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This article proposes a linear power assist unit for pedal-driven wheelchair and its control algorithm. The proposed power assist unit is controlled explicitly taking into consideration mechanical impedances of both a human operator and environment. In the proposed system, impedance variations for the environment are treated as a plant perturbation, but those of the human operator are considered to be identified to adapt the controller to the variations. For the first step, an identification method using Kalman filter is proposed, and evaluation results for a basic deterministic controller performance are reported. Simulation results showed that the proposed indemnification method is able to track impedance parameter varying about a frequency of 0.1 Hz. Experimental results showed that the prototype has robustly worked well against perturbations of the environment, keeping the power assist control bandwidth about 1 to 3 Hz.