Statically-balanced mechanisms have been widely used for passive compensation of gravity loads in many applications including neurological rehabilitation and micro-/reduced-gravity simulation. For these applications, it is desirable that the used mechanism has minimal impedance the interacting human can feel. Impedance of a statically-balanced mechanism is contributed by many factors including the payload on the end-effector and the joint friction. This paper studies the relation between the end-effector impedance and the load-dependent joint friction for statically-balanced mechanisms. In the study a load dependent joint friction force model was developed. With the model, contribution of the end-effector load or payload on the joint friction can be evaluated, from which the end-effector impedance of the mechanism caused by the joint friction can be computed. The study results are applied to the analysis of a reduced-gravity simulator to evaluate the effect of the joint friction on the end-effector impedance of the mechanism. The findings of the study can help the assessment of the dynamic performance and also help the optimal design of statically-balanced mechanisms.

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