Capacitive touch controls are now finding their way into many applications, from mobile phones to television sets. These interfaces are both robust and practical, and increasingly affordable. The designer’s quest for the ideal human interface is increasingly answered with electronic touch controls that increase consumer appeal and improve usability.

In 2002, Slovenian kitchen appliance manufacturer Gorenje approached Quantum Research for assistance in the design of a new series of cooktops, refrigerators and ovens. The objective was to create an appealing design that would help move Gorenje upmarket and strengthen its image as a design and technology trendsetter. Working with both Quantum Research and Pininfarina design studios, Gorenje realized this objective. (See Fig. 1.)

The result of this collaboration was Gorenje’s Pininfarina series of kitchen appliances introduced in 2005, which promptly won three design awards, including the November 2006 Manufacturer’s Award from The Designer magazine. These appliances use touch controls from Quantum Research, specifically, touch button and slider chips. As a result of the commendations Gorenje received for this product series, as well as the gratifying consumer response, Gorenje has made a further commitment to use touch controls on many more appliances currently in development.

Cooking control

Fig. 2. Gorenje’s QT401 based slider circuit module uses a discrete resistor chain along discrete metal electrodes that are interpolated for high resolution.

An interesting type of user interface based on a touch slider has been implemented in three of Gorenje’s Pininfarina cooktops. For these controls, Gorenje made use of Quantum’s QT401 QSlide™ sensor chips. These cooktops also use up to 10  additional discrete touch buttons, provided by QT1100A Quantum chips. In operation, the buttons and sliders on these cooktops permit a much faster and more satisfying control of heat settings when compared with slew setting buttons normally associated with touch controls.

In a conventional slew control, the user touches an UP or a DOWN key to make the heat setting rise or fall. While this only requires two touch areas and can thus be compact, it has the disadvantage of requiring a frustrating amount of time to set the heat level. Worse, a short moment of distraction can lead to setting overshoot, thus requiring the user to slew in the other direction again to compensate.

As a result, slew controls are usually restricted in the range of possible settings in order to minimize setting time. For example, if the settings ranged from 0 to 5, a slew control might be acceptable. But a slew control that spans a range of 0 to 20 or more might meet consumer resistance due to the amount of patience required to operate it. A conventional mechanical knob on the other hand can be set with a twist in a fraction of a second. What Gorenje designers desired was a better control that could be set with the speed and ease of a knob.

Enlarge this picture Fig. 3. Interleaved copper sections also can be used to create a slider, eliminating the need for resistors.

Gorenje found a solution in the QSlide technology, which allows a linear sensing strip to output an absolute position with no perceptible delay. Fine tuning of the setting can be accomplished by sliding the finger over the linear track as required, using a motion that is quite similar to what a user would do with a knob. These features are offered in Quantum’s QT401 and the later QT411 and QT1106 devices.
Gorenje focused on the slider and touch button solution after considerable study of available technologies and design concepts and made its decision for the following reasons:

• QSlide and QTouch technologies were established and tested, allowing Gorenje to focus on the industrial design and module development rather than the core technology.
• The slider design concept was fresh; at the time, no other company had produced such a design.
• The design concept was shown to be very user-friendly and less frustrating to use compared with other touch solutions.
• The total cost was attractive compared to other solutions.

The market response to the new appliances confirmed Gorenje’s decision. As a result, Gorenje is further expanding the product offerings in its Pininfarina series.

Technology package

Enlarge this picture Fig. 4. The QT401 chip connects to the ends of the slider strip.

Quantum provides established solutions that do not require the purchase of components that need special programming. The QT401 slider chip and QT1100A button chips both provide predefined, tested and stable solutions for applications ranging from MP3 players to microwave ovens. Chips using this technology have been designed into well over a thousand products and are now sold in the tens of millions each year. Both chips use charge-transfer sensing technology coupled with spread-spectrum signal modulation to enhance performance and dramatically reduce noise problems. And both offer automatic signal calibration and drift compensation for a trouble-free life. Using Quantum-recommended circuit designs and layouts, the designer merely needs to wire the parts to a host microcontroller and configure the sensing electrodes. These two chips connect to a host device via a simple serial SPI protocol in order to control various setting parameters and to recover touch information.

The QT401 offers a 7-bit (0 to 127) absolute position result with a response time of 50 ms. It responds to either a tap or a slide motion, and reports back with the position information over the serial interface. The device’s sense fields readily penetrate glass more than 5 mm thick. Simultaneous measurements are made from both ends of the sensing strip, and the signals are then processed to provide a ratiometric result that is independent of signal strength, provided that there is a minimum amount of touch signal present, so that the reported position is the same for either a small or large finger. A newer pin compatible device, the QT411 is available now with high thermal tracking capability.

The QT1100A 10-key chip is a 48-pin device that sports patented Adjacent Key Suppression™ (AKS™), which is used to resolve touch ambiguity when more than one key is touched unintentionally, as might occur on tightly spaced keypads. The device also offers a unique failure-mode detection to help designs become compliant under IEC and UL rules (EN60730 / UL60730 and EN60335). These rules are especially critical for cooking appliances where the OFF button must be guaranteed to work at all times.

Slider strip

Fig. 5. Gorenje slider module showing both the slider strip as well as the power-level indicator LEDs. The brass parts are key spring electrodes that are pressed against the front panel; these connect to the QT1100A 10-key touch chip.

QSlide chips use a simple sensing strip that can be created in one of two ways. One is to use a resistive element similar to a potentiometer along the sensing track that is behind the control panel. The resistive element can be made of screen-printed resistive ink, or more conventionally a series of copper rectangles interconnected by discrete resistors. This latter method was the one chosen by Gorenje. (See Fig. 2).

A second way is to use an interleaved conductive electrode set (see Fig. 3) that creates the necessary field gradient; no resistors are required for this method. The electrode set can be made of an etched PCB copper area. A small disadvantage is that the no-resistor pattern is slightly more difficult to design in CAD than the resistive type, but both perform equally well. Both slider types connect to the chip via connections made from the ends of the strip to the chip. (See Fig. 4).

Close to the slider area are a series of square LEDs that act like an old-style VU-meter to indicate the setting value in an intuitive analog fashion. (See Fig. 5). Moreover, these LEDs track the user’s finger with both single-touch and sliding motions. In addition, there is a two-digit LED display, one for each burner to indicate the power level digitally.

In Gorenje’s design, the slider electrode board is connected to the main control board using spring-loaded spacers and 0.5 mm FFC cable connections. (See Fig. 6). The entire assembly is mounted in the appliance chassis in such a way that, when the glass is assembled, the electrode board and the discrete key touch electrodes are compressed onto the glass to create the keys. As is common with glass-ceramic cooking surfaces, the underside of the surface is rippled to increase strength. When the electrode board is pressed against this rippled surface, air pockets are created in the surface valleys. With most capacitive sensors, these gaps would require filling with a compound to aid in the transmission of the capacitive fields, which are heavily attenuated by air. Gorenje found it unnecessary to use a filler compound because the QT401 chip has enough sensitivity to reliably detect touch through the air gaps as well as the glass-ceramic surface.

Fig. 6. Springs are used to push the sensor PCB and the electrode PCB onto the glass. The springs provide compliance and positive pressure.

This control also uses formed brass pieces that act as contact springs to implement the discrete touch key electrodes. (See Fig. 5.) Like the slider electrode PCB, these brass spring pieces make contact with the inside surface of the glass-ceramic surface to create touch areas. No adhesive is used between the control module and the glass, making the unit simple to assemble and easy to replace.

The user sees only the net effect of the construction methods, which is an intuitive control that provides both a relative at-a-glance power-level indication along with a direct-acting, finger-touch input, and a numerical readout. (See Fig. 7). The ease of development with this technology is evident by the fact that the design was implemented entirely by Gorenje with little input from Quantum, even though it was Gorenje’s first ever internally led touch-control design.

Successful project

Fig. 7. The user’s view of the panel. The LED “VU meter” clearly indicates relative power level at a glance; the 7-segment LEDs indicate a more precise power level reading.

According to Joze Dermol, lead engineer of Gorenje’s R&D department, Gorenje will extend the use of Quantum touch buttons and sliders on many more products.

“The positive experiences customers report about these touch controls made us think, research and develop user interfaces in a different, fresh way,” Dermol says. “Therefore, with these devices, we see that there are no longer any problems for us to create any kind or shape of appliance front panel, solving a major design barrier. We realize now that touch sensor based user interfaces can meet any design demand.”

Commenting about the success of the Pininfarina series, Bill Miller, sales director for Gorenje U.K., notes, “Customers are very impressed by the distinctive design and innovative features. The Gorenje Pininfarina collection has proved to be a big hit with U.K. kitchen designers, high-end developers and discerning consumers.”

Miller credits the use of high-tech touch controls as being a significant part of the success of the Pininfarina series.

For more information email: george.east@qprox.com