Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
1997-06-09
2001-05-29
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C324S260000
Reexamination Certificate
active
06239596
ABSTRACT:
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=nd&phgr;/dt.
Therefore, the total flux (&phgr;) is proportional to the integral of the voltage over time and inversely proportional to the number of turns in the coil:
&phgr;=1
∫Edt+&phgr;
0
.
In conventional fluxmeters the meter is set to zero before the measurement is made so that &phgr;
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'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.
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Callahan Rian M.
Hoffman Todd R.
Stupak Michael
Stupak, Jr. Joseph J.
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