Control of Pneumatic Artificial Muscle Actuated Two Degrees-of-Freedom Robot Using PD-Based Pulse Width Modulation Strategy With Feed-Forward Outer Control Loop

This work presents a novel approach for the design and control of a two degrees-of-freedom (DOF) robotic manipulator driven by one pneumatic artificial muscle (PAM) and one passive spring for each of its DOFs. The required air pressure is supplied to the PAMs using fast-switching on/off type pneumatic flow control valves. The proposed control architecture uses a proportional-derivative (PD) controller with a feed-forward term in the outer control loop to correct the position errors using an approximate model of the system dynamics and approximate PAM force-contraction characteristics. An inner pressure regulator loop tracks the reference pressure signals supplied by the outer loop using a pulse width modulation (PWM) scheme to control the pneumatic valves based on the approximated inflation-deflation characteristics for the given pneumatic flow circuit. The proposed controller is unique for PAM actuated robots that simultaneously consider three levels of complications, namely, coupled dynamics of multi-degrees-of-freedom system, non-linearities in the force-contraction characteristics of PAMs, and non-linearities involved in the use of on/off type pneumatic flow control valves. Experiments carried out using a laboratory prototype validate the effectiveness of the proposed control scheme.