Control design for a lower-limb paediatric therapy device using linear motor technology

Chrif, Farouk; Nef, Tobias; Lungarella, Max; Dravid, Raja; Hunt, Kenneth J. (2017). Control design for a lower-limb paediatric therapy device using linear motor technology. Biomedical signal processing and control, 38, pp. 119-127. Elsevier 10.1016/j.bspc.2017.05.011

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Background
Rehabilitation robots support delivery of intensive neuromuscular therapy and help patients to improve motor recovery. This paper describes the development and evaluation of control strategies for a novel lower-limb paediatric rehabilitation robot, based on linear-motor actuator technology and the leg-press exercise modality.

Methods
A functional model was designed and constructed and an overall control strategy was developed to facilitate volitional control of pedal position based on the cognitive task presented to the patient, together with automatic control of pedal forces using force feedback and impedance compensation.

Results
Each independent drive for the left and right legs can produce force up to 288 N at the user's foot. During dynamic testing, the user maintained a variable target position with root-mean-square tracking error (RMSE) of 3.8 ° with pure force control and 2.8 ° with combined force/impedance control, on a range of periodic motion of 20–80 °. With impedance compensation, accuracy of force tracking was also slightly better (RMSE of 9.3 vs. 9.8 N, force/impedance vs. force control only).

Conclusions
The control strategy facilitated accurate volitional control of pedal position and, simultaneously, accurate and robust control of pedal forces. Impedance compensation showed performance benefits. Control accuracy and force magnitude are deemed appropriate for rehabilitation of children with neurological impairments, but, due to current levels required, linear motor technology may not be suitable for applications where higher force is needed. Further work is required to validate the device within the target population of impaired children and to develop appropriate patient-interface software.

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