Perception Accuracy of a Multi-Channel Tactile Feedback System for Assistive Technology

Abstract

Assistive technology uses multi-modal feedback devices, focusing on the visual, auditory, and haptic modalities. Tactile devices provide additional information via touch sense. Perception accuracy of vibrations depends on the spectral and temporal attributes of the signal, as well as on the body parts they are attached to. The widespread use of AR/VR devices, wearables, and gaming interfaces requires information about the usability of feedback devices. This paper presents results of an experiment using an 8-channel tactile feedback system with vibrators placed on the wrists, arms, ankles, and forehead. Different vibration patterns were designed and presented using sinusoidal frequency bursts on 2, 4, and 8 channels. In total, 27 subjects reported their sensation formally and informally on questionnaires. Results indicate that 2 and 4 channels could be used simultaneously with high accuracy, and the transducers' optimal placement (best sensitivity) is on the wrists, followed by the ankles. Arm and head positions were inferior and generally inadequate for signal presentation. For optimal performance, signal length should exceed 500 ms. Furthermore, the amplitude level and temporal pattern of the presented signals have to be used for carrying information rather than the frequency of the vibration.

Keywords: assistive technology; haptics; perception; tactile feedback; vibrating transducers.

Figure 1

Example of a 2-s test signal pattern that has 250 ms of sinusoidal bursts of a given frequency followed by 250 ms of silence.

Figure 2

Patterns P1 to P5 used in the experiment for sessions 33–41.

Figure 3

Linear vibrator frequency characteristics of the motors according to the manufacturer’s data sheet [61].

Figure 4

The vibrating motor fixed with double-sided tape.

Figure 5

Fixing points on the human body.

Figure 6

Graphical representation of the test sessions. Each column represents one subject, and each row corresponds to a session. Black dots refer to mandatory sessions evaluated by all 27 subjects. Red dots refer to non-mandatory sessions distributed among the subjects equally. Each 2-channel and 4-channel session was evaluated by 18 subjects, and each 8-channel session by 9 subjects. The last row shows the total number of sessions for each subject varying between 19 and 27.

Figure 7

All sessions used in the experiment. M denotes mandatory sessions and C is for continuous signal. The first number in each cell corresponds to the frequency, followed by the length of the bursts and pauses in milliseconds. Colored characters help orientation in the table by highlighting the same signal. In sessions 33 to 41, P1 to P5 are the patterns based on Figure 2.

Figure 8

A bone conduction headphone using the bones in the jaw and upper cheek to transmit vibrations directly into the inner ear during playback (left) and Mutalk, the latest development for the Metaverse: a microphone with a mute function that prevents one’s voice from leaking out (right). Using a helmet instead of glasses will not leave space for further devices to be attached to the head.

Figure 9

Vest-like feedback device with 4 × 30 vibrators [35,65].