Impedance Control of a Pneumatic Actuator for Contact Tasks

This paper presents a method for the impedance control of a pneumatic linear actuator for tasks involving contact interaction. The method presented takes advantage of the natural compliance of pneumatic actuators such that a load cell, typically used in impedance control, is not required. The central notion of the method is that by departing from a stiff actuation system, low-bandwidth acceleration measurements can be used in lieu of high-bandwidth force measurements. The control methodology presented contains an inner loop to control the pressure on two sides of a pneumatic cylinder, while an outer loop enforces an impedance relationship between external forces and motion and commands desired pressures to the inner loop. The inner loop enforces the natural compliance of the pneumatic actuator by controlling both the sum and difference of the pressures on both sides of the pneumatic actuator. This is accomplished by utilizing two three-way proportional spool valves instead of a four-way valve typically used in fluid power control. Experimental results are shown demonstrating the pressure tracking control of the inner loop. Experimental results are also shown that demonstrate the impedance tracking of the outer loop for free motion and the transition from free motion to contact.