EP0442275A2 - Device for detecting a vessel put in the heating zone of a cooking or heating apparatus - Google Patents

Abstract

The pot detection system operates with an inductive sensor (22) which is arranged close to the heated region of the cooking apparatus (11). The sensor signals are evaluated as a function of the signal rate of change in order to compensate for the poor coil quality of the high-temperature-resistant coils. The evaluation takes place digitally with frequency counting, and a bridging device (50) is provided in order to be able to use the apparatus even without pot detection. <IMAGE>

Description

Attempts have already been made to create pan detection systems which only switch on a cooking appliance when there is a pot in the heating zone. Systems with optical sensors and partially with brightness comparison (DE-A 35 33 997 and 33 27 622) and with inductive sensors (DE-A 37 11 589 and 37 33 108) are known. The arrangement of the sensor in the heated zone was problematic in all of these systems. Sensors that can withstand the high temperatures are usually too insensitive to separate useful and interference signals, especially because the pots behave very differently in the sensor field.

The object of the invention is to create a device in which the arrangement of the sensor in the heating zone is unproblematic and in the most varied Operating conditions allows a clear detection of a set pot. This object is solved by claim 1.

The dependency of the detection on the signal change avoids the setting of a certain absolute value for the switching point, so that changing basic requirements, e.g. B. changed by the influence of temperature characteristics of the sensor can be taken into account. The dependency on the rate of change makes it possible to select the response speed of the pot detection to be greater than the rate of change of the basic values. This device enables the detection of pots which cause such small changes in sensor signals that they are no larger or even smaller than the change in the sensor characteristics. However, since these characteristics change much more slowly than the pot position to be recognized, a clear distinction is possible.

This opens up a wide range of possible sensors that were previously difficult to use. In practice, inductive sensors could only be used with poor results because they had to be temperature-shielded and therefore had to be too far away from the actual hotplate. According to the invention, they can be arranged largely directly at the heating point, for. B. at the edge or in the middle of the heating zone and especially just below the actual cooking surface, closer to this than z. B. radiant heating elements. A material which has proven to be particularly advantageous for an inductive sensor as a material which is resistant to high temperatures has hitherto not been considered suitable for such purposes, namely an electrically insulating oxidized heating conductor material, for example a chromium-nickel alloy Art Ni Cr 7030. This material, which is known as a heating conductor material, was considered unusable due to its high resistance value for induction coils, above all because a ferromagnetic coil core must be dispensed with for temperature reasons. Temperatures up to 1300 K (approx. 1000 degrees Celsius) can occur in the area of the induction coil, while conventional coil materials can only withstand a fraction of these temperatures.

According to the invention, the evaluation means can work analogously and determine the rate of change by differentiating the output sensor signal. However, the evaluation means can particularly advantageously work digitally, the starting point being a comparison of the pulses of a sensor resonant circuit frequency counted over a certain gate time with a comparison number, which is kept at a distance in each case from the sensor-dependent pulse number by a certain threshold value. The comparison number is adapted to the actual value of the number dependent on the sensor frequency in each case in a predetermined time sequence, so that the threshold value has a certain size or, depending on the case, also an absolute value of the sensor signal in all operating states. The sign of the threshold value is changed depending on the detection (pot present / not available). If the difference between the sensor-dependent value and the comparison value becomes too large, the readjustment to the threshold value, which is supposed to take place slowly in order to be able to use even weak values, could be accelerated by shortening the readjustment time. This can be achieved by a readjustment speed that is directly dependent on the size of the respective difference value. With digital training, a simple and nevertheless very good one can be Generate the adjustment in such a way that after the initial readjustment with a constant adjustment speed, this takes place suddenly, if the adjustment has not been completed by a predetermined point in time.

A microcontroller is particularly suitable for implementation, i. H. a programmable module that works digitally like a computer, as is often used in control systems. At the same time, it could also contain the functions of an adjustable power control device, a temperature-dependent control device and / or other control functions, such as for a parboil, for temperature limitation, etc., so that apart from control sensors, normally only a code transmitter for manual setting and a power Switching component (relay, triac or the like) are necessary to implement the entire control of the cooking device.

The setup is possible with various sensor systems, including capacitive, optical or similar sensors. With some sensor types, for example the inductive one, certain cooking vessel materials are not detected. Therefore, the device should have a bypass or a switch-off device that enables the cooking device to be operated independently of the pan detection. It can be time-controlled so that it switches back to automatic pot detection after a certain time.

In the case of a device that processes the signals in analog fashion, the previously described functional sequence could also be used. Instead of the pulse counting and the difference formation from these values, there would be a difference (beat) between the sensor frequency and one correspondingly adjusted comparison frequency can be used. The device can also be implemented as a user-specific integrated circuit (ASIC).

In addition to the ease of use, the invention provides further advantages, above all increased safety, because it prevents a hotplate from being operated in idle mode after the cooking appliance has been removed. The inductive version, which is dependent on a ferromagnetic material in the cooking vessel, has the additional safety advantage that it does not respond, for example, when a plastic container is placed on the hotplate, which would be possible with optical devices.

Fig. 1 und 2

je einen schematischen Teilschnitt durch ein Kochgerät mit Glaskeramikplatte und einem Strahlheizkörper und

Fig. 3

ein schematisches Blockschaltbild einer Topferkennungseinrichtung, wobei die einzelnen Blöcke mit erläuternden Funktionssymbolen versehen sind.

These and further features of preferred developments of the invention are evident from the claims and also from the description and the drawings, the individual features being implemented individually or in groups in the form of subcombinations in one embodiment of the invention and in other areas, and can represent advantageous and protectable versions for which protection is claimed here. Embodiments of the invention are shown in the drawing and are explained in more detail below. Show it:

1 and 2

each a schematic partial section through a cooking appliance with glass ceramic plate and a radiant heater and

Fig. 3

is a schematic block diagram of a pot detection device, wherein the individual blocks are provided with explanatory function symbols.

1 shows part of a cooking appliance 11 with radiant heater 13 arranged under a glass ceramic plate 12. It contains, in a sheet metal carrier shell 14, heat-resistant insulation 15 with a peripheral edge 16 supported on the glass ceramic plate 12 and an annular recess 17 on the bottom thereof for example, radiant heating resistors 18 designed as a heating winch are arranged spirally surrounding a central zone 19. Several radiant heaters 13 are pressed resiliently onto the underside of a glass ceramic plate 12 and form individual heating zones 20. They are also suitable for heating or other purposes.

The central zone 19 is formed by an upwardly projecting section of the insulation 15. A recess 21 is provided in it, in which a sensor coil 22 is located. The recess is closed at the top by a disk 23 made of a temperature-resistant insulating material which is firmer than the insulating material 15 and which is supported on the underside of the glass ceramic plate 12. The coil 22 is therefore in a room shielded from direct heat radiation from the radiators. The also electrically insulating disk 23 ensures that contact with live parts is excluded, since glass ceramic becomes conductive at operating temperatures. It also protects the edges of the central zone from damage.

It can be seen that the coil is thus just below the glass ceramic plate and closer to it than the heating resistors 18 and also at a central point.

Fig. 2 differs from Fig. 1 only in that there Insulation 15 of the radiant heater 13 has a bowl or shell shape without a protruding central zone. The sensor coil 22 extends all the way around the radiant heater and is arranged in a circumferential groove 24 provided from the outside in the upper part of the edge 16 of the insulation. An annular disk 23 is inserted between the glass ceramic plate 12 and the rim 16 and has a mechanical and electrical protective function. The groove could also be an angular edge recess, ie without the interposition of part of the insulating body 15 between the coil and the disk.

Here too, the coil is protected against direct exposure to the radiant heating. Nevertheless, considerable temperatures occur there. For this reason, the coil is made of a material that, including its insulation, is resistant to over 1300 K (approx. 1000 degrees C). It is preferably a chromium-nickel alloy of the type Ni Cr 7030. It is electrically insulated by oxidizing its outer surface. However, this material has a rather high electrical resistance. It can therefore have only a few revolving turns, in particular in the case of an embodiment according to FIG. 2. Because of the lack of a ferromagnetic coil core, the coil quality is therefore low. After all, this material enables use directly in the area of the heating zone, possibly even closer to the heating resistors or between them and the glass ceramic plate.

The coil 22 is the sensor of a device for recognizing a cooking vessel 25 set up in the heating zone, which also includes objects to be heated, roasting, heating or other vessels. The sensor responds to such cooking vessels if they are off a material that changes its inductance (ferromagnetic material) or contain it.

3, the pot detection system is explained. The sensor coil 22 generates an output signal in the form of an inductance change when there is a change in the induction of its surroundings caused by the installation of a cooking vessel 25. It is part of an oscillating circuit whose remaining parts, for example a capacitance, are contained in a signal input element 26. The signal is then converted into a square wave signal in a signal converter 27, i.e. a square wave frequency is produced from the sinusoidal oscillation frequency, which is more suitable for further digital processing. In the subsequent frequency measuring device 28, the number of pulses of the square-wave signal and thus a number representing an oscillation frequency is determined and stored via a specific gate time specified by a timer 29.

This pulse number, which is dependent on the sensor frequency, is fed to a difference-forming device 30, where it is compared with a corresponding comparison number, which comes from a comparison number memory and is formed there, as described later. Once per gate time, a signal corresponding to the resulting difference is sent to logic logic 32, including the sign of the difference. The logic logic 32 also contains a memory for a desired distance or threshold value, below which an output signal is given to the switching means 33, possibly via a control or control device 34 explained later. In practice, depending on the current operating state (cooking vessel available / not available) or cooking appliance on / off) corresponding to the threshold value Number added or subtracted from the difference, so that a corresponding enable signal is generated at each zero crossing.

This is done in the rhythm of the goal time, which can be fractions of a second.

The comparison number, which is stored in the memory 31, is the respective actual value, i.e. the number corresponding to the sensor frequency, adapted or tracked. With the aim of obtaining a certain target distance or a target difference. For this purpose, when the actual value and thus the difference value change, a certain amount is added or subtracted from the comparison number in the memory 31 per cycle (gate time interval) via an adaptation device 35 (depending on the +/- sign in the memory of the logic logic 32). The comparison number is thereby adjusted in the direction of the actual value, i.e. tracked until the differential setpoint is reached. As a result, the same response threshold is always achieved regardless of the absolute size of the signal present.

Should this adaptation, which is in any case slower than the corresponding actual value change which is to trigger a circuit, be too slow to have reached the differential setpoint in a predetermined time, which is determined in a timer 36, then a comparison is carried out Jump device 37 the comparison value jumped to the customs differential value. A resetting device 38 resets the timer 36 to the beginning if the target difference was reached before the time expired.

With the exception of the switching means 33 and the regulating or control device 34, the described switching means belong to the evaluation means 40, as symbolized in FIG. 3 by the dashed frame. In the exemplary embodiment, most work digitally. They can be part of a microcontroller 41 or microcomputer, including the regulating or control device 34. The individual devices and elements described for explanation in FIG. 3 are not physically contained therein, but are replaced by appropriate programming in order to carry out the functions described. This also applies to the function of the regulating or control device 34, which also carries out the switching on / off also functions such as power setting, temperature, monitoring and / or regulation etc. It also receives an output signal from the evaluation means 40, possibly signals from a code transmitter 42, which e.g. can be a binary transmitter actuated by a setting button, and / or by a temperature measuring and / or switching device 44. The switching means 43 switch the voltage of the household network 45 to the heating resistors 18 on the high current side and can contain a mechanical relay or corresponding electronic components.

The facility works according to the following procedure:
If the cooking appliance is ready for operation but its heating is not switched on, the resonant circuit containing the sensor coil 22 is in operation. It generates its specific frequency, which means that the frequency measuring and storage device detects a specific number of pulses during the gate time. The associated comparison number from the comparison number memory 31 is a predetermined difference value away from it.

If the inductance of the oscillating circuits operated at a relatively high frequency of, for example, 100 kHz to 1 mHz changes now by placing a cooking vessel, the actual number also changes, which is determined by the frequency measuring device during the gate time and is supplied to the difference 30. If this exceeds the threshold value, then in the logic logic 32 there is a zero crossing in the manner described above, and for example a positive output signal is generated which switches on the heating 18 via the control unit 34 and the switching means 33.

A gradual, relatively slow adjustment of the comparison value to the current actual values is now carried out per cycle via the adaptation device 35. If, for example, a very strongly ferromagnetic pot was used, which caused a large change in inductance, the setpoint distance may not be reached within a predetermined time set by the timer 36, so that the jump device 37 makes an abrupt adjustment by changing the comparison value to the specified inch distance from the actual value is set. This means that even after a relatively short time, the evaluation device is again able to respond even to smaller changes in inductance, for example after a strongly ferromagnetic pot has been removed by placing a little ferromagnetic pot on it.

The inductance properties of the sensor coil 22 change greatly as a result of heat and other environmental influences. In particular, the high-temperature-resistant trace material has a strongly positive resistance characteristic, which leads to a significant drift in the inductance values without spatial changes in the cooking pot / heating zone assignment leads. Since these changes in the absolute values take place in a time range that differs significantly from the placement or removal of a pot, the adjustment of the comparison value via the adjustment device 35 can easily follow this change and set the respective threshold value interval again without that the evaluation unit is triggered. It only reacts to changes that occur faster than the adaptation, so that the sensitivity of the device can also be predetermined via the adaptation speed.

When a pot is removed, the same thing happens again, except that in this case the difference formation shows a different sign, so that the logic logic also delivers and stores an appropriately polarized output signal. The direction of the adaptation also depends on this polarity.

The evaluation means also contain a temporary switch-off device 50 which was actuated by the user, for example via a push button 51. With it, the user can deactivate the evaluation device for a time specified by a timer 53 in its effect on the switching means 33, for example if he wants to cook with a glass ceramic tableware. The schematic circuit diagram indicates that the output signal of the logic logic 32 is suppressed. However, this switch-off device could also be implemented in another way, for example by switching off the entire evaluation device, by bridging the switching means 33 or the like on the high current side. However, it is important that after a certain time (timer 53) this switch-off of the pot detection is canceled again is going to automatic pot detection to return and thus to restart the advantageous function and safety effect. Manual control can also be done using a conventional on / off switch, which is automatically reset after the given time. Since the automatic pot detection can not only lead to increased operational safety, but also to considerable energy savings, it is very suitable not only for domestic stoves, but above all also for commercial kitchens. There, the usual running through of the cooking appliances is avoided throughout the day and, in conjunction with a low-capacity heating, the same result is achieved without delay for the cook. An additional advantage is the lower heat development and thus improved working conditions for the kitchen staff.

Claims (10)

Device for recognizing a cooking vessel (25) set up in a heating zone (20) of a cooking or heating device (11) with a sensor (22) which emits a sensor signal which changes when the cooking vessel is set up or removed, and with evaluation means (40) , which emit an output signal as a function of the sensor signal, characterized in that the evaluation means (40) generate the output signal as a function of the rate of change of the sensor signal. Device according to Claim 1, characterized in that the sensor (22) is an inductive sensor which is arranged in or immediately adjacent to the heating zone (25), preferably on the underside of a plate (which forms the cooking surface of the cooking or heating device (11) 22), such as a glass ceramic plate, the sensor (22) preferably on part of a heat-resistant insulating body (15), such as the edge (16) or a central projection (19) of an insulating shell of a radiant heater (13), and in particular in a recess (21, 24) of the insulating body, is arranged. Device according to one of the preceding claims, characterized in that the sensor (22) is a coil without ferromagnetic coil core with only a few turns, which consists of a high-temperature-resistant material, in particular an electrically insulating oxidized heating conductor material, such as a chromium-nickel alloy. Device according to one of the preceding claims, characterized in that the sensor (22) is part of an oscillating circuit whose oscillation frequency changes as a function of the influence on the sensor inductance. Device according to one of the preceding claims, characterized in that the evaluation means work analogously and possibly include a differentiation of the sensor signal. Device according to one of claims 1 to 4, characterized in that the evaluation means (40) work digitally. Device according to one of claims 4 to 6, characterized in that the evaluation means (40) comparison means (30, 32) for comparing a value dependent on the sensor resonant circuit frequency with a comparison value and adaptation means (35, 37) for changing the comparison value have a distance up to a predetermined threshold value in the direction of the sensor-dependent value, the adaptation means (35, 37) preferably changing the comparison value as a function of time, in particular with a rate of change dependent on the size of the distance between the sensor-dependent value and the comparison value, the adaptation means (35, 37) start with a constant rate of change and then carry out an abrupt adjustment if the threshold value distance is not reached in a predetermined time unit. Device according to one of the preceding claims, characterized in that switching means (33) are provided which, depending on the output signal, switch the cooking or heating device (11) on / off or change its operating state. and / or an actuatable switch-off device (50) is provided, which allows a time-independent actuation of the cooking or heating device (11) independent of the detection. Device according to claim 8, characterized in that the evaluation means (40) is assigned a regulating or control device (34) for operating states of the cooking or heating device. Device according to one of the preceding claims, characterized in that the evaluation means (40) are at least partially contained in a microcontroller (41) or an integrated circuit which is preferably used to carry out further control and regulating functions for the cooking or heating device (11). is trained.

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