FIELD OF THE INVENTION

This invention relates to an apparatus and method for detecting a malfunction in an appliance heater, for example an electrical heating element in a cooker. This invention also relates to a heating device such as an appliance with a heating element, for example a cooker.

BACKGROUND OF THE INVENTION

Hitherto, control of the temperature of an electrical heating appliance has been performed using a temperature sensor. However, if the sensor malfunctions, the heater may dangerously overheat the appliance or may cause the heater to switch off so that no heating occurs.

It is also known that heating devices such as slow cookers, yoghurt makers and other domestic appliances include a heater element, typically electrically driven, with a temperature sensor in intimate thermal contact with the element to detect its operating temperature. The temperature sensor provides a temperature signal as a function of the heater element temperature, and a control processor that is responsive to the temperature signal controls the operational temperature of the heater by controlling an electrical drive current fed to the heater from e.g. a mains supply. The control processor may include a user interface which allows the user to set the desired operating temperature so that the control processor can compare the temperature signal from the temperature sensor with the set temperature and control the heater drive current in a feedback loop.

A problem with this known arrangement is that if the temperature sensor is in an unsatisfactory, non- intimate thermal contact with the heater element, the temperature sensor will under-record the operating temperature of the heater element, which can result in over-heating of the device that could result in a fire, melting of the appliance or the release of poisonous smoke, particularly when the heating device is configured for use over long pertiods e.g. overnight, such as a slow cooker or a yoghurt maker. SUMMARY OF THE INVENTION

In one aspect the invention provides an apparatus for detecting a malfunction in an appliance heater, comprising a sensor to provide a sensor signal (T _rea d) as a function of the temperature of the heater, a controller responsive to the sensor signal to control power supplied to the heater so as to maintain the heater at an operating temperature within a working temperature range for the normal user operation, the controller being operable to compare the sensor signal (T _rea d) with data corresponding to upper and lower limits (T _wor k_max Twork min) of the working temperature range for the normal user operation and to disable normal user operation for the heater in the event that the temperature signalled by the sensor signal falls outside of the working temperature range wherein the controller comprises a digital processor with an associated non- volatile memory, the processor being operable to store a disable signal in the non- volatile memory when the comparison indicates that the sensor signal falls outside the upper and lower limits of the working temperature range, the processor being configured to disable the supply of power to the heater in response to the disable signal stored in the non- volatile memory.

Thus according to the invention, the apparatus is configured to cause the heater to be disabled for normal operation in response to a detected temperature sensor malfunction, so that the apparatus cannot be restarted until e.g. a service engineer has determined the fault. For example if the sensor has failed, it can be replaced to bring the apparatus back into operation.

With such an arrangement, the service engineer can re-set the data in the nonvolatile memory once the apparatus has been repaired e.g. by replacement of the sensor.

The apparatus may include a display to indicate when normal user operation has been disabled by the controller, indicating that the service engineer needs to be called.

In order to ensure that the heater is only disabled when a genuine fault has occurred, the controller may be configured to disable the heater in the event that the temperature indicated by the sensor falls outside of the working temperature range for more than a given time, for example between 0.5 and 5 seconds, typically 1 second.

The invention also includes a method of detecting a malfunction in an appliance heater, comprising using a sensor to provide a sensor signal (T _rea d) as a function of the temperature of the heater, controlling the power to the heater in response to the sensor signal to maintain the heater at an operating temperature within a working temperature range for the normal user operation, comparing the sensor signal (T _rea d) with data corresponding to upper and lower limits (T _wor k _ _max , T _wor k_min) of the working temperature range for the normal user operation, storing a disable signal in a non volatile memory in response to the temperature signalled by the sensor falling outside of the working temperature range for normal user operation, and disabling the heater for normal user operation in response to the disable signal stored in the memory.

The invention further includes a computer program to be run by a processor to perform the aforesaid method.

In another aspect, the invention provides a heating device comprising a heater element, a temperature sensor to be in thermal contact with and provide a temperature signal as a function of the temperature of the heater element, and a control device configured to compare the temperature signal with a given reference upon the heater element being operated to perform a given change in temperature and in response to provide an indication of the quality of thermal contact between the temperature sensor and the heater element.

Thus, in accordance with the invention, power to the heater element can be shut down when the control device indicates poor thermal contact between the heater element and the sensor which could give rise to overheating.

In one embodiment, the control device is configured to compare the temperature signal that occurs after powering the heater for a predetermined test duration and to compare the temperature indicated by the temperature signal with a predetermined value at the end of said test duration. The control device can be configured to signal an alarm condition if at the end of the predetermined test duration the value of the temperature signal is less than the predetermined value.

The test duration (D _test ) may run from power-up of the heater element so that the testing can be performed when the device is switched on to reduce the risk of overheating during use.

The predetermined test duration can correspond to powering the heater element to reach a threshold temperature which is less than a temperature at which unsafe operation of the heating device will occur, for example less than 100 °C.

The invention also includes a method of testing thermal contact between a heater element and a temperature sensor configured to provide a temperature signal as a function of the temperature of the heater element, comprising: causing the heater element to underdo a predetermined change in temperature, comparing the temperature signal with a given reference after the temperature change, and providing an indication of the quality of thermal contact between the sensor and the heater element as a function of the comparison. BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully understood an embodiment thereof will now be described by way of illustrative example with reference to the accompanying drawings in which:

Figure 1 is a vertical sectional view of a slow cooker;

Figure 2 is a schematic diagram of electrical circuitry included within the slow cooker;

Figure 3 is a flow diagram of steps performed by a program controlled processor illustrated in Figure 2;

Figure 4 is a schematic block diagram of a heating device;

Figure 5 is a graph of the temperature signal produced by a temperature sensor shown in Figure 4 over time when the sensor is in good, intimate thermal contact with the heater element i.e. correctly fixed in place;

Figure 6 corresponds to Figure 5 in a situation where the temperature sensor is badly fixed in place, i.e. in poor thermal contact with the heater element;

Figure 7 is a flow diagram of a process performed by the control processor to determine the quality of thermal contact between the temperature sensor and the heater element; and

Figure 8 is a flow diagram of another process to be performed by the processor to determine the quality of thermal contact.

DETAILED DESCRIPTION

Referring to Figure 1 an electrical appliance in the form of a slow cooker comprises a housing 1 which is designed to sit on a kitchen counter and has a generally disc shaped electrical heater 2 mounted on its upper side in a horizontal configuration to receive a cooking pot 3 having a flat, disc shaped base 4, a cylindrical sidewall 5 and a removable lid 6.

A stirring blade 7 is coupled to a vertically extending axial column member 8 connected by axial coupling pieces 9, 10 to an electric stirring motor 11 mounted in the housing 1. An AC electrical supply is coupled through plug P to the motor 11 and also to the electrical heater element 2.

Control of the electrical supply to the heater 2 and motor 11 is controlled by a controller 12 mounted on a circuit board 13 within the housing 1. The controller 12 receives a temperature signal T from a heater temperature sensor 14 mounted on the heater element 2 so as to detect its working temperature.

Referring to Figure 2, the controller 12 includes a control processor 15 which provides a heater drive signal that is fed to a heater driver circuit 16. The heater driver circuit 16 feeds the AC mains supply from plug P in a duty cycle to the heater element 2 and the duty cycle is controlled as a function of the heater drive signal from the control processor 15 in order to control the temperature of the heater element 2.

The sensor 14 conveniently comprises a thermistor for example a PTC, NTC,

PT100 or the like. The sensor 14 is driven by a sensor drive and control circuit 17 which derives an electrical signal T as a function of the sensed temperature of the heater element 2.

The control processor 15 repeatedly samples the signal T to derive a sampled signal T _rea d as a function of the sensed temperature.

The control processor 15 receives a set temperature T _set , which may be set by the user to determine the desired set temperature for the heater element 2. The control processor 15 thus controls the duty cycle of the heater driver circuit 16 to maintain the sampled temperature T _rea d towards equality with T _set in a feedback loop.

The control processor 15 has an associated program memory 18 which stores programs for controlling the heater to perform different heating routines for different cooking recipes at selectable, different temperatures T _set .

The program memory 18 also includes a program for checking for malfunctions of the sensor 14. If the sensor 14 becomes damaged, the value of T _rea d may go towards zero and as a result, the processor 15 will cause the heater element 2 to be driven at full power in an uncontrolled manner, which may produce scorching or overcooking.

Conversely, if the sensor 14 fails in such a way that T _rea d adopts a very high value, the heater element 2 may be turned off or operate at very low power so that no cooking occurs. To overcome this problem, the cooker is provided with a sensor malfunction detection program in memory 18, which is run by the processor 15 to check for malfunctions of the sensor 14.

To this end, the characteristics of the sensor 14 are selected so that the normal temperature range over which it can operate to sense the temperature of heater 2 is greater than the maximum and minimum allowed working temperature for the heater. Thus, the

characteristics of sensor 14 give rise to the following relationship: re _a d min ^ T _WO rk min ^ T _WO rk nrax ^ T _rea d _max (1) Tread is the current read temperature

T read min is the minimum possible read temperature

T work min is the minimum allowed working temperature

T work max is the maximum allowed working temperature

T read max is the maximum possible read temperature

Thus a malfunction of sensor 14 can be detected by determining whether the value of T _rea d from the sensor falls within one of two ranges defined as follows:

Low Range: T _rea d min≤ T _rea d≤ T_ _wor k_min (2)

High Range: T _wor k _max ≤ T _rea d≤ T _rea d max (3) In the event that the control processor 15 determines that the sensor temperature signal t _rea d falls within the low range or high range defined by the inequalities (2) and (3) set out above, the drive signal to the heater driver 16 is disabled so that no ac mains supply is fed to the heater 2. Also, a disable signal is stored in a non-volatile memory 19, conveniently an EEPROM, so as to permanently disable normal user operation of the cooker. Also, an error message is displayed on display device 20 in response to the detected malfunction under the controller processor 15. The error message may indicate that a service engineer needs to be called.

The service engineer may then replace the sensor 14. This will clear the error display and restore the device ready for normal user operation.

Figure 3 illustrates in more detail the process steps performed by the processor

15 under the control of the malfunction detection program held in memory 18.

Referring to Figure 3, at step SI, the processor 15 samples the temperature signal T produced on a continuous basis by sensor electronics 17 to produce a current sample of the sensed temperature T _rea d.

At step S2, the processor checks to see whether the current value of T _rea d falls within the normal working range and thus implicitly checks whether the value of T _rea d falls within the low or high ranges defined in equalities (2) and (3).

If the current value of T _rea d falls within the normal operating range i.e. not within the aforesaid low or high ranges, the process continues according to the normal temperature control process in which the drive signal provided by the control processor 15 to the heater driver 16 varies the duty cycle of AC power supplied to the heater element 2 so as to maintain the heater temperature to the value T _set . This is indicated at step S3 in Figure 3. A control parameter N is set to value zero at step S4.

If the value of the sensor temperature sample T _rea d falls outside of the normal working range as tested at step S2, the sensor temperature reading is tested a second time after a predetermined delay in order to ensure that the system has not been spuriously triggered to indicate a malfunction. With the current value of N=0 (previously set at step S4), the process moves through step S5 to a delay step S6 of for example between 0.5 and 5 seconds e.g. 1 second, after which the parameter N is incremented to N+l at step S7 and the temperature T is sampled in a second occurrence at step SI . If the second sample of T _rea d is determined at S2 also to be outside of the normal working range, then, at step S5, it is determined that N>0 (N=l from step S7) and so an alarm is provided on display 20 at step S8, the disable signal is written into non- volatile memory 19 at step S9 and the processor 15 commands the heater driver 16 to switch off the mains power from the heater 2 at step S10.

Many modifications and variations of the described system are possible. For example whilst the described example of the invention relates to a slow cooker, it will be appreciated that the sensor malfunction detecting apparatus can be used for heaters in other appliances such as cookers and tumble dryers, and also appliances which use heaters that are not necessarily electrically driven e.g. a gas stove.

Another aspect will now be described with reference to Figures 4 to 8.

Referring to Figure 4, a heating device in the form of an electrical cooking appliance includes a heater element 101 that comprises a metal plate 102 with an associated electrical heating coil 103. The plate 102 may be disk shaped and configured to receive a cooking vessel 104 on its upper surface as illustrated in dotted outline. The heater coil 103 is in this example driven from a domestic AC electrical supply 105 that is fed to a heater driver circuit 106 which controls the on period of the AC supply fed through cable 106a to the heater coil 103, so as to permit control of its operating temperature.

The operating temperature of the heater element 101 is detected by a temperature sensor 107 which conveniently comprises a thermistor for example a PTC, NTC, PT100 or the like. However, other temperature sensors could be used such as a

thermocouple, thermostat, or a thermal elongation sensor. The sensor 107 is driven by a sensor driver and a control circuit 108 which derives an electrical signal T as a function of the sensor temperature of the heater element 101. The thermistor in the sensor 107 is conveniently received within a thermally conductive housing e.g. made of ceramics material or for example metal, received in intimate contact with the heater plate 102 within a recess 109 formed in its underside.

Control processor 110 conveniently comprises a microprocessor which repeatedly samples the signal T from the sensor electronics 108 to derive a sampled signal read as a function of the sensor temperature over time. The control processor 110 also receives a set temperature T _set from a user interface 111. The control processor 110 thus controls the duty cycle of the heater driver circuit 106 to maintain the sampled temperature T _r ead towards equality with T _set in a feedback loop. The control processor 110 has an associated program memory 112 which stores programs for controlling the heater element 101 to perform different heating routines for different cooking recipes at selectable different temperatures T _set . The user interface 111 also allows the cooking duration D _coo k to be set by a user to control the duration of cooking performed by the device.

The control processor 110 has an associated program memory 112, a non- volatile memory 113 such as an EEPROM for storing data and an error display 114 such as a warning light for indicating an error condition.

The program memory 112 includes a program for checking that the sensor 107 is in intimate thermal contact with the heater element i.e. intimately received within the recess 109 of the heater plate 102. The program can conveniently be run at start up of the device so as to avoid overheating due to bad thermal contact of the sensor 107 with plate 102. The program is configured to check the rate at which the sensed temperature T _rea d increases during a test duration D _tes t following power-up of the heater element 103. If the thermal contact of sensor 107 with plate 102 is poor, the sensor 107 will under-report the actual temperature of the heater element 101. This can be seen from the graphs of Figures 5 and 6.

Figure 5 illustrates the rise in temperature of the heater element 101 following start up of the heater element 101. At time t = 0, the heater element 101 is at ambient temperature T _a and when power is initially switched on to the heater element 101, its temperature increases over time as illustrated by curve 115. In the example shown in Figure 5, the element 101 reaches a functional temperature Tf = 60°C after a time t = 28 seconds. Also, it can be seen from the graph of Figure 5 that the temperature of element 101 reaches a threshold temperature T _tri at time t = 40 seconds at which the safety of the apparatus is comprised. In the example shown in Figure 4, the threshold temperature T _tri may be equal to 90°C i.e. just below the boiling point of water i.e. 100°C. In the graph of Figure 5, the values of temperature are accurately sampled in the data T _rea d from the sensor 107 since the sensor is in good thermal contact with the heater element 101. However, if the sensor 107 is in bad thermal contact, the values of T _rea d under- report the actual temperature as illustrated by curve 116 in Figure 6. Here, the functional temperature Tf as reported by the values of T _rea d is not reached until time t=64 seconds, at which time the true temperature T of the heater element 101 has reached 150°C which is significantly above the safe threshold temperature T _tri as illustrated by curve 115 for the actual temperature, which could result in dangerous overheating of the device.

To avoid such overheating, the control processor 110 runs the program held in memory 112 to check the quality of data provided by the sensor 107. One example of the routine is illustrated in Figure 7.

The process starts at step S4.1 at which time the controller 110 commands the heater driver 106 to apply mains power through cable 106a to the heater element 101 on a full duty cycle for a test period D _tes t. In this example, a period D _tes t corresponds to the time taken for the heater element 101 to reach a predetermined test temperature T _tes t. In this example, the value of test temperature T _tes t may correspond to the threshold temperature T _tri for the heater element 101 above which unsafe operation may occur. However, the value of Ttest may be selected to be some other value less than T _tri , by suitably adjusting the duration of test period D _tes t. Thus, in the example illustrated in Figures 5 and 6, the test period D _tes t corresponds to 40 seconds. If an intimate thermal contact is established between the sensor 107 and the plate 102, the value of T _rea d at the end of the test period D _tes t = 40 seconds will exceed the temperature T _tes t = 60°C in this example as illustrated by curve 115 in Figure 6.

However, if poor thermal contact occurs between the sensor 107 and plate 102, the value of T _rea d at the end of D _tes t will be represented by curve 116 and will be less than the value of Ttest-

At step S4.4 as shown in Figure 7, the value of T _rea d at the end of the period Dtest is compared with the reference value corresponding to T _tes t. In this example the value of Ttest corresponds to the functional temperature Tf for convenience but this need not be the case and T _tes t can be selected independently of Tf .

If the value of T _rea d is less than T _tes t, this indicates that the sensed temperature is following curve 116 shown in Figure 6 and is indicative of poor thermal contact between the sensor 107 and the heater element 101, as indicated at step S4.5.

The routine may be run during an initial power-up of the device during manufacture as part of a quality control procedure for checking that the manufactured device is fully operational. Further quality checks of other operational aspects of the device may be disabled at step S4.6 in response to the fault condition indicated at step S4.5. The fault condition may be recorded in EEPROM 113 illustrated in Figure 4 and also an error display may be provided by display 114.

However, if the test performed at step S4.4 indicates that T _rea d is greater than

Ttest, this indicates that the value of T _rea d is changing generally as illustrated in curve 115 of Figure 6, corresponding to good, intimate thermal contact between the sensor 107 and the heater plate 102 and so the device is considered to have passed the test so that it is worthwhile performing further quality control checks for the device as shown at step S4.7.

As previously explained, the process illustrated in Figure 7 may be carried out as a quality control procedure at the time of manufacture of the heating device. Another version of the program held in program memory 1 12 is illustrated in Figure 8 may be run during normal use of the device at start up.

The process starts at step S5.1 and the control parameter N is initialised at step S5.2. At step S5.3, the controller 110 samples the sensor temperature reading T _rea d.

Then, having checked at step S5.4 that the parameter N is initialised, the process checks at step S5.5 whether the sample value of T _rea d is less than a relatively low start temperature T _star t which for example may correspond to the initial ambient temperature T _a shown in Figure 5. In this way, it can be determined whether the device has cooled down sufficiently from a previous use for the program to be run successfully and check the change of temperature that occurs at power-up of the device.

If T _rea d is less than T _star t, a timer is started at step S5.6 and run for a period corresponding to the duration D _tes t. Also, the control parameter N is set to value 1 at step S5.7.

At step S5.8, a check is carried out to determine if the timer has timed out.

When this occurs, the process reverts to step S5.3 and a further a reading of T _rea d is taken by the processor 110. At this point, N >0 and so the process moves to step S5.9 which corresponds to step S4.4 in Figure 8 to determine whether the value of T _rea d is less than the temperature T _tes t at the end of the test period D _tes t. If the temperature T _rea d exceeds the value Ttest, the normal feedback loop control process previously described with reference to Figure 4 is carried out to control the operating temperature of the heater element 101 during cooking, as illustrated at step S5.10.

However, if the value of T _rea d tested at step S5.9 does not exceed the temperature T _tes t, a bad thermal contact has developed between the sensor 107 and heater element 101 so an alarm is provided at step S5.11 on display 1 14 shown in Figure 4 and also an error message is written into the EEPROM 1 13 at step S5.12. At this point, the control processor instructs a heater driver 106 to disconnect the main supply 105 from the heater element in order to avoid overheating. The processor 1 10 may not permit power to be restored to the heater element 101 until a service engineer has repaired the device and cancelled the error message stored in EEPROM 1 13.

Many modifications and variations of the described system can be evident to those skilled in the art and it will understood that the particular temperatures given in the foregoing example may be altered from the values stated hereinbefore.

Also, it will be understood that in the foregoing examples the rate at which the temperature signal T _read varies as the heater element is energised is measured between two fixed points in time, namely the time at which the device is powered up and the end of the test period D _test . However, those skilled in the art will appreciate that other methods of determining the change can be used. For example the slope of the graph shown in Figures 5 and 6 can be can be measured, for example by the use of multiple test sampling points over time at which the temperatures are measured successively.

Also, the test could be performed during a period when the heater element cools down rather than during initial heating. Many other modifications and variations will be evident to those skilled in the art.

Aspects:

1. A heating device comprising:

a heater element (101),

a temperature sensor (107) to be in thermal contact with and provide a temperature signal (T _read ) as a function of the temperature of the heater element, and

a control device (1 10) configured to compare the temperature signal (T _read ) with a given reference (T _test ) upon the heater element being operated to perform a given change in temperature, and in response to provide an indication of the quality of thermal contact between the temperature sensor and the heater element. 2. A heating device according to aspect 1 wherein the control device (1 10) is configured to compare the temperature signal that occurs after powering the heater element (101) for a predetermined test duration (D _test ) and to compare the temperature indicated by the temperature signal with a predetermined value (T _test ) at the end of said test duration. 3. A heating device according to aspect 2 wherein the control device (110) is configured to signal an alarm condition if at the end of the predetermined test duration the value of the temperature signal is less than the predetermined value (T _tes t). 4. A heating device according to aspects 2 or 3 wherein control device (110) is configured to provide the test duration (D _tes t) from power-up of the heater element (101).

5. A heating device according to aspects 2, 3 or 4 wherein the predetermined duration (D _tes t) corresponds to powering the heater element (101) to reach a threshold temperature (T _th ) which is less than a temperature at which unsafe operation of the heating device will occur.

6. A heating device according to aspect 5 wherein the threshold temperature is less than 100 °C.

7. A heating device according to any preceding aspect wherein the control device includes an electrical processor (110).

8. A heating device according to any preceding aspect wherein the heater element (101) is electrically driven and the control device (110) is operable to control electrical heating power supplied to the heater element as a function of the temperature signal ( _r ead) so as to tend to cause the temperature of the heater element to be be maintained at a set temperature (T _set ). 9. A heating device according to aspect 8 including a heater driver (106) operable to control the duty cycle of an electrical alternating current supply supplied to the heater element (101).

10. An electrical appliance including a heating device according to any preceding aspect.

11. A method of testing thermal contact between a heater element (101) and a temperature sensor (107) configured to provide a temperature signal (T _rea d) as a function of the temperature of the heater element, comprising: causing the heater element to underdo a predetermined change in temperature, comparing the temperature signal (T _rea d) with a given reference (T _tes t) after the temperature change, and

providing an indication of the quality of thermal contact between the sensor and the heater element as a function of the comparison.

12. A method according to aspect 11 performed during a quality checking procedure on manufacture of the device, including disabling predetermined quality control checks in the event that the thermal contact is indicated to be in adequate.

13. A computer program to be run by a processor to test thermal contact between a heater element (101) and a temperature sensor (107) configured to provide a temperature signal (T _rea d) as a function of the temperature of the heater element, the program when run by the processor being operative to:

cause the heater element to underdo a predetermined change in temperature, compare the temperature signal (T _rea d) with a given reference (T _tes t) after the temperature change, and

provide an indication of the quality of thermal contact between the sensor and the heater element as a function of the comparison.

It will be appreciated that herein, the term "comprising" does not exclude other elements or steps and that the indefinite article "a" or "an" does not exclude a plurality. A single processor may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to an advantage. Any reference signs in the claims should not be construed as limiting the scope of the claims.

Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combinations of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the parent invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of features during the prosecution of the present application or of any further application derived therefrom.

Other modifications and variations falling within the scope of the claims hereinafter will be evident to those skilled in the art.