GB2392500A - Method of operating a fluid contaminant sensor - Google Patents

Abstract

A method for measuring the contaminant level in a fluid using an annular test passage 48, which gets progressively occluded or blocked by contaminants, and a variable-volume chamber 47. The following steps are carried out at least twice: directing a flow of fluid through the test passage into the chamber; measuring the rate of change of volume of the chamber using LVDT 44; determining the fluid flow; determining the volume of fluid to have passed through the test passage in a given period; calculating the cumulative volume of fluid which has passed through the passage and estimating the total volume of fluid which will flow through the passage before it gets blocked completely. The level of contamination is related this estimated total volume. The estimate is based upon the ratio of the initial and final values of the measured flow therefore eliminating errors due to fluid temperature.

Description

METHOD OF OPERATING A

FLUID CONTAMINANT SENSOR

t - Technical Field

The present invention relates generally to fluid-powered systems, and, more 5 particularly, to an improved method of operating a fluidcontaminant sensor for determin ing the level of particulate contaminants in a system fluid.

Background Art

Contaminated fluids can cause erratic operation or catastrophic failure of pumps, valves and actuators used in hydraulic control systems. Many aircraft fuel systems use 10 servo-based technology, and contamination can lead to dangerous failures. This is often exacerbated by the fact that these are typically "total loss" failures.

Fluid cleanliness is difficult to monitor. Lubricating oil systems for high-cost plant and machinery, such as gas and steam turbines, must be maintained to an exception ally high standard if damage and loss of production is to be avoided. Traditional ways 15 of monitoring oil cleanliness are inconvenient and expensive, and often the results are only available after considerable delay.

-The contamination sensitivity of various elements of a fluid power system is fairly well established. For pumps and motors, there is a critical range of particulate size which will cause most damage. However, a servovalve can experience erratic operation 20 and accelerated wear due to particles as small as a few microns impacting on, and accu mulating around, the lands of a valve spool. These potential failure modes are addressed and guarded against by specific design features that are built into the servovalve. The same is true ofthe other components within the system. However, experience has shown that there is still a strong correlation between the cleanliness of the fluid and the reliabil 25 ity of the system in which it is used.

Advances in fluids, filtration and component design have resulted in improve ments. As a result, stable hydraulic systems are generally reliable, but problems arise when parameters change. For example, a pump might fail, sending a cloud of debris through the system. The fluid may overheat and create a multitude oftiny hard particles.

1 '' 1

-2 New lubricating oil, added to the tank, might not be quite as clean as was thought. I;ucl oil (e.g., aircraft fuel) does not re-circulate through the system, so there is not the oppor tunity for it to sequentially pass through various filters. Such occurrences can wreak havoc with even a well-designed system, and, being unexpected and random in nature, 5 can jeopardize both plant and safety in an unpredictable manner.

Traditionally, cleanliness has been monitored by taking a sample ofthe fluid, and then assessing the number and size of contaminant particles in such sample. This can be done either manually (e.g., by using a microscope) or automatically (e.g., usually em ploying some sort of lightblockage technique). Both techniques are essentially labora 10 tory procedures. Although it is true that most of these instruments are portable, and, in some cases, can even be connected directly to the fluid system to be tested, none is regarded as being small or robust enough to be permanently installed.

Accordingly, there is believed to be a need for a contaminant sensor which is small, robust and suitable for permanent installation in high- cost and/or safety-critical 15 equipment. A system potentially offering these features has been developed by others (see, e.g., published British Pat. Application No. 2,361,548), but owing to the necessary size of the flow-measuring piston required, it is still awkwardly large.

An improved apparatus which permits the size of a volume-or flowmeasuring piston-and-cylinder arrangement to be significantly reduced is disclosed in our published 20 British patent application No. 2369441. The apparatus has a source of pressurized fluid (P') to be tested; a sump (R); a test passage which is substantially free of occluding contaminants at the beginning of the test period and which is adapted to be supplied with a flow of fluid from the source. The passage,which is preferably annular, is so configured and arranged as to be progressively occluded by contaminants in the fluid flow. The apparatus 25 includes a device defining a chamber. The volume of this chamber is variable between a minimum value and a maximum value.

Disclosure of the Invention

30 The present invention relates to an improvement in the operation of such apparatus for testing the level of contaminants in a fluid during a test period.

-3 The improvement provides a method of operating such a senucntia!-sam. pling apparatus for measuring the contaminant level in a fluid by passing a volume of the fluid through an annular test passage, so configured and arranged as to be progressively occluded by contaminants in the fluid, into a variable-volume chamber, comprising 5 the steps of: (a) directing a first sample flow of the fluid through the test passage into the chamber; (b) measuring a first rate of change of volume of the chamber attributable to the first sample flow; (c) determining the first sample flow from the measured first rate of change of volume of the chamber; (d) calculating a first volume of fluid passing through the passage during a first sample period that includes a first time during which 10 the first sample flow is directed into the chamber; (e) directing a second sample flow of the fluid through the test passage into the chamber; (I) measuring a second rate of change of volume of the chamber attributable to the second sample flow; (g) determining the second sample flow from the measured second rate of change of volume ofthe chamber; (i) calculating a second volume of fluid passing through the passage during the second 15 sample period that includes a second time during which the second sample flow is di rected into the chamber; (I) adding the second volume to the first volume to establish an accumulated volume; (k) calculating an estimated total volume of contaminated fluid that would be passed through the passage before it became completely occluded, by multiply ing the accumulated volume by the ratio of the first sample flow to the difference be 20 tween the first and second sample flows; (1) directing a next sample flow of the fluid through the passage into the chamber; (m) measuring a next rate of change of volume of the chamber attributable to the next sample flow; (n) determining the next sample flow from the measured next rate of change of volume of the chamber; (o) calculating a next volume of fluid passing through the passage during the next sample period that includes 25 a time during which the next sample flow is directed into the chamber; (p) adding the next volume to the previous accumulated volume to establish a next accumulated vol ume, (q) calculating a next estimated total volume of contaminated fluid that would be passed through the passage before it becomes completely occluded, by multiplying the next accumulated volume by the ratio of the first sample flow to the difference between 30 the first and next sample flows, and (r) repeating steps (1) through (q) a plurality oftimes to successively calculate the estimated total volume of contaminated fluid that would be

-4 passed through the test passage before it becomes completely occluded; thereby to deter mine the contaminant level in the fluid as a function of the last estimated values.

The method may include the additional step of: (s) tenninating the repetition of steps (1) though (q) when the difference between the last estimate and the just-previous 5 estimate is less than a predetermined value. The plurality may be a predetermined num ber. I he level of contamination in the fluid may be determined as a function of the flow through the test passage during the first cycle and the number of cycles required for such flow to be reduced to a predetermined minimum value. The test passage may be 10 defined between the facing surfaces on a valve spool land and a cylindrical bushing mounted on the body. The device may have a piston mounted for sealed sliding movement within a cylinder. The valve may be a relief valve. The return mechanism may be fluid powered, and the controller may cause the chamber to expand or contract cyclically as a function of the flow through the passage.

Brief Description of the Drawing

Fig. I is a schematic block diagram of a fluid contaminant sensor, this view showing the contaminant-sensing spool valve, the flow-measuring piston, the recycling

valve and the flush valve.

Fig. 2 is a plot of flow (ordinate) vs. volume (abscissa) according to the practice of the improved method.

Description of the Preferred Embodiments

5 At the outset, it should be clearly understood that like reference numerals are intended to identify the same structural elements, portions or surfaces consistently throughout the several drawing figures, as such elements, portions or surfaces may be further described or explained by the entire written specification, of which this detailed

description is an integral part. Unless otherwise indicated, the drawings are intended to

10 be read (e.g., cross-hatching, arrangement of parts, proportion, degree, etc.) together with the specification, and are to be considered a portion of the entire written description of

this invention. As used in the following description, the terms "horizontal", "vertical",

"left", "right", "up" and "down", as well as adjectival and adverbial derivatives thereof (e.g. "horizontally", "rightwardly", "upwardly", etc.) , simply refer to the orientation of 15 the illustrated structure as the particular drawing figure faces the reader. Similarly, the terms "inwardly" and "outwardly" generally refer to the orientation of a surface relative to its axis or elongation, or axis of rotation, as appropriate.

Referring now to the drawings, and more particularly to Fig. 1 thereof, a fluid contaminant sensor is generally indicated at 20. Sensor 20 is shown as broadly including 20 a solenoid-operated flush valve 21, a contaminantsensing spool valve 22, a flow-measur ing piston-and-cylinder assembly 23, and a recycle valve 24. Pressurized fluid, contain ing an unknown quantity of contaminants, from a suitable source is supplied to the system as supply pressure Ps. Supply pressure is provided via line 25 to flush valve 21, and via line 26 to recycle valve 24. Line 26 is shown as having a restricted orifice 28 25 therein. Flush valve 21 and recycle valve 24 are both electrically-operated two-position solenoid-type valves, and are schematically shown as being de-energized in Fig. 1.

Contaminant-sensing valve 22 is shown as having a valve spool mounted for sealed sliding movement within a body. This valve has a rightward sealed land 29, a leeward sealed land 30, and an intermediate non-sealed silting land 31 arranged in 30 closely-spaced facing relation to an inwardlyfacing cylindrical body or bushing surface.

-6 A spring 32 is operatively arranged n the spool left end chamber, and acts between the body and the valve spool. This spring continuously urges the valve spool to move rightwardly relative to the body until a nub 33, extending rightwardly from the spool right end face, abuts the leftwardlyfacing end wall of the right end chamber.

5 Supply pressure Ps in line 26 is also provided via line 34 to the annular space of the contaminant-sensing valve between intermediate silting land 31 and right land 29.

Contaminated fluid from the source may then flow through the annular orifice 48 defined between silting land 31 and the facing body surface into the annular chamber between silting land 31 and left land 30. This chamber communicates with a fluid sump at return 10 pressure R via a conduit 35, which contains a relief valve 36. Fluid in line 35 is supplied via line 38 to the right end chamber 47 ofthe flow-measuring piston-and-cylinder assem bly 23. This assembly includes a piston 39 mounted for sealed sliding movement within a cylinder40. The left end chamber ofthis flow-measuring piston-and-cylinder assembly 23 communicates via line 41 with recycle valve 24, which in turn communicates via line 15 42 with flush valve 21. Line 43 communicates flush valve 21 with the spool right end chamber of contaminant-sensing valve 22. Line 42 communicates with the fluid return R via lines 45 and 35. Line 46 communicates the spool left end chamber of the contami nant sensing valve with line 45.

The position of piston 39 relative to cylinder 40 is continually monitored by a 20 position sensing device, such as a linear variable differential transformer ("LVDT") 44.

The operation of the improved device will now be described.

Assume first that recycle valve 24 and flush valve 21 are both deenergized, as shown, such that flow from P,past these valves will be blocked. In this condition, pres surized contaminated fluid from the source IS will flow via lines 26 and 34 to the annular 25 chamber to the right of silting land 31 in the contaminant-sensing valve, and then through the annular test passageway 48 to the return via relief valve 36. However, relief valve 36 has a spring biasing a ball to a closed position, such that flow through the valve will produce a slightly-elevated back pressure upstream of the valve. Hence, when recycle valve 24 is deenergized such that the left end chamber of flow-measuring valve 23 30 communicates with return via communicating lines 41,42 and 45, a differential pressure across piston 39 will cause fluid in line 35 to flow into the flow-measuring cylinder

assembly right end chamber 47, rather than flowing to return The press', rP diferentia! drives flow-measuring piston 39 leftwardly. The position of this piston is continuously monitored by LVDT 44, which provides an electrical signal via line 49 to a controller 50.

The controller selectively provides output signals to the recycle valve and the flush valve 5 via lines 51 and 52, respectively.

When the flow-measuring piston 39 has been displaced leftwardly toward the end of its stroke, controller 50 energizes the solenoid of the recycle valve to its alternative position. In this condition, supply pressure from the source is applied via lines 26 and 41 to the left end chamber of the flow-measuring piston. Such pressure will urge the 10 flow- measuring piston to move rightwardly, forcing fluid in the right end chamber 47 out through line 38 and relief valve 36 to the retum. This causes the pressure in line. 35 to increase to the previously-mentioned slightly-elevated back pressure. Since this back pressure is very small compared to the pressure drop across the test passageway 48, there will not be a significant decrease in the pressure drop across silting land 31. Hence, there l 5 will not be a significant change in the flow of contaminated fluid, or the rate of occlusion.

Once flow-measuring piston 39 has been sufficiently displaced to the right, controller 50 de-energizes the solenoid of recycling valve 24, allowing the recycle valve to return to the position shown. In this arrangement, fluid passing through the orifice 48 will again flow via conduits 35, 38 to enter chamber 47 and drive the flow-measuring piston 20 leftwardly, forcing fluid in the left end chamber to flow to return via now-connected conduits 41, 42 and 45. Thus, controller 50 may cause the flow-measuring piston to oscillate back and forth, thereby cyclically expanding and contracting the volume of chamber 47 during a test period. A timer 58 is arranged to continually sense the time of these oscillations. Thus, as the orifice 48 becomes progressively occluded by the pres 25 ence of contaminants in the fluid, the time required to increase the volume of chamber 47 from a first volume to a second volume will progressively increase. This reflects the diminished flow through the progressively-occluded orifice 48.

The device operates cyclically in this manner until the cycle time indicates that the flow through the orifice 48 has reached a predetermined minimum value, at which 30 point the test may be stopped and the contamination level calculated from the values of initial flow and final flow and the number of cycles in the test.

-8 The test may be rerun by clearing or flushing the occluded passageway and repeating the test cycles. To accomplish this, the controller sends an appropriate signal to the solenoid of flush valve 21. This causes the flush valve to move to the alternative position, which provides supply pressure via now-communicating lines 25, 43 to the 5 contaminant-sensing valve spool right end chamber. This drives the contaminant-sensing valve spool leftwardly within its body, overcoming the opposing bias of spring 32. By moving silting land 31 to a larger-diameter region 53 of the body, the occlusions caused by the accumulated contaminants from the previous cycle are permitted to be flushed and removed toward the return. Thus, the flush valve operates to substantially remove all 10 contaminants from the orifice 48, and to ready the device for the next series of cyclical flow-measurements. While the flow through the annular clearance test orifice 48 is dependent on fluid temperature, this effect is essentially eliminated from the calculation of contamination level by basing the calculation on the ratio of the initial value to the final value of the 15 measured flow, rather than on the absolute values of flow, and also by keeping the mea suring time relatively short so that the temperature is constant.

Under fixed conditions, there is a linear relationship between the change in flow as the test orifice is progressively occluded by contaminant and the volume of fluid which has passed through the orifice. This is shown graphically by Fig. 2, in which the flow 20 through the orifice at any elapsed time during the test Qua is plotted versus the total, or accumulated, volume Vc of fluid passed through the orifice up to that time. The maxi murn volume of fluid passed, termed the contaminated volume Vc, is the total volume of fluid accumulated when the occluded flow reaches zero. While the starting value of flow Q. and the slope of the plot will change with temperature and viscosity, all such plots will 25 meet at the value of the contaminated volume Vc.

This fact can be utilized to determine empirically how long it is necessary to continue the cycling sequence to obtain a reliable estimate of Vc and hence ofthe contam ination level of the tested fluid. If an initial estimated value of Vc is calculated from the initial flow measurement, and a first value of elapsed flow and the associated value of 30 accumulated volume, and successive estimates arc similarly calculated, it will be found that successive estimated values will rapidly converge. This will happen in a small

-9- fraction of the time that it would take to allow the test oriBlce to become completely occluded, thus permitting the test to be terminated.

Modifications The present invention contemplates that many modifications may be made. For 5 example, while the apparatus has been shown schematically as including two-position solenoid operated valves, this valving arrangement could readily be changed or modified as desired. The structure and operation of the various components of the improved sensor may be changed or modified as desired. It should be clearly understood that the accompanying drawing is used to illustrate the principle of operation of the improved 10 sensor, without intending to limit the structure that might perform the necessary functions defined by the various claims.

Therefore, while the presently-preferred method of operating the contaminant sensor has been shown and described, and several modifications thereof discussed, persons skilled in this art will readily appreciate that various additional changes and 15 modifications may be made without departing from the scopeofthe invention, as defined and differentiated by the following claims.

Claims (4)

-lo- Claims

1. A method of operating a sequential-sampling apparatus for measuring the contaminant level in a fluid by passing fluid through an annular test passage, so config 5 urea and arranged as to be progressively occluded by contaminants in said fluid, into a variable-volume chamber, comprising the steps of: (a) directing a first sample flow of said fluid through said test passage into said chamber; (b) measuring a first rate of change of volume of said chamber attributable to said 10 first sample flow; (c) determining said first sample flow from said measured first rate of change of volume of said chamber; (d) calculating a first volume of fluid passing through said passage during a first sample period that includes a first time during which said first sample flow is directed I S into said chamber; (e) directing a second sample flow of said fluid through said test passage into said chamber; (I) measuring a second rate of change of volume of said chamber attributable to said second sample flow; 20 (g) determining said second sample flow from said measured second rate of change of volume of said chamber; (i) calculating a second volume of fluid passing through said passage during said second sample period that includes a second time during which said second sample flow is directed into said chamber;. -.

25 (I) adding said second volume to said first volume to establish an accumulated volume; (k) calculating an estimated total volume of contaminated fluid that would be passed through said passage before it becomes completely occluded, by multiplying said accu mulated volume by the ratio of said first sample flow to the difference between said first

-11 and second sample flows; (1) directing a next sample Dow of said fluid through said passage into said chamber; (m) measuring a next rate of change of volume of said chamber attributable to said next sample flow; 5 (n) determining said next sample flow from said measured next rate of change of volume of said chamber; (o) calculating a next volume of fluid passing through said passage during said next sample period that includes a time during which said next sample flow is directed into said chamber; 10 (p) adding said next volume to the previous accumulated volume to establish a next accumulated volume; (q) calculating a next estimated total volume of contaminated fluid that would be passed through said passage before it becomes completely occluded, by multiplying said next accumulated volume by the ratio of said first sample flow to the difference between 15 said first and next sample flows; and (r) repeating steps (1) through (q) a plurality of times to successively calculate the estimated total volume of contaminated fluid that would be passed through said test passage before it becomes completely occluded; thereby to determine the contaminant level in said fluid as a function of said last 20 estimated values.

2. The method as set forth in claim 1, and further comprising the additional step of: (s) terminating the repetition of steps (1) though (q) when the difference between the last estimate and the just-previous estimate is less than a predetermined value.

3. The method as set forth in claim I wherein said plurality is a predetermined 25 number.

4. The method as set forth in claim 1, substantially as described with reference to the accompanying drawings.