In this research, we developed an ankle-foot assist device for foot rehabilitation and walking assistant purpose. In order to support all 6 DOFs human foot motion (the rotations of human foot and the displacement of rotation axes), a six air cylinders drive Stewart Platform Mechanism was used. In our previous work, we have proposed a measuring and calculating method to get the posture of human foot and estimate the instantaneous rotation axis of ankle joint [1]. In this paper, we further propose a new method to control the position, force and stiffness of this device. The position can be controlled even if the potentiometers do not measure the length of air cylinders directly. The force can be controlled by changing the air pressures in two chambers of each cylinder, without using any force sensors. The stiffness can also be controlled which is controlled by changing the stiffness of all cylinders, by changing the pressures in chambers. These control methods are tested in experiments for one cylinder and for developed assist device. The results show the accuracy and validity of the device and the method in the control of position, force and stiffness.

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