A Direct-Drive Permanent-Magnet Motor Selective Compliance Assembly Robot Arm: Modeling, Motion Control, and Trajectory Optimization Based on Direct Collocation Method

Most selective compliance assembly robot arms (SCARAs) adopt a conventional indirect-drive structure consisting of servo motors and gearings, which presents drawbacks, including low precision, limited lifespan, and complex structure. In contrast, a direct-drive SCARA (DDSCARA) is compact and delivers superior positioning accuracy and velocity. However, the direct-drive structure is more prone to resonance. In this article, we use real-time filters to eliminate resonance and add the dynamics feedforward (DFF) obtained by decoupling the DDSCARA's dynamics into the control system to improve position accuracy. The experimental results show that the DFF reduces the position error by about ten times. In addition, a key contribution of this article is the comparison of the DDSCARA with SCARA. The general kinematic and dynamic models of both robot arms are established. They verify the strong coupling of the DDSCARA. We use the direct collocation method (DCM) to optimize the trajectory of both SCARA with reducers and the DDSCARA. We show that for SCARA with reducers, the impact of DCM is limited compared to trajectories widely used. On the contrary, for the DDSCARA, the DCM reduces the power losses significantly. This is validated by experiments that reveal a reduction of the power loss by 50.3% for a motion time of 0.3 s, and a decrease in the mean absolute error of both rotor joints' position by 52.4% and 67.8%, respectively.