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Grip Amplifier: Design of a Residual Force Control Strategy to Support Pinch Grip with a Minimalistic Hand Exoskeleton


Hand exoskeletons could potentially improve hand use after stroke, but are typically obtrusive, and lack intuitive control. Here we propose a grip force strategy suitable for a minimalistic hand exoskeleton based on three key concepts. First, people achieve substantial hand functionality when using only pinch grip. Second, people who have a stroke retain isometric force control ability. Third, force generation is highly correlated between fingers after a stroke. From these observations we developed a control strategy based on measuring the isometric flexion force produced by digits 3-5 against the palm to control the force of an exoskeleton assisting in pinch grip. We implemented this “residual force control” strategy (RFCS) using the FINGER exoskeleton with ten unimpaired participants. Participants performed five sets of five lifts with no exoskeleton, with the exoskeleton donned on but powered off, and with the RFCS using two amplification gains. When using the RFCS participants were asked to maximize the contribution of the robot to object force, and we displayed the percent robot contribution using a computer monitor after each set. Participants were able to use the RFCS to lift the object, significantly increasing the contribution of force by the robot as the sets progressed. However, the grip force became larger and less dynamic compared to when participants were not wearing the exoskeleton. This suggests that unimpaired subjects could intuitively use this strategy to pick up an object and learn to amplify their grip force with practice. However, the strategy alters the normal grip force control strategy.

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