P-type ILC with phase lead compensation for a pneumatically driven parallel robot

Iterative learning control is a popular method for accurate trajectory tracking of systems that repeat the same motion many times. This paper presents a P-type iterative learning control for a fast two-degree-of-freedom parallel robot driven by two pairs of pneumatic muscle actuators. The robot consists of a light-weight closed-chain structure with four moving links connected by revolute joints. The two base joints are active and driven by pairs of pneumatic muscles by means of a toothed belt and a pulley, respectively. The proposed control has a cascade structure. The internal pressure of each pneumatic muscle is controlled by a fast underlying control loop. Hence, the control design for the outer control loop, with the angles of the active joints and their velocities as controlled variables, can be simplified by considering these controlled muscle pressures as ideal control inputs. The suggested ILC algorithm is employed to modify the desired trajectory of the outer control loop. Experimental results from an implementation on a test rig show an excellent control performance.