Touch, MultiTouch and something else

MultiTouch is a rapidly gaining popularity technology that allows devices such as touch panels and screens to recognize multiple touches simultaneously. Many consider it the technology of the future, which will bring the user interface to a new level of interaction. The first mass mutlitouch device, was the Apple IPhone, released in 2007.
But the iPhone is not the only multi-touch alive, in this article I want to talk about the different approaches to the implementation of Touch interfaces from the technical side, but without delving into super-complex and incomprehensible to the common man nuances.

A bit of history

But not Apple invented the MultiTouch, work in this direction has been going on since the mid-80s. In 1984, Bell Labs developed a multi-touch screen on which images could be manipulated with more than one hand, but this development did not see the future.
In 1999, FingerWorks released several MultiTouch devices, such as the iGesture Pad's external gesture touchpad and the highly exotic TouchStream keyboard. After years of being in their own product niche, FingerWorks was acquired by Apple.

Technological side

For a simple user, everything you can poke with a stylus with your finger is a touch, and then you can poke it with several fingers at the same time - a multi-touch . But from a technical point of view, there are many completely different approaches to the implementation of touch surfaces.
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Resistive technology

The technology is relatively old, and becoming already "unfashionable." The touchpad consists essentially of two parts: a flexible layer on top (thin polyester film , etc. ), and a bottom rigid layer (usually glass), both layers are conductors and have electrical resistance. They are separated by air or another dielectric. Finger touching the top layer, bends it so that it contacts with the bottom lining. The voltage at the point of contact is measured and the coordinates of the touch are calculated.
Surfaces can be transparent and used in touchscreens , PDAs , etc. For greater accuracy, you can use the stylus, or any stylus-like object.

This technology has many disadvantages, such as low sensitivity, mechanical wear, relatively large physical dimensions, susceptibility to accidental touches.
But Nokia, using resistive technology from its touch concepts and the new model 5800, achieved tactile feedback from the screen. That is , it feels like pressing a button on such a screen is similar to pressing a real physical button.

Capacitive technology

Mutual capacity

The sensitive surface is applied like ordinary printed circuit boards. It consists of an upper insulating layer and a grid of conductor tracks below it. When a finger touches the surface, it changes the capacitance between adjacent tracks, based on which it is possible to calculate its position and area of ​​contact.

This approach is more resistive, more durable due to the lack of moving parts, more accurate, and thanks to manufacturing technology, can be applied to almost any surface. Most modern manufacturers (Synaptics, Apple, others) use it. Conductor tracks can be made of transparent materials, which also allows the use of capacitive technology in touch-screens . Also, the technology allows you to recognize several touches at the same time, but only responds to human fingers (and other objects with similar electrical characteristics). The use of classic styluses, or improvised means in their quality - is impossible, but there are special styluses on the market, in effect imitating the properties of a human finger.

Own capacity

The technology is similar to the previous one, but instead of two separate layers of conductors, one layer of electrodes connected to a capacitive-sensitive circuit is used.

The technology may have a higher accuracy compared to mutually capacitive , but it is poorly applicable for devices with a large touch-surface area due to the high susceptibility to parasitic capacitances appearing on the electrodes themselves.
In the iPhone, according to unconfirmed data, both approaches are used.
The DuoSense ™ technology introduced by N-Trig for Microsoft products is essentially the same mutual capacitive technology, with support for multi-touch and special styluses (Apple products, however, too.).

Other approaches to MultiTouch

Dispersion of total internal reflection

This approach with a very abstruse name, is very promising for creating MultiTouch-surfaces of a large area.
An image is projected onto an acrylic screen using projectors located on the opposite side of the user. When an object touches the surface, the light emitted by the LEDs placed around the screen perimeter is scattered at the point of contact and hits the reflection sensors, which are used to calculate the location of the touch point using software methods.

Infrared light propagates inside the acrylic, not going outside, but if the finger touches the surface, the light scatters from it and hits the sensors. The use of flexible material to cover the screen will allow you to measure the force of pressure, watching the change in the nature of the dispersion of rays.

Microsoft Surface uses a similar approach, but the infrared light source is located next to the projector, and the refractions are recognized by infrared cameras.

Other, less common technologies

Infrared positioning

There are two fundamentally different methods:

The first

relies on the change in electrical resistance of the surface after a change in temperature. This method is inaccurate, slow and requires warm hands.

Second

consists in placing a grid of infrared emitters and sensors around the perimeter, in front of the screen. When touched, the object shields the rays with it and thus makes it possible to determine the position. This method is used in military applications requiring a touch panel.

Recognition of surface acoustic waves

Ultrasonic waves propagate along the touch-surface , when touched by an external object, they are distorted and the touch position is calculated on this basis.

Tensiometers

Tensiometer - a device for determining mechanical deformations in a solid and converting them into electrical impulses. When combining such a device with a screen, an inexpensive, inaccurate, but durable and reliable touchscreen is obtained. Used in ticket vending machines , account replenishers , etc. because of its vandal resistance.

Optical recognition

Relatively new and promising approach. Two or more optical sensors are placed at the edges of the screen (usually at the corners). From opposite sides, in the zone of visibility of the cameras - infrared illumination. When a third-party body is in contact with the screen, a shadow appears from the backlight, the cameras shoot it from different angles. The information is mapped, and allows you to calculate the location.

Signal dispersion

The touch point is calculated by sensors with respect to vibrations and deformations in the glass. It is claimed that this technology is resistant to dust, dirt and scratches. Also, due to the small number of additional devices, it can be used on ordinary glass (windows, countertops , etc. ). But since mechanical vibrations are monitored, this system cannot identify an immovable object.

Acoustic Recognition

The technology is similar to the signal dispersion method, but mechanical vibrations are converted into an audio signal and compared with the prerecorded profiles of each point on the screen. The advantages and disadvantages are the same as in the dispersion method.

Pixel touchscreen displays

Sharp has announced ultra-sensitive Touch-screens. The essence of the novelty is that each pixel of the screen includes an optical sensor. Thus, there is no need for additional coverage on top of the screen, and a cleaner image can be achieved compared to other touch displays. Such a screen can track the touch, movement, as well as be used as a scanner.
(This info was provided by krmolot )

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The text is collected from various sources, written with bare hands in PyRoom , marked up in Vim and finally typed in Typographer .
^{I decided to zakosit advertising at the end of the post :)}