Reaction times to constraint violation in haptics: comparing vibration, visual and audio stimuli

In teleoperation and in particular in surgical robotics, it is important to avoid getting closer to certain forbidden areas typically limited by virtual constraints. In this paper we compare different sensory modalities, vibratory, auditory and visual, to convey information about constraint violation to the operator. We focus on which of these modalities can elicit the fastest reaction time on the user. An experiment was devised in which subjects were asked to slowly insert a virtual tool by means of a haptic interface, and retract it as soon as they hit an obstacle; such event triggered an alert signal. We evaluated different signals: auditory, vibrotactile, and visual, with two amplitude levels for audio and vibration. Lower reaction times were observed on the strong vibrotactile modality, followed by the weak vibrations and the loud auditory tone, although the latter was described as uncomfortable by the subjects. The vibrotactile feedback was described as pleasant by most subjects and appears promising for future developments.