.88 10.13 0.82 12.88 8.35 0.65 Case 13 Case 12 0.35 0.30 1.05 1.80 25.00 9.05 11.88 10.13 0.82 0.36 25.00 9.05 0.36 0.44 Case 14 Case 13 0.35 0.35 1.30 1.05 18.88 8.29 18.88 8.29 0.44 0.55 Case 15 Case 14 0.35 0.35 1.55 1.30 15.88 8.69 Case 15 0.35 1.55 15.88 8.69 0.55 Case 16 0.35 1.80 14.38 9.90 0.69 Case 16
0.35
1.80
(a)
14.38
9.90
0.69
(b)
Figure 10. Maximum pressurepressure (a) and maximum deflection (b)deflection with design of round-groove Figure 10. Maximum (a) and maximum (b)parameters with design parameterstype. of round-groove type.
Figure 10 shows the maximum pressure and deflection at varied thickness and width. Figure 10a indicates that as the round-groove thickness increases and width decreases, the maximum pressure Figure 10 shows the maximum pressure and deflection at varied thickness and width. Figure 10a prior to yielding becomes larger. The change rates affected by both parameters are almost equal. indicates that as the round-groove thickness increases and width decreases, the maximum pressure Figure 10b shows that the round-groove width affects the deflection much more than the prior to yielding becomes larger. The change rates affected by both parameters are almost equal. round-groove thickness but does not show a consistent trend when thickness increased. So it would Figure 10b shows that the round-groove width affects the deflection much more than the round-groove be better to use the sensitivity calculated in Table 1 as an indicator to compare how sensitive the thickness but does not show a consistent trend when thickness increased. So it would be better to diaphragm is to pressure. The thinner and wider one is more sensitive to pressure change. The larger use the sensitivity calculated in Table 1 as an indicator to compare how sensitive the diaphragm is to
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pressure. The thinner and wider one is more sensitive to pressure change. The larger width increases width increases the sensitivity much more than the thickness but lowers the measurable maximum the sensitivity much more than the thickness but lowers the measurable maximum pressure too much, pressure too much, as shown in Figure 10a. as shown in Figure 10a. Based on the measurement requirements of pressure and sensitivity, the design parameters of Based on the measurement requirements of pressure and sensitivity, the design parameters of diaphragm can be selected. In this study the round-groove diaphragm with a groove 0.2 mm thick diaphragm can be selected. In this study the round-groove diaphragm with a groove 0.2 mm thick and and 1.05 mm wide is chosen to investigate its performances throughout the experiment. 1.05 mm wide is chosen to investigate its performances throughout the experiment.
5. Sensor 5. SensorPerformance PerformanceEvaluation Evaluationthrough throughPressure PressureMeasurement MeasurementTest Test Applied Pressure Pressure 5.1. Measurements of Diaphragm Deflection According to Applied It is necessary to examine how well the diaphragm deflection deflection relates relates to the applied applied pressure. pressure. Figure 11 shows a setup of an apparatus composed of a pressure chamber, dead weight tester and Figure 11 shows a setup of an apparatus composed of a pressure chamber, dead weight tester and half half bridge transformer (HBT) forexperiment. the experiment. bridge transformer (HBT) for the
Figure equipment with with pressure pressure varied varied by by dead dead weights. weights. Figure 11. 11. Diaphragm Diaphragm deflection deflection measurement measurement equipment
The deflection is measured by a HBT with pressure increased by 2.5 MPa increments up to 20 The deflection is measured by a HBT with pressure increased by 2.5 MPa increments up to MPa. To evaluate the repeatability of diaphragm deflection in response to changes in pressure, the 20 MPa. To evaluate the repeatability of diaphragm deflection in response to changes in pressure, measurement experiments were performed six times with a single specimen. The measurement the measurement experiments were performed six times with a single specimen. The measurement results are listed in Table 2 and displayed in Figure 12, which shows almost the same behavior. results are listed in Table 2 and displayed in Figure 12, which shows almost the same behavior. Table 2. 6 times measured results of diaphragm deflection according to varied pressure Table 2. 6 times measured results of diaphragm deflection according to varied pressure
Pressure Test 1 Test 2 Test 3 Test 4 1 Test 2(μm) Test 3 (μm) Test 4 (MPa) Pressure (μm) Test(μm) (MPa)
0.0
0.00
(µm)
0.26
12.5
0.0 0.5 0.57 2.5 2.85 5.0 7.5 5.70 10.0 12.58.60 15.011.55 17.5 20.014.27
0.00 0.570.82 2.85 3.14 5.70 8.606.05 11.55 14.278.90 16.14 11.81 17.69 18.9414.4
15.0
16.14
16.22
0.5 2.5 5.0 7.5 10.0
(µm)
0.26 0.82 3.14 6.05 8.90 11.81 14.4 16.22 17.71 18.94
(µm)
0.00
0.00 0.56 0.56 2.82 2.82 5.72 5.72 8.59 11.47 8.5914.15 11.4716.01 17.50 14.1518.80
16.01
(µm)
0.09
0.09 0.68 0.68 3.00 3.00 5.94 8.82 5.94 11.7 8.82 14.3 16.11 11.7 17.58 14.3 18.8
16.11
Test 5 Test 5 (μm) (µm)
0.00
Test 6
Standard
Average Test(μm) 6 Standard Deviation Average (µm) Deviation 0.02
0.00 0.59 0.59 2.85 2.85 5.70 8.58 5.70 11.48 8.58 14.16 16.00 11.48 17.47 14.16 18.76
0.02 0.61 0.61 2.93 2.93 5.85 8.72 5.85 11.63 14.218.72 16.0211.63 17.50 18.7614.21
16.00
16.02
0.06 0.64 2.93 5.83 8.70 11.61 14.25 16.08 17.57 18.83
0.06
0.10 0.64 0.10 0.12 2.93 0.15 5.83 0.14 0.13 8.70 0.10 11.61 0.09 0.11 14.25 0.09
16.08
0.10 0.10 0.12 0.15 0.14 0.13 0.10 0.09
17.5 17.69 17.71 curve 17.50 17.58 up to17.47 17.50 17.57 0.11 The deflection-pressure appears linear about 12.5 MPa lower than the simulated
yielding MPa and to remain even up18.83 to 20 MPa, 0.09 although 20.0 pressure 18.94of 13.75 18.94 18.80the elasticity 18.8 seems 18.76 18.76 the deflection rate against pressure becomes lower. This would be due to work hardening of the round-groove of the diaphragm. Through regression analysis linear relationship between The deflection-pressure curve appears linear up to about 12.5the MPa lower than the simulated
yielding pressure of 13.75 MPa and the elasticity seems to remain even up to 20 MPa, although the deflection rate against pressure becomes lower. This would be due to work hardening of the round-groove of the diaphragm. Through regression analysis the linear relationship between
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diaphragm deflection (δ) and pressure (P), δ (μm) = 1.142P (MPa) + 0.087, is of such good fitness that (δ) andof pressure δ (μm) = 1.142P (MPa) + 0.087, is of such good fitness that adiaphragm coefficient deflection of determination 0.999 is(P), obtained. diaphragm deflection (δ) and pressure δ (µm) = 1.142P (MPa) + 0.087, is of such good fitness that a a coefficient of determination of 0.999(P), is obtained. coefficient of determination of 0.999 is obtained.
Figure 12. Plot of measured diaphragm deflection with pressure. Figure 12. Plot of measured diaphragm deflection with pressure. Figure 12. Plot of measured diaphragm deflection with pressure.
5.2. Assembled Pressure Sensor 5.2. 5.2.Assembled AssembledPressure PressureSensor Sensor Figure 13 shows a cross section view of an assembled pressure sensor developed for this research. Figure of an an assembled assembledpressure pressuresensor sensordeveloped developedfor forthis thisresearch. research. Figure13 13shows showsaa cross cross section section view view of
Figure 13. 13. Cross Cross section section view view of of an an assembled assembled pressure pressure sensor. Figure sensor. Figure 13. Cross section view of an assembled pressure sensor.
Its round-groove a round-grooved diaphragm was welded on round-groovemachined machinedininboth bothfaces facesbybyturning, turning, a round-grooved diaphragm was welded Itsone round-groove machined in both faces by turning, a round-grooved diaphragm was welded the one end of housing. An An insulator waswas placed between diaphragm and and target plateplate to prevent heat on the end of housing. insulator placed between diaphragm target to prevent on the one end housing. An insulator was between diaphragm and target plate to prevent transfer from theofdiaphragm since the coil impedance is affected by temperature. heat transfer from the diaphragm since the coilplaced impedance is affected by temperature. heat transfer from the diaphragm since the coil impedance is affected by temperature. 5.3. Performance Evaluation Evaluation of of an an Assembled 5.3. Performance Assembled Pressure Pressure Sensor Sensor According According to to Applied Applied Pressure Pressure 5.3. Performance Evaluation of an Assembled Pressure Sensor According to Applied Pressure The performance of assembled sensor same equipment equipment as in Figure Figure 11 11 with with The performance of an an assembled sensor was was tested tested on on the the same as in aa HBT probe replaced by a multi-meter to measure the pressure output (voltage) depicted in Figure 3. performance of aanmulti-meter assembled to sensor was the tested on theoutput same equipment as in Figure 11 with HBTThe probe replaced by measure pressure (voltage) depicted in Figure 3. The measurement results are summarized in Table 33the andpressure the output is displayed displayed in Figure Figure 14a. a HBT probe replaced byare a multi-meter toin measure output (voltage) depicted in Figure 3. The measurement results summarized Table and the sensor sensor output is in 14a. The sensor output is linearly well related with input pressure up to 10 MPa and has maximum linearity The sensor measurement are summarized Tableinput 3 andpressure the sensor is displayed in maximum Figure 14a. The output results is linearly well relatedinwith upoutput to 10 MPa and has errors lesserrors than 0.16%, seen0.16%, in well Figure 14b, is much smaller than commercialized sensors. The sensor output linearly related with input pressure up is toother 10 MPa and hasthan maximum linearity lessisas than as seenwhich in Figure 14b, which much smaller other linearity errors less than 0.16%, as seen in Figure 14b, which is much smaller than other commercialized sensors. commercialized sensors.
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(a)
(b)
Figure 14. 14. Pressure sensor error(b) (b) along with applied pressure Figure Pressure sensoroutput output(a) (a) and and linearity linearity error along with applied pressure Table 3. Measurement sensoraccording according to varied pressure. Table 3. Measurementresults resultsof ofthe the assembled assembled sensor to varied pressure.
Pressure Pressure (MPa) (MPa) Impedance Change Impedance Change (mΩ) (mΩ) 0 1 2 3 4 5 6 7 8 9 10 6. Conclusions
Sensor Output (V) Sensor Output (V) Error (%)
0.000.00 0 0.00 0.00 1 11.66 0.98 0.98 11.66 2 23.45 1.99 23.45 1.99 3 35.29 2.99 35.29 4 47.14 3.99 2.99 5 58.96 4.99 3.99 47.14 6 70.75 5.99 58.96 4.99 7 82.52 6.99 70.75 8 94.30 7.99 5.99 9 106.14 8.99 6.99 82.52 10 118.10 10.00 94.30 7.99 * Error (%) = (Sensor output (V)—Expected value(V))/(10V) ˆ 100. 106.14 8.99 118.10 10.00
0.00 0.13 0.15 0.12 0.09 0.08 0.10 0.13 0.16 0.13 0.00
Error (%) 0.00 0.13 0.15 0.12 0.09 0.08 0.10 0.13 0.16 0.13 0.00
* Error (%) = (Sensor output (V)—Expected value(V))/(10V) × 100. An eddy current type pressure sensor with an outer diameter of 10 mm was developed and its characteristic was investigated so as to get appropriate values of the design parameters of the sensor’s 6. Conclusions two key components, i.e., diaphragm and coil. Some important findings are summarized as follows.
An eddy current type pressure sensor with an outer diameter of 10 mm was developed and its
(1) The coil designed in this research hasget a good linear relationship between the impedance change characteristic was investigated so as to appropriate values of the design parameters of the and the to targeti.e., platediaphragm up to 500 µm. sensor’s two keydistance components, and coil. Some important findings are summarized (2) A round-grooved diaphragm is much better for stable measurement of deflection even with much as follows. less deflection compared to a flat type one. (1) (3) The Depending coil designed in this research has aand good linear relationship between theparameters impedanceofchange on the required pressure sensitivity measurement, the design a and round-grooved the distance todiaphragm target plate up to 500 μm. can be selected. (2) (4) A round-grooved betterlinearity for stable measurement deflection even The developed diaphragm sensor showsisamuch very good up to 10 MPa withof linearity errors less with much less deflection compared to a flat type one. than 0.16%.
(3) Depending on the required pressure and sensitivity measurement, the design parameters of a Acknowledgments: This work was supported by the Human Resource Training Program for Regional round-grooved diaphragm can be selected. Innovation and Creativity through the Ministry of Education and National Research Foundation of (4) Korea The developed sensor shows a very good linearity up to 10 MPa with linearity errors less than (2013H1B8A2032233). 0.16%. Author Contributions: H.R. Lee and G.S. Lee designed and performed the experiments; H.R. Lee and J.H. Ahn analyzed the data and wrote the paper; G.S. Lee and H.Y. Kim built up the research project.
Acknowledgments: This was supported byofthe Human Resource Training Program for Regional Conflicts of Interest: Thework authors declare no conflict interest. Innovation and Creativity through the Ministry of Education and National Research Foundation of Korea (2013H1B8A2032233). Author Contributions: H.R. Lee and G.S. Lee designed and performed the experiments; H.R. Lee and J.H. Ahn analyzed the data and wrote the paper; G.S. Lee and H.Y. Kim built up the research project. Conflicts of Interest: The authors declare no conflict of interest.
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