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issue: January 2007 APPLIANCE European Edition

Touch Controls
Simplicity is the Art of Technology

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by Martin Zapf, product manager, Controls & Sensors, Cherry GmbH

The evolution of infrared (IR) touch control technology has helped advance today’s cooking appliances.

When the first IR touch control for a cooktop—a warming zone switch—entered the market in 1994, many found it hard to believe how rapidly touch-control technology for the operation of cooktops had developed. Today in Western Europe, more than 2 million touch controls annually are sold and assembled in built-in cooktops.

Technology Basics

Figure 1.

Technology has been the driving force of IR touch controls since the 1980s, such as a patented, integrated self-calibrating process from Cherry.
The physical principle is simple. An IR transmitter sends invisible light through the glass. The finger of a user, operating the sensor, reflects the light at a specific rate back through the glass into the photo-sensor (see Figure 1). The sensitive area of a focused IR touch sensor is roughly an 8-mm diameter circle around the center of the sensor.
The major advantages of IR technology can be categorized as follows:

• no electrostatic discharge (ESD) damage potential, as the reflecting operating finger is fully isolated against the sensor potential
• no malfunction caused by bad user conductivity to ground
• no influence of water on sensors
• no operation errors caused by pots placed above sensors (pots above the sensor switches the control off)
• detailed failure indication
• hardware self-check integrated into every sensor, so that damaged components will be precisely detected and indicated

IR Focus Technology
• concentrates IR light on finger for a good coupling coefficient
• excellent immunity against ambient light  

Low-Resistance Sensor Circuit
• high EMI immunity
• no problems with humidity under the glass
• low thermal and humidity isolation efforts
• easy hob construction
• low system costs  

Development of IR technology continues. Today’s fourth generation of IR sensors have proven their reliability with millions of sensors in the market. New vitroceramic materials are enabling a variety of new color options such as white, blue, grey, and silver, and IR technology can be adapted to nearly all of these glass colors. In addition, new and more cost-effective IR components are available, which has brought down the price of IR touch controls considerably.
The idea of creating a line of sensors is as old as the idea of the touch control technology itself. The first concepts were available in 1998; however, for cost reasons, the technology was not feasible at that time. Solutions based on alternative technologies, e.g. capacitive technology, were available in some products, yet were also vulnerable to the same disadvantages as capacitive sensors—sensitivity against EMI, ESD susceptibility, humidity sensitivity, or sensitivity dependant on user-grounding.
Significant drops in optoelectronic and other electronic component prices has enabled engineers to integrate these concepts into cooktop controls. With this work, IR technology has also progressed. New components and intelligent sensor arrangements, as well as advanced software algorithms that interpret the detected signals, have all helped move touch control technology forward.
Applied in a line array, today’s IR sensors allow the realization of a touch control with sliding operation. Slider operation allows direct input by touching the level related area in the span of the slider. It’s that simple: There is no need to explain this in a consumer operating manual. Well-designed glass printing with obvious symbols show the user how to operate the appliance.
The arrangement of the sensor array is flexible. A slider-operated control can be realized in many different layouts and orientations. The easiest and most popular arrangement is in a line or in a circle. Most convenient is a slider length of 6 cm to 8 cm for 10 levels [levels 0 to 9]. Shorter arrays can change levels by variation of finger pressure on the glass, and longer designs need more components and are, therefore, less competitive.
The latest optical component slider arrangements use a cross light coupling. This means that every IR LED is neighboured by phototransistors. The operation of a sensor directly on top of the IR LED sends a balance reflection signal to phototransistors on both sides (see Figure 2). Sliding the finger to either side changes the balance of this signal to the neighbouring phototransistors and, thus, indicates even the smallest movement. Intelligent readout routines prevent misinterpretation of signals, such as those caused by a pot placed on the sensor or a cleaning cloth on the cooktop surface.   

Figure 2.

Also, operation by two fingers indicating two different cooking levels will not confuse the system, but instead, will be detected. For example, if a user unintentionally (or intentionally) uses one finger to indicate a cook setting of 2 while another finger indicates a cook setting of 7, the system will realize the error and ignore the command. In fact, a perfectly adjusted IR sensor system could even measure the operator’s pulse; however, this is not required for any cooktop application so far.
Sophisticated software using barycentre algorithms indicates the width of the finger and double operation on the slider. The small sensitive operating area of the focused sensors enables precise and outstanding linearity of slider sensing. But, of course, the concept is not that simple. Software algorithms for the finger detection and interference suppression are core elements to the design.
The task for the application engineer is to always find the best possible compromise between cost, reliability and usability. A new, reliable technology is one thing, but the integration of suitable and desired advantages into a product is another story.

 Figure 3. Backlit version

Addressing Usability and Simplicity

If the costs are acceptable, usability becomes the next major criteria by which every new operating concept will be judged. As decreasing semiconductor prices reduce the cost of a touch control to the level of electromechanical rotary heater controls, they become attractive for the higher volumes of the combination cooktop segment. In the past, optical sensing technology enabled the designs of complete hob controls by using up to 10 discrete sensors. However, for a convenient operation, a minimum of one decrement and one increment sensor is necessary for each heater. However, this lengthens the time it takes the user to switch between cook settings (e.g., changing from cook setting 4 to cook setting 5). Either the user touches the decrement sensor in a sequence, touching it four or five times, or the user has to keep his or her finger on the sensor for a longer period to activate the automatic augmentation. Both can be laborious and tedious. So there is still room for increased comfort and improved usability.
Consequently, a completely new generation of cooktop operating sensors was required. Slider technology, for example, uses an IR sensor array that enables both the selection of the heater and the choice of the heat setting with the single touch of a finger. By sliding a finger on the operating line, the heat setting can be varied constantly without having to enter repeated commands to turn the heater up or down (see Figure 3). Thus, the patented technology is not just easy, but self-explanatory for the user. It adopts the latest IR touch control technology and is projected to the user requirements for a traditional cooktop operation.
The use of touch control technology enables a completely smooth surface, but is also as intuitive as a conventional rotary knob. The technology’s simple design is also important for the application engineer (see Figure 4.). The design is based on a modular and freely configurable construction (i.e., hardware and firmware adjustments), assuring that each module fulfils engineer requirements.

 Figure 4.

About the Author

Martin Zapf is a graduate in electronics engineering from the University of Erlangen-Nuernberg. He worked 7 years as a development engineer for touch controls and head of pre-development for inductive sensing at Cherry GmbH before he changed to the position of product manager, Controls & Sensors in 2002. If you would like to contact Zapf, e-mail editor@appliance.com


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