GB2466219A

GB2466219A - Thick film heating element

Info

Publication number: GB2466219A
Application number: GB0822739A
Authority: GB
Inventors: Robin Keith Moore; Robert Henry Hadfield
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 2008-12-12
Filing date: 2008-12-12
Publication date: 2010-06-16
Also published as: WO2010040981A2; EP2346615A2; CN203245059U; GB0822739D0; CN102176850A; WO2010040981A3; CN202154509U

Abstract

A thick film heating element 12 comprising a substrate 21 with a heating track 23 and temperature sensing track 30 formed thereon. The temperature sensing track 30 has one or more first and second sections made of first and second materials with high and low resistivity respectively. In one embodiment, the heating track 23 comprises at least a pair of parallel heating sections connected at one end with the temperature sensing track 30 extending between the parallel sections and comprising parallel sensor sections. In a further embodiment, the heating track 23 and temperature sensing track 30 are formed of the same thick film material. There is also an independent claim for a thick film heating element comprising a substrate with a thick film heating track and a steam sensor formed thereon.

Description

Thick Film Heating Element The present invention relates to a thick film heating element, to a thermal control for controlling the heating element, to a control method for the control and to a computer program product for implementing the control method.

Background to the Invention

Thick film heating elements generally comprise one or more heating tracks that are printed as an ink or paste onto an insulating substrate and fired to form tracks of high electrical resistivity. Connecting tracks may be printed in a separate layer, and fired to form connecting tracks of low resistivity. The insulating substrate may be of an electrically insulating material, such as ceramic, or may be metallic with an insulating surface layer. Such thick film elements are typically used in liquid heating vessels, flow-through heaters, electric irons and other domestic appliances. In liquid heating vessels, the substrate is typically a substantially flat steel plate that forms the bottom of a liquid reservoir, with the tracks deposited on the underside, to form an underfloor heating element.

Heating elements usually require some form of element protection, arranged to disconnect the heating current when the element begins to overheat. In thick film elements, protection may be provided by a mechanical thermostat such as a bimetal in thermal contact with the element. However, in many applications it is preferable to integrate the element protection within the thick film element. An example of this approach is the E-fast' (RTM) element protection disclosed for example in WO 2006/083162 Al, in which the thick film element in deposited on a dielectric comprising first and second dielectric layers with an electrically conductive layer in between. When the element overheats, a leakage current is detected between the thick film element and the electrically conductive layer, in response to which the heating current is disconnected. The E-fast' element protection has proved successful, but for some applications there is a need for a lower-cost solution.

Patent publication EP-A-286 215 discloses a thick film heater for a hob, including a thick film temperature sensor track of high temperature coefficient. The heating track and the sensor track may be manufactured in the same process. To detect local hot spots, the sensor track may be arranged closely to follow the path of the associated heating track so as to cover a large area of the substrate.

Patent publication EP-A-485 211 discloses a thick film heater comprising a conductive heating track formed of pure nickel and a resistive track formed of standard thick film resistor material, interdigitated with the heating track and acting as a thermistor. The thick film heater may be incorporated in a kettle having a steam sensor in an upper part of the kettle reservoir.

Patent publication GB-A-2269980 discloses a kettle with a thick film heater element and first and second thick film temperature sensing tracks arranged to detect tilting of the kettle by detecting a difference in resistance between the first and second temperature sensors.

Statement of the Invention

According to one aspect of the present invention, there is provided a thick film heating element according to claim 1. According to another aspect of the present invention, there is provided a thick film heating element according to claim 8. According to another aspect of the present invention, there is provided a thick film heating element according to claim 9. According to another aspect of the present invention, there is provided a thick film heating element according to claim 10.

In an embodiment of the present invention, there is provided a printed or thick film element having one or more additional tracks. The additional tracks may be of a PTC (positive temperature coefficient) material. The additional track may be used to monitor temperature of the element, such as during normal use, to detect overheating such as in a dry boil condition, to monitor scale build up or to identify if the heater is being used to heat liquid other than water. The additional track may be arranged to act as a steam sensor by monitoring a localised increase in temperature or by monitoring a localised increase in humidity/moisture. A signal from the additional track may be sensed and enhanced by employing a dithering' process to improve the useful temperature measurement resolution.

Brief Description of the Drawings

Embodiments of the invention will now be described with reference to the drawings identified below.

Figure 1 is a schematic diagram of a kettle incorporating a thick film element plate according to the first or second embodiment of the invention.

I

Figure 2 is a plan view of a thick film element plate incorporating a temperature sensor in a first embodiment of the invention.

Figure 3 is a plan view of a thick film element plate incorporating a temperature sensor in a second embodiment of the invention.

Figure 4 is a schematic diagram of a kettle incorporating the thick film element plate of the third or fourth embodiment of the invention.

Figure 5 is a plan view of a thick film element plate incorporating a steam sensor in a third embodiment of the invention.

Figure 6 is a plan view of a thick film element plate incorporating a steam sensor in a fourth embodiment of the invention.

Figure 7 is a plan view of a capacitative steam sensor in an embodiment of the present invention.

Figure 8 is a circuit diagram of a sensor circuit for use with the first embodiment of the invention.

Figure 9 is a graph showing digital conversion values of an analog signal, such as generated by the temperature sensor or steam sensor in embodiments of the invention.

Figure 10 is a graph showing digital conversion of an analog signal, such as generated by the temperature sensor or steam sensor in embodiments of the invention, with a moving average of the digital conversion values being taken as a digital measurement signal.

Figure 11 is a graph showing digital conversion of an analog signal, such as generated by the temperature sensor or steam sensor in embodiments of the invention, with an dithering signal being applied thereto before digital conversion, and a moving average of the digital conversion values being taken as a digital measurement signal.

Detailed Description of Embodiments

Liquid Heating Vessel with Thermistor Figure 1 shows schematically a jug kettle with an electronic control, as an example of a liquid heating vessel to which the first and second embodiments of the invention may be applied. In this example, the kettle is a cordless kettle comprising a vessel body 1 and a power base 2 having respective body and base cordless connectors 3 and 4, such as 360° cordless connectors of the type described in patent publication WO-A-94/06185 and/or as sold by Otter Controls Ltd. under the CS4/CS7 (power base socket) and CP7 (appliance plug) references. The power base is connectable by a power cord 13 to an electrical power outlet (not shown).

The vessel body 1 comprises a reservoir 5 for containing water to be heated, and a base section 6, as well as a spout 7, a lid 8 and a handle 9. Water is heated by an element plate 12 forming the base of the reservoir 5, and including a heating element on the underside (i.e. facing towards the base section 6). The element plate 12 may be fitted into the vessel body using the Easifix (RTM) fitting as described in WO 99/1 7645. The element comprises a thick film element. Preferably, the element plate 12 is composed of stainless steel. The element plate may be circular, oval, rectangular or any shape required to fit the vessel body. The element plate 12 is described in more detail below.

The base section contains an electronic control 10 for controlling the operational state of the vessel, as will be described in more detail below. A user interface 11 allows the user to operate the vessel, and may provide a display of the operational state of the vessel. The control electronics may be divided between the user interface 11 and the control 10 as desired.

A thermistor 14 is arranged to sense the temperature of water in the reservoir 5 through the element plate 12, and is preferably thermally isolated from the heating element. In this example, there is no steam tube to carry steam from the top of the reservoir 5 to the control 10, since boiling is detected from the input of the thermistor 14 rather than by sensing steam, as will be described in more detail below. The thermistor 14 may have a negative temperature coefficient (NTC). In some of the embodiments described below, boiling is detected by means other than temperature sensing, so that the thermistor 14 is not required.

The vessel may have one or more additional features, some of which are described in more details below. However, to avoid repetition, some of these features will be outlined here.

Additional heating features of the vessel may include a keep warm' feature, in which the liquid is maintained around a predetermined temperature, preferably after boiling; this may be done by intermittent activation of the main heating element, or by intermittent or continuous activation of a secondary heating element. The predetermined temperature may be just below boiling point, or a lower temperature such as 80°C, and may be selectable by the user.

Another heating feature is a sub-boil feature, in which the liquid is heated up to a predetermined temperature below boiling, such as 80°C for making coffee, and the heating power is then switched off or reduced, for example to activate a keep warm mode. The predetermined temperature may be selectable by the user.

Another heating feature is a prolonged boil feature, whereby the liquid is heated to boiling and then boiled for at least a predetermined time, such as 30 seconds to 2 minutes, to sterilize the liquid.

The user interface 11 may comprise a remote control, as described for example in The element plate 12 may comprise one or more elements connectable to a plurality of electrical sockets, for example as described in PCT/GBO8/002808.

Thick Film Element Plate Each of the embodiments described below comprises a thick film element plate 12 for a liquid heating vessel, comprising a substantially circular steel substrate 21, preferably of stainless steel. The heating elements in these embodiments are rated at 3.1 kW for a 240 VAC power supply.

A dielectric layer 22 is deposited on the substrate 21. Thick film heating track portions 23 of high resistivity material are deposited on the dielectric layer 22, and contacts 24a, 24b of low resistivity material are deposited on the dielectric layer 22 and in electrical contact with the ends of the heating track portions 23. Connecting portions 25 are formed from the same low resistivity material, to connect heating track portions 23 together in series. Bridges 26 of the same low conductivity material may be formed over bends in the heating track portions 23, to avoid overheating. Next, a protective glaze may be deposited over at least some parts of the element plate 12, to provide electrical insulation andlor to protect parts from corrosion. External contacts, such as contacts 24a, 24b, 31, 32a and 32b are left unglazed so as to allow electrical connection thereto.

The thermistor 14 may be mounted in the central area of the element plate, shown as a central blank area in the drawings, and connected to contacts 31 and 32b. The heating I' track 23 is designed to make as much use of the area around the outside of the element as possible. The central area is kept free of heating track 23 to reduce direct conduction of heat from the heating track 23 to the thermistor 14, which therefore tracks the water temperature more accurately.

First Embodiment In the first embodiment, a sensor track 30 is deposited on the dielectric 22.

Advantageously, the seusor track 30 is deposited at the same time as the heating track 23, is therefore substantially coplanar with, and forms part of the same layer as the heating track 23. The sensor track 30 is preferably formed from the same resistive track material as the heating track 23, which is generally much cheaper than high resistivity/TCR material (63 pence per gram versus 805 pence per gram at December 2008). However, this material has a relatively low resistivity (e.g. 20-200 mWsquare, for example lO0mWsquare) for the purposes of temperature sensing, and a relatively low temperature coefficient of resistance (TCR), which results in a relatively low signal strength.

The sensor track 30 is designed to be as long and as thin as possible in order to maximise the resistance and hence the strength of the signal derived from the sensor track 30, such as a voltage signal. Moreover, the sensor track 30 is distributed over the majority of the element plate 12 in order to be able to sense overheating in substantially any area of the element plate 12, and therefore be suitable for detection of spots, boil-dry caused by tilting, and scale build up. To achieve these advantages,' the sensor track extends along gaps between the heating track portions 23 and doubles back on itself, such that the sensor track 30 includes substantially parallel sections cormected continuously at their ends by narrow bends of substantially 180°. These ends may be located adjacent to bends in the heating track portions 23.

Sensor contacts 32a, 32b are connected to either end of the sensor track 30, to allow electrical connection of the sensor track 30 to a sensor circuit or to the electronic control for detecting a temperature condition of the element plate 12. Specific embodiments of the sensor circuit and the electronic sensor are described in more detail in the separate sections below.

Second Embodiment The second embodiment, as shown in Figure 3, differs from the first embodiment in that the sensor track 30 includes sensing sections 33 of a high resistivity, high TCR material.

S

The resistivity may typically be in the range 1-100 Wsquare, for example 10 Il/square.

The TCR may typically be in the range 1500-3000 ppm. These sections 33 are connected in series by discrete sections of the sensor track 30, made of the same material as the heater track 23 as in the first embodiment; this is cheaper than connecting the sensing sections 33 with the low resistivity material from which the contacts 24a, 24b, 31, 32a and 32b are made, which typically contains silver. In addition, the material, having a PTC, will contribute to the change in signal with temperature.

The sensing sections 33 are placed in areas that are likely to experience a high rate of temperature rise in a dry-boil condition. In addition, they are distributed around the element in order to detect a tilted boil-dry regardless of the direction of tilt.

The sensing sections 33 provide higher signal strength, but are more expensive in both material and labour than the first embodiment, because an additional material is required for the sensing sections 33. The lengths of the sensing sections 33 can be selected to provide the desired trade-off of cost against performance.

Liquid Heating Vessel with Steam Sensor Figure 4 shows schematically a jug kettle with an electronic control, as an example of a liquid heating vessel to which the first and second embodiments of the invention may be applied. The kettle is similar to that shown in Figure 1; similar parts are indicated by the same reference numerals, and their description is not repeated here. However, instead of using the thermistor 14 for boil detection, there is provided a steam tube 17 that conveys steam from above the water level to a steam chamber 18 beneath the element plate 12 when the liquid boils, as shown by the dashed line in Figure 4. A steam sensor 40 is provided on the underside of the element plate 12 within the steam chamber 18, as described in more detail below. The steam chamber may provide a volume for the containment of steam in the space beneath the element plate 12, and may at least partially segregate the steam sensor 40 from other electrical components, such as the electronic control 10. In this way, damage to other electrical components, caused by ingress of steam, may be prevented or mitigated.

Figure 4 shows the steam sensor 40 formed at the centre of the element plate 12 and the steam tube 1 7 passing through an aperture at an edge of the element plate 12. This arrangement requires a relatively long steam chamber 18 under the element plate 12 to convey steam to the steam sensor 40. Alternatively, the steam tube 17 could pass through an aperture in the central part of the element plate 12, adjacent the steam sensor 40, so that less space is occupied by the steam chamber 18. Alternatively, the steam sensor 40 could be positioned in any suitable area of the element plate 12 and advantageously may be positioned towards the edge of the element plate 12 for easier access to a steam tube 17 positioned to one side of the reservoir 5, or outside the reservoir 5.

Third Embodiment The third embodiment differs from the first embodiment in that a steam sensor 40 is formed on the element plate using thick film printing. The steam sensor 40 comprises a long, thin track of higher resistive material, typically in the range 1-100 Wsquare, for example 10 1/square. The TCR may typically be in the range 1500-3000 ppm. The track is arranged in a labyrinthine or serpentine arrangement so as to be compressed into a small area. The steam sensor 40 is electrically connected between contacts 31 and 32b. When the liquid boils, steam is directed through the steam tube 17 onto the steam sensor 40, which senses a sudden or step change in local humidity, for example by detecting a change in resistance or capacitance. In response to this change, the electronic control 10 reduces or switches off heating power in the liquid heating vessel.

An alternative embodiment of the steam sensor 40 is shown in more detail in Figure 7.

Instead of comprising a continuous track, the steam sensor 40 comprises a pair of interdigitated silver electrodes 40a, 40b formed on a barium titanate (BaTiO3) thick film layer 42 deposited on the dielectric layer 22. The capacitance between the electrodes 40a, 40b depends on the humidity and or level of moisture between the electrodes 40a, 40b.

Preferably, the steam sensor 40 is left unglazed so as to be more sensitive to humidity.

Selection of the material of the steam sensor 40 is very important to prevent silver migration. For example, the silver electrodes may include a high level of palladium.

Preferably, the steam sensor 40 is dried out after sensing a boil condition. A discrete heating track may be provided to dry out the steam sensor 40, or the steam sensor 40 may be dried out by ambient heating or heat conducted through the element.

Alternatively, a low current may be provided through the steam sensor 40 for detecting humidity until a boil condition is detected, and a higher current may be passed through the steam sensor 40 after boiling is detected, so as to dry out the steam sensor 40.

Before the steam sensor 40 has dried out, it may be unable to detect whether the liquid in the liquid heating vessel is boiling. Therefore, if a heating current is manually switched on soon after boiling, the heating element may be re-energised for a predetermined time, such as 15 seconds, and then switched off without determining whether boiling has occurred.

Fourth Embodiment The fourth embodiment, as shown in Figure 6, differs from the third embodiment in that the steam sensor 40 is sensitive to temperature rather than humidity; for example, the resistivity of steam sensor 40 may be sensitive to temperature. Steam impinging on the steam sensor 40 of this embodiment causes a sudden change in the sensed temperature, in response to which the electronic control 10 reduces or switches off heating power in the liquid heating vessel. The steam sensor 40 may be calibrated so that a change in temperature corresponding to the impingement of steam may be reliably detected.

Sensor Circuit As shown in Figure 8, the sensor track 30 may be placed in series with a resistor 16 across a low supply voltage V, and a change in voltage Vi across the resistor 16 may be detected by the electronic control 10 as an indication of the change in resistance R2 of the sensor track 30. The heating current may be disconnected or reduced when either a resistance threshold has been reached or a rate of resistance change has been reached for the resistance of the sensor track 30. The sensitivity of the sensor circuit may be improved by using an amplifier.

Digital Conversion For input to the electronic control 10, the voltage Vi will typically be converted to a digital value by an analog-to-digital converter (ADC). This results in a quantised digital measurement value which may have a poor resolution if the signal is small relative to the input range of the ADC, as shown for example in Figure 9.

The resolution may be enhanced by averaging the digital measurement values. For example, Figure 10 shows the result of taking a moving average of 10 measurements as the input value. Although the transitions between quantised measurement values are smoother than the result shown in Figure 9, the measurement values are still highly quantised.

The resolution of the measurement values may be greatly improved by adding a periodic dither' signal to the analog signal to be measured, before input to the ADC, and taking a moving average of 10 measurements by the ADC as the measurement values, as shown in Figure 11. The amplitude of the dither signal is preferably at least equal to the resolution of the ADC, and more preferably twice as large, so that the measured analog signal transitions between at least two digital output values of the ADC, even if the analog signal to be measured is static. As a result, the dithered' average measurement values have a much greater resolution than that shown in Figures 9 and 10.

Alternative Embodiments In addition to the variants mentioned above, other variants are envisaged as falling within the scope of the invention. For example, the present invention is not limited to kettles and heating elements therefor; aspects of the invention may be applied to wasserkochers, coffee makers such as moka makers, Turkish tea makers, samovars, water boiling urns, pans, sauce makers, hobs, steamers, chocolate fountains, fondues, steamers, slow cookers and milk frothers, for example.

The above embodiments illustrate, but do not limit, the present invention. Alternative embodiments, which may occur to the skilled reader on reading the above description, may also fall within the scope of the invention.

Claims

Claims 1. A thick film heating element comprising a substrate having a thick film heating track and a temperature sensor comprising a thick film temperature sensing track formed thereon, wherein the temperature sensing track comprises one or more first sections of a first material having a high resistivity and/or a high temperature coefficient of resistance, and one or more second sections of a second material having a low resistivity and/or a low temperature coefficient of resistance.
2. The element of claim 1, wherein the one or more second sections are of the same material as the heating track.
3. The element of any preceding claim, wherein the resistivity of said first sections is in the range 1-100 u/square.
4. The element of any preceding claim, wherein the temperature coefficient of resistance of the first sections is in the range 1500-3000 ppm.
5. The element of any preceding claim, wherein the resistivity of said second sections is in the range 20-200 m /square.
6. The element of any preceding claim, wherein the second sections have a positive temperature coefficient of resistance.
7. The element of any one of claims I to 6, wherein the sensing track comprises a plurality of said first sections, mutually distributed around the element.
8. A thick film heating element comprising a substrate having a thick film heating track and a temperature sensor comprising a thick film temperature sensing track formed thereon, wherein the heating track comprises at least a pair of substantially parallel heating sections connected at one end, wherein the temperature sensing track extends between said pair of substantially parallel sections and comprises substantially parallel sensor sections connected adjacent said one end.
9. A thick film heating element comprising a substrate having a thick film heating track and a temperature sensor comprising a thick film temperatureSsensing track formed thereon, wherein the heating track and the temperature sensing track are formed of the same thick film material.
10. A thick film heating element comprising a substrate having a thick film heating track and a steam sensor formed integrally thereon.
11. The element of claim 10, wherein the steam sensor is formed as a thick film.
12. The element of any one of claims 10 to 11, wherein the steam sensor is sensitive to humidity.
13. The element of claim 12, wherein the steam sensor is a capacitative sensor.
14. The element of claim 13, wherein the steam sensor comprises a plurality of interdigitated electrodes.
15. The element of claim 14, wherein the electrodes comprise silver.
16. The element of claim 15, wherein the electrodes include a high level of palladium.
17. The element of any one of claims 10 to 11, wherein the steam sensor is sensitive to temperature.
18. The element of claim 17, wherein the steam sensor comprises a long, thin, thick film track confined to a small area of the substrate.
19. The element of any one of claims 10 to 18, including means for drying the steam sensor.
20. The element of claim 19, wherein said means comprises a second, discrete heating track.
21. The element of claim 19, wherein said means is arranged to pass a heating current through the steam sensor.
22. The element of any one of claims 10 to 21, including means for directing steam onto the steam sensor.
23. The element of claim 22, wherein said means includes a steam tube.
24. The element of claim 22 or 23, wherein said means includes a steam chamber arranged to segregate the steam sensor from its surroundings.I
25. The element of any preceding claim, wherein said substrate is substantially planar.
26. The element of any preceding claim, wherein said substrate is substantially circular.
27. The element of any preceding claim, wherein said substrate comprises steel with a dielectric surface layer.
28. The element of claim 27, wherein said steel comprises stainless steel.
29. The element of any preceding claim, including a sensing circuit connected to the sensor so as to generate a sensing signal.
30. The element of claim 29, wherein said sensing circuit comprises an analog to digital converter.
31. The element of claim 30, wherein the sensing circuit is arranged to apply a periodic dithering signal to the sensing signal prior to digital conversion.
32. The element of claim 30 or 31, wherein the sensing circuit is arranged to calculate a moving average of the digitally converted sensing signal.
33. A liquid heating vessel including an element according to any preceding claim.
34. A flow-through heater including an element according to any one of claims 1 to 32.
35. An element substantially as herein described with reference to and/or as shown in any one of Figures 2, 3, 4 and 5 of the accompanying drawings.