Ultrahaptics’ core mid-air, haptic technology uses ultrasound (i.e. frequencies beyond the range of human hearing) to project tactile sensation directly onto the hands. Ultrahaptics has developed and patented algorithms that realise the required ultrasonic waves and modulation techniques needed to do this, using relatively little computing power and enabling our unique, haptic technology to run on readily accessible hardware.
Ultrahaptics uses phased arrays of ultrasonic speakers to transmit waves timed to coincide at a point in space.
How does the hand feel invisible, mid-air sensations?
A continuous, unchanging wave creates a focal point at a frequency that is too fast to feel or perceive as sensation. To create a haptic point that you can feel, the focal point must change, either by pulsing its intensity – Amplitude Modulation – or moving it – Spatiotemporal Modulation. As an example, the 40kHz ultrasound intensity can be modulated with a much lower frequency sine wave, so that the pressure at the focal point varies. This is shown in the diagram:
The Ultrahaptics array can change the position and strength of this focal point, or Control Point, at a fast-enough rate to create haptic sensation. In addition, multiple control points can be created simultaneously.
The surface of the skin is covered with sensitive nerve endings called mechanoreceptors, that respond to touch, pressure and stretching. High concentrations of mechanoreceptors are on hairless skin, i.e. the palms of the hand and front and sides of the fingers.
In combination with a 3D, hand tracking camera (Ultrahaptics development kits come with a Leap Motion® camera module) we can track the hand and position the control points on the palm or fingers. It is these haptic control points that stimulate the mechanoreceptors and create tactile sensation. More complex shapes, like lines and circles, can be made by rapidly moving the control point or by using multiple control points.
Amplitude Modulation and Spatiotemporal Modulation
The example above shows how a control point’s intensity can be modulated using a sine wave envelope. This is called Amplitude Modulation . Amplitude Modulation is not the only way to create a haptic sensation. Another is to keep the control point intensity constant and move it rapidly across the surface of the skin. Because the control point is moving over each point on a path, a bit like the bow wave of a ship, the pressure is effectively modulated and can be felt. This is called Spatiotemporal Modulation.
Spatiotemporal Modulation takes our haptic technology to the next level, making it possible to render increasingly complex shapes and other effects in mid-air.
You can see both techniques illustrated in the example where a circle sensation is created using Amplitude Modulation (a) and Spatiotemporal Modulation (b). With Amplitude Modulation we use groups of control points, pulsed around the circle; with Spatiotemporal Modulation, one control point moves around the circle fast enough to create sensation.
Projecting ultrasound onto an oil bath allows you to ‘see’ the shapes created by Ultrahaptics’ haptic technology and the difference between Amplitude Modulation and Spatiotemporal Modulation.
Here you can see the discrete focal points created by amplitude modulation.
Here you can see how Spatiotemporal Modulation enables you to render shapes.
NB: Spatiotemporal modulation is not supported by the TOUCH Development Kit (UHDK5).
The Ultrahaptics Solver
At the heart of the Ultrahaptics technology is an algorithm used to calculate the strength and phase of each transducer for all control points. When multiple control points are used, their fields can destructively interfere with each other and reduce their strength. Since the phase of the 40 kHz ultrasound signal is unimportant to the haptic sensation, the Ultrahaptics technology uses a solver algorithm to optimise the phases of each control point so that fields constructively interfere as much as possible to give the strongest sensations.