Noncontact Thermal and Vibrotactile Display Using Focused Airborne Ultrasound
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In a typical mid-air haptics system, focused airborne ultrasound provides vibrotactile sensations to localized areas on bare skin. Herein, a method for displaying heat sensations to hands where gloves are worn is proposed. The gloves employed in this study are commercially available gloves with sound absorption characteristics, such as cotton work gloves without any additional devices such as Peltier elements. The method proposed in this study can also provide vibrotactile sensations by changing the ultrasonic irradiation pattern. In this paper, we report basic experimental investigations on the proposed method. By performing thermal measurements, we evaluate the local heat generation on the surfaces of both the glove and the skin by focused airborne ultrasound irradiation. In addition, we performed perceptual experiments, thereby confirming that the proposed method produced both heat and vibrotactile sensations. Furthermore, these sensations were selectively provided to a certain extent by changing the ultrasonic irradiation pattern. These results validate the effectiveness of our method and its feasibility in mid-air haptics applications.
KeywordsHeat sensation Vibrotactile sensation Airborne ultrasound
Recently, mid-air haptics technologies have attracted substantial interest because they can produce tactile sensations without any physical contact or the need for wearing any devices. Airborne ultrasound phased arrays (AUPAs) are one of the most practical devices in mid-air haptics. They can produce vibrotactile sensations on bare skin based on acoustic radiation pressure [1, 2]. The stimulus area, a focal point generated by AUPAs, can be down to wavelength, i.e., approximately 8.5 mm at 40 kHz (for typical AUPAs). It can be generated at an arbitrary position and controlled electronically. Complex stimulus patterns, such as the shapes of various objects based on multi-foci  and focal points with time-division , can be produced via the proper drive control of the transducers in AUPAs.
Most existing studies that employ AUPAs have developed applications that can only produce vibrotactile sensations. The realization of other types of haptic sensations using AUPAs, in addition to vibrotactile sensations, can expand the range of applications of AUPAs and contribute to the evolution of mid-air haptics technologies. In this paper, we propose a method to produce heat sensations using AUPAs, in addition to producing vibrotactile sensations.
The proposed method requires users to wear gloves to produce heat sensations, whereas existing methods in mid-air haptics do not have such requirements. This is a limitation of the proposed method; however, the gloves that are required in the proposed method are ordinary ones, such as cotton work gloves that absorb ultrasound, without any additional devices such as Peltier elements. Additionally, vibrotactile sensations can be produced by changing ultrasonic irradiation patterns. We can find some practical applications where wearing gloves are acceptable. For example, at numerous factories, workers wear cotton work gloves while they work. Our method can be employed to prevent the inadvertent intrusion of workers’ hands into dangerous zones by imparting a hot sensation as a danger alert. Our method can also be applied to surgery support by improving the surfaces of surgical gloves to absorb sound, where the glove remains disposable and battery-less.
Thermal displays in mid-air haptics, such as methods that employ infrared lasers  and thermal radiation , have been proposed. A method that uses lasers can also display a tactile sensation similar to a mechanical tap when an elastic medium is attached to the skin . A generic comparison of the proposed method with the aforementioned laser-based methods is not straightforward; however, it is certain that the proposed method is the easiest, with no additional cost, with regard to application in a scenario where a worker wearing cotton gloves is already being aided by ultrasound mid-air haptics. A method for providing a cold sensation using AUPAs has been proposed; however, providing warm and hot sensations was beyond the scope of this method .
Herein, we report basic experimental investigations regarding the proposed method. We conducted two kinds of experiments: the first involves temperature measurements on the glove and the skin surface when the glove was exposed to ultrasound. The second is a perceptual experiment for confirming that our method provides heat and vibrotactile sensations and that it can display these sensations selectively by changing the ultrasonic irradiation patterns.
2 Proposed Method
These sensations can be provided selectively by two modes of irradiation: static pressure (SP) mode where constant-amplitude ultrasound is irradiated and amplitude modulation (AM) mode where the ultrasound is modulated at 150 Hz. The acoustic absorption coefficient of the glove and the acoustic power at the focal point determine the temperature of the glove exposed to ultrasound. In SP mode, the ultrasound generates heat on the glove while inducing no vibrotactile sensations. In AM mode, the modulated ultrasound produces vibrotactile sensations. The modulation frequency of 150 Hz is selected so that the mechanoreceptors are excited efficiently . The glove temperature also rises in AM mode. However, it is possible to adjust the amplitude and duration of the ultrasound to produce only vibrotactile sensations without heat sensation. The irradiation duration and amplitude should be adjusted to generate only vibrotactile sensations.
We confirmed the feasibility of the selective stimulation in the following experiments.
3.1 Heat Generation on Glove Surface and Skin Surface
First, we measured the temperature elevation of a cotton work glove that is in contact with a human palm. The glove was exposed to ultrasound, and the temperature was measured both on the surface of the glove and the surface of the palm.
3.2 Distinguishing Vibrotactile and Heat Sensations
Next, we conducted perceptual experiments to confirm whether or not our method can generate both heat and vibrotactile sensations, as well as that can switch the generated sensation by the SP-AM mode alternation.
Average results for 10 participants. The values in bracket are standard errors in each result.
Heat & vibration
In conclusion, the proposed method generated both heat and vibrotactile sensations. Additionally, these sensations were selectively generated to a certain extent by using different ultrasonic radiation patterns.
The perceptual experiment shows that AM mode could provide vibrotactile sensation; however, this mechanism is not obvious. This mechanism has three possibilities, as follows: The first is that the acoustic radiation pressure of ultrasound passing through the mesh of the fabric stimulates skin. The second is that the acoustic radiation pressure vibrates the fabric, which taps skin. The third is that an elastic wave in fabric excited by irradiation ultrasound propagates to the skin. The actual mechanism needs further investigation.
The limit range of displaying heat sensation from the AUPAs is decided by acoustic energy at the focal point. The acoustic energy transmitted from AUPAs is inversely proportional to the square of the distance from AUPAs, assuming that energy attenuation in the air is ignored. The limit range in the setup of this paper is approximately 300 mm assuming that the temperature of the skin surface reaches 40 ℃.
In this paper, we proposed a noncontact method for generating heat and vibrotactile sensations using focused airborne ultrasound. The proposed method provides heat sensations to a localized area by irradiating airborne ultrasound to the surface of a hand wearing a glove having sound absorption characteristics. This method also provides vibrotactile sensations by changing the ultrasonic irradiation pattern. It was confirmed that the proposed method locally provided both heat and vibrotactile sensations. In addition, our method generated these sensations selectively to a certain extent by varying the ultrasonic radiation pattern.
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