sensors Article
Design of Novel FBG-Based Sensor of Differential Pressure with Magnetic Transfer Guohui Lyu 1 , Guohang Che 1 , Junqing Li 2 , Xu Jiang 3, *, Keda Wang 1 , Yueqiang Han 1 and Laixu Gao 1 1
2 3
*
Research Center for Fiber Optic Sensing Technology National Local Joint Engineering, Electronic Engineering College, Heilongjiang University, Harbin 150080, China; [email protected] (G.L.); [email protected] (G.C.); [email protected] (K.W.); [email protected] (Y.H.); [email protected] (L.G.) Physics Department, Harbin Institute of Technology, Harbin 150001, China; [email protected] College of Information Science and Technology, Heilongjiang University, Harbin 150080, China Correspondence: [email protected] or [email protected]; Tel.: +86-132-045-10451
Academic Editors: Christophe Caucheteur and Tuan Guo Received: 14 December 2016; Accepted: 9 February 2017; Published: 15 February 2017
Abstract: In this paper, a differential pressure sensor with magnetic transfer is proposed, in which the non-electric measurement based on the fiber Bragg grating (FBG) with the position limiting mechanism is implemented without the direct contact of the sensing unit with the measuring fluid. The test shows that the designed sensor is effective for measuring differential pressure in the range of 0~10 kPa with a sensitivity of 0.0112 nm/kPa, which can be used in environments with high temperature, strong corrosion and high overload measurements. Keywords: FBG; magnetic transmission; differential pressure sensor; beam of uniform strength
1. Introduction The differential pressure sensor has been widely used in differential pressure signal measurements of fluid. When the fluid flows through the point to be measured, the tiny pressure difference between the two sampling points can be measured by the sensor [1]. Traditional electronic sensors are vulnerable to electromagnetic interference and are not suitable for applications in flammable and explosive places. In recent years, with the development of sensing technology, FBG has become one of the candidates to replace those traditional electronic sensors [2–5]. For example, different strategies have been adopted successfully in order to obtain feasible and reliable temperature sensing [6]. Figure 1 describes the typical structure of a differential pressure sensor including the FBG. In differential pressure sensing, a pressure-transferring diaphragm is required to have contact with the medium in both directions. Usually the sensing unit should be immersed in the measured fluid, which may result in interference to the measurement. In order to guarantee the sensitivity and durability of the sensor, special approaches should be taken in real measurement environments with high temperatures, high pressures, overload [7,8] and corrosion [9,10].
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Figure sensor in in which which an an FBG FBG may may be be used. used. Figure 1. 1. The The typical typical structure structure of of aa differential differential pressure pressure sensor
2. Designs of Differential Pressure Sensor with Magnetic Transfer It should be pointed out that an FBG-sensitive element must be protected effectively, although it is extremely hard to encapsulate the FBG in a sensor in reality. In a real design, the advantage over 2.1. The Overall Design Figure 1. Theand typical a differential pressure sensor in FBGaccount. may be used. traditional sensors the structure effective of protection of the FBG should bewhich takenaninto In order to sense the differential pressure effectively and to protect the sensor from invalidity with Transfer caused by the contact of thePressure FBG with the fluid toMagnetic be measured, we propose a sensor structure with a 2. Designs of Differential Sensor Transfer magnetic transfer mechanism. As seen in Figure 2, four parts are included in this sensor structure as 2.1. The Overall Overall Design 2.1. The Design follows: the pressure-transferring part comprised of three strong magnets and an elastic corrugated diaphragm; the input part for differential pressure with aand high input channel a low In effectively and topressure protect sensor fromand invalidity In order order to to sense sense the the differential differential pressure pressure effectively to protect the the sensor from invalidity pressure input channel; the part in which a triangle cantilever beam is used to convert the caused by by the the contact contact of of the the FBG FBG with with the the fluid fluid to to be be measured, measured, we we propose propose aa sensor sensor structure structure with with caused aa differential pressure to the wavelength shift of the FBG; and a limiting mechanism which is used for magnetic transfer mechanism. As seen in Figure 2, four parts are included in this sensor structure as magnetic transfer mechanism. As seen in Figure 2, four parts are included in this sensor structure as resistingthe overload [11]. A structurepart design for insulating the fluid magnets and FBG and is thean most critical aspect. follows: the pressure-transferring part comprised of three three strong magnets and an elastic corrugated follows: pressure-transferring comprised of strong elastic corrugated diaphragm; the the input input part part for for differential differential pressure pressure with with aa high high pressure pressure input input channel channel and and aa low low diaphragm; pressure input channel; the part in which a triangle cantilever beam is used to convert the differential pressure input channel; the part in which a triangle cantilever beam is used to convert the pressure to pressure the wavelength shift of theshift FBG;ofand limiting which is used differential to the wavelength theaFBG; and amechanism limiting mechanism whichfor is resisting used for overload [11]. A structure design for insulating the fluid and FBG is the most critical aspect. resisting overload [11]. A structure design for insulating the fluid and FBG is the most critical aspect.
Figure 2. The structure of the FBG-based differential pressure sensor with magnetic transfer.
2.2. Sensing Principle of a Cantilever Beam of Uniform Strength Figure 3 presents a structure of a triangle cantilever beam as an elastic sensitive element with the rightFigure end fixed and the other end movable,differential on whichpressure an FBGsensor is fixed. 2. structure of FBG-based with magnetic transfer. transfer. Figure 2. The The structure of the the FBG-based differential pressure sensor with magnetic 2.2. Sensing Principle of a Cantilever Beam of Uniform Strength 2.2. Sensing Principle of a Cantilever Beam of Uniform Strength Figure 3 presents a structure of a triangle cantilever beam as an elastic sensitive element with Figure 3 presents a structure of a triangle cantilever beam as an elastic sensitive element with the the right end fixed and the other end movable, on which an FBG is fixed. right end fixed and the other end movable, on which an FBG is fixed.
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b0
bx
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O
x
l
Figure 3. Structure Structure of the the triangle triangle cantilever cantilever beam. beam.
As shown shown in in Figure Figure 3, 3, when when aa force force F acts acts on on the the point point O O near near the the free free end, end, which which is is just just the the top top As point of of the thetriangle triangleand andlocated locatedatata adistance distance l from fixed of the beam, the deformation point of of l from thethe fixed endend of the beam, the deformation ε of ε of the beam caused by the force can be expressed as: the beam caused by the force can be expressed as:
εε(x) (x) =
xx) 66(ll− F E · hh · A A·F E
(1) (1)
where E, E,hhand andAAindicate indicate elastic modulus, thickness of beam the beam and transverse area, where thethe elastic modulus, thickness of the and transverse sectionsection area, which which is a product of thebwidth bx and thickness h of a position certain position x with to respect O,point the top is a product of the width h of a certain x with respect O, thetotop of x and thickness point of triangle. From Equation (1), the deformation is a function of x . triangle. From Equation (1), the deformation is a function of x. Therefore, A can be expressed expressed as: as: lx A(x) h b h b l − x (2) A( x ) = h · bx x = h · b00 · l (2) l where b0 denotes the beam width at the fixed end, l means the height of the triangle. Substituting where b0 denotes the beam width at the fixed end, l means the height of the triangle. Substituting Equation (2) into Equation (1), one can obtain: Equation (2) into Equation (1), one can obtain: 6l ε 6 · l 2 F (3) ε = E b0 h 2 · F (3) E · b0 · h From Equation (3), the deformation is equal everywhere when the point O at the movable end (3),a the deformation is the equal everywhere when the pointtoOfix atthe theFBG movable end of of theFrom beamEquation is applied force F; therefore, requirement for the position is released. the beam is force F; therefore, requirement foron thethe position the FBG released. It It should beapplied pointeda out that the force the F must be applied point to O fix which is theisintersection should be pointed out that the force F must be applied on the point O which is the intersection between between the two hypotenuses of the triangle. the two hypotenuses of the triangle. The deflection D of the movable end of the beam is [12] The deflection D of the movable end of the beam is [12] 6 l3 D 6 · l3 3 F (4) D = E b0 h 3 · F (4) E · b0 · h From the Bragg conditions we have: From the Bragg conditions we have: λB 2neff Λ (5) λ B = 2ne f f Λ (5) where neff , Λ represent the effective refractive index and grating period of the FBG. The Bragg where ne f f , Λ represent grating periodperiod. of the When FBG. The Bragg wavelength changes withthe theeffective effectiverefractive refractiveindex indexand and the grating a force is wavelength changes with the effective refractive index and the grating period. When a force is applied applied to the movable end of the beam, the FBG pasted on the surface of the beam may convert the to the movableofend the beam, theshift FBGof pasted on the surface of the beam convert deformation deformation theofbeam to the the central wavelength of themay FBG. After the controlling the of the beam to the shift of the central wavelength of the FBG. After controlling the temperature or temperature or compensating for the effect of the temperature on the FBG (the temperature
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compensating for the effect of the temperature on the FBG (the temperature compensation design of compensation of the sensor will section), be introduced in a later section), the sensor willdesign be introduced in a later the relationship between the the relationship deformationbetween and the the deformation and thecan wavelength of the wavelength of the FBG be simplified as:FBG can be simplified as:
Δλ K ∆λ BB = Kεε · εε
(6) (6)
where Kε is a coefficient. Combining Equations (3) and (4) into Equation (6), we obtain: where Kε is a coefficient. Combining Equations (3) and (4) into Equation (6), we obtain:
Δλ l 2 D ∆λ BB · l 2 D = K h Kε · h
(7) (7)
ε
As movable beam As seen seen in in Equation Equation (7), (7), the the deflection deflection of of the the movable beam end end D D is is proportional proportional to to the the Δλ of the FBG. wavelength shift ∆λ of the FBG. B B 2.3. Structure Structure Design Design of of Magnetic Magnetic Transfer Transfer 2.3. Usually, ititisisnot to separate the the FBGFBG fromfrom the measured fluid. In order solve to thissolve problem, Usually, noteasy easy to separate the measured fluid. Intoorder this we introduced a special mechanism with magnetic transfer to isolate the FBG from the measured fluid, problem, we introduced a special mechanism with magnetic transfer to isolate the FBG from the where three identical magnetsstrong are used, described as A,described B and C,asrespectively, Figure 4a. measured fluid, wherestrong three identical magnets are used, A, B and C, in respectively, Magnet A is fixed, and the S pole of A faces the N pole of B. While magnets B and C are movable, in Figure 4a. Magnet A is fixed, and the S pole of A faces the N pole of B. While magnets B and C are magnet B is linked rigidly with the center of the elastic corrugated diaphragm; the S pole of movable, magnet B is linked rigidly with the center of the elastic corrugated diaphragm; the BS faces pole theBNfaces polethe of C, C ismagnet fixed toCthe movable end of the beam. and C A, areBattracted to of N and polemagnet of C, and is fixed to the movable end ofA, theB beam. and C are each other and must keep equal distances of about 3~5 mm between each other. Actually, among them attracted to each other and must keep equal distances of about 3~5 mm between each other. there must be an them isolating structure as a polymer spacer) to prevent them spacer) from close contact. Actually, among there must be(such an isolating structure (such as a polymer to prevent The FBG is close fixed contact. on a beam uniform strength. It is of insulated and bottom surfaces them from TheofFBG is fixed on a beam uniformbetween strength.the It top is insulated between the of the diaphragm. When there is a difference in the pressure between the top and bottom surfaces of top and bottom surfaces of the diaphragm. When there is a difference in the pressure between the the and diaphragm, the center diaphragm leading the movement of magnet top bottom surfaces of of thethe diaphragm, themoves centerup of or thedown, diaphragm moves up or down, leading B through the rigid link, therefore synchronously driving magnet C by means driving of the magnetic the movement of magnet B through the rigid link, therefore synchronously magnet force. C by The side of the S pole of magnet C is connected to the movable end of the cantilever beam; therefore, means of the magnetic force. The side of the S pole of magnet C is connected to the movable end of the cantilever difference beam; of the pressure transferred toof the FBG. the therefore,isthe difference the pressure is transferred to the FBG.
Figure of sensitive sensitive unit unit with withmagnetic magnetictransfer; transfer;(b) (b)the therelationship relationshipofofmagnets magnetsA,A,B Figure 4. 4. (a) (a) The The structure structure of B and C. and C.
2.4. The Position Limiting Mechanism 2.4. The Position Limiting Mechanism In the real design, we enabled the sensor to resist overload by introducing a position limiting In the real design, we enabled the sensor to resist overload by introducing a position limiting mechanism [13]. In our structure, two points are needed in the careful design, as seen in Figure 5; mechanism [13]. In our structure, two points are needed in the careful design, as seen in Figure 5; one is for magnet B to move and the other is for the elastic corrugated diaphragm. The two points one is for magnet B to move and the other is for the elastic corrugated diaphragm. The two points are are limited to prevent the damage of the diaphragm and magnet and to avoid the invalidity of the limited to prevent the damage of the diaphragm and magnet and to avoid the invalidity of the sensor. sensor.
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Figure Figure5.5.The Theposition positionlimiting limitingmechanism mechanismofofthe thesensor. sensor.
2.5. 2.5.The TheTemperature TemperatureCompensation Compensation When Whenaaforce forceisisapplied appliedtotothe thebeam, beam,its itsupper uppersurface surfaceexperiences experiencesaastretching, stretching,corresponding corresponding negative deformation, while the lower surface is compressed and experiences positive negative deformation, while the lower surface is compressed and experiences positivedeformation, deformation, but butthe thedeformation deformationofofthe theupper upperand andlower lowersurfaces surfacesisisequal equalininabsolute absolutevalue. value.Therefore, Therefore,when whentwo two FBGs temperature sensitivity coefficient areare pasted on on thethe upper andand lower surfaces of FBGswith withthe thesame same temperature sensitivity coefficient pasted upper lower surfaces the beam, respectively, the wavelength shifts of the two gratings change to the opposite direction of the beam, respectively, the wavelength shifts of the two gratings change to the opposite direction with withan anidentical identicalvalue valueofofthe theshifts. shifts.ItItshould shouldbe bepointed pointedout outthat thattwo twofiber fibergratings gratingsare arelocated locatedinin the and the thedifference differencebetween betweenthe the shifts gratings be thesame same temperature temperature environment, environment, and shifts of of twotwo gratings willwill be kept kept unchanged, although the change of the temperature may cause an individual shift for each unchanged, although the change of the temperature may cause an individual shift for each grating. grating. Thatthe means the temperature effect on the grating is and eliminated and gets the agrating gets a That means temperature effect on the grating is eliminated the grating compensation compensation in temperature. in temperature. 3.3.Simulations SimulationsofofPressure PressureCorrugated CorrugatedDiaphragm Diaphragmand andTest Testof ofSensor Sensor 3.1.The TheSimulations Simulationsofofthe theElastic ElasticCorrugated CorrugatedDiaphragm Diaphragm 3.1. To obtain obtain the of the and toand verify consistency of the diaphragm To theoptimal optimalparameters parameters of diaphragm the diaphragm to the verify the consistency of the design with the actual case, we adopted ANSYS 14.0 (ANSYS Inc., Pittsburgh, PA, USA) to simulate the diaphragm design with the actual case, we adopted ANSYS 14.0 (ANSYS Inc., Pittsburgh, PA, pressure diaphragm. ANSYS a general analysis software on the analysis finite element method which United States) to simulate the is pressure diaphragm. ANSYSbased is a general software based on allows us to carry out the study of the structure with heat, sound, fluid and electromagnetic fields. the finite element method which allows us to carry out the study of the structure with heat, sound, The electromagnetic 1:1-scale three-dimensional (3D) model of the elastic corrugated diaphragm is imported into fluid and fields. ANSYS first, as three-dimensional shown in Figure 6. In thismodel figure,ofthe of the full-size diaphragm is 100 mm, The at 1:1-scale (3D) thediameter elastic corrugated diaphragm is imported and the corrugation type is chosen as “triangle waveform” (6 rings) with a corrugation depth of 2 mm. into ANSYS at first, as shown in Figure 6. In this figure, the diameter of the full-size diaphragm is Themm, material diaphragmtype is stainless steel And the parameters the diaphragm are listed 100 and of thethe corrugation is chosen asfilm. “triangle waveform” (6 of rings) with a corrugation in Table depth of 1. 2 mm. The material of the diaphragm is stainless steel film. And the parameters of the Then its mechanical property in a certain range of pressure differences (0~10 kPa). diaphragm we are simulated listed in Table 1. As an example, Figure 7 gives the deformation and force distribution of the diaphragm under a differential pressure of 10 kPa when the thickness is 0.2 mm, where the pressure difference may be reflected through stress at the center of the diaphragm.
Figure 6. The 1:1 scale 3D model of the elastic corrugated diaphragm.
the finite element method which allows us to carry out the study of the structure with heat, sound, fluid and electromagnetic fields. The 1:1-scale three-dimensional (3D) model of the elastic corrugated diaphragm is imported into ANSYS at first, as shown in Figure 6. In this figure, the diameter of the full-size diaphragm is 100 mm, and the corrugation type is chosen as “triangle waveform” (6 rings) with a corrugation Sensorsdepth 2017, 17, of 375 2 mm. The material of the diaphragm is stainless steel film. And the parameters of the 6 of 9 diaphragm are listed in Table 1.
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Table 1. Parameters of the diaphragm.
Parameters Value Figure 6. The scale 3Dmodel model of of the diaphragm. Figure 6. The 1:11:1 scale 3D the elastic elasticcorrugated corrugated diaphragm.
Diameter of full size 100 mm Edge width 2 mm Table 1. Parameters of the diaphragm. Number of corrugation 6 Depth of corrugation 2 mm Parameters Value Spacing of corrugation 6 mm Diameter of full size 100 mm material stainless steel Edge width 2 mm Number of corrugation 6 Then we simulated its mechanical property in a certain range of pressure differences (0~10 Depth of corrugation 2 mm kPa). As an example, Figure 7 gives the deformation and force distribution of the diaphragm under Spacing of corrugation 6 mm a differential pressure of 10 kPa when the thickness is 0.2 mm, where the pressure difference may material stainless steel be reflected through stress at the center of the diaphragm.
(a)
(b) Figure 7. (a) Deformation diaphragm; (b) distribution of diaphragm. Figure 7. (a) Deformation of of diaphragm; (b)Force Force distribution of diaphragm.
In the real design, the edge of a diaphragm of 2 mm is reserved in advance to link with the of the the sensor and of to isolate the fluids of at both sides the diaphragm. That means the with the In theother realpart design, edge a diaphragm 2 mm is ofreserved in advance to link distribution at thetoedge is kept unchanged. other part force of the sensor and isolate the fluids at both sides of the diaphragm. That means the force distribution at the edge is kept unchanged.
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In order to get the best diaphragm thickness, we simulated diaphragms with thicknesses of 0.2 mm, 0.1 mm, 0.05 mm, 0.025 mm, and 0.0125 mm, respectively. The simulation results are listed Sensors 2017, 17, 375 7 of 9 in Table 2. In order to get the best diaphragm thickness, we simulated diaphragms with thicknesses of 0.2 Table 2. The simulation results. mm, 0.1 mm, 0.05 mm, 0.025 mm, and 0.0125 mm, respectively. The simulation results are listed in Table 2. Maximum Deformation Maximum Pressure on the Diaphragm Thickness of Diaphragm Center of Diaphragm Table 2. The simulation results.
0.200 mm 0.035 mm Diaphragm Thickness Maximum Deformation of Diaphragm 0.100 mm 0.094 mm 0.200 mm 0.035 mm 0.050 mm 0.172 mm 0.100 mm 0.094 mm 0.025 mm 0.300 mm 0.050 mm 0.172 mm 0.0125 mm 0.300 mm 0.025 mm 0.300 mm 0.0125 mm
0.300 mm
10.899 MPa 26.495 MPa 10.899 MPa 35.854 MPa 26.495 MPa 69.720 MPa 35.854 MPa 136.830 MPa 69.720 MPa
Maximum Pressure on the Center of Diaphragm
136.830 MPa
The simulation results indicate a tradeoff between the thickness of the diaphragm and the The simulation results indicate a tradeoff between the thickness of the diaphragm and the measurement range. When the diaphragm is thick, the sensitivity of the diaphragm is low, which leads measurement range. When the diaphragm is thick, the sensitivity of the diaphragm is low, which to anleads enlarged of measurement with a lower When the diaphragm is too thin, despite to anrange enlarged range of measurement withresolution. a lower resolution. When the diaphragm is too the enhanced sensitivity, the measurement range is reduced and results in a degradation in endurance thin, despite the enhanced sensitivity, the measurement range is reduced and results in a to the impact of in a high pressure difference, resulting damage to theresulting diaphragm. According to degradation endurance to the impact ofeven a high pressureindifference, even in damage to the measurement and resolution, we chose a diaphragm that is mm thick to fabricate the diaphragm.range According to the measurement range and resolution, we0.15 chose a diaphragm that is and 0.15 mm to fabricate and the conduct Figure 8 presents conduct tests.thick Figure 8 presents real tests. picture of the sensor. the real picture of the sensor.
Figure (a)The Thereal real sensor; sensor; (b) inside thethe sensor. Figure 8.8.(a) (b)The Thestructure structure inside sensor.
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3.2. 3.2. Test Systemofofthe theSensor Sensor 3.2.Test TestSystem AA test system was used evaluatethe thelinearity linearityofofthe thesensor, sensor,asasshown 9. Atest testsystem systemwas wasused usedtotoevaluate shownininFigure Figure9.9.
Figure 9. The test system of sensor. Figure 9.9. The test system ofof sensor. Figure The test system sensor.
The The test system included the air pressure difference–generating device, the air pressure display Thetest testsystem systemincluded includedthe theair airpressure pressuredifference–generating difference–generatingdevice, device,the theair airpressure pressuredisplay display device, device, computer, FBGdemodulator demodulator andthe thesensor sensortotobebetested. Theair airpressure–generating pressure–generating device,a aacomputer, computer,ananFBG demodulatorand tested.The pressure–generating device made ofofof stainless steel, which can provide ananaccurate and pressure source. The device was made stainless steel, which cancan provide accurate andstable stable pressure source. The devicewas was made stainless steel, which provide an accurate and stable pressure source. sensor totobebetested on ofofthe the sensor tested wasmounted mounted onananinterface interface thepressure pressure source,and and thecalibrated calibrated pressure The sensor to be was tested was mounted on an interface of the source, pressure source, and the pressure calibrated meter was mounted value ofofvalue the pressure meter meter was mounted onanother another interface ofthe thesource. source. The value thecalibrated calibrated pressure meter pressure meter was on mounted oninterface another of interface of theThe source. The of the calibrated pressure was displayed asasa areference value. The signal from the sensor was analyzed by was displayed reference value. Thelight lightlight signal from thethe sensor was analyzed by the meter was displayed as a reference value. The signal from sensor was analyzed bythe the demodulator demodulator and displayedon onthe thePC. PC. demodulatorand anddisplayed Two produces Twoindependent independentpressure-generating pressure-generatingdevices devicesare aresupposed supposedtotobebeused usedgenerally: generally:one one produces one produces high low pressure atat the same time. However, inin this test, we high pressure and the other produces low pressure the same time. However, this test, we only highpressure pressureand andthe theother otherproduces produces low pressure at the same time. However, in this test, weonly only used used one generator provide high pressure, while the low pressure was fixed simplify the test usedone onegenerator generatortoto toprovide providehigh highpressure, pressure,while whilethe thelow lowpressure pressurewas wasfixed fixedtoto tosimplify simplifythe thetest test system. It was easy to generate differential pressure ranging from 0 to 10 kPa. system. system. ItIt was was easy easy to to generate generate differential differential pressure ranging from 0 to 10 kPa. 4.Results Results 4.4. Results Thetest testresults results curve) are shown in in Figure 10. 10. We fitted the data to a straight The calibration curve) are shown We the data totoa a The results(the (thecalibration calibration curve) are shown inFigure Figure 10.have Wehave havefitted fitted the data line with λBwith =with 0.0112 P + 1568.780, with a linear coefficientcoefficient of 0.994, and we obtained the straight line λλ B B=× × ×PP+ +1568.780, with a alinear ofof0.994, and straight line =0.0112 0.0112 1568.780, withcorrelation linearcorrelation correlation coefficient 0.994, andwe we sensitivity of the sensor as 0.0112 nm/kPa. The relations between the central wavelength shift of the obtained the sensitivity of the sensor as 0.0112 nm/kPa. The relations between the central obtained the sensitivity of the sensor as 0.0112 nm/kPa. The relations between the central FBG and the differential pressure were almost linear. wavelength shift ofofthe and differential pressure wavelength shift theFBG FBG andthe the differential pressurewere werealmost almostlinear. linear.
Figure 10. The differential pressure sensor linearity curve. Figure 10. The differential pressure sensor linearity curve. Figure 10. The differential pressure sensor linearity
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5. Conclusions In this paper, a differential pressure sensor with magnetic transfer was proposed, and a non-electric measurement method based on an FBG was introduced, which can avoid the direct contact of the sensing unit with the measuring fluid. In this design, the position limiting mechanism and temperature compensation design were also considered. The test results demonstrated that the sensor is effective to measure differential pressure in the range of 0~10 kPa with a sensitivity of 0.0112 nm/kPa, and it can operate in high temperature, strong corrosion and high overload conditions. Acknowledgments: The research is supported in part by the Science and Technology Project of Hei Long Jiang Province Science and Technology Department (GZ11A306). Project (1252CGZ-H11) in the Hei Long Jiang Province Department of Education, Harbin Technology Bureau. Project (2013AE1BW050), released by the Chinese Patent Office: Patent (No. 2014105889286) and (2014105889286). Author Contributions: G.L. and X.J. conceived and designed the experiments; G.C. and K.W. performed the experiments; J.L., Y.H. and L.G. analyzed the data; G.C. contributed analysis tools; K.W. and X.J. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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Hu, H.; Zhong, L.; Zhou, Q. The present situation and the development of differential pressure sensor technology. Mach. Tool Hydraul. 2013, 11, 187–190. Tong, C.; Yang, J.; Liu, Z.; Yuan, L. Double spring tube fiber differential pressure sensors. Acta Photon. Sin. 2004, 10, 1172–1176. Dong, G.; Li, D.; Hao, R. Study on Micro Differential Pressure Sensor Based on Ferrofluid. Chin. J. Sens. Actuators 2009, 1, 13–16. Tong, W.; Li, X. The application of differential pressure sensors in liquid level measurement. Sci. Technol. Wizard 2010, 29, 50–54. Xu, G.; Xiong, D. Fiber Bragg grating sensing technology application in engineering. Chin. Opt. 2013, 3, 306–313. Scarcia, W.; Palma, G.; Falconi, M.C.; de Leonardis, F.; Passaro, V.M.N.; Prudenzano, F. Electromagnetic Modelling of Fiber Sensors for Low-Cost and High Sensitivity Temperature Monitoring. Sensors 2015, 15, 29855–29870. [CrossRef] [PubMed] Tang, H.; Sun, H.; Liu, H.; Zheng, D.; Zhang, G.; Chen, X.; Zheng, Y. Diffusion silicon multifunctional integration differential pressure sensor. Instrum. Tech. Sens. 2004, 7, 6–8. You, C.; Zhang, W.; Wang, W. A piezoresistive micro differential pressure sensor design and stress free. Instrum. Tech. Sens. 2009, 4, 105–107. Li, P.; Cao, Y.; Fan, Y. Semiconductor research application of differential pressure sensor. J. Data Acquis. Process. 1992, 3, 202–209. Chen, B.; Hu, X.; Sun, H. Capacitance differential pressure sensor is designed based on AD7745. Transducer Microsyst. Technol. 2011, 6, 71–76. Fu, W. Three differential pressure sensor center diaphragm structure design of the diaphragm. Instrum. Tech. Sens. 1997, 2, 35–38. Wang, Q.; Yan, N. Design and Study of Fiber Sensor Based on Cantilever Beam of Uniform Strength. Microcomput. Inf. 2010, 4, 107–108. Liu, J.; Li, Y.; Liu, B.; Zhang, N.; Zhang, Z.; Liu, Q. The improvement of the two-way overload differential pressure sensor error. Instrum. Tech. Sens. 2009, 2, 94–96. © 2017 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 Novel FBG-Based Sensor of Differential Pressure with Magnetic Transfer Guohui Lyu 1 , Guohang Che 1 , Junqing Li 2 , Xu Jiang 3, *, Keda Wang 1 , Yueqiang Han 1 and Laixu Gao 1 1
2 3
*
Research Center for Fiber Optic Sensing Technology National Local Joint Engineering, Electronic Engineering College, Heilongjiang University, Harbin 150080, China; [email protected] (G.L.); [email protected] (G.C.); [email protected] (K.W.); [email protected] (Y.H.); [email protected] (L.G.) Physics Department, Harbin Institute of Technology, Harbin 150001, China; [email protected] College of Information Science and Technology, Heilongjiang University, Harbin 150080, China Correspondence: [email protected] or [email protected]; Tel.: +86-132-045-10451
Academic Editors: Christophe Caucheteur and Tuan Guo Received: 14 December 2016; Accepted: 9 February 2017; Published: 15 February 2017
Abstract: In this paper, a differential pressure sensor with magnetic transfer is proposed, in which the non-electric measurement based on the fiber Bragg grating (FBG) with the position limiting mechanism is implemented without the direct contact of the sensing unit with the measuring fluid. The test shows that the designed sensor is effective for measuring differential pressure in the range of 0~10 kPa with a sensitivity of 0.0112 nm/kPa, which can be used in environments with high temperature, strong corrosion and high overload measurements. Keywords: FBG; magnetic transmission; differential pressure sensor; beam of uniform strength
1. Introduction The differential pressure sensor has been widely used in differential pressure signal measurements of fluid. When the fluid flows through the point to be measured, the tiny pressure difference between the two sampling points can be measured by the sensor [1]. Traditional electronic sensors are vulnerable to electromagnetic interference and are not suitable for applications in flammable and explosive places. In recent years, with the development of sensing technology, FBG has become one of the candidates to replace those traditional electronic sensors [2–5]. For example, different strategies have been adopted successfully in order to obtain feasible and reliable temperature sensing [6]. Figure 1 describes the typical structure of a differential pressure sensor including the FBG. In differential pressure sensing, a pressure-transferring diaphragm is required to have contact with the medium in both directions. Usually the sensing unit should be immersed in the measured fluid, which may result in interference to the measurement. In order to guarantee the sensitivity and durability of the sensor, special approaches should be taken in real measurement environments with high temperatures, high pressures, overload [7,8] and corrosion [9,10].
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Figure sensor in in which which an an FBG FBG may may be be used. used. Figure 1. 1. The The typical typical structure structure of of aa differential differential pressure pressure sensor
2. Designs of Differential Pressure Sensor with Magnetic Transfer It should be pointed out that an FBG-sensitive element must be protected effectively, although it is extremely hard to encapsulate the FBG in a sensor in reality. In a real design, the advantage over 2.1. The Overall Design Figure 1. Theand typical a differential pressure sensor in FBGaccount. may be used. traditional sensors the structure effective of protection of the FBG should bewhich takenaninto In order to sense the differential pressure effectively and to protect the sensor from invalidity with Transfer caused by the contact of thePressure FBG with the fluid toMagnetic be measured, we propose a sensor structure with a 2. Designs of Differential Sensor Transfer magnetic transfer mechanism. As seen in Figure 2, four parts are included in this sensor structure as 2.1. The Overall Overall Design 2.1. The Design follows: the pressure-transferring part comprised of three strong magnets and an elastic corrugated diaphragm; the input part for differential pressure with aand high input channel a low In effectively and topressure protect sensor fromand invalidity In order order to to sense sense the the differential differential pressure pressure effectively to protect the the sensor from invalidity pressure input channel; the part in which a triangle cantilever beam is used to convert the caused by by the the contact contact of of the the FBG FBG with with the the fluid fluid to to be be measured, measured, we we propose propose aa sensor sensor structure structure with with caused aa differential pressure to the wavelength shift of the FBG; and a limiting mechanism which is used for magnetic transfer mechanism. As seen in Figure 2, four parts are included in this sensor structure as magnetic transfer mechanism. As seen in Figure 2, four parts are included in this sensor structure as resistingthe overload [11]. A structurepart design for insulating the fluid magnets and FBG and is thean most critical aspect. follows: the pressure-transferring part comprised of three three strong magnets and an elastic corrugated follows: pressure-transferring comprised of strong elastic corrugated diaphragm; the the input input part part for for differential differential pressure pressure with with aa high high pressure pressure input input channel channel and and aa low low diaphragm; pressure input channel; the part in which a triangle cantilever beam is used to convert the differential pressure input channel; the part in which a triangle cantilever beam is used to convert the pressure to pressure the wavelength shift of theshift FBG;ofand limiting which is used differential to the wavelength theaFBG; and amechanism limiting mechanism whichfor is resisting used for overload [11]. A structure design for insulating the fluid and FBG is the most critical aspect. resisting overload [11]. A structure design for insulating the fluid and FBG is the most critical aspect.
Figure 2. The structure of the FBG-based differential pressure sensor with magnetic transfer.
2.2. Sensing Principle of a Cantilever Beam of Uniform Strength Figure 3 presents a structure of a triangle cantilever beam as an elastic sensitive element with the rightFigure end fixed and the other end movable,differential on whichpressure an FBGsensor is fixed. 2. structure of FBG-based with magnetic transfer. transfer. Figure 2. The The structure of the the FBG-based differential pressure sensor with magnetic 2.2. Sensing Principle of a Cantilever Beam of Uniform Strength 2.2. Sensing Principle of a Cantilever Beam of Uniform Strength Figure 3 presents a structure of a triangle cantilever beam as an elastic sensitive element with Figure 3 presents a structure of a triangle cantilever beam as an elastic sensitive element with the the right end fixed and the other end movable, on which an FBG is fixed. right end fixed and the other end movable, on which an FBG is fixed.
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b0
bx
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O
x
l
Figure 3. Structure Structure of the the triangle triangle cantilever cantilever beam. beam.
As shown shown in in Figure Figure 3, 3, when when aa force force F acts acts on on the the point point O O near near the the free free end, end, which which is is just just the the top top As point of of the thetriangle triangleand andlocated locatedatata adistance distance l from fixed of the beam, the deformation point of of l from thethe fixed endend of the beam, the deformation ε of ε of the beam caused by the force can be expressed as: the beam caused by the force can be expressed as:
εε(x) (x) =
xx) 66(ll− F E · hh · A A·F E
(1) (1)
where E, E,hhand andAAindicate indicate elastic modulus, thickness of beam the beam and transverse area, where thethe elastic modulus, thickness of the and transverse sectionsection area, which which is a product of thebwidth bx and thickness h of a position certain position x with to respect O,point the top is a product of the width h of a certain x with respect O, thetotop of x and thickness point of triangle. From Equation (1), the deformation is a function of x . triangle. From Equation (1), the deformation is a function of x. Therefore, A can be expressed expressed as: as: lx A(x) h b h b l − x (2) A( x ) = h · bx x = h · b00 · l (2) l where b0 denotes the beam width at the fixed end, l means the height of the triangle. Substituting where b0 denotes the beam width at the fixed end, l means the height of the triangle. Substituting Equation (2) into Equation (1), one can obtain: Equation (2) into Equation (1), one can obtain: 6l ε 6 · l 2 F (3) ε = E b0 h 2 · F (3) E · b0 · h From Equation (3), the deformation is equal everywhere when the point O at the movable end (3),a the deformation is the equal everywhere when the pointtoOfix atthe theFBG movable end of of theFrom beamEquation is applied force F; therefore, requirement for the position is released. the beam is force F; therefore, requirement foron thethe position the FBG released. It It should beapplied pointeda out that the force the F must be applied point to O fix which is theisintersection should be pointed out that the force F must be applied on the point O which is the intersection between between the two hypotenuses of the triangle. the two hypotenuses of the triangle. The deflection D of the movable end of the beam is [12] The deflection D of the movable end of the beam is [12] 6 l3 D 6 · l3 3 F (4) D = E b0 h 3 · F (4) E · b0 · h From the Bragg conditions we have: From the Bragg conditions we have: λB 2neff Λ (5) λ B = 2ne f f Λ (5) where neff , Λ represent the effective refractive index and grating period of the FBG. The Bragg where ne f f , Λ represent grating periodperiod. of the When FBG. The Bragg wavelength changes withthe theeffective effectiverefractive refractiveindex indexand and the grating a force is wavelength changes with the effective refractive index and the grating period. When a force is applied applied to the movable end of the beam, the FBG pasted on the surface of the beam may convert the to the movableofend the beam, theshift FBGof pasted on the surface of the beam convert deformation deformation theofbeam to the the central wavelength of themay FBG. After the controlling the of the beam to the shift of the central wavelength of the FBG. After controlling the temperature or temperature or compensating for the effect of the temperature on the FBG (the temperature
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compensating for the effect of the temperature on the FBG (the temperature compensation design of compensation of the sensor will section), be introduced in a later section), the sensor willdesign be introduced in a later the relationship between the the relationship deformationbetween and the the deformation and thecan wavelength of the wavelength of the FBG be simplified as:FBG can be simplified as:
Δλ K ∆λ BB = Kεε · εε
(6) (6)
where Kε is a coefficient. Combining Equations (3) and (4) into Equation (6), we obtain: where Kε is a coefficient. Combining Equations (3) and (4) into Equation (6), we obtain:
Δλ l 2 D ∆λ BB · l 2 D = K h Kε · h
(7) (7)
ε
As movable beam As seen seen in in Equation Equation (7), (7), the the deflection deflection of of the the movable beam end end D D is is proportional proportional to to the the Δλ of the FBG. wavelength shift ∆λ of the FBG. B B 2.3. Structure Structure Design Design of of Magnetic Magnetic Transfer Transfer 2.3. Usually, ititisisnot to separate the the FBGFBG fromfrom the measured fluid. In order solve to thissolve problem, Usually, noteasy easy to separate the measured fluid. Intoorder this we introduced a special mechanism with magnetic transfer to isolate the FBG from the measured fluid, problem, we introduced a special mechanism with magnetic transfer to isolate the FBG from the where three identical magnetsstrong are used, described as A,described B and C,asrespectively, Figure 4a. measured fluid, wherestrong three identical magnets are used, A, B and C, in respectively, Magnet A is fixed, and the S pole of A faces the N pole of B. While magnets B and C are movable, in Figure 4a. Magnet A is fixed, and the S pole of A faces the N pole of B. While magnets B and C are magnet B is linked rigidly with the center of the elastic corrugated diaphragm; the S pole of movable, magnet B is linked rigidly with the center of the elastic corrugated diaphragm; the BS faces pole theBNfaces polethe of C, C ismagnet fixed toCthe movable end of the beam. and C A, areBattracted to of N and polemagnet of C, and is fixed to the movable end ofA, theB beam. and C are each other and must keep equal distances of about 3~5 mm between each other. Actually, among them attracted to each other and must keep equal distances of about 3~5 mm between each other. there must be an them isolating structure as a polymer spacer) to prevent them spacer) from close contact. Actually, among there must be(such an isolating structure (such as a polymer to prevent The FBG is close fixed contact. on a beam uniform strength. It is of insulated and bottom surfaces them from TheofFBG is fixed on a beam uniformbetween strength.the It top is insulated between the of the diaphragm. When there is a difference in the pressure between the top and bottom surfaces of top and bottom surfaces of the diaphragm. When there is a difference in the pressure between the the and diaphragm, the center diaphragm leading the movement of magnet top bottom surfaces of of thethe diaphragm, themoves centerup of or thedown, diaphragm moves up or down, leading B through the rigid link, therefore synchronously driving magnet C by means driving of the magnetic the movement of magnet B through the rigid link, therefore synchronously magnet force. C by The side of the S pole of magnet C is connected to the movable end of the cantilever beam; therefore, means of the magnetic force. The side of the S pole of magnet C is connected to the movable end of the cantilever difference beam; of the pressure transferred toof the FBG. the therefore,isthe difference the pressure is transferred to the FBG.
Figure of sensitive sensitive unit unit with withmagnetic magnetictransfer; transfer;(b) (b)the therelationship relationshipofofmagnets magnetsA,A,B Figure 4. 4. (a) (a) The The structure structure of B and C. and C.
2.4. The Position Limiting Mechanism 2.4. The Position Limiting Mechanism In the real design, we enabled the sensor to resist overload by introducing a position limiting In the real design, we enabled the sensor to resist overload by introducing a position limiting mechanism [13]. In our structure, two points are needed in the careful design, as seen in Figure 5; mechanism [13]. In our structure, two points are needed in the careful design, as seen in Figure 5; one is for magnet B to move and the other is for the elastic corrugated diaphragm. The two points one is for magnet B to move and the other is for the elastic corrugated diaphragm. The two points are are limited to prevent the damage of the diaphragm and magnet and to avoid the invalidity of the limited to prevent the damage of the diaphragm and magnet and to avoid the invalidity of the sensor. sensor.
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Figure Figure5.5.The Theposition positionlimiting limitingmechanism mechanismofofthe thesensor. sensor.
2.5. 2.5.The TheTemperature TemperatureCompensation Compensation When Whenaaforce forceisisapplied appliedtotothe thebeam, beam,its itsupper uppersurface surfaceexperiences experiencesaastretching, stretching,corresponding corresponding negative deformation, while the lower surface is compressed and experiences positive negative deformation, while the lower surface is compressed and experiences positivedeformation, deformation, but butthe thedeformation deformationofofthe theupper upperand andlower lowersurfaces surfacesisisequal equalininabsolute absolutevalue. value.Therefore, Therefore,when whentwo two FBGs temperature sensitivity coefficient areare pasted on on thethe upper andand lower surfaces of FBGswith withthe thesame same temperature sensitivity coefficient pasted upper lower surfaces the beam, respectively, the wavelength shifts of the two gratings change to the opposite direction of the beam, respectively, the wavelength shifts of the two gratings change to the opposite direction with withan anidentical identicalvalue valueofofthe theshifts. shifts.ItItshould shouldbe bepointed pointedout outthat thattwo twofiber fibergratings gratingsare arelocated locatedinin the and the thedifference differencebetween betweenthe the shifts gratings be thesame same temperature temperature environment, environment, and shifts of of twotwo gratings willwill be kept kept unchanged, although the change of the temperature may cause an individual shift for each unchanged, although the change of the temperature may cause an individual shift for each grating. grating. Thatthe means the temperature effect on the grating is and eliminated and gets the agrating gets a That means temperature effect on the grating is eliminated the grating compensation compensation in temperature. in temperature. 3.3.Simulations SimulationsofofPressure PressureCorrugated CorrugatedDiaphragm Diaphragmand andTest Testof ofSensor Sensor 3.1.The TheSimulations Simulationsofofthe theElastic ElasticCorrugated CorrugatedDiaphragm Diaphragm 3.1. To obtain obtain the of the and toand verify consistency of the diaphragm To theoptimal optimalparameters parameters of diaphragm the diaphragm to the verify the consistency of the design with the actual case, we adopted ANSYS 14.0 (ANSYS Inc., Pittsburgh, PA, USA) to simulate the diaphragm design with the actual case, we adopted ANSYS 14.0 (ANSYS Inc., Pittsburgh, PA, pressure diaphragm. ANSYS a general analysis software on the analysis finite element method which United States) to simulate the is pressure diaphragm. ANSYSbased is a general software based on allows us to carry out the study of the structure with heat, sound, fluid and electromagnetic fields. the finite element method which allows us to carry out the study of the structure with heat, sound, The electromagnetic 1:1-scale three-dimensional (3D) model of the elastic corrugated diaphragm is imported into fluid and fields. ANSYS first, as three-dimensional shown in Figure 6. In thismodel figure,ofthe of the full-size diaphragm is 100 mm, The at 1:1-scale (3D) thediameter elastic corrugated diaphragm is imported and the corrugation type is chosen as “triangle waveform” (6 rings) with a corrugation depth of 2 mm. into ANSYS at first, as shown in Figure 6. In this figure, the diameter of the full-size diaphragm is Themm, material diaphragmtype is stainless steel And the parameters the diaphragm are listed 100 and of thethe corrugation is chosen asfilm. “triangle waveform” (6 of rings) with a corrugation in Table depth of 1. 2 mm. The material of the diaphragm is stainless steel film. And the parameters of the Then its mechanical property in a certain range of pressure differences (0~10 kPa). diaphragm we are simulated listed in Table 1. As an example, Figure 7 gives the deformation and force distribution of the diaphragm under a differential pressure of 10 kPa when the thickness is 0.2 mm, where the pressure difference may be reflected through stress at the center of the diaphragm.
Figure 6. The 1:1 scale 3D model of the elastic corrugated diaphragm.
the finite element method which allows us to carry out the study of the structure with heat, sound, fluid and electromagnetic fields. The 1:1-scale three-dimensional (3D) model of the elastic corrugated diaphragm is imported into ANSYS at first, as shown in Figure 6. In this figure, the diameter of the full-size diaphragm is 100 mm, and the corrugation type is chosen as “triangle waveform” (6 rings) with a corrugation Sensorsdepth 2017, 17, of 375 2 mm. The material of the diaphragm is stainless steel film. And the parameters of the 6 of 9 diaphragm are listed in Table 1.
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Table 1. Parameters of the diaphragm.
Parameters Value Figure 6. The scale 3Dmodel model of of the diaphragm. Figure 6. The 1:11:1 scale 3D the elastic elasticcorrugated corrugated diaphragm.
Diameter of full size 100 mm Edge width 2 mm Table 1. Parameters of the diaphragm. Number of corrugation 6 Depth of corrugation 2 mm Parameters Value Spacing of corrugation 6 mm Diameter of full size 100 mm material stainless steel Edge width 2 mm Number of corrugation 6 Then we simulated its mechanical property in a certain range of pressure differences (0~10 Depth of corrugation 2 mm kPa). As an example, Figure 7 gives the deformation and force distribution of the diaphragm under Spacing of corrugation 6 mm a differential pressure of 10 kPa when the thickness is 0.2 mm, where the pressure difference may material stainless steel be reflected through stress at the center of the diaphragm.
(a)
(b) Figure 7. (a) Deformation diaphragm; (b) distribution of diaphragm. Figure 7. (a) Deformation of of diaphragm; (b)Force Force distribution of diaphragm.
In the real design, the edge of a diaphragm of 2 mm is reserved in advance to link with the of the the sensor and of to isolate the fluids of at both sides the diaphragm. That means the with the In theother realpart design, edge a diaphragm 2 mm is ofreserved in advance to link distribution at thetoedge is kept unchanged. other part force of the sensor and isolate the fluids at both sides of the diaphragm. That means the force distribution at the edge is kept unchanged.
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In order to get the best diaphragm thickness, we simulated diaphragms with thicknesses of 0.2 mm, 0.1 mm, 0.05 mm, 0.025 mm, and 0.0125 mm, respectively. The simulation results are listed Sensors 2017, 17, 375 7 of 9 in Table 2. In order to get the best diaphragm thickness, we simulated diaphragms with thicknesses of 0.2 Table 2. The simulation results. mm, 0.1 mm, 0.05 mm, 0.025 mm, and 0.0125 mm, respectively. The simulation results are listed in Table 2. Maximum Deformation Maximum Pressure on the Diaphragm Thickness of Diaphragm Center of Diaphragm Table 2. The simulation results.
0.200 mm 0.035 mm Diaphragm Thickness Maximum Deformation of Diaphragm 0.100 mm 0.094 mm 0.200 mm 0.035 mm 0.050 mm 0.172 mm 0.100 mm 0.094 mm 0.025 mm 0.300 mm 0.050 mm 0.172 mm 0.0125 mm 0.300 mm 0.025 mm 0.300 mm 0.0125 mm
0.300 mm
10.899 MPa 26.495 MPa 10.899 MPa 35.854 MPa 26.495 MPa 69.720 MPa 35.854 MPa 136.830 MPa 69.720 MPa
Maximum Pressure on the Center of Diaphragm
136.830 MPa
The simulation results indicate a tradeoff between the thickness of the diaphragm and the The simulation results indicate a tradeoff between the thickness of the diaphragm and the measurement range. When the diaphragm is thick, the sensitivity of the diaphragm is low, which leads measurement range. When the diaphragm is thick, the sensitivity of the diaphragm is low, which to anleads enlarged of measurement with a lower When the diaphragm is too thin, despite to anrange enlarged range of measurement withresolution. a lower resolution. When the diaphragm is too the enhanced sensitivity, the measurement range is reduced and results in a degradation in endurance thin, despite the enhanced sensitivity, the measurement range is reduced and results in a to the impact of in a high pressure difference, resulting damage to theresulting diaphragm. According to degradation endurance to the impact ofeven a high pressureindifference, even in damage to the measurement and resolution, we chose a diaphragm that is mm thick to fabricate the diaphragm.range According to the measurement range and resolution, we0.15 chose a diaphragm that is and 0.15 mm to fabricate and the conduct Figure 8 presents conduct tests.thick Figure 8 presents real tests. picture of the sensor. the real picture of the sensor.
Figure (a)The Thereal real sensor; sensor; (b) inside thethe sensor. Figure 8.8.(a) (b)The Thestructure structure inside sensor.
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3.2. 3.2. Test Systemofofthe theSensor Sensor 3.2.Test TestSystem AA test system was used evaluatethe thelinearity linearityofofthe thesensor, sensor,asasshown 9. Atest testsystem systemwas wasused usedtotoevaluate shownininFigure Figure9.9.
Figure 9. The test system of sensor. Figure 9.9. The test system ofof sensor. Figure The test system sensor.
The The test system included the air pressure difference–generating device, the air pressure display Thetest testsystem systemincluded includedthe theair airpressure pressuredifference–generating difference–generatingdevice, device,the theair airpressure pressuredisplay display device, device, computer, FBGdemodulator demodulator andthe thesensor sensortotobebetested. Theair airpressure–generating pressure–generating device,a aacomputer, computer,ananFBG demodulatorand tested.The pressure–generating device made ofofof stainless steel, which can provide ananaccurate and pressure source. The device was made stainless steel, which cancan provide accurate andstable stable pressure source. The devicewas was made stainless steel, which provide an accurate and stable pressure source. sensor totobebetested on ofofthe the sensor tested wasmounted mounted onananinterface interface thepressure pressure source,and and thecalibrated calibrated pressure The sensor to be was tested was mounted on an interface of the source, pressure source, and the pressure calibrated meter was mounted value ofofvalue the pressure meter meter was mounted onanother another interface ofthe thesource. source. The value thecalibrated calibrated pressure meter pressure meter was on mounted oninterface another of interface of theThe source. The of the calibrated pressure was displayed asasa areference value. The signal from the sensor was analyzed by was displayed reference value. Thelight lightlight signal from thethe sensor was analyzed by the meter was displayed as a reference value. The signal from sensor was analyzed bythe the demodulator demodulator and displayedon onthe thePC. PC. demodulatorand anddisplayed Two produces Twoindependent independentpressure-generating pressure-generatingdevices devicesare aresupposed supposedtotobebeused usedgenerally: generally:one one produces one produces high low pressure atat the same time. However, inin this test, we high pressure and the other produces low pressure the same time. However, this test, we only highpressure pressureand andthe theother otherproduces produces low pressure at the same time. However, in this test, weonly only used used one generator provide high pressure, while the low pressure was fixed simplify the test usedone onegenerator generatortoto toprovide providehigh highpressure, pressure,while whilethe thelow lowpressure pressurewas wasfixed fixedtoto tosimplify simplifythe thetest test system. It was easy to generate differential pressure ranging from 0 to 10 kPa. system. system. ItIt was was easy easy to to generate generate differential differential pressure ranging from 0 to 10 kPa. 4.Results Results 4.4. Results Thetest testresults results curve) are shown in in Figure 10. 10. We fitted the data to a straight The calibration curve) are shown We the data totoa a The results(the (thecalibration calibration curve) are shown inFigure Figure 10.have Wehave havefitted fitted the data line with λBwith =with 0.0112 P + 1568.780, with a linear coefficientcoefficient of 0.994, and we obtained the straight line λλ B B=× × ×PP+ +1568.780, with a alinear ofof0.994, and straight line =0.0112 0.0112 1568.780, withcorrelation linearcorrelation correlation coefficient 0.994, andwe we sensitivity of the sensor as 0.0112 nm/kPa. The relations between the central wavelength shift of the obtained the sensitivity of the sensor as 0.0112 nm/kPa. The relations between the central obtained the sensitivity of the sensor as 0.0112 nm/kPa. The relations between the central FBG and the differential pressure were almost linear. wavelength shift ofofthe and differential pressure wavelength shift theFBG FBG andthe the differential pressurewere werealmost almostlinear. linear.
Figure 10. The differential pressure sensor linearity curve. Figure 10. The differential pressure sensor linearity curve. Figure 10. The differential pressure sensor linearity
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5. Conclusions In this paper, a differential pressure sensor with magnetic transfer was proposed, and a non-electric measurement method based on an FBG was introduced, which can avoid the direct contact of the sensing unit with the measuring fluid. In this design, the position limiting mechanism and temperature compensation design were also considered. The test results demonstrated that the sensor is effective to measure differential pressure in the range of 0~10 kPa with a sensitivity of 0.0112 nm/kPa, and it can operate in high temperature, strong corrosion and high overload conditions. Acknowledgments: The research is supported in part by the Science and Technology Project of Hei Long Jiang Province Science and Technology Department (GZ11A306). Project (1252CGZ-H11) in the Hei Long Jiang Province Department of Education, Harbin Technology Bureau. Project (2013AE1BW050), released by the Chinese Patent Office: Patent (No. 2014105889286) and (2014105889286). Author Contributions: G.L. and X.J. conceived and designed the experiments; G.C. and K.W. performed the experiments; J.L., Y.H. and L.G. analyzed the data; G.C. contributed analysis tools; K.W. and X.J. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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