A vibrotactile belt to display precise directional information for visually impaired
Abstract: This paper presents a vibrotactile belt to display precise directional information for visually impaired. Considering the characteristics of tactile perception, the torso-related transfer function was used to arrange actuators on the belt, and a coding algorithm using vibrotactile funneling illusion was proposed to display precise directional information. A psychophysical experiment was performed to evaluate the validity of the belt in displaying precise directional information.
The experimental results indicated that the vibrotactile belt using our proposed coding algorithm achieves a resolution of 7.5 degrees with a high recognition accuracy of up to 91%. The current work provides valuable guidance for the design of vibrotactile navigation aids.
Keywords: haptic device, vibrotactile display, torso-related transfer function, vibrotactile funneling illusion Classification: Circuits and modules for electronics display，vibration motor.
1 Introduction
The favorable effects of vibrotactile (vibratory tactile) displays on navigation performance, situational awareness, and workload reduction have been shown for blind people, pilots and drivers [1, 2, 3].
Vibrotactile displays have been widely used to provide directional navigation cues .
such as guiding commands: backward, stop and forward [4]; turn-left, turn-right, and go-forward [5]; or cardinal directions: frontal, back, left, right [6].
However, most of vibrotactile displays were used to provide simple directional information. Vibrotactile displaying of precise directional information is rarely studied. The skin covering the torso is capable of precisely encoding information since it contains hundreds of mechanoreceptors [7]. A belt-type display can be worn under a coat without attracting public attention [8]. Therefore, we designed a vibrotactile belt to display precise directional information.
The first belt-type vibrotactile display ever reported in the literature is the “ActiveBelt”, designed to convey directional information using 8 vibrators [9]. Recently, a vibrotactile belt with 8 vibration motors arranged evenly around the torso was developed to provide directional guidance for blind walkers [10]. Although these vibrotactile belts could provide guiding information for visually impaired, they can not be used to cue precise directional information. The precise directional information is required to guide blind people to their destination accurately, since a small error in cueing direction may cause a large deviation of the traveler’s orientation relative to the desired path [11, 10]. Besides in a vibrotactile navigation system, the geomagnetic sensor with high precision (e.g., TMC3000NF by NEC Tokin) is generally used to detect direction, which then is converted into vibration patterns.
Therefore the vibrotactile displays without fine resolution cannot take full advantage of the precision of the geomagnetic sensor. In applicable belt-type vibrotactile displays which have been reported, the minimum recognizable directional deviation in the horizontal plane of the waist is approximately 15 degrees [11]. However a resolution of 15 degrees may not precise enough to cue slighter deviations from the intended path for visually impaired. From the standpoint of vibrotactile perception, a resolution of approximately 10 degrees in the horizontal plane of the torso is feasible [12]. According to study of Cholewiak et al., subjects’ overall accuracy of localization performance for 6 or 8 vibrotactile stimuli presented to sites around the torso was higher than 92% [13]. The goal of current work is to achieve a resolution of less than 10 degrees with recognition accuracy of more than 90%。
The factors that influence the precision of vibrotactile cueing direction include the arrangement and number of tactors set around the torso [13], and coding algorithm of displaying directional information [14]. In most vibrotactile displays, tactors were located around the torso with same degrees between each adjacent pair [7, 10]. However previous psychophysical experiments of vibrotactile perception on torso indicated that the equally spaced tactors will not be perceived as equally distributed [12]. The bias between the tactor angle and perceived angle changes as a function of tactor angle. This function was called torso-related transfer function (TRTF). The implementation of TRTF in applications that require high precision of cueing directions is suggested [12]. The vibrotactile funneling illusion has also been reported as effective in improving the resolution of the cueing direction [15, 5]. By activating two adjacent vibration tactors simultaneously, it is possible to elicit a vibrotactile funneling sensation just like a virtual tactor vibrating at midpoint of them [16]. The perceived tactile sensation of the virtual tactor is almost equivalent to that of a real tactor [17]. In this work, we implemented the torsorelated transfer function to determine the arrangement of tactors around the torso, proposed a coding algorithm using vibrotactile funneling illusion to cue precise directional information. A psychophysical experiment was performed to evaluate the validity of the belt in displaying precise directional information.
2 Design of vibrotactile belt
Displaying rich directional information intuitively with as few tactors as possible
is a general principle of designing vibrotactile displays. In this section we will
describe the number and arrangement of tactors set on the torso, and how the
precise directional information was intuitively encoded with vibration patterns. As
shown in Fig. 1, the hardware system of vibrotactile belt is consisted of controller,
geomagnetic sensor and actuator. The controller receive directional signal detected
by the geomagnetic sensor and activate actuators to vibrate. The actuators used in
current work are coin motors in regular cell phones (Fig. 1 right).
Compared to
other types of vibrotactile actuators (e.g., linear resonant actuator and piezoelectric
actuator), the coin motors are size-small, low-price and energy efficient [18], which
make them an ideal choice for vibrotactile belt. The C1234B016F flat coin motor was selected as tactor to evoke vibrotactile sensation (http://www.vibrationmotor.net). The vibration intensity produced by the motor can be controlled by PWM duty.
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