Sandeep.G Thursday, March 1, 2012Touchscreen Working Principle

AbstractIn todays society, the way in which one physically interacts with electronic devices influences the technological researches. This has led to many great advances, including the development of touch screen technology. Through the use of touch screen technology, the operator is given an alternative method of how he can interact with a device. This technology operates currently in seven distinct ways: resistive systems, capacitive systems, infrared systems, Surface Acoustic Wave (SAW) systems, strain gauge systems, optical imaging systems and dispersive signal technology. The objective of the report titled Touchscreen working principle is to briefly explain the principles involved in the working of these technologies. The main focus will be on the resistive and capacitive touchscreen systems while briefly introducing the other mentioned touch sensing technologies. 1. Introduction A touch screen is a two dimensional sensing device display that can detect the presence and location of a touch within the display area. The term generally refers to touching the display of the device with a finger or hand. Touchscreens can also sense other passive objects, such as a stylus. Touchscreens are common in devices such as game consoles, all-in-one computers, tablet computers and Smart phones. A brief history of touch screensThroughout the past century, technology has improved in many ways. The way in which humans interact with technology is one of the most important ways technology is changing. By using touch screen technology, the user is able to manipulate a digital environment by only the touch of their finger, or another input device, on the screen. This report discusses the different technologies that make this possible: infrared, resistive, and capacitive touch screens, as well as their qualities in modern devices.Touch screen technology first entered the public eye in 1971, with the invention of the Elograph, by Elographics, Inc. This company was created to produce Graphical Data Digitizers for use in research and industrial applications [1]. This technology set the stage for many devices to come. One of the next devices to be invented was the HP-150, the first touch screen computer. Hewlett Packard invented this device in 1983. This technology is important because it had infrared touch-screen capability, allowing for creation of ATM (Automated Teller Machine)-like applications. These are two of the most important devices in the development of touch screen technology. As time progressed, touch screen devices have become increasingly more complex and sustainable, providing the user with greater accuracy and more features to improve the quality of life.The touchscreen has two main attributes. First, it enables one to interact directly with what is displayed, rather than indirectly with a pointer controlled by a mouse or touchpad. Secondly, it lets one do so without requiring any intermediate device that would need to be held in the hand (other than a stylus, which is optional for most modern touchscreens). Such displays can be attached to computers, or to networks as terminals. They also play a prominent role in the design of digital appliances such as the satellite navigation devices, palmtop computer, mobile phones, and video games etc. How Does a Touch screen Work? A basic touch screen has three main components: a touch sensor, a controller, and a software driver. The touch screen is an input device, so it needs to be combined with a display and a PC (Personal Computer) or other device to make a complete touch input system.1.1 Touch SensorA touch screen sensor is a clear glass panel with a touch responsive surface. The touch sensor/panel is placed over a display screen so that the responsive area of the panel covers the viewable area of the video screen. There are several different touch sensor technologies on the market today, each using a different method to detect touch input. The sensor generally has an electrical current or signal going through it and touching the screen causes a voltage or signal change. This change is used to determine the location of the touch to the screen.1.2 ControllerThe controller is a small PC card that connects between the touch sensor and the PC. It takes information from the touch sensor and translates it into information that PC can understand. The controller is usually installed inside the monitor for integrated monitors or it is housed in a plastic case for external touch add-ons/ overlays. The controller determines what type of interface/connection you will need on the PC. Integrated touch monitors will have an extra cable connection on the back for the touch screen. Controllers are available that can connect to a Serial/COM port (PC) or to a USB port (PC or Macintosh). Specialized controllers are also available that work with DVD players and other devices.1.3 Software Driver The driver is a software update for the PC system that allows the touch screen and computer to work together. It tells the computer's operating system how to interpret the touch event information that is sent from the controller. Most touch screen drivers today are a mouse-emulation type driver. This makes touching the screen the same as clicking your mouse at the same location on the screen. This allows the touch screen to work with existing software and allows new applications to be developed without the need for touch screen specific programming. Some equipment such as thin client terminals, DVD players, and specialized computer systems either do not use software drivers or they have their own built-in touchscreen driver. Types of Touch Screen TechnologySome of the different existing and developing types of the touchscreen techniques are Resistive touchscreen Capacitive touchscreen Infrared touchscreen Surface Acoustic Wave (SAW) touchscreen Strain gauge touchscreen Optical imaging touchscreen Dispersive signal technology touchscreen2. Resistive touchscreen Resistive touch screen systems are the most common type of touch screen technology in todays market. These devices are used in many applications, such as cell phones, handheld games, GPS navigation devices, and even some digital cameras. A resistive touchscreen panel comprises several layers consist of a glass or acrylic panel that is coated with electrically conductive and resistive layers made with indiumtin oxide (ITO), the most important of which are two thin, transparent electrically-resistive layers separated by a thin invisible spacer (Fig 2.1). These layers face each other, with a thin gap between. One resistive layer is a coating on the underside of the top surface of the screen. Just beneath it is a similar resistive layer on top of its substrate. One layer has conductive connections along its sides, the other along top and bottom [2].

Fig. 2.1 Structure of Resistive Touch PanelsWhen an object, such as a fingertip or stylus tip, presses down on the outer surface, the two layers touch to become connected at that point. The panel then behaves as a pair of voltage dividers, one axis at a time. For a short time, the associated electronics (device controller) applies a voltage to the opposite sides of one layer, while the other layer senses the proportion (think percentage) of voltage at the contact point. That provides the horizontal [x] position. Then, the controller applies a voltage to the top and bottom edges of the other layer (the one that just sensed the amount of voltage); the first layer now senses height [y]. The controller rapidly alternates between these two modes. As well, it sends position data to the CPU in the device, where it's interpreted according to what the user is doing.Different configuration of resistive touch screen are available, namely 4-wire, 5-wire, 6-wire, 7-wire, and 8-wire systems where 8-wire is similar to 4-wire and the others are similar among themselves. The most popular are 4-wire and 5-wire systems. 2.1 4-wire resistive touch screenFig 2.2 shows the layer construction of resistive touch screen. The 4 wire resistive touchscreen (Fig 2.3) uses a glass panel with a uniform conductive ITO coating on the one-side surface. A PET (Polyethylene terephthalate) film is tightly suspended over the ITO coating surface of a glass panel. The glass substrate and the PET film are separated by tiny, transparent insulating dot spacers. The PET film has a hard coating on the outer side and a conductive ITO coating on the inner side. The structure is film-glass process. The early process is film-film-glass structure.

Fig. 2.2 Resistive touchscreen layer construction

Fig. 2.3 4-Wire Resistive Touch screen The x and y coordinates of a touch on a 4-wire touch screen can be read in two steps. First, Y+ is driven high, Y is driven to ground, and the voltage at X+ is measured. The ratio of this measured voltage to the drive voltage applied is equal to the ratio of the y coordinate to the height of the touch screen obtained from the equation (2.1). The y coordinate can be calculated as shown in Fig 2.4 from the equation (2.2). The x coordinate can be similarly obtained by driving X+ high, driving X to ground, and measuring the voltage at Y+. The ratio of this measured voltage to the drive voltage applied is equal to the ratio of the x coordinate to the width of the touch screen. This measurement scheme is shown in Fig 2.2. Fig. 2.4 Coordinate capturing (a) Capturing the touch, (b) Capturing the touch (2.1)

(2.2)

When the screen is touched, it pushes the conductive ITO coating on the PET film against the ITO coating on the glass. That results the electrical contact, producing the voltages. It presents the position touched. 2 The pins (X left) and (X right) are on the glass panel, and the pins (Y up) and (Y down) are the PET film. 3 The microprocessor applies +5V to pin (X left) on the glass panel, and the voltage is uniformly decreasing to pin (X right) for 0V because of the resistive ITO coating on the ITO Conductive Coating Glass Substrate PET Film Insulating Dot Spacer Hard Coating glass substrate, and the PET film is grounded. When the touchscreen is not touched, the controller detects the voltage on the PET film is zero. The next electric cycle, the microprocessor applies +5V to pin (Y up) on the PET film, and the voltage is uniformly decreasing to pin (Y down) for 0V. When the touchscreen is not touched, the controller detects the voltage on the glass panel is zero. 4. When the touchscreen is touched, a voltage on the glass substrate proportional to the X (horizontal) position of the touch appears on the PET film. This voltage is digitized by the A/D Converter and subjected to an averaging algorithm. Then it is stored and transferred to the host. Hence, the X position is produced. The next electric cycle, a voltage on the PET film proportional to the Y (vertical) position of the touch appears on the glass substrate. This voltage is digitized by the A/D Converter and subjected to an averaging algorithm. Then it is stored and transferred to the host. Hence, the Y position is produced.2.2 5-wire touch screenThe constructions of the panels are similar with 4-wire technology, but for a 5-wire touch screen all four bus bars are connected to the lower, non-flexible layer of the screen. The flexible layer is always used as a sense layer to read the voltage connection point to the bottom layer. A uniform voltage gradient is applied to one sheet. Whenever the second sheet touches the other sheet, the second sheet measures the voltage as a distance along the first sheet. This combination of voltage and distance provide X coordinate. After the X coordinate is located, the process repeats itself by applying uniform voltage gradient to the second sheet in order to find the Y coordinate. This entire process happens in a matter of milliseconds, oblivious to human eye [3].

Fig. 2.5 5-Wire Resistive Touch screenIn the Fig 2.5 , the five-wire design, one wire goes to the coversheet (E) which serves as the voltage probe for X and Y. Four wires go to corners of the back glass layer (A, B, C, and D). The controller first applies 5V to corners A and B and grounds C and D, causing voltage to flow uniformly across the screen from the top to the bottom. Upon touch, it reads the Y voltage from the coversheet at E. Then the controller applies 5V to corners A and C and grounds B and D, and reads the X voltage from E again.So, a five-wire touchscreen uses the stable bottom layer for both X- and Y-axis measurements. The flexible coversheet acts only as a voltage-measuring probe. This means the touchscreen continues working properly even with non-uniformity in the coversheet's conductive coating. The result is an accurate, durable and more reliable touchscreen.To get the "X" touch position the controller sets the pin corresponding to Y+ and Y- to 5V (Vref) and 0V(GND) respectively, now the position is read by the controller by converting the voltage to a number and sends it to the host computer. Similarly to get the "Y" touch position the controller set X+ and X- to 5V and 0V (GND), and voltage corresponding is sent to the host computer.A touch screen system has a touch panel (Fig. 2.3), a touch-screen controller, and a host processor. The touch screen or touch panel is the "resistive sensor" of the system. The topology of the touch-screen controller includes a driver for the panel, a multiplexer, and an ADC. The driver in the touch-screen controller independently powers both coordinates of the touch panel to ON or OFF. The amount of current conduction through the touch panel is approximately equal to the power-supply voltage divided by the touch-panel resistance. The ADC inside the touch-screen controller measures the touch position and pressure, by converting the analog voltage from the touch screen into digital code. Typically, the ADC topology is a successive approximation register (SAR) with resolutions of 8-, 10-, or 12-bits. There are two interfaces in the touch screen system an analog interface between the panel and the touch-screen controller, and a digital interface between the touch-screen controller and host. The touch-screen controller uses the 4-wire analog interface between the touch panel (Fig 2.2) and the touch-screen controller to power the panel and execute coordinate measurements. During a given X- or Y-coordinate measurement (Fig. 2.2), the touch-screen controller provides power through two wires (X+ and X-) of the analog interface to one ITO layer of the panel, and senses the coordinate location of the stylus using the second ITO layer with the other two wires (Y+ and Y-) (Fig. 2.4). 2.2 8-wire touch screen8-wire touch screens compensate for drift by adding 4 additional reference lines. This allows the voltage to be measured directly at the touch screen bus bars. Note: you can use an 8-wire touch screen in 4-wire mode by connecting the drive and reference lines together. Use of this type of touch screen wont eliminate the need for an initial calibration of the touch screen but should eliminate the need for any subsequent calibrations. Figure 3 shows the construction for making the measurements.In comparison to a 4-wire touch screen, an 8-wire touch screen adds sense wires to the end of each of the conductive bars. This allows any voltage offset created by the wiring or drive circuitry to be calibrated out during operation. An 8-wire touch screen is calibrated by measuring voltage extremes on either coordinate. First, Y+ drive is driven high and Y drive is driven low. The corresponding voltages measured at Y+ sense and Y sense are denoted VYmax and VYmin. A similar procedure yields VXmax and VXmin. These are the maximum and minimum possible voltages across each coordinate. The coordinates of a touch on an 8-wire touch screen can be read by first driving Y+ drive high, driving Y drive to ground, and reading the voltage at X+ sense. Using the maximum and minimum results obtained during calibration, the y coordinate can be calculated as shown in the equations in Figure2.6. The x coordinate can be obtained by driving X+ drive high, driving X drive to ground, and reading the voltage at Y+ sense. This process is shown in Fig 2.6.The digital communication between the processor and touch-screen controller includes an interrupt signal and a serial digital bus (SPI or I2C) (Fig. 2.4). The processor can ignore the touch-screen controller and focus on performing other tasks, if there is no panel touch event. As soon as the panel is touched, the interrupt from the touch-screen controller informs the processor. The processor then reads the touch-screen data from the touch-screen controller through the serial bus.

Fig. 2.6 8-Wire Resistive Touch screen

Fig. 2.7 Resistive touch screen system block diagram Resistive Touchscreen IntegrationResistive Touch Screens are in effect a lamination of circuit layers made from varying materials. Although in itself extremely robust some care should be taken when integrating a resistive Touch Screen into a bezel or housing.Two fundamentally different Touch Screen constructions are, A Film-On-Glass construction (Fig 2.8) consists of two circuit layers, whereby the bottom layer is an ITO coated piece of glass, to which the sensing circuitry has been applied. The top circuit on the other hand is made from an ITO coated Polyester sheet. Both circuits are bonded via an adhesive layer in the perimeter of the Touch Screen. This construction, as shown in Fig 2.8 offers the highest transmission and optical clarity.

Fig. 2.8 Typical Film-On-Glass constructionFor those applications where special glass is required a Poly Laminate construction (Fig 2.9). In this type of Touch Screen the bottom circuit consists of ITO coated Polyester, which is laminated onto a glass plate by means of an optical adhesive. The glass can therefore be thermally or chemically strengthened offering up to three to four times the strength of normal glass. The fact that the circuit consists of a non-breakable polymer material means that even in the event of the glass cracking the sensor will remain electrically functional.

Fig. 2.9 Typical Poly-Laminate constructionThere are two fundamental ways in which a Touch Screen can be integrated into a bezel. The simplest method is to clamp a Touch Screen between a bezel and the display. This integration method is referred to as rear mount integration. The Touch Screen is hereby mounted behind a bezel with a rectangular cut out in the dimensions of the Touch Screens viewing area.The Touch Screen is positioned behind this cut out and typically clamped between some mounting plates or directly by the LCD. Rubber or foamed rubber gaskets hereby ensure that the screen is environmentally sealed and cannot be displaced. Fig 2.10 shows typical side view of a rear mount Touch Screen integration.Fig. 2.10 Typical side view of a rear mount Touch Screen integrationAlternatively you may choose front mount Touch Screen integration. The Touch Screen hereby is assembled into a sill that is molded or machined into the front of the bezel. A flexible graphic overlay is then applied to the surface of the bezel and the Touch Screens perimeter in order to seal the gap between the bezel and the Touch Screen. Fig. 2.8 shows the side view of a typical front mounted Touch Screen. Fig. 2.11 Typical front mounts Touch Screen (covered by a graphic overlay)

Resistive technology is cheap, making it viable for high volume applications. But wider acceptance is limited due to disadvantages like mechanical weakness, few design options, the need for a bezel, thickness of the screen, poor optical performance, and the need for user calibration. Also, it cant sense an approaching finger or multiple fingers. Advantages: Cost effective and durable Can works in all climate conditions and in any environment like the restaurants, hospitals, etc. Input can be made using a stylus or finger. However, stylus is better suited. Less sensitive to scratches as compared to capacitive screensDisadvantages: Normally does not support multi-touch though Nokia has announced that on its handsets, multi-touch would be supported on both resistive and capacitive touch screens Recalibration might be required over time as resistive touch screens are known to drift over time. However, this is a simple process and can be performed by users by a simple dot mapping. While using fingers, slightly more pressure needs to be put on the screen Provides only 75% optical transparency which means that its clarity is lower than capacitive touch screen3. Capacitive touchscreenA capacitive touchscreen consists of glass panel with a capacitive (charge storing) material coating its surface. Circuit located at corners of the screen measure the capacitance of a person touching the overlay. Frequency changes are measured to determine the X and Y co-ordinates of the touch event. A capacitive touchscreen panel consists of an insulator such as glass, coated with a transparent conductor such as indium tin oxide (ITO). As the human body is also an electrical conductor, touching the surface of the screen results in a distortion of the screen's electrostatic field, measurable as a change in capacitance.Derivations of capacitive touch screens include: Surface Capacitance Projected Capacitance Mutual Capacitance Self Capacitance 3.1 Surface CapacitanceCapacitive touch panels offer outstanding accuracy, optics durability and touch accuracy. Slim border design allows better integration for new generation flat panel displays. With excellent optical transmission, low reflection, and minimized color distortion, the capacitive touch panel consists of multilayer coatings on a glass panel. The layered structure is shown in (Fig. 3.1). Transparent conductive coatings are coated on both sides of the glass panel. Specially designed electrodes are laid around the panels edge on top of the front-side conductive coating to evenly distribute a low voltage across the front side conductive coating, creating a uniform electric field. The backside conductive coating is used for electromagnetic interference (EMI) shielding. A hard coat layer is laid on top of the front-side conductive coating to provide protection to the front-side conductive coating. The touch screen coated material that stores electrical charges, when touched allows a small amount of charge to be drawn to the point of contact. Circuits located at each corner of the panel measure the charge and send the information to the controller for processing. Capacitive touch screen panels must be touched with a finger unlike resistive and surface wave panels that can use fingers and stylus.

Fig 3.1 Surface Capacitance The sensor's controller can determine the location of the touch indirectly from the change in the capacitance as measured from the four corners of the panel. As it has no moving parts, it is moderately durable but has limited resolution, is prone to false signals from parasitic capacitive coupling, and needs calibration during manufacture. Small amount of voltage is applied to the electrodes on the four corners. A human body is an electric conductor, so when you touch the screen with a finger, a slight amount of current is drawn, creating a voltage drop. The current respectively drifts to the electrodes on the four corners. Theoretically, the amount of current that drifts through the four electrodes should be proportional to the distance from the touch point to the four corners. The controller precisely calculates the proportion of the current passed through the four electrodes and figures out the X/Y coordinate of a touch point.3.2 Projected capacitive TouchscreenThe Projected capacitive touchscreens (Fig.3.2) use a glass substrate with a tin oxide coating that is charged with a slight electrical current. When a conductive stylus or finger touches the surface, it creates a capacitive coupling that causes a current draw at that point. The x- and y-coordinates can then be determined a capacitive touchscreen sensor consists of a large array of indium tin oxide (ITO) conductors on one or more layers of glass or polyethylene terephthalate (PET) plastic. The good optical clarity and low resistivity of ITO make it the perfect conductor for creating a touchscreen. When the ITO sensor is connected to a capacitive sensing chip with a suitably high signal-to-noise ratio (SNR), it can accurately sense minute changes in capacitance. A finger's presence for instance is on the order of a picoFarad (10-12 Farads). It is typically accompanied by background capacitances of 10's of nanoFarads (10-9Farads). This situation makes the sensing environment challenging and mandates an exceptionally high SNR. Charge transfer technology is well suited to high SNR capacitive sensing systems. It allows the capacitive system to sense minute changes in capacitance even from a finger as it approaches the phone before it touches it or from the touch of a fingernail.Charge transfer technology enables high SNR by using a pair of sensing electrodes for each capacitive channel. One is a transmit electrode into which a charge consisting of logic pulses is driven. The receive electrode couples to the emitter via the overlying panel dielectric. When a finger touches the panel, the field coupling is reduced and touch is detected. Most charge signal acquisition techniques leave the charge lines hot (sensitive to the touch) during signal conversion. The current on the sensor edge wiring can be included as part of the position calculation, introducing positional inaccuracy to the measurement. The contribution of the edge wiring increases with the length of the routing between the sensor and the driver chip and becomes seriously problematic if the distance exceeds a few centimeters. The charge transfer technique holds the receive lines at zero potential during the charge acquisition process and solves this problem, effectively restricting the transfer of charges to those between the transmitter X and receiver Y electrodes at the point of interest in the main sensor area.This "charge-transfer" signal-acquisition technique uses individual resistive one-dimensional stripes to create a touchscreen. These stripes can be read either in parallel or sequentially, since the connections to these stripes are independent of one another. There is an interpolated coupling between adjacent lumped electrode elements and an object such as a finger.. Fig 3.2 Projected capacitive touchscreenProjected capacitive touch (PCT) technology is a capacitive technology which allows more accurate and flexible operation, by etching the conductive layer. An X-Y grid is formed either by etching one layer to form a grid pattern of electrodes, or by etching two separate, perpendicular layers of conductive material with parallel lines or tracks to form the grid comparable to the pixel grid found in many liquid crystal displays (LCD).3.3 Mutual capacitanceIn mutual capacitive sensors, there is a capacitor at every intersection of each row and each column. A 16-by-14 array, for example, would have 224 independent capacitors [3]. A voltage is applied to the rows or columns. Bringing a finger or conductive stylus close to the surface of the sensor changes the local electrostatic field which reduces the mutual capacitance. The capacitance change at every individual point on the grid can be measured to accurately determine the touch location by measuring the voltage in the other axis. Mutual capacitance allows multi-touch operation where multiple fingers, palms or styli can be accurately tracked at the same time.3.4 Self-capacitanceSelf-capacitance sensors can have the same X-Y grid as mutual capacitance sensors, but the columns and rows operate independently. With self-capacitance, the capacitive load of a finger is measured on each column or row electrode by a current meter. This method produces a stronger signal than mutual capacitance, but it is unable to resolve accurately more than one finger, which results in "ghosting", or misplaced location sensing.Advantages: Higher clarity display (up to 90% optical transparency) Supports multi-touch High touch resolution High sensitivityDisadvantages: Needs a human finger to register input. It is not possible to use this screen wearing gloves Cannot be used in all weather scenarios4. InfraredInfrared Touch Screen technology relies on the interruption of an infrared light grid in front of the display screen. The touch frame or opto-matrix frame contains a row of infrared LEDs and photo transistors, each mounted on two opposite sides to create a grid of invisible infrared light.Fig 4.1 Infrared touchscreen An infrared touchscreen uses an array of X-Y infrared LED and photo detector pairs around the edges of the screen to detect a disruption in the pattern of LED beams. These LED beams cross each other in vertical and horizontal patterns. This helps the sensors pick up the exact location of the touch. A major benefit of such a system is that it can detect essentially any input including a finger, gloved finger, stylus or pen. It is generally used in outdoor applications and point of sale systems which can't rely on a conductor (such as a bare finger) to activate the touchscreen. Unlike capacitive touchscreens, infrared touchscreens do not require any patterning on the glass which increases durability and optical clarity of the overall system. The frame assembly is comprised of printed wiring boards on which the opto-electronics are mounted and is concealed behind an infrared-transparent bezel (Fig. 4.1). The bezel shields the opto-electronics from the operating environment while allowing the infrared beams to pass through. The infrared controller sequentially pulses the LEDs to create a grid of infrared light beams. When a stylus, such as a finger, enters the grid, it obstructs the beams. One or more phototransistors detect the absence of light and transmit a signal that identifies the x and y coordinates. Infrared Touch Screens are often used in manufacturing and medical applications because they can be completely sealed and operated using any number of hard or soft objects. Fig 4.2 Infrared touchscreen TIRInfrared touchscreen utilizes the concept of Total Internal Reflection (Fig. 4.2). The surface of the table is a thin diffuser, which has infrared light and a projector reflected on its underside. Infrared cameras pick up on any objects such as fingers, paintbrushes over the surface, when touching the display.Advantages Excellent clarity of image Less prone to vandalism(scratching etc)Disadvantage Expensive to manufacture (number of Light Emitting Diodes (LEDs) required) Overly sensitive, touch can be sensed before screen touched. That is the major issue with infrared is the seating of the touch frame is slightly above the screen. Consequently, it is susceptible to early activation before the finger or stylus has actually touched the screen. The cost to manufacture the infrared bezel is also quite high

5. Surface Acoustic Wave (SAW) TouchscreenSurface waves are readily absorbed when a soft object such as a fingertip touches the substrate. SAW Touch Screen use pure glass with transmitting and receiving piezoelectric transducers for both the X and Y axes. The touch screen controller sends an electrical signal to the transmitting transducer, which converts the signal into ultrasonic waves within the glass. When you touch the screen, you absorb a portion of the wave traveling across it. The received signal is then compared to the stored digital map, the change recognized, and a coordinate calculate.On the pure glass substrate, there are four piezoelectric transmitting and receiving transducers on the three corners for both the X and Y axes. Around the glass, there are four 45-degree reflectors around the glass, divert the ultrasonic bust across the touchscreen.The SAW controller sends a 5 MHz electrical signal to the X-axis and Y-axis transmitting transducers. They convert the signal into ultrasonic waves to the reflectors. These waves are changed direction across the front surface of the touchscreen by a 45-degree array of reflectors. The 45-degree reflectors on the opposite side gather and re-direct the waves to the X-axis and Y- axis receiving transducers, which reconvert them into an electrical signal. The signal is represented by a wavy curve on an oscillo-graph. When the touchscreen is touched, the finger absorbs a portion of the wave passing cross the surface of the panel. The signal received by the receiving transducers is then compared to the wavy curve that is produced when the touchscreen is not touched. The microprocessor in the

Fig 5.1 Surface Acoustic Wave (SAW) Touchscreencontroller recognizes the change of the wave and calculates a coordinate. This process happens independently for both the X and Y axes. The coordinates are transmitted to the host for processing.Advantages: Better clarity of image than Resistive and CapacitiveDisadvantages: Must use soft object to enable touch. Cannot be completely sealed (affected by water and dust). Surface contaminants cause dead spots.6. The Strain-gauge Touch Screen A strain gauge is a device that is used to measure the strain that occurs in an object. The device was invented in the year 1938 by Edward E. Simmons and Arthur Ruge. The device is still being used in many electronic circuits mainly as the principle sensing element for sensors like torque sensors, pressure sensors, load cells and so on.A strain gauge consists of a foil of resistive characteristics, which is safely mounted on a backing material. When a known amount of stress is applied on a resistive foil the resistance of the foil changes accordingly, thus, there is a relation between the change in the resistance and the strain applied. This relation is known by a quantity called gauge factor. The change in the resistance can be calculated with the help of a Wheatstone bridge. The strain gauge used is connected to the Wheatstone bridge with the help of an adhesive called cyanocrylate [9].As shown in the Fig 6.1, the imbalance is detected by the voltmeter in the center of the bridge circuit. The resistance R2 will be a rheostat and hence adjustable. The value of this resistance is made equal to the strain gauge resistance without the application of any force. The resistances R1 and R3 will have equal values. Thus, according to the Wheatstone bridge principle the entire circuit will be balanced and the net force will be zero. Thus the strain will also be zero. Now provide a compression or tension on the conductor and the circuit will be imbalanced. Thus you will get a reading at the voltmeter. Thus, the strain produced in response to the measured variable (mechanical force), is known as a quarter-bridge circuit (Fig. 6.1). The Strain-gauge Touch Screen has pressure sensors that measure at each corner the stresses that a touch to the screen produces. The ratio of the four readings indicates the touch point coordinates. The platform touch screen doesnt use a screen. Instead, the monitor or display device rests on a platform with force measurement sensors at the corners of the base. A touch to the display device translates to forces at the platforms base corners.

Fig 6.1 Strain gauge Wheatstone bridge The platforms controller performs the vector calculations that determine the touch point from the four force measurements through rigid body mechanics. The controller tracks out static forces, such as gravity, and repetitive forces, such as vibration. This type also has no glass panel construction that may reduce visibility of the display. The platform type is a good concept in theory because there is no integration of touch components into the display. You need only set the display on the touch base, calibrate and go. Practically, problems occur when the display is moved only a very small amount on the platform base or if even the display is tipped up or down for different heights of viewing. This throws off the base vector values as initially calibrated and therefore the calibration. The life span is excellent (ideally infinity).

7. Optical Imaging touch screenThis is a brand-new touch technology, in which two optical sensors are placed around the top left and right corner of the screen (Fig. 7.1). Two optical sensors track the movement of any object close to the surface by detecting the interruption of an infra-red light source which is placed in the cameras field[4][5].Optical Imaging touch screen technology revolutionizes the way we interface with computer technology. Unlike many touch screen displays, the entire screen, corners included, is sensitive to the touch. This technology uses optical components. No surface coatings are used on the screens - hence images are kept crystal clear. Any method can be used to touch the screen: a finger, a gloved hand or any pointer (Fig 7.1).

Fig 7.1 Optical Imaging touch screenOnly a light touch is required. Optical imaging technology provides touch sensitivity over the whole screen, including the corners. With over 400,000 touch points, accuracy is guaranteed. Optical imaging technology touch screens are calibrated just once at commissioning time. The technology provides continuous operation with no drift, no need to recalibrate. Because optical imaging technology solutions don't employ surface coatings, the customer-facing screen is not affected by scratches or contamination. Also, the products can be easily sealed for resistance to dirt, dust and moisture, making them ideal for demanding public environments. The modular touch screen system can be replaced in the event of failure or damage, provides two-touch capability, middle mouse-key scrolling and object size recognition. Diagnostic utilities are also available. VarTechs optical imaging touch solutions do not require special software drivers; they incorporate HID compliant USB plug-and-play interfaces.Advantages Strengths of this approach include compatibility with bare fingers, gloves, or standard styluses, as well as support for multi-touch input and high light transmittance similar to that of an SAW touch panel. Structure can be easily extended to larger panels and high durability, since no direct contact with the sensor components occurs.Disadvantage This technology requires a thick bezel, since the image sensor is installed outside the monitor area. Sensing precision is also susceptible to influence from external light. Sensing errors can occur if strong light is reflected from the screen or if users attempt touch operation with glossy objects.8. Dispersive Signal TechnologyDispersive Signal Technology, specifically developed for interactive digital signage applications, sets new large-format touch standards for fast, accurate repeatable touch response. In addition, Dispersive Signal Technologys operation is unaffected by contaminants, static objects or other touches on the touch screen [5]. Other key characteristics of this patented technology are exceptional optics, ease of integration, and input flexibility.8.1 Key Technology Characteristics Fast, accurate and repeatable touch response Operation unaffected by surface damage, including Scratches Input flexibility, accepting touch from finger, pencil, credit card, fingernail, or almost any type of stylus Operates with static objects or other touches on the screen Exceptional optical characteristicsDispersive Signal Technology determines a touch point by measuring the mechanical energy (bending waves) within a substrate created by a finger or stylus touching the surface of the glass [6]. Bending waves differ from surface waves in that they traverse through the thickness of the panel rather than the surface of the material, which provides several important advantages including enhanced palm rejection and superior scratch resistance. When the touch implement impacts the screen, bending waves are induced that radiate away from the touch location. As the wave travels outwards, the signal spreads out over time due to the phenomena of dispersion.Piezoelectric sensors positioned in the corners on the backside of the glass convert this smeared mechanical impulse into an electrical signal. The distance from each sensor determines the extent to which the signal is dispersed. Namely, the further away the touch point is from the sensor, the more the signal is smeared.9. Conclusion The advancement in the field of Touchscreen technology has created a new sense in the life style of people. Touch screens are used in hospitals, ATM, in operating units of heavy industries or controlling the machines, touch screen locks. Touch screen is widely used in computer and TV displays and its demand is tremendously increasing.Touch screen technology will increase in significance as an I/O technique for user oriented embedded systems. Vendors have been steadily reducing or eliminating the weaknesses in touch sensors as well as adding new capabilities. This combination of steady improvement punctuated by innovation will continue to broaden the range of applications that touch screens can serve.Moving towards the future, consumers will continue to see the growth of the touch screen industry, due to extensive engineering advancements in user interfaces. The ability to physically touch a screen is easier than searching for a specific key in a sea of buttons. Society, for these reasons, has found touch screens to be the future of many devices. The social norm of today includes walking down the street surfing the web on an iPhone or sifting through music on an iPod Touch. No additional buttons are necessary, just the small, portable device in ones pocket until needed, society will continue to see the development of touch screen technology as human-device interaction is perfected.

"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius -- and a lot of courage -- to move in the opposite direction."- Albert Einstein

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