The Whirlpool Velos SpeedCook oven, introduced last month, is an innovative appliance, not just for its multimodal cooking technologies, but also for its novel industrial design, which was made possible, in part, by a new interactive touch screen technology.

The Velos oven is the first appliance to utilize a transparent, glass capacitive touch control placed in front of an LCD, creating a virtually indestructible touch screen. The new sensing method gives appliance makers a new tool in the quest for clean, functional, reliable and aesthetically pleasing products, and enables radical new appliance concepts.

The Whirlpool Velos employs these characteristics to maximum effect, breaking new ground in appliance design by using a single-piece, hinged-glass oven door, which contains a touch screen LCD plus additional fixed touch buttons. The usual fixed-control keypad typically found to the right of the door is conspicuous by its absence. The entire interface is within the glass door itself. (See Fig. 1).

The technology used was developed by Quantum Research Group, Hamble, U.K., and is sold under the trademark QField. Quantum already is a major supplier of charge-transfer-based capacitive controls to the appliance industry, counting among its customers GE Appliances, LG Electronics, Samsung Electronics, Electrolux, and many other OEMs who have designed glass touch controls into their products. The dominant technology Quantum uses for kitchen applications is based on its capacitive matrix devices that sense touch keys using passive X-Y scanning. The Velos breakthrough came in the development of an affordable, clear sensing film that can be bonded to the rear surface of the glass panel over an LCD, without the need to cut an opening in the glass. Quantum was able to create a modification of its capacitive matrix key scanning IC (QMatrix) to allow the sensing of the clear touch screen film, as well as additional touch keys to the sides of the LCD, all with one chip.

In the Velos design, Quantum provided a solution with 10 menu keys over the LCD, and an additional 22 fixed keys to the sides of the LCD.

The QField technology provides both the durability and transparency of solid, unbroken glass, resulting in a product that survives attack by sharp objects, harsh chemicals, and scouring compounds. The Velos design exploits these advantages by using nothing more than the smooth, uncut, slightly curved glass surface of the oven door itself as the touch surface. No seals are needed around the touch screen. The LCD is merely positioned behind the glass, significantly saving on assembly costs, while eliminating all the field failure mechanisms of resistive touch screens. As a bonus, the touch chip is mounted on a single-sided PCB for low-cost construction.

Advantages

Fig. 2. The top layer of a typical resistive screen is a thin layer of PET plastic film, coated with indium tin oxide, and spaced apart from a fixed ITO-coated PET layer. Pressing on the top layer forces the two ITO layers into contact.

Appliance designers are constantly facing the challenge of creating fresh designs that differentiate their products while providing ease of operation and a unique customer experience. As kitchen products become more sophisticated, operability becomes an increasing challenge. Customers like advanced features but resist having to run for the instruction manual each time they try to do something beyond reheating leftovers.

Touch screens are a great way to simplify the operation of an appliance by offering context-sensitive menu options. Unfortunately, resistive screens come with several drawbacks: bezel dirt traps, poor chemical and scour tolerance, heavy light loss, and low contrast, as well as vulnerability to sharp objects.

“The cleanability of the control is a great aspect of the design,” says Melodie A. Nakhle, associate brand manager for Whirlpool brand cooking. “With the fully integrated control, it is easy to wipe the surface, even if you use the controls with messy hands. Beyond that, the display is bright and easy to read, while the logic tree behind the display lets users easily select what they want to do.”

LCD touch screens have held out the promise of providing simplicity plus operability, but have been tantalizingly out of reach for kitchen use due to a long laundry list of problems. But within an appliance budget, to date the only touch screen technology that has been available has been resistive. As a result, the number of surviving kitchen appliance designs using LCD touch screens is small and declining.

The first problem facing resistive screens is robustness. The top layer of a typical resistive screen is a thin layer of PET plastic film, coated with indium tin oxide, and spaced apart from a fixed ITO-coated PET layer. Pressing on the top layer forces the two ITO layers into contact. (See Fig. 2). Separation of the layers is usually accomplished via a large number of microdot spacers. If a sharp object strikes the top layer, it can become dented, or its ITO layer can crack, resulting in screen failure due to a short or open circuit. The pliable PET top layer of these screens is also affected negatively by scouring compounds or other forms of abrasion, and an expensive oven with a scratched and crazed screen is a poor recipe for customer satisfaction or loyalty.

A second weakness of resistive touch screens is poor optical properties. The two layers of the screen reduce light transmission and contrast due to multiple PET-air and ITO-air boundary reflections, as well as the light absorption characteristics of the ITO. Suppliers of resistive screens may try to make them more robust by making the ITO layers thicker or adding hardcoat layers, but these efforts diminish contrast even more. Poor optical properties require that the LCD backlight must be made brighter, and more focus has to be put on the quality of the LCD itself. All of this costs money, yet does not completely solve the problem.

A third weakness of resistive screens is that, since their operation relies on flexing of the top PET layer, that layer must be directly accessible, and cannot be placed behind glass or hard plastic barrier. This access requires a cut panel opening, bezel frame, and sealing. The cost of the hole and gasketing is not trivial in the design of the product. Many OEMs report that the cost of the hole is every bit as much as the cost of the resistive screen itself. One OEM reports that the cost of the hole was ultimately twice as much as the resistive touch screen. In addition to the cost, the necessary bezel creates a dirt trap which fastidious homeowners might try to scrape clean periodically. This type of cleaning can break down the seal, or, even worse, crack the ITO layer, which would result in total failure.

Finally, while resistive screens add tremendous functionality, they lack aesthetic appeal, and their appearance only becomes worse with age, as they gradually acquire more scratches and crazing. Cost is also a paramount subject matter when dealing with kitchen appliances. The QField technology demonstrates advantages here also because it uses a single sensing chip and low cost, commonly available materials and construction techniques. When comparing the total system cost of QField versus a resistive screen, QField costs less money.

Operation

Fig. 3. Quantum’s matrix chips operate by pumping a charge into one electrode of a two-electrode key. The electric fields from this charge arc through the overlying glass panel into a receiving electrode, which transfers the charge back to the chip.

Charge-transfer, capacitive-sensing technology has become the sensing technology of choice for kitchen appliance controls for many OEMs. Quantum’s matrix chips operate by pumping a charge into one electrode of a two-electrode key. (See Fig. 3.) The electric fields from this charge arc through the overlying glass panel into a receiving electrode, which transfers the charge back to the chip. When the exterior of the glass panel is touched, a portion of the charge is absorbed by the human body, resulting in a reduction in signal strength. This signal change can be easily detected by the chip and determined to be a human touch. Interestingly, water films have the opposite effect as a human touch, in that they actually increase coupling between the electrodes. As a result, the signal change due to a water film moves in the wrong direction. The chip detects this, and so the water effects are suppressed.

This charge-transfer process is carried out in a burst mode using a microprocessor-controlled switching sequence. Spread-spectrum modulation dramatically increases signal-to-noise ratio, resulting in lower power, faster response time, and substantial EMC improvements. An RFI tolerance of 100V/m is routinely possible with this technology. These devices also continually compensate for changes in environmental conditions over the life of the equipment, making them highly reliable, even with large temperature or humidity swings. All of these aspects have been perfected in Quantum’s QMatrix devices and have been in routine use for several years. The innovation behind the Velos screen is the use of QMatrix charge-transfer technology to drive a new type of clear matrix film that is bonded to the back of the glass panel. In the Velos design, a QMatrix chip drives an array of capacitive touch-screen keys etched into two layers of patterned, clear ITO on PET film. (See Fig. 4.)

Fig. 5. Developed for touch screen applications, Quantum’s QT6C11 matrix chip is capable of sampling up to 32 touch keys in spread spectrum mode with two independent groupings of AKS.

The ITO pattern on each layer crosses at right angles with opening structures at each key location to allow the fields to propagate towards the touch surface. This ITO stack is then bonded with clear adhesive to the interior of the glass. The ITO stack has a connection tail that is plugged into the main, single-sided PCB, which is bonded to the glass with industrial sheet adhesive. (See Fig. 5.)

The innovative aspect of this arrangement lies in the use of a new, etched ITO pattern that allows simple, fixed touch-key construction, and in a modification of the QMatrix chip to handle the increased capacitive load provided by the film. The capacitance increases dramatically with the film over standard touch keys because the X and Y scanning tracks have much larger amounts of opposing surface area, creating large parallel-plate capacitances that are normally difficult to drive. The single sensor chip also drives all the adjacent fixed, panel touch keys. The matrix chip seamlessly reports the touches in either area via a SPI interface. Quantum’s patented AKS feature provides assured touch control by determining which key has the strongest touch signal, eliminating any confusion when a finger happens to touch two or more closely spaced keys. The dominant key is always the only key that reports a detection.

While the ITO screen touch keys operate in the same basic way as other matrix panel keys, they have different coupling characteristics and capacitive loading parameters than conventional matrix keys. As a result, they need to be driven a bit differently. For these touch screen applications, Quantum developed a special matrix chip, the QT6C11. This device is capable of sampling up to 32 touch keys in spread spectrum mode with two independent groupings of AKS. (See Fig. 6.) The 32 keys can be entirely on the touch screen surface if desired.

The technology also continuously senses and reports back to the main controller any opens or shorts on the sensor board, tracks, and the ITO film. This reporting is crucial for full FMEA validation. This feature is also found in the Velos project.

The ITO film stack used in the Velos is a first generation release. Future versions close to release will use single-layer ITO in order to increase clarity and reduce cost even further. A full X-Y analog-reporting, single-layer film is also in development and is expected to release in the second quarter of this year.

Fig. 6. The ITO stack has a connection tail that is plugged into the main, single-sided PCB, which is bonded to the glass with industrial sheet adhesive.

Conclusion

Appliance designers are always looking for ways to add features, yet make their products more stylish and operator friendly. The look and quality feel of glass touch controls has always commanded a premium in the appliance market, as consumers see value it its durability and quality. This technology is now migrating into the broader mass-market range of appliances. Capacitive touch screens are the obvious next step for mid-market and high-end appliances.

The introduction of QField technology in the Whirpool Velos oven ushers in a new generation of touch screen technology that is technically and commercially superior to resistive screens, yet competitively priced. It will allow the realization of appliance designers’ dreams: the use LCD touch screens being welcome in the kitchen. This new design option will be increasingly seen in new product concepts in the years ahead.