A body powered prehensor with variable mechanical advantage.

The purpose of this research was to improve body powered, voluntary closing (VC) prosthetic prehension. A prototype prehensor with variable mechanical advantage was fabricated and tested. The device operates at low mechanical advantage during sizing of an object to reduce cable excursion requirements. It shifts to high mechanical advantage during gripping to allow high prehensile forces to be generated with reduced cable tension. The prototype provides a mechanical advantage of 2.4, nearly five times that of conventional VC devices. The prototype also acts as a holding assist; after grip forces are applied, they can be maintained with a cable tension of only 3 lb (13.34N). Field testing indicated that the device performs well in many tasks. The mechanism allows greater range of motion while an object is grasped than standard voluntary closing prehensors. However, the device performed poorly in grasping very compliant objects. To address this problem, a switch has been incorporated into the prototype to allow it to be used in a free-wheel mode.