Proceedings of the 001 IEEE International Conference on Robotics & Automation Seoul, Korea May 1-6, 001 Dome Shaped Touch Sensor Using PZT Thin Film Made by Hydrothermal Method Guiryong KWON*, Fumihito ARAI*, Toshio FUKUDA**, Kouichi ITOIGAWA***, and Yasunori THUKAHARA*** * Department of Micro System Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan, Phone +81-(5)-789-3116 ** Center for Cooperative Research in Advanced Science and Technology, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan, Phone +81-(5)-789-4478 *** Tokai Rica Co.,LTD., Oguchi-cho, Niwa-gun, Aichi-Ken 480-0195, Japan, Phone +81-(578)-95-704 E-mail : kwon@robo.mein.nagoya-u.ac.jp, arai@mein.nagoya-u.ac.jp Abstract We propose a dome shaped touch sensor unit, which is small and is applicable in high temperature environment. The PZT thin film was made on the halfround Ti substrate, and the electrodes were deposited on its surface to form the sensor and driving actuator. The PZT thin film was made on the curved surface of the substrate by the hydrothermal method. The actuator part is driven by the high frequency voltage at the resonant frequency [6.kHz] and the sensing part can detect the impedance change before and after the contact. The sensor works with the low voltage [5V]. It works under the high temperature [over 50 C]. The structure is simple and easy to miniaturize. This sensor can be used as the control touch-pad of the car instead of the conventional capacitive type computer touch pad, because it can resist high temperature and disturbance from the electrostatic inductance change. The basic property of the sensor is shown. Keyword: Touch sensor, PZT thin film, Micro-sensor, Vibration, Hydrothermal method 1. Introduction Today, a role of the automobile is not only a vehicle but also a moving office combined with the communication tools, such as a cellular phone and car navigation system with online information. Since the driver must concentrate on driving as much as possible, the role of the human interface or command input device is very important. There are a lot of research works on the intelligent human machine interface, such as a verbal communication system to reduce the burden. However, there is a permanent demand on the switch or key input device for secure operation. In future, the number of those input device will increase and more compact input device will be required. The conventional switch is based on the mechanical contact and has difficulty in miniaturization. So, we need a new switching device which is suitable for miniaturization and robust against the disturbance. The inside temperature of the car will range from 85 C to 0 C depends on the season and location. So, the sensor must resist such a wide temperature variation. Recently various kinds of touch sensors were developed [1-5]. Touch sensors that have been developed until now can be classified as follows. (1) Switch conductor type () Resistance change type (3) Polymer type (Ex: pressure sensitive conductor material) (4) Capacitance change type (5) Optical type (measurement of intensity of light) (6) Thermal type (7) Vibration type (Ex: piezo-electric material) The switch conductor type sensor was fabricated by MEMS (Micro Electro Mechanical System) technology. But it has the problem of the structural rigidity. Nilsson [] proposed the skin-type touch sensor, which adopted the resistance change. The main characteristic of this sensor is its structural flexibility. Niihara et al. developed a new polymer type touch sensor using rubber composite [3]. This sensor has good quality of sensing, but works under 50 C, so it is not suitable for usage in the hazard environment. A touch pad based on the capacitance change is very popular, but is not suitable for the car because of high inside temperature in summer. Yamada et al. developed a touch sensor, which can sense slip motion [4]. However, it has difficulty in sensing the contact state of the object. Optical type has the problem in miniaturization. Higuchi et al. fabricated a tactile prove sensor [5]. This sensor can also be used as the proximity sensor. However, it is not suitable for application of sensing soft object like a finger, since the shape of the contact point is so sharp. In order to use the sensor in harshness environment such as a car for all seasons, we need to develop a new touch 0-7803-6475-9/01/$10.00 001 IEEE 577
sensor, which is gentle for the human finger and not so fragile, can endure in high or low temperature, and strong for electrostatic disturbance. In this paper, we propose a dome shaped touch sensor unit, which is small and is applicable in high temperature environment. The PZT thin film was made on the halfround Ti substrate, and the electrodes were deposited on its surface to form the sensor and driving actuator. Figure 1 shows the outline of the sensor. The PZT thin film was made on the curved surface of the substrate by the hydrothermal method. The actuator part is driven by the high frequency voltage around the resonant frequency [6.kHz], and the sensing part can detect the impedance change before and after the contact. The sensor works with the low voltage [5V]. In general, the PZT and its composite shows a stable property under the temperature around 80 C. So, the PZT material is considered to be suitable for the touch sensor in harshness environment. It works under the high temperature [over 50 C]. The structure is simple and easy to miniaturize. This sensor can be used as the control touch-pad of the car instead of the conventional capacitive type computer touch pad, because it can resist high temperature and disturbance from the electrostatic inductance change. The basic concept of the sensor is shown in fig. 1. Finger methods such as the Sol-Gel method, Screen-Press, MOCVD, Sputtering and so on [6]. By these methods, it is difficult to make the PZT thin film on a threedimensional structure. Moreover, reaction temperature is higher than 500 C. By using the hydrothermal method, we can avoid such manufacturing difficulties. The advantages of the hydrothermal method are summarized as follows [6]. PZT thin film can be fabricated on the threedimensional structure. This method grows the PZT thin film in the solution. Thickness of PZT film can be controlled by repeating the crystal growth process. This method consists of two processes, namely the nucleation process and crystal growth process, we can control the thickness of the film by repeating the crystal growth process. PZT thin film can be fabricated only on the titanium substrate. PZT thin film is deposited at relatively low temperature (below 00 C). This method doesn t need the polarization process. The process to fabricate the PZT thin film by the hydrothermal method had been researched by Shimomura and Ohba et al. [7]. The improved process was proposed by Higuchi et al. [5]. In the autoclave, potassium hydroxide, lead nitrate, zirconium oxychloride, titanium substrate, and titanium tetrachloride are ionized in solution. Chemical reaction formulas in each solution are written as follows. Potassium hydroxide + KOH K + OH (1) Fig. 1 Outline of the dome shaped touch sensor. Hydrothermal Method In these days, many researchers have interest in piezoeletric material, because it can transfer the mechanical deformation to the electric charge and transfer the electric charge to the mechanical deformation. Piezoelectric material has been applied to the various kinds of sensor, for example, a touch probe sensor [5] and vibration gyroscope [6]. Those sensors use the PZT thin film as a sensor and actuator. To form the PZT thin film on the dome shaped substrate, we used the hydrothermal method. In the past, PZT thin film was made by the various Lead nitrate Pb ( NO NO () + 3) Pb + 3 Pb (3) + + + OH H + HPbO Zirconium oxychloride ZrOCl ZrO + + Cl (4) ZrO (5) + + + 4OH H + Zr( OH ) 5 Titanium substrate Ti + 4OH Ti( OH Titanium tetrachloride ) 4 TiCl 4 + 4OH Ti( OH ) 4 + Cl (6) 4 (7) 578
Finally, the formulas of mixed solution in autoclave is written as follows. HPO Pb( Zr + 0.5Zr ( OH ) 0.5 Ti 0.48 ) O 3 5 + 0.48Ti( OH ) + H 4 O + 1.5OH (8) The concentration of KOH was the same (4 mol - 44ml) for both processes. But the other solutions are changed for each process. Table 1 shows the condition of the hydrothermal method for each process. Figure shows the photograph of crystal shape after the growth process. The temperature of the nucleation process is 150 C and that of the crystal growth process is 10 C. The reaction temperature was controlled by the electric heating of the oil bath, which contains the silicon oil and the autoclave. The autoclave was rotated in the bath by the AC motor in order to blend the solvent inside it. The rotation velocity of the autoclave is 7 [rpm]. It is considered that the suitable rotation velocity ranges from 7 [rpm] to 10 [rpm][5],[7]. Table 1. Process condition of the hydrothermal method Pb(NO ) 3 Nucleation Process 0.5mol 8ml Crystal Growth Process 0.776mol 8ml 3. Vibration Analysis of Curvature Beam The shape of the sensor unit is half-round. There are 4 electrodes on inside and outside surface of it. The length of each electrode is one third of the length of the quarter of the circle length. The inside electrode is used as a sensor part and the outside electrode is used as a driving part. When the voltage is applied in the driving part, the top of the touch sensor moves up and down. 3.1 Simplified model u Fig.3 Model of the touch sensor To derive the dynamical equation of this sensor, we assume the following conditions. (1) The touch sensor is symmetric and its top moves in vertical direction. () The touch sensor is modeled as the one-dimensional spring-mass model. (3) The finger is assumed as the visco-elastic material. The finger is fixed firmly and the mass of the finger does not move. Based on these assumptions, the model of this sensor is shown in Fig. 3. Then, the dynamical equation of motion of this sensor is written as follows. Fo k 0 C0 EA L C F pzt ZrOCl H O TiCl 4 8 0.815mol 8ml 1.6mol 1.4ml 1.179mol 8ml 1.6mol.1ml EA mu&& + ( c + c ) u& + ( + k ) u = F + F PZT L 0 0 0 (9) Here, m is total mass. c and c 0 are damping coefficients. E, A, and L are the Young s modulus, the cross section of the plate, and the plate length of the sensor, respectively. F pzt is force caused by the PZT thin film actuator. F0 is the external force. It is obvious that the impedance of the total system is changed before and after the contact of the finger. So, if the sensor is sensitive enough to detect the impedance change, it can detect the contact. Fig. Photograph of crystal shape by the hydrothermal method In order to analyze the property of the sensor, we need equivalent coefficients of the model including the finger. However, it is very difficult to obtain those parameters precisely. So, next we analyze the vibration property of the dome shaped structure by the finite element method (FEM) to design a sensitive sensor. 3. Vibration modal analysis by FEM 579
We need to know the vibration mode and resonant frequency to determine the driving frequency of the actuator. We did modal analysis by the FEM. We considered the effect of the piezo-electric effect in vibration analysis by FEM. We used the ANSYS program. For input data, elastic modulus of substrate is 110Gpa, poisson s ratio is 0.33, and applied voltage of the driving part is 5[v]. Specific inductive capacity is set e / e 33 0 = 1700 and e / e 11 0 = 1730. Thickness of the titanium substrate is 5 µ m. Since we assume the sensor is symmetric, we used the half-length model for the finite element analysis as shown in Fig. 4. (b) Second vibration mode (6. KHz) Fig. 5 Vibration mode of the sensor by FEM analysis FEM Model Electrode h d Fig. 6 Resonance point by FEM (a) Configuration of the model (side view) Figure 5 shows the vibration mode by the FEM analysis. The first resonant frequency is about.1[khz]. The second resonant frequency is about 6.[kHz]. Figure 6 shows the admittance change according to the driving frequency. From this analysis, we can evaluate the second resonant frequency around 6. [khz]. We used the -nd resonant frequency for the touch sensing. 5. Experiment Result (b) Half-length model Fig. 4 Model for finite element analysis (a) First vibration mode (.1 KHz) 5.1 Touch sensing experiment We fabricated the sensor unit by the hydrothermal method. The process condition of the hydrothermal method is shown in Table 1. The size of titanium substrate is 3 10 0.05 [mm]. The height of the curved sensor unit is 3.5 [mm] (h in Fig. 4 (a)), and its width is 6.1 [mm] (d in Fig. 4 (a)). After finishing the hydrothermal process, we made electrodes at both sides of sensor unit by the vacuum evaluator. Electrodes inside the sensor unit are used for the driving part, and electrodes outside the sensor unit are used for the sensing part. Sensing part is connected to the lock-in Amplifier. Figure 7 is the electric circuit diagram of sensor pad. Figure 8 is photograph of the sensor unit. 580
Doppler vibration measurement device. The amplitude of vibration from the analysis and experiment is almost the same. The displacement amplitude is about 1 µm. When we changed the thickness of the titanium substrate from 0.1[mm] to 0.01[mm], we found that output voltage of thin substrate is bigger and clearer than that of the thick one. Fig. 7 Electric circuit of the sensor Table Comparison of output signal before and after the contact Output Voltage [mv] Phase Difference [deg] Before contact After contact.3 66.5 0.4 57.3 Displacement [10-6 m] - 1 0.8 0.6 0.4 0. 0 Fig. 8 Photograph of the sensor unit No -contact Contact 5 5.5 6 6.5 7 Frequency [khz] We also did an experiment of touching by the nonconductive material, like wood. The result of this experiment is almost the same as the previous experiment touching by the finger. This means that output voltage was influenced mainly by the variation amplitude. So, we conclude that the main factor to determine the sensitivity of the sensor is the variation amplitude. Next, we linked three sensors with the parallel circuit. The sensor array is shown in Fig. 10. The sensor unit B located at the center was touched by the finger. The result is shown in Fig. 11. The output from the sensor B decreased after the contact. On the other hand, the output from the sensor A did not decrease after the contact. This means the sensor unit is independent. This result suggests us that we will be able to develop sensor array with fine special resolution in future. Fig. 9 Difference of displacement of the sensor before and after the contact We tested the sensor with driving voltage 5 [V] and vibration frequency 6. [khz]. The sensor at this condition was very sensitive. Table is the experiment result of this sensor before and after the contact. When the finger contacts the top of the sensor, the displacement and output voltage decreased together. At the same time, phase difference between the driving frequency and output frequency occurred. Figure 9 shows the difference of displacement according to the driving frequency when the finger touched the sensor. We obtained the result in Fig. 9 by using the Laser Fig. 10 Photograph of parallel linked sensor array. 581
5. Conclusion We fabricated a new touch sensor using the PZT thin film, which is made by the hydrothermal method, in order to develop a new type touch sensor, which can be miniaturized easily and is applicable in hazard environment, such as a car. The prototype sensor detected the difference before and after the contact. The signal level was several mv. In future, we plan to evaluate this sensor in high and low temperature environment. (a) Before contact We also evaluated the response to the force input. The change of the signal was very sharp and it is not practical for force sensing of the finger at present. However, if we modify the configuration, the sensor has possibility to measure the force with good precision. We will expand the application of this sensor in future. References (b) After contact Fig. 11 Difference of output signal before and after the contact (Oscilloscope monitor) 5. Force sensing experiment We tested the relationship between the applied force and output voltage of the sensor. Figure 1 shows the experiment result. From this result, the proposed touch sensor is very sensitive for tactile sensing. On the other hand, this sensor is not suitable for force sensing, since the output decays suddenly when the force exceeds 0.3 mn. To use this sensor for force measurement, we need to modification. Amplitude [mv.5 1.5 1 0.5 0 0 5 10 15 0 Force [e-4n] [1] H.Meixner and R. Jones, Sensors, A Comprehensive Survey, Micro and Nanosensor Technology/Trends and Markets, VCH, Vol.8, pp.3-13, 1995 [] M. Nilsson, Tactile Sensing with Minimal Wiring Complexity, Proceeding of the IEEE ICRA, Vol.1, pp.93-98, 1999 [3] K. Niihara, Y.H. Choa, M. Hussain, Y. Hamahashi, H. Kawahara, Y.Okamoto and H.Nishida, New Contact Sensor in Organic/Inorganic Composite System (in Japanese), Materials Integration, Vol.1, No.5, pp.47-53, 1999. [4] Y. Yamada, M. Furukawa, S. Nishi, K. Imai and Y. Umetani, Primary Development of Viscoelastic Robot Skin with Vibrotactile Sensation of Pacinian / Non-Pacinian Channels, Proceedings of the 3rd International Conference on Advanced Mechatronics, pp.879-885, 1998. [5] T.Kanda, T.Morita, M.K.Kurosawa, T.Higuchi, A Rod-Shaped vibro touch Sensor using PZT thin film, IEEE Transaction on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 46, No.4, pp.875-88, 1999 [6] T. Fukuda, H. Sato, F. Arai, H. Iwata, and K.Itoigawa, Parallel Beam Micro Sensor/Actuator Unit Using PZT Thin Films and Application Examples, Proceeding of the IEEE ICRA, Vol., pp.1498-1503, 1998 [7] Y. Ohba, M. Miyauchi, T. Tsurumi, M. Daimon, Analysis of Bending Displacement of Lead Zirconate Titanate Thin Film Synthesized by Hydrothermal Method, Journal of J.Appl. Phys., Vol.3, pp.4095-4098,1993 Fig. 1 Relationship between applied force and output voltage of sensor 58