Touch Sensor Responds to Contact Pressure
A pressure sensor for a mechanical hand gives better feedback of the gripping
force and more-sensitive indication of when the hand contacts an object.
Optical fibers bring light into cells on the gripping surface. Light is
reflected from a flexible covering into other fibers leading to detectors.
Distortion due to tactile pressure changes the amount of reflected light.
The new device is superior to previous sensors. For example, television
or other direct-viewing systems are not sensitive to contact pressure, and
the contact area is often hidden from view. Electrical sensors are subject
to electrical noise, especially at the low signal levels associated with
low contact pressure. Optical sensors have been used to detect proximity
or contact but not contact pressure. The new optical sensor is illustrated
in the figure. The sensing surface of the hand is divided into cells by
opaque partitions. An optical fiber brings light into each cell from a lamp,
light-emitting diode, or other source. Another fiber carries light from
the cell to a detector; for example, a photodiode or phototransistor. The
cells are covered by an elastic material with a reflective interior surface.
The rest of the cell is coated with a nonreflective material. As shown in
the figure, pressure against a cell cover causes a distortion, which changes
the internal reflection of light. The change is sensed by the detector,
and the output signal informs the operator of contact. The greater the pressure
and distortion, the greater is the change in light reflection. Thus, grip
pressure can be sensed using analog circuitry. If only a touch indication
is desired, a threshold detector can be included in the electronics. In
an automatic manipulator, the detector signal could control the manipulator
movements. The cells can be arranged such that those in each row share one
light source, while those in each column share one detector. This reduces
the number of sources and detectors and facilitates scanning. For example,
a 10-by-10 matrix would have 100 sensing points while requiring only 10
sources and 10 detectors. The array can be scanned by sequentially pulsing
sources and detectors.
Point of Contact:
Antal Bejczy
Mail Stop 198-219
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109
818-354-4568
bejczy@telerobotics.jpl.nasa.gov
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Maintained by: Dave Lavery
Last updated: May 10, 1996