sensors Article

Design and Performance Analysis of an Intrinsically Safe Ultrasonic Ranging Sensor Hongjuan Zhang, Yu Wang, Xu Zhang, Dong Wang and Baoquan Jin * Key Laboratory of Advanced Transducers and Intelligent Control Systems, Ministry of Education, Taiyuan University of Technology, No. 79 Yingzexi Street, Taiyuan 030024, China; [email protected] (H.Z.); [email protected] (Y.W.); [email protected] (X.Z.); [email protected] (D.W.) * Correspondence: [email protected]; Tel.: +86-138-3515-5702 Academic Editor: Piervincenzo Rizzo Received: 24 February 2016; Accepted: 7 June 2016; Published: 13 June 2016

Abstract: In flammable or explosive environments, an ultrasonic sensor for distance measurement poses an important engineering safety challenge, because the driving circuit uses an intermediate frequency transformer as an impedance transformation element, in which the produced heat or spark is available for ignition. In this paper, an intrinsically safe ultrasonic ranging sensor is designed and implemented. The waterproof piezoelectric transducer with integrated transceiver is chosen as an energy transducing element. Then a novel transducer driving circuit is designed based on an impedance matching method considering safety spark parameters to replace an intermediate frequency transformer. Then, an energy limiting circuit is developed to achieve dual levels of over-voltage and over-current protection. The detail calculation and evaluation are executed and the electrical characteristics are analyzed to verify the intrinsic safety of the driving circuit. Finally, an experimental platform of the ultrasonic ranging sensor system is constructed, which involves short-circuit protection. Experimental results show that the proposed ultrasonic ranging sensor is excellent in both ranging performance and intrinsic safety. Keywords: ultrasonic ranging sensor; driving circuit; impedance matching circuit; intrinsically safe circuit; dual level protection

1. Introduction Non-contact distance measurement is widely used for aircraft altimetry, ship docking, vehicle safety reversing and some other location detection systems. However, non-contact distance measurements in flammable and explosive environments such as coal mines, and the chemical, oil, gas industries and some other circumstances have always been a technical challenge. In general, there are several mature non-contact distance measurement technologies, such as capacitive [1], laser [2,3], infrared [4,5], and ultrasonic [6,7] methods, etc. The capacitive sensor is normally used in near-distance measurements whose range is within 0.3 m [8]. Laser distance measuring sensors show advantages of high accuracy, anti-electromagnetic interference, and their measurement distance can be up to 1000 m. They are often considered suitable for distance measurement in harsh environments. The infrared distance measuring sensor is based on the characteristics of non-proliferation when infrared light is spread. With its advantages of high speed, high resolution and high security, an infrared distance measuring sensor can be used to take a distance measurement of up to hundreds of meters. However, the above two methods have some disadvantages. They both have a small beam-angle, which will lead to a limited direction coverage, and they cannot replace ultrasonic devices in some applications such as an ultrasonic parking assistance systems. The beam-angle of an ultrasonic transducer varies from 7˝ to 30˝ , which is a large direction range. The ranging principle of ultrasonic distance measurement systems is based on the time-of-flight Sensors 2016, 16, 867; doi:10.3390/s16060867

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measurement of reflected echoes [9,10]. Ultrasonic sensors are increasingly popular compared to other devices in the field of distance measurement up to a few meters, because they have advantages of low cost, easy implementation, anti-electromagnetic interference, and strong ability to adapt all kinds of targets and light intensities [11,12]. For this reason, ultrasonic sensors are widely used in target distance measurement tasks [13,14]. However, in certain flammable and explosive environments, the concentration of flammable gas can be high. It is dangerous to let a distance measuring system to be exposed to this environment, so all the electrical devices need to be installed in explosion-proof shells, which are heavy and limits their use in the field. It is much safer when an ultrasonic ranging sensor is designed with an intrinsically safe circuit. Furthermore, the design makes the intrinsically safe equipment have smaller size and lower cost [15,16]. Intrinsically safe circuits are widely used in flammable and explosive environment applications, such as water level measurement [17] and power supplies [18]. Unfortunately, the applications of currently existing ultrasonic distance measurement systems in potentially flammable or explosive environments are limited due to their non-intrinsic safety. Therefore, it is necessary to design an intrinsically safe ultrasonic ranging system. Intrinsic safety is a protection method based on the principle of limiting energy in an electrical circuit, which is more favorable than the other existing safety concepts [19,20]. In general, there are two categories of intrinsically safe and associated apparatus, which are “ia” and “ib” respectively. The categories differ in two main aspects: one is the number of specified faults and the other is the value of safety factor [21]. In this paper, the circuit is designed according to the standard of “ia”, which has more stringent requirements. When the system is operating, an intrinsically safe circuit in an electrical apparatus of category “ia” should not cause ignition in each of the following circumstances: (a) (b) (c)

normal operation; normal operation and with one countable fault; normal operation and with two countable faults.

Based on the considerations and thorough analyses of the conventional ultrasonic ranging sensor, a novel ultrasonic ranging sensor has been designed and fabricated to achieve intrinsically safe characteristics and to be usable in flammable or explosive environment applications, while maintaining (almost) the same performance and function levels as conventional sensors. Besides the necessary designs to meet the precision and range demands of ultrasonic distance measurement, the intrinsically safe circuit is laid out and evaluated. Moreover, an energy limiting circuit is specially designed to achieve dual over-voltage and over-current protection for the whole sensor system. Finally, a series of experiments are performed to verify the performance of the proposed intrinsically safe ultrasonic ranging sensor. 2. An Analysis of Ultrasonic Ranging Sensors and Their Impedance Matching Circuits The block diagram of the ultrasonic ranging sensor is illustrated in Figure 1. According to the demands of ultrasonic ranging equipment, such as a reversing ultrasonic alert system, the system should have at least three transducer circuits at the same time due to the directivity characteristics of a single ultrasonic transducer, if not, it is difficult to detect a target outside the beam-angle. In Figure 1, only one transducer circuit is shown. The implementation of the other two is the same as this. As can be easily seen, the whole system is broadly composed of a power supply circuit, driving circuit, transducer, echo receiving circuit, ultrasonic ranging and data processing, DS18B20 temperature compensator (Dallas Semiconductor Inc, Dallas, TX, USA), ranging and alarm control unit, visualization and sound-light alarm.

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Figure 1. Block ultrasonicranging ranging sensor. Figure Blockdiagram diagramof of the the ultrasonic sensor. Figure 1. 1. Block diagram of the ultrasonic ranging sensor.

power supply circuitprovides provides all all kinds kinds of thethe sensor system. The The The The power supply circuit of voltage voltagegrades gradesforfor sensor system. The power supply circuit provides all kinds of voltage grades for the sensor system. The ranging ranging module is made up of an ultrasonic transmitting circuit, an echo receiving module and data ranging module is made up of an ultrasonic transmitting circuit, an echo receiving module and data processing module. GM3101(Chengdu (Chengdu Goldtel Goldtel Co., LTD, Chengdu, module is made up ofThe anThe ultrasonic transmitting circuit,Microelectronics an echo receiving module and dataChina) processing processing module. GM3101 Microelectronics Co., LTD, Chengdu, China) ultrasonic ranging IC(Chengdu (integratedcircuit, circuit,IC) IC) in charge charge of 40 40 kHz pulse signals and and module. The GM3101 Goldtel Microelectronics Co., LTD, Chengdu, China) ultrasonic ultrasonic ranging IC (integrated isis in oftransmitting transmitting kHz pulse signals transforming the echo signals into distance information. The alarm module consists of two parts: one ranging IC (integrated circuit, IC) is in charge of transmitting 40 kHz pulse signals and transforming transforming the echo signals into distance information. The alarm module consists of two parts: one part is the ranging and alarm unit used to send ranging and alarm commands, and the other part is the into information. The alarm and module consists of two parts: one part partecho is thesignals ranging anddistance alarm unit used to send ranging alarm commands, and the other part is is the visualization and sound-light alarm module. the ranging and alarm unit used to send ranging and alarm commands, and the other part is the the visualization and sound-light alarmsafe module. In the proposed intrinsically ultrasonic ranging sensor, a waterproof piezoelectric visualization and sound-light alarm module. In the proposed intrinsically safe ranging a waterproof piezoelectric transducer with integrated transceiver is ultrasonic selected as the energysensor, transducing element because it is In the proposed intrinsically safe ultrasonic ranging sensor, a waterproof piezoelectric transducer transducer integrated transceiversuch is selected as theThe energy transducing because it is suitablewith for complex environments, as coal mines. appearance and theelement equivalent circuit with integrated transceiver is selected as as the energy transducing element because it is suitable for diagram of the waterproof piezoelectric transducer with integrated transceiver are equivalent schematically suitable for complex environments, such coal mines. The appearance and the circuit complex environments, as coal mines.transducer The appearance and the equivalent circuit of the shown Figure 2a,b.such piezoelectric diagram of in the waterproof with integrated transceiver arediagram schematically waterproof piezoelectric transducer with integrated transceiver are schematically shown in Figure 2a,b. shown in Figure 2a,b.

Figure 2. (a) The adopted ultrasonic transducer; (b) Internal equivalent circuit of ultrasonic transducer; (c,d) Equivalent circuit transformation when parallel resonant.

Figure The adopted ultrasonic (b) Internal equivalent circuit structure of transducer; ultrasonic Figure 2.2.(a)(a) The adopted ultrasonic transducer; Internal equivalent circuit ultrasonic Figure 2a shows the transducer has atransducer; metal(b) shell, which can insulate theof internal from transducer; (c,d) Equivalent circuit transformation when parallel resonant. water and dust circuit in thetransformation complex environment. Moreover, when the transducer works in resonant (c,d) Equivalent when parallel resonant. state, its equivalent circuit can be depicted as in Figure 2b, where the static capacitance, the dynamic

Figure 2a shows the transducer has a metal shell, which can insulate the internal structure from Figure 2a shows the transducer has a metal shell, which can insulate the internal structure from water and dust in the complex environment. Moreover, when the transducer works in resonant water and dust in the complex environment. Moreover, when the transducer works in resonant state, its equivalent circuit can be depicted as in Figure 2b, where the static capacitance, the dynamic state, its equivalent circuit can be depicted as in Figure 2b, where the static capacitance, the dynamic

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capacitance, the dynamic inductance, and the dynamic resistance of the piezoelectric transducer are defined as C0, C, L and R, respectively. Sensors 2016, 16, 867 4 of 17 When the transducer’s parallel branch is resonant, in other words, working at a parallel resonant angular frequency, the reactance of L, C and C0 will be counteracted and the transducer capacitance, the dynamic inductance, the dynamic resistancethe of the piezoelectric transducer are may be equivalent to Figure 2c. C′0, C′,and L′ and R′ are respectively static capacitance, the dynamic defined as C , C, L and R, respectively. capacitance,0the dynamic inductance, and the dynamic resistance of the piezoelectric transducer the transducer’s parallelThen, branch is resonant, in other working at a parallel resonant after When equivalent transformation. the equivalent circuitwords, can be simplified into Figure 2d angular the reactance of L, C and will be counteracted and the transducer may be through frequency, further equivalent transformation [22].CRelevant calculations are expressed as follows: 0 equivalent to Figure 2c. C1 0 , C1 , L1 and R1 are respectively the static capacitance, the dynamic C  C 0 resistance of the piezoelectric transducer capacitance, the dynamic inductance, and wthe dynamic (1) p LCC 0 after equivalent transformation. Then, the equivalent circuit can be simplified into Figure 2d through further equivalent transformation [22]. Relevant calculations are expressed as follows: 1 1 d  Zp  jw C 1 C ` C p 0  jw C  jwp C0 (1) 1 wp “ p 0 LCC 1 wp2 LC  jwp CR 0 R  jwp L  jwp C 

1 “ Zp “ CR 1 wp2 LC1  jw 1 p` jwp C0 R`jwp L`

jwp C

jwp C

1

2 LC`jw CR 1´wp p

2 jwp (C  C0 - wp2 LCC 0 ) - wp CC 0 R 1 wp2 LC` jwp CR 1 “ jw pC`1C0 ´ˆwp2 LCC0 q´wp2 CC0 R  2 2 - pj 1w C 1 wp C“ 0 R 2 2 p ´0 j wp C0 w C R

` jwp C0

(2) (2)

p 0

where, w wpp is is the the parallel parallel resonance resonance angular angular frequency frequency [22], [22], and andZZpp is the where, is the the total total impedance impedance of of the transducer when when working working at at the the parallel parallel resonance resonance angular angular frequency. frequency. transducer After combining Equation (2) and Figure 2d, some conclusions can be be drawn drawn as as follows: follows: After combining Equation (2) and Figure 2d, some conclusions can

1 1 R  R1 “ wp22C 022 R wp C0 R

(3) (3)

C C00 CC1 00 “

(4) (4)

The In order order to to improve The piezoelectric piezoelectric transducer transducer is is capacitive capacitive at at resonance. resonance. In improve the the vibration vibration amplitude of the surface and the work efficiency of piezoelectric transducer, the problem impedance amplitude of the surface and the work efficiency of piezoelectric transducer, theofproblem of matching be solved first. Generally, there arethere various impedance matching circuits such impedancemust matching must be solved first. Generally, are various impedance matching circuits as parallel inductance, series inductance, capacitance-inductance, T network, them, such as parallel inductance, series inductance, capacitance-inductance, T network,etc. etc. Among Among them, the matching circuit circuit is is the the most for it the capacitance-inductance capacitance-inductance matching most practical, practical, for it can can not not only only match match the the transducer but also wipe off the harmonic from the power source. As can be seen from Figure 3, transducer but also wipe off the harmonic from the power source. As can be seen from Figure 3, L1 is Lthe is the matching inductance and C is the matching capacitance. 1 matching inductance and C1 is the 1 matching capacitance.

Figure 3. LC matching circuit diagram of the piezoelectric transducer. Figure 3. LC matching circuit diagram of the piezoelectric transducer.

To simplify the calculations, the total shunt capacitance of both ends of the transducer is To simplify the total capacitance of bothofends of the transducer is expressed expressed as CSthe = calculations, C1 + C0. Hence, the shunt total input impedance piezoelectric transducer can be as C = C + C . Hence, the total input impedance of piezoelectric transducer can be derived S 1 0 derived by substituting the CS expression into Equation (2). The following equations can by be substituting obtained: the CS expression into Equation (2). The following equations can be obtained: Z1 p “

1 1 ` jpwp L1 ´ q wp Cs wp2 c2s R

(5)

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Z p 

1 wp2cs2 R

 j(wp L1 

1 ) wpCs

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where, Z′p is the total impedance after the impedance matching. Then the imaginary part is set as 0, the relationship between the matching capacitance and the matching inductance is obtained, as is where, Z1 p is the total impedance after the impedance matching. Then the imaginary part is set as shown below: 0, the relationship between the matching capacitance and the matching inductance is obtained, as is 1 1 wp   shown below: (6) 1L C L (C 1 C0 ) wp “ ? 1 s “ a 1 1 (6) L1 Cs L1 pC1 ` C0 q Equation (6) shows that the parameters of L1 and C1 only depend on C0, the static capacitor of Equationtransducer, (6) shows that the the parameters of L1 and C1 only depend on Cin of 0 , the piezoelectric when piezoelectric transducer is working thestatic statecapacitor of parallel piezoelectric transducer, when the piezoelectric transducer is working in the state of parallel resonance. resonance. Relevant parameters of the selected piezoelectric transducer and matching circuit can be Relevant parameters of the selected piezoelectric transducer and matching can beofseen indriving Table 1. seen in Table 1. Moreover, all of the parameters are obtained under thecircuit condition 48 V Moreover, all of the parameters are obtained under the condition of 48 V driving voltage and are voltage and are allowed for an intrinsic safety design. The experimental studies will be conducted allowed for an intrinsic safety design. The experimental studies will be conducted and analyzed in and analyzed in next section. next section. Table 1. Relevant parameters. Table 1. Relevant parameters. Parameter Value Uint Parameter Value R 200.00 ΩUint L 166.64 mH R 200.00 Ω 0.66 mH L1 L 166.64 mH C 0.10 nF L1 0.66 mH 1.90 nF C0 C 0.10 nF C0 1.90 nF 22.0 nF C1 C1 22.0 nF

3. Analysis of Non-Intrinsically Safe Ultrasonic Driving Circuit 3. Analysis of Non-Intrinsically Safe Ultrasonic Driving Circuit Compared to the non-waterproof transducer, the waterproof piezoelectric transducer with an Compared to the non-waterproof transducer, the waterproof piezoelectric transducer withand an integrated transceiver can be used in harsh environments. However, it has lower transmitting integrated transceiver canThe be used in harshpiezoelectric environments. However, it has transmitting and receiving efficiency [23]. waterproof transducer with anlower integrated transceiver receiving [23]. Theby waterproof piezoelectric transducer with an integrated cannot cannot beefficiency driven directly a microprocessor. Therefore a specially designed transceiver driving circuit is be drivenFor directly by a microprocessor. Therefore specially designed drivingcircuit circuitare is required. required. the ultrasonic ranging sensor, a powerasupply circuit and a driving the main For the ultrasonic power of supply circuit and a drivingranging circuit are the main energy energy circuits. Aranging typical sensor, drivinga circuit conventional ultrasonic sensor is shown in circuits. A typical driving circuit of conventional ultrasonic ranging sensor is shown in Figure 4. Figure 4.

Figure 4. Typical driving circuit of conventional ultrasonic ranging sensor. Figure 4. Typical driving circuit of conventional ultrasonic ranging sensor.

In Figure 4, B1 is the special intermediate frequency transformer to provide enough driving In Figure B1 V) is the intermediate frequency transformer provideofenough driving voltage (about4,48 for special the ultrasonic transducer. It also has the to function matching the voltage (about 48 V) for the ultrasonic transducer. It also has the function of matching the impedance. impedance. The inductance coil of B1’s high-voltage side and the capacitance of transducer can form The inductance coilHowever, of B1 ’s high-voltage side and the capacitance of transducer form inductance a resonant a resonant circuit. the intermediate frequency transformer has suchcan a large circuit. However, the intermediate frequency transformer has such a large inductance (7.8 ˘ 0.2 mH)

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(7.8 ± 0.2 mH) that the driving circuit is difficult to meet the requirement of an intrinsically safe that the driving circuit is difficult to meet the requirement of an intrinsically safe circuit. A specific circuit. A specific energy feature analysis is shown below: energy feature analysis is shown below: 1 1  138.58  R   12 2  (7) 3 2 9 2 1 200 “ 138.58 Ω R1 “ 2wp2Cs R “ ( 2    40  10 ) 2  ( 23.9  10 )  (7) 2 wp Cs R p2 ˆ π ˆ 40 ˆ 103 q ˆ p23.9 ˆ 10´9 q ˆ 200

1 ZZp “ RR1´ jj 1 “ (138.58 166.48)Ω p138.58  ´ jj166.48q p wC wpp Css

(8) (8)

In In thethe stage of of ultrasonic wave transmission, thethe current in in each branch of of thethe driving circuit and stage ultrasonic wave transmission, current each branch driving circuit theand current through the inductor are calculated as follows: the current through the inductor are calculated as follows:

U00 48 U 48 A“ 0.141 ` j 0.170 )A A  (p  (p0.141  j0.170q IItt “ “ q) A Z 138 . 58 j166 .48  Zpp 138.58 ´ j166.48 48 U0 U 48 IIRR “ R 0 “  (p 2 ˆ 1000 q) A A“  00.024 . 024 A A R11 2  1000 11 IL “ It ` IR “ p0.165 ` j0.170q A “ 0.236=46.0˝ A

(9) (9)

(10) (10) (11)

I  I t  I R  (0.165  j0.170) A  0.23646.0 A (11) where, IL , IR , and It are theLcurrent of each branch, respectively. IL is also the current through the inductor. U is the driving voltage about 48 V. R is the current limiting resistance. In order to improve where, IL0, IR, and It are the current of each branch, respectively. IL is also the current through the 11 reliability of U the the current should by acurrent safety limiting factor k (2.0), whichInis:order to inductor. 0 iscircuit, the driving voltage aboutbe 48multiplied V. R11 is the resistance. improve reliability of the circuit, the current should be multiplied by a safety factor k (2.0), which is: IL1 “ k ˆ IL “ p2.0 ˆ 0.236q A “ 0.472 A (12) I L  k  I L  (2.0  0.236) A  0.472 A (12) According to the critical ignition curve of an inductance circuit [21], we can find the minimum According to the critical ignition curve of an inductance circuit [21], we can find the minimum igniting current 300 mA mA when whenthe thevoltage voltageisis3636V.V.Hence, Hence,it it believed = 300 is is believed igniting currentfor for8 8mH mHinductance inductanceisisIIBB = 1 is much higher than the minimum igniting current under the condition of 48 V voltage. that the I L that the IL′ is much higher than the minimum igniting current under the condition of 48 V voltage. Therefore, thethe typical to have have non-intrinsically non-intrinsicallysafe safefeatures features Therefore, typicaldriving drivingcircuit circuithas hasbeen been proved proved to byby thethe calculation of of current. calculation current. 4. 4. Analysis DrivingCircuit Circuit AnalysisofofIntrinsically IntrinsicallySafe Safe Ultrasonic Ultrasonic Driving 4.1.4.1. Energy Limiting Energy LimitingCircuit Circuit In In thethe proposed sensorsystem, system, energy limiting circuit is designed proposeddistance distance measurement measurement sensor anan energy limiting circuit is designed to to develop develop the theintrinsically intrinsicallysafe safecharacteristic characteristic sensor. energy limiting circuit is shown of of thethe sensor. TheThe energy limiting circuit is shown in in Figure 5. 5.

Figure 5. Energy limiting circuit. Figure 5. Energy limiting circuit.

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Over-Voltage Protection Over-Voltage Protection

As can be seen from Figure 5, the transistor Q1 is an on-off component that is controlled by the . .InInnormal state, the the basebase of theoftransistor Q1 is inQlow so the transistor comparator UU1A 1A and andUU normal state, the transistor 1 is level, in low level, so the 1B1B Q1 is at anQON-state. D15 is TL431, voltage stabilizer, can which keep Ucan voltage of transistor 1 is at an ON-state. D15 ais3-terminal TL431, a 3-terminal voltage which stabilizer, keep U9 (the 9 (the positive input end ofinput U1A )end at a of holding potential of 2.5 V. Namely, U9V.= Namely, 2.5 V. Moreover, voltage voltage of positive U1A) at a holding potential of 2.5 U9 = 2.5UV. Moreover, 8 (the of8 inverting input end of U1A ) canend be represented as:represented as: U (the voltage of inverting input of U1A) can be

U  U8 “

UOO ˆ RR13 U 13 R R12 ` RR13 13

(13) (13)

is 12.5 12.5 V. V. After matching the In the proposed sensor system, the threshold of input voltage UII is value of ofRR1212with withR13 R,13the , the divider resistors achieve 2.5 V when the input voltage suitable value divider resistors U8 U will achieve 2.5 V when the input voltage UI is 8 will U is 12.5 V. If the input voltage is higher than 12.5 V, it will lead to the result that U is greater 12.5 V. If the input voltage is higher than 12.5 V, it will lead to the result that U 8 is greater than U9, I 8 thanthat U9 , the andoutput that the output voltage of the comparator U1A will to low. and voltage level of thelevel comparator U1A will change fromchange high tofrom low.high Hence, the Hence, theinput inverting end U1B will be pulled down through the circuits of DD917and D17 , inverting end input voltage ofvoltage U1B willofbe pulled down through the circuits of D9 and , which which will the cause theof base Q1 turning to a high voltage. Then, the transistor at an OFF-state, will cause base Q1 of turning to a high voltage. Then, the transistor Q1 isQ1atisan OFF-state, in in which power supply will cutoffofftotorealize realizean anover-voltage over-voltageprotection. protection.From From Figure Figure 6, 6, we can which thethe power supply will bebe cut will be be 00 V V when when the the input input voltage UII is is higher higher than than 12.45 12.45 V. V. know that the output voltage UO O will

Figure Figure 6. 6. Over-voltage Over-voltage protection protection curve. curve.

In the proposed sensor, the threshold current of system is 100 mA. R4, whose value is 2 Ω, is the In the proposed sensor, the threshold current of system is 100 mA. R4 , whose value is 2 Ω, is the resistance for over-current detecting. Moreover, because D3 is an accurate voltage stabilizer TL431 resistance for over-current detecting. Moreover, because D3 is an accurate voltage stabilizer TL431 (Texas Instruments, Dallas, TX, USA), the voltage between cathode and anode of D3 is 2.5 V. Hence, (Texas Instruments, Dallas, TX, USA), the voltage between cathode and anode of D3 is 2.5 V. Hence, the positive input end voltage of U1B can be expressed as: the positive input end voltage of U1B can be expressed as: 2 .5  R U 5  U I  U R5  U I  2.5 ˆ R55 (14) U5 “ UI ´ UR5 “ UI ´ R5  R6 (14) R5 ` R6 where R5 and R6 are the divider resistors, respectively, UI is the input voltage, and UR5 is the voltage where R5 and R6 are the divider resistors, respectively, UI is the input voltage, and UR5 is the voltage of R5. As can be seen from Equation (14), by matching the values of R5 with R6, the voltage of UR5 is of R5 . As can be seen from Equation (14), by matching the values of R5 with R6 , the voltage of UR5 is stabilized at 0.2 V. stabilized at 0.2 V. As the rated current of the proposed sensor is about 70 mA, the voltage of R4 is lower than 0.2 V As the rated current of the proposed sensor is about 70 mA, the voltage of R4 is lower than 0.2 V under normal circumstances, so for the comparator U1B (LM339, Texas Instruments, Dallas, TX, under normal circumstances, so for the comparator U1B (LM339, Texas Instruments, Dallas, TX, USA), USA), the voltage of its positive input end is lower than that of inverting input end, which will make the voltage of its positive input end is lower than that of inverting input end, which will make the the output voltage of U1B become low and Q1 be at an ON-state, but when short circuits and other faults occur, the system current is more than 100 mA. Furthermore, the voltage of R4 will be higher than 0.2 V, which will lead to the output becoming high and Q1 being at an OFF-state. Then, the

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output voltage of U1B become low and Q1 be at an ON-state, but when short circuits and other faults of 17 current is more than 100 mA. Furthermore, the voltage of R4 will be higher8than 0.2 V, which will lead to the output becoming high and Q1 being at an OFF-state. Then, the power power willoff beso cutasoff so as toover-current realize over-current protection. Figurethe 7 shows the maximum supply supply will be cut to realize protection. Figure 7 shows maximum allowable allowable current of sensor is 0.101 A. current of sensor is 0.101 A. Sensors 16, 867 occur,2016, the system

Figure curve. Figure 7. 7. Over-current Over-current protection protection curve.

4.2. Safe Driving Driving Circuit Circuit 4.2. Design Design and and Evaluation Evaluation of of an an Intrinsically Intrinsically Safe To solve the the above above issue, issue, aa transducer transducer driving driving circuit circuit with with aa high-speed high-speed opto-coupler opto-coupler has has been been To solve designed. The proposed driving circuit is shown in Figure 8, where IC 1 is the high-speed designed. The proposed driving circuit is shown in Figure 8, where IC1 is the high-speed opto-coupler, opto-coupler, and its can switching speed can grade. reach Once microsecond grade.opto-coupler Once the closes, high speed and its switching speed reach microsecond the high speed it can opto-coupler closes, it can isolate the power supply from an echo signal completely. Therefore it is isolate the power supply from an echo signal completely. Therefore it is suitable for transmitting suitable for transmitting impulse signal and weak echo signal. At the same time, there is no energy impulse signal and weak echo signal. At the same time, there is no energy storage element in the storage the proposed driving circuit. It proposed can be concluded that meets the proposed driving proposedelement drivingin circuit. It can be concluded that the driving circuit the requirement circuit meets the requirement of an intrinsically safe circuit based on the above calculation and of an intrinsically safe circuit based on the above calculation and analysis. The sparks safety assessment analysis. The sparks safety assessment of transducer’s matching circuit and drive circuit can be seen of transducer’s matching circuit and drive circuit can be seen as follows: as follows: 1 1 R1 “ 2 2 1 “ “ 138.58 Ω (15) 1 Rwp Cs2 R 2 p2  ˆ π ˆ 40 ˆ 1033 q22 ˆ p23.9 ˆ 109´92q2 ˆ 200 138.58  (15) Sensors 2016, 16, 867

wp Cs R

(2    40 10 )  (23.9 10 )  200

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First, under normal condition, the normal work current I1 of the designed driving circuit is expressed as:

I1 

U0 48 ( ) mA  117.48 mA R15  R13  R 135  135  138.58

(16)

where U0 is the driving voltage about 48 V. The current limiting resistances R13 and R15 are both 135 Ω. I1 is the current of flowing through the inductor L1. In order to improve reliability of the circuit, the current should be multiplied by a safety factor k (2.0), that is:

I1  k  I1  (2.0  117.48) mA  234.96 mA

Figure with high-speed high-speed opto-coupler. opto-coupler. Figure 8. 8. Intrinsically Intrinsically safe safe driving driving circuit circuit with

(17)

According to the critical ignition curve of inductance circuit [21], we can find the minimum Second, when the0.33 short circuit occurs is in1.1 theA, transducer, the current of the234.96 proposed igniting current IB of mH inductance which is much higherI2 than mA. driving So, the circuit is calculated as follows: designed circuit meets the requirements of the desired intrinsic safety circuit.

I2 

U0 48 ( ) mA  177.78 mA R13  R15 135  135

where I2 is the shorted current of L1. If the current I2 is multiplied by a safety factor k (2), that is:

(18)

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First, under normal condition, the normal work current I1 of the designed driving circuit is expressed as: 48 U0 “p q mA “ 117.48 mA (16) I1 “ R15 ` R13 ` R1 135 ` 135 ` 138.58 where U0 is the driving voltage about 48 V. The current limiting resistances R13 and R15 are both 135 Ω. I1 is the current of flowing through the inductor L1 . In order to improve reliability of the circuit, the current should be multiplied by a safety factor k (2.0), that is: I11 “ k ˆ I1 “ p2.0 ˆ 117.48q mA “ 234.96 mA

(17)

According to the critical ignition curve of inductance circuit [21], we can find the minimum igniting current IB of 0.33 mH inductance is 1.1 A, which is much higher than 234.96 mA. So, the designed circuit meets the requirements of the desired intrinsic safety circuit. Second, when the short circuit occurs in the transducer, the current I2 of the proposed driving circuit is calculated as follows: I2 “

48 U0 “p q mA “ 177.78 mA R13 ` R15 135 ` 135

(18)

where I2 is the shorted current of L1 . If the current I2 is multiplied by a safety factor k (2), that is: I21 “ k ˆ I2 “ p2.0 ˆ 177.78q mA “ 355.56 mA

(19)

As it can obviously seen, IB " I21 , which meets the requirements of the desired intrinsic safety circuit. If the current I2 is multiplied by a safety factor k (1.5), that is: 2

I2 “ k ˆ I2 “ p1.5 ˆ 177.78q mA “ 266.67 mA

(20)

According to the critical ignition curve of resistance circuit [21], we can find the minimum igniting current IB corresponding to the 48 V power supply is 275.00 mA, which is higher than 266.67 mA. Therefore, the above spark ignition analyses and calculation results indicate that the circuit is intrinsically safe. The transmitting pulses produced by the novel approach of driving current are shown in Figure 9. Due to the resistance distribution of current limiting resistor and opto-coupler, a pulse string, composed of about 16 V and 40 kHz pulse signals which is controlled by GM3101, will be sent to the anode of the transducer. The time of the stage A, when the sensor is transmitting pulses, is TA = 16 ˆ 25 µs = 400 µs. After transmitting the pulses, the microprocessor will turn off the opto-coupler and the transducer will go to the stage of aftershock (C) before the echo receiving stage. In order to distinguish the aftershock signal and the echo signal, a period of aftershock shielding time is necessary, after which the proposed sensor begins to detect an echo. In Figure 9, B represents the stage of aftershock shielding. Obviously, TB is much larger than TC . After designing and implementing the driving circuit, we take a waterproof piezoelectric transducer with an integrated transceiver for experiments to test its driving ability. Tests have been done in various distances of the target object, and the echo waveform in 185 cm is shown in Figure 10. We can find that the echo voltage is still very large (about 1.75 V) in such a long distance, which shows that the proposed novel driving circuit has a pretty good performance in driving the waterproof piezoelectric transducer with integrated transceiver.

I 2  k  I 2  (1.5  177.78) mA  266.67 mA

(20)

According to the critical ignition curve of resistance circuit [21], we can find the minimum igniting current IB corresponding to the 48 V power supply is 275.00 mA, which is higher than 266.67 mA. Therefore, the above spark ignition analyses and calculation results indicate that the Sensors 2016, 16, 867 10 of 17 circuit is intrinsically safe.

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the stage of aftershock shielding. Obviously, TB is much larger than TC. After designing and implementing the driving circuit, we take a waterproof piezoelectric transducer with an integrated transceiver for experiments to test its driving ability. Tests have been done in various distances of the target object, and the echo waveform in 185 cm is shown in Figure 10. We can find that the echo voltage is still very large (about 1.75 V) in such a long distance, which shows that the proposed novel driving circuit has a pretty good performance in driving the waterproof piezoelectric 9. The measured drive pulse of transducer. transducer with integrated Figure transceiver. The transmitting pulses produced by the novel approach of driving current are shown in Figure 9. Due to the resistance distribution of current limiting resistor and opto-coupler, a pulse string, composed of about 16 V and 40 kHz pulse signals which is controlled by GM3101, will be sent to the anode of the transducer. The time of the stage A, when the sensor is transmitting pulses, is TA = 16 × 25 μs = 400 μs. After transmitting the pulses, the microprocessor will turn off the opto-coupler and the transducer will go to the stage of aftershock (C) before the echo receiving stage. In order to distinguish the aftershock signal and the echo signal, a period of aftershock shielding time is necessary, after which the proposed sensor begins to detect an echo. In Figure 9, B represents

Figure 10. 10. The The measured measured echo echo voltage voltage of of transducer. transducer. Figure

5. Experimental Demonstration and Discussion 5. Experimental Demonstration and Discussion 5.1. The The Realization Realization of of the the Distance Distance Measurement Measurement Sensor Sensor 5.1. The digital digital circuit circuit hardware hardware selection selectionisisshown shownin inTable Table2.2. The Table 2. 2. The The digital digital circuit circuit hardware hardwareselection. selection. Table Number Number 1 1

2 2

Designation Designation Distance measurement Distance chip measurement chip Microprocessor Microprocessor

MSP430F149

Display

3 4

Display Sound and light alarm Sound and light alarm

GM3101 GM3101

‚ ‚

MSP430F149 ‚

3

4

Version Version

‚

LCD12864 Active buzzer and‚ LCD12864 LED light-emitting diode Active buzzer and LED ‚ light-emitting diode ‚

Function Function  send and receive ultrasonic signals  calculate and send thesignals distance send and receive ultrasonic information calculate and send the distance information  control the work of ranging sensor and receive its measurement results control the work of ranging sensor  control the work of LCD and sound and receive its measurement results and light alarm device control the work of LCD and sound  light display thedevice distance information of and alarm target object display the distance information of  Emit a shining light signals target object  Sound alarm Emit a shining light signals Sound alarm

A software program for the MSP430F149 microprocessor (Texas Instruments, Dallas, TX, USA) has been developed. The flowchart of the software is given in Figure 11. The program was written in C language on the platform of Keil uVision 4 IDE (Advanced RISC Machines, Cambridge, UK). After the system was initialized, the microprocessor sent the command of measuring distance to GM3101 first and then was ready to receive the distance information from GM3101. For the information from GM3101, the microprocessor determines whether it is correct distance

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A software program for the MSP430F149 microprocessor (Texas Instruments, Dallas, TX, USA) has been developed. The flowchart of the software is given in Figure 11. The program was written in C language on the platform of Keil uVision 4 IDE (Advanced RISC Machines, Cambridge, UK). After the system was initialized, the microprocessor sent the command of measuring distance to GM3101 first and then was ready to receive the distance information from GM3101. For the information from GM3101, the microprocessor determines whether it is correct distance information. If the information is right, it will be analyzed and calculated so that we can get the accurate distance S of the obstacle. Sensors 2016, 16, 867 11 of 17

Figure 11. Flowchart of the software. Figure 11. Flowchart of the software.

Next, the distance S is compared with the setting thresholds in order to decide whether it is Next,tothe distance S is compared with the setting thresholds orderofto decide whether is necessary trigger the alarm. In the proposed system, there are threeinlevels alarm. When S ≤ 0.5itm, necessary to trigger thethe alarm. In the proposed system, there are three levels0.5 ofm alarm. ď 0.5 m, the LED will be lit and buzzer will beep at the frequency 10 Hz. When < S ≤ When 0.75 m,S the LED the LED will be lit and the buzzer will beep at the frequency 10 Hz. When 0.5 m < S ď 0.75 m, the LED will be lit and the buzzer will beep at the frequency 5 Hz. When 0.75 m < S ≤ 1 m, the LED will be lit will and the buzzer beep at the frequency 5 Hz.S When 0.75sound-light m < S ď 1 m, the LED be and be thelitbuzzer will beep will at the frequency 2 Hz. When >1 m, the alarm will will not be lit and theMoreover, buzzer willthe beep at theinformation frequency 2will Hz.be When S >1 m, not be triggered. distance displayed onthe thesound-light LCD12864. alarm As canwill be seen in triggered. Moreover, the distance information will be displayed on the LCD12864. As can be the Figure 11, after initializing the I/O ports and LCD12864, the microprocessor will transformseen the in the Figurewhere 11, after portsdisplay and LCD12864, the microprocessor transform coordinates theinitializing characters the andI/O images on the LCD screen into the will corresponding the coordinates where the characters and images display on the LCD screen into the corresponding coordinates of the LCD screen inside, and then feeds the coordinates into LCD screen through coordinates the LCD screen and then sends feeds the intoimages LCD screen throughobstacle parallel parallel I/O of ports. Finally, the inside, microprocessor thecoordinates characters and that express I/O ports.toFinally, the through microprocessor the characters and images that express obstacle distances distances LCD still parallelsends I/O ports. to LCD still through parallel I/O ports. 5.2. Experimental Setup and System Errors 5.2. Experimental Setup and System Errors To test and verify the effectiveness of the novel transducer circuit and the practicality of the To test and verify the effectiveness of the novel transducer circuit and the practicality of the real-time measurement sensor, an experimental measurement has been performed with the real-time measurement sensor, an experimental measurement has been performed with the intrinsically intrinsically safe ultrasonic ranging sensor. The entire experimental setup is shown in Figure 12. safe ultrasonic ranging sensor. The entire experimental setup is shown in Figure 12. An ultrasonic transducer and target object were placed on an optical bench (300 cm in length) with an easily readable distance scale so that the actual distance value was controlled and read precisely by a simple way. In order to avoid the effect of other objects, the transducer and target objects simulating explosion-proof enclosure (25 cm × 25 cm) were fixed at an altitude of 50 cm by two metal rods. The 12 V switching power source provided basic electrical power for the proposed distance measurement sensor. Three types of measurements were performed. The schematic diagram of the experiment platform (side view) is shown in Figure 13. From Figure 13, it can be observed that the value read from the standard gauge block is not the actual distance between ultrasonic transducer and target board, due to the errors of test platform. In fact,

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Figure 12. Experimental setup. Figure 12. Experimental setup.

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An ultrasonic transducer and target object were placed on an optical bench (300 cm in length) with an easily readable distance scale so that the actual distance value was controlled and read precisely by a simple way. In order to avoid the effect of other objects, the transducer and target objects simulating explosion-proof enclosure (25 cm ˆ 25 cm) were fixed at an altitude of 50 cm by two metal rods. The 12 V switching power source provided basic electrical power for the proposed distance measurement sensor. Three types of measurements were performed. The schematic diagram of the experiment platform (side view) is shown in Figure 13. From Figure 13, it can be observed that the value read from the standard gauge block is not the actual distance between ultrasonic transducer and target board, due to the errors of test platform. In fact, there is a system error between them, and the actual distance can be expressed as follows: Da “ Ds ´ De

(21)

where Da is the actual distance between transceiver and target board, Ds is the standard distance that Figure 12. system Experimental setup. read from the standard gauge De diagram is the In these experiments De is 5 cm. Figure 13. The block, schematic of the error. experiment platform (side view).

5.3. Resolution and Blind Area For the proposed intrinsically safe ultrasonic ranging sensor, the resolution is mainly determined by the GM3101 distance measurement chip. The details of the GM3101 can be found in the GM3101 datasheet, which indicates a resolution of 5 cm. Although there are higher resolution ultrasonic ranging sensors in existing products, the resolution is acceptable in the practical application considering the purpose of the developed equipment. A test has been done in order to evaluate the different measuring performance between the proposed intrinsically safe ultrasonic ranging sensor and conventional ultrasonic ranging sensor. We chose several sample points and measured the distance of every sample points by using two categories of ranging sensor respectively. The measured results show that the blind is 30 cm and the maximum measuring range is 300 cm for the proposed intrinsically safe sensor. Within the measurement range of 30 cm to 300 cm, the same measurement results were obtained by adopting two categories of ranging sensor respectively. 5.4. Linearity

Figure platform (side (side view). view). Figure 13. 13. The The schematic schematic diagram diagram of of the the experiment experiment platform

Linearity is used to evaluate the performance of the distance measurement sensor. In the 5.3. Resolution and Blind Area experiment, the target object distance is decided by a 300 cm long optical bench, whose precision is 1 proposed intrinsically ultrasonic sensor, the resolution is distance mainly mm. For Eachthe time, the distance measuredsafe by the proposedranging sensor will be compared with the determined by the GM3101 distance measurement chip. The details of the GM3101 can be found in the GM3101 datasheet, which indicates a resolution of 5 cm. Although there are higher resolution ultrasonic ranging sensors in existing products, the resolution is acceptable in the practical application considering the purpose of the developed equipment.

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5.3. Resolution and Blind Area For the proposed intrinsically safe ultrasonic ranging sensor, the resolution is mainly determined by the GM3101 distance measurement chip. The details of the GM3101 can be found in the GM3101 datasheet, which indicates a resolution of 5 cm. Although there are higher resolution ultrasonic ranging sensors in existing products, the resolution is acceptable in the practical application considering the purpose of the developed equipment. A test has been done in order to evaluate the different measuring performance between the proposed intrinsically safe ultrasonic ranging sensor and conventional ultrasonic ranging sensor. We chose several sample points and measured the distance of every sample points by using two categories of ranging sensor respectively. The measured results show that the blind is 30 cm and the maximum measuring range is 300 cm for the proposed intrinsically safe sensor. Within the measurement range of 30 cm to 300 cm, the same measurement results were obtained by adopting two categories of ranging sensor respectively. 5.4. Linearity Linearity is used to evaluate the performance of the distance measurement sensor. In the experiment, the target object distance is decided by a 300 cm long optical bench, whose precision is Sensors 2016, 16, 867 13 of 17 1 mm. Each time, the distance measured by the proposed sensor will be compared with the distance measured by optical bench. According shown in Section we know that the measured by optical bench. Accordingto to the the interpretation interpretation shown in Section 4, we4,know that the sensorsensor has ahas period of time forfor aftershock (TBB).).InInthis this paper, ultrasonic transducer a period of time aftershockshielding shielding (T paper, the the ultrasonic transducer NU40A25TR-1 (Shenzhen JINCI Technology CO., CO., LTD., China) has has beenbeen used,used, consider NU40A25TR-1 (Shenzhen JINCI Technology LTD.,Shenzhen, Shenzhen, China) consider of the characteristics of it, T B was set as 1.7 ms. Hence, the measuring blind area can be obtained as of the characteristics of it, TB was set as 1.7 ms. Hence, the measuring blind area can be obtained follows: as follows: 340 ˆ 1.7 CˆT DB “D  C BTB“p( 340 1.7 ) m q m “ 0.289 m (22) (22) B 2 22 ˆ 1000  0.289 m 2 1000 where DB is the distance of measuring blind area, C is the speed of sound and TB is the aftershock where DB is the distance of measuring blind area, C is the speed of sound and TB is the aftershock shielding time.time. Experimental results testinina ameasuring measuring range 30tocm 300 shielding Experimental resultsofofthe thelinearity linearity test range of 30ofcm 300tocm arecm are shownshown in Figure 14. In Figure 14, it can be seen that the maximum error of this test is about ´6 in Figure 14. In Figure 14, it can be seen that the maximum error of this test is about −6 cm or cm or 2.22%2.22% within the full scale span (FSS). within the full scale span (FSS).

Figure Experimentalresults results of a range of 300 cm. cm. Figure 14.14. Experimental of linearity linearityinin a range of 300

The following conclusions can be drawn from the above. Although the sensor is not able to

The following conclusions can be drawn and froma the above. Although theitsensor notsatisfy able to display display the merits of the high precision wide measuring range, is ableisto the requirement of distance measurement in measuring practical applications. the most important the merits of the high precision and a wide range, it isMoreover, able to satisfy the requirement of advantage is that the is intrinsically safe. the most important advantage is that the distance measurement inproposed practicalsensor applications. Moreover, proposed sensor is intrinsically safe. 5.5. Repeatability

In the experiment, the repeatability of the proposed sensor was also tested specially. Repeatability is the variation of measured value when a surveyor repeatedly measuring the identical characteristic on the same part using a specific measuring instrument. The relative error of repeatability is used for reliability evaluation of measurement sensor. For the proposed sensor, the relative error of repeatability can be expressed as:

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5.5. Repeatability In the experiment, the repeatability of the proposed sensor was also tested specially. Repeatability is the variation of measured value when a surveyor repeatedly measuring the identical characteristic on the same part using a specific measuring instrument. The relative error of repeatability is used for reliability evaluation of measurement sensor. For the proposed sensor, the relative error of repeatability can be expressed as: L ´ LiMin Ei “ iMax (23) Li where Ei is the relative error of repeatability of the sensor, LiMax is the maximum measured value of the i-th sample points, LiMin is the minimum measured value of the i-th sample points, and Li is the standard value of the i-th sample points. In the experiment, at each fixed target object distance, ten measurements are carried out repeatedly. It is observed that there is a small variation among the measurement results. After the measured data are processed, the relative error of repeatability of each sample point can be obtained and is shown in Figure2016, 15, respectively. Sensors 16, 867 14 of 17

Figure test. Figure 15. 15. Experimental Experimental result result of of the the repeatability repeatability test.

As shown in Figure 15, we have selected 15 sample points and carried out 100 measurements in As shown in Figure 15, we have selected 15 sample points and carried out 100 measurements in the repeatability experiment. With the increase of the measured distance, in general, the relative the repeatability experiment. With the increase of the measured distance, in general, the relative error error of repeatability of the sensor decreases gradually. The maximum repeatability error is 5 cm or of repeatability of the sensor decreases gradually. The maximum repeatability error is 5 cm or 10.64% 10.64% of actual value, when the actual value is 47 cm. The minimum repeatability error is 0 cm at of actual value, when the actual value is 47 cm. The minimum repeatability error is 0 cm at several several sample points. The relative error of repeatability at close range is seemingly relatively high, sample points. The relative error of repeatability at close range is seemingly relatively high, because because the resolution of the proposed sensor is 5 cm. Moreover, it is necessary to mention that the the resolution of the proposed sensor is 5 cm. Moreover, it is necessary to mention that the maximum maximum absolute error of repeatability is 5 cm in a long distance (from 30 cm to about 190 cm). In absolute error of repeatability is 5 cm in a long distance (from 30 cm to about 190 cm). In the range of the range of 190 cm to 300 cm, the maximum absolute error of repeatability is 10 cm. Therefore, for 190 cm to 300 cm, the maximum absolute error of repeatability is 10 cm. Therefore, for such a wide such a wide measurement range, this repeatability of the proposed ranging sensor is excellent. measurement range, this repeatability of the proposed ranging sensor is excellent. 5.6. Short-Circuit Test 5.6. Short-Circuit Test In order to verify the intrinsically safe feature of the proposed ranging sensor, we have carried In order to verify the intrinsically safe feature of the proposed ranging sensor, we have carried out a short-circuit test. The output terminal of the energy limiting circuit is selected as the out a short-circuit test. The output terminal of the energy limiting circuit is selected as the short-circuit short-circuit point because the power source short circuit is the worst case. The test scheme is shown point because the power source short circuit is the worst case. The test scheme is shown in Figure 16. in Figure 16.

5.6. Short-Circuit Test In order to verify the intrinsically safe feature of the proposed ranging sensor, we have carried out a short-circuit test. The output terminal of the energy limiting circuit is selected as the short-circuit Sensors 2016, 16,point 867 because the power source short circuit is the worst case. The test scheme is shown 15 of 17 in Figure 16.

Figure Figure 16. 16. Short-circuit Short-circuit testing testing of of the the proposed proposed intrinsically intrinsically safe safe ultrasonic ultrasonic ranging ranging sensor. sensor.

In Figure 16, the part indicated with red font is the schematic diagram of the short-circuit In Figure 16, the part indicated with red font is the schematic diagram of the short-circuit testing apparatus. K is a button switch. The sensor works normally when K is disconnected. testing apparatus. K is a button switch. The sensor works normally when K is disconnected. However, the output terminal of the energy limiting circuit will be shortened when K is pressed However, the output terminal of the energy limiting circuit will be shortened when K is pressed down. The voltmeter and ammeter in testing apparatus are used to measure the voltage and current down. The voltmeter and ammeter in testing apparatus are used to measure the voltage and current of of the circuit. An oscilloscope is utilized to observe the state of voltage and current in the the circuit. An oscilloscope is utilized to observe the state of voltage and current in the short-circuit short-circuit process. The measured results are presented in Figure 17, respectively. Sensors 2016, 16, measured 867 15 of 17 process. The results are presented in Figure 17, respectively.

Figure curves in in the the short-circuit short-circuit protection protection testing. testing. Figure 17. 17. Voltage Voltage and and current current curves

The transient responses of the short-circuit voltage and current are presented in Figure 17. As The transient responses of the short-circuit voltage and current are presented in Figure 17. As can can be seen, the operating current was about 70 mA and the voltage was about 12 V under the be seen, the operating current was about 70 mA and the voltage was about 12 V under the normal normal state. At the time of 0 s, when the button switch S was pressed down, the measured voltage state. At the time of 0 s, when the button switch S was pressed down, the measured voltage decreased decreased to about 0 V within 0.4 μs. Furthermore, the current of sensor system surged to 1.5 A to about 0 V within 0.4 µs. Furthermore, the current of sensor system surged to 1.5 A within 0.4 µs, within 0.4 μs, and then reduced to 0 A quickly (within 35 μs). Consequently, the test proves that the and then reduced to 0 A quickly (within 35 µs). Consequently, the test proves that the proposed sensor proposed sensor system can be protected under short-circuit condition. In conclusion, the system can be protected under short-circuit condition. In conclusion, the performance of the proposed performance of the proposed intrinsically safe ultrasonic ranging sensor is summarized in Table 3. intrinsically safe ultrasonic ranging sensor is summarized in Table 3. Table 3. Performance of the proposed sensor. Specification Resolution Measuring range Maximum nonlinearity error Maximum repeatability error Critical value of over-voltage protection Critical value of over-current protection Short-circuit response time

6. Conclusions

Value 5 300 2.22 10 12.45 0.101 0.4

Unit cm cm % FSS cm V A μs

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Table 3. Performance of the proposed sensor. Specification

Value

Unit

Resolution Measuring range Maximum nonlinearity error Maximum repeatability error Critical value of over-voltage protection Critical value of over-current protection Short-circuit response time

5 300 2.22 10 12.45 0.101 0.4

cm cm % FSS cm V A µs

6. Conclusions In this paper, an intrinsically safe ultrasonic ranging sensor was successfully designed and demonstrated according to the design requirements of intrinsically safe equipment. The design of the proposed sensor was a comprehensive work of digital circuit design, analog circuit design, and software compilation. The hardware and software of the sensor are developed, with which an intrinsically safe ultrasonic ranging sensor is fabricated. Additionally, as the key energy circuit of sensor, the driving circuit of the transducer is the mainly research object. The analysis of the existing driving circuits of transducers showed with a conventional driving circuit it is difficult to meet the intrinsic safety requirements due to the existence of a large energy storage inductor. Finally, a novel driving circuit approach based on a high-speed opto-coupler was designed. Moreover, its intrinsically safe characteristics were analyzed through detailed derivation and calculation. The calculation results show that the circuit is intrinsically safe. In order to improve the reliability of the sensor system, the energy limiting circuit is designed to provide dual over-voltage and over-current protection for the whole sensor so that the circuit will be protected immediately if the current or voltage exceed a critical value. Through the verification experiments in three aspects: over-voltage, over-current and short-circuit, the critical voltage and critical current are 12.45 V and 0.101 A respectively, and the short-circuit response time is 0.4 µs. Compared to the traditional ultrasonic ranging sensor, it can be concluded from the experiments that the proposed ultrasonic ranging sensor has good performance and function, and it has an innovative intrinsically safe design. Therefore the proposed sensor is suitable for distance measurements and can be applied in complex hazardous environments such as coal mines, chemical, oil, gas plants and other circumstances. Accordingly, this proposed sensor extends the application of ultrasonic ranging sensors and provides a high security level with easy implementation. An improved version will be further developed to provide simpler circuits, better accuracy, and a smaller blind area. Acknowledgments: This project is supported by the National Natural Science Foundation of China (No. 51504161). Author Contributions: Hongjuan Zhang and Baoquan Jin provided the idea of intrinsically safe ultrasonic ranging sensor, performed the experiments, provided data for the paper and wrote the paper. Yu Wang and Xu Zhang analyzed the data and prepared the literature survey. Dong Wang revised and improved the manuscript and were involved in the design of the experimental platform. Conflicts of Interest: The authors declare no conflict of interest.

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