Colloquia

Summary

Flexure based mechanisms are very suited for the development of precision manipulators. This is due to their deterministic behaviour resulting from low hysteresis and lack of backlash. However, the use of flexures also presents challenges. Since a flexure depends on elastic deformation, it can only provide a limited range of motion. In addition, the generation of sufficient support stiffness in a flexure-based design is subject to a lot of research. In particular, it is challenging to maintain the support stiffness under deflection. At the same time, the driving stiffness should be as low as possible such that actuator saturation limits are respected.

In this study, a flexure based 2DoF 3RRR Redundantly Actuated Parallel Kinematic Manipulator (RA-PKM) test setup is specifically redesigned in view of the properties and limitations associated with flexure hinges. The redesign is focused on maximization of the work area, without deteriorating the dynamic properties in terms of parasitic eigenfrequencies. Therefore, a new workspace definition is introduced, which reduces the required range of motion of the flexure hinges with respect to the reachable work area of the end effector. The reduced range of motion enables the optimization to shorten the length of flexure elements, which in turn increases the support stiffness.  This compensates for the increased mass of the extended arms resulting from the workspace optimization.

The parallel kinematic redundancy is utilized to reduce the effective driving stiffness by use of pre-tensioned clock-springs. The stiffness balancing effect is explained by the reduction of potential energy variation when moving the end effector throughout its workspace. To demonstrate the effect of this balancing action on the test setup, motion control is developed to determine the required actuator effort for a grid of set-points within the work area. For this, a PID-controller is developed in conjunction with an analytical model-based stiffness feed-forward controller.

As a result of the optimization, the redesigned manipulator can cover a 2.2 times larger work area compared to the previous workspace optimization study. At the same time, the dynamic performance is maintained. Furthermore, it is demonstrated by means of motion control on the test setup that the required actuator torques are reduced by a factor of 1.9 due to the stiffness balancing effect of the pre-tensioned clock-springs.