Abstract:
A magnetic flux measuring device includes a coil of conductive material for sensing magnetic flux and producing a voltage in the presence of a changing magnetic field. An analog to digital converter is coupled to an output of the sensing coil for sampling the voltage produced by the magnetic flux sensing coil and for converting that voltage to digital data. A digital integrating device determines the total magnetic flux sensed by the coil over an event time interval by integrating the digital data over that same time interval.

Description:
BACKGROUND OF THE INVENTION 
     The following invention relates to a fluxmeter and more particularly to a fluxmeter having digital circuitry to provide a high degree of stability and accuracy. 
     Fluxmeters are devices which measure total magnetic flux linking a sensor coil. The fluxmeter measures total flux, which is the integral of flux density times area over the area of the coil. The output of the fluxmeter is proportional to the number of turns on the coil as well. Typically, a fluxmeter uses a coil of electrically conductive wire which is often chosen by the user for the particular purpose at hand. 
     For a coil circling a flux path, the Faraday Induction Law states that: 
     
       
         
           E=ndφ/dt. 
         
       
     
     Therefore, the total flux (φ) is proportional to the integral of the voltage over time and inversely proportional to the number of turns in the coil: 
     
       
         φ=1 /n∫Edt+φ   0 . 
       
     
     In conventional fluxmeters the meter is set to zero before the measurement is made so that φ 0 =0. The output may also include a multiplier correction for the number of turns in the coil or scale changes for the output display. 
     Conventional fluxmeters are analog devices and typically employ an operational amplifier with a shunt capacitor to integrate the voltage over time. Two different problems are encountered with this type of device. The first is that the input impedance of the meter varies with the output scale setting. If the input impedance is low enough, and if the sensor coil has significant resistance, a correction is required which must be computed and which is different for each output scale setting. Additionally, analog integrators are highly susceptible to drift. Any small offset voltage produces a gradual change in the output, even with no actual change in flux. The offset voltage may arise as a result of electrical and magnetic noise. The drift may be so large, however, that it may be difficult or impossible to obtain accurate, reliable and repeatable readings. 
     A typical fluxmeter is shown in a paper by Sasaki “A Simple Precision Fluxmeter”, Nuclear Instruments and Methods 76, North Holland Publishing Company (1969) pp. 100-102. The Sasaki fluxmeter states that it employs an integrating digital voltmeter, but in reality Sasaki&#39;s integrator is a conventional analog integrator with digital processing of the integrated output. This device is therefore susceptible to the type of drift discussed above. Another type of fluxmeter is shown in the U.S. Patent to Krause, U.S. Pat. No. 5,506,500. Krause appears to show the use of a digital voltmeter as a fluxmeter, however, the voltmeter is not truly a digital device, but instead contains a precision analog integrator comprising an OP amp and capacitor. It is therefor susceptible to the problem of drift as outlined above. Furthermore, Krause relies upon a stepper motor to move a sample between a coil pair. The mechanical dimensions of the device are carefully controlled as are the starting and stopping of the motor which controls the timing of a measurable event. With manually operated fluxmeters however, there is no such degree of control, and events of interest must be determined by the instrument itself which must discriminate frequently between the measurement of magnetic flux and readings caused by noise, such as noise from nearby AC-lines, fluorescent lighting or machinery. 
     SUMMARY OF THE INVENTION 
     A magnetic flux measuring device constructed according to the invention includes a coil of conductive material for sensing magnetic flux and producing a voltage in the presence of a changing magnetic field. An analog to digital converter is coupled to an output of the sensing coil for sampling the voltage produced by the magnetic flux sensing coil and for converting that voltage to digital data. A digital integrating device determines the total magnetic flux sensed by the coil over an event time interval by integrating the digital data over that same time interval. 
     Digitizing the output of the coil and integrating the data digitally makes the instrument much less susceptible to the common drift experienced by analog type integrating devices. It also permits the filtering and correction of the data prior to integration, and allows for more accurate discrimination between noise and events of interest. 
     The analog to digital converter may be coupled to a recirculating memory which continuously refreshes data, writing over the oldest stored data. Connected in parallel to the recirculating memory is an event detector. The event detector is coupled to a noise detector which averages the steady-state output of the analog to digital converter in the environment of use, but away from the sample to be tested. The noise detector calculates an average ambient noise level. The event detector compares the average ambient noise level with the output of the analog to digital converter and when the output rises above the noise threshold for a predetermined period of time, an event is detected. The detection of an event triggers a counter which captures the data in the recirculating memory from a time just prior to the detection of an event until the event has stopped. The memory addresses thus detected are downloaded and the digital data is sent to an event analyzer. The event analyzer contains filtering, offset correction and a digital integrator. 
     Because an event may last longer than the dynamic space available in the recirculating memory, the counter may include a rate control which slows down the sampling rate of the analog to digital converter. Thus, data from a predetermined time before event start is not overwritten in the memory. This provides for less resolution in the event data, but this is an acceptable trade-off given the fact that the event data at this point is well above the noise threshold. 
     The user may provide input to the event analyzer which includes selective filtering, optimization based upon a previous event, output scale factoring or change of units and selections based upon the physical coil configuration such as scaling for the number of turns in the coil. The event analyser also includes a digital integrator. The digital integrator calculates flux by multiplying each digital voltage sample by the time interval over which the sample was taken and summing the products over a well defined period. 
     This approach results in a very high fixed input impedance which does not require that drift and the effects of noise be removed completely. Having the information in digital form allows for easy transmittal to other instruments, computers, printing or storage. Filtering techniques and noise correction methods may be updated by changes in software and do not require any new circuitry to implement. 
     The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block schematic drawing of a magnetic flux detecting device. 
     FIG. 2 is a block schematic diagram of the event analyzer of FIG.  1 . 
     FIG. 3A is a first portion of a schematic flow-chart diagram of a magnetic flux detecting device employing microprocessor based architecture. 
     FIG. 3B is a second portion of the schematic flow-chart diagram referred to in the description of FIG.  3 A. 
     FIG. 4 is a schematic flow-chart diagram of the noise level detection function in the device of FIG.  3 . 
     FIG. 5 is a schematic flow-chart diagram of the event start detection function of the device of FIG.  3 . 
     FIG. 6 is a schematic flow-chart diagram of the event end detection function of the device of FIG.  3 . 
     FIG. 7 is a schematic flow-chart diagram of the output calculation function of the device of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 a flux measuring device includes a detector coil  12  which is coupled to an analog to digital converter  14 . The analog to digital converter  14  has an output that is coupled to a recirculating memory  16 , an event detector  18  and a noise detector  20 . The noise detector  20  includes a noise memory  22 . The event detector is coupled to a counter  24  which has an output connected to the recirculating memory and a rate control output  26  coupled to the analog to digital converter  14 . A reset switch  28  is also coupled to the recirculating memory  16  which clears the memory when pressed. The noise detector  20  includes a noise display unit  30 . The output of the counter  24  is coupled to a location memory  32  and to an event analyzer  34 . The event analyzer  34  is also coupled to the recirculating memory  16 . A user input unit  36  having various programmable functions and user options is coupled to the event analyzer  34 . The output of the event analyzer  34  is connected to an output display unit  38  which may be any type of visual display, such as an LCD device. 
     The signal from the detector coil is sampled by the ADC  14  at a nominal or predetermined rate and converted to digital data. The data is stored in the recirculating memory  16  so that the freshest data overwrites the oldest data previously stored in the memory. The output of the ADC  14  is also provided to the event detector  18  and to the noise detector  20 , whose functions will be described below. 
     Because of the high amount of noise inherent in the environment in which the system is typically used (e.g., AC line noise, fluorescent lighting, inductive loads) it is necessary to discriminate between ambient noise and the detection of a genuine event of interest. An event is detected when the signal from the detector coil  12  raises significantly above the background noise level and remains there for a predetermined amount of time. Once this occurs, the counter  24  is started, which preserves data collected in the recirculating memory  16  from a time before the event started until after the stopping point of the event. Thus, the counter  24  prevents memory from being overwritten from a time prior to the start of an event. If the counter determines that the memory  16  is half full with information which may not be overwritten and, if the event detector  18  reports that the event is still continuing, then the counter  24  signals the ADC  14  on line  26  to sample at a lower rate, albeit at reduced resolution, so that the entire event may be preserved. The sampling could be lowered still more if the event continues as the remaining available memory is reduced again by half or by some other fixed ratio. 
     The address location of the event start, which will usually be later than the recorded event&#39;s starting address as determined by the counter  24 , the event stop detection of zero crossing (below the noise threshold) the signal start location and the memory locations at which the rate changes occurred are kept in the location memory  32 . At some fixed time after the event detector  18  signals that the event is complete, the counter  24  stops the recirculating memory  16  and this data is provided to the event analyzer  34  for filtering, offset correction, and integration. 
     At this point in the cycle the data in the event analyzer  34  is data which has not been filtered or corrected, but is simply raw data as picked up by the detector coil  12 . The user input  36  may provide any number of options for data correction and filtering. Options include filters that compensate for 60 cycle noise. This also includes multiples of the 60 cycle per second line frequency common in electrical wiring. A high frequency cut-off filter filters noise which may be caused by fluorescent lighting. 
     The event analyzer  34  includes an integrator  40 , a 60 cycle filter  42  which controls the time interval of the integrator  40  in accordance with the description below, a DC offset correction  44 , and a low pass filter  46 . The 60 cycle filter  42  and the DC offset  44  are coupled to the location memory  32  and the counter  24 . The low pass filter  46  is coupled to the recirculating memory  16  and to the user input  36 . 
     Noise that occurs at 60 cycles and at multiples of 60 cycles may be filtered by extending the time event interval over which integration takes place in the integrator  40  so that the event interval is an integral multiple of the 60 cycle period. Since 60 cycle noise is caused by AC lines (which may also include three phase lines), the noise is symmetrical about the zero crossing point. Integration, therefore, reduces the noise to zero as long as the integration takes place over the proper event interval. The 60 cycle filter  42  therefore extends the integration time interval so that it equals the next integral multiple of the period at 60 hz. 
     The low pass filter may include any conventional digital filter that rejects high frequency noise. Such digital filters are well known in the art. 
     The DC offset correction circuit  44  calculates a DC offset by averaging the ambient signal present from a time prior to the detection of an event. This data is kept in the noise memory  22 . A predetermined number of data points prior to the event are averaged and this average represents an offset to the recovered data. 
     Referring to FIGS. 3A and 3B (collectively referred to hereinafter as FIG. 3) a preferred embodiment of the invention shows a digital fluxmeter employing microprocessor-based architecture. FIG. 3 shows, in schematic form, a functional flow-chart of the operation of the microprocessor and associated memory. 
     A coil  50  for sensing magnetic flux is coupled to an A/D converter  52 . The A/D converter  52  samples voltage in the coil  50  at a rate determined by a clock  54  to develop a digital signal E t . The digital voltage samples E t  are evaluated by a noise level detection function  56 , event start detection function  58  and an event end detection function  60 . Data is stored in a memory  62  which is a recirculating memory similar to recirculating memory  16  (refer to FIG.  1 ). 
     The noise level detection function  56  is shown in FIG.  4 . The digital voltage samples E t  have their absolute values summed and averaged over an interval t. This creates the value E n . A nominal noise value, E n0 , is stored at block  64 . The decision at node  66  selects either E n  or E n0  depending upon the time interval. If the time interval is less than t ni  then a new value E′ t  is made equal to E n0 , otherwise E′ t  is made equal to E n . This means that the value used for ambient noise is either a user selected value for a brief startup period or is the average over a predetermined time interval. The noise value E′ t  is multiplied at block  68  by a constant K f  and is multiplied at block  70  by another constant K i . Thus E′ t K i  forms an input to the event start detect block  58  and E′ t K f  is provided as an input to the event end detect block. 
     The event start detect function  58  is shown in FIG.  5 . To determine the start of an event the sampled digital voltage E t  is compared with the noise level times a constant for a predetermined number of cycles. If the comparison is true a time T s  is set. At node  72  a time constant T i  is subtracted from T s  and is stored in the memory  62 . This marks a location in memory where event evaluation is to begin. This operation therefore captures all samples E t  stored at addresses beginning at time T s −T i . 
     The detection of the end of the event is calculated in much the same fashion as shown in FIG.  6 . The event end detection function  60  compares the sampled digital voltage E t  with a constant times the noise level for a predetermined number of cycles. If the condition is true, a time T e  is set. A constant T f  at block  76  is added to T e  to designate the location in the memory  62  marking the end point of the values of E t  which are of interest. The values of E t  that occur between times T s −T i  and T e +T f  are then provided to the output calculation block  78 . 
     The output calculation function is shown in FIG.  7 . The data E t  is first filtered by filters  80  and then integrated over time between T s  and T f  in integrator block  82 . The output calculation  78  is provided to an output format block  84  which is in turn connected to an output display  86  which may be a digital display. A front panel  88  is provided wherein the user may enter commands such as reset  90 , format control  92  as well as selective activation of filters such as filter A  94  or filter B  96 . Filters A and B  94 ,  96  function like low pass filters  46  and  44  in FIG.  2 . The other functions of the embodiment of FIG. 3 are otherwise in all respects similar to those described in connection with FIG.  1 . 
     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.