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A parallelogram chain designed to measure human joint motion Cousins, Steven J.

Abstract

This work is concerned with the problem of measuring the motion of a human joint under dynamic conditions. Past and present solutions to this problem are examined in a literature search. Criteria are established for the evaluation of designs, based on one year's clinical experience with a locally built device. A parallelogram chain is chosen from generated design alternatives using the established evaluation criteria. A prototype of the parallelogram chain is built. The chain casting has thin flexible plastic hinges. As each parallelogram scissors, unwanted translations are absorbed while three mutually perpendicular rotations pass through the chain unchanged. This allows the potentiometer motica transducers to be self-aligning with a joint. This will reduce patient fitting time and increase the reproduceability of results compared to previous devices. The chain is applicable to any large joint of the upper or lower limbs. A test rig, simulating human joint motion, is built. Test results indicate that the parallelogram chain can record dynamic motions of walking and slow running. The chain has a definable working volume within which low error and reproduceable results can be obtained. The chain produced best results, comparable or better than those obtained by other workers, when only one of the three measuring potentiometers was aligned with the corresponding joint axis of rotation. Errors caused by the joint simulator are explained and will facilitate redesign. The main source of measurement error is the hinge geometry and not the material properties of the chain. As a result, a new chain design is presented that should give reduced measurement errors.

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