Recent Advances in the Design and Control of the Michigan Arm
D. S. CHILDRESS*, E. C. GRAHN, T. G. MANHART,J. S. STRYSIK, AND C. W. HECKATHORNEChicago, Illinois
Two sizes of the Michigan Arm have been developed for the Area Child Amputee Center in Grand Rapids, Michigan for children with bilateral upper ameba. The prostheses are an outgrowth of an earlier coordinated arm. The present device has been scaled up in size for use by children ages eight to thirteen. The arm couples the elbow and the wrist to maintain the terminal device, the Michigan Hook, in a fixed attitude relative to the user's body through the entire range of elbow flexion, of particular importance during eating. Several arms have been fitted. They are controlled by three electrical switches that can be activated by pushing motions within the socket. Two switches control flexion and extension of the elbow and one activates the hook. Switch control, a form of velocity control, is a simple and inexpensive control method for powered prosthetic joints; however, better control modalities are available.
A different control scheme has been applied experimentally. The method involves extended physiological proprioception, a principle introduced by Dr. David Simpson (Edinburgh). The shoulder is force-coupled to a position servo that controls flexion/extension of the elbow so that elbow position corresponds to shoulder position, elbow velocity to shoulder velocity, and elbow acceleration is related to shoulder acceleration. Experimental results indicate this control to be superior to velocity control methods and suggest that we may be able to build limbs in which multi-functional control can be achieved at subconscious levels, with little mental load on the user.
*Northwestern University, 345 East Superior Street, Chicago, IL 60611