Abstract:
A low cost flame sensing system having at last one floating point. For instance, the system may have two grounds. There may be a flame sensing rod for detecting a flame which has a model circuit which appears upon the existence of the flame proximate to the sensing rod. The sensing rod may function without an explicit or dedicated excitation source connected to it. There may be diagnostics in the system for detecting leakage or shorts of the sensing rod to ground. Also, the system may have AC grounding phase detection.

Description:
BACKGROUND  
       [0001]     The invention pertains to sensors, and particularly to flame sensors. More particularly, the invention pertains to circuitry for flame sensors.  
         [0002]     The present application is related to the following indicated patent applications: attorney docket no. 1161.1224101, entitled “Dynamic DC Biasing and Leakage Compensation”, U.S. application Ser. No. ______, filed ______; attorney docket no. 1161.1225101, entitled “Leakage Detection and Compensation System”, U.S. application Ser. No. ______, filed ______; and attorney docket no. 1161.1228101, entitled “Adaptive Spark Ignition and Flame Sensing Signal Generation System”, U.S. application Ser. No. ______, filed ______; which are all incorporated herein by reference.  
       SUMMARY  
       [0003]     The invention may include a flame sensor for a control system having at least one floating reference point and diagnostics relating to the system. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0004]      FIG. 1  is a circuit of a flame sensing system;  
         [0005]      FIG. 2  is another circuit of the flame sensing system;  
         [0006]      FIG. 3  is a graph of flame sensing signal relative to a ground and flame on and off signals;  
         [0007]      FIG. 4  is a diagnostic circuit for the flame sensing system; and  
         [0008]      FIG. 5  is a graph of two out-of-phase signals from half-wave rectified power input signals. 
     
    
     DESCRIPTION  
       [0009]     Hydrocarbon flames may have certain electrical properties. A commonly used electrical flame model may be a diode in series with a resistor and a leakage resistor in parallel with the diode and resistor combination. Many flame detectors rely on the flame diode behavior. These detectors may have a relatively high voltage AC signal coupled to the flame (detector) through a capacitor. When a flame exists, because of the flame diode effect, a DC offset voltage may appear. Flame detection may be realized by detecting the existence and amplitude of the DC offset component. When the flame is weak, the series resistance (according to the flame model) may be quite large, resulting in the generating of a very small DC component and then making flame detection more difficult. To compensate for the reduced DC component, the device for detecting a weak flame may have to be very sensitive, or the AC excitation voltage may need to be increased up to several hundred volts. If a standard line voltage is used, then filtration of the low-frequency AC component may require high ohm filter resistors that slow a circuit&#39;s detection of a flame and add vulnerability to leakage. If a high-frequency voltage AC signal is generated locally to avoid the problems of high ohm resistors, then the cost of the flame sensing system may increase significantly. The present invention may provide a solution to the noted problems by utilizing the leakage resistor of the flame model rather than the diode. Leakage may be used for diagnostic purposes. The phases between certain components and one of the grounds may have a synch or out-of-synch relationship. This relationship may also be used for diagnostic purposes. There may be other leakage detected.  
         [0010]      FIG. 1  reveals a flame sensing system that does not have a flame excitation signal at the flame sensing rod. Instead, the sensing system uses the voltage difference between an earth ground  11  and a control ground  12  to detect the current path provided by the flame. The flame sensing system, without circuit to generate the excitation signal, may be of very low system cost. The system may have a system reference point  12  (i.e., the control ground) floating relative to the earth ground  11 . An AC power supply  13  may be common line power or 24 volts AC from a transformer or other power source. One end of the AC power supply  13  may be connected to the earth ground  11  which may also be regarded as an appliance ground. The ground  11  connected to one end of the AC supply  13  may be designated as a C phase. The other end  14  of the supply  13  may be designated as an R phase. The anode of diode  15  and the cathode of diode  16  may be connected to a lead  14  of the AC supply  13 . The anode of diode  17  and the cathode of diode  18  may be connected to lead  65  of the supply  13 . The cathodes of diodes  15  and  17  may be connected to each other. The lead  65  and ground  11  may be commonly connected. The anodes of diodes  16  and  18  may be connected together and to a circuit or control ground  12 . Diodes  15 ,  16 ,  17  and  18  may form a full-wave rectifier  19 . A load resistor  21  may have one end connected to the cathodes of diodes  15  and  17  and the other end connected to the anodes of diodes  16  and  18 . The ends of resistor  21  may look at a full-wave DC output of rectifier  19  which is a rectification of the AC output of supply  13 . Resistor  21  may represent a control system load, such as for example, supporting electronics and/or a microcontroller  40 .  
         [0011]     A first flame resistor  22  may have an end connected to the appliance or earth ground  11 . A second flame resistor  23  may have an end connected to the ground  11 . A flame diode  24  may have a cathode connected to the other end of resistor  22  and an anode connected to the other end of resistor  23 . The flame diode  24 , the first flame resistor  22  and the second flame resistor  23  may make up a model circuit or network  25  that indicates a presentation of a flame.  
         [0012]     A resistor  26  may have one end connected to a flame rod  62 . The other end of resistor  26  may be connected to a terminal  29 . One end of a resistor  27  may be connected to the terminal  29  and the other end of the resistor  27  may be connected to the circuit ground  12 . Also shown is a dashed-line resistor symbol  53  representing a leakage current path from rod  62  to ground  11 . Resistor  26  and resistor  27  may form a flame detection interface circuit  31 . Resistors  26  and  27  may form a voltage divider. Resistor  26  may provide current limiting of flame detection signals to an analog-to-digital (A/D) converter input which is connected to the terminal  29 . The resistor  27  may help to convert the flame current into a flame voltage. Also, resistor  27  may pull down the A/D input at terminal  29  when there is no signal present to the A/D input. Optionally, a capacitor (not shown) may be connected in parallel with resistor  27  to filter out any induced noise at terminal  29 . A flame signal from circuit  25  may go via resistor  26  and node or terminal  29  to the A/D converter of a microcontroller  40 .  
         [0013]      FIG. 2  shows a circuit configuration  20  which may be partially different than that of circuit  10  in  FIG. 1 . Source  13  is like that of circuit  10  in that it may be a line voltage of about 115 or 220 volts at 50 or 60 Hz or so. It may instead be 24 volts or some other low voltage. The source  13  may be a secondary winding of a transformer. The source  13  may have one side connected to the appliance ground  11 . If an AC voltage that is used is about 100 volts or higher, then a low cost flame sensing approach may be implemented (e.g., a voltage increaser might not be needed). One end of a capacitor  61  may be connected to the R-phase line  14 . Capacitor  61  may be a DC blocking capacitor. The other end of capacitor may be connected to resistor  26  of network  31  and to a sensing flame rod  62  which is connected to a representative or model circuit  25  which appears electrically when a flame is sensed. When a flame is not present, the electrical equivalent circuit  25  may appear as open or non-existent concerning diode  24  and resistors  22  and  23 . However, current leakage may remain in absence of a sensed flame, as its path may be represented by a resistor symbol  53 . The cathode of diode  24  and one end of the resistor  23 , when model circuit  25  appears during the sensing of a flame, may be connected to the earth or appliance ground  11 . Leakage path  53  likewise may connect flame rod  62  to ground  11 .  
         [0014]     Resistor  26  may be part of a voltage divider that includes a resistor  27 . An optional capacitor  28  (shown) may be connected in parallel with resistor  27 . The other end of resistor  27  may be connected to the circuit or control ground  12 . An output  29  of the network  31  may go to an A/D converter of a microcontroller or processor  40 . The controller or processor may be electrically referenced on or tied to a circuit or control ground  12 . The circuit or control ground  12  may float relative to the appliance or earth ground  11 .  
         [0015]     Resistor  27  and capacitor  28  may be selected such that a time constant of resistor  27  and an optional capacitor  28  equals to about 0.3 to 1.0 portion of a half-cycle of time of the AC power supply  13  output. With this time constant value, the peaks of the flame signal may appear at about the zero-crossing time of the C phase pulses (i.e., &lt;90 degrees out of phase), and the peak-to-peak value of the flame signal may be attenuated very little. One set of exemplary values may include resistor  26  as one megohm, resistor  27  as one megohm, and the optional capacitor as 4700 picofarads.  
         [0016]     The leakage of the flame rod  62  may occur due to, for example, old or weak insulation. There may be cross-leakage or other kinds of leakage. The leakage may be measured for calibration purposes. A leakage component may be used to detect a flame rod short, open, or leakage to something such as one of the grounds or components. Leakage may range from the nanoampere to the microampere range. For instance, there may be a one microampere of leakage current and the flame sensor may be usable for flame detection purposes despite a 200 nanoampere signal indicating a flame. Flame indication currents may range from hundreds of nanoamperes to several tens of microamperes. If the leakage current is beyond a level where the system can not be comfortably relied on, the system may be calibrated relative to the leakage (e.g., with a leakage current magnitude subtracted from a flame indication signal).  
         [0017]      FIG. 3  reveals waveforms of the C phase pulses  32 , a flame on time  33  and off time  34 , and a flame signal  35  at the A/D input terminal  29 . The C phase peaks  32  may be about 33 volts for a 24 volt AC powered system and about 162 volts for a 115 volt AC powered system. The floor  36  of the C phase pulses  32  may be about one diode drop below the circuit ground  12  level  54 .  
         [0018]     There may be several situations involving flame rod sensor leakage: no flame and no leakage; no flame and some leakage; a flame and no leakage; and a flame and some leakage. These combinations may be apparent on the signal at the terminal  29  to the A/D converter of the controller or processor  40 . When a flame exists, the flame leakage resistor  23  may provide a current path from the C phase to the interface circuit  31 . The resulting current may produce a flame voltage signal at the A/D input  29 . The micro controller  40  may note the peak-to-peak value of the flame voltage signal and determine if a flame exists and if so whether the flame is strong enough. When a flame does not exist, the current path may be open and no flame signal is present at the A/D input  29 . Consequently, the flame diode  24  and the series flame resistor  22  appear to have little or no effect on the flame leakage detection mechanism. Inherently, the flame circuit  25  appears to be sensitive to current leakage from the earth ground  11  to flame rod  62 .  
         [0019]     When there is no flame, the circuit  25  is open or at that time non-existent. However, there may be current leakage of the flame rod  62  when there is no flame, which may be represented by a resistance  53  as shown in circuit  20  in  FIG. 2 . This resistance  53  and resultant leakage may exist even when there is no flame. In  FIGS. 1 and 2 , rod leakage resistor  53  appears in parallel with flame resistor  23 . Therefore, resistor  53  may produce the same signal as shown by waveform  35  in  FIG. 3 . Waveform  35  shows the C-phase signal appearing at A/D input if flame resistance  23  or leakage resistance  53  exists. Waveform  35  may be of a circuit without the capacitor  28  in the interface circuit  31 . The noted waveforms in  FIG. 3  are example representations of the signals for illustrative purposes. These representations may vary in shape, magnitude and timing due to various circuit elements, component values, and signal and element parameters.  
         [0020]     As the rod leakage resistance  53  may produce the same signal as flame resistance  23  can, one may need to take necessary precautions to limit the leakage path and check for leakage during operation. A printed circuit board (PCB) of the system may be laid out such that resistor  26  is well isolated from earth ground  11  connections. The flame rod and flame wire should likewise be well insulated. The leakage may and should be checked during each heating cycle involving a sensed flame. Before a flame is lit, the signal caused by leakage may be measured and the peak-to-peak value checked against a predetermined threshold. If the value is too high, then the flame sensing circuit may be unreliable because of high leakage. There may be a device with a warning indicating such. Otherwise, the peak-to-peak value of the leakage signal may be used as an offset value and be subtracted from the flame signal  35  when the flame is on as indicated by signal  33 .  
         [0021]     This approach may also be used to detect the presence of a short circuit between the flame rod  62  and the earth ground  11 , such as an appliance ground, which may be a nuisance problem common during related appliance servicing. When the flame rod  62  is shorted to the appliance or earth ground  11 , a very large C-phase component may be noticed at the A/D input  29 . This peak value may be compared with a measured value for the C-phase and a determination may be made if the flame rod is shorted, or not, to the earth ground  11 . If the flame rod  62  is determined to be shorted, then a control system may annunciate some kind of a problem alert to a service person.  
         [0022]     This approach may also be used to detect which phase of a low voltage transformer of a source  13  is connected to earth ground  11 . For example, if a circuit  30  of  FIG. 4  is directly connected to one of the transformer  41  connections  45  or  46 , it may compare the phase (R or C) of that connection with the signal measured by the flame sense input. If the flame sense signal is in phase with the reference transformer  41  connection, it may be assumed that the R-phase is connected to the earth ground  11 . Otherwise, if the flame sensor signal may be more out of phase with the referenced transformer connection, it may be assumed that the C-phase of the transformer is grounded. As shown by the reference phase (R phase) waveform  37  and the flame detector phase (C phase) waveform  38  in  FIG. 5 , which are not in phase with each other, it may be determined that the reference phase is not connected to the earth ground  11 .  
         [0023]     Circuit  30  that may be utilized for determining which phase of a low voltage transformer  41  is earth grounded, as described above. Transformer  41  may have an AC input to leads  42  and  43  of its primary winding. The transformer  41  may provide isolation between the circuit  30  and an AC supply  44 . The secondary winding may output a 24 volt AC signal at leads  45  and  46 . The output of the transformer  41  may go to a full-wave bridge rectifier  19 . Control electronics  40  may be connected across the rectifier  19 . Control electronics  40  may include input analog-to-digital converter (ADC)  63  and ADC  64 .  
         [0024]     Lead  45  may be connected to an anode of diode  17  and a cathode of diode  18 . Lead  46  may go to an anode of diode  15  and a cathode of diode  16 . The cathodes of diodes  15  and  17  may be connected together. The anodes of diodes  16  and  18  may be connected to a circuit ground  12 . Lead  46  of the secondary winding may be connected to an earth or appliance ground  11 . A resistor  66  may have one end connected to lead  45 , and have the other end connected to one end of a resistor  67 . The other end of resistor  67  may be connected to circuit ground  12 . The connection between resistors  66  and  67  may be a reference point  47 . Resistors  66  and  67  may constitute a network  51 . Point  47  may reveal a signal of ground  11  relative to ground  12  since the ADCs  63  and  64  may use a circuit ground  12  reference.  
         [0025]     A resistor  27  may have one end connected to the circuit ground  12 . The other end of resistor  27  may be connected to one end of a resistor  26 . The other end of resistor  26  may be connected to flame rod  62  which in turn is connected to lead  46  of transformer  41  and ground  11  through flame resistor  23  when a flame exists. The connection between resistors  27  and  26  may be regarded as a flame sense point  48 . Resistors  27  and  26  may constitute a network  52 . A reference point  47  of network  51  may be connected to ADC  63  and flame sense point  48  of network  52  may be connected to ADC  64  of control electronics  40 . The signal to ADC  63  may indicate a phase sensing and the signal to ADC  64  may indicate a flame sensing signal imposed on a phase signal relative to ground  12 . The signals to ADC  63  and ADC  64  may be about 180 degrees out of phase relative to each other under normal circumstances.  
         [0026]     In the present specification, some of the matter may be of a hypothetical or prophetic nature although stated in another manner or tense.  
         [0027]     Although the invention has been described with respect to at least one illustrative example, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.