Abstract
A variable-friction tactile display (VFTD) is a haptic device that can control the lateral friction on a fingertip touching and exploring the tactile display. An adjustable friction can be induced by the ultrasonic flexural standing waves producing the squeeze-film effect on a thin tactile plate. Because the reduction in lateral friction monotonically increases as the amplitude of vibration increases, the topology of the friction coefficient on the VFTD surface resembles the level surface of vibration amplitude on the same surface. Hence the existence of stationary nodal lines limits the use of the entire surface as a tactile display. This paper presents an isofrequency mode switching technique between the two-fold degenerate vibration modes to prevent a user's finger as much as possible from moving across or along the nodal lines. The finite element simulations had been conducted to design a VFTD prototype and to determine its free vibration characteristics. The simulations also helped to predict the availability of dual or multiple degenerate mode shapes at certain resonance frequencies. The computational results were verified by the experimental results with the measured resonance frequencies and corresponding nodal line configurations. The proposed mode switching techniques may contribute to the advancement of haptic displays utilizing squeeze-film damping effects.
