Copyright @ IFAC Intelligent Components and Instruments for Control Applications, Budapest, Hungary, 1994
FERROPIEZOELECTRIC TACTILE SENSOR ARRAY V.TODOROVA, S.MILCHEV Higher Institute of Mechanical and Electrical Engineering,4,H. Dimiter St. , 5300 Gabrovo, Bulgaria Abstract A ferroelectric array sensor of a functional type for primary processing of tactile information, representing a homogeneous field of ~nsors in an active PZT ceramic substrate, has been developed. The results. obtained by physical-mathemallcal slmulallono~ certain array sensor element, which is a non resonance piezoelectrical transformer with . a runnmg bulk acoustIc wave (NRPT WIt RBAW) taking into accound its interaction with the homogeneous common ferroelectnc medIUm are pr~ented. TaclIl~ arrays are designed for using for identification of tactile images to be applied to intelligent robots and other specIalized means of mformatlOn inputting. Key Words. Delay. Electrical Sensing Devices, Identification. Image Processing, Mathematical Models, Robotics. Sensors
tile effect information carrier by comparing the delay time or (tj = t - or) and the RBAW propagation co-ordinates r in a certain point or area of the piezowafer:
1. INTRODUCTION Tactile transducers are a perspective trend in the field of modern information collection and processing systems. A wide range of principles, .materials and technologies can be used (Skopahc et al. 1984, Yardley and Backer 1986, Todorova et al. 1987). The application of electromechanical type of sensors, i.e. transducers of mecha~ical action into an electrical signal, is predetermmed by tactile principle of information inputting.
= r / vac.
Electromechanical processes in NRPT with pulse exciting RBA W were presented by Golubenko et al. (1984) and Zavadski (1989) by means of one- and two-dimensional models. NRPT require rigid fixing when assembled. It allows homogeneous array fields of rigidly fixed to each other and to the base NRPT to be created in a PZT ceramic substrate. The NRPT are realized by n output electrodes.
The results of the research and development of a functional piezoelectric tactile array sensor have been presented in this paper. The object of study is an array of non resonance piezoelectrical tran~ formers (NPRT) with a running bulc acoustic wave (RBAW). The NRPT differ from the re~o nance piezoelectric transformers (RPT), studied by Meson (1966), Lavrinenco (1975) and Kartashov and Marchenko (1978), in the type of fixing and excitation.
2. MATHEMATICAL SIMULATION AND CHOOSING OF CONSTRUCTION V ARIANTS OF TACTILE ARRAYS The theoretical consideration of the one-dimensional model is sufficient for choosing the construction variants of the array sensor. The theory is reduced to the following formulations: 1) description of the electromechanical process of excitation and RBAW formation in the excitation section zone of the NRPT is carried out according to Kolski (1953) by a differential equation:
NRPT operate normally in a pulse excitation mode. Square pulse of electric voltage causes non stationary acoustic transverse oscillations z(t) in a limited volume of the NRPT active medium at the expense of the reverse piezo~ffect: At ea~h moment of time, a volume of medIUm different m location is activated due to the displacement of the excited RBAWat a constant velocity vac' That eliminates energy accumulation in the active medium and enables us to consider RBAW as a tac131
where: z(t) is the amplitude of the excited acoustic transverse oscillations; n is the internal fric tion coefficient; wo2(z+ yz3) is the symmetric elastic characteristic of an anisotropy solid; Cl is a general coefficient of the piezoelectric material; AI and h are design parameters of NRPT the exciting electrode area and the piezowafer thickness, respectively; U I (t) is the exciting electric pulse; /
2) description of the RBAW propagation along the wafer, taking into account the losses: Z(t-'t,x)=z(t)e-!1X ,
where!.l is the loss coefficient;
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tions have opposite polarity. The array with sectional exciting electrode of Fig. 2 is exciting in X direction only. The arrays shown in Fig. 1 are applicable to tactile effect registration concentrated in a region commensurable with the area of a single sensitive element, i.e. for point effects. The accuracy obtained by such tactile array is defined within :t one discrete. But when recognizing objects with tactile images with outlines or arbitrary area, the deviation from the real object outlines might reach a considerable value. This effect is a consequence of the obtained "hidden zones". For example, when two or more adjacent sensors, diagonally activated, are 'available, they immediately form an area of x2 points, which prevents the ambiguous identification of the touched points causing the response. This effect is a result of the excitation and propagation of RBA W separately in X and Y directions, neglecting their mutual influence.
Fig. I. An array construction with exciting in X and Y directions
3) description of the output electric signal received from the corresponding activated output electrode at the expense of the direct piezoeffect:
where C 2 is a general coefficient of the piezoelectric material.
The mathematical simulation of the processes in NRPT with RBAW has been carried out by means of program package "TUTSIM". The electrophysical parameters of the piezoelectric materials, the exciting pulse parameters and the design parameters of the array related to the output signal shape and amplitude have been optimized. In accordance with the simulation results, two constructions of tactile array sensors have been designed, shown in Figures 1 and 2.
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00000000 00000000 Fig. 2. Sectional tactile array
This problem is solved by the construction of Fig. 2. It is a row-aray with one exciting and n output electrodes which is technologically multiplied in rows - m in number. The use of such array is more justified from the point of view of the reliability of the one-dimensional mathematical model of RBA W processes in NRPT since only under certain limiting conditions concerning the electrode width, it could be assumed, that RBAW propagates only in one direction and those conditions are:
Standard piezoceramic material of the type PZT nKM - 061 has been selected with wafer thickness of h = 0,35 mm and dimensions of the array field of output electrodes - (8 x 8) elements. The optimum parameters of exciting square pulse related to the selected design are: amplitude U = 20 V; pulse duration - 0,7 !AS; repetition period over 2 !AS; slope steepness - below 50 ns. In the design variant of the array in Fig. 1, two exciting electrodes forming RBAW in X and Y directions have been used and pulses sent in these direc-
where bI , bO are the exciting and output electrode widths, respectively; }..o is the length of excited RBAW, }..o= vac / fO . For the selected ceramics and design of tactile array }..o = 2,4.10- 3 m. Therefore if b I and bo are less than 1,2 mm, the construction shown in Fig. 2 guarantees the reliability of the one-dimentional mathematical model of the processes in NRPT as well. This construction has a shortcoming compared to the one shown in Fig. 1 because the sectional tactile array has (m - 2) more terminals and their number will grow when the dimensions of the array are increased in the Y direction whereas tactile array in Fig. 1 has a fixed number of terminals regardless of its dimensions. This is a technologi- . cal problem which reflect on the array cost since a package with more than four pins has to be used.
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However, this construction has an advantage: the exact co-ordinates of each activated sensitive element can be obtained irrespective of its location in relation to the rest of the activated elements. The number of the scans is m - 2 and the information word has a length of n bits and is grid copy of the tactile image of one row with accuracy within tone discrete. 3. EXPERIMENTAL RESULTS The variants of tactile arrays showw in Fig. 1 and Fig. 2 have been developed by means of a thickfilm technology. The electrode system has been formed by offset printing of Ag paste followed by baking. After polarization of the NRPT exciting and output sections and assembling into a package, the arrays have been experimentally tested. Fig. 4 shows an oscillogram of the exciting and output electric signals for an activated output electrode of the array shown in Fig. 2. As a comparison, Fig. 3 shows the mathematical simulation result from a row-array of the same construction. Table 1 shows a comparison of the signal timedelays obtained from the mathematical simulation and experiment for the row-array with 8 output electrodes. A comparatively satisfactory compliance of the results (within ±1 %) has been observed.
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Table 1. The signal time-delays I
Results from mathematical simulation
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Experimental results Fig. 3. Mathematical simulation data of the processes in the NRPT with RBA W
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According to the autors, the theoretical and experimental results obtained cofirm the opportunity of practical application of the developed constructions of tactile arrays. Moreover, they can be used for solving both direct (inputting) and the reverse (recognition) problems for tactile data processing. For example, tactile fields with dimensions (p x q) arrays can be developed based on the construction shown in Fig. 1. They might be applied to the inputting information systems. Unlike them, the construction shown in Fig. 2 can be used for tactile image recognition which makes them of practical interest for modem robots and other devices of the same type.
Fig. 4. The experimental oscillogram of the exciting and output electric sign 133
Taking into account the operating principle of the discussed constructions of ferropiezoelectric tactile arrays the problems of secondary processing of tactile information can be definitely formulated.
5. ACKNOWLEDGEMENTS The authors are grateful to V. Zavadski from Kiev Polytechnic (Ukraine) and R . Radev from HIMEE - Gabrovo for their help in the physical and mathematical simulation of the electromechanical processes in NRPT with RBAW.
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