In-line metallic debris particle detection system

An arrangement for detecting metallic particles carried by a fluid includes a metallic probe member which surrounds an elongated passage through which the fluid flows and which is constituted by a split tube having two marginal portions delimiting a gap which extends along said passage and completely separates the marginal portions from one another. Capacitors are arranged at the gap and alternating electric current is caused to flow in the probe member around the passage so that the probe member and the capacitors form a tank circuit having resonance characteristics that are influenced by any inclusion then present in the passage in a manner dependent on the electromagnetic properties of the inclusion. The character of any metallic particle then present in the passage is determined from variations in the alternating electric current that reflect the influence of such metallic particle on the resonance characteristics.

Description 
1. Technical Field 
The present invention relates to particle detection in general, and more 
particularly to arrangements for detecting metallic particles carried by a 
fluid, especially lubricating oil. 
2. Background Art 
There are already known various constructions of detecting arrangements 
capable of detecting the presence of metallic particles in a flow of a 
fluid. So, for instance, the British patent application No. 2,101,330 A, 
published on Jan. 12, 1983, discloses a system for detecting particles in 
flowing fluids. This system utilizes two inductive coils which are spaced 
from one another along a section of the path of flow of the fluid and each 
of which surrounds a portion of this detecting path section. As 
ferromagnetic particles and other inclusions entrained in the fluid pass 
through the detecting path section, they cause changes in the electrical 
impedance of such coils and these changes are then evaluated. The coils 
and the evaluating circuitry together constitute a detector arrangement 
which, because it is designed and operated to suppress detection of 
non-ferromagnetic particles and bubbles on the basis of the sense of the 
phase shift between the alternating electric current flowing through the 
coils and the alternating electric field supplied to such coils, 
determines the presence and sizes of ferromagnetic particles. 
However, this detecting arrangement is capable of detecting the 
ferromagnetic particles only when the total inductance change caused by 
such particles then present in the detecting path section is above a 
predetermined threshold, so that small ferromagnetic particles, if 
permitted to travel through the detecting path section individually or in 
a haphazard manner, would not be detected. To avoid this undesirable and 
potentially dangerous situation, such small ferromagnetic particles are 
accumulated upstream of this detecting path section and released to be 
carried by the fluid into the detecting path section, either from time to 
time or when it is determined that the total amount of such accumulated 
ferromagnetic particles is sufficient to be detected by the 
above-mentioned detecting arrangement. However, this approach has the 
disadvantage that the passage of the thus released batch of small 
ferromagnetic particles through the detecting path section could coincide 
with, and thus obscure, the passage of relatively large non-ferromagnetic 
particles, so that the system could furnish unreliable or even misleading 
results. Also, the coils generate substantial electric fields in the 
detecting path section and this further detracts from the accuracy of the 
obtained results in that such electric fields, in contradistinction to 
magnetic fields, are highly susceptible to the influence thereon of 
bubbles entrained in the liquid. This influence, in turn, changes the 
phase of the alternating electric current flowing through the coils in a 
manner which could mask the passage of ferromagnetic particles through the 
detecting path section. 
Experience with systems of the above type has confirmed that, in addition 
to their relatively low sensitivity, they also suffer of the important 
disadvantage of responding not only to entrained ferromagnetic particles, 
but also to other kinds of particles and air bubbles, in a manner which 
does not permit the detecting system reliably to distinguish between 
ferromagnetic particles and other, non-ferromagnetic, inclusions, or 
between various kinds of such other inclusions. This constitutes a serious 
or even life-threatening drawback in applications, such as in an aircraft 
lubricant monitoring application, where it is crucial not only to 
determine the presence of inclusions in the flow of the fluid, such as 
lubricating oil, and the size of any of such inclusions, but also to 
distinguish between inclusions which are relatively innocuous (as, for 
instance, air bubbles or even dielectric particles entrained in the flow 
of lubricating oil would be), on the one hand, and usually metallic but 
not necessarily ferromagnetic particles (for instance those whose presence 
and/or size is indicative of dangerous deterioration of bearings, 
transmission gears or the like) which might be indicative of an imminent 
failure of the aircraft or similar equipment. 
Accordingly, it is a general object of the present invention to avoid the 
disadvantages of the prior art. 
More particularly, it is an object of the present invention to provide a 
particle detecting arrangement which does not possess the disadvantages of 
the known arrangements of this kind. 
Still another object of the present invention is so to develop the particle 
detecting arrangement of the type here under consideration as to maximize 
its sensitivity to metallic particles and to minimize its sensitivity to 
non-metallic inclusions. 
It is yet another object of the present invention to devise a particle 
detecting arrangement of the above type which renders it possible to 
reliably detect even quite minute individual metallic particles and also 
to distinguish between ferromagnetic and non-ferromagnetic metallic 
particles. 
A concomitant object of the present invention is an arrangement of the 
above type designed in such a manner as to be relatively simple in 
construction, inexpensive to manufacture, easy to use, and yet reliable in 
operation. 
DISCLOSURE OF THE INVENTION 
In keeping with these objects and others which will become apparent 
hereafter, one feature of the present invention resides in an arrangement 
for detecting metallic particles carried by a fluid. The detecting 
arrangement includes means for bounding at least one elongated passage for 
the flow of the fluid therethrough and a metallic probe member which is 
stationary with respect to the bounding means and is constituted by a 
split tube which extends around the passage and has two marginal portions 
delimiting a gap which extends along the passage and completely separates 
the marginal portions from one another. The arrangement further includes 
capacitor means which is arranged at the gap and includes at least two 
mutually facing capacitor surfaces each electrically connected with one of 
the marginal portions of the probe member, and at least one dielectric 
layer interposed between the capacitor surfaces. There is further provided 
means for causing alternating electric current to flow in the probe member 
around the passage between the two capacitor surfaces so that the probe 
member and the capacitor means form a tank circuit having resonance 
characteristics that are influenced by any inclusion then present in said 
passage in a manner dependent on the electromagnetic properties of the 
inclusion. Last but not least, the arrangement of the invention includes 
means for determining the character of at least any metallic particle then 
present in the passage from variations in the alternating electric current 
that reflect the influence of such metallic particle on the resonance 
characteristics. 
A particular advantage of the arrangement as described so far is that, as a 
consequence of the construction of the probe member as a split tube and of 
the positioning of the capacitor means at the gap of the split tube, the 
above tank circuit exhibits an extremely high Q factor. This, coupled with 
the operation of the above tank circuit at resonance, results in a 
situation in which even very small metallic particles individually passing 
through the passage surrounded by the probe member cause an immediate 
pronounced or perceptible change in the resonance characteristics of the 
tank circuit, with the sense of the change depending on whether the 
metallic particle is or is not ferromagnetic. At the same time, the split 
tubular configuration of the probe member results in only a minimum 
electric field in the passage so that bubbles and other dielectric 
inclusions have only a minimum, if any, influence on the resonance 
characteristics of the tank circuit and, consequently, the passage of such 
dielectric inclusions through the interior of the probe member will hardly 
be perceived by the determining means, if at all.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to the drawing in detail, and first to FIG. 1 thereof, it may 
be seen that the reference numeral 10 has been used therein to identify a 
tubular probe housing or pipe section. The pipe section 10, which is of a 
non-conducting unity magnetic permeability material, bounds a passage 11 
for the flow therethrough of a fluid that is to be examined for the 
presence therein of various inclusions, such as magnetic and non-magnetic 
metallic particles. 
A probe member 12 of a highly electrically conductive material, such as 
copper, is arranged in such a manner as to be stationary relative to the 
pipe 10 and to circumferentially surround the passage 11. For instance, 
the probe member 12 may be embedded or potted in the pipe section 10. The 
probe member 12 is constituted by a split tube or sleeve and it has 
respective marginal portions 13 and 14 which bound a gap 15 with one 
another. The split tubular probe member 12 may have an axial length of, 
for instance, 11/8" and a diameter of about 0.71", and may be made of 3 
mil thick copper sheet. It will be appreciated, though, that the above 
dimensions, while they have been carefully chosen for a particular 
construction of the detecting arrangement of the present invention, may be 
altered without departing from the present invention, so long as the 
altered dimensions satisfy the operating criteria that will be discussed 
below. 
In the probe member construction illustrated in FIG. 1, the marginal 
portions 13 and 14 overlap one another, and a capacitor arrangement 16 is 
interposed between the overlapping regions of the marginal portions 13 and 
14. The capacitor arrangement 16 may include merely a layer or slab of 
dielectric material, in which case the overlapping regions of the marginal 
portions 13 and 14 constitute respective capacitor plates. However, more 
often than not, the surface areas of the overlapping regions of the 
marginal portions 13 and 14 are insufficient to provide the required 
capacitance. In such a case, in accordance with the present invention, the 
capacitor arrangement 16 may be constituted by a single multilayer 
capacitor device, or preferably by a number of such multilayer devices 
which are distributed at predetermined, such as substantially identical, 
intervals along the gap 15 between the overlapping regions of the marginal 
portions 13 and 14 of the tubular probe member 12. 
In an alternative construction of the probe member 12 which is not 
illustrated in the drawing, the marginal portions 13 and 14 are turned up 
as considered in FIG. 1 of the drawing, thus giving the gap 15 which 
accommodates the capacitor arrangement 16 a vertical orientation. Another 
alternative construction of the probe member 12, which is also not shown 
in the drawing, has the marginal portions 13 and 14 aligned with one 
another in the circumferential direction and separated from each other by 
the gap 15, and the capacitor arrangement 16 being located at the exterior 
of the probe member 12 and spanning the gap 15. In either case, if the 
probe member 12 is embedded in the housing or pipe section 10, so may be 
the capacitor arrangement 16. 
The multilayer capacitor device, or each of such devices, which is used in 
the capacitor arrangement 16 of the above-mentioned type, may include, as 
is well known to those familiar with design and manufacture of capacitors 
two sets of capacitor plate layers. The capacitor plate layers of each set 
are electrically connected with one another and the capacitor plate layers 
of one of the sets are interleaved with those of the other set. Each 
adjacent two of the capacitor plate layers, which belong to different 
sets, are separated from each other by an intervening dielectric 
separation layer. The capacitor plate layers of one of the sets are 
electrically connected with the marginal portion 13, while the capacitor 
plate layers of the other set are electrically connected with the marginal 
portion 14. 
As further shown in FIG. 1 of the drawing, the marginal portions 13 and 14 
have respective electric leads 17 and 18 connected to them. The electric 
leads 17 and 18 serve to supply alternating electric current to the 
marginal portions 13 and 14. When this occurs, the split tubular member 12 
forms a parallel tank circuit with the capacitor arrangement 16. Because 
of the split tubular configuration of the probe member 12 and the location 
of the capacitor arrangement 16 at the gap 15, that is, as close as 
physically possible to the marginal portions 13 and 14, this tank circuit 
has the highest Q factor achievable at the frequency chosen for the 
alternating electric current. It will be appreciated that an important 
criterion in altering the dimensions of the probe member 12 (and/or the 
capacitance of the capacitor arrangement 16) is the preservation of this 
high Q factor at the selected operating frequency. 
The existence of this high Q factor means that, when the electric current 
supplied to the marginal portions 13 and 14 through the electric leads 17 
and 18 alternates at such a frequency that the tank circuit operates at or 
close to resonance in the absence of any inclusions from the fluid present 
in the passage 11, any change in the characteristic response of the 
contents of the passage 11 caused, for instance, by the presence of 
metallic particles in the fluid flowing through the passage 11, will 
introduce an imbalance into the operation of this tank circuit in a manner 
dependent on the electromagnetic properties of such inclusions. More 
particularly, ferromagnetic metallic particles influence the 
electromagnetic field generated by the probe member 12 and thus the 
electric current flowing in the tank circuit differently than dielectric 
particles or other dielectric inclusions, and metallic non-ferromagnetic 
particles influence the electromagnetic field differently than either the 
metallic ferromagnetic particles or the dielectric inclusions, resulting 
in a different phase shift in each instance, while the magnitude of the 
change depends, by and large, on the size of the respective particle or 
inclusion. At the same time, however, the split tubular configuration of 
the probe member 12 results in a situation where the electric field within 
the probe member 12 is as low as possible, so that air bubbles which are 
frequently encountered in lubricants will affect the operation of the 
aforementioned tank circuit only to an insignificant extent, if at all. 
The phase shift response of the tank circuit constructed in accordance with 
the present invention to changes in the electromagnetic properties of the 
contents of the passage 11 is diagrammatically depicted in FIG. 2 of the 
drawing in which the point of origin 0 represents the conditions 
encountered when the passage 11 is filled with lubricating oil devoid of 
any inclusions. If the dielectric constant of the fluid present in or 
flowing through the passage 11 changes, which may occur, for instance, due 
to replacement of the original dielectric fluid by another dielectric 
fluid, both the relative resistivity (.DELTA.R/R) and the relative 
impedance (.DELTA.L/L) of the overall tank circuit (which includes the 
fluid present in the passage 11 in addition to the aforementioned tank 
circuit proper that is constituted by the split-tube probe member 12 and 
the capacitor arrangement 16) change generally to the same relatively 
small degree. This is indicated in FIG. 2 by the point A located on a 
straight line D, the distance OA being representative or the worst case 
scenario involving complete replacement of lubricating oil by air. It may 
be seen that the above distance is rather small, and it will be 
appreciated that in actual operation of the detecting arrangement only a 
limited amount of air will usually be present in the lubricating oil so 
that the actual deviation from the point 0 along the line D on account of 
air bubbles will be so small as to be negligible. 
On the other hand, when a ferromagnetic particle enters the internal 
passage 11 that is surrounded by the split-tube probe member 12, they both 
the relative impedance and the relative resistivity change in dependence 
on the size of the particle so as to be located on a curved line F which 
is applicable when the ferromagnetic particle is substantially spherical. 
As an example, point B of the curve F may be reached when the spherical 
ferromagnetic particle is about 7 mils in diameter, and the distance on 
the curve F from the point 0 will be lesser for smaller and greater for 
larger spherical ferromagnetic particles. For other shapes of the 
ferromagnetic particles, other curves akin to curve F and forming a family 
therewith apply, but all such curves are always located in the first 
quadrant of the graph depicted in FIG. 2. Thus, it may be seen that the 
values located in the first quadrant are indicative of the ferromagnetic 
character of the respective particle, and that the extent of deviation 
from the point 0 is indicative of the size of the respective ferromagnetic 
particle. 
In contradistinction thereto, when the particle entering the internal 
passage 11 of the split tubular probe member 12 is metallic but 
non-ferromagnetic, the relative resistivity still changes in the positive 
sense, but the relative impedance changes in the negative sense, in 
accordance with the representative curve N of a curve family akin to that 
mentioned above, with all curves of this family this time being always 
located in the fourth quadrant of the FIG. 2 graph, and the distance along 
the respective curve, such as N, being again indicative of the size of the 
respective metallic non-ferromagnetic particle. Thus, when it is 
determined that the value lies in the fourth quadrant, then the particle 
must be metallic and non-ferromagnetic, while the distance from the point 
of origin 0 gives the size of such particle. 
A circuit constructed in accordance with the present invention to gather 
and decipher the above information is presented in FIG. 3 of the drawing 
where the same reference numerals as before have been used to identify 
corresponding parts (i.e. their electrical equivalents). The lead 18 from 
the tank circuit 12 and 16 is shown to be grounded, while the lead 17 is 
connected to one end of one transformer winding 19 of a driving and pickup 
transformer 20. The transformer 20 further includes another transformer 
winding 21 whose one end is grounded while the other end thereof is 
supplied with an alternating electric current from a voltage controlled 
oscillator (VCO) 22. The alternating electromagnetic field generated by 
the other transformer winding 21 induces a correspondingly alternating 
electric current in the one transformer winding 19, and this latter 
electric current drives the tank circuit 12 and 16. The frequency of the 
alternating electric current issued by the oscillator VCO is such that the 
tank circuit 12 and 16 operates at or close to resonance at least when the 
passage 11 (FIG. 1) is filled exclusively with lubricating oil. 
The alternating electric current is also supplied directly to one input of 
a first mixer 23, and through a 90.degree. phase shifter 24 to one input 
of a second mixer 25. A line 26 supplies an alternating electric current 
derived from the one coil 19 and thus representative of the alternating 
electric current flowing through the one coil 19 and thus into and out of 
the tank circuit 12 and 16 to a pre-amplifier 27 from where the amplified 
electric current is supplied to another input of the first mixer 23, as 
well as to another input of the second mixer 25, where the respective 
incoming alternating electric currents are mixed with one another, with 
the result that respective in-phase and quadrature error signals 
indicative of the difference between the output frequency of the VCO 22 
and the frequency encountered in the tank circuit 12 and 16 appear at 
respective outputs 28 and 29 of the mixers 23 and 25. These error signals 
are then filtered by respective low-pass filters 30 and 31 to obtain 
respective resistive (in-phase) and reactive (quadrature) error signals. 
The reactive error signal is supplied to a reactive error amplifier 32 
which amplifies this reactive error signal, and this amplified reactive 
error signal is then supplied to an input of the VCO 22 which changes its 
operating (output) frequency in dependence on the magnitude of the 
amplified reactive error signal. Similarly, the resistive error signal is 
fed to an input of a resistive error amplifier 33 which amplifies this 
resistive error signal, and this amplified resistive error signal is then 
supplied to an input of a voltage controlled resistor (VCR) 34 which is 
interposed between the other end of the one transformer winding 19 and the 
ground and whose resistance varies in dependence on the magnitude of the 
amplified resistive error signal. The resistive and reactive error 
amplifiers 33 and 32 are constructed to operate with a relatively large 
time constants, so that the resistance of the VCR 34 and the frequency of 
the VCO 22 change gradually in response to relatively long-term changes, 
especially those due to temperature variations, of the resonance 
characteristics of the tank circuit and/or of the characteristic 
properties of the contents of the passage 11. On the other hand, 
short-lived changes in such characteristic properties, such as those 
caused by the passage of individual metallic particles through the 
interior of the probe member 12, will leave the performance of the VCO 22 
and of the VCR 34 virtually unaffected. 
The output signals of the low-pass filters 30 and 31 are also supplied to 
an evaluating circuit 35 which is constructed to evaluate the reactive and 
resistive error signals to determine therefrom the character and size of 
any metallic particle then present in the passage 11. A quite simple 
exemplary implementation of the evaluating circuit 35 is shown in FIG. 3 
of the drawing, but it is to be understood that the evaluating circuit 35 
may have other configurations, depending on needs, or requirements for 
accuracy. The illustrated implementation of the evaluating circuit 35 
incorporates a voltage divider 36 and a plurality of comparators 37a to 
37n (n being any arbitrarily chosen integer) each of which has two inputs 
one of which is connected to an associated section of the voltage divider 
36 while the other input is supplied with the filtered resistive error 
signal appearing at the output of the low-pass filter 30. Thus, the 
comparators 37a to 37n compare the filtered resistive error signal voltage 
with various reference voltage levels derived from the voltage divider 36, 
and that or those of the comparators 37a to 37n at which the filtered 
resistive error voltage exceeds the respective reference voltage issues an 
output signal or issue respective output signals which is or are then 
supplied to a drive circuit 38 of any known construction which drives a 
display 39. Furthermore, the filtered reactive error signal appearing at 
the output of the low-pass filter 32 is also supplied to the drive circuit 
38 and is used to drive the display 39 accordingly. 
It will be appreciated that, in the construction of the evaluating circuit 
35 depicted in FIG. 3, the drive circuit 38 and the display 39 may be 
constructed in any well-known manner to present a numerical indication of 
the value of the resistive error signal which, as a reference to FIG. 2 
will reveal, is indicative of the size of the respective metallic 
particle, whether such particle is ferromagnetic or non-ferromagnetic, and 
to present a simple, for instance on/off, indication of the sign of the 
reactive error signal to distinguish ferromagnetic metallic particles from 
non-ferromagnetic ones. However, it ought to be realized that it is also 
contemplated by the present invention to provide other constructions of 
the evaluating circuitry 35 and/or of the display 36, which present more 
sophisticated and/or more accurate results. So, for instance, the reactive 
and resistive error signals from the outputs of the filters 30 and 32 have 
been supplied to an oscillograph for recording thereat, and the thus 
recorded traces of the reactive and resistive error signals have been 
compared and evaluated in view of one another to determine both the size 
and the magnetic properties of respective particles. Of course, it is also 
contemplated to automate this cross-referencing procedure to determine the 
exact location of the response to the respective particle on the graph of 
FIG. 2 with attendant more precise determination of the characteristics 
(size, magnetic properties) of the respective particle. 
FIG. 4 of the drawing, in which the same reference numerals as before but 
supplemented with a prime have been used to identify corresponding parts, 
illustrates an alternative construction wherein the probe member 12' 
includes a tubular inlet portion 12'a toroidal central portion 12'b, and a 
tubular marginal portion 12'c. The gap 15' in this instance is arranged at 
the top of the toroidal central portion 12'b and extends all along a 
circle between the marginal portions 13' and 14' that are 
circumferentially aligned with one another, while the capacitor 
arrangement 16, which is shown to consist of a plurality of equidistantly 
arranged separate capacitor devices of the type mentioned earlier, is 
arranged at the exterior of the central portions 12'b and spans the gap 
15'. This particular construction of the probe member 12' has the 
advantage that the magnetic field in the portion of the passage 11' that 
is situated within the central portion 12'b is much more uniform and lower 
than in the construction of FIG. 1, due to the absence of end edges from 
the toroidal central portion 12'b in which the passage of the metallic 
particles is being detected, with attendant additional reduction of 
sensitivity of the detecting arrangement to bubbles and other dielectric 
inclusions. 
While the present invention has been illustrated and described as embodied 
in a particular construction of a metallic particle detection arrangement, 
it will be appreciated that the present invention is not limited to this 
particular example; rather, the scope of protection of the present 
invention is to be determined solely from the attached claims.