Cooker hoods are known which are controlled by e. g. a humidity sensor or a temperature sensor, commencing extraction of fumes or increasing the rate when the humidity or temperature reaches a pre-determined level. It is also known to use a number of sensors, each sensitive to particular compounds in gas form, to control a cooker hood. It appears that the known constructions which depend on sensors for chemicals all only react to a very specific range of chemical susbstances, and they are inherently very sensitive to contamination which means that their performance and service life in a kitchen environment is less than desired.

Cooker hoods functioning according to the principles outlined above will be found among the following patent publications: DE30339346, DE 25 18 750, US 36 25 135, EP892221.

It is the purpose of the invention to provide a cooker hood which does not display the disadvantages associated with prior known control systems and which is able to provide automatic fume extraction in a wide range of foods being prepared on the cooker. This is obtained in a cooker hood which is particular in that a multitude of sensor elements are used, and that the combined responses of said sensor elements provides a controlling indication of the intensity of the cooking process.

In an advantageous embodiment of the invention, each sensor has a sensitivity to a separate broad range of chemicals, the instantaneous rate of extraction being determined by a specific combination of sensor signals.

In a further advantageous embodiment, three sensors are used, the first sensor being sensitive to water vapour but not sensistive to grease and fat vapours, the second sensor being sensitive to both water vapour and to fat vapour, with opposing reactions, and the third having a high sensitivity to fat vapours, and a low sensitivity to water vapour.

In a further advantageous embodiment the output from each sensor is compared to corresponding data stored in a permanent memory.

In a further advantageous embodiment the data stored in permanent memory is obtained during a calibration run with foods of predetermined compositions being heated with a predetermined amount of heat at a predetermined rate.

The invention will be described further in the following with reference to the drawing, in which Fig. 1 shows a disposition of a cooker hood in relation to a pan on a cooker, and Fig. 2 shows a block diagram of the control for a cooker hood employing the invention.

From a condition of rest, the cooking of food in conjunction with a cooker hood according to the invention proceeds as follows: the heating provided by the cooker heats the food, and a series of chemical processes are activated, each creating vapours, the mixture and concentrations of which may be detected as specific to the particular process by means of dedicated sensors. At low temperatures water vapour is given off, and at higher temperatures certain oxidation of some of the components in the food takes place, and at still higher temperatures fatty components are also given off as vapours, and from parts of the food covered with the most heat tolerant components of the fat, various forms of decomposition may also occur. All of these processes provide quite specific volatile compounds. Known sensors for cooker hood control are directed to the detection of such compounds and to activate the fan or blower. According to the invention this is not required, but instead the signals from a number of sensors are combined as defined in the patent claims, in order to obtain a

classification of compounds and an intensity which activates the fan. When the fan is activated, there will at first be an increase in the concentration of the vapours present at the sensors, and this is very beneficial, because it creates a use-dependent delay or hysteresis which ensures that the fan is activated sufficiently to perform in the manner intended-to provide a fume-free environment in the kitchen.

In the embodiment shown, three semiconductor gas sensors (marked smell sensors) are mounted generally above the cooking area, and above the grease filter and below the impeller of the fan. In the present embodiment they are semiconductor sensors which change their electrical resistance when influenced by the vapours they respond to. Sensor 1 is sensitive to water vapour but not sensitive to grease and fat. Sensor 2 is sensitive to both water vapour and to fat vapour but influence the resistance in opposite directions. Sensor 3 has a high sensitivity to fat vapour but lower sensitivity to water vapour.

When power is first applied to the control, the sensors are monitored for a period of time (typically 5 minutes), this in order to make sure that the sensors are thermally and electrically stable and to gain a knowledge (to be stored) of the sensor output pattern that represents a clean kitchen atmosphere. After this time the monitoring programme starts. All the resistance values (signal levels) of all three sensors are continously monotored. The signals are read into RAM memory every 30 seconds (optional), and this value is compared to a rolling average over 5 minutes (optional).

Both of times indicated are pre-programmed and adjusted to obtain a reliable and speedy response, with a minimum of false starts, nuisance speed changes and false shut downs.

Sensor 1 and 3 have a high signal stability over time under clean air conditions and are therefore mainly used to detect and confirm the existence of and return to clean air conditions. Sensor 2 does not have a stable signal over time in clean air, bur responds quickly to levels of fat vapour and is therefore mainly used to select fan speed changes, rather than to switch the fan on or off. The following are the control parameters of the present embodiment:

The fan switches from offto speed level I-the ratio (sensorl at 30 sec signal)/ (sensorl at 5 min average) is < 0.9 and the ratio (sensor3 at 30 sec signal)/ (sensor3 at 5 min average) is < 0.85.

The fan switches from level 1 to level 2-the ratio (sensorl at 30 sec signal)/ (sensorl at 5 min average) is <0.75 and the ratio (sensor3 at 30 sec signal)/ (sensor3 at 5 min average) is < 0.5, or sensor 2 drops below the value 100kohm.

The fan switches from level 2 to level 3-the ratio (sensor3 at 30 sec signal)/ (sensor3 at 5 min average) is < 0.3, or sensor 3 drops below 50kohm.

Clean air-the fan is switched off Light steam/water vapour-fan speed 1 Heavy steam/water vapour or light fat vapours-fan speed 2 Heavy fat vapours-fan speed 3 After cessation of the cooking and a clean kitchen has been detected and achieved, the fan runs for 5 minutes at speed 1.

In a further embodiment, the fan can switch directly from level 0 to level 2 or level 3, and it can also switch back from level 3 to level 1 or level 2 before switching off, in dependence of the precise inputs provided by the sensors.