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A proposal of new layer sensor based on PVDF film for material identification
Sensors and Actuators A 161 (2010) 23–28
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
A proposal of new layer sensor based on PVDF film for material identification Akira Kimoto a,∗ , Shintarou Fujisaki b , Naoki Sugitani b a b
Department of Electrical and Electronic Engineering, Saga University, Honjyo 1, Saga 840-8502, Japan Department of Advanced Systems Control Engineering, Graduate School of Saga University, Honjyo 1, Saga 840-8502, Japan
a r t i c l e
i n f o
Article history: Received 22 July 2009 Received in revised form 5 March 2010 Accepted 26 April 2010 Available online 5 May 2010 Keywords: Layer sensor PVDF film Transparent conductive electrode Material identification
a b s t r a c t This paper proposes a new layer sensor for material identification under the visible light. In the proposed sensor, the optical sensor and PVDF (polyvinylidene fluoride) film with the transparent conductive electrodes are layered. The optical properties of the material are measured by a light emitting diode (LED) and a phototransistor, and the electrical properties are measured by a pair of the transparent conductive electrodes of PVDF films arranged on the surfaces of the LED and phototransistor. In addition, the vibrational property is obtained by PVDF film. Therefore, the optical, electrical and vibrational properties of the material are measured by the proposed layer sensor. To test the sensor experimentally, eleven material samples – clear, white, blue, green and black acrylic, clear glass, white polytetrafluoroethylene, silver aluminum and sponge with three types of hardness – were prepared. In the contactless measurement under the visible light, the identification of their sample materials and the detection of distance between the sensor and the surface of material were demonstrated by using the measurement values of the optical and electrical properties in the proposed sensor. In addition, the hardness was detected by the vibrational property at the contact between the proposed sensor and the material. The results indicated the usefulness of the proposed layer sensor as the proximity and touch sensors for the material identification such as optical, electrical and hardness properties, although there are still some problems that must be addressed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction A touch, or proximity sensor is one of the most important sensors in the industrial field, especially, in robot arm. Several tactile sensors have been investigated for material discrimination, shape recognition, detection of hardness, and so on. Tactile sensors based on the pressure of conductive rubber [1,2], a strain gage [3], an acoustic sensor [4], the principle of the piezoelectric resonance [5], a capacitor [6], and using piezoelectric transducers, strain gages [7], pneumatic actuation [8], and optical fibers [9] have been developed. In addition, fingertip type tactile sensors have been implemented by strain gage and polyvinylidene fluoride (PVDF) film for material discrimination [10], PVDF and a pressure variable resistor [11], a multi-modal tactile sensor based on polymer materials and metal thin film sensors [12], and by a high-speed vision sensor for robotic grasping [13]. In proximity sensor including the contactless sensor, several sensors such as optical sensors, which include computer vision [14–18], ultrasonic sensors [19,20], electromagnetic sensors [21–23], electrical capacitive sensors [24,25] and microwave sensors [26,27], have been developed for the detection of position or distance, identification of materials including thickness
∗ Corresponding author. Tel.: +81 952288637; fax: +81 952288651. E-mail address: [email protected] (A. Kimoto). 0924-4247/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2010.04.013
and moisture, and for the measurement of mechanical properties. The aim of our research is to develop a multifunctional sensor for material recognition. We proposed a new contactless sensor [28]. In the proposed sensor, the optical properties of the object are measured using a light emitting diode (LED) and a phototransistor, and the electrical properties are measured using indium tin oxide (ITO) films with the transparent conductive properties coated on each surface of the LED and phototransistor. Thus, the optical and electrical properties are simultaneously measured by the proposed sensor. In actually, the material identification and the distance between the sensor and material were detected from their measured values. In this paper, we proposed a new layer sensor for material identification including the hardness under the visible light. The proposed sensor is the layer structure of the optical sensors and PVDF (polyvinylidene fluoride) films with the transparent conductive electrodes. The optical properties of the material are measured by LED and a phototransistor, and the electrical properties are simultaneously measured by a pair of the transparent conductive electrodes. In addition, the vibrational property is obtained by PVDF film. Therefore, the optical, electrical and vibrational properties of the material are measured by the layered sensor. In the experiment, eleven material samples – clear, white, blue, green and black acrylic, clear glass, white polytetrafluoroethylene (PTFE), silver aluminum, and orange sponge with three types
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Fig. 1. Proposed layer sensor. Measurements of: (a) optical property (b) electrical property and (c) vibrational property.
of hardness – were prepared. The identification of their sample materials including hardness and detection of distance between the sensor and the surface of material were demonstrated under the visible light. 2. Method The proposed sensor is layered by the photo sensor and PVDF film with ITO film electrodes as the transparent conductive electrodes. A schematic diagram of the proposed layer sensor is shown in Fig. 1. As shown in Fig. 1(a), the reflected light as the optical property of the object is measured by the light emitting diode (LED) and the phototransistor. Capacitance as the electrical property is measured by using ITO film electrodes pasted on each surface of the LED and phototransistor. In addition, the voltage as vibrational property is measured by the PVDF film at the contact to the material. In this way, the optical and electrical properties of the object are simultaneously measured by the proposed sensor as the contactless sensor. Moreover, the electrical and vibrational properties are obtained by the same sensor as the touch sensor. Therefore, it is possible to identify the material including the surface color and the hardness by measuring the voltages induced by the reflected light and vibration, and capacitance.
Fig. 2. Schematic diagram of proposed layer senor.
the thickness of 8 mm were fixed to the upper side of the stainless plates. A light emitting diode (LED) with 935 nm peak wavelength was arranged at the center of the stainless pipe and four phototransistors with 880 nm maximum sensitive wavelength and 90◦ angle were arranged around the LED. Their diameters are 5 mm. IR filters (Kodak Corp. no. 87C) with 8 mm × 8 mm were pasted on surfaces of LED and the phototransistors for cutting off visible light. In addition, the PVDF film (Kureha Co. Ltd.; thickness 40 m), which consists of ITO films as the transparent conductive electrodes, with 8 mm × 8 mm was layered on each IR filter. The surface resistance of the PVDF film is approximately 150 /sq and the all-optic transmittance is 80%. The piezoelectric constants, d31 and d32 are also
3. Experimental Fig. 2 shows a schematic diagram of the layer sensor used for this experiment. Fig. 3 shows the picture of the sensor. A stainless halfpipe with an outer diameter of 30 mm and a length of 35 mm was fixed to the stainless plate (45 mm × 50 mm × 2 mm). In addition, two acrylic plates (35 mm × 40 mm × 10 mm) with the sponge with
Fig. 3. Picture of proposed layer sensor.
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Table 1 Characteristics of sample materials. Material
Color
Relative permittivity
Hardness (JIS no.)
Acrylic
White Black Blue Green Clear White Clear Silver Orange
3.3
124 HR
2 7 – 1.5–1.6
50–55 HS 6.5 Mohs scale (15 levels) 68 HB 100 200 K6252 300
PTFE Glass Aluminum 5052 Al–Mn Sponge
measured by the digital stragescope. In this time, the sinusoidal vibration of 1 mm amplitude with 35 Hz was given at the interval of 1 s. The measurement was automatically carried out by a general purpose interface bus (GPIB) and stored in a PC. In the experiment, eleven material samples – clear, white, blue, green and black acrylic, white polytetrafluoroethylene (PTFE), clear glass, silver aluminum, and sponge with three types of hardness – were prepared. Table 1 shows the property of each material. The size of each sample is 60 mm × 60 mm × 10 mm. The identification of them including the hardness and detection of the distance from 0 mm to 3 mm between the sensor and the material were demonstrated from capacitance, the detected photo voltage and the detected vibrational voltage.
Fig. 4. Schematic diagram of measurement system. (a) Robot arm with the proposed sensor. (b) Measurement circuit.
25 ± 5 × 10−12 C/N and 4.5 ± 0.5 × 10−12 C/N at 10 Hz, respectively. The silicone rubber with 0.5 mm thickness was pasted onto the surface of each PVDF film to protect the damage of the sensor at contact between the sensor and the measurement material. In this time, capacitance between ITO film electrodes on the surface of each PVDF film, A and B and that between A and C, and the detected voltage between them of photo sensors under the visible light were respectively measured when the sensor was closing to the material. In addition, the voltage induced by the vibration at the PVDF film, A was measured at the contact between the sensor and the material. Fig. 4 shows a schematic diagram of the measurement system. The proposed sensor was fixed to the head of the robot arm (DENSO VS-6354DM). The measurement material was set on the black color acrylic plate with a thickness of 50 mm. Capacitance was measured by the LCR meter (Agilent, 4284A) at 1 V with a frequency of 100 kHz when the sensor moved the distance from 3 mm to 0 mm. In terms of optical properties, the LED emitted light at 1.4 V and the voltage induced to each phototransistor by the reflected light was measured by a digital multimeter when the sensor was moved as well as capacitance measurement. After the contact between the sensor and material (d = 0 mm), the voltage induced by the vibration was
Fig. 5. Experimental results of detected voltage. (a) Detected voltages between photosensors, A and B. (b) Detected voltages between photosensors, A and C.
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4. Results Fig. 5 shows the measurement results of detected voltage. Vab and Vac show the detected voltages between photo sensors, A and B and those of A and C, respectively. Each measurement value is the average value of 15 measured values, which were obtained by five measurements at each setting. The standard deviation of detected voltage was less than ±2.0% from 15 measurement data. This error would be mainly caused by the difference in the setting of the distance between the proposed sensor and the material. It is found that the change of detected voltage between A and B is different to that between A and C. It is also found that the detected voltage is changed by the surface color of the material and the distance. Therefore, the results exhibit that the detection of the surface color and that of distance between 0 mm and 3 mm are possible by the proposed sensor under the visible light. It is found that the detected voltage in acrylic at the 3 mm distance becomes smaller in the order, white, green, blue, clear and black because the reflected light becomes smaller. In addition, it is also found that the detected voltage decreases as the distance between 0 mm and 3 mm becomes smaller because the reflected light does not insert to the phototransistor except for clear, green and blue color materials. We suppose that detected voltages in clear, green and blue color materials increase as the sensor closes to the material due to the scattering and transmission of the light. As next step, we would have to address these phenomena clearly. Fig. 6 shows the measurement results of capacitance values. Cab and Cac are capacitance values between ITO film electrodes on each surface of PVDF films A and B and those of ITO film electrodes A
Fig. 7. Experimental results of detected voltage. (a) Hardness material (b) sponge.
and C, respectively. Each measurement value is expressed by the average value of 15 measured values, which were obtained by five measurements at each setting. In this time, the standard deviation of capacitance was less than ±2.5% from 15 measurement data. This error would be also mainly caused by the difference in the setting of the distance between the proposed sensor and the material. Capacitance in aluminum at the contact (d = 0) was reflected by the relative permittivity of the silicon rubber pasted on the surface of the PVDF film. It is found that capacitance value becomes larger as relative permittivity of material becomes larger or in conductive material within approximately 0.5 mm distance. Meanwhile, it is found that capacitance value becomes smaller as relative permittivity of material becomes larger or in conductive material over approximately 2.0 mm distance since the electric field is spread to the material by larger relative permmittivity of the measurement material than air and the distance of electric field is equivalently longer [29]. After that, capacitance closes to higher capacitance value of air as the distance becomes farther. Thus, we suppose that minimum value of capacitance is caused by the relative permittivity. It is also found that the change of capacitance value between A and B is different to that between A and C. Therefore, the results exhibit that electrical property of the object and detection of distance between 0 mm and 3 mm are possible from measured capacitance values. Fig. 7 shows the results of the vibrational voltage measured by the PVDF film when the sensor was moved from 0 mm to −1 mm. From results, maximum voltages among the hard materials such as aluminum, glass PTFE and acrylic were almost same. Meanwhile, it is found that maximum voltage of sponge is changed by the hardness. Therefore, we suppose that the hardness is detected from the maximum value of the vibrational voltage. 5. Discussion
Fig. 6. Experimental results of capacitance values. (a) Capacitance values between ITO electrodes, A and B. (b) Capacitance values between ITO electrodes, A and C.
Fig. 8 shows the material identification and detection of distance based on capacitance values, Cab , Cac and detected voltage Vab , Vac as the contactless sensing in the proposed sensor [28]. Here, d1 and d3 show 1 mm distance between the material and the sensor and the 3 mm distance, respectively. We suppose that the material and the distance can be estimated by the measured values, Cab(m) , Cac(m) ,
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Fig. 9. Relationship between Vmax and Cab at the contact.
Fig. 8. Estimated method of material identification and detection of distance. The relationship between (a) Vab and Cab or (b) Vac and Cac .
Vab(m) and Vac(m) at one of eleven sample materials and unknown distance between 1 mm and 3 mm. For example, when Cab0 and Vab0 are measured at one of eleven material samples with unknown distance, two combinations of material and distance, (glass, 1.8) and (sponge 200, 1.7) are obtained as shown by the dotted lines of Cab0 ,Vab0 and the × mark of Fig. 8(a). We cannot decide the material and distance although the material is decided if distance is known. In addition, when Cac0 and Vac0 are measured, one combination of them, (glass, 1.8) is estimated as shown by the dotted line of Cac0 and Vac0 and the × mark of Fig. 8(b). Therefore, The combination of material and distance, (glass, 1.8) is decided. Meanwhile, when Cac1 and Vac1 are measured, the material and distance can not be decided because three combinations of material and distance, (acrylic clear, 1.6), (sponge 300, 1.7)and (sponge 200, 1.2) are obtained as shown by the dashed lines of Cac1 ,Vac1 and the mark of Fig. 8(b). However, the combination (acrylic clear, 1.6) is decided from measured values, Cab1 and Vab1 as shown by the dashed lines of Cab1 ,Vab1 and the mark of Fig. 8(a). The identification of the material and detection of the distance from 0 mm to 1 mm were also possible by same estimated method based on the detected voltage and capacitance value. Therefore, the identification of the material such as relative permittivity, color and detection of the distance from 0 mm to 3 mm
are possible by the detected voltage and capacitance values under the visible light condition in the proximity sensing of the proposed sensor. Fig. 9 shows the relationship between the maximum voltage of the vibration and the capacitance value obtained between A and B at the contact. Maximum voltage was derived from the maximum amplitude after the voltage wave as shown in Fig. 7 was passed through the band pass filter between 10 Hz and 30 Hz. Each measurement value was expressed by the average value of nine measured values, which were obtained by three measurements at each setting. From Fig. 9, it is possible to detect the hardness of the material by the vibration voltage although it is difficult to discriminate the hard material such as PTFE, acrylic, glass, and aluminum. In addition, it is possible to identify the material by capacitance. Therefore, it is found that the identification of material and the detection of the hardness of sponge are possible by the vibration voltage and capacitance at the contact. The results exhibited that the proposed senor was used as the touch sensor. From these results, it is suggested that identification of material such as the relative permittivity, the surface color and hardness and the detection of the distance are possible by proximity or touch sensing of the proposed sensor. In this time, it was difficult to estimate different material from eleven sample materials in the experiments and its distance although identification of eleven sample materials including the color and hardness and detection of distance were demonstrated. In the estimation of the unknown material and unknown distance by the proposed sensor, the electrical and optical properties and the distance would be estimated by using the numerical calculation method such as the finite element method (FEM) from detected voltage and capacitance. In addition, the vibration property of material is analyzed from hardness including the viscosity. As the next step, we try to estimate the unknown material and the distance in the proximity sensor of the proposed sensor and unknown hardness including the viscosity and the surface rough condition in the touch sensor. In addition, we must analyze the behavior of measurement results, especially, the behavior between the measurements in A–B and A–C, and the detection capability of the proposed sensor.
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6. Conclusion This paper proposed a new layer sensor to identify the material including the hardness. In the proposed sensor, an optical sensor and a PVDF film with ITO electrodes as the transparent conductive electrodes were layered. In the proximity sensing, the voltage by the reflected light of the material as the optical property was measured by a LED and a phototransisitor under the visible light condition, and capacitance as the electrical property was measured using a pair of ITO films pasted on each surface of PVDF films. In addition to capacitance measurement as the touch sensing, the vibrational voltage was measured by the PVDF film. Accordingly, the proposed layer sensor can simultaneously measure the optical and electrical properties in the proximity sensor and measure the vibrational and electrical properties at the same sensor in the touch sensor. For the experiment, eleven material samples – clear, white, blue, green and black acrylic, white PTFE, clear glass, silver aluminum and sponge with three types of hardness – were prepared, and identification of them and detection of the distance between the sensor and the material were demonstrated from detected voltage and capacitance. In addition, the detection of hardness was demonstrated from the vibrational voltage. These results indicated that the measurement of optical, electrical and vibrational properties at the same sensor was possible using the proposed sensor, although there were some problems that could be overcome to improve performance. In addition, it was shown that the identification of eleven material samples such as relative permittivity, color and hardness and the detection of the distance under the visible light condition are possible by the proposed sensor as the proximity or touch sensor. References [1] M. Shimojo, A. Namiki, M. Ishikawa, R. Makino, K. Mabuchi, A tactile sensor sheet using pressure conductive rubber with electrical-wires stitched method, IEEE Trans. Sens. J. 4 (2004) 589–596. [2] J. Yuji, K. Shida, A new multi-functional tactile sensing technique for simultaneous discrimination of material properties, in: Proc. IMTC 98, vol. 2, 1998, pp. 1029–1032. [3] A.S. Fiorillo, A. Piezoresistive, Tactile Sensor, IEEE Trans. Instrum. Meas. 46 (1997) 15–17. [4] K. Teramoto, K. Watanabe, Acoustical tactile sensor utilizing multiple reflections for direct curvature measurement, in: Proc. 41st SICE Annual Conf., l, 2002, pp. 309–312. [5] M.G. Krishna, K. Rajanna, Tactile sensor based on piezoelectric resonance, IEEE Trans. Sens. J. 4 (2004) 691–697.
[6] T. Hoshi, H. Shinoda, Tactile sensing using nonlinear elasticity, Proc. SICE Annual Conf. (2005) 2978–2981. [7] S. Omata, S. Murayama, C.E. Constantinou, Real time robotic tactile sensor system for the determination of the physical properties of biomaterials, Sens. Actuators A 112 (2004) 278–285. [8] M. Shikida, T. Shimizu, K. Sato, K. Itogawa, Active tactile sensor for detecting contact force and hardness of an object, Sens. Actuators A 103 (2003) 213–218. [9] Y. Tada, K. Hosoda, M. Asada, Sensing ability of anthropomorphic fingertip with multi-modal sensors, Proc. 8th Conf. Intelligent Autonomous Syst. (2004) 1005–1012. [10] C. Byungjune, C.R. Hyouk, K. Sungchul, Development of tactile sensor for detecting contact force and slip, in: Proc. Intelligent Robots Syst. 2005, IROS 2005, 2005, pp. 1977–1982. [11] J. Engel, J. Chen, Z. Fan, C. Liu, Polymer micromachined multimodal tactile sensors, Sens. Actuators A 117 (2005) 50–61. [12] A. Namiki, T. Komuro, M. Ishikawa, High-speed sensory–motor fusion for robotic grasping, Meas. Sci. Technol. 13 (2002) 1767–1778. [13] T. Oiwa, H. Nishitani, Three-dimensional touch probe using three fibre optic displacement sensors, Meas. Sci. Technol. 15 (2004) 84–90. [14] M.K. Ramasubramanian, R.A. Chandini, V. Mallapragada, Optical sensor for noncontact measurement of lignin content in high-speed moving paper surfaces, IEEE Sens. J. 5 (5) (2005) 1132–1139. [15] C. Berger, Strain measurement of moving periodic objects with optical sensors, IEEE Trans. Instrum. Meas. 51 (4) (2002) 616–621. [16] S. Sayli, D.V. Kerns, S.E. Kerns, Comparison of contactless measurement and testing techniques to a all silicon optical test and characterization method, IEEE Trans. Instrum. Meas. 54 (5) (2005) 2082–2089. [17] B. Culshaw, G. Pierce, J. Pan, Non-contact measurement of the mechanical properties of materials using an all-optical technique, IEEE Sens. J. 3 (1) (2003) 62–70. [18] S. Chakravarthy, R. Sharma, R. Kasturi, Noncontact level sensing technique using computer vision, IEEE Trans. Instrum. Meas. 51 (2) (2002) 353–361. [19] U. Grimaldi, M. Parvis, Noise-tolerant ultrasonic distance sensor based on a multiple driving approach, Measurement 15 (1995) 33–41. [20] P. Purnell, T.H. Gan, D.A. Hutchins, J. Berriman, Noncontact ultrasonic diagnostics in concrete: a preliminary investigation, Cement Concrete Res. 34 (2004) 1185–1188. [21] B. Legrand, Y. Dordet, J.Y. Voyant, J.P. Yonnet, Contactless position sensor using magnetic saturation, Sens. Actuators A 106 (2003) 149–154. [22] S.C. Mukhopadhyay, A novel planar mesh-type microelectromagnetic sensor. Part II. Estimation of system properties, IEEE Sens. J. 4 (3) (2004) 308–312. [23] G. Rieger, K. Ludwig, J. Hauch, W. Clemens, GMR sensors for contactless position detection, Sens. Actuators A 91 (2001) 7–11. [24] R.F. Wolffenbuttel, J.A. Foerster, Noncontact capacitive torque sensor for use on a rotating axle, IEEE Trans. Instrum. Meas. 39 (6) (1990) 1008–1013. [25] M. Gasulla, L. Xiujun, G.C.M. Meijer, H. Van Brussel, R. Puers, A contactless capacitive angular-position sensor, IEEE Sens. J. 3 (5) (2003) 607–614. [26] L.F. Root, L. Kaufman, Noncontacting low-cost instrument for film thickness measurement, IEEE Trans. Instrum. Meas. 41 (6) (1992) 1014–1019. [27] T. Hinz, F. Menke, R. Eggers, R. Knöchel, Development of a microwave moisture sensor for application in the food industry, Lebensm.-Wiss. U.-Technol. 29 (4) (1996) 316–325. [28] A. Kimoto, S. Fujisaki, K. Shida, A proposal of new contactless layered sensor for material identification, Sens. Actuators A 155 (2009) 33–38. [29] M. Seto, K. Shida, The property analysis of a plane type capacitance sensor for object detection, IEEJ Trans. Electron. Inform. Syst. 123 (5) (2003) 913–918.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
A proposal of new layer sensor based on PVDF film for material identification Akira Kimoto a,∗ , Shintarou Fujisaki b , Naoki Sugitani b a b
Department of Electrical and Electronic Engineering, Saga University, Honjyo 1, Saga 840-8502, Japan Department of Advanced Systems Control Engineering, Graduate School of Saga University, Honjyo 1, Saga 840-8502, Japan
a r t i c l e
i n f o
Article history: Received 22 July 2009 Received in revised form 5 March 2010 Accepted 26 April 2010 Available online 5 May 2010 Keywords: Layer sensor PVDF film Transparent conductive electrode Material identification
a b s t r a c t This paper proposes a new layer sensor for material identification under the visible light. In the proposed sensor, the optical sensor and PVDF (polyvinylidene fluoride) film with the transparent conductive electrodes are layered. The optical properties of the material are measured by a light emitting diode (LED) and a phototransistor, and the electrical properties are measured by a pair of the transparent conductive electrodes of PVDF films arranged on the surfaces of the LED and phototransistor. In addition, the vibrational property is obtained by PVDF film. Therefore, the optical, electrical and vibrational properties of the material are measured by the proposed layer sensor. To test the sensor experimentally, eleven material samples – clear, white, blue, green and black acrylic, clear glass, white polytetrafluoroethylene, silver aluminum and sponge with three types of hardness – were prepared. In the contactless measurement under the visible light, the identification of their sample materials and the detection of distance between the sensor and the surface of material were demonstrated by using the measurement values of the optical and electrical properties in the proposed sensor. In addition, the hardness was detected by the vibrational property at the contact between the proposed sensor and the material. The results indicated the usefulness of the proposed layer sensor as the proximity and touch sensors for the material identification such as optical, electrical and hardness properties, although there are still some problems that must be addressed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction A touch, or proximity sensor is one of the most important sensors in the industrial field, especially, in robot arm. Several tactile sensors have been investigated for material discrimination, shape recognition, detection of hardness, and so on. Tactile sensors based on the pressure of conductive rubber [1,2], a strain gage [3], an acoustic sensor [4], the principle of the piezoelectric resonance [5], a capacitor [6], and using piezoelectric transducers, strain gages [7], pneumatic actuation [8], and optical fibers [9] have been developed. In addition, fingertip type tactile sensors have been implemented by strain gage and polyvinylidene fluoride (PVDF) film for material discrimination [10], PVDF and a pressure variable resistor [11], a multi-modal tactile sensor based on polymer materials and metal thin film sensors [12], and by a high-speed vision sensor for robotic grasping [13]. In proximity sensor including the contactless sensor, several sensors such as optical sensors, which include computer vision [14–18], ultrasonic sensors [19,20], electromagnetic sensors [21–23], electrical capacitive sensors [24,25] and microwave sensors [26,27], have been developed for the detection of position or distance, identification of materials including thickness
∗ Corresponding author. Tel.: +81 952288637; fax: +81 952288651. E-mail address: [email protected] (A. Kimoto). 0924-4247/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2010.04.013
and moisture, and for the measurement of mechanical properties. The aim of our research is to develop a multifunctional sensor for material recognition. We proposed a new contactless sensor [28]. In the proposed sensor, the optical properties of the object are measured using a light emitting diode (LED) and a phototransistor, and the electrical properties are measured using indium tin oxide (ITO) films with the transparent conductive properties coated on each surface of the LED and phototransistor. Thus, the optical and electrical properties are simultaneously measured by the proposed sensor. In actually, the material identification and the distance between the sensor and material were detected from their measured values. In this paper, we proposed a new layer sensor for material identification including the hardness under the visible light. The proposed sensor is the layer structure of the optical sensors and PVDF (polyvinylidene fluoride) films with the transparent conductive electrodes. The optical properties of the material are measured by LED and a phototransistor, and the electrical properties are simultaneously measured by a pair of the transparent conductive electrodes. In addition, the vibrational property is obtained by PVDF film. Therefore, the optical, electrical and vibrational properties of the material are measured by the layered sensor. In the experiment, eleven material samples – clear, white, blue, green and black acrylic, clear glass, white polytetrafluoroethylene (PTFE), silver aluminum, and orange sponge with three types
24
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Fig. 1. Proposed layer sensor. Measurements of: (a) optical property (b) electrical property and (c) vibrational property.
of hardness – were prepared. The identification of their sample materials including hardness and detection of distance between the sensor and the surface of material were demonstrated under the visible light. 2. Method The proposed sensor is layered by the photo sensor and PVDF film with ITO film electrodes as the transparent conductive electrodes. A schematic diagram of the proposed layer sensor is shown in Fig. 1. As shown in Fig. 1(a), the reflected light as the optical property of the object is measured by the light emitting diode (LED) and the phototransistor. Capacitance as the electrical property is measured by using ITO film electrodes pasted on each surface of the LED and phototransistor. In addition, the voltage as vibrational property is measured by the PVDF film at the contact to the material. In this way, the optical and electrical properties of the object are simultaneously measured by the proposed sensor as the contactless sensor. Moreover, the electrical and vibrational properties are obtained by the same sensor as the touch sensor. Therefore, it is possible to identify the material including the surface color and the hardness by measuring the voltages induced by the reflected light and vibration, and capacitance.
Fig. 2. Schematic diagram of proposed layer senor.
the thickness of 8 mm were fixed to the upper side of the stainless plates. A light emitting diode (LED) with 935 nm peak wavelength was arranged at the center of the stainless pipe and four phototransistors with 880 nm maximum sensitive wavelength and 90◦ angle were arranged around the LED. Their diameters are 5 mm. IR filters (Kodak Corp. no. 87C) with 8 mm × 8 mm were pasted on surfaces of LED and the phototransistors for cutting off visible light. In addition, the PVDF film (Kureha Co. Ltd.; thickness 40 m), which consists of ITO films as the transparent conductive electrodes, with 8 mm × 8 mm was layered on each IR filter. The surface resistance of the PVDF film is approximately 150 /sq and the all-optic transmittance is 80%. The piezoelectric constants, d31 and d32 are also
3. Experimental Fig. 2 shows a schematic diagram of the layer sensor used for this experiment. Fig. 3 shows the picture of the sensor. A stainless halfpipe with an outer diameter of 30 mm and a length of 35 mm was fixed to the stainless plate (45 mm × 50 mm × 2 mm). In addition, two acrylic plates (35 mm × 40 mm × 10 mm) with the sponge with
Fig. 3. Picture of proposed layer sensor.
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25
Table 1 Characteristics of sample materials. Material
Color
Relative permittivity
Hardness (JIS no.)
Acrylic
White Black Blue Green Clear White Clear Silver Orange
3.3
124 HR
2 7 – 1.5–1.6
50–55 HS 6.5 Mohs scale (15 levels) 68 HB 100 200 K6252 300
PTFE Glass Aluminum 5052 Al–Mn Sponge
measured by the digital stragescope. In this time, the sinusoidal vibration of 1 mm amplitude with 35 Hz was given at the interval of 1 s. The measurement was automatically carried out by a general purpose interface bus (GPIB) and stored in a PC. In the experiment, eleven material samples – clear, white, blue, green and black acrylic, white polytetrafluoroethylene (PTFE), clear glass, silver aluminum, and sponge with three types of hardness – were prepared. Table 1 shows the property of each material. The size of each sample is 60 mm × 60 mm × 10 mm. The identification of them including the hardness and detection of the distance from 0 mm to 3 mm between the sensor and the material were demonstrated from capacitance, the detected photo voltage and the detected vibrational voltage.
Fig. 4. Schematic diagram of measurement system. (a) Robot arm with the proposed sensor. (b) Measurement circuit.
25 ± 5 × 10−12 C/N and 4.5 ± 0.5 × 10−12 C/N at 10 Hz, respectively. The silicone rubber with 0.5 mm thickness was pasted onto the surface of each PVDF film to protect the damage of the sensor at contact between the sensor and the measurement material. In this time, capacitance between ITO film electrodes on the surface of each PVDF film, A and B and that between A and C, and the detected voltage between them of photo sensors under the visible light were respectively measured when the sensor was closing to the material. In addition, the voltage induced by the vibration at the PVDF film, A was measured at the contact between the sensor and the material. Fig. 4 shows a schematic diagram of the measurement system. The proposed sensor was fixed to the head of the robot arm (DENSO VS-6354DM). The measurement material was set on the black color acrylic plate with a thickness of 50 mm. Capacitance was measured by the LCR meter (Agilent, 4284A) at 1 V with a frequency of 100 kHz when the sensor moved the distance from 3 mm to 0 mm. In terms of optical properties, the LED emitted light at 1.4 V and the voltage induced to each phototransistor by the reflected light was measured by a digital multimeter when the sensor was moved as well as capacitance measurement. After the contact between the sensor and material (d = 0 mm), the voltage induced by the vibration was
Fig. 5. Experimental results of detected voltage. (a) Detected voltages between photosensors, A and B. (b) Detected voltages between photosensors, A and C.
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4. Results Fig. 5 shows the measurement results of detected voltage. Vab and Vac show the detected voltages between photo sensors, A and B and those of A and C, respectively. Each measurement value is the average value of 15 measured values, which were obtained by five measurements at each setting. The standard deviation of detected voltage was less than ±2.0% from 15 measurement data. This error would be mainly caused by the difference in the setting of the distance between the proposed sensor and the material. It is found that the change of detected voltage between A and B is different to that between A and C. It is also found that the detected voltage is changed by the surface color of the material and the distance. Therefore, the results exhibit that the detection of the surface color and that of distance between 0 mm and 3 mm are possible by the proposed sensor under the visible light. It is found that the detected voltage in acrylic at the 3 mm distance becomes smaller in the order, white, green, blue, clear and black because the reflected light becomes smaller. In addition, it is also found that the detected voltage decreases as the distance between 0 mm and 3 mm becomes smaller because the reflected light does not insert to the phototransistor except for clear, green and blue color materials. We suppose that detected voltages in clear, green and blue color materials increase as the sensor closes to the material due to the scattering and transmission of the light. As next step, we would have to address these phenomena clearly. Fig. 6 shows the measurement results of capacitance values. Cab and Cac are capacitance values between ITO film electrodes on each surface of PVDF films A and B and those of ITO film electrodes A
Fig. 7. Experimental results of detected voltage. (a) Hardness material (b) sponge.
and C, respectively. Each measurement value is expressed by the average value of 15 measured values, which were obtained by five measurements at each setting. In this time, the standard deviation of capacitance was less than ±2.5% from 15 measurement data. This error would be also mainly caused by the difference in the setting of the distance between the proposed sensor and the material. Capacitance in aluminum at the contact (d = 0) was reflected by the relative permittivity of the silicon rubber pasted on the surface of the PVDF film. It is found that capacitance value becomes larger as relative permittivity of material becomes larger or in conductive material within approximately 0.5 mm distance. Meanwhile, it is found that capacitance value becomes smaller as relative permittivity of material becomes larger or in conductive material over approximately 2.0 mm distance since the electric field is spread to the material by larger relative permmittivity of the measurement material than air and the distance of electric field is equivalently longer [29]. After that, capacitance closes to higher capacitance value of air as the distance becomes farther. Thus, we suppose that minimum value of capacitance is caused by the relative permittivity. It is also found that the change of capacitance value between A and B is different to that between A and C. Therefore, the results exhibit that electrical property of the object and detection of distance between 0 mm and 3 mm are possible from measured capacitance values. Fig. 7 shows the results of the vibrational voltage measured by the PVDF film when the sensor was moved from 0 mm to −1 mm. From results, maximum voltages among the hard materials such as aluminum, glass PTFE and acrylic were almost same. Meanwhile, it is found that maximum voltage of sponge is changed by the hardness. Therefore, we suppose that the hardness is detected from the maximum value of the vibrational voltage. 5. Discussion
Fig. 6. Experimental results of capacitance values. (a) Capacitance values between ITO electrodes, A and B. (b) Capacitance values between ITO electrodes, A and C.
Fig. 8 shows the material identification and detection of distance based on capacitance values, Cab , Cac and detected voltage Vab , Vac as the contactless sensing in the proposed sensor [28]. Here, d1 and d3 show 1 mm distance between the material and the sensor and the 3 mm distance, respectively. We suppose that the material and the distance can be estimated by the measured values, Cab(m) , Cac(m) ,
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Fig. 9. Relationship between Vmax and Cab at the contact.
Fig. 8. Estimated method of material identification and detection of distance. The relationship between (a) Vab and Cab or (b) Vac and Cac .
Vab(m) and Vac(m) at one of eleven sample materials and unknown distance between 1 mm and 3 mm. For example, when Cab0 and Vab0 are measured at one of eleven material samples with unknown distance, two combinations of material and distance, (glass, 1.8) and (sponge 200, 1.7) are obtained as shown by the dotted lines of Cab0 ,Vab0 and the × mark of Fig. 8(a). We cannot decide the material and distance although the material is decided if distance is known. In addition, when Cac0 and Vac0 are measured, one combination of them, (glass, 1.8) is estimated as shown by the dotted line of Cac0 and Vac0 and the × mark of Fig. 8(b). Therefore, The combination of material and distance, (glass, 1.8) is decided. Meanwhile, when Cac1 and Vac1 are measured, the material and distance can not be decided because three combinations of material and distance, (acrylic clear, 1.6), (sponge 300, 1.7)and (sponge 200, 1.2) are obtained as shown by the dashed lines of Cac1 ,Vac1 and the mark of Fig. 8(b). However, the combination (acrylic clear, 1.6) is decided from measured values, Cab1 and Vab1 as shown by the dashed lines of Cab1 ,Vab1 and the mark of Fig. 8(a). The identification of the material and detection of the distance from 0 mm to 1 mm were also possible by same estimated method based on the detected voltage and capacitance value. Therefore, the identification of the material such as relative permittivity, color and detection of the distance from 0 mm to 3 mm
are possible by the detected voltage and capacitance values under the visible light condition in the proximity sensing of the proposed sensor. Fig. 9 shows the relationship between the maximum voltage of the vibration and the capacitance value obtained between A and B at the contact. Maximum voltage was derived from the maximum amplitude after the voltage wave as shown in Fig. 7 was passed through the band pass filter between 10 Hz and 30 Hz. Each measurement value was expressed by the average value of nine measured values, which were obtained by three measurements at each setting. From Fig. 9, it is possible to detect the hardness of the material by the vibration voltage although it is difficult to discriminate the hard material such as PTFE, acrylic, glass, and aluminum. In addition, it is possible to identify the material by capacitance. Therefore, it is found that the identification of material and the detection of the hardness of sponge are possible by the vibration voltage and capacitance at the contact. The results exhibited that the proposed senor was used as the touch sensor. From these results, it is suggested that identification of material such as the relative permittivity, the surface color and hardness and the detection of the distance are possible by proximity or touch sensing of the proposed sensor. In this time, it was difficult to estimate different material from eleven sample materials in the experiments and its distance although identification of eleven sample materials including the color and hardness and detection of distance were demonstrated. In the estimation of the unknown material and unknown distance by the proposed sensor, the electrical and optical properties and the distance would be estimated by using the numerical calculation method such as the finite element method (FEM) from detected voltage and capacitance. In addition, the vibration property of material is analyzed from hardness including the viscosity. As the next step, we try to estimate the unknown material and the distance in the proximity sensor of the proposed sensor and unknown hardness including the viscosity and the surface rough condition in the touch sensor. In addition, we must analyze the behavior of measurement results, especially, the behavior between the measurements in A–B and A–C, and the detection capability of the proposed sensor.
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6. Conclusion This paper proposed a new layer sensor to identify the material including the hardness. In the proposed sensor, an optical sensor and a PVDF film with ITO electrodes as the transparent conductive electrodes were layered. In the proximity sensing, the voltage by the reflected light of the material as the optical property was measured by a LED and a phototransisitor under the visible light condition, and capacitance as the electrical property was measured using a pair of ITO films pasted on each surface of PVDF films. In addition to capacitance measurement as the touch sensing, the vibrational voltage was measured by the PVDF film. Accordingly, the proposed layer sensor can simultaneously measure the optical and electrical properties in the proximity sensor and measure the vibrational and electrical properties at the same sensor in the touch sensor. For the experiment, eleven material samples – clear, white, blue, green and black acrylic, white PTFE, clear glass, silver aluminum and sponge with three types of hardness – were prepared, and identification of them and detection of the distance between the sensor and the material were demonstrated from detected voltage and capacitance. In addition, the detection of hardness was demonstrated from the vibrational voltage. These results indicated that the measurement of optical, electrical and vibrational properties at the same sensor was possible using the proposed sensor, although there were some problems that could be overcome to improve performance. In addition, it was shown that the identification of eleven material samples such as relative permittivity, color and hardness and the detection of the distance under the visible light condition are possible by the proposed sensor as the proximity or touch sensor. References [1] M. Shimojo, A. Namiki, M. Ishikawa, R. Makino, K. Mabuchi, A tactile sensor sheet using pressure conductive rubber with electrical-wires stitched method, IEEE Trans. Sens. J. 4 (2004) 589–596. [2] J. Yuji, K. Shida, A new multi-functional tactile sensing technique for simultaneous discrimination of material properties, in: Proc. IMTC 98, vol. 2, 1998, pp. 1029–1032. [3] A.S. Fiorillo, A. Piezoresistive, Tactile Sensor, IEEE Trans. Instrum. Meas. 46 (1997) 15–17. [4] K. Teramoto, K. Watanabe, Acoustical tactile sensor utilizing multiple reflections for direct curvature measurement, in: Proc. 41st SICE Annual Conf., l, 2002, pp. 309–312. [5] M.G. Krishna, K. Rajanna, Tactile sensor based on piezoelectric resonance, IEEE Trans. Sens. J. 4 (2004) 691–697.
[6] T. Hoshi, H. Shinoda, Tactile sensing using nonlinear elasticity, Proc. SICE Annual Conf. (2005) 2978–2981. [7] S. Omata, S. Murayama, C.E. Constantinou, Real time robotic tactile sensor system for the determination of the physical properties of biomaterials, Sens. Actuators A 112 (2004) 278–285. [8] M. Shikida, T. Shimizu, K. Sato, K. Itogawa, Active tactile sensor for detecting contact force and hardness of an object, Sens. Actuators A 103 (2003) 213–218. [9] Y. Tada, K. Hosoda, M. Asada, Sensing ability of anthropomorphic fingertip with multi-modal sensors, Proc. 8th Conf. Intelligent Autonomous Syst. (2004) 1005–1012. [10] C. Byungjune, C.R. Hyouk, K. Sungchul, Development of tactile sensor for detecting contact force and slip, in: Proc. Intelligent Robots Syst. 2005, IROS 2005, 2005, pp. 1977–1982. [11] J. Engel, J. Chen, Z. Fan, C. Liu, Polymer micromachined multimodal tactile sensors, Sens. Actuators A 117 (2005) 50–61. [12] A. Namiki, T. Komuro, M. Ishikawa, High-speed sensory–motor fusion for robotic grasping, Meas. Sci. Technol. 13 (2002) 1767–1778. [13] T. Oiwa, H. Nishitani, Three-dimensional touch probe using three fibre optic displacement sensors, Meas. Sci. Technol. 15 (2004) 84–90. [14] M.K. Ramasubramanian, R.A. Chandini, V. Mallapragada, Optical sensor for noncontact measurement of lignin content in high-speed moving paper surfaces, IEEE Sens. J. 5 (5) (2005) 1132–1139. [15] C. Berger, Strain measurement of moving periodic objects with optical sensors, IEEE Trans. Instrum. Meas. 51 (4) (2002) 616–621. [16] S. Sayli, D.V. Kerns, S.E. Kerns, Comparison of contactless measurement and testing techniques to a all silicon optical test and characterization method, IEEE Trans. Instrum. Meas. 54 (5) (2005) 2082–2089. [17] B. Culshaw, G. Pierce, J. Pan, Non-contact measurement of the mechanical properties of materials using an all-optical technique, IEEE Sens. J. 3 (1) (2003) 62–70. [18] S. Chakravarthy, R. Sharma, R. Kasturi, Noncontact level sensing technique using computer vision, IEEE Trans. Instrum. Meas. 51 (2) (2002) 353–361. [19] U. Grimaldi, M. Parvis, Noise-tolerant ultrasonic distance sensor based on a multiple driving approach, Measurement 15 (1995) 33–41. [20] P. Purnell, T.H. Gan, D.A. Hutchins, J. Berriman, Noncontact ultrasonic diagnostics in concrete: a preliminary investigation, Cement Concrete Res. 34 (2004) 1185–1188. [21] B. Legrand, Y. Dordet, J.Y. Voyant, J.P. Yonnet, Contactless position sensor using magnetic saturation, Sens. Actuators A 106 (2003) 149–154. [22] S.C. Mukhopadhyay, A novel planar mesh-type microelectromagnetic sensor. Part II. Estimation of system properties, IEEE Sens. J. 4 (3) (2004) 308–312. [23] G. Rieger, K. Ludwig, J. Hauch, W. Clemens, GMR sensors for contactless position detection, Sens. Actuators A 91 (2001) 7–11. [24] R.F. Wolffenbuttel, J.A. Foerster, Noncontact capacitive torque sensor for use on a rotating axle, IEEE Trans. Instrum. Meas. 39 (6) (1990) 1008–1013. [25] M. Gasulla, L. Xiujun, G.C.M. Meijer, H. Van Brussel, R. Puers, A contactless capacitive angular-position sensor, IEEE Sens. J. 3 (5) (2003) 607–614. [26] L.F. Root, L. Kaufman, Noncontacting low-cost instrument for film thickness measurement, IEEE Trans. Instrum. Meas. 41 (6) (1992) 1014–1019. [27] T. Hinz, F. Menke, R. Eggers, R. Knöchel, Development of a microwave moisture sensor for application in the food industry, Lebensm.-Wiss. U.-Technol. 29 (4) (1996) 316–325. [28] A. Kimoto, S. Fujisaki, K. Shida, A proposal of new contactless layered sensor for material identification, Sens. Actuators A 155 (2009) 33–38. [29] M. Seto, K. Shida, The property analysis of a plane type capacitance sensor for object detection, IEEJ Trans. Electron. Inform. Syst. 123 (5) (2003) 913–918.