Abstract:
A device for determining the level of particulate contamination in a fluid power or lubrication system. Between the inlet and the outlet of the device, working fluid passes through a narrow orifice ( 26 ) defined between a cylindrical spool ( 22 ) and surrounding sleeve ( 20 ). As the orifice becomes obstructed by particulate contamination, fluid flow through the device reduces. The contamination level is assessed by measuring the characteristic reduction in fluid flow rate over a period of time. Following the measurement cycle, fluid flow through the device is reversed causing the spool to move relatively within the sleeve to an area of increased bore ( 40 ) whereby the enlarged gap allows the release of all trapped contaminants. Following this flushing cycle, the fluid is reverted to the original flow direction, the spool returns to the position within the spool that defines the narrow orifice and another measurement cycle begins.

Description:
CROSS-REFERENCE TO OTHER APPLICATIONS 
     Not applicable. 
     BACKGROUND—FIELD OF INVENTION 
     This invention relates to fluid power systems, specifically to a device for the determination of the level of solid particulate contaminants within the working fluid in a fluid power system or lubrication system. 
     BACKGROUND—DESCRIPTION OF PRIOR ART 
     Although fluid power systems and lubrication systems generally have the reputation of high reliability, the cost of a system failure can be significant, through damage, down-time and, in extremis, danger to personnel. There is overwhelming evidence that associates a fluid power system&#39;s reliability to the level of particulate contamination present in the working fluid. It has been establish,ed that clean systems suffer fewer problems with component wear, seal breakdown awl, most important, with catastrophic failures such as valve seizure. It follows that, by determining the particulate contamination level of a fluid system, component and system health can be monitored. Fluid contamination monitoring not only establishes when fluid and filters should be cleaned or replaced but, also, forecasts impending component failure—thereby allowing predictive maintenance procedures to be initiated. 
     One of the most common techniques for assessing fluid continuation is the Patch Test. A small sample of working fluid, previously drained from the fluid system, is forked through a filter membrane (patch). The degree of contamination is indicated by the discoloration of the patch. The problem with this procedure is that the small fluid sample is unlikely to represent overall system contamination level. Additionally, the test cannot readily distinguish particulate sizes and quantitative measurements of contamination levels (in terms of ISO, NAS, or ASTM numbers) are impossible. Finally, the procedure typically takes two hours or more and must be carried out under laboratory conditions to avoid extraneous contaminants from entering the sample during measurement. 
     Another common technique for contamination monitoring is based on optical counter technology. Here a beam of light (white or laser) is directed through a sample of the working fluid whereupon it impinges on a photo-detector arrangement that senses both the size and occurrence (concentration) of the particulates. Particulate counters are available as static, offline monitors (requiring a sample of fluid to be tested under laboratory conditions) or as portable on-line monitors (temporarily integrated with the fluid power system). Optical counter technology has several problems: Firstly, at high concentrations of particles, monitor response becomes saturated. Secondly, sensor collaboration is set against a standard contaminant such as AC dust or glass spherical particles; however, wear debris is rarely spherical, giving rise to spurious results. Furthermore, optical counters cannot distinguish between water and solid particlates and may report that a system is contaminated when it is not. Entrained air and translucent particles, such as quartz and glass, also cause significant problems. Finally, optical counters are delicate in construction, expensive, and are highly sensitive to vibration and pressure ripple which affect the optical light-patch and may cause anomalous readings. 
     A further approach utilizes sensors to measure he pressure differential across a filter medium. The level of fluid contamination is estimated by the time taken to reach a pre-determined differential as filter blockage takes place. It is a problem with this method that it is highly sensitive to changes in pressure, flow rate, viscosity and temperature in the fluid upstream from the monitor. Accordingly, some inventors have included a second reference filter assembly to provide a computer compensated output, as in U.S. Pat. No. 4,685,066 to Hafele et al (1987). This adds significantly to monitor expense and complexity. Also, filters cannot be flushed adequately for continued operation. Therefore, filters must be changed following each measurement cycle thereby confining their adaptability and increasing running costs. 
     To redress some of these deficiencies, U.S. Pat. No. 3,050,987 to Osgood et al (1962) assesses contamination using an on-line monitor hat passes the working fluid through a narrow orifice (gap) between two flat displaceable surfaces. When the gap blocks with particulates, one surface is forced to move in relation to the other and the nature of the resulting movement is used to assess the degree of continuation. It is an advantage of the blockage technique in that it gives a direct indication of the contamination that is likely to cause problems in fluid power systems because the sensor gap is sized to be representative of the most critical gaps in the system. However, although Osgood&#39;s method indicates the presence of contaminants, it has the problem characterized by poor repeatability because the relative movement between two blocked plates is also highly dependent upon oil viscosity, temperature, pressure fluctuations, and contaminant shape and substance The method is, therefore, highly unpredictable in nature and unlikely to give quantitative measurement of contamination level. Due possibly to these reasons and others, Osgood&#39;s method has not been commercially developed and, to our knowledge, has never been available as a marketable product. 
     U.S. Pat. No. 4,468,954 to Lanctot et al (1984), and U.S. Pat. No. 4,495,799 to Fisher et al (1985) again assesses contamination using the “blockage technique”. The method is characterized by passing the working fluid through a narrow annular gap and using the pressure differential across the gap to trigger a counter at a pre-determined contamination level. The cycling rate of the counter is used as an indication of the particulate concentration. It is a problem with the method that pressure within a fluid power system is rarely constant and working pressure changes, machinery vibration, as well as ripple effect, would cause the counter to operate, thereby producing false readings. Working pressure changes would also cause relative movement between the two elements of the gap, thereby releasing trapped particles prematurely and causing erroneous results. Correction of the results produced by this method to compensate for such effects would require substantial additions in both complexity and cost. Due possibly to these reasons and others, Lanctot&#39;s method has not been commercially developed and, to our knowledge, has never been available as a marketable product. 
     U.S. Pat. No. 4,599,893 (1986), continued by U.S. Pat. No. 4,663,966, (1987) both to Fisher et al, also employ the blockage technique. These patents are characterized by the method of introducing a sample of working fluid of pre-determined volume into a cylinder (typically 1 liter) and forcing a piston, accommodating the annular gap, from one end of the cylinder towards the other. When the gap blocks, the progress of the piston is halted. The distance traveled by the piston may be used to assess fluid contamination level, This method has several problems: In order to measure at clean concentration levels (NAS 3/ASM 0), the monitor must be typically over one meter in length and weigh as much as 80 lbs., thereby severely limiting its portability and application. Furthermore, its measurement cycle time is typically over one hour, which also limits its use as a portable device. Also, the means used to communicate force to the piston to move it through the fluid is prone to flexing, tending to distort the gap thereby causing poor repeatability of results. Finally, the distance traveled by the piston is not only related to the level of contamination but, also, to the degree of force used to move the piston. This force may not be constant or repeatable over time. Due possibly to these reasons and others, Fisher&#39;s method has not been developed commercially and, to our knowledge, has never been available as a marketable product. 
     U.S. Pat. No. 5,095,740 to Hodgson et al (Mar 1992) is a simplification of Fisher&#39;s design (U.S. Pat. No. 4,663,966). Working fluid is passed through a filter membrane al system pressure and the volume collected therethrough. Volume to block is then read directly. Hodgson&#39;s method is characterized by poor repeatability. Filter membranes are composed of random material providing an extremely random range of cavities for trapping debris. Also, the volume of the collected fluid used to back-flush the filter is inversely proportional to the contamination level. Therefore, should the filter block quickly with highly contaminated fluid, the flushing volume available is small and unlikely to flush the membrane adequately in preparation for the next cycle; furthermore, back-flushing a filter is known not to be fully effective. Finally, unless the filters are replaced often (thereby reducing repeatability), such a method is also highly prone to membrane failure. Should the filter burst, or become permanently blocked, full system feed pressure is routed directly to the return side of the fluid power system having significant effects upon the system. 
     SUMMARY 
     In accordance with the current invention, a device for determining the level of particulate contamination in a fluid power system comprising an inlet and outlet for working fluid, a silting device to collect contaminants, and a flow rate detector adjacent to the silting device. 
     Objects and Advantages 
     Our invention employs the blockage technique and flow rate disk discrimination. Several objects and advantages of the patent invention are to provide a contaminant monitor that: 
     a) operates on-line as an integral part of the fluid system and in capable of continuous and uninterrupted operation; 
     b) provides repeatable measurements over a wide range of contamination levels and has a rapid operating cycle time since fall blockage is not necessary; 
     c) is compact (typically less than 20 cms.), easily portable and sufficiently rugged for field use; 
     d) is light (typically less than 15 lbs.) and compatible to a wide range of operational uses; 
     e) gives a direct indication of the solid particulate contamination most likely to cause problems (regardless of shape or substance) in a given fluid power system; 
     f) is unaffected by entrained air, water, vibration, and ripple; 
     g) is unaffected by working fluid changes or variations in fluid flow rate, viscosity or temperature; 
     h) is not prone to sensor gap distortions or premature release ol collected particles; 
     i) the flushing volume is independent to the level of contamination, thereby allowing full flushing each cycle. 
     Further objects and advantages are to provide a monitor that is simple and safe to use and inexpensive to manufacture; that is integrated on-line either as a permanent installation on a fluid power system or, in a form that is fully portable (hand-held); that warns when a pre-determined (but variable) contamination level is reached; that may be used in conjunction with and controlled by, a portable computer; that is sufficiently small and of low weight so as to be applicable to airborne (aircraft) use. Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings. 
    
    
     DRAWING FIGURES 
     The invention will now be described, by way of example, with reference to the accompanying drawings in which: 
     FIG. 1 is a schematic diagram showing a section through the device according to one embodiment of the design (showing the measuring cycle) 
     FIG. 2 is a schematic diagram showing a section through the device according to one embodiment of the design (showing the flushing cycle) 
     FIG. 3 is a schematic diagram showing a typical positioning of the device within a fluid power system. 
     FIG. 4 is a schematic diagram showing an alternative positioning of the device within a fluid power system. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 Reference Numerals in Drawings 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 12 
                 Silting device 
               
               
                   
                 14 
                 Flow meter 
               
               
                   
                 16 
                 Control valve 
               
               
                   
                 18 
                 Control/Display unit 
               
               
                   
                 20 
                 Sleeve 
               
               
                   
                 22 
                 Spool 
               
               
                   
                 24 
                 Land 
               
               
                   
                 26 
                 Orifice 
               
               
                   
                 28 
                 Drilling 
               
               
                   
                 30 
                 Cone 
               
               
                   
                 32 
                 Drilling 
               
               
                   
                 34 
                 Drilling 
               
               
                   
                 36 
                 Cone 
               
               
                   
                 38 
                 Drilling 
               
               
                   
                 40 
                 Increased bore region 
               
               
                   
                 42 
                 Device 
               
               
                   
                 44 
                 Pump 
               
               
                   
                 46 
                 Main Supply line 
               
               
                   
                 48 
                 Low pressure return line 
               
               
                   
                 50 
                 Receptacle 
               
               
                   
                   
               
             
          
         
       
     
    
    
     DESCRIPTION 
     FIGS.  1 ,  2  and  3  Preferred Embodiment 
     The foregoing and other objects are achieved by this invention which provides a measurement arrangement to determine the level of solid particulate contaminants within a working fluid. The contaminant monitor of this invention makes use of the known properties of a contaminant-sensitive aperture whereby the flow rate of a contaminated fluid tends to vary predictably with the accumulation of contaminants at such an aperture. This “silting characteristic” is often associated with industrial servo-valves and is normally highly undesirable; however, for the purposes of this invention, it is exploited since gives a direct indication of fluid contamination when assessed using fluid flow rate. 
     A preferred embodiment of the device of the present invention is illustrated in FIG. 1 (measuring cycle) and FIG. 2 (flushing cycle). FIG. 1 shows the device in its normal state. The device comprises a silting device  12  attached to a flow meter  14  each having communication with a control valve  16 , and a control/display unit  18 . 
     Silting device  12  comprises a sleeve  20  which has a hollow cylindrical interior, within which it carries a spool  22 , including a land  24  which defines an orifice  26  between a the spool and the sleeve. Spool  22  is a close fit a inside sleeve  20  but may slide longitudinally within it. The critical dimension of orifice  26  is typically sized to be representative of the most critical clearances in the fluid power system (nominally within the range 5 μm to 10 μm). A drilling  28  formed in the right-band side of the spool allows axial communication of fluid between the right-band side of spool  22  and the clearance at land  24 . The access to drilling  28  is in the form of a cone  30  to encourage fluid to enter spool  22  and to act as a pressure surface to facilitate moving the spool to the measuring position. Communication between control valve  16  and drilling  28  is through a drilling  32  formed in the right-hand end of sleeve  20 . Similarly, a drilling  34  formed in the left-hand side of spool  22  allows axial communication of fluid between the left-hand side of the spool and the clearance at land  24 . The access to drilling  34  is in the form of a cone  36  to encourage fluid to enter spool  22  and to act as a pressure surface to facilitate moving the spool to the flushing position. Communication between flow meter  14  and drilling  34  is through a drilling  38  formed in the left-hand end of sleeve  20 . 
     Sleeve  20  is formed with an increased bore region  40  so that when spool  22  is in its flushing position (fully right) land  24  is registered with increased bore region  40  as shown by FIG.  2 . When spool  22  is in its measuring position (fully left) land  24  is not registered with increased bore region  40  thereby having orifice  26  set as shown by FIG.  1 . 
     Control/display unit  18  is electrically connected to control valve  16  to affect control and timing functions. Control/display unit  18  is also electrically connected to flow meter  14  so that when the control/display unit operates it records the instantaneous flow readings and thereafter computes and displays contamination levels through visual means. In the preferred embodiment of the invention, control/display unit  18  may incorporate an automatic timing function which may be used to cycle through sequential measuring and flushing cycles thereby allowing a plurality of measurements to be made over prolonged periods of time. 
     In the preferred embodiment of the invention, the contamination level of solid contaminants may be derived upon comparison with a pre-determined set of flow rate responses (signature flow rate characteristics). By comparing initial with successive flow rates (and comparing these over time with known and calibrated flow rate models) contamination levels may be established without full blockage of the orifice occurring—thereby improving response time. In another embodiment of the invention, the contamination level of solid contaminants may be derived form measuring the time between two fixed and predetermined flow rate values (initial system flow rate and zero flow rate (full blockage) for example). 
     For optimum results, it is desirable (but not essential) that the device of the kind shown in FIGS. 1 and 2 (referenced as  42  in FIG. 3) should be connected to the supply of the parent system fluid carrying the majority of fluid flow. In the preferred embodiment, FIG. 3, a pump  44  supplies pressurized fluid to a main supply line  46  of the parent fluid power system. A feed from this line passes directly to the inlet port of the device. The outlet port of the device is connected to a low-pressure return line  48  of the parent system and then to tank. In this configuration, continuous operation is available. 
     FIG.  4 —Alternative Embodiment 
     FIG. 4 shows an alternative way to connect the device within a fluid power system. Main supply line  46  is identical to that of FIG.  3 . However, the outlet port of device  42  is connected to a receptacle  50  which may have such capacity so as to bold one or a plurality of cycles of system fluid. In this configuration, access to the low-pressure line is not required and operation is possible on a small sample basis adding to the invention&#39;s portability 
     Advantages 
     From the description above, a number of advantages of our contaminant monitor become evident: 
     a) The orifice/flow meter configuration provides a very compact unit (typically less than 20 cms.) easily portable and sufficiently rugged for field use. Its simplicity also makes the unit very cost effective and low maintenance (there is only one moving part). 
     b) The orifice/flow meter configuration provides a light (typically less than 15 lbs.) monitor, making the unit highly portable and compatible with a wide variety of applications—both permanent and temporary. 
     c) The device may be located in a fluid power system so that it becomes an integral, on-line part, capable of continuous operation 
     d) The orifice/flow meter configuration provides repeatable measurements over a wide range of contamination levels and has a rapid operating cycle time since full aperture blockage is not necessary. 
     e) The orifice/flow meter configuration provides a direct indication of the solid particulate contamination most likely to cause problems (regardless of shape or substance) in a given fluid power system. 
     f) The orifice/flow meter configuration is unaffected by entrained air, water, vibration, and ripple. It is also unaffected by working fluid changes or variations in flow rate, viscosity or temperature. Thereby spurious readings are avoided that could lead to unnecessary system maintenance. 
     g) Finally, the orifice/flowmeter configuration is not prone to sensor gap distortions or premature release of collected particles. Therefore, accurate and repeatable measurements are maintained. 
     Operation—FIGS.  1  and  2   
     FIG. 1 shows the device at initial conditions. One cycle of operation of the device begins with control valve  16  de-activated, spool  22  in the fully left position, and spool land  24  registered with sleeve  20  so that the correct orifice  26  is in effect to entrap contamination. As control valve  16  is de-activated (on command from control/display unit  18 ), the measuring phase is initiated permitting a proportion of the system fluid at system pressure containing solid particulates to flow between inlet I anti port A. The fluid passes into silting device  12  in the first direction via drilling  32 , cone  30 , drilling  28 , and through orifice  26  formed between land  26  and sleeve  20 . As that flow takes place, particles of contaminant present in the fluid become lodged in orifice  26  and will progressively block it thereby reducing fluid flow. Fluid exiting orifice  26 , exits silting device  12  via drilling  38 , cone  36 , and drilling  34  to pass though flowmeter  14  where the flow rate is assessed. System pressure in the first direction is sufficient to hold spool  22  in the fully left position throughout the measuring cycle. Fluid exiting flow meter  14  is ported through control valve  16  via port B and to outlet O to tank. Should spool  22  be in the fully right position at the initiation of the measuring cycle, then system pressure exerted against cone  30  will force spool  22  to move axially within sleeve  20  to the fully left position. 
     At predetermined intervals of time, control/display wait  18  records the instantaneous fluid flow rate measured by flow meter  14  and, through calculation or otherwise, determines fluid contamination level (since contamination level is directly proportional to the reduction in flowrate over time). In the preferred embodiment, the control/display unit records the progressive decrease in flow rate per unit time and compares these measured values with a stored library of empirical flow/time curves, each curve representing a particular contamination level. When the measurement cycle duration has produced sufficient flow rates for a correlation to be made, the contamination level will be displayed. 
     When sufficient time has elapsed to allow contamination measurement, control/display unit  18  activates control valve  16  to initiate the flushing cycle. 
     FIG. 2 shows the device in the flushing condition. The flushing cycle of operation of the device begins with control valve  16  activated permitting a proportion of the system fluid at system pressure to flow between inlet I and port B of control valve  16 . Fluid enters the silting device via flow meter  14  (now disabled). Thereafter, fluid passes through silting device  12  in the second direction through trilling  38 . System pressure is exerted against the surface of cone  36  causing spool  22  to move axially within sleeve  20  to the fully right position. At the fully right position, spool land  24  becomes registered with sleeve  20  so that increased bore region  40  is in effect. As that flow takes place, system pressure flows unimpeded around exposed land  24  causing previously trapped particulates to become dislodged and flushed away through drilling  28 , cone  30  and drilling  32 , System pressure in the second direction is sufficient to bold spool  22  in the fully right position throughout the flushing cycle. Fluid exiting silting device  12  is ported through control valve  16  via port A and to outlet O to tank. 
     Although the invention has been described in terms of specific embodiments and applications, these should not be construed as limiting the scope of the invention blat as merely providing illustrations of some of the presently preferred embodiments of the invention. 
     Conclusions Ramifications, and Scope 
     Accordingly, the reader will see that the Contamination Monitor of this invention can be used to detect microscopic fluid-borne debris in fluid power systems as an integral, on-line unit. It provides continuous operation and repeatable measurements over a wide range of contamination levels and has a rapid operating cycle time since full aperture blockage is not necessary. Moreover, the unit is light, compact, easily portable and sufficiently rugged for field use. Its simplicity also makes the unit very cost effective and low maintenance, The invention provides a direct indication of the solid particulate contamination most likely to cause problems and is unaffected by entrained air, water, vibration, and tipple. It is also unaffected by working fluid changes or variations in flow rate, viscosity or temperature. In addition, the orifice/flowmeter configuration is not prone to sensor gap distortions or premature release of collected particles; thereby, spurious readings are avoided that could lead to unnecessary system maintenance. Furthermore, the Contamination Monitor has the additional advantages in that: 
     a) its on-line capability will obviate the need to install and remove the device during operation with the inherent risk of introducing more contaminants than were previously present; 
     b) its continuous operation allows the detection of contamination at the earliest opportunity thereby allowing maintenance to be timely and to avoid component failure (since failing parts themselves release contaminants into the working fluid); 
     c) its inherently rugged construction allows the unit to be installed directly on dynamic and vibrating machinery, such as military vehicles, simulators, and robots, with no undue effects on measuring accuracy; 
     d) its light weight (typically less than 15 lbs.) and small size is adaptable to airborne use where no contaminant monitor has been used before. This is particularly significant since it allows the unit to be installed on aircraft fluid power systems (including control surface hydraulic actuation systems) where contamination problems can, and have, proven fatal. 
     Although the description above contains may specificities, these should not be construed as limiting the scope of the invention but merely providing illustrations of some of the presently preferred embodiments of this invention. For example, the unit could be coupled to a fluid filtration rig used to scrub the working fluid and to automatically de-activate the filter system when the desired level of fluid cleanliness was achieved The flow meter may be positioned either upstream or downstream of the silting device with equal effect, or it could be bypassed throughout the flushing phase to increase flow and improve flushing effectiveness. Furthermore, the flow meter described may operate by coriolis mass, vortex displacement, or other means with the appropriate accuracy and sensitivity. The spool and sleeve configuration could be adapted so that the cones were not required or that the fluid could enter the orifice from the outside of the spool and return back though the inside of the spool (or vice versa) to reduce length and weight further. The land  24  could also be arranged to pass a “squeegee” arrangement to physically remove debris and, therefore, improve flushing performance. Although the device is designed specifically to determine the levels of contamination in fluid power systems such as hydraulic actuation systems, it is equally effective in determining contamination levels in lubrication oil systems, heavy oil and fuel oil systems, electrical transformer oil cooling systems and similar devices. 
     Thus, the appended claims and their legal equivalents should determine the scope of the invention, rather an the examples given.