In the velvet hand illusion (VHI), a person rubs his/her hands together on either side of wires strung through a frame, producing the sensation of rubbing a very smooth and soft surface like velvet. We focus on VHI to determine the specifications of an actuator for a tactile display enhanced by VHI. We investigate the relationship between wire distance and intensity of illusionary sensation using a series of psychophysical experiments under active and passive touch. According to the experiments using Thurstone's Paired Comparison, the strength of VHI depends on the distance between two adjacent wires, and VHI caused by passive touch is considerably stronger than that caused by active touch. This result suggests that VHI is controlled by mechanical external stimulation using tactile displays. We think the mechanism of VHI is as follows: although the area bounded by two wires moves relative to the hands, tangential force does not occur on the hand surface except for wire-passing portion, causing operators to experience the illusion of touching a smooth virtual film with a zero coefficient of friction. Since VHI becomes weaker with a small distance between two adjacent wires, excessive tangential stimulation prevents the occurrence of VHI. Therefore, VHI control requires two actuations: one is for normal actuation on the operator's finger surface to generate a touch feeling; the other is appropriate tangential actuation to generate a moving-edge feeling on the operator's finger surface.
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