GB2165650A - Method and apparatus for the quantative and qualitative measurement of small metal particles - Google Patents

Abstract

A particle sample 7 to be measured is placed in a divergent alternating magnetic field created by an excitation coil 2 and the outputs from two pick-up coils 1A and 1B are compared. The extra emf induced by the sample 7 appears as a difference signal which is amplified by an amplifier 5 and after further amplification stages is passed through a phase sensitive detector 13 and an indication of the quality or quantity of particles in the sample is displayed on a unit 15. Zeroing of the device, prior to introduction of the sample 7 can be achieved either by varying the position of a metal slug 6 on the axis of the unit or by adjusting a variable resistor 4A. <IMAGE>

Description

SPECIFICATION Method and apparatus for the quantative and qualitative measurement of small metal particles This invention relates to the detection of, or measurement of, the quality and quantity of small metallic particles. The terms quality and quantity refer to such parameters as the total mass, volume, type of metal and magnetic or electric permeability of the particle samples.

Frequently in industrial and scientific applications it is considered necessary to detect or measure the quality and quantity of small metallic particles. One such application is that of condition monitoring, which relates to the analysis of wear of moving parts in machinery where the metal particles may vary in size from a few millimetres down to a few hundredths of a micron. It may be necessary to measure the parameters of the particles in fluid suspension, deposited on a non-metallic substrate or in isolation from any fluid or substrate.

Quantative measurements of the size and nature of metallic particles are frequently made using known techniques; these include transmitted light techniques, chemical analysis, spectrometric analysis, oscillating sample magnetometers and electronic inductive bridges.

Present magnetometers rely on moving or oscillating samples requiring analysis in a strong static magnetic field and consequentiy have attendant problems, some of which are of a very severe nature. They may be listed as follows: 1. Feedback through mechanical vibration through the samplespecimen holder drive apparatus.

2. Feedback through the electronic amplifiers if the drive apparatus is motivated electronically or electrically.

3. A tendency of the electronic amplifier to pick up 50 or 60HZ from the mains supply, since mechanical drive systems operate at low frequencies only.

4. The inherent difficulty in designing and building a low inertia specimen drive apparatus which is also robust and easily operated.

All these prior methods have limitations and are either relatively expensive and time-consuming to operate, and may thus be considered to be primarily laboratory techniques, or lack the required sensitivity and adaptability of usage.

According to the present invention, there is provided a particle quantity measurement device comprising a unit for creating a divergent alternating magnetic field, a pair of detector coils positioned coaxially on either side of the unit in the divergent field path, means for positioning a particle sample adjacent to one of the coils and a circuit for comparing the outputs of the two coils.

The introduction of metallic particles to one side of one of the pick-up coils causes a local realignment of the magnetic field and so creates a change in the emf induced in that coil. The emf's from the pair of pick-up coils are compared or mixed in opposition; hence the particle samples form a differential signal output which is proportional to the quality and quantity.

The operating frequency of the magnetic field as well as the intensity of the field are related to the differential signal by: Vs = Ky.M.B.f where Vs is the difference voltage signal K is a constant of proportionality M is the total mass of the particle or particles B is the magnetic field generated f is the frequency of oscillation of the field and L is the magnetic permeability of the particle or particles The actual value of B.f will depend upon the type of core material chosen and the frequency of the oscillating field. There are certain advantages in choosing a high value of f, the operating frequency, but for practical purposes f 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 sig nal or signals is transmitted. A detector stage follows which may consist of an analogue AC to DC convertor 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 coil.

For most purposes the difference voltage signal should be exactly zero before sample particles are placed in position so as to facilitate the user in taking the desired measurements. The zeroing of the apparatus is achieved by exactly matching the emf's induced in the pick-up coils so that at the mixer stage they cancel out. One method of achieving this is to finely adjust one or maybe both resistors placed in series with the two pick-up coils and the mixer stage and so alternate the induced emf's until an equality is obtained.

Another or additional method involves altering the position of a small slug of a suitable metal in relation to one of the pick-up coils, so as to induce a small balance restoring emf.

The device may include positioning means for positioning the particle sample alternately adjacent to one or the other of the two detec tor coils. If this positioning means comprises a plunger incorporating a particle trap and movable within a tube between the two positions for the particle sample, then it is possible to carry out automatic measurement on a sample such as by inserting the plunger into a flow passageway for a particle contaminated fluid to collect a new sample.

The invention also extends to a method of measuring the quantity of particles in a sample using a device of this invention as hereinbefore defined, and which comprises positioning the sample adjacent one of the coils, and comparing the outputs from the coils with outputs achieved in the absence of the sample, during induction of the alternating divergent magnetic field by the unit.

The invention may be performed in various ways and preferred embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows in cross-section an excitation and pick-up coil assembly of a device of the invention with sample particles in position; Figure 2 is a schematic representation of the coil assembly, attenuating resistors and subsequent electronic signal processing circuitry for the device of Figure 1; Figure 3 is a schematic diagram of phase detectors and a final display staged for a modified circuit; Figure 4 illustrates a variation of the circuit of Figure 3; Figure 5 shows an additional sample and hold feature for the circuit of Figure 4; Figure 6 is a cross-section through a modified device of the invention for automatic monitoring of a sample; and Figures 7 and 8 illustrate a part of the device of Figure 6 in different stages of operation.

In figure 1, a pair of pick-up coils, 1 A and 1 B, and an excitation coil 2 are coaxially arranged so that the excitation coil lies between the two pick-up coils. The pick-up coils are spaced a small distance from the excitation coil so as to be influenced by a strong alternating, symmetrical, divergent magnetic field.

In order to provide the required sensitivity the excitation coil 2 has a core of magnetic material 3 to concentrate the magnetic field.

To take a reading of measurements with sample particles, it is necessary to adjust the apparatus in the absence of these particles.

One method of reducing the difference signal to a minimum is by adjusting the comparative values of resistors 4A and 4B leading into a mixer amplifier 5 (Figure 2). A second and further means of zero control may be madeby adjusting the relative axial position of a metal slug 6 (Figures 1 and 2). When a satisfactory zero is obtained, readings or measurements with the sample particles in place can be taken. Sample particles are placed in position at the point marked 7 in Figure 1. The extra emf induced by these samples appears as a difference signal which is initially amplified by the amplifier 5, and then further amplified and extraneous noise filtered by amplifiers 8, 9, 10 and 11. Figure 2 also shows an electronic oscillator 12 by means of which the excitation coil 2 is driven.

Figure 3 is a diagrammatic illustration of a circuit which is able to distinguish between ferrous and nonferrous particles in a given sample. Ferrous particles give rise to signals which are in phase with the oscillating magnetic field, while non-ferrous particles give rise to signals 90" out of phase with the same field. Thus, signals amplified and filtered by preceding stages are examined by two phase sensitive detectors 13 and 14, which convert the signals due to ferrous and non-ferrous particles from an oscillating nature (A.C.) to a steady state nature (D.C.). Signals relating to ferrous and non-ferrous particles may be examined visually on a final display section 15, by closing and opening switches 16 and 17.

The sum of the signals may be displayed by closing both switches so that the signals are added in a summing amplifier 18. Digital displays by their nature require a reference signal of some sort and in this embodiment of the invention, this is taken from the excitation oscillator 12 via an A.C. to D.C. convertor 19, which measures the amplitude of oscillation of the excitation field.

The variable resistor 4A can be operated automatically to create the necessary zeroing function by taking an output signal from the phase detector 13 which will indicate either positive or negative value enabling a servomotor to drive the variable resistor 4A in the right direction to achieve zeroing before particle measurement commences. The apparatus tends to be highly sensitive and so there can be a continuous drift off from zeroing. Hence regular automatic adjustment is advisable.

In the circuits illustrated in Figures 4 and 5, a single switch 20 is provided so that signals from either one of the phase sensitive detectors can be fed to the display unit 15, which can incorporate a computation section to analyse the signals and calculate a combined value reading for ferrous and non-ferrous particles if desired. Figure 5 illustrates a sample and hold circuit to be used for a different method of operation of the apparatus of Figure 1. The particles present at the position 7 shown in Figure 1 produce an output potential Va at capacitors 21, 22 when switches 23, 24 close. Switch 23 then opens leaving capacitor 21at charge at Va, switch 24 remaining closed. The particles are then removed to a new position adjacent to the pick-up coil 1B of Figure 1 (but at the same distance as from pick-up coil 1A previously). A new output signal potential Vb is now produced at capacitor 22. The original signal potential on capacitors 21, 22 (Va-Va) determines the zero signal on a meter of unit 15 as the outputs from the two capacitors are fed to the two input terminals of the meter. The new signal potential between capacitors 21, 22 (Vb-Va) determines the amplitude signal.

Movement of the particles between the pick-up coils 1 A and 1B can be achieved automatically by a special version of the device shown in Figure 6. This has a non-metallic tube, or cylinder 25, positioned axially through the coil assembly 2, 3. The particles 7, to be quantitized, are positioned axially, firstly adjacent to the coil 1 A and then adjacent to the coil 1 B. When this unit is used in conjunction with the circuits of Figures 4 and 5, again the reading (Vb-Va) gives a measure of the quantity of particles being measured.

Vb is generally of the same magnitude as Va, but since it is obtained via the phase detector 13 it is of opposite sign. Hence when the signals from the capacitors 21 and 22 are applied to the terminals of the meter of unit 15, the output is twice that of the simple non-inline instrument and the error signals have been cancelled out. When switch 20 is moved to the alternative position the instrument will register signals passing through the 90" phase sensitive detector 14 rather than the detector 13 so as to check for non-ferrous particles.

A means of conveying the particles from one position 1 A to the other 1B is by means of a nonmetallic piston trap 26, which may move along the axis of the tube, or cylinder 25 as shown in Figure 7. The piston may oscillate between these two positions taking the entrapped particles 7 with it, thereby enabling a number of different readings to be taken in quick succession, so that an average result may be obtained.

A method of readily collecting particles is shown in Figure 8. The piston trap of the preceding section is moved along the axis of the cylinder 5, to a position opposite a flow passageway 29 orthogonal to it. When a magnet or electromagnet 28 is brought up to the piston trap 27, and there is a flow of a medium such as a lubricating oil along the passageway 29 through the chamber formed by the two parts of the piston trap 27, then these particles are pulled out of suspension.

After a determined amount of liquid has gone through, the piston trap 27 is made to take its original position as in Figure 7. The particles 7 may be released from entrapment when the piston 27 is moved into the passageway 29, the magnet or electromagnet being moved away or reduced in magnitude so as no longer to hold the particles. The particles are then removed naturally by fluid flow.

A complete cycle of events would consist of: collection of particles in the piston trap (Figure 8); moving piston 26 slightly up the cylinder 25; removing the electromagnetic field 28 taking a reading of the unit 15 with particles at position 1 A; taking a second reading with particles at position 1B (these two readings may be repeated a number of times as required); return of the piston 26 to the position of Figure 8 when particles are flushed away.

It is also possible to separate parts of the methods outlined above with reference to Figures 6 to 8 and use them in isolation, i.e. not as in an inline instrument, but more as a special version of the original instrument shown in Figure 1.

Where, as illustrated in the circuits of Figures 3 and 4, two phase sensitive detectors, 13 and 14, are used, zeroing of the unit when signals are passed through the 90" phase sensitive detector 14 can be achieved by a nonmetallic slug on the axis of the device similar to the metallic slug 6 shown in Figure 1.

Claims (1)

1. A particle quantity measurement device comprising a unit for creating a divergent alternating magnetic field, a pair of detector coils positioned coaxially on either side of the unit in the divergent field path means for positioning a particle sample adjacent to one of the coils and a circuit for comparing the outputs of the two coils.

2. A device according to claim 1, wherein the unit comprises an excitation coil, preferably with a central magnetic coil.

3. A device according to claim 1 or claim 2, wherein the detector coils are connected in parallei from a common zero -potential point through respective resistors to a common mixing point.

4. A device according to claim 3, wherein one or both of the resistors is variable.

5. A device according to claim 4, wherein a servomotor is connected to drive the variable resistor to achieve an automatic zeroing of the output at the common mixing point prior to positioning of a sample for measurement.

6. A device according to claim 5, wherein theservomotor is connected for control by a signal derived through a phase sensitive detector having an input from the common mixing point.

7. A device according to any one of claims 1 to 6, wherein a magnetic slug is movably positioned on the axis of the unit.

8. A device according to any one of claims 1 to 7, wherein the circuit for comparing the outputs from the two coils incorporates 0 and 90" phase sensitive detectors which may be switched in independently of one another.

9. A device according to claim 8, wherein a slug of non-magnetic material is movably positioned on the axis of the unit.

10. A device according to any one of claims 1 to 9, wherein positioning means are provided for positioning the particle sample alternately adjacent to one or the other of the two detector coils.

11. A device according to claim 10, wherein said positioning means comprises a plunger incorporating a particle trap and movable within a tube between the two positions for the particle sample.

12. A device according to claim 11, wherein the tube communicates with a flow passageway for a particle-contaminated fluid into which the plunger can pass to collect and/or discard a particle sample.

13. A device according to claim 12, including a unit for creating a variable magnetic field in the region of the conjunction of the tube with the flow passageway.

14. A device according to any one of claims 10 to 13, wherein the circuit for comparing the outputs of the two coils includes means for comparing signals created in the coils when the particle sample is in each of its two positions.

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

16. A method of measuring the quantity of particles in a sample using a device according to any one of claims 1 to 15, which comprises positioning the sample adjacent one of the coils, and comparing the outputs from the coils with outputs achieved in the absence of the sample, during induction of the alternating divergent magnetic field by the unit.

17. A method of measuring the quantity of particles in a sample substantially as herein described with reference to the accompanying drawings.

21. A method of obtaining a measurement of the quality and quantity of small metallic particles whereby they are located within a suitable electric or magnetic field, and the resulting changes in the electric or magnetic field are electronically detected or processed to provide the required quantification or qualification.

22. A method as in claim 21, in which the suitable electric or magnetic field and the pickup coils form the essential part of the invention.

23. A method as in claim 22, in which the suitable electric or magnetic field is an oscillating field, where the frequency of oscillation may lie between 30Hz and 50MHz.

24. A method as in preceding claims, whereby the suitable field is generated by an excitation coil and this is driven by a suitable oscillator.

25. A method as in claim 24, whereby the excitation or driving coil contains a core of magnetic material to concentrate the magnetic or electric field.

26. The method of claim 24, whereby the excitation or driving coil is located between two pick-up coils so that an electromotive force (emf) induced by the field in the said pick-up coils.

27. The method of claim 26, whereby the pick-up coils are wound and connected in such a manner that the emf's induced in the two coils are of an equal or near equal magnitude, are of an opposite polarity and thus tend to cancel or nearly cancel each other out when mixed.

28. A method of claim 27 and claim 28, whereby the pick-up coils are slightly spaced from the excitation coils so as to be influenced by a diverging field and thereby improve the sensitivity of the apparatus.

29. A method of claim 27, whereby the value of the emf's can be equalised by adjusting the value of two alterable resistances positioned between each pick-up coil and a common mixing point.

30. A method of claim 27, whereby through adjusting the position of a small metal slug in proximity to one of the pick-up coils, a more fine condition of equality can be obtained between the emf's induced in the two pick-up coils.

31. A method of preceding claims, whereby the sample particles are placed by one of the pick-up coils and the local realignment of the field causes an addition or otherwise change in the emf induced in that pick-up coil.

32. A method of claim 17 and claim 21, whereby the emf's from the two pick-up coils are mixed in opposition so as to produce a difference signal which is proportional to the quality and quantity of the sample metal particles.

33. A method of claim 22, whereby the difference signal is subjected to electronic processing.

34. A method of claim 22, 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.

35. A method of claim 22 and claim 13 and claim 14, whereby electronic processing can be used to detect and measure the phase and magnitude of all or any part of the differential signal.

36. A method of claim 22, 23, 24, and 25, whereby the difference signal may be electronically processed to provide an analogue or digital index or measure of the quality or quantity of the sample metal particles.

37. A method of claims 22, 23, 24 and125, 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.

41. A method of obtaining two separate readings from metal particles, having moved from one position and then a second relative to the coil assembly, and comparing the difference in these readings.

42. A method of claim 41, whereby a sample and hold circuit compares the outputs of circuit (Fig. 4) produced when the particles move from the first to the second position. A typical sample and hold circuit is shown in Fig. 5.

43. A method of claims 41 and 42, whereby particles in a coaxially mounted tube or chamber relative to the coil assembly are located and then transferred from one position to another as in Fig. 6.

44. A method of claims 43, 42 and 43, whereby the particles are conveyed from position to position by means of a special piston called an 'entrapment piston'.

45. A method of claims 41, 42, 43 and 44, whereby the metal particles may be filtered and collected by a magnetic or electromagnetic field in the special piston and then transferred, so that the method of claim 43 may be carried out.

46. A method of claim 45, of releasing the particles from entrapment so that a future cycle of events can take place.

47. A method whereby methods 41 to 46 may be used separately and, if necessary, in isolation.