GB2072334A

GB2072334A - Temperature responsive apparatus

Info

Publication number: GB2072334A
Application number: GB8009806A
Authority: GB
Original assignee: Thorn Domestic Appliances Electrical Ltd
Current assignee: Electrical and Musical Industries Ltd
Priority date: 1980-03-24
Filing date: 1980-03-24
Publication date: 1981-09-30

Abstract

Apparatus 5 responds to the temperature of the underside of cooking utensil P heated on hot plate 1 of a cooker. The apparatus includes infrared radiation detector 6 which generates electrical signals in dependence on the heat reflected by the underneath of the utensil. These signals may be utilised to control the heat supplied to the hot plate to maintain a constant temperature. The radiation may be transmitted to the detector by one or more optical fibres 7. <IMAGE>

Description

SPECIFICATION Cooking apparatus The present invention relates to cooking apparatus, such as electric cookers and electric cooking hobs.

In conventional electric cookers, a bi-metallic switch is often used to interrupt the electrical supply to a hot plate when the hote plate has reached the temperature set on a user-set control knob. Because of the variation, inter alia, of the thermal load presented by particular a saucepan or frying pan and the material which it contains, there is not a precise one to one relationship between the temperature of the hot plate and the temperature of the pan. Probably the most accurate msasure of the temperature of the material in the pan would be made using a probe placed in the pan. This, however, has obvious practical disadvantages not the least of which being the safety aspect arising from the need for wires trailing from the probe over the side of the pan.

According to the present invention, we provide means for detecting the temperature of a cooking utensil being heated on a hot plate by providing means for detecting infrared radiation from the underside of the utensil and for producing an electrical signal representative of the temperature of the underside.

A particularly convenient means of detecting infrared radiation in these circumstances is an infrared sensitive photodiode. In order to avoid the need for placing the photodiode or other infrared detector in close proximity to the heat source of the hot plate, it is convenient to place the detector at a location remote from the hot plate and transmit the infrared radiation to it via one or more optical fibres. In the case of a spiral electrical heating element, the input end of the optical fibre arrangement can conveniently be disposed in a reflector located at the centre of the spiral.

The signal derived from the detector can be used for closed-loop temperature control and/or to exercise a thermal tripping function.

The invention will be further described with reference to the accompanying drawings in which: Figure 1 is a very schematic perspective view, partially in cross-section of an embodiment of the present invention; and Figure 2 is a schematic circuit diagram of the embodiment of Figure 1.

Shown in Figure 1 is the spiral heating element of an electric cooker on which is disposed a saucepan P, the contents of which are to be heated.

Referring to Figure 2 it will be seen that the heating element 1 is connected in series across the mains supply with a triac 2 and a burst fire controller 3 is arranged to deliver gating pulses to the triac 2 in dependence upon the setting of a user-operable control, namely a potentiometer 4.

The principle of burst fire control in which a variable, integral number of half-cycles of the mains supply are supplied across the load, i.e. the heating element 1 and specific burst fire control circuitry are both very well known and need not, therefore, be described in detail here. Broadly, the burst fire controller 3 includes a ramp generator producing a ramp waveform having a period of several seconds and a comparator which compares this ramp waveform with the voltage Vjn applied to an input of the burst fire controller 3.

The comparator is connected to a suitable driving circuit so that the triac 2 is gated on for each part of the ramp cycle during which the ramp voltage is less than Vjn.

The illustrated cooker is provided with an arrangement 5 which measures the temperature of the underside of the saucepan P by detecting the infrared radiation from the underside of the saucepan P and converts it to a voltage Vet which varies linearly with temperature.

Broadly, this arrangement comprises an infrared-sensitive photodiode 6 which is optically coupled by a bundle of optical fibres 7 to a parabolic reflector assembly 8 which is intended to concentrate infrared radiation from the central area of the underside of pan P onto the input end of the optical fibres; the reflector 8 may also be shaped and dimensioned to shied the optical fibre input from direct radiation from the heater 1. The photodiode 6 has associated with it suitable circuitry for producing the signal VT which is proportional to temperature.

In operation, infrared radiation from the centre of the underside of saucepan P brought about by its heating by heating element 1 is concentrated by the reflector 8 into the input end of the optical fibre bundle and transmitted to the photodiode 6 which is arranged, along with the other control circuitry shown in Figure 2 in an assembly 9 located remote frorn the hot plate so as not to be subject to undue heating from the heating element 1. The assembly 9 can aiso accommodate the circuitry' associated with the other, of for example three other, hot plates of the cooker which are also provided with similar photodiodes 6.

The resulting output signal VT is compared by a differencing amplifier 10 with the voltage picked off from potentiometer 4 which represents the desired temperature and the difference between the signals constitutes an error signal which is supplied as the voltage Vjn to the input of burst fire controller 3. The sign of the subtraction performed by the amplifier 10 is, of course, suitably chosen so that overall the circuitry acts as a negative feedback closed ioop control system controlling the temperature of the underside of the saucepan to maintain a temperature substantially equal to the setting on the user-operable control 4.

The reflector 8 may be covered with a cover plate 11 of a glass suitable to transmit infrared radiation, so as to prevent dirt accumulating in the reflector 8. The cover plate 11 can, of course, be wiped clean.

It will be appreciated that instead of and/or in addition to the control of the pan temperature, the arrangement 5 can be used to carry out a thermal tripping function so that the burst fire controller 3 is disabled and the heating element 1 thus turned off in the event that the temperature of the underside of the pan P as detected by the arrangement 5 exceeds a desired maximum value.

This function can be achieved by comparing the signal VT (which where only the thermal tripping function is required need not necessarily vary linearly with temperature) with a reference voltage indicating a desired maximum temperature of the saucepan P.

Claims

1. An apparatus responsive to the temperature of a cooking utensil being heated on a hot plate comprising means for detecting infrared radiation from the underside of the utensil and for producing electrical signals representative of the temperature of the underside.

2. An apparatus according to Claim 1 including one or more optical fibres for transmitting infrared radiation to the detecting means.

3. An apparatus according to Claim 2 including reflector means, located below the hot plate, for reflecting infrared radiation to the input end of a said optical fibre.

4. An apparatus according to any one of Claims 1 to 3 wherein the means for detecting infrared radiation is an infrared radiation sensitive photodiode.

5. An apparatus according to any one of the preceding claims including circuit means for responding to said electrical signals to regulate the heat supplied to the hot plate.

6. An apparatus according to Claim 5 wherein the circuit means comprises means for comparing the electrical signals with a preselectable signal indicative of a desired temperature of the hot plate, and in dependence on the comparison generating an output signal appropriate for so regulating the power supplied to the hot plate to reduce a difference between said electrical and preselectable signals.

7. An apparatus substantially as hereinbefore described by reference to and as illustrated in the accompanying drawings.