Design of a Hybrid Hydraulic Actuation Mechanism for the Delft Cylinder Hand Prosthesis

Abstract

The development of the Delft Cylinder Hand (DCH) demonstrated the design of a lightweight and functional hydraulic body-powered (BP) hand prostheses. The low friction losses of the hydraulics make it an attractive alternative to a classical mechanic transmission using rigid linkages and Bowden cables. There are benefits and trade-offs associated with BP and myoelectric prostheses. For example, improved sensory feedback is a benefit of BP prostheses. In this paper we set out to design a hybrid hydraulic actuation system for the body-powered DCH by extending the BP system with electro-hydraulic assistance, attempting to combine benefits of both BP and electrically actuated prostheses. We designed the hydraulic circuit, using a miniature external gear pump driven by a brushless DC (BLDC) motor in combination with solenoid valves to control the hydraulic flow. Furthermore, we designed a custom circuit board with a microcontroller, connected to pressure sensors and tactile sensors on the fingertips, to control the valves and the pump by a PD controller. Finally, we designed a 3D printed forearm structure, supporting the components, that connects to the hand through a wrist mechanism, allowing a pronation angle of 90°. We developed the hybrid prototype and verified its functioning by conducting several experiments. The prototype required an activation force of 53.5 N and 280 N mm of work done, at the input cylinder, to achieve a pinch force of 15 N, which is an improvement compared to commercial BP prostheses. Furthermore, the prototype was able to exert a pinch force of 22.5 N at an activation force of 100 N, at limited motor power, which is not as high as some commercial BP prostheses. Finally, the closing time of the prototype is 233 ms for a full close and 165 ms for the fingers to touch the thumb. The mass of the full prosthesis system is 901 g, including the battery pack, and could be reduced to an estimated 650 g. Future steps include optimization and miniaturization of hydraulic and electronic components, and mechanical structure of the prototype, reducing its mass to an acceptable level. Finally, extensive user testing is required to further validate the design direction.