NOVEMBER 15, 2011 by Murray Slovick
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Consumer electronics and home appliances are of little use if they cannot easily be controlled. There are a variety of ways to do so and we are familiar with most: switches, push buttons, keyboards, knobs and slider controls are all part of our everyday life.
Recently, however, touch interfaces have become popular and are now widely adopted in a variety of consumer-facing products ranging from smartphones and tablets, to ATMs and even plasma TVs. Touch elements do not contain moving parts, and unlike conventional control solutions, no holes or other openings are necessary where dust and dirt can enter the device.
Two technologies, capacitive and resistive, comprise the vast majority of today’s touchsensing applications. We’ll look at both technologies to explore how they detect touch and weigh their advantages and disadvantages.
Capacitive Touch
Capacitive touch sensors are used in consumer products such as digital audio players, laptop track pads, computer displays and, increasingly, mobile phones. In contrast to mechanical contacts, capacitive touch sensors do not require force to trigger a response. The science principal behind capacitive touch is pretty straightforward: when two electrically conductive objects come near one another without touching, their electric fields interact to form capacitance. When a user touches a screen or pad with his finger, the charge on the capacitive layer changes. This change can be detected by a measurement circuit and converted into a signal sent to a microcontroller for processing.
Capacitive sensors are constructed on a single glass layer with a conductive and transparent coating such as indium tin oxide (ITO) on one side. In touchscreen applications the sensing pads are placed above the display, so the user can touch the screen directly.
The capacitive system transmits about 90 percent of the light from the monitor. On the whole, capacitive touch sensors are not as accurate as resistive touch sensors (covered next) and are relatively more expensive. Environmental factors (temperature, humidity) also can affect the capacitance value. But sensors often are calibrated to account for these and other variations. What’s more, electrical interference and stray capacitance can hamper capacitive touch sensing performance, so the sensing electronics are usually placed as close as possible to the sensing spot and conductive material is kept away from the sensor.
Multi-touch input, defined as the ability to recognize two or more simultaneous touch points on a single display, is widely available for capacitive-type touch sensing. This allows users to pinch and zoom for ease of readability, which was popularized in devices such as iPods and iPhones. (Incidentally, multitouch was invented in 1982 at the University of Toronto and not by Apple, as many people believe.)
Resistive Touch
Resistive touchscreens account for more than 75 percent of the touchscreen market, mainly because they have simpler structures than capacitive touch. Resistive touchscreens have a flexible top layer and a rigid bottom layer. The top flexible layer is coated on its inside with a conductor and the bottom supporting layer of glass is also coated with a conductor (again, indium tin oxide).
In its static state the two layers are not in contact with each other but are separated by a thin space consisting of a non-conductive separator material. When a user presses on the top sheet, pressure causes the conductive sides to come into contact with one another, effectively closing a circuit. The resulting voltages can be measured in more than one direction and a controller can calculate the x-y coordinate of the point of touch.
In touchscreen applications the existence of two layers also results in a loss of brightness, usually between 10 and 20 percent. Another disadvantage of resistive touch technology is its shorter lifespan due to mechanical wear and tear.
Until recently, unlike capacitive touch alternatives, resistive technology has not been able to support multi-touch. But this may be changing. Rohm Semiconductor has produced a resistive touch controller that can handle multi-touch, albeit with a maximum of two touches registered at once, enough for simple gesture recognition, including pinching, spreading and rotating. And Texas Instruments offers a resistive touchscreen controller designed to work with low-power handheld products. Unlike other resistive touch controllers, TI’s TSC2020 can detect at least three touches simultaneously and supports up to a three-by-five-inch touchscreen array.
As noted, touch technology requires a sensor IC to detect touch events and a microcontroller (MCU) to receive detection data and process the resulting information. Sometimes one IC can handle both tasks.
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