A Digital Goniometer based on encoders for measuring knee-joint position in an orthosis

Angular measurements of knee joint are a key factor in orthoses design, actually is very important for human gait analysis in order to obtain a deep knowledge of the musculoskeletal system performance. Video-based motion analysis systems have been used as measurement tools to quantify the angle and movement of knee joint. Though this technique is good, it requires special equipment and high computational cost. This work introduces a novel method of knee joint measurement. The design is mainly based on the contactless magnetic absolute encoder AS5040 (Austria Microsystems^®, USA), and the microprocessor type PIC16F877A. The developed device has a resolution of 0.35 degrees, quite enough for an orthosis that requires knee position measurements. The reliability of the system was evaluated by making: 1) Static measurements by using a mechanical goniometer as reference, in which a correlation up to 0.999 was obtained. 2) Dynamical measurements by performing flexion-extension movements by a healthy subject and comparing the records with a commercial motion analysis system (APAS^®, USA). A correlation of 0.9943 was obtained. Besides, trials were performed by three subjects under natural gait.