Surplus heat drive actuators using MH alloys for assistive devices

A metal hydride (MH) actuator that uses the reversible reaction between the heat energy and mechanical energy of a hydrogen absorbing alloy is characterized by its small size, low weight, noiseless operation and mechanical compliance. Though the moving speed of the MH actuator is slow, this actuator is a suitable as a power source for rehabilitation or assistive devices. Prototype assistive devices using the MH actuator were reported, such as a wheelchair with a seat lifter and a robotic transfer aid to assist sit-to-stand motion. The main characteristic of the MH actuator includes a heat drive. This allows the MH actuator to be driven by surplus heat present in the surrounding environment. When only a high-temperature heat source is available, the surplus heat drive of the MH actuator requires rapid hydrogen absorbing to the MH alloy. This paper describes the means to improve the hydrogen absorption characteristics of the MH alloys. Three compositions of MH alloys were tested. One of these alloys obtained sufficient hydrogen absorption under natural cooling of the MH module. This means that the surplus heat source provides repeating absorption and release of hydrogen on the MH alloy. Maintaining this cycle forms an MH air compressor in which the air pressure is produced by converting the hydrogen pressure in the MH actuator. The advantages of this air compressor are that the small size and easily altered shape have the potential for the realization of unprecedented assistive technology devices for persons with disabilities.