Force-Reflecting Anthropomorphic Handmaster Requirements

This paper summarizes the results of a four-year study of force-reflecting handmaster exoskeleton technology and design requirements. The state of the art in anthropomorphic force-reflecting hand exoskeletons is characterized by a small number of independent efforts in government, academic, and commercial laboratories. Efforts are in widely varying stages of development, and have been slowed by the serious design limitations of this application, and by a dearth of suitable actuators. Topics covered include component technologies such as actuators, a study of human capabilities, and a discussion of handmaster requirements. Existing force-reflecting handmaster systems will be used as examples throughout the paper. Based upon published data and kinematic analysis, a maximum isometric metacarpal-phalangeal (base knuckle) torque of 5 N-m is calculated for the human middle finger, and lesser figures are calculated for other finger joints. The author recommends that handmaster designers specify a peak force capability 20-30% of the human maximum, and a continuous force capability 3-15% of the human maximum. This paper also references new experimental data on maximum human finger joint speeds. Citing the difficulty of designing adequate transmission systems for placement of actuators remotely from the hand, as well as deficiencies in actuator technology, the paper investigates the potential of novel actuators (magnetostrictive and piezoelectric) for co-location on the hand exoskeleton joints themselves. Motors using piezoelectric and magnetostrictive technologies and having torque/mass ratios in excess of 100 N-m/kg, with a mass of less than 57 grams, are presented. This capability far exceeds the 35-55 N-m/kg determined to be necessary for a co-located exoskeleton motor.