Non-back-drivable rotary mechanism with intrinsic compliance for robotic thumb abduction/adduction

Duplicating the complexity of the human thumb with a robotic one is a difficult task for engineers, when taking into consideration the necessary miniaturization and robustness requirements. Miniaturization is required to fit all the components within the size of a human hand; however, the reduction in size of the components affects the strength of the transmission. Several robotic hands with active thumbs were designed. Most of these had the abduction/adduction joint rigidly connected to the electrical motor and gearhead. This is a non-optimal solution because although miniaturized, the transmission is back-drivable and thus the motor gearhead is exposed to shocks. In this paper, we present the design of a non-back-drivable rotary mechanism with intrinsic compliance for a robotic thumb ab/adduction joint. The mechanism enables switch off of the power supply once a desired posture is stable, thus avoiding accidental dangerous releases of the grasped object, absorbs the impact forces generated at the instant of grasping and dissipates the repeated strains that are induced during manipulation. The mechanism was developed and tested. Detailed kinematic, static and stiffness analyses are presented as well as experimental measurements which demonstrate suitable performance for our application (max efficiency ~0.85, critical load 1.2 Nm, energy absorbed 1.5J).