Scientists at Imperial College London have developed a sensor system that enables control of a robotic hand by detecting mechanical rather than electrical signals. This may eliminate some technological challenges associated with prosthetic movement.

A number of robotic prosthetics currently in development are connected to the user via a socket, which seals around the stump and detects electrical signals from muscle movements. One challenge is maintaining an electrical connection to the muscle via sensors placed on the skin inside the socket. The connection can be lost due to sweat and other factors. Researchers Dr. Ravi Vaidyanathan and Samuel Wilson, both from Imperial College's Department of Mechanical Engineering, have developed a prototype sensor system that they say avoids this by detecting mechanical signals from vibrations produced by muscle fibers that move when muscles flex. These vibrations can be sensed and passed to a robot hand to make it move in response to the user’s muscles.

The technology includes a motion sensor system to detect arm movements. This enables a user to control how the robotic hand grasps different-sized objects through a simple sequence of muscle flexes and arm movements. The researchers carried out a demonstration of the prototype with Alex Lewis, who lost both legs, his right arm and use of the other arm following a rare infection.

To operate the robotic hand, Lewis had a small arm band placed around his bicep. The band had a muscle sensor and motion-tracking electronics embedded into it. When he flexed his bicep, the vibrations were detected by the sensor, then were interpreted as signals and transmitted to a computer. Software executed a mathematical algorithm designed to isolate Lewis’s muscle signal and filter other arm motions and sounds, converting the signal into a command for the robotic hand.

Sensors detect mechanical signals from vibrations produced when muscles flex. Image credit: Imperial College London.Sensors detect mechanical signals from vibrations produced when muscles flex. Image credit: Imperial College London.Lewis had the option of activating two different grip modes while the prosthetic lay detached from his arm. The first was a three-fingered pinch that could enable movements such as picking up a small object, such as a set of keys. The other was a power grip that could enable the prosthetic to grasp a larger object, such as a glass of water.

The research engineers caution that they have not yet carried out patient clinical trials with the technology and have a number of refinements to complete before the sensor and motion-tracking system can be commercialized. However, they say their work is a step forward in making robotic prosthetics more robust in their design, which could widen their usefulness for patients.

Future refinements are expected to include isolating the range of vibration interference that may make the hand open or close unexpectedly. The researchers also plan to refine the device so that it is more portable and enables the user to self-calibrate. This would end the need for engineers to take part in the set-up process. Adding more sensors also would expand the range of commands, so that the prosthetic could perform more complex grasping tasks.

In the longer term, the pair predicts that the sensor system could be adapted for use in controlling other technologies and appliances around the home. They also are working to establish a start-up company to market the sensor and motion-tracking technology.

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