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Although much equipment for physical therapy has been developed, equipment to improve the quality of physical therapy is scarce. We propose a robotic biofeedback exercise device that can display human joint torque and muscle force during training without a problematic electromyogram (EMG). The purpose is to increase the therapeutic value by understanding a person's condition during exercise and to provide an incentive to improve performance. The manipulator supports lower limb rehabilitation in the sagittal plane. With its ability to adjust the maximum speed and the time constant, the manipulator provides simultaneous and safe isokinetic exercise for the knee and hip joints. This paper describes the estimation of the human joint torque and muscle force. The display of the joint torque and the muscle force is realized during exercise of the knee joint using the developed manipulator. The estimation of the muscle force from Crowninshield's method and Hase's method generally agrees with the EMG.

It is desirable to realize a nominal system which can control acceleration in order to realize fast and precise servo system, even if servo system has parameter variation and suffers from disturbance. We have shown that compensating disturbance and parameter variation by using disturbance observer can realize an ideal linear system which can control acceleration. In this paper, first, we transform a plant with disturbance compensation loop by disturbance observer into a two-degrees-of-feedom control system with a feedforward controller, which indicates that the disturbance observer system includes a series compensator such as P, PI, etc. depending on order of the disturbance observer. By clarifying the feature of the series compensator, we describe the bandwidth and the stability (the attenuation) of the system and show some simulation results, considering influence of the measurement noise.