IROS2019国际学术会议论文集0821

Development of Flexible Dual-type Proximity Sensor with Resonant Frequency for Robotic Applications Taeseung Kim1, Jiho Noh1, Tien Dat Nguyen1and Hyouk Ryeol Choi1Fellow, IEEE AbstractThis paper presents a fl exible dual-type proximity sensor for robotic applications such as human collaborative robots(HCRs) to detect the surroundings. The sensor consists of two parts; sensing transducer and a shielding layer. To amplify the sensing performance, a resonant frequency is formed by an inductive(L-type) electrode and two capacitive(C- type) electrodes, which are placed in coplanar with an LCR circuit. An optimal frequency range is suggested to amplify the proximity detecting performance with a consistent response of impedance change. The developed sensor has a size of 100 x 120 x 2.8 mm3 . To obtain the fl exibility for various robot surfaces attachment, the electrode layer is made of Flexible Printed Circuit Board(FPCB). Combined with the grounded shielding layer, the sensor can detect objects up to 300 mm in 10 mm resolution when attached to a grounded surface. The sensor is evaluated in diverse circumstances to validify for practical use on robotics. I. INTRODUCTION Human-robot interaction has become an essential part of many industrial fi elds and our daily-lives 1. Especially, the demand for robots that are capable of working with the human in the same space has increased greatly. A workability, time and space effi ciency are the advantages of human collaborative robots(HCRs) if safety is guaranteed. For the safe collaboration with HCRs, the robot should be able to detect its surroundings. Sensors play a key role in recognizing contact and approaching objects 2 4. Using the data from the sensor, HCRs can avoid critical accidents with their surroundings such as humans. Thus, diverse safety sensors are essential for safe usage of HCRs. Some sensors focus on detecting collision by measuring the force exerted by the worker using a torque sensor 5. However, to minimize the damage, detecting the human before the collision is preferable, thus, the necessity of proximity sensor arose. Fig. 1(a) illustrates the safety response of the HCRs when working with a human in the same space 6 7. When the HCR is close enough to the human, the sensor detects its surroundings and from the sensor data, the HCR decides whether to stop moving or intelligently avoid the collision. Recently, many safety sensors have been studied to ob- tain safety and reliability between workers and the HCRs. Hiroaki et al. developed a net-structured proximity sensor *This work was supported by the Robot Industry Core Technology Development Program, 20000944, Development of skin type proximity sensing technology for detecting human at 20 cm distance to prevent collision funded by Ministry of Trade, Industry and Energy (MI, Korea) 1Authors arewithSchoolofMechanicalEngineering, SungkyunkwanUniversity,Suwon,K; ; tiendatme.skku.ac.kr; hrchoime.skku.ac.kr Fig. 1.Flexible proximity sensor for robotic applications. (a) Intelligent human-robot interaction. (b) Dual-type proximity sensor. which uses refl ection of infrared light 8. The sensor is capable of sensing a large area with 1 ms response time of circuitry computation. However, the sensor is diffi cult to fabricate due to its complicated fabrication process and has insuffi cient durability for collision with humans. It also has short proximity sensing range of 70 mm. Wenqing et al. developed a proximity sensor using a combination of inductive and capacitive sensor 9. The sensor has an advantage of distinguishing different materials while sensing the distance. However, the detecting distance is 5 mm, which is not suffi cient for collision prevention. In addition, its rigid form makes it diffi cult to be implemented on a curved HCRs surfaces. Zhang et al. designed a transparent fi lm sensor using silver wires which can detect tactile and proximity values 10. It is capable of measuring 260 kPa pressure, but the tactile sensing sensitivity is 0.4 MPa and can detect a short distance of 3 cm. A versatile and modular capacitive tactile and proximity sensor was studied by Alagi et al. 11. However, the detectable distance of the sensor is 30 mm and 2019 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS) Macau, China, November 4-8, 2019 978-1-7281-4003-2/19/$31.00 2019 IEEE8288 Fig. 2. The working principle of the sensor. (a) The sensor confi guration. (b) Inductive proximity sensing principle. (c) Capacitive proximity sensing principle. Fig. 3.Circuitry schematic of the dual-type proximity sensor. the rigidness of the sensor makes it diffi cult to attach on curved robot surfaces. A highly sensitive fl exible proximity tactile array sensor with carbon micro coil was reported 12. It can detect 150 mm of proximity distance and 330 kPa pressure, however, due to the absence of shielding system, it is vulnerable to disturbances. In this paper, we propose a fl exible dual-type proximity sensor with high spatial resolution and wide detecting range that can be applied to complex robotic surfaces, as shown in Fig. 1(b). The unique structure of the sensor enables detection of diverse materials in long distance. Effect of electrical resonance and optimal excitation frequency is sug- gested for amplifi cation of proximity sensing. A prototype sensor is fabricated and experimented to evaluate feasibility and validity for practical usage on HCRs. The rest of the paper is organized as follows. Section II describes the design and sensing principle of the dual-type proximity sensor. In Section III, the effect of resonant fre- quency and an optimal excitation frequency is suggested. In Section IV, we discuss the experimental setup and evaluation results. The conclusion and future plans are fi nally mentioned in Section V. II. DESIGN OFDUAL-TYPEPROXIMITYSENSOR A. Sensor Confi guration The dual-type proximity sensor confi guration is illus- trated in Fig. 2(a). The sensing transducer consists of an inductive(L-type) electrode and capacitive(C-type) elec- trodes. The L-type electrode is placed outside because hollow spiral pattern has a high quality factor. In addition, the spiral coil electrode is designed to have a narrow trace width because it maximizes the inductance due to a large number of turns and also, suppresses the high-frequency skin effect and proximity effect 13. The two semicircular C-type electrodes are placed in the coreless middle area. The areal ratio of the C-type electrodes and the L-type electrode is determined by considering the purpose of the proximity sensor which is detecting the human body rather than other materials. The sensing transducer is fabricated with Flexible Printed Circuit Board(FPCB) with a size of 100 x 120 x 0.12 mm3 to easily attach on diverse surfaces. The L-type and C-type electrodes are connected in parallel with an embedded LCR circuit to induce resonance in order to amplify the proximity detecting performance. The shielding layer consists of a dielectric layer and a conductive layer. The dielectric layer is to prevent a decrease in proximity sensing performance when the sensor is directly attached to a grounded surface. The conductive layer is electrically grounded and attached to shield the electromagnetic disturbance from the rear side of the sensor. The overall sensor has a size of 100 x 120 x 2.8 mm3. The circuitry schematic of the dual-type proximity sensor is shown in Fig. 3. The inductive and capacitive electrodes are connected in parallel and they are connected in series with a parallel LCR circuit. Hence, the impedance of the sensor system can be expressed by the following equations. Zsystem= Zpassive+Zsensor(1) where Zpassiveis the impedance of the passive LCR circuit, and Zsensoris the impedance of the sensing transducer. The Zpassiveand the Zsensorcan be calculated by the following formulas. Zpassive= Rp+(Lp2Cp3Rp2Cp +Lp)j (1CpLp2)2+Rp2Cp22 (2) Zsensor= Zsensor1+Zsensor2+Zsensor3+Zsensor4 ( 1 2Cs2 2Ls Cs +2Ls2)(RLs2+2RLsRCs+RCs2) (3) Zsensor1= Cs 2Ls2RC s4+(CsLsRCsCsLsRLs)2+RLs Cs22 (4) 8289 Fig. 4.Analysis of frequency. (a) The resonant frequency shift of the sensor. (b) Optimal excitation frequency range. Zsensor2= CsRLsRCs 2+(CsRL s2+Ls)RCs+LsRLs Cs (5) Zsensor3= (CsLsRCs 2+CsLsRL sRCs)2RLsRCsRLs2 Cs j (6) Zsensor4= (Cs2LsRLsRCs+CsLs2)2CsRLsRCsLs Cs2 j (7) where Lp, Cp, and Rpare the inductance, capacitance, and resistance of the LCR circuit, respectively. Lsis the inductance of the transducer, and Csis the capacitance of the transducer. RLsis the resistance of the L-type electrode, and RCsis the resistance of the C-type electrodes. B. Principle of Dual-type Proximity Sensing Inductive and capacitive sensing methods are commonly used individually for proximity sensing. However, the pro- posed dual-type proximity sensor measures the change of impedance. Thus, both the inductive and capacitive sensing methods are used to amplify the impedance change. As shown in Fig. 2(b), the L-type sensor dominantly gen- erates a magnetic fi eld when an AC current fl ows through the Fig. 5. Experimental setup. (a) For fl at surface. (b) For curved surface. coplanar coil electrode. When a conductive object enters the magnetic fi eld, a circulating current(eddy current) is induced on the surface of the object. The eddy current generates its own magnetic fi eld which cancels out the original magnetic fi eld from the sensor. This causes the inductance of the sensor to change. The C-type sensor dominantly generates an electric fi eld by fringe effect, which forms a mutual capacitance. As illustrated in Fig. 2(c), when a human body approaches the sensor, the electric fi eld is absorbed into the body, causing the mutual capacitance to change. When the dual-type proximity sensor is attached directly to a grounded surface, which is the most frequent case of HCRs links, high capacitance is formed between the sensor and the grounded surface. This is because the distance between the sensor and surrounding objects is larger than the distance between the sensor and the attached grounded surface, a stronger electromagnetic fi eld is formed between the sensor and the grounded surface, meaning higher capacitance is formed. To prevent the sensor to form the electromagnetic fi eld with the grounded surface, the dielectric layer with a low dielectric constant is inserted between them. This increases the distance between the sensor and the grounded 8290 surface, enabling the sensor to detect its surroundings more sensitively 14 15. The sensor radiates electromagnetic fi eld in all directions, however, the proximity sensing capability should be limited to the front direction of the sensor. By adding the grounded conductive layer at the very bottom of the sensor, the electromagnetic fi eld from the rear side of the sensor is absorbed to the layer, preventing the sensing from the rear side of the sensor. III. ANALYSIS OFFREQUENCY To amplify the proximity sensing performance, the sensor is adjusted to form a resonant frequency and an optimal frequency range is derived. As explained in Fig. 4(a), as the distance between the sensor and the object gets closer, the resonant frequency shifts to a higher value. This is because the resonant frequency is formed by both inductive and capacitive elements of the sensor, hence, change in both quantities results change in the resonant frequency. If a fre- quency higher than the initial resonant frequency is used as the excitation frequency, the impedance change according to the distance between the sensor and the object is inconsistent, as shown in Fig. 4(a) Case 2. As the object gets closer to the sensor, the impedance magnitude changes in 1, 2, and 3 order. It shows that the impedance magnitude increases and then decreases, which is not consistent. However, if a frequency lower than the initial resonant frequency is used, the impedance change acts consistently, as illustrated in Fig. 4(a) Case 1. As the object gets closer to the sensor, the impedance magnitude changes in 1, 2, and 3 order. It shows that the impedance magnitude gradually decreases, which is consistent. Fig. 4(b) illustrates impedance values when there is no object and when the object is 200 mm away from the sensor. The subtraction of the two impedance values when the frequency is lower than the initial resonant frequency is also illustrated. Only the frequency range below the initial resonant frequency is examined to maintain consistency of impedance change. It is visible from Fig. 4(b) that at 99 % of the initial resonant frequency shows the highest change of impedance, and at 80 % of the initial resonant frequency the impedance change is about 10 kOhm, which is assumed to be the minimum impedance value for an impedance reading chip to process. Hence, 80 99 % of the initial resonant frequency is suggested as the optimal excitation frequency range to amplify the proximity detection performance. IV. EXPERIMENT A. Experimental Setup To evaluate the performance of the dual-type proximity sensor, a single axis linear stage is designed, as shown in Fig. 5. A jig is designed to slot different material objects and manufactured by a 3D printer (Object24, Stratasys Co., Ltd.) to avoid any electromagnetic effect on the sensor. Two different surfaces, a fl at surface and a curved surface with 90 mm diameters, where the sensor is attached, is placed under the jig. The distance between the jig and the Fig. 6.Comparison of three types of sensing transducers. (a) Capacitive, inductive, and dual-type transducers. (b) Comparison results. Fig. 7.Proximity sensing performance on different surfaces. sensor is controlled by a motor (Ezi-SERVO Plus-R Motor, FASTECH Co., Ltd.), and the maximum distance is 400 mm. The data from the sensor is obtained by an LCR meter (E4980A, Agilent Technologies) and analyzed by Matlab and LabVIEW programs. B. Experiments of Dual-type Proximity Sensor The proximity sensor is examined by measuring the impedance change when the distance between an object changes. However, since the obtained data is raw data from the LCR meter, it is necessary to set a criterion to distinguish the impedance change from the distance change and from noise disturbance. In this work, the impedance 8291 Fig. 8. Proximity sensing performance. (a) Effect of the optimal frequency. (b) Proximity performance with various target objects. (c) Shielding performance. (d) Repeatability performance. change is assumed to be changed according to the distance if CNR(Contrast-to-noise ratio, a measure of image quality based on a contrast) value, which can be defi ned by A/N, is higher than 3 16. Three types of sensing transducers(capacitive, inductive, and dual-type transducers) were evaluated to validify the effectiveness of dual-type transducer in proximity sensing. As shown in Fig. 6(a), the three types of transducers were manufactured to have the same outer diameter of 70 mm. A human hand was shifted from 300 mm to 90 mm in 30 mm decrement, and the impedance change was examined. For consistent condition, the initial impedance values for the three transducers were set to the same value. As illustrated in Fig. 6(b), the experimental result showed the dual-type proximity sensor is the most adequate transducer to detect proximity distance, for the impedance change was the great- est. To examine the sensing performance on diverse surfaces, the sensor was attached to three different surfaces; non- conductive fl at surface, grounded fl at surface and grounded curved surface. The impedance change is demonstrated in Fig. 7 while the distance between the sensor and a human hand changed from 200 mm to 170 mm. The impedance magnitude when the distance is 200 mm is different for each case. Sensitivity also shows a difference for each case; the highest value of 220 Ohm when the sensor is attached to the non-conductive fl at surface, and the lowest of 1.2 Ohm when attached to the conductive curved surface. This is due to the effect of the attached surface on the magnetic fi eld from the sensor. However, for the three cases, the sensor is capable of detecting the human hand at 200 mm distance. To evaluate the effect of the optimal frequency, the sensor was excited in two different frequencies; an optimal fre- quency of 900 kHz and a non-optimal frequency of 300 kHz. A human hand approached to the sensor from 300 mm to 100 mm in 10 mm decrement while measuring the impedance magnitude. As shown in Fig. 8(a), by using the optimal frequency, the detecting distance was farther and the 10 mm 8292 distance resolution was more detectable with less noise. Also, the sensitivity, the amount of impedance change according to the distance change, was 30 times greater when the sensor was excited with the optimal frequency. Therefore, using the optimal frequency range is concluded to signifi cantly enhance the sensing performance. Various objects(hand, aluminum plate, plastic plate, and the jig without any object) were experimented to evaluate the proximity sensing performance for different materials, which is shown in Fig. 8(b). Each object moved from 1 mm to 300 mm while examining the impedance magnitude. In case of hand, the sensor could detect up to 300 mm and the impedance magnitude changed from 6.33 kOhm to 5.92 kOhm. For 150 x 150 mm2sized aluminum plate, the sensor can detect up to 210 mm. The sensor is capable of detecting non-conductive plastic plate with the same size of the alu- minum plate up to 120 mm with very low sensitivity. From the result, it can be concluded that the sensing performance is the greatest for the hand. The reason for this phenomenon is as follow; the human hand has electrical properties such as conductivity due to its ionized water composition, and it is partly connected to the earth ground which enables the sensor to detect better 17. R