Upper limb amputees, using a body-powered prosthesis, generate energy with their shoulders which leads to opening or closing of the prosthesis. The preference for one of these types, voluntary opening (VO) or voluntary closing (VC), differs for individuals and for task specifics. Research is, therefore, done regarding terminal devices which provide both types. Earlier presented hybrid devices are heavy, do not provide mechanical advantage or present a difficult switching procedure. A novel VO/VC device was presented, with the promising features of easy switching and good mechanical performance. However, in user tests this device did not perform well, mainly because of its finger shape.
The goal of this study is to improve the grasping performance of the VO/VC device. Changing fingers did not create a device which could fulfil all the requirements. Instead, another idea was embodied, prototyped and tested.
This device is able switch between VO and VC, by the use of two separate pivot points. Switching between the two is done with a bi-stable mechanism, which is activated by the intact hand. Results show that the device can be used in both modes and that users are able to switch with the bi-stable mechanism, but the mechanical properties do not meet the requirements. Additionally, the device performs less than expected on the user test, due to the bad mechanical properties and misalignments in the 3D-printed prototype. The device created is a good proof of principle for using two pivot points to shift between VO and VC. However, the bi-stable mechanism did not function as expected and a lot of friction is present leading to high forces in the cable. It is recommended to change the material, focus on using other springs which allow for larger opening widths and generate less friction and change the switching system to increase usability and safety.