Tech Briefs

These head-mounted transducers are used to aid soldiers in orientation, navigation, and communication.

T he human sensory systems most frequently used in military displays (vision and audition) are sometimes overloaded. When soldiers rely on visual or auditory displays, their situation awareness can be degraded due to the need to attend to the display. A proposed alternative sensory channel for soldier displays is the tactile channel. In military applications, VT displays are primarily being used to aid in orientation, navigation, and communication. Tactile displays can also be used to impart more abstract information such as in command-and-control applications.

Figure 1. A side view of the Tactor Locations. The study’s right and left side sites are in green and the midline sites are in yellow (shown in both drawings).
By presenting information to more than one sensory channel, performance can be improved over using any single channel in isolation. Tactors can be mounted on the head with a dedicated harness or it might be feasible (if acceptable to the appropriate system proponent) to integrate them into headgear already worn, such as helmets or caps.

Prior work investigated and obtained perception thresholds for tactors mounted at seven sites on the head. The sites used were from the set used for electroencephalogram (EEG) recordings. The selected sites were F3, Cz, Pz, O2, T3, T4, and F8 (see Figure 1). Even-numbered sites are right-hemisphere sites, and odd-numbered sites are left-hemisphere sites. Sites with a “z” are midline sites. In Figure 1, the study’s right and left side sites are in green and the midline sites are in yellow (shown in both drawings). The sites were chosen so that all major non-facial skull bone regions (frontal, parietal, occipital, and temporal bones) were represented.

A pilot study determined that at frequencies above 64 Hz, a bone-conducted auditory signal was present in addition to the desired tactile signal. In order to isolate the tactile effects, thresholds were obtained for 32, 45, and 63 Hz, where these auditory signals were absent.

Results of the threshold measurements were that locations Pz, O2, and T3 had significantly lower thresholds than Cz, F3, and F8 (T4 was not significantly different than either group). Also, thresholds at 45 and 63 Hz were significantly higher than 32 Hz. To build on this earlier study’s results, the same seven tactor locations and three frequencies (with substitution of 63.5 Hz for 63 Hz for instrumentation reasons) were used in this localization study.

Seven C-2 tactors were used to generate the VT stimuli. With C-2 tactors, both vibration amplitude and frequency can be controlled. The C-2 tactor is 1.2" in diameter, 0.31" thick, and weighs 17 grams. A piston of 0.3" in the center of the tactor housing moves perpendicularly to the plane of the tactor housing to create the VT signal.

Figure 2. The “Halo” Experimental Fixture used an Advanced Combat Helmet shell as a base.
The tactors were mounted in an experimental “halo” fixture (see Figure 2). The halo was created using a size large Advanced Combat Helmet shell as a base. The entire halo assembly was counterweighted via a weight suspended with a cord through an overhead pulley so that less than 1 oz of weight was applied to the head. This was done so that the weight of the helmet did not cause greater contact pressure for the top tactors.

The response methodology entailed the subject touching a mannequin head at the analogous location they perceived the origin of the tactile signal on their head. The mannequin was marked with duct tape at the seven tactor locations and positioned in front of the participant. Manual data collection was used to record the location indicated by the subject. The tactor stimulation was designed to present a rapid “tap-tap-tap” sensation to the subject; the “taps” were presented at one of the experimental frequencies (32, 45, and 63.5 Hz). Tactor activation was controlled by computer.

Based on the results of this study, and the prior threshold results, the circumferential sites of O2, T3, and T4 have promise for a head-mounted tactile display. The use of 32 Hz as a tactor excitation frequency would be preferred based on its lower detection threshold. A display employing circumferential tactors would allow for information flow such as cueing the location of snipers or providing navigational instructions. The ability to determine active tactor location could also be used to impart abstract information by, for example, activating tactors in certain patterns or combinations.

This work was done by Mary S. Binseel and Joel T. Kalb of the ARL’s Human Research and Engineering Directorate. ARL-0154

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Localization of Head-Mounted Vibrotactile Transducers (reference ARL-0154) is currently available for download from the TSP library.

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