Abstract:
A system for operating a flame sensing device to obtain readings of increased accuracy without degrading the life of the sensor. There may be levels of a flame requiring a precise measurement. One improvement of accuracy uses higher voltage on the sensor, but this degrades the sensor and thus shortens it life. Further improvement may be achieved by limiting the time that the sensor is operated at a higher voltage. Readings, as if the sensor were operated at a higher voltage, may be inferred from actual readings of the sensor operated at a lower voltage.

Description:
[0001]    The present application is a continuation-in-part of U.S. patent application Ser. No. 10/908,467, filed May 12, 2005, and entitled “Adaptive Spark Ignition and Flame Sensing Signal Generation System”. U.S. patent application Ser. No. 10/908,467, filed May 12, 2005, and entitled “Adaptive Spark Ignition and Flame Sensing Signal Generation System”, is hereby incorporated by reference. 
         [0002]    The present application is a continuation-in-part of U.S. patent application Ser. No. 12/368,830, filed Feb. 10, 2009, and entitled “Low Cost High Speed Spark Voltage and Flame Drive Signal Generator”, which in turn is a continuation-in-part of U.S. patent application Ser. No. 11/773,198, filed Jul. 3, 2007, and entitled “Flame Rod Drive Signal Generator and System”. U.S. patent application Ser. No. 12/368,830, filed Feb. 10, 2009, and entitled “Low Cost High Speed Spark Voltage and Flame Drive Signal Generator”, is hereby incorporated by reference. U.S. patent application Ser. No. 11/773,198, filed Jul. 3, 2007, and entitled “Flame Rod Drive Signal Generator and System”, is hereby incorporated by reference. 
       RELATED APPLICATIONS 
       [0003]    The present application is related to the following indicated patent applications: U.S. patent application Ser. No. 11/741,435, filed Apr. 27, 2007, and entitled “Combustion Instability Detection”; U.S. patent application Ser. No. 11/276,129, filed Feb. 15, 2006, and entitled “Circuit Diagnostics from Flame Sensing AC Component”; U.S. patent application Ser. No. 11/306,758, filed Jan. 10, 2006, and entitled “Remote Communications Diagnostics Using Analog Data Analysis”; U.S. patent application Ser. No. 10/908,466, filed May 12, 2005, and entitled “Flame Sensing System”; U.S. patent application Ser. No. 10/908,465, filed May 12, 2005, and entitled “Leakage Detection and Compensation System”; U.S. patent application Ser. No. 10/908,463, filed May 12, 2005, and entitled “Dynamic DC Biasing and Leakage Compensation”; and U.S. patent application Ser. No. 10/698,882, filed Oct. 31, 2003, and entitled “Blocked Flue Detection Methods and Systems”; all of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0004]    The invention pertains to sensors and particularly to flame sensors. More particularly, the invention pertains to optimization of flame sensing. 
       SUMMARY 
       [0005]    The invention is a system for operating a flame sensing device to obtain readings of increased accuracy without degradation of the life of the sensor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0006]      FIG. 1  is a diagram of a spark voltage and flame signal generation circuit; 
           [0007]      FIG. 2  is a graph showing flame current from four different flame rod configurations over a wide voltage range; 
           [0008]      FIG. 3  is a graph showing an approach for improved accuracy of flame sensing without a need for continuous high voltage; 
           [0009]      FIG. 4  is a flow diagram of a control system for flame sensing; 
           [0010]      FIG. 5  is a graphic example of the voltage adjustment of the control system described in  FIG. 4  based on a typical appliance run cycle; and 
           [0011]      FIG. 6  is a graphic example of the control sampling of the flame signal at various times or zones during an appliance run cycle. 
       
    
    
     DESCRIPTION 
       [0012]    The flame current sensed in an ignition system may depend on the applied voltage. In particular, the relationship between AC voltage and flame current at a given frequency may be different for each application. Not only does this result in less accurate flame readings, but could create a safety concern if not handled properly. In addition, using too high of an AC voltage may cause excessive build-up of contamination on a flame rod, increased energy consumption that generates extra heat, and also stress associated electronic circuitry unnecessarily. 
         [0013]    One possibility for more accurately measuring the flame signal at a given frequency may be to increase the AC voltage when accuracy is critical. It appears that higher voltages reduce the overall differences between different flame rod configurations. Once a flame has been established, the AC voltage may be adjusted to a lower level to avoid excessive component stress, energy consumption, increased electrical noise, and contamination build-up. 
         [0014]    Another approach may be to vary the AC voltage in order to generate a curve of flame readings for a particular flame rod configuration. Once this curve or ratio between different voltages has been determined at a given flame level, a lower AC voltage may be used and the flame sensed value can be scaled as needed. 
         [0015]    An electronic circuit with adjustable AC voltage supply may be used to generate the different voltage levels. This may be accomplished using a resonant circuit such as an inductor-capacitor combination driven at varying duty cycles with a feedback network used to fine-tune the voltage level. The software in an embedded microprocessor may then adjust the AC voltage to the highest level required, say 250Vpk, for most accurate flame sensing, and can readjust to a lower level, say 170Vpk or 90Vpk, to sense less critical flame levels and help extend the life of the system. Other voltage levels may be used, depending on the particular flame sensing apparatus. 
         [0016]    Alternatively, the microprocessor may switch between different voltage levels very quickly and compare the flame readings at each level to determine a ratio factor. Using this ratio factor, the measured flame current at lower voltage levels may be scaled to an equivalent higher voltage reading or via a predetermined lookup table, based on empirical or calculated data, for greater accuracy. 
         [0017]    Either method may limit the amount of time using the highest voltage levels, thus reducing component stress and noise, limiting energy consumption, and improving life of the flame rod with reduced contamination build-up. 
         [0018]      FIG. 1  is a diagram of a spark and flame signal generation circuit  10 . Transistors  11  and  12  and diode  13  form a push-pull drive. DC_voltage  14  relative to a reference terminal or ground  39  may be rectified 24VAC. Voltage  14  may be in the range of 20 to 40 volts. When FlameDrivePWM  15  is at a resonant frequency of the LC circuit  16  containing an inductor  17  and capacitor  18 , a high voltage near sinusoidal waveform may be generated as an output  57  at the common node of inductor  17  and capacitor  18 . The common node or output of circuit  16  may be also regarded as an output terminal  57 . Inductor  17  may have value of about 18 millihenries and capacitor  18  may have a value of about 10 nanofarads. A duty cycle of FlameDrivePWM  15  may be changed with pulse width modulation to control the amplitude of the near sinusoidal waveform. The waveform may be sent to ToFlameRod terminal  19  connected via a D.C. blocking capacitor  36  and current limiting resistor  37 . The waveform may proceed from terminal  19  via a line  65  to a flame rod  44  for flame sensing. Capacitor  36  may have a value of about 2,200 picofarads. Resistor  37  may have a value of about 100 K-ohms. 
         [0019]    A high level voltage does not necessarily exist anywhere in the drive circuit  40  (a 1.5 K-ohm resistor  21 , a 2 K-ohm resistor  22 , diode  23 , diode  24 , diode  13 , transistor  11  and transistor  12 ). So these components may be implemented for low voltage applications and have a low cost. 
         [0020]    Diode  23  and diode  24  may be added to provide current path when the resonant current of the LC network  16  is not in perfect synchronization with the drive signal. To generate a spark voltage on capacitor  25  quickly, the drive may need to be rather strong, and diode  23  and diode  24  may be added to improve the network efficiency and reduce the heat generated on the drive components. 
         [0021]    A spark voltage circuit  50  may include components  25  and  26 . Diode  26  may rectify the AC output voltage from circuit  16  so as to charge up a capacitor  25 . Capacitor  25  may be charged up to a high voltage level for spark generation. Typically, capacitor  25  may be 1 microfarad and be charged up to about 170 volts or so for each spark. 
         [0022]    An output  67  of circuit  50  may go to a spark circuit  68 . Output  67  may be connected to a first end of a primary winding of a transformer  69  and to a cathode of a diode  71 . An anode of diode  71  may be connected to a second end of the primary winding. The second end of the primary winding may be connected to an anode of an SCR  72 . A cathode of SCR  72  may be connected to a reference voltage or ground  39 . A gate of SCR  72  may be connected to controller  43  through a 1 K-ohm resistor  76 . A first end of a secondary winding of transformer  69  may be connected to a spark terminal  73 . A second end of the secondary winding of transformer  69  may be connected to ground or reference voltage  39 . 
         [0023]    When capacitor  25  is charged up, a signal from controller  43  may go to the gate of SCR  72  to turn on the SCR and discharge capacitor  25  to ground or reference voltage  39  resulting in a high surge of current through the primary winding of transformer  69  to cause a high voltage to be across the secondary winding to provide a spark between terminal  73  and ground or reference voltage  39 . 
         [0024]    A diode  38 , a 470 K-ohm resistor  27 , a 35.7 K-ohm resistor  28  and a 0.1 microfarad capacitor  29  may form a circuit  60  for sensing flame voltage from output  57  of LC circuit  16 . Circuit  60  may provide an output signal, from the common connection of resistors  27  and  28  to microcontroller  43 , indicating the voltage amplitude of the drive signal to flame rod  44 . 
         [0025]    A 200 K-ohm resistor  32 , a 200 K-ohm resistor  33 , a 0.01 microfarad capacitor  34  and a 0.01 microfarad capacitor  35  may form a circuit  70  having an output at the common connection of resistor  32  and capacitor  34  for flame sensing which goes to controller  43 . At least a portion of circuit  70  may incorporate a ripple filter for filtering out the AC component of the flame rod drive signal so as to expose the DC offset current of flame rod  44 . The DC offset current may be indicated at the output of circuit  70 . When a flame is present, flame rod  44  may have a corresponding DC offset current. A resistor connected in series with a diode having its cathode connected to ground may be an equivalent circuit of flame rod  44  sensing a flame. When no flame is present, flame rod  44  may have no or little DC offset current. Resistor  31  may be a bias element. Microcontroller  43  may provide a bias  75  input (e.g., about 4.5 volts) to circuit  70  via a 200 K-ohm resistor  31 . As the flame current is flowing from flame rod  44  out to the flame, generating a negative voltage at capacitor  34 , a positive bias  75  is necessary to pull the voltage at capacitor  34  above ground or reference voltage  39  for microcontroller  43  to measure the flame. 
         [0026]    At first power up, a microcontroller  43  may drive a FlameDrivePWM signal at an input  15  with a nearly square waveform shape. The frequency of the FlameDrivePWM signal at terminal  15  may be varied and the flame voltage at line  57  be monitored to find the resonant frequency of the LC network  16 . After that, the drive is generally kept at this frequency, and the duty cycle may be changed so that capacitor  25  can be charged to the required level within the predetermined time interval. This duty cycle may be stored as SparkDuty. The duty cycle may be changed again to find a duty cycle value at which the flame sensing signal is at the desired level, for example, 180 volts peak. This duty cycle value may be saved as FlameDuty. The frequency of the PWM signal  15  may be changed to fine tune the signal amplitude at the output of LC network  16 . 
         [0027]    One may note that if the DC_Voltage  14  changes, the duties may need adjustment. This adjustment may be done continuously and slowly at run time. At spark time, the FlameDrivePWM signal may stay at the SparkDuty value and the spark voltage be monitored. The SparkDuty value may be adjusted as necessary during spark time. 
         [0028]    At flame sensing time, capacitor  25  is to be overcharged some 10 to 20 volts higher than the flame voltage, so that capacitor  25  will not present itself as a burden or heavy load on the LC network  16  and thus the flame voltage at line  57  can be varied quickly. 
         [0029]    The flame sensing circuit  70  may support a high flame sensing rate, such as 60 samples per second. Sixty samples/second may be limited by the fact that the drive and flame signal itself carries a line frequency component, not limited by the circuit. 
         [0030]      FIG. 2  is a graph showing an example of typical flame readings (taken at one flame level) from four different flame rod configurations over a wide voltage range. Data may be empirically obtained by taking flame readings at various voltages for each of the several configurations, and plotted on a graph like that in  FIG. 2  or recorded and arranged in another manner. The flame readings versus peak-to-peak (Pk-Pk) voltage for configurations  1 ,  2 ,  3  and  4  are plotted as revealed by curves  81 ,  82 ,  83  and  84 , respectively. A high voltage flame circuit as described in  FIG. 1  may be used to generate the high voltage needed for flame rectification. As the graph shows, expected accuracy at a flame excitation voltage of 320V pk-pk is about +/−20 percent. At 520V pk-pk, the accuracy improves to better than +/−5 percent at area  85 . Whenever accuracy of the flame readings is critical, the highest excitation voltage could be used. When flame readings are high and accuracy is less critical, lower excitation voltages may be used to reduce power consumption and noise, extend life of electrical components, and reduce contamination build-up on the flame rod  44 . 
         [0031]      FIG. 3  is a graph showing an approach to gain improved accuracy without the need for continuous flame sensing at a high excitation voltage. The approach includes measuring the flame at a lower voltage and scaling the flame readings to an equivalent higher voltage flame level. A current ratio to 520V readings versus lower Pk-Pk voltages at a given flame level is graphed in  FIG. 3  for four different flame rod configurations. To determine which scaling factor to use, a comparison of the flame readings at two different voltages may be done resulting in a “current ratio.” For example, in this graph, configuration  1  has a current ratio between 320V pk-pk and 520V pk-pk of just over 0.80, as shown by curve  86 , while configuration  2  has a ratio of just less than 1.30, as shown by curve  87 . The ratios for configurations  3  and  4  are shown by curves  88  and  89 . Data in the graph of  FIG. 2  may be used to determine the ratios plotted in the graph of  FIG. 3 . These current ratios may be used to directly scale a lower voltage flame reading to their equivalent higher voltage levels. Another implementation of this scaling may include dividing the current ratios into predetermined groups  1  through  3 , as shown in  FIG. 3 . Group 2 may include both configurations  3  and  4 , represented by curves  88  and  89 , respectively, since their current ratios are very close, and as expected in  FIG. 2  their actual flame readings are very close. Group 1 may include curve  87  and group  3  may include curve  86 . Additional data may be taken and other calculations made for plotting points on the graphs in  FIGS. 2 and 3  for different flame rod configurations. Since the ratios in  FIG. 3  are based on 520 volts pk-pk readings, the ratios of the configurations converge to one at that level as indicated at area  80 . Additional current levels other than those shown in  FIGS. 2 and 3  may be used for calculating the flame scaling ratios. These measurements can be referenced by any equivalent voltage units as appropriate, such as pk-pk, pk or rms. Since the ratios shown are for one particular flame level, additional ratios may be calculated to cover the entire operating range of flame currents for greatest accuracy. 
         [0032]    The approach for using low voltages to obtain high voltage-like readings may require an initial calibration period when the voltage levels are quickly changed between high and low levels; but once the respective current ratio is established, control may be allowed to run at a low excitation voltage and result in reduced stress on components as noted herein. 
         [0033]    A formula may be used for various calculations related to flame sensing. R H1  may be regarded as a relatively accurate flame reading of a flame sensor, for example, configuration  1  at a designated high voltage. V H  may represent the designated high voltage for the sensor at a flame reading in the area  85  of  FIG. 2 , which may be regarded as a relatively accurate area of flame readings from flame sensors of various configurations. R L1  may be a flame reading of a flame sensor of the configuration  1  taken at a sensor voltage V L  which would have a magnitude less than that of V H . A flame reading divided by the sensor voltage may be a ratio. For example, r L1  may represent the ratio for R L1 /V L  and r H1  may represent the ratio for R H1 /V H  involving a flame sensor of configuration  1 . A current ratio relative to the V H  flame reading for configuration  1  may be designated as r C1  which may equal r L1 /r H1  or (R L1 /V L )/(R H1 /V H ). 
         [0034]    For instance, to calculate the reading-to-voltage ratio (r L1 ) for configuration  1  at a reading for a pk-pk voltage of 320 (V L ), one may note a flame reading of 800 units (R L1 ), as shown by point  121  on curve  81  in  FIG. 2 . A reading-to-voltage ratio (r H1 ), and for a pk-pk voltage of 520 (V H ), one may note a reading of about 1600 units (R H1 ) at point  122  on curve  81 . One may divide 800 units by 320 volts to obtain 2.50 units per volt (r L1 ), and divide 1600 units by 520 volts to obtain about 3.08 units per volt (r H1 ). To obtain the current ratio for the readings of configuration  1  at 320 volts and 520 volts, one may divide the 2.50 flame reading units per volt at the 320 volt reading by the 3.08 flame reading units per volt at the 520 volt reading to obtain a current ratio of about 0.8125 (r C1 ). This ratio may be plotted as point  123  as part of plot or curve  86  for configuration  1  on the graph in  FIG. 3 . The flame reading at 520 volts may be regarded as the most precise reading (e.g., a touchstone) since the readings of all the configurations may converge at area  85 . With the current ratio (r C1 ) for a flame reading from a flame sensor of configuration  1  at a low 320 volt level, one may calculate, scale or extrapolate a relatively precise flame reading at a high 520 volt level. One may take the r C1  equation and derive R H1 =(R L1 V H )/(r C1 V L ). If a low voltage reading (V L ) is 800; calculating for the reading R H1  as it should be with the high sensor voltage V H , one may get (800×520)/0.8125×320)=1600. One may convert other readings at the low voltage for obtaining readings as they would be if obtained at the high voltage. The present approach may be used for obtaining readings for other configurations and voltages. This portion of the approach may be in a look-up table, program, or other form of control. The general approach may be in a look-up table, program, input, or other form of stored control or processing. An advantage of the approach is that without actually running a flame rod and associated components at the high voltage, one may still obtain high-voltage precision readings and avoid excessive component stress, energy consumption and contamination build-up which would occur when obtaining flame readings using high voltage on the flame sensor. 
         [0035]    Similar calculations for current ratios may be done for other flame readings at other voltages for the flame sensor or sensing rod  44  ( FIG. 1 ) of configuration  1 . Flame readings may be taken for configurations  2 ,  3  and  4  as shown in the graph of  FIG. 2 . Calculations may be performed to obtain current ratios for flame sensor or sensing rod configurations  2 ,  3  and  4 , and be plotted as shown in the graph of  FIG. 3 . Data and calculations may be obtained and plotted for other configurations. The voltages used may also be different. In summary, the information of  FIGS. 2 and 3  may be used for obtaining flame readings measured at lower voltages which are nearly as accurate as if these readings were measured at optimally higher voltages.  FIGS. 2 and 3  were plotted for one flame level (i.e., 0.7 micro amp). At other flame current levels, the curves may be different. Thus,  FIGS. 2 and 3  may be plotted for other flame levels. 
         [0036]      FIG. 4  is a diagram  90  of control system of a high level example of the operational flow for an approach of changing between three flame excitation voltage levels—high, nominal, and low. The control may typically operate at the nominal voltage level unless the flame drops below a critical threshold, at which time the excitation voltage may adjust to a higher level for greatest accuracy as shown in  FIG. 2 . On the other hand, if the flame increases to a higher, less critical level, the excitation voltage may adjust down to a lower level and reduce stress on components. Nominal may be regarded as between low and high. 
         [0037]    Flow diagram  90  in  FIG. 4  of a control system which may be run by controller  43  of  FIG. 1  may begin with a symbol  91  which asks whether the flame is in a critical range. If the answer is yes, then the flame voltage is a high voltage at block  92 , which means the flame scaling is high as indicated in block  93 . Then the system may return to symbol  91  to inquire again whether the flame is in the critical range. If the answer is no, then the system may go to symbol  94  which asks whether the flame is greater than the high flame threshold. If the answer is yes, then the flame voltage is equal to a low voltage as indicated by block  95 , which means that the flame scaling is low as indicated in block  96 . Then the system may return to symbol  91  to inquire again whether the flame is in the critical range. If the answer is no, then the system may go to symbol  94  which asks whether the flame is greater than the high flame threshold. If the answer is no, then the flame voltage is equal to the nominal voltage as indicated by block  97 , which means that the flame scaling is nominal as indicated in block  98 . The system may return to symbol  91  and repeat the inquiries and indications about the flame, voltage and scaling. 
         [0038]      FIG. 5  is a diagram of a graphic example of the voltage adjustment of the control system described in diagram  90  of  FIG. 4  based on a typical appliance run cycle. The top curve  100  shows the flame current of an appliance as it slowly increases at first through the beginning zone  101 , the critical zone  102  and nominal zone  103 , stabilizes at a high zone  104  level, and then drops off during zones  105  and  106  at the end of the cycle. The control flame voltage is shown on the bottom curve  110  and may be adjusted depending on whether the flame is in the critical, nominal, or high zone or range  102 ,  103  or  104 , respectively. 
         [0039]      FIG. 6  is a diagram of a graphic example of the control sampling  111  of the flame signal at various times, durations or zones  101 ,  102 ,  103 ,  104 ,  105  and  106 , during a typical appliance run cycle. Since the flame signal may be inherently unstable, especially in appliances that have a lot of air movement, it is important to take enough samples to accurately sense the flame. During generally normal running conditions such as in zones  103 ,  104  and  105 , the flame just needs to be sampled periodically  111  to maintain normal operation, for example only 20 percent or some of the time, thus reducing stress on the flame components. If the flame has reached a critical level in zone  102  or  106 , the flame sampling  111  may become continuous to ensure the flame is sensed accurately and quickly. 
         [0040]    In the present specification, some of the matter may be of a hypothetical or prophetic nature although stated in another manner or tense. 
         [0041]    Although the present system 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 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.