How do touch screen monitor work?
We talk about how the simplest way to interact with electronics is arranged – a touch screen.
iPhone 2G was the first mobile phone, the management of which was completely based on interaction with the touch screen. More than ten years have passed since its presentation, but many of us still do not know how the Touchscreen works. But we are faced with this intuitive input tool not only in smartphones, but also in ATMs, payment terminals, computers, cars and airplanes – literally everywhere.
Prior to the touch screen monitor, 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 touch screen is similar to the keyboard according to the operating principles may 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, the same thing happens when each key is pressed. An electric circuit is closed under a button on a computer board, the computer registers the passage of current at this point in the circuit, “understands” which key is pressed, and executes the corresponding command. In the case of the touch screen, almost the same thing happens.
There are about a dozen different types of touch screens monitors, but most of these models are either outdated and not used long ago, or are experimental and are unlikely to ever appear in mass-produced devices. First of all, I will talk about the device of relevant technologies, those of which you constantly interact with, or at least may encounter in everyday life.
Resistive touch screen monitor
Resistive touch screens were invented back in 1970 and have not changed much since then.
In displays with such sensors, a pair of additional layers is located above the matrix. However, I’ll make a reservation, the matrix is not obligatory here. The first resistive touch devices were not screens at all.
The lower sensor layer consists of a glass base and is called a resistive layer. A transparent metal coating is applied to it, which transfers the current well, for example, from a semiconductor such as indium tin oxide. The top layer of the touchscreen with which the user interacts by tapping the screen is made of a flexible and elastic membrane. It is called a conductive layer. An air gap is left in the space between the layers, or it is evenly strewn with microscopic insulating particles. Four, five or eight electrodes are connected to the sensor layer along the edges, connecting it with sensors and a microcontroller. The more electrodes, the higher the sensitivity of the resistive touchscreen, since the voltage change on them is constantly monitored.
Here’s a resistive touch screen on. Nothing is happening yet. An 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, makes contact. This is followed by a change in voltage at once on all electrodes of the screen.
The touchscreen controller detects voltage changes and reads readings from the electrodes. Four, five, eight meanings and all are different. The microcontroller will calculate the X-coordinate of the pressing by the difference in the readings between the right and left electrodes, and by the differences in the voltage on the upper and lower electrodes, it will determine the Y-coordinate and, thus, tell the computer the point at which the layers of the touch layer of the screen are in contact.
Resistive touch screens boast 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. Due to the need for a layer between the sensor layers, the matrices of such screens noticeably lose in 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 because of their resistance to pollution, the ability to use gloves and, most importantly, low cost.
Such input tools are universally mounted in low-cost mass devices, such as information terminals in public places, and are still found in outdated gadgets, such as cheap MP3 players.
Infrared touch screen
The next, far less common, but, nevertheless, relevant version of the touch screen is an infrared touchscreen. It has nothing to do with a resistive sensor, although it performs similar functions.
The infrared touch screen is constructed from arrays of LEDs and photosensitive photocells located on opposite sides of the screen. LEDs illuminate the surface of the screen with invisible infrared light, forming on it something like a web or grid. This resembles a burglar alarm, which is shown in spy action films or computer games.
When something touches the screen, it doesn’t matter if it’s a finger, a gloved hand, a stylus, or a pencil, two or more rays are interrupted. Photocells record this event, the touchscreen controller finds out which of them receive infrared light and, by their position, calculates the area of the screen in which the obstacle occurs. The rest is to compare the touch with which interface element is on the screen in this place – the software task.
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, Amazon Kindle Touch and Sony Ebook. In addition, because of simplicity and maintainability, devices with similar sensors liked the military.
Capacitive touch screen
If the computer registers a change in conductivity following a tap on the screen directly between the layers of the sensor in resistive touchscreens, then capacitive sensors record the touch directly.
The human body, skin are good conductors of electricity and have an electric charge. Usually, you notice this by walking on a woolen carpet or taking off your favorite sweater, and then touching something metallic. We are all familiar with static electricity, have experienced its effects on ourselves, and have seen tiny sparks bursting from our fingers in the dark. The weaker, inconspicuous exchange of electrons between the human body and various conductive surfaces occurs constantly and it is precisely this that capacitive screens fix.
The first such touchscreens were called surface-capacitive and were a logical development of resistive sensors. They have only one conductive layer, similar to the one used previously, installed directly on top of the screen. Sensitive electrodes were also attached to it, this time around the corners of the touch panel. Sensors monitoring the voltage at the electrodes and their software were made much more sensitive and could now detect the slightest changes in the flow of electric current across the screen. When a finger (another current-conducting object, for example, a stylus) touches a surface with a surface-capacitive touch screen, the conductive layer immediately begins to exchange electrons with it, and the microcontroller notices this.
The appearance of surface-capacitive touchscreens was a breakthrough, but due to the fact that the conductive layer applied directly on top of the glass was easily damaged, they were not suitable for new generation devices.
To create the first iPhone, projection-capacitive sensors were required. This type of touchscreen quickly became the most common in modern consumer electronics: smartphones, tablets, laptops, all-in-ones 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 a grid. At first, they were superimposed on top of each other in two layers, then they began to be placed on the same level to reduce the thickness of the screen.
Made of semiconductor materials, including the already mentioned indium tin oxide, these conductive hairs create an electrostatic field at their intersections.
When the finger touches the glass, due to the electrical conductive properties of the skin, it distorts the local electric field at the nearest intersections of the electrodes. This distortion can be measured as a change in capacitance at a single grid point.
Since the array of electrodes is made quite small and dense, such a system is able to track the touch very accurately and without problems captures several touches at once. In addition, the absence of additional layers and interlayers in a sandwich made of a matrix, a sensor and a protective glass positively affects the image quality. However, for the same reason, broken screens, as a rule, are completely replaced. Once assembled together, a screen with a projection-capacitive sensor is extremely difficult to repair.
Now the advantages of projective-capacitive touchscreens do not sound like something surprising, but at the time of the presentation of the iPhone they provided tremendous success, despite objective disadvantages – sensitivity to pollution and humidity.
Pressure Sensitive Touch Screens – 3D Touch
The original predecessor of pressure-sensitive touch screens was Apple’s proprietary technology called Force Touch, which was used in the company’s smartwatches, MacBook, MackBook Pro and Magic Trackpad 2.
Having tested on these devices interface solutions and various scenarios for the use of pressure recognition, Apple began to introduce a similar solution in its smartphones. In the iPhone 6s and 6s Plus, recognition and measurement of pressure has become one of the functions of the touchscreen and has received the commercial name 3D Touch.
Although Apple did not hide the fact that the new technology only modifies the usual capacitive sensors and even showed a scheme that outlined the principle of its operation, details about the device of touch screens with 3D Touch appeared only after the first new-generation iPhones were dismantled by enthusiasts.
In order to teach the capacitive touch screen to recognize pressures and distinguish several degrees of pressure, the Cupertino engineers needed to rebuild the touch screen sandwich. They made changes to its individual parts and added another new layer to the capacitive one. And, interestingly, when doing this, they were clearly inspired by obsolete resistive screens.
The grid of capacitive sensors remained unchanged, but it was moved back closer to the matrix. An additional array of 96 separate sensors was integrated between the set of electrical contacts that monitor the place of touching the display and the protective glass.
Its task was not to locate a finger on the iPhone screen. The capacitive touchscreen still did a great job with this. These plates are necessary to detect and measure the degree of bending of the protective glass. Apple specifically for the iPhone ordered Gorilla Glass to design and produce a protective coating that retains its original strength and, at the same time, is flexible enough to allow the screen to respond to pressure.
On this development, it was possible to finish the material telling about touch screens, if not for another technology, which several years ago had a great future.
Wave touch screens
Unexpectedly, but they do not use electricity and do not even have anything to do with light. Surface Acoustic Wave system technology uses surface acoustic waves that propagate along the surface of the screen to determine the point of contact. The ultrasound generated by the piezoelectric elements in the corners is too high for the human hearing to catch. It spreads back and forth, repeatedly reflected from the edges of the screen. Sound is analyzed for anomalies created by objects touching the screen.
There are not many disadvantages in wave touch screens. They begin to make mistakes after severe contamination of the glass and in conditions of strong noise, but at the same time, in screens with such a sensor there are no additional layers that increase the thickness and affect the image quality. All sensor components are hidden under the display frame. In addition, wave sensors can accurately calculate the area of contact of the screen with a finger or other object and indirectly calculate the force of pressing the screen from this area.
We are unlikely to encounter this technology in smartphones due to the current fashion for frameless displays, but several years ago Samsung experimented with the Surface Acoustic Wave system in monoblocks, and panels with acoustic touchscreens are sold as accessories for gaming machines and advertising terminals. now
Instead of a conclusion
In a very short time, touchscreens have conquered the world of electronics. Despite the lack of tactile response and its other drawbacks, touch screens have become a very intuitive, understandable and convenient method of entering information into computers. Last but not least, they owe their success to a variety of technical implementations. Each with its own advantages and disadvantages, suitable for its class of devices. Resistive touch screen monitor 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 screen design should be left intact. In conclusion, it remains only to state that touch screens are with us for a long time, no replacement is expected in the near future.