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It is known that touchscreen is actually a combination of LCD screen and touch sensors. Theory of LCD screen display didn`t change too much throughout years as it reflect the light so that pattern can be obtained and with optical or digital filter colored pattern can be obtained. So the reason why there are so many touchscreen technologies available on the market is because of different types of sensors applied. Those sensors apply different sensing methods which include resistive, Surface acoustic wave, capacitive, infrared, strain gauge, optical imaging, dispersive signal and acoustic pulse recognition. Some of the above sensing method has been used in mass production, but some are still in the early experimental stage. In this report resistive, capacitive and infrared touchscreen will be introduced in detail as they are the most common sensing method and can be experiment on.
Resistive touchscreen is a type of passive technology because the screen responds to physical pressure. Two flexible sheets are utilized, separated by air gaps and covered by resistive material. A uniform voltage gradient is applied to one sheet. Whenever the second sheet touches the first sheet, the second one measures the voltage as a distance along the first sheet. So this combination of the voltage and distance provides X coordinate. The same method can be applied to locate the Y coordinate. Using the X-Y coordinates, the precise location of the input can be determined.
Resistive touch-screen has an advantage compared to other touchscreen technology. The cost of resistive touch-screen is lower and also supports multi-touch input. But this technology also has some limitations. Due to the nature of passive touch-screen design, the touching region cannot be large.
Capacitive touch-screen has a panel, which consists of an insulator coated with a transparent conductor. Since the human body is also a conductor. When a user touches the surface, it will distort electrostatic field and measure by a change in capacitance. Different technologies may be applied to register the location of the touch area. For instance, in the surface capacitance, only one side of the insulator is coated with a conductive layer. A small voltage is applied to the layer, which results in a uniform electrostatic field. When a conductor touches the uncoated surface, a capacitor is formed. The sensorÂ¡Â¯s controller can determine the location of the touch from the change in the capacitance.
Compared with resistive touch-screen, capacitive touch-screen is faster and more responsive. Another advantage is that capacitive touch-screen can be cleaned with clothes because it only responds to conductive materials. But it also has some disadvantage. It is more expensive to manufacture. Compared with resistive touch-screen, the accuracy of the intended location is low.
We will talk about the infrared touch-screen in the last section. An infrared touch-screen 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. LED means light-emitting diode that is a semiconductor light source. It doesn't rely on a conductor, so the user can touch it with gloves.
In this report three type of touch sensing method will be experimented.
Due to limited resources we might use simplified method and some visual effect to explain the theory.
Experiment 1: Resistive type touch sensor matrix
Equipment: 8051 micro controller kitset (see appendix for detail)
8051 micro controller kitsetMethod: Resistive type sensor matrix used in modern touch screens has thousands of columns and rows for better point recognition but it is relatively more complex in data processing. The software and hardware design in the data processing are not revealed by company due to confidentiality, however several techniques were developed to cut the hardware cost. Most common techniques are line reversal technique, Row scanning method and time multiplexing technique. In this experiment a 4 X 4 touch sensor matrix and line reversal technique is introduced for better understanding and easier experiment.
A virtual diagram in the right hand side is created to illustrate how line reversal technique is used. One end of all row and columns are connected to 5V, the other end connect to microcontroller bidirectional pin.
Graphical illustration A program is written to continuously set columns and rows to toggle between output 0V and input alternatively so that at any point of time either column pins or row pins output 0V and the others are set to be input for a short duration until next system clock comes.
When a switch between row 3 and column 3 is open, the switch resistance is infinity and 5V will appear at input duration for both row and column.
When switch is closed and either column or row pins output a 0V, switch resistance will goes to zero and the opposite input pins will be short to 0V.
Then system can obtain an x-y coordinates by reading 0V at respective rows and columns in a high speed. Using line reversal technique (see appendix for detail) to implement the program can grant any system to perform x-y coordinates reading action with minimum numbers of microcontroller pin. At last, system will display the character of input coordinates on LCD screen.
Outcome: The respective key character will display on the respective position of screen when pressed.
Advantage: Save number of pins required to read all the input data. It can read any pointing device or stylus object which in other sensing method can`t read. This type of matrix can be very cheap and ready for mass production.
Disadvantage: Transparency of matrix remains at 80% because of the density of the columns and rows. It is not ideal transparency for touchscreen. A physical press action, which affects the endurance of matrix and unnecessary work done for user, must be done at surface of sensing matrix in order to trigger the matrix.
Experiment 2: Capacitive type touch
Equipment: Oscilloscope, Perfboard, PC and Multi-purpose chipset using Hitachi H8/2633 microcontroller developed by Yuan Shenghai.
Method: Touchscreen is actually using ITO (Indium tin oxide) to do the sensing part in Iphone. Here prefboard is used to replace the ITO due to large quantity of minimum order set by supplier and also for better observation. Engineers from Apple Computer are actually put large numbers of ITO pieces on the LCD screen and different generations have different way of arrangement. C:\Users\Snake\Desktop\New Folder (2)\IMG_1076.JPGC:\Users\Snake\Desktop\New Folder (2)\IMG_1081.JPG
In this experiment only 4 pieces of prefboards arranging in matrix manner is used to show the theory without using overwhelming complex circuit design. When prefboard is touched by finger, a charging and discharging waveform will appear in the oscilloscope. C Program can only be written by proper interpret of waveform. In programming stage, inputs from prefboard are sampled and listed as an integer value in micro controller. Higher the voltage is, the higher the number is. If nothing touches, output will be at 0V. So the Prefboard is said to be touched when output voltage reading is bigger than a threshold value or voltage change respect to time is large than a differential threshold value. So whenever a point is touched, the respective part in computer screen will react.
A charging and discharging waveform will appear on the screen if prefboard sensing finger. Otherwise waveform will be grounded. As shown above on the left hand side.
Once prefboard board sensing is confirmed working. Four smaller prefboard will be placed in matrix manner and program shown in the screen will display their status.
The advantage of capacitive touchscreen is they are more responsive as flow of electron is more likely to trigger compare to work done in resistive sensing and the main disadvantage is they are more expensive compare to resistive type.
The material for a common touchscreen can be roughly divided into 3 layers. The top layer is a protective hard coat which is made of glass. The bottom layer is the LCD (Liquid Crystal Display) display layers. The core material for the touch screen material is a material called ITO (Indium tin oxide) which is the mixture of indium oxide (In2O3) and tin oxide (SnO2), typically 90% In2O3, 10% SnO2 by weight.
ITO is widely used in the touch screens. It has many advantages compared to other materials. It has two chief properties: high electrical conductivity and high optical transparency. High concentration of charge carriers (electrons and poles) will increase the material's conductivity, but it will decrease the transparency of the material. So it's hard to find a material like ITO which has both high conductivity and high transparency at the same time. This is the main reason why ITO can't be replaced by other material. Solid ITO is typically yellowish to grey depending on its degree of oxidation. But it is transparent and colorless in thin layers. In fact, the material shows a high transmittance over 80% in visible range. Normally the layer will have a resistivity of 2Î©cm. But the resistivity will change accordingly if the temperature changes.
The figure shown is a simple capacitive touch screen structure. The top layer has ITO columns and bottom layer has ITO rows, separated by spacing dots. This is a multi-touch screen. The difference between a capacitive and a resistive screen is that the resistive don't have the space dots layer. So it can only sense single touch movement. Capacitive screen has an accuracy of 1% of screen dimension; its light transmission is around 88% at a depth of 550nm. While resistive screen's accuracy is 2%; the light transmission is also a bit lower with 82% overall. Their operating temperatures are also different. Capacitive screen can work in temperature range of -15â„ƒ to ï¼‹70â„ƒ and resistive screen is -20â„ƒ to ï¼‹50â„ƒ.
The touch screen material requires good sensitivity. The sensitivity is measured in terms of latency which is the delay between the initial movement and the detectable signal. Usually latency below 20ms is ideal. Then the screen is very sensitive which is more comfortable for the user to operate.
Capacitive sensing is a technology for detecting proximity, position, etc., based on capacitive coupling effects. Capacitive sensing as a human interface device (HID) technology, for example to replace the computer mouse, is becoming increasingly popular. Capacitive sensors are used in devices such as laptop trackpads, MP3 players, computer monitors, cell phones and others. More and more engineers choose capacitive sensors for their flexibility, unique human-device interface and cost reduction over mechanical switches. Capacitive touch sensors have become a predominant feature in a large number of mobile devices and MP3 players.
The most important element of touchscreen technology is the way it determines the position of the input entered by the user. The report discusses two very important touchscreen technologies. They are resistive and capacitive. The objectives of the report are to differentiate the differences in the two technologies and find out the most suitable environment and condition that each operates best in.
The result from the experiment shows that the input are correctly displayed on their respective intended position hence indicating the capability of the resistive touchscreen in terms of accuracy. It also shows that through the manipulation of the sensors using a matrix, the space required to register the number of inputs can be reduced significantly allowing the device to be smaller and this matrix costs very little. However the results show that energy is needed to produce the input which causes less responsiveness and slow in detecting input.
From the experiment, the capacitive touchscreen show by manipulating the prefboard, the device is able to detect the position of the inputs accurately even when using a finger that has larger touching surface. Capacitive surface has no physical change in shape hence there is no energy wastage in operating this touch screen device. This implies that it has faster response time compare to resistive touchscreen.
Overall, the experiment indicates that capacitive touchscreen is more suitable to the public in modern times due to its characteristics of high responsiveness and faster detection time.
There is a rise of patents for hybrid touchscreen that incorporates both resistive and capacitive touch sensors. With this type of touchscreen, it will radically change the way touchscreen perform. It has a variety of capabilities like fast responsiveness to finger touch, support for various gestures, high precision and stylus character entry capabilities. By combining the advantages of both technologies and eliminating the disadvantages, the future of touchscreen will be more user-friendly. Few products patented are the new iMac and Blackberry touchscreen.
RIM Blackberry hybrid touchscreen patent Apple hybrid touchscreen patent
Scientist touching a hologram
User using SkinputFor conclusion, the report discusses the touchscreen technology using a surface as a input medium. Many revolutionized ideas are proposed to change the way touch screen works like Skinput whereby the human skin is used as the medium and using optical LCD embed with tiny sensors to recognize gestures instead of touch to detect input. Hence, the future of touch may change the way human interact with digital contents maybe making them as "life-like" as us. http://www.calvin.edu/~dsc8/images/BiDiScreen.jpghttp://2.bp.blogspot.com/_GFo2NhVEkEk/TEVpXqGWf8I/AAAAAAAAJog/fSJaKr5BCtQ/s1600/touchableholograms.JPGskinput-0
Optical touchscreen using Bidirectional (BiDi) screen