AuICORR2005 - An EMG-position controlled system for an...

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Abstract —Although below-knee prostheses have been commercially available for some time, today’s devices are completely passive, and consequently, their mechanical properties remain fixed with walking speed and terrain. To improve the current performance of below-knee prostheses, we study the feasibility of using the amputee’s residual limb EMG signals to control the ankle position of an active ankle-foot prosthesis. We propose two control schemes to predict the amputee’s intended ankle position: a neural network approach and a muscle model approach. We test these approaches using EMG data measured from an amputee for several target ankle movement patterns. We find that both controllers demonstrate the ability to predict desired ankle movement patterns qualitatively. In the current implementation, the biomimetic EMG-controller demonstrates a smoother and more natural movement pattern than that demonstrated by the neural network approach, suggesting that a biologically-motivated, model-based approach may offer certain advantages in the control of active ankle prostheses. I. I NTRODUCTION lthough the potential benefit of powered prostheses for both upper and lower extremity amputees has been well documented, most of the research and commercial activity has focused on active upper limb devices [1]-[4]. Today, commercially available ankle-foot prostheses are completely passive, and consequently, their mechanical properties remain fixed with walking speed and terrain [5]. In distinction, normal human ankle stiffness varies within each gait cycle and also with walking speed [6][7]. Furthermore, some studies have indicated that one of the main functions of the human ankle is to provide adequate energy for forward progression of the body [6]-[8]. Not surprisingly, below-knee amputees that use passive ankle- foot prostheses exhibit non-symmetric gait patterns and higher metabolic ambulatory rates [5]. Thus, in order to mimic the behaviour of the human ankle and to increase gait symmetry and walking economy, a prosthetic ankle-foot device should be able to actively control joint impedance, motive power, and joint position. Samuel. K. Au is with MIT Media Lab, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. (corresponding author to provide phone: 617-324-1701; e-mail: [email protected] mit.edu). Paolo Bonato, is with the Department of Physical Medicine and Rehabilitation, Harvard Medical School and Spaulding Rehabilitation Hospital, Boston, MA 02134 USA. (e-mail:[email protected]) Hugh Herr is with MIT Media Lab and MIT-Harvard Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. He is also with Spaulding Rehabilitation Hospital, Boston, MA 02134 USA. (e-mail: [email protected]) When developing an active ankle-foot prosthesis, a key challenge that needs to be addressed is how to measure and respond to the amputee’s movement intent. For some time, researchers have attempted to use electromyographic (EMG) signals measured from the residual limb as control commands for an external prosthesis [9]-[13]. However,
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