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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to sensing devices. It is particularly concerned
with tactile sensors that can detect touch and detect and quantitate
applied and reactive forces and applied force components normal to the
sensor surface and the distribution of those forces on the sensor surface.
Principal uses of such sensors are in robotics, teleoperation, industrial
automation and prosthetics.
2. Prior Art
There has been a great deal of development effort in the past with regard
to apparatus and devices, such as industrial robots and sub-systems for
use with them, that will accomplish desired work objects. The field of
industrial robots, for example, has expanded rapidly but further progress
is presently limited by the absence of low cost adequate sensor perception
and feedback control systems. Technology presently exists to permit design
of robots able to respond to control signals and to move even as little as
a few thousandths of an inch to accomplish their designed tasks. Such
robots are usually engineered to accomplish specific jobs in well-defined
work environments, and they are usually complex and costly.
A great deal of emphasis has previously been placed on artificial vision
systems that will serve as "eyes" for a robot in the detection of objects
or circumstances and that will provide signals for responsive operation of
the robot. Consequently, the artificial vision systems have become quite
sophisticated, while tactile sensors that have not received the same
design emphasis have generally remained rather crude, often having coarse
spatial resolution and slow response times.
Tactile sensors can synergistically complement visual systems by becoming
the controlling system at the time contact is made between a gripper of
the robot and an object or objects being gripped, this being a time when
vision is often obscured. The potential importance of tactile information
is evident when it is realized that the information can readily operate a
robot hand to function in a manner comparable to the way a blindfolded
person can sense objects and perform simple functions such as the
threading of a nut onto a bolt.
It is believed that continuing advances in industrial robotics will require
robots capable of performing tasks that will place a premium on the
tactile sensory perception capability of the machines. It is also believed
that relatively low cost touch responsive sensors are required for use
with such robots. A number of motion control and force sensor devices have
been heretofore disclosed. Representative of these devices are those shown
in U.S. Pat. Nos. 3,307,393; 3,416,365; 4,098,001; 4,155,169; 4,156,835;
and 4,094,192. These known devices may be useful for their specific design
purposes and they are generally designed for use in complex mechanical
systems, with the chief objective of the systems being to obtain high
precision. However, because in most circumstances sensory feedback is
lacking, the degree of precision obtained is not as high as desired.
Other force sensors have been developed, and when such individual force
sensing elements are placed in an array, force distribution or tactile
sensing can be achieved. One such design uses a sandwich structure of two
linear arrays of electrodes separated by a thin material, the electrical
conductivity of which changes with pressure. By constructing the array in
such a manner that the two arrays of electrodes intersect diagonally, a
force-sensitive cell is defined at the intersection. The dimensions of the
cell are defined by the width and separation of the electrodes.
Another force sensor concept employs small "windows" that are arranged in a
matrix. The "windows" are electrodes that are surrounded by a common
grounding medium. When a conducting material covering the electrodes is
deflected with sufficient force current will flow from the electrodes to
the grounded material.
Still another variation of a force-sensor array consists of utilizing a
sheet of silicone rubber and an etched circuit board. The rubber sheet is
made of alternating conducting and nonconducting layers with the
conducting layers being silver or graphite impregnated rubber. The
conducting layers of the rubber sheet are placed in a perpendicular
relationship to circuit board conductors. The contact points at each
intersection then form pressure sensors. Complex data output and readout
circuitry are required to obtain and use this force-sensor array.
Other devices utilize conducting rubber in association with a semiconductor
material. The limitations of these devices center around the potential
contamination of the semiconductor material as well as abrasion of the
rubber surface. In addition, as with other devices utilizing conducting
rubber, the response indicated between the deformation of the rubber and
the pressure applied is often non-linear. In linearizing the response of
the rubber, means are provided to digitize the pressure response. One
method of linearizing employs a triangle-shaped device with a series of
electrodes along two sides of the triangle. Progressively larger forces
applied to a rubber surface resting on a corner of the triangle will cause
the corner to penetrate into the rubber and to contact an increasing
number of electrodes as the penetration continues and increases.
An optical device, made by the Lord Corporation of North Carolina, U.S.A.
employs a small force-sensing device consisting of a small shutter that
controls a light beam in conjunction with a light emitting diode and
photodetector. Each element requires individual calibration.
While the prior art devices discussed above may each be suitable for a
specific use for which it is designed, they are not widely adaptable for
use because of their expense, complexity, nonlinear readouts and/or
limited performance when used generally. It has been found, however, that
an ultrasonic distance measuring device which measures the thickness of a
compressible medium meets the general requirements necessary to provide a
tactile sensor suitable for a wide range of uses. If a resilient medium is
used, the device is adaptable for repeated uses.
The use of sound in the measuring of distance has long been known. In many
applications ultrasonic pulse echoes or mechanical vibrations, are used
for precision measurement of various properties of a given piece of
material. Attention is directed to U.S. Pat. Nos. 4,033,244; 3,994,154;
3,688,565; 3,228,232; 3,108,469; 3,745,833; 4,044,606; 3,315,520;
3,540,265; 3,416,365; and 3,942, 381. As illustrated in these patents both
thickness and acoustic properties of the material can be measured.
SUMMARY OF THE INVENTION
The present invention comprises a sensor which detects contact and/or
measures forces occurring between the sensor and an object. An ultrasonic
pulse generated by a transducer travels through a medium, is reflected
from a surface of the medium, returns through the medium, and is detected
at the transducer. If the material at the other side of the interface with
the medium has a lower acoustic impedance than does the medium itself,
such as occurs when air is present at the reflecting surface, versus, for
example, a rubber medium, the echo is inverted, i.e., undergoes a
180.degree. phase shift upon reflection. If, however, the medium is in
contact with an object having a greater acoustic impedance the echo will
be reflected, but will not undergo a 180.degree. phase shift.
Consequently, the ultrasonic pulse is reflected whether or not an object
is in contact with the sensor medium.
Since whether the signal is inverted or not depends upon whether a
contacted object has a greater acoustic impedance than the rubber, this
characteristic can be used in identifying the nature of the object.
However, if the material contacting the surface has an acoustic impedance
similar to the rubber, then the sound will be poorly reflected unless an
acoustic "mirror" is incorporated into the interface by use of a medium
having a different acoustic impedance. This can be done, for example, by
building in an air gap or a metal layer at the interface.
Since it is possible to calculate the speed of travel of an ultrasonic
pulse through a medium or composite media and since the pulse will be
reflected by an interface, the thickness of the medium or media can be
determined by measuring the two-way travel time of generated pulses.
Forces applied to the surface of the sensor by an object can be calculated
from the measurement of the deformation of the medium by the object with
knowledge of the force necessary to deform the medium a known amount.
The composition and configuration of the medium may take on many forms for
different applications. For example, it may be an elastomeric pad used on
the fingers of a robot gripper; it may be a device where the sensor
measures movement of a spring-loaded rigid plate through an intermediate
medium such as air, fluids or gels; or a nonresilient medium might be used
to measure the maximum force applied.
It has been found that for transducers used in sensors of the invention,
certain characteristics are deemed desirable: First, the transducer must
operate at a high enough frequency to provide good distance resolution, to
simplify detection and to avoid interference between a transmitted pulse
and the returning echo of the pulse within very short distances that are
involved. Second, the transducer and associated apparatus preferably will
be small and light enough not to make the sensor too large for use with
other structures. Third, the transducers ideally are adaptable for use in
compact arrays. To achieve these characteristics a small, high frequency
transducer must be used. While other transducers can be used, it has been
found that polyvinylidene fluoride (PVDF), a thin film polymer exhibiting
excellent piezoelectric characteristics, has been found desirable as a
transducer material. PVDF is inexpensive, rugged and flexible and can be
molded to fit various surfaces. It is thin enough to not add significantly
to the bulk of a sensor array and has a high dampening factor to avoid
ringing in generating the transmission pulse and receiving the echo pulse.
In addition, the acoustic impedance of PVDF is relatively close to that of
rubber and to some epoxy adhesives allowing strong coupling of the
ultrasonic pulse into these materials. Also, PVDF has low acoustic
cross-coupling through the material, which provides acoustic isolation of
array elements. Finally PVDF can be used in conjunction with
photo-reduction and photo-lithographic techniques for mass production of
array or electrode patterns. While PVDF has low efficiency as an
ultrasound transmitter and a limited upper operating temperature, these
characteristics are not severe limitations for the device. In the event
that PVDF material is not satisfactory for a particular sensor, other
transducers, for example, such as those made of quartz, lead
titanate-zirconate or other ceramic or piezo-electric polymers can be
used.
OBJECTS OF THE INVENTION
A principal object of the present invention is to provide a sensor that is
adaptable to a wide range of uses in sensing devices.
Another object is to provide a device that is capable of sensing contact
with great sensitivity.
Yet another object of the present invention is to provide a sensing device
that is capable of accurately determining shape by good spatial resolution
of force distributions.
Still another object of the present invention is to provide a measuring
device capable of sensing force with suitable precision and sensitivity.
Another principal object of the present invention is to provide a force
measuring device that is capable of sensing the distribution of force
applied to a given surface.
FEATURES OF THE INVENTION
Principal features of the invention include one or more ultrasonic
transducers arranged to generate pulses into a suitable deformable medium.
In the presently preferred embodiment, an ultrasonic transducer generates
pulses into an elastic pad made of a compressible substance such as
rubber. Each transducer generates a signal used to measure a change in the
thickness of the elastic pad, which is representative of the force applied
to the pad in the vicinity of the transducer. With the transducers
arranged in either a one or a two-dimensional array, and electronically
connected to appropriate circuitry, direct and reactionary forces applied
to the overall surface area of the pad can be measured and the sensor can
detect the force distribution. It is then possible to determine the shape
of an object coming in contact with the pad surface overlying the array.
Conventional pulse-echo, time-of-flight measuring circuitry or
conventional phase-shift circuitry can be used with the sensor of the
invention to signal contact of a sensor with an object and to provide
signals indicative of a plurality of contacts that will provide
information regarding shape of an object being contacted.
While many types of deformable media may be used, natural, synthetic and
silicone rubbers may be found highly satisfactory for the purposes of the
invention.
In one preferred embodiment of the invention, a plurality of transducers is
covered by a single sheet of deformable material. In an alternate
embodiment of the invention, each transducer used has its individual
deformable pad. The choice between the alternative embodiments is based on
economics and the specific sensing requirements involved. In either of the
alternative embodiments, the array itself can be flat or may be shaped,
i.e., curved, to match the shape of a particular object being handled.
Since the sensor of the invention, in addition to sensing contact,
effectively measures shape and force distribution, it also measures
changes in force distribution caused by movement or object slippage.
Feedback from the sensor can be used to initiate any desired, appropriate
corrective response.
Other objects and features of the invention will become apparent from the
following detailed description and drawing disclosing what are presently
contemplated as being the best modes of the invention.
THE DRAWING
In the drawing:
FIG. 1 is a perspective view of a typical transducer as used in the
invention;
FIG. 2, a greatly enlarged sectional view through a sensor of the
invention, with arrows showing typical reflected signals; and
FIG. 3, a typical electrode arrangement for a sensor of the invention.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring now to the drawing:
In the illustrated, preferred embodiment, a typical four-element sensor
array of the invention is shown generally at 10. In this embodiment a
transducer, comprising a thin film sheet of polyvinylidene fluoride (PVDF)
11, has extremely thin sheets of aluminum-tin foil 12 on opposite surfaces
thereof. Individual sensing elements 12a and arrays in any desired pattern
arrangement are constructed by removing, as by etching away, portions of
one or both of the metal sheets 12 to provide an appropriate pattern on
the transducer material 11. Alternatively, the desired pattern can be
deposited by vacuum deposit technique or other known methods. Electrical
lead wires 13 and 13a are respectively electrically bonded at 14 to the
remaining metal electrodes 12a and the other metal sheet 12, and are
electrically connected to a suitable circuit 13b.
An elastomeric sheet of deformable, resilient material 15, having
well-known mechanical and speed of sound characteristics, is placed over
the sensor electrodes 12a. The object to be contacted by sensor array 10
deforms the exposed surface of the elastomeric sheet.
As shown in FIG. 2, the elements 12a, which serve as electrodes, excite the
PVDF 11 to transmit signals S through the elastomer 15 that are reflected
by (1) air, and (2) any object that is pressed against the elastomer 15
back to the sensing elements 12a. Conventional circuitry can measure the
echo time from each individual electrode 12a, or can average the echo
response times from several electrodes, depending upon the specific
application.
In addition, the echo polarity, as determined by the acoustic impedence of
a contacted object with respect to the impedence of the sheet 15 can be
determined using conventional electrical circuits. With differing polarity
sensing, the force sensor 10 can be readily calibrated to differentiate,
for example, between metal, plastic, fabric or other materials, as
necessary to the particular use of the invention. Such echo polarity
determination can take place without any appreciable deformation of the
elastomer 15, thereby greatly increasing the sensitivity and consequently
the usefulness of the sensor.
Various arrangements of electrode patterns can be used in practicing the
invention, depending upon the use to be made of the device and other
governing criteria. The electrode pattern may be determined by the size of
the sensor surface or the sensitivity desired as well as by the tasks to
be performed by the sensor.
In the preferred embodiment of the invention, PVDF is used because of the
low acoustic cross-coupling and its impedence match with elastomers. It is
clear, however, that transducers made of other materials, such as
ceramics, can be utilized should they prove advantageous under given
circumstances.
Conventional pulse-echo circuitry or phase-shift circuitry may be used as
circuitry 13b to determine the ultrasonic transit time, which is
proportional to the thickness of the deformable medium used.
With the thickness determined at each transducer, a pattern of force
(either direct or reactionary) applied to the face of the deformable
member can be measured and sensing of a contacted or contacting object can
be determined, as well as the shape of such object.
The transducers can be individually coupled into the circuit used, or a
method of cross-point switching may be used. As shown best in FIG. 3, a
PVDF transducer 20 has spaced parallel electrode strips 21 on one face
thereof and spaced, parallel electrode strips 22 on the opposite face
thereof. The strips 22 extend normal to the strips 21, and each strip 21
and each strip 22 are connected to electrical leads. If a particular strip
21 is switched on by the circuitry used at the same time a particular
strip 22 is switched on, the volume of the PVDF between the strips
indicated approximately by the dotted line box 23 in the embodiment shown,
will be activated. Such an arrangement greatly reduces the number of
electrical leads necessary for operation of a sensor having many
transducers. If individual transducers are used, having their own wires,
then for n rows and m columns of transducers, the number of leads required
is n.times.m. However, with cross-point switching as described above, the
number of leads required is only n+m.
It will be apparent that in addition to indicating contact of any portion
of the surface of the deformable medium, and the areas of such contact,
the sensor can measure force applied to such surface and the distribution
of such force. Further, if x and y axes are established at the contact
face of the deformable medium, torques applied about such axes can be
readily measured.
Although a preferred form of our invention has been herein disclosed, it is
to be understood that the present disclosure is by way of example and that
variations are possible without departing from the subject matter coming
within the scope of the following claims, which subject matter we regard
as our invention.
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