Abstract:
A system and method for determining a remaining useful life of a portion of a wet clutch system is provided. The system comprises a first clutch assembly, a proportional valve, a sensor, and a controller. The proportional valve regulates a pressure applied to the first clutch assembly. The sensor measures a response of the first clutch assembly during one of engaging and disengaging a first portion of the first clutch assembly with a second portion of the first clutch assembly. The controller controls the proportional valve, calculates a mean coefficient of friction of the first clutch assembly based on the sensed response of the first clutch assembly, and determines a remaining useful life of a portion of the wet clutch system based on the mean coefficient of friction of the first clutch assembly.

Description:
RELATED APPLICATION 
       [0001]    The present application claims the benefit of U.S. Provisional Application No. 61/874,637 filed on Sep. 6, 2013, which is incorporated herein in its entirety by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to vehicle monitoring and prognostics systems and, more particularly, to a monitoring and prognostics system for use with wet clutches. 
       BACKGROUND OF THE INVENTION 
       [0003]    Wet clutches are well-known and used products. Wet clutches are utilized in many applications, including by way of example, wet-plate transmissions for off-highway vehicles and axle brakes. Typically, wet clutches are used to engage a portion of a driveline with a vehicle output or portions of a transmission to engage a specific gear ratio. 
         [0004]    Friction separator discs, or plates, are a critical part of wet clutch shifting. During wet clutch shifting, the friction plates gradually degrade as their friction layer is worn off from physical and chemical processes. When the friction plates degrade, a torque transmission capability and a reaction of the wet clutch can be significantly reduced. 
         [0005]    Currently, a remaining useful life of a clutch may be calculated in a static manner based on a number of shifts performed and/or a total time the clutch is used. As a result, the friction plates may be changed either too early, which results in unnecessary servicing of the wet clutch, or too late, which results in damage to the wet clutch. 
         [0006]    At least one system is commercially available that is used for monitoring and prognostics of automatic transmission fluids for on-highway vehicles, known as Allison prognostics, supplied by Allison Transmission, Inc. While the details of the operation of Allison prognostics are not fully known, such a system does not appear to provide any information on a mean coefficient of friction for a clutch monitored by the system. 
         [0007]    Therefore, the development of a monitoring and prognostics system for the friction plates is of paramount importance to achieve an optimal maintenance strategy for vehicles equipped with shifting wet clutch systems. 
         [0008]    It would be advantageous to develop a monitoring and prognostics system used with a wet clutch system having friction plates that implements an optimal maintenance strategy based on information available from existing sensors used with the wet clutch system. 
       SUMMARY OF THE INVENTION 
       [0009]    Presently provided by the invention, a monitoring and prognostics system used with a wet clutch system having friction plates that implements an optimal maintenance strategy based on information available from existing sensors used with the wet clutch system, has surprisingly been discovered. 
         [0010]    In one embodiment, the present invention is directed to a method for determining a remaining useful life of a portion of a wet clutch system. The wet clutch system comprises a first clutch assembly rotatingly disposed in a housing, a first portion of the first clutch assembly drivingly engaged with an input member and a second portion of the first clutch assembly drivingly engaged with an output member, a proportional valve for regulating a pressure applied to the first clutch assembly, a controller controlling the proportional valve, and a sensor for measuring a response of the first clutch assembly. The method comprises the steps of providing the wet clutch system, actuating the first clutch assembly to one of engage and disengage the first portion of the first clutch assembly with the second portion of the first clutch assembly, sensing a response of the first clutch assembly during one of engaging and disengaging the first portion of the first clutch assembly with the second portion of the first clutch assembly, calculating a mean coefficient of friction of the first clutch assembly based on the sensed response of the first clutch assembly, and determining a remaining useful life of a portion of the wet clutch system based on the mean coefficient of friction of the first clutch assembly. 
         [0011]    In another embodiment, the present invention is directed to a method for determining a remaining useful life of a portion of a wet clutch system. The wet clutch system comprises a first clutch assembly rotatingly disposed in a housing, a first portion of the first clutch assembly drivingly engaged with an input member and a second portion of the first clutch assembly drivingly engaged with an output member, a second clutch assembly rotatingly disposed in a housing comprising a first portion drivingly engaged with an input member and a second portion drivingly engaged with an output member, a proportional valve for regulating a pressure applied to the first clutch assembly and the second clutch assembly, a controller controlling the proportional valve, and a sensor for measuring a response of the first clutch assembly and the second clutch assembly. The method comprises the steps of providing the wet clutch system, actuating the first clutch assembly to one of engage and disengage the first portion of the first clutch assembly with the second portion of the first clutch assembly, actuating the second clutch assembly to one of engage and disengage the first portion of the second clutch assembly with the second portion of the second clutch assembly, sensing a response of one of the first clutch assembly and the second clutch assembly in response to actuation of one of the first clutch assembly and the second clutch assembly, calculating a mean coefficient of friction of one of the first clutch assembly and the second clutch assembly based on the sensed response of one of the first clutch assembly and the second clutch assembly, and determining a remaining useful life of a portion of the wet clutch system based on the mean coefficient of friction of one of the first clutch assembly and the second clutch assembly. 
         [0012]    In yet another embodiment, the present invention is directed to a system for determining a remaining useful life of a portion of a wet clutch system. The system comprises a first clutch assembly, a proportional valve, a sensor, and a controller. The first clutch assembly is rotatingly disposed in a housing and a first portion of the first clutch assembly is drivingly engaged with an input member and a second portion of the first clutch assembly is drivingly engaged with an output member. The proportional valve regulates a pressure applied to the first clutch assembly. The sensor measures a response of the first clutch assembly during one of engaging and disengaging the first portion of the first clutch assembly with the second portion of the first clutch assembly. The controller controls the proportional valve, calculates a mean coefficient of friction of the first clutch assembly based on the sensed response of the first clutch assembly, and determines a remaining useful life of a portion of the wet clutch system based on the mean coefficient of friction of the first clutch assembly. 
         [0013]    Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic illustration of a wet clutch system according to the present invention; 
           [0015]      FIG. 2  is a graph which illustrates an exemplary degradation of the mean coefficient of friction of a portion of a wet clutch system with respect to a number of cycles of use of the wet clutch system; 
           [0016]      FIG. 3  is a graph which illustrates a pressure profile of an engaging hydraulic piston associated with the wet clutch system illustrated in  FIG. 1 ; and 
           [0017]      FIG. 4  is a graph which illustrates a pressure profile of an ongoing hydraulic piston and an offgoing hydraulic piston associated with a wet clutch system similar to the wet clutch system illustrated in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    It is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification are simply exemplary embodiments of the inventive concepts defined herein. Hence, specific dimensions, directions or other physical characteristics relating to the embodiments disclosed are not to be considered as limiting, unless expressly stated otherwise. 
         [0019]    A remaining useful life (RUL) clutch monitoring and prognostic system and method which can be used on a telematics and control platform of a vehicle that uses existing sensors (for example, speed sensors, pressure sensors, temperature sensors, etc.) which are typically present in a wet clutch system is described herein. 
         [0020]    The clutch monitoring and prognostic system provides an estimation of a mean coefficient of friction (COF) of the clutch for at least servicing the wet clutch system. The estimated coefficient of friction can also be used for other purposes as well. Further, the coefficient of friction estimation may also be used with clutch control, as the coefficient of friction estimation allows a more accurate calculation of the transmitted torque. 
         [0021]    The clutch monitoring and prognostic system is based on an online estimation of the mean coefficient of friction. The estimation method for the mean COF uses new methods, which are described hereinbelow. Three of these methods can be used during an overlap shift of a transmission and a remaining method can be used during a single clutch engagement (for example, during a vehicle launch) of the transmission. 
         [0022]      FIG. 1  illustrates a wet clutch system  10  that may be used with the transmission. The wet clutch system  10  is an electrohydraulically actuated wet multi-plate clutch system. The wet clutch system  10  is an electrohydraulically actuated wet plate clutch system. The wet clutch system  10  comprises a sump  12 , a high pressure pump  14 , an electroproportional valve  16 , an accumulator  18 , a piston assembly  20 , a clutch assembly  22 , a controller  24 , and a plurality of fluid conduits  26 . It is understood that the wet clutch system  10  may include additional clutch assemblies (not shown). The high pressure pump  14  is in fluid communication with the sump  12  and the electroproportional valve  16 . The piston assembly  20  is in fluid communication with the electroproportional valve  16  and the accumulator  18 . The clutch assembly  22  is disposed adjacent to and may be placed in contact with a portion of the piston assembly  20 . The controller  24  is in communication with the electroproportional valve  16 . A signal from a sensor  25  in communication with the controller  24 , which may be integrated into the electroproportional valve  16  is used to facilitate the estimation of the mean COF of the clutch assembly  22 . The sensor  25  may be a speed sensor, a pressure sensor, a temperature sensor, or a torque sensor. Further, it is understood that the sensor  25  may comprise a plurality or combination of sensors. When the sensor  25  is a speed sensor, for example, the sensor  25  is configured to measure rotational speeds of an input and an output associated with the clutch assembly  22 . 
         [0023]    The sump  12  is a container in which a hydraulic fluid is stored. The sump  12  is in fluid communication with the high pressure pump  14 . One of the fluid conduits  26  affords fluid communication between the sump  12  and the high pressure pump  14 . A filter  28  forms a portion of the fluid conduit  26  between the sump  12  and the high pressure pump  14 . The sump  12  includes a breather  30 , to facilitate fluid communication between an ambient environment of the wet clutch system  10  and an interior of the sump  12 . 
         [0024]    The high pressure pump  14  is a fixed displacement hydraulic pump. The high pressure pump  14  is in fluid communication with the sump  12  and the electroproportional valve  16 . As a non-limiting example, the high pressure pump  14  may generate a pressure of about 20 bar. One of the fluid conduits  26  affords fluid communication between the high pressure pump  14  and the electroproportional valve  16 . A filter  32  forms a portion of the fluid conduit  26  between the high pressure pump  14  and the electroproportional valve  16 . A pressure relief valve  33  is present to limit a pressure difference across the filter  32  created by the high pressure pump  14 , such as if the filter  32  becomes obstructed. Further, it is understood that the high pressure pump  14  may also be in fluid communication with a pressure limiting valve (not shown). The pressure limiting valve limits a pressure within the fluid conduit  26  between the high pressure pump  14  and the electroproportional valve  16 . 
         [0025]    The electroproportional valve  16  is a hydraulic valve in fluid communication with the high pressure pump  14 , the piston assembly  20 , and the accumulator  18 . The electroproportional valve  16  is in electrical communication with the controller  24 . The electroproportional valve  16  is supplied with a pulse width modulated signal to apply a current to a solenoid  34  forming a portion of the electroproportional valve  16 . Upon receipt of the pulse width modulated signal, the electroproportional valve  16  may be placed in at least a partially open position. In the open position, the electroproportional valve  16  affords fluid communication between the fluid conduit  26  between the high pressure pump  14  and the electroproportional valve  16  and a fluid conduit  26  between the electroproportional valve  16 , the piston assembly  20 , and the accumulator  18 . It is understood that the controller  24  may adjust the pulse width modulated signal to adjust a pressure within the fluid conduit  26  between the electroproportional valve  16 , the piston assembly  20 , and the accumulator  18  by placing the electroproportional valve  16  in at least the partially open position. As shown in  FIG. 1 , the electroproportional valve  16  includes a draining orifice  36 . A flow of hydraulic fluid through the draining orifice  36  is dependent on a pressure within the electroproportional valve  16 , but also a viscosity of the hydraulic fluid and a temperature of the hydraulic fluid. 
         [0026]    The accumulator  18  is a hydraulic device that dampens rapid changes in pressure (such as pressure drops or pressure peaks) within the fluid conduit  26  between the electroproportional valve  16  and the piston assembly  20 . The accumulator  18  facilitates smooth operation of the clutch assembly  22 . The accumulator  18  is in fluid communication with the piston assembly  20  and the electroproportional valve  16 . As shown in  FIG. 1 , the accumulator  18  includes a draining orifice  38 . A flow of hydraulic fluid through the draining orifice  38  is dependent on a pressure within, the fluid conduit  26  between the electroproportional valve  16  and the piston assembly  20 , but also a viscosity of the hydraulic fluid and a temperature of the hydraulic fluid. 
         [0027]    The piston assembly  20  comprises a housing  40 , a piston  42 , a piston rod  44 , and at least one return spring  46 . The housing  40  is a hollow, cylindrical member in fluid communication with the electroproportional valve  16  through the fluid conduit  26  between the electroproportional valve  16 , the piston assembly  20 , and the accumulator  18 . The piston  42  is a cylindrical member sealingly and slidingly disposed within the housing  40 . The piston rod  44  is an elongate member in driving engagement with the piston  42 . The piston rod  44  is sealingly and slidingly disposed through the housing  40 . The at least one return spring  46  is a biasing member disposed between the piston  42  and the housing  40 . When pressure at or above an engagement threshold is applied to the housing  40  by the electroproportional valve  16 , the pressure within the housing  40  urges the piston  42  and the piston rod  44  towards the clutch assembly  22 , while also compressing the at least one return spring  46 . When pressure at or below a disengagement threshold is present within the housing  40 , the at least one return spring  46  urges the piston  42  and the piston rod  44  into a starting position. 
         [0028]    The clutch assembly  22  comprises a housing  50 , a first plurality of plates  52 , a second plurality of plates  54 , and a pressure plate  56 . The housing  50  is a hollow member into which a transmission fluid is disposed. The first plurality of plates  52  and the second plurality of plates  54  are rotatingly disposed within the housing  50 . The pressure plate  56  is disposed adjacent the first plurality of plates  52  and the second plurality of plates  54  and may be urged towards the first plurality of plates  52  and the second plurality of plates  54  by the piston rod  44 . The first plurality of plates  52  is interleaved with the second plurality of plates  54 . Within the clutch assembly  22 , an input member (not shown) is drivingly engaged with one of the first plurality of plates  52  and the second plurality of plates  54  and an output member (not shown) is drivingly engaged with a remaining one of the first plurality of plates  52  and the second plurality of plates  54 . A pressure in which the piston rod  44  contacts the pressure plate  56  and where additional pressure would result in at least variable driving engagement between the first plurality of plates  52  and the second plurality of plates  54  is known as a kiss pressure. At pressures greater than the kiss pressure, torque is able to be transferred from the first plurality of plates  52  to the second plurality of plates  54  or from the second plurality of plates  54  to the first plurality of plates, depending on a configuration of the clutch assembly  22 . When pressure at or above the engagement threshold is applied to the housing  40  by the electroproportional valve  16 , the pressure within the housing  40  urges the piston  42  and the piston rod  44  towards the clutch assembly  22 , applying a pressure to the first plurality of plates  52  and the second plurality of plates  54  through the pressure plate  56 . In response to the pressure, the first plurality of plates  52  becomes at least variably drivingly engaged with the second plurality of plates  54 , causing the input member to be at least variably drivingly engaged with the output member. 
         [0029]    The basis of a remaining useful life determination for the wet clutch system  10  is formed by an estimation of the mean coefficient of friction of the plurality of plates  52 ,  54 .  FIG. 2  illustrates an exemplary degradation of the mean coefficient of friction of the plurality of plates  52 ,  54  with respect to a number of cycles of use of the wet clutch system. 
         [0030]    In several applications only one clutch, such as the clutch assembly  22 , has to be engaged, without the need to simultaneously release a second clutch. For example, such a process occurs with the clutch assembly  22  is an axle brake clutch or when the clutch assembly  22  forms a portion of a transmission and is used for a vehicle launch. During engagement of the clutch assembly  22 , a pressure profile is imposed on the piston  42  of the piston assembly  20 , which is shown in  FIG. 1 . An exemplary pressure profile is shown in  FIG. 3 . The pressure profile consists of several intervals: filling the housing  40 , synchronize the clutch assembly  22 , and finally a locking-up of the clutch assembly  22 . The pressure profile shown in  FIG. 3  is divided into sections A, B, C, and D, representing different stages of the engagement of the clutch assembly  22 . Such a method can be applied during the synchronization phase of the step of actuating the clutch assembly  22 . The synchronization phase of the clutch assembly  22  occurs in section D and beyond. A torque transmitted by the clutch assembly  22  during a slipping condition of the clutch assembly  22  is proportional to:
       An area and a number of the plurality of plates  52 ,  54     A net pressure on the plurality of plates  52 ,  54  (hydraulic pressure minus a force applied by the return spring  46 )   The mean coefficient of friction the plurality of plates  52 ,  54         
 
         [0034]    As torque transmitted by the clutch assembly  22  is proportional to the COF and a net pressure imposed on the piston  42 , a first estimate is made by dividing a torque value by the net pressure and filtering the resulting value. The torque value can be obtained from a torque converter look-up table (turbine torque follows from the lookup-table and a measured speed ratio), or can be estimated by an angular displacement estimation method (the angular displacement of two non-slipping ends of a shaft is proportional to the applied torque and the torsional stiffness of the shaft), or by any other torque estimation or measurement. 
         [0035]    In several applications a power shift, also known as an overlap shift, is performed where one clutch assembly  22  is engaged while another clutch assembly (not shown) is simultaneously disengaged. An exemplary pressure profile for both an ongoing and an offgoing clutch during the overlap shift is shown in  FIG. 4 . 
         [0036]    Three methods, which are discussed hereinbelow, may be used to estimate the mean COF during the overlap shift. Each of these methods may be used in a different part of the engagement process during the overlap shift. Each of these methods are noted with corresponding numbers, which appear in  FIG. 4 . 
         [0037]    The pressure profile for the overlap shift comprises several intervals: filling the housing  40 , a stabilization period of the offgoing clutch, and an inertial or synchronization phase of the clutch assembly  22 . The pressure profile shown in  FIG. 4  is divided into the sections Filling, Torque, and Inertia, representing different stages of the overlap shift. 
         [0038]    A first method can be applied during the inertial or synchronization phase of the clutch, indicated in  FIG. 4  by number  1 . As torque transmitted by the ongoing clutch is proportional to the COF and the net pressure on the piston, a first estimate is made by dividing a torque value by the net pressure and filtering the resulting value. The torque value can be obtained using various methods, such as measurement or estimation. 
         [0039]    In a second method, during the unloading of the offgoing clutch, the offgoing clutch releases and starts slipping at a certain pressure, which is dependent on the torque transmitted and the COF. This slipping point is indicated in  FIG. 4  by number  2 . The slipping point can be characterized using a slip speed and a measured pressure. Using the speed and pressure sensors, a release time (when the slip speed differs from a value of zero) and a release pressure can thus be used to estimate the mean COF. 
         [0040]    In a third method, during the stabilization period of the offgoing clutch, which is indicated in  FIG. 4  by number  3 , the pressure is kept constant or the slip speed is controlled prior to fully unloading the offgoing clutch, and a method similar to the first method can be used. 
         [0041]    Due to the noise in the measurements, the measurements need to be filtered. For each clutch assembly, there will be estimations for the mean COF available using each of the different methods. The filtered results need to be combined to give a single estimation value. Combination of the filtered results is performed by weighing the values in an appropriate way. 
         [0042]    The information from the speed, temperature and pressure sensors from every engaging and disengaging clutch assembly  22  in the transmission, as well as estimated or measured torque signals, are sent to the controller  24  (or another CPU) to perform the estimation of the mean COF. The estimated values are aggregated in the controller  24  and compared to a learned or stored relationship between the mean COF and a wear of the friction plates. The controller  24  then computes the remaining useful life of the clutch assembly  22  and sends data regarding a current condition of the clutch assembly  22 , the remaining useful life of the clutch assembly  22 , and a next estimated maintenance of the clutch assembly  22  to a user interface of the vehicle. The user interface can be any kind of interface, including a dashboard or a telematics interface. 
         [0043]    The estimated value of the mean COF for the clutch assembly  22  can be used in other applications as well. For example, one use would be in the controller  24 , where the mean COF may be used to derive appropriate slip or pressure reference values for the clutch assembly  22 . 
         [0044]    The application of the monitoring and prognostics system for the clutch assembly  22  increases a productivity of a vehicle the wet clutch system  10  is incorporated in. More particularly, productivity of the vehicle is increased by increasing a time between vehicle servicing intervals, since a remaining useful life of the clutch assembly  22  can be monitored and known. Under such circumstances, when a remaining useful life of the clutch assembly  22  is near its end, the clutch assembly  22  can be serviced or replaced at a convenient time to the owner or operator of the vehicle. Further, the monitoring and prognostics system prevents a possibility that the clutch assembly  22 , or other vehicle parts, may be damaged when the clutch assembly  22  unexpectedly reaches an end of its useful life. 
         [0045]    Moreover, the estimated value of the mean COF can be used to monitor the current state of a transmission in many different fields, such as industrial robots and machine tools, for example. One of such possible uses is to detect efficiency losses and shortened service life due to component wear. For example, engine components and energy conversion devices, such as pumps and motors, often include gears, bearings, and other movable components which translate and rotate with respect to a fixed surface. Therefore, a monitoring of the friction between movable parts which causes the degradation, for example wear of, the movable components, is a very important task. One of the estimation techniques may be used to identify the mean COF based on a real time knowledge of an axis or joint displacement and a torsional stiffness. 
         [0046]    In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiments. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.