The approach used to implement mechanical impedance can be generally separated into the following two types: an impedance control system and an admittance control system. When an impedance control system reproduces a hard environment, large vibrations are generated. In contrast, an admittance control system is excellent at reproducing a high-stiffness environment for utilizing the inverse of the environmental impedance. However, stability analyses have not been conducted on admittance control systems. Therefore, in this study, a stability analysis for an implementation system of mechanical impedance is conducted using a pole placement. The analysis results indicate that the pole placement of the impedance control system is moved toward the direction of an imaginary axis based on the influence of the environmental stiffness. Hence, large vibrations occur when the environmental stiffness is determined to be a large value. On the other hand, for the admittance control, when the environmental stiffness is dominant, pole placement is not affected by the environmental stiffness. The superiority of the admittance control system in a high-stiffness environment was shown through an experiment.
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