An actuator is required to change its speed and force depending on the situation. Using multiple actuators for one driving axis is one of the possible solutions; however, there is an associated problem of output power matching. This study proposes a new active joint mechanism using multiple actuators. Because the actuator is made of a flexible bag, it does not interfere with other actuators when it is depressurized. The proposed joint achieved coordinated motion of multiple actuators. This report also discusses a new actuator which has dual cylindrical structure. The cylinders are composed of flexible bags with different diameters. The joint torque is estimated based on the following factors: empirical formula for the flexible actuator torque, geometric relationship between the joint and the actuator, and the principle of virtual work. The prototype joint mechanism achieves coordinated motion of multiple actuators for one axis. With this motion, small inner actuator contributes high speed motion, whereas large outer actuator generates high torque. The performance of the prototype joint is examined by speed and torque measurements. The joint showed about 30% efficiency at 2.0 Nm load torque under 0.15 MPa air input.

This study proposes a new accordion door shape actuator and an active joint mechanism moved by coordinated motion of multiple actuators. The proposed actuator is made of a flexible bag which is called hydraulic skeleton. The actuator has accordion door shape for strong driving force by increasing its inner pressure. It becomes very flexible when it is depressurized. The proposed joint is equipped one driving axis with multiple pairs of the actuators. Although such arrangement is difficult in general, the proposed mechanism enables coordinated motion of multiple actuators because depressurized actuators are flexible enough and they do not disturb other actuator motions. The proposed joint changes the strength and speed by the number of using actuator depending on the load. This performance is enhanced by specialized actuators such as high strength type and high speed type. Free-rotation is also possible without additional mechanism such as a clutch.