Abstract
Manual wheelchair propulsion is known to be inefficient and causes upper extremity pain, fatigue, and injury. Power-assisted wheelchairs can mitigate these effects through motors that reduce users' effort and load during propulsion. Among the different control strategies proposed to govern the user-wheelchair interaction, impedance control-based ones appear to be the most natural and effective. It can change the apparent dynamical properties of the wheelchair, particularly mass and friction, and automatically compensate for external disturbances such as terrain conditions. This study investigates the advantages and disadvantages of this control strategy employing predictive simulations of locomotion with power-assisted wheelchairs in different scenarios. The simulations are generated using a biomechanically realistic model of the upper extremities and their interaction with the power-assisted wheelchair by solving an optimal control problem. Investigated scenarios include steady-state locomotion vs. a transient maneuver starting from rest, movement on a ramp vs. a level surface, and different choices of reference model parameters. The results reveal that the investigated impedance control-based strategy can effectively reproduce the reference model and reduce the user's effort, with a more significant effect of inertia in the transient maneuver and of friction in steady-state locomotion. However, the simulations also show that imposing a first-order, linear reference model with constant parameters can produce disadvantageous locomotion patterns, particularly in the recovery phase, leading to unnecessary energy dissipation and consequent increase in energy consumption from the batteries. These observations indicate there is room for improvement, for instance, by exploring energy regeneration in the recovery phase or by switching reference model nature or parameters along the cycle. To the best of our knowledge, this is the first investigation of impedance control-based strategies for power-assisted wheelchairs using predictive simulations and a realistic, nonlinear model of the user-wheelchair system.
Highlights
The main objective of this study is to evaluate, through predictive simulations of wheelchair locomotion at different typical conditions, the performance of assistance based on impedance control and the effects of considering in the control law a first-order dynamics as the reference model
The model allowed for taking the dynamic effects of arm motion, the intrinsic muscle properties, and the varying system dynamics in the propulsion and recovery phases into account
The results confirm the advantages of the studied impedance control strategy, including automatic compensation of gravity forces in inclined terrains and the possibility of naturally adjusting assistance by manipulation of physical parameters such as mass and damping
Summary
Wheelchair locomotion is common among people with physical disabilities and can help them have a more independent living. In order to mitigate these effects, wheelchairs with partial motor assistance are drawing increasing interest, and studies compare the performance of commercially available models in different circumstances (Karmarkar et al, 2008). It has electric motors that assist the person in manually propelling the wheelchair, enabling the user to exercise while avoiding excessive muscle effort and upper extremity loads and, reducing the risk of muscle fatigue and joint injuries (Kloosterman et al, 2012). This type of assistance is important in conditions where large torques are required, such as on ramps or rough terrain (Kloosterman et al, 2012). It can be helpful on descents where the assistance torque can act as a brake to ensure safer locomotion (Seki et al, 2009)
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