Mechanical and psychophysical studies of surface wave propagation during vibrotactile stimulation

Abstract

Vibrotactile displays are based on mechanical stimulation delivered using an array of motors to communicate with the user. The way in which the display's motors are spaced and positioned on the body can have a significant impact on the effectiveness of communication, especially for tactile displays used to convey spatial information. The objective of the present research was to determine how the surface waves induced by vibrotactile stimulation of the skin varied as a function of the site on the body where the motors were mounted, and how these waves influenced the ability to localize vibrotactile stimulation. Three locations on the body were selected for study: the palm, the forearm, and the thigh. A flexible printed circuit board containing 3-axis micro-accelerometers was fabricated to measure the amplitude and frequency of surface waves produced by a vibrating motor at each body site. Results of these experiments showed significant differences in the frequency and amplitude of vibration on the glabrous skin on the palm as compared to the hairy skin on the arm and thigh. The palm had the highest frequency and lowest amplitude surface waves, and the forearm and thigh were very similar with lower frequency higher amplitude surface waves. No anisotropies were found from surface wave measurements. Most wave attenuation occurred within the first 8 mm from the motor, but there were still detectable amplitudes at a distance of 24 mm from the motor, which suggests that motor spacing should be at least 24 mm for this type of motor when used for precise spatial localization. A series of psychophysical experiments was conducted using a three-by-three array of motors in which the ability of subjects to localize the point of stimulation in an array was determined at each of the three body locations. The results from these experiments indicated that the palm had the highest localization accuracy (81% correct) as compared to the forearm and thigh which had similar localization accuracies (49% correct on forearm, 45% correct on thigh). Accuracy on the palm and forearm improved when the motor spacing increased from 8 mm to 16 mm, but increased spacing did not improve accuracy on the thigh. The results also showed that subjects were more able to identify the column of activation as opposed to the row of activation, which suggests a higher spatial acuity along the mediallateral as opposed to proximal-distal axis. The localization experiments indicate that glabrous skin is better suited for precise spatial localization than hairy skin, and that precise spatial localization requires an inter-motor spacing of more than 16 mm at these sites.