Experimental Investigations of Model-Free Control Approaches on Control of Shape Memory Alloy (SMA) Spring Actuated Parallel Manipulator

Abstract

Robots in the industry are mostly used for assembly operations. Robot assembly is a difficult operation that requires additional compliance to reduce the position error between two mating parts. Flexible parallel manipulators, owing to their accuracy in positioning and inherent compliance, are suitable for use as orienting mechanisms. This paper describes a two-degree-of-freedom flexible parallel manipulator driven by Shape Memory Alloy (SMA) springs, designed to function as an auxiliary orientation unit for a tabletop Cartesian robot. While SMA springs provide advantages such as compact integration, simple actuation, and high power density, their nonlinear characteristics significantly increase the complexity of the control process. Apart from handling nonlinearities and disturbances, the controller should be capable of controlling the co-contraction of actuators in the mechanism. Model-free Sliding Mode Control (SMC) with linear and nonlinear sliding surfaces is chosen for the servo control, combined with the Time Delay-based Estimation (TDE) as a secondary control for the implementation and performance analysis. This study presents an experimental comparison of nonlinear control strategies for trajectory tracking in the developed redundant parallel manipulator. Efficient Trajectory tracking characteristics are crucial for any motion platform, and hence, real-time experiments with complex trajectories are conducted on the prototype in the presence of external disturbances, and the results of the experimentation are presented, which demonstrate the superior nature of nonlinear SMC over other controllers. The inclusion of the secondary term (TDE) in the design of control is found to linearize the closed-loop system.