This paper is concerned with trajectory tracking control of a two-axis arm driven by pneumatic artificial muscles, in which it is important for the plant output to track general reference inputs other than step signals. To comply with this control objective, this paper applies the plant-variable-optimal (PV-optimal) servo system derived in the framework of two-degree-of-freedom optimal control, and shows that trajectory tracking can be achieved with high accuracy by taking the intrinsic features of the arm into account in the design. That is, we first apply a minor feedback loop based on the pressure measurements about the pneumatic muscles, and then derive a plant model taking account of the relationship between the link angles and pressure measurements. We then adjust the weighting matrices for the PV-optimal servo system according to the physical meaning of the plant state, and design an observer that uses pressure measurements as well as link angle measurements. It is demonstrated with experiments that trajectory tracking with high accuracy can be achieve with such design procedures.
