A cam-displacement transducer device for measuring small two-degree of freedom inter-component motion in a prosthesis

Abstract

An electromechanical system is described that comprises a cam and two linear displacement transducers for measuring two-degree of freedom. This is aimed at quantifying the extent and pattern of both transverse rotation and longitudinal translation that takes place during amputee gait at what is sometimes termed a “torque-shock absorbing adapter” located between the prosthetic socket and foot. Both types of motion are measured independently by mounting the cam around the adapter and the two transducers orthogonal to one another with their spring loaded plungers moving separately against the cam’s perimeter and distal, flat face. The cam geometry should be an arc of a spiral and not a circle for the angular displacement measurement to be linear in response to rotation. Over a range of ±18° for transverse rotation the maximum deviation of the ideal cam geometry away from an arc of a circle is less than ±0.5% of the transducers’ 10 mm stroke. The transducer’s linearity is ±2% and it could therefore be assumed that this would be the dominant source of error. The theoretical predictions are supported by the calibration results, in which the pattern of errors that are found in the transducers’ calibration can also be seen in the calibration of the cam.