EP3059496B1

EP3059496B1 - Measuring arrangement for a gas burner, gas burner and method for operating the gas burner

Info

Publication number: EP3059496B1
Application number: EP16150776.9A
Authority: EP
Inventors: Gerwin Langius; Erwin Kupers
Original assignee: Honeywell Technologies SARL
Current assignee: Garrett Motion SARL
Priority date: 2015-02-23
Filing date: 2016-01-11
Publication date: 2018-10-10
Anticipated expiration: 2036-01-11
Also published as: EP3059496A1

Description

The present patent application relates to a measuring arrangement for a gas burner, to a gas burner and to a method for operating a gas burner.
EP 1 460 338 B1 discloses a measuring arrangement for a gas burner comprising a flame rod, a power supply device for applying a supply voltage to the flame rod, wherein the power supply device can be switched on and off thereby switching on and off the supply voltage to the flame rod. The supply voltage can also be called measuring voltage. The measuring arrangement further comprises a measuring device for measuring a flame ionization current at the flame rod with the aid of the supply voltage applied to the flame rod, wherein the flame ionization current can be measured with the supply voltage being switched on and with the supply voltage being switched off. A comparison device of the measuring arrangement compares an earth potential with a frame potential of the measuring arrangement, wherein the flame ionization current is determined when the frame potential corresponds to the earth potential.
EP 2 667 097 A1 discloses a method for operating a gas burner. During burner-on phases a mixing ratio of gas and air of a gas/air mixture can be calibrated to different gas qualities on basis of a signal provided by a flame rod positioned downstream of the mixing device within the burner chamber. The flame rod is used to measure the flame ionization current and to calibrate the gas/air mixture to different gas qualities on basis of the flame ionization current. The control of the gas/air mixture over the modulation range of the gas burner is independent from the flame ionization current.
EP 2 549 187 A2 discloses a method to control the defined gas/air mixture over the modulation range of the gas burner on basis of a signal provided by an ignition rod and on basis of a signal provided by separate flame rod. The flame rod is used to measure the flame ionization current. The ignition rod is used to measure a voltage resulting from Edison-Richardson effect which is also often called thermionic effect. Both, namely the flame ionization current provided by the flame rod and the thermionic voltage provided by the ignition rod are used to control the gas/air mixture and thereby the λ-value over the modulation range of the gas burner. EP 2 549 187 A2 requires to make use of two different rods for the measurement of the flame ionization current and of the thermionic voltage. The method of EP 2 549 187 A2 uses an absolute value of the thermionic voltage requiring a time constant of a few seconds to measure a stable absolute value of the thermionic voltage.
Document EP 2 549 187 A2 discloses a measuring arrangement according to the preamble of claim 1. Document US 5 971 745 A discloses a method according to the preamble of claim 12. Against this background a novel measuring arrangement for a gas burner according to claim 1, a novel gas burner according to claim 11 and a novel method for operating a gas burner according to claim 12 are provided.
The measuring arrangement for a gas burner according to the present application is defined in the claim 1. The measuring arrangement comprises an input definition circuit configured to switch the measuring arrangement between a relative low impedance measuring circuit and a relative high impedance measuring circuit, wherein the measuring arrangement is adapted to measure the flame ionization current at the flame rod when the measuring arrangement is switched by the input definition circuit to a relative low measuring impedance circuit; and wherein the measuring arrangement is adapted to measure a thermionic voltage at the flame rod when the measuring arrangement is switched by the input definition circuit to a relative high impedance measuring circuit. The invention provides the ability to measure the flame ionization current and the thermionic voltage making use of the same flame rod and the same measuring arrangement.
According to a preferred embodiment, the measuring arrangement comprises a first connection terminal, wherein the first connection terminal is connected to the microcontroller, and wherein the microcontroller switches the first connection terminal between a first terminal status and a second terminal status.
The measuring arrangement is switched to a relative low impedance measuring circuit so that the same is adapted to measure the flame ionization current at the flame rod when the first connection terminal is switched by the microcontroller to the first terminal status. The measuring arrangement is switched to a relative high impedance measuring circuit so that the same is adapted to measure the thermionic voltage at the flame rod when the first connection terminal is switched by the microcontroller to the second input terminal status. This design of the measuring arrangement is simple and provides a reliable solution that the flame ionization current and the thermionic voltage can both be measured at different impedance statuses of the measuring arrangement making use of the same flame rod.
Preferably, the measuring arrangement comprises a second power supply device and a ground potential, wherein the input definition circuit further comprises a second connection terminal through which the input definition circuit is connected to the power supply; a third connection terminal through which the input definition circuit is connected the ground potential; a fourth connection terminal through which the input definition circuit is connected to the flame rod. A first resistor of the measuring arrangement is connected between the first connection terminal and the fourth connection terminal. A second resistor of the measuring arrangement is connected between the second connection terminal and the fourth connection terminal. A third resistor of the measuring arrangement is connected between the third connection terminal and the fourth connection terminal. The second resistor and the third resistor have both a bigger electric resistance than the first resistor. This provides a simple and reliable solution to measure the flame ionization current and the thermionic voltage making use of the same flame rod.
Preferably, the microcontroller comprises an electronic switch, wherein the electronic switch is configured to by operated in such a way that when the electronic switch is closed the first connection terminal is connected to the second power supply device so that the same is switched to the first terminal status, and that when the electronic switch is opened the first connection terminal is disconnected from the second power supply device so that the same is switched to the second terminal status. This provides a simple and reliable solution to measure the flame ionization current and the thermionic voltage making use of the same flame rod.
The electronic switch may be provided by a transistor having a drain, a gate and a source, wherein the drain of the transistor and the second connection terminal of the input definition circuit are both connected to the second power supply device, wherein the source of the transistor is connected to the first connection terminal of the input definition circuit, and wherein the gate of the transistor is configured to by operated in such a way that when a first defined voltage is applied to the gate the first connection terminal is switched by the microcontroller to the first terminal status and that when a second defined voltage is applied to the gate the first connection terminal is switched by the microcontroller to the second terminal status.
The gas burner of the present application is defined in claim 11. The gas burner comprises the measuring arrangement according to the present invention.
The present application further relates to a method for operating a gas burner according to claim 12.
The method according to the present invention provides a calibration of the gas/air mixture based on at least two calibration steps, namely a coarse calibration step and fine calibration step.
During the coarse calibration step the flame rod is used to measure the flame ionization current and/or the thermionic voltage, wherein the gas/air mixture is made richer during the coarse calibration step by increasing the gas amount of the gas/air mixture relative to the air amount of the same until a flattening or a maximum in a dynamic behaviour of the flame ionization current and/or until a minimum in a dynamic behaviour of the thermionic voltage is detected. During the fine calibration step following the coarse calibration step the same flame rod is used to measure at least the thermionic voltage, wherein the gas/air mixture is made leaner during the fine calibration step after the flattening or the maximum in the dynamic behaviour of the flame ionization current is detected and/or after the minimum in the dynamic behaviour of the thermionic voltage is detected by decreasing the gas amount of the gas/air mixture relative to the air amount of the same until an increase in the dynamic behaviour of the thermionic voltage is detected. The gas/air mixture for which the increase of the thermionic voltage is detected corresponds to a gas/air mixture providing combustion with a λ-value equal 1.
Preferred developments of the invention are provided by the dependent claims and the description which follows. Exemplary embodiments are explained in more detail on the basis of the drawing, in which:

Figure 1: shows a schematic view of a gas burner;
Figure 2: shows a measuring arrangement for a gas burner according to the present invention; and
Figure 3: shows a diagram illustrating the method according to the present invention.

Figure 1 shows a schematic view of a gas burner appliance 10. The same comprises a gas burner chamber 11 with a gas burner surface 25 in which combustion of a defined gas/air mixture having a defined mixing ratio of gas and air takes place during burner-on phases of the gas burner. The combustion of the gas/air mixture results into flames 12 monitored by a flame rod 13.
The defined gas/air mixture is provided to the burner chamber 11 of the gas burner by mixing an air flow with a gas flow. A fan 14 sucks in air flowing through an air duct 15 and gas flowing though a gas duct 16. A gas regulating valve 18 for adjusting the gas flow through the gas duct 16 and a gas safety valve 19 are assigned to the gas duct 16.
The defined gas/air mixture having the defined mixing ratio of gas and air is provided to the burner chamber 11 of the gas burner. The defined gas/air mixture is provided by mixing the air flow provided by an air duct 15 with a gas flow provided by a gas duct 16. The air flow and the gas flow become preferably mixed by a mixing device 23. Such a mixing device can be designed as a so-called Venturi nozzle.
The quantity of the air flow and thereby the quantity of the gas/air mixture flow is adjusted by the fan 14, namely by the speed of the fan 14. The fan speed can be adjusted by an actuator 22 of the fan 14.
The fan speed of the fan 14 is controlled by a controller 20 generating a control variable for the actuator 22 of the fan 14.
The defined mixing ratio of the defined gas/air mixture is controlled by the gas regulating valve 18, namely by a pneumatic controller 24 of the same. The pneumatic controller 24 of the gas regulating valve 18 controls the opening/closing position of the gas valve 18.
The position of the gas valve 18 is adjusted by the pneumatic controller 24 on basis of a pressure difference between the gas pressure of the gas flow in the gas pipe 16 and a reference pressure. The gas regulating valve 18 is controlled by the pneumatic controller 24 in such a way that at the outlet of the gas valve 18 the pressure is equal to the reference pressure.
In Figure 1, the ambient pressure serves as reference pressure. However, it is also possible to use the air pressure of the air flow in the air duct 15 as reference pressure. The pressure difference between the gas pressure and the reference pressure is determined pneumatically by pneumatic sensor of the pneumatic controller 24.
Alternatively, it is possible to determine the pressure difference between the gas pressure of the gas flow in the gas pipe and the reference pressure electronically by an electric sensor (not shown). In this case, the gas valve 18 would be controlled by an electronic controller, e.g. by the controller 20.
In any case, the mixing ratio of the defined gas/air mixture is controlled is such a way that over the entire modulation range of the gas burner the defined mixing ratio of the defined gas/air mixture is kept constant.
A modulation of "1" means that the fan 14 is operated at maximum fan speed and thereby at full-load of the gas burner. A modulation of "5" means that the fan 14 is operated at 20% of the maximum fan speed and a modulation of "10" means that the fan 14 is operated at 10% of the maximum fan speed.
By changing the fan speed of the fan 14 the load of the gas burner can be adjusted. Over the entire modulation range of the gas burner the defined mixing ratio of the defined gas/air mixture is kept constant.
As described above, the mixing ratio of the defined gas/air mixture is controlled during burner-on phases so that over the entire modulation range of the gas burner the defined mixing ratio of the gas/air mixture is kept constant.
During burner-on phases the defined mixing ratio of gas and air of the defined gas/air mixture can be calibrated to different gas qualities.
The calibration is performed by adjusting a position of a throttle 17. The throttle position can be adjusted by an actuator 21 assigned to the throttle 17. The controller 20 controls the actuator 21 and thereby the throttle position of the throttle 17. The calibration can be performed at selected times, namely immediately after installation of the sensor and/or immediately after restart of the gas burner and/or immediately after a reset. Alternatively, the calibration can be performed in a modulating range of the gas burner close to full-load operation of the same, e.g. between 50% (corresponds to a modulation of "2") and 100% (corresponds to a modulation of "1") of full full-load operation.
The controller 20 of the gas burner comprises a measuring arrangement 43. A preferred embodiment of the measuring arrangement 43 is shown in Figure 2.
The measuring arrangement 43 comprises the fame rod 13.
The measuring arrangement 43 comprises further a first power supply device 26 for applying a supply voltage - also often called measuring voltage - to the flame rod 13, wherein the first power supply device 26 is configured to be switched on and off thereby switching on and off the supply or measuring voltage to the flame rod 13.
A power supply source is connectable to connection terminals 30 of the first power supply device 26. The power supply at the connection terminals 30 usually provides a supply voltage of around 200 Volt.
The measuring arrangement 43 comprises further a measuring device 27 for measuring a flame ionization current, preferably a dynamic behaviour of the flame ionization current, with the aid of the supply voltage applied to the flame rod 13 by the first power supply device 26. The flame ionization current can be measured with the supply voltage being switched on and with the supply voltage being switched off. The flame rod 13 is connected to connection terminal 29 of the measuring device 27.
The measuring arrangement 43 comprises further a comparison device 18 comparing an earth potential 32 with a frame potential 33 of the measuring arrangement, wherein the flame ionization current is determined when the frame potential 33 corresponds to the earth potential 27. The gas burner surface 25 or a different part of the gas burner is connectable to the connection terminal 31 of the comparison device 18.
The measuring arrangement 43 of the present invention comprises an input definition circuit 34 configured to switch the measuring arrangement 43 between a relative low impedance measuring circuit and a relative high impedance measuring circuit. The input definition circuit 34 preferably is part of the measuring device 27.
The measuring arrangement 43 is adapted to measure the flame ionization current at the flame rod 13 when the measuring arrangement 43 is switched by the input definition circuit to a relative low measuring impedance circuit 43. So, the measuring arrangement 43 is used to measure the flame ionization current when the measuring arrangement is switched by the input definition circuit 34 to a relative low impedance measuring circuit. For measuring the flame ionization current the supply voltage provided by the first power supply device 26 is preferably switched on and off making use of a defined switching rate.
The measuring arrangement 43 is adapted to measure a thermionic voltage at the flame rod 13 when the measuring arrangement 43 is switched by the input definition circuit 34 to a relative high impedance measuring circuit 43. So, the measuring arrangement 43 is used to measure the thermionic voltage based on the thermionic effect when the measuring arrangement 43 is switched by the input definition circuit 34 to a relative high impedance measuring circuit. For measuring the thermionic voltage the supply voltage provided by the first power supply 26 is preferably permanently switched off.
The thermionic voltage is a voltage resulting from the Edison-Richardson effect which is also often called thermionic effect. It is based on the effect that when two conductive metals are in the flame at different temperatures, an electrical voltage is generated depending on the temperature difference. One of these metals is provided by the flame rod 13, the other one of the metals is provided by the burner surface 25 or a different metallic part within the burner chamber.
Depending on the status of the input definition circuit 34 and thereby on the status of the measuring arrangement 43, the measuring arrangement 43 is either adapted to measure the flame ionization current or to measure the thermionic voltage making in both statuses use of the same flame rod 13.
Preferably, the switching of the input definition circuit 34 between a relative high impedance circuit and a relative high impedance circuit happens at another defined switching rate. Said switching between the between a relative high impedance measuring circuit and a relative high impedance measuring circuit is controlled by a microcontroller 44 of the measuring arrangement 43. The microcontroller 44 preferably is part of the measuring device 27.
The input definition circuit 34 comprises a first connection terminal 39, wherein the measuring arrangement 43 is switched to a relative low impedance measuring circuit when the first connection terminal 39 is switched to or operated in a first terminal status also often called output terminal status.
The measuring arrangement 43 is switched to a relative high impedance measuring circuit when the first connection terminal 39 is switched to or operated in a second terminal status also often called input terminal status.
The microcontroller 44 switches the first connection terminal 39 between the first (output) terminal status and the second (input) terminal status as discussed later below.
The input definition circuit 34 comprises further a second connection terminal 37 through which the input definition circuit 34 is connected to a second power supply device 45 of the measuring arrangement 43. The second power supply device 45 at the second connection terminal 37 usually provides a supply voltage of around 5 Volt. The second power supply device 45 is also connected to the microcontroller 44.
The input definition circuit 34 comprises further a third connection terminal 38 through which the input definition circuit 34 is connected to ground potential 46.
The input definition circuit 34 comprises further a fourth connection terminal 36 through which the input definition circuit 34 is connected to the flame rod 13.
The microcontroller 44 comprises an electronic switch, wherein the electronic switch is according to Figure 2 provided by a transistor 47. The first connection terminal 39 is connected to a first port of the electronic switch and the second power supply device 45 is connected to a second port of the electronic switch.
The electronic switch is configured to by operated in such a way that when the electronic switch is closed, the first connection terminal 39 is connected to the second power supply device 45 so that the same is switched to the first terminal status, and that when the electronic switch is opened, the first connection terminal 39 is disconnected from the second power supply device 45 so that the same is switched to the second terminal status.
The transistor 47 shown in Figure 2 has a drain D, a gate G and a source S. The drain D of the transistor 47 and the second connection terminal 37 of the input definition circuit 34 are both connected to the second power supply device 45. The source S of the transistor 47 is connected to the first connection terminal 39 of the input definition circuit 34.
The gate G of the transistor 47 is configured to by operated in such a way that when a first defined voltage is applied to the gate G the first connection terminal 39 is switched by the microcontroller 44 to the first terminal status and that when a second defined voltage is applied to the gate G the first connection terminal 39 is switched by the microcontroller 44 to the second terminal status.
So, when the electronic switch is closed, especially when gate G of the transistor 47 is operated by other components of the microcontroller 44 in such a way that the transistor 47 is conductive by applying the first defined voltage to the gate G, the supply voltage provided by the second connection terminal 37 is present at the first connection terminal 39. In this status the first connection terminal 39 is switched by the microcontroller 44 to the first terminal status and the input definition circuit 34 switches the measuring arrangement 43 is to a relative low impedance measuring circuit in which the same is adapted to measure the flame ionization current at the flame rod 13. When the electronic switch is opened, especially when the gate G of the transistor 47 is in operated by other components of the microcontroller 44 in such a way that the transistor 47 is non-conductive by applying the second defined voltage to the gate G, the supply voltage provided by the second connection terminal 37 is not present at the first connection terminal 39 but the first connection terminal 39 is floating. In this status the first connection terminal 39 is switched by the microcontroller 44 to the second terminal status and the input definition circuit 34 switches the measuring arrangement 43 is to a relative high impedance measuring circuit in which the same is adapted to measure the thermionic voltage at the flame rod 13. Details how a microcontroller 44 controls the status of the electronic switch to be opened or closed, especially how the transistor 47 can be made conductive or non-conductive by applying the defined voltages to the gate G, are well known to the person skilled in the art and a part of a typical push-pull stage of a microcontrollers input/output port.
In the shown embodiment, the transistor 47 is provided by a MOSFET transistor.
The flame rod 13 is also connected to the microcontroller 44, preferably through the the fourth connection terminal 36. The fourth connection terminal 36 is preferably connected to an analog/digital converter input of the microcontroller 44.
A first electrical resistor 42 of the measuring arrangement 43 is connected between the first connection terminal 39 and the fourth connection terminal 36. A second electrical resistor 40 of the measuring arrangement 43 is connected between the second connection terminal 37 and the fourth connection terminal 36. A third resistor 41 of the measuring arrangement 43 is connected between the third connection terminal 38 and the fourth connection terminal 36. The second resistor 40 and the third resistor 41 have both a bigger electric resistance than the first resistor 42. Preferably, the electric resistance of second resistor 40 and the third resistor 41 is at least 10 times higher than the electric resistance of the first resistor 42. The second resistor 40 and the third resistor 41 preferably have an electric resistance of in the magnitude of Mega Ohms (e.g. of 1MΩ). The first resistor 42 has preferably an electric resistance of in the magnitude of Kilo Ohms (e.g. 56kΩ).
In the above descripted low impedance mode the supply voltage of e.g. +5V provided by the second power supply device 45 is present at connection terminal 39. In this case resistor 42 is connected to said supply voltage of e.g. 5V, where also resistor 40 is connected to. Resistor 40 (e.g. having a magnitude of 56 kΩ) and resistor 42 (e.g. having a magnitude of 1MΩ) in parallel provide together a resistor of 53kΩ. Looking from connection terminal 36, the input definition circuit 34 has an impedance of 50kΩ (1MΩ in parallel to 53kΩ). In the above descripted low impedance high impedance mode the supply voltage of e.g. +5V provided by the second power supply device 45 is not present at connection terminal 39. This means that resistor 42 is not effective, meaning that resistor 42 does not have a function. Only resistors 40 and 41 are effective, both having a magnitude of e.g. 1 MΩ. Looking from connection terminal 36, the input definition circuit 34 has an impedance of 500kΩ (1MΩ in parallel to 1MΩ).
The measuring arrangement of the present invention comprising the input definition circuit 34 can switch between a low impedance flame ionization measurement circuit to a high impedance thermionic voltage measurement circuit.
The same arrangement with the same flame rod can be used for both measurements. The measuring arrangement of the present invention allows a very accurate calibration of the defined gas/air mixture to different gas qualities.
The present application provides further a unique calibration method for calibrating the gas/air mixture to different gas qualities. The calibration is based on a signal provided by the flame rod 13 positioned downstream of the mixing device 23 within the burner chamber 11 making use of the above described measuring arrangement of the present invention. The calibration of the gas/air mixture of the present invention is performed in at least two calibration steps, namely in a coarse calibration step and a fine calibration step.
In the coarse calibration step the measuring arrangement as well as the flame rod 13 are both used to measure the flame ionization current and/or the thermionic voltage. During said coarse calibration step the gas/air mixture is made richer by increasing the gas amount of the gas/air mixture relative to the air amount of the same until a flattening or a maximum in a dynamic behavior of the flame ionization current is detected and/or until a minimum in a dynamic behavior of thermionic voltage is detected.
As mentioned above, both measurements can in principle be used for the coarse calibration step. In the embodiment of Figure 3 only the flame ionization current measurement is used during the coarse calibration. It would also be possible to use the thermionic voltage for the coarse calibration. For the measurement of the flame ionization current the measuring arrangement 43 is switched by the input definition circuit 34 to a relative low measuring impedance circuit and the supply voltage provided by the first power supply device 26 is preferably switched on and off making use of a defined switching rate.
The curve 43 of Figure 3 illustrates the dependence of the dynamic behavior of the flame ionization current I provided by the flame rod 13 during the coarse calibration step from the throttle position X₁₇ of the throttle 17 assigned to the gas duct 16 used for the calibration.
The arrow 44 in Figure 3 illustrates that the gas/air mixture is made richer during the coarse calibration step. For the coarse calibration step the throttle position X₁₇ is changed through the actuator 21 and the controller 20 in order to increase the gas flow while keeping the fan speed of the fan 14 and the air flow constant thereby increasing the gas amount of the gas/air mixture relative to the air amount of the same (see arrow 44 in Figure 3). The throttle position X₁₇ is preferably continuously changed with a first throttle position changing speed in order to continuously increase the gas amount of the gas/air mixture relative to the air amount of the same until a flattening or a maximum of the signal I provided by the flame rod 13 is detected.
Figure 3 shows a throttle position X_17-COARSE determined during the coarse calibration step.
After a flattening or a maximum in the dynamic behavior of the flame ionization current I has been detected during the coarse calibration step, the fine calibration step will be performed. For the fine calibration step the circuit arrangement as well as the flame rod 13 are used to measure the thermionic voltage based on the thermionic effect. During said fine calibration step the gas/air mixture is made leaner by decreasing the gas amount of the gas/air mixture relative to the air amount of the same until an increase of the thermionic voltage U is detected.
As mentioned above, the thermionic voltage measurement is used during the fine calibration. For the measurement of thermionic voltage the measuring arrangement 43 is switched by the input definition circuit 34 to a relative high measuring impedance circuit and the supply voltage provided by the first power supply device 26 is preferably permanently switched off.
The curve 45 of Figure 3 illustrates the dependence of the dynamic behavior of the thermionic voltage U provided by the flame rod 13 during the fine calibration step from the throttle position X₁₇ of the throttle 17 assigned to the gas duct 16.
The arrow 46 in Figure 3 illustrates that the gas/air mixture is made leaner during the fine calibration step. For the fine calibration step the throttle position X₁₇ is changed starting from throttle position X_17-COARSE through the actuator 21 and the controller 20 in order to decrease the gas flow while keeping the fan speed of the fan 14 and the air flow constant thereby decreasing the gas amount of the gas/air mixture relative to the air amount of the same (see arrow 46 in Figure 3). The throttle position X₁₇ is preferably continuously changed with a second throttle position changing speed in order to continuously decrease the gas amount of the gas/air mixture relative to the air amount of the same until an increase of the signal U provided by the flame rod 13 is detected.
Figure 3 shows a throttle position X_17-FINE determined during the fine calibration step.
The first throttle position changing speed during the coarse calibration step is higher than the second throttle position changing speed during the fine calibration step.
The throttle position X_17-FINE provides a gas/air mixture providing a combustion with a A-value of 1,0. The calibrated throttle position X_17-CAL is calculated from the throttle position X_17-FINE determined during the fine calibration step by adding an offset value ΔX₁₇ to the throttle position X_17-FINE. The following formula is used: $X_{17 - CAL} = X_{17 - FINE} + Δ X_{17}$
The calibrated throttle position X_17-CAL is used for the further operation of the gas burner.
The coarse calibration step and a fine calibration step both make use of the same flame rod 13. In some gas burner this flame rod 13 also serves as ignition rod.
It has to be noted that in the shown embodiment the throttle 17 which is used for the calibration is assigned to the gas duct 16. However, it should be understood that the throttle 17 which is used for the calibration can alternatively be assigned to the mixing device 23.
In the above described calibration method the dynamic behavior of the flame ionization current I and the dynamic behavior of the thermionic voltage U are used for the calibration, not the absolute values of the same. The used dynamic behavior is detected based on a changing gas amount in the gas/air mixture and thereby on a changing A-value. So, it is not necessary to consider a time constant of a few seconds to measure a stable absolute value of e.g. the thermionic voltage U. Due to the fact that the method does not make use of the absolute value of e.g. the thermionic voltage, but of the dynamic behaviour of the same, this thermionic voltage doesn't have to stabilize. The transition that happens is big enough to see it immediately even if the signal has a natural time constant. So, the change of throttle position e.g. during the fine calibration step can be done continuously as described above.
Only the calibration of the defined gas/air mixture depends on the flame ionization current and on the thermionic voltage, wherein the control of the defined gas/air mixture over the modulation range of the gas burner 11 is independent from the flame ionization current and from the thermionic voltage. The control of the defined gas/air mixture over the modulation range of the gas burner 11 depends on a pressure difference between the gas pressure of the gas flow in the gas pipe 16 and a reference pressure, wherein either the air pressure of the air flow in the air duct 15 or the ambient pressure is used as reference pressure, and wherein the pressure difference between the gas pressure of the gas flow in the gas pipe 16 and the reference pressure is determined either pneumatically by pneumatic sensor or electronically by an electric sensor.
In Figure 1 the defined gas/air mixture is controlled by the pneumatic controller 24 making use of a pneumatic sensor to determine the pressure difference between the gas pressure and the reference pressure.

List of reference signs

10: gas burner appliance
11: gas burner chamber
12: flame
13: flame rod
15: air duct
16: gas duct
17: throttle
18: gas valve / regulating valve
19: gas valve / safety valve
20: controller
21: actuator
22: actuator
23: mixing device
24: pneumatic controller
25: gas burner surface
26: first power supply device
27: measuring device
28: comparison device
29: connection terminal
30: connection terminal
31: connection terminal
32: earth potential
33: frame potential
34: input definition circuit
35: filter device
36: connection terminal
37: connection terminal
38: connection terminal
39: connection terminal
40: resistor
41: resistor
42: resistor
43: measuring arrangement
44: microcontroller
45: second power supply device
46: ground potential
47: transistor

Claims

Measuring arrangement (43) for a gas burner, comprising
a flame rod (13);

a first power supply device (26) configured to apply a supply voltage to the flame rod (13), wherein the first power supply device (26) is configured to be switched on and off thereby switching on and off the supply voltage to the flame rod (13);

a measuring device (27) configured to measure a flame ionization current at the flame rod (13) with the aid of the supply voltage applied to the flame rod (13);

a microcontroller (44) configured to receive the flame ionization current from the flame rod (13);

characterized in that

the flame ionization current is measurable with the supply voltage being switched on and with the supply voltage being switched off, and in that the measuring arrangement further comprises an input definition circuit (34) configured to switch the measuring arrangement between a relative low impedance measuring circuit and a relative high impedance measuring circuit, wherein
the measuring arrangement (43) is adapted to measure the flame ionization current at the flame rod (13) when the measuring arrangement (43) is switched by the input definition circuit (34) to a relative low measuring impedance circuit;

the measuring arrangement (43) is adapted to measure a thermionic voltage at the flame rod (13) when the measuring arrangement (43) is switched by the input definition circuit (34) to a relative high impedance measuring circuit.
Measuring arrangement of claim 1, characterized in that the input definition circuit (34) comprises a first connection terminal (39), wherein the first connection terminal (39) is connected to the microcontroller (44), and wherein the microcontroller (44) is configured to switch the first connection terminal between a first terminal status and a second terminal status.
Measuring arrangement of claim 2, characterized in that the measuring arrangement (43) is switched to a relative low impedance measuring circuit when the first connection terminal (39) is switched by the microcontroller (44) to the first terminal status, and wherein the measuring arrangement (43) is switched to a relative high impedance measuring circuit when the first connection terminal (39) is switched by the microcontroller (44) to the second input terminal status.
Measuring arrangement of claim 2 or 3, characterized by a first resistor (42), wherein the first resistor (42) is connected to the first connection terminal (39).
Measuring arrangement of one of claims 2 to 4, characterized by a second power supply device (45), wherein the input definition circuit (34) further comprises a second connection terminal (37) through which the input definition circuit (34) is connected to the second power supply device (45).
Measuring arrangement of one of claims 2 to 5, characterized by a ground potential (46), wherein the input definition circuit (34) further comprises a third connection terminal (38) through which the input definition circuit (34) is connected to the ground potential (46).
Measuring arrangement of one of claims 2 to 6, characterized in that the input definition circuit (34) further comprises a fourth connection terminal (36) through which the input definition circuit (34) is connected to the flame rod (13).
Measuring arrangement of claim 7, characterized by a filter device (35), wherein the filter device (35) is connected between the fourth connection terminal (36) of the input definition circuit (34) and the flame rod (13).
Measuring arrangement of one of claims 5 to 8, characterized by
a second resistor (40), wherein the second resistor (40) is connected between the second connection terminal (37) and the fourth connection terminal (36);

a third resistor (41), wherein the third resistor (41) is connected between the third connection terminal (38) and the fourth connection terminal (36);

wherein the second resistor (40) and the third resistor (41) have both a bigger electric resistance than the first resistor (42).
Measuring arrangement of one claims 5 to 9, characterized in that the microcontroller (44) comprises an electronic switch, wherein the electronic switch is configured to by operated in such a way that when the electronic switch is closed the first connection terminal (39) is connected to the second power supply device (45) so that the same is switched to the first terminal status, and that when the electronic switch is opened the first connection terminal (39) is disconnected from the second power supply device (45) so that the same is switched to the second terminal status.
Gas burner, comprising
a burner chamber (11) with a burner surface (25);

a measuring arrangement of one of claims 1 to 10,

wherein the flame rod (13) of the measuring arrangement positioned within the gas burner chamber (11) adjacent to a gas burner surface (25).
Method for operating the gas burner of claim 11, wherein during burner-on phases a defined gas/air mixture having a defined mixing ratio of gas and air is provided to a burner chamber (11) of the gas burner for combusting the defined gas/air mixture within the burner chamber (11), wherein the defined gas/air mixture is provided by a mixing device (23) mixing an air flow provided by an air duct (15) with a gas flow provided by a gas duct (16), and wherein during burner-on phases the defined mixing ratio of gas and air of the defined gas/air mixture can be calibrated to different gas qualities on basis of a signal provided by a flame rod (13) positioned downstream of the mixing device (23) within the burner chamber (11), characterized in that the calibration of the gas/air mixture is performed in at least two calibration steps, namely in a coarse calibration step and a fine calibration step, namely in such a way that
in the coarse calibration step the flame rod (13) of the gas burner is used to measure a flame ionization current and/or a thermionic voltage, wherein the gas/air mixture is made richer during the coarse calibration step by increasing the gas amount of the gas/air mixture relative to the air amount of the same until a flattening or a maximum in a dynamic behavior of the flame ionization current and/or until a minimum in a dynamic behavior of the thermionic voltage is detected,

in the fine calibration step following the coarse calibration step the flame rod (13) is used to measure at least the thermionic voltage, wherein the gas/air mixture is made leaner during the fine calibration step after the flattening or the maximum in the dynamic behavior of the flame ionization current is detected and/or after the minimum in the dynamic behavior of the thermionic voltage is detected by decreasing the gas amount of the gas/air mixture relative to the air amount of the same until an increase in the dynamic behavior of the thermionic voltage is detected.
Method as claimed in claim 12, characterized in that the gas/air mixture is made richer during the coarse calibration step by increasing a throttle position of a throttle (17) assigned to a gas duct (16) or to a mixing device (23) of the gas burner making use of a first throttle position changing speed, and that the gas/air mixture is made leaner during the fine calibration step by decreasing the throttle position of said throttle (17) making use of a second throttle position changing speed, wherein the first throttle position changing speed is higher than the second throttle position changing speed.
Method as claimed in claim 12 or 13, characterized in that
only the calibration of the defined gas/air mixture depends on the thermionic voltage, wherein the control of the defined gas/air mixture over the modulation range of the gas burner (11) is independent from the thermionic voltage, and/or

only the calibration of the defined gas/air mixture depends on the flame ionization current, wherein the control of the defined gas/air mixture over the modulation range of the gas burner (11) is independent from the flame ionization current.
Method as claimed in claim 14, characterized in that the control of the defined gas/air mixture over the modulation range of the gas burner (11) depends on a pressure difference between the gas pressure of the gas flow in the gas pipe and a reference pressure, wherein either the air pressure of the air flow in the air duct or the ambient pressure is used as reference pressure, and wherein the pressure difference between the gas pressure of the gas flow in the gas pipe and the reference pressure is determined either pneumatically by pneumatic sensor or electronically by an electric sensor.