A compliant and redundantly actuated 2-DOF 3RRR PKM: Less is more

The development of a compliant (or flexure-based) manipulator with redundant actuation has been considered before, showing that the redundancy can be exploited to increase the support stiffness and reduce actuator loads. In this previous design the manipulator’s workspace has been defined to encompass all kinematically accessible end effector positions. In this paper we reconsider the design philosophy. It appears that limiting the workspace (“less”) ultimately results in a better performance in a larger area (“more”) as before.
The dynamic performance of the manipulator is evaluated with a flexible multibody model. The links are assumed to be rigid. The SPACAR software is used as its flexible beam element can describe the non-linear behaviour of the flexure joints well at rather large deflections. This numerically efficient model is well-suited for design optimisation which aims at the largest workspace area while assuring a minimal parasitic natural frequency and limiting the local stresses throughout the full workspace. Furthermore, the simulations show that preloading of the flexures results in smaller required actuator torques.
An optimised design has been build. A control system has been synthesised that handles the actuator redundancy by minimising the 2-norm of the driving torques. It is demonstrated that the setup’s behaviour is similar to the model and that in particular the preloading significantly lowers the required actuator torques.