Patent Publication Number: US-7723992-B2

Title: Detector for an ultraviolet lamp system and a corresponding method for monitoring microwave energy

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to ultraviolet lamp systems and, more particularly, to detection of microwave energy from microwave-excited ultraviolet lamp systems. 
   BACKGROUND OF THE INVENTION 
   Ultraviolet (UV) lamp systems are commonly used for heating and curing materials such as adhesives, sealants, inks, and coatings. Certain ultraviolet lamp systems have electrodeless light sources and operate by exciting an electrodeless plasma lamp with microwave energy. In an electrodeless ultraviolet lamp system that relies upon excitation with microwave energy, the electrodeless lamp is mounted within a metallic microwave cavity or chamber. One or more microwave generators, such as magnetrons, are coupled via waveguides with the interior of the microwave chamber. The magnetrons supply microwave energy to initiate and sustain a plasma from a gas mixture enclosed in the electrodeless lamp. The plasma emits a characteristic spectrum of electromagnetic radiation strongly weighted with spectral lines or photons having ultraviolet and infrared wavelengths. 
   To irradiate a substrate, the ultraviolet light is directed from the microwave chamber through a chamber outlet to an external location. The chamber outlet is capable of blocking emission of microwave energy while allowing ultraviolet light to be transmitted outside the microwave chamber. A fine-meshed metal screen covers the chamber outlet of many conventional ultraviolet lamp systems. The openings in the metal screen transmit the ultraviolet light for irradiating a substrate positioned outside the microwave chamber; yet substantially block the emission of microwave energy. 
   In order to protect operators of the ultraviolet lamp systems, RF sensing devices are placed between the operator and the lamp. These RF sensing devices are connected to microwave energy detectors, which are set, based on OSHA standards (similar to standards for microwave ovens), to detect microwave energy in excess of a predetermined amount, e.g., about 5 mW/cm 2 . If microwave energy output levels exceed this amount, the microwave energy detector is configured to shut down the system to limit the exposure of the operator to the microwave energy. 
   Microwave energy detectors may contain some components that are radiation sensitive. The microwave energy detectors can become damaged and not accurately report excessive microwave energy emissions if one or more of the radiation sensitive components fails. For example, the mesh screens that are used to cover the chamber outlets and block microwave energy are typically made of metals such as tungsten wire and are fairly delicate so they may be easily damaged, allowing microwave energy out of the microwave cavity. In extreme cases, attempts to operate the ultraviolet lamp system may be made with the protective screen removed. In these cases the detector should prevent the operation of the lamp system. However, the detector and supporting circuitry may be damaged by the excess microwave energy and damaged in such a way that the detector allows continued operation of the lamp system as damaged components become open or short circuits. 
   SUMMARY OF THE INVENTION 
   The invention provides an ultraviolet lamp system including a microwave energy generator and an electrodeless lamp configured to emit ultraviolet light when excited by microwave energy generated from the microwave energy generator. The ultraviolet lamp system further includes a detector for detecting an excessive amount of microwave energy. The detector includes a first circuit configured to detect microwave energy, where the first circuit includes at least one radiation sensitive component capable of failing upon exposure to the excessive amount of microwave energy. A second circuit is coupled to the first circuit and is configured to intermittently test whether the radiation sensitive component has failed. 
   The second circuit is configured to supply a known signal to the first circuit to test the radiation sensitive component. The known signal may be an RF signal, a low frequency AC signal, or a DC signal. The second circuit is further configured to temporarily suspend the detection of the microwave energy by the first circuit to test the radiation sensitive component. The second circuit tests the radiation sensitive component by detecting an open or short circuit across the radiation sensitive component, which may be a detector diode or resistor. In some embodiments, the microwave energy generator for the ultraviolet lamp system is a magnetron. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the principles of the invention. 
       FIG. 1  is a block diagram of an Ultraviolet Lamp System. 
       FIG. 2  is a block diagram of an exemplary embodiment of a detector circuit, including a test circuit, for a microwave energy detector in the UV Lamp System of  FIG. 1  consistent with the invention. 
       FIG. 3  is a schematic diagram of an exemplary embodiment of the test circuit of  FIG. 2 . 
       FIG. 4  is a flowchart of an exemplary embodiment of the invention illustrating the test circuit working in conjunction with the detector circuit of the microwave energy detector. 
   

   DETAILED DESCRIPTION 
   Various embodiments disclosed herein provide a microwave energy detector for an ultraviolet lamp system including a test circuit coupled with a detector circuit. The test circuit is operable to check the integrity of one or more radiation sensitive components in the detector circuit, and to provide additional safeguards to an operator of the ultraviolet lamp system. In some embodiments, the test circuit may suspend the operation of the microwave energy detector to test the radiation sensitive component(s) in the detector circuit by introducing known signals to the detector circuit and comparing the microwave energy detector output against theoretical values based on the known test signals. A test result that indicates failure of the radiation sensitive component(s) may cause the ultraviolet lamp system to shut down, as would a detection of excessive energy from the microwave energy detector. The term circuit in the embodiments is used to refer to both an individual collection of electrical elements forming an electrical circuit and a collection of electrical circuits that perform a particular function. 
   While radiation sensitive components may fail in various other ways, one failure mechanism for such components relates to the fine mesh screens commonly used to cover the aperture of a UV lamp system. As discussed above, fine mesh screens, typically provided on the front of the ultraviolet lamp, are used to minimize the amount of microwave energy coming from the microwave chamber through the aperture with the electromagnetic radiation (ultraviolet light). The microwave energy is continually monitored by microwave energy detectors as discussed above, which may shut down the microwave energy generator and operation of the lamp if the microwave energy exceeds preset safety levels. It has been found, however, that operation of an ultraviolet lamp system with the fine mesh screen removed or damaged can expose radiation sensitive components in the detector circuit of the microwave energy detector to excessive microwave energy, and cause those components to fail, preventing the detector circuit from reporting excessive output of microwave energy. 
   Referring now to the drawings where like numbers denote like components among the several views,  FIG. 1  is a block diagram of an ultraviolet lamp system  10  that relies upon excitation of an electrodeless lamp  12  with microwave energy. The electrodeless lamp  12  is mounted within a metallic microwave chamber  14 . One or more microwave energy generators  16   a ,  16   b , e.g., magnetrons, are coupled via waveguides  18   a ,  18   b  with the interior of the microwave chamber  14 . The microwave energy generators  16   a ,  16   b  supply microwave energy to the electrodeless lamp  12  in order to generate ultraviolet light  20 . The ultraviolet light  20  is directed from the microwave chamber  14  through a chamber outlet  22  to an external location through a fine-meshed metal screen  24  which covers the chamber outlet  22  and is capable of blocking emission of microwave energy, while allowing the ultraviolet light  20  to be transmitted outside the microwave chamber  14 . 
   An RF sensing device  28 , which is coupled to a microwave energy detector  30 , protects operators  26  of the ultraviolet lamp system  10  from exposure to excessive levels of microwave energy. The RF sensing device  28  is placed between the operator  26  and the lamp  10 . The microwave energy detector is configured to detect microwave energy in excess of a predetermined amount, e.g., about 5 mW/cm 2 . If the UV lamp system  10  outputs microwave energy levels exceeding the predetermined amount, the microwave energy detector  30  is configured to directly or indirectly shut down the UV lamp  10  to limit the exposure of the operator  26  to the microwave energy. 
     FIG. 2  is block diagram providing additional detail of an exemplary embodiment of a microwave energy detector  30  having a diode detector circuit  40  with a test circuit  60 - 66  consistent with the invention. An RF sensing device  28 , such as an antenna  29 , is placed between the lamp  10  and an operator  26  as a protection measure for the operator  26  as illustrated in  FIG. 1 . Receptacles  29   a - 29   c  ( FIG. 3 ) of the antenna  29  receive microwave energy generated by a microwave generator  16   a ,  16   b  and radiated from the microwave cavity  14  through the aperture  22 . The antenna  29  is electrically connected to a first circuit configured as a diode detector circuit  40 , which may contain radiation sensitive components, such as a detector diode  70  ( FIG. 3 ). 
   The diode detector circuit  40  acts as a comparator circuit, comparing the incoming microwave energy against preset values. Comparison may occur in the diode detector circuit  40  itself or, as an additional example, the detector circuit  40  may be electrically connected to a microcontroller  42 , which performs the comparisons between the received microwave energy and the preset values. If the microwave energy exceeds the preset values, the microcontroller  42  may be used in some embodiments to shut down the operation of the UV lamp system  10 . Preset comparison values are on the order of 5 mW/cm 2 , which is based on current microwave oven standards and OSHA requirements. Other embodiments of the detector circuit may have preset values that are based on other standards that differ from those above. In some embodiments, the microcontroller  42  may not directly shut down the UV lamp system  10 , but rather set a “Trip” condition that signals other circuitry in the UV lamp system  10  to shut down. Adjusting the trip condition for detection with a trip adjustment potentiometer  44  is part of the initial calibration performed at the manufacturer. 
   Microwave energy received by the antenna  29  is value shifted so that an input voltage is always being measured by the diode detector circuit  40 . In some embodiments, the voltage shift may be approximately 2 volts for a zero energy input. The shift level may be adjusted by the trip adjust potentiometer  44  or by other reference circuits  46 . 
   In some embodiments, the microcontroller  42  communicates with other portions of the UV lamp system  10  such as the circuitry and components labeled communications and power supply connector  48  in  FIG. 2 . The microcontroller  42  may communicate through serial communications  50 , or other types of communications for other embodiments. The communications and power supply connector supplies power  52  to the microcontroller  14  and is responsible for determining and/or setting the reference voltage levels in the reference circuits  46 . 
   To improve the safety of the UV lamp system  10 , embodiments include a second circuit, configured as a test circuit  60 - 66  that tests the integrity of any radiation sensitive components in the diode detector circuit  40 . Tests on the components of the diode detector circuit  40  may be conducted intermittently, for example, approximately every 10 to 30 seconds. In other embodiments, tests may be performed more or less frequently. For purposes of this application, intermittent means periodic if the tests are performed at regular intervals, or non-periodic if the intervals between the tests are variable and/or at the discretion of the operator  26 . The microcontroller  42  temporarily suspends the detection of microwave energy by the diode detector circuit  40  in order to perform the tests. After suspension of the microwave energy detection, the microcontroller  42  activates a first analog switch  60 , which allows a test signal to be passed through a low pass filter  62  to the diode detector circuit  40  in front of the radiation sensitive component(s). If the radiation sensitive component(s) pass the test, the microcontroller  42  then deactivates the first analog switch  60 , and activates a second analog switch  64 . Again a test signal is passed from the switch  64  through a low pass filter  66  to the diode detector circuit  40  behind the radiation sensitive component(s). If the radiation sensitive components also pass this test, the microcontroller  42  deactivates the second analog switch  64  and restores the operation of microwave energy detection to the diode detector circuit  40 . The low pass filters  62 ,  66  may be used to isolate the test circuit  60 - 66  from the microwave energy received from the antenna  29 . 
   The radiation sensitive components of the detector circuit  40  may consist of a detector diode  70  and a resistor  72  in parallel as shown in the schematic diagram in  FIG. 3 . Failure of either of these components from exposure to excessive microwave energy may affect the ability of the diode detector circuit  40  to accurately detect microwave energy levels. During the first test, the microcontroller  42  activates the first analog switch  60 , passing a DC reference voltage  74  through the analog switch  60  and the low pass filter  62  to supply a voltage from the antenna  29  to ground at capacitor  76 . A voltage measurement may be taken at capacitor  76 , which may be higher than the threshold voltage of about 2 volts as discussed above. In other embodiments, a voltage measurement from another part of the diode detector circuit  40  may be taken, e.g. in a resistor network  78 . The microwave energy detector may also consist of two sections, the diode detector circuit  40  and a digital logic circuit  80 . The section containing the diode detector circuit  40  uses the detector diode  70 , in conjunction with the resistor network  78  to provide a voltage that is compared against a reference as discussed above and is currently known in the art. The digital logic circuit  80  includes additional digital components  82  with the microcontroller  42  which communicates with the UV lamp system control and in some embodiments, may be operable to switch between the detection of microwave energy and the detection of test signals. 
   During the second test, the microcontroller  42  deactivates the first analog switch  60  and activates the second analog switch  64 . This allows the reference voltage  74  to be applied behind the diode  70  to ground at capacitor  76 . Again, a voltage measurement may be taken at capacitor  76 , which may be less than the voltage from the first test because of the voltage drop across the diode. If the diode  70  is shorted, the voltage may be the same. If the diode  70  is an open circuit, the voltage may be even lower, indicating a fault in the diode detection circuit  40 . Similar voltage measurements may indicate a failure of the resistor  72 . Alternate embodiments of the test circuit  60 - 66  may only test the voltage across the resistor  72  or the diode  70 . 
   The detection  40  and test  60 - 66  circuits described above in  FIGS. 2 and 3  operate in two testing loops as illustrated in the flowchart of  FIG. 4 . After the initialization of the microcontroller  42  and other hardware in block  100 , a diagnostic timer is loaded with the time periods for testing the diode detector circuit  40  in block  102 . If the timer for testing has not expired (“no” branch of decision block  104 ), the loop for the diode detection circuit  40  executes. The diode detection circuit  40  loop first reads the trip value set from the calibration potentiometer  44  in block  106 . The circuit  40  then measures an RF voltage from the microwave energy received on the antenna  29  and processes the RF voltage through the diode detector circuit  40  in block  108 . The RF voltage is then compared to the trip value obtained from the calibration potentiometer  44  in block  110 . If the measured RF voltage exceeds the trip value (“yes” branch of decision block  112 ), the microcontroller  42  may set the trip condition in block  114  and then proceed with another detection cycle. As discussed above, other circuitry may then receive the trip signal and shut down the operation of the UV lamp system  10 . If the measured RF voltage does not exceed the trip value (“no” branch of decision block  112 ), the microcontroller  42  will clear the Trip condition in block  116  and proceed with another detection cycle. This series of steps  104 - 116  continues until the timer for testing has expired (“yes” branch of decision block  104 ). 
   When the timer expires, as discussed above, the microcontroller  42  may temporarily suspend the microwave energy detection before testing the diode detector circuit  40 . The test begins in block  120  when the microcontroller  42  enables the first analog switch  60 . The reference voltage  74  applied from the first analog switch  60  is measured as discussed above and compared to a theoretical voltage value in block  122 . If the measured test voltage does not match the theoretical value within an acceptable window (“no” branch of decision block  124 ), a fault condition is set and the first analog switch  60  is disabled in block  126 . The process then returns to microwave energy detection operation, where, as with the diode detector circuit  40  above, other circuitry may receive the fault condition and shut down or take other appropriate action with the UV lamp system  10 . 
   If the measured voltage does match the theoretical value within an acceptable window (“yes” branch of decision block  124 ), the first analog switch  60  from the first test is disabled and the second analog switch  64  for the second test is enabled in block  128 . The reference voltage  74  applied from the second analog switch  64  is measured and compared to a theoretical value in block  130 . Similar to above, if the measured voltage does not match the theoretical value within an acceptable window (“no” branch of decision block  132 ), a fault condition is set and the second analog switch is disabled in block  134 . The process then returns to microwave energy detection operation where, similar to a failure from the first test and as with the diode detector circuit  40  above, other circuitry may receive the fault condition and shut down or take other appropriate action with the UV lamp system  10 . If the measured voltage matches the theoretical value within an acceptable window (“yes” branch of decision block  132 ) the fault condition is cleared and the second analog switch  64  is disabled in block  136 . The diagnostic timer for the test circuit is then reset and loaded with a new time value in block  102  and microwave energy detection resumes in blocks  104 - 116 . 
   Though the embodiments discussed above describe a system comprising two test conditions, other embodiments may intermittently employ more or less than two test conditions. Similarly, while the embodiments described above test only a resistive component  72  and a diode component  70 , one or more other components of the diode detector circuit  40  may be tested alternatively or additionally. The embodiments discussed above utilize low pass filters  62 ,  66  to isolate the test circuit  60 - 66  from the diode detection circuit  40 , which allow test signals at DC or low frequency AC. RF test signals may also be employed which may require isolation methods other than the use of low pass filters as discussed above. 
   Additionally, embodiments of the invention may also be used to detect a drift in the calibration of the microwave energy detector  30 . During calibration of the UV lamp system  10 , a calibration test voltage is taken across the radiation sensitive component(s) when there is no microwave energy present and is stored in a nonvolatile memory by the microcontroller  42 . During subsequent operation, the microcontroller  42  will first determine if microwave energy is present. If so, the microcontroller  42  will intermittently suspend the detection of microwave energy and supply test signals as described in the embodiments above. If no microwave energy is present, the microcontroller  42  compares the voltage measured across the radiation sensitive component(s) with the calibration test value that was saved during the calibration to determine if there is a drift in the calibration. 
   While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicants&#39; general inventive concept.