iPhone 2G was the first mobile phone, the management of which was completely based on interaction with the touch screen. Since its presentation, more than ten years have passed, but many of us still do not know how touchscreen works. But we are faced with this intuitive means of input not only in smartphones but also in ATMs, payment terminals, computers, cars, and airplanes – literally everywhere.
Before touchscreens, the most common interface for entering commands into electronic devices was various keyboards. Although, it seems that they have nothing to do with touchscreens, in fact, how much the touchscreen is similar in principle to the keyboard, can surprise. Let’s look at their device in detail.
The keyboard is a printed circuit board on which several rows of switch-buttons are installed. Regardless of their design, membrane or mechanical, each key presses the same thing. On the computer board, the button closes the electrical circuit, the computer registers the current flow at this point in the circuit, “understands” which key is pressed and performs the corresponding command. In the case of a touchscreen, almost the same thing happens.
Type of Touchscreens: How Touchscreen Works
There are about a dozen different types of touch screens; however, most of these models are either outdated and not used, or are experimental, and are unlikely to ever appear in serial devices. First of all, I will talk about the design of current technologies, those of them that are constantly interacting or at least can face in everyday life.
Resistive touch screen
Resistive touchscreens were invented back in 1970 and had changed little since then. In the displays with such sensors, there is a pair of additional layers of the matrix. However, I will make a reservation, the matrix here is not required at all. The first resistive sensory devices were not screens at all.
The lower sensor layer consists of a glass substrate and is called a resistive layer. It is coated with a transparent metallic coating, a good current transfer, for example, from a semiconductor such as indium-tin oxide. The top layer of the touchscreen, with which the user interacts by pressing the screen, is made of a flexible and elastic membrane. It is called the conducting layer. In the space between the layers an air layer is left, or evenly it is covered with microscopic insulating particles.
At the edges of the sensor layer, four, five or eight electrodes are connected, connecting it with sensors and a microcontroller. The more electrodes, the higher the sensitivity of the resistive touchscreen, because the voltage change on them is constantly monitored.
Here is a screen with resistive touchscreen enabled. So far nothing has happened. The electric current flows freely through the conductive layer, but when the user touches the screen, the membrane bends from above, the insulating particles part, and it touches the lower layer of the touchscreen, comes into contact. This is followed by a change in voltage across all the electrodes of the screen.
The touchscreen controller detects voltage changes and reads the readings from the electrodes. Four, five, eight values and all different. On the difference in the readings between the right and left electrodes, the microcontroller will calculate the X-coordinate of the depression, and by differences in voltage at the top and bottom electrodes, will determine the Y-coordinate and, thus, inform the computer of the point at which the layers of the touch screen layer come into contact.
Resistive touch screens can boast of a long list of shortcomings. So, in principle they are not able to recognize two simultaneous clicks, not to mention a larger number. They behave badly in the cold. Because of the need for an interlayer between the layers of the sensor, the matrices of such screens noticeably lose brightness and contrast, tend to glare in the sun, and generally look noticeably worse. Nevertheless, where image quality plays a secondary role, they continue to be used due to resistance to contamination, the possibility of using gloves and, most importantly, low cost.
Such input tools are universally mounted in inexpensive mass devices, like information terminals in public places and are still found in obsolete gadgets, such as cheap MP3-players.
Infrared touch screen
The next, much less common, but an actual variant of the touch screen is the infrared touchscreen. It has nothing to do with the resistive sensor, although it performs similar functions.
The infrared touchscreen is constructed from arrays of light-emitting diodes and photosensitive photocells located on opposite sides of the screen. LEDs illuminate the screen surface with invisible infrared light, forming on it something like a web or a grid. It resembles a burglar alarm, as it is shown in spy insurgents or computer games.
When something touches the screen, it does not matter whether it’s a finger, a gloved hand, a stylus, or a pencil, two or more beams are interrupted. Photocells fix this event, the touchscreen controller finds out which ones are not receiving infrared light and by their position calculates the area of the screen in which the obstacle has arisen. The rest – to compare the touch with what element of the interface is on the screen in this place – the task of the software.
Today, infrared touch screens can be found in those gadgets whose screens have a non-standard design, where adding additional touch layers is technically difficult or impractical – in e-books based on E-link displays, for example, the Amazon Kindle Touch and Sony Ebook. In addition, the devices with similar sensors due to simplicity and maintainability attracted the military.
Capacitive Touch Screen
If the computer registers a change in conductivity in the resistive touchscreens, which follows the screen pressing, directly between the sensor layers, then the capacitive sensors fix the touch directly.
The human body, the skin are good conductors of electricity and have an electric charge. Usually, this is noticed walking on a wool carpet or taking off your favorite sweater, and then touching something metallic. We are all familiar with static electricity, have experienced its effect on ourselves and have seen tiny sparks breaking off our fingers in the dark. A weaker, inconspicuous exchange of electrons between the human body and various conducting surfaces occurs constantly, and it is this capacitive screen that fixes it.
The first such touchscreens were called surface-capacitive and were the logical development of resistive sensors. In them, only one conductive layer, similar to the one used previously, was installed directly on top of the screen. Sensitive electrodes were also attached to it, this time around the corners of the touch panel.
Sensors tracking the voltage on the electrodes and their software were made much more sensitive and could now detect the slightest changes in the current flow across the screen. When a finger (another conductive object, such as a stylus) touches a surface with a capacitive touchscreen, the conductive layer immediately begins to exchange electrons with it, and the microcontroller notices it.
The appearance of surface-capacitive touchscreens was a breakthrough; however, because the conductive layer applied directly over the glass was easily damaged, they were not suitable for new generation devices.
To create the first iPhone, capacitive sensors were required. This type of touchscreen quickly became the most common in modern consumer electronics: smartphones, tablets, laptops, monoblocks and other household devices.
The top layer of the screen with a touchscreen of this type performs a protective function and can be made of tempered glass, for example, the famous Gorilla Glass. Below are the finest electrodes that form the grid. At first, they were superimposed on each other in two layers, then to reduce the thickness of the screen, they were placed on one level.
It made of semiconductor materials, including the indium-tin oxide already mentioned, these current-carrying hairs create an electrostatic field at their intersections.
When the finger touches the glass, due to the electrically conductive properties of the skin, it distorts the local electric field at the places of the nearest intersections of the electrodes. This distortion can be measured as a change in capacitance at a single point in the grid.
Since the array of electrodes is sufficiently small and dense, such a system can track the touch very accurately and without difficulty catches several touches at once. In addition, the absence of additional layers and layers in the sandwich from the matrix, sensor and protective glass positively affects the image quality. However, for the same reason, broken screens, as a rule, are completely replaced. Once assembled, the screen with a projection-capacitive sensor is extremely difficult to repair.
Now the advantages of the projection-capacitive touchscreens do not sound like something amazing, but at the time of the presentation of the iPhone, they provided the technology with enormous success, despite the objective disadvantages – sensitivity to pollution and humidity.
Touch sensitive touch screens – 3D Touch
The ideological predecessor of pressure-sensitive touchscreens was Apple’s proprietary technology called Force Touch, used in the company’s smartwatches, the MacBook, MacBook Pro and Magic Trackpad 2.
After testing the interface solutions and various scenarios for using the recognition of the force of pressing on these devices, Apple began the introduction of a similar solution in their smartphones. In the iPhone 6 and 6s Plus, the recognition and measurement of pressure became one of the touchscreen functions and was given the commercial name 3D Touch.
Although Apple did not hide the fact that the new technology only modifies the capacitive sensors that we are familiar with and even showed a scheme explaining the principle of its operation in general terms, details about the device of touch screens with 3D Touch appeared only after enthusiasts dismantled the first new generation iPhone.
To teach the capacitive touchscreen to recognize the presses and distinguish between several degrees of pressure, the engineers from Cupertino needed to reassemble the touchscreen sandwich. They made changes to its parts and added one more, a new layer to the capacitive one. And, interestingly, doing this, they were clearly inspired by outdated resistive screens.
The grid of capacitive sensors remained unchanged, but it was moved back, closer to the matrix. Between the set of electrical contacts monitoring the place of contact with the display and the protective glass, an additional array of 96 individual sensors was integrated.
His job was not to locate the finger on the iPhone screen. With this, the capacitive touchscreen still perfectly handled. These plates are necessary for detecting and measuring the degree of bending of the protective glass. Apple specifically for the iPhone ordered from Gorilla Glass the development and production of such a protective coating that would maintain the same strength and, at the same time, was flexible enough to allow the screen to react to pressure.
In this development, it was possible to finish the material that tells about touch screens, if not for another technology, which several years ago was predicted for a great future.
Wave touch screens
Unexpectedly, they do not use electricity and do not even have anything to do with light. The Surface Acoustic Wave system uses a surface acoustic wave to propagate along the screen surface to determine the touch point. The ultrasound produced by the piezoelectric elements in the corners is too high for human ears to catch. It spreads back and forth, repeatedly reflected from the edges of the screen. The sound is analyzed for the anomalies created by objects touching the screen.
Disadvantages of wave touch screens are not many. They begin to make mistakes after heavy contamination of the glass and in conditions of strong noise, but, in this case, there are no additional layers that increase the thickness and affect the image quality in screens with such a sensor. All components of the sensor are hidden under the display frame. In addition, wave sensors allow accurate counting of the contact area of the screen with a finger or other object and by this area indirectly calculate the force of pressing the screen.
We are unlikely to encounter this technology in smartphones because of the current fashion for frameless displays, but a few years ago, Samsung experimented with the Surface Acoustic Wave system in monoblocks, and as a component for gaming machines and advertising terminals panels with acoustic touchscreens are sold and now,
For a very short time, the touchscreens have won the world of electronics. Despite the lack of tactile response and other disadvantages, touch screens have become very intuitive, understandable and convenient method of entering information into computers. Last but not least, they owe their success to the variety of technical implementations.
Each with its own advantages and disadvantages, suitable for its class of devices. Resistive screens for the cheapest and most popular gadgets, capacitive screens for smartphones and tablets and desktop computers with which we interact every day and infrared touchscreens for those cases where the design of the screen should be left intact. In conclusion, it remains only to state that the touch screens are with us for a long time, there will be no substitutions shortly.