Selective control of a subtask of walking in a robotic gait trainer(LOPES)

Robotic gait trainers are used all over the world for the rehabilitation of stroke patients, despite relatively little is known about how the robots should be controlled to achieve the optimal improvement. Most devices control complete joint trajectories and assume symmetry between both legs by either a position or an impedance control. However we believe that the control should not be on a joint level but on a subtask level (i.e. foot clearance, balance control). To this end we have chosen for virtual model control(VMC) to define a set of controllers that can assist in each of these tasks. Thus enabling the exoskeleton to offer selective support and evaluation of each substask during rehabilitation training. The aim of this explorative pilot study was to assess the performance of a VMC of the step height and to assess if selective control of the step height left the remaining of the walking pattern unaffected. Four young healthy subjects walked on a treadmill with their legs and pelvis attached to the lopes exoskeleton in 3 different conditions: (1) providing minimal resistance, (2) control of the left step height with a low stiffness (3) control of the step height with a large stiffness. We have shown that it is possible to exert a vertical forces for the support of foot clearance during the swing phase. The higher stiffness of the VMC resulted in a greater change of the step height, which was achieved by a larger increase of the maximal hip and knee flexion compared to the low stiffness condition. The control of the step height resulted in minor changes in the cycle time and swing time. The joint angles also showed only minor changes. The preliminary results suggest that we were able to control a subtask of walking, while leaving the remaining walking trajectory largely unaffected. In the near future, control of other subtask will be implemented and evaluated in isolation and in conjunction with each other.