Abstract:
A pipeline pig signal made according to this invention houses one or more shielded magnetometer sensors and a microcontroller with adaptive thresholding detection means for reducing the likelihood of false alarms. The adaptive thresholding detection means removes outlier data from the magnetic flux data stream and then passes the outlier-free data stream through four low pass filters. A smoothed magnitude of the data stream is compared to detection limits and, if a passage event has occurred, a recent detection is indicated, a counter of a display unit is incremented, a time of passage is recorded, and both statistics are displayed on the display unit. Because a single object may produce multiple magnetic fields, the detector may be locked-out for a predetermined period of time after the passage event to prevent a second detection of the same object as it passes the detection device.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to pipeline monitoring systems and more particularly concerns a sensor for detecting passage of an object, such as a pipeline pig, through a pipeline. 
     Various means have been developed for detecting the passage of a pig through a pipeline. “Dumb pigs” or “smart pigs” may be detected by intrusive mechanical devices such as threaded adapters with spring-loaded shafts. The shaft has an exposed end with a spring loaded-lever or flag and an opposing end that extends into the interior space of the pipe. Intrusive detection devices, however, require making a hole or hot-tapping into an active piping, an often costly and inconvenient process for a pipeline operator. As a result, non-intrusive detection devices, which are fully located outside of the pipeline and do not require additional hot-tapping or welding, are often preferred by the pipeline operator. 
     There are two main types of non-intrusive detection devices: acoustic/ultrasonic detectors, which detect a change in sound, and electromagnetic detectors, which detect a change in ambient magnetic field. Passive acoustic detectors can detect a change in sound caused by an object travelling through a pipeline but cannot easily distinguish between this sound change and that caused by a surrounding noise such as a pump or motor vehicle. Active acoustic detectors can eliminate this problem by transmitting an ultrasonic signal, but these devices are costly, require a high level of power and, because of the power requirements, limit or prevent battery-power options. 
     Electromagnetic detectors often use one or more coils to detect a change in magnetic flux over time. A change in the ambient magnetic field inducts a voltage in the coil or coils proportional to the change of the magnetic field over time. As a result, a slow travelling ferromagnetic object may not generate enough voltage in the coils to generate a detection event. 
     Magnetometers—which determine a change in magnetic flux by measuring the instantaneous flux over time—are not object-speed dependent. Magnetometers, therefore, can detect any object causing a change in the electromagnetic field regardless of object speed. Magnetometers, however, can be subject to false alarms. Therefore, appropriate methods must be used for noise cancellation, signal processing, and shielding of ambient magnetic fields. 
     SUMMARY OF THE INVENTION 
     A system and method for detecting the passage of an object in a pipeline includes a non-intrusive detection device that houses one or more shielded magnetometer sensors and a microcontroller with adaptive thresholding detection means. An AC/DC power source with a backup battery source is employed to provide power to the device. The battery backup power source is preferably configured to break electrical contact prior to exposure to ambient environment, thereby making the detection device suitable for use in explosion-proof zones. 
     The magnetometer sensors are preferably magnetic flux sensors using a variable permeability material to directly measure flux and are arranged orthogonal to one another. The inner shield surrounding the sensors is an electrically insulating material. The outer shield is a magnetic permeable material. A display unit in communication with the microcontroller displays various statistics, including the number of objects detected and the time of their passage. 
     The detection device may have an adjustable end for orienting the display unit and positioning the magnetometer sensors near the exterior surface of the pipeline. A reed switch or other means for locking out the detection device may be used when positioning the device on the pipeline or when moving it to a different location on the pipeline. Once the detection device is in its proper position and unlocked, the magnetometer sensors and microcontroller, which process the input magnetic flux data stream, can signal a detection. 
     The adaptive thresholding detection means employed first removes the outlier data from the magnetic flux data stream. This outlier-free data stream is then passed through four low pass filters. The first low pass filter estimates a baseline by removing a bias value from the magnetic flux data stream and limiting the data stream to a value no greater than an outlier limit. The second low pass filter then uses the baseline estimate to produce a noise estimate. The third low pass filter is a boxcar filter that provides a smoothed magnitude of the data stream. The smoothed magnitude is compared to a set of upper and lower detection limits and then passed through the fourth low pass filter to determine the length of a passage event. If a passage event has occurred, a counter of the display unit is incremented and a time of passage is recorded. Because a single object may produce multiple detections or detection events, the detector may be locked-out for a predetermined period of time after the passage event to prevent a second detection of the same object as it passes the detector. A Bayesian lockout estimator is preferred for this purpose. 
     A better understanding of the invention will be obtained from the following detailed description of the preferred embodiments taken in conjunction with the drawings and the attached claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of an embodiment of a non-intrusive detector made according to this invention. A sensor board (see  FIGS. 8 to 11B ) that includes a microcontroller and at least one magnetometer sensor (“sensor”) is located in a lower portion of the sensor housing. The sensor housing threads into a base structure, which is detachably secured to the pipe, until the bottom of the housing contacts the pipe surface. A display housing, located at the upper end of a conduit assembly section, houses a digital display that indicates the detection of an object within the pipeline as it passes under the detector. A field wiring conduit box is provided for connecting the detector to an AC/DC power source and for accommodating wiring to communicate with, for example, a control room that is controlling the opening and closing of pipeline valves. 
         FIG. 2  is a side view of the embodiment illustrated in  FIG. 1 . The display housing houses an assembly that includes a digital display insert and a DC power source (see  FIGS. 6 &amp; 7 ). As the threaded cap at the rear of the display housing is being unthreaded, the DC power source is disconnected, thereby providing a safe environment for use of the detector in explosion-proof zones. 
         FIG. 3  is a front view of the display panel of  FIG. 1 . A magnetic reed switch located on the front of the panel resets a timer on the display. A second magnetic reed switch places the detector in locked or unlocked mode and allows an operator to scroll through an object detection history. When in locked mode, the device is prevented from detecting objects and the detector may be moved between locations on the pipeline without resulting in an undesired detection. A battery icon displays battery life if the detector is battery-powered. 
         FIG. 4  is another front view of the display panel of  FIG. 1  indicating that the detector is hard wired to an AC/DC power source (e.g., 24V power source) and the batteries are not in use. Visual indicators are provided to display statistics such as the number of objects that have been detected, time since detection of any object in the history, and time elapsed since reset. 
         FIG. 5  is an isometric view of the display insert illustrated in  FIG. 2 . The display panel and digital display is located at the upper end of the assembly. Symmetrical slots are provided about the periphery of the insert body to retain the insert in the display housing and to ensure correct orientation of the display and provide adequate internal wiring access. 
         FIG. 6  is a rear isometric view of the detector illustrated in  FIG. 1  with the rear cap removed from the display housing. Because the batteries engage spring-loaded contacts within the display unit, all electrical contact with the batteries is broken prior to the rear cap being completely removed. A sheet of insulating padding, preferably affixed to the underside of the rear cap, resides between the batteries and the cap to prevent electrical contact with the cap. 
         FIG. 7  is a partial, rear isometric view of the display housing illustrated in  FIG. 1 . The batteries reside in a cradle/battery holder that is received by the insert body. 
         FIG. 8  is a view of the sensor housing. The sensor housing receives the sensor board that includes the microcontroller and the magnetometer sensor. 
         FIG. 9  is a view of the shielding for the sensor board of  FIG. 8 . The inner shield is preferably an electrically insulating material. The outer shield is preferably a magnetic permeable material. 
         FIG. 10  is a view of the sensor board of  FIG. 8 . The sensor board includes an interface for communication with the display (which could be a remote located display) or a control system, a microcontroller that runs an adaptive thresholding algorithm (see  FIGS. 12 to 19 ), and a sensor. The sensor is preferably a magnetic flux sensor using a variable permeability material to directly measure magnetic flux. 
         FIGS. 11A &amp; 11B  illustrate alternate embodiments of the sensor board of  FIG. 8 . Two sensors are arranged orthogonal to one another. The sensor board may use a 1-D, 2-D, 3-D or n-D array of sensors which may differ in orientation and separation of the sensor elements. Orthogonal orientations, however, are preferred. 
         FIG. 12  is a flowchart of a signal processing algorithm implemented by the microcontroller for processing data collected by an n-D array of sensors. The detection scheme is an adaptive thresholding algorithm utilizing a real-time noise estimate for the sensor or sensors. 
         FIG. 13  is a flowchart of the outlier removal step of the algorithm illustrated in  FIG. 12 . 
         FIG. 14  is a flowchart of the baseline estimation step of the algorithm illustrated in  FIG. 12 . 
         FIG. 15  is a flowchart of the noise estimation step of the algorithm illustrated in  FIG. 12 . 
         FIG. 16  is a flowchart of the boxcar or input smoothing step of the algorithm illustrated in  FIG. 12 . 
         FIG. 17  is a flowchart of the detector step of the algorithm illustrated in  FIG. 12 . 
         FIG. 18  is a flowchart of the time discriminator step of the algorithm illustrated in  FIG. 12 . 
         FIG. 19  is a flowchart of the Bayesian lockout step of the algorithm illustrated in  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The magnetometer-based detector that is described below is not limited in its application to the details of the construction, arrangement of the parts, and process flows illustrated in the accompanying drawings. The invention is capable of other embodiments and of being practiced or carried out in a variety of ways. The phraseology and terminology employed herein are for purposes of description and not limitation. 
     Elements shown by the drawings are identified by the following numbers: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 10 
                 Detector 
               
               
                 20 
                 Sensor housing/module 
               
               
                 21 
                 Upper end of 20 
               
               
                 23 
                 Lower end of 20 
               
               
                 25 
                 Threaded portion of 20 
               
               
                 30 
                 Conduit assembly section 
               
               
                 31 
                 Fill plug 
               
               
                 33 
                 Field wiring conduit box 
               
               
                 35 
                 Tee fitting 
               
               
                 37 
                 Upper end of 30 
               
               
                 40 
                 Mounting base 
               
               
                 41 
                 Brackets 
               
               
                 43 
                 Shackle pin 
               
               
                 45 
                 Turnbuckle 
               
               
                 47 
                 Chain 
               
               
                 50 
                 Sensor board 
               
               
                 51 
                 Micro-controller 
               
               
                 53 
                 Oscillator 
               
               
                 55 
                 Communication interface 
               
               
                 60 
                 Magnetometer sensor 
               
               
                 61 
                 Inner insulating shield 
               
               
                 63 
                 Outer shield 
               
               
                 70 
                 Display insert/module 
               
               
                 71 
                 Digital display 
               
               
                 72 
                 Insert body 
               
               
                 73 
                 Object count cumulative indicator 
               
               
                 74 
                 Slot in 72 
               
               
                 75 
                 Object count history indicator 
               
               
                 77 
                 Object icon 
               
               
                 79 
                 Time since last reset indicator 
               
               
                 81 
                 Time since object passage in history indicator 
               
               
                 83 
                 Power status indicator 
               
               
                 85 
                 Locked/unlocked status indicator 
               
               
                 87 
                 Light emitting diode 
               
               
                 89 
                 Reed switch 
               
               
                 90 
                 Display housing 
               
               
                 91 
                 Front cap 
               
               
                 93 
                 Rear cap 
               
               
                 94 
                 Battery pack 
               
               
                 95 
                 Cradle/battery holder 
               
               
                 97 
                 Batteries 
               
               
                 99 
                 Insulating pad 
               
               
                 100 
                 Detection algorithm 
               
               
                 110 
                 Outlier removal algorithm 
               
               
                 130 
                 Baseline estimate algorithm 
               
               
                 150 
                 Noise estimate algorithm 
               
               
                 170 
                 Input smoothing algorithm 
               
               
                 190 
                 Detection event algorithm 
               
               
                 210 
                 Time discriminator algorithm 
               
               
                 230 
                 Lockout discriminator algorithm 
               
               
                   
               
             
          
         
       
     
     Referring first to  FIGS. 1 ,  10  &amp;  12 , a detector  10  that is located external to a pipeline section P employs a magnetometer sensor  60  and a detection algorithm  100  to detect the presence of an object in the pipeline. The object may be in motion inside the pipeline section P with detector  10  stationary on the pipeline. The object in the pipeline may be a “pig” used for pipeline maintenance or inspection. The object may carry a magnetic source or intrinsic properties of the object may allow detection. An example of an intrinsically marked object would be a “brush” pig or a pig containing a sizable amount of ferromagnetic material. 
     Measurements from the magnetometer sensor  60  are processed by detection algorithm  100 , which is an adaptive thresholding algorithm, to produce a “passage event.” This event may be displayed and/or counted by a digital display  70  or light emitting diode  87 . The event can also trigger outputs used to signal remote devices such as a control system for controlling the opening and closing of valves in the pipeline. 
     As illustrated in  FIGS. 1 &amp; 2 , detector  10  is detachably secured to pipeline section P by way of a mounting base  40  that receives a threaded portion  25  of sensor housing  20 . This arrangement provides the capability to (1) adjust the distance between the lower end  23  of sensor housing  20  and the external wall surface of pipeline section P and (2) orient the direction of display housing  90 . Threaded portion  25  is preferably threaded into base  40  until the lower end  23  of threaded portion  25  makes contact with the external wall surface of pipeline section P. Two opposing brackets  41  of base  40  each receive a shackle pin  43  and an end of chain  47 . Chain  47 , along with turnbuckle  45 , is used to secure base  40  in a desired location on pipeline section P. The above adjustability feature of detector  10  provides the ability to position magnetometer sensor  60  for maximum detection capability. 
     Located at the upper end  37  of conduit assembly section  30  is a display housing  90 . Display housing  90  is preferably detachably secured to conduit assembly section  30 . Display housing  90  receives a display insert  70  that provides various indicators and statistics (see text below discussing  FIGS. 3 to 5 ). Symmetrical slots  74  located on the periphery of the insert body  72  ensure the correct orientation of the display  71  and provide necessary wiring access. A front cap  91  of display housing  90  provides a window for the digital display  71  of display insert  70 . 
     Conduit assembly section  30  is located at the upper end  21  of sensor housing  20 . The conduit assembly section  30  includes a tee fitting  35  for connecting detector  10  to a field wiring conduit box  33 . Conduit box  33  may include wiring for placing detector  10  in communication with an AC/DC power supply, for hardwiring detector  10  to a control room, or for providing wiring to a remote display insert  70 . A fill plug  31  may also be provided to add packing, filler, potting compound or sealant. 
     Referring now to  FIGS. 3 to 5 , display insert  70  may include a digital display  71  that displays various indicators and statistics. In a preferred embodiment, digital display  71  displays an object count indicator  73 , an object passage history indicator  75 , an object icon  77 , a time since last reset indicator  79 , and a time since last object in history indicator  81 . A microcontroller (not shown) on display  71  receives information from the microcontroller  51  and magnetometer sensor  60  located on sensor board  50  (see  FIG. 8 ). 
     Display insert  70  also includes a power source indicator  83  that indicates whether detector  10  is operating under battery power ( FIG. 3 ) or AC/DC power ( FIG. 4 ). When on battery power, power source indicator  83  preferably displays a battery icon  83  that indicates battery life. 
     Digital display  71  also includes a locked/unlocked status indicator  85 . A magnetic reed switch  89 B places detector  10  in the locked or unlocked mode, thereby providing the ability to control unwanted detection. When in locked mode, detector  10  is prevented from detecting objects and may be moved between locations on the pipeline. Reed switch  89 B also allows a user to interact with the statistics and scroll through the object history as indicated by indicators  75  and  81 . A second magnetic reed switch  89 A resets timer  79 . Display unit  70  also includes light-emitting diode indicators  87  to indicate whether a recent passage occurred. Light-emitting diode indicators  87 A and  87 B light up when reed switches  89 A and  89 B are triggered, respectively. 
     As illustrated in  FIGS. 6 &amp; 7 , display unit  70  preferably includes a battery pack  94 . The cradle/battery holder  95  in which the batteries  97  resides is received by an interior space of the display insert  70 . Spring-loaded contacts (not shown) within display insert  70  urge against the distal end of battery pack  94  so that as a user unthreads the rear cap  93  of display housing  90 , battery pack  94  is urged away from the contacts and toward the retreating rear cap  93 . The connection between the spring-loaded contacts and battery pack  94  is, therefore, broken prior to rear cap  93  being completely removed from housing  90 . This feature provides for use of detector  10  in explosion-proof zones. An insulating pad  99  is provided between the proximal end of battery pack  94  and rear cap  93 . 
     Referring now to  FIGS. 8 to 11B , sensor housing  20  houses a sensor board  50 . A microcontroller  51  on the sensor board  50  receives data collected by magnetometer sensor  60  and runs detection algorithm  100  (see  FIG. 12 ) to determine whether a passage event has occurred. Microcontroller  51 , which is of a type well known in the art, is in communication with the display unit  70  or other systems by way of a communication interface  55 . In a preferred embodiment, interface  55  is a RS485 interface. Board  50  also includes an oscillator  53  made up of a comparator, an analog switch array and an AND gate array. 
     Detector  10  may use a 1-D, 2-D, 3-D, or n-D array of magnetometer sensors  60  which may differ in orientation relative to one another, separation of the individual sensor elements, or both. Orthogonal orientations, as illustrated in  FIGS. 11A  &amp; B, are preferred when multiple sensors  60  are used. Sensors  60  with a common orientation but that are offset may be used to improve the detection process using coincidence (or correlation) algorithms. 
     Various magnetometer technologies may be employed for sensor  60 . In a preferred embodiment, sensor  60  is a magnetic flux sensor using a variable permeability material. Changes in the flux alter the effective inductance of the magnetometer. A flux sensor manufactured by PNI Corporation, Inc. (Santa Rosa, Calif.) is an effective magnetic flux sensor  60 . 
     Digital signal processing is essential to the detection process and a digital, adaptive detection algorithm is the preferred signal processing algorithm. As illustrated in  FIG. 12 , the detection scheme is an adaptive thresholding detection process  100  based on a real-time noise estimate for the sensor(s)  60 . Parameterization allows process  100  to be adjusted for the widest application with a minimum of false alarms and a high probability of detection. Preferred range and values for critical parameters are indicated in the description of process  100  below. 
     Detection process  100 , which is implemented by microcontroller  51 , may include all of the following processing steps: automatic elimination of outliers, computation and removal of the measurement offset, estimation of the measurement noise, establishing threshold(s) with and without hysteresis, sequential detection, and event time discrimination/detection. Sensor(s)  60  collect magnetometer data  101  (“mag data” or mag) for processing and detection event  103  is determined by a number of criteria including but not limited to amplitude, duration, and previous events. The fine structure of the response of sensor(s)  60  may also be accounted for by using pattern recognition techniques. 
     Mag data  101  is first processed by processing step  110 , elimination of outliers:
 
mag i =min(mag i −baseline estimate i-1 *signum(mag i ), outlier limit)  (Eq. 1)
 
See  FIG. 13  &amp; sub-steps  113 - 117 . After removal of the offset (baseline) from the measurement, the input is limited to less than or equal to the outlier limit. The sign is preserved over the operation. The “signum” function returns the sign of the argument. The “min” function returns the arithmetical minimum of the argument list. The outlier-free magnetometer data are then returned to process  100  and passed through a low pass filter to estimate the baseline.
 
     Processing step  130 , estimation of the baseline, is:
 
baseline estimate i =baseline estimate i-1 +(mag i −baseline estimate i-1 )* f/ 65536  (Eq. 2)
 
See  FIG. 14  &amp; sub-steps  131 - 137 . Processing step  130  is a low pass filter which is used to estimate the offset from zero for the measurement. The low pass filter is a simple exponential type. The transfer function for the filter is:
 
                         H   1     ⁡     (   z   )       =         b   0     +       b   1     ⁢     z     -   1         +       b   2     ⁢     z     -   2               a   0     +       a   1     ⁢     z     -   1         +       a   2     ⁢     z     -   2               ⁢     
     ⁢     where   ⁢     :       ⁢     
     ⁢     b   =     [           f   65536         0       0         ]       ⁢     
     ⁢     a   =     [         1         (       f   65536     -   1     )         0         ]               (     Eq   .           ⁢   3     )               
The value for time constant “f” may be in the range of 1 to 4096. The preferred value for “f” is 128. The baseline estimate is:
 
baseline estimate= H   1(z) *mag  (Eq. 4)
 
     Processing step  150  provides a noise estimate: 
                     noise   i     =     max   (       noise     i   -   1       +     (         abs   ⁡     (       (       mag   i     -     baseline   ⁢           ⁢     estimate     i   -   1           )     -     noise     i   -   1         )       *     fn   65536       ,     min   ⁢           ⁢   noise       )                 (     Eq   .           ⁢   5     )               
See  FIG. 15 , sub-steps  151 - 163 . This is a low pass filter which produces a noise estimate for the adaptive threshold selection required by the detection process. The noise level is always positive and is bounded by a minimum. The transfer function for the filter is:
 
                         H   2     ⁡     (   z   )       =         b   0     +       b   1     ⁢     z     -   1         +       b   2     ⁢     z     -   2               a   0     +       a   1     ⁢     z     -   1         +       a   2     ⁢     z     -   2               ⁢     
     ⁢     where   ⁢     :       ⁢     
     ⁢     b   =     [           fn   65536         0       0         ]       ⁢     
     ⁢     a   =     [         1         (       fn   65536     -   1     )         0         ]               (     Eq   .           ⁢   6     )               
The value for time constant “fn” may be in the range of 1 to 256. The preferred value for “fn” is 32. The noise estimate is:
 
noise= H   2(z) *(abs(mag−baseline))  (Eq. 7)
 
     The input magnetometer values are then smoothed using a “boxcar” low pass filter in processing step  170 . See  FIG. 16 , sub-steps  171 - 181 . The structure of the filter is: 
                     H     3   ⁢     (   z   )         =       1   n     ⁢       ∑     i   =   0       n   -   1       ⁢     (       rectangular     ⁢           ⁢     window   ⁡     [   i   ]       *     z     -   i         )                 (     Eq   .           ⁢   8     )               
This filter is used to shape the response after magnitude processing (absolute value). The length of the rectangular window may be in the range of 2 to 128. In the preferred configuration the length is 32.
 
     Following input smoothing, the detection process occurs in processing step  190 . See  FIG. 17 , sub-steps  191 - 201 . The detection process is a sequential process which includes the previous value of the detector. If the current value of the detector is “off” and the smoothed magnitude is greater than the upper threshold, the detector is switched “on.” If the detector is “on” and the smoothed magnitude is less than the lower threshold, the detector is switched “off.” Other conditions for the input result in no change in the detector value. In equation form:
 
if ( H   3(z) *(abs(mag i )−baseline estimate i )≧upper threshold i ) and (detector i-1 =0) then detector i =1  (Eq. 9a)
 
If ( H   3(z) *(abs(mag i )−baseline estimate i )≦lower threshold i ) and (detector i-1 =1) then detector i =0  (Eq. 9b)
 
else detector i =detector i-1   (Eq. 9c)
 
The determination of the upper and lower detection thresholds is:
 
upper threshold i   =p 1*noise i   (Eq. 10a)
 
lower threshold i   =p 2*noise i   (Eq. 10b)
 
The value for p1 and p2 may be in the range of 1 to 10. The preferred value for p1 is 3. The preferred value for p2 is 1.
 
     Following a detection event, processing step  210  uses time to determine the extent of the event. See  FIG. 18 , sub-steps  211  to  221 . One or more filters of the following type may be used to characterize the event to determine if single or multiple events are present: 
                       H     4   ⁢     (   z   )         =       1   m     ⁢       ∑     i   =   0       n   -   1       ⁢     rectangular   ⁢           ⁢     window   ⁡     [   i   ]       *     z     -   i               ⁢     
     ⁢   where   ⁢     
     ⁢     m   ≤   n             (     Eq   .           ⁢   11     )               
This transfer function operates on the detected output, whose value is 0 or 1. Various lengths of rectangular windows can be used to discriminate between short and long events. The longest window, passing the detection limit, indicates the extent of a single event:
 
event detect= H   4(z) *detect  (Eq. 12).
 
     Because a single pig or object may present multiple magnetic fields, processing step  230 , lockout discriminator, may be employed to prevent multiple passage events being detected for a single object as the object passes by detector  10 . See  FIG. 19 , sub-steps  231  to  253 . In a preferred embodiment, processing step  230  employs a Bayesian lockout estimator. If there is a passage event, then a detection timer is incremented and further detection is locked-out for a predetermined time period. Once the predetermined time period is exceeded, the detector is unlocked and initialized and the lockout timer is stopped and cleared until the next detection event. Similar to reed switch  89 B, processing step  230  provides the ability to control unwanted detection. 
     While detector  10  and process  100  have been described with a certain degree of particularity, many changes may be made in the details of construction and the arrangement of components or steps without departing from the spirit and scope of this disclosure. The invention, therefore, is not limited to the embodiments set forth herein for purposes of exemplification, but is to be limited only by the scope of the attached claims, including the full range of equivalency to which each element thereof is entitled.