GB1588802A

GB1588802A - Flame monitor circuit arrangements

Info

Publication number: GB1588802A
Application number: GB3420/78A
Authority: GB
Original assignee: Landis and Gyr AG; LGZ Landis and Gyr Zug AG
Current assignee: Siemens Building Technologies AG; Landis and Gyr AG
Priority date: 1977-02-02
Filing date: 1978-01-27
Publication date: 1981-04-29
Also published as: DE2707120A1; DE2707120C3; DE2707120B2; FR2379769A1; CH604086A5

Description

(54) FLAME MONITOR CIRCUIT ARRANGEMENTS (71) We, LGZ LANDIS & GYR ZUG AG, a body corporate organised and existing under the laws of Switzerland, of CH-6301 Zug, Switzerland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following state ment:- This invention relates to flame monitor circuit arrangements.

Photocells which respond to ultra-violet light, hereinafter called UV-cells, are frequently used for monitoring large permanently-operating oil or gas furnaces.

Their main advantage is that they only react to the ultra-violet light produced by the flame and not to the radiation emitted by red hot firebricks. However, such UV-cells undergo a slow change in their characteristics over a period of time and can therefore reach a state in which they are no longer able to determine the extinction of a flame, that is they conduct even in the absence of UVradiation.

According to the present invention there is provided a flame monitor circuit arrangement comprising: a flame sensor including a circuit having supply terminals for connection to an a.c.

source and containing a radiation-sensitive cell arranged in use to be exposed to radiation from a flame, and a first diode connected so that said circuit supplies half-wave pulses when said cell is exposed to said flame; screen means for cyclically interrupting said radiation from falling on said cell, the light/dark periods of said cyclic interruption being controlled by the frequency of said source; means operative in dependence on the number of said half-wave pulses developed in said circuit in each said light cycle and which produces a predetermined oscillatory signal in synchronism with the cycle of said screen means; an amplifier means to which said predetermined oscillatory signal is supplied and which derives a reference signal and power supply from said source; and a relay to be energised by the output of said amplifier means and operative to control said flame; said amplifier means being operative to energise said relay only when supplied with said predetermined oscillatory signal.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a diagrammatic view of a circuit arrangement according to the invention; Figure 2 shows a first amplifier; Figure 3 shows a further amplifier; Figure 4 shows a circuit using sequential logic; and Figure 5 shows a circuit using a microcomputer.

In Figure 1, there is shown a flame 1 of a burner 2, the fuel supply to which is controlled by a solenoid valve 3. The radiationsensitive cell of a flame sensor 4 is a UV-cell 5. However, it would also be possible to use some other suitable radiation, and in particular light-sensitive element, such as, for example, a photoelectric cell or a photoconductive cell. The UV-cell 5 is disposed in a common housing 8 with a rotary shutter 7 driven by a synchronous motor 6, the shutter 7 periodically interrupting the radial tion to the UV-cell 5. The shutter 7 thus serves as a screen for producing a light/dark cycle. The UV-cell 5 is connected in series with a diode 9 and a resistor 10 across an a.c. voltage source 11 which also supplies the synchronous motor 6.A voltage derived from across the resistor 10 forms the input to a monostable trigger circuit 12, to which are coupled an integrator 13 for determining the presence of the maximum or minimum number of half-wave pulses necessary for the production to occur of an oscillatory signal 21 (Figure 2), and a threshold switch 14 driving an amplifier 17.

Trigger circuit 12, integrator 13 and threshold switch 14 are supplied by a common d.c. voltage source 15, which in turn derives power from the a.c. source 11. The output of threshold switch 14 after amplification by the amplifier 17 actuates a flame-control relay 16. By means of the contact 18, the relay 16 controls the solenoid valve 3 and can also carry out other operating functions not mentioned here.

The supply voltage of the amplifier 17, which simultaneously serves as a reference voltage is derived from the a.c. source 11 by way of a diode 19.

A fuse 40 is located in a common return line 27 for all the elements connected to the a.c. source 11.

A first form of amplifier 17 is shown in Figure 2. It comprises two transistors 22 and 23 switched alternately by the oscillatory signal 21 supplied to the input 20, the emittercollector paths of the transistors 22 and 23 being supplied with a half-wave rectified supply voltage which simultaneously serves as the reference voltage. The potential of the base 24 of the transistor 22 is controllable by means of a voltage divider formed by two resistors 25 and 26 connected between the input 20 and the line 27. The collector 28 of the transistor 22 is connected to the supply voltage via a collector resistor 29 and to the base 30 of the second transistor 23, whilst the emitter 39 is connected to the line 27. The emitter-collector path of the second transistor 23 controls a charging circuit to a capacitor 32 via a first winding 31 of the relay 16.To this end, collector 33 of the second transistor 23 is directly connected to the supply voltage, whilst a further diode 35 is provided between its emitter 34 and one terminal of the first winding 31. A second terminal of the capacitor 32 is connected to the line 27. A second winding 36 is also provided which acts on relay 16 in the same direction as the charging current when discharging the capacitor 32.

The winding 36 is connected to the common point of the first winding 31 and capacitor 32, and via two series-connected diodes 37 and 38 to the collector 28 of the first transistor 22 (and to the base 30 of the second transistor 23), so that the discharge current for the capacitor 32 can pass via the emitter-collector path of the first transistor 22.

The common points between the two diodes 37 and 38 and between the emitter 34 of the second transistor 23 and the diode 34 are interconnected.

The alternative form of amplifier 17 shown in Figure 3 has two transistors 41 and 42 which vary their constantly opposite switching states in synchronism with the oscillatory signal 21. The base 43 of the first transistor 41 is coupled via an input resistor 44 to receive the oscillatory signal 21. A series path, connected between the line which supplies the half-wave rectified supply voltage and the line 27, comprises the emittercollector path of the second transistor 42, a charging capacitor 47 and a first diode 48 leading to the line 27. Via the line 27, the series path forms a charging current path for charging the capacitor 47 in one half cycle of the oscillatory signal 21. The base 49 of the second transistor 42 is controlled by the first transistor 41, and is connected to the collector 50.A resistor 57 is connected between the collector 50 and the line which supplies the half-wave rectified supply voltage.

In order to form a discharge circuit in the other half cycle of the oscillatory signal 21, a further series path is provided based on the capacitor 47, whereby a second diode 51 is connected in anti-parallel with the baseemitter path of the second transistor 42.

This is followed by the emitter-collector path of the first transistor 41. The emitter 52 is connected to the line 27 and this forms the continuation of the series path to relay 54, and a second capacitor 55 connected in parallel with the relay 54. The series path returns to the capacitor 47 by way of a third diode 56.

The block circuit diagrams of Figures 4 and 5, include the a.c. source 11 with the series-connected diode 9, which together provide the supply voltage for the flame sensor 4. In the example of Figure 4 first inputs 58 and 59 of an EXCLUSIVE-OR gate 60 and an AND gate 61 respectively are connected to a voltage divider comprising two resistors Rl and R2 of the circuit containing the series connection with the UVcell 5. Second inputs 62 and 63 of gates 60 and 61 respectively are connected to a voltage divider comprising two further resistors R3 and R4 supplied by the half-wave rectified supply voltge and providing the reference voltage.

The two gates 60 and 61 belong to a sequential logic comprising a reversible counter 64 and two inputs 66 and 65 for counting up and down respectively, and a digital switch 67 which on reaching predetermined upper and lower pulse counts changes condition. The output of the EX CLUSIVE-OR gate 60 feeds the down input 65 and the output of the AND gate 61 the up input 66 of the counter 64. A return line 68 between the switch 67 and the counter 64 is used so that switching over the switch 67 causes the simultaneous stopping of the counting process in the existing direction.

Due to its switching over, the switch 67 produces square wave pulses as oscillatory signal 21, which are evaluated by the seriesconnected amplifier 17 and operate the relay 16 or 54.

For the continuous determination of the number of half-wave pulses occuring during the light or dark phases, in the example of Figure 5 a micro-computer 69 is used which supplies an alternating signal 21 to the following amplifier 17 within predetermined limits for the permitted number of pulses.

As in the example of Figure 4 for the inputs of the gates 60 and 61, two corresponding inputs 70 and 71 are provided for the microcomputer 69 whereof one input 71 scans as the reference voltage the half-wave pulses occuring in the circuit of the UV-cell 5 and the other input 70 scans the half-wave pulses produced by the half-wave rectified supply voltage. As described with reference to Figure 4, inputs 70 and 71 are connected for this purpose in the same way.

The arrangement described functions in the following manner: When a flame is present and with shutter 7 rotating, a light/dark cycle of for example 3 Hz and a light/dark ratio of for example 1 to 1 are obtained at UV-cell 5. These values remain constant due to the use of the synchronous motor 6 for driving the diaphragm 7.

However, an asynchronous motor could also provide usable values, but account would then have to be taken of increased voltage and temperature sensitivity.

In place of a rotary shutter 7 for obtaining a periodic screening of the UV-cell 5, it would also be possible to use other means, for example a liquid crystal cell disposed in front of the UV-cell 5.

In the case of a correctly functioning UVcell 5 and a 50 Hz mains frequency, the UVcell 5 conducts approximately 7 to 8 times during each light phase, so that a group of 7 to 8 half-wave pulses are developed followed by a similar period without pulses.

In the example of Figure 1, the voltage pulses obtained via the resistor 10 are differentiated by the monostable trigger circuit 12, converted into pulses of clearly defined pulse duration and fed to the integrator 13. Without pulses, the integrator 13 has its full voltage at its output. Each incoming pulse is integrated according to height and duration by the integrator 13 and reduces the output voltage of the integrator 13. The integrator 13 therefore also acts as a signal reversal stage. In the absence of pulses the output voltage rises again. The drop in the integrator voltage resulting from a pulse is greater than the rise in voltage in the absence of a pulse. With pulses coming in at regular intervals the integrator 13 attains its minimum outout voltage (full modulation) after a predetermined constant number of pulses. Further pulses maintain this output voltage.In the absence of pulses the output voltage rises again. By dimensioning the gradients of the integrator appropriately the monitoring quality can be selected. As transient conduction of the UV-cell 5 when there is no light present and transient nonconduction when there is light present must not have a prejudicial effect, full driving of the integrator 13 with four successive pulses has proved satisfactory for the frequencies indicated above. The output voltage of the integrator 13 acts on the threshold switch 14, the first threshold point of which is located somewhat above the minimum value and the second threshold point of which is located somewhat below the maximum value of the integrator output voltage.When a flame is present and the monitoring system is functioning correctly, the oscillatory signal 21 is formed at the threshold switch output and acts on the following amplifier 17, so that when the UV-cell 5 is screened, a signal voltage is present, and when the UV-cell is illuminated it is not.

The monostable trigger circuit 12 used as a differentiating element and pulse shaper, and the threshold switch 14 can be omitted, but the use of these elements in the described manner means that the relay response values are less dependent on the intensity of the flame radiation and mains voltage fluctuations.

The amplifier of Figure 2 functions as follows: When the voltage of the oscillatory signal 21 is zero, the first transistor 22 is nonconductive and the second transistor 23 is conductive. The half-wave pulses of the amplifier supply voltage charge the capacitor 32 via the diode 35 and the first winding 31 of the relay 16. The charging current energises the relay 16. The direct connection between the two diodes 35 and 37 retards the deenergization of the relay 16, so that it does not flutter due to the half-wave rectified supply voltage during the time in which transistor 22 is rendered non-conductive.

When the voltage of the oscillatory signal 21 is positive, the first transistor 22 is conductive and the second transistor 23 is nonconductive. A discharge circuit is formed for the capacitor 32 via the second winding 36 of the relay 16, the current direction of which further ensures energisation of the relay 16, via the diodes 37 and 38, the collectoremitter path of the first transistor 22 and the line 27 back to the capacitor 32.

The amplifier of Figure 3 operates in the following manner: When the voltage of the square-wave oscillatory signal 21 is zero, the first transistor 41 is non-conductive and the second transistor 42 is conductive. By means of the half-wave rectified supply voltage and via the collector-emitter path of the second transistor 42, as well as via the capacitor 47 and the first diode 48 to the line 27, a charging current for the capacitor 47 is formed which charges to the positive peak value of the supply voltage whilst the third diode 56 prevents a flow of current to the relay 54.

When the voltage of the oscillatory signal 21 is positive, the first transistor 41 is rendered conductive and the second transistor 42 nonconductive. The capacitor 47 transmits part of the charge thereon to the capacitor 55, so that the relay 54 is then energised over the following path: the second diode 51, the collector 50 and the emitter 52 of the first transistor 41, the line 27, through the parallel circuit of the capacitor 55 and the relay 54, and back through the third diode 56. The charge of capacitor 55 is now sufficient to maintain the relay 54 energised during the next zero voltage portion of the oscillatory signal 21.

As will be seen, the amplifiers 17 of Figures 2 and 3 only react to a permanently varying oscillatory signal and specifically within the limits of the oscillatory signal switch-on relationship given by the charging and discharging circuits. This relationship is given by the quotient of the on period and the sum of the on and off periods amounting to approximately 0.5. However, as was stated hereinbefore transient non-conductions of the UV-cell 5 are permitted; they can bring about a slight displacement of the switch-on relationship but must not lead to dc-energisation of the relay 16 or 54.

In addition, both amplifiers 17 are con structed in such a way that a d.c. signal or a signal at input 20 of the amplifier 17 which varies with the mains frequency bring about de-energisation of the relay 16 or 54 due to the fact that a half-wave rectified voltage is used for supplying the amplifier 17. The half-wave pulses of this supply voltage which serve as a refercnce voltage have the same polarity as the oscillatory signal 21 at input 20 of the amplifier 17 varying between approximately zero and a maximum value.

As in the dark phase, that is when the UVcell 5 is screened by the shutter 7, the oscil latory signal 21 is present, and as the second transistor 23 or 42 of the amplifier 17 must be rendered non-conductive in the dark phase, the positive part of an oscillatory signal 21 appearing for any reason with the mains frequency at amplifier input 20 will always render transistor 23 or 42 nonconductive if the supply voltage for the amplifier 17 is present. The relay 16 or 54 does not then receive the voltage required for energisation. This case can for example occur if due to a defect, the integrator 13 no longer integrates and instead merely functions as a switch.

The arrangments according to Figures 4 and 5 function in the following manner: In both cases, the signals produced by the UVcell 5 and the shutter 7 in flame sensor 4 are derived from voltage divider Rl, R2 whilst at the second voltage divider R3, R4 constant half-wave pulses are derived for comparison purposes. In the example of Figure 4 with the sequential logic, the first pulse entering the voltage divider of the sensor circuit starts a counting program. The counter 64 only counts upwards when pulses also appear at the second voltage divider R3, R4. After five successive pulses, the digital switch 67 changes its state and stops the counter 64 via the return line 68. The counter 64 waits until a change occurs at the input, which happens with the first missing pulse in the dark phase.The counter 64 counts downwards and the switch 67 again changes its state and again stops the counter 64, so that the alternating signal 21 is formed at the amplifier input. The example of Figure 5 functions in exactly the same way as that of Figure 4, but in this case the micro-computer 69 carries out the counting process.

As a result of the arrangements described, all the circuit components including the amplifier are monitored in such a way that any short-circuit or interruption of a component either leads to the blowing of fuse 40 (Figure 1) or to de-energisation of the relay 16 or 54, no matter whether a flame is present or not. This leads to the necessary intrinsic safety.

WHAT WE CLAIM IS: 1. A flame monitor circuit arrangement comprising: a flame sensor including a circuit having supply terminals for connection to an a.c.

2. An arrangement according to claim 1 wherein said screen means comprises a synchronous motor and a rotary shutter driven by said motor.

3. An arrangement according to claim 1 or claim 2 wherein said means for producing said predetermined oscillatory signal includes an integrator operative to prevent production of said predetermined oscillatory signal unless the number of said half-wave

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

of the charge thereon to the capacitor 55, so that the relay 54 is then energised over the following path: the second diode 51, the collector 50 and the emitter 52 of the first transistor 41, the line 27, through the parallel circuit of the capacitor 55 and the relay 54, and back through the third diode 56. The charge of capacitor 55 is now sufficient to maintain the relay 54 energised during the next zero voltage portion of the oscillatory signal 21.

As will be seen, the amplifiers 17 of Figures 2 and 3 only react to a permanently varying oscillatory signal and specifically within the limits of the oscillatory signal switch-on relationship given by the charging and discharging circuits. This relationship is given by the quotient of the on period and the sum of the on and off periods amounting to approximately 0.5. However, as was stated hereinbefore transient non-conductions of the UV-cell 5 are permitted; they can bring about a slight displacement of the switch-on relationship but must not lead to dc-energisation of the relay 16 or 54.

In addition, both amplifiers 17 are con structed in such a way that a d.c. signal or a signal at input 20 of the amplifier 17 which varies with the mains frequency bring about de-energisation of the relay 16 or 54 due to the fact that a half-wave rectified voltage is used for supplying the amplifier 17. The half-wave pulses of this supply voltage which serve as a refercnce voltage have the same polarity as the oscillatory signal 21 at input

20 of the amplifier 17 varying between approximately zero and a maximum value.

As in the dark phase, that is when the UVcell 5 is screened by the shutter 7, the oscil latory signal 21 is present, and as the second transistor 23 or 42 of the amplifier 17 must be rendered non-conductive in the dark phase, the positive part of an oscillatory signal 21 appearing for any reason with the mains frequency at amplifier input 20 will always render transistor 23 or 42 nonconductive if the supply voltage for the amplifier 17 is present. The relay 16 or 54 does not then receive the voltage required for energisation. This case can for example occur if due to a defect, the integrator 13 no longer integrates and instead merely functions as a switch.

The arrangments according to Figures 4 and 5 function in the following manner: In both cases, the signals produced by the UVcell 5 and the shutter 7 in flame sensor 4 are derived from voltage divider Rl, R2 whilst at the second voltage divider R3, R4 constant half-wave pulses are derived for comparison purposes. In the example of Figure 4 with the sequential logic, the first pulse entering the voltage divider of the sensor circuit starts a counting program. The counter 64 only counts upwards when pulses also appear at the second voltage divider R3, R4. After five successive pulses, the digital switch 67 changes its state and stops the counter 64 via the return line 68. The counter 64 waits until a change occurs at the input, which happens with the first missing pulse in the dark phase.The counter 64 counts downwards and the switch 67 again changes its state and again stops the counter 64, so that the alternating signal 21 is formed at the amplifier input. The example of Figure 5 functions in exactly the same way as that of Figure 4, but in this case the micro-computer 69 carries out the counting process.

As a result of the arrangements described, all the circuit components including the amplifier are monitored in such a way that any short-circuit or interruption of a component either leads to the blowing of fuse 40 (Figure 1) or to de-energisation of the relay 16 or 54, no matter whether a flame is present or not. This leads to the necessary intrinsic safety.

WHAT WE CLAIM IS: 1. A flame monitor circuit arrangement comprising: a flame sensor including a circuit having supply terminals for connection to an a.c.

source and containing a radiation-sensitive cell arranged in use to be exposed to radiation from a flame, and a first diode connected so that said circuit supplies half-wave pulses when said cell is exposed to said flame; screen means for cyclically interrupting said radiation from falling on said cell, the light/dark periods of said cyclic interruption being controlled by the frequency of said source; means operative in dependence on the number of said half-wave pulses developed in said circuit in each said light cycle and which produces a predetermined oscillatory signal in synchronism with the cycle of said screen means; an amplifier means to which said predetermined oscillatory signal is supplied and which derives a reference signal and power supply from said source; and a relay to be energised by the output of said amplifier means and operative to control said flame; said amplifier means being operative to energise said relay only when supplied with said predetermined oscillatory signal.
2. An arrangement according to claim 1 wherein said screen means comprises a synchronous motor and a rotary shutter driven by said motor.
3. An arrangement according to claim 1 or claim 2 wherein said means for producing said predetermined oscillatory signal includes an integrator operative to prevent production of said predetermined oscillatory signal unless the number of said half-wave

pulses developed in said circuit in each said light cycle falls within a predetermined range.
4. An arrangement according to claim 1, claim 2 or claim 3 wherein said means for producing said predetermined oscillatory signal comprises a resistor connected in series with said cell, a monostable trigger circuit connected across said resistor, an integrator connected to the output of said monostable trigger circuit, and a threshold switch connected to the output of said integrator, said amplifier being connected to the output of said threshold switch.
5. An arrangement according to any one of the preceding claims wherein said amplifier means comprises first and second transistors connected so as to be switched alternately and oppositely in synchronism with said predetermined oscillatory signal, a half-wave rectified supply voltage source connected to the collector of said second transistor and by way of a collector resistor to the collector of said first transistor, the collector of said first transistor also being connected to the base of said second transistor, a voltage divider to which said oscillatory signal is supplied and from which said predetermined oscillatory signal is supplied to the base of said first transistor, a capacitor, a charging circuit for flow of charging current for said capacitor and controlled by the emittercollector path of said second transistor, said charging circuit being connected by way of a first winding of said relay to said capacitor, and a second winding of said relay connected to enable flow of discharge current for said capacitor with said discharge current flowing in the same direction as said charging current, one end of said second winding being connected to the common point of said first winding and said capacitor and the other end being connected via at least one further diode to the collector of said first transistor, whereby said discharging current can be controlled by the emitter-collector path of said first transistor.
6. An arrangement according to any one of claims 1 to 4 wherein said amplifier means comprises first and second transistors connected so as to be switched alternately and oppositely in synchronism with said pre determined oscillatory signal, a half-wave rectified supply voltage source connected to the collector of said second transistor and by way of a collector resistor to the collector of said first transistor, the collector of said first transistor also being connected to the base of said second transistor, means to couple said predetermined oscillatory signal to the base of said first transistor, a first capacitor, a second diode connected in series with said first capacitor, a charging path for said first capacitor comprising the emittercollector path of said second transistor, and a discharging path for said first transistor comprising the emitter-collector path of said first transistor, a third diode connected in anti-parallel with the base-emitter path of said second transistor, a fourth diode, and a second capacitor connected in parallel with a winding of said relay.
7. An arrangement according to claim 5 or claim 6 wherein said power supply for said amplifier means is derived from said source by way of a further diode thereby to produce half-wave pulses to form said reference signal, and the voltage of said predetermined oscillatory signal supplied to said amplifier means varies between zero and a maximum with the same polarity as said half-wave pulses derived from said source.
8. An arrangement according to claim 1 or claim 2 wherein said means for producing said predetermined oscillatory signal comprises a reversible counter having an up input, a down input and an output, a digital switch connected to said output of said counter and which changes its condition when said counter reaches predetermined upper and lower pulse counts whereby said digital switch produces said predetermined oscil latory signal and stops the counting process ot said counter in the existing direction, an EXCLUSIVE-OR gate having two inputs and an output, the output being connected to said down input, a first voltage divider connected in series with said cell and arranged to supply inputs to one input of each of said gates, and a second voltage divider connected to derive a half-wave rectified supply from said source for supply to the other inputs of each of said gates.
9. An arrangement according to claim 1 or claim 2 wherein said means for producing said predetermined oscillatory signal comprises a micro-computer having two inputs and an output, a first voltage divider connected in series with said cell and arranged to supply an input to one input of said micro-computer, and a second voltage divider connected to derive a half-wave rectified supply from said source for supply to the other input of said micro-computer, said predetermined oscillatory signal being supplied at said output of said micro-computer.
10. An arrangement according to any one of the preceding claims wherein said cell is a UV-cell.
11. A flame monitor circuit arrangement substantially as hereinbefore described with reference to the accompanying drawings.