Method for controlling a gas mixture using a gas sensor and a gas mixture sensor

Abstract

Method for controlling a gas mixture (8, 107) formed from a gas (2, 104) and a fuel gas (1, 103) in a fuel gas-operated heating appliance, wherein the gas mixture is generated by via a first actuator (4, 108) a quantity of gas and A quantity of fuel gas is supplied and mixed via a second actuator (3, 102), wherein a microthermal gas sensor (7, 105) which detects at least one material property of the gas (2, 104), is exposed to the gas and transmits a sensor signal dependent on the respective gas to a control unit (10, 112), wherein a microthermal gas mixture sensor (9, 106, 109), which detects at least one material property of the gas mixture (8, 107), is exposed to the gas mixture and continuously transmits a sensor signal dependent on the respective gas mixture to a control unit (10, 112), wherein, when the detected sensor signal of the gas sensor changes, the newly detected sensor signal of the gas sensor is compared with reference values ​​measured in the laboratory and stored in a table of values ​​in the control unit (10, 112), and a target value of the sensor signal of the gas mixture sensor (9, 106, 109) is determined from this, without changing the mixing ratio of the gas mixture (8, 107) of fuel gas and gas. wherein the setpoint of the sensor signal of the gas mixture sensor (9, 106, 109) is adjusted by the control unit (10, 112) depending on the composition of the gas or fuel gas and the adjustment of the setpoint of the sensor signal of the gas mixture sensor (9, 106, 109) is carried out by a calibration process, and wherein The calibration process is carried out in particular by an ionization current control of a flame signal of a burner (110) of the heating device (200) until an ionization setpoint is reached.

Description

[0001] The invention relates to a method for controlling a gas mixture in a fuel gas-operated heating device.

[0002] Generic processes are known from the prior art, for example from the disclosure in document WO2006 / 000366A1.

[0003] State-of-the-art combustion control also includes the so-called SCOT method, in which the air volume supplied to the heater's burner is controlled according to the burner output. A flame signal is measured using an ionization sensor, and the gas-air mixture is regulated to a target ionization value stored in a characteristic curve. However, the SCOT method has the disadvantage that the flame signal drops significantly at low burner outputs, making control unreliable. Furthermore, the adaptation effort, particularly for adjusting the burner geometry, is high, and the burner output can only be determined imprecisely via the fan speed of a fan that supplies the air flow for the gas-air mixture.

[0004] Furthermore, the prior art discloses an electronic mixture control system based on a thermal gas mass flow sensor, which can be used to detect the fuel gas properties. The gas mass flow sensor measures the fuel gas volume flow, and a control unit uses thermal conductivity to determine the fuel gas type from a reference table. The required air volume is then calculated and adjusted based on the determined air requirement. However, this is complex because all input variables—i.e., the gas volume flow, the air volume flow, and the fuel gas properties—must be measured and monitored.

[0005] The invention is based on the object of providing a method for controlling a gas mixture in a fuel gas-operated heating device, which detects changes in the properties of the gas supplied to the fuel gas and specifically adapts control parameters.

[0006] This object is achieved by the combination of features according to claim 1.

[0007] According to the invention, a method for controlling a gas mixture formed from a gas and a fuel gas in a fuel gas-operated heating device is proposed, in which the gas mixture is generated by providing and mixing a quantity of gas via a first actuator and a quantity of fuel gas via a second actuator. A microthermal gas sensor, which detects at least one material property of the gas, is exposed to the gas and transmits a sensor signal dependent on the respective gas to a control unit. In addition, a microthermal gas mixture sensor, which detects at least one material property of the gas mixture, is exposed to the gas mixture and continuously transmits a sensor signal dependent on the respective gas mixture to the control unit.If the detected sensor signal of the gas sensor changes, the newly detected sensor signal of the gas sensor is compared with reference values ​​measured in the laboratory and stored in a value table in the control unit, and from this a target value of the sensor signal of the gas mixture sensor is determined without changing the mixing ratio of the gas mixture of fuel gas and gas.

[0008] In practice, it is often the case with heaters that only the properties of the gas change, for example the relative humidity, without this resulting in a new mixture ratio for gas and fuel gas that is necessary to ensure continued clean combustion. Nevertheless, the sensor signal from the gas mixture sensor changes due to this change in the material properties of the gas. To avoid unnecessary adjustment of the mixture ratio, the change in the material properties of the gas is recorded via the gas sensor and only the setpoint of the sensor signal from the gas mixture sensor is adjusted. The mixture ratio of fuel gas and gas remains unchanged. The control of the gas mixture (ratio of fuel gas and gas) via the modulation range of the heater is then continued with the new setpoint of the sensor signal from the gas mixture sensor.

[0009] This gas mixture control is based on monitoring the signal value of the gas mixture sensor. For this purpose, the control unit compares the continuously recorded sensor signal of the gas mixture sensor with a target value of the sensor signal of the gas mixture sensor. If the recorded sensor signal of the gas mixture sensor deviates from the target value of the sensor signal, it controls at least one of the first and second actuators to thereby adjust the gas mixture by increasing or decreasing the gas quantity and / or increasing or decreasing the fuel gas quantity until the target value of the sensor signal of the gas mixture sensor is reached.

[0010] A key aspect is the measurement of at least one material property of the gas mixture and at least one material property of the gas. A change in the gas quantity or the fuel gas quantity would be immediately detected by a change in the material properties at the gas mixture sensor. A change in the material properties of the gas mixture at the gas mixture sensor can be immediately corrected via the control unit. A change in the material properties of the gas is immediately addressed to adjust the target value of the sensor signal of the gas mixture sensor.

[0011] The material property of the gas mixture detected by the microthermal gas mixture sensor is preferably the thermal conductivity or the thermal diffusivity of the gas mixture. However, several of these material properties can also be detected, allowing a more precise assignment of the majority of properties to the gas mixture. The material property of the gas detected by the microthermal gas sensor corresponds to that of the gas mixture sensor.

[0012] Another possibility is based on the functional principle of ultrasonic measurement to determine the specific speed of sound present in each gas mixture. A corresponding sensor is also used as a gas sensor.

[0013] In a further development, the method is characterized in that the target value of the sensor signal is adjusted by the control unit depending on the composition of the gas or the fuel gas. If the composition of the fuel gas changes (e.g., from propane to butane), the measured properties of the gas mixture change. In addition, different fuel gas compositions also require different air quantities for optimal combustion. Therefore, a new mixture ratio between gas and fuel gas is also required.

[0014] Such an adjustment of the sensor signal setpoint is performed through a calibration process. For this purpose, the control unit adjusts the first gas flow actuator or the second fuel gas flow actuator until the desired result is achieved. The original setpoint is replaced by the new measured sensor signal for further mixture control.

[0015] Specifically, the calibration process involves regulating the ionization current based on a flame signal from a heater burner until an ionization setpoint is reached. To do this, stoichiometric combustion of the heater burner is first set. An ionization probe detects the flame signal from the heater burner and thus a corresponding ionization current. During stoichiometric combustion, the ionization current is at its maximum. From this ionization current value, a laboratory-determined percentage is used to calculate an ionization setpoint and save it as the future ionization current setpoint that must be achieved for the desired combustion. Subsequently, the gas flow is reduced by a predetermined factor to operate the burner with the desired gas mixture at the predetermined ionization setpoint.

[0016] The method is further characterized in that, upon reaching the ionization target value, at least one material property of the gas mixture is measured using the gas mixture sensor and stored in the control unit as the new target value of the sensor signal. The new target value is used for further control and replaces the previous target value.

[0017] The calibration process is preferably performed when the sensor signal from the gas mixture sensor is implausible or at cyclic, predetermined intervals. In one embodiment, the detection of implausible sensor signals occurs when the heater is started by initially supplying only the known gas and subjecting the gas mixture sensor to this pressure. An implausibility occurs when the sensor signal measured by the gas mixture sensor for a material property, e.g., thermal conductivity or thermal diffusivity, does not correspond to a sensor signal for the known gas. In heaters, the gas typically used is ambient air, whose material properties are known.

[0018] Different types of fuel gas or fuel gas families (natural gas, LPG) influence the material properties of the gas mixture in different ways. For example, when LPG is added to air, the thermal conductivity decreases, while when natural gas is added to air, the thermal conductivity increases. In a further development of the method, it is therefore provided that when the heater is started, only the known gas, preferably air, is initially supplied and the gas mixture sensor is exposed to it. The fuel gas is then supplied, the gas mixture is generated, and the gas mixture sensor is exposed to the gas mixture. The gas type of the fuel gas is determined from the change in the sensor signal during the supply of the fuel gas. The control unit then adjusts the gas mixture depending on the detected gas type of the fuel gas until the target value of the sensor signal is reached.Advantageously, the starting power can be controlled to a favorable starting point immediately after the gas family has been recognized by the control unit via the position of the actuator of the fuel gas, and the ignition mixture for the burner start is achieved more quickly and precisely.

[0019] The method also utilizes the above-described effect of the increase or decrease in the sensor signal for different fuel gases. It provides that the direction of the control effect is determined from the change in the sensor signal during the supply of the fuel gas. This determines whether the supplied fuel gas quantity should be increased or decreased to achieve the target value of the sensor signal. The goal is always clean combustion with the required gas mixture.

[0020] The gas is preferably air, the fuel gas is preferably liquefied petroleum gas or natural gas or any fuel gas mixture.

[0021] A further development of the method further comprises an embodiment for the case where the thermal properties of the fuel gas are too close to the thermal properties of the air and reliable mixture control is not possible because any change in either the air quantity or the gas quantity does not result in a signal change at the gas mixture sensor. This case can arise, for example, when mixed fuel gases are used for combustion which happen to have the same physical properties as air. This condition is detected by the control unit both when the heater is started and during calibration through a plausibility check. This is because no significant change in the sensor signal is measured regardless of the air or gas quantity.In this case, the control unit can temporarily deactivate mixture control via the gas mixture sensor and, with a reduced modulation range, control it exclusively via the ionization current control described for calibration. As soon as the fuel gas properties allow control via the gas mixture sensor positioned in the gas mixture, mixture control via the gas mixture sensor will resume.

[0022] In one embodiment, it is provided that the material properties of the gas detected by the gas sensor are continuously checked for changes and the control unit continuously adjusts the setpoint value of the sensor signal of the gas mixture sensor.

[0023] Other advantageous developments of the invention are characterized in the subclaims or are presented in more detail below, together with the description of the preferred embodiment of the invention, with reference to the figures. They show: Fig. 1 a basic structure for carrying out the procedure, Fig. 2 a structure of a heating device for carrying out the process, Fig. 3 a control characteristic of the sensor signal of the gas mixture sensor, Fig. 4 a control characteristic of the sensor signal of the gas mixture sensor, Fig. 5 Control characteristics before and after a change in gas properties, Fig. 6 Control characteristics before and after a change in the fuel gas properties, Fig. 7 a characteristic curve of the ionization current control.

[0024] In Fig. Figure 1 shows a basic setup for implementing the process. In the following description of the figures, air is always assumed to be the gas, although other gases could theoretically be used.

[0025] The control unit 10 controls the respective opening positions of the actuator 4 for supplying a controllable amount of air 2 and the actuator 3 for supplying a controllable amount of fuel gas 1 in order to generate the gas mixture 8 in a specific fuel gas-air mixture ratio. The gas mixture sensor 9 is positioned in the area of ​​the gas mixture 8 and is supplied with the gas mixture 8. The gas sensor 7, which is supplied exclusively with air, is positioned in the area of ​​the regulated air 6. The air, whose quantity is regulated, and the fuel gas 5, whose quantity is regulated, are homogeneously mixed in the area of ​​the gas mixture sensor 9. Each of the two sensors 7, 9 is designed to measure the physical material properties, e.g., thermal conductivity, thermal diffusivity and / or, with other sensors, the speed of sound. The control unit 10 and the control system are monitored via a process monitoring unit 11.In addition, the signal lines to and from the control unit 10, which processes the control of the gas mixture 8, are shown by arrows.

[0026] Fig. Figure 2 shows a specific embodiment of a fuel gas-operated heater 200 with a gas safety valve 101, a gas control valve 102 as an actuator for the amount of fuel gas 103, and a mixing fan 108 for drawing in air 104 and mixing it with the fuel gas 103 to generate the gas mixture 107. The air volume can be adjusted via the speed of the mixing fan 108; it thus represents the actuator for the air supply. The heater 200 includes the microthermal gas mixture sensor 106, with a second gas mixture sensor 109 shown as an alternative installation position in the exhaust area of ​​the mixing fan 108. In principle, however, a second gas mixture sensor is not required. The heater 200 further includes the microthermal gas sensor 105 in the air supply area. The mixing fan 108 conveys the gas mixture 107 to the burner 110, on which the ionization electrode 111 is installed to monitor the burner flame.In addition, the signal lines to and from the control unit 112, which processes the control of the gas mixture 107, are shown by arrows.

[0027] In the following, the components of the basic structure according to Fig. 1, which, however, refers directly to the heater 200 according to Fig. 2 are transferable.

[0028] In Fig. Figure 3 shows a simplified linear relationship used for control purposes in a diagram 30 between the sensor signal 31 detected by the gas mixture sensor 9 for pure air 2 (reference numeral 34 corresponds to 100% air) and the sensor signal 32 for pure fuel gas 1 (reference numeral 36 corresponds to 100% fuel gas). For the gas mixture 8 (reference numeral 35 corresponds to 60% air and 40% fuel gas), the sensor signal 33 lies in between. The quantities of air 2 and fuel gas 1 are adjusted via the respective actuators 3 and / or 4 until the mixture properties of the desired mixture ratio required by the process are detected by the gas mixture sensor 8. Fig. 3 shows a linear characteristic curve of the sensor signal, but non-linear characteristic curves are also possible, which, for example, enable control of the corresponding positions of the actuators 3, 4 via value tables.

[0029] According to Fig. 3, the sensor signal decreases the more fuel gas 1 is supplied. The sensor signal is shown as a function of the thermal conductivity as a material property of the gas mixture 8, where the fuel gas is, for example, liquefied petroleum gas, and the thermal conductivity of liquefied petroleum gas is lower than that of air. However, there are also gas types where the direction of action of the control is reversed, as in Fig. 4. Here, the fuel gas 1 is natural gas, whose thermal conductivity is higher than that of air. In diagram 40 according to Fig. 4 shows a simplified linear relationship used for the control between the sensor signal 41 detected by the gas mixture sensor 9 for pure air 2 (reference numeral 44 corresponds to 100% air) and the sensor signal 42 for pure fuel gas 1 (reference numeral 46 corresponds to 100% fuel gas / natural gas). For the gas mixture 8 (reference numeral 45 corresponds to 75% air and 25% fuel gas / natural gas), the sensor signal 43 lies in between, but close to, the sensor signal 41 for pure fuel gas 1. For control with natural gas, the control unit 10 determines the direction of action of the control from the signal change of the gas mixture sensor 9 when the fuel gas quantity increases and uses this as the basis for further mixture control.

[0030] The determination of the setpoint value of the sensor signal of the gas mixture sensor 10 is shown in diagram 50 according to Fig. 5 is shown schematically. A new sensor signal setpoint is determined from the material property measured by the gas sensor 7, e.g. when the relative air humidity changes. The mixture ratio 57 of fuel gas 2 and air 1, however, remains unchanged. If, for example, with pure air 1 (reference numeral 56 corresponds to 100% air; reference numeral 58 pure fuel gas) the sensor signal changes from the original value (reference numeral 51) to a new value (reference numeral 54) due to a change in the quality, e.g. when the relative air humidity changes, the control unit 11 extrapolates the mixture sensor setpoint from the old value (reference numeral 53) to the new value (reference numeral 55) using the signal value of the gas sensor 7 or corrects it with values ​​from a value table stored in the control unit 11. The reference numerals 52 and 54 denote the sensor signals for pure air and pure fuel gas, respectively.

[0031] Fig. Figure 6 shows a diagram 60 illustrating the calibration when, for example, the nature of the fuel gas 1 changes such that a new gas mixture composition is required to ensure optimal combustion. Fig. 6, the fuel gas changes from propane to butane, for example. Reference numerals 66 and 68 determine the range between 100% air and 100% fuel gas, with signal value 61 being present at 100% air. During calibration, the gas mixture 8 is changed from the original mixture ratio 67 with the associated signal value 63 to the new mixture ratio 69 with the associated signal value 64. By recording the material properties of both the air 2 and the fuel gas 1 via separate sensors, the direction of action of the control can be predetermined, and the gas mixture 8 can be preset for the subsequent control of the fuel gas quantity and / or air quantity. Fig. 5 and Fig. 6 the situation of the original composition of the fuel gas is shown in dashed lines, the new situation is shown with solid lines and arrows.

[0032] Fig. Figure 7 shows a diagram 20 for calibration by means of ionization current control with a characteristic curve of the ionization signal (Io signal) detected by the ionization electrode in the burner flame versus the fuel gas-air ratio λ. Since the basic structure according to Fig. 1 does not show an ionization electrode, the heater 200 is subsequently switched on according to Fig.2. During burner operation, the control unit 112 controls the amount of air 105 to a predetermined value, measures the ionization signal at the ionization electrode 111 of the burner 110, and increases the amount of fuel gas 103 until the ionization signal has risen from the original ionization value 21 at a fuel gas-to-air ratio of 24 to the maximum 22. From this value, the ionization setpoint 23 is calculated using a laboratory-determined percentage and stored as the future ionization current setpoint, which must be used to achieve the desired fuel gas-to-air ratio 25 with a higher excess air.

Claims

[1] Method for controlling a gas mixture (8, 107) formed from a gas (2, 104) and a fuel gas (1, 103) in a fuel gas-operated heating device, wherein the gas mixture is produced by via a first actuator (4, 108) a quantity of gas and a quantity of fuel gas is provided and mixed via a second actuator (3, 102), wherein a microthermal gas sensor (7, 105) which detects at least one material property of the gas (2, 104), is exposed to the gas and transmits a sensor signal dependent on the respective gas to a control unit (10, 112), wherein a microthermal gas mixture sensor (9, 106, 109), which detects at least one material property of the gas mixture (8, 107), is supplied with the gas mixture and continuously transmits a sensor signal dependent on the respective gas mixture to a control unit (10, 112), wherein, in the event of a change in the detected sensor signal of the gas sensor, the newly detected sensor signal of the gas sensor is compared with comparison values ​​measured in a laboratory and stored in a value table in the control unit (10, 112), and a target value of the sensor signal of the gas mixture sensor (9, 106, 109) is determined therefrom, without changing a mixing ratio of the gas mixture (8, 107) of fuel gas and gas, wherein the setpoint value of the sensor signal of the gas mixture sensor (9, 106, 109) is adjusted by the control unit (10, 112) as a function of a composition of the gas or the fuel gas, and the adjustment of the setpoint value of the sensor signal of the gas mixture sensor (9, 106, 109) is carried out by a calibration process, and wherein the calibration process is carried out in particular by ionization current control of a flame signal of a burner (110) of the heater (200) until an ionization setpoint is reached. [2] Method according to claim 1, characterized bythat the control unit (10, 112) compares the continuously detected sensor signal of the gas mixture sensor (9, 106, 109) with a target value of the sensor signal of the gas mixture sensor (9, 106, 109) and, in the event of a deviation between the detected sensor signal of the gas mixture sensor (9, 106, 109) and the target value of the sensor signal, controls at least one of the first and second actuators and thereby adapts the gas mixture (8, 107) by increasing or decreasing the gas quantity and / or increasing or decreasing the fuel gas quantity until the target value of the sensor signal of the gas mixture sensor (9, 106, 109) is reached. [3] Method according to claim 1 or 2, characterized by that the material property of the gas mixture detected by the microthermal gas mixture sensor (9, 106, 109) is / are the thermal conductivity and / or the thermal conductivity of the gas mixture. [4] Method according to one of the preceding claims 1-3, characterized bythat the material property of the gas detected by the microthermal gas sensor (7, 105) is / are the thermal conductivity and / or the thermal diffusivity of the gas. [5] Method according to one of claims 1-4, characterized by that when the ionization target value is reached, the at least one material property of the gas mixture (8, 107) is measured by means of the gas mixture sensor (9, 106, 109) and stored as a new target value of the sensor signal in the control unit (10, 112). [6] Method according to one of claims 1-5, characterized by that the calibration process is carried out in case of implausibilities of the sensor signal of the gas mixture sensor (9, 106, 109) or cyclically. [7] Method according to the preceding claim, characterized bythat the implausibilities of the sensor signal are determined when the heater is started by first supplying only the gas (2, 104) and applying it to the gas mixture sensor (9, 106, 109), wherein an implausibility exists if the sensor signal measured by the gas mixture sensor (9, 106, 109) does not correspond to a sensor signal for the known gas (2, 104). [8] Method according to one of the preceding claims, characterized bythat when the heater is started, initially only the gas (2, 104) is supplied and the gas mixture sensor (9, 106, 109) is acted upon thereby, wherein subsequently the fuel gas (1, 103) is supplied, the gas mixture (8, 107) is generated and the gas mixture sensor (9, 106, 109) is acted upon with the gas mixture (8, 105), wherein the gas type of the fuel gas (1, 103) is determined from a change in the sensor signal when the fuel gas is supplied and the control unit (10, 112) adapts the gas mixture (8, 107) depending on the determined gas type of the fuel gas (1, 103) until the target value of the sensor signal is reached. [9] Method according to the preceding claim, characterized by that the direction of action of the control is detected from the change in the sensor signal during the supply of the fuel gas (1, 103) and it is determined from this whether the supplied fuel gas quantity is increased or decreased in order to achieve the setpoint value of the sensor signal. [10] Method according to claim 8 or 9, characterized by that the material properties of the gas detected by the gas sensor are continuously checked for changes and the control unit (10, 112) continuously adapts the setpoint value of the sensor signal of the gas mixture sensor (9, 106, 109). [11] Method according to one of the preceding claims, characterized by that the gas (2, 104) is air and the fuel gas (1, 103) can be selected as desired.

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