Active preload control of a redundantly actuated Stewart platform for backlash prevention

• An online active preload control method with actuation redundancy was proposed to prevent backlash on a general Stewart platform.
• Placing the redundant leg into the robot inner space demonstrated effective active preload distribution efficiency.
• The preload control method significantly mitigated backlash limit cycles and hence allow higher bandwidth control implemented on the robot.
• The proposed method was designed to account for large six degrees of freedom interactive loads.

There is an increasing trend to use Stewart platforms to implement ultra-high precision tasks under large interactive loads (e.g. machining and material testing) mainly due to their high stiffness and high load carrying capacity. However, the backlash or joint clearance in the system can significantly degrade the accuracy and bandwidth. This work studied the application of actuation redundancy in a general Stewart platform to regulate the preloads on its active joints for the purpose of backlash prevention. A novel active preload control method was proposed to achieve a real-time approach that is robust to large six degrees of freedom interactive loads. The proposed preload method applies an inverse-dynamics based online optimization algorithm to calculate the desired force trajectory of the redundant actuator, and uses a force control scheme to achieve the required force. Simulation and experimental results demonstrate that this method is able to eliminate backlash inaccuracies during application of large interactive loads and therefore ensure the precision of the system.