As the basic proportional-integral-derivative (PID) control with off-the-shelf control hardware is still the widely used standard of industrial control practice, it is important to find some practical ways to increase the contouring accuracy without a need to apply a model-based control scheme which requires tedious dynamic modeling and leads to high computational cost. This paper applied three model-free approaches, namely decentralized PID control with linear PID gain scheduling, decentralized PID control with velocity trajectory input, and decentralized PID control with high-order polynomial motion profile, to a geometrically nonsymmetric parallel kinematics machine (PKM) having 3PRR topology (3 = number of legs, P = prismatic joint, R = revolute joint) to improve its contouring accuracy. The experiments showed that the linear PID gain scheduling does not provide a significant improvement in the PKM contouring accuracy, whereas the velocity trajectory input and the higher-order motion profiles significantly improve the PKM contouring accuracy.

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