Electronics and Communication Department. Jaypee Institute of Information Technology. Sector 128, Noida, India, 201307 email: arun.sinha@jiit.ac.in.
Measurement of Piezoelectric based Tactile sensor using low C o st S etup Arun Kumar Sinha Electronics and Communication Department Jaypee Institute of Information Technology Sector 128, Noida, India, 20 1 3 07 email: arun.sinha@j iit.ac.in spatial resolution among devices. Table I shows the specification of the chips based on the two processes, and WPVDF and LpYDF refer to width and length of PVDF-TrFE between two metal electrodes.
Abstract-In this paper we have presented measurement results of a piezoelectric based tactile sensor. We have used customized signal conditioning board to record the sensor output. And linear actuator to apply mechanical displacement on the sensor, during our experiments we have used non-transparent silicone rubber sheet to cover the tactile sensor. The results show that linear actuator can successfully test the sensor at low frequencies and hence can be a suitable cost effective setup. The overall setup is very cheap, compared to the setup used in the past for similar purposes.
In our work we have used the POSFET chip based on a CMOS process [ 1 0] . Fig. l (a) shows the top view of this chip and the inset shows a simplified view of a POSFET unit. Each such 4 x 4 device unit makes a sensor chip which needs to be biased separately during sensing application. Fig. 1 (b) shows the electrical equivalent of the POSFET unit with resistance Rs to bias the sensor in common drain configuration. Also in a row all the drain terminals were commonly connected and maintained at the same supply potential, and the sensor chip
Keywords-Linear actuator, measurement, signal conditioning, tactile sensor
I.
INTRODUCTION
TABLE I SOME IMPORTANT SPECIFICA nON s OF POSFET CHIPS
Piezoelectric semiconductor devices had been widely reported for application as a transducer for ultrasonic and force sensing [ 1 ] - [4] . With further advancement in the technology they have been reported for tactile sensing elsewhere [5], [6] . In recent times Dahiya et al. , have designed and fabricated a type of piezoelectric semiconductor devices called by the name Piezoelectric Semiconductor Field Effect Transistors (i.e., POSFETs) for tactile sensing by a humanoid robot [7] . In this paper POSFET will refer to such devices: they are usually arranged in a square (rectangular) or triangular arrays. A sensor chip of POSFET devices has been realized by a technology derived from a non-standard Ion Sensitive Field Effect Transistor (i.e., ISFET) based process [8] . In the p-well, an NMOS transistor is fabricated having inter-digitized gate of very large aspect ratio. The gate oxide consists of a double layer of Si02/Si3N4 dielectrics; the combined thickness being equal to 45 nm. A 4 f.tm AI metal contact (i.e., lower electrode) is made on top of the NMOS gate and a layer of piezoelectric polymer i.e., PVDF-TrFE (65/3 5) is spin coated to a thickness of 2.5 f.tm, and then a top metal layer (i.e., upper electrode) is implemented with an alloy of evaporated gold. The polarization of the piezoelectric polymer present between top gold metal, and lower Al metal was done by applying a high DC potential to electrically activate the polymer. This method is called poling procedure and was performed on the sensor chip through external accessible contacts via metal layer [9] .
PROCES S
Masks levels
CMOS 8 J.lm and 3 . 5 kD!sq.
PVDF-TrFE NMOS gate
Structural descriptions
11
8
MOS physical L = 12 J.II11 , W = 7500 J.lm structure on WPVDF J.lm,
=
730 J.lm, LpVDF
=
L = 12 J.lm, W = 7268 J.lm
730 WPVDF = 626 J.lm, LpVDF 834 J.lm
Taxel size : 1 x I mm' Taxel centre to centre distance: 1 . 5 mm Chip size : 1 . 5 x 1 . 5 cm' Number of POSFETS devices : 25
(al
The NMOS devices for the POSFET sensor chip have been realized based on two different processes, namely NMOS and CMOS. In both processes all the fabrication steps to make NMOS devices are same as well as deposition and poling of polymer; but they differ in the structure i.e., dimension and
978-1 -4799-1 607-8/1 3/$31 .00©201 3 I E E E
NMOS
p-well junction depth and sheet 4.76 J.lm and 3 . 5 kD!sq. resistance
=
Taxel size : 0.9 x 0.9 mm' Taxel centre to centre distance: 1 mm Chip size: 0 . 8 x 1 cm' of Number POSFETS devices: 16
(b)
Fig. I. (a) View of a CMOS based POSFET chip, in the inset is shown a unit of the sensor. A polymer layer is on the top of the NMOS gate insulator; inset also shows the alignment of charges when polymer is acted upon by a dynamic force. (b) An electrical representation of POSFET unit, a resistance Rs is used to bias the sensor at common drain configuration.
407
•
works under single supply voltage.
•
In this work we will present a measurement system to measure the response of the sensor using a very low setup cost. We will use customized signal conditioning board to record and a linear actuator to apply mechanical force on the sensor. In the past electromechanical setup a.k.a., shaker was used to perform similar test [ 1 1 ] . Though the shaker is powerful and can be used in a wide frequency range, it is costly, complex to control and mounting a sensor chip on the testing platform could be risky for possible damages to the sensor unit. The shaker assembly also makes a strong magnetic field which can induce noise in the dynamic response of the sensor while measuring. For this purpose, we made use of electric linear actuator [ 1 2] which is less expensive than the shaker, easy to handle and can demonstrate sufficient test for sensor characterization. As reported in [ 1 3 ] about the influence of noises on POSFET sensor, we have covered the sensor chip with a black sheet of silicone rubber [ 1 4] to improve protection from noises; while in the past a transparent sheet of silicone rubber i.e., PolyDimethylSiloxane (PDMS) was used to cover POSFET sensor chip [ 1 1 ] .
•
•
• •
•
SIGNAL CONDITIONING UNIT
ANALOG STAGE
--_I
i_DIGITAL STAGE
ENOB of ADC (N): 12 bits Resolution of ADC (l LSB):
!'!. =
N Vref . ( l /2 )
Sampling rate of one channel: 2fH Scanning rate of 1 6 channels : 1 6 . 2fH EBR (Effective bit rate) : Number of channels ' 2fH . N kbps.
III.
The overall architecture used for data acquisition is shown in Fig. 2, and consist mainly of analog stage and digital stage. The shown architecture is typically used for sensors data acquisition but we have designed and customized the system to suit for recording response by this tactile sensor. The system for analog stage consists of POSFET board and analog unit whose specification is given in Table II. The specification for the digital stage is as follows,
,------�
Reference voltage to the ADC (Vref) : VDD (depends on the type of converter therefore not a strict constraint)
As given in Table II the signal conditioning unit (i.e., analog unit) has 1 6 channels which can acquire data from 4 x 4 arrays of POSFETs simultaneously. These data can be plotted and saved on Personal Computer (PC). The system is powered by the Universal Serial Bus (USB) of the PC; therefore it does not require an external supply voltage. The POSFET board has arrangements to fix the passive devices for biasing the sensors in common drain configuration. The 16 channels of signal conditioning unit were realized by using the minimum number of passive components and have been mentioned elsewhere [ 1 7] . The gain response of analog unit was band limited between 2 Hz to 1 kHz within the supply voltage rails. For acquiring and plotting the data on PC, we have used PicoLog1 2 1 6 data logger from pico ® technology, more information regarding which can be found in [ 1 8 ] . The PicoLog supply to the POSFET board and analog unit was 2.5 V. The developed data acquisition system is attachable in parts and hence can be separated, to facilitate testing and transporting of this system.
The layout o f this paper i s as follows: In Section II we have briefly described our signal conditioning unit for data acquisition. In Section III we have described a linear actuator based on stepper motor and to use it for applying controlled displacements over the sensor. Then in Section IV we have presented results of our test and measurements. Finally in Section V we have made conclusions of this work. II.
Single supply operation [ 1 5 ] , [ 1 6] : 0 - VDD (V)
MECHANICAL SETUP
In this section we will briefly describe the setup and interpretation of the travel profile made by the linear electric actuator. We have used a linear actuator L4 1 1 8 S 1 404-T6x l A 5 0 from Nanotec ® c o . [ 12] to apply controlled displacements over the POSFET sensor. The actuator can apply controlled displacements at a constant thrust of 200 N up to a velocity of 20 mmls. Fig. 3 shows the arrangement of setup while performing mechanical test; the actuator was fixed at a distance using a clamp above the DUT. The actuator is controlled by personal computer using software and hardware provided by Nanotec ® co.
------+
Fig. 2. The overall architecture. TABLE II
SPECIFICATION OF AN ANALOG UNIT NUMBER OF CHANNELS IN MxN ANALOG STAGES
GAIN OF THE AMPLIFIER (Ay)
4 x 4 = 16
Ay = Yo / Yin = YDD I (2 'Yin)
HPF (FL, ORDER)
LPF (FH, ORDER)
BAND WIDTH OF ANALOG CHANNEL
2 Hz/ 1 st
I kHz/2nd
998 Hz
Fig. 3 . Anangement of setup to apply mechanical displacement on the cover layer present over the POSFET sensor.
408
(during which acceleration and deceleration takes place) happens within the thickness of the cover layer. Therefore d was adjusted to some suitable values, so that the probe applies �d within d - d4, where d4 is the adjusted distance when the probe just touches the surface of the cover layer.
Electric Linear Actuator
time
- dO dl
I I I I I I
L
Ad
Table III shows various specifications and their values for this actuator, during the run similar settings were used to adjust the values of acceleration and deceleration time �t and displacement �d. The acceleration time in Table III is also an indication of the mechanical frequency applied by the actuator (i.e., f = 1/ �t) on the cover layer. The piezoelectric polymer responds to dynamic mechanical input, so we have to adjust the actuator parameters to get frequency response from the sensor. During our measurements the POSFET device was present at an approximate distance of 1 0 . 5 5 mm from the probe tip.
I
I
L�::�::��}----n���=-�������� d3 o L\ t1 _ 1112 -+ 13 1"- .
0.034 0.032
0.03
0 02 . 8.2
0.22
0.24
0.26
0. 2 8 Ad rom
0.3
0.32
0.34
0.36
Fig. 6. Plot of voltage versus displacement for 0 . 5 mm thick cover layer at 80 ms.
409
o.o6 ,----,-----.---r---,---,,--.--,
of author. Author would also like to thanks Prof. Daniele D. Caviglia, Professor in DITEN, University of Genova, Italy, for supervising his work.
0 . 055
�
:>�
0.05
REFERENCES
0.045
[I]
0.04
0 .035
[2]
0.03
0 . 025"-----: 8 ----:0� 9--: . 0:-: . 1--: . 1-:0'-: .0"" 0"'= 1 2,--"" 0.-';1 ----:0:-' 1 ::- 0-'; 1 5:---;:. 1-:;. ·1 7 3 -: 4-:6 --;O:-' 0-';0 .� 0-'; . :-: . 1-;-
M (rrun)
[3]
Fig. 7. Plot of voltage versus displacement for 0 . 5 mm thick cover layer at 24 ms. [4] 0.06 ,----,-----.---r---,---,,--.--,
[5]
0.055
�
:>&
0.05
[6]
0.045 0.04
[7]
0.035
1r.03
0.03
[8]
6d (rrun) Fig 8. Plot of voltage versus displacement for 0 . 5 mm thick cover layer at 12 ms. 0° '
0.03 5
0.04
0.04 5
0.05
0.055
0.06
0.065
0.07
0.075
0.08
[9]
83.3 Hz. Fig. 6 - 8 shows the results of measurements obtained after recording the sensor response . The calculated values of sensitivities from Fig. 6 - 8 are : 0. 1 VImm, 0.3 VImm and 0 . 6 VImm, which shows that sensitivity increases with the increase in transient displacement frequency. Hence this proves that piezoelectric material responds more to high frequency mechanical displacement and hence validated our proposed mechanical setup for test applications on the sensor. However this setup has limitation to realize high frequency displacements. As shown by Fig. 8 that with the decrease in time period, the displacement step decrease that can be realized within the maximum permissible velocity by this actuator. Also sensor began to exhibit spatial distribution in response plot, which can be due to j erks in actuator when applying such displacements at high frequency. V.
[ 1 0]
[I I]
[ 1 2] [13] [ 1 4] [ 1 5]
CONCLUSIONS
[ 1 6]
In our work we have presented a less costly and an effective setup for measuring piezoelectric sensor at low frequency. The cost of this overall setup is many times less than an electromechanical setup. However the proposed setup has some limitation at high frequencies which can be overcome by improving the specification of linear actuator. For the present purpose it can be good device at low cost for mechanical test.
[ 1 7]
[ 1 8] [ 1 9]
ACKNOWLEDGMENT
This work was partially supported by Regione Liguria P.O. c.R.O. FSE 2007-20 1 3 PhD program, during doctoral research
41 0
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