An Orientation Estimator for the Wheelchair´s Caster Wheels

Wheelchair ergometers are a highly valuable tool in the study of the biomechanics of manual wheelchair propulsion. However, current ergometers have some drawbacks that affect their level of realism. For example, the moment of inertia of the wheelchair-user system and the caster wheels´ orientation are usually neglected, despite their high influence on the wheelchair´s behavior. Taking these factors into account requires a complex dynamic model, and the calibration of such a model requires on-the-field recordings of the caster wheels´ orientation, which are currently difficult to obtain. In this paper, we have proposed an open-loop observer that estimates each caster wheel´s orientation (CWO) based only on the rear wheels´ kinematics. The model was validated by propelling the wheelchair on three different floors (vinyl, carpet, and concrete) with five different normal forces between the caster wheels and the ground. Comparison between the estimated CWO and a reference one recorded by an optoelectronic device gave an accuracy error of less than ±8°. This error reduced to ±5° when the wheelchair was propelled following straight or slightly curved patterns. This observer has implications in the design of better wheelchair ergometers and simulators, as well as in the control of electric wheelchairs.