Capacitive touch-control technologies are considered by many to be the workhorses of the electronics industry, having been used in applications such as cash registers, kiosks, cooking appliances, medical equipment, consumer electronics, and more. The robustness and versatility of capacitive-based touch control has been under development for more than 80 years, and its uses continue to grow.

This touch-control panel features a touch screen, wheel slider, and buttons.

A capacitive touch control with a touch screen uses a surface coated with a conductive material such as indium tin oxide that can store a charge. The material conducts an electrical current across the panel along the X- and Y-axes. When touched by something conductive, such as a finger, that controlled field is altered and the location of the touch along the horizontal and vertical axes can be determined. In an application with button-key touch locations, a discrete sensor is placed under that particular key location, and when the sensor’s field is disturbed, the system notes the touch and the location. Capacitive technologies have a number of advantages, including high-touch resolution and the ability to use touch surfaces that provide high image clarity, and resistance to dirt, grease or moisture. Of course, it is not necessarily a panacea for all touch-control applications. One disadvantage of capacitive technology is that to actuate the screen or panel, a finger must touch it. This is unlike some other technologies such as resistive controls or acoustic signal processing systems that can be activated with a pen, a stylus, a corner of a credit card or other item. Some experts also say that traditional capacitive systems are susceptible to electrostatic discharge and electromagnetic interference.

Enlarge this picture QTouch sensors can drive single or multiple keys.

The technology has traditionally also cost more to use than some alternatives. It has been expensive to use because of the complex signal processing electronics. For instance, when developing a human machine interface with multiple discrete touch points, each touch point has to be directly tied to a sensor. Ten buttons require 10 sensors.

One of the early players in this field was the U.K.-based Quantum Research Group whose technology is based on the capacitance method, but whose technology has evolved to keep the advantages of the system while mitigating some of the disadvantages. Founded in 1996, the company’s technology has been successfully incorporated into a range of devices from home appliances to consumer electronics. In many of these applications, the touch controls can be subjected to moisture, temperature changes, chemicals, electrical noise, and a host of other things that have the potential to confuse a user-interface control system.

“Capacitive sensing dates back to the 1920’s, but it has taken a long time to make it sufficiently robust and cost effective to the point where designers feel comfortable enough to use it,” says Chris Ard, director of business development for Quantum. “Over the last 10 years of our company, the technology has been perfected and now we are extending the technology and its applications.”

Enlarge this picture QMatrix detects a touch using a scanned, passive matrix.

Quantum has focused on developing integrated circuit technology that is based on charge-transfer capacitive sensing. The company’s earliest products were based on the QTouch™ system, which was patented in 1999. Among its first applications were touch and proximity controls for kitchen refrigerator water dispensers in the U.S., as well as biomedical equipment in the UK. QTouch ICs are designed to detect touch using a single connection between the sensor chip and a simple key electrode. These chips are best suited for low-key count applications of up to 10 keys.

The QTouch devices work by charging a sense electrode of unknown capacitance to a known potential. The resulting charge is transferred into a measurement circuit. By measuring the charge after one or more charge-and-transfer cycles, the capacitance of the sense plate can be determined. Placing a finger on the touch surface introduces external capacitance that affects the flow of charge at that point. This registers as a touch. QTouch microcontrollers can also be programmed to detect the proximity of a finger, rather than absolute touch.

Signal processing in the decision logic module makes QTouch robust and reliable, Ard says. False triggering due to electrostatic spikes or momentary unintentional touch or proximity is eliminated. QTouch sensors can drive single or multiple keys. Where multiple keys are used, each key can be set for an individual sensitivity level. Keys of different sizes and shapes can be used to meet both functional and aesthetic requirements.

The Samsung microwave features Quantum’s touch scroll-wheel technology.

While QTouch has been a company staple for years, today Quantum offers even more advanced technology such as the QSlide™ and QWheel™. These products, which are based on the QTouch technology, are touch sensors designed to replace conventional resistive linear sliders or rotary controls.

QSlide works with a linear resistive element that can consist of discrete resistors connected in series, a resistive thick-film layer, or an optically-clear indium tin oxide film for use over LCDs. Dedicated algorithms are used to determine the touch position independent of signal strength.

For QWheel the electrode consists of a simple resistive ring element that can be placed behind any dielectric panel. Three capacitive QTouch channels are connected to this ring.

This touch control system features a wheel slider and seven independent keys.

The company’s newest and most sophisticated technology is called QMatrix™. This technology is designed to detect touch using a scanned, passive matrix of electrode sets. It can power a large number of touch keys driven by a single chip. QMatrix circuits offer excellent signal-to-noise ratios, high levels of immunity to moisture films, extreme levels of temperature stability, superb low power characteristics, ease of wiring, and small IC package sizes for a given key count, according to Ard.

QMatrix uses a pair of sensing electrodes. One is an emitting electrode into which a charge consisting of logic pulses is driven in burst mode. The other is a receive electrode that couples to the emitter via the overlying panel dielectric. When a finger touches the panel the field coupling is reduced, and touch is detected.

Electrodes are typically areas of copper on a printed circuit board, but can also be areas of clear conductive indium tin oxide (ITO) on a glass or plastic touch screen. A single QMatrix device can drive a large number of keys, enabling a very low cost-per-key to be achieved, Ard says.

A touch-control board featuring a 10-key touch sensor.

High key counts, he says, are made possible via the configuration of pulse-driven rows and charge-receiving columns. For each key, a row couples into the driven electrode and a column connects to the receive electrode. If there are 8 rows and 4 columns, 32 possible keys can be wired into the matrix. Quantum’s QMatrix devices currently go up to an array of 8 rows and 6 columns, for a maximum of 48 keys in one chip.

But, it is not just the number of keys that are important, says Ard, it is the ability of the designer to place them where needed. QMatrix based keys can be located anywhere on a panel up to a meter apart from each other, all controlled by a single chip. They can also be a variety of sizes and shapes, mixing small and large keys together in one design. A single QMatrix chip can also be used for sense sliders, wheels and touch screens along with discrete buttons.

To help ensure that unintentional touches do not occur, the company has developed a number of features. A change in signal strength can indicate a valid touch or it could be a false actuation – for example a glitch due to electrical noise. To be certain one way or the other, the signal needs to be verified repeatedly, but quickly, as soon as a likely detection event has occurred. To make sure the signal is not merely electrical noise, Quantum’s chips operate at multiple frequencies, using digital spread spectrum technology to skip around noise frequencies and make sure that there really is a touch. The process of noise suppression uses what Quantum calls a “detection integrator,” a type of filter that employs consensus over a number of signal samples, for example at different frequencies, to prevent false detections. This type of filter can require that 10 consecutive signal samples all confirm a touch; even one drop out among the ten samples will cause the signal confirmation to fail.

A cooktop from Gorenje, a Slovenian kitchen appliance manufacturer, features both the QSlide and QTouch technologies.

Ard says that when the system is turned on, the chip takes a measurement of its “capacitive surroundings” on each key. The result is considered the resting count or signal reference level. “Signal changes relative to the reference level are the important things to look for. They are a function of moisture, interference, temperature drift, and of course touch,” he says.

If the long term signal level varies in a particular direction from the reference level for a period of time, for example due to temperature effects, the system will alter the reference level slowly in the direction of the signal change using a feature called drift compensation. This feature only works when the signal changes are small and known not to be the result of human touch.

The company’s algorithms can also help ensure that the correct key is detected among tightly spaced keys, through its Adjacent Key Suppression (AKS®) algorithm. This process runs constantly, helping to discern the user’s actual intent. If, due to an analysis of keys’ signal strengths, it believes that the actual button being pushed is the letter “D” for example, it will lock out the letters “C” and “E” and any other nearby keys.

Adding to the robustness of many of Quantum’s QMatrix products is the self-diagnostic capability it has through a built-in Failure Mode and Effects Analysis program. This capability has been incorporated into certain QMatrix chips to provide an extra level of certainty, says Ard.

“FMEA is particularly interesting in cooking appliances,” says Ard. “You wouldn’t want the appliance going on by itself in the middle of the night.”

An FMEA enabled QMatrix chip does constant self-diagnostics. It checks for shorts and opens in the matrix, and probes the matrix itself using special diagnostic acquisitions to make sure keys are still there and working correctly. Via the chip’s serial interface, the host appliance controller can interrogate the QMatrix device for complete error information, and as a result take an informed decision on what to do – for example, to shut down the appliance, in the case of a critical key failure.

Because of these innovations, the company’s product range has expanded from chips for single touch buttons, to multitudes of buttons, keys, slides and wheels, projected capacitive touch screens, and combinations of these features. The technology can sense through any non-conductive material such as plastic, glass, and even wood, and the substrates can be of sizes as thick as 5 mm or more.

This versatility, and multiple capabilities, can be seen in some recent applications. For example, Samsung offers a microwave oven with Quantum’s touch scroll-wheel. The oven uses the company’s QT511 QWheel rotary-scrolling chip that consists of a ring electrode array placed behind the plastic panel. Three capacitive QT sensing channels are connected to this ring, and the resulting signals are processed to provide 128 positions. The result is output on an SPI serial interface. The device can be set to sense through panels up to 3 mm thick and even works through gloves.

For eight decades, capacitive technology has been a versatile industry workhorse. The microwave oven is just one application in which capacitive technology is being used. Today, Quantum’s devices are found in mobile phones, home theater equipment, computer peripherals and many other products. And, tomorrow’s applications may even be broader as touch screens become even more content driven. A cooking recipe, for example, could appear on a stove’s touch screen and the choices that are made can help program the oven.

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