Keywords: vibrotactile displays

Vibro-tactile displays convey messages by presenting vibration to the user?s skin. In recent years, the interest in and application of vibro-tactile displays is growing. Vibratory displays are introduced in mobile devices, desktop applications and even in aircraft [1]. Despite the growing interest, guidelines on the design of vibro-tactile displays are still lacking. Existing guidelines are mainly concerned with passive displays, such as Braille labels on controls, nibs on keyboards and notches on smart cards [2, 3, 4]. In this paper we focus on active displays, either consisting of a single vibrating element (used in for example mobile phones and computer mice) or numerous elements (used in for example active Braille displays and body suits [5, 6]).

This paper discusses a first set of guidelines, dealing with the basic vibro-tactile parameters. The set is mainly derived from neurophysiological and psychophysical data. The guidelines indicate the relevant parameters as well as possible pitfalls. As such they can serve as a point of departure for interface designers. Important expansions of the set can come from user evaluation studies and examples of best practices.

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However, the potential information transfer capacity of the tactile channel is much larger than the 1-bit message ''your phone is ringing''.

Four primary parameters have been studied: amplitude, frequency, timing and location.

-> Encoding information by using different intensity levels is possible [14], although the number
of levels is small: not more than 4 different levels should be used between the detection threshold and the comfort / pain threshold.
-> The subjective magnitude of a stimulus can be enlarged either by increasing the intensity or by enlargning the area of stimulation.
-> coding information by frequency. No more than 9 different levels of frequency should be used for coding information. Difference between the levels should be at least 20%.
-> coding information by temporal patterns. The temporal sensitivity of the skin is very high (close to that of the auditory system and larger than that of the visual system). When using a single actuator of a tactile display to encode information in some kind of temporal pattern, the time between signals must be at least 10 ms.Depending on the type of actuator and the load, a vibratory stimulus will take time to reach the set frequency
-> coding information by location. In a multi-element display, information can be coded by location. Actuator density is an important parameter in the design of multiple elements tactile displays. When a high density is needed, only certain body parts have a sufficiently high spatial resolution

Irrespective of the parameter choice, it is important to make tactile messages self-explaining.

...On the other hand, the temporal sensitivity of the vibro-tactile channel is rather large. For example, breaks as small as 5.5 ms in a further steady signal can be detected.

The four primary parameters may also interact and change the percept in an undesirable way. For example, the amplitude threshold changes with frequency.