Low-cost capacitive touch screen technology drives system design innovation

Capacitive touch technology has been used in kitchen appliances for a few years, with split buttons behind opaque glass panels in ovens and frying pans, for example. These touch control keys are gradually replacing mechanical buttons, which have problems with short lifespan, lack of hygiene, and the associated cost of cutting holes in the panel to install the buttons.

Figure 1: Schematic diagram of the "charge transfer" technology principle.

Capacitive sensing technology has gradually become the technology of choice for touch control due to its durability and ease of low-cost implementation. In addition, due to its scalability, this technology can also provide user functions that other technologies cannot achieve. The user interface is provided in the form of soft keys on the display screen, which is often referred to as a touch screen. Quantum Research's developments in touch screen technology have opened the door to the design of new devices that will increasingly become mainstream in the market.

The device user input functions and application range that can be realized by using Quyan's capacitive touch sensing technology include: separate touch button applications with a small number of buttons (up to 10), which are generally used as simple switch function buttons, but can also be customized. "Gesture" type input, such as slide-to-open functions to reduce accidental operations; matrix touch buttons (up to 48) for multi-button applications, generally used in applications that require many user function buttons, and can also be used to develop low-resolution Rate scroll function device; spatial dimension control device, such as a linear slide key or a dial; used to increase/decrease temperature or time settings, you can use absolute devices (one touch to set a specific value), or relative devices (sensing the movement of a finger) to increase or decrease a set point); fixed button touch screen for soft keys; provides transparent fixed position/size buttons on the display. Can be implemented as a combination of transparent/non-transparent keys, driven by a chip; for analog touch screens with fully reconfigurable soft keys, fully reconfigurable keys (size and position) on the display report the touch position in the xy direction, with Subsequent processing in the main controller.

Figure 2: Typical separate button implementation schematic using QT240 chip.

This article will mainly introduce the implementation and application of capacitive touch screens. In addition, it also introduces traditional capacitive touch functions.

technical background

Many capacitive sensing technologies have been successfully applied to varying degrees. Quantum Technology's capacitive sensing technology is based on "charge transfer" technology. This technology offers a very high degree of robustness, and fully digital sensing is ideal for kitchen environments because of its superior ability to discern signals among background noise. This means that charge transfer sensing technology works very well even with very thick front panels.

Quantity's matrix chip works by drawing charge into one electrode of a two-electrode button (Figure 1). The electric field generated by the charge arc penetrates the upper glass panel and enters the receiving electrode, which transfers the charge to the chip. When touching the surface of the glass panel, part of the charge is absorbed by the human body, resulting in a reduction in signal strength. This way the chip can easily detect when human contact occurs. When the human body comes into contact, the water film has the opposite effect, actually increasing the coupling between the electrodes. This is because the water film moves in the opposite direction to what the chip can detect, so the water effect that often occurs in kitchen environments is inhibition.

Figure 3: Composite surface driving effect on the touch surface of the mixer

The charge transfer process occurs in a burst mode via a microprocessor-controlled switching sequence. Spread spectrum modulation greatly increases the signal-to-noise ratio, resulting in lower power, faster response time, and significant EMC improvements. RFI tolerance of 100V/m can be easily achieved using this technology. These devices also continuously compensate for changes in environmental conditions over the life of the device, allowing them to achieve high reliability even under high temperature and humidity changes.

An additional feature Quantitative Technology has integrated into its device-oriented chips is continuous FMEA (Failure Mode and Effects Analysis) sensing and reporting, where the system host processor can use the QMatrix chip to self-diagnose open circuits or open circuits in the entire sensor module. short circuit condition. This report is crucial to the FMEA confirmation of the entire system.

Touch buttons based on Quyan capacitive sensing technology

Split and matrix key chips have been developed and perfected over the past few years, providing a convenient way to implement touch keys on the back of the device's front panel. When connected to the QProx chip, any conductive material on the back side of the dielectric material can act as a sensor button. QProx board designs are very simple, usually single-sided, and can use low-cost materials like CEM-1 or similar. Implementation with a single panel is usually by gluing the board directly to the back of the front panel - the electrodes are on the opposite side of the PCB, sensing through the PCB, the glue layer, and the front panel.

Figure 4: QWheel rotation touch sensor circuit layout.

More relevant construction techniques can be used when specific user effects are required. A good example is Jenn-Air's Attrezzi mixer. The blender's mold interior decoration technology uses a conductive ink to provide a sensing layer on the back of the curved surface (Figure 3). Log in to www.qprox.com/background/blender_construction.php to learn about this construction technology.

Implementing rolling devices using capacitive sensing technology

Devices with touch input scrolling/pointing functions, such as the click wheel on iPod music players, have gained widespread acceptance in the consumer market and are gradually appearing in the consumer device market. There are two basic types of rolling devices: the absolute reporting type, which provides a direct position output report, and the relative type, which provides direct reporting of increasing or decreasing a value.

Quantitative Research has developed these two types of rolling devices for setting time (Figure 5) and temperature.

Recently, capacitive charge transfer technology has been used again to provide signal acquisition solutions for rolling devices. For absolute devices, scroll keys are made into straight lines or circles, or any other shape defined by the impedance sensor element, using a three-terminal connection (non-closed shapes like straight lines or arcs can be thought of as cut-outs) circular shape, with both ends of the circle serving as common electrodes). The resistive element can be a series resistor connected to a pad on the PCB, or it can be implemented in transparent form using ITO on a transparent plastic (PET) resistive trace. Relative reporting devices use repeating key strips to determine and report orientation by measuring touch sequences.

Touchscreen for kitchen environments

For the first time, consumer electronics manufacturer Whirlpool Corporation has introduced a capacitive LCD touch screen technology that is difficult to destroy. New sensing methods enabling this application provide manufacturers with a tool to develop products that are clean, functional, reliable and enjoyable, inspiring entirely new device concepts. Whirlpool Corporation's Velos(tm) oven uses these new features to optimize its industrial design. The company is breaking new ground in kitchen appliances with a single glass oven door that itself contains a touchscreen LCD and fixed touch buttons. In their designs, the common fixed control keypad is disappearing, and everything is enclosed inside a glass door.

Figure 5: Samsung microwave oven uses QWheel control technology.

Quantitative Research has developed a new QField technology for this design. The mainstream technology for kitchen applications is based on the company's capacitive matrix devices, which use passive XY scanning as described earlier to sense key touches. Velos product innovation was achieved by developing an affordable sensing film that can be attached to the back of the glass panel on the LCD without the need for opening holes. Quyan has improved its capacitive matrix key scanning chip ('QMatrix'), integrating key sensing functions into the transparent touch screen film, and integrating other touch keys onto both sides of the LCD, all using only one chip . In the design of Velos products, Quangyan provided a solution that supports 10 menu buttons on the LCD and 22 fixed buttons on the side of the LCD.

QField technology is non-destructible and highly transparent, so products designed using this technology are not easily damaged by sharp objects, corrosive chemicals and cleaning mixtures. Whirlpool's innovative LCD touch screen can be viewed through a seamless, fully curved glass oven door, which itself becomes the touch surface. There is no need to seal around the touch screen, and the LCD is placed behind the glass, which greatly saves the assembly cost of the product and eliminates possible failures in the application of resistive touch screens. Moreover, the touch chip only needs to be installed on a single-sided PCB, which reduces product costs.

The obvious fact is that although resistive touch screens add many functional features, they can affect the overall design aesthetics. Many devices using resistive touch screens may look cheap, but that doesn't match their actual cost. Resistive touch screens age quickly when scratched and cracked.

In addition, the cost of kitchen equipment is also critical. QField technology has no technical barriers to implementation because it uses a sensing chip and low-cost common materials and construction methods. QField technology offers lower overall system cost compared to resistive touch screens.