Error mapping controller: a closed loop neuroprosthesis controlled by artificial neural networks

BACKGROUND: The design of an optimal neuroprostheses controller and its clinical use presents several challenges. First, the physiological system is characterized by highly inter-subjects varying properties and also by non stationary behaviour with time, due to conditioning level and fatigue. Secondly, the easiness to use in routine clinical practice requires experienced operators. Therefore, feedback controllers, avoiding long setting procedures, are required. METHODS: The error mapping controller (EMC) here proposed uses artificial neural networks (ANNs) both for the design of an inverse model and of a feedback controller. A neuromuscular model is used to validate the performance of the controllers in simulations. The EMC performance is compared to a Proportional Integral Derivative (PID) included in an anti wind-up scheme (called PIDAW) and to a controller with an ANN as inverse model and a PID in the feedback loop (NEUROPID). In addition tests on the EMC robustness in response to variations of the Plant parameters and to mechanical disturbances are carried out. RESULTS: The EMC shows improvements with respect to the other controllers in tracking accuracy, capability to prolong exercise managing fatigue, robustness to parameter variations and resistance to mechanical disturbances. CONCLUSION: Different from the other controllers, the EMC is capable of balancing between tracking accuracy and mapping of fatigue during the exercise. In this way, it avoids overstressing muscles and allows a considerable prolongation of the movement. The collection of the training sets does not require any particular experimental setting and can be introduced in routine clinical practice

Swing-through gait with free-knees produced by surface functional electrical stimulation

The swing-through gait is often the gait of choice for those crutch walkers who can perform it. However, a practical (sufficiently low energy and sufficiently fast) gait is usually not achievable by paraplegic individuals with thoracic lesions. Functional electrical stimulation (FES) was used to assist three spinal cord injured (SCI) subjects with complete thoracic lesions at T11, T11 and T6 to ambulate with a swing-through gait patten. Eight channels of surface stimulation were used to bilaterally stimulate knee extensors, knee flexors, hip extensors and hip flexors. The stimulation sequence was controlled by a computer that implemented a finite-state, rule-based control strategy according to sensor inputs. Over a long, level walkway, the T11 subjects averaged 0.40 m/s and 0.38 m/s for distances of 56 m and 51 m; the T6 subject averaged 0.30 m/s for 43 m. Using a motion analysis system, the gait patterns of two of the subjects were compared to those of a trained, non-impaired subject. The SCI subjects spent more time in both double support phases (when both crutches and both feet contact the floor) than did the non-impaired subjects, leading to a loss of momentum and hence a slower and less efficient gait. In conclusion, an FES assisted swing-through gait is shown to be a potentially useful mode of FES gait