Understanding Touchscreen Technology : How Capacitive Sensing Works

Touchscreen Technology   Resistive Touchscreens are the most common touchscreen technology. Th ey are used in high-tra...

Touchscreen Technology  

Resistive Touchscreens are the most common touchscreen technology. Th ey are used in high-traffic applications and are immune to water or other debris on the screen. Resistiv e touchscreens are usually t he lowest cost touchscreen implementation. Because they react to pressure, they can be ac tivated by a finger, gloved hand, stylus or other object like a credit card.
A resistive screen consists of a number of layers. When the screen is pressed, the outer later is pushed onto the next layer — the technology senses that pressure is being applied and registers input. Resistive touchscreens are versatile as they can be operated with a finger, a fingernail, a stylus or any other object.Aug 6, 2010



How Capacitive Sensing Works

 Capacitive sensing is a very old technology. Mature readers may remember novel room lamps that could be turned on by touching a growing plant, and every reader has probably used capacitive elevator buttons at least once in his or her life. These primitive capacitive- sensing applications typically used a solid- state timer (such as an NE555 integrated circuit, first available in 1971) that “clicked” at a steady rate as determined by the time constant of an external resistor-capacitor (RC) network. A microcontroller was then pro- grammed to monitor the clicks from the timer and when the rate increased or decreased, it would react. A wire (or piece of ivy, in the case of the novel lamp) was routed to a touch point and when a human touched it, additional body capacitance was added to the RC network which, in turn, altered the click rate and caused a touch to be detected (see Fig. 1). Now, over 30 years later, the same function is typically accomplished by using a simple capacitive switch IC.



Surface Capacitive Touchscreens provide a much clearer display than the plastic cover typically used in a resistive touchscreen. In a surface capacitive di splay, sensors in the four corners of the display detect capacitance changes due to touch. These touchscreens can only be activat ed by a finger or other conductive object.

Sensing Method
The same phase voltage is imposed to the electrodes on the four corners, then a uniform electric field will be forming over the panel. When a finger touches on the panel, electrical current will flow from the four corners through the finger. Ratio of the electrical current flowing from the four corners will be measured to detect the touched point. The measured current value will be inversely proportional to the distance between the touched point and the four corners. 
Source: http://www.dmccoltd.com/english/museum/touchscreens/technologies/Surface.asp



Projected Capacitive Touchscreens are the latest entry to the marke t. This technology al so offers superior opti cal clarity, but it has significant advantages over surface capacitive screens. Pr ojected capacitive sensors require no positional calibration and provide much higher positional accuracy. Projected capacitive touchscreens are also very exciting because they can detect multiple touches simultaneously.

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RajEngineer.com - Analog, Digital, Mixed-Signal, PCB, Electrical Design Engineering: Understanding Touchscreen Technology : How Capacitive Sensing Works
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