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Clippard EV Valves Help Interface Brain with Artificial Leg


Application Story Published by Clippard, May, 1983

An artificial leg that moves on command from the brain of the user is now a reality thanks in part to electronic/pneumatic interface valves from Clippard Instrument Laboratory.

For the last three years, a team of bioengineers, computer scientists and physicians at the Rehabilitation Hospital in Philadelphia have been designing this myoelecric prosthesis. It uses min-computers and electrodes to transmit the body’s natural electrical signals to Clippard's electronic (EV Series) valves that actuate leg motion.

The researchers are from Moss, Drexel University and the Temple University School of Medicine under the direction of Dr. Gordon D. Moskowitz, Senior Research Scientist at Moss.

According to Howad Hillstrom, a research scientist for the project, it has been well understood since the 1950s how the electrical activity from the brain signals muscles to contract and move the leg in a coordinated motion. “As muscles are activated, they give off electrical impulses as they contract. That’s what happens in an intact muscle,” he pointed out.

“In an amputee, the electrical signals still are sent down the remaining portion of the thigh muscles,” Hillstrom continued. “The only difference is that he doesn’t have the limb.”

The researchers embedded electrodes in a plastic socket that covers the stump. The electrodes pick up signals from nine muscles that accomplish most of the leg motions. Six of the muscles are in the thigh and three in the hip area. The signals can be detected on the surface of the skin.

“The purpose of these electrodes is to detect the desire of the amputee to move the leg. The motion of the knee, for example, is a result of the pattern of electrical activity of all nine muscles recorded about the hip and knee joints,” Hillstrom explained. “When all nine electrodes receive the signal, a computer can interpret that as a signal from the brain for the knee to move.”

The electrodes amplify those electrical impulses between 1,000 and 10,000 times, and also filter out extraneous electrical noises from the atmosphere. Upon conditioning the signals, so as to form estimates of muscle force, they are digitized for computer processing.

According to the signals it interprets, the computer sends 24-volt signals to Clippard electronic/pneumatic interface valves located in the actuating assembly that is an integral part of the prosthesis itself. They EV valves, operating at low pressure, actuate a double-acting cylinder.

Depending on the signal input from the mini-computer, the cylinder rod extends a desired distance. This stroke creates swing action required for walking or other dynamics.

The big advantage of the Drexel-Moss myoelectric system over its predecessors, according to Hillstrom, is that it allows not just for normal walking, but for conscious maneuvers such as obstacle avoidance and stumble recovery.

“The miniaturization of the Clippard components will be of major assistance in our attaining the most crucial task of the project, making the entire apparatus self-contained so the amputee can get out of the lab and into active life with “the leg,” Hillstrom emphasized.

At present, the system utilizes a pressure reservoir that is recharged by piston action during the extension phase of the swing cycle. An additional pressure source is required, however, to maintain an adequate level for efficient operation. How to achieve this on a portable self-contained basis remains a major challenge, together with the problem of creating an on-person microprocessor.

According to Hillstrom, the Clippard EV valves were specified not only because of their small size, lightweight, cool operation and low power requirements, but also because of their fast response of 5 milli-seconds. This enables them to respond well within the signal tolerance of computer input.

The research team says that it hopes to have a clinical model of the myoelectric limb ready in three to five years. Their goal is to produce a device that is reliable, functional and economical.