Flame rectification circuit using operational amplifier

A method for detecting a flame is provided. The method includes the step of providing alternating current to a flame rectification probe to produce a first voltage as an input for a sense circuit, wherein the flame rectification probe is placed in proximity to the flame. The method further includes the step of conditioning the first voltage using the sense circuit to produce a second voltage. Additionally, the method includes the steps of outputting the second voltage to a microcontroller, and determining with the microcontroller whether the flame is present based on a magnitude of the second voltage.

FIELD OF THE INVENTION

This invention generally relates to flame detection and more particularly to a device and method that uses flame rectification to detect the presence of a flame.

BACKGROUND OF THE INVENTION

Direct spark ignition (DSI) controls use the principal of flame rectification to determine if a burner flame is present during a call for heat cycle. The DSI controls utilize a flame probe that is designed in such a way that a small DC current will flow in one direction when an AC voltage is applied to the probe. The DSI control then senses this small current to determine if a flame is present. If a flame is not present, the control may do several ignition attempts and then go into a safety shutdown mode to shut off the gas flow.

Since the flame conducts the majority of current only in one direction, the AC voltage developed in the network has a negative bias if current is present and zero bias if there is no flame current. The average DC signal is negative, but the operating range of the DSI circuit is positive (generally between 0V and +5V). This presents a problem with finding a circuit that will operate when the input is beyond the circuit supply voltage range. In addition, the flame current is very low (in the microampere range), so the sensing circuit has to be very high impedance so it does not affect the flame sense signal.

Conventionally, to achieve the two goals (sensing below 0V and high impedance), flame sense circuits used a junction field effect transistor (JFET). JFETs are normally on with zero gate voltage, and they switch off when a negative gate voltage is applied. Additionally, JFETs have very high impedance. However, to sense in the microampere range, the JFET has to be a non-standard, sorted device with a tighter gate threshold voltage. Further, since the introduction of integrated controllers and microcontrollers, the market for discrete JFETs has decreased, and many previous suppliers no longer manufacture these devices. Thus, because the part has to be sorted, satisfactory JFETs have become even more difficult to obtain.

Even if satisfactory JFETs can be found, they still tend to have a wide switching threshold. Generally, the JFETs can have a gate-source cutoff voltage with the maximum value being five times larger than the minimum value. To improve the switching threshold range, the JFETs must be sorted to even tighter tolerances, which makes finding suitable JFETs still more difficult. Additionally, JFETs are only off or on such that the actual flame current level is unknown.

Embodiments of the invention provide a flame sense circuit that is designed to address the disadvantages of conventional JFET flame sense circuits. In particular, the sense circuit employs readily available components that can accept a negative input voltage and output a positive voltage within the range of standard microcontroller devices. Moreover, the sense circuit can provide information about the condition of the flame beyond simply determining whether a flame is present or not. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.

BRIEF SUMMARY OF THE INVENTION

In one aspect, a method for detecting a flame is provided. The method includes the steps of providing alternating current to a flame rectification probe to produce a first voltage as an input for a sense circuit. The flame rectification probe is placed in proximity to the flame. The method also includes the steps of conditioning the first voltage using the sense circuit to produce a second voltage, outputting the second voltage to a microcontroller, and determining with the microcontroller whether the flame is present based on a magnitude of the second voltage.

In embodiments of the method, the step of conditioning the first voltage includes the steps of reversing the polarity of the first voltage and modifying the magnitude of the first voltage.

In another embodiment of the method, the step of conditioning the first voltage is performing using an operational amplifier. In such an embodiment, the method can include the additional step of performing a component fault check to determine if any of the components of the sense circuit have failed. In a further embodiment, the sense circuit includes an operational amplifier and performing a component fault check includes providing a flame check reset circuit and providing a flame check gain circuit. The flame check reset circuit is operably connected to an inverting input of the operational amplifier and wherein the flame check gain circuit is operably connected to a non-inverting input of the operational amplifier. Still further, the method can further include the steps of sending a first signal from the flame check reset circuit such that the inverting input of the operational amplifier senses a positive voltage and outputting a zero voltage from the sense circuit to the microcontroller. The microcontroller detects a component fault if an output voltage other than zero is provided by the sense circuit.

In another embodiment, the component fault check further includes the steps of sending a first signal from the flame check reset circuit such that the inverting input of the operational amplifier senses a positive voltage, sending a second signal from the flame check gain circuit such that the non-inverting input of the operational amplifier senses an input voltage, and outputting a differential voltage from the sense circuit to the microcontroller. The microcontroller detects a component fault if the differential voltage does not match an expected differential voltage.

In a particular embodiment, the step of providing a flame check reset circuit further comprises providing a flame check reset circuit including a PNP transistor. Further, an emitter of the PNP transistor can be connected to a circuit rail voltage. A base of the PNP transistor can be connected to a reset input in which the reset input is capable of providing a reset signal, and a collector of the PNP transistor can be connected to the inverting input of the operational amplifier. During the step of determining whether a flame is present, the reset signal can be provided to the base of the PNP transistor such that the PNP transistor enters a cutoff mode.

In another aspect, a flame sense circuit configured to receive a first voltage produced by an AC signal source and a flame rectification probe is provided. The flame sense circuit includes an operational amplifier having an inverting input adapted to receive the first voltage, a non-inverting input, and an output. The flame sense circuit also includes a first resistor (R1) connected on a negative feedback loop between the output and the inverting input of the operational amplifier, and a second resistor (R2), R2being connected to the inverting input. The flame sense circuit further includes a microcontroller configured to detect a second voltage from the output of the operational amplifier.

In embodiments of the flame sense circuit, the first voltage is negative in polarity and has a first magnitude when a flame is present at the flame rectification probe. Further still, the second voltage can be positive in polarity and has a second magnitude that is smaller than the first magnitude. Moreover, the second magnitude of the second voltage can be approximately equal to R2/R1multiplied by the first magnitude of the first voltage.

In other embodiments, the flame sense circuit further comprises a flame check reset circuit. The flame check reset circuit includes a transistor having a base, a collector, and an emitter in which the collector is operably connected to the inverting input of the operational amplifier. A flame check reset output provides a third voltage to the base of the transistor, and a fourth voltage is supplied to the emitter. The transistor operates in the cut-off mode when the third voltage is higher than the fourth voltage, and while the transistor is in the cut-off mode, the second voltage of the operational amplifier that is output to the microcontroller depends on the presence and quality of the flame. The transistor operates in the forward-active mode when the third voltage is less than the fourth voltage, and while the transistor is in the forward-active mode, the second voltage of the operational amplifier is compared to a predetermined reset check expected voltage in the microcontroller to determine if a component has failed.

In embodiments, the flame sense circuit also includes a flame check gain circuit. The flame check gain circuit can include a flame check gain output that provides a fifth voltage to the non-inverting input of the operational amplifier while the transistor is in the forward-active mode. In such an embodiment, the second voltage of the operational amplifier can be compared to an expected gain check voltage in the microcontroller to determine if a component has failed.

In other embodiments of the flame sense circuit, R2/R1is less than 1. In still other embodiments of the flame sense circuit, a first capacitor is placed in parallel to R2on the negative feedback loop. In yet another embodiment of the flame sense circuit, a second capacitor is operably connected between a node in common with R1and ground. The flame sense circuit of claim11, wherein the second voltage output by the operational amplifier is constrained to a rail voltage of the microcontroller.

DETAILED DESCRIPTION OF THE INVENTION

In general, a flame sense circuit including an operational amplifier is provided. Using the operational amplifier, the flame sense circuit is able to invert the negative input voltage resulting from flame rectification of the AC signal at the flame probe. Additionally, by adjusting the gain of the operational amplifier, the sense circuit can output to a microcontroller a signal that is within the typical rail voltage of the microcontroller. Further, the gain of the operational amplifier can be manipulated such that the output signal varies linearly with the flame current. As such, the microcontroller can determine not only whether a flame is present but also what the condition of the flame is. While described primarily in the context of a sense circuit for a burner, a person having ordinary skill in the art will appreciate from the disclosure that the flame sense circuit is applicable in other applications.

In order to provide an overall context for the flame sense circuit within an ignition control system,FIG. 1provides a flame sense block diagram10. In the flame sense block diagram10, an AC source12provides the initial signal for operating the ignition control system. Via an AC coupling network14that includes a capacitor, the AC source12is connected to a flame probe impedance circuit16and a bias network18that provides DC averaging. The flame probe impedance circuit16is connected to a flame probe20that is placed in proximity to a burner22. The flame probe20is placed in such proximity to the burner22that, when the burner22is lit, the flame probe20is disposed within the flame24. Because of flame rectification when a flame24is present at the burner22, only positive current from the AC source12is passed through the flame24. This results in a net negative voltage accumulating at the capacitor of the AC coupling network14. The magnitude of the voltage at the AC coupling network14depends on the size and quality of the flame. The bias network18averages this negative DC voltage from the AC coupling network14.

The bias network18is operably connected to a sense circuit26. In turn, the sense circuit26is operably connected to a microcontroller28, which controls the gas valve30for the burner22. The negative DC voltage detected by the sense circuit26is communicated to the microcontroller28. Since the voltage sensed by the sense circuit26is generally large in magnitude and negative in polarity, the sense circuit26does not communicate this sensed voltage directly to the microcontroller28. Instead, the sense circuit26reduces the magnitude of the voltage to the magnitude of the rail voltage of the microcontroller28and switches the polarity from negative to positive. Typically, the voltage detected will be between −25V and 0V. For example, when no flame24is present, the voltage will be 0V, and when a particularly robust flame24is present, the voltage can be up to approximately −25V.

Once the sensed voltage is inverted and reduced to the desired microcontroller input voltage, the microcontroller28determines the status of the flame24at the burner22based on the voltage received. Depending on whether the voltage received indicates that a flame is present or not, the microcontroller will attempt to ignite the gas at the burner22if no flame is sensed or simply monitor the condition of the flame24if a flame24is sensed. If no flame24is sensed after a predetermined number of ignition attempts and/or after a predetermined amount of time, the microcontroller28will signal a gas valve control30to shut off the flow of gas to the burner22to allow any accumulated gas to dissipate.

A basic configuration of the sense circuit26is provided inFIG. 2. As can be seen, the sense circuit26utilizes an operational amplifier (“op-amp”) U1to invert the incoming voltage signal to a positive voltage while reducing the voltage to the rail voltage of the microcontroller28. Resistors R3, R4and capacitor C1form a voltage divider bias circuit such that the voltage at node A is between −25V and 0V. The capacitor C1helps eliminate power supply and pick-up noise that could be coupled into the sense circuit26. Resistor R1helps set the op-amp gain and limits the magnitude of the current entering the op-amp U1. Resistor R1also provides a known high impedance load for the bias network18. As shown inFIG. 2, the input voltage enters the inverting input of the op-amp U1, which causes the op-amp U1to invert the signal. In the basic configuration of the sense circuit26shown inFIG. 2, the non-inverting input of the op-amp U1is connected to ground. Further, as shown inFIG. 2, the power supply pins of the op-amp U1limit the output of the op-amp U1to the rail voltage of the microcontroller28. InFIG. 2, the op-amp U1is provided with negative feedback by coupling resistor R2to the inverting input of the op-amp U1. In this way, the gain of the op-amp U1is set by the ratio of resistor R2to R1(i.e., gain=R2/R1). In the embodiment depicted, the input voltage is larger in magnitude than the rail voltage, and therefore, the gain of the op-amp U1is designed to be less than one for this embodiment.

Exemplary components usable in one embodiment of the basic configuration circuit ofFIG. 2are summarized in Table 1. As mentioned above, the gain of the op-amp U1is less than one for the sense circuit26depicted inFIG. 2. In an exemplary embodiment, resistor R1is selected to be 10 MΩ and resistor R2is selected to be 4.7 MΩ. Thus, the gain of the op-amp U1is 0.274. Accordingly, for example, an input voltage of −10V will result in an output voltage Voutof 2.74V. The gain can be adjusted to give a linear output over the desired range, i.e., for the typical input range of −25V to 0V. The output voltage Voutcan then sent to an analog-to-digital converter (not shown) of the microcontroller28to get a digital representation of the flame sense current. For the sense circuit26depicted inFIG. 2, the output is linear for flame currents of 0 μA to 3.6 μA. Based on the output, and the threshold and hysteresis levels for a flame present condition can be determined in software.

The sense circuit26depicted inFIG. 2has several properties. Initially, as mentioned above, the input bias current is kept low (in the nanoampere range) so that leakage currents do not affect readings. Further, the input common mode range is below zero (−0.3V) because the input is at virtual ground (0V). The output is rail-to-rail, i.e., the sense circuit26has a typical output range of 0V to 5V. In further embodiments, the sense circuit26is characterized by other properties. For instance, in preferable embodiments, the op-amp input pins have phase reversal protection in case the inputs go below zero. In such a case without phase reversal protection, the op-amp U1could invert the output, which could cause a false signal level. In a further preferably embodiment, the op-amp input clamps at −0.7V and handles small input current (<10 μA) without any detrimental effects if the input pin goes below zero volts.

In a further embodiment of the sense circuit26, the sense circuit26is preferably capable of performing a component fault detection method. A control diagram for performing the component fault check method is provided inFIG. 3. The control diagram considers a flame sense input signal resulting from a flame current of 2 μA, which results in approximately a 2.35V output for the flame sense circuit26(i.e., output of the op-amp U1). As can be seen inFIG. 3, normal operation of the burner occurs with the flame_chk_gain set low and the flame_chk_reset set high. When flame_chk_reset goes low, the negative flame input voltage signal gets reset to positive 1V, which sets the output of the op-amp U1to 0V. When flame_chk_gain goes high and flame_chk_reset is low, the op-amp U1then acts as a difference amplifier, resulting in an output of the op-amp U1of approximately 1.2V (for the exemplary configuration of the circuit). By using these checks, component faults can be detected if the op-amp U1does not output the expected value, and the control will go into a lockout or safe condition if a fault occurs. As shown inFIG. 3for the exemplary embodiment, a flame is determined to be present if the signal is above 0.4V.FIG. 3also provides exemplary times for the flame_chk_reset, flame_chk_gain, and flame detection timers.

The component fault detection method is also illustrated schematically in the state diagram provided inFIG. 4. The flow diagram begins with step100in which power is provided to the system. At that time, the flame detection status and check timers are reset. In step110, the system is initialized to begin detecting a flame and to begin performing the component checks. In step120, flame detection begins, and the flame sense input from the sense circuit26(as shown inFIG. 5, discussed below) is read. Flame detection is performed until the flame detection timer expires. At that time, the flame_chk_reset output34(FIG. 5) is enabled and the reset timer is started. In step130, the reset timer expires, and the circuit check timer begins. In step140, the circuit check is performed. During the circuit check, the flame sense input is read, and if a flame is detected, an error is set. Referring back toFIG. 3, during the circuit check, no flame should be sensed because the signal should be 0V as shown in the bottom portion of the control diagram.

As shown inFIG. 4, after the circuit check timer expires, the flame_chk_gain output38(FIG. 5) is enabled and the gain check timer begins. In step150, the gain check is initialized. When the gain check initialization timer expires, the gain check occurs in step160. During the gain check, the flame sense input is read, and an error is set if the gain does not match the expected value. As discussed above with respect to the exemplary embodiments, a value of approximately 1.2V is expected when flame_chk_gain is high and flame_chk_reset is low. This condition is also depicted in the bottom control diagram ofFIG. 3. Returning toFIG. 4, the flame_chk_gain output38(FIG. 5) and the flame_chk_reset output34(FIG. 5) are disabled after the gain check timer expires, and the recovery timer is started. Then, in step170, the reset recovery takes place. During this step, the flame sense input is read, and an error is set if the flame sense input is greater than a predetermined voltage. When the recovery timer expires, the reset recovery step170concludes, and the flame detect timer begins. The method then returns to the flame detection step120. As stated above, by performing this component check method, the controller can monitor the flame status and determine if a component has failed based on whether the output of the sense circuit26(FIG. 5) matches the expected output.

FIG. 5depicts the flame sense circuit26of the ignition control system including components to perform the component fault check method described above. As can be seen inFIG. 5, the components of the basic configuration of the sense circuit26are still present. The flame_chk_reset portion32of the sense circuit26is added at node A of the sense circuit26. The flame_chk_reset portion32includes a PNP transistor Q1with a base connected to the flame_chk_reset output34and resistor R5. The emitter is connected between resistor R6and resistor R7. Resistor R6is connected to the rail voltage, and resistor R7is connected to ground. Because of the split resistor biasing, the voltage at the emitter will be the rail voltage multiplied by R7/(R6+R7). Resistor R8is provided on the connection between the collector of transistor Q1and node A of the flame sense circuit. As discussed above, the flame_chk_reset output34is high during normal operation. In that condition, the base voltage is higher than both the emitter and collector voltage, which puts the transistor Q1in the cut-off mode. As such, the voltage at the inverting input of the op-amp U1will be the voltage from the bias network18, which is typically between −25V and 0V.

As also shown inFIG. 5, the flame_chk_gain portion36of the sense circuit26is connected to the non-inverting input of the op-amp U1. Specifically, the non-inverting input of the op-amp U1is connected between resistor R9and resistor R10. The voltage at the non-inverting input of the op-amp U1is the voltage derived from the split resistor biasing of the source voltage, i.e., the flame_chk_gain output38, and ground. As discussed above, the flame_chk_gain output38is zero during normal operation, so the voltage at the non-inverting input is also zero during normal operation.

Capacitor C2provides filtering of the negative feedback through resistor R2. Filtering is also provided by capacitor C1. Advantageously, the combination of filtering from capacitors C1and C2provides quicker filtering and faster response times to flame status than using just a single, larger capacitor C1. Capacitor C3also provides filter of the source voltage for the op-amp U1so as to avoid ripples in the power supply to the op-amp U1such that the op-amp U1is able to accurately output a voltage to the microcontroller28.

The voltage output of the op-amp U1is provided to the microcontroller28. As shown inFIG. 5, the output of the op-amp U1is split between a first path having resistor R11and a second path having resistor R12. The paths are redundant, and in other embodiments, a single path or three or more paths could also be provided. Using these paths, the microcontroller28receives a signal that corresponds to the presence of the flame as well as the condition of the flame. In an embodiment, the output of the op-amp U1scales linearly between 0V and 5V for a flame current of up to 3.6 μA. In this way, the sense circuit26is able to convert a large, negative voltage created by flame rectification in a DSI control system into a signal usable by the microcontroller28to provide more information about the flame at a burner than conventional DSI control systems.