touch screen

touchscreen (excerpted from zh/en-wiki ):

触控屏幕（英文:Touch panel ,Touchscreens）又称为触控面板，是个可接收触头（无论是手指或胶笔尖等）等输入讯号的感应式液晶显示装置，当接触了屏幕上的图形按钮时，屏幕上的触觉反馈系统可根据预先编程的程式驱动各种连结装置，可用以取代机械式的按钮面板，并借由液晶显示画面制造出生动的影音效果。

而简单说是指种可触控式的屏幕，通常是在半反射式液晶面板上覆盖一层压力板，其对压力有高敏感度，当物体施压于其上时会有电流讯号产生以定出压力源位置，并可动态追踪。

Resistive touchscreens are composed of two flexible sheets coated with a resistive material and separated by an air gap or microdots. When contact is made to the surface of the touchscreen, the two sheets are pressed together, registering the precise location of the touch. Because the touchscreen senses input from contact with nearly any object (finger, stylus/pen, palm) resistive touchscreens are a type of "passive" technology.

For example, during operation of a four-wire touchscreen, a uniform, unidirectional voltage gradient is applied to the first sheet. When the two sheets are pressed together, the second sheet measures the voltage as distance along the first sheet, providing the X coordinate. When this contact coordinate has been acquired, the uniform voltage gradient is applied to the second sheet to ascertain the Y coordinate. This operation occurs instantaneously, registering the exact touch location as contact is made.

Advantages
Resistive touchscreens typically have high resolution (4096 x 4096 DPI or higher), providing accurate touch control. Because the touchscreen responds to pressure on its surface, contact can be made with a finger or any other pointing device.

Disadvantages
Due to the nature of passive touchscreen design, when "inking" (taking handwritten notes with a stylus), the user cannot press a large hand down on the screen while writing [2] [3]. This is the tradeoff between having a dedicated implement (stylus) versus the ability to use one's fingers as a stylus. A few modern tablets recognize both fingers and a stylus, and avoid this problem by deactivating recognition for non-stylus input when the stylus makes contact.

For people who must grip the active portion of the screen or must set their entire hand down on the screen, alternative touchscreen technologies are available, such as active touchscreen in which only the stylus creates input and touches from the hand are rejected. However, there are now newer touchscreen technologies which allow the use of multi-touch without the aforementioned vectoring issues. [4]

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]
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Projected capacitance

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 multi-touch operation. The greater resolution 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. However, conductive smudges and the like can seriously interfere with the resolution, making multiple touches necessary to get the desired functionality. Such conductive smudges come mostly from sticky or sweaty finger tips, especially in high humidity environments. Collected dust, which adheres to the screen due to the moisture from fingertips, is a serious drawback for the long-life operation of PCT.

A capacitive touchscreen panel consists of an insulator such as glass, coated with a transparent conductor such as indium tin oxide (ITO).[2][3] As the human body is also a conductor, touching the surface of the screen results in a distortion of the body's electrostatic field, measurable as a change in capacitance. Different technologies may be used to determine the location of the touch. The location can be passed to a computer running a software application which will calculate how the user's touch relates to the computer software.