VISHVESHWARAIAH TECHNOLOGICAL UNIVERSITY S.D.M COLLEGE OF ENGINEERING AND TECHNOLOGY A seminar report on TOUCH SCREEN Submitted by Roshan Kamath 2SD06CS078 8 th semester DEPARTMENT OF COMPUTER SCIENCE ENGINEERING 2009-10 1
VISHVESHWARAIAH TECHNOLOGICAL UNIVERSITY S.D.M COLLEGE OF ENGINEERING AND TECHNOLOGY DEPARTMENT OF COMPUTER SCIENCE ENGINEERING CERTIFICATE Certified that the seminar work entitled Touch Screen is a bonafide work presented by Roshan Kamath bearing USN NO 2SD06CS078 in a partial fulfillment for the award of degree of Bachelor of Engineering in Computer Science Engineering of the Vishveshwaraiah Technological University, Belgaum during the year 2009-10. The seminar report has been approved as it satisfies the academic requirements with respect to seminar work presented for the Bachelor of Engineering Degree. Staff in charge H.O.D CSE Name: Roshan Kamath. USN:2SD06CS0078 2
INDEX 1. Introduction 4 2. History 4 3. Capability 5 4. Components that act as Backbone for this technology 5 4.1 Touch sensor 4.2 Controller 4.3 Software Driver 5. Technology 5.1 Infrared 5.2 Resistive 5.3 Surface acoustic wave (SAW) 5.4 Projected capacitative 5.5 Surface capacitance 7 6. Construction 12 7. Comparison of technologies 13 8. Understanding how touch screen accepts input using resistive technology as an example. 13 9. Conclusion 10. References 18 18 3
1. Introduction: TouchScreens are display as well as input devices. These are electronic visual devices that are sensitive to pressure thus detect the presence and location of a touch within the display area. The screens are sensitive to pressure; a user interacts with the computer by touching pictures or words on the screen. The term Touch generally refers to touch or contact to the display of the device by a finger or hand. However, if the object sensed is active, as with a light pen, the term touch screen is generally not applicable. The ability to interact physically with what is shown on a display (a form of "direct manipulation") typically indicates the presence of a touchscreen. 2. History: Touch screens originally emerged from academic and corporate research labs in the second half of the 1960s. In 1971, the first "touch sensor" was developed by Doctor Sam Hurst (founder of Elographics) while he was an instructor at the University of Kentucky. This sensor was called the "Elograph" and was patented by The University of Kentucky Research Foundation. One of the first places where they gained some visibility was in the terminal of a computer-assisted learning terminal that came out in 1972 as part of the PLATO project. They have subsequently become familiar in kiosk systems, such as in retail and tourist settings, on point of sale systems, on ATMs and on PDAs where a stylus is sometimes used to manipulate the GUI and to enter data. The popularity of smart phones, PDAs, portable game consoles and many types of information appliances is driving the demand for, and the acceptance of, touchscreens. The HP-150 from 1983 can be considered as the world's earliest commercial touch screen computer. It doesn't actually have a touch screen in the strict sense, but a 9" Sony CRT surrounded by infrared transmitters and receivers which detect the position of any nontransparent object on the screen. Until the early 1980s, most consumer touch screens could only sense one point of contact at a time, and few have had the capability to sense how hard one is touching. With commercialization of touchscreens the technology used changed to multipoint technology from dingle point. Historically, the touchscreen sensor and its accompanying controller-based firmware have been made available by a wide array of after-market system integrators and not by display, chip or motherboard manufacturers. With time, however, display manufacturers and chip manufacturers worldwide have acknowledged the trend toward acceptance of touchscreens as a highly desirable 4
user interface component and have begun to integrate touchscreen functionality into the fundamental design of their products. 3. Capability: The Touch Screens come with a variety of definite advantages over normal /conventional input-output devices. Some of them are discussed below. Easy to use: This provides for a rich user interface experience as this supports for a very intuitive easy to use environment and is facilitated by just a touch. Saves space: In this world where cost of real estate [i.e., property prices] are sky rocketing intelligent utilization of space is of great importance. Thus touch screens facilitate for this by saving space of keyboard and this finds many application in day today activities. Speed and Reliability: While laptops do come with a mouse pad and a USB port to allow you to attach an external mouse to your laptop for easier navigation, the amount of time spent to do simple navigations with these devices are extremely slow as compared to simply touching the screen and pointing directly at the option. Having a touchscreen laptop would make navigation extremely faster and more reliable. No need to worry about clicking the wrong option, especially if you are making transactions over the Internet. 4. Components that act as Backbone for this technology: A basic touchscreen has three main components: a touch sensor, a controller, and a software driver. The touchscreen is an input-output device, so it needs to be combined with a display and a PC or other device to make a complete touch input system. 4.1 Touch Sensor A 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 5
change. This voltage change is used to determine the location of the touch to the screen. 4.2. Controller The 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 touchscreen. 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. 4.3.Software Driver The driver is a software update for the PC system that allows the touchscreen 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 mouseemulation type driver. This makes touching the screen the same as clicking your mouse at the same location on the screen. This allows the touchscreen to work with existing software and allows new applications to be developed without the need for touchscreen 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 touch screen driver. SOFTWARE DRIVER CONTROLLER TOUCHSCREEN SENSOR SETUP Computer [Can also mean processor of hand held devices] Figure 4.3.1 6
5. Technologies: There are a number of types of touchscreen technologies. Some of them are 5.1 Infrared In this technology infrared(ir) light-emitting diodes(leds) are placed at the opposite edges [ aks. Bezel edges] to analyze the system and detect the touch event.the LED and photosensor pairs create a grid of light beams across the display. An object (such as a finger or pen) that touches the screen interrupts the light beams, causing a measured decrease in light at the corresponding photosensors. The measured photosensor outputs can be used to locate a touch-point coordinate. Widespread adoption of infrared touchscreens has been hampered by two factors: the relatively high cost of the technology compared to competing touch technologies and the issue of performance in bright ambient light. This latter problem is a result of background light increasing the noise floor at the optical sensor, sometimes to such a degree that the touchscreen s LED light cannot be detected at all, causing a temporary failure of the touch screen. This is most pronounced in direct sunlight conditions where the sun has a very high energy distribution in the infrared region. However, certain features of infrared touch remain desirable and represent attributes of the ideal touchscreen, including the option to eliminate the glass or plastic overlay that most other touch technologies require in front of the display. In many cases, this overlay is coated with an electrically conducting transparent material such as indium-tin oxide (ITO), which reduces the optical quality of the display. This advantage of optical touchscreens is extremely important for many device and display vendors since devices are often sold on the perceived quality of the user display experience. Another feature of infrared touch which has been long desired is the digital nature of the sensor output when compared to many other touch systems that rely on analog-signal processing to determine a touch position. These competing analog systems normally require continual recalibration, have a complex signal-processing demand (which adds cost and power consumption), demonstrate reduced accuracy and precision compared to a digital system, and have longer-term system-failure modes due to the operating environment. 7
Figure 5.1.1 5.2 Resistive A resistive touchscreen panel is composed of several layers, the most important of which are two thin, metallic, electrically conductive layers separated by a narrow gap. When an object, such as a finger, presses down on a point on the panel's outer surface the two metallic layers become connected at that point: the panel then behaves as a pair of voltage dividers with connected outputs. This causes a change in the electrical current which is registered as a touch event and sent to the controller for processing. 8
Figure 5.2.1 5.3 Surface acoustic wave Surface acoustic wave (SAW) technology uses ultrasonic waves that pass over the touchscreen panel. When the panel is touched, a portion of the wave is absorbed. This change in the ultrasonic waves registers the position of the touch event and sends this information to the controller for processing. Surface wave touch screen panels can be damaged by outside elements. Contaminants on the surface can also interfere with the functionality of the touchscreen[1]. 9
Figure 5.3.1 5.4 Projected Capacitive Projected Capacitive Touch (PCT) technology is a capacitive technology which permits more accurate and flexible operation, by etching the conductive layer. An XY array is formed either by etching a single 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 LCD displays). Applying voltage to the array creates a grid of capacitors. Bringing a finger or conductive stylus close to the surface of the sensor changes the local electrostatic field. The capacitance change at every individual point on the grid can be measured to accurately determine the touch location[5]. The use of a grid permits a higher resolution than resistive technology and also allows multitouch operation. The greater resolutions of PCT allows operation without direct contact, such that the conducting layers can be coated with further protective insulating layers, and operate even under screen protectors, or behind weather and vandal-proof glass. 10
Figure 5.4.1 5.5 Surface capacitance In this basic technology, only one side of the insulator is coated with a conductive layer. A small voltage is applied to the layer, resulting in a uniform electrostatic field. When a conductor, such as a human finger, touches the uncoated surface, a capacitor is dynamically formed. 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. It is therefore most often used in simple applications such as industrial controls and kiosks[4]. 11
Figure 5.5.1 6. CONSTRUCTION: There are several principal ways to build a touchscreen. The key goals are to recognize one or more fingers touching a display, to interpret the command that this represents, and to communicate the command to the appropriate application. In the most popular techniques, the capacitive or resistive approach, there are typically four layers; 1. Top polyester layer coated with a transparent metallic conductive coating on the bottom 2. Adhesive spacer 3. Glass layer coated with a transparent metallic conductive coating on the top 4. Adhesive layer on the backside of the glass for mounting. When a user touches the surface, the system records the change in the electrical current that flows through the display. Dispersive-signal technology which 3M created in 2002, measures the piezoelectric effect the voltage generated when mechanical force is applied to a material that occurs chemically when a strengthened glass substrate is touched. 12
7. Comparison of touch screen technologies: 8. Understanding how touch screen accepts input resistive technology as an example : [5][6][7] This part can be considered as a case study where we shall explore resistive screen technology and understand how exactly the exact location of touch will be calculated by the controller [AD7879] circuit. 13
Figure 8.1 Figure 8.1 Shows a basic diagram of the construction and operation of a touch screen. The screen is formed by two plastic films, each coated with a conductive layer of metal usually indium tin oxide (ITO) that are separated by an air gap. One plate, the X-plate in the diagram above, is excited by the supply voltage. When the screen is touched, the two conductive plates come together, creating a resistor divider along the X-plate. The voltage at the point of contact, which represents the position on the X-plate, is sensed through the Y+ electrode, as shown in Figure 8.2. The process is then repeated by exciting the Y-plate and sensing the Y position through the X+ electrode. 14
Figure 8.2 Next, the supply is placed across Y+ and X, and two further screen measurements are made: Z1 is measured as the voltage at X+, and Z2 is measured as the voltage at Y. These measurements can be used to estimate the touch pressure in one of two ways. If the resistance of the X-plate is known, the touch resistance is given by: 15
Wake Up on Touch The AD7879 can be configured to start up and convert when the screen is touched and to power down after release. This can be useful for battery-powered devices where power conservation is important. After each conversion sequence, the AD7879 delivers an interrupt to the host microcontroller, waking it from its low-power mode to process the data. Thus, the microcontroller also draws little power until the screen is touched. Figure 3 shows the setup for the wake-up-on-touch function. When the screen is touched, the X- and Y-plates connect, pulling the deglitch input low and waking the AD7879, which then starts converting. An interrupt is sent to the host at the end of the conversion. 16
Touch screen acquisition flowchart: Initially the touch screen controller s pen interrupt (PENIRQ) is held high. When the user touches the screen this line is made low indicating the drivers to read X, Y and if needed Z coordinate. Drivers for X, Y and Z are activated in succession to take the reading. In the averaging stage the inputs which are read are checked to fall within a permissible limit and if found to fall outside this limit then these are discarded and are to be read again. This is computed for all three coordinates. Thus the end result of this reading is the coordinate where the touch has been sensed. 17
9. Conclusion: Touch screen can be considered as the future on which all new gadgets shall bank on. We are already in the era that has seen many ground breaking technologies emerge and touch screen is one amongst them which has changed the way users interact with their gadgets to a whole new level. Its needless to say that the further improvement in this technology is inevitable and this can change the way we think what input-output devices are. Our world today has already started to see the emergence of visual screen that are flexible, can be worn on wrist like a wrist watch,gadgets with gait recognition all thanks to ground breaking technology of touchscreens that set the ball rolling. 10. References: 1. Shneiderman, B. Touch screens now offer compelling uses. IEEE Software, 8, 2, (March 1991) 93-94, 107. 2. Potter, R., Weldon, L. and Shneiderman, B. Improving the accuracy of touch screen: An experimental evaluation of three strategies. Proc. CHI'88. (Washington, D.C., May, 1988), ACM Press, 27-32. 3.Sears, A., Plaisant, C., and Shneiderman, B. A new era for high precision touchscreens. In Advances in Human-Computer Interaction, 3 (1992), Hartson, R and Hix, D. (Eds.), Ablex, NJ, 1-33. 4. Sears, A. and Shneiderman, B. High precision touchscreen: Design strategies and comparison with a mouse. Int. J. of Man-Machine Studies, 34 (1991), 593-613. 5. Rick Downs: Using resistive touch screens for human/machine interface. 6. Pratt, Susan. Ask The Applications Engineer 35, "Capacitance Sensors for Human Interfaces to Electronic Equipment." Analog Dialogue. Volume 40, Number 4. www.analog.com/library/analogdialogue/archives/40-10/cap_sensors.html. 7. Kearney, Paul. "The PDA Challenge Met by the AD7873 Resistive-Touch-Screen Controller ADC." Analog Dialogue. Volume 35, Number 4. www.analog.com/library/analogdialogue/archives/35-04/touchscreen. 18
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