WO1990007851A1

WO1990007851A1 - Cook top

Info

Publication number: WO1990007851A1
Application number: PCT/EP1989/001575
Authority: WO
Inventors: Albert Kiessling
Original assignee: Industrieelektronik Dr.Ing. Walter Klaschka Gmbh & Co.
Priority date: 1988-12-23
Filing date: 1989-12-20
Publication date: 1990-07-12
Also published as: DE3843460A1; EP0374868A1; JPH03502852A

Abstract

A cook top has an automatic pot detector in the form of a capacitative presence sensor consisting of the base (34) of the pot and two electrodes (14, 28) arranged at a distance apart on the lower surface of the glass ceramic supporting plate (10) of the cook top.

Description

Cooktop

description

The invention relates to a hob according to the preamble of claim 1.

Such cooking fields typically have a support plate made of ceramic which is permeable to heat radiation and under which an infrared radiator is arranged. The support plate is usually colored so that it cannot be easily recognized whether the infrared radiator is in operation or not. On cookers for commercial kitchens, it is also often desired that the hob automatically switches on when a pan is placed on top or roast pieces or the like are placed on top. For this reason, it has already been proposed to arrange an inductive sensor under the support plate of such a hob, which responds to the placement of a pot made of metal. The attachment of inductive sensors under the support plate of a hob is difficult, however, since such sensors are not designed for high temperatures. In addition, the sensor must be able to distinguish between pots and metal cutlery that is accidentally left on the hob. In addition, inductive sensors are relatively expensive and for this reason the use of occupancy sensors has not previously been considered in domestic hobs.

The present invention is therefore intended to create a hob according to the preamble of claim 1, which allows the reliable detection of pots on the hob with a mechanically simple structure. This object is achieved according to the invention by a hob according to claim 1.

In the hob according to the invention, two electrodes are provided on the underside of the support plate, which together with a pot base form two capacitors which are connected in series. The pot base forms the counter electrodes for the two spaced electrodes attached to the underside of the field and at the same time the electrical connection which connects the two capacitors in series. Connections to an external operating and evaluation circuit that responds to the change in capacitance between the connection terminals need only be provided on the underside of the support plate, that is to say in the area protected against contamination. The top of the support plate and thus the hob is unchanged from a standard hob.

Advantageous developments of the invention are specified in the subclaims.

With the development of the invention according to claim 2 it is achieved that the presence sensor only responds at the same time when the pot placed substantially coincides with the axis of the hob. Only then is there a sufficient overlap between the bottom of the pot and the electrodes arranged on the underside of the support plate. A sharp "directional characteristic" of the capacitive occupancy sensor is advantageous because you can move a relatively small amount

Pot away from the center of the hob can switch off the hob. The hob does not need a large one

To have "parking spaces" for pots that are no longer to be heated; these pots, which sometimes together with their contents also represent large masses, do not need to be lifted off the hob to switch it off the .

The development of the invention according to claim 3 has the advantage that heat radiation can also pass through to the bottom of the pot through the surfaces of the support plate underside occupied by the electrodes.

The development of the invention according to claim 4 is in

With regard to simple and inexpensive manufacture of the electrodes of the capacitive presence sensor, this is advantageous.

In a hob according to claim 5, there is only a very small ohmic short-circuit path across the measuring capacitor path, even if the support plate is heated to a very high temperature.

The development of the invention according to claim 6 is also advantageous with regard to small ohmic losses in the measuring capacitor. Since the individual wires or bars of the electrodes are only in linear contact with the underside of the support plate, there is a very high contact resistance between the electrodes and the support plate.

The development of the invention according to claim 7 allows the electrodes to be made of relatively thick wire or

to form in grids with relatively thick bars, without larger portions of the heat rays emitted by the heat radiation source are absorbed by the electrodes. Thicker wires or thicker bars

are advantageous with regard to greater mechanical inherent stability of the electrodes and allow the electrodes to be arranged in a self-supporting manner at a distance below the support plate. This means ohmic losses in between

the bottom of the pot and the electrodes formed capacitor kept small even at high temperatures. If one chooses the distance between the electrodes below the support plate according to claim 8, then on the one hand contact points between the electrodes and the underside of the support plate are reliably avoided, which could result from low residual residues of the electrodes, on the other hand, the bottom of the pot and the electrodes are still reliably recognizable with simply constructed electronics Capacities. Arranging the two electrodes at a relatively small distance, which is desirable in order to be able to switch on the hob even with small pots, creates additional capacities between the edges of the neighboring electrodes, which are not switched by placing the pot, but are continuously present . Such capacities would in themselves deteriorate the response of the capacitive occupancy sensor. With the measure specified in claim 9, however, it is achieved that the stray capacitances between the two adjacent electrodes have no influence on the presence detection.

The development of the invention according to claim 10 is advantageous with regard to a particularly simple and inexpensive qualitative evaluation of the capacitance between the two electrodes. If there is a pot over the two electrodes, an oscillator circuit is completed so that it starts to oscillate. This swinging can be easily determined using simple switching elements.

In a hob according to claim 11, one can of the

Make use of the fact that the conductivity of the support plate greased from glass ceramic increases noticeably with increasing temperature to switch off the predetermined temperature inevitably, the electrodes, which are primarily parts of a pot sensor, now simultaneously forming parts of a temperature sensor which comprises the temperature-dependent dielectric of the support plate between the pot base and electrodes as a temperature-sensitive medium.

As already explained, support plates made of glass ceramic have a very high but finite electrical resistance, which decreases with increasing temperature. This ohmic resistance is parallel to the capacitive measuring resistor and affects the sensitivity of the capacitive pot presence sensor. With the development of the invention according to claim 12, this ohmic interference resistance of the glass ceramic material is thermally coupled to the support plate by an additionally provided

Resistance compensated.

The development of the invention according to claim 13 is again advantageous with regard to inexpensive and simple manufacture of the hob. You can easily apply the compensation resistor together with the electrodes to the underside of the support plate in one operation. The development of the invention according to claim 14 is also advantageous with regard to good response behavior of the capacitive presence sensor.

The invention is explained in more detail below using an exemplary embodiment with reference to the drawing. In this show:

Figure 1: a plan view of the underside of a hob, which is equipped with a capacitive pan detection; Figure 2: a transverse section through the hob shown in Figure 1 along the section line II-II;

Figure 3: an electrical circuit diagram of the capacitive

Pot detection of the hob according to Figures 1 and 2; FIG. 4: a top view of a modified electrode arrangement;

Figure 5: a vertical section through part of a

Hob, which contains the electrode arrangement according to Figure 4; and

Figure 6: a modified circuit for capacitive

Pot detection, which can be used together with the electrode arrangement according to FIG. 4.

In the drawing, 10 denotes a support plate made of glass ceramic, under which an infrared heating coil 12 is arranged, which is only indicated schematically in FIG.

An outer annular electrode 14 is applied to the underside of the support plate 10. The electrode 14 has a connecting lug 16 and is interrupted at 18. An annular shield electrode 20 is arranged concentrically to the circular electrode 14. The latter has a connecting lug 22 which is passed through the interruption 18 of the electrode 14. The shield electrode 20 has an interruption 24 through which a connecting lug 26 a concentric circle shaped inner electrode 28 is passed.

In order to ensure that the ohmic resistance between the electrodes is high even at high temperatures of the support plate 10, an electrically insulating layer 29 is inserted between the electrodes and the underside of the support plate 10. This can be, for example, a layer consisting of SiO ₂ , which has been produced by printing on a frit and subsequent heat treatment, by sputtering or the like. Such a SiO ₂ layer is permeable to the thermal radiation emitted by the infrared heating coil 12 and can therefore be drawn over the entire underside of the support plate 10 for the sake of simplicity. Furthermore, a thin layer is on the underside of the support plate 10. ider 30 applied.

The application of the electrodes 14, 20 and 28 and the thin film resistor 30 to the underside of the support plate 10 can e.g. by steaming gold or silver on the underside of the support plate 10 or by printing a frit in the screen printing process with the desired pattern on the underside of the support plate 10 and then subjecting the printed material to a heat treatment, in which case a continuous metallic conductive film is formed. As shown at 32 for the outer electrode 14, the electrodes do not form a continuous surface, but rather a grid with an area coverage of typically 10 to 20%, so that the heat radiation generated by the IR heating coil 12 also in the region of the electrodes through the support plate 10 can step through.

Instead of grid-shaped printed or evaporated electrodes, electrodes made of wire material can also be used, preferably wire mesh material. Also this is permeable to the heat radiation generated by the heating coil 12.

The heat radiation thus also reaches the bottom of a pot placed on the hob in the electrode area, as shown at 34 in FIG.

The pot base made of electrically conductive material forms, together with the annular electrode 14, an annular capacitor C ₁ and together with the central circular electrode 28 a circular capacitor C ₂ . Through the electrically conductive bottom 34, these two capacitors are simultaneously connected in series, as indicated by a line 36. The series connection of the capacitors C ₁ and C ₂ can via the connection lugs

16 and 26 are connected to an oscillator circuit 38, as is shown in more detail in FIG. 3.

In Figure 3, in addition to the capacitors C ₁ and C _2, a resistor 40 is shown, which represents the ohmic losses in the dielectric of the support plate 10. These losses typically increase with increasing temperature of the base plate material. A switch 42 symbolizes the switching bridge, which is also formed by the pot bath 34.

A differential amplifier 44 has its positive input connected via a resistor 46 to a positive supply rail 48 and via a resistor 50 to a negative supply rail 52. Furthermore, the positive input of the differential amplifier 44 is connected to the amplifier output via an adjustable resistor 54.

This resistance is set so that the differential amplifier does not yet start to oscillate. The negative input terminal of differential amplifier 44 is also connected to the amplifier output via a resistor 56. The combination of the capacitors C ₁ , C ₂ , the resistor 40 and the switch 42 which forms the capacitive pot presence sensor is denoted overall by 58 in FIG. This presence sensor is connected on the one hand to the output of the differential amplifier 44, and on the other hand via the thin film resistor 30 to the positive input of the differential amplifier 44. The two input terminals of the differential amplifier 44 are also connected to one another via a resistor 60, and the negative input terminal of the differential amplifier 44 is connected to the negative supply rail 52 via a capacitor 62, which is parallel to the resistor 50 and together with this a predetermined frequency of the oscillator RC element forms. Diodes 64, 66, which are poled in opposite directions, limit the maximum voltage present at the input terminals of the differential amplifier 44.

The oscillator circuit 38 described above operates roughly so that the oscillator does not oscillate as long as the switch 42 is open (there is no pot on the hob). As soon as the switch 42 is closed, there is an increased feedback via the capacitors C ₁ and C ₂ , and the oscillator circuit 38 begins to oscillate. The alternating signal provided on the output line 68 of the oscillator circuit can then, if necessary after rectification or other evaluation, be used to switch on the IR heating coil 12.

If the pot is removed from the cooktop or pushed clearly away from the center of the cooktop, the capacitive feedback is weakened or completely eliminated, and the oscillator circuit 38 no longer oscillates. The alternating signal on the output line 68 then disappears and the IR heating coil 12 is automatically switched off. With a somewhat different connection to the oscillator circuit 38, the above-described electrode arrangement can also be used for cooking material detection, as shown in broken lines in FIG. 3. In this case, the connection lugs 16 and 26 are connected to the negative input of the differential amplifier or the negative supply rail 52. One then has a small stray capacitance C ₃ between the two electrodes and a loss resistance 70, which characterizes the dielectric losses in the food.

With its closed state, the switch 42 again represents the presence of cooking or roasting material on the upper side of the support plate 10. In this variant, which is already being carried out in the factory for hobs intended for roasting or can be set by the user using a changeover switch, you get a signal on the output line 68 when 10 Kochbzw on the top of the support plate. Fried food is located.

In the exemplary embodiment according to FIGS. 4 and 5, parts of the hob, which have already been explained above, are again provided with the same reference symbols. These parts need not be described again in detail.

The outer electrode 14 and the inner electrode 28 as well as the shield electrode 20 lying between them now have a square shape when viewed from above.

The electrodes 14 and 28 are flat, planar electrodes which are punched out of expanded metal made of molybdenum. The ribbed metal has a diamond shape Meshes with a mesh length of approximately 5 mm, a mesh width of 3 mm and a web width of 1 mm. The webs themselves have a V-shaped cross section. The electrodes are bare and reflective, resulting in a total heat loss of less than 0.5% on the electrodes.

The shield electrode 20 has the shape of a lower square shaft with a vertical axis. Each of the electrodes is supported by four ceramic supports 72, 74 and 76, respectively, which are arranged on suitable supports

Connected to the electrode material, e.g.

by mechanical positive connection such as snap connections or by adhesive connection with a heat-resistant ceramic material.

One of the supports 72, 74, 76 is hollow and receives a connecting conductor 78, 80, 82, which is connected to the electrode under consideration.

The supports 72, 74, 76 are in turn inserted into a tub part 84, which is made of porous ceramic material and at the same time carries and surrounds the IR heating coil 12.

As can be seen from FIG. 6, the electrode 14, the electrode 2B, the shield electrode 20 and the base of a pot placed on the hob together form three variable capacitances C14-T (T = pot), C20-T and C28-T, where " variabei "both the change in capacity by touchdown or

Losing weight from a pot also involves changing the capacity due to temperature change.

These capacities are via shielded cable 86 with the

Primary winding of a transformer 90 connected via a capacitor 88 is connected to the secondary winding thereof

is. The transformer 88 has the polarity indicated by dots. The secondary winding of transformer 90

is at one end with the base terminal of a transistor

92 connected, which is part of an amplifier stage.

Its base bias is predetermined by a transistor 94, the base of which is connected to the supply voltage via a resistor 96 and the emitter of which is connected to ground via a resistor 98. The collector of transistor 94 is connected to the second terminal of the secondary winding of transformer 90.

The collector of transistor 92 is supplied with the supply voltage via an adjustable resistor 100 and at the same time is connected to the primary winding of a feedback transformer 102. Its secondary winding is on the

Connected resonant circuit, which is designated by the primary winding of the transformer 90 and 104 in total

Measuring capacitor arrangement is formed.

The emitter of transistor 92 is through a resistor

106 connected to ground, and the one tapped at the emitter

Signal is via a coupling capacitor 108 on a

Output line 110 provided.

After rectification, smoothing and optionally amplification, the output signal can then be used to control a relay or a thyristor in order to switch the IR heating coil on and off.

The feedback of the oscillator circuit 38 shown in FIG. 6 can be adjusted by adjusting the resistor 100. This setting is made so that the oscillator at a predetermined operating temperature of the hob, which, for example, can be at 400 ° C, stops vibrating. This is because the ohmic losses in the glass ceramic are now greater and the capacitance of the measuring capacitor arrangement is reduced, so that the resonance condition is no longer met. Since no signal is now received at the output of the oscillator circuit, the IR heating coil is switched off by a relay controlled by this output signal or by a thyristor controlled as a function of this output signal. Likewise, you do not get an output signal of the oscillator circuit that switches on the heating coil if there is no pot on the hob above the electrodes, since this too

If the measuring capacitor arrangement does not have a capacitance which enables the resonant circuit to oscillate.

Claims

1. Hob with one that is permeable to heat radiation

Support plate (10), with a heat radiation source (12) arranged under the support plate and with a contactless presence sensor (58) assigned to the support plate (10), characterized in that the presence sensor (5S) has at least two sensors arranged on the underside of the support plate (10) spaced electrodes (14, 28).

2. Hob according to claim 1, characterized in that it has two concentric electrodes (14, 28) with a preferably circular edge contour.

3. Hob according to claim 1 or 2, characterized in that the electrodes (14, 28) are each permeable to heat radiation.

4. Hob according to one of claims 1 to 3, characterized in that the electrodes (14, 28) are thin-film electrodes which are produced by vapor deposition of conductive material or printing of a paste with conductive material and subsequent sintering of the paste material.

5. Hob according to claim 4, characterized in that between the electrodes (14, 28) and the support plate

(10) an insulating layer (29) permeable to heat radiation is laid.

6. Hob according to one of claims 1 to 3, characterized

characterized in that the electrodes (14, 28) consist of wire or grid material, in particular a wire mesh or expanded rib metal.

7. Hob according to claim 6, characterized in that the electrodes (14, 28) have a reflective surface, in particular consist of a non-oxidizing and non-tarnishing metal such as molybdenum even at the working temperature of the hob or are coated with such a material.

8. Hob according to claim 6 or 7, characterized in that the electrodes (14, 28) are at a small distance of the order of magnitude 0.5 - 2 mm below the support plate (10).

9. Hob according to one of claims 1 to 8, characterized in that a ground electrode (20) is provided between the adjacent edges of the two electrodes (14, 28).

10. Hob according to one of claims 1 to 9, characterized in that öie two electrodes (14, 28) are part of an oscillator circuit (3B).

11. Hob according to claim 10, characterized in

that the feedback path of the oscillator circuit has an adjustable component (100), so that the feedback can be adjusted in such a way that the oscillator circuit does not start when there is no pan on the hob, and does not start when a pan is on the hob stands, but the temperature of the support plate is above a predetermined temperature.

12. Hob according to claim 10, characterized by a

thermally coupled to the support plate (10) temperature-dependent resistor (30), which caused the thermal Change in resistance of the material of the support plate (10) is compensated and is preferably connected to the feedback path of the oscillator circuit (38).

13. Hob according to claim 12, characterized in that the compensation resistor (30) is a thin-film resistor applied to the underside of the support plate (10) by vapor deposition or printing.

14. Hob according to one of claims 10 to 13, characterized in that the input terminals of the oscillator circuit (38) are earth-symmetrical.