Patent Publication Number: US-10782023-B2

Title: Flame scanner with photodiode coupled to a signal conditioner to generate an output signal emulating an output signal of an ultraviolet tube flame scanner

Description:
TECHNICAL FIELD 
     The subject matter disclosed herein relates generally to the field of flame scanners, and more particularly, to a flame scanner having a photodiode for flame detection. 
     BACKGROUND 
     Flame scanners are used to detect the presence of a flame in equipment such as furnaces, boilers, etc. Many existing flame scanners use an ultraviolet (UV) tube to sense the presence of a flame. The UV tube generates a pulsed output, where the pulse frequency is proportional to the intensity of the UV light that hits the UV tube. The pulses are used by a controller as an indicator of flame presence or not. 
     The use of a UV tube in a flame scanner has several drawbacks. One drawback is that the UV tube has a short life span (1-10 years). Another drawback is that the UV tube can have an unsafe failure mode. A common failure mode is “runaway” triggering, which indicates a flame is present where there is not a flame present. 
     BRIEF DESCRIPTION 
     According to one embodiment, a flame scanner includes terminals for connection to a controller, the flame scanner comprising: a photodiode to generate a detection signal; and a signal conditioner coupled to the photodiode, the signal conditioner to generate an output signal across the terminals, the output signal emulating an output of an ultraviolet tube flame scanner. 
     In addition to one or more of the features described above, or as an alternative, further embodiments may include wherein the signal conditioner comprises a pulse generator, the pulse generator generating the output signal in response to the detection signal. 
     In addition to one or more of the features described above, or as an alternative, further embodiments may include wherein the signal conditioner comprises an amplifier to receive the detection signal and generate a voltage in response to the detection signal. 
     In addition to one or more of the features described above, or as an alternative, further embodiments may include the signal conditioner comprises a voltage-to-frequency converter to receive the voltage and generate a waveform in response to the voltage. 
     In addition to one or more of the features described above, or as an alternative, further embodiments may include wherein the pulse generator generates the output signal in response to the waveform. 
     In addition to one or more of the features described above, or as an alternative, further embodiments may include wherein the pulse generator comprises a switching element that opens and closes in response to the waveform, the switching element connected across the terminals. 
     In addition to one or more of the features described above, or as an alternative, further embodiments may include wherein the pulse generator comprises a voltage limiting element to control voltage across the terminals. 
     In addition to one or more of the features described above, or as an alternative, further embodiments may include wherein the signal conditioner comprises a voltage converter to convert a voltage from the terminals to a supply voltage for the amplifier and voltage-to-frequency converter. 
     In addition to one or more of the features described above, or as an alternative, further embodiments may include wherein the output signal oscillates between a high value and a low value with a frequency proportional to an intensity of flame sensed at the photodiode. 
     In addition to one or more of the features described above, or as an alternative, further embodiments may include wherein the signal conditioner operates on a high voltage applied across the terminals. 
     In addition to one or more of the features described above, or as an alternative, further embodiments may include wherein the high voltage is about 300 volts. 
     In addition to one or more of the features described above, or as an alternative, further embodiments may include wherein the high voltage is substantially the same voltage used for the ultraviolet tube flame scanner. 
     In addition to one or more of the features described above, or as an alternative, further embodiments may include wherein the signal conditioner draws a low current when a flame is not present at the photodiode. 
     In addition to one or more of the features described above, or as an alternative, further embodiments may include wherein the low current is about 100 microamps. 
     In addition to one or more of the features described above, or as an alternative, further embodiments may include wherein the low current is lower than a flame presence trigger limit of the controller. 
     Technical effects of embodiments of the disclosure include a flame scanner that includes a photodiode for detecting flame presence and a signal conditioner that generates an output signal that is similar to that of a UV tube flame scanner. 
     These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a flame sensing system in an embodiment; and 
         FIG. 2  is a schematic diagram of a flame scanner in an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of a flame sensing system  10  in an embodiment. The flame sensing system  10  includes a flame scanner  12  including a photodiode  14  and a signal conditioner  20 . The photodiode  14  generates a detection signal in the presence of a flame. The photodiode  14  may be implemented using known photodiodes (e.g., silicon, silicon carbide, indium gallium arsenide, etc.). The photodiode  14  may selectively generate a detection signal in response to certain wavelengths of light, such that the photodiode  14  only generates a detection signal when a flame is present. 
     The signal conditioner  20  receives the detection signal from the photodiode  14  and generates an output signal that is used by controller  50  to determine the presence of a flame. The signal conditioner  20  receives power from the controller  50  and operates on substantially the same high voltage that would be typically provided to a UV tube (e.g., about 300 volts AC or DC). The signal conditioner  20  also generates an output signal that is similar to the output signal of a UV tube. Therefore, the flame scanner  12  can replace an existing UV tube without any modifications to controller  50 . 
     The signal conditioner  20  draws low current (e.g., about 100 microamps) when a flame is not present. The controller  50  will trigger and indicate a flame is present if a current exceeding a limit is drawn by the signal conditioner  20 . Hence, the signal conditioner  20  runs on a very small amount of current, so as not to cause a false trigger at controller  50 . 
       FIG. 2  is a schematic diagram of the flame scanner  12  in an embodiment. The signal conditioner  20  includes an amplifier  22  which receives the detection signal from the photodiode  14 . The amplifier  22  may be a transimpedance amplifier (i.e., current to voltage converter) that generates a voltage in response to the detection signal (i.e., a current) from the photodiode  14 . The magnitude of the voltage output by amplifier  22  is proportional to the current output by the photodiode  14 . The output of amplifier  22  is provided to a pulse generator  30 . The pulse generator  30  produces an output signal at terminals  32 , which connect the flame scanner  12  to the controller  50 . 
     The pulse generator  30  includes a voltage-to-frequency converter  24  that generates a waveform (e.g., sinusoid, square wave, etc.) having a frequency proportional to the voltage output by the amplifier  22 . The flame scanner  12  is powered by controller  50  at terminals  32 , receiving a high voltage (e.g., about 300 volts AC or DC) across terminals  32  and drawing a low current (e.g., about 100 microamps). The pulse generator  30  includes a switching element  34  (e.g., a transistor) that opens and closes in response to the waveform from the voltage-to-frequency converter  24 . The switching element  34  is connected across terminals  32 , so that the output signal at terminals  32  will oscillate between a high value when switching element  34  is open (e.g., about 300 AC or volts DC) to a low value (e.g., about 170 volts AC or DC) when switching element  34  is closed. The frequency of the output signal at terminals  32  is proportional to the intensity of the flame sensed at photodiode  14  (e.g., the larger the output current at photodiode  14 , the higher the frequency of the output signal at terminals  32 ). 
     The voltage across terminals  32  is prevented from being zero by voltage limiting elements, including a zener diode  36  and resistance  38 . By selecting values for the zener diode  36  and resistance  38 , the output signal at terminals  32  emulates the output signal of a UV tube flame scanner. An example UV tube flame scanner may produce pulses that oscillate between about 300 volts and about 170 volts. This pulse train is emulated by the pulse generator  30 , so that the controller  50  does not require any modification to work with the flame scanner  12 . Components of the pulse generator  30  may be adjusted to emulate different types of UV tubes. 
     The signal conditioner  20  includes a voltage converter  40  used to power the amplifier  22  and voltage-to-frequency converter  24 . The voltage converter  40  receives input power at terminals  32  (e.g., about 300 volts AC or DC) and converts the input power to a supply voltage suitable for use by the amplifier  22  and the voltage-to-frequency converter  24  (e.g., 5 or 12 volts DC). The voltage converter  40  consumes low current (e.g., about 100 microamps). If excess current is drawn by the signal conditioner  20 , the controller  50  will indicate this as the presence of a flame, resulting in a false trigger. The current drawn by the signal conditioner  20  should be lower than a flame presence trigger limit of the controller  50 . 
     Embodiments provide a solid state flame scanner having a much longer life span than conventional UV tube flame scanners. If the photodiode fails, it fails to produce a detection signal, which means the flame scanner indicates that no flame is present (i.e., safe failure mode). The solid state flame scanner generates an output signal that emulates a UV tube flame scanner, and as such, no modifications are needed to the controller to replace the UV tube flame scanner with the solid state flame scanner. 
     While the disclosure has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the disclosure is not limited to such disclosed embodiments. Rather, the disclosure can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the disclosure. Additionally, while various embodiments of the disclosure have been described, it is to be understood that aspects of the disclosure may include only some of the described embodiments. Accordingly, the disclosure is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.