Analog integrated circuit pressure sensor

An integrated array of pressure transducers capable of producing an analog output voltage representative of the applied pressure is proposed. The individual transducing elements (16) are defined by a three-layer structure including a thin layer of piezoelectric material (10) disposed between a reference potential plate (12) and a plurality of electrodes (15) contained in a silicon substrate (14). A force applied to a localized portion of the reference plate will cause a deflection of the piezoelectric material towards the electrodes on the substrate, inducing a capacitive charge on the electrode in the localized area. This capacitance is stores at a node A associated with the transducing element, and may be interrogated by a sensing circuit (18) located in the silicon substrate. Since the induced charge is directly proportional to the applied force, a measurement of the output voltage from node A will yield a direct indication of the localized force applied to the sensor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a monolithic tactile sensor and, more 
particularly, to a monolithic tactile sensor which combines considerable 
spatial resolution with pressure sensing over a range of two orders of 
magnitude. 
2. Description of the Prior Art 
The ability of a robot to pick up and manipulate mechanical parts can be 
greatly improved if the grasping surfaces are able to sense pressure. This 
type of sensor can be used for object identification as well as for 
insertion and mating tasks associated with assembly. The preferred 
location of the sensor is on the extremity of a movable finger. 
Consequently, the sensor must be compact and rugged and should provide 
spatial resolution of better than 0.1 inch .sup.2 /element (one hundred 
sensing elements per square inch) to be compatible with industrial 
applications. While some progress has been made in developing tactile 
(touch) sensors, these sensors are crude in comparison to the available 
image and audio transducers. Two general methods are used for tactile 
sensing: force and torque measurements at either the manipulator wrist or 
at the work piece, and direct "finger-tip" pressure sensing. 
One type of force sensor is disclosed in U.S. Pat. No. 4,286,459 issued to 
W. S. N. Trimmer et al on Sept. 1, 1981. As disclosed, the Trimmer et al 
force sensor includes at least one strip of flexible piezoelectric 
material having an electrically conductive coating on opposite sides 
thereof. When a force is applied to the piezoelectric strip causing it to 
stretch, the strip will oscillate at a frequency that is determined by the 
magnitude of the applied force. Accordingly, the magnitude of the applied 
force can be determined by measuring the frequency of the resulting 
oscillation. 
U.S. Pat. No. 4,306,148 issued to C. G. Ringwall et al on Dec. 15, 1981 
discloses an alternative tactile sensor. The Ringwall et al arrangement 
relates to a tactile area sensor for robots which has an array of 
pneumatic flow passages. The air flow in each passage is dependent on a 
localized force excited by an object pressing against an elastic pad on 
the face of the sensor. The air flow impinges on a metallic tab and its 
angular displacement is sensed by directing a light beam from an optical 
fiber onto the tab and monitoring the quantity of light reflected to a 
paired optical fiber. 
An ultrasonic transducer array and imaging system is disclosed in U.S. Pat. 
No. 3,979,711 issued to M. G. Maginness et al on Sept. 7, 1976. The array 
comprises a plurality of transducer elements each having a major face 
capable of bidirectionally transmitting and receiving ultrasonic energy. 
The opposite face of each element comprises means for bidirectionally 
providing and receiving electrical energy across each transducer element. 
In the imaging system, the array may be selectively scanned to provide a 
fine detail structure image over an extended area. 
The provision of "finger-tip" tactile sensing for application to the real 
time sensory feedback control of mechanical manipulators remains largely 
an unsolved problem. Progress in this area has been impeded primarily by 
the lack of an adequate transducer. Factors such as nonlinearity, 
hysteresis and limited dynamic range have for the most part limited 
tactile sensors to a simple binary response. The problem remaining in the 
prior art, therefore, is to provide a "finger-tip" tactile sensor with 
more than a simple contact/no contact response. 
SUMMARY OF THE INVENTION 
The problem remaining in the prior art has been solved in accordance with 
the present invention, which relates to a monolithic tactile sensor and, 
more particularly, to a monolithic tactile sensor which combines 
considerable spatial resolution with pressure sensing over a range of two 
orders of magnitude. 
It is an aspect of the present invention to provide an array of pressure 
transducers, formed by means of a thin layer of piezoelectric material 
sandwiched between a reference potential plate and a silicon substrate, 
which will provide linear sensing over a dynamic range of at least four 
bits. 
It is another aspect of the present invention to provide a monolithic array 
of pressure transducers suitable for "finger-tip" tactile sensing 
applications where the array structure is compatible with conventional MOS 
integrated circuit processing. 
Other and further aspects of the present invention will become apparent 
during the course of the following description and by reference to the 
accompanying drawings.

DETAILED DESCRIPTION 
A cross-sectional view of the basic structure of an exemplary tactile 
sensing array formed in accordance with the present invention is 
illustrated in FIG. 1. A layer of pressure sensitive piezoelectric 
material 10 is disposed between a ground (or dc) plane 12 and a silicon 
integrated circuit substrate 14. A plurality of individual transducing 
elements 16 are defined by electrodes formed on substrate 14 and these 
electrodes are connected via interconnections 17 to a sensing circuit 18 
(not shown) which are integrated in substrate 14. Sensing circuit 18 
defines the array size and number of the individual pressure sensitive 
cells and provides the necessary circuitry to scan and multiplex the 
signals from each cell to a common output. Each cell is thus a capacitor 
where the instantaneous charge is dependent on both the histories of force 
applied to the cell and readout of the cell by the associated electronics. 
A bonding layer 22 connects piezoelectric layer 10 to substrate 14 and the 
entire device is covered by a compliant cover 20 which converts vertical 
displacement to pressure and also provides mechanical protection. 
In accordance with the present invention, piezoelectric layer 10 of FIG. 1 
should be thin and locally conformable so as to localize the influence of 
pressure applied in the region of a single transducing element 16. 
Piezoelectric layer 10 should also provide as high an electromechanical 
coupling as possible. One example of such material is the piezoelectric 
polymer polyvinylidene fluoride (PVF.sub.2). 
A top view of an exemplary arrangement of the present invention comprising 
a matrix arrangement of electrodes is illustrated in FIG. 2. The 
individual electrodes 16.sub.1,1 -16.sub.N,M are arranged in an NxM matrix 
format, with row interconnections (X) 17.sub.x and column interconnections 
(Y) 17.sub.y to sensing circuit 18. It is to be understood that such an 
arrangement is exemplary only, and the individual electrodes may be 
disposed in any suitable arrangement in accordance with the present 
invention. In the arrangement of FIG. 2, sensing circuit 18 interrogates 
transducer elements 16.sub.1,1 -16.sub.N,M by their row x and column y 
address to ascertain the change in force applied thereto. It is to be 
understood that within the capabilities of the silicon integrated circuit 
technology, transducing elements 16.sub.1,1 -16.sub.N,M may assume a 
variety of planar geometries, and the individual elements themselves may 
be of arbitrary shape and size. For a "finger-tip" tactile sensor, a 
square array of, for example, 16.times.16 elements integrated in an area 
of one square centimeter is appropriate for providing effective spatial 
resolution. 
As mentioned hereinbefore, each sensing element may be viewed as a 
capacitor with piezoelectric layer 10 forming the dielectric. A change in 
force applied to compliant cover 20 is transferred through piezoelectric 
layer 10 and induces a charge on a transducer element 16.sub.i,j directly 
under the area of compliant cover 20 which received the force. If the 
associated transducer element 16.sub.i,j is connected to a high impedance 
node in sensing circuit 18, the charge can be accumulated over a 
predetermined interval of time and then sensed to determine the net change 
in force applied to transducer element 16.sub.i,j. For example, for a 30 
.mu.m thick piezoelectric layer 10 of PVF.sub.2, a pressure of 9.8 
N/cm.sup.2 will induce a charge of approximately 3.times.10.sup.-10 
Coul/cm.sup.2. If transducer element 16.sub.i,j is isolated from 
significant parasitic capacitance, the resulting voltage induced will be 
approximately 0.75 volts. However, in a practical device, parasitics may 
reduce this voltage level by as much as an order of magnitude. 
An exemplary 16.times.16 array of transducer elements 16.sub.1,1 
-16.sub.N,M may typically be scanned at a rate of at least 100 
times/second. At this rate, each transducer element 16.sub.1,1 -16.sub.N,M 
is interrogated every 10 msec, and thus, a total of 40 .mu.sec would be 
available for sensing an individual transducer element 16.sub.i,j. The 
leakage discharge time constant for the charge storage node associated 
with a transducer element 16.sub.i,j must be large in comparison with the 
scan cycle time. To avoid errors due to the long term accumulation of 
leakage charge, only the change in charge during a single cycle is sensed. 
This is accomplished by discharging the lower transducer electrode to a 
reference potential at the end of the sensing interval. 
An arrangement of sensing circuit 18 which may be used to interrogate the 
individual elements in a tactile sensing array formed in accordance with 
the present invention is illustrated in FIG. 3. In this low-frequency 
representation, an exemplary transducer element 16.sub.i,j is modeled by a 
first capacitor 30 and a current source 32. A second capacitor 34 
represents the parasitic capacitance between the lower electrode of 
transducer element 16.sub.i,j and substrate 14. A change in pressure on 
the portion of compliant cover 20 above transducer element 16.sub.i,j 
results in an injection of charge proportional to that change in pressure 
into a node A common to capacitors 30 and 34. 
Access to an individual transducer element 16.sub.i,j from a column line j 
is provided by a switch 36. When switch 36 is closed, the charge stored on 
capacitors 30 and 34 is redistributed between them and a column line 
parasitic capacitance 38. Switch 36 may be controlled via sensing circuit 
18 either by both row (x) and column (y) addressing or simply by column 
addressing. In the latter case, an entire column of transducer elements is 
accessed at one time. A plurality of low-noise column preamplifiers 
40.sub.1 -40.sub.N interconnect a plurality of switches 36.sub.1,1 
-36.sub.N,M to a plurality of N column output lines of the array, where, 
for example, preamplifier 40.sub.j is connected to transducer elements 
16.sub.j,1 -16.sub.j,N. Preamplifiers 40.sub.1 -40.sub.N function to 
convert the charge applied as an input to a voltage level at the output. 
The outputs from preamplifiers 40.sub.1 -40.sub.N are subsequently 
multiplexed onto the input of an A/D converter 42 via a plurality of 
multiple switches 44.sub.1 -44.sub.N. The output of A/D converter 42 is 
subsequently transferred to robotic sensing circuitry (not shown) which 
controls the manipulation of the device containing a tactile sensor formed 
in accordance with the present invention. 
To obtain as large a dynamic range as possible, a balanced sensing 
configuration may be emloyed in association with the present invention. It 
is to be understood that such a balancing arrangement is exemplary only, 
and merely affects the dynamic range of operation of the present 
invention. In the arrangement illustrated in FIG. 3, a dummy switch 37 and 
a dummy reference cell 39 are included to achieve the balanced 
configuration, where one such dummy cell and dummy switch are included in 
each column of the array. Dummy reference cell 39 is identical in 
structure to the above-described transducer 16.sub.i,j except that its 
associated storage node A is set to the cell dc reference potential prior 
to sensing. 
An exemplary arrangement of a transducer 16.sub.i,j and column preamplfier 
40.sub.j is illustrated in FIG. 4. Prior to accessing the transducer cell, 
switch 36 is open and an amplifier switch 46 is closed, thus, feedback 
capacitor 48 is discharged and the column line is effectively clamped to 
an equivalent differential ground. Upon access to transducer element 
16.sub.i,j, amplifier switch 46 is opened and cell switch 36 is closed. In 
this condition, transducer cell node A is discharged to ground through 
capacitor 34, and its charge is transferred to a feedback capacitor 48. It 
is the action of operational amplifier 40.sub.j that serves to drive the 
column line associated therewith to ground. 
Prior to the access of transducer element 16.sub.i,j, a voltage V.sub.T and 
corresponding charge Q.sub.T, where Q.sub.T =(C.sub.30 +C.sub.34) V.sub.T 
and C.sub.30 is the capacitance value of capacitor 30 and C.sub.34 is the 
capacitance value of capacitor 34, have been induced on the cell node A. 
Following the access of element 16.sub.i,j, the output of amplifier 
40.sub.j will settle to a voltage 
##EQU1## 
Thus, the voltage V.sub.out depends, to first order, only on the charge 
induced in transducer element 16.sub.i,j and feedback capacitor 48. After 
access to transducer element 16.sub.i,j, the position of switches 36 and 
46 are reversed and the process is repeated.