A flexible link of a manipulator has an advantage over a rigid link in the sense that, not only is it light-weighted and thus can move fast using a small-sized actuator, but also that it is safer when it comes into contact with its environment, in particular with humans. However, the vibration due to the flexibility of the link makes it difficult to control the position of the end-point with precision, and when the link is in contact with its environment the problem becomes further complicated. On the other hand, if an actuator can deliver enough force while maintaining proper compliance, it would be advantageous for the sake of safety. An artificial pneumatic muscle-type actuator is an adequate choice in this case. However, the dynamic characteristics of this particular actuator possess strong nonlinearity and load-dependency, and thus a number of problems need to be resolved for its successful application as an actuator. In this work, the position and force control problem of a two-d.o.f. arm system having a flexible second link with artificial pneumatic muscle-type actuators is addressed. A composite controller design method is proposed in the framework of the singular perturbation method. Various robust control schemes are designed in order to meet with payload variation, parameter uncertainty, unmodelled vibration mode and actuator dynamics, both in the slow and the fast subsystems. Simulations and experimental results confirm the effectiveness of the suggested composite control scheme.
