GB2222278A - Control of electric heating - Google Patents

Abstract

An electronic energy regulator is disclosed comprising a bistable relay 3 for controlling the supply of electrical power to an electric heating device, e.g. a cooking appliance hot-plate 2, a user input control 1 for affording an output indicative of the desired set temperature and a control circuit 5 which includes a computation circuit (6, Fig. 2) for affording a computed output which corresponds to the estimated temperature of the hot-plate 2 and a comparator (8, Fig. 2) for comparing the computed output with output of the user input control 1 for controlling the energy supply to the hot-plate. The relay 3 may be switched to one state or the other depending on the direction of current pulses gated by a triac 4. The circuit 6 may comprise counters which count up when power is supplied and count down when power is not supplied to the hot-plate 2. Hysteresis may be provided in the regulator. <IMAGE>

Description

Electronic Control Systems This invention relates to electronic control systems and is especially applicable to electronic energy regulators for use, for example, in electric cooking appliances.

Conventional, electro-mechanical type energy regulators as used in domestic cooking appliances for controlling the power applied, for example, to a hot-plate thereof, make use of a normally closed pair of contacts which supply power to the hot-plate, and a bimetallic strip which has a heater winding which is energised when the hot-plate is energised, thereby causing the bimetallic strip to be heated until a temperature is reached at which it operates to cause the normally closed pair of contacts to open, thereby removing the power supply to the hot-plate. The hot-plate and the bimetallic strip both then cool down until the bimetallic strip operates in the opposite sense to allow the pair of contacts to close again to re-connect the power supply to the hot-plate.The hot-plate and the bimetallic strip thus both cycle ON and OFF dependent upon the temperature setting of the bimetallic strip. This temperature setting is normally adjustable by means of a mechanical control which operates in conjunction with the bimetallic strip to allow the power supply to the hot-plate to be regulated between an OFF position and a MAXIMUM position.

Such an electro-mechanical regulator is advantageous in that the bimetallic strip effectively "models" the hot-plate so that when the energy regulator is initially set to a "high" setting, power is applied continuously to the hot-plate until the required temperature is reached. Similarly, on reducing the setting of the energy regulator, power is disconnected from the hot-plate until the required temperature is reached. Prior art forms of electronic energy regulator operate on a switching cycle system in which when a "high" setting is selected, power is sequentially connected to and disconnected from the hot-plate so that the hot-plate takes a long time to reach its required temperature setting.Similarly, when a reduced setting is selected, power is still sequentially connected to and disconnected from the hot-plate, albeit at a lower level, so that again the hot-plate takes a long time to reach the required lower setting. These time delays to reach the required temperature are inconvenient and in some cases are wasteful of power.

It is one object of the present invention to provide an improved electronic energy regulator whereby the time delays to achieve new settings are optimised.

One disadvantage with the conventional electromechanical energy regulator results from the so-called "hysteresis" effect of the bimetallic strip. It will be appreciated that the bimetallic strip operates at one temperature in order to open the normally closed contacts and then cools down to a second temperature at which it operates in the reverse sense- in order to allow the contacts to close again. This difference in temperature is conveniently referred to as "hysteresis", and in electro-mechanical energy regulators is constant over the range of the regulator.

When this "hysteresis" is chosen to give acceptable cycle times at high power settings, at low power settings it results in undesirably long cycles and large excursions of hot-plate temperature.

It is another object of the present invention to provide an improved electronic energy regulator in which the "hysteresis" is changed over the range of the regulator and is lower at lower power settings and higher at higher power settings.

According to one aspect of the present invention there is provided an electronic control system comprising switch means for controlling the supply of electrical power to an output heating device, a user control for affording an output indicative of the temperature to be attained by said output heating device, computation means for affording a computed output which corresponds to the estimated temperature of said output heating device, and comparator means for comparing said computed output with said user control output for controlling said switch means.

In one preferred arrangement for carrying out the invention according to the aforesaid first aspect, it will be arranged that said switch means takes the form of a bistable relay, conveniently a remanent relay or a permanent magnet relay, and it may preferably be arranged that bidirectional switch means, conveniently in the form of a TRIAC, is provided for controlling said bistable relay.

In another preferred arrangement for carrying out the invention according to the aforesaid first aspect, it will be arranged that means is provided for adjusting said user control output in dependence upon whether electrical power is being applied to said output heating device, said comparator means being effective for comparing said computed output with said user control output when electrical power is being applied to said output heating device, and said comparator means being effective for comparing said computed output with said adjusted user control output when electrical power is not being applied to said output heating device.

In accordance with another aspect of the present invention there is provided a bistable relay for controlling the supply of electrical power to an output heating device, and bidirectional switch means, conveniently in the form of a TRIAC, for controlling said bistable relay.

An exemplary embodiment of the invention will now be described, reference being made to the accompanying drawings, in which: Fig. 1, is a partially block schematic diagram of an electronic control system in the form of an electronic energy regulator in accordance with the present invention; Fig. 2, is a partially block schematic diagram of the control circuit of Fig. 1; Fig. 3, is a partially block schematic diagram of an alternative form of control circuit of Fig. 1; and Fig. 4, depicts various waveforms existing at different points in the control circuit of Fig. 3.

The electronic energy regulator depicted in Fig. 1 of the drawings comprises a user input control 1, in the form of a potentiometer which is used to regulate the power applied to a hot-plate element 2 of an electric cooking appliance (not shown). The hot-plate element 2 is connected in series with a normally closed pair of contacts P1 between the live L and neutral N terminals of a 50Hz mains supply, the contacts P1 forming part of a bistable relay 3. The bistable relay 3, which may, for example, take the form of a remanent relay or a permanent magnet relay, has the facility of being switchable between two states dependent upon the direction of the current that is supplied to it; in one state the contacts Pl being closed and in the other state the contacts P1 being open. Switching of the bistable relay 3 is effected by a bidirectional switching device, conveniently in the form of a TRIAC 4, which connects the bistable relay between the live L and neutral N terminals of the mains supply, the direction of current being determined by causing the TRIAC 4 to be rendered conductive during either a positive half-cycle or a negative half-cycle of the mains supply. Thus, if the TRIAC 4 is rendered conductive during, say, a positive half-cycle of the mains supply, the bistable relay 3 may be switched into its contacts (P1) closed state, and if the TRIAC 4 is rendered conductive during a negative half-cycle of the mains supply, the bistable relay 3 may be switched into its contacts (P1) open state.

Control of the TRIAC 4 is effected by means of a control circuit 5, which will be described later in greater detail, which applies a suitably timed "FIRE" signal to the control electrode of the TRIAC 4.

The control circuit 5 has also connected to it the user input control potentiometer 1 and also the live L and neutral N terminals of the mains supply from which it derives a 50Hz reference timing signal and also a low voltage e.g. +5 volt. power supply.

The function of the control circuit 5 of Fig. 1 is to control the energisation of the hot-plate element 2 in dependence upon the setting of the user input control potentiometer 1 and in a way which "models" the temperature behaviour of the hot-plate element 2 so that when, for example, a user first selects a high setting, power is applied continuously to the hot-plate element 2 until a computational process estimates that the required temperature has been reached, whereafter an on-off cycle will maintain the "modelled" temperature close to its setting. Similarly, when a reduced setting is selected, power is disconnected from the hot-plate element 2 until a computational process estimates that the hot-plate element 2 temperature has fallen to its required setting, whereafter an on-off cycle will maintain the "modelled" temperature close to its setting.

The control circuit 5 of Fig. 1 is shown in greater detail in Fig. 2 of the drawings. The control circuit 5 of Fig. 2 comprises a computation circuit 6 which is effective for generating a numerical value which corresponds to an estimated temperature of the hot-plate element 2 (Fig. 1). The computation circuit 6 comprises an 8-stage counter A which is connected to a 6-stage counter B, thereby forming a 14-stage counter AB, and a 7-stage counter C. Basically the computation circuit 6 has a numerical value AB stored in the counters A and B and this numerical value is arranged to be increased exponentially when power is being applied to the hot-plate element 2 (Fig. 1) and is arranged to be decreased exponentially when power is not being applied to the hot-plate element 2 (Fig. 1).

Thus the numerical value contained in the counters A and B closely "models" the temperature of the hot-plate element 2 (Fig. 1) and is used in conjunction with an output derived from the user input control 1 (Fig. 1) in order to control the switching of the bistable relay 3 (Fig. 1), thereby to control the power supplied to the hot-plate element 2 (Fig. 1).

The computation circuit 6 of Fig. 2 is operated in conjunction with a sequence counter 7 which has 8 states, labelled PO to P7, a first clock signal, CLOCK 1, being provided typically of 15KHz for controlling the counting and sequencing operations and a second clock, CLOCK 2, being provided typically of 4Hz for controlling the sampling rate of the circuit 6.

During state PO of the sequence counter 7, the value of counter B in computation circuit 6 is copied into counter C thereof.

During state P1, CLOCK 1 pulses are applied to counters A and B and to counter C to cause them to count downwards until counter C reaches zero value. It is to be noted that state P1 cannot progress to state P2 until counter C reaches zero.

During state P2, a PULSE signal, typically of +5v is applied to one end of the user input control potentiometer 1, the other end of which is grounded and the wiper of which is connected via a resistor R1 to the input COM2 of a comparator 8.

During states P1, P2 and P3 an output RAMPC is generated via OR gate 9 which is applied via a resistor R3 to a capacitor C causing it to be charged. The voltage Vc developed across the capacitor C at the end of state P2 is closely related to the numerical value held in counters AB and this voltage Vc is applied to the input COM1 of the comparator 8. The comparator 8 compares the voltage Vc derived from RAMPC with the output of the user input control potentiometer 1 and derives a logic output COMPIN which is dependent upon the comparison.

During state P3 the output COMPIN is applied to a LATCH circuit 10 which affords an output HEAT which is used to control the supply of power to the hot-plate element 2 (Fig. 1) as will later be described.

During state P4, if the latched value of COMPIN is such that HEAT is true (i.e. power is being supplied to the hot-plate element 2) a numerical value of 32 is added to counters A and B.

States P5 and P6 are used to synchronise the operation of the sequence counter 7 with the CLOCK 2 (4Hz) signal and the computation and comparison sequence described above is repeated at the CLOCK 2 rate, i.e. four times per second.

Thus, it will be appreciated that if the HEAT output is true, i.e. power is being supplied to the hot-plate element 2 (Fig. 1), then the numerical value held in counters A and B is increased by 32 at the CLOCK 2 rate (e.g. four times per second) and if the HEAT output is not true i.e. power is not being applied to the hot-plate element 2 (Fig. 1), then the numerical value held in the counters A and B is reduced by the value held in counter B at the CLOCK 2 rate. The computation of the numerical value held in counters A and B can be mathematically defined by discrete difference equations as follows: AB := AB - (AB divided by 256) + 32 (1) if HEAT is true, or AB := AB - (AB divided by 256) (2) if HEAT is false.

The HEAT output from LATCH 10, as has already been mentioned, is used to control the supply of power to the hot-plate element 2 (Fig. 1). It does this by means of a firing circuit Il to which the HEAT signal is applied. The firing circuit 11 also has a 50Hz mains supply signal applied to it and generates at appropriate positive or negative half-cycles of the mains supply signal, dependent upon which current direction the TRIAC 4 is to be rendered conductive in, a FIRE output signal which is applied to the control electrode of the TRIAC 4 of Fig. 1.

In the arrangement thus far described, the comparator 8 of Fig. 2 simply compares the output Vc derived from the RAMPC signal with the output derived from the user input control potentiometer 1 in order to derive the logic signal COMPIN, and there is no inherent "hysteresis" in the system. It is very desirable to include "hysteresis" in the system and this is achieved in the control circuit 5 of Fig. 2 by proportionately decreasing the output from the user input control potentiometer 1 when the HEAT signal is false. Thus when the HEAT signal is true the actual output from the user input control potentiometer 1 is applied to the comparator 8 and when the HEAT signal is false, a proportionately decreased output is applied to the comparator 8.This is achieved in the control circuit 5 of Fig. 2 by providing a further resistor R2 connected to the resistor R1 and the input COM2 of the comparator 8, and by arranging that the free end of the resistor is grounded via transistor 12 whenever the HEAT output is false. The resistors R1 and R2 thus constitute a potential divider connected across the output of the user input control potentiometer 1, thereby providing a proportionately decreased output only when the HEAT output is false. This arrangement has the advantage that the "hysteresis" thereby provided is not constant over the range of the user input control potentiometer 1 but increases as the user input control is increased.

It has been found that although the control circuit 5 which has been described with reference to Fig. 2 operates satisfactorily, it does not lend itself to being fabricated using integrated circuit techniques.

In Fig. 3 of the drawings there is shown an alternative form of control circuit 5 which does lend itself to being fabricated using integrated circuit techniques.

The control circuit shown in Fig. 3 of the accompanying drawings differs from the control circuit of Fig. 2 in that the counter arrangement (counter A, counter B, counter C) of the computation circuit 6 of Fig. 2 has been replaced by a shift register/arithmetic unit as will hereinafter be described.

In Fig. 3 of the drawings, the computation circuit 6 comprises a shift register 15, typically 13 bits in length, which can be considered in two parts AH typically 5 bits, and AL typically 8 bits, part AH being at the most significant end of the shift register 15 and part AL being at the least significant end of the shift register 15. The shift register 15 is provided with an intermediate output IO between the two parts AH and AL. The intermediate output is applied as one input to a SELECT circuit 16, to a second input of which is connected the HEAT output from latch 10. The output from the SELECT circuit 16 is connected via an inverter 17 as one input to a full adder circuit 18. A second input to the full adder circuit is derived from the least significant bit of the shift register 15.A D-type flip-flop 19 is connected between the CARRY output of the full adder circuit 18 to a further input thereof. The SUM output from the full adder circuit 18 is fed back as the input to the shift register 15. The arrangement thus far described operates as follows: Assume that the shift register 15 has a 13-bit number stored in it which corresponds to an estimate of the temperature of a hotplate which is being controlled. It is required that the numerical value in the shift register 15 be increased exponentially when power is being applied to the hotplate i.e. HEAT is true, and be decreased exponentially when power is not being applied to the hotplate element i.e. HEAT is false.To achieve this the numerical value in the shift register 15 is varied in accordance with the aforementioned difference equations (1) and (2) as follows: AB := AB - (AB divided by 256) + 32 (1) if HEAT is true, or AB := AB - (AB divided by 256) (2) if HEAT is false, where AB is the numerical value stored in the shift register 15.

This is achieved in the circuit arrangement of Fig. 3 by arranging that the SELECT circuit 16, under the control of a "TOP 5" signal, selects, by means of the intermediate output IO, the contents of the part AH of the shift register, which part corresponds to the numerical value "AB divided by 256".

Instead of subtracting this from the value AB stored in the shift register 15, it is inverted by means of the inverter 17 and is added to the contents of the shift register 15 together with a "carry" bit derived from the D-type flip-flop 19. After the 5 bits of the part AH of the shift register have been selected by the SELECT circuit 16, it switches from the output IO to the HEAT output applied to it and applies a further 8 bits to the inverter 17, the 8 bits being logic 1 if the HEAT output is true and logic 0 if the HEAT output is false.

Consider the following examples: Assume the numerical value AB in the shift register 15 to be: 0101000000000 which is the binary equivalent of 2560.

If HEAT is true the number: 1111111101010 is passed to the inverter 17 in which it is inverted to: 0000000010101 The full adder 18 then carries out the following summation: 1 = CARRY 0 1 0 1 0 0 0 0 0 0 0 0 0 = AB = 2560 0 0 0 0 0 0 0 0 1 0 1 0 1 = AB/256 = 10 0 1 0 1 0 0 0 0 1 0 1 1 0 = 2582 = 2560 - 10 + 32 Thus the numerical value 2560 in the shift register 15 has been reduced by 10 (i.e. 2560 divided by 256) and increased by 32 because HEAT is true, in accordance with difference equation (1).

If HEAT is false, the number: 0000000001010 is passed to the inverter 17 in which it is inverted to: 1111111110101 The full adder 18 then carries out the following summation: 1 = CARRY 0 1 0 1 0 0 0 0 0 0 0 0 0 = AB = 2560 1111111110101 0 1 0 0 1 1 1 1 1 0 1 1 0 = 2550 = 2560 - 10 Thus the numerical value AB = 2560 in the shift register 15 has been reduced by 10 (i.e. 2560 divided by 256) because HEAT is false, in accordance with difference equation (2).

The overall operation of the arrangement of Figure 1 will now be described with the aid of the various waveforms depicted in Figure 2, some of which are generated by a sequence controller 20.

The arrangement has an operating sequence: LOAD : TEST : COMPUTE During the LOAD sequence a SET pulse is generated by the sequence controller 20 which sets the D-type flip-flop 19 to apply a CARRY to the full adder 18 and also causes a timer register 21 to be set to correspond to the contents of the part AH of the shift register 15.

During the TEST sequence a PULSE output from the sequence controller 20 is applied to resistor R3 and capacitor C to cause a ramp voltage Vo to be developed across the capacitor C, and at the same time is applied to the user input control potentiometer 1. The output Vo and the output from the potentiometer 1 are applied to a comparator 8 which affords an output COMPIN if the potentiometer is greater than the voltage Vo.

Coincident with the leading edge of the PULSE output a 32-pulse TIME signal is applied to the timer register 21 which affords a LATCH output signal when the contents of the timer register are reduced to zero.

The LATCH signal is applied to the latch 10 to cause it to latch the COMPIN signal from the comparator 8 that exists at that time and to afford the HEAT output.

The HEAT output is fed back to the SELECT circuit 16 of the computation circuit 6 and during the COMPUTE sequence it is used to increase or decrease the numerical value stored in the shift register 15 under the control of the C13 pulses and the TOP 5 signal, as has already been described.

It will be appreciated that the bit length of the shift register 15 and the bit lengths of the parts AH and AL thereof may be varied to suit any particular application.

It will be appreciated that the embodiments which have been describe have been given by way of example only and may be modified to suit any particular application. For example, the input user control 1 of Fig. 1 may take any convenient form, such as, for example, a rotary control, a membrane switch, or a touch sensitive electronic switch.

The electronic energy regulator described may also be used in appliances other than electric cookers.

In addition, the use of a bistable relay in conjunction with a bidirectional switching device to control power to an electric heating device, may have application in other forms of energy regulators.

Claims (24)

1. An electronic control system comprising switch means for controlling the supply of electrical power to an output heating device, a user control for affording an output indicative of the temperature to be attained by said output heating device, computation means for affording a computed output which corresponds to the estimated temperature of said output heating device, and comparator means for comparing said computed output with said user control output for controlling said switch means.

2. A system as claimed in claim 1, in which the computation means comprises counting means for storing a numerical value which corresponds to the estimated temperature of said output heating device.

3. A system as claimed in claim 1, in which the computation means comprises shift register means for storing a numerical value which corresponds to the estimated temperature of said output heating device.

4. A system as claimed in claim 2 or claim 3, in which said numerical value is successively increased when electrical power is being applied to said output heating device, and is successively decreased when electrical power is not being applied to said output heating device.

5. A system as claimed in claim 4, in which the increase and decrease in said numerical value is determined in accordance with respective predetermined difference equations.

6. A system as claimed in any preceding claim, in which said switch means comprises a bistable relay.

7. A system as claimed in claim 6, in which the bistable relay takes the form of a remanent relay.

8. A system as claimed in claim 6, in which the bistable relay takes the form of a permanent magnet relay.

9. A system as claimed in any of claims 6 to 8, comprising bidirectional switch means for controlling said bistable relay.

10. A system as claimed in claim 9, in which the bidirectional switch means takes the form of a TRIAC.

11. A system as claimed in claim 10, in which said TRIAC is actuated in one current sense to cause said bistable relay to operate to apply electrical power to said output heating device, and is actuated in the opposite current sense to cause said bistable relay to operate to remove the electrical power applied to said output heating device.

12. A system as claimed in claim 11, in which said TRIAC is actuated in said one current sense by causing it to actuate in one half-cycle of an alternating supply which is applied to said TRIAC, and is actuated in said opposite current sense by causing it to be actuated in an opposite polarity half-cycle of said alternating supply.

13. A system as claimed in any preceding claim, in which means is provided for adjusting said user control output in dependence upon whether electrical power is being applied to said output heating device, said comparator means being effective for comparing said computed output with said user control output when electrical power is being applied to said output heating device, and said comparator means being effective for comparing said computed output with said adjusted user control output when electrical power is not being applied to said output heating device.

14. A system as claimed in claim 13, in which the adjustment of said user control output is varied in dependence upon the setting of said user control.

15. A system as claimed in claim 14, in which the adjusted user control output is a proportion of said user control output.

16. An electronic control system comprising a bistable relay for controlling the supply of electrical power to an output heating device, and bidirectional switch means for controlling said bistable relay.

17. A system as claimed in claim 16, in which the bidirectional switch means takes the form of a TRIAC.

18. A system as claimed in claim 17, in which said TRIAC is actuated in one current sense to cause said bistable relay to operate to apply electrical power to said output heating device, and is actuated in the opposite sense to cause said bistable relay to operate to remove the electrical power applied to said output heating device.

19. A system as claimed in claim 18, in which said TRIAC is actuated in said one current sense by causing it to be actuated in one half-cycle of an alternating supply which is applied to said TRIAC, and is actuated in the opposite current sense by causing it to be actuated in an opposite polarity half-cycle of said alternating current supply.

20. A system as claimed in any of claims 16 to 19, in which the bistable relay takes the form of a remanent relay.

21. system as claimed in any of claims 16 to 19, in which the bistable relay takes the form of a permanent magnet relay.

22. An electronic control system as claimed in any preceding claim and substantially as hereinbefore described with reference to the accompanying drawings.

23. An electronic control system substantially as hereinbefore described with reference to the accompanying drawings.

24. An electrical cooking appliance including an electronic control system as claimed in any preceding claim.