A Two-Axis Shoulder Position Transducer for Control of Orthotic/Prosthetic Devices

A shoulder position transducer has been designed and built for use by quadriplegic patients as a control signal source. It provides control signals suitable for controlling two-axis assistive devices utilizing proportional control. The signals correspond to shoulder elevation/depression and protraction/retraction. The device is small [measuring 2 in (5 cm) by 2 in (5 cm) by 5/8 in (1.6 cm) thick] and is held over the sternum by a lightweight harness or double-sided tape. A spring wire extends from the package to the shoulder where it is attached. Flexures of the wire in the vertical and horizontal directions are detected by semiconductor strain elements. Signal amplitudes of 1 to 2 V are obtained for shoulder movements of 1 in. Channel crosstalk is less than 5 percent (dependent on the accuracy of strain element mounting). To accommodate individual characteristics and postural changes, the device utilizes an automatic rezeroing circuit. This circuit enables any point in the coordinate system to be defined as the resting position and the center of control upon command by the operator.