WO1989002083A1

WO1989002083A1 - A high resolution magnetometer

Info

Publication number: WO1989002083A1
Application number: PCT/GB1987/000594
Authority: WO
Inventors: Dennis Amerena Parker; Richard Vincent Parker
Original assignee: Dennis Amerena Parker; Richard Vincent Parker
Priority date: 1987-08-24
Filing date: 1987-08-24
Publication date: 1989-03-09
Also published as: JPH02504669A; EP0377555A1

Abstract

A particle sample (6) to be measured is placed in the alternating field of one of a pair of special transducers (1 and 2). Each transducer is comprised in part of a high permeability magnetic body (5), shaped to have a central limb, around which is positioned an excitation coil (4), and an outer return limb. The rest of the magnetic path consists of an air gap in which a detector coil (3) is positioned to be co-axial with the excitation coil. The outputs of the pair of transducers is compared. The extra emf induced by the sample appears as a difference signal which is amplified by an amplifier (7). After further amplification stages the signal is passed through a phase sensitive detector and an indication of the number of particles in the sample is displayed on a display unit. Zeroing of the device, prior to introduction of the sample (6) can be achieved either by varying the position of two metal masses (8) (one ferrous, one non-ferrous), on the axis of the unit and/or by adjusting a variable resistor (9).

Description

"A High Resolution Magnetometer" This invention relates to a magnetometer whose purpose is to measure such parameters as the total mass, volume and magnetic or electric properties such as permeability, magnetic moments, Curie temperatures, etc., of a sample particle or particles. Existing magnetometers usually rely on oscillating samples in a strong magnetic field, and consequently have problems arising from the feedback of mechanical vibration, pick¬ up of mains frequency and other inherent problems. Present magnetometers are primarily used in connection with laboratory techniques, and not easily transportable or lack the sensitivity and adaptability that may be required.

According to the present invention there is provided an electro magnetic device for sensing or measuring metallic samples of material, the device comprising one or more transducers, consisting of an excitation coil for creating an alternating gradient magnetic field in the gap between the coil and the limb or limbs of a path defined by a body of magnetically permeable material, a detector coil positioned in the gap of the path, means for positioning a sample adjacent to the detector coil of one transducer and reading means for indicating any changes to the output of the transducer due to the positioning of a sample adjacent to the detector coil and/or for comparing in opposition the outputs from two such transducers.

Because of the protected closed magnetic path of the device, substantial immunity from stray magnetic fields and from the influence of unstable objects on the underside of the transducer is achieved.

One application for the present device is that of condition monitoring, whereby the analysis of metal particles, worn from moving parts in machines, in terms of type of metal, size and total mass, may give an insight into the present and future wear patterns.

It may be necessary to measure the parameters of the sample or samples in a fluid suspension of gas or liquid - or else deposited on a non-metallic substrate or in solution from any fluid or suspension.

A sample may be subjected to measuring by positioning it in proximity to the sensing area of one of a pair of inductive transducers. The two transducers form a balanced, differential pair of elements in such a manner that when no particles are present their electromagnetic signal outputs are of equal or near equal magnitude, and of opposite polarity.

An alternating magnetic field is created by an alternating current in the excitation coil and this is situated within the permeable limb so that the lines of force are largely contained within the path, except for a relatively short air gap. It is into this air gap that the pick-up coil is positioned so that the electromotive force (emf) is induced in it by the alternating field. The introduction of a sample in proximity to the detector coil causes a local realignment of the magnetic field and so creates a change in the emf induced in the coil which may be detected or measured.

Great precision is achieved by use of two near- identical transducers and comparing or mixing the outputs from their respective pick-up coils so that when no sample is present their outputs are equal or near equal, are of opposite polarity and hence cancel each other out. Placing the sample above one transducer will then result in a differential signal output which is proportional to the parameters of the sample.

The operating frequency of the magnetic field, the intensity of the field, the permeability of the magnetic limbed path, as well as the properties of the particles are all related to the differential signal.

There are advantages in choosing a high value for the frequency of the magnetic field, but for practical purposes it may have values from 30Hz to 50MHz. The difference signal output is then electronically processed. In one, but by no means only embodiment of the design, the difference signal is amplified by an initial amplifier, the output of which is then passed through a series of further amplifiers which constitute band pass filters, so that only the desired signal or signals is transmitted. A detector stage follows which may consist of an analogue AC to DC converter which can drive an analogue meter or a digital panel meter. In these a reference signal may be taken from an amplitude detector monitoring the amplitude of oscillation of the field in the excitation coils. For most purposes the difference voltage signal should be exactly zero before a sample is placed in position so as to facilitate the accurate measurement of the sample's parameters. This zeroing of the apparatus is achieved by exactly matching e fs induced in the two detector coils so that at the mixer stage they cancel out. There are two possible methods of achieving this. The first method is to adjust one or possibly both resistors placed in series with the two detector coils and the mixer stage, and so alter the relative signals until equality is obtained between them. A second method involves altering the position of two small masses, one of non-ferrous, and the one of ferrous metal, in relation to one of the detector coils, so as to induce a small balance restoring emf. These methods may be used independently or together.

The invention may be performed in various ways and preferred embodiments thereof will now be described with reference to the accompanying drawings, in which:-

Figure 1 illustrates, in cross-section, a transducer for a sample measuring device of this invention; Figures 1A, IB and 1C are alternative embodiments of the transducer, in cross-section;

Figures ID and IE are, respectively, plan views of the transducers of Figures lAand IB and of Figure 1C;

Figures IF and 1G are cross-sectional and plan views respectively of a further embodiment of a transducer of this invention;

Figures 2A and 2B illustrate alternative methods of use of a dual transducer system of a measuring device of the invention; Figure 3 illustrates excitation or detection circuitry for use with the device of Figure 2;

Figures 4 and 5 show in cross-section and plan view two proposed types of sample collecting cell for use with the measuring device shown in previous drawings; Figures 6 and 7 are side and plan views of a further type of sample collecting cell; and ^''

Figures 8 and 9 illustrate two still further possible types of sample collecting cell.

Figure 1 illustrates a transducer wherein an annular detector coil 3 is shown arranged co-axially above an excitation coil 4. An alternating magnetic field is created in a low reluctance magnetic path 5 by means of an alternating current in the drive coil 4. The detector coil is positioned in such a way that it is influenced by a strong gradient magnetic field between the centre pole and the peripheral pole or poles of the magnetic path, with the result that the detector coil has an emf induced in it. Introduction of a sample 6 of particles in the position illustrated causes a local realignment of the magnetic field and hence a change in the emf induced in the detector coil 3.

Figures 1A to 1G illustrate a number of possible variations in the design of the transducer. In each of these examples, the geometry of the permeable limb pole is similar to that of Figure 1. However the outer low reluctance limb 5 varies from one example to another giving a variation of the magnetic field pattern, providing flexibility of design for particle measurement instruments where the particles are distributed in various ways on substrates. As shown in Figure 2, the e fs from the two detector coils of a pair of transducers 1,2 are compared or mixed in opposition. When no sample is present the voltage signals at the mixing point should be equal and of opposite polarity to cancel exactly. In order that this equality can be obtained prior to a reading taken, two masses 8, one of non-ferrous and the other of ferrous etal, are adjusted in relation to one of the transducers so as to change the emf induced within the detector coils. Adjustable resistors 9A and 9B can also be used to adjust the voltage signals from the two transducers and so zero the mixing point.

After zeroing, a sample can be positioned as shown and the differential voltage signal is taken as being proportional to the parameter of the sample. The difference signal is electronically processed and filtered by a main amplifier 7 and a series of further amplifiers constituting band-pass filters and leading to. a detector and a signal display unit, as shown in Figures 2 and 3. Figure 3 shows a. sinusoidal drive device 10 required to provide the excitation field in transducers 1 and 2.

Figure 2A illustrates a procedure whereby particles are located in one place 6A, relative to one of the transducers 1, in which condition a reading from the magnetometer is taken. The particles are then moved up to a second position 6B from which a second reading from the magnetometer is taken. The difference between the two readings is computed by the circuit of Figures 2A and 3.

Figure 2B illustrates a different procedure wherein the • particles are located initially at 6A around the vertical axis of the transducer 1 and a reading is taken from the magnetometer. The particles are then taken to a second position 6B on the axis of transducer 2 when a second reading from the magnetometer is taken. The difference in the two readings is then computed by the circuit of Figures 2B and 3. The results achieved by the method of Figure 2B is twice the amplitude of that achieved when using the circuit of Figure 2A.

The remaining drawings illustrate two methods of gathering particles relating to the use of the magnetometer sensors.

Figure 4 indicates a filter capsule made of non- metallic and non-conductive components. Fluid flows into an inlet 13 and out through an outlet 14. A fine filter 15 of porous membrane of paper or of some other material is provided, together with a perforated support plate 16. A known amount of fluid is passed through the capsule. Metal particles 17 are thereby trapped by the filter 15 and are collected in a flat container 11. A removable cover 12 allows for cleaning after use. Figure 5 indicates a filter capsule wherein a fluid containing particles flows via inlet 22 and outlet 23 through a container 21. The particles are separated and trapped by an external magnetic or electric field source represented at 24. A predetermined amount of fluid, is passed through- the container 21 and the magnetic field causes the particles to be trapped therein. When the external magnetic field is removed the particles may be measured by the magnetometer. Subsequently, the capsule may be cleaned and made ready for re-use by pumping a flushing fluid through the capsule. The filter capsules of Figures 4 and 5 may be used in conjunction with the methods of metal particle measurement described in connection with Figures 2A and 2B.

As a further aid to metal particle measurements either of the filter capsules of Figures 4 and 5 may be attached to or built into a movable carrier for ease of movement in relation to the transducers, as shown in Figures 6 and 7, Figure 8 or Figure 9. This method may be used for the method of inline fluid flow particle measurement.

Claims

C AIMS

1. An electro magnetic device for sensing or measuring metallic samples of material, the device comprising one or more transducers, consisting of an excitation coil for creating an alternating gradient magnetic field in the gap between the coil and the limb or limbs of a path defined by a body of magnetically permeable material, a detector coil positioned in the gap of the path, means for positioning a sample adjacent to the detector coil of one transducer and reading means for indicating any changes to the output of the transducer due to the positioning of a sample adjacent to the detector coil and/or for comparing in opposition the outputs from two such transducers.

2. A device according to claim 1, wherein the excitation coil is formed around one of the limbs of the body so as to generate an effective magnetic field.

3. A device according to claim 1 or claim 2, wherein the geometry of a permeable magnetic path and its associated variable reluctance circuit, and hence the nature of the gradient magnetic field, and the position of the detector coil and the adjacently placed sample are so defined as to obtain a near maximum linear signal emf response in the detector coil from any particular amount of sample.

4. A device according to any one of claims 1 to 3, including means for electronically balancing signals from the_. detector coil of a transducer so as to provide a null signal prior to placing the sample adjacent to the detector coil.

5. A device according to claim 1 and substantially as herein described with reference to the accompanying drawings.

6. A method of measuring the mass of a particle sample using apparatus according to any one of claims 1 to 5, wherein the output of the detector coil with no sample adjacent to the detector coil, is compared to the output when a sample is present.

7. A method according to claim 6, wherein the output from the detector coils of two transducers are compared in opposition so that they cancel or tend to cancel and form a differential combined output signal, and the output signal is measured both with a sample present and absent from a position adjacent one of the coils.

8. A method according to claim 7, wherein ferrous and non-ferrous masses are brought into proximity to one of the two transducers so as to alter the gradient field strength and so as to cause a change in the differential combined signal output to enable an initial null value to be obtained before introduction of the sample.

9. A method according to claim 8, wherein the total mass of a sample is measured by comparing the differential output from two detector coils, when the sample is positioned adjacent to one coil, to that of the differential output produced when the sample is positioned adjacent to the other .coil.

10. A method according to any one of claims 7 to 9, wherein the signals from the two transducers are exactly balanced by altering the value of one or both current resistors placed between the outputs of one or both of the two transducers and a common mixing point.

11. A method according to any one of claims 7 to 10, wherein the differential voltage signal is measured, filtered or otherwise electronically processed to provide a measure of the magnetic parameters of the sample.

12. A method according to claim 11, wherein the electronic processing includes stages of amplification, filtering and the conversion of the signal or signals, from an oscillatory to a steady state nature.

13.. A method according to claim 12, wherein the electronic processing is used to detect and measure the phase and magnitude of all or any part of the differential signal.

14. A method according to any one of claims 11 to 13, wherein the difference signal is electronically processed to provide an analogue or digital index or measure of the quality or quantity of the metal particles sample.

15. A method according to any one of claims 11 to

14, wherein the difference signal is electronically processed or conditioned so as to provide a suitable input signal for a computer, microprocessor or other digital equipment.

16. A method according to any one of claims 6 to

15, wherein the magnetic or electric field is an oscillating field, and the frequency of oscillation is between 30Hz and 50MHz.

17. A method according to any one of claims 6 to

16, wherein metal particles are filtered by a filter capsule, through which a fluid flow passes, and are entrapped by a replaceable porous membrane.

18. A method according to claim 17, wherein the filter capsule through which fluid passes is placed in the immediate vicinity of an external magnetic or electric field causing the ferrous metal particles to be separated from the fluid flow and thereby entrapped.

19. A method according to claim 17 or claim 18, wherein the filter capsule is attached to or built in a movable carrier for ease of particle measurement.

20. A method of measuring the mass of a particle sample substantially as herein described with reference to the accompanying drawings.