Two-way Actuator for Arm Mounted Exoskeleton

Abstract

Human exoskeleton designs have in recent years started adopting the use of flexible and pneumatic powered actuators, because of their ease of use and inherent properties. A disadvantage however is that they have to be set up in antagonistic pairs to create two-way movement on a load. A fluid-driven, bidirectional actuator inhabiting similar properties could be a step forward in simplifying today's exoskeleton design.

A prototype actuator was designed and assembled using two different pneumatic artificial muscles; one commercially available - providing contraction - the other built in-house using a design from a pre-project - creating extension movement. Mathematical expressions for the actuator's volumetric change, potential force and stiffness were derived, and static tests were performed - using both pneumatic and hydraulic input.

The results yielded a thorough characterization of the unit, summarized in a data sheet. Linear functions for its force, stiffness and length development were estimated using graphical tools, and their accuracy were assessed and compared to the derived expressions. Hydraulic actuation appeared to be favorable in terms of both controller implementation and sizing.

The prototype seems a feasible option in replacing an antagonistic pair for two-way actuation. Further design improvements are necessary; especially to better its stroke length - being the limiting factor now - but it still gave sufficient results for its intended application. In addition, its asymmetrical force property makes it well-suited to be used in conjunction with human joint control.