When a mechanism is overconstrained it leads to the generation of internal forces. These forces do no work but have an effect on the apparent structural stiffness of the mechanisms. The degree of this effect depends on the magnitude or the mode of the internal forces and the geometry of the mechanisms. Internal forces may result from external or gravitational forces or the inertial loads during motion. If a mechanism has a closed chain structure, the redundant actuation may generate internal forces as well. -. . 'o The authors analyse the apparent structural stiffness of the closed mechanisms due to redundant actuation and the forces arising during dynamic motion.>

A control scheme for pneumatic drive systems is proposed. In general, static friction, Coulomb friction, and other nonlinear disturbances crucially worsen the performance of pneumatic drive systems. In the proposed control scheme, to guarantee robustness against such disturbances and stability of the motion, position feedback loops and pressure feedback loops are hierarchically connected. The stability of the motion is investigated by using a Lyapunov function. As a result, it is mathematically proven that the motion of pneumatic drive systems converges to a fixed desired point as time tends to infinity, even if the disturbances affect the motion. The effectiveness of this control scheme is demonstrated through some experimental results.>