The applications for infrared energy are wide ranging and include imaging and cooking, nondestructive tests and home heating, motion sensors and remote controls for consumer electronics. For at least two decades, infrared has also been used in touch-control technology. Here, too, infrared technology’s uses may grow as new innovations emerge, such as improved slide controllers.

Infrared-based touch controllers are especially useful for appliances that require robustness such as cooking appliances or clothes dryers, which can face heat, cleansing agents, grease and other challenging conditions, says Martin Zapf, product manager of controls and sensors for Cherry GmbH, a German-based company with North American offices in Pleasant Prairie, Wis. In the 1980s, Cherry was one of the first companys to develop touch controls for cooktops and is one of the companies utilizing IR technology in an expanding number of touch-control uses.

Infrared touch controls operate by using a transmitter to send a beam of infrared light through a glass surface. When a finger touches a keypad, the infrared light is reflected off the finger to an IR receiver and a microprocessor analyzes the reflection. (See Fig. 1.) To ensure the touch is intentional, Cherry uses various methods, including the use of barycenter algorithms to determine if the finger falls within the sensitive operating area of the focused sensors. The software also looks at other factors, including reflection speed rate and plausibility factors, such as whether two buttons were to be pressed simultaneously in a specific scenario. According to Zapf, accidental switching caused by cleaning, children or pets, pans boiling over, or outside light sources is almost impossible.

Fig. 2. The IR transmitter emits light, which is reflected back and is captured by photosensors on either side of the transmitter.

Cleaning the cooktop is a good example of how the controls can be touched, but not activated. “Cleaning the glass above the sensor area with a cloth will not affect the heat settings of the control.”

Zapf says that Cherry’s IR Focus Technology concentrates IR light on the finger and creates a  “good coupling coefficient,” even if the user is wearing rubber gloves. This also provides immunity against ambient light and permits low resistance sensor circuits. The low resistance sensor circuit features high EMI immunity, no problems with heat or humidity under the glass on the electronics, easy cooktop construction, and low system costs.

The optical principle upon which the control system is based means that it is extremely insensitive to electromagnetic interference, says Zapf, which can be a problem with other technologies such as traditional capacitive sensors. This is also true of electrostatic discharge; while traditional capacitive sensors can be susceptible to ESD, the IR technology cannot be affected by electrostatic discharge because the finger that actuates the control is fully isolated from the sensor.

Fig. 3. Here movement is detected as the IR light is reflected to the photosensor in the direction of movement.

The company offers a variety of standard and customized products including the standard comfort modules (SCM I, II, III and Quattro). These standard modules consist of two PCBs. The first board includes the power supply, relays, a buzzer and connectors that are used to connect the various burners, the power supply and an optional main relay to isolate the secondary circuit from the main. In addition, a pot detection module can be connected using edge connectors. The second control board contains the infrared emitters, photosensors, display elements and the microcontroller. The microcontroller processes the data from the photosensors and controls the displays, the buzzer and the relays on the power board.

The two PCBs are attached to each other by springs, which press the upper sensor board against the glass ceramic surface. The travel of the springs compensates to a certain degree for mechanical tolerances. The module can be mounted in the cooktop by plastic posts.

The display elements used for the burners are 13-mm, seven-segment displays that can be read from a considerable distance. The various different modules have additional displays for timer, dual zones and other features.

Slider technology controls four burners.

Zapf adds that a touch control panel’s touch point diameter ranges from 6 mm up to 10 mm. Touch controls can be used in conjunction with every IR-transmissive material. For Cherry’s Touch Controls, an infrared transparency of about 30 percent is sufficient. The substrate materials can be tinted or come in a variety of color options such as white, blue, grey and silver. He adds that glass is used at a thickness of about 3 mm to 4 mm, but with the aid of a mechanical adapted mounting sleeve, which helps to focus the IR, the use of thicker materials are possible. While the system can see through light-transmissive material, it cannot be used with other materials such as metal as can other technologies such as acoustic systems.

The company’s newest product is its slider touch control. The most advanced version of its optical component sliders use cross-light coupling. This means that every IR transmitter has photosensors on either side to help determine the direction of finger movement. (See Fig. 2.)

“Sliding the finger to either side changes the balance of the signal to the neighboring photosensors and thus indicates even the smallest movements,” says Zapf.

Fig. 4. Here is one example of a slider design. Cherry offers many different types of slider designs.

By sliding a finger along the operation line, the heat setting can be varied constantly without having to enter repeated commands to turn the heat up or down, as one would with a conventional slew control system. In addition, using the slider technology enables both the selection of the burner and the choice of the heat setting with the single touch of a finger. Zapf says that most users find the patented technology to be self-explanatory because of well-designed symbols that point the user to the correct operation.

The technology provides for “high flexibility,” he says, as the arrangement of the sensor array is flexible. A slider-operated control can be realized in many different layouts and orientations. Zapf says the easiest and most popular arrangement is in a line or circle with a slider length of 6 cm to 8 cm.

Zapf adds that the technology offers a “kind of simplicity” in terms of the system’s modularity and configurability that allows for a freedom of design using the slider technology. (See Fig. 3.)

This flexibility is helping the company expand the applications on which the technology can be used (See Fig. 4). Zapf says that they are presently working with several customers to create touch controls for other household appliances, home entertainment devices, medical and other applications.

For more information, email: andreas.kohlmeyer@cherry.de