sensors Article

An Improved Measurement Method for the Strength of Radiation of Reflective Beam in an Industrial Optical Sensor Based on Laser Displacement Meter Youngchul Bae Department of Electrical and Semiconductor Engineering, Chonnam National University, Daehak-ro 50 Yeosu, Jeonnam 59626, Korea; [email protected]; Tel.: +82-61-659-7315 Academic Editors: Suk-Seung Hwang, Euntai Kim, Sungshin Kim and Keon Myung Lee Received: 18 April 2016; Accepted: 17 May 2016; Published: 23 May 2016

Abstract: An optical sensor such as a laser range finder (LRF) or laser displacement meter (LDM) uses reflected and returned laser beam from a target. The optical sensor has been mainly used to measure the distance between a launch position and the target. However, optical sensor based LRF and LDM have numerous and various errors such as statistical errors, drift errors, cyclic errors, alignment errors and slope errors. Among these errors, an alignment error that contains measurement error for the strength of radiation of returned laser beam from the target is the most serious error in industrial optical sensors. It is caused by the dependence of the measurement offset upon the strength of radiation of returned beam incident upon the focusing lens from the target. In this paper, in order to solve these problems, we propose a novel method for the measurement of the output of direct current (DC) voltage that is proportional to the strength of radiation of returned laser beam in the received avalanche photo diode (APD) circuit. We implemented a measuring circuit that is able to provide an exact measurement of reflected laser beam. By using the proposed method, we can measure the intensity or strength of radiation of laser beam in real time and with a high degree of precision. Keywords: laser ranger finder; optical sensor; reflect signal; return signal; radiation intensity; phase delay; strength of radiation

1. Introduction An optical sensor such as a laser range finder (LRF) remotely measures distances by using the laser wavelength that is reflected and returned from a target. The optical sensor has been mainly used to measure the distance between a launch position and the target. Helicopters, tanks, and armored vehicles of military and defense forces have been equipped with them because the optical sensor comparatively provides exact measured distance to the target. Typically, an optical sensor techniques based on laser displacement meter (LDM) can be divided into the following types: time-of-flight [1–4], triangulation [5,6], frequency modulation continuous-wave (FMCW) phase measurement [7,8], phase shift [9–14], interferometry method [15–19] and others [20–22]. Among these types, interferometry method is well-known for providing the highest resolution [15–19]. However, the optical sensor has a technical difficulty for making it into an industrial product because industrial products require more precise technique compared to products used in military. The measuring-distance range for the military is more than several km, and, in accordance with the precise military standard, the measuring-error parameters of the optical sensors are within 5 m to 10 m. Although price unit of the optical sensor for military rather than for industry is much more expensive, there is no problem for the prices of laser and parts for the optical sensors in the military. However, the prices of the laser and its parts to manufacture the optical sensors are relatively expensive in industrial products compared to military products. Sensors 2016, 16, 752; doi:10.3390/s16050752

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The efforts to transit the optical sensor into a commercial product, from the military to industry, has continued. To commercialize a product in the optical-sensor industry, it is necessary to manufacture a product with an increased precision, a high reliability, a low price, and a small size that is lightweight. The optical sensor can be applied in automated systems in the field including those commercial product that provide the distance measurements that protects a ship [23] from the damage as it is positioned alongside a pier, unmanned over-speed sensors and the collision-avoidance systems of vehicles [24]. Typically, an industrial optical sensor requires a measuring distance within 1 km and a measuring error within 1 mm to 10 mm to be compatible with industry, due to the characteristics that are required for unmanned industrial systems. An optical sensor can measure precise distances using high re-flexibility and straight features, both of which are laser-beam characteristics. However, the strength of radiation of reflected laser beam at the target rapidly changes according to the distance and the surface status of the reflective object. As a consequence, the variation range of the measurement errors is largely influenced by the intensity (strong and weakness) of this reflected laser beam. In this case, to reduce measurement error, we have to greatly increase the number of measurements and then calculate the average value during a short time, within microseconds; as a result, the measurement-error variation range can be reduced. However, this is not reasonable because the measurement time is proportionally longer than the number of repeated measurements. Therefore, in a high-speed operation, a different method to reduce error is needed. The errors in the industrial optical sensor are numerous and various. Many researchers focus on reduction of errors in the industrial optical sensor based on laser displacement meter. There are many sources of errors in the optical sensor such as statistical errors, drift errors, cyclic errors, alignment errors and slope errors in LRF or LDM. Among these errors, an alignment error is the most serious error in industrial optical sensors, and is caused by the dependence of the measurement offset upon the strength of radiation of returned beam incident upon the focusing lens from the target. The alignment error contains measurement error for the strength of radiation of laser beam. Perchet et al. [14] proposed magnification of the phase-shift for LDM. This system uses two coherent indirect frequency synthesizers, with the reference signal and photoelectrical signal; however, this method also does not provide the circuit for error measurement for returned laser beam signal from target. Hashemi et al. [20] studied the source of error in LRF, and they demonstrate several errors such as statistical error, drift error, cyclic error, alignment error and slope error. However, they did not propose the circuit for error measurement. In order to measure the error for the strength of radiation of laser beam, we have to compensate the difference of phase delay in the APD. However, the optical demodulated signal for measuring are different from each other at the focusing lens, so it is hard to measure the strength of radiation of reflective laser beam. In this case, because the reflected signal level and phase difference in the target is not constantly reflected according to the strength of radiation of laser beam, variation factor of reflected signal level of laser beam take place according to the strength and amount of radiation of laser beam. In accordance with this variation factor, the error for result of measurement in LDM occurs. The variation factor of reflected signal level can largely be divided into two types. One is variation by surface re-flexibility in the target with identical distance. The other is that variation for distance differences exists in the target, even though they have the same surface re-flexibility. Therefore, we require a compensating technique for error by the variation of reflective signal level of laser beam in the industrial optical sensor. In this paper, our sole focus is the reduction of the measurement error for the strength of radiation of returned laser beam from the target among alignment errors in the industrial optical sensor based on LDM. The reduction of measurement error in the industrial optical sensor is important to make a precise industrial optical sensor. This measurement error causes the phase delay that occurs in the APD output signal and its extent depends on the strength of the reflective laser beam that is

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returned from the target of the object. Even though this measurement error is very significant, an exact measuring-circuit technique to measure the error has not yet been developed. The difficulty of this situation is compounded because we are not only unable to measure an error, but also we cannot try to reduce the incidence of errors. To compensate for an error is difficult, because the difference of the phase delay occurs after the intensity of the reflective laser beam is measured. Additionally, it is difficult to measure the exact radiation intensity due to the differences between the optical modulated waves. In this paper, in order to solve these problems, we propose a novel method for the measurement of the output of DC voltage that is proportional to the strength of radiation of returned laser beam in the received APD circuit. We therefore implemented a measuring circuit that is able to provide an exact measurement of reflected laser beam. By using the proposed method, we can measure the intensity or strength of radiation of laser beam in real time and with a high degree of precision. 2. Optical Sensor In this section, we describe the basic and application principles of an optical sensor, configuration of industrial optical sensor for high precision, heterodyne technique, current-to-voltage converter and detection of phase difference. 2.1. Basic Principle of an Optical Sensor Basically, the optical sensor refers to the LDM. The most important parts in the optical sensor are the laser rangefinder and the displacement meter of laser that can be represented as the external interferometer. The optical sensor is basically organized by the laser diode, lens, beam splitter, mirror and photodetector, which are called the optical transducer. This device of the optical sensor can be used to measure the distance to an object by measuring the variation of the frequency, which is the difference of phase displacement of laser beam in the optical frequency, using an optical laser. In other words, the optical sensor measures the distance by measuring the phase difference between the returned signals reflected back from the target and mirror, in which both signals refer to the transmitted modulation signal generated from a laser diode (LD). Generally, the principle of the LDM can be divide into two techniques: homodyne or heterodyne. When the homodyne technique is used, variation of the original frequency of laser beam and measured frequency is identical. On the other hand, the heterodyne technique uses nonlinearly mixed frequency with an original frequency and the scattered beam with almost close-by constant frequency. Basically, Michelson interferometer and Mach-Zehnder interferometers are the common systems of homodyne and heterodyne systems, respectively. 1 that Figure 1 shows the basic principle of an optical sensor based on the LDM. The beam ( ) emanates from the LD travels to the target that is the measuring object. Because this beam meets the beam splitter when travelling, the beam is divided into two paths by the beam splitter. Then, they travel separately along two paths that are reflective mirror and target. 2 (typically less than 3% to 5% of the total beam generated from LD) is One part of the beam ( ) selected for one path and sent to the reflective mirror through the beam splitter. This beam approaches the reflective mirror and is then reflected by the mirror so that it is returned again to the beam splitter. 3 from the reflective mirror meet again at the beam splitter, When the returned reflective beam ( ) this beam splitter also has same function; it divides the reflective beam into two directions, LD and 3 4 photodetector. Among the total returned beam from the reflective mirror ( ), about 95% to 97% ( ) 5 travels to LD through the of the strength of radiation travels to photodetector and rest (3% to 5%) ( ) beam splitter. Because this returned signal to the photodetector, which is called photo diode (PD), is used as a reference to measure the beam returned from another direction, we call this the “reference signal beam”.

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Figure1.1.Basic Basicprinciple principleofofthe thelaser laserdisplacement displacement meter Figure meter(LDM). (LDM). 6 (typically more than 95% to 97% of Another part of the beam, involving most of the beam ((⑥)

) the total beam generated from LD), operates quite similar to the processing in the reflective mirror used as as reference referencesignal. signal.This Thisbeam beam goes target through the beam splitter and focusing lens used goes to to thethe target through the beam splitter and focusing lens from from the original LDisand also reflected the target. this beam is returned (⑦) to again the 7 again the original LD and alsoisreflected off theoff target. Then, Then, this beam is returned ( ) the to beam beam splitter through the focusing lens and separated is also separated into photodetector LD.this When this splitter through the focusing lens and is also into photodetector and LD.and When beam is beam is off reflected off the it is scattered into the area. surrounding Basically,beam this has returned reflected the target, it is target, scattered into the surrounding Basically,area. this returned to be beam hasinto to be the focusing lens from the thethe beam is scattered, the collected thecollected focusing into lens from the target. Because the target. beam isBecause scattered, focusing lens cannot focusing lens cannot collect the entire beam, and as a result, the strength of the radiation of the collect the entire beam, and as a result, the strength of the radiation of the beam will be weakened and beam will beThus, weakened and not cause constant. Thus, these behaviors cause the generation of error. not constant. these behaviors the generation of error. Among the total returned beam from Among theabout total 95% returned beam target,ofabout 95% travel to 97%to(⑧) theabout strength of5% radiation 8 from 9 of the target, to 97% ( ) of thethe strength radiation LD of and 3% to ( ) travel to LDofand about 3% to to 5%another (⑨) of photodetector, the strength ofwhich radiation travel another photodetector, the strength radiation travel is called theto“avalanche photo diode” which is called photo beam” diode”or(APD). Thus,signal we call this “target signal beam” or (APD). Thus, we the call “avalanche this “target signal “measuring beam”. “measuring beam”. 5 ) 9 The twosignal returned signal beams ( , arrive in the photodetector, which is organized by TheAPD. two returned signal beamssignal (⑤, ⑨) arrive in the photodetector, is organized PD PD and These two returned beams have a phase differencewhich between reference by signal and APD. returned a phase difference signal beam beam and These target two signal beam signal due tobeams a timehave delay according to thebetween distancereference differences between and mirror target signal beam due to two a time delay according to the( , distance differences between mirror 5 ), 9 the and object. With returned signal beams we can determine thethe distance and object. two returned signaldifferences beams (⑤,between ⑨), wethe canreference determine the beam distance the 5 through through the With calculation of the phase signal ( ) that comes calculation of the phase between thesignal reference signal beam (⑤) from that comes from 9 that from the reflective mirrordifferences and the reflected target beam ( ) comes the target in the reflective mirror and the reflected target beam (⑨) that comesthe from target in the photodetector. However, when the laser beamsignal is reflected through the target, laserthe beam is scattered photodetector. However, when laser beam reflected theradiation target, the beam is and the intensity of radiation willthe be reduced. As is a result, the through strength of thatlaser arrives at the scattered and is thedifferent intensity of radiation beofreduced. a result, the strength that photodetector according to thewill kind surface, As shape and material mediaofofradiation the reflective arriveswhich at the causes photodetector according to the of surface, shape and material media target, an error is indifferent the distance measured by kind the optical sensor. of the reflective target, which causes an error in the distance measured by the optical sensor. 2.2. Configuration of Industrial Optical Sensor for High Precision 2.2. Configuration of Industrial Optical Sensor for High Precision Figure 2 shows the configuration of an industrial optical sensor based on LDM for high precision.

It consists of 2a field programmable gate array (FPGA) including the digital splitter, Figure shows the configuration of an industrial optical sensormixer, baseda beam on LDM for APD, high LD, PD, pre-amplification part, control part and its connecting microprocessor that controls the entire precision. It consists of a field programmable gate array (FPGA) including the digital mixer, a beam displacement thePD, optical sensor. splitter, APD,ofLD, pre-amplification part, control part and its connecting microprocessor that Twothe laser beams enter into the digital mixer through APD and PD. The reference signals come controls entire displacement of the optical sensor. into the digital mixer from PD and the measuring (reflective) signal enters into the digital mixer from APD (in Figure 2). The reference signal and measuring signal can describe by Equations (1) and (2), respectively.

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Figure 2. Basic Basic configuration configuration of the industrial optical sensor with high precision.

Two laser beams enter into the digital mixer“through and PD. The reference signals come Sre f erence A sin ωtAPD (1) 1 into the digital mixer from PD and the measuring (reflective) signal enters into the digital mixer Smeasur B sin ωtsignal 2 from APD (in Figure 2). The reference signal and “ measuring can describe by Equations (1) and (2) (2), respectively. From Equations (1) and (2), we can rearrange into Equation (3) through each of the digital mixers. = sin (1) Dm “ A sin ωt1 ˆ B sin ωt2 (3) (2) = sin Equation (3) can be (4). From Equations (1)rewritten and (2), as weEquation can rearrange into Equation (3) through each of the digital mixers. AB rcos pωt1 ´ ωt2 q ´ cospωt1 ` ωt2 qs (4) Dm “ 2 (3) = sin × sin From Equation (4), can get aas3-kHz frequency Equation (3) can bewe rewritten Equation (4). by adjusting A and B of the amplitude coefficient. Because coefficient A is fixed and coefficient B is variable, coefficient B affects Equations (3) and (4). (4) ) − cos(ω = [cos( in − + )]B amplitude. Hence, we get the necessary reflective signal the LDM as controlling 2 However, in this case, the reflected signal level and phase difference in the target is not constantly Fromaccording Equationto (4), westrength can get of a 3-kHz frequency adjusting and B ofcaused the amplitude coefficient. reflected the radiation of laserby beam; thus,A variation by reflected signal Because coefficient A is fixed and coefficient B is variable, coefficient B affects Equations (3) and (4). level of laser beam take place according to the strength and amount of radiation of the laser beam. Hence, we get the necessary reflective the LDMofasmeasurement controlling B amplitude. In accordance with this variation, thesignal errorin for result in LDM occurs. The variation However, in this case, the reflected signal level and difference the target is not caused by the reflected signal level can largely be divided intophase two types. One isin variation by surface constantly reflected according to the strength of radiation of laser beam; thus, variation caused re-flexibility in the target with identical distance. The other is that the variation for difference by of reflected signal level of laser beam take place according to the strength and amount of radiation of distance exists in the target, even though they have the same surface re-flexibility. Therefore, we the laser beam. In accordance with this variation, the error for result of measurement in LDM require a compensating technique for the error by the variation of reflective signal level of laser beam occurs. The variation caused by the reflected signal level can largely be divided into two types. One in the industrial optical sensor. is variation by surface re-flexibility in the target with identical distance. The other is that the variation for difference 2.3. Heterodyne Technique of distance exists in the target, even though they have the same surface re-flexibility. Therefore, we require a compensating technique for the error by the variation of The heterodyne technique uses the heterodyne-detection principle, whereby a nonlinear optical reflective signal level of laser beam in the industrial optical sensor. process—its frequency is nonlinearly mixed with a reference signal from a local oscillator—is used. Figure 3 shows Technique the basic heterodyne technique that consists of a beam splitter and a local oscillator (LO). 2.3. Heterodyne The laser beam modulates the frequency signal that is proportionate to the maximum The heterodyne uses thebeam heterodyne-detection principle, wherebyin aheterodyne. nonlinear measurement distance,technique and the reflected from the target is then demodulated optical process—its frequency nonlinearly a reference signal a local The heterodyne can measure the is distance after it mixed uses thewith original modulated signalfrom to detect the oscillator—is used. Figure 3 shows the basic heterodyne technique that consists of a beam splitter phase difference. and a local oscillator (LO).

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Figure 3. Heterodyne technique. Figure 3. Heterodyne technique.

The modulated proportionate to that the maximum measurement is laser beam frequency modulatesthat theisfrequency signal is proportionate to the distance maximum represented bydistance, Equationand (5), the as follows: measurement reflected beam from the target is then demodulated in heterodyne. The heterodyne can measure the distance after it uses the original modulated signal to detect the c fm “ Hz (5) phase difference. 2d The modulated frequency that is proportionate to the maximum measurement distance is represented Equation (5), as follows: where c is thebyspeed of beam 3 ˆ 108 m{s and d is the maximum measurement distance. When we directly detect the phase difference between the original modulated signal and the Hz demodulated signal, the error range is extended. = We2 therefore need to transform the frequencies of(5) the two signals into a lower frequency that makes it easy to measure the phase. where is the speed of beam 3 × 10 m/s and is the maximum measurement distance. The lower frequency is called the “intermediate frequency” and can be represented by Equation When we directly detect the phase difference between the original modulated signal and the (6), as follows: demodulated signal, the error range is extended. We therefore need to transform the frequencies of fi “ fl ˘ fm (6) the two signals into a lower frequency that makes it easy to measure the phase. The frequency frequency is called and the f “intermediate where f l islower local oscillation frequency. and can be represented by m is maximum frequency” Equation (6),acquire as follows: We can the intermediate frequency in the demodulated circuit at any time after the local oscillation frequency ( f l q is found. In this paper,=we use the ± the modulated frequency that transforms(6) same frequency, and the intermediate frequency ( f i q serves as a reference signal. where is reference local oscillation frequency and isfrequency maximumasfrequency. Let the f re f equal the intermediate in Equation (7), and let the ratio of the We can acquire the intermediate frequency in the demodulated frequency divide pNq be obtained according to Equation (8), as follows: circuit at any time after the local oscillation frequency ( ) is found. In this paper, we use the modulated frequency that transforms the same frequency, and the intermediate ( ) serves as a reference signal. (7) f re f “ ffrequency i Let the reference equal the intermediate frequency as in Equation (7), and let the ratio of fm the frequency divide ( ) be obtained according N “to Equation (8), as follows: (8) fi = (7) 2.4. Current-to-Voltage Converter = (8) There are two methods for the amplification of the signal of modulated weak laser beam. One is a simple voltage-amplification method, and the other is the current-to-voltage-converter method that is also called “trans-impedance 2.4. Current-to-Voltage Converter amplification” (TIA). The simple voltage-amplification method has the disadvantage of linear characteristics of output voltage compared to the TIA method. The TIA There are two methods for the amplification of the signal of modulated weak laser beam. One transforms the current signal into a voltage signal using an operational amplifier; Figure 4 shows a basic is a simple voltage-amplification method, and the other is the current-to-voltage-converter method TIA circuit. Generally, the TIA can be used to amplify the photo detectors to a usable voltage in the that is also called “trans-impedance amplification” (TIA). The simple voltage-amplification method optical sensor. Because current-to-voltage converters that are used in the TIA have a current response has the disadvantage of linear characteristics of output voltage compared to the TIA method. The that is linear to the voltage response, it is more popular to use than a simple voltage-amplification TIA transforms the current signal into a voltage signal using an operational amplifier; Figure 4 method. Thus, in this paper, we adapted the TIA method. shows a basic TIA circuit. Generally, the TIA can be used to amplify the photo detectors to a usable For this paper, we implemented a modified TIA for which the basic circuit of Figure 4 was used, voltage in the optical sensor. Because current-to-voltage converters that are used in the TIA have a as is shown in Figure 5. current response that is linear to the voltage response, it is more popular to use than a simple voltage-amplification method. Thus, in this paper, we adapted the TIA method. For this paper, we implemented a modified TIA for which the basic circuit of Figure 4 was used, as is shown in Figure 5.

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Figure 4.4. Basic circuit ofof the trans-impedance amplification (TIA). Figure 4. Basic circuit of the trans-impedance amplification (TIA). Figure Basic circuit the trans-impedance amplification (TIA).

Figure 5.Modified Modified circuit of of the TIA TIA from Figure Figure 4. Figure Figure 5. 5. Modified circuit circuit of the the TIA from from Figure 4. 4.

From Figure Figure 5, 5, we we can can arrange arrange an an input input resistance resistance and and frequency frequency by by following following Equations Equations (9) (9) From From Figure 5, we can arrange an input resistance and frequency by following Equations (9) and and(10), (10),respectively. respectively. and (10), respectively. V RF = IN = = (9) (9) R I N “= “ (9) + I I N 111+ ` Avol 1 11 = (10) f ´3dB “= (10) (10) 2 2 2πR I N C I N The dynamics dynamics range range is is represented represented by byEquation Equation(11) (11)as asfollows: follows: The =M = De “ P where M M and and PP represents represents maximum maximum input input current current and and peak peak noise noise current, current, respectively. respectively. where where M and P represents maximum input current and peak noise current, respectively.

(11) (11) (11)

2.5. Detection Detection of of Phase Phase Difference Difference 2.5. 2.5. Detection of Phase Difference To detect detect the the phase phase difference, difference,we we used usedEquation Equation(12), (12),as asfollows: follows: To we used Equation (12), as follows: To detect the phase difference, (12) × (12) Tx2 × 2 ˆ DM (12) 2 where is the the phase phase difference difference with with the the reference reference signal signal and and is the the distance distance of of maximum maximum where is is measurement. The precision of and are dependent upon the counter clock and the the measurement. The precision of and are dependent upon the counter clock and wavelength of of the the modulated modulated signal, signal, respectively. respectively. wavelength

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where Tx is the phase difference with the reference signal and D M is the distance of maximum measurement. The precision of Tx and D M are dependent upon the counter clock and the wavelength Sensors 2016, 16, 752 8 of 13 of the modulated signal, respectively.

3. Measurement 3. MeasurementCircuit Circuitfor forStrength Strength of of Radiation Radiation of of Reflective Reflective Laser Laser Beam Beam In this this section, section, we we describe describe an an error error source, source, principle principle of of measurement, measurement, measurement measurement circuit circuit for for In strength of of radiation radiation of of returned returned laser laser beam beam and and then then explain explainthe theexperimental experimentalresult resultand andreview. review. strength 3.1. Error Source errors in in the the industrial industrial optical optical sensor based on the LDM LDM are are numerous numerous and and The reasons for errors various. Many researchers carry out research focused on reduction of errors in the industrial optical various. in the industrial optical sensor. There are many sources of errors in the optical sensor such as statistical errors, drift errors, cyclic errors, alignment errors and and slope slope errors. errors. Among these errors, the alignment error is is caused caused dependence of the measurement offset upon the the width, width, position, and strength of radiation radiation of by the dependence focusing lens. Generally, the biggest error in the optical sensor the returned beam incident upon the focusing based on LDM is the measurement error by the difference difference of of phase phase delay delay of of APD. APD. This This error error caused caused 1 the APD APD′ss output signal according to the strength of radiation of the by the phase delay occurring in the the focusing focusing lens lens through through the the target. target. Nevertheless, Nevertheless, this error is very very large, and and we we returned beam at the cannot make makeany anyeffort effort to reduce this error as well as measure the error because the exact to reduce this error as well as measure the error because the exact measuring measuring circuits have not been developed. circuits have not been developed. In order order to tomeasure measurethe theerror, error,we wehave havetotocompensate compensate the difference phase delay. However, the difference of of phase delay. However, as as the optical demodulated signals measuring different other at focusing the focusing the optical demodulated signals for for measuring are are different fromfrom eacheach other at the lens,lens, it is it is hard to measure the strength of radiation of reflective beam. hard to measure the strength of radiation of reflective beam. In this paper, to solve this problem, we need to develop circuit that that can can measure measure the the precise precise In this paper, to solve this problem, we need to develop aa circuit reflective beam beam from from the the target. target. Therefore, Therefore, we we propose propose aa novel novel precise precise measurement measurement circuit circuit of of the the reflective reflective beam beam based based on on when when we we measure measure the the output output of of DC DC that that is is proportional proportional to to the the strength strength of of reflective radiation in the receiving circuit of APD. We can measure the strength of radiation in real time. radiation in the receiving circuit of APD. We can measure the strength of radiation in real time. 3.2. Principle of Measurement for Strength of of Radiation Radiation of of Returned Returned Laser Laser Beam Beam When the strength strength of ofthe theradiation radiationofofthe thelaser laser beam that is returned from target is high, beam that is returned from thethe target is high, the the output voltage of the APD receiving circuitisislow lowand andthe themaximum maximum value value of of the received DC DC output voltage of the APD receiving circuit received modulated signal the reflective beam is signal becomes becomeslarge. large.Conversely, Conversely,when whenthe thestrength strengthofofradiation radiationofof the reflective beam low, thethe DCDC output voltage of theofAPD received circuitcircuit is highisand theand maximum value of value the received is low, output voltage the APD received high the maximum of the modulated signal becomes These principles are shownare in shown Figure in 6. Figure 6. received modulated signal small. becomes small. These principles

Figure 6. 6. Principle for the the strength strength of of radiation radiation of of reflective reflective beam. beam. Figure Principle of of measurement measurement for

Generally, the maximum value of a real received modulated signal has only about 4% for the value of output DC level. In Figure 6, however, the output signal of the received modulated signal is large compared to the magnitude of the output DC level for an easier understanding process. 3.3. Measurement Circuit Although a very similar method for measurement of the strength of radiation of the reflective laser beam [25] was proposed, this method does not always provide precise measuring results: it

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Generally, the maximum value of a real received modulated signal has only about 4% for the value of output DC level. In Figure 6, however, the output signal of the received modulated signal is large compared to the magnitude of the output DC level for an easier understanding process. 3.3. Measurement Circuit Sensors 2016, 16, 752 of 13 Sensors 2016, 16, 752 of 13 Although a very similar method for measurement of the strength of radiation of the reflective99laser

beam [25] was proposed, this method does not always provide precise measuring results: it depends depends on and weak signal. Because this method does have method for depends on noise noise weak signal.this Because this method doesa not not have aa preprocessing preprocessing for on noise and weakand signal. Because method does not have preprocessing method formethod DC output DC output voltage including filtering for noise reduction or amplification for magnifying weak DC output voltage including filtering for noise reduction or amplification for magnifying weak voltage including filtering for noise reduction or amplification for magnifying weak signals, it cannot signals, constantly measure the strength of the radiation of reflective signals, it it cannot cannot constantly measure strength the radiation of the the reflective laser laser beam. beam. constantly measure the strength of thethe radiation ofof the reflective laser beam. By using the measurement principal without preprocessing method [25], we proposed an By using using the the measurement measurement principal principal without without preprocessing preprocessing method method [25], [25], we we proposed proposed an an By improved amplification circuit with MOSFET with preprocessing method for DC output voltage improved amplification circuit with MOSFET with preprocessing method for DC output voltage improved amplification circuit with MOSFET with preprocessing method for DC output voltage and we also implemented it as circuit of for the of of and we wealso alsoimplemented implemented it the as the the circuit of measurement measurement the strength strength of radiation radiation of the the and it as circuit of measurement for thefor strength of radiation of the reflective reflective laser beam, as shown in Figures 7 and 8. reflective laser beam, in as Figures shown in Figures laser beam, as shown 7 and 8. 7 and 8.

Figure strength of of radiation radiation of of reflective reflective laser beam. Figure 7. 7. Measurement Measurement circuit circuit for for measuring measuring the the strength reflective laser laser beam. beam.

Figure 8. Implemented circuit for measuring the strength of the radiation of the reflective laser beam Figure Figure 8. 8. Implemented Implemented circuit circuit for for measuring measuring the the strength strength of of the the radiation radiation of of the the reflective reflective laser laser beam beam based on Figure 7. based based on on Figure Figure 7. 7.

3.4. Experiments and Review 3.4. Experiments and Review The amplification circuit in Figures 7 and 8 are composed of a combination that comprises the The amplification circuit in Figures 7 and 8 are are composed composed of of aa combination combination that that comprises comprises the function of an I-V converter and an amplification circuit with a high gain. The output current of the gain. function of an I-V converter and an amplification circuit with a high gain. The output current of the APD is emitted as the MOSFET output voltage and the gain is approximately 60 dB. APD APD is is emitted emitted as as the the MOSFET MOSFET output output voltage voltage and and the the gain gain is is approximately approximately 60 60 dB. dB. In this paper, when there is no the strength of radiation of the reflective laser In this paper, when there is no the strength of radiation of the reflective laser beam, beam, we we set set the the DC DC voltage voltage of of the the drain drain of of the the MOSFET MOSFET (( ), ), as as follows follows (Equation (Equation (13)): (13)):

2 2 × ×3 3 In order to satisfy the condition of Equation (13), In order to satisfy the condition of Equation (13), we we set set the the resistor resistor = =

(13) (13) = = 100 kΩ 100 kΩ and and

= =

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In this paper, when there is no the strength of radiation of the reflective laser beam, we set the DC voltage of the drain of the MOSFET (Vd ), as follows (Equation (13)): Vd “ Vcc ˆ

2 3

(13)

In order to satisfy the condition of Equation (13), we set the resistor R1 “ 100 kΩ and R2 “ 30 kΩ and we also set R L “ 100 kΩ so that the value of the drain current Id flows 15 mA to 20 mA, as shown Sensors 2016, 16, 752 10 of 13 in Figures 8 and 9.

Figure 9. DC-offset output voltage shown as: (a) DC 3.3 V; (b) DC 3.1 V; and (c) DC 2.5 V, respectively, Figure DC-offset output voltage shown as: DC are: 3.3 (a) V; none; (b) DC V; and and(c) (c)strong. DC 2.5 V, for when9.the strength of radiation of reflective laser(a) beam (b) 3.1 weak; respectively, for when the strength of radiation of reflective laser beam are: (a) none; (b) weak; and (c) strong.

In this experiment, we obtainedVcc “ `5 V, Id “ 17 mA, and R L “ 100 Ω. When there is no strength of the radiation of reflective laser beam, the drain voltage (Vd q was measured as 3.3 V. In this experiment, we obtained = +5 V, = 17 mA, and = 100 Ω. When there is no The relationship of the gate current and the output voltage is described in Equation (14), as follows: strength of the radiation of reflective laser beam, the drain voltage ( ) was measured as 3.3 V. The relationship of the gate current and the output voltage is described in Equation (14), Vd “ 60 dB, pG “ 1000q (14) as follows: Ig

= 60 dB, ( = 1000)

(14)

When is ±1 μA, we get that is 1 mV. Typically, because the dynamics range of APD is ±20 μA to 50 μA, becomes ±20 mV to 50 mV. However, the strength of radiation of laser beam

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When Ig is ˘1 µA, we get that Vd is 1 mV. Typically, because the dynamics range of APD is ˘20 µA to 50 µA, Vd becomes ˘20 mV to 50 mV. However, the strength of radiation of laser beam transforms the APD-output offset current because the measuring beam of LDM applies to direct optical modulation. As a result, the APD offset current coincides exactly with the strength of radiation of the incident beam. We know that the offset currents that are separated by modulated-signal variation, Sensors 2016, 16, 752 11 of 13 appear proportional to the intensity of the incident beam at MOSFET Vd .

3.5.Experimental ExperimentalResult Result 3.5. Figure99represents representsthe theoutput-voltage output-voltage level that is acquired through implemented circuit Figure level that is acquired through the the implemented circuit for for measuring the strength of radiation of reflective laser beam (Figure 8). Figure 9 shows DC measuring the strength of radiation of reflective laser beam (Figure 8). Figure 9 shows the DCthe offset offset voltage of thethat drain as (Figure 3.3 V, (Figure 3.1 (Figure V and (Figure 2.5 V voltage of the drain as that (Figure 9a) DC9a) 3.3DC V, (Figure 9b) DC9b) 3.1DC V and 9c) DC9c) 2.5 DC V when whenisthere is (a)(b) none, (b)and weak (c) strengths strong strengths of radiation of reflective laser beam. there (a) none, weak (c) and strong of radiation of reflective laser beam. Figure10 10shows showsthe thereceived receiveddemodulated demodulatedoutput outputsignal signalthat thathave have250 250kHz kHzas asfrequency frequencyand and Figure (Figure10a) 10a)15.8 15.8mV mVand and(Figure (Figure10b) 10b)129 129mV mVasasmaximum maximumvoltage, voltage,respectively. respectively. (Figure

Figure10. 10.Output Output signal of received modulated with frequency 250and kHz, and maximum Figure signal of received modulated wavewave with frequency 250 kHz, maximum voltage: voltage: (a) and 15.8 (b) mV; and (b) 129 mV. (a) 15.8 mV; 129 mV.

We recognize that Figures 9 and 10 are an exact match for the computational results from We recognize that Figures 9 and 10 are an exact match for the computational results from Equations (13) and (14). Equations (13) and (14). 3.6. Discussion of Experimental Result From Figures 9 and 10, we recognize that we acquire similar results compared with previous results without preprocessing [25]. The previous method cannot always provide the same quality. However, the proposed method with preprocessing always provides the same results under certain circumstance, such as with noise and weak signal of DC output voltage. Therefore, we expect that the proposed method can be applied in the optical sensor based on LDM with constantly excellent quality. As an experimental result with implemented circuit to measure the strength of radiation of the

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3.6. Discussion of Experimental Result From Figures 9 and 10 we recognize that we acquire similar results compared with previous results without preprocessing [25]. The previous method cannot always provide the same quality. However, the proposed method with preprocessing always provides the same results under certain circumstance, such as with noise and weak signal of DC output voltage. Therefore, we expect that the proposed method can be applied in the optical sensor based on LDM with constantly excellent quality. As an experimental result with implemented circuit to measure the strength of radiation of the laser beam that is returned from the target, we acquire the following results. 1. 2. 3. 4. 5.

The DC offset voltage of the drain is DC 3.3 V when there is no strength of radiation of reflective laser beam. The DC offset voltage of the drain is DC 3.01 V when the strength of radiation of reflective laser beam is weak. The received demodulated output signal has 250 kHz frequency and 8 mV maximum voltage. The DC offset voltage of the drain is DC 2.50 V when the strength of radiation of reflective laser beam is strong. The received demodulated output signal has 250 kHz frequency and 129 mV maximum voltage.

From these results, we know that we can improve the performance of optical sensor. 4. Conclusions In this paper, we proposed a novel method for the measurement of the output of direct current (DC) voltage that is proportional to the strength of radiation of returned laser beam in the received APD circuit. We also implemented a measuring circuit that is able to provide an exact measurement of reflected laser beam. By using the proposed method, we can measure the intensity or strength of radiation of laser beam in real time and with a high degree of precision. From these results, as discussed in the previous section, we confirm that theoretical and experimental results are identical. This conclusion will provide the reduction of measurement error in the optical sensor based on LDM. Therefore we can apply these results to make a real optical sensor based on LDM. Acknowledgments: This study was supported by the Ministry of Knowledge Economy (MKE) through the Regional Innovation Centre Programme. Conflicts of Interest: The author declares no conflict of interest.

References 1. 2. 3. 4. 5. 6. 7.

Markus-Christian, A.; Thierry, B.M.L.; Risto, M.; Marc, R. Laser ranging: A critical review of usual techniques for distance measurement. Opt. Eng. 2001, 40, 10–19. Erkki, I.; Viktor, K. Pulsed Time-of-Flight Laser Range Finder Techniques for Fast, High Precision Measurement Application; Oulu University Press: Oulu, Finland, 2004; pp. 24–29. Yuriy, R. Investigation and Calibration of Pulsed Time-of-Flight Terrestrial Laser Scanners; Roy. Inst. Tech: Stockholm, Sweden, 2006. Kilpelä, A.; Pennala, R.; Kostamovaara, J. Precise pulsed time-of-flight laser range finder for industrial distance measurements. Rev. Sci. Instrum. 2001, 72, 2197–2202. [CrossRef] Bickel, G.; Haulser, G.; Maul, M. Triangulation with expanded range of depth. Opt. Eng. 1985, 24, 975–977. [CrossRef] Dorsch, R.G.; Hausler, G.; Herrmann, J.M. Laser triangulation: Fundamental uncertainty in distance measurement. App. Opt. 1994, 33, 1306–1314. [CrossRef] [PubMed] Dupuy, D.; Lescure, M.; Hélène, T.B. Analysis of an avalanche photodiode used as an optoelectronic mixer for a frequency modulated continuous wave laser range finder. J. Opt. A Pure Appl. Opt. 2002, 4, S332–S336. [CrossRef]

Sensors 2016, 16, 752

8. 9.

10. 11. 12. 13.

14.

15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

13 of 13

Journet, B.; Bazin, G. A low-Cost laser range finder based on an FMCW-Like method. IEEE Trans. Instrum. Meas. 2000, 49, 840–843. [CrossRef] Heesun, Y.; Jinpyo, H.; Seonggu, K. A multiple phase demodulation method for high resolution of the laser range finder. In Proceedings of the 3rd International Conference on Sensing Technology, Tainan, Taiwan, 30 November–3 December 2008; pp. 324–328. Shahram, M.N.; Kiazand, F.; Saeed, O. Modified Phase-Shift Measurement Technique to Improve Laser-Range Finder Performance. J. Appl. Sci. 2008, 8, 316–321. Journet, B.; Poujouly, S. High resolution laser range-finder based on phase-shift measurement method. Proc. SPIE 1998, 3520, 123–132. Liu, S.-Y.; Tan, J.-B.; Binke, H. Multicycle synchronous digital phase measurement used to further improve phase-Shift laser range finding. Meas. Sci. Technol. 2007, 18, 1756–1762. [CrossRef] Hu, P.; Tan, J.; Yang, H.; Zhao, X.; Liu, S. Phase-Shift laser range finder based on high speed and high precision phase-Measuring techniques. In Proceedings of the 10th International Symposium of Measurement Technology and Intelligent Instrument, Daejeon, Korea, 29 June–2 July 2011. Perchet, G.; Lescure, M.; Bosch, T. Magnification of the phase-shift for laser distance measurement. In Proceedings of the IEEE Instrument and Measurement Technology Conference, Ottawa, ON, Canada, 19–21 May 1997; pp. 617–621. Lawall, J.; Kessler, E. Michelson interferometry with 10 p.m. accuracy. Rev. Sci. Instrum. 2000, 71, 2669–2676. [CrossRef] Wu, C.; Su, C. Nonlinearity in measurements of length by optical interferometry. Meas. Sci. Technol. 1996, 7, 62–68. [CrossRef] Gonda, S.; Doi, T.; Kurosawa, T.; Tanimura, Y. Real-time, interferometrically measuring atomic force microscope for direct calibration of standards. Rev. Sci. Instrum. 1999, 71, 2669–2676. [CrossRef] Donati, S.; Giuliani, G.; Merlo, S. Laser Diode Feedback Interferometer for Measurement of Displacements without Ambiguity. IEEE J. Quantum Electron. 1995, 31, 113–119. [CrossRef] Giuliani, G.; Norgia, M. Laser Diode Linewidth Measurement by Means of Self-Mixing Interferometry. IEEE Photonics Technol. Lett. 2000, 12, 1028–1030. [CrossRef] Hashemi, K.S.; Hurst, P.T.; Oliver, J.N. Source of error in a laser rangefinder. Rev. Sci. Instrum. 1994, 65, 3165–3171. [CrossRef] Hausler, G.; Heckel, W. Light sectioning with large depth and high resolution. Appl. Opt. 1988, 27, 5165–5169. [CrossRef] [PubMed] Kim, D.; Cha, H.; Song, K.; Yang, K. Development of Time Counter for Range Finder Using Pulsed Diode Laser. J. KASBIR 2003, 3, 205–211. Hong, T. Building a fusion information system for safe navigation. Int. J. Fuzzy Log. Intell. Syst. 2014, 14, 105–112. [CrossRef] Fu, L.; Wu, W.; Zhang, Y.; Klette, R. Unusual motion detection for vision-based driver assistance. Int. J. Fuzzy Log. Intell. Syst. 2015, 15, 27–34. [CrossRef] Bae, Y.; Cho, E.; Lee, H.; Kim, S.; Kim, H. The measurement method of reflected intensity of radiation for high precision laser ranger finder. J. Adv. Navig. Technol. 2009, 13, 34–40. © 2016 by the author; 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/).