Robustness comparative study of two control schemes for 3-DOF flexible manipulators

This article describes a comparative study of two control schemes designed for a new three-degree-of-freedom flexible arm. This arm has been built with light links, has most of its mass concentrated on the tip, and its special mechanical configuration uncouples tip motions in spherical coordinates. This special configuration simplifies the dynamic modeling and control of the arm. A compliance matrix is used to model the oscillations of the structure. A consequence of this simple dynamics is that minimum sensing effort is required (only direct motor and tip measurements), and the use of complex observers is avoided because the state of the system can be very easily obtained from these measurements; then its control becomes very simple. Two two-nested control loop schemes are used to control the tip position, by using a joint position and tip acceleration feedback, measured with accelerometers placed at the tip (first control scheme), or tip deflexions feedback, measured with strain gauges placed at the bars of the mechanism near the joints (second control scheme). Both control systems can be considered as equivalents when nominal payload is used for designing them. It can be proved that the use of strain gauges is more robust than the use of accelerometers as tip sensors if the tip mass differs from the nominal one. Simulated results are presented for both control schemes and different payload conditions. Comparative results between the controlled and non-controlled tip responses are also shown.