Design and Fabrication of a Gravity Balanced Planar Mechanism Using Auxiliary Parallelograms

Abstract

Abstract Exercise machines are becoming increasingly popular in rehbilitation and physical therapy. Gravity balancing is an important feature that is sought after in rehabilitative machines in order to make these accessible to patients with low muscular ability. Often, gravity loads in a mechanism are compensated by feed-forward actuator efforts that quasi-statically balance the mechanism in a configuration and are applied in addition to the actuator efforts required to perform a task. Such a scheme works but places large demands on the actuators. Also, actuator efforts are required at every joint in order to keep the mechanism balanced. This paper describes the design and fabrication of a gravity balanced planar mechanism, where auxiliary parallelograms are used to identify the center of mass of the augmented mechanism. A sliding carriage positions a counterweight directly above the center of mass in order to make the system gravity balanced. The sliding carriage uses joint encoder data to compute the location of the center of mass. Experiment results are presented which demonstrate the effectiveness of the approach.