Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
1998-04-29
2001-07-31
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
Reexamination Certificate
active
06268725
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to flux-gate magnetometers. More particularly, the present invention pertains to the reduction of the effects of electromagnetic interference (EMI) in the measurement of magnetic fields using flux-gate magnetometers.
BACKGROUND OF THE INVENTION
Flux-gate magnetometers have significant advantages in size, weight, power consumption, and reliability for use in the measurement of magnetic fields, particularly small magnetic fields. Generally, a flux-gate magnetometer includes one or more sensors which include a magnetizable core and at least one coil wound around the core. The flux-gate magnetometer senses the magnetic field by stimulating the sensor with a known signal. The known signal is used to drive the core in and out of saturation. The nonlinear magnetic properties of the core cause the second harmonic of the frequency of the known drive signal to be generated. The magnitude of the external magnetic field, i.e., the magnetic field to be measured, is proportional to or can be determined as a function of the second harmonic. For example, in the absence of any component of an external magnetic field, the peaks detected in an output voltage generated across a sensor of the flux-gate magnetometer may be uniform. On the other hand, in the presence of an external magnetic field, the voltage peaks may vary in a manner which may be measured by applying the output voltage to signal conditioning circuitry so as to provide a measurement signal representative of the external magnetic field to be measured.
In other words, the measurement of the external magnetic field is performed through modulation of a core of variable permeability. The modulated field is detected with the coil wound about the core. A change in permeability is accomplished with the known drive signal, e.g., drive current, in the coil wound about the core in such a fashion as to saturate the core during part of the cycle of the drive waveform. Modulation of the magnetic field to be sensed occurs only at even harmonics of the drive waveform due to the symmetry of the magnetization curve. Generally, the second harmonic is used as the measure of the external magnetic field.
Problems may occur in flux-gate magnetometers if operated in high noise environments, e.g., such as in an automobile or around other noisy equipment producing EMI. For example, if the frequency of the EMI is twice that of the drive signal, i.e., equal to the second harmonic frequency, the EMI will be undesirably sensed by the sensor(s) and interpreted as all or a part of an external magnetic field.
In the past, differential circuitry has been used to reduce the effects of EMI. For example, such differential techniques may involve the use of two sensors oriented opposite to one another in a magnetic field to be measured such that one sensor would provide a second harmonic signal which is inverted with respect to the other sensor. Using subtraction of the two signals, noise which is common to both of the sensors (i.e., common mode noise) can be canceled. However, in high noise environments, such differential circuit techniques do not provide adequate EMI immunity.
Conventionally, the drive signal used for driving the core in and out of saturation is a periodic and repetitive drive signal. For example, drive signals which have been used in the past to drive the sensors in and out of saturation include repetitive and periodic triangular waveforms and other repetitive and periodic waveforms, such as those waveforms having a constant duty cycle and/or a constant frequency over time. Flux-gate magnetometers are well known in the art, some examples of which may be found in the issued patents and references listed in Table 1 below.
TABLE 1
Patent No.
Inventor(s)
Issue Date
Articles
3,638,074
Inouye
25 January 1972
“Sensor Noise in
5,530,349
Lopez, et al.
25 June 1996
Low-Level Flux-Gate
Magnetometers,”
by D.C. Scouten,
IEEE Transactions
on Magnetics,
Vol. Mag-8, No. 2,
(June 1972)
All references listed in Table 1 above are herein incorporated by reference in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, the Detailed Description of the Embodiments, and the claims set forth below, any of the devices or methods disclosed in the references of Table 1 may be modified advantageously by using the teachings of the present invention.
SUMMARY OF THE INVENTION
The present invention has certain objects. That is, various embodiments of the present invention provide solutions to one or more problems existing in the prior art with respect to the effects of EMI on flux-gate magnetometers. Such problems are present when flux-gate magnetometers are used in a high noise environment, such as in automobiles or around other equipment that generates noise. For example, if the frequency of EMI in the environment is twice that of the drive signal for the flux-gate magnetometer, the EMI will be sensed by the sensor(s) of the magnetometer and undesirably be interpreted as an external magnetic field. As such, inaccuracies in the measurement of external magnetic fields to be measured will result. While differential circuitry reduces the effects of EMI for flux-gate magnetometers in noisy environments, the reduction is inadequate for the high sensitivity desired for many magnetometer applications.
In comparison to known techniques for reducing EMI effects in flux-gate magnetometers, various embodiments of the present invention provide for further reduction in the effect of EMI on magnetic field measurement. The various embodiments of the present invention may provide one or more of the following advantages: allow the detection of very small magnetic fields in a very high noise environment; provide for further EMI immunity in a noisy environment when combined with the use of differential circuits; provide for a second harmonic signal that would not likely be followed by EMI in the noisy environment, and reduce EMI emissions.
Some embodiments of the invention include one or more of the following features: a drive signal generator operatively connected to drive one or more sensors of a flux-gate magnetometer, wherein the drive signal has a characteristic that varies over time; a drive signal that has a characteristic that is pseudo-randomly varied over time (e.g., a duty cycle of the signal that pseudo-randomly varies over time, a frequency that varies over time, or phase shift that varies over time); a drive signal that has a frequency that varies over time; a drive signal that has a frequency that is non-randomly varied over time such as a frequency varied over time in a predetermined pattern, e.g., not in a pseudo-random manner; translation circuitry operatively connected to sensor(s) of a flux-gate magnetometer to provide a measurement output representative of the external magnetic field based on the output from one or more sensors of a flux-gate magnetometer; differential circuitry for canceling common mode noise present at multiple sensors of a flux-gate magnetometer; a drive signal generator that includes a triangle wave oscillator having a controllable capacitance, wherein the controllable capacitance is used to vary the frequency of a triangle wave drive signal over time (e.g., introduce time jitter into the drive signal); and/or a drive signal generator that includes a triangle wave oscillator having a controllable current, wherein the controllable current is used to vary the frequency of a triangle wave drive signal over time (e.g., introduce time jitter into the drive signal).
REFERENCES:
patent: 3626280 (1971-12-01), Van Englehoven et al.
patent: 3638074 (1972-01-01), Inouye
patent: 4037149 (1977-07-01), Foner
patent: 4107607 (1978-08-01), Kirkland
patent: 4277251 (1981-07-01), Lawson et al.
patent: 4321530 (1982-03-01), Rhodes
patent: 4447776 (1984-05-01), Brown
patent: 4933637 (1990-06-01), Ueda et al.
patent: 5442290 (1995-08-01), Crooks
patent: 5530349 (1996-06-01), Lopez et al.
patent: 5831432 (1998-11-01), Mohri
Ripka, P, “Review of f
Tyler Larry E.
Vernon Scott D.
Medtronic Inc.
Patidar Jay
Patton Harold R.
Woods Thomas F.
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