Active Control of Robot Structure Deflections

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Abstract
The characteristic beam-like structure of most robots leads to significant bending deflections, which in turn lead to poor endpoint positioning accuracy. One solution is to actively counteract this bending using a “straightness servo.” In this paper, such a system is proposed comprising two parallel beams, one to act as the manipulator link and one to carry only the bending loads. These beams are clamped together at one end and joined at the other by an hydraulic actuator. A single variable straightness control system is presented and analyzed. Simulations based upon a realistically sized prototype indicate that attenuation of force disturbances (compliance reduction) to less than 0.006 cm/kN (e.g., 0.001 in./100 lb.) is possible over a 10 Hz bandwidth. Also, system stability can be maintained even when additional mass (which changes the system parameters) is placed at the endpoint. There results are verified by experiments on a full size prototype. Greater improvements in system bandwidth will require state variable control because of the high dynamic order of the structure.

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