UltLever: Ultrasound-Driven Passive Haptic Actuator Based on Amplifying Radiation Force Using a Simple Lever Mechanism.

A lightweight haptic display that does not interfere with the user's natural movement is required for an immersive haptic experience. This study proposes a lightweight, powerful, and responsive passive haptic actuator driven by airborne focused ultrasound. This 6.2 g completely plastic passive device amplifies an applied ultrasound radiation force by a factor of 35 using a simple lever mechanism, presenting an amplified force of 0.7 N to the user's finger pad. 2-30 Hz vibration can also be presented. Since the radiation force is presented at the speed of sound, the amplified force is presented at high speed even with the high amplification rate of a lever, achieving such strong force and vibration presentation. Physical measurements showed that the amplified force was 0.7 N for the 20 mN input radiation force, and the amplitude of the presented vibration was over 0.1 N at 2-30 Hz. A psychophysical experiment showed that the vibration and force were perceivable with a device output level of -7.7 dB. In the future, we will explore methodologies around device design to present desired tactile sensations.

Noncontact Haptic Rendering of Static Contact With Convex Surface Using Circular Movement of Ultrasound Focus on a Finger Pad.

A noncontact tactile stimulus can be presented by focusing airborne ultrasound on the human skin. Focused ultrasound has recently been reported to produce not only vibration but also static pressure sensation on the palm by modulating the sound pressure distribution at a low frequency. This finding expands the potential for tactile rendering in ultrasound haptics as static pressure sensation is perceived with a high spatial resolution. In this study, we verified that focused ultrasound can render a static pressure sensation associated with contact with a small convex surface on a finger pad. This static contact rendering enables noncontact tactile reproduction of a fine uneven surface using ultrasound. In the experiments, four ultrasound foci were simultaneously and circularly rotated on a finger pad at 5 Hz. When the orbit radius was 3 mm, vibration and focal movements were barely perceptible, and the stimulus was perceived as static pressure. Moreover, under the condition, the pressure sensation rendered a contact with a small convex surface with a radius of 2 mm. The perceived intensity of the static contact sensation was equivalent to a physical contact force of 0.24 N on average, 10.9 times the radiation force physically applied to the skin.

Sufficient Time-Frequency Resolution for Reproducing Vibrotactile Sensation.

In vibrotactile stimuli, it is essential to reproduce realistic tactile sensations to enhance the immersiveness of applications. To reproduce more realistic tactile experiences, various tools have been proposed to fine-tune and design vibrotactile sensations. Considering the situation where users adjust parameters manually, providing tactile sensations with fewer parameters is desirable. This study examines the coarsest resolution in the time and frequency dimensions necessary to present tactile sensations as realistic as vibrations recorded by the sensor. Time and frequency are fundamental parameters to express vibrations as a spectrogram, and we considered it important to investigate how much coarser the resolution could be without changing perception. We focus on the textural vibrations and the preliminary experiment compared actual texture vibrations with the reconstructed vibration as coarse as possible in the frequency dimension. The result showed that the frequency resolution above 172 Hz makes it difficult to distinguish between the vibrations. The main experiment, a similar discrimination experiment, verified the time resolution using the averaging filter of vibration intensity over time. The results indicate that with the update interval set to 30 ms, the discrimination rate compared to the original vibration is approximately 60%. This percentage is below the chance level of 75%, indicating that distinguishing between the two is difficult. Based on our experiments, it is necessary to have a frequency resolution of at least 172 Hz and a time resolution that updates intensity at a rate of 30 fps or higher to recreate tactile sensations comparable to actual vibrations.

Sound Pressure Field Reconstruction for Airborne Ultrasound Tactile Display Encountering Obstacles.

Airborne ultrasound tactile display (AUTD) is used to provide non-contact tactile sensations with specific foci sound fields through the optimization of transducer phases. However, most existing optimization approaches are not directly applicable in case of an inhomogeneous medium, such as in the presence of obstacles between the AUTD and objective sound field. Certain methods can perform optimizations by considering the sound-scattering surfaces of the obstacles to compute the transmission matrix, which requires several complex measurements. This study proposed two methods to reconstruct the sound field under an inhomogeneous medium, wherein the need to calculate the impact of the obstacles was eliminated. The two methods are Bayesian optimization and greedy algorithm with brute-force search. Further, the process of the foci field generation was assumed as a black box. The proposed methods require only the pressure intensity at the control point generated by the input phases, discarding the need for transmission matrix in the presence of obstacles. Moreover, these methods offer the advantage of optimization of the phases in the presence of obstacles. This study explains the working of proposed methods in different forms of foci fields encountering obstacles.

Focusing Reflected Ultrasound Using Boundary Element Model for Mid-Air Tactile Presentation.

Ultrasound focusing with curved reflectors has various advantages in mid-air tactile presentation. First, tactile sensations can be presented from various directions without placing a large number of transducers. It also avoids conflicts in the arrangement of transducer arrays with optical sensors and visual displays. Furthermore, the blurring of the focus can be suppressed. We propose a method for focusing reflected ultrasound by solving the boundary integral equation for the sound field on a reflector divided into elements. This method does not require a prior measurement of the response to each transducer at the tactile presentation point, as in the previous method. It enables real-time focusing on arbitrary locations by formulating the relationship between the transducer input and the reflected sound field. This method also enhances the focus intensity by incorporating the tactile presentation's target object into the boundary element model. Numerical simulations and measurements showed that the proposed method could focus ultrasound reflected from a hemispherical dome. A numerical analysis was also performed to determine the region where focus generation with sufficient intensity was possible.

Remote Friction Reduction on Polystyrene Foam Surface by Focused Airborne Ultrasound.

In recent years, various tactile displays having the ability to change their surface friction have been proposed. These displays can express many types of textures and shapes that the materials used for them do not possess. In our study, we found that the ultrasound converged on the surface of polystyrene foam reduces the surface friction. This method has potential applications in disposable and three-dimensional tactile displays. In this study, physical and psychophysical experiments were conducted to verify the effectiveness of the proposed method and to examine the basic conditions under which it is perceived. As a result, we confirmed that the surface friction was reduced on the polystyrene foam, which may be due to the squeeze film effect caused by the external ultrasound excitation of the surface.

Non-Vibratory Pressure Sensation Produced by Ultrasound Focus Moving Laterally and Repetitively With Fine Spatial Step Width.

Focused airborne ultrasound provides various noncontact spatiotemporal pressure patterns on the skin. However, the presentation of static force remains an untouched issue because the static radiation force by ultrasound is too weak for the human hand to perceive. Hence, creatable sensations have been limited to vibrations or some dynamically changing feelings. This study demonstrates that a non-vibratory pressure sensation is presented by low-frequency Lateral Modulation (LM) with a fine spatial step width. LM is a pressure modulation method that moves a single ultrasound focus laterally and repetitively along the skin surface. The produced sensation in this study was not perfectly static, but the vibratory perception contained in the stimulus was significantly suppressed under a condition while maintaining its intense perception. We found the condition was 5 to 15 Hz in the LM frequency with a motion step width of less than 1 mm. In a comparison test in the most vibration-suppressed case, the participants reported 0.21 N as an equivalent force to the LM stimulus, significantly higher than the 0.027 N force physically applied by the ultrasound. The statistical analysis also showed that the step width of the LM had a significant effect on its vibratory sensation but not on the intensity of the evoked pressure sensation.

Spatiotemporal Pinpoint Cooling Sensation Produced by Ultrasound-Driven Mist Vaporization on Skin.

In this study, we achieved a noncontact tactile display that presents a pinpoint and instantaneous cooling sensation on the skin surface with no devices directly in contact with the user's body. We employed ultrasound phased arrays to generate a focused ultrasound, which locally and instantaneously expedites the vaporization of room-temperature water mist floating near the surface of the user's skin, offering a sudden pinpoint cooling sensation. In this article, we describe the physical configuration of the proposed method and show the measurement results, demonstrating how the user's skin surface was cooled. During the experiments, we discovered that a part of the skin exposed to a focused ultrasound within the floating mist was selectively cooled with negligible delay. Our prototype system offers a cooling spot of approximately 15 mm in diameter, which causes a temperature decrease of 4.6 K in 1 s and 3.3 K in the first 0.5 s on a hand situated 500 mm away from the device. Additionally, the ultrasound-driven cooling spot can be controlled on the skin surface, which is felt as a cool moving spot. Such a position-free cooling system with a high spatiotemporal resolution will open the door to unprecedented practical tactile applications.

Remote Friction Reduction on Resonant Film Surface by Airborne Ultrasound.

We propose a film device that can be attached to flat surfaces, including touch panels, to remotely reduce surface friction by irradiating airborne ultrasound. In this article, we present a film-air resonance structure that produces large-amplitude surface vibrations excited by airborne ultrasound. We confirmed via simulation that the surface amplitude increases to a level sufficient to reduce friction at the designed frequency. It was also observed in an experiment using a prototype that the friction between a finger and film surface is sharply reduced, and the surface vibrates with sufficient amplitude when touched with a finger.

Radiation Pressure Field Reconstruction for Ultrasound Midair Haptics by Greedy Algorithm With Brute-Force Search.

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%.

AUTD3: Scalable Airborne Ultrasound Tactile Display.

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 µs and the phase and amplitude can be specified at 8 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 mm above the array surface.

Reducing Amplitude Fluctuation by Gradual Phase Shift in Midair Ultrasound Haptics.

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 article 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.

Tactile Stimulation by Repetitive Lateral Movement of Midair Ultrasound Focus.

We report a new vibrotactile modulation method of midair ultrasound focus, namely, lateral modulation (LM), in which the focus quickly moves along a small cyclic trajectory and provides stronger and clearer vibrotactile stimuli than those by the conventional amplitude modulation (AM) method. Midair ultrasound haptics has an essential technical advantage of offering remote, non-contact, and pinpoint tactile stimuli on device-free bare skin. On the other hand, lack of clarity in the presented vibrotactile sensation has often been pointed out, and until now, an AM focus has been valid only on glabrous skin. Our main scientific contribution of the article is to verify the LM method, with the following experimental findings newly obtained. We confirmed that with the same maximum output amplitude of the ultrasound phased arrays, LM stimuli with circular focal trajectories were sensed stronger than AM stimuli by glabrous skin and hairy skin in a modulation frequency of 10-200 Hz. We also found that the detection threshold in glabrous skin mainly depended on the focal speed, whereas the tendency in hairy skin was different from that. With these results, we discuss a basis of perceptional mechanism that responds to LM stimuli, along with practical aspects of potential applications.

Midair Haptic Pursuit.

In human vision, smooth pursuit eye movement is the basic ability to visually follow a moving object by keeping it at the sight center. In this study, we validate that a human hand has a similar ability to track a midair haptic stimulus, i.e., a human palm exposed to a point vibration by a noncontact ultrasound tactile display can follow the continuous movement of the stimulation point. The experimental results show that the trackable velocity limit is 10 cm/s for motion parallel to the palm, when the initial velocity is zero. This ability of motion tracking by hand can be applied to haptic guidance for visually impaired people or for evacuation navigation, where no devices are needed to be equipped by users.

Acoustical boundary hologram for macroscopic rigid-body levitation.

In previous studies, acoustical levitation in the far-field was limited to particles. Here, this paper proposes the "boundary hologram method," a numerical design technique to generate a static and stable levitation field for macroscopic non-spherical rigid bodies larger than the sound wavelength λ. This paper employs boundary element formulation to approximate the acoustic radiation force and torque applied to a rigid body by discretizing the body surface, which is an explicit function of the transducer's phase and amplitude. Then, the drive of the phased array is numerically optimized to yield an appropriate field that stabilizes the body's position and rotation. In experiments, this paper demonstrates the levitation in air of an expanded polystyrene sphere with a diameter of 3.5 λ and a regular octahedron with diagonal length of 5.9 λ, both located 24 λ from the acoustic elements, by a 40 kHz (λ = 8.5 mm) ultrasonic phased array. This method expands the variety of objects that can be levitated in the far-field of an ultrasonic phased array.

Inter-IC for Wearables (I2We): Power and Data Transfer Over Double-Sided Conductive Textile.

We propose a power and data transfer network on a conductive fabric material based on an existing serial communication protocol, Inter-Integrated Circuit (I2C). We call the proposed network inter-IC for wearables. Continuous dc power and I2C-formatted data are simultaneously transferred to tiny sensor nodes distributed on a double-sided conductive textile. The textile comprises two conductive sides, isolated from each other, and is used as a single planar transmission line. I2C data are transferred along with dc power supply based on frequency division multiplexing. Two carriers are modulated with the clock and the data signals of I2C. A modulation and demodulation circuit is designed such that off-the-shelf I2C-interfaced sensor ICs can be used. The novelty of this paper is that a special filter to enable passive modulation is designed by locating its impedance poles and zeros at the appropriate frequencies. The term "passive modulation" herein implies that the sensor nodes do not generate carrier waves by themselves; instead, they reflect only the externally supplied careers for modulation. The proposed scheme enables the flexible implementation of wearable sensor systems in which multiple off-the-shelf tiny sensors are distributed throughout the system.

Body Sensor Networks Powered by an NFC-Coupled Smartphone in the Pocket.

This paper proposes a body sensor network (BSN) on clothing that is wirelessly powered by a smartphone in a pocket. The network consists of a host device and multiple sensor nodes, which are distributed on a wear and are electrically connected with conductive threads. The smartphone with a built-in near field communication (NFC) feature powers the host, which is fixed at the pocket. These devices are wired to a special cloth embroidered with conductive threads by using a special connector consisting of a pin & socket without one-to-one wiring. In the proposed BSN, the host device and the smartphone are coupled via NFC radio within the pocket. Energy harvesting with NFC radio wave requires maintaining antennas within several centimeters to obtain enough power. Positioning and fixing of the smartphone is required within the pocket. A proposed host device can expand the range of energy harvesting by using multiple antennas and a power aggregation circuit. The experimental results demonstrate the feasibility of the batteryless BSNs system.

Relationship between vection and motion perception in depth.

This study examined the effects of cues to motion in depth - namely, stereoscopic (i.e., changing-disparity cues and interocular velocity differences) and changing-size cues on forward and backward vection. We conducted four experiments in which participants viewed expanding or contracting optical flows with the addition of either or both cues. In Experiment 1, participants reported vection by pressing a button whenever they felt it. After each trial, they also rated the magnitude of the vection (from 0 to 100). In Experiments 2 and 3, the participants rated the perceived velocity and motion-in-depth impression of the flows relative to standard stimuli, respectively. In Experiment 4, the participants rated the perceived depth and distance of the display. We observed enhancements in vection, motion-in-depth impression, and perceived depth and distance when either or both types of cues indicated motion-in-depth, as compared to those when the cues did not (Experiments 1, 3, and 4). The perceived velocity changed with cue conditions only for the high velocity condition (Experiment 2). Correlational analyses showed that the vection can be best explained by the motion-in-depth impression. This was partially supported by the multiple regression analyses. These results indicate that the enhancement of vection caused by cues is related to the impression of motion-in-depth rather than the perceived velocity and perceived three-dimensionality.

Midair Ultrasound Fragrance Rendering.

We propose a system that controls the spatial distribution of odors in an environment by generating electronically steerable ultrasound-driven narrow air flows. The proposed system is designed not only to remotely present a preset fragrance to a user, but also to provide applications that would be conventionally inconceivable, such as: 1) fetching the odor of a generic object placed at a location remote from the user and guiding it to his or her nostrils, or 2) nullifying the odor of an object near a user by carrying it away before it reaches his or her nostrils (Fig. 1). These are all accomplished with an ultrasound-driven air stream serving as an airborne carrier of fragrant substances. The flow originates from a point in midair located away from the ultrasound source and travels while accelerating and maintaining its narrow cross-sectional area. These properties differentiate the flow from conventional jet- or fan-driven flows and contribute to achieving a midair flow. In our system, we employed a phased array of ultrasound transducers so that the traveling direction of the flow could be electronically and instantaneously controlled. In this paper, we describe the physical principle of odor control, the system construction, and experiments conducted to evaluate remote fragrance presentation and fragrance tracking.

Aerial Vibrotactile Display Based on MultiUnit Ultrasound Phased Array.

In this paper, we report on an airborne vibrotactile display with a multiunit ultrasound phased array synthetic aperture. The system generates an ultrasound field with a location-tunable focus in the air, which exerts time-variant acoustic radiation pressure on the user's skin, resulting in perceivable localized vibrotactile stimuli. The paper contains three major new contributions from previous related works. The first is an experimental validation of large-aperture focusing with improved synchronization offering an enlarged workspace in which sufficient acoustic power concentration is guaranteed. From the experiments, it is expected that perceivable vibrotactile focus can be generated 1 m away from a four-unit array system. The second is an experimental evaluation of the presented pressure for producing a broad variety of tactile perception, which shows that the generated ultrasound focus can serve as an vibrotactile actuator that has flat frequency characteristics in the domain of perceptual stimuli. The third is a psychophysical result of the detection threshold curve for sinusoidal stimuli offered by the system. The obtained curve shows similarity with conventionally known results, which have minimum values at approximately 200 Hz.