Control of Independent Crank Ergometer with Adaptive Phase Shift for Cycling Rehabilitation

Robotic therapy is becoming common these days. Lower-limb rehabilitation is highly demanded in stroke patients. Instead of gait training, cycling rehabilitation using non-impact cyclic movement is safer for patients from falling. Conventional ergometers have a mechanical constraint between their crank arms to be always 180 degrees apart, so any assistive or resistive command affects both the non-target and target legs. The concept of independent crank ergometer with adaptive phase shift is presented in this paper. Each crank arm has its actuator to assist or resist movement independently from another. The non-target leg can face a high resistive moment while the target leg experiences lower constraint when doing rehabs. A synchronizing pedal control is applied to create a virtual spring connecting both crank arms where the stiffness and the equilibrium offset can be adjusted. If one pedal is moved faster than another, the virtual spring moment will resist the leader crank arm while pulling the follower crank forward to maintain the desired crank offset. The proposed control system was simulated via MATLAB Simulink to validate the crank motor torque when the phase shift between both crank arms varies periodically. The alpha prototype of the independent crank ergometer using T-Motor AK10-9 brushless motors was built. For different crank stiffnesses, the virtual spring moment varying against the sinusoidal phase differences was validated experimentally. For observing advantage on rehabilitation, the influence of adaptive phase shift on cycling behaviors will be observed in healthy participants and patients in future studies.