Independent Stiffness and Force Control of Pneumatic Actuators for Contact Stability during Robot Manipulation

This paper proposes a control approach that controls the stiffness and force of pneumatic actuator independently. This independent control of stiffness and force removes a primary cause of contact instability when utilizing stiffness or impedance based force control during interaction with stiff environments. Specifically, in typical stiffness or impedance control, since the force is defined in terms of motion variables, the desired force term appears as high gain position feedback when a manipulator is in contact with a stiff environment. In the proposed approach, since stiffness and force are controlled independently, the force appears as an exogenous input rather than as high gain position feedback, and therefore this source of instability is removed. The paper describes the control approach for independent stiffness and force control, which is based on an MIMO sliding mode controller design. Experimental results are presented that demonstrate the effectiveness of the control approach.