GB2143053A - Boiling control for hotplate - Google Patents

Abstract

Vibration in a vessel 214 : detected while it is being heated and a control means determines and alters the level of energy applied to the vessel, according to the level of vibration in the vessel. As shown a probe 216 contacts the base of a pan 214. On heating, vibration in the pan is detected by the probe and a signal is generated through a transducer 218, an amplifier 220 and sent to a microprocessor 222 which determines the maximum level of vibration in the pan corresponding to the boiling point of liquid in the pan. The microprocessor 222 then adjusts a valve 224 which reduces the amount of energy applied to the pan 214. <IMAGE>

Description

SPECIFICATION Boiling control for hotplate The present invention relates to boiling control for hotplates. The invention particularly concerns the control of gas hotplates.

Thermostats for use with hotplates are known.

These generally contact a pan base, detect its temperature and maintain it by control of the heat applied to the pan, within a narrow range of temperature. The problem of such devices is that they are inaccurate because of variations in degree of contact, pan size material and load in the pan. The device does not allow the pre-setting of a simmering, slow boiling or a fast boil temperature for any type of pan or load with any degree of accuracy.

It has been observed that a pan containing water on a hotplate to which a constant level of heat is applied will gradually vibrate with increasing temperature. The vibration will reach a maximum level as boiling approaches and then fall away slightly to a state of constant boiling. However, with maximum heat input even constant boiling is generally violent causing excess of steam or the liquid to be expelled from the pan unnecessarily. The present invention uses the phenomenon of pan vibration to control the level of heat input immediately after boiling has been reached.

According to the present invention there is provided a device for detecting vibration in the pan and a control means for determining the level of energy applied to a hotplate according to the level of vibration in the pan.

The hotplate may use either a gas burner or an electric hotplate. The present invention is particularly suitable for controlling the amount of heat to a gas burner because of the almost instantaneous increase or decrease of heat levels achieved by adjustment of the burner. It will also be effective in electric heating means which allows relatively fast changes in heat, for example, electric induction heaters. With electric hotplates in general, the rate of heat control is slower because of residual heat of an electric heating element. However, the present invention may be used with such an element, although control may not be as effective.

The detection means is preferably a spring-loaded probe which contacts the base of a pan and conveys movement or vibration detected within the pan to an electrical transducer, for example, a piezo crystal, electrostatic or electromagnetic types, or a strain gauge. Whilst the electrical transducer may directly operate on the control means, it is preferred that the signal generated from the detector is amplified and fed to a microprocessor which can interpret the changing signal and adjust the controlling means accordingly.

The present invention will be further described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a graph showing the level ofvibration as against time when heat is applied to a pan containing liquid.

Figure 1A shows the effect of different pans or loads.

Figures 2 and 2A are schematic arrangements of a particular embodiment of the present invention; Figures 3 and 3A show embodiments of the vibration detector.

Figure 4 shows a proportioning valve in the form of a stepper motor and rotary valve.

Figure 5 shows a proportioning solenoid valve.

Figure 6 shows a bimetal proportioning valve.

Figure 7shows a hot wire proportioning valve.

The concept of this invention will be described in relation to Figure 1. As indicated above, when a pan of liquid is placed onto a hotplate and heat is applied at constant level, the level of vibration in the pan increases with temperature and therefore time. As heat is applied, there is first a low level of noise, or vibration as air bubbles etc. form in the pan. This noise, or vibration, increases gently until it produces a 'singing stage' between times t1 and t2. As the temperature of the pan increases, the vibration of the pan becomes more vigorous increasing rapidly to a peak output at boiling. Peak output occurs for a short time during which air is expelled from the water. On expulsion of such air, a lower level of vibration is reached.If the level of heat input to the pan remains the same, a steady fast boiling of the liquid occurs which is generally rather violent such that it is necessary to reduce the heat input. This is shown as level A in Figure 1. If the heat input is reduced, a lower level of vibration shown by level B in Figure 1 may be reached at which slow boiling occurs. If the heat is further reduced from the peak output, then the pan may simmer with vibration occurring at level C as shown in Figure 1. Various pans and loads will give different levels A, B, C, etc., but the ratios B/A, C/A are similar in each case (see Figure 1A).

The microprocessor detects boiling by sensing the peak of the curve. This may be done by the microprocessor determining the maximum voltage and hence the maximum level of vibration at two or more consecutive times t3, t4 etc. These may be the same or may reduce after a maximum level has been detected. When such values have been determined, the microprocessor causes the proportioning valve to close to maintain the boiling level at a level set by the user.

A specific embodiment of the invention will now be described. Referring to Figure 2, a pan 210 is placed on a gas burner 212. Heat is applied through the gas burner 212 to the pan containing liquid 214.

A probe 216 contacts the base of the pan and is connected to a piezo unit 218 and an amplifier 220.

The amplifying unit 220 is connected via a microprocessor 222 to a proportioning valve 224. Gas input is via a gas tap 226 through the proportioning valve 224 to the burner 212. The proportioning valve may or may not embody complete gas shut-off. The gas tap 226 also includes the boiling level to be set by the user (see Figure 2A).

In operation, maximum heat is normally applied to the pan 210 from the burner 212 to cause the liquid 214 in the pan to boil. Lightly contacting the base of the pan 210, is the probe 216 which detects any vibration in the pan and transmits it to the piezo unit 218. The piezo unit 218 is an electrical transducer which on movement of the probe 216 generates a signal corresponding to the level of movement. This signal is applied to the amplifier 220. The signal is amplified and can be represented on the graph of vibration against time as amplified output in volts against time when constant heat is applied (see Figure 1). After time t1 and beforetimet2the'singing stage' of the pan occurs such that the light vibration is detected by the probe 216 producing at the amplifier.After time t2, the output potential of the amplifier increases more rapidly with the time until the onset of boiling is reached at time t3. Between time t2 and time t3, the output potential from the amplifier increases steeply compared with the increase of potential up to time t2. The microprocessor is programmed to disregard vibration levels below the trigger level tx (Figure 1). Above this level, it detects boiling by recognising eitherthe plateau (at t3) or the subsequent decline in level. For example, it can be programmed to average vibration levels at fixed time intervals, and then react if (a) a number of successive averages are similar (plateau), or (b) successive averages are falling. Because the pan and contents may vary in load or viscosity, the characteristics of the boiling curves shown in Figure 1A may vary slightly.A pan containing highly viscous material, such as porridge, ora heavy load such as potatoes may have a constant vibration level on boiling or such a small drop that it is difficult to detect a reduction in the vibration level. The microprocessor is able to reduce the boiling level because it can recognise eitherthe plateau or decline in vibration level. Thus, the pan material and its contents do not affect the operation of the present invention. The microprocessor 222 causes the stepper motor 427 to rotate the plug of the proportioning valve 424 which restricts the flow of gas and therefore reduces the violent boiling to a reasonable level.The microprocessor can be programmed such that it proportionally closes the valve to reduce the heat sufficiently such that a desired boiling or simmering level can be maintained without attention, using e.g. a potentiometer set by the control knob shown in Figure 2A. The knob and tap can be arranged to give both manual and auto-control of the burner e.g. by turning the knob in opposite directions.

The pan 210 and liquid 214 contained within the pan 210 will have different vibration characteristics according to the materials used to construct the pan or the viscosity of the liquid in the pan (see Figure 1A), but as the form of the curves is similar, boiling can be detected in each case and control to a pre-set fraction of peak vibration will provide the required state (e.g. slow boil or simmer in each case).

Figures 3 and3A show alternative embodiments of the vibration detector. A probe 316 contacts a leaf.

spring mounted on a block 321 and a resistive strain gauge 323 is affixed to the leaf spring. On vibration of the leaf spring, a change in resistance is detected in the circuit 323 which is subsequently amplified. In the piezo unit shown in Figure 3A, the probe is attached to a leaf spring 325 mounted on a block 327 and connected to a piezo unit 329, in turn mounted on a inertial bracket 331. On detection of vibration from the pan a voltage is induced in the piezo unit which is amplified.

Various embodiments of proportioning valves are shown in Figure 4 to 7. In Figure 4, a rotary valve 424 is controlled by a stepper motor 427 which operates on a signal from the microprocessor 222 of Figure 2 to restrict the flow of gas to the burner.

In Figure 5, the proportioning valve 524 is a solenoid valve in which the input from the microprocessor is applied via known interface circuitry to a coil 525. An armature 527 is connected to a control pin 529 which restricts the flow of gas from the gas outlet 531 to the burner 212. A non-linear reaction spring 533 allows the armature position, and hence the gas flow, to be held at any desired point. Such solenoid valve provides a fast reaction time for restricting the flow of gas from the gas inlet 526 to the gas outlet 531.

Figure 6 shows a bimetal valve 624 having a gas inlet 526 to the gas outlet 531 to the burner 212 of Figure 2. A bimetal strip 635 is directly heated by known interface circuitry controlled by the microprocessor. On applying an electric current to the bimetal strip 635, the thin bimetal heated by the current flowing in it bends and closes a valve 637 restricting flow of gas from the gas outlet to the burner 631. Such a device can have a fast reaction time.

Figure 7shows a hot wire valve in which a gas input 726 is restricted by a valve 737 operated by a thin wire 735 which can be electrically heated by known interface circuitry controlled by the microprocessor. As the wire is heated, it expands, and lowers the valve 737 which restricts the flow of gas to the outlet 731 from the valve. Other known means of mechanically amplifying the wire's expansion may be used.

Claims (13)

1. A device for detecting vibration in a vessel while it is being heated, including a control means for determining and altering the level of energy applied to the vessel, according to the level of vibration in the vessel.

2. A device as claimed in claim 1, wherein the control means detects the maximum level of vibration in the vessel and reduces the level of energy applied to a predetermined input level.

3. A device as claimed in claim 2, wherein the control means includes a vibration detection means and a transducer capable of generating a signal proportional to the level of vibration; and a microprocessor for detecting the maximum vibration level and producing a signal to operate a proportioning means to reduce the energy applied to a predetermined input level.

4. A device as claimed in claim 3, in which the vibration detection means is a spring-loaded probe which contacts the base of the vessel and conveys movement or vibration detected within the pan to the electrical transducer.

5. A device as claimed in claim 3 or claim 4, wherein the device is for use with a gas ring and includes a proportioning means in the form of a gas proportioning valve.

6. A device as claimed in claim Sin which the proportioning valve is a rotary valve controlled by a stepper motor operated by a signal from the microprocessor to reduce or stop the flow of gas.

7. A device as claimed in claim 5, in which the proportioning means is a solenoid valve operated by a signal from the microprocessor to reduce or stop the flow of gas.

8. A device as claimed in claim 5, in which the proportioning valve is a bimetal valve operated by a signal from the microprocessor such that a current causes a bimetal strip to reduce or stop the flow of gas.

9. A device as claimed in claim 5, in which the proportioning valve is a hot wire valve operated by a signal from the microprocessor such that a current causes the hot wire to expand and reduce or stop the flow of gas.

10. A device as claimed in any one of claims 3 to 9, wherein the microprocessor detects a signal above a predetermined level to determine maximum vibration level.

11. A device as claimed in any one of claims 4 to 10, wherein the signal proportional to the level of vibration is a voltage which is compared by the microprocessor at successive time points to determine the voltage level such that the same voltage at consecutive time points will cause the microprocessorto operate the proportioning means.

12. A device as claimed in any one of claims 3 to 11, in which control means includes an amplifier to amplify the signal generated by the transducer for the microprocessor.

13. A device as claimed in claim 1, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.