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Natural movements with artificial legs

Prosthesis users find it very difficult to cope with everyday situations: artificial legs do not always work as they should and movements often tend to be fairly awkward. Researchers from the Fraunhofer Institute for Manufacturing Engineering and Automation IPA in Stuttgart are to present a motion control system that enables prosthesis users to intuitively move the prosthesis in the same way as a natural leg. The new system will be presented at the Sensor+Test 2010 exhibition held in Nuremberg between 18th and 20th May 2010.

Florian Dennerlein walks up the stairs step by step, stair by stair: he lifts his right leg, bends his knee, moves the leg slightly forward, sets it down, puts his weight on it and shifts the weight to the left leg. Then he repeats the process. Twelve stairs to the top – and Florian can do this totally unaided and relatively quickly. For Florian, who only has one natural leg, this is nothing short of a miracle. Florian’s prosthesis makes his life much easier and more comfortable. Before he had the prosthesis, he found it extremely difficult and awkward to climb stairs. The new prosthesis system enables him to intuitively control the prosthesis in the same way as people who have two natural legs: walking, running and climbing stairs are natural movements that occur without thought because the brain intuitively controls the muscles, enabling them to contract when required at the appropriate intensity.

The sensor array incorporates a special pressure sensing technology in the prosthesis shaft. © Fraunhofer IPA

Researchers from the Fraunhofer Institute for Manufacturing Engineering and Automation IPA in Stuttgart will present a new motion control system at the Sensor+Test 2010 exhibition. This new system enables prosthesis users to intuitively control artificial legs. The prosthesis shaft contains sensors that transfer signals from functioning muscles and transfer them into prosthesis movements. In the past, the major disadvantage of intuitively controlled prostheses was that when a prosthesis user unconsciously tensed a leg muscle, the artificial leg would react to this tension and the prosthesis knee joint would extend.

Unintentional movements might sometimes look very strange but, for people like Florian, the unintentional movement of a leg can become extremely dangerous if it happens when the person is climbing stairs. Obstacles such as doorsteps appear without warning and can be a major problem for prosthesis users if the user does not see the obstacle in good time. Prosthesis users are only able to adjust the movement of the prosthesis when they have plenty of advance notice.

"Our new control system does away with such dangers and enables prostheses to be controlled much more intuitively," said Harald von Rosenberg, the project leader. The system is based on a very sophisticated technology: the prosthesis shaft contains a sensor array that measures the activity signals of the remaining leg muscles. The system then determines the ideal muscle signal from signalling patterns. "This means that the system must be able to recognise the intention of the prosthesis user," explains von Rosenberg adding, "the sensors simultaneously register each specific movement of the prosthesis user". The prosthesis thus "knows" whether its user is sitting, lying down, walking, standing upright or running. It also registers when the user is bending or kneeling down. "This is done through pressure sensors that are located in a sandwich-like fashion below the electronic sensors. They recognise, amongst other things, when the amputee shifts his/her weight to the prosthetic leg.

These two pieces of information, i.e. the sensing of muscular activity and state of movement, are then turned into an intuitive signal by the motion control system. This signal then prompts the prosthesis to move as required at the same time as adapting its damping to the movement. It is important to point out that the intuitive signal is determined in real-time. "This means that artificial extremities are now functioning in a range that is closer and closer to natural movements," said von Rosenberg.

The researchers also envisage using their invention in a broad range of other applications, given that the system is a human-machine interface that could also be used to improve the operation of electrical and other devices. For example, joggers who listen to music on their usual morning run would be able to tense their upper arm muscles to select the next song, rather than having to fumble for the buttons of their MP3 player in their trouser pockets. The researchers are planning to use a pin-ball machine to demonstrate how their new control system works at the upcoming Sensor+Test 2010 exhibition: Sensors attached to the forearm register the tension of the player's muscles, enabling him/her to control the ball by thought without having to pull levers and press buttons.

Further information:
Dipl.-Ing. Harald von Rosenberg
Fraunhofer Institute for Manufacturing Engineering and Automation IPA
Nobelstr. 12
70569 Stuttgart
Tel.: +49 (0)711 970-3648 
Fax: +49 711 970-953648 
Website address: https://www.gesundheitsindustrie-bw.de/en/article/press-release/natural-movements-with-artificial-legs