GB2109603A - A cooking control and cooking apparatus including such a control - Google Patents

Abstract

A cooking control has a gas supply control (A) and a flame failure device, the gas supply control comprising a main valve, a supply port and an actuator (9) moveable to allow gas to flow to a burner through the supply port initially at a first rate. A temperature sensor, responsive to the temperature of gas being burnt at the burner, is arranged to cause the main valve to be held open while simultaneously supplying gas to the burner at a second higher rate; there is also provided a logic device (116) to automatically move the actuator to allow gas through the supply port long enough to allow the main valve to be held open by the temperature sensor to thereby supply gas to the burner at the second rate. The flame failure device activates the ignitor (117) in cooperation with the logic device (116). <IMAGE>

Description

SPECIFICATION A cooking control and cooking apparatus including such a control This invention relates to a cooking control to regulate the supply of gas to a burner, and also to gas cooking apparatus including such a control.

According to the present invention there is provided a cooking control comprising a gas supply control and a flame failure device, the gas supply control comprising a main valve, a supply port and an actuator which is capable of being moved to allow gas to flow to a burner through the supply port initially at a first rate, a temperature sensor capable of being responsive to the temperature of gas being burnt at the burner and arranged to cause the main valve to be held open while simultaneously allowing gas to be supplied to the burner at a second rate which is higher than the first rate, and circuit control means to automatically move the actuator to allow gas through the supply port for a time period long enough to allow the main valve to be held open by the temperature sensor to thereby supply gas to the burner at the second rate, the flame failure device comprising means to initiate a new ignition sequence in co-operation with the circuit control means if no flame is detected at the burner.

Further according to the present invention there is provided cooking apparatus including at least one cooking control as described in the immediately preceding paragraph.

The circuit control means may be provided with a timer which predetermines the time period or alternatively with a second sensor which senses when the temperature sensor has responded adequately to the temperature of gas being burnt at the burner to hold the main valve open, and generates a signal to move the actuator Preferably, the temperature sensor is arranged to cause the main valve to be held open by causing an electrical signal to be supplied to energise an electro-magnet which in turn attracts and holds open the main valve when the temperature sensor has sensed a particular temperature of the gas being burnt at the burner. The gas supply control may be such that the actuator is operable on the main valve to open the main valve when the actuator is depressed, allowing gas only to flow to the burner through the supply port until the electro-magnet holds the main valve open.At this moment the circuit control means may be such as to allow the actuator to return to its initial position and simultaneously open a second valve which allows gas to be supplied to the burner at a higher rate. Gas may flow through the second valve and the supply port which is a bypass port (i.e. leads to the burner by a line bypassing the second valve) to provide said second rate of gas flow, or alternatively, as the second valve is opened the supply port may be closed so that the second rate of flow takes place through the second valve only.Alternatively, the arrangement may be such that when the actuator is moved to a first position opening the main valve the supply port is partially blocked allowing gas to the burner at a low rate, and when the actuator is moved to a second position (after the temperature sensor has caused the main valve to be held open) the supply port is fully open allowing gas to the burner at a higher rate. The temperature sensor may be of a type such as a thermocouple which generates an electrical signal directly to energise the electro-magnet. Alternatively, the temperature sensor may be arranged to connect and disconnect a power supply to the electro-magnet at appropriate times.

Where a second sensor is provided as aforesaid and an electrical signal is caused to be supplied to an electro-magnet as aforesaid, the circuit control means preferably comprises a switch controlled by the second sensor, the second sensor being arranged to sense when the electro-magnet is sufficiently energised to hold the main valve open, and to control the switch in order to release the actuator after said time period.

The flame failure device comprises means to cut-off the gas supply to the burner should the flame of the burner be extinguished and preferably said means comprises or at least includes the temperature sensor and aforesaid electro-magnet so that if the flame is extinguished the electro-magnet is deenergised to close the main valve or alternatively a new ignition sequence is initiated.

The circuit control means may include a thermostat switch which overrides the timer or switch controlled by the second sensor, when a cooking space or vessel has reached a certain temperature measured by the thermostat, such that the actuator is moved to cut down the gas supply to the burner to a rate lying below said second rate and which may be equal to said first rate.

Alternatively the circuit control means may include a logic device to provide logic control of the actuator, timer and ignition of the gas, the electro-magnet being de-energised and the flow of gas cut off by relay contacts.

In order that the invention may more readily be understood a description is now given, by way of example only, with reference to the accompanying schematic drawings in which: Figure 1 shows a sectional view of a conventional gas supply control and flame failure device; Figure 2 is a simplified diagram of cooking apparatus incorporating the control and device of Figure 1; Figure 3 shows an embodiment of cooking control according to the present invention; Figure 4 shows the embodiment including a first modified form of circuit control means; Figure 5 shows the embodiment including a second modified form of circuit control means; Figures 6 and 7 show part of the circuit control means illustrated in Figure 3 and Figure 5 respectively with possible forms of time switches;; Figure 8 shows part of the circuit control means as illustrated in Figure 4 with a possible form of relay switch and sensor; Figure 9 shows a further modified arrangement of that shown in Figure 7; Figures 10 to 1 3 show further modified arrangements for the circuit control means; Figure 14 shows an embodiment including alternative circuit control means incorporating a logic device, and Figures 1 5 to 18 show parts of the circuit control means illustrated in Figure 14; Figure 19 shows the circuit diagram of another embodiment of the present invention; Figures 20 and 21 show the circuit diagram of two components used in the embodiment of Figure 19; Figure 22 shows the circuit diagram of another embodiment of the present invention; Figures 23 to 26 show the circuit diagrams of some components used in the embodiment of Figure 22;; Figure 27 shows the circuit diagram of a further embodiment of the present invention; Figures 28 to 30 show the circuit diagrams of some components used in the embodiment of Figure 27; Figure 31 shows the circuit diagram of a further embodiment of the present invention; and Figures 32 and 33 show the circuit diagrams of some components used in the embodiment of Figure 31.

Figure 1 shows a conventional form of gas supply control A and thermo-electric flame failure device and so will not be described in great detail. The gas supply control includes a main valve 1 comprising a valve member 1 a resiliently urged against a valve seat 1 b by a spring 1 c. In the position as shown in Figure 1 the gas supply to a burner 2 (see Figure 2) is cut off by the main valve 1. An actuator in the form of spring-loaded plunger 3 can be moved axially to the left in Figure 1 or depressed to move the valve member 1 a from its seat 1 b to open the main valve 1. As the plunger 3 is moved to the left valve member 8a seals against seat 8b to close a second valve 8. When the plunger 3 opens the main valve 1 gas is supplied to the burner 2 through a bypass port 4 and bypass line 5 at a first rate which is low.The gas is then ignited at the burner 2 by a suitable ignition device such as an electronic spark igniter. A temperature sensor in the form of a thermocouple 6 (see Figure 2) is positioned adjacent the burner 2 and generates an electrical signal in response to heat at the burner. After the plunger 3 has been depressed for about fifteen seconds the electrical signal generated is such as to energise an electro-magnet 7 to overcome the resilient force of spring 1 c and hold the main valve 1 open by attracting the valve member 1 a. The plunger 3 can then be released to thereby open the second valve 8 to increase the supply of gas to the burner to a second or full rate, the gas being able to pass to the burner through valve 8 and bypass line 5.If the flame at the burner is extinguished the electrical signal generated by the thermocouple 6 drops, de-energises the eiectro-magnet, and the force of the spring 1 c returns the valve member 1 a onto its seat 1 b to close the main valve 1 and cutoff the supply of gas to the burner 2.

Figure 3 shows circuit control means B in combination with the conventional gas supply control A. The control means B includes a solenoid thruster 9 which is positioned to control axial movement of the plunger 3. The circuit includes a double pole switch 10 opened and closed by a thermostat setting knob T (see Figure 2) whenever the burner 2 in the oven is switched on. As shown in Figure 2 the burner is a horizontal cylindrical burner at the bottom of the back of the oven, but it may alternatively be a round hotplate burner.

A second double pole switch 11 is opened and closed by a clock (not shown) on which may be pre-set an automatic cooking time period. Switches 10 and 11 are both shown in the open or "off" position and switch 11 can be manually closed or moved to an "on" position so that there is no pre-set cooking time period. An electrical connection 1 2 from the thermocouple 6 to the electro-magnet 7 is also connected to the circuit control means B through the double pole switches 10 and 1 When either or both of the switches 10, 11 is/are in the open position the thermocouple 6 is connected to earth, to ensure that the electro-magnet 7 is de-energised and the main valve 1 closed. In this position no current can pass to an ignition device 13 or to the solenoid thruster 9.

In operation of the circuit control means B, switches 10 and 11 are closed to connect the A.C.

mains supply to the control means B across live and neutral terminals L and N respectively, and also isolate the thermocouple 6 from earth. The ignition device 1 3 is energised and the solenoid thruster 9 is also energised by a diode 14 and a time switch 1 5. Rod 9a of solenoid thruster 9 is thereby moved to the left in Figure 3 to depress the plunger 3 and open the main valve 1. Thus the gas at the burner 2 from the bypass line 5 is lit by the ignition device, which generates a spark to light the gas and which ceases to spark as soon as the burner is lit. The time switch 1 5 is pre-set to energise the solenoid thruster 9 for a time period long enough to allow the thermocouple 6 to energise the electro-magnet 7 sufficiently to hold the main valve 1 open. On completion of this time period the time switch 1 5 moves to an "off" position and de-energises the solenoid thruster 9 to release the plunger 3, allowing it to move to the right and open the second valve 8 to supply gas to the burner 2 at the full rate. Switch 1 5 remains "off" until it is re-set by switches 10 and 1 The circuit control means B also includes a thermostat switch 1 6 across the time switch 1 5. When a set temperature of, for example, a cooking space or cooking vessel is reached switch 1 6 is closed by a thermostat (not shown) located in the vicinity of the space or vessel to again energise the solenoid thruster 9 and push the plunger 3 to close the second valve and thereby allow gas to be supplied to the burner only through the bypass line 5.The thruster 9 remains energised until the thermostat indicates that that temperature it is measuring has dropped sufficiently to open the thermostat switch 1 6 again.

Figure 14 shows a first modified form of circuit control means B' which is similar in operation to control means B except that the solenoid thruster 9 is no longer energised for a period determined by a timer. Time switch 1 5 in Figure 3 is replaced by a relay switch 1 7 and a sensor 1 8 which senses the strength of the magnetic field in the electro-magnet 7 or the current passing through it or the voltage developed across it. The relay switch 1 7 keeps the thruster 9 energised until the sensor 1 8 opens the switch 1 7 by sensing when the magnetic field of the electro-magnet 7 is strong enough to hold the main valve 1 open.

Figure 5 shows a second modified form of circuit control means B" which may include the aforedescribed time switch 1 5 as illustrated or the aforedescribed sensor 1 8 and relay switch 1 7.

Control means B" includes a charging capacitor 1 9 in order that the size of the solenoid thruster 9 can be reduced considerably. The thruster 9 is initially energised by the capacitor 1 9 and subsequently the rod 9a is held in a position depressing the plunger 3 by a greatly reduced current flowing through a resistor 20, connected to the output of diode 14. In this instance, the time switch 1 5 or relay switch 1 7 are arranged to delay current flowing to the thruster 9 until enough charge has built up on the capacitor 19.

Figure 6 shows one particular form of time switch 1 5. In this instance switch 1 5 is a thermal switch and comprises two heating elements 21 and 22 which are energised simultaneously when switches 10 and 11 are closed. Energisation of the elements 21 and 22 causes an adjacent bimetal strip 23 to absorb heat from the elements and open the switch 1 5 after the required pre-set time period. When the switch 1 5 is opened heating element 22 is de-energised but element 21 is still energised and gives out enough heat to prevent switch 1 5 closing again until the double pole switches 10or11 are opened.

Figure 7 shows an electronic timer 1 5 which provides a short time delay before switching on in order to allow the capacitor 1 9 to charge. Current to the solenoid thruster 9 is switched on by a thyristor circuit 24 and off by a thyristor circuit 25. When current is switched on to the thruster 9 the gate 26a of thyristor 26 is triggered through a neon bulb 27 whose triggering voltage is matched to the resistance network 28, 29, 30 so that the thyristor 26 cannot fire until the voltage across capacitor 1 9 is adequate to energise the solenoid thruster 9.

At the same time current can flow through resistance 31 to capacitor 32, and the values of resistance 31 and capacitor 32 are chosen so that the voltage on capacitor 32 does not exceed the breakdown voltage of a neon bulb 33 until the pre-set time period has elapsed. Neon bulb 33 then discharges capacitor 32 and triggers gate 34a of thyristor 34. A capacitor 35 is rapidly discharged and since current momentarily stops flowing through thyristor 26 triggering of thyristor 26 is halted.

The solenoid thruster 9 is thus de-energised by thyristor circuit it 25 but thyristor 34 remains conducting thus suppressing the triggering of both thyristors until such time as the switches 10 or 11 are switched off and then on again to reset the electronic switch 1 5.

Figure 8 shows a particular form of relay switch and sensor 1 7, 18, which can be used in the Figure 4 circuit, including a reed switch 36, which closes when placed in a magnetic field of a predetermined strength. The reed switch 36 may be placed in the magnetic field of electro-magnet 7 or 21ternatively spaced from the electro-magnet 7 within a coil of wire connected to the thermocouple 6.

The arrangement is such that the reed switch 36 is only closed after the electro-magnet 7 is energised sufficiently to hold the main valve open.

Initially the thruster 9 is energised when contacts 37 of the relay 38 are closed. The reed switch 36 senses the magnetic field around it (either from the electro-magnet or coil of wire surrounding it as aforesaid) and when the electromagnet 7 is energised sufficiently to hold the main valve 1 open the reed switch 36 closes and energises the relay 38 thereby opening the contacts 37 and de-energising the thruster 9.

Figure 9 shows an arrangement similar to Figure 7 but further includes a reed switch 39 to trigger thyristor 34 instead of neon bulb 33, through a resistance network 40 and 41.

If the capacitor 1 9 is omitted from the circuit control means shown in Figure 9 then the current through the thyristor 26 will be zero during half of each cycle and it can therefore be switched off by earthing the gate 26a.

This allows considerable simplification as shown in Figure 10 which also includes a circuit 42 for the ignition device 1 3 which will only be energised when the thermocouple 6 is cool or when a separate spark is generated when demand button 43 is depressed.

The thruster 9 is energised through thyristor 26 which is triggered through resistance capacitance network 28, 29, 30, 44 and 45.

The ignition device is energised through resistances 28 and 46 and diode 47. A capacitor 48 is charged and suddenly discharged through gas discharge gap 49 and the primary windings of a transformer 50. This produces sparks at the ignition electrodes via the high voltage secondary windings of the transformer 50. The capacitor 48 is repeatedly charged through 28, 46 and 47 and discharged through 49 and 50, producing a succession of sparks.

When the thermocouple 6 has reached a temperature which will hold open the main valve then the reed switch 39 closes, earthing both the gate 26a of the thyristor 26 and the supply to the ignition device. The thruster 9 is de-energised and the cooking control continues to operate as described before.

Sparks may be produced at any time for lighting other burners by depressing the "spark on demand" button 43.

In Figure lithe reed relay of Figure 10 has been replaced by solid state devices which amplify the thermocouple voltage and trigger a thyristor when this voltage has reached a value which will hold the main valve 1 open.

The circuit control means shown also controls its own ignition device. In general the components are numbered and their functions are identical with those in Figure 10. The points of difference are as follows: (a) The reed relay 38 is replaced by a thyristor 55 which is triggered from an amplifier and comparator 59.

(b) A simple balanced power supply is provided for the amplifier and comparator at 58.

(c) The amplifier amplifies the thermocouple voltage and the comparator sends a pulse to thyristor 55 when this thermocouple voltage exceeds the value at which the electro-magnet 7 will hold the main valve 1 open.

(d) The action of thyristor 55 in de-energising both the thruster 9 and the ignition device 42 is similar to that of the reed switch of Figure 10 except that some smoothing is introduced by capacitor 56 resistance 57 and diode 60 so that the thyristor 55 does not switch off during the negative half cycle.

Figure 12 shows an arrangement including a smoothed, low voltage DC supply indicated at 62 and an integrated circuit timer, (type 555 for instance) indicated at 61.

A timer trigger 64 is connected to a resistance capacitor circuit which ensures that a timing pulse is received when the unit is first switched on and that no further trigger pulses are sent until the unit is switched off and then on again.

The duration of the time delay is determined by a resistance capacitor circuit connected to 65 and is of sufficient duration to ensure that the electromagnet 7 will hold in at the end of it.

An output 66 triggers the thyristor 26 through a suitable network and so keeps the thruster 9 energised during the timing period.

The output 66 can also be used, through a suitable circuit to control the ignition as described previously.

A timer control 67 is connected to neutral via a capacitor, and the reset and supply 68 is connected to the positive of the DC supply.

As an alternative the integrated circuit 61 may be replaced by a latching timer circuit made up of components as shown in Figure 13.

Low voltage, smoothed DC is provided by resistance 70 and capacitance 71. When first switched on the point 79 is at zero voltage while point 80 is at a higher voltage, and neither of transistors 72 or 73 can conduct. In this condition the thyristor 26 is triggered and the thruster 9 is energised.

Resistances at 76 and 77 and the resistance capacitor circuit 75, 74 are so chosen that the voltage at point 79 exceeds the voltage at 80 and the end of the required timing period. The transistors 72 and 73 then begin to conduct, so dropping the voltage at 80 and increasing their conduction. Point 80 is brought to zero potential and the thyristor ceases to conduct, thus de-energising the thruster 9.

Out put at 78 might be used to control ignition as previously described but through additional circuitry.

Figure 14 shows an alternative embodiment of the invention incorporating a logic device 11 6 to provide logic control of solenoid thruster 9, timer 11 5 and igniter 11 7. The embodiment has the same mechanical devices as shown in Figure 1, the solenoid thruster 9 gas supply control A and the thermoelectric flame failure device whose valve is held open by a current from a thermocouple 6 (not shown in Figure 14) flowing in wire 97 and returning in sheath 98. However electro-magnet 7 (not shown) is de-energised and the flow of gas is cut off by relay contacts 104 in series with conductor 97 instead of parallei switches 10 and 11 in Figure 3.

The other main elements of the electronic control consist of a relay 103 which controls the contacts 104, a timer 11 5 with clock input 108 and reset input 109 and an output 110 which goes from logic "0" to "1" 20 seconds after the reset 109 goes to zero providing that the clock 108 is running, a logic device 1 6, an ignitor 1 7 which provides ignition for the oven or for other burners by operation of a "spark on demand" push button 124, a power supply 1 8 which provides smooth controlled DC current, a thermostat 1 9 consisting of an amplifier 1 32 which amplifies the signal from a thermocouple 6 and a comparator 1 33 which compares the signal with a value set on a potentiometer 120, and a clock-switch 1 30 which is a double throw switch connected so that when the main circuit is switched off a secondary circuit is connected via a "Hold-warm" switch 101 to provide a low voltage signal at input 102 of the logic device 116.

Igniter 11 7 has three inputs, namely: 113 from the logic device to suppress oven ignition at appropriate times, 1 23 from thermostat switch 129 and 124 from "spark on demand" button 124a.

Igniter 117 has two outputs, namely: 121 for delaying the start of the timer until a flame is established and 122 which energises the power supply 11 8. The signal which is amplified by the thermostat 11 9 is fed via output 125 to input 126 of the logic device. The clock-switch 1 30 is normally operated automatically by the clock at the start and finish of an automatic cooking cycle but it can be set manually if desired.

For manual control, the clock switch 130 must be set in the "manual" position with connection to contact C. When the thermostat potentiometer 120 is set the switch 1 29 will be closed. This will energise the igniter 11 7 and the power supply 118. In the absence of flame or a signal from the logic device 116, the igniter will spark and it will also send a signal via 121 and 127 to stop the timer 115 until flame is established. The relay 103 is energised, closing contacts 104 so that the flame failure device will hold its valve open as soon as its thermocouple is hot. 50 Hz pulses at input 105, and a steady DC signal at input 106 to the logic device 11 6, are provided. The supply is connected to the solenoid 9 which can then be energised through thyristor 29.The logic device will then maintain ignition, provide pulses for the timer clock and energise the solenoid, thus admitting gas to the burner at a low rate. When the low rate gas has been ignited, the igniter stops sparking and sends a signal via 121, 127, 128 to 109 on timer 115 indicating that the flame is established. The timer, therefore, starts to run and after 20 seconds the=solenoid is de-energised. Provided the thermocouple is hot enough to hold in the main valve the control will now run at full rate and the ignition sequence is concluded.

If the flame failure device fails the flame will be extinguished and the igniter 11 7 will start to spark again sending a signal via 121 and 109 to restart the timer. The sequence will therefore continue until successful ignition occurs.

After the oven has run for a time at full rate the thermostat 11 9 will cease to call for heat and will send a signal via 125 to input 126 on the logic device 116, which will then energise solenoid 9, thus reducing the burner flame to a low rate. The burner will continue to operate from high to low rate under the influence of the thermostat 11 9 until the thermostat is turned off thus de-energising the whole circuit via switch 129, including contacts 104 via relay 103.

If at the end of the cooking period, the cooked food needs to be kept hot, "Hold warm" switch 101 should be closed and clock-switch 130 moved from contact C to contact D. The ignition 117 and solenoid 9 will then be energised, via the logic device 11 6, and the pulses to the timer 11 5 will be stopped whether the thermostat calls for heat or not. The burner will therefore be continuously on the lowest rate.

For automatic cooking, the clock switch 1 30 is first set to position D, the thermostat is set to the desired cooking temperature and the "hold-warm" switch 101 is on, if desired. Automatically, at the start of the desired cooking period, the clock switch will move from D to C, and at the end of cooking period it will move from C back to D again. During the waiting period, before the start of the cooking period, the igniter 117 and power supply 118 will be energised through switch 129 and power will be available for the solenoid 9 when energised through thyristor 29. When the clock switch 1 30 moves from D to C at the start of the cooking cycle, the conditions are the same as the start of the manual cooking cycle and ignition will take place.The oven will be controlled by the thermostat and, at the end of the automatic cooking cycle, the clock switch will move from C to D opening switch 104 via relay 103.

If the "hold-warm" switch 101 is open, the gas is turned off. If the "hold-warm" switch is closed, then the ignition and solenoid are operative and energised. If flame is lost then the igniter 11 7 will relight the flame.

The timer 115 is a standard 12 bit binary counter used to count pulses at 50 Hz applied to the clock input at all stages when the timer is required to run. The timer is prevented from starting until flame is established or re-started, if flame is lost, by applying a signal to the re-set input 1 09 via the logic device 11 6.

Figure 1 5 shows a logic device consisting entirely of CMOS logic elements, which may be in the configuration shown or in any other configuration producing a similar result.

Figure 1 6 shows an arrangement for an igniter which is energised by applying mains AC voltage to either input 1 24a for "spark on demand" or input 123 for the electronically controlled oven burner. If either or both of these inputs are energised then half wave rectified AC will be the output from 1 22 to feed the power supply 1 18.

A conventional spark generator circuit comprising resistors R8, R9 and R10, capacitor C4, transformer T1, thyristor TH 1 and spark gaps G 1 to G6 will also be energised if either of inputs 123 or 1 24a is energized. The spark generator will produce a spark every time a pulse is received at the gate of thyristor TH 1. Such pulses are provided by astable pair NOR2 and NOR3 via capacitor C5, the frequency depending on the values of C6 and R1 1. The pulses are only produced if input E to NOR3 stands at logic "0", which means ignition is required. If it becomes logic "1", which means that ignition is not required, then the output of NOR 3 is always zero and both pulses and sparking stop. It follows that if either input J or K of NOR1 goes to logic "1", then sparking will start.

Considering now input 124a, resistors R1 and R2 are so chosen that the small DC current flowing into C1 is sufficient to produce a logic "1" at input J of NOR 1. Sparking will therefore always take place when input 1 24a is energised.

Similarly, when input 123 is first energised a logic "1" is applied to the input H of AND 1. If, and only if, ignition control input 11 3 also carries a logic "1" then logic "1" is applied to K and sparking occurs. The logic "1" applied to K is also applied to output 121, but only after a short time delay provided by R7 and C3. Alternatively H can be biassed to logic "1" by being connected to the positive supply through a high value resistor. The diode D2 can then be deleted.

Oven burner spark gap G1 is also connected via its transformer coil and resistance R6 to the junction of resistors R4 and R5. When a flame is present, the rectifying effect of the electrode burner pair will cause a DC current to flow from R4 into R6. The values of R3, R4, R5 and R6 are so chosen that this DC current is sufficient to neutralise that component passing through diode D2 with a result that the logic "1" which was present at input H is reduced to logic "0" when flame is present. Input K is also reduced to "0" and sparking will cease. The logic "0" at K will also be repeated, after a short delay, at output 121.

"Spark gap" S provides a low impedance return path for the current when spark gap G 1 is actually producing a spark. Diode D3 protects the thyristor TH 1 from voltage surges when switching the transformer primary and diode D4 protects the gate of the thyristor from negative pulses through capacitor C5.

Figure 1 7 shows an arrangement for a power supply which is provided by half wave rectification of the mains through diode D1 and resistance R12 two a smoothing capacitor C7. Zener diode ZD1 provides over-voltage protection and a conventional voltage regulator 1 31 maintains the output at a level sufficiently steady for the requirements of the CMOS logic and of the thermostat amplifier.

Referring now to Figure 18, which shows an electronic oven thermostat with cold junction compensation, the circuit includes an operational amplifier 1 32 and a comparator 133 combined into a single integrated circuit. Resistors R13 and R14 and the resistance of thermistor 134 which is placed near the cold junction of thermocouple 6, are chosen so that the input to the operational amplifier 1 32 is biassed to be sufficiently above zero for the gain to be reasonably linear.

The changes in resistance of the thermistor 134 with temperature cause changes in bias voltage VAT which compensate for the cold junction temperature of the thermocouple 6. This compensation is reasonably correct over the small range of temperatures likely to be experienced by the cold junction.

The output of the operational amplifier 132 is compared with a reference voltage set on potentiometer R1 5. This reference voltage defines the temperature setting of the thermostat.

If the operational amplifier output is low, the output of the comparator 1 33 is zero and the thermostat calls for heat. If the output from the operational amplifier is high, the output of the comparator 1 33 is logic "1" and the thermostat ceases to call for heat.

Capacitors C8, C9 and C10 reduce unwanted ripple or oscillation.

An advantage of the embodiment shown in Figures 14 to 1 8 is that it replaces mechanical devices of current designs and of other suggested modifications by electronic circuitry. Furthermore most of this circuitry is CMOS logic and can be produced as a single integrated circuit incorporating all the logic functions of the system including those of the thermostat and of the igniter.

It is to be appreciated that further modified arrangements may be provided to control one or more burners on cooking apparatus. In particular the solenoid thruster 9 of Figure 3 and the gas supply control A may be part of the same unit, for example, the rod 9a being integral with plunger 3.

Another embodiment, which is shown in circuit diagram in Figure 19, includes a simplified startup unit 130 (replacing the timer) which latches any "no flame" signal (FS "1") for a delay period long enough to allow ignition and flame sensing to be established by the FFD, the latched signal being fed to the modified logic unit 1 16' through input SU referenced 131.

The start-up unit 130 (see Figure 20) has a latch, formed by NOR 1 and NOR 2, to latch the input state of the flame sense signal FS into the output SU whenever it is logic "1" i.e. no flame. An oscillator timing device, comprising NOT1, NOT2, R1, R2, R3, D1, C1 and C2, sends a short pulse after a delay period long enough to allow the thermocouple of the FFD to gain sufficient heat to hold open the FFD.

These pulses are repeated indefinitely at regular intervals. Each pulse re-sets the output of the latch to "O". If there is no flame or the flame is extinguished by the FFD dropping out, then output SU is immediately set to "1 " again and the start-up sequence continues. If flame is present then FS is "O" and SU remains "0" until flame is lost again for any reason. When SU is "0" the burner can be run at full rate under the control of the thermostat. Diode D1 ensures that the positive pulse is very much shorter than the delay period. Capacitor C1, connected to earth, biasses the oscillator to produce an output "1" when switched on.

Also, in this embodiment the "hold-warm" input H referenced 102 is derived by direct switching from the supply rather than via the clock switch 134 (now a single pole, single throw type).

To prevent the oven burner being set to the "low, hold warm" setting during the wait period before cooking, the logic unit 11 6' generates a value H' which is always "0" when powered up and only takes the value "H" after the cooking cycle has begun.

Logic unit 1 16' has an input P numbered 133 to indicate when relay RLA is energised, i.e. when cooking is required.

Logic unit 116' (see Figure 21) has a latch comprising NOR1 and NOR2 and is biassed by R1 to produce output H' which is "0" when switched on. If H is "O" at any time the output, (via NOT1 and NOR2) is "O". If H is 1 then output H' remains "0" until P is "1" when H' is "1". H' remains latched as "1" when P is 0 again. It can be re-set to "O" by switching H to "O" or by switching off power to the logic circuit and then switching it on again.

The truth table of logic unit 1 6' is as follows: INPUTS OUTPUTS S H' P SU= IG L T' Stat Latched Power Start Up O Low Line O=Heat "Hold to Signal Inhibits Flame number 1=LowHeat Warm" Relay 7=Low lgnition =1 Remarks

1 0 0 0 0 0 O Wait before or after cooking 2 0 0 0 1 0 0 - cycle 3 0 0 1 0 1 0 During cook period, high rate 4 u u 1 1 1 1 During start-up period 5 0 1 0 0 1 1 During "hold warm" period 6 0 1 0 1 1 1 During "hold warm" period redundant start 7 0 1 1 0 1 0 Cook period, hold enabled 8 0 1 1 1 1 1 Start-up for cook period, hold enabled

9 1 0 0 0 0 0 Wait before or after cook 10 1 0 0 1 0 0 cycle 11 1 0 1 0 1 1 Cook period, t'stat satisfied 12 1 0 1 1 1 1 Start-up period t'stat satisfied 13 1 1 0 0 1 1 Hold warm period t'stat satisfied 14 1 1 0 1 1 1 Hold warm period redundant start 1 5 1 1 1 0 1 1 Cook period, t'stat satisfied 16 1 1 1 1 1 1 Start-up t'stat satisfied For manual control, clock switch 1 34 must be closed (i.e. in its manual setting) and once the thermostat potentiometer 1 20 is set then the switch SW is moved to its closed position to energise the circuitry.

The flame sense FS signal, initially "1" (no flame), is latched by the start-up unit 130 and fed to input 131 (SU) of the logic unit 1 16' for the duration of a delay period. The other inputs to logic unit 116' are P="1", H=as set by switch 132 (and the value will be transmitted to H' since P is "1") and S="0" since the oven is cold the thermostat calls for heat, the logic inputs corresponding to lines 4, 8 and possibly 12 or 16 of the truth table. In all cases the ignition is enabled and the solenoid 135 energised, thus starting up at low rate. At the end of the delay period the start-up unit sets SU="O". If flame is not established the flame sense signal FS immediately sets SU to "1" again and a further ignition attempt is made.If a flame is sensed then SU remains "0" (the input logic then corresponding to lines 3 or 7 of the truth table), resulting in the solenoid 135 being de-energised and the burner rate being increases to full-on.

When the oven reaches the set temperature, S becomes "1" (the input logic corresponding to lines 11 or 15) resulting in the solenoid 135 being energised and the burner rate being reduced to low.

The oven will continue to cycle between the "flame-sensed" and "set-temperature" conditions just mentioned, thereby maintaining its temperature in the region set on the thermostat.

The oven can be turned off at any time by turning the thermostat "off" thus opening switch SW and de-energising the circuitry.

To hold food warm after cooking, the "hold warm" switch 132 is closed and clock switch 134 is opened. In this condition H' is "1" and P is "O". Thus the input logic corresponds with lines 5, 6, 1 3 or 14 of the truth table. In all cases both ignition and the solenoid 135 are energised and the oven burner will continue at low rate until the thermostat switch SW or the "hold warm" switch 132 is switched off.

For automatic cooking, the clock switch 134 is automatically switched on at the start of the programmed "cook" period and off at the end of it. More specifically, at the start of an automatic cooking cycle and during the "wait" period the clock switch 1 34 is set open (its automatic setting) and the "hold warm" switch 132 is closed if desired. The thermostat is set to the temperature required, thus closing switch SW and energising ignition and the logic circuits.

All inputs to the logic unit 11 6' will be "0" except SU and possibly S, thereby corresponding with lines 1 or 2 and possibly 9 or 10 of the truth table. In all cases both outputs are "0" and the burner is turned off.

At the end of the wait period switch 1 34 is closed automatically by the clock, so that the logic inputs change with P becoming "1", H' taking on the value of H which depends on the position of the "hoidwarm" switch 1 32 and SU remaining "1" for the delay period since no flame is established. Thus the input logic corresponds to lines 4, 8, 12 or 1 6 of the truth table and both ignition and the solenoid 1 35 will be energised, thus igniting on low flame as described before. The oven then runs under the control of the thermostat as in the manual mode.

At the end of the cooking period, the clock switch 1 34 is automatically switched off thereby changing P from "1" to "O". If the "hold" switch 132 is open then H equals H' which is "0" and the input logic to 11 6' corresponds to lines 1, 2, 9, or 10 of the truth table. In each case both outputs are zero and, since the relay RLA has also been de-energised by the switch 1 34, the gas will be cut off from the burner. If the "hold" switch 1 32 is closed then H equals H' which is "1" and the logic input corresponds to lines 5, 6 13 or 14. In each case both ignition and solenoid 135 are energised and the burner will continue to burn at low rate indefinitely.

If flame is lost at any time the relight igniter 117 will immediately start to produce sparks and the flame will normally relight immediately without change of rate. If there is no flame after a short time delay igniter 11 7 generates a logic output of "1" at FS which is transmitted to start-up unit 130 thereby immediately changing the output SU to "1" and initiating a new start-up sequence, reducing the rate of the burner to a low level until the thermo-electric FFD holds the main gas valve open.

If, due to a failure of the electronic logic, the main valve is left open in the absence of flame, then it will eventually be closed when the thermocouple has cooled sufficiently to release the electromagnet and to allow the valve to close under the action of its spring.

Figure 22 shows the circuit diagram of a further embodiment in which the thermocouple is eliminated, its function being taken over by the sensing electrode of the igniter. The modification obviates the problem of accurately positioning the thermocouple in the flame at both high and low rates and provides instantaneous sensing of the flame enabling the elimination of the time delay circuits for allowing the thermocouple time to reach its working temperature. In order to minimize the risk of no action being taken upon flame failure, this embodiment includes the following safety features (described in greater detail iater):-- (i) an inherently safe gas control; (ii) constant electronic surveillance of electronic circuits for failures which might cause danger: and, (iii) a system of automatic checks on the electronic circuitry before the burner is ignited.

One potentially dangerous situation arises if the circuitry which produces the flame sense signal FS (FS as "0" indicating a flame sensed, and FS as "1" indicating no flame) produces FS as "0" when there is in fact no flame; this is overcome by testing the circuitry during the start-up test, before any attempt is made to ignite the burner.

Another such situation arises if thyristor 1 45 fails effecting a short circuit so that the magnet 147 is energised even though the circuit logic may have signalled "0" to V. This can be overcome by testing the thyristor during the start-up test by switching V to "1" and then to "0" again and monitoring P and T, with the system going to lockout (i.e. the ignition sequence is stopped and locked in a condition which can only be re-started by human intervention) if failure has occurred. No gas will escape since valve 1 will not lift when only coil 147 is energised.

Another potentially dangerous situation arises if thyristor 145 fails during the cooking cycle and the flame is lost so that, if no precautions had been taken gas at full rate would escape through the burner. The remedy is to energise the solenoid 135 and the igniter, thus passing gas at low rate and continuing to attempt to ignite it. This is done by the logic system of the Fault Finder 142 which sends a "1" signal to both outputs X and Y indicating that the remedy is to switch both L and 1G to "1".

If thyristor TA fails producing an open circuit, neither ignition at low rate nor the last-mentioned remedy can occur. To overcome this, after testing thyristor 145 start-up tester 140 switches the thyristor TA on and then off again during the start-up test. If it fails open circuit the start-up procedure is halted with all outputs "0" (lockout type A). If it fails short circuit then the solenoid will already be energised and gas will be passing at low rate. The ignition is also energised and start-up halted (lockout type B).

In this embodiment, coil 147 of the FFD 136 is wound to be energized from the mains electricity supply with half wave rectification and "flywheel" diode as shown. The coil 147 produces approximately the same magnetic field under these conditions as the thermocouple produced in previous embodiments.

If energised with valve 1 closed, coil 147 has insufficient force to open it; only if valve 1 has already been opened by solenoid 135, will coil 147 generate sufficient magnetic force to hold it open.

The embodiment includes resistance/capacitance circuits to sense and signal the presence of a voltage at certain key points in the circuit. Thus, circuit 148 signals to input CT of logic unit 160 the presence of a voltage at the anode of thyristor TA; circuit 149 sends a signal to P when power is switched on to the FFD magnet coil 147; circuit 1 50 sends a signal T when a voltage appears at the anode of thyristor 145.

Fault finder 142 (shown in greater detail in Figure 23) monitors constantly certain outputs, and if their condition indicates any risk of incorrect working of the system it selects the more appropriate of two remedies A or B to forestall any dangerous situation and suitably commands the output control 141. The following outputs are monitored: V+: the positive supply to the logic circuits; C: the voltage at the anode of thyristor TA; L: the gate signal to thyristor TA; T: the voltage at the anode of thyristor 145; V: the gate signal to thyristor 145; P: voltage applied to the FFD magnet coil 147; H': the "hold warm" signal latched after the oven has been lit; FS: Flame sensing signal is "1 " for no flame; SU: Start-up test signal is "1" for successful completion of tests.

Remedy A consists of setting outputs L, 1G and V to zero, and remedy B consists of setting V to zero but energizing L and IG, thereby maintaining gas flow at the low rate but continuing to attempt ignition.

The operation of Fault Finder 1 42 is shown in the following table:

Condition monitored Fault Remedy CLTZ0} Thyristor TA is short circuit B CT=1} Thyristor TA is open circuit A L=1 V=0 Thyristor 145 is short circuit (See below) P=1 T=1 Thyristor 145 is open circuit A V=1 P=O Flame is sensed when none should be there A Remedy B is used when thyristor 145 is short circuit only if a successful start-up sequence has been completed and SU is "1", because the valve may be held in the open position by the coil 147 and cannot be switched off due to the short circuiting of thyristor 145.

A diagnostic unit can also be plugged in which indicates which fault has been detected, the unit being fitted with an over-ride push button so that a complete ignition sequence can be run under supervision by shorting the outputs X and Y to "O".

The start-up tester 140, (shown in greater detail in Figure 24) applies a switching sequence to its outputs IG' L and V such that any fault is detected by the fault finder 142 before it could produce a dangerous situation. When the start-up test is successfully complete a signal is sent via the line SU to the logic unit allowing it to proceed with normal ignition and control of the burner. Start-up tester 140 also commands the output control 141 to pass signals from the logic unit 1 60 for execution by the devices which they control (IG to ignition, V to the magnet coil and L to the low flame control).

More specifically, the start-up test sequence is initiated at the start of the cook period, when logic input P goes from "0" to "1". The tester switches the output V from "0" to "1" and then, after a short delay, back to "0" again. This energises the FFD magnet coil 147 and then de-energises it again. The coil has insufficient power to lift the valve, so no gas escapes, but if no faults appear during this test then the correct operation of thyristor 1 45 is proved and the test continues. If faults appear the system goes to the appropriate lockout condition.

The tester then switches 1G and L to "1" thus energising the solenoid 1 35 and the ignition, so admitting gas at the low rate and igniting it. If flame is sensed before ignition is energised the test is stopped. When flame is sensed after ignition, the solenoid 1 35 and ignition are de-energised and a check is carried out to ensure that flame is no longer sensed and that the thyristor TA has switched off.

Then the signal SU "1" is sent, thereby signifying that the start-up test is complete. An indicator lamp is illuminated when the self test routine is in progress and when operation of the tester has been halted by a fault.

In Figure 24, latches NOT1 +NOR1 and NOT2+NOR2 are set when powered up through capacitors to V+. When P is switched to "1" then V is "1" through the gate AND1. The latch is re-set after a short delay and V is "0" again.

The latch signals gate AND 2 and provided no faults have been found the signal continues to gate AND 3 which checks that latch 2 is set, that P is "1" and that FS is "1" i.e. no flame sensed before switching on L and 1G When flame is sensed the latch 2 is re-set by gate AND 4 thus switching off L and 1G and sending a signal to gate ANDS. This checks that no further faults have appeared, that no flame is sensed and then re-sets a further latch (NOT3+NOR3) which sends and holds SU is "1" which signals that the start-up test is complete. A diagnostic plug-in unit with manual over-ride can be inserted to find out where a sequence has been halted and to run it through under supervision.

Output control 141 (shown in greater detail in Figure 25) accepts and analyses signals from the fault finder 142, the start-up tester 140 and the logic device 160, allocates a priority to these signals and transmits an appropriate output to 1G (ignition), V (magnet coil) and L (low flame solenoid). It gives absolute priority to signals from the fault finder 1 42. If a fault is signalled via lines X or Y, the output control 141 sets the outputs to an appropriate setting and locks them there so that they cannot be changed except by turning the oven off and then on again or by plugging in a special diagnostic circuit.

The start-up tester 140 has second priority. Once it has completed its test sequence it sends an appropriate signal via the SU line causing the second priority to pass to the logic unit 1 60.

More specifically, during the start-up test, SU is "0", and outputs from the logic unit are blocked by gates AND1 , AND2 AND3. If no faults have been found X and Y are "0" and latch X is energised with an output "1" (biassed by capacitor C1), latch Y is energised with an output "0" (biassed by capacitor C2).

Therefore control signals from the start-up unit can pass to the outputs V, 1G and L via OR1, OR2, OR3, AND4, ANDS, AND6, OR4 and OR5.

When the start-up test is complete then SU is "1" and all other signals (V, 1G and L) from the start-up tester are "0".

Signals from the logic unit 1 60 can then pass to the output via AND 1, AND2, AND3, OR1 . OR2, OR3, AND4, ANDS, AND6, OR4 and OR5. If at any time a fault is detected then input X is "1" and the latch X resets to an output "0". This stops all signals to the output at gates AND4, ANDS and AND6. If the fault is of the type requiring remedy B then latch Y is set and its output becomes "1" which is transmitted to 1G and L via gates OR4 and OR5. Latches X and Y ensure that the lockout condition is held until power is switched off and then switched on again.A short time delay is introduced into the two latches in capacitors C1 and C2 and by resistors R3 and R4. This is to avoid nuisance tripping by transients. A "fault indicator" LED is driven from the output of latch X to show when the cooking has been stopped by the presence of a fault.

Logic unit 1 60 (shown in detail in Figure 26) differs from the previous logic units in that it utilizes the flame sensing for flame failure protection. When SU is 0 the appliance is either in a "wait" or on "off" period or is carrying out a start-up test, so that the control from the logic unit is disabled and the value of the output is of no consequence. Such situations are marked as Ow in the following truth table for logic unit 1 60. This truth table is only concerned with switching of the ignition (IG) the low flame solenoid 10 (L) and the FFD magnet, coil 1 47 (V) to produce a smooth ignition at low rate and thereafter to effect control from the thermostat and the "hold" switch as in previous embodiments. It also initiates a new "low flame" ignition sequence if flame is lost at any time.

INPUTS OUTPUTS

0 0 0 0 o : E 0 0 nE LL P 0 E0 LL 6 o 'o- ' : 0 0 0 E E t: a, C = t 0 0 (0 0 0 LL 'nO LL LL 0 C X ill T Il n II II II N II C') w E o =0 'no II II Il 0 1 0 0 0 0 0 &commat; Q 03 Wait penoa, before cooking 2 0 0 0 0 1 1i 8 1i3 flame sense illegal 3 O O O 1 O O O O Off period 3 0 0 0 1 0 0 0 0 after cooking 4 O O O 1 1 O O O flame sense illegal 5 0 0 1 O O &commat; lib 1 During } start-up 6 0 0 1 0 1 &commat; &commat; test 7 0 0 1 1 0 1 0 1 Cook cycle-high rate 8 0 0 1 1 1 1 1 1 - Cook cycle, ignition, low rate 9 0 1 0 0 0 3 Q &commat; HzWait before cook, flame sense 10 0 1 0 0 1 23 (B (B illegal 11 0 1 0 1 0 1 1 O Hold warm, low rate 12 0 1 0 1 1 1 1 O Hold warm,~low rate flame lost 13 0 1 1 0 0 23 &commat; Cookcyclestart-up tests 14 0 1 1 0 1 &commat; &commat; (3 Cook cycle start-up tests 15 0 1 1 1 0 1 0 1 Cook cycle, high rate 1 6 0 1 1 1 1 1 1 1 Cook cycle, low rate, ignition 17 1 0 0 0 0 &commat; &commat; HzWait before cook cycle illegal flame sense 18 1 0 0 0 .1 &commat; 23 HzWait before cook cycle 19 1 O 0 0 O O O O 0 Offaftercookcycle, illegal flame sense 20 1 0 0 1 1 0 0 O Off after cook cycle 21 1 0 1 0 0 23 &commat; &commat; Cook cycle start-up tests 22 1 0 1 0 1 &commat; by Hz Cook cycle start-up tests 23 1 0 1 1 0 1 1 1 Cook cycle at low rate stat satisfied 24 1 0 1 1 1 1 1 1 Cook cycle at low rate stat flame out 25 1 1 0 0 0 &commat; &commat; &commat; Wait before cook, illegal flame sense 26 1 1 0 0 1 &commat; &commat; &commat; Wait before cook 27 1 1 0 1 0 1 1 O Hold warm, low rate ,,., UTS--------- INPUTS --OUTPUTS

(0 C (0 CO (0 - E a, o Q (0 0 C 0 E : (0 1L (0 E (00 = oa, E m m (0(0 (0 (0 (0 - Hz g E E o - m 0 (0 a, C (0 C') 0 LL LL = 0 C a' (0 = I II II II II II C') w E C II II C') 0 II II II 1 1 1 1 I Hold warm, low rate :j 'no LL 0 28 1 1 0 1 1 1 O Hold warm, low rate flame out 29 1 1 i 1 O 0 (B; i &commat; 1g3 Start up tests cook cycle-stat satisfied 30 1 1 1 0 1 (B &commat; 8 Start up tests cook cycle-stat satisfied 31 1 1 1 1 0 1 1 1 Cook cycle, low rate, stat satisfied 32 1 1 1 1 1 1 1 1 Cook cycle, low rate, st satisfied flame out The CMOS logic used in this and other embodiments switches in less than a micro-second.

Within this sort of time period "fault" conditions may appear and yet be harmless-for instance the voltage sensors at 148, 149 and 1 50 will take several cycles to respond to a change in thyristor gate voltage and the flame will linger at the burner (and be sensed) after the current to the FFD coil has been switched off.

To prevent nuisance tripping the two latches in the output control 141 have built-in time delay so that the unit only goes to lockout if fault conditions persist.

Furthermore a delay is introduced into the flame sensing circuit and in the circuit associated with it in the fault finder so that the presence or absence of flame in any circumstances is not interpreted as a fault unless it endures for some time. The igniter however responds immediately and will start or stop sparking without loss of time. It follows that the delays at each step of the start-up test sequence are long enough to allow the fault finder and output control to interpret fault conditions.

In the embodiment whose circuit diagram is shown in Figure 27, the function of the external clock timer (which operated switch 1 34 in previous embodiments) is taken over by an internal electronic timer.

An advantage of this embodiment is that the circuits for the timer can be combined with some or all of the other logic circuits in one or more integrated circuits.

The main functions of FFD 136, solenoid 135, Igniter 117, power pack 118, relay RLA 103 with contacts 104 and electronic thermostat 11 9 remain unchanged. The relay 103 which is used to close valve 1 at the end of the cooking period is controlled through thyristor 145 by a signal from the logic unit.

The first stage 1 61 of the timer measures time by counting pulses at 50 Hz supplied through output CLK on the logic unit 1 60 and provides pulses at 20 sec and intervals for the start up unit 1 63 and pulses at intervals of 56.25 seconds for the second stage 162 of the timer.

The second stage 1 62 produces an output signal, corresponding to the time elapsed, as a six bit binary number whose least significant bit changes every 1 5 minutes, the most significant bit changing every 8 hours so that the maximum measured time delay is approximately 1 d hours.

The duration of the cooking period (C) and the total delay (D) required before the end of the cooking period are set manually on dials 1 64 and 1 65 these settings being processed by an arithmetic unit to produce the time delay before the start of the cooking cycle (DA) and the time delay to the end of the cooking cycle (D) both being in binary digital form compatible with the output of the second stage 1 62 of the timer.

The output from second stage 1 62 is taken to one channel of each of two magnitude comparators where the value of this six bit number (T) is compared with the "cook start" time and "cook end" time which have been set on dials 1 64 and 1 65. If T is greater than or equals D-C then a signal "1' is sent to the input Tc of the logic unit 160, otherwise the signal is "O". The second channel compares the time T from the cooker with the "finish cook" time D and if T is greater than or equals D it sends a signal "1" to input TE of logic unit 160, otherwise the signal sent is "O".

The start-up unit 1 63 (shown in greater detail in Figure 29) receives a pulse from the timer every 20 seconds and a signal from the logic unit indicating whether flame is sensed or not (FS). If flame is not sensed at the start of a 20 second period between pulses then the output (SU) is held at "1" for the full duration of the 20 seconds. If flame is sensed at the start of the 20 second period the output, SU is set to "0" until, at any time, the flame is no longer sensed, in which case it returns to "1".

A latch consisting of NOR1 +NOR2 is set by the flame sense signal FS, which is "1" when there is no flame (i.e. output SU is 1). This latch is reset through gate AND 1 when all digits 7, 8, 9, 10 of timer 161 are "1". This occurs for approximately one second every 20 seconds, and during the one second pulse output SU is "O". If flame has been established during the 20 second period then FS is "0" and the latch will remain reset and the output will remain latched with SU being "0". If flame is lost at any time then FS is "1" and the latch will set to output SU as "1". It will remain latched at SU as "1" for 20 seconds after the last one second pulse from AND 1 to NOR2 and the cycle will be repeated until ignition is established and the FFD does not drop out.

If a "ho!d warm" period is required at the end of a cooking cycle then switch 1 32 is closed, thus sending a signal "1" to input H of the logic unit.

If switch 1 68 is closed so that a signal "1" is sent to the input MA of the logic unit 1 60, the other controls are over-ridden and, on closing switch SW the oven lights immediately; the oven is then under control of the thermostat until turned off. V, CLK and 1G are always "1". If, while the "manual" switch 1 68 is closed, the "hold-warm" switch 132 is also closed then the burner will be turned to the lowest rate and kept there.

The switch 1 68 may conveniently be incorporated in the setting controls 1 64 and 1 65 by providing a special setting position for "manual" control.

In a similar way to previous embodiments the logic unit 1 60, shown in greater detail in Figure 30, inputs MA, SU, S, H, FS, Tc TE and CL and uses logic to control the outputs CLK, V, 1G and L.

For manual operations, the "Manual" switch 1 68 is closed and the "hold-warm" switch 132 is opened. The thermostat is set to the required temperature. Switch SW is closed and the following are energised:-- igniter 117, power pack 118, logic unit 160, timer stages 161 and 162, start-up unit 163 and thermostat 119. The manual input MA to the logic unit makes it ignore the signals Tc and TE and start up the oven at once. This activates the start-up unit 1 63 which ensures that the burner is on "low" for at least 20 seconds after a "no flame" situation has been sensed.

When flame is established SU is "0" and the burner continues under the influence of the thermostat until either the thermostat is turned off, when the burner is extinguished, or the "holdwarm" switch 132 is closed, when the burner is turned to "low flame" and kept there.

For automatic operation "Manual" switch 1 68 is opened and "hold-warm" switch 132 closed if the oven is required to remain heated at low rate at the end of the cooking period. Otherwise switch 132 is opened.

Dials 1 64 and 1 65 are suitably set and the thermostat is set to the oven temperature required.

This closes switch SW and energises igniter 117, power pack 118, logic unit 160, timer stages 161 and 162, start-up unit 1 63 and thermostat 11 9. However as MA is not energised the logic unit sets outputs V, 1G and L to "0" and the cooker is in the "wait" period.

The timer runs until its output reaches the "start cook" value output by the setting unit. At this time Tc is "1" and the logic unit 160 energises V and 1G and controls L (low flame) from signals received from the start-up unit 1 63 and the thermostat 11 9. This situation remains for the "cook" period.

At the end of the cook period the timer output reaches the "finish cook" time and TE is "1". This signal then causes the oven burner to be turned off unless the "hold-warm" switch 1 32 is closed. If switch 132 is closed the logic unit energises L and 1G only thus leaving the burner on low rate indefinitely. It also stops the timer to prevent the remote risk of its re-cycling after 16 hours.

In all embodiments previously described there is a lower limit below which the temperature of the oven cannot be reduced while under the control of the thermostat. This limit is set by the lowest rate at which the burner flame remains stable. One way of overcoming this limitation is to exercise thermostatic control between an "on" and an "off" position. However if when the burner is on, it is operating at the full rate, there are the following disadvantaq.es:- a) igniting at full rate at every thermostat operation is undesirable; b) when the oven is set to a low average temperature, occasional burst of full oven flame may result in local hot spots so that the temperature of some part of the oven greatly exceeds the set temperature on the thermostat.

Thus, in a further embodiment whose circuit diagram is shown in Figure 31 thermostatic control is effected by a "high-low" device which changes its mode of operation to "low-off" when set to temperatures which are too low to be attained by the "high-low" mode. Moreover the oven temperature when in the "hold-warm" mode can be controlled in the same way.

In order to conserve energy and/or to assist pyrolytic cleaning of a gas oven the amount of thermal insulation round the outside of the oven may be considerably increased. The provision of the "low-off" thermostat operation will then be essential to achieve acceptable cook and "hold-warm" temperatures towards or at the bottom of the temperature range because the minimum stable rate of the burner, if burning continuously, may give unacceptably high temperatures.

An advantage of this embodiment is that the re-ignition when switching from "off" to "low" is at low (ignition) rate and demands no special ignition routine.

The thermostat in Figure 31 has two additional outputs: SL=which indicates, when at "1", that the thermostat is in the "special low" (i.e. low-off) rate mode of operation.

TH=which acts as a thermostat output for the "hold-warm" condition. When TH is "1", it calls for low heat, when TH is "O" it turns the burner off.

The relay RLA 103 is now controlled by thyristor 145, through output V of the logic unit 1 70 as well as by switch 13.

The logic unit 1 70 is modified to have the following inputs and outputs: H: Hold warm control, is "1" if "holdowarm" is required after cooking cycle and is "0" if burner is to be turned off after cooking; H1: value of H after start of cooking cycle. Always is "0" before start of cooking cycle; SU: "start-up" signal. Remains at "1" for 20 seconds after last signal when no flame was sensed.

Thereafter becomes "0" for as long as flame is sensed; S: Thermostat output signal. When S is "0" the thermostat calls for more heat, when S is "1" the thermostat calls for less heat whether it is normal "high-low" mode or in "low-off" mode; TH: Thermostat output when in "hold-warm" condition TH being "1" means set to low heat, TH being "0" means extinguish the burner; SL: is "1" means "special low" mode of thermostat operation (i.e. "low-off"); SL1: Value of SL after the thermostat output S has been "1"; P: Power-on input. P is "1" when both switches SW and 13 are closed, P is "O" if either switch SW or 13 is opened.

V: Drives the thyristor 145. When V is "1" the thyristor is on and the relay RLA 103 is energised if switches SW and 13 are closed; IG When 1G is "1" the ignition is enabled; L: When L is "1" the solenoid is energised and gas is on low rate for ignition or thermostat operation.

In a typical cooking cycle the thermostat is set to the "super low" position. After start up the oven burner runs at full rate until the thermostat is satisfied. At this point the burner is turned off completely and thereafter cycles between the low rate and off under the control of the thermostat.

In logic circuit 1 70 shown in Figure 32, IG is "1" if either V or L is "1", i.e. ignition is always enabled if gas be fed to the burner, also V is "1" through AND 3 if P is "1" and SL' is "0", i.e. full rate is enabled only during a cook cycle and if the burner is not under control of a "super-low" thermostat setting.

To set L first H1 is set by latch 1 to the value of the H after P is "1" as in previous embodiments, and similarly SL1 is set by latch 2 to the value of SL after S is "1".

For start-up conditions P is "1", SU is "1" then L is "1" via AND9 and ANDS. When start up is complete SU is "0" and L is "0" allowing the burner to go at full rate.

For "hold-warm" conditions P is "0", H' is "1". The value of H1 is therefore signalled to AND4 via AND2 and L comes under direct control of the thermostat TH, i.e. L is "1" when TH is "1" and L is "O" whenTH is "O".

Other signals are prevented from reaching L by applying signal "0" to an input of ANDS.

For thermostatic control with high setting P is "1", SL is "O". Since SL1 is "0" the thermostat signal S is "O" or "1" is passed through AND6 in the "gear change" and then through ANDS to L. Other signals to L are blocked at AND2 by the P input. Therefore L comes under direct control from S, i.e.

when S is "1", L is "1', when S is "0", L is "O".

Thermostatic control with low setting can only operate after the oven is up to temperature, i.e.

after S is "1" for the first time. Then latch 2 sets 5L1 to be "1" if SL is "1".

This switches off AND3 and sets V to "0", i.e. burner cannot go to full rate. When SL' is "1" then AND9 is also switched off, so that L cannot be controlled by SU. Since P is "1" then AND2 is switched off and L cannot be controlled by H1 and TH.

When SL1 is "1" the "gear change" passes S via AND7 and ANDS to L. L is therefore under control from the thermostat S, but inverted from the high setting, i.e.

Call for heat: S is "0" L is "1" burner on low No heat: S is "1" Liy"0" burner off.

As shown in Figure 33 the output of the oven temperature sensing thermocouple is corrected for cold junction temperature by a resistance network R1, R2 containing a thermistor RT. This output is amplified by operational amplifier Al and fed to two operational amplifiers A2 and A4.

Amplifier A2 is a comparator which compares the amplified output with a voltage set on potentiometer P2 by the user, a voltage which corresponds with the desired oven temperature. The output of amplifier A2 will therefore be "1" when oven temperature is higher than the set temperature and "0" when it is lower than the set temperature. This is the thermostat output S.

Amplifier A4 compares the amplified temperature signal with the voltage set on a potentiometer P3 and this voltage corresponds with the low temperature at which food will be "held-warm". Due to reversal of inputs compared with amplifier A2, when the oven temperature is lower than the set temperature the output of amplifier A4 is "1", when it is higher than the set temperature the output is "O". This is the "hold" thermostat output TH.

The amplifier A3 compares the setting of potentiometer P2 with that of P 1. P 1 is set to a voltage which corresponds with the oven temperature which would be attained if the burner were left continuously on "low" and thus represents the point where the mode of operation of the thermostat must change from "high-low" to "low-off".

When P2 is set to a higher voltage than P 1 then the output of A3 is "O". When P2 is set lower than P 1 then the output of A3 is "1". This output is the signal to determine the thermostat mode which we have called SL.

While certain embodiments as presently described (for example those described with reference to Figures 1 to 7, 9 and 11 to 13), do not include the feature of initiating a new ignition sequence should there be no flame detected at the burner, each embodiment can readily be modified such that it incorporates that feature. For example, in the embodiment described with reference to Figure 3, suitable modification would involve adding a device, preferably controlled electrically or electronically, to activate the igniter 1 3 when there is a drop in the electrical signal generated by thermocouple 6 indicating the absence of a flame. Likewise, appropriate modifications to the other embodiments mentioned above would be evident to a man skilled in the art.

Claims (10)

Claims

1. A cooking control comprising a gas supply control and a flame failure device, the gas supply control comprising a main valve, a supply port and an actuator which is capable of being moved to allow gas to flow to a burner through the supply port initially at a first rate, a temperature sensor capable of being responsive to the temperature of gas being burnt at the burner and arranged to cause the main valve to be held open while simultaneously allowing gas to be supplied to the burner at a second rate which is higher than the first rate, and circuit control means to automatically move the actuator to allow gas through the supply port for a time period long enough to allow the main valve to be held open by the temperature sensor to thereby supply gas to the burner at the second rate, the flame failure device comprising means to initiate a new ignition sequence in co-operation with the circuit control means if no flame is detected at the burner.

2. A control according to claim 1 wherein the circuit control means includes a thermostat switch which, in use, overrides the timer or switch controlled by the second sensor, when a cooking space or vessel has reached a certain temperature measured by the thermostat, such that the actuator is moved to cut down the gas supply to the burner to a rate lying below said second rate and which may be equal to said first rate.

3. A control according to claims 1 to 2, wherein the circuit control means includes a logic device to provide logic control of the actuator, the timer or the ignition of the gas, or any combination of these.

4. A control according to any one of the preceding claims, comprising one or more relay switches to de-energise the electro-magnet and cut off the flow of gas.

5. A control according to any one of the preceding claims comprising means to initiate a start up sequence which latches any signals indicating the absence of a flame for a time period sufficient to allow ignition.

6. A control according to any one of the preceding claims, wherein the function of determining the presence or absence of a flame is effected by a sensing electrode of an igniter.

7. A control according to any one of the preceding claims comprising an internal electronic timer.

8. A control substantially as hereinbefore described with reference to and as illustrated in Figure 8, or in Figures 10 to 13, or in Figures 10 to 18, orion Figures 19 to 21, or in Figures 22 to 26, or in Figures 27 to 30, or in Figures 31 to 33 of the accompanying drawings.

9. A cooking control substantially as described with reference to and as illustrated in any one or more of Figures 1 to 7, 9, 11 to 13, when modified such that, in use, a new ignition sequence is initiated if no flame is detected at the burner.

10. Cooking apparatus including at least one cooking control as claimed in any one of claims 1 to 9.