Ordinal force-feedback devices mainly employ electromagnetic motors (EMMs), and are excellent at expressing springy sensations. However, it is not easy to express a realistic sense of hardness and roughness using such devices. On the other hand, an actuator system (AS) using multilayered piezoelectric actuators exhibits performance characteristics that are opposite to those of an ordinal AS using an EMM. The objective of the present study is to develop an ideal AS for a haptic display. A two-degree-of-freedom AS utilizing a pair of hybrid AS units, each consisting of an EMM, an ultrasonic motor and a piezoelectric clutch, is proposed and evaluated. Such an arrangement allows the different types of actuators to complement each other, thus compensating for their individual weaknesses. This hybrid AS can expand the range of representable sensations. The results show that the proposed AS can realistically express both hardness and softness by switching between actuator combinations.
