Abstract:
A metal detector used for identifying contaminants in packages on a conveyor. The detector includes coils a search head and an analog to digital converter generating a reactive signal and a resistive signal in response to the presence of a contaminant. During a learning mode a sample product passes through the metal detector providing a representative product effect signal which is stored by the reactive learn memory and the resistive learn memory. The learn memory values provide a reference value subtracted from each product signal during a normal production cycle, canceling the product effect caused by contaminants in individual packages. The product effect is monitored during successive cycles composed of a series of packages undergoing normal inspection by the metal detector. A tracking processor averages the product effect signal produced by the individual packages and continuously updates a product effect trend signal that is subtracted from each product signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention pertains generally to the field of radio frequency metal detectors, and more particularly to the calibration of such a device. 
     2. Description of Prior Art 
     Metal detectors are used in the food processing industry, for example, to detect contaminants within a product. The unwanted material may include very small metallic particles having differing compositions. As seen in  FIG. 2 , the typical metal detector is housed in an enclosure  26  containing a longitudinal aperture  25  through which the product under test  23  is transported, usually by means of a conveyor belt, in the direction of arrow  24 . The metal detector includes a radio frequency transducer or oscillator that radiates a magnetic field by means of some arrangement of coils that serve as a radio frequency antenna. An example of such a metal detector operating in the radio frequency range is disclosed in U.S. Pat. No. 5,994,897, entitled FREQUENCY OPTIMIZING METAL DETECTOR, issued on Nov. 30, 1999 to King. 
     The typical metal detector enclosure  26  includes both radiating and receiving coils formed to surround the aperture  25  through which the product travels. The oscillator coil is a continuous wire loop formed within the search head. The oscillator coil surrounds the aperture  25  and receives radio frequency excitation from an oscillator circuit. The enclosure  26  also includes an input coil connected to produce a zero input signal when no metal is present. 
     A disturbance in the radiated magnetic field is sensed by the input coil and processed in order to detect a metal contaminant within the product passing through the detector aperture. Modern digital signal processing techniques resolve the input signal into two signal components, one component being resistive and the other signal component being reactive.  FIG. 1  depicts a typical signal processing scheme used in such a metal detector. The coils  1  are connected to the search head  2  that contains a radio frequency transmitter and receiver. When the coils  1  receive an electromagnetic signal the search head  2  divides the received signal into a reactive (X) component  11  and a purely resistive component  12 . The signals  11  and  12  are in an analog form and so are forwarded to the analog to digital (A/D) converter  3  where the signal  11  is converted into a digital reactive component signal  13  and a digital resistive component signal  14 . An example of a metal detector using digital signal processing techniques is disclosed in U.S. Pat. No. 7,432,715, entitled METHOD AND APPARATUS FOR METAL DETECTION EMPLOYING DIGITAL SIGNAL PROCESSING, issued on Oct. 7, 2008 to Stamatescu. 
     A nonzero input coil signal is due to either mechanical imbalances in the construction of the search head, inherent electrical changes in the circuitry such as frequency drift, metal being introduced into the aperture, or the effect of the product itself. The “product effect” is caused by the product passing through the aperture and is due primarily to electrical conduction via salt water within the product, the electrical conduction causing large magnitude resistive signals and relatively smaller reactive signals. 
     Calibration of a metal detector including compensation for the effect of the product is usually accomplished by the user of the detector. This process is dependent on operator skill and experience, and results in inconsistent results between different operators using the same machine. An example of a manually operated interactive metal detection calibration process is disclosed in U.S. Pat. No. 6,816,794, entitled APPARATUS AND METHOD FOR DETECTING CONTAMINATION OF OBJECT BY A METAL, issued on Nov. 9, 2004 to Alvi. 
     An attempt to directly address the effect of the product is disclosed in U.S. Pat. No. 6,636,827, entitled FOREIGN MATTER DETECTOR AND FOREIGN MATTER DETECTING SYSTEM, which was issued to Sakagami on Oct. 21, 2003. The Sakagami system relies on a library of stored product effect parameters that are manually selected by the equipment operator in order to reduce the sensitivity of the metal detector to the effect of the product. 
     A related patent is U.S. Pat. No. 5,045,789, DETECTOR FOR DETECTING FOREIGN MATTER IN OBJECT BY USING DISCRIMINANT ELECTROMAGNETIC PARAMETERS, issued on Sep. 3, 1991 to Inoue, et al, which discloses the concept of defining a set of parameters or values which define the detecting envelope, and thus the border between an acceptable product and one containing metal. 
     U.S. Pat. No. 4,719,421, entitled METAL DETECTOR FOR DETECTING PRODUCT IMPURITIES, issued on Jan. 12, 1988 to Kerr discloses the use of an adjustable phase shifter (element  10  in  FIG. 2 ). The phase shifter is adjusted to provide a null or linear output in response to product variations that might otherwise erroneously indicate the presence of metal. More specifically, if a nonlinear output is produced more than a given number of times in succession, thereby indicating that the nonlinearity is characteristic of that particular product, then the phase shifter is adjusted to produce a linear output. 
     There are numerous disadvantages to the metal calibration methods just described. In general, the operator of the metal detector does not have a clear understanding of the concept and function of the metal detection process. This lack of understanding leads to misuse of detection calibration controls and necessarily to a reduction in metal detector sensitivity due to improper settings of the metal detector system. A need therefore exists to employ a method of metal detector product effect compensation which permits substantially all product effect corrections to be performed automatically by the metal detector. 
     SUMMARY OF THE INVENTION 
     The current invention relates to improvements in compensating for the effect of a product when the product itself is undergoing inspection by a metal detector by using a time synchronized digital subtraction technique. The present invention improves the simultaneous sensitivity of the detector for metals of different groups by eliminating the need to eliminate signals having a phase component that is also a characteristic phase produced by the product under test. A second aspect of the present invention includes the tracking of trends or changes in the product effect characteristic during the normal flow of multiple products through the metal detector. A change in the average effect generated by the products being examined is used to modify the reference signal employed by the metal detector when the time synchronized digital subtraction technique is active. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of the prior art signal processing scheme of a metal detector; 
         FIG. 2  is a perspective view of an enclosure and aperture arrangement used in the metal detector depicted in  FIG. 1 ; 
         FIG. 3  is a block diagram of the metal detector signal processing protocol of the present invention; 
         FIG. 4  is a timing diagram depicting the product effect tracking feature depicted in  FIG. 3 ; and 
         FIG. 5  is a flow chart showing the signal processing protocol utilized by the product effect tracking feature depicted in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 3 , a block diagram of a metal detector constructed according to the principles of the present invention is shown generally at  40 . The signals  11  and  12  generated by the search head  2  is processed by the analog to digital converter  3 . The resultant digital reactive component signal  13  and resistive component signal  14  are forwarded to a high pass filter  4  for further signal processing. The metal detector  40  includes the capability to measure the instantaneous conveyor speed  33  via conveyor speed sensor  9 , the conveyor speed  33  being substantially equal to the speed of any article being introduced into the region of the detector coils  1 . The metal detector  40  includes a speed filter  30 , which contains a signal processing hardware or software that provides the necessary information to the metal detection algorithm processors  10  and  41  which are capable of determining the presence or absence of a contaminant such as metal. 
     The speed filter  30  receives data from the conveyor speed sensor  9  in order to provide a correction or adjustment signal to the high pass filter  4  and the low pass filter  8  depending on the value of certain variables including the conveyor speed  33 . The speed filter  30  also receives data derived from the digitized reactive output signal  13 , thereby providing a basis for the speed filter to correlate the frequency response of the search head  2  with the conveyor speed  9 . For each conveyor speed  9 , an optimum frequency exists at which the signal processing algorithm best detects the presence of a contaminant within the region of the coils  1 . In order to determine the optimum frequency, contaminant frequency learn processor  6  receives the signal  13  that is derived from the data produced by the search head  2  and correlates the signal  13  with other constant parameters including the physical configuration of the coils  1 , and the physical dimensions of the case housing the metal detector  40  and the conveyor speed  9  which is a variable that is dependent on the nature of the specific type of product being introduced into the region of the coils  1 . 
     The speed fitter correction data  42  is forwarded to both the high pass filter  4  and the low pass filter  8 . The high pass filter  4  receives and filters both the reactive component input signal  13  and resistive component input signal  14  received from the analog to digital converter  3 . The high pass filter  4  generates a filtered reactive component output signal  17  and a filtered resistive component output signal  18 . The filtered data enters the product effect compensation section  16  of the metal detector  40 , which includes a sample product effect learn mode that is activated when a sample product having nominal characteristics is introduced into and examined by the search head  2 . When such a nominal sample product is being inspected, the filtered reactive component output signals  17  are forwarded to reactive component learn memory  19  where the nominal reactive component output signal is stored as reactive reference data  32 . When the metal detector  40  is functioning in a production mode and thereby continuously processing numerous products in a serial fashion, the reactive reference data  32  that is stored in the reactive component learn memory is forwarded to the reactive effect reference processor  20 . 
     As each product is inspected during the production mode, the reactive component residing in the reactive effect reference processor is sent to the reactive component synchronization processor  21  and subtracted from the reactive signal produced by the specific product under test. The reactive component synchronization processor  21  receives a signal  22  from the infeed photo sensor  27  in order to initiate subtraction of the reactive reference data  32  from the signal actually being generated in response to a product under inspection which can occur only when such a product is present. 
     When a nominal sample product is being inspected, the filtered resistive component output signal  18  is simultaneously forwarded to the resistive component learn memory  28  where the nominal resistive component output signal is stored as resistive reference data  29 . When the metal detector  40  is functioning in a production mode and continuously processing numerous products in a serial fashion, the resistive reference data  29  that is stored in the resistive component learn memory is forwarded to the resistive effect reference processor  35 . 
     As each product is being inspected during the production mode, the resistive component data  29  residing in the resistive effect reference processor  35  is sent to the resistive component synchronization processor  34  via path  88  and subtracted from the resistive signal produced by the specific product under test. The resistive component synchronization processor  34  also receives a signal  22  from the infeed photo sensor  27  in order to subtract the resistive reference data  32  from the signal actually being generated in response to a product under inspection which occurs only when such a product is present in the region of the coils  1 . 
     The reactive component synchronization processor  21  generates an output signal  36  that represents the difference signal between the product under test and the reactive reference product effect signal  32 . Similarly, the resistive component synchronization processor  34  generates an output signal  37  that represents the difference between the product actually undergoing inspection and the resistive reference product effect signal  29 . Both of these difference signals  36  and  37  are forwarded to the low pass filter  8 , thereby creating a filtered reactive output signal  38  and a filtered resistive output signal  39 . 
     The filtered reactive output signal  38  is forwarded to reactive component detection algorithm processor  10  which generates a reactive detection output signal  43 . Similarly, the filtered resistive output signal  39  is forwarded to the resistive component detection algorithm  41  to create a resistive detection output signal  44 . Summing processor  45  adds the two component detection signals  43  and  44  to create a single output signal  46  that serves as the input to the metal detection processor  47 . 
     As the flow of products through the metal detector  40 , the trend of the individual product effect signals is monitored by tracking processor  48  which is able modify the reference signals  29  and  32  as necessary. Referring also to  FIG. 4 , the product effect tracking function performed by tracking processor  48  may be better understood. Only the reactive component signal tracking function is depicted in  FIG. 4 , with the resistive component tracking being accomplished in a substantially similar manner. 
     Each series of packages, composed of packages  49 ,  50  and  51 , for example, triggers the beginning  52  and end  53  of a product tracking cycle  54 . The elapsed time since the beginning  56  (time=0) of the product inspection period is increasing in the direction of arrow  55 . Since each package generates its own individual product effect signal, the first package  49  generates a first product effect signal  57 , the second package  50  generates a second product effect signal  58  and the third package  51  generates a third product effect signal  59 . Each of the product effect signals  57 ,  58  and  59  is forwarded to the tracking processor  48  which generates a reactive trend signal  60  as well as a resistive trend signal  61 . The tracking processor  48  receives each of the individual product effect signals  57 ,  58  and  59  and divides the sum of the signals by three, thereby creating a reactive trend signal  60  which is the average of the individual product effect signals of all of the packages  49 ,  50  and  51  that have been monitored during the cycle  54 . The reactive trend signal  60  is sent to the reactive component learn memory  19  where it is compared to the original reference signal generated by a test package prior to the beginning of any product tracking cycle. Only packages not having a metal contaminant as determined by the metal detection processor  47  are used to determine product effect trends. The original reference signal is subtracted from the reactive trend signal and then forwarded for use by the reactive component reference processor  20  at the end of the monitoring cycle  54 . After monitoring cycle  54  is completed, the next cycle  62  begins. 
     The number of packages per product tracking cycle is not limited to three. In the general case, the number of packages monitored during each product tracking cycle is N, and the reactive trend signal  60  is expressed as
 
 R =( P   1   +P   2   +. . . +P   N )/ N , where
 
     R is the average inherent product effect characteristic, 
     P is a product effect of each individual package being inspected and 
     N is the total number of packages inspected. 
     As best seen in  FIG. 5 , when utilizing the product effect tracking feature the initial step  63  is to convert the raw analog data  11  and  12  into a digitized reactive component  13  and a digitized resistive component  14 , both of which are then processed by the high pass filter  4  at step  80 . The filtered component signals  17  and  18  are then sent at step  64  to the tracking processor  48 , which performs a tracking cycle at step  65  that includes a predetermined number N of packages, such as packages  49 ,  50  and  51 . The tracking processor  48  functions simultaneously on both a reactive component calculation path  81  and a resistive component calculation path  82 . Each filtered reactive signal component  17  in any given tracking cycle is stored and averaged at step  66 , with the averaged value being used to update the reactive product effect reference signal at step  68 . Once the reference signal update has occurred a new cycle is initiated at step  70 . The averaged reactive value calculated at step  68  is subtracted at step  72  from the reactive signal component  17  generated by each individual product undergoing inspection, with the difference value being filtered by the low pass filter  8  at step  74 . The filtered signal produced at step  74  is then processed by the reactive detection algorithm processor  10  at step  76 . 
     Similarly, each filtered resistive signal component  18  in each tracking cycle is stored and averaged at step  67 , with the averaged value being used to update the resistive product effect reference signal at step  69 . Once the resistive reference signal update has occurred a new cycle is initiated at step  70 . The averaged resistive value calculated at step  69  is subtracted at step  73  from the resistive signal component  18  generated by each individual product undergoing inspection, with the difference value being filtered by the low pass filter  8  at step  75 . The filtered signal produced at step  75  is then processed by the resistive detection algorithm processor  41  at step  77 . The two detection signals  43  and  44  representing the reactive and resistive signal components, respectively, are added at step  78  and forwarded to the metal detection processor  47  at step  79 . The output signal  86  of the metal detection processor  47  is forwarded to logical IF processor  83  which determines if a metal contaminant was detected in the inspected product inspected. If metal is detected, path  85  sends the product effect data for that package to be discarded at step  87 . If no metal is detected, path  84  causes the product effect data for that package to be retained for the product effect calculation of step  66 . 
     The foregoing improvements embodied in the present invention are by way of example only. For example, the product effect tracking feature just described may be omitted or used selectively in response to the expected variation in product effect for successive package during a given inspection period. Those skilled in the metal detecting field will appreciate that the foregoing features may be modified as appropriate for various specific applications without departing from the scope of the claims.