Patent Application
20060257802
The invention pertains to sensors, and particularly to flame sensors. More particularly, the invention pertains to circuitry for flame sensors.
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.
The invention may include a flame sensor for a control system having at least one floating reference point and diagnostics relating to the system.
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'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.
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.
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.
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.
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., <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.
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).
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.
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
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.
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.
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
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.
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.
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.
In the present specification, some of the matter may be of a hypothetical or prophetic nature although stated in another manner or tense.
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.
