Improved "Smart" Robot Hand

NASA's Jet Propulsion Laboratory, Pasadena, California

An improved version of a developmental "smart" robot hand is equipped with a bidirectional, wide-band optical-fiber link for the transmission of digitized strain-gauge force- and torque- sensor signals from the hand and for the transmission of command signals to the motor drive unit on the hand. The rate of transmission of data to and from the hand is approximately 10 kHz, which is about 100 times that of previous versions. The higher data-collection speed makes it possible to perform advanced processing of the sensor data in a host processor; that is, in a computer located elsewhere than on the hand. In a typical previous version of the "smart" robot hand, a microprocessor has performed the data-collection and communication functions. This feature tends to limit the speed at which data can be collected. Furthermore, the strain gauge excitations have been limited to about 12 Vdc because higher applied voltages would heat the strain gauges excessively. However, higher excitations would be desirable because they would increase the signals received from the gauges while leaving noise levels constant. The new design addresses both the speed and the signal-to-noise- ratio heating issues. The improved robot hand (see figure) includes an electronic unit called the "state machine," which is a high-speed, custom-designed circuit that generates clock signals and synchronizes and formats incoming command signals and outgoing sensor data signals. The state machine serves as the interface between the data bus on the hand and the bidirectional, highspeed opticalfiber link. Because of the high bandwidth of the optical link, it is no longer necessary to process the data locally in the hand. All dataprocessing functions are performed at the host processor, permitting the applicable software to be written in a familiar and convenient development environment. Because the strain gauges in the improved hand are excited by pulses of only 5-5s duration, the excitation signals can be made as large as 100 V without causing excessive heating. This increase results in a tenfold increase in the signal- to-noise ratios of the sensor outputs. Furthermore, the strain-gauge excitations can be varied via software to vary the full- scale force or torque range. By use of this feature, the digitized sensor outputs can be maintained at 12-bit accuracy, no matter what range is selected. As the flgure also shows, the hand resembles a motorized vise in which two jawlike fingers move along a linear bearing. The motor is mounted in the middle, between the left-handed and right-handed threads at opposite ends of a spindle, which drives the fingers in opposite directions. This is a directdrive configuration. Its prime advantage is that the motor is removed from the housing of the electronics to reduce the electrical noise and give more room for the circuits. The fingers are nonbackdrivable, but a backdrivable version could also be built using a ball screw. The advantage of nonbackdrivability is that after grasping, it is not necessary to maintain motor current to hold the object. This results in less dissipation of heat in both the motor and its driver circuit. The use of more sensitive strain-gauge sensor electronics makes it possible to increase the sizes of the beams of the force-and-torque-sensor portion of the hand. This makes the sensor portion sturdier, so that limit screws are no longer needed. In the original grip-force sensors. the strain gauges were mounted in the middle of the bending beams, where the sensitivity was small. In the new grip-force sensor, the strain gauges are offset to the ends, where the stresses are greatest.

Point of Contact:
Zoltan F. Szakaly,
Zoltan Vigh,
Timothy Ohm,
Antal Bejczy
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109
818-354-2013
bejczy@telerobotics.jpl.nasa.gov

Telerobotics Program page

Please email the site webmaster with any comments, criticisms or corrections for this page.
Maintained by: Dave Lavery
Last updated: May 10, 1996