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

- Controllers for a novel lower-limb paediatric rehabilitation robot were developed.
- The device uses linear motor actuators and the leg-press exercise modality.
- A functional model was designed, constructed and tested.
- Force feedback and impedance compensation strategies were employed.
- Accurate control of pedal position (volitional) and force (automatic) was achieved.

BackgroundRehabilitation 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.MethodsA 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.ResultsEach 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).ConclusionsThe 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.