Capacitive screen working principle. Capacitive touch screen

Information input device, which is a screen that responds to touching it. There are many different types of touch screens that operate on different physical principles. But we will consider only those that are found in mobile phones and other portable equipment.

How resistive touch screens work

Resistive touch screens are of two types, four-wire and five-wire. Let's consider the principle of operation of each of the types separately.

Four-wire resistive shield

How the 4-wire resistive touchscreen works

The resistive touchscreen consists of a glass panel and a flexible plastic membrane. Both the panel and the membrane are coated with a resistive coating. The space between the glass and the membrane is filled with micro-insulators, which are evenly distributed over the active area of ​​the screen and reliably insulate conductive surfaces. When the screen is pressed, the panel and membrane are closed, and the controller using analog-to-digital converter registers the change in resistance and converts it to touch coordinates (X and Y). In general terms, the reading algorithm is as follows:

1. A voltage of + 5V is applied to the upper electrode, the lower one is grounded. The left and right are short-circuited and the voltage across them is checked. This voltage corresponds to the Y-coordinate of the screen.
2. Similarly, + 5V and "ground" are supplied to the left and right electrodes, the X-coordinate is read from the top and bottom.

5-wire resistive shield

The 5-wire shield is more reliable due to the fact that the resistive coating on the membrane is replaced by a conductive one (the 5-wire shield continues to work even with a cut membrane). The rear window has a resistive coating with four electrodes at the corners.

How the 5-wire resistive touchscreen works

Initially, all four electrodes are grounded, and the membrane is "pulled up" by a resistor to + 5V. Diaphragm voltage level is constantly monitored analog-to-digital converter... When nothing touches the touch screen, the voltage is 5 V.

As soon as the screen is pressed, the microprocessor senses the change in membrane voltage and begins to calculate the coordinates of the touch as follows:

1. The two right electrodes are supplied with a voltage of + 5V, the left ones are grounded. The voltage on the screen corresponds to the X-coordinate.
2. The Y-coordinate is read by connecting both top electrodes to + 5V and both bottom electrodes to ground.

How capacitive touch screens work

A capacitive (or surface-capacitive) screen exploits the fact that a large object conducts alternating current.

How the capacitive touchscreen works

A capacitive touchscreen is a glass panel covered with a transparent resistive material (usually an indium oxide / tin oxide alloy). The electrodes located at the corners of the screen apply a small alternating voltage to the conductive layer (the same for all corners). When you touch the screen with a finger or other conductive object, current leakage occurs. In this case, the closer the finger is to the electrode, the lower the resistance of the screen, which means that the current is greater. The current in all four corners is registered by the sensors and transmitted to the controller, which calculates the coordinates of the touch point.

Earlier models of capacitive screens used direct current - this simplified the design, but with poor user contact with the ground, it led to malfunctions.

Capacitive touch screens are reliable, about 200 million clicks (about 6 and a half years of clicks with an interval of one second), do not pass liquids and perfectly tolerate non-conductive pollution. Transparency at 90%. However, the conductive coating is still vulnerable. Therefore, capacitive screens are widely used in machines installed in a protected room. Does not respond to gloved hands.

How Projected Capacitive Touch Screens Work

A grid of electrodes is applied to the inside of the screen. The electrode together with the human body forms a capacitor; the electronics measures the capacity of this capacitor (gives a current pulse and measures the voltage).

How the Projected Capacitive Touch Screen Works

The transparency of such screens is up to 90%, the temperature range is extremely wide. Very durable (bottleneck - complex electronics that handle pressing). Glass with a thickness of up to 18 mm can be used on PEE, which leads to extreme vandal resistance. They do not react to non-conductive contaminants, conductive ones are easily suppressed by software methods. Therefore, projected capacitive touch screens are used in vending machines installed outdoors. Many models react to a gloved hand. In modern models, designers have achieved very high accuracy - however, vandal-resistant designs are less accurate.

PEE even react to the approach of a hand - the trigger threshold is set by software. Pressing by hand is distinguished from pressing with a conductive stylus. Some models support multi-touch. Therefore, this technology is used in touchpads and multi-touch screens.

It should be noted that due to differences in terminology, surface and projected capacitive screens are often confused. According to the classification used in this article, the iPhone screen is projected-capacitive.

Conclusion

Each of the types of touch screens has its own advantages and disadvantages, for clarity, consider the table.

The article was written based on the materials of the site

There is a constant debate about which phone has the best screen. Especially between Apple technology owners and those who prefer Android devices.

This simple infographic beautifully shelves the benefits of a particular type of touchscreen. I hope that when buying another smartphone, it will help you make the right choice and not overpay a tidy sum.

So, there are three types of touch screens: Resistive, Capacitive and Infrared

Resistive

Resistive Screen Phones: Samsung Messager Touch, Samsung Instinct, HTC Touch Diamond, LG Dare

How do they work? Small dots separate several layers of material that carries current. When the upper flexible layer presses on the lower layer, the electric current changes and the place of impact, that is, touch, is calculated.

How much does it cost to make? The manufacturing costs of resistive touch screens are not very high - $ .

Screen material. A layer of flexible material (usually polyester film) is placed on top of the glass.

Impact instruments. Fingers, gloved fingers or stylus.

Outdoor visibility. Poor visibility in sunny weather.

Possibility of multi-gestures. No.

Durability. For its cost, the screen lasts long enough. Scratches easily and is susceptible to other minor damage. It wears out rather quickly and needs to be replaced.

Capacitive

Capacitive touchscreen phones: Huawei Ascend, Sanyo Zio, iPhone, HTC Hero, DROID Eris, Palm Pre, Blackberry Storm.

How do they work? The current is broadcast from the corners of the screen. When the finger touches the screen, it changes the direction of the current and thus the place of the touch is calculated.

How much does it cost to make? Expensive enough - $$ .

Screen material. Glass.

Impact instruments. Only fingers without gloves.

Outdoor visibility. Visibility is good on a sunny day.

Possibility of multi-gestures. There is.

Durability.

Infrared

Infrared touchscreen phones: Samsung U600 (warm), Neonode N2 (optical).

How do they work? In order for the heat-sensitive screen to react, you need to touch it with a warm object. The optical screen uses a grid of invisible sensors directly above the screen. The tangency point is calculated based on the point where the x-y axis was violated.

How much does it cost to make? Very expensive - $$$ .

Screen material. Glass.

Impact instruments. Optical - fingers, gloves and stylus. Heat Sensitive - Warm fingers without gloves.

Outdoor visibility. Visibility in sunny weather is good, but strong sunlight affects productivity and accuracy.

Possibility of multi-gestures. Yes.

Durability. Serves long enough. Glass breaks only from serious damage.

A touchscreen is a device for inputting and outputting information through a pressure- and gesture-sensitive display. As you know, the screens of modern devices not only display the image, but also allow you to interact with the device. Initially, familiar buttons were used for such interaction, then the no less well-known “mouse” manipulator appeared, which greatly simplified the manipulation of information on the computer display. However, the "mouse" for work requires a horizontal surface and is not very suitable for mobile devices. This is where an addition to the usual screen comes to the rescue - Touch Screen, which is also known as Touch Panel, touch panel, touch film. That is, in fact, the touch element is not a screen - it is an additional device installed on top of the display from the outside, protecting it and serving to enter the coordinates of touching the screen with a finger or other object.

Usage

Today, touch screens are widely used in mobile electronic devices. Initially, the touchscreen was used in the design of pocket personal computers (PDA, PDA), now communicators, mobile phones, players, and even photo and video cameras hold the championship. However, the technology of controlling the finger through virtual buttons on the screen turned out to be so convenient that almost all payment terminals, many modern ATMs, electronic information kiosks and other devices used in public places are equipped with it.

Touchscreen laptop

It should be noted also laptops, some models of which are equipped with a rotatable touch screen, which gives the mobile computer not only wider functionality, but also more flexibility in operating it on the street and on the weight.

Unfortunately, there are not so many similar laptop models, popularly called "transformers", but they exist.

In general, the touch screen technology can be described as the most convenient when you need instant access to device control without preliminary preparation and with stunning interactivity: the controls can replace each other depending on the function being activated. Anyone who has ever worked with a sensor device understands the above.

Types of touch screens

In total, several types of touch panels are known today. Naturally, each of them has its own advantages and disadvantages. Let's highlight the main four constructions:

• Resistive
• Capacitive
• Projected capacitive

In addition to these screens, matrix screens and infrared are used, but due to their low accuracy, their field of application is extremely limited.

Resistive

Resistive touch panels are among the simplest devices. At its core, such a panel consists of a conductive substrate and a plastic membrane with a certain resistance. When the membrane is pressed, it closes with the substrate, and the control electronics determines the resulting resistance between the edges of the substrate and the membrane, calculating the coordinates of the pressure point.

The advantage of a resistive screen is its low cost and simplicity of the device. They have excellent resistance to dirt. The main advantage of resistive technology is sensitivity to any touch: you can work with your hand (including gloves), stylus (pen) and any other hard blunt object (for example, the top end of a ballpoint pen or the corner of a plastic card). However, there are also quite serious drawbacks: resistive screens are sensitive to mechanical damage, such a screen is easily scratched, therefore, a special protective film is often purchased additionally to protect the screen. In addition, resistive panels do not work very well at low temperatures, and also have low transparency - they transmit no more than 85% of the luminous flux of the display.

Using a touch screen pen

Application

• Communicators
• Cell Phones
• POS terminals
• Tablet PC
• Industry (control devices)
• Medical equipment

Communicator

Capacitive

Capacitive touchscreen technology is based on the principle that a large object (in this case, a person) is capable of conducting electrical current. The essence of the capacitive technology is to apply an electrically conductive layer to the glass, while a weak alternating current is applied to each of the four corners of the screen. If you touch the screen with a large grounded object (finger), current will leak. The closer the point of contact (and hence the leakage) to the electrodes in the corners of the screen, the greater the leakage current, which is recorded by the control electronics, which calculates the coordinates of the point of contact.

Capacitive screens are very reliable and durable, their resource is hundreds of millions of clicks, they perfectly resist dirt, but only those that do not conduct electric current. Compared to resistive ones, they are more transparent. However, the disadvantages are still the possibility of damage to the electrically conductive coating and insensitivity to touch with non-conductive objects, even with gloved hands.

Information kiosk

Application

• In guarded premises
• Information kiosks
• Some ATMs

Projected capacitive

Projected capacitive screens are based on measuring the capacitance of a capacitor formed between the human body and a transparent electrode on the glass surface, which in this case is a dielectric. Due to the fact that the electrodes are applied on the inner surface of the screen, such a screen is extremely resistant to mechanical damage, and taking into account the possibility of using thick glass, projection-capacitive screens can be used in public places and outdoors without special restrictions. In addition, this type of screen recognizes the pressure of a gloved finger.

Payment terminal

These screens are quite sensitive and distinguish between finger and conductive pen presses, and some models can recognize multiple presses (multitouch). The features of a projected capacitive screen are high transparency, durability, and immunity to most contaminants. The disadvantage of such a screen is not very high accuracy, as well as the complexity of the electronics that processes the coordinates of pressing.

Application

• Electronic kiosks in the streets
• Payment terminals
• ATM machines
• Laptop touchpads
• iPhone

With the definition of surface acoustic waves

The essence of the touch panel with the definition of surface acoustic waves is the presence of ultrasonic vibrations in the thickness of the screen. When touching the vibrating glass, the waves are absorbed, while the point of contact is recorded by the sensors on the screen. The advantages of the technology are high reliability and pressure recognition (as opposed to capacitive screens). The disadvantages are that they are poorly protected from environmental factors, therefore, screens with surface acoustic waves cannot be used outdoors, and in addition, such screens are afraid of any pollution that blocks their work. They are rarely used.

Other rare types of touch screens

• Optical screens. Glass is illuminated with infrared light, as a result of touching such glass, light is scattered, which is detected by the sensor.
• Induction screens. Inside the screen there is a coil and a grid of sensitive wires that react to touch with an active pen powered by electromagnetic resonance. It is logical that such screens respond to pressing only with a special stylus. Used in expensive graphics tablets.
• Strain gauges - react to screen deformation. Such screens have low accuracy, but they are very durable.
• Infrared Reticle is one of the earliest technologies to recognize screen touches. The grid consists of a plurality of light emitters and receivers located at the sides of the screen. It reacts to blocking the corresponding rays by objects, on the basis of which it determines the coordinates of pressing.

• Move two fingers together - reduce the image (text)
• Spread two fingers to the sides - Zoom
• Movement with several fingers at the same time - scrolling of text, pages in the browser
• Rotating with two fingers on the screen - rotate the image (screen)

The benefits and drawbacks of touch screens

Touch screens have been around for a long time in handheld devices. There are several reasons for this:

• The ability to make the minimum number of controls
• Simplicity of the graphical interface
• Ease of management
• Prompt access to device functions
• Expanding multimedia capabilities

However, there are more than enough disadvantages:

• Lack of haptic feedback
• Frequent need to use a pen (stylus)
• Possibility of screen damage
• Fingerprints and other dirt on the screen
• Higher energy consumption

As a result, it is not always possible to completely get rid of the keyboard, because it is much more convenient to type text using the familiar keys. But the touch screen is more interactive, thanks to faster access to menu items and settings of modern gadgets.

We hope this material will help you when choosing a touchscreen device.

Discuss on the forum

The universal type of touch screens has not yet been developed, and the technologies used today have both advantages and disadvantages. Read about the pros and cons of the main types of touch screens in this article.

The use of touch screens is most useful in small portable devices. Firstly, this is due to the inconvenience of using a mouse, keyboard and other input devices in phones and other small electronics. Secondly, the rejection of hardware buttons allows you to seriously increase the screen area. Thirdly, the production of touch panels is not cheap, and their use in large screens is still at least economically unprofitable.

However, having started with small devices such as PDAs, touchscreens have already made their way to the medium format (tablets and some laptops), and it is only a matter of time before they appear on the big screen.

There are only a few types of touch screens. Below we will talk about the three most common technologies, as well as several of its varieties.

RESISTIVE PANELS

The sensory part of such screens consists of two layers, separated by a small space, each of which has an array of resistive or conductive elements (depending on the specific implementation).

When you press your finger, stylus (or any other object) on the surface of the screen, these layers touch, the elements are closed, and the screen "understands" where they touched it.

Considering that contact between the two layers is possible only when using a flexible material that will bend under pressure, resistive screens are usually covered with a special flexible film, not glass. This leads to scratches and more frequent damage to the screen if you apply excessive pressure with the stylus.

The technology is one of the simplest, which is why it first appeared in touch devices. It still has certain advantages, but there are more disadvantages than other types of touch screens.

Advantages

In addition to the low price (the cost of such displays is approximately two times lower than capacitive ones), the accuracy of resistive screens also depends little on the state of the upper layer, therefore, in the event of its contamination or wetting, the responsiveness of the sensor practically does not change.

Despite the age of the technology, it still allows for the most accurate touchpads. In a properly calibrated display, you can actually hit a specific pixel with the stylus thanks to the dense array of resistive elements.

Flaws

Despite the fact that there are exceptions to this rule, most resistive screens do not recognize multitouch, that is, the screen understands only one touch (the very first, or the strongest), which significantly limits the interface control options. Even in devices where multitouch is implemented, fewer simultaneous touches are still recognized than in the most common capacitive screens.

Using multiple layers will reduce the contrast and brightness of the screen. The light transmission coefficient is ~ 75%, which is ~ 15% lower than in capacitive screens. Thus, in devices with a resistive sensor, the contents of the screen are more difficult to view in direct sunlight or under strong artificial light.

The use of two layers, separated by a small gap, is an indirect reason for the decrease in the accuracy of the sensor. If you hold the stylus perpendicular to the screen, then the accuracy can be one, but at an angle, the discrepancy will be several pixels due to the fact that the point on which the stylus presses is not directly above the desired pixel (parallax effect).

Protection against accidental input in resistive screens is a certain pressure that must be overcome in order for the device to count the command. Consequently, it is more difficult to equip resistive screens with an additional protective coating, which will only increase the response threshold. Paired with a plastic coating, which is necessary for the flexibility of the touch layer, resistive screens are more susceptible to damage than others, especially scratches, and if handled incorrectly (pressing hard with a sharp object), they can simply crack.

Despite the fact that the number of clicks at each specific point is estimated at 30 million, resistive screens still fail earlier than other types and are the most unreliable in this indicator.

Conclusion

Low cost and resistance to contamination (or rather, maintaining the accuracy of input when contaminated), coupled with all of the above disadvantages, have led to the fact that resistive screens are slowly being replaced from use, although they were able to gain a foothold in some niches, for example, in the sector of terminals for fast payment.

Styluses

A characteristic feature of devices with a resistive sensor is the widespread use of a stylus, the contact area of ​​which with the surface is smaller than that of a finger, and the pressure force is greater, which leads to more accurate input.

The presence of a stylus is desirable, although not necessary for screens with a small diagonal (mainly phones, and a few years ago, PDAs), however, in tablets, sufficient accuracy can be achieved with the help of fingers.

After a few years ago, PDAs were completely supplanted by smartphones and other devices, it seemed that the stylus had left the scene with them, but now you can increasingly find their reincarnation, especially in devices of intermediate sizes between smartphones and tablets.

As resistive screens are used less and less now, styluses have changed a bit too. Adapting to modern realities, they began to be produced with special nozzles at the end, which are recognized by capacitive screens.

CAPACITIVE PANELS

The principle of operation of capacitive screens is that a small voltage is applied to a special layer of an electrical conductor located on the outer surface of the screen, which forms a uniform electrostatic field. When a finger is applied to the screen, which is a conductor of electricity, the properties of the field change due to the appearance of a leak (the user acts as a ground electrode and "steals" the current from the screen). By changing the capacity, you can determine the presence of a contact and its coordinates.

To determine the coordinates, electrodes are installed in the corners of the screen to measure the leakage current, and the stronger it is on each specific sensor, the closer the pressure is. By defining specific values, you can calculate the click coordinates very accurately.

A subclass of capacitive screens are projection-capacitive screens, the principle of which is also to measure capacitance, however, the basic elements in them are located not on the outside of the screen, but on the inside, which increases the security of the sensor. It is these screens that are now used everywhere in smartphones.

Unlike resistive panels, which use flexible material, capacitive sensors are covered with glass. This better protects them from scratches, although they are more likely to crack when hit or dropped.

Dignity

The absence of several layers of additional materials not only increases the brightness of the screen (transparency to light is approximately 90%), but also reduces the distance between the screen surface and the image, which allows you to more accurately hit the desired pixels. Even if the gain is not large, it is still noticeable, especially when the device is at a certain angle relative to the axis of view, that is, in those moments when the difference between the real position of the desired pixel on the screen and the point to be hit is displaced as much as possible relative to each other. friend.

Samsung's Super AMOLED displays further reduce screen thickness by eliminating the need for an additional layer of capacitive elements. In this type of screens, they are embedded directly into the matrix.

Capacitive screens are much more durable than resistive ones (by almost an order of magnitude) when it comes to the number of clicks before the sensor elements fail. The number of such repetitions is estimated at 200+ million times.

Flaws

Capacitive screens are more expensive to manufacture than resistive screens and require that the material touching their surface must have conductive properties. Therefore, using any convenient object or working with ordinary gloves with capacitive screens will not work. In this regard, special capacitive styluses and gloves for working with touch panels in cold weather are becoming widespread.

The accuracy of capacitive screens is slightly lower than that of resistive screens, although in practical problems this difference is not very noticeable, since it is literally 1-3 pixels, and given that in most cases the program interface is already sharpened to eliminate these errors, it is difficult to call it a disadvantage ...

Conclusion

Capacitive panels, in terms of their characteristics and price, are best suited for screens of mobile devices, and therefore they now dominate in this sector.

INFRARED PANELS

Despite the fact that infrared sensors began to appear in devices later than other types of panels, they should not be considered more advanced. They have several advantages, however, most likely, like resistive screens, they will remain niche and will not be able to squeeze capacitive panels.

Optical

The main difference between infrared sensors and all others is that special sensors are located not on the surface of the screen, but along the edges of it and form a series of horizontal and vertical infrared rays directly above the display. When the object touches the screen, the rays are broken and thus the place of contact is determined.

Thermal

A variety of infrared screens are screens with thermal sensors. In order for them to respond to touch, the object must be warm.

As with capacitive panels, devices with infrared sensors use a protective glass coating, which accounts for the same advantages and disadvantages: better scratch resistance, but more likely to crack when hit hard.

Dignity

The location of the sensors on the sides of the matrix eliminates the need for an intermediate layer on the LCD matrix, which improves the brightness of the picture (the transparency of the coating is almost 100%), reduces the gap between the real image and the screen surface, makes the display more resistant to damage, and also allows you to work with dirty screen, provided that dirt does not interfere with the free propagation of infrared rays.

Infrared (optical) screens can be operated with gloves or using any other convenient items.

Flaws

Any contamination on the edges of the sensor that obscures the infrared signal sources will cause the sensors to malfunction. Problems also arise with small curvatures of the device, when the rays leave a plane parallel to the screen.

However, one of the most common problems with infrared sensors is false alarms. Since users do not need to physically touch the screen, sometimes the sensors are activated even when the finger is sufficiently close to the screen or during its movement from one point to another.

Despite the fact that infrared sensors are often used in devices with a relatively low cost (for example, in e-books), screens with an infrared sensor themselves are more expensive than both resistive and capacitive screens.

Conclusion

If resistive and capacitive screens could be conditionally attributed to, respectively, dying and dominant types of screens, then infrared sensors are a technology of marginal devices, since they are used in little-known models of portable electronics. An exception is e-books such as the Nook Touch.

INSTEAD OF EPILOGUE

There are still many innovations (flexible matrix, new protective coatings) in the near future for touch and conventional displays, but when it comes to technologies responsible for input recognition, there are no revolutionary alternatives on the horizon, so capacitive sensors will continue to dominate. as the most convenient and relatively inexpensive compared to other types of sensors.