Touch screen technologies. Infrared touch technology

Development of sensor technologies

Touch technologies are actively invading Russian computer market. The debut of these systems took place more than four years ago, but the rapid growth of the market began only this summer, when touchscreen information kiosks appeared at Moscow metro stations, large hotels and train stations. Some of them were installed as part of the Urban Information system Moscow", the other - as projects of individual companies.

All of these kiosks have a convenient and truly user-friendly interface that allows even an inexperienced user to easily manage a complex information system.

Touch screens allow you to achieve such simplicity and convenience. Touch technology first appeared more than 25 years ago, when specialists from the American company ELO TouchSystems developed resistive electrode technology, which allows them to achieve a rare combination of high reliability and guaranteed accuracy with amazing adaptability. This development gave impetus to the development of sensor technologies. Touch screens using the principle of surface acoustic waves (ELO TouchSystems), changes in distributed capacitance (MicroTouch), infrared waves and 4-electrode resistive technology (a number of Taiwanese companies) began to appear on the market.

Let's consider the features of various types of touch interface implementations.

Resistive 5-electrode technology

The space between glass and plastic is separated by micro-insulators ("dots"), patented by ELO TouchSystems, which are evenly distributed over the active area of ​​​​the screen. They reliably insulate conductive surfaces. When pressed, these surfaces come into contact with each other. The change in resistance is recorded by the controller and transmitted to the computer. The advantage of AccuTouch is its high reliability. The screen is completely insensitive to dirt and aggressive environments. The AccuTouch touch screen connects to a controller that processes signals from the screen surface and converts them into touch coordinates (X and Y) that are transmitted to the system bus computer and are processed as standard signals"mice".

Principle of surface acoustic waves (SAW)

A screen based on this principle (IntelliTouch) is made in the form of a glass panel with piezoelectric transducers located in the corners of the screen. A special controller sends a high-frequency electrical signal to them, which is converted into acoustic waves. The waves are reflected by an array of sensors located along the edges of the panel. Receiving sensors collect the reflected waves and send them back to transducers, which convert the received data into an electrical signal that is analyzed by the controller. The peculiarity of this technology is that the touch coordinate is calculated not only along the X and Y axes, but also along the Z axis.

The principle of changing distributed capacity

The screen is made in the form of a glass panel with a conductive layer applied to it, i.e. the surface of the screen is a distributed capacitance that changes when touched. These changes are registered and processed by the controller, which then calculates the coordinate of the touch.

Infrared wave technology

The screen is made in the form of a frame with rows of infrared emitters that create a lattice. The appearance of a foreign object within the grid is registered by the controller, processed and transmitted to the computer.

Structurally, touch screens are made in the form of a glass base that follows the curvature of the surface of a cathode ray tube or liquid crystal matrix of a monitor. There are spherical, FST, cylindrical and flat screens on the market, allowing you to choose best option for any monitor.

The exception is screens that use infrared waves and “vandal-resistant” SecureTouch screens from ELO. The first, as already mentioned, are made in the form of a frame that is placed on the monitor. The second ones are installed in front of the monitor. This is due to the fact that SecureTouch is a touch screen increased strength. Developed with surfactant technology, SecureTouch is designed to withstand harsh impacts. It will continue to work despite scratches that would mar any other touchscreen, and can withstand impacts from heavy objects. SecureTouch is based on annealed or tempered glass, 0.25 or 0.5 inches thick.

Touchscreens in this class are tested according to the UL specification (UL-1950). A one-kilogram steel ball is dropped several times onto the screen surface from a height of 51.5 inches (approximately 131 cm). SecureTouch passes the test without damage or scratches to the surface.

At the beginning of this year, another type of touch screen appeared. These are Scribex screens from ELO. Scribex makes it possible to enter handwritten information into computer system. This solves the pressing problems of banking and trading applications. The new solution helps users avoid difficulties that arise when authorizing access and filling out various documents using the keyboard. The screens are made using 5-electrode resistive technology. High resolution and high scanning speed allow you to enter a signature with a quality sufficient for identification by most programs.

They completely emulate a standard mouse. The driver allows you to set response modes to pressing, releasing, double-tapping, and even the right mouse button. Currently, drivers are available for DOS, Windows 3.x, Windows 95, Windows NT and a number of UNIX systems, OS/2, Apple Macintosh.

There are many types of controllers available touch screens, differing from each other in the way they communicate with the computer. PC-Bus controllers are inserted into the expansion slot motherboard, serial - connect to the serial port. The latter can be either external or internal, built directly into the monitor. A series of PCMCIA controllers are available for use in laptop PCs.

Touch input technology has a number of properties that make it indispensable in many applications. The first of them is the implementation of the genetically inherent attitude of “touching an object of interest.” It is natural for humans to touch an object to obtain more information about it. This happens intuitively and does not lead to the internal conflict that traditional input means sometimes cause. This property ideally solves the problem of a user-friendly interface in reference and information systems designed for mass access.

Second property - maximum protection from operator errors. Many people probably remember the taped-over keyboards on cash registers in stores. Irrational placement of keys and high loads lead to input errors. Therefore, cashiers found a simple solution and covered rarely used keys with matchboxes.

When using touch input, the keyboard on the monitor screen is generated by software. This allows you to avoid overloading the operator and display only those keys that are used in this moment. In addition, you can choose optimal size and the color of the keys.

Touch input also reduces the likelihood of hacking and unauthorized activities on a computer system.

These and other features make touch technology optimal for use as POS terminals, in medicine, in industry (process control terminal), in mass access applications, in the security business (system for identifying and targeting tracking devices), in financial applications.

Already now, solutions based on touch input are successfully used in various organizations in Moscow, St. Petersburg and many other cities.

QUARTA Touch Systems Company, http://www.quarta.msk.ru/ucc/ -

Number of different electronic devices, equipped with touch displays, is increasing every year. However, not all touchscreens are created equal. Currently, there are several options for implementing such solutions. In this article we will look at the features and scope of application various technologies, used to create touch displays.

It may be hard to believe, but the history of touchscreen displays began almost four decades ago. Back in 1971, University of Kentucky employee Sam Hurst designed a touch panel, which was patented under the name “elograph”. To develop and promote devices of this type, Sam Hirst founded the company Elographics. In 1974, its employees managed to create a prototype of a display equipped with a transparent touch panel. In 1977, Elographics received a patent for a five-wire resistive touch panel design, a solution that remains very popular more than three decades later. The company still operates, albeit under a different name: in 1994 it was renamed Elo TouchSystems, and subsequently became part of the Tyco Electronics holding.

With this we will complete the brief historical excursion and move on to consider various solutions, allowing you to implement the touch input function.

Resistive technology

The overview opens with resistive technology. By by and large It is what contributed to the current popularity of portable electronic devices with touch screens. Even with the advent of more advanced designs, resistive technology still occupies a leading position in the market. touch panels. According to the analytical agency DisplaySearch, at the end of 2009, the share of touch panels based on resistive technology in quantitative terms amounted to 50% of the total global supply.

Currently, there are two main options for implementing resistive touch panels - four and five-wire.

First, let's look at the operating principle of a resistive panel based on four-wire technology. Above the glass or plastic substrate is a thin, flexible membrane made of transparent material. The surfaces of the membrane and the substrate facing each other have a transparent coating that conducts electric current. The contact of the membrane with the substrate is prevented by miniature insulators located between them. Pairs of metal electrodes located on opposite sides are attached to the substrate and membrane (Fig. 1). In this case, the membrane electrodes are placed perpendicular to the substrate electrodes.

Rice. 1. Diagram of a four-wire resistive panel

When you press on the surface of the touch screen, the membrane in this place comes into contact with the substrate, resulting in electrical contact between the conductive layers (Fig. 2). Reading of the coordinates of the pressing point is performed sequentially. First, one of the substrate electrodes is connected to a DC source and the other is grounded. The membrane electrodes are short-circuited (Fig. 3), and the controller measures the voltage on them, thus determining one of the coordinates (in in this case- horizontal). Current is then applied to the membrane electrodes, and the controller measures the voltage across the connected substrate electrodes, recording the second coordinate.

Rice. 2. When pressed, the membrane bends and closes
with backing at the point of contact

Rice. 3. Reading horizontal (top)
and vertical coordinates of the pressing point
from a four-wire resistive panel

In the case of a five-wire panel, electrodes are installed on each side of the substrate, and the fifth is connected to the membrane (Fig. 4). When pressed, the membrane comes into contact with the substrate; the controller alternately supplies constant pressure into pairs of electrodes corresponding to the horizontal and vertical axis (Fig. 5). Based on the voltage on the electrode connected to the membrane, the controller determines the coordinates of the pressing point.

Rice. 4. Diagram of a five-wire resistive panel

Rice. 5. Electrical circuit for horizontal reading (top)
and vertical coordinates of the pressing point from a five-wire resistive panel

There is also an eight-wire technology (in this case, electrodes are attached to each of the four sides of the substrate and membrane), but this solution is used quite rarely due to its higher cost.

Touch panels based on resistive technology have a simple design and low cost - these are the factors that determine the popularity of such solutions. In addition, resistive panels respond solely to the pressure exerted by an object on the touch surface. Thanks to this, you can control the interface using both your fingers (including those wearing gloves) and various objects - a stylus, a match, etc. Such panels have a low response delay (about 10 ms) and remain operational even in the presence of various types of contamination on the surface. touch surface. We also note that it is possible to produce resistive touch panels with both a glossy and matte finish. The former provide more high definition images, but at the same time they glare a lot, and when you press the touch surface with your fingers, they also quickly lose their neat appearance. The matte finish effectively neutralizes glare and is less susceptible to fingerprints. True, the image in this case looks less clear and contrasty.

If we talk about the differences between four and five-wire technologies, then the first wins in terms of cost, and the second provides more high resource(up to tens of millions of clicks at one point). Eight-wire technology provides higher accuracy in determining the coordinates of the pressing point, however, as already mentioned, the production of such panels is much more expensive compared to four and five-wire designs.

Of course, resistive panels also have certain disadvantages. They are more susceptible to mechanical damage than other structures - after all, to operate, you need to apply a certain force and it’s easy to overdo it. The most vulnerable element of the structure is the flexible membrane, which is regularly subject to deformation. If the integrity of the membrane is damaged (a tear or cut appears), the panel fails.

Resistive panels are inferior to a number of devices in terms of accuracy in determining the coordinates of the point of pressure and, moreover, require periodic recalibration. Even the best examples of resistive panels have a light transmittance of about 85%, thus reducing the original brightness and contrast of the image. Due to the presence of several surfaces between the display screen and the viewer (substrate, membrane and protective layer), the use of a resistive touch panel inevitably leads to a deterioration in image clarity (this disadvantage is more inherent in designs with a matte finish).

Until recently, the disadvantages of screens based on resistive technology included the inability to perceive pressure at several points simultaneously. However, thanks to recent developments, this limitation has been overcome. For example, resistive touch panels from Fujitsu Components America, demonstrated during the SID 2010 forum, are capable of accepting up to 32 clicks at different points on the screen simultaneously.

Currently, touch screens based on resistive technology are widely used in PDAs, mobile phones, portable media players, POS terminals, as well as in industrial and medical equipment.

Capacitive technology

For quite some time now, scientists have found out that from the point of view of electrical engineering, the human body is a capacitor, and of a fairly large capacity. It is this property of our body that is used in touch screens based on capacitive or, as it is sometimes called, electrostatic technology.

Touchpad of this type is made on a transparent (glass or plastic) substrate. The outer surface of the plate is covered with a conductive layer, and an electrode connected to the controller is fixed in each of its four corners (Fig. 6). During operation, the controller supplies weak pulses to the electrodes. alternating current. If you touch the surface of the touch screen with your finger (connect a capacitor), current will leak. The amount of leakage current is inversely proportional to the distance from the point of pressure to the electrode. By comparing the magnitude of the leakage current through each of the four electrodes, the controller calculates the coordinates of the pressing point.

Rice. 6. Diagram of the capacitive panel device

Due to the absence of flexible membranes, capacitive panels have higher reliability compared to resistive ones (the resource is several hundred million clicks). In addition, thanks to less optical elements capacitive panels have a higher light transmittance (about 90%). The main disadvantage of panels of this type is the need to provide electrical contact between the surface and the human body. For example, if you press on such a screen with a stylus made of dielectric material or with a gloved finger, it will not work. In addition, the normal operation of the capacitive panel may be disrupted if the surface is contaminated with substances that conduct electrical current.

Currently, touch panels based on capacitive technology are used in the displays of information kiosks and ATMs, as well as in industrial equipment.

Projected capacitive technology

At the moment, this solution ranks second in the popularity rating of touch technologies, second only to resistive panels. Structurally, the panel based on projection-capacitive technology consists of two glass plates, between which there is a grid of thin electrodes (Fig. 7). During operation, the controller sends short pulses to each of the electrodes. When a finger is near the touch surface, an effect occurs similar to connecting a large capacitor (the role of which in this case is played by the human body) to nearby electrodes. By measuring the magnitude of the voltage drop (resulting from current leakage through the capacitor), the controller determines the coordinates of the touch point.

Rice. 7. Diagram of the projected capacitive panel

Touch panels based on projected capacitive technology have whole line advantages that contributed to a significant increase in their popularity in last years. In particular, they are durable, have a high light transmittance (about 90%), are resistant to dirt and mechanical damage to the working surface, and are able to function in a wide temperature range.

Projection-capacitive technology is capable of providing very high accuracy in determining the coordinates of the pressing point, however, it must be borne in mind that this parameter directly depends on the thickness of the protective layer. The thicker it is, the less accurate it is, and vice versa.

In addition, touch panels of this type allow you to perceive clicks at several points on the screen simultaneously. Depending on the controller settings, the panel can respond not only to touch, but also to a finger brought to the working surface. Accordingly, it is possible to control it with a gloved hand.

The main disadvantage of projection capacitive panels is complexity electronic components to process information about clicks, and therefore a fairly high production cost. In addition, the cost of projected capacitive panels increases markedly as the size and resolution of the screen increases. The listed factors prevent the spread of touch panels of this type in inexpensive devices, as well as in devices with screens big size.

Projected capacitive panels are good at detecting point pressure, but are not the best for functions that involve dragging GUI objects or drawing on the screen. As with resistive panels, devices of this type require periodic recalibration.

Currently, touch panels based on projected capacitance technology are used in cell phones, digital media players, information kiosks and touchpads on portable PCs. The popularity of this solution is growing rapidly. Thus, according to the DisplaySearch agency, last year the share of touch panels based on projection capacitive technology amounted to 31% of the total number of products supplied.

Optical technologies

A separate group of touch screens consists of devices based on optical technologies. The popularity of such solutions is still low: according to last year’s results, the share of optical touch panels was only 3% of the total global supply. However, the potential of such devices has not yet been fully revealed.

IR sensor with an array of fixed optocouplers

The operating principle of this solution is quite simple. The module framing the screen contains lines of IR LEDs with focusing lenses on both sides, and lines of photodiodes or phototransistors are located on opposite sides (Fig. 8). When the LEDs are turned on, an invisible grid formed by infrared rays is formed above the surface of the screen. When any object approaches the surface of the screen, it blocks the rays intersecting at that point. The absence of a beam is detected by photosensitive elements of optocouplers, by changing the state of which the controller determines the coordinates of the touch point.

Rice. 8. Diagram of an IR sensor with an array of fixed optocouplers

Such sensors are used mainly in display panels with large size screen. The fact is that the resolution of such sensors is limited physical dimensions optocoupler elements and focusing lens parameters. As a rule, the optical grid pitch is about 2-3 mm, and even when installed on a 32-inch display, the resolution of a sensor of such a design will not exceed 320x240 pixels.

However, IR sensors with an array of fixed optocouplers also have undeniable advantages. Since there is no interference between the display screen and the observer (glass, additional conductors, etc.), the installation of such a sensor does not affect indicators such as brightness, contrast, clarity and color accuracy. In addition, a sensor of this type can be manufactured in the form of a removable module attached to any display panel with a screen of the appropriate size (unlike capacitive and resistive panels, which, as a rule, are combined into a single module with a display).

For obvious reasons, an IR sensor with fixed elements does not require calibration. In addition, you can use your fingers and any suitable sized objects to control interface elements.

The disadvantages include the rather high cost of such devices, as well as the need to regularly clean the optical elements from dust and dirt to ensure the stability of their operation. This type of touch screen may not function properly if exposed to direct sunlight on the photocells.

There is one more nuance. For many models of IR sensors, the plane in which the optocoupler elements lie is located at some distance from the surface of the screen. As a result, when using an object that is not strictly perpendicular to the screen plane, errors occur in determining the coordinates.

Currently, LCD and plasma panels with IR sensors are used in presentation equipment, in educational institutions, situation centers, etc.

IR sensor with beam sweep mechanism

A development of the idea of ​​contactless touch registration using IR rays has become IR technology with a moving beam. Instead of an array of optocouplers, a single source of IR radiation is used (LED or semiconductor laser) and a scanning mechanism that ensures the movement of the beam, with high speed scanning the working surface. If there is no obstacle, the beam is scattered. If there is any obstacle in the path of the beam, the beam is reflected from it and captured by a photodiode. Based on a change in the state of the photodiode, the controller records the touch at the corresponding point.

Unlike IR sensors with fixed optocouplers, the described design can be implemented in the form of a very compact module - which, in turn, allows it to be easily used in portable devices. A unique feature of this technology is the ability to use it with projected images, and the size work area can vary over a fairly wide range. Due to the absence of interference, the operation of the optical sensor does not affect the image characteristics. In addition, the cost of such sensors is low.

Among the disadvantages, we note the not very high resolution, limited opportunities for recognizing several touches at the same time and a rather large error in determining the coordinates of the touch point at the edges of the screen, where the angle of incidence of the beam is minimal.

The first commercial devices to use optical sensors with a sweep mechanism were virtual keyboards (Fig. 9). The lighter-sized device allows you to replace a hardware keyboard when working with a laptop or pocket PC. IN Lately Developers of multimedia projectors, as well as portable devices with built-in projectors, are showing increased interest in such sensors (Fig. 10).

Rice. 9. Wireless virtual keyboard for PDAs
and mobile phones

Rice. 10. IR sensor with scanning mechanism
beam allows you to implement the touch input function
for projected images

NextWindow IR sensor

This technology was developed by NextWindow and is used in its touch panels. Unlike the pair of solutions described above, where the touch surface is virtual, NextWindow technology involves the use of a physical object in this capacity - a glass or plastic plate. On three sides, at the ends of the plate, IR radiation sources (lines of LEDs) are installed, and on two sides upper corners there are optical sensors operating in the IR range (Fig. 11).

Rice. 11. Diagram of the NextWindow IR sensor

When you touch the surface with a finger or any object, the pattern of IR radiation propagation changes. These changes are recorded by optical sensors, based on changes in the readings of which the controller calculates the coordinates of the touch point.

The advantages of this solution are the high light transmittance of the panel (more than 92%), the ability to register touches at two points simultaneously and high resolution. Sensors of this type are highly stable and do not require periodic calibration during operation.

Among the disadvantages, one can note the rather complex design of the controller and, accordingly, not the lowest cost of such devices.

Touch panels of this design are best suited for equipping displays with large screen sizes (from 20 inches diagonally or more). Based on NextWindow technology, both display panels with an integrated touch screen and removable modules are produced.

Optical sensors based on video cameras

In devices where the image on the screen is formed using the rear projection method, an optical sensor based on digital video camera. In the simplest case, one video camera operating in the IR range is used (Fig. 12). The image on the screen in this case is not a hindrance, since it is projected in the visible range and the camera simply perceives it.

Rice. 12. Diagram of an optical sensor with a video camera in devices
the image on the screen is formed by the method of rear projection

The inner surface of the screen is illuminated with infrared rays. In the absence of any objects on the surface of the screen, IR rays pass through the glass without hindrance. If the rays touch the surface, they are reflected from the obstacle that appears and the video camera captures a spot (or several spots) on a uniform background. The resulting image is processed by software that calculates the coordinates of the touch points.

Such a sensor may also contain several video cameras - this makes it possible to increase its reliability and implement additional features. For example, in the Microsoft Surface device (Fig. 13), five video cameras are installed to service a sensor of this type. In addition to registering touches and gestures, they provide an object recognition system. To do this, miniature black and white marks are applied to the underside of objects used with this device, reminiscent of the numbers on dominoes. Using these tags, the software can determine the type of object and automatically perform the action associated with it - open a document with a description, launch an application, etc.

Rice. 13. The Microsoft Surface feature
touch input is implemented using video cameras,
installed inside the housing

The optical sensor with the video camera does not have any effect on the quality of the image on the screen. Other advantages of this solution include the ability to process multiple touches simultaneously; using both fingers and various objects (and in any combination) to work with graphical interface. The resolution of such a sensor can vary widely depending on the resolution of the video camera and optical system used. In addition, the same sensor with minimal modification can be used to work with screens of different sizes.

Due to their high cost and large dimensions, optical sensors based on a video camera are unsuitable for use in portable devices. The system requires careful calibration after installation and regular adjustments to ensure acceptable accuracy.

As already mentioned, camera-based optical sensors are only suitable for use in rear-projection displays, and this greatly limits their scope of application. Currently, this class of devices is very small: the demand for projection TVs is rapidly declining, and devices like the Microsoft Surface are produced in microscopic quantities.

Technologies based on the properties of acoustic waves

So far, none of the technologies that use the properties of acoustic waves to implement the touch input function have become widespread. Nevertheless, such solutions are interesting not only because of their original operating principle, but also because of a number of important advantages.

Surface Acoustic Wave Technology

As the name suggests, the operation of this solution is based on the properties of the propagation of surface acoustic waves (SAW). A surfactant-based touch panel is a glass plate that is mounted in front of the display screen with a small gap. Piezoelectric transducers (PETs) and receiving sensors are installed in the corners of the plate, and reflectors are installed at the edges (Fig. 14). During operation, the controller supplies a high-frequency electrical signal to piezoelectric transducers, which, in turn, excite surface acoustic waves in the ultrasonic range (with a frequency of the order of several megahertz) in the glass plate. These waves are evenly distributed by reflectors throughout the thickness of the plate and are then captured by receiving sensors, which convert them into an electrical signal read by the controller. When you touch the touch surface, part of the energy of surface acoustic waves is absorbed (a finger or other object in this case acts as a damper, preventing the free propagation of waves). By changing the signals read by the receiving sensors, the controller determines the coordinates of the touch point.

Rice. 14. Diagram of a touch panel based on surfactant technology

Touch panels based on surfactant technology are distinguished by their reliability (they can withstand tens of millions of clicks at one point), high light transmittance (more than 90%) and responsiveness to touches made by both fingers and various objects. In some implementations, this technology allows you to determine not only the coordinates, but also the pressing force.

Among the disadvantages of touch panels of this type, it is necessary to note the sensitivity to contamination of the working surface (dirt affects the propagation of acoustic waves) and the not very high accuracy of determining the coordinates of the point of pressing. It is also possible for the touch panel to malfunction under conditions of strong noise and vibration, which significantly limits the ability to use devices of this type outdoors.

There are several options for implementing surfactant-based touch panels - IntelliTouch, SecureTouch, iTouch, etc. The main area of ​​application of touch panels based on surfactant technology is currently information kiosks, terminals, etc. Due to the technical features of this solution, it is advisable to use it in displays with large screen sizes (19 inches or more).

Acoustic pulse recognition technology

Acoustic Pulse Recognition (APR) technology, created by Elo TouchSystems, is further development ideas used in surfactant-based panels. However, the operating principle of touch panels based on APR technology differs significantly from devices based on surfactants.

The touch surface is a glass plate. Four piezoelectric transducers are installed on its sides, converting those spreading throughout the thickness of the glass sound waves into an electrical signal (Fig. 15).

Rice. 15. Diagram of a touch panel based on APR technology

The operating principle of the APR panel is based on the fact that the sound generated when you touch each point on the touch surface is unique. When you touch the touch surface, a sound pulse is generated that travels along the glass panel. Having reached the edge of the panel, the pulse acts on the probe, which converts it into an electrical signal and transmits it to the controller. The latter compares the signals coming from the sensors with the reference signals stored in memory, recorded when touching the various points panels. If the sound picture does not match the standards stored in memory, the controller does not register the press - this is how it is implemented efficient system filtering external noise and vibrations.

Touch panels based on APR technology provide higher (compared to surfactant-based devices) accuracy in determining the coordinates of the touch point and are much less susceptible to influence extraneous noise and vibrations. Pressing can be done either with fingers or with various objects. Such panels have a high light transmission rate (more than 90%) and remain operational even in the presence of scratches and dirt on the touch surface. Touch panels based on APR technology provide high stability and do not require recalibration during operation. This solution is highly scalable: it can be used in display panels with both small and large screen sizes.

Today, the main application area for APR technology is digital kiosks and POS terminals. Deliveries of commercial solutions with touch displays based on APR technology began relatively recently - at the end of 2006.

Ultrasound technology

To operate this type of touch screen, a special pen is used, which contains a generator, an ultrasonic wave emitter and a miniature power source. Two sensors that respond to ultrasound are mounted on the display frame near the upper corners of the screen (Fig. 16). When the pen tip touches the surface of the screen, a switch is activated and the pen begins to emit ultrasonic waves. The controller records the response time of each sensor and, based on the difference of these values, calculates the coordinates of the touch point.

Rice. 16. Diagram of a display device with an ultrasonic sensor

The main advantages of this solution are ease of implementation (no changes to the design of the display panel are required), low cost, and the absence of interference affecting image quality. This design has good scalability: a sensor of this type can be used with screens various sizes(only minor changes to the controller program are required).

The main disadvantage is the need to use a special pen. Besides, this decision provides not very high accuracy in determining the coordinates of the pressing point (±0.5 mm) and requires additional space to place sensors on the frame around the screen. Thus, ultrasonic sensor practically unsuitable for use in portable devices.

As an example serial device, equipped with an ultrasonic touch input system, can be cited as a 17-inch LCD monitor released in early 2006 Samsung SyncMaster 720TD (Fig. 17). The sensor sensors in this model were made in the form of cylindrical washers located in the upper corners of the monitor frame.

Rice. 17. The SyncMaster 720TD LCD monitor is equipped with a
touch input based on ultrasonic technology

Electromagnetic resonance technology

In conclusion, it is worth mentioning the electromagnetic resonance technology developed by Wacom for use in graphics tablets (digitizers). In 1998, the first LCD display model with a built-in graphics tablet, the Cintiq 18sx, appeared in the company's product line. Wacom currently produces two series of touch screen displays - Cintiq and PL (Fig. 18).

Rice. 18. Wacom Cintiq series LCD equipped with
built-in graphics tablet

Touch panels based on electromagnetic resonance technology provide very high positioning accuracy and also allow you to receive Additional information from the built-in pen sensors - this way you can record the pressure, tilt angle, tip type, etc.

This design allows you to track the location of the pen even when its tip is at a distance of 1-2 cm from the working surface. Thanks to this, the touch panel can be installed under the LCD display module - without thereby compromising optical characteristics display.

Unfortunately, there are a number of shortcomings. Touch panels based on electromagnetic resonance technology only work with a special stylus and require periodic calibration during operation. In addition, due to the complexity of the design, such products are quite expensive to manufacture, and the price increases significantly as the screen size increases.

Touch panels based on this technology consume a lot of electricity and are a source of electromagnetic interference that can damage normal work located nearby wireless equipment(mobile phones, access points, etc.).

Apparently, in the coming years, electromagnetic resonance technology will remain a solution focused mainly on the small segment of expensive touch displays used for working with professional applications ( graphic editors, 3D modeling systems, CAD systems, etc.).

Touch technologies May 27th, 2011

More convenient buttons and wheels

I wonder if you guessedHenry Edward RobertsAndMartin Cooper,creating the world's first personal computerand a cell phone, ohthatP Will it be about half a century and the already familiar use of communication devices - keyboards, mice and joysticks - will fade into the background?

Today, a completely different way of interaction between a person and a stationary or laptop computer- This sensor technologies, which have also found active use in self-service touch information kiosks and payment terminals and have significantly simplified the process of “communication” between the consumer and high-tech equipment. Modern touchscreen equipment has become so attractive and intuitive that even untrained users can operate it.

Touch technologies are based on the influence of four basic types of waves: resistive, surface acoustic, surface capacitive and infrared and allow a person to take direct (contact) participation in requesting information, making payments and orders, etc.

As practice shows, it is important for our clients to know more about touch technologies, so on our website we publish a description of the basic touch technologies that formed the basis for the development of touch screens:

Resistive touch technology.

The operating principle of a resistive screen is based on the action of resistive waves. This screen has a multilayer structure and consists of glass panel and flexible plastic membrane, whereand the panel and 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 the conductive surfaces. When you press the membrane, the resistive coatings close and a special controller registers the change in resistance between the electrodes, converting this change into coordinates.

There are four- and five-wire resistive screens. On the five-wire membrane

the resistive coating is replaced by a conductive one. This allows the resistive screen to remain operational even if the membrane is cut; such a screen is considered the most reliable.

Resistive touch screens have proven themselves in the service sector as part of POS terminals, industry, medicine, and transport. They have maximum resistance to contamination, are reliable and durable. The screen can withstand 35 million touches on one point.

Surface acoustic sensor technology (SAS).

Such screens operate based on surface acoustic wave technologyAnd are a glass panel, which allows you to get the highest quality image on the touch screen.

Such screens are built on the principle of using miniature piezoelectric sound emitters, inaudible to humans, installed in three corners of the screen. This signal is converted into an ultrasonic acoustic wave directed along the surface of the screen, and the screen itself is presented to the control program touch sensors in the form of a digital matrix, each value of which corresponds to a specific point on the screen surface. Special reflectors distribute the acoustic wave over the entire surface of the screen. Touching the screen changes the pattern of propagation of acoustic vibrations, which is recorded by sensors. By changing the nature of the vibrations, you can calculate the coordinates of the disturbances and the pressing force.

Touch screen based on surface acoustic wave technology provides maximum transparency and high quality images, works even with scratches, fixes exact coordinates and touch strength, has an anti-reflective coating. The touch screen can respond to the touch of a finger, a gloved hand and a stylus.

Infrared touch technology.

Infrared touch panels work using two very complex techniques.

The first technique is based on using changes in the generated heat on the surface of the panel. This method is not very practical as it requires that your hands are always warm.

Another technique involves placing infrared sensors around the entire perimeter of the panel, which detect interruptions in the flow of light rays above the surface of the screen when touched. If one of the infrared rays is blocked by the rays that fall within the range of foreign object, the beam stops reaching the receiving element, which is immediately recorded by the microprocessor controller. This way the touch coordinate is calculated. Note that it does not matter which object (finger, pen, glove) is placed in the workspace of the infrared touch screen.

Infrared touch panels are considered to have the most durable surface, and are most often used in educational institutions (as large interactive panels), medical, government and government organizations. slot machines, as well as for military purposes.

Capacitive(electrostatic) or surface capacitive technology.

There are two types of capacitive screens: surface capacitive and projected capacitive. In both cases, control is carried out not by pressing, but by touching the screen. Technology is based on the human ability to conduct electric current.

The capacitive (electrostatic) touch screen has some electric charge. By touching the touch screen, a person slightly changes the charge pattern, transferring part of the charge to the point of pressing. Screen sensors are located in all four corners and monitor the flow of charge on the screen, determining the coordinates of the touch.

Capacitive screens are also distinguished by their reliability and high degree of transparency and durability - the possibility of up to a billion clicks in the same place. However, as a rule, when working with such a screen, you cannot use an auxiliary object (stylus, glove, etc.) - only your finger. Although there already exist capacitive screens where it is possible to work from a specially made this type screen with a stylus.

Capacitive touch monitors have good transparency and are durable, so they are intensively used in crowded places: shopping and entertainment centers, supermarkets, air and railway ticket offices, on the street, etc.

There are also other emerging sensor technologies, e.g. multi-touch with the function of touch input systems, which simultaneously determines the coordinates of two or more touch points.

Recently, contactless operation schemes with touch screens have begun to be actively developed and applied. Modern touch screen sensors respond to heat, hand movements, and it is not necessary to touch the screen at all. This sensor system detects finger movement at a distance of up to two centimeters above the screen surface.

The use and development of sensor technologies today gives a new impetus to the development of medicine, automotive industry, education, banking, smart home technology, games and entertainment, service and trade, and much more are being transformed.

The screens of modern devices can not only display images, but also allow you to interact with the device through sensors.

Initially, touch screens were used in some pocket computers, and today touch screens are widely used in mobile devices, players, photo and video cameras, information kiosks, and so on. Moreover, each of the listed devices can use one or another type of touch screen. Currently, several types of touch panels have been developed, and, accordingly, each of them has its own advantages and disadvantages. In this article we will look at what types of touch screens there are, their advantages and disadvantages, and which type of touch screen is better.

There are four main types of touch screens: resistive, capacitive, with the detection of surface acoustic waves and infrared . In mobile devices, only two are most widespread: resistive and capacitive . Their main difference is the fact that resistive screens recognize pressure, while capacitive screens recognize touch.

Resistive touch screens

This technology is most widespread among mobile devices, which is explained by the simplicity of the technology and low production costs. The resistive screen is LCD display, on which two transparent plates are superimposed, separated by a dielectric layer. The top plate is flexible, as the user presses on it, while the bottom plate is rigidly fixed to the screen. Conductors are applied to surfaces facing each other.

Resistive touch screen

The microcontroller supplies voltage in series to the electrodes of the top and bottom plates. When the screen is pressed, the flexible top layer flexes and its inner conductive surface touches the lower conductive layer, thereby changing the resistance of the entire system. The change in resistance is recorded by the microcontroller and thus the coordinates of the touch point are determined.

The advantages of resistive screens include simplicity and low cost, good sensitivity, and the ability to press the screen with either a finger or any object. Among the disadvantages, it is necessary to note poor light transmission (as a result, you have to use more bright backlight), poor support for multiple clicks (multi-touch), cannot determine the force of pressing, as well as fairly rapid mechanical wear, although in comparison with the life of the phone, this drawback is not so important, since it usually faster phone fails than the touch screen.

Application: cell phones, PDAs, smartphones, communicators, POS terminals, TabletPC, medical equipment.

Capacitive touch screens

Capacitive touch screens are divided into two types: surface-capacitive and projected-capacitive . Surface capacitive touch screens They are glass on the surface of which a thin transparent conductive coating is applied, on top of which a protective coating is applied. Along the edges of the glass there are printed electrodes that apply low-voltage alternating voltage to the conductive coating.

Surface capacitive touch screen

When you touch the screen, a current pulse is generated at the point of contact, the magnitude of which is proportional to the distance from each corner of the screen to the point of contact, thus, it is quite simple for the controller to calculate the coordinates of the point of contact and compare these currents. The advantages of surface capacitive screens include: good light transmission, short response time and great resource touches. Among the disadvantages: the electrodes placed on the sides are not suitable for mobile devices, they are demanding on external temperature, they do not support multi-touch, you can touch them with your fingers or a special stylus, and they cannot determine the pressing force.

Application: Information kiosks in secure areas, at some ATMs.

Projected capacitive touch screens They are glass with horizontal leading lines of conductive material and vertical defining lines of conductive material applied to it, separated by a layer of dielectric.

Projected capacitive touch screen

Such a screen works as follows: a microcontroller sequentially applies voltage to each of the electrodes in the conductive material and measures the amplitude of the resulting current pulse. As the finger approaches the screen, the capacitance of the electrodes located under the finger changes, and thus the controller determines the location of the touch, that is, the coordinates of the touch are intersecting electrodes with increased capacitance.

The advantage of projected capacitive touch screens is their fast touch response speed, multi-touch support, more accurate coordinate determination compared to resistive screens, and pressure detection. Therefore, these screens are used to a greater extent in devices such as the iPhone and iPad. It is also worth noting the greater reliability of these screens and, as a result, a longer service life. Among the disadvantages, it can be noted that on such screens you can only touch with your fingers (drawing or writing by hand with your fingers is very inconvenient) or with a special stylus.

Application: payment terminals, ATMs, electronic kiosks on the streets, touchpads of laptops, iPhone, iPad, communicators and so on.

SAW touch screens (surface acoustic waves)

The composition and operating principle of this type of screen is as follows: piezoelectric elements are placed at the corners of the screen, which convert the electrical signal supplied to them into ultrasonic waves and direct these waves along the surface of the screen. Reflectors are distributed along the edges of one side of the screen, which distribute ultrasonic waves across the entire screen. On the opposite edges of the screen from the reflectors there are sensors that focus ultrasonic waves and transmit them further to the transducer, which in turn converts the ultrasonic wave back into an electrical signal. Thus, for the controller, the screen is represented as a digital matrix, each value of which corresponds to a specific point on the screen surface. When a finger touches the screen at any point, the waves are absorbed, and as a result, the overall pattern of propagation of ultrasonic waves changes and as a result, the transducer produces a weaker electrical signal, which is compared with the one stored in memory digital matrix screen, and thus the coordinates of touching the screen are calculated.

SAW touch screen

The advantages include high transparency, since the screen does not contain conductive surfaces, durability (up to 50 million touches), and surfactant touch screens allow you to determine not only the coordinates of pressing, but also the pressing force.

Among the disadvantages, we can note the lower accuracy of determining coordinates than capacitive ones, that is, you won’t be able to draw on such screens. A big disadvantage is malfunctions when exposed to acoustic noise, vibrations or when the screen is dirty, i.e. Any dirt on the screen will block its operation. Also, these screens only work correctly with objects that absorb acoustic waves.

Application: SAW touch screens are mainly found in secure information kiosks, educational institutions, gaming machines and so on.

Infrared touch screens

The design and operating principle of infrared touch screens is quite simple. Along two adjacent sides of the touch screen there are LEDs that emit infrared rays. And on the opposite side of the screen there are phototransistors that receive infrared rays. Thus, the entire screen is covered with an invisible grid of intersecting infrared rays, and if you touch the screen with your finger, the rays overlap and do not hit the phototransistors, which is immediately registered by the controller, and thus the coordinates of the touch are determined.

Infrared touch screen

Application: Infrared touch screens are mainly used in information kiosks, vending machines, in medical equipment, etc.

Among the advantages we can note the high transparency of the screen, durability, simplicity and maintainability of the circuit. Among the disadvantages: they are afraid of dirt (therefore they are used only indoors), they cannot determine the force of pressing, the accuracy of determining coordinates is average.

P.S. So, we have looked at the main types of the most common sensor technologies (although there are also less common ones, such as optical, strain gauge, induction, and so on). Of all these technologies, resistive and capacitive ones are most widespread in mobile devices, as they have high accuracy determining the point of contact. Of them best characteristics have projected capacitive touch screens.

The text was prepared based on materials from open sources by Technological methodologists Karabin A.S., L.V. Gavrik, S.V. Usachev

Due to the widespread use of mobile devices, as well as various consumer electronics, in particular pocket personal computers, portable navigators and game consoles, touch displays are increasingly occupying their own niche in many aspects of our lives.

There are several types of touch displays currently in use, but the four most widely used technologies are:

Resistive;

Infrared;

Capacitive;

Surface acoustic wave (SAW).

All of these technologies have their own distinctive features, benefits, advantages and disadvantages.

Resistive touch screen technology

A resistive touch screen has a multilayer structure consisting of two conductive surfaces separated by a special insulating compound distributed over the entire active area of ​​the screen.

When you touch the outer layer, made of thin transparent plastic, its internal conductive surface is combined with the conductive layer of the main plate (can be made of glass or polyester), which plays the role of a frame of the structure, due to which the resistance of the entire system changes. This change is recorded by a microprocessor controller that transmits the coordinates of the touch point control program computer.

Triggering occurs when pressed with a finger or other hard object. Resistive touch screens are resistant to dirt, dust, grease and many liquids (such as water, acetone, beer, tea, coffee, etc.), including some chemically caustic ones.

Main features of resistive touch screens (touchscreen):

excellent quality indicators;

excellent technical characteristics;

entering information with both a stylus and a finger;

typical transparency is 80%.

Resistive products are the most attractive in terms of price, as they are quite inexpensive. Also, the advantages of resistive displays include a high resolution, the ability to use a regular metal or plastic stylus, resistance to influences such as dust, dirt, water and intense lighting. However, this type of product also has its drawbacks. For example, the image clarity of this type of touch screen is not high enough. And the displays themselves need regular calibration due to the fact that the system’s reaction location begins to mismatch with the press location. Sometimes it is also possible that a resistive display can respond synchronously to more than one press. In addition to all of the above, such displays are quite fragile, which significantly limits their use.

Capacitive touch screen technology

The sensing element of a capacitive touch screen is glass, on the surface of which a thin transparent conductive coating is applied. Along the edges of the glass are narrow printed electrodes that distribute the low-voltage electric field evenly across the conductive coating. A protective coating is applied over the conductive layer. When you touch the screen, a capacitive coupling is formed between the finger and the screen, which causes a pulse of current to the point of contact. The electrical current from each corner of the screen is proportional to the distance to the point of contact, so the controller simply compares these currents to determine where the touch is made. Result - transparent screen with short response time, high strength and durability.

Today, the touch screen with ThruTouch technology is the unique and only touch screen designed for use in street payment terminals or information kiosks.

This technology was originally used in models such as cellular iPhone phones and LG Prada. In this case, the sensor was located under a layer of mineral glass, which gave it additional protection from scratches, and, consequently, increasing its reliability. The electrical properties of the conductors undergo changes as soon as the finger approaches the display. That's why iPhone responds great to even light touches. Projected capacitive displays allow you to record multiple taps at the same time. For example, the iPhone uses two-finger gestures to zoom.

The iPhone, thanks to its popularity, managed to become the progenitor of the characteristic design for most “touch” phones.

A distinctive feature was the elegant candy bar with a large touch display and a minimum number of buttons.

The iPhone screen has an excellent pixel resolution (320x480). The picture on the display is lively and bright, with a wide viewing angle and, moreover, impeccable behavior in the sun. The screen backlight changes quickly depending on the level of illumination.

The iPhone display also has motion sensors, allowing it to automatically change its orientation when you rotate the phone.

There is no stylus for the iPhone, and the device does not respond to it. However, the ease of working with the display does not suffer from this.

The iPhone is convenient primarily for working with the Internet, so most of the features are designed to work in the browser. These include, for example, optimizing the size of Internet pages by double-clicking.