On Smooth Time-Optimal Trajectory Planning in Twisted String Actuators

Abstract

Twisted string actuators (TSA) are efficient, compliant cable actuators with high power density that have found their way into many engineering and robotics applications in recent years. They are generally used in the scenarios involving comparatively slow positioning, since quick motions in TSAs often result in large overshoot values, undesired oscillations and loss of cable tension. This work discusses time optimal trajectory generation for point-to-point transitions in TSA. We propose a method to generate smooth trajectories by directly solving an optimal control problem while respecting constraints on motor torque and speed and preventing tension loss on cables. We have conducted an experimental evaluation demonstrating that, in comparison to classical, constrained 'bang-bang' transitions, the proposed trajectories result in much faster settling time, nearly-zero overshoot and require almost 60% less motor power for tracking. The proposed method can help TSAs find their way into highly-dynamical applications that have been previously deemed to be too demanding, which include parallel manipulators and antagonistically-controlled systems.