Phase switching dynamics for body-powered hand prostheses

Abstract

Body-powered prosthetic hands used by below-elbow amputees function in two phases. There is the opening and closing motion of the hand (Motion Phase) and the phase when the hand is in contact with an object, and the fingers apply a pinching force (Pinching Phase). Each phase requires a different set of dynamics to properly function. In the Motion Phase, a small amount of applied force must result in a large translation of the fingers, in the Pinching Phase a small amount of force must result in a large pinching force and a small translation. The problem is that body-powered prostheses have fast motion given a low force input during the motion phase but cannot achieve a high pinching force with a low force input during the pinching phase. The goal of the project is to design a mechanism that passively switches between the dynamics of the motion phase and pinching phase for body-powered active closing hook prostheses. In the motion phase a small force input must result in a large output translation and in the pinching phase a small force input must result in a larger force output and a small output translation. In the motion phase a minimum cable force of 10 N is required and in the pinching phase a pinching force of 30 N must be achieved with an input force of 40 N or less The prototype consists of a hydraulic telemanipulation system with a master (shoulder), operated by the shoulder harness, and slave (hand), operates the hand prosthesis, cylinder connected to each other via a booster mechanism. The booster is inactive in the motion phase and active in the pinching phase. The results show that in the motion phase an input force of 5 N can perform the full translation of the prosthesis. If the hand is met with resistance the input 5 N input can increase up to 24 N until the pinching phase activates. The translation in the pinching phase is too small to observe any change. The pinching phase activates at a pinching force of 12 N and an input force of 24 N. In the pinching phase the pinching force reaches 35 N with an input force of 32 N input force. The dynamics of the phases show to be unaffected by different object stiffnesses and lowering the activation force of the pinching phase only shifts the pinching phase along the motion phase. In conclusion the 5 N minimum input force is too low. An input force below 10 N result in inferior control for the user. In the pinching phase an input force of 32 N can reach a pinching force of 35 N, which meets the set requirements of 30 N pinching force with an input force of 40 N or less. A larger translation is required in the pinching phase to compare the translation dynamics between the motion and pinching phase. More research is required to properly define the desired activation force, how much translation is required in the pinching phase, and to find optimal spring properties for the return springs in the hydraulic cylinders.