New-Tech Europe | Q2 2020 | Digital Edition

Alternatively, non-contact approaches are being explored, where ultrasonic waves generate a local pressure field causing the user to feel a light sensation. This ‘mid-air haptics’ allows users to interact with objects without touching them. Or to interact with consumer applications in a more hygienic way – a measure which is receiving great attention due to the recent COVID-19 outbreak. This mid-air haptic feedback can be combined with gesture recognition techniques. This way, the system can follow and track your hand, and beam back to the precise position of your fingers. Think about remote surgery, where the combination of the two techniques can give the surgeon a precise ‘feeling’ of what he touches from a distance. Mid-air haptic feedback: how does it work? To turn ultrasound into touch, you need an array of multiple ultrasound sources. Each of these ‘drums’ launches acoustic energy in all directions, like a spherical wave going out. The energy of one source generates the same feeling at different positions around the sphere and can give an imprecise user interaction. But by properly organizing phase shifts between the different sources, ultrasound energy can be focused on one or more spots, while being cancelled at other spots. This way, the ultrasound energy cloud can be shaped in three dimensions, like a full acoustic hologram. Ultrasound energy with frequencies above 20kHz can however not be ‘felt’ by our finger receptors. In a next step, the frequency of the acoustic hologram needs to be down-modulated to arrive at frequencies below 500Hz in order to be perceived by our fingers.

Figure 1: Mid-air haptics allows users to interact with smart systems without touching them.

oscillate at the same frequency and produce ultrasound. The thickness of the piezoelectric elements determines its dominant resonant frequency. First applications making use of these classical ultrasound transducers (or UTs) have already been commercialized, but since they are thick and bulky, the frequency is limited to about 40kHz, impeding a precise interaction with the user. During the last decade, the idea of using micromachined ultrasound transducers (or MUTs) has become very attractive. MUTs use miniaturized MEMS-based structures for emitting the ultrasound energy. Having a smaller dimension, a wider frequency range and a larger integration potential, these MUTs promise to outperform classical ultrasound transducers. They favour the development of large 2D arrays of transducers and enable close integration with the supporting electronics. Two families of MUTs have been proposed: the piezoelectric MUT or pMUT – which is the micromachined equivalent of the classical piezoelectric transducer – and the capacitive MUT or cMUT. In the latter solution, electrostatic forces cause vibration of a membrane that is part of a parallel plate capacitor.

The precision with which we feel the acoustic hologram depends on the carrier frequency of the ultrasound wave. The higher this frequency, the finer the interaction with our finger receptors. For example, for a carrier frequency of 40kHz, the resolution of the acoustic hologram is in the centimetre range. This improves to millimetre range when the carrier frequency is in the MHz range. At these higher frequencies, however, ultrasound waves are more easily absorbed in air. When trading off resolution with absorption, it turns out that frequencies around 400kHz are most interesting for mid-air haptic feedback sensations at about 1cm distance from the source. From bulky to micromachined ultrasound transducers On the technology side, haptic feedback can be generated by ultrasound transducers which convert electrical signals (AC voltage) into ultrasound (and vice versa). So far, first implementations have relied on classical piezoelectric transducers, using bulky ceramic piezoelectric elements. These elements change size and shape when a voltage is applied. AC voltage makes them

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