A mechanism has been developed to serve as an end effector for one of the legs of the Legged Excursion Mechanical Utility Robot (LEMUR) — a walking robot designed for demon- strating robotic cap- abilities for maintenance and repair. [The LEMUR was described in “Six-Legged Experimental Robot” (NPO-20897), NASA Tech Briefs, Vol. 25, No. 12 (December 2001), page 58.] Through controlled actuation of this mechanism, the tip of the leg can become either (1) a foot for stable support during walking or (2) the robotic equivalent of a simple hand tool — a ballend hexagonal driver for a standard hexagonal-socket machine screw. More specifically, the foot can be extended to enable walking, or can be retracted to enable cameras that are parts of the robot to view the insertion of the tool bit in a socket. Retraction of the foot also enables the tool to be used in confined spaces in which the foot cannot fit.
The mechanism (see figure) includes a hollow lead screw. The foot includes a nut that engages the lead screw. A compact, light- weight motor with a speed-reducing, torque-multiplying gear head is housed inside the lead screw. An external guide prevents rotation of the foot when such rotation is not desired. In preparation for retraction of the foot, the guide is initially locked against rotation of the foot by use of a pin and slot that mate in only one position. When the motor rotates the lead screw in one direction, the foot is retracted along the lead screw, exposing the tool bit.
When an axial compressive force is applied to the tool (as when the tool bit is inserted in a socket in a machine screw), interface pins between the tool bit and the guide transmit some of this force to the guide, thereby pushing the guide out of the slot that locks it and the foot against rotation. Now the guide and foot begin to rotate along with the lead screw and retraction of the foot ceases. Because the tool bit is mated with the lead screw in such a way that when the lead screw is rotating, the bit is also rotating, the lead screw can now be rotated in either direction to cause the tool to rotate the socket and thereby tighten or loosen the machine screw. The output torque of the motor, and hence the torque applied to the machine screw, is governed by a current- feedback motor controller.
Once the tool is relieved of the axial compressive force (as when the tool bit is removed from the socket), the guide and foot continue to rotate until the pin engages the slot. At this point, the guide and foot are once again constrained against rotation; hence, rotation of the lead screw in the appropriate direction causes extension of the foot, restoring the walking configuration.
This work was done by Jennifer Knight and Stephen Askins of Caltech for NASA’s Jet Propulsion Laboratory.