Mike Schwert
Cherry Electrical Products
Pleasant Prairie, Wis.

Electric ranges and cooktops are about to get a new look. A combination of microprocessors and sensors will soon make these appliances more efficient, while simultaneously making them safer and easier to use.

One such advance results from a joint effort of Cherry Electrical Products, Pleasant Prairie, Wis., and Schott Glas, Mainz, Germany. It applies a microprocessor-controlled sensor/feedback system to a vitroceramic cooktop unit. The sensor/feedback system consists of a user-interface module, a network of sensors, and a sensing electronics module.

Sensors let the stove automatically detect the presence or absence of a cooking pot and automatically start or stop cooking.

The system has the ability to automatically detect the size of a cooking pot and adjust the burner size accordingly. With electronic control also comes numerous programmable features and other capabilities aimed at safety and convenience.

The user-interface is the first thing most cooks will notice about ranges using this new technology. It includes a touch-sensitive panel and LED readouts of heat range.

Electronics control the entire cooktop operation. One module can control as many as four heat zones on the stove top with up to two dual zones. The module also contains all required power switching electronics so no additional electromechanical switches, relays, or triacs are needed.

Microprocessor control gives the user-interface module the ability to monitor and control each burner unit independently with high accuracy. If the combination of pot placement and user input makes no sense to the interface, the electronics turn all burners off. This safety feature keeps a child or pet from accidentally activating the cooktop. The interface module also turns off all burners if it sees an interruption in ac power. This protects the cooktop and its associated electronics from damage. Automatic self-calibration compensates for changes in room ambient lighting as well as aging of the stove electronics and optical components.

The user-interface module consists of two PC boards attached with springs. The springs press the upper PC board against the underside of the vitroceramic cooktop surface. This upper PC board contains the controller electronics, the touch-panel sensors, and the LED displays. Touch-panel sensors operate on infrared (IR) technology, sensing the presence of the cook’s finger via an IR transmitter/receiver combination.

The lower PC board contains a power supply, power relays for controlling the burner units, and provisions for connection to the primary ac power input. This PC board can screw-mount to the metal frame of the cooktop unit.

Because vitroceramic cooktop units can develop humidity under the glass, it is generally necessary to seal off their electronics. But the Cherry user-interface module needs no separate casing, as it is designed and manufactured with low susceptibility to moisture.

Sensor-based detection
The system’s ability to detect cooking pots and measure their temperature comes from a network of sensors screen-printed on the vitroceramic surface’s underside. These sensors are 2-mm-wide tracks of gold alloy, and their location under the glass coincides with that of the heating elements.

The sensing system is based on ac induction. Sensors transmit and receive electromagnetic signals whose amplitudes are affected by the presence or absence of a cooking pot. Sensing electronics use this information as feedback for controlling the cooktop. Because the sensing system is based on transmission and the resulting pick-up of an ac signal, each basic heating area requires two sensors, or tracks.

The sensing system works as follows: A 12-MHz signal is sent into the transmitter track. The amplitude of the signal seen at the receiving track is determined by the degree of electromagnetic damping. The quantity of conductive material (in other words, the size and location) sitting on the heat zone surface controls the amount of damping. Thus a sufficiently small or large received signal denotes the presence or absence of a cooking pot. Note that only conductive pots (aluminum, copper, and iron) can be detected by the sensors. Also, this system is purely static and does not require that the pot be moved to be sensed.

Heating areas intended for large-diameter pots use a dual-zone system. The larger heating area requires one transmitter track and two receiver tracks for coverage.

All sensor tracks run to a common area for attachment to ribbon cable connectors. Schott Glas has developed special adhesives and methods for attaching the connector to the glass.

The materials used in the gold alloy tracks and connector adhesive have been life-tested. The tracks last 2,000 hr at 595°C, the maximum operating temperature of the glass.

The ribbon cable itself can withstand a maximum temperature of 165°C, and a maximum relative humidity of 95%.

Smart stove electronics
In all, the sensing electronics can detect the presence/absence of up to eight cooking pots. Cooks can manually switch on a burner even if no pot has been detected for cooking with glass or ceramic cookware. Sensors recalibrate each time power comes on. The integrity of sensor readings is assured by automatic testing of glass and sensor tracks to look for cracks and defects.

To detect the vitroceramic glass heat-zone temperature, the sensing electronics measure the specific resistance of the gold-alloy sensor track. The resistance change is directly proportional to temperature change, making it easy to perform high-accuracy temperature measurement.

In contrast, conventional cooktops use an electromechanical assembly known as a rod limiter as a temperature control for the burner unit. The rod limiter is used to prevent the glass from reaching 595°C, where permanent weakening of the glass occurs. When the burner reaches a preset temperature of approximately 525°C, the rod limiter activates and disconnects the burner.

Primitive rod limiters have several shortcomings. First, they are not highly accurate and cannot be relied upon to operate at the same temperature each time. As a result, they must be set to activate at a temperature that provides a margin for error, 525 versus 595°C, to prevent hitting excessive temperatures.

Second, the ordinary controls measure the temperature of the burner unit, not the temperature of the glass itself which is a much more accurate gauge of the temperature in the pot.

Of course, the Cherry/Schott system uses sensors attached directly to the cooktop glass and so measures the actual glass temperature. This means the burners safely operate at temperatures higher than 525°C. Pots heat faster and liquids boil more quickly. Eliminating rod limiters also simplifies wiring and lowers manufacturing costs.

The sensing electronics that replace rod limiters consist of a power supply and power transformer, microcontroller with internal watchdog timer, an external watchdog timer, transmitter and receiver electronics, and permanent memory (EEPROM).

Redundant internal and external watchdogs shut down the system if a function “times out” or there are other questionable conditions suggesting a component has failed.

Combining a microprocessor with high-accuracy temperature measurement opens the door to highly useful features. For example, the processor can interpret a rapid rise in the temperature of the pot as an indication the pot has boiled dry. The burner unit can then be turned off automatically.

The EEPROM is a permanent memory that can store programming for up to 16 different cooktop systems. This is of interest to cooktop OEMs. They can use the same sensing electronics in different products via straightforward programming changes, simplifying their design and manufacturing.

Looking ahead
The cooktop system will be made available in two phases. By the end of 1999, systems will provide automatic calibration mode, automatic cooktop shut-off if the glass cracks, pot recognition for five ordinary and three dual-heat zones, and indication of residual heat in three levels.

The second quarter of next year will see the addition of temperature monitoring, elimination of rod limiters, and provisions for both analog and digital temperature indication.

Future systems may combine sensing and user-interface electronics into one unit. Additional features and capabilities are a possibility as well.

The sensing and interface modules are standard products, but can be customized for specific requirements.