Open-loop Force Control of A Three-finger Gripper Through PWM Modulated Pneumatic Digital Valves

This paper proposes an open-loop force control system for a three-finger gripper using small digital solenoid valves modulated using PWM (Pulse-Width-Modulation) technique. These valves are good candidates for this robotic application because of their small dimensions, low weight and cost, simple structure and easy operation by means of on/off signals. A linear model for pressure control in the pneumatic actuator's thrust chamber was developed and tested experimentally. A large dead-band was introduced in the PWM driver in order to investigate a suitable method for its correction. Good results were obtained by correcting the static characteristic curve of the three-way equivalent valve that controls the flow rate exchanged with the pneumatic cylinder thrust chamber. These results can be applied to similar solutions with larger digital valves. Gripping mechanism efficiency and the effects of friction forces were taken into account in developing the force control.