Because of the elderly society in Japan, a passive exercise device that can be used in homes or hospitals without assistance is desirable. In the device, a soft actuator that can move with both longer strokes and a larger force is useful. In the previous study, a pneumatic linear stepping actuator (PLSA) that can push and pull a flexible rod while changing the gripping position of a tube was developed. In this study, a variable step type PLSA is proposed and tested. Moreover, the development of a passive exercise device for the hip joint using three PLSAs and one connecter connected with three flexible rods and a knee supporter is described. The tracking position control of the knee supporter based on an analytical model is also described. The control system using developed actuators is also introduced.

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