GB2060329A

GB2060329A - Cooking hobs

Info

Publication number: GB2060329A
Application number: GB8032832A
Authority: GB
Original assignee: Thorn Domestic Appliances Electrical Ltd
Current assignee: Electrical and Musical Industries Ltd
Priority date: 1979-10-11
Filing date: 1980-10-10
Publication date: 1981-04-29

Abstract

A ceramic or glass type cooking hob has associated with each hot plate a heating element 3 and an array of proportional or tripping temperature sensors 7, the outputs of which are scanned by a MPU controlling heat triacs for the purposes of closed loop temperature control or thermal tripping in an overtemperature condition. The use of an array of sensors 7, each bonded to the underside of the hot plate area, helps avoid the occurrence of hot spots which might otherwise damage the glass or ceramic material. <IMAGE>

Description

SPECIFICATION Cooking hobs The present invention relates to cooking hobs.

There are two main varieties of ceramic and glass cooking hobs. In the first, the cooking hob comprises a plate in which are embedded a number of heating elements which are used to heat pans and other utensils placed on the plate.

In the second type, the heating surface is a ceramic or glass plate below which are located a number of heaters which radiate heat onto the plate which conducts it to the pan being heated.

Both of these two arrangements have the advantage that the surface of the hob is smooth and therefore easy to clean but particularly the second, radiant heating version suffers from the disadvantage that the material used for the plate tends to be a relatively poor conductor and under certain circumstances overheating, particularly of the underside of the plate, can occur which may shorten its useful life. The rate at which the plate surface will absorb the radiated energy from the electrical heating element without a dangerous temperature being reached varies according to the effectiveness with which the cooking utensil absorbs heat energy. If, for example, the cooking utensil is in close contact with the ceramic, it can very effectively absorb the radiated energy.If, however, the cooking utensil is distorted and only in partial contact with the plate, the plate cannot effectively conduct heat to the utensil and overheating of the plate may occur.

According to the present invention, there is provided a cooking hob comprising at least one hot plate, the or each hot plate having at least one associated heating element and at least one temperature sensor at the heated area of the hot plate and in thermal contact with it and control circuitry arranged to limit or to control the power supply to the heating element or elements to keep the temperature of the hot plate to a safe level.

Preferably a number of temperature sensors are distributed over the heated area. This is useful if the material of the plate is a poor conductor of heat. Thus, while a single sensor might be used, the difficulty with this would be that although the sensor would be effective to prevent overheating at the actual point where it is located, the temperature conditions across the plate might vary so that another part might reach a harmful temperature. One circumstance in which this could occur would be if a pan were placed offcentre on the heating area. Thus, while the sensor might be located below a part of the heated area in contact with the pan and thus register a safe temperature, the absence of the pan over another part of the heated area would slow the removal of heat from that part which could thus result in overheating.

The temperature sensors may be two- or multistate devices such as, for example, bimetallic switches or proportional devices such as thermistors, thermocouples etc. Where proportional sensors are used, it is possible to use the sensors to carry out closed-loop control of the power supply to the heaters so that the plate reaches a desired temperature set by the user.

This could, of course, also be done when bimetallic switches are used but it would then be more difficult to make the operating temperature adjustable. With bimetallic switches and the like, the temperature sensors can be used to indicate that overheating is occurring and the control circuitry can be arranged so that in those circumstances the power supply to the heater concerned is interrupted until a safe temperature is reached.

The temperature sensors may be distributed in a wide number of configurations over the heated area. One possibility is to have the temperature sensors arranged around the circumference of one or more concentric circles. Other possibilities include their being arranged in a square or rectangular grid fashion. Clearly if the primary purpose of the temperature sensors is to prevent over heating of the plate, the distances between adjacent temperature sensors should be chosen having regard to the thermal conduction characteristics of the plate, the temperature distributions over the heated area which are likely to occur in practice and other such factors so as to avoid the possibility of "hot spots" occurring at points away from the temperature sensors themselves.

The invention will be further described with reference to the accompanying drawings in which: Figure 1 is a cross-sectional view through one embodiment of the present invention; Figure 2 is a plan view of one of the heated areas of the hot plate to an enlarged scale; and Figure 3 is a block diagram of the control circuitry of the cooking hob.

The cooking hob 1 shown in Figure 1 has a base plate 2 on which rests a number, for example, four, spiral heating elements 3 which are spaced from the base plate 1 by insulating pads 4.

Above the base plate 2 and parallel to it is a plate 5 which constitutes the heating surface of the hob on which pans and other utensils are stood to be heated. The plate 5 is made of glass, a suitable glass ceramic or other material. The hot plate areas 6 are indicated on the plate by, for example, suitable markings on the plate 5 or by other means such as raised ridges. Over each of the areas 6 are distributed a number of temperature sensors 7 which are used to sense the temperature of the glass plate 5 and appropriately control the energisation of the heating elements 3 so that the above-mentioned over temperature conditions are avoided. The distribution of the sensors 7 is decided on the basis of the criteria mentioned above.These temperature sensors 7 may be proportional devices such as thermistors, thermocouples etc. or two- or multi-state devices such as bimetallic switches. The temperature sensors 7 are either bonded to the plate 5 or embedded in it so that in either event they are in thermal contact with the plate 5.

Control circuitry which may be used where the temperature sensors are two-state devices will now be described in connection with Figure 3. As indicated in Figure 3, the heating elements 3 are each provided with a respective triac 10, these triacs being supplied with gating signals by a micro-processor (MPU) 11 which is appropriately programmed to burst-fire the triacs 10 so that each ring dissipates an amount of power dependent upon the user setting.

Also associated with the MPU 11 are a number of user operable controls 12 to adjust the heat setting for each ring, a number of displays 13 to display the heat settings for each ring and the temperature sensors 7.

The user operable control for each ring may be a count-up/count-down control as described and claimed in our British Patent No. 1,464,093. The controls may comprise touch control pads on, for example, the upper surface of the cooking hob 1.

The MPU 11 may be appropriately programmed so that the relationship between the indicated values on the displays 13 and the heat output from the rings 3 is logarithmic so as to provide a wide range of heat settings with a reasonably fine control at the low-power end of the range so that the rings are more easily set for simmering.

The temperature sensors 7 are arranged in a matrix, part of which is shown in Figure 3 and each is connected between one input and one output line of the MPU 11 in a similar fashion to the one conventionally used to scan the state of keyboard contacts in for example pocket calculators. Briefly, this involves the MPU producing a signal on one of the output lines of the matrix at a time and identifying the input lines on which this produces a signal. The temperature sensors 7 are set to operate at a desired temperature to avoid damaging overheating of the plate 5. Thus, when the MPU 11 detects that one or more of the sensors 7 associated with one of the rings 3 indicates an over-temperature condition on the plate 5, it interrupts the supply to that ring until such time as the over-temperature condition is eliminated.It will be appreciated that the MPU 11 both in correcting over-temperature conditions and setting the power level of each ring 3 may take account of the thermal time constant of the ring 3 and plate 5.

Under normal conditions, the MPU 11 will burst-fire each triac 10 so that the mean power delivered to the associated ring 3 is in accordance with the heat setting set by the user. To speed the response of the rings 3 to changes in heat setting, the MPU 11 may be programmed so that for an interval after change in heat settings, the associated ring is either turned on full or turned off, depending on whether the change in setting was up or down.

It will be appreciated that such interfacing circuitry as may be required to interface the controls, displays, temperature sensors and triacs etc. with any particular micro-processor have been omitted from Figure 3. If the temperature sensors were proportional devices, such as thermistors, they could be interfaced with a microprocessor by means of an analaog to digital converter. Because of the present expense of such converters, it might be appropriate to connect the temperature sensors to an analog time-division multiplexer which would feed samples of the outputs of each temperature sensor to the A to D converter.

Where the temperature sensors 7 are proportional devices, they may be used to carry out closed-loop control of the ring 3 so that a desired temperature could be set. In those circumstances, of course, the MPU 11 will be programmed so as to compare the temperature set by the user on a control 12 with the temperature detected by the sensors 7 associated with the ring in question and appropriately control the ring to give the desired temperature.Where the temperature sensors are proportional and it is desired to detect when a threshold temperature is reached, the need for a multiple-bit analog to digital converter at the input to the MPU may be avoided by generating the threshold signal as an analog signal - this could be done for example by a simple digital to analog converter using a resistor array connected to outputs of the MPU and to compare this analog threshold signal with the outputs of the various temperature sensors by use of a number of comparators which, of course, give digital outputs which can be applied to the micro-processor.

The MPU can, of course, be programmed so that both the over-temperature protection and temperature control functions are achieved.

Where a seven-segment or similar display is used to display the heat setting, the MPU can be programmed so that when a ring is turned off the display illuminates the symbol "H" until such time as the sensors 7 detect that the plate is safe to touch.

The control circuitry need not of course be implemented using a microprocessor. Custom l.C.s or other devices may be used.

Although the invention has been described by way of example with reference to the count up/count down system of control, other systems may of course be used. These might include, a direct selection of the digital setting by touching "1." "2." "3." tabs as appropriate, a knob indicating either an arbitrary scale or indeed, temperature.

Claims

1. Cooking hob comprising at least one hot plate, the or each hot plate having at least one associated heating element and at least one temperature sensor at the heated area of the hot plate and in thermal contact with it and control circuitry arranged to limit or to control the power supply to the heating element or elements to keep the temperature of the hot plate to a safe level.

2. A cooking hob according to Claim 1 wherein there is a multiplicity of temperature sensors distributed over the heated area of the or each hot plate.

3. A cooking hob according to Claim 2 wherein the temperature detectors are arranged in a rectangular or circular array.

4. A cooking hob according to Claim 1, 2 or 3 wherein the or each hot plate comprises a panel of glass or ceramic material below which the associated heating element or elements is disposed to heat the panel by radiant heating.

5. A cooking hob according to Claim 4 wherein there are a plurality of hot plate areas provided on a common panel and each having an associated independently controllable heating element.

6. A cooking hob according to any one of the preceding Claims wherein for the or each hot plate, the or each associated temperature sensor is a proportional sensor.

7. A cooking hob according to any one of the preceding Claims wherein the control circuitry is arranged to control the temperature of the associated hot plate in accordance with the temperature as detected by the associated sensor or sensors and the setting of a user operable control.

8. A cooking hob according to any one of Claims 1 to 5 wherein the or each temperature sensor is a bimetallic trip.

9. A cooking hob constructed and arranged to operate substantially as hereinbefore described with reference to the accompanying drawings.