Through nonlinear effects, airborne ultrasound phased arrays enable mid-air tactile presentations, as well as auditory presentation and acoustic levitation. To create workplaces flexibly, we have developed a scalable phased array system in which multiple modules can be connected via Ethernet cables and controlled from a PC or other host device. Each module has 249 transducers and the software used can individually specify the phase and amplitude of each of the connected transducers. Using EtherCAT for communication, the system achieves high accuracy synchronization among the connected modules. In this article, we describe the details of the hardware and software architecture of the developed system and evaluate it. We experimentally confirmed the synchronization of 20 modules within an accuracy of $0.1~\mathrm{{\mu }s}$ and the phase and amplitude can be specified at $8~\mathrm{bits}$ resolution. In addition, using nine modules, we confirmed that we could make a focal point of the size consistent with the theory at $500~\mathrm{mm}$ above the array surface.

Ultrasound emitted from an array of transducers can produce various tactile sensations by temporally controlling the phase and amplitude of the transducers. However, the controllability in haptic applications has not been well examined. This study clarifies a phase shift of the driving signal causes amplitude fluctuation of emitted ultrasound, even under a constant driving amplitude. We demonstrate theoretically that this problem exists in general resonant systems with various quality factors, and point out that it produces radiation force decrease and audible noise. We also show a method to reduce the fluctuation and quantitatively evaluate the effectiveness. The results provide the measure of the displayed force fluctuation by a fast focus movement and enable silent haptic stimulation.

Recently, the applications of airborne ultrasound technology have garnered considerable attention; however, high-speed ultrasound field measurements in air have not been well researched. This study demonstrates that thermal responses to high-intensity airborne ultrasound can be used to visualize airborne ultrasound fields. Thermal images on a mesh screen enable the rapid quantitative measurement of two-dimensional ultrasound fields. Additionally, based on the temperature distribution on a surface that reflects ultrasound waves, it is possible to determine the ultrasound focus position with a resolution much finer than the wavelength. The results demonstrate that thermography observations based on ultrasound-temperature conversion, which are conventionally time-consuming or difficult to perform, can be used to visualize acoustic fields within 0.5 s. These findings will advance engineering applications that use strong airborne ultrasound, such as midair haptics and acoustic levitation, and provide scientific and technical applications for detecting surface properties and composition, including those of cells and fragile materials.

Non-contact tactile presentation using ultrasound phased arrays is becoming a powerful method for providing haptic feedback on bare skin without restricting the user's movement. In such ultrasonic mid-air haptics, it is often necessary to generate multiple ultrasonic foci simultaneously, which requires solving the inverse problem of amplitudes and phases of the transducers in a phased array. Conventionally, matrix calculation methods have been used to solve this inverse problem. However, a matrix calculation requires a non-negligible amount of time when the number of control points and the number of transducers in the array are large. In this article, we propose a simple method based on a greedy algorithm and brute-force search to solve the field reconstruction problem. The proposed method directly optimizes the desired field without matrix calculation or target field phase optimization. The empirical results indicate that the proposed method can reproduce the target sound with an accuracy of more than 80%.

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.