Generating Airborne Ultrasonic Amplitude Patterns Using an Open Hardware Phased Array

Abstract

Holographic methods from optics can be adapted to acoustics for enabling novel applications in particle manipulation or patterning by generating dynamic custom-tailored acoustic fields. Here, we present three contributions towards making the field of acoustic holography more widespread. Firstly, we introduce an iterative algorithm that accurately calculates the amplitudes and phases of an array of ultrasound emitters in order to create a target amplitude field in mid-air. Secondly, we use the algorithm to analyse the impact of spatial, amplitude and phase emission resolution on the resulting acoustic field, thus providing engineering insights towards array design. For example, we show an onset of diminishing returns for smaller than a quarter-wavelength sized emitters and a phase and amplitude resolution of eight and four divisions per period, respectively. Lastly, we present a hardware platform for the generation of acoustic holograms. The array is integrated in a single board composed of 256 emitters operating at 40 kHz. We hope that the results and procedures described within this paper enable researchers to build their own ultrasonic arrays and explore novel applications of ultrasonic holograms.

OptiTrap: Optimal Trap Trajectories for Acoustic Levitation Displays

“…This setup uses two opposed arrays of 16×16 transducers controlled by an FPGA and an OptiTrack tracking system (Prime 13 motion capture system at a frequency of 240Hz). The design of the arrays is a reproduction of the setup in [Morales et al 2021], modified to operate at 20Vpp and higher update rates of 10kHz. The device generates a single twintrap using the method described in [Hirayama et al 2019], allowing for vertical and horizontal forces of 4.2 • 10 −5 N and 2.1 • 10 −5 N, respectively, experimentally computed using the linear speed tests in [Hirayama et al 2019] (i.e., 10cm paths, binary search with 9 out 10 success ratios, with particle mass 𝑚 ≈ 0.7 • 10 −7 kg).…”

Mid-Air Haptics: Future Challenges and Opportunities

“…Initially, the acoustic focus, which forms the basis for mid-air haptics, was amplitude modulated (AM) to create a localized vibrotactile sensation, while later the focus was moved around in space to create small lateral modulations (LM) [8] or to trace out larger tactile shapes using so-called Spatio-temporal modulation (STM) [9]. Techniques that use acoustic holography [10], multiple focal points [11], or a blend of AM and STM [12] have also emerged and appear to be more suitable at delivering different haptic sensations in different settings. However, none of these modulation techniques adapt or take into consideration the heterogeneity of the human skin, the density, and types of mechanoreceptors being targeted, nor any effects of wave interference on the skin surface [13].…”

Theoretical Zero-Thickness Broadband Holograms Based on Acoustic Sieve Metasurfaces

“…If the desired image size is set too large, the paraxial approximation will not be satisfied or the information content [21] of the hologram will be less than that of the image. Therefore, in general, for acoustic holographic imaging, the desired image size is set to be equal to or similar to the hologram size in most research [3,[15][16][17][18][19][21][22][23][24][25][26][27][28].…”

“…Due to its powerful and flexible capability of arbitrary sound beam shaping and predesigned sound field reconstruction, acoustic holography has recently received increased attention in many interdisciplinary fields related to acoustics, including medicine [3][4][5][6], engineering [7,8], and biology [9][10][11], in addition to the pure acoustic field. Many applications have been made possible by acoustic holography, including acoustic fabrication [7], beam shaping [12][13][14][15][16][17][18][19][20], acoustic holographic imaging [3,[15][16][17][18][19][20][21][22][23][24][25][26][27][28], volumetric display [29,30], volumetric haptics [31,32], particle manipulation [11,21,26,33,34], 3D ultrasound imaging [8], cell manipulation [9,10], characterizing medical ultra...…”

“…The algorithms used include the iterative angular spectrum approach [21], the iterative backpropagation algorithm [33], the weighted Gerchberg-Saxton algorithm [20], the pseudo-inverse method [36], the conjugate field method [37], the deep learning method [35], and so on. Physically, these calculated maps are realized by either discrete and independently driven elements of active phasedarray emitters [3][4][5][27][28][29][30][31][32][33][34] (such as loudspeakers in the air or piezoelectric transducers in 2 of 13 water) or meta-pixels of passive acoustic metasurfaces [6][7][8][9][12][13][14][15][16][17][18][19][20][21][22][23][24][25]38], or a combination of both [26]. In the reconstruction process, these acoustic holographic devices with the required phase and/or amplitude distributions diffract sound waves to form the desired real image in the pre-designed image plane.…”