Dual-loop control strategies for vibration attenuation of a flexible parallel manipulator

Most of the current robots employed in industrial applications are classified according to two main types: serial link and parallel link manipulators. Despite the advantage of larger access to the workspace by the serial manipulators, parallel manipulators can provide improved relations between strength and weight and higher accuracy. The flexible parallel manipulator (PM) appears as an interesting alternative due to its positive characteristics, such as lightweight, higher speed, energy efficiency, enhanced mobility, and great payload/arm weight ratio compared to PMs with rigid links. However, the flexible dynamics of the PMs can generate uncertainties and undesired vibrations during the motion, requiring novel joint and robust dual-loop control strategies. This manuscript investigates dual-loop control strategies using a 3RRR prototype with flexible links. Initially, only the camera loop is assessed as a reference. Then, the design of a dual-loop control, combining the camera and strain loops, is investigated. Each dual-loop control is formed by two control loops: position-based visual servo and strain-based feedback control schemes. While the position-based employs an oCam-5CRO-U for acquiring the images, the strain-based loop combines the strain output and filters to remove the DC offset and signal noise. In addition, different trajectories and masses are employed to investigate the overall performance of the control strategies. Compared with the full visual servo methodology, the enhanced dual-loop control can considerably attenuate the overshoot of the end-effector response by reaching up to 54.83% for the step in x-direction and up to 40.83% for the step in y-direction and oscillations up to 19.95%.