Intrinsically safe mechanisms represent an innovative solution to develop physical human-robot interactions. These systems are characterized by low masses, inertia and torques. In this paper, an innovative actuation strategy is presented, focused on safety concerns. The system is first statically balanced to compensate gravity forces in any configuration. Our contribution then lies in the design of a mechanism that modifies the system balancing, making it possible to follow a planned trajectory or to remain in contact with a moving environment, without developing large forces. This principle is illustrated with an elementary one degree of freedom arm. The whole design procedure is described, so as to define properly the arm parameters for a given task. A closed loop position control strategy is then proposed in order to drive the mechanism. It uses a proportional-derivative controller with configuration dependent gains, whose efficiency is illustrated by trajectory following and interaction simulations.

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