Patent Publication Number: US-9849412-B2

Title: Detecting the clogging of a fluid filter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. application Ser. No. 12/364,754 filed Feb. 3, 2009, the entire contents of which is incorporated herein by reference. U.S. application Ser. No. 12/364,754 claims the benefit of priority from prior French Application No. 08 50744 filed Feb. 6, 2008. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of fluid filters. 
     More particularly, the invention relates to a method and a device for detecting the clogging of a fluid filter. 
     BACKGROUND OF THE INVENTION 
     A fluid circuit is generally fitted with a filter that serves to filter the fluid, i.e. to remove impurities and residues therefrom that might otherwise damage the equipment in which the fluid flows. By way of example the fluid may be a lubricant such as oil, or it may be a fuel. 
     In use, by recovering such impurities and residues, the filter clogs up progressively, and ends up preventing fluid from passing through the filter. In order to avoid that situation, the branch of the fluid circuit having the filter mounted therein includes a bypass circuit in parallel. This configuration is shown in  FIG. 1 , which shows the state of the art. In the description below, the terms “upstream” and “downstream” are defined relative to the normal flow direction of the fluid in the circuit. The filter  20  is mounted in a main branch  10  of a fluid circuit. Upstream from the filter, the main branch  10  splits into two branches, a first branch  12  extending the main branch  10  and having the filter  20  mounted therein, and a second branch  14  having a check valve device  40  mounted therein (a “bypass” device). Downstream from the bypass device  40 , the second branch  14  rejoins the first branch  12  downstream from the filter  20  so as to reform the main branch  10 , such that the bypass device  40  is mounted in parallel with the filter  20 . Consequently, the fluid pressure upstream from the bypass device  40  is substantially equal to the pressure P upstream from the filter  20 . In  FIG. 1 , the fluid flows from left to right, as represented by arrow F. 
     When the filter is not clogged with impurities, i.e. at the beginning of its operation, the pressure P f  immediately downstream from the filter  20  (in the first branch  12 ) is substantially equal to the pressure P upstream from the filter  20 . Similarly, the pressure P dc  immediately downstream from the bypass device  40  (in the second branch  14 ) is substantially equal to the pressure P upstream from the bypass device  40 . All of the fluid flows in the branch  12 , and the fluid in the upstream portion of the branch  14  is blocked by the bypass device  40 . This bypass device  40  behaves like a valve that allows fluid to pass only when the pressure difference ΔP=P−P dc  from upstream to downstream across the bypass device  40  is greater than a critical value ΔP C . Such a bypass device  40  is known in the prior art. For example it may be a device comprising a ball valve, operating in the manner recalled briefly above. 
     A device with a ball valve comprises a container containing a ball and a spring. When the ball valve device is in its closed position, the ball is urged by the spring to block the upstream end of the container and thereby prevents the fluid in the portion of the branch situated upstream from the container from penetrating therein. When the pressure of the fluid upstream from the container increases relative to the pressure of the fluid downstream from the container, the force exerted by said fluid by the ball, and thus against the spring, increases in proportion. At a certain critical value ΔP C  for the pressure difference between upstream and downstream of the ball valve device, the reaction force from the spring is exactly counterbalanced by the force generated by the pressure ΔP C  on the ball. When this pressure exceeds the critical value ΔP C , the ball is pushed downstream and thus allows the fluid to pass through the ball valve device, which is then in its open position. 
     It will be understood that as the filter  20  becomes clogged as a result of impurities accumulating thereon, the upstream to downstream pressure difference ΔP=P−P f  across the filter  20  increases. The pressure difference across the bypass device  40  also increases since this difference is substantially equal to the pressure difference across the filter, since the first branch  12  including the filter  20  and the second branch  14  including the bypass device are in parallel. The bypass device  40  is calibrated in such a manner as to switch to its open position when the filter  20  becomes completely clogged. Thus, the critical value ΔP C  is referred to as the clogging value (or the “clogging pressure difference”). 
     Once the filter  20  is completely clogged, the fluid thus passes via the second branch  14  of the circuit, through the bypass device  40 , and is therefore no longer filtered. The fluid therefore continues to reach the downstream mechanism for which it is intended (e.g. an engine if the fluid is a fuel, a ball bearing or any other moving part if the fluid is an oil), but it does so filled with impurities. The impurities run the risk of damaging the mechanism, and in practice it is therefore essential to replace the filter  20  before it becomes completely clogged. 
     That is why the filter  20  is used in association with a pressure detector that informs the user (e.g. the pilot) that the filter  20  is about to become clogged and needs to be replaced quickly.  FIG. 1 , showing the prior art, shows one such detector  60 . The detector  60  is mounted in a third branch  16  that is connected to the first branch  12  on either side of the filter  20 . The detector  60  is a device that is known in the prior art (e.g. it is of the pressure contact type) and its structure is not described in detail here. It operates on the following principle: when the pressure difference ΔP m  across the detector  60  reaches a threshold value ΔP S′  the pressure contact sends a signal (typically an electrical signal) to the user to inform the user that the pressure difference ΔP m  across the detector  60  has crossed the threshold value ΔP S . The threshold value ΔP S  is selected to be less than the clogging critical value ΔP C  (e.g. 90% of said critical value). Thus, when the signal coming from the detector  60  is received by the user, the user knows that the filter  20  needs to be replaced soon, and sufficient time remains for the replacement to take place before the filter  20  becomes completely clogged. This information is passed on to the maintenance crew who perform the change in good time during a subsequent maintenance operation. 
     When the filter  20  is still not completely clogged, merely replacing the filter suffices prior to putting the circuit back into operation. However, it can happen that the maintenance crew performs this maintenance operation late, possibly after the filter  20  has become completely clogged. Under such circumstances, the filter  20  will have been bypassed and fluid will have continued to flow in the circuit, taking impurities through the bypass device  40  into the entire circuit downstream from the filter  20 . Under such circumstances, it is essential for the circuit to be inspected exhaustively and cleaned, which operations are much more burdensome than replacing a filter. When the maintenance crew are performing a maintenance operation, it is therefore necessary for them to know whether or not the filter has already become clogged and has been bypassed. 
     That is why the portion of the second branch  14  that is situated downstream from the bypass device  40  is provided with a visual indicator  70  (see  FIG. 1 ) that shows whether the fluid has flowed in the downstream portion of the second branch  14 , i.e. whether the fluid has passed through the bypass device  40 . The visual indicator  70  reveals such passage of fluid when the bypass device  40  is inspected visually. The visual indicator  70  may, for example, be a pop-up type indicator, i.e. it has a tongue that is initially contained inside the visual indicator  70  (the tongue is therefore not visible from the outside). Such a visual indicator  70  is known in the prior art and its operation is not described in detail herein. When the fluid passes through the portion of the second branch  14  that is situated downstream from the bypass device  40  and where the visual indicator  70  is located, the visual indicator  70  causes the tongue to move so that it can be seen by the maintenance crew. Thus on looking at the bypass device  40  (and its visual indicator  70 ), the crew can tell immediately whether the filter  20  has become clogged and whether or not it has been bypassed. 
     That clogging detection system associated with the filter  20  nevertheless presents the drawback that it is necessary to inspect the visual indicator  70  in order to know whether the filter has become completely clogged or has been clogged only partially. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The invention seeks to propose a method of detecting the clogging of a filter that makes it possible to eliminate the need to use the visual indicator and to inspect it when performing a maintenance operation. 
     This object is achieved by the fact that the method of detecting clogging of the filter comprises measuring, at successive sampling instants, the pressure difference ΔP from upstream to downstream across the filter, then calculating at different instants the first time derivative {ΔP}′ of the pressure difference ΔP, and then sending a clogging signal S C  representative of the filter being clogged when the first derivative {ΔP}′ is equal to zero. 
     By means of these provisions, the user (e.g. the pilot of the airplane if the filter is in a circuit of a turbojet) and the maintenance crew are informed that the filter is clogged. When the filter is completely clogged, the pressure difference ΔP across the filter becomes constant and remains constant. The first derivative {ΔP}′ of the pressure difference ΔP then becomes zero and the clogging signal S C  is sent. 
     Advantageously, prior to sending the clogging signal S C , the method includes sending a threshold signal S S  when the pressure difference ΔP exceeds a threshold value ΔP S  less than the pressure difference ΔP C  corresponding to the filter being clogged. 
     Thus, a single sensor is used for detecting both crossing the threshold value ΔP S  and crossing the clogging value ΔP C  that indicates that the filter is completely clogged, instead of using a detector and a visual indicator. This reduces costs both on purchase, and during maintenance of the filter, as explained above. The user and the maintenance crew are informed both of the fact that the filter is partially clogged beyond a level corresponding to the threshold value, and that the fact the filter is or is not completely clogged, without it being necessary to inspect the visual indicator. 
     The invention also provides a detector device for detecting clogging of a fluid filter, the device being mounted on a circuit for said fluid. 
     According to the invention, the detector device comprises: a sensor connected to the circuit upstream and downstream from the filter, and suitable for measuring, at successive sampling instants, the upstream to downstream pressure difference ΔP across the filter; and a processor suitable for calculating at different instants the first time derivative {ΔP}′ of the pressure difference ΔP, and suitable for sending a clogging signal S C  representative of clogging of the filter when the first derivative {ΔP}′ is equal to zero. 
     Advantageously, prior to sending the clogging signal S C , the processor is suitable for sending a threshold signal S S  when the pressure difference ΔP exceeds a threshold value ΔP S  less than the clogging pressure difference ΔP C  corresponding to the filter being clogged. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood and its advantages appear more clearly on reading the following detailed description of an embodiment given by way of non-limiting example. The description refers to the accompanying drawings, in which: 
         FIG. 1  is a diagram of a prior art filter and method for detecting clogging of the filter; 
         FIG. 2  is a diagram of a filter of the invention and of the method of the invention for detecting clogging of the filter; and 
         FIG. 3  is a graph showing the variation over time in the pressure difference ΔP across the filter, the variation in the first derivative of said pressure difference ΔP, and the variation in the second derivative of said pressure difference ΔP. 
     
    
    
     MORE DETAILED DESCRIPTION 
       FIG. 2  shows a filter  20  mounted in a main branch  10  of a fluid circuit. By way of example, the filter  20  may be mounted in a fluid circuit of a turbomachine. The fluid may be constituted, for example, by a lubricant such as oil, or by a fuel. Upstream from the filter  20 , the main branch  10  splits into two branches, a first branch  12  that extends the main branch  10  and that has the filter  20  mounted therein, and a second branch  14  that has a bypass device  40  mounted therein. Downstream from the bypass device  40 , the second branch  14  rejoins the first branch  12  downstream from the filter  20  so as to reform the main branch  10 , so that the bypass device  40  is mounted in parallel with the filter  20 . In  FIG. 2 , the fluid flows from left to right as represented by the arrow F. 
     A sensor  160  is connected to the first branch  12  containing the filter  20 , on either side of the filter  20 . Thus, a third branch  16  of the fluid circuit connects the first branch  12  upstream from the filter  20  to the second branch  12  downstream from the filter  20  via the sensor  160 , which is mounted in said third branch  16 . The connection of the third branch  16  upstream from the filter  20  is thus located downstream from the connection of the second branch  14  that is upstream from the filter  20 , and the connection of the third branch  16  downstream from the filter  20  is thus located upstream from the connection of the second branch  14  that is downstream from the filter  20 . It would also be possible to connect the third branch  16  to the main branch  10  upstream from the filter  20  and upstream from the location where the second branch  14  is connected to said main branch  10 , and to connect the third branch  16  to the main branch  10  downstream from the filter  20  downstream from the location where the second branch  14  is connected to said main branch  10 . Whatever the configuration, the first branch  12  including the filter  20  and the third branch  16  including the sensor  160  are in parallel, and the pressure difference across the ends of the sensor  160  is substantially equal to the pressure difference ΔP (head loss) across the ends of the filter  20 . The solution shown in  FIG. 2  (the third branch  16  in parallel with the first branch  12 , i.e. closer to the ends of the filter  20 ) is preferable since the pressure difference measured by the sensor  160  is then closer to the pressure difference ΔP across the ends of the filter, given that the head loss due to the length of the branches in the circuit has no influence on the measurement. The sensor  160  is also connected to a processor  165  capable of processing a signal, typically an electrical signal. The sensor  160  is a device that is known in the prior art, as is the processor  165  associated therewith. Their structures are therefore are not described in detail below. 
     The sensor  160  is suitable for continuously measuring the pressure difference ΔP across the filter  20  in the third branch  16 . Thus, the pressure difference ΔP is measured at successive time intervals, these instants at which the measurement is taken constituting sampling over time. Ideally, these intervals are substantially regular, and the instants at which the measurement is taken are sufficiently close together to reproduce reliably the variations over time in the pressure difference ΔP. This variation in the difference ΔP is shown in  FIG. 3 . The pressure difference ΔP increases continuously from zero up to a value ΔP C  corresponding to the filter  20  being completely clogged and to the bypass device  40  opening and allowing fluid to pass therethrough in the second branch  14 . Once the filter  20  has become clogged, the difference ΔP remains constant and equal to the clogging value ΔP C . 
     The sensor  160  sends a threshold signal S S  (typically an electrical signal) to the user (e.g. the pilot) when the pressure difference ΔP across the sensor  160  (i.e. across the filter  20 ) reaches a threshold value ΔP S , i.e. at the instant t S  in  FIG. 3  (or the measurement instant immediately following t S ). The threshold value ΔP S  is selected to be less than the clogging value ΔP C , as shown in  FIG. 3  (e.g. 90% of said critical value). Thus, when the signal coming from the sensor  160  is received by the user, the user knows that the filter  20  needs to be replaced soon. This information is forwarded to the ground crew who then perform this replacement in good time during a subsequent maintenance operation. 
     In addition, because the pressure difference ΔP is measured continuously by sampling, the processor  165  can make use of this continuous measurement for calculating, at least each sampling instant, or at a plurality of said instants, the derivative d/dt (ΔP) of the pressure difference ΔP relative to time (this derivative is the derivative of the measurement signal delivered by the sensor  160  to the processor  165 , the measurement signal being proportional to the pressure difference ΔP). This first derivative is written {ΔP}′. Thus, the derivative 
               d     d   ⁢           ⁢   t       ⁢     (     Δ   ⁢           ⁢   P     )           
is calculated at an arbitrary instant t 1  (written
 
                   d     d   ⁢           ⁢   t       ⁢     (     Δ   ⁢           ⁢   P     )       ]     1         
or {ΔP}′ 1 ) can be calculated, e.g. by using two measurements ΔP 1  and ΔP 2  of the pressure difference ΔP at the instant t 1  and at the following instant t 2  by applying the following formula:
 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     The closer together the instants t 1  and t 2 , the more accurate the calculation of the derivative. Ideally, the instants t 1  and t 2  are therefore selected to be two successive sampling instants. This therefore produces a continuous measurement of the derivative {ΔP}′ of the pressure difference across the filter  20 . 
     When the filter  20  is completely clogged (i.e. at instant t c  in  FIG. 3 ), the pressure difference ΔP across the filter  20  becomes constant and remains constant (prior to the filter becoming clogged, the pressure difference increases over time, so its derivative is strictly positive). The derivative {ΔP}′ of the pressure difference then becomes zero, as shown in  FIG. 3 , which plots variation in the derivative {ΔP}′ as a function of time. When the derivative {ΔP}′ becomes zero (i.e. at a measurement instant equal to or immediately greater than t c ), the processor  165  sends a clogging signal S C  to the user who is then informed that the filter  20  has become completely clogged. 
     Thus, by using a single sensor (the sensor  160 ) and a processor  165 , it is possible to inform the user (and thus the maintenance crew) that the filter  20  is clogged either partially or completely, without it being necessary to use a visual indicator on the bypass device  40 . 
     The processor  165  may be incorporated in the sensor  160  or it may be separate therefrom. Under such circumstances, the processor  165  may be included in the engine monitoring unit (EMU) or in the airplane maintenance unit. 
     In order to confirm the fact that the filter is completely clogged, it is possible to configure the processor  165  so that the clogging signal S C  is delivered only when the first derivative {ΔP}′ is equal to zero on at least two successive instants. The processor  165  could calculate a first derivative of zero in error, or the calculation could take place at the extremum of a fluctuation in the pressure difference ΔP (and thus give a first derivative that is zero). The probability of such an error occurring at two distinct instants is minimal, so it then becomes certain that the filter is completely clogged if the first derivative {ΔP}′ is equal to zero at two successive instants (e.g. at two successive sampling instants). 
     It is also possible to confirm the fact that the filter is completely clogged by calculating the second derivative 
                 d   2       d   ⁢           ⁢     t   2         ⁢     (     Δ   ⁢           ⁢   P     )           
of the pressure difference ΔP. When the first derivative is zero over a time interval of non-zero duration (and not just at a single point), the second derivative is zero over that interval. Thus, by using the processor  165  to calculate both the first derivative {ΔP}′ of the pressure difference ΔP at a given instant t 1  and its second derivative (written {ΔP}″) at said instant t 1 , it is possible to ensure that the pressure difference ΔP is constant over a time interval around the instant t 1 . In present circumstances, this means that the pressure difference ΔP is constant at all times greater than t 1 , and thus that the filter  20  is completely clogged. It can be seen in  FIG. 3  that the second derivative of the pressure difference ΔP is zero at all times  t  greater than the instant t c  when the filter becomes completely clogged. The second derivative {ΔP}″ at an arbitrary instant t 1  (written
 
                     d   2       d   ⁢           ⁢     t   2         ⁢     (     Δ   ⁢           ⁢   P     )       ]     1         
or {ΔP}′ 1 ) can be calculated, e.g. by using two measurements ΔP 1  and ΔP 2  of the pressure difference at the instant t 1  and at the following instant t 2  (e.g. the sampling instant following the instant t 1 ), and by applying the following formula together with formula (I):
 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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                       1 
                     
                     
                       
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                   ( 
                   II 
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     Thus, in order to confirm that the filter is completely clogged, it is possible to configure the processor  165  to calculate the second time derivative {ΔP}″ of the pressure difference ΔP at different instants, and for the clogging signal S C  to be sent to the user only when the second derivative {ΔP}″ and the first derivative {ΔP}′ are equal to zero. This situation can only happen at instants that are later than the clogging instant t c , so it is certain that the filter is completely clogged. 
     Alternatively, given that when the first derivative is zero over a time interval of non-zero length, the derivative of order N (where N is greater than 2) is zero over said interval, and by calculating at a given instant t 1 , simultaneously the first derivative {ΔP}′ of the pressure difference ΔP and its N th  order derivative (written {ΔP} N ), it is possible to ensure that the pressure difference ΔP is constant at any instant greater than t 1 , and thus that the filter  20  is completely clogged. Calculation of the N th  order derivative is trivial by iterating the formula (II). 
     The invention also provides a device for detecting clogging of the filter  20 , the device comprising a sensor  160  and a processor  165  operating as stated above.