GB2367172A

GB2367172A - Flame detection apparatus

Info

Publication number: GB2367172A
Application number: GB0010024A
Authority: GB
Inventors: George Morgan
Original assignee: PEKTRON GROUP Ltd
Current assignee: PEKTRON GROUP Ltd
Priority date: 2000-04-26
Filing date: 2000-04-26
Publication date: 2002-03-27
Anticipated expiration: 2020-04-26
Also published as: GB2367172B; GB0010024D0

Abstract

A monitoring arrangement 22 applies an alternating current across a gap 18 to detect the presence or absence of a flame within the gap. This is detected by a detector circuit 28 which monitors and times zero crossing of the voltage at the base of transistor 36, to detect symmetry or asymmetry of the waveform. A distorted, asymmetrical waveform indicates the presence of a flame. A symmetrical waveform indicates the absence of a flame.

Description

Detection A Detection Apparatus and a Method of Detection The present invention relates to apparatus and methods for detection of flames, such as, for example, flame detection within a gas burner of an appliance.

An important safety feature of the control system for gas-fired appliances is to be able to detect the presence or absence of a gas flame at the gas burner, so that the gas valve supplying the burner can be closed in the event that the flame has been extinguished by virtue of a fault or for another reason. This removes a possible source of gas leaks.

The present invention provides a method of flame detection in which an alternating current is applied across a gap, the alternating current having a waveform which changes substantially without discontinuities and the resultant alternating voltage across the gap is monitored to detect if a flame is present in the gap, the alternating voltage being monitored to detect a change in the symmetry of the voltage waveform about a reference voltage.

Preferably the current waveform is symmetrical, the voltage waveform being monitored to detect asymmetry. The reference voltage is preferably 0 volts. Preferably the voltage is monitored to determine when the waveform crosses the reference voltage. The time between consecutive crossings of the reference voltage is preferably measured, asymmetry being detected by time measurements which are not constant. The time is preferably measured by reference to a timebase having a frequency which is high relative to the frequency of the alternating current. The time may be measured by counting cycles of the timebase.

Monitoring is preferably based on measurements taken over a plurality of cycles of the waveform. Monitoring may be based on average measurements taken over a plurality of cycles. Monitoring may be based on measurements taken over each of the plurality of cycles of the waveform, each measurement being assessed to produce a result indicating the presence or absence of a flame, and the results being combined by a majority logic arrangement to yield a combined result.

The alternating voltage is preferably converted to a square wave before being monitored. The alternating voltage may be applied to an analogue-todigital converter before being monitored.

The invention also provides flame detection apparatus comprising voltage monitoring means operable to monitor an alternating voltage appearing across a gap to which an alternating current is applied, during use, the alternating current having a waveform which changes substantially without discontinuities, the voltage being monitored to detect if a flame is present in the gap, the monitoring means being operable to detect a change in symmetry of the waveform of the alternating voltage, about a reference voltage.

Preferably the current waveform is symmetrical, the voltage waveform being monitored to detect asymmetry. Preferably the reference voltage is zero volts. Preferably the monitoring means is operable to determine when the waveform crosses the reference voltage. Preferably the monitoring means is operable to measure the time between consecutive crossings of the reference voltage, asymmetry being detected by time measurements which are not constant. The monitoring means preferably comprises a timebase having a frequency which is high relative to the frequency of the alternating current, the monitoring means being operable to measure time by reference to the timebase.

Preferably the monitoring means is operable to measure time by counting cycles of the timebase.

Preferably the monitoring means is operable to take measurements over a plurality of cycles of the waveform and to base the result of the monitoring on those measurements. The monitoring means may be operable to form average measurements taken over a plurality of cycles and to base the output of monitoring on the average measurement. Alternatively, the monitoring means may be operable to take a measurement over each of a plurality of cycles of the waveform, and to assess each measurement to produce a result indicating the presence or absence of a flame, the monitoring means comprising a majority logic means operable to yield a combined result in response to the results from each of the plurality of cycles.

The monitoring means may incorporate conversion means operable to convert the alternating voltage to a square wave before being monitored. The monitoring means preferably comprises an analogue to digital converter for conversion of the alternating voltage to digital form before being monitored.

Embodiments of the present invention will now be described in more details, by way of example only, and with reference to the accompanying drawings, in which: Fig. l is a highly schematic diagram of a gas burner modified to operate in accordance with the present invention: Fig. 2 is a circuit diagram showing an equivalent circuit for the gas burner arrangement, and a circuit of a flame detector according to the present invention: Fig. 3 shows waveforms present within the circuit of Fig. 2 under various conditions.

Fig. 1 shows a gas burner 10 for a gas-fired appliance (not shown). The burner 10 incorporates a gas nozzle 12 at which a flame 14 is supported when gas is supplied from the nozzle 12, and the gas has been ignited. The flame 14 may be a burner, or a pilot light used to ignite a main burner.

An electrode or probe 16 is located a short distance above the nozzle 12 to define a gap 18 between the nozzle 12 and probe 16. The flame 14 is present within the gap 18, when alight.

A voltage source 20 is connected between the probe 16 and the nozzle 12, i. e. across the gap 18 and is operable to create a spark across the gap 18, in order to ignite the flame 14. The voltage source 20 may be a piezoelectric device, for instance.

Thus, in the event that the flame 14 is not alight and is to be lit, gas will be supplied to the nozzle 12 and the source 20 will create a spark across the gap 18, to ignite the flame. Thereafter, the presence or absence of the flame 14 is monitored by apparatus which will now be described, so that the gas supply to the nozzle 12 can be closed in the event that the flame 14 has been extinguished by reason of fault or otherwise.

Flame detection is achieved by a monitoring arrangement 22 indicated very simply in Fig. 1. The arrangement 22 includes an alternating current source 24 which, in use, applies current to the nozzle 12 and probe 16, i. e. across the gap 18, through a capacitor 26 connected in series between the source 24 and the probe 16. For reasons which will be described, this results in a voltage across the gap 18, which is detected by a detector circuit 28, the output 30 of which indicates whether or not a flame 14 is present in the gap 18.

Fig. 2 illustrates the electrical arrangement in more detail. In Fig. 2, a series resistor 32 is illustrated between the capacitor 26 and the probe 16, representing resistance within the arrangement. Operation of the current source 24 applies current across the gap 18, having a waveform which changes smoothly, i. e. without discontinuities. This creates a voltage V across the gap 18, as will be explained. This voltage is applied to an analogue to digital converter 34 within the circuit 28, and in the form of a common emitter amplifier formed by a transistor 36 and a resistor 38. The voltage V is applied to the base of the transistor through a current limiting resistor 40. A reverse diode 42 is provided in parallel with the base-emitter junction of the transistor 36, to protect the junction against reverse voltages occurring during negativegoing excursions of the voltage V.

Output from the converter 34 is taken from the collector 44 of the transistor 36. The transistor 36 will either turn hard on or hard off as the voltage V oscillates, so that the output at 34 will be a square wave with transitions between logic high and logic low levels. The term"square wave"is used figuratively to refer to a waveform having logic high and logic low levels and substantially discontinuous, instantaneous transitions between them, but not to indicate that those transitions are necessarily regularly spaced in time.

Indeed, it is an important feature of the present invention that they may not be, as will now be described with reference to Fig. 3.

Fig. 3 shows a solid waveform 46 for the voltage V, in the event that no flame is present in the gap 18. It can be seen from Fig. 3 that the waveform 46 is symmetrical about the zero volts reference level, being a sinusoid following the sinusoidal form of the current source 24. The waveform 46 gives rise to an output at 44 which is shown at 48, being at the logic high level while the waveform 46 is in the positive-going part of the cycle, and at logic low level during the negative-going part of the waveform 46. The transition between these logic levels occurs at each zero crossing 50 of the waveform 46. This results in a square wave 48 which has transitions 52 which are equally spaced in time.

This position changes when a flame 14 is present in the gap 18. Strong ionisation present within the flame as a result of the combustion process causes the flame 14 to act in accordance with an equivalent circuit which may be represented as a large resistance and a"leaky"diode connected in a series across the gap 18, with the conducting direction of the diode being from the probe 16 to the nozzle 12. The term"leaky"is used to indicate a diode which does not complete surpress current flow when reverse-biased.

Accordingly, when a flame 14 is present in the gap 18, the voltage V adopts a distorted waveform 54 (shown in broken lines in Fig. 3) which is no longer symmetrical about the zero volts reference. In particular, it can be seen from Fig. 3 that the positive going excursions 56 are smaller in magnitude and shorter in duration than the negative going excursions 58. This is reflected in the square wave output 60 at 44. The square wave 60 still consists of high and low logic levels with transitions 62 therebetween, but the length of the peaks is now shorter than the length of the troughs, by virtue of the asymmetry of the waveform 54.

Consequently, the symmetry or asymmetry of the waveform 46,54, reflected in the square wave 50,60, can be used as an indicator of the presence or absence of a flame 14. This is achieved by means of a counter 64 which is provided with a reference frequency at 66 from a timebase 68. The counter 64 operates to determine the duration of the peaks and troughs of the waveforms 46,54, by counting cycles of the reference frequency 66 which occur between adjacent transitions in the waveforms 46,54. If these counts are constant, the waveform is symmetrical, indicating that no flame 14 is present in the gap 18.

This is indicated at the counter output 30, allowing appropriate action to be taken, particularly to close the gas supply valve to the nozzle 12, in the event that it is open.

In the event that counts between transitions do not remain constant, a flame 14 is indicated. An appropriate output will be provided at 30, allowing appropriate action to be taken, such as to allow the gas supply to the nozzle 12 to be maintained.

The counter 64 may operate in various ways to assess the asymmetry of the waveform 54. In a simple arrangement, the length of a peak and an adjacent trough will be measured and if a difference is detected, this will be used to indicate the presence of a flame. A threshold value may be used to represent a minimum difference before indicating a presence of a flame.

In a more sophisticated arrangement, average values for the duration of peaks and troughs could be calculated over a series of cycles and then compared to determine if the average values are the same (perhaps within a specified tolerance) or different. Taking averages in this way will help provide resilience against interference, such as mains hum.

In a further alternative, each peak and its adjacent trough are compared to produce a result indicating the presence or absence of a flame, and the results of a series of such comparisons are used as inputs to a majority logic detection arrangement so that, for instance, if most results indicate the presence of a flame, but one indicates the absence of a flame, the minority result will be ignored. This helps to ensure that rogue results arising from interference, voltage spikes or the like, can be ignored.

It will be apparent from an understanding from the explanation set out above that the apparatus exhibits a fail-safe property, in that if the gap becomes open circuit or short circuit, a no flame signal will be given at 70, thus resulting in the gas supply being shut off as a result of this fault.

Many variations and modifications of the apparatus described above can be made without departing from the scope of the present invention. In particular, many other detection circuits could be devised, operating according to various detection criteria. Detection could in principle be implemented by means of a microprocessor or similar device. The detection gap has been described above as the gap between a spark ignition probe and the burner nozzle, which is particularly convenient in minimising the number of additional components required to implement the invention. However, a probe separate from the ignition probe could be used, and the gap could be defined between two probes, rather than between a probe and the nozzle.

The shape of the waveforms has been shown as being substantially sinusoidal but other waveforms could be used. However, it is desirable for the current waveform to have a profile without discontinuities, i. e. to be other than a square wave, such as a sinusoid, saw tooth, triangular or other regular waveform. It has been found that these will create a change in the symmetry of the voltage waveform, but that the discontinuities of a square wave will prevent the time of zero crossings from changing.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

CLAIMS 1. A method of flame detection in which an alternating current is applied across a gap, the alternating current having a waveform which changes substantially without discontinuities and the resultant alternating voltage across the gap is monitored to detect if a flame is present in the gap, the alternating voltage being monitored to detect a change in the symmetry of the voltage waveform about a reference voltage.
2. A method according to claim 1, wherein the current waveform is symmetrical, the voltage waveform being monitored to detect asymmetry.
3. A method according to claim 1 or 2, wherein the reference voltage is 0 volts.
4. A method according to any preceding claim, wherein the voltage is monitored to determine when the waveform crosses the reference voltage.
5. A method according to claim 4, wherein the time between consecutive crossings of the reference voltage is measured, asymmetry being detected by time measurements which are not constant.
6. A method according to claim 5, wherein the time is measured by reference to a timebase having a frequency which is high relative to the frequency of the alternating current.
7. A method according to claim 5 or 6, wherein the time is measured by counting cycles of the timebase.
8. A method according to any preceding claim, wherein monitoring is based on measurements taken over a plurality of cycles of the waveform.
9. A method according to claim 8, wherein monitoring is based on average measurements taken over a plurality of cycles.
10. A method according to claim 8 or 9, wherein monitoring is based on measurements taken over each of the plurality of cycles of the waveform, each measurement being assessed to produce a result indicating the presence or absence of a flame, and the results being combined by a majority logic arrangement to yield a combined result.
11. A method according to any preceding claim, wherein the alternating voltage is converted to a square wave before being monitored.
12. A method according to any preceding claim, wherein the alternating voltage is applied to an analogue-to-digital converter before being monitored.
13. A method substantially as described above, with reference to the accompanying drawings.
14. Flame detection apparatus comprising voltage monitoring means operable to monitor an alternating voltage appearing across a gap to which an alternating current is applied, during use, the alternating current having a waveform which changes substantially without discontinuities, the voltage being monitored to detect if a flame is present in the gap, the monitoring means being operable to detect a change in symmetry of the waveform of the alternating voltage, about a reference voltage.
15. Apparatus according to claim 14, wherein the current waveform is symmetrical, the voltage waveform being monitored to detect asymmetry.
16. Apparatus according to claim 14 or 15, wherein the reference voltage is zero volts.
17. Apparatus according to any of claims 14 to 16, wherein the monitoring means is operable to determine when the waveform crosses the reference voltage.
18. Apparatus according to claim 17, wherein the monitoring means is operable to measure the time between consecutive crossings of the reference voltage, asymmetry being detected by time measurements which are not constant.
19. Apparatus according to claim 17 or 18, wherein the monitoring means comprises a timebase having a frequency which is high relative to the frequency of the alternating current, the monitoring means being operable to measure time by reference to the timebase.
20. Apparatus according to claim 17,18 or 19, wherein the monitoring means is operable to measure time by counting cycles of the timebase.
21. Apparatus according to any of claims 14 to 20, wherein the monitoring means is operable to take measurements over a plurality of cycles of the waveform and to base the result of the monitoring on those measurements.
22. Apparatus according to claim 21, wherein the monitoring means is operable to form average measurements taken over a plurality of cycles and to base the output of monitoring on the average measurement.
23. Apparatus according to claim 21 or 22, wherein the monitoring means are operable to take a measurement over each of a plurality of cycles of the waveform, and to assess each measurement to produce a result indicating the presence or absence of a flame, the monitoring means comprising a majority logic means operable to yield a combined result in response to the results from each of the plurality of cycles.
24. Apparatus according to any of claims 14 to 23, wherein the monitoring means incorporate conversion means operable to convert the alternating voltage to a square wave before being monitored.
25. Apparatus according to any of claims 14 to 24, wherein the monitoring means comprises an analogue to digital converter for conversion of the alternating voltage to digital form before being monitored.
26. Flame detection apparatus substantially as described above, with reference to the accompanying drawings.
27. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.