Modeling and control of transverse and torsional vibrations in a spherical robotic manipulator: Theoretical and experimental results

The present work addresses modeling and control issues pertaining to the positioning and orientating of rigid body payloads as they are being manipulated by flexible spherical robotic manipulators. A general approach, to systematically derive the equations of motion of the robotic manipulator, is used herein. The objective of the controller is to yield a desired rigid body response of the arm while damping out the transverse and torsional vibrations of the compliant link. Note that the control objective has to be achieved by solely relying on the existing joint actuators whose bandwidths are far below the natural frequencies of the torsional modes. The current work demonstrates that, in spite of the physical limitations of the system, the controller can actively damp out the torsional vibrations by relying on the coupling terms between the torsional vibrations and the remaining degrees of freedom of the arm. Moreover, a gain scheduling procedure is introduced to continuously tune the controller to the natural frequencies of the flexible link whose length is varied by the prismatic joint. The digital simulation results demonstrate the capability of the ''rigid and flexible motion controller (RFMC)'' in drastically attenuating the transverse and torsional vibrations during point-to-point (PTP) maneuvers of the arm. Further more, the gain scheduling procedure is shown to significantly reduce the degradations in the RFMC performance that are brought about by having the flexible link connected to a prismatic joint. A limited experimental work has also been conducted to demonstrate the viability of the proposed approach.