Caregiver-machine collaborative manipulation with an advanced hydraulically actuated patient transfer assist device

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Humphreys, Heather
Book, Wayne J.
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A significant need has been identified for an improved device to assist in transferring mobility limited patients, particularly those who are heavier or bariatric. Typical transfers include moving between a bed, wheelchair, chair/couch, toileting chair or toilet, car, or the floor. Currently, clinicians suffer more disabling workplace injuries than construction workers or firefighters, many of which are attributable to moving patients. A new, cost effective, hydraulically actuated prototype patient transfer assist device has been developed and fabricated; hydraulic actuation has advantages in terms of force density over electrical actuators that are typically used at this power scale. More generally, improved methods have been developed for control of machines that work collaboratively with humans, sharing a task and a workspace. Furthermore, this work investigates how hydraulic power, specifically electro-hydraulic pump controlled actuation, can be utilized in the human power scale. It also aims to overcome some of the control challenges with these actuation systems in this type of application, such as non-ideal characteristics of the low-cost actuation systems and management of a machine with large force capability operating in a home or clinical environment with humans in its workspace. A needs assessment has been performed, and the results indicate several needed improvements over current market patient lifts. A new prototype mobile patient transfer assist device (PTAD) has been designed and fabricated, with four actuated degrees of freedom (DOFs) fully functional. Each DOF is actuated by an electro-hydraulic pump control system. A simple, intuitive caregiver interface has been implemented, which provides coordinated rate control using an operator input from a force sensing handle mounted near the patient. This machine enables the exploration of controls and operator interfaces that have potential to transform healthcare. With a powerful machine working in a relatively delicate environment, it is necessary for the controller to manage any external interaction forces, to keep them in a safe range, in addition to smoothly controlling motion. A significant challenge lies in implementation of interaction control with these electro-hydraulic pump controlled actuators, which are intrinsically stiff, have slow dynamics, and have many nonlinear or non-ideal features. An impedance control framework has been formulated and implemented, using redundant sensing of obstacles, with feedback of both external interaction forces and proximity. Operator experiments were performed, using a mannequin representing the patient, including transfer operations between various locations in a simulated home/clinical environment. Some transfer experiments included obstacles to evaluate the control performance in unwanted collisions. Results indicate improvements over current market lifts in terms of operator ratings, even with this first generation prototype, and they show that comparable stages of the transfer operations can be performed considerably faster with a single operator using the PTAD than with the current market patient lift. Operator control experiment results show that the interaction control results in statistically significant reductions in collision forces by an average of 53%, and greater reductions in cases where the machine is moving faster. Similar software input experiments testing interaction control performance show reductions in collision forces of 87%. Overall, the results demonstrate that the new features of the patient transfer assist device make it easier, more efficient and safer to use, as compared with current market patient lifts. Beyond the patient transfer application, this project aims to make steps toward improving control in the broader application set of machines that work collaboratively with humans, sharing a task and a workspace; for example, in construction, manufacturing, or distribution.
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