Actuation and control for robotic physiotherapy

Abstract

Over the last few years researchers have developed prototype robots that are capable of administering physiotherapy, however, these devices tend to be complex and
\nexpensive. The use of modem pneumatic servo systems as actuators would make such devices simpler and cheaper. This thesis assesses the feasibility of a pneumatically actuated robotic physiotherapy device through the implementation of force and position control strategies.
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\nTraditional pneumatic servo systems consist of a pneumatic cylinder prone to stiction effects and a single spool valve. Here the performance of modem pneumatic
\nservo system, consisting of a low friction pneumatic cylinder and two electropneumatic proportional valves has been evaluated. The increased linearity of the modem pneumatic system enabled a self-tuning pole-placement controller to be implemented that would be unsuitable for conventional pneumatic systems. The selftuning pole-placement controller enabled consistent and accurate position control.
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\nOther researchers have achieved force control of pneumatic systems, however their force models are not applicable on this modem configuration. Accurate control of the
\nservo system force output, while the position of the cylinder piston is fixed, has been achieved through an open-loop force controller, however applications for fixed
\nposition force control are limited. The servo system force output, during motion, has been found to be a function of the piston velocity and input control signal.
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\nA pneumatic robot has been designed and fabricated with a position workspace that enables the average male to perform upper limb reach and retrieve exercises when attached to the robot. The pneumatically actuated robot, combined with a simple three degree-of-freedom force sensor, form a device capable of administering upper-limb robotic physiotherapy. Impedance control has been identified as the most suitable force and position control strategy for implementing physiotherapy.
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\nApplying the impedance control strategy, to a single link of the robot, resulted in accurate implementation of the desired force and position relationship.
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\nExtending the controller to two and three degrees of freedom has resulted in degradation of the controller performance due to limitations of the three degree-offreedom
\nforce sensor. The controller performance is also found to be dependent upon selection of the impedance characteristics. Low stiffness and high damping, along
\nwith high stiffness and high damping have been identified as particular low points in controller performance due to the requirement for the system to provide large forces
\nwith little resulting motion.
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\nIt was concluded that the pneumatic robot and impedance control strategy have the potential to administer physiotherapy. However, further work incorporating a force
\nsensor with greater accuracy that is robust to torque inputs and a rigorous stability analysis would be required before the device could be clinically evaluated.