Modeling and force control of a pneumoelectric end-effector for robotic continuous contact operations

The force-controlled end-effectors for industrial robots usually face the problems of low output force, poor control accuracy, or slow dynamic response. This paper presents a pneumoelectric force-controlled end-effector (PFE) for industrial robots to perform continuous contact operations. The end-effector possesses the advantages of both the pneumatic drive having large force-mass ratio and the electric direct drive having high force control accuracy and fast dynamic response. Through dynamic modeling and analyses, a force coordination control method based on impedance control is proposed for the PFE. The pneumatic cylinder is actuated by a semi-closed loop, while the voice coil motor (VCM) is controlled through a closed loop to improve the output force, accuracy, and dynamics. Impedance controller is utilized to reduce the impact caused by the critical contact between the tool and workpiece. Simulation analyses show that the force-mass ratio has increased, while the friction influence and overshoot has been reduced. In addition, prototype experiments verify the effectiveness of force control method and demonstrate that the PFE is able to achieve good performances such as control accuracy, step response, and dynamic bandwidth. Compared with the traditional pneumatic end-effector, the force control hysteresis is almost reduced from 41.9 to 0% without apparent creeping phenomenon. The rise time of the step response also decreases from 435 to 6.5 ms, and the bandwidth reaches 47 Hz. The PFE with force coordination control shows great potential in robotic deburring, grinding, and polishing applications.