Accelerometers keep prosthetic limbs in step

Technology used in smartphones could make artificial limbs easier to operate by helping users to orient them.

Researchers at the Norwegian University of Science and Technology (NTNU) are working to incorporate accelerometers, used to determine which way up something is, into electronic prostheses.

Accelerometers detect changes in gravity or velocity and are widely used in electronic devices, for example to help a smartphone know if you want look at the screen in portrait or landscape format.

Associate professor Øyvind Stavdahl and PhD student Anders Fougner have shown that accelerometers can help an amputee to keep their prosthetic limb oriented when attempting new tasks the device is unfamiliar with.

‘Prostheses are driven by the remaining muscles in the severed limb,’ said Stavdahl. ‘Sometimes, however, the prosthesis receives atypical signals from the muscles, which can confuse the system and lead to the prosthesis performing the wrong movement.

‘Knowing the orientation of the arm is essential for normal prosthetic function. It simply makes it a lot easier to operate.’

An amputee operates his or her prosthesis by moving a muscle, which sends out a kind of electrical signal. Electrodes placed on the surface of the skin detect these “myoelectric” signals and convert them to movement using a microprocessor and motor.

Activating specific groups of muscles will lead to specific movements, such as opening or closing the hand, or rotating the wrist. Amputees can run into difficulty when attempting new tasks that they haven’t trained for.

‘A simple task, like picking up a cup of tea, might prove difficult,’ said Stavdahl. ‘If you try to do this when the arm is in an awkward position, that is, a position that you haven’t practiced with before, the muscles will start sending unfamiliar signals to the prosthesis.

‘This will often lead to the prosthesis doing the opposite of what the user intended – and the cup will fall to the ground.’

An accelerometer helps to compensate for these unfamiliar signals, reducing the user’s frustration.

Stavdahl and Fougner’s research has shown that without any training, the user’s prosthesis will make the wrong move around 30 per cent of the time. With an accelerometer and proper training, this falls to 5 per cent.

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engineering precision


December 13, 2010



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