NASA Langely Research Center, Hampton, Virginia
The variable truss (VGT) is a truss mechanism that functions like a two axis gimbal in a truss manipulator arm. The VGT provides joint rotations about two axes just as if there were an actual two axis gimbal operating at the joint in the manipulator arm.
These VGT joints may be used in any number of different configurations. For instance, in one application for a dexterous arm on a mobile robot vehicle for DoE, the truss part of the arm consists of a VGT joint connected to a three longeron truss which in turn connects to another VGT joint. In applications requiring even greater dexterity, VGT joints may be chained together, one after another.
The variable geometry truss manipulator arm is a truss structure that uses a VGT to provide joint rotations. The VGT is a truss mechanism with three variable length battens in it's mid-plane. The three variable length battens form a triangle and connect to two other triangular planes (one on each side of the mid-plane) by struts called cross longeron. Six cross longerons, in a Stewart platform configuration, connect each triangular end plane to the triangular mid-plane so that there are twelve cross longerons. The two triangular planes on each side of the variable length battens mid-plane contain three fixed length battens. Moving in the longitudinal direction along the VGT, first three fixed length battens are connected by six cross longerons to the three variable length battens. Next the three variable length battens are connected by six cross longerons to the remaining three fixed length battens.
The three variable length battens allow the orientation of one end plane with respect to the other end plane to be changed in a controlled manner. This is accomplished by solving the inverse kinematics of the VGT which relate the lengths of the variable length battens to the relative orientation of the two end planes. These equations have been programmed in C and executed on a 486 based PC system. Given a command corresponding to the desired relative orientation of the two ends, the controller changes the length of the variable length battens to accomplish this orientation.
Point of Contact:
Garnett C. Horner
Mail Stop 230
NASA Langley Research Center
4b West Taylor Street
Hampton, VA 23681-0001
804-864-6489
g.c.horner@larc.nasa.gov
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Maintained by: Dave Lavery
Last updated: May 10, 1996