Method of measuring magnetic field characteristics of magnetic materials

• US 4,340,861 A
• Filed: 03/03/1980
• Issued: 07/20/1982
• Est. Priority Date: 03/03/1980
• Status: Expired due to Term

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Raytheon Company (Rtx Corporation)
• Inventors: Sparks, Richard A.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US06/126,260
• Time in Patent Office: 869 Days
• Field of Search: 324/200, 324/202, 324/205, 324/244, 324/250, 324/260
• US Class Current: 324/205
• CPC Class Codes: G01R 33/24 for measuring direction or magnitude of magnetic fields or magnetic flux

Abstract

Bodies of a ferrimagnetic material are disposed within small cavities provided in a magnetometer. The magnetometer is initially calibrated utilizing an electromagnet to produce an applied magnetic field of either known field strength, H_DC, or known field distribution. Radio frequency energy is fed equally to each cavity and each material and results in resonance frequency output signals which may be coupled to a visual display. When the calibrating magnetic field is of known uniform strength, H_DC, the orientation of the bodies is changed until each of the resonance signals have a frequency substantially given by f_c =γH_DC where γ=2.8 MHz/oersted. When the calibrating magnetic field has a known field distribution the orientation of bodies is changed until such resonance signals have frequencies related to one another in accordance with the relative magnetic field strengths passing through the cavities. After calibration, the electromagnet is replaced by a test fixture including a magnet, the magnetic properties of which are to be measured. Radio frequency energy signals are reintroduced into the cavities. The frequency (f_M) of each of the output signals is then measured. The strength of magnetic field (H_M) of the permanent magnet passing through the cavities is calculated in accordance with H_M =f_M /γ. The distribution of the magnetic field of the magnet under evaluation is determined by comparing frequencies of the output signals.

Claims

1. A method of measuring a magnetic field comprising the steps of:

(a) passing a magnetic field of a calibrating magnet through a cavity in an apparatus having disposed therein a material adapted to produce ferromagnetic resonance;

(b) coupling radio frequency energy into said cavity;

(c) positioning said material within said cavity to produce a first output signal having a frequency related to the magnetic field of the calibrating magnet passing into the cavity and through said material;

(d) passing a second magnetic field to be measured into the cavity and through the positioned material;

(e) coupling radio frequency energy into said cavity to generate a second output signal; and

(f) comparing the frequency of the first output signal with the frequency of the second output signal to provide a measure of the second magnetic field.

2. A method of measuring magnetic field distribution comprising the steps of:

(a) placing apparatus having a plurality of cavities with materials adapted to produce ferromagnetic resonance disposed in each cavity in a magnetic field of a calibrating magnet having a known magnetic field distribution across the cavities;

(b) coupling radio frequency energy into the plurality of cavities and through the materials;

(c) positioning the material within said cavities to produce a plurality of first output signals having frequencies related to the relative magnetic field distribution of the calibrating magnet passing through the cavities;

(d) substituting a magnet whose magnetic field distribution is to be measured for the calibrating magnet;

(e) coupling radio frequency energy into the plurality of cavities and through the positioned materials to generate a plurality of second output signals;

(f) measuring the relative frequencies of the plurality of second output signals across said cavities and comparing the frequencies of such signals with the relative frequencies of the plurality of first output signals to determine the magnetic field distribution of the magnet being measured.

3. A method of measuring magnetic field strength comprising the steps of:

(a) placing apparatus having a cavity with a material adapted to produce ferromagnetic resonance disposed therein in a magnetic field of a calibrating magnet having a predetermined magnetic field strength (H_DC);

(b) coupling radio frequency energy into said cavity and through the material to generate a first output signal having a frequency related to the orientation of the material in said cavity;

(c) positioning the material to a calibrated orientation position resulting in first output signals having a frequency (f_c) defined by the equation (f_c =γ

H_DC) where γ

=the gyromagnetic ratio of the material (2.8 MHz/oersted);

(d) substituting the magnetic field of a magnet having an unknown magnetic field strength (H_M) with the material in a calibrated position for the pedetermined magnetic field strength of the calibrating magnet;

(e) coupling the radio frequency energy into the cavity and the material in the calibrated position to generate a second output signal having a measured frequency (f_M); and

(f) determining the magnetic field strength (H_M) in accordance with the equation (H_M =F_M /γ

).

4. A method of measuring a magnetic field comprising the steps of:

(a) placing apparatus having a cavity with a material adapted to produce ferromagnetic resonance disposed therein in a magnetic field of a calibrating magnet;

(b) transmitting radio frequency energy at a preselected frequency to display video means to generate a reference signal pulse;

(c) coupling radio frequency energy in a predetermined range of frequencies including said reference frequency into said cavity to generate first output signals;

(d) converting said first output signals to video frequencies and transmitting such video signal frequencies to means for display as video signal pulses;

(e) adjusting the orientation of said material within said cavity until the first output signal pulses are in predetermined alignment with the reference signal pulse on the video display means, indicating the calibrated orientation of the material in the cavity relative to the magnetic field of the calibrating magnet passing through said cavity;

(f) substituting a magnet whose magnetic field is to be measured with the material in a calibrated orientation relative to the magnetic field of the calibrating-magnet;

(g) coupling the radio frequency energy into said cavity with the material in the calibrated position to generate second output signals for display on said video means; and

(h) comparing the second video output pulses with the reference signal pulse to provide an indication of the magnetic field properties of the magnet being measured.

5. A method of measuring magnetic field strength and distribution comprising the steps of:

(a) placing apparatus having a plurality of cavities with material adapted to produce ferromagnetic resonance disposed in each cavity in a magnetic field of a calibrating magnet having known properties (H_DC);

(b) transmitting radio frequency energy at a preselected frequency to video display means to generate a reference signal pulse (f_c);

(c) coupling modulated and sweeping radio frequency energy in a predetermined range of frequencies including said reference frequency into each of said cavities to generate first output signals;

(d) converting said signals to video frequencies and transmitting said first video output signals to video display means for display as pulses;

(e) adjusting the orientation of said material within said cavities until the first output signal pulses are in a predetermined array and alignment relative to said reference signal pulse on the video display means, indicating the calibrated orientation of the material in the cavities relative to the magnetic field of the calibrating magnet passing through each of said cavities, whereby the frequency (f_c =γ

H_DC) where γ

=the gyromagnetic ratio (2.8 MH_z /oersted) and H_DC =the applied magnetic field of the calibrating magnet;

(f) substituting a magnet whose magnetic-field (H_M) is to be measured with the material in a calibrated orientation for the calibrating magnet;

(g) coupling the radio frequency energy into said cavities to generate second output signals, (f_M);

(h) converting said second output signals to video frequencies and transmitting said second video output signals to video display means for display as pulses; and

(i) visually comparing the arrangement of the displayed second output signal pulses relative to first output signal pulses to ascertain the comparative magnetic field distribution of the magnet being measured relative to the field distribution of the calibrating magnet; and

(j) measuring the magnetic field strength by application of the equation (f_M =γ

H_M) where (f_M =the ferromagnetic resonance of the material in the calibrated orientation in the magnetic-field of the magnet being evaluated;

γ

=gyromagnetic ratio (2.8 MH_z /oersted) and H_M =applied magnetic field strength of the magnet to be measured.

6. The method according to claim 5 wherein said calibrating magnet comprises an electromagnet having a variable DC voltage source.

7. The method according to claim 5 wherein said material adapted to produce ferromagnetic resonance comprises yttrium iron garnet.

8. The method according to claim 5 wherein said magnet being measured is mounted adjacent to an annular ring of a temperature compensating ferrimagnetic material.