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  1. Free, publicly-accessible full text available July 1, 2024
  2. Free, publicly-accessible full text available October 1, 2024
  3. Despite the ubiquitous presence of tactile actuators (tactors) in mobile devices, there is a continuing need for more advanced tactors that can cover the entire frequency range of human tactile perception. Broadband tactors can increase information transmission and enrich sensory experience. The engineering challenges are multifold in that the ideal tactors should exhibit an effective bandwidth of at least 300 Hz, small form factor, robustness, power efficiency and low cost. For wearable applications, there are the additional challenges of ease of mounting and maintaining adequate skin contact during body movements. We propose an approach to interleave narrowband tactile stimuli to achieve broadband effects, taking advantage of the limited spatial resolution of the skin on the torso and limbs. Three psychophysical experiments were conducted to assess the validity of this approach. Participants performed pairwise discriminations of two broadband stimuli delivered using one or two tactors. The broadband stimuli consisted of one mid-frequency and one high-frequency component delivered through one tactor by mixing the two components, or through two tactors (one component per tactor). The first two experiments revealed extraneous cues such as localization and mutual masking of mid- and high-frequency components that were subsequently eliminated in the third experiment. Results from 12 participants confirmed that performance on pairwise comparisons was below the discrimination threshold, confirming that broadband haptic effects can be achieved through narrowband tactors placed within the skin’s two-point limen. 
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  4. Free, publicly-accessible full text available October 1, 2024
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  6. Stand-alone devices for tactile speech reception serve a need as communication aids for persons with profound sensory impairments as well as in applications such as human-computer interfaces and remote communication when the normal auditory and visual channels are compromised or overloaded. The current research is concerned with perceptual evaluations of a phoneme-based tactile speech communication device in which a unique tactile code was assigned to each of the 24 consonants and 15 vowels of English. The tactile phonemic display was conveyed through an array of 24 tactors that stimulated the dorsal and ventral surfaces of the forearm. Experiments examined the recognition of individual words as a function of the inter-phoneme interval (Study 1) and two-word phrases as a function of the inter-word interval (Study 2). Following an average training period of 4.3 hrs on phoneme and word recognition tasks, mean scores for the recognition of individual words in Study 1 ranged from 87.7% correct to 74.3% correct as the inter-phoneme interval decreased from 300 to 0 ms. In Study 2, following an average of 2.5 hours of training on the two-word phrase task, both words in the phrase were identified with an accuracy of 75% correct using an inter-word interval of 1 sec and an inter-phoneme interval of 150 ms. Effective transmission rates achieved on this task were estimated to be on the order of 30 to 35 words/min. 
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  7. Current wearable haptic display technology is limited by the lack of broadband tactors capable of delivering rich haptic effects across the entire perceptible frequency range. Audio speakers are often used in laboratory studies as broadband tactors, but it is difficult to attach them to skin and maintain contact during movement. Commercially-available narrowband tactors are small, low in cost and power efficient. We investigate the idea of interleaving narrowband tactile stimuli to achieve broadband effects. Twelve participants performed pairwise discrimination of two stimulus alternatives using two broadband tactors. One alternative was a broadband vibration composed of the sum of a mid- and a high-frequency vibration, delivered by a single tactor. The other alternative consisted of the mid-frequency component delivered by one tactor and the high-frequency by the other. The upper arm was chosen for stimulation because the tactors can be placed within the two-point limen of the skin. The sensitivity index results were significantly below 1.0, the criterion for discrimination threshold, thereby confirming that broadband haptic effects can be achieved by placing narrowband tactors with mid and high resonant frequencies within the skin’s spatial resolution. We provide guidelines and examples of applying our findings to the design of wearable haptic displays. 
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