Patent Publication Number: US-7909733-B2

Title: Clutch end-of-fill detection strategy

Description:
TECHNICAL FIELD 
     This disclosure relates generally to systems and methods for enabling robust clutch fill control and calibrating a hydraulic transmission clutch and, more particularly, to systems and methods for calibrating the flow of a pressurized operating medium within a clutch-controlled transmission. 
     BACKGROUND 
     Hydraulic clutches are well known in general, and can be found in many systems and devices. In one implementation, a set (plurality) of hydraulic clutches are used to facilitate shifting of a transmission between differing input/output gear ratios or ratio ranges. More generally, a transmission typically includes an input shaft, an output shaft, and a collection of interrelated gear elements, such as in a planetary arrangement or otherwise, usable to selectively couple the input and output shafts. The clutches may be used to select gear ratios in a discrete transmission, and to select gear ratio ranges in a continuous transmission. Both types of coupling will be referred to herein as “ratios.” 
     The selection of a gear ratio at the output shaft is executed via one or more clutches that affect the rotations and/or interrelationships of the gear elements. The clutches are typically hydraulically actuated to engage band or disk torque transfer elements. Shifting from one gear ratio to another normally involves releasing or disengaging an off-going clutch or clutches associated with the current gear ratio and applying or engaging an on-coming clutch or clutches associated with the desired gear ratio. By way of example, although many different clutch arrangements are possible within such transmissions, one possible arrangement is a two-clutch shifting transmission. In this arrangement, two clutches are required to hold a specific gear in said transmission. Typically, this entails a primary clutch, often a rotating clutch element, which is retained for an upcoming gear, and a secondary clutch that is disengaged in order to shift into the upcoming gear. The secondary clutch for this shift condition is referred to in the art as the off-going clutch. This clutch is replaced by a new clutch, the “on-coming” clutch, required to actuate the transmission into the new gear. In other words, a shift is executed by deactivating a single “off-going” clutch, activating a single “on-coming” clutch, and holding a third clutch for both the old and new gears. In other arrangements, multiple on-coming and\or off-going clutches are employed, increasing the complexity and criticality of clutch actuation timing. 
     Each hydraulic clutch is typically driven via an electrically controlled solenoid valve. Such solenoid valves are electrically modulated to control hydraulic fluid pressure to the clutch and hence to control the clutch piston movement during the clutch fill phase. 
     The phasing of the on-coming and off-going clutch element can have a substantial impact on the perceived shift quality. For example, if the off-going clutch disengages prematurely, the engine speed may surge briefly before the on-coming clutch, still in the fill phase, possesses sufficient torque capacity. Furthermore, if the on-coming clutch fills prematurely, the clutch element has sufficient torque capacity before the off-going clutch is ready to commence torque transfer. This can lead to a three-way clutch tie up which is detrimental to the transmission&#39;s useful life in a mild case, and often results in mechanical damage to the transmission in an extreme case. Conversely, in the event of a late clutch fill, the off-going clutch hands off torque to the on-coming clutch before the on-coming clutch has sufficient torque capacity, and the transmission slips as the on-coming clutch does not have sufficient time to lock with adequate torque capacity to hold the specific gear in question. The end result is a slip phenomenon in the clutch discs, also an undesirable event as this tends to produce high clutch energies resulting from excessive heat generation produced by the higher clutch relative velocities of the rotating clutch discs. In addition to creating an unpleasant user experience, badly timed shifting will over time, impact the efficiency and service life of the transmission. To this end, it is desirable to actuate the clutches with precision such that a smooth shift occurs throughout the entire operating speed range of the transmission during its entire useful life. 
     Known methods for calibrating transmission clutches tend to be empirical rather than contemporaneous. In other words, the behavior of the clutch may be observed at some point, and conclusions may be drawn as to how the clutch reacts to hydraulic flow. These observations are then used to periodically “calibrate” the clutch. However, the condition and operating environment of a clutch can change substantially between calibration intervals, resulting in a degradation of shift quality. 
     Although the resolution of deficiencies, noted or otherwise, of the prior art has been found by the inventors to be desirable, such resolution is not a critical or essential limitation of the disclosed principles. Moreover, this background section is presented as a convenience to the reader who may not be of skill in this art. However, it will be appreciated that this section is too brief to attempt to accurately and completely survey the prior art. The preceding background description is thus a simplified and anecdotal narrative and is not intended to replace printed references in the art. To the extent an inconsistency or omission between the demonstrated state of the printed art and the foregoing narrative exists, the foregoing narrative is not intended to cure such inconsistency or omission. Rather, applicants would defer to the demonstrated state of the printed art. 
     SUMMARY 
     In one aspect, the disclosure pertains to a method of controlling a transmission having a plurality of hydraulic clutches for shifting between one or more transmission ratios. In this aspect, the method comprising executing a shift of the transmission by commanding a decrease of hydraulic pressure to an off-going clutch element to begin disengagement of the clutch and commanding a flow of hydraulic fluid to an on-coming clutch to fill a clutch chamber of the second said hydraulic clutch. The method further entails detecting a pressure rise greater than a predetermined magnitude in the chamber of the second hydraulic clutch and determining based on the detected pressure rise that the clutch chamber is filled. Thereafter a clutch modulation phase is initiated to fully engage the on-coming hydraulic clutch, enabling it to fully accept torque transfer from the off-going clutch element. 
     In another aspect, the disclosure pertains to a transmission control system for controlling a transmission having a plurality of hydraulic clutches. The system comprises a transmission controller for controlling a flow of hydraulic fluid to an on-coming clutch and an off-going clutch, and a ‘solenoid valve’ associated with each clutch. Each solenoid valve has a coil element linked to the transmission controller usable to control a flow of hydraulic fluid through the solenoid valve. Each solenoid valve further comprises a fluid inlet, a fluid outlet, and a pressure sensor fixed to the solenoid valve, in fluid communication with the outlet and the clutch chamber. The pressure sensor is adapted to sense a pressure within the solenoid valve and to transmit a signal indicative of a sensed pressure to the transmission controller for causing the transmission controller to modify operation of the solenoid valve. 
     In yet a further aspect, the disclosure pertains to a solenoid valve for use in a hydraulic transmission, the solenoid valve comprising a valve body, a valve spool, a spring biasing the valve spool, a pressure chamber biasing the valve spool in an opposite direction. The solenoid valve further includes an inlet, an outlet, and a pressure sensor linked to the valve body operable to sense a hydraulic fluid pressure within a cavity of the valve body and to transmit an electrical signal based on the sensed pressure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a hydraulic clutch controllable in accordance with the disclosed principles; 
         FIG. 2  is a schematic diagram of a hydraulic clutch control system in accordance with the disclosed principles; 
         FIG. 3  is a cross-sectional view of an electrohydraulic clutch pressure control valve in accordance with the disclosed principles; 
         FIG. 4  is an idealized clutch pressure timing plot illustrating a hydraulic pressure spike usable to detect an end of fill in accordance with the disclosed principles; and 
         FIG. 5  is a flow chart illustrating a process of a controlling a hydraulic clutch in accordance with the disclosed principles. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure relates to the operation of transmissions that employ hydraulic clutches to control the timing of transmission ratio or range shifts. The disclosed principles provide a mechanism for configuring and controlling a clutch so that the end of fill event of the clutch can be known precisely, improving the shift quality.  FIG. 1  is a simplified schematic view of a hydraulic clutch  1 . A hydraulic clutch  1  typically comprises a cylinder  2  defining a chamber  3 , for retaining hydraulic fluid. The chamber  3  also contains a cooperating fitted piston  4  or other movable member for transmitting the pressure of the fluid from an associated extension  5  to a friction member  6 , e.g., a stack of clutch plates. When the fluid volume within the chamber  3  reaches a level that the friction member  6  has moved into its final position, e.g., the stack of clutch plates is fully touching their interleaved transfer elements, not shown, the clutch  1  is said to be “filled.” Between the empty and filled state of the clutch  1 , the piston  4  may move a short distance, e.g., about 4 mm. 
     Once the clutch  1  is filled, the continued introduction of fluid into the chamber  3  will cause a pressure rise within the chamber  3 . This translates into an increased force by the fluid against the piston  4 , and a corresponding increase in friction between the friction member  6  and its counterpart, e.g., the interleaved transfer elements. At a certain pressure level, which may be unique to the clutch  1 , the friction between the between the friction member  6  and its counterpart fully overcomes the resistance of a load attached to the counterpart, e.g., a machine transmission etc., and the clutch  1  “locks” so that the friction member  6  and its counterpart move together and torque is fully transferred through the clutch  1 . 
     In the environment of a multi-clutch transmission, the timing with which the clutches lock and unlock is important. For example, if an on-coming clutch locks before an off-going clutch unlocks, severe damage to the transmission or machine may result. Even if damage is avoided, the machine operator may nonetheless experience rough shifting and discomfort. 
     Typically, a clutch-specific and empirically-determined point in time at which the clutch  1  is thought to be filled is used to change the introduction of fluid into the chamber  3  from one mode, i.e., pulse phase, to another mode, i.e., ramp phase. Thus, the timing of the fill point is important to shift quality. As noted above, existing clutch timing schemes use an estimated fill point because of the difficulty of instrumenting the chamber  3  to detect the actual fill point, as well as other related impediments. In an embodiment of the disclosed principles, a novel system is used to detect, in real time, the filling of a clutch, thus avoiding the estimation and calibration errors inherent in existing static systems. 
     In an embodiment, a machine transmission system  10  employs one or more electrohydraulic clutch pressure control (ECPC) valves. An example of an ECPC valve  12  is shown schematically in  FIG. 2  within a typical transmission system  10  operating environment. In the illustrated example, the ECPC valve  12  receives an input of pressurized fluid from a fluid source such as a hydraulic pump  11 . The pressurized fluid is described herein as hydraulic fluid; however, those of skill in the art will appreciate that any fluid capable of meeting implementation requirements in a given system will be suitable. 
     The ECPC valve  12  receives electrical control signals, e.g., a current or voltage signal, from a transmission controller  13  to actuate the valve spool which causes the ECPC valve  12  to provide an output of fluid at a pressure set by the control signals to the clutch  1 . In this manner, the transmission controller  13  is able to control the pressure of fluid provided to the clutch, and hence to control the operation of the clutch. In an embodiment, the transmission controller  13  controls the clutch  1  so that the clutch fills at one or more first predetermined pressures to avoid a rough “touch up” at the end of fill point, after which the clutch pressure increases to one or more second predetermined pressures, e.g., substantially greater than the one or more first predetermined pressures. In this manner, once the clutch chamber is filled and the clutch is ready to transmit torque, the transmission controller  13  initiates clutch modulation to maximum clamp pressure, which prepares the clutch for the torque transfer phase. 
     As noted above, the timing of clutch transitions can greatly influence the quality of a shift between transmission ratios. In order to determine more precisely when to switch from a pressure suitable for filling the clutch  1  (i.e., a “clutch fill pressure”) to a pressure suitable for locking the clutch  1  (i.e., a “clutch lock pressure”), the transmission controller  13  determines the point in time at which the clutch  1  has completed filling (i.e., the “end of fill point”). In one example, the transmission controller  13  determines the end of fill point by monitoring a pressure in the hydraulic fluid within the ECPC via a pressure switch or transducer. In particular, it has been discovered that at the end of fill point, a perturbation in fluid pressure feeds back from the clutch  1  into the ECPC valve  12 , and that this perturbation may be harnessed to identify the end of fill point with precision. 
     An ECPC implementation consistent with this insight is illustrated schematically in  FIG. 3 . In overview, the ECPC valve  12  of  FIG. 3  comprises a valve body  20  having a plurality of orifices and chambers arranged to regulate a flow of pressurized hydraulic fluid from a source inlet  21  to a clutch outlet  22  responsive to a solenoid  23 . The ECPC valve  12  includes a valve spool  24  that moves linearly within the body  20  under the influence of two forces, namely the force of a compression spring  25  as well as an oppositely directed displacement force caused by pressure chamber  26 . 
     The solenoid  23  comprises an actuator  27  within a coil unit  28 . When energized, the coil unit  28  forces the actuator  27  toward the body  20  with a force that is at least approximately a function of a current applied to the coil unit  28  of the solenoid  23 , e.g., by an electronic control module (ECM), e.g., transmission controller  13 . As the actuator  27  is forced toward the body  20 , a stop  29  on the actuator  27  cooperates with a pressure chamber orifice  30  to regulate the flow of fluid out of the pressure chamber  26 . This in turn regulates a hydraulic pressure on the valve spool  24  to oppose the compression spring  25 , thus regulating the linear position of the valve spool  24  within the body  20 . 
     As the valve spool  24  moves within the body  20 , a cylindrical projection  31  on the valve spool  24  cooperates with a land  32  on the body  20  to regulate the introduction of fluid from the source inlet  21  into a valve plenum  33  in fluid communication with the clutch outlet  22 . As a result of the described interactions, the fluid pressure supplied at the clutch outlet  22  is controllable via a current applied to the coil unit  28  of the solenoid  23  by the transmission controller  13 . This allows the transmission controller  13  to control the position and pressure of one or more clutches associated with the ECPC valve  12 . 
     However, as noted above, it is difficult to measure the actual position of clutch components relative to their fully engaged position, e.g., their position when the clutch is fully transferring torque. As such, it is also difficult to coordinate an on-coming clutch with an off -going clutch with sufficient accuracy to avoid suboptimal shift behavior. To overcome this deficiency and to allow real-time positioning of the clutch components based on real-time conditions rather than historical data, the ECPC valve  12  further comprises a pressure switch  34  in fluid communication with the valve plenum  33 . The pressure switch  34  may be for example a switch-to-ground (SWG) input that may be either normally on (closed) or normally off (open). 
     The pressure switch  34  is linked to the transmission controller  13  in order to transmit one or more electrical signals to the controller. In response to the transmitted signal, the transmission controller  13  changes the manner in which it energizes the solenoid  23  in order to optimize the shift timing. In particular, the switch  34  responds to a predetermined pressure change pattern in the valve plenum  33  indicative of the clutch end of fill point. The end of fill point corresponds to the maximum travel of the piston  4 , and when this point is reached, the volume of the clutch chamber  3  reaches its maximum and stops. When the clutch chamber  3  suddenly stops expanding at the end of fill point, the fluid flowing within the system continues to flow into the fixed clutch chamber  3  at substantially the same rate for a brief period of time due to its inertia. 
     This flow imbalance causes a momentary pressure rise or spike in the clutch chamber  3  at the end of fill point, and this pressure spike feeds back into the control side of the ECPC valve  12 . As the end of fill pressure spike reaches the ECPC valve  12 , the pressure in the valve plenum  33  rises briefly, and the switch  34  detects this rise. At this point, the switch  34  transmits a signal indicative of the pressure spike to the transmission controller  13 , and the transmitted signal is interpreted by the transmission controller  13  as signaling the end of fill point. 
     It has been observed that in one arrangement the end of fill pressure spike may have an amplitude of about 10 psi and last for a duration of about 4 ms. Thus, it is desirable in this embodiment to use a switch that triggers at or below 10 psi. However, it will appreciated that there may be a trade-off between shift quality and sensor cost. The larger the required spike, the rougher the shift could be. However, the lower the required spike, the higher the sensor cost, due to increased resolution. At the same time, the sensitivity of the switch  34  should be such that the switch  34  will not trigger on system noise such as may be present at an amplitude of about 5 psi or less. The sensitivity of the switch  34  may vary depending upon the implementation. In particular, it will be appreciated that an end of fill pressure spike may be greater or less than 10 psi and the system noise level may be greater or less than 5 psi depending upon the system in which the disclosed principles are implemented. 
     Given the pressure spike duration of about 4 ms, the switch  34  should have a response time low enough to respond on this order of time. In addition, although many ECMs operate with a loop time (time between re-execution of control flow) of about 10 ms, this loop time is too long to ensure that the pressure spike is observed. In particular, if the pressure spike occurs between loops, it may go undetected. For this reason, in an embodiment, the transmission controller  13  loop time is about 2.5 ms or less, ensuring that the pressure spike is detected whenever it occurs. 
     Despite taking precautions regarding the switch response time and sensitivity and transmission controller  13  loop time, it is possible that the clutch pressure spike will go undetected or that a false trigger will occur prior to the clutch pressure spike. For example, the clutch pressure spike in the clutch chamber  3  may occur at substantially the same time as another source of pressure variation in the control side of the pertinent valve. In such circumstances, the pressure spike from the clutch chamber  3  may not feed back intact to the valve plenum  33 , and may thus go undetected. For this reason, in a further embodiment the transmission controller  13  may end the clutch fill phase and begin a clutch modulation phase, i.e., to ensure the torque transfer and lock up the clutch  1 , if the clutch fill phase has been ongoing for longer than a clutch-specific empirically predetermined amount of time without detection of an end of fill pressure spike. The predetermined amount of time depends upon the implementation environment, but in an example, the predetermined amount of time is set at about 625 ms. It will be appreciated that the clutch fill time is a function of the clutch volume, as well as the hydraulic fluid temperature and viscosity. 
     Similarly, to avoid premature triggering of the switch  34 , the switch  34  is disabled in an example, or its output ignored, for a predetermined interval after the clutch fill phase begins. This ensures that for most of the fill phase, noise-induced pressure fluctuations in the control side of the pertinent valve will not be able to trigger the switch prematurely. Although the magnitude of the predetermined interval depends upon the implementation environment, the predetermined amount of time is set at about 450 ms in an example. 
     An example plot  40  showing a representation of a pressure rise and associated pressure spike is shown in  FIG. 4 . It will be appreciated that the pressure switch  34  will sense the illustrated spike  41  but will typically not sense the rest of the pressure curve  42 . However, in an embodiment, a pressure sensor or transducer may be used in lieu of switch  34 , in which case such sensor may detect the various pressure levels of the pressure curve  42 . The pressure curve  42  represents the hydraulic pressure in the control side of the ECPC valve  12 , e.g., within the valve plenum  33 , and shows a relatively constant pressure during the fill phase onset  43  to the end of fill point  44 , beyond a transient initial stage. At the end of fill point  44 , the pressure spikes, e.g., rises on the order of 10 psi, in the manner described above. The spike  41  is transient and subsequently fades as the fluid pressures within the control side equilibrate. As noted above, the pressure is used by the transmission controller  13  to identify the end of fill event and thus to start the next phase, e.g., a modulation phase during period  45 . 
     The flow chart of  FIG. 5  illustrates an exemplary process  50  for clutch management, including end of fill detection, in accordance with the principles described above. For purposes of describing the process  50 , it will be assumed that the system architecture is as described in  FIGS. 1-3 . It will also be assumed that the machine transmission under discussion is executing a two-clutch shift. However, these assumptions are made merely for ease of understanding and are not required conditions for all embodiments. 
     At stage  51  of the process  50 , the transmission controller  13  determines that a transmission shift is required. This requirement may be due to conditions such as increasing or decreasing machine speed and/or load, or operator action, such as increased or decreased use of auxiliary devices, etc. The transmission controller  13  commands a hydraulic pressure decrease to an off-going clutch associated with the current transmission ratio at stage  52 . 
     At stage  53 , which is begun at a predetermined time relative to (before, at, or after) the commencement of stage  52 , the transmission controller  13  begins a fill phase for an on-coming clutch associated with the new desired transmission ratio. In an embodiment, the fill phase comprises commanding a clutch fill pressure via solenoid  23 . During the fill phase, the transmission controller  13  monitors the switch  34  to detect an end of fill pressure spike at stage  54 . Simultaneously in stage  55 , the transmission controller  13  monitors the time elapsed since the commencement of the fill phase. If at stage  56  the transmission controller  13  determines that either a pressure spike has been detected via switch  34  or a predetermined amount of time has elapsed during the fill phase, the transmission controller  13  moves to stage  57 . Otherwise, the process  50  returns to parallel stages  54  and  55 . 
     At stage  57 , the transmission controller  13  ceases the fill stage and initiates a clutch modulation phase, i.e., to increase the torque transfer and lock up the clutch  1 . Typically this phase entails increasing the clutch pressure until the clutch no longer slips and fully transfers torque. Once the clutch  1  reaches lock up, the shift is complete. It will be appreciated that in the case of multiple on-coming and multiple off-going clutches, the foregoing principles are equally applicable for each clutch. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is applicable to hydraulic transmissions, i.e., transmissions that employ hydraulic clutches to control the timing of transmission ratio or range shifts. In particular, the disclosed principles provide a mechanism for configuring and controlling a clutch  1  so that the end of fill event of the clutch  1  is known precisely, improving the shift quality. This system may be implemented in on-highway or off-highway machines, construction machines, industrial machines, etc. Although many machines that may benefit from the disclosed principles will be machines used at least occasionally for transport of goods, materials, or personnel, it will be appreciated that hydraulic transmissions are used in other contexts as well, and the disclosed teachings are likewise broadly applicable. 
     Using the disclosed principles, a transmission controller  13 , e.g., an ECM, is able to determine the point in time at which a clutch has reached its limit of travel toward engagement. Using this determination, the transmission controller  13  is then able to precisely time the onset of the clutch modulation to avoid delayed or premature lock-up of the clutch  1 . In a further aspect, the disclosed system provides a back-up mechanism in the event that the transmission controller  13  for any reason fails to detect the end of fill time. In particular, in an embodiment, the transmission controller  13  initiates the clutch modulation stage if a predetermined period of time has expired from the onset of the fill phase. Moreover, because system noise may trigger the pressure switch  34  used to detect the end of fill time, the controller may disable or ignore the pressure switch  34  for a predetermined amount of time after the onset of the fill phase. 
     Although the examples described above employ a pressure switch or transducer for each solenoid valve, this is not a requirement for implementing the disclosed principles. Rather, it will be appreciated that the foregoing teachings also apply in environments wherein a single pressure switch or transducer is associated with a plurality of solenoid valves. In an embodiment, a pressure switch or transducer may be multiplexed among two or more solenoid valves. 
     It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated. 
     Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. 
     Accordingly, this disclosure includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context.