A new, advanced patient transfer device is being developed for moving mobility limited patients, for example, from a wheelchair to a bed or a floor into a chair. Current market patient lift devices are antiquated and insufficient for customer needs, with only one actuated degree of freedom. The high power to size ratio of hydraulic actuation makes it suitable for moving larger, heavier patients. We have developed a prototype pump-controlled hydraulically actuated patient transfer device that is more maneuverable and agile, using multiple actuated degrees of freedom. We are also working toward developing a more intuitive and safe caretaker interface and control strategy.
We have performed an extensive needs assessment; these are a few associated key design requirements relevant to this presented work. A compact package is needed for ease of maneuvering the patient around obstacles in an uncertain environment. The device is capable of producing large forces, so it is desirable for the controller to minimize any unintentional large external contact forces, or provide force feedback. In this system, the caretaker and device work together to maneuver a complex payload, a human body; the operator’s mental workload must be minimized. With humans in the device workspace, safety and stability are necessary, including environment interactions.
This new application presents several challenges related to the hydraulic control. First, we are using a separate bidirectional fixed displacement pump with a reversible brushed DC motor for each degree of freedom. The low level control involves obtaining desirable response from each motor-pump-actuator system, while compensating for significant static friction. At a higher level, we are testing several approaches to attain the desired intuitive control and desired dynamics, and minimize the operator workload. First, we are developing a coordinated control using a force input, such that the operator simply pushes on the device in the desired direction of motion. We are testing several different controllers to attain the desired dynamics. This paper presents the design of the machine, the proposed control structures as applied to this application, operator interface options, some preliminary results, and future work.