GB2186978A - A high resolution magnetometer for sensing metallic samples - Google Patents

Abstract

A particle sample (6) to be measured is placed in the alternating field of one of a pair of transducers 1 and 2. Each transducer (Figure 1) is comprised of in part a high permeability magnetic path (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 are compared (Figure 3). The e.m.f. induced by the sample appears as a difference signal which is further amplified by an amplifier (7) and after further amplification stages is passed through a phase sensitive detector (13) and an indication of the number of particles in the sample is displayed on a unit (14). Zeroing of the device, prior to introduction of the sample 6 can be achieved either by varying the position of two metal masses one ferrous, one non ferrous, on the axis of the unit or by adjusting a variable resistor coupling the detector coils to amplifier (7). <IMAGE>

Description

SPECIFICATION Thigh resolution magnetometer This invention relates to a new type of magnetometerforthe measurement of such parameters as the total mass, volume and magnetic or electric prop ertiessuch as permeability, magnetic moments, Curietemperatures, etc., of a sample particle or particles.

Existing magnetometers usually rely on oscillating samples in a strong magneticfield - and consequ entry have problems arising from thefeedbackof mechanical vibration, pick-up of mains frequency and other inherent problems.

Present magnetometers are primarily laboratory techniques, not easily transportable or lack the sen- sitivityand adaptability that may be required.

Another type of existing magnetometer is that described in U.K. patent 8509040. The present invention has none of the problems of the mechanical magnetometers and is easily transportable and also has a number of advantages over the previous patent which may be listed asfollows: 1. Greater sensitivity and stability.

2. Because of the protected closed magnetic path, afar greater immunity from stray magnetic fields and the influence of unstable objects on the underside of the transducer.

One application for the present method isthatof condition monitoring,where by the analysis of metal particles warn of moving parts in machines in terms oftype 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 anyfiuid or suspension.

According to the present invention, there is a method of measuring the parameters of a sample by positioning it in proximity to the sensing area of one of a pairofinductivetransducers.Thetwotrans- ducersform a balanced, differential pairofelements in such a mannerthatwhen no particles are present their electromagnetic signal outputs are of equal or near equal magnitude, and of opposite polarity.

Each inductive transducer consists of a high permeability limbed path, annular excitation coil and an annular detector coil. An alternating magnetic field is created by an alternating currentintheexcitation 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 pro ximityto the detector coil causes a local realignment of the magnetic field and so creates a change in the e.m.f. induced in the coil which may be detected or measured.

Amore precise method isto usetwo near identical transducers and to compare or mixthe 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 ofthe 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 thefrequencyofthe magneticfield, but for practical purposes it may have values from 30Hzto 50MHz.

The difference signal output is the 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 offurtheramplifierswhich constitute band pass filters, so that only the desired signal or signals istransmitted.Adetectorstagefol- lows which may consist of an analogue AC to DC converterwhich can drive an analogue meterora 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 differencevoltagesignal should be exactly zero before a sample is placed in position so as to facilitate the accurate measurement of the samples parameters. This zeroing of the apparatus is achieved by exactly matching e.m.f.s.

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 isto firmly adjust one or possibly both resistors placed in series with the two detector coils and the mixer stage, and so alter the relating signals until equality is obtained between them.

The second additional method involves altering the position of two small masses, one of non ferrous, and the one of ferrous metal, in relation to one ofthe detector coils, so as to induce a small balance restoring e.m.f.

Byway of example, one specfic embodiment of the invention will be described with reference madeto drawings.

Figure2 is a schematic representation of two transducers with particles and balancing masses shown adjacent to their respective transducers; the output from each is fed into a mixer amplifier so that the difference of the two e.m.f.s. only is amplified.

Figure 1 shows in cross-section atransducerwith particle in position for measurement.

Figure 3 is a schematic representation of two transducers connected together with associated excitation and detection circuitry.

In Figure 1,an annular detector coil (3) is shown arranged co-axially above an excitation coil 4. An alternating magnetic field is created in the 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, hence the detector coil has an e.m.f. induced in it. An introduction of a sample (6) of particles in the position causes a local realignment of the magneticfield and hence a change in the e.m.f. induced in the det ector coil.

The e.m.f.s. from two detector coils from a pair of transducers are compared or mixed in opposition as shown in Figure 2. When no sample is present the voltage signals at the mixing point should be equal and of opposite polarity to exactly cancel. In order that this equality can be obtained priorto a reading taken, two masses one of non ferrous and the other offerrous metal (8) are adjusted in relation to one of the transducers so as to change the e.m.f. induced within the detector coils. Adjustable resistors 9a and 9b can also be used to alternate the voltage signal from the two transducers and so zero the mixing point.

After zeroing a sample can be positioned as shown and the differential voltage signal taken as being pro portional to the parameter of the sample. The differ ence signal is electronically processed and filtered by amplifiers (7) as in Figures 2 and 3.

Figure 3 shows the sinusoidal drive circuitry (10) required to provide the excitation field in transducers land 2.

Figures 1 a, 1 band 1 c illustrate variations of Figure 1.

In these examples,the geometry of the permeable limbs pole are similartothat of Figure 1; however the outer low reluctance limb (5) varies from one example to another giving variation of the magnetic field pattern providing flexibility of design for particle measurement instruments wherethe particles are distributed in varies ways on substrates.

Figure 2a illustrates the 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 upto a second position (6b) from which a second reading from the magnetometer is taken.

The difference in the two readings is computed by the circuit of 2a and 3.

Figure 2b illustrates a different procedure whereby the particles located at (6a) initially around the vertical axis of the transducer (1) when a reading is taken from the magnetometer.

The particles are then taken to a second position (6b) to the axis oftransducer (2) when a second read ing from the magnetometer is taken.

The difference in the two readings is computed by the circuit of 2b and 3. The results from the method of Figure 2b is twice the amplitude of that shown in Figure 2a.

The second section herein illustrates two methods of gathering particles relating to the use of the mag netometer sensors as described in the application 8524945 and further described herein.

Figure4indicatesafiltercapsule made of non- metallic and non-conductive components. Fluid flows into an inlet (3) and outfrom 4. Item 5 is afine filter of porous membrane of paper or of somme other material. Item 6 is a perforated support plate.

A known amount of fluid is passed through the capsule. Metal particles (7) are thereby trapped by the filter (5) and are so gathered in theflatcontainer (1). Item 2 is removeablecoverto allowforcleaning afteruse.

Figure 4 indicates a filter capsule wherein a fluid containing particles flows via inlet (2) and outlet (3) through a container (1); the particles are seperated and trapped by an external magnetic or electricfield source represented by 4.

A determined amount offluid is passed through the container and the magmeticfield causes the part iclesto 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 3 and 4 may be used in conjunction with the methods of Figures 4 and 5 for metal particle measurement.

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 and methods of Figures 2a and 2b. This method may be used forthe method of inline fluid flow particle measurement.

Claims (20)

1. An electro magnetic device for sensing or measuring metallic samples of material comprised oftransducerortransducers, consisting of an excitation coil for creating an alternating gradient mag neticfield in the gap between the limb or limbs of a path of magnetically permeable materials, a detector coil positioned in the gap, means for positioning a sample adjacent to the detector coil of such a transducer or comparing in opposition the outputs from two such transducers.

2. A device according to claim 1 wherein the excitation coil informed around oneofthe limbs so as to generate an effective magnetic field.

3. A method according to claims 1 to 2wherein 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 ofthe detector coil and the adjacently placed sample are so defined as to obtain a maximum linear signal e.m.f. response in the detectorcoil from any particular amount of sample.

4. A device according to claims 1 and 3 wherein the signals from the detector coil of a transducer can be electronically balanced so as to provide a null signal prior to placing the sample adjacent to the detector coil.

5. A method of claims 1 to 4whereby the mass of the sample can be measured, by comparing the output of the detector coil with no sample adjacentto the detector coil, to the output when a sample is not present.

6. A method of claims 1 to 5 wherein the output from detector coils from two transducers can be compared in opposition so thatthey cancel ostend to cancel and form a differential combined output signal.

7. A method of claims 1 to 6 whereby ferrous and non ferrous masses may be brought into proximity to one of the two transducers so as to alterthe gradientfield strength and so asto cause a change in the differential combined signal output.

8. A method of claims 1,5-7 whereby moving the proximity offerrous and non ferrous masses the differential combined signal output can be adjusted to zero.

9. A method according to the claims 1 to 8 hereby the total mass of sample can be measured bycomparing the differential output from detector coils of two such transducers when the sample is positioned adjacent to one coil to that ofthe differential output when the sample is not so present.

10. A method according to claims 1 to 8 wherein the total mass of a sample can be measured bycom- paring 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 to the other coil.

11. A method as in Claims 1 to 10 wherebythe magnetic or electric field is an oscillating field, where the frequency of oscillation may be between 30Hz and 50MHz.

12. A method as in Claims 1 to 11 wherebythe signals from the two transducers can be exactly balanced by means of altering the value of one or possiblytwo current resistors placed between the outputs of the two transducers and a common mixing point.

13. Amethod as in Claim 11 wherebythedifferential voltage signal can be measured, filtered or otherwise electronically processed to provide a measure of the samples magnetic parameters.

14. A method of Claim 12, whereby the electronic processing may include stages of amplification, filtering and the conversion of the signal or signals, from an oscillatory to a steady state nature.

15. Amethod of Claims 13,14, whereby elec- tronic processing can be used to detect and measure the phase and magnitude of all orany partofthedifferential signal.

16. A method of Claims 13, 14 and 15whereby the difference signal may be electronically processed to provide an analogue or digital index or measure ofthe quality orquantity ofthe sample metal particles.

17. AmethodofClaimsl3,14,15and16, whereby the difference signal may be electronically processed or conditioned so as to provide a suitable input signal for a computer, microprocessor or other digital equipment.

18. A method of Claims 1 to 17 whereby metal particles may be filtered by a filtercapsulethrough which a fluid flow passes and by employing are placeable porous membrane which entraps the particles.

19. An alternative method to Claim 18,whereby the filter comprises a capsulethrough which fluid may pass and is placed in the immediate vicinity of an external magnetic or electric field causing thefer- rous metal particles to be separated from the fluid flow and thereby entrapped.

20. A method of Claims 1 to 19 wherebythefilter capsules may be attached to or built in a moveable carrierforease of particle measurement. This method could be employed for conditions of incline particle separation from the fluid flow.