Method and apparatus to control clutch pressures in an electro-mechanical transmission

A method for reducing occurrence of clutch slip in electromechanical transmission adapted to selectively transmit mechanical power to an output member through selective application of a hydraulically actuated clutch includes monitoring operation of said clutch, identifying an indication of clutch wear based upon said monitoring said operation, and increasing a minimum clamping force applied to said clutch based upon said indication of clutch wear.

TECHNICAL FIELD

This disclosure pertains to control systems for electromechanical transmissions.

BACKGROUND

Known powertrain architectures include torque-generative devices, including internal combustion engines and electric machines, which transmit torque through a transmission device to an output member. One exemplary powertrain includes a two-mode, compound-split, electromechanical transmission which utilizes an input member for receiving motive torque from a prime mover power source, preferably an internal combustion engine, and an output member. The output member can be operatively connected to a driveline for a motor vehicle for transmitting tractive torque thereto. Electric machines, operative as motors or generators, generate a torque input to the transmission, independently of a torque input from the internal combustion engine. The electric machines may transform vehicle kinetic energy, transmitted through the vehicle driveline, to electrical energy that is storable in an electrical energy storage device. A control system monitors various inputs from the vehicle and the operator and provides operational control of the powertrain, including controlling transmission operating state and gear shifting, controlling the torque-generative devices, and regulating the electrical power interchange among the electrical energy storage device and the electric machines to manage outputs of the transmission, including torque and rotational speed.

Operation of the above devices within a hybrid drive vehicle require management of numerous torque bearing shafts or devices representing connections to the above mentioned engine, electrical motors, and driveline. Various control schemes and operational connections between the various aforementioned components of the hybrid drive system are known, and the control system must be able to engage and disengage the various components in order to perform the functions of the hybrid drive system. Engagement and disengagement is known to be accomplished through the use of a transmission employing clutches. Clutches are devices well known in the art for engaging and disengaging shafts including the management of rotational velocity and torque differences between the shafts. Engagement or locking, disengagement or unlocking, operation while engaged or locked operation, and operation while disengaged or unlocked operation are all clutch states that must be managed in order for the vehicle to operate properly and smoothly.

Implications to vehicle operation related to perceptible jerks or abrupt changes to vehicle acceleration are collectively described as drivability. One source of perceptible jerks affecting drivability is slip, or relative rotational movement between the connective surfaces of a clutch. Slip occurs whenever the reactive torque transmitted through the clutch exceeds the actual torque capacity. Clutches can be designed to operate with some level of controlled slip in asynchronous operation, or clutches can be designed to operate with little or preferably no slip in synchronous operation. This disclosure deals with clutches designed primarily for synchronous operation. Slip in a transmission in synchronous operation results in unintended loss of control within the transmission and adverse effects upon drivability.

Clutches are known in a variety of designs and control methods. One known type of clutch is a mechanical clutch operating by separating or joining two connective surfaces, for instance, clutch plates, operating, when joined, to apply frictional torque to each other. One control method for operating such a mechanical clutch includes applying a hydraulic control system implementing fluidic pressures transmitted through hydraulic lines to exert or release clamping force between the two connective surfaces. In an exemplary hydraulically actuated clutch, capacity of the clutch to transfer reactive torque is created by the applied clamping force compressing and creating friction force between the clutch connective surfaces. Applied clamping force is reacted by hydraulic pressure acting within an actuation device, such as a piston driven cylinder, translating pressure through the piston into a force. Operated thusly, the clutch is not operated in a binary manner, but rather is capable of a range of engagement states, from fully disengaged and desynchronized, to synchronized with no clamping force applied, to engaged but with only minimal clamping force, to engaged with some maximum clamping force. This variable control of clutches allows for smooth transition between locked and unlocked states and also allows for managing slip in a locked transmission.

The capacity of the clutch to transmit reactive torque is a function of the magnitude of the clamping force applied to the clutch and the coefficients of friction between the connective surfaces being applied. Coefficients of friction between two surfaces are known in the art to include a static coefficient of friction and a kinetic coefficient of friction. Through the life of a clutch, wear causes gradual degradation of the clutch torque capacity for a given clamping force. As a result, a clutch pressure originally sufficient to create a particular torque capacity within the clutch can eventually fail to create the same torque capacity.

A method to operate a powertrain comprising modulating clutch clamping force based upon clutch slip would be beneficial to adjust for degradation in clutch torque capacity.

SUMMARY

A method for reducing occurrence of clutch slip in electro-mechanical transmission adapted to selectively transmit mechanical power to an output member through selective application of a hydraulically actuated clutch includes monitoring operation of said clutch, identifying an indication of clutch wear based upon said monitoring said operation, and increasing a minimum clamping force applied to said clutch based upon said indication of clutch wear.

DETAILED DESCRIPTION

Referring now to the drawings, wherein the showings are for the purpose of illustrating certain exemplary embodiments only and not for the purpose of limiting the same,FIGS. 1 and 2depict an exemplary electro-mechanical hybrid powertrain. The exemplary electro-mechanical hybrid powertrain in accordance with the present disclosure is depicted inFIG. 1, comprising a two-mode, compound-split, electro-mechanical hybrid transmission10operatively connected to an engine14and first and second electric machines (‘MG-A’)56and (‘MG-B’)72. The engine14and first and second electric machines56and72each generate power which can be transmitted to the transmission10. The power generated by the engine14and the first and second electric machines56and72and transmitted to the transmission10is described in terms of input torques, referred to herein as TI, TA, and TBrespectively, and speed, referred to herein as NI, NA, and NB, respectively.

The exemplary engine14comprises a multi-cylinder internal combustion engine selectively operative in several states to transmit torque to the transmission10via an input shaft12, and can be either a spark-ignition or a compression-ignition engine. The engine14includes a crankshaft (not shown) operatively coupled to the input shaft12of the transmission10. A rotational speed sensor11monitors rotational speed of the input shaft12. Power output from the engine14, comprising rotational speed and output torque, can differ from the input speed, NI, and the input torque, TI, to the transmission10due to placement of torque-consuming components on the input shaft12between the engine14and the transmission10, e.g., a hydraulic pump (not shown) and/or a torque management device (not shown).

The exemplary transmission10comprises three planetary-gear sets24,26and28, and four selectively engageable torque-transmitting devices, i.e., clutches C170, C262, C373, and C475. As used herein, clutches refer to any type of friction torque transfer device including single or compound plate clutches or packs, band clutches, and brakes, for example. A hydraulic control circuit42, preferably controlled by a transmission control module (hereafter ‘TCM’)17, is operative to control clutch states. Clutches C262and C475preferably comprise hydraulically-applied rotating friction clutches. Clutches C170and C373preferably comprise hydraulically-controlled stationary devices that can be selectively grounded to a transmission case68. Each of the clutches C170, C262, C373, and C475is preferably hydraulically applied, selectively receiving pressurized hydraulic fluid via the hydraulic control circuit42.

The first and second electric machines56and72preferably comprise three-phase AC machines, each including a stator (not shown) and a rotor (not shown), and respective resolvers80and82. The motor stator for each machine is grounded to an outer portion of the transmission case68, and includes a stator core with coiled electrical windings extending therefrom. The rotor for the first electric machine56is supported on a hub plate gear that is operatively attached to shaft60via the second planetary gear set26. The rotor for the second electric machine72is fixedly attached to a sleeve shaft hub66.

Each of the resolvers80and82preferably comprises a variable reluctance device including a resolver stator (not shown) and a resolver rotor (not shown). The resolvers80and82are appropriately positioned and assembled on respective ones of the first and second electric machines56and72. Stators of respective ones of the resolvers80and82are operatively connected to one of the stators for the first and second electric machines56and72. The resolver rotors are operatively connected to the rotor for the corresponding first and second electric machines56and72. Each of the resolvers80and82is signally and operatively connected to a transmission power inverter control module (hereafter ‘TPIM’)19, and each senses and monitors rotational position of the resolver rotor relative to the resolver stator, thus monitoring rotational position of respective ones of first and second electric machines56and72. Additionally, the signals output from the resolvers80and82are interpreted to provide the rotational speeds for first and second electric machines56and72, i.e., NAand NB, respectively.

The transmission10includes an output member64, e.g. a shaft, which is operably connected to a driveline90for a vehicle (not shown), to provide output power, e.g., to vehicle wheels93, one of which is shown inFIG. 1. The output power is characterized in terms of an output rotational speed, NOand an output torque, TO. A transmission output speed sensor84monitors rotational speed and rotational direction of the output member64. Each of the vehicle wheels93, is preferably equipped with a sensor94adapted to monitor wheel speed, VSS-WHL, the output of which is monitored by a control module of a distributed control module system described with respect toFIG. 2, to determine vehicle speed, and absolute and relative wheel speeds for braking control, traction control, and vehicle acceleration management.

The input torques from the engine14and the first and second electric machines56and72(TI, TA, and TBrespectively) are generated as a result of energy conversion from fuel or electrical potential stored in an electrical energy storage device (hereafter ‘ESD’)74. The ESD74is high voltage DC-coupled to the TPIM19via DC transfer conductors27. The transfer conductors27include a contactor switch38. When the contactor switch38is closed, under normal operation, electric current can flow between the ESD74and the TPIM19. When the contactor switch38is opened electric current flow between the ESD74and the TPIM19is interrupted. The TPIM19transmits electrical power to and from the first electric machine56by transfer conductors29, and the TPIM19similarly transmits electrical power to and from the second electric machine72by transfer conductors31, in response to torque commands for the first and second electric machines56and72to achieve the input torques TAand TB. Electrical current is transmitted to and from the ESD74in accordance with whether the ESD74is being charged or discharged.

The TPIM19includes the pair of power inverters (not shown) and respective motor control modules (not shown) configured to receive the torque commands and control inverter states therefrom for providing motor drive or regeneration functionality to achieve the input torques TAand TB. The power inverters comprise known complementary three-phase power electronics devices, and each includes a plurality of insulated gate bipolar transistors (not shown) for converting DC power from the ESD74to AC power for powering respective ones of the first and second electric machines56and72, by switching at high frequencies. The insulated gate bipolar transistors form a switch mode power supply configured to receive control commands. There is typically one pair of insulated gate bipolar transistors for each phase of each of the three-phase electric machines. States of the insulated gate bipolar transistors are controlled to provide motor drive mechanical power generation or electric power regeneration functionality. The three-phase inverters receive or supply DC electric power via DC transfer conductors27and transform it to or from three-phase AC power, which is conducted to or from the first and second electric machines56and72for operation as motors or generators via transfer conductors29and31respectively.

FIG. 2is a schematic block diagram of the distributed control module system. The elements described hereinafter comprise a subset of an overall vehicle control architecture, and provide coordinated system control of the exemplary powertrain described inFIG. 1. The distributed control module system synthesizes pertinent information and inputs, and executes algorithms to control various actuators to achieve control objectives, including objectives related to fuel economy, emissions, performance, drivability, and protection of hardware, including batteries of ESD74and the first and second electric machines56and72. The distributed control module system includes an engine control module (hereafter ‘ECM’)23, the TCM17, a battery pack control module (hereafter ‘BPCM’)21, and the TPIM19. A hybrid control module (hereafter ‘HCP’)5provides supervisory control and coordination of the ECM23, the TCM17, the BPCM21, and the TPIM19. A user interface (‘UI’)13is operatively connected to a plurality of devices through which a vehicle operator controls or directs operation of the electromechanical hybrid powertrain. The devices include an accelerator pedal113(‘AP’) from which an operator torque request is determined, an operator brake pedal112(‘BP’), a transmission gear selector114(‘PRNDL’), and a vehicle speed cruise control (not shown). The transmission gear selector114may have a discrete number of operator-selectable positions, including the rotational direction of the output member64to enable one of a forward and a reverse direction.

The aforementioned control modules communicate with other control modules, sensors, and actuators via a local area network (hereafter ‘LAN’) bus6. The LAN bus6allows for structured communication of states of operating parameters and actuator command signals between the various control modules. The specific communication protocol utilized is application-specific. The LAN bus6and appropriate protocols provide for robust messaging and multi-control module interfacing between the aforementioned control modules, and other control modules providing functionality such as antilock braking, traction control, and vehicle stability. Multiple communications buses may be used to improve communications speed and provide some level of signal redundancy and integrity. Communication between individual control modules can also be effected using a direct link, e.g., a serial peripheral interface (‘SPI’) bus (not shown).

The HCP5provides supervisory control of the powertrain, serving to coordinate operation of the ECM23, TCM17, TPIM19, and BPCM21. Based upon various input signals from the user interface13and the powertrain, including the ESD74, the HCP5generates various commands, including: the operator torque request (‘TO—REQ’), a commanded output torque (‘TCMD’) to the driveline90, an engine input torque command, clutch torques for the torque-transfer clutches C170, C262, C373, C475of the transmission10; and the torque commands for the first and second electric machines56and72, respectively. The TCM17is operatively connected to the hydraulic control circuit42and provides various functions including monitoring various pressure sensing devices (not shown) and generating and communicating control signals to various solenoids (not shown) thereby controlling pressure switches and control valves contained within the hydraulic control circuit42.

The ECM23is operatively connected to the engine14, and functions to acquire data from sensors and control actuators of the engine14over a plurality of discrete lines, shown for simplicity as an aggregate bi-directional interface cable35. The ECM23receives the engine input torque command from the HCP5. The ECM23determines the actual engine input torque, TI, provided to the transmission10at that point in time based upon monitored engine speed and load, which is communicated to the HCP5. The ECM23monitors input from the rotational speed sensor11to determine the engine input speed to the input shaft12, which translates to the transmission input speed, NI. The ECM23monitors inputs from sensors (not shown) to determine states of other engine operating parameters including, e.g., a manifold pressure, engine coolant temperature, ambient air temperature, and ambient pressure. The engine load can be determined, for example, from the manifold pressure, or alternatively, from monitoring operator input to the accelerator pedal113. The ECM23generates and communicates command signals to control engine actuators, including, e.g., fuel injectors, ignition modules, and throttle control modules, none of which are shown.

The TCM17is operatively connected to the transmission10and monitors inputs from sensors (not shown) to determine states of transmission operating parameters. The TCM17generates and communicates command signals to control the transmission10, including controlling the hydraulic control circuit42. Inputs from the TCM17to the HCP5include estimated clutch torques for each of the clutches, i.e., C170, C262, C373, and C475, and rotational output speed, NO, of the output member64. Other actuators and sensors may be used to provide additional information from the TCM17to the HCP5for control purposes. The TCM17monitors inputs from pressure switches (not shown) and selectively actuates pressure control solenoids (not shown) and shift solenoids (not shown) of the hydraulic circuit42to selectively actuate the various clutches C170, C262, C373, and C475to achieve various transmission operating range states, as described hereinbelow.

The BPCM21is signally connected to sensors (not shown) to monitor the ESD74, including states of electrical current and voltage parameters, to provide information indicative of parametric states of the batteries of the ESD74to the HCP5. The parametric states of the batteries preferably include battery state-of-charge, battery voltage, battery temperature, and available battery power, referred to as a range PBAT—MINto PBAT—MAX.

Each of the control modules ECM23, TCM17, TPIM19and BPCM21is preferably a general-purpose digital computer comprising a microprocessor or central processing unit, storage mediums comprising read only memory (‘ROM’), random access memory (‘RAM’), electrically programmable read only memory (‘EPROM’), a high speed clock, analog to digital (‘A/D’) and digital to analog (‘D/A’) circuitry, and input/output circuitry and devices (‘I/O’) and appropriate signal conditioning and buffer circuitry. Each of the control modules has a set of control algorithms, comprising resident program instructions and calibrations stored in one of the storage mediums and executed to provide the respective functions of each computer. Information transfer between the control modules is preferably accomplished using the LAN bus6and SPI buses. The control algorithms are executed during preset loop cycles such that each algorithm is executed at least once each loop cycle. Algorithms stored in the non-volatile memory devices are executed by one of the central processing units to monitor inputs from the sensing devices and execute control and diagnostic routines to control operation of the actuators, using preset calibrations. Loop cycles are executed at regular intervals, for example each 3.125, 6.25, 12.5, 25 and 100 milliseconds during ongoing operation of the powertrain. Alternatively, algorithms may be executed in response to the occurrence of an event.

The exemplary powertrain selectively operates in one of several operating range states that can be described in terms of an engine state comprising one of an engine on state (‘ON’) and an engine off state (‘OFF’), and a transmission state comprising a plurality of fixed gears and continuously variable operating modes, described with reference to Table 1, below.

Each of the transmission operating range states is described in the table and indicates which of the specific clutches C170, C262, C373, and C475are applied for each of the operating range states. A first continuously variable mode, i.e., EVT Mode I, or MI, is selected by applying clutch C170only in order to “ground” the outer gear member of the third planetary gear set28. The engine state can be one of ON (‘MI_Eng_On’) or OFF (‘MI_Eng_Off’). A second continuously variable mode, i.e., EVT Mode II, or MII, is selected by applying clutch C262only to connect the shaft60to the carrier of the third planetary gear set28. The engine state can be one of ON (‘MII_Eng_On’) or OFF (‘MII_Eng_Off’). For purposes of this description, when the engine state is OFF, the engine input speed is equal to zero revolutions per minute (‘RPM’), i.e., the engine crankshaft is not rotating. A fixed gear operation provides a fixed ratio operation of input-to-output speed of the transmission10, i.e., NI/NO, is achieved. A first fixed gear operation (‘FG1’) is selected by applying clutches C170and C475. A second fixed gear operation (‘FG2’) is selected by applying clutches C170and C262. A third fixed gear operation (‘FG3’) is selected by applying clutches C262and C475. A fourth fixed gear operation (‘FG4’) is selected by applying clutches C262and C373. The fixed ratio operation of input-to-output speed increases with increased fixed gear operation due to decreased gear ratios in the planetary gears24,26, and28. The rotational speeds of the first and second electric machines56and72, NAand NBrespectively, are dependent on internal rotation of the mechanism as defined by the clutching and are proportional to the input speed measured at the input shaft12.

In response to operator input via the accelerator pedal113and brake pedal112as captured by the user interface13, the HCP5and one or more of the other control modules determine the commanded output torque, TCMD, intended to meet the operator torque request, TO—REQ, to be executed at the output member64and transmitted to the driveline90. Final vehicle acceleration is affected by other factors including, e.g., road load, road grade, and vehicle mass. The operating range state is determined for the transmission10based upon a variety of operating characteristics of the powertrain. This includes the operator torque request, communicated through the accelerator pedal113and brake pedal112to the user interface13as previously described. The operating range state may be predicated on a powertrain torque demand caused by a command to operate the first and second electric machines56and72in an electrical energy generating mode or in a torque generating mode. The operating range state can be determined by an optimization algorithm or routine which determines optimum system efficiency based upon operator demand for power, battery state of charge, and energy efficiencies of the engine14and the first and second electric machines56and72. The control system manages torque inputs from the engine14and the first and second electric machines56and72based upon an outcome of the executed optimization routine, and system efficiencies are optimized thereby, to manage fuel economy and battery charging. Furthermore, operation can be determined based upon a fault in a component or system. The HCP5monitors the torque-generative devices, and determines the power output from the transmission10required to achieve the desired output torque to meet the operator torque request. As should be apparent from the description above, the ESD74and the first and second electric machines56and72are electrically-operatively coupled for power flow therebetween. Furthermore, the engine14, the first and second electric machines56and72, and the electromechanical transmission10are mechanically-operatively coupled to transmit power therebetween to generate a power flow to the output member64.

Clutch torque capacity is a function of the clamping force applied to the clutch and the relevant coefficient of friction for the connective surfaces of the clutch. When the clutch connective surfaces are synchronized and locked, meaning that the connective surfaces are in contact and moving with zero relative angular velocity, a static coefficient of friction describes the clutch torque capacity resulting from the clamping force. When the clutch connective surfaces are not locked and the clutch is slipping, a kinetic coefficient of friction, lower than the static coefficient of friction, describes the torque between the connective surfaces resisting the slip. If this torque resisting the slip is greater than the torque being transmitted through the clutch, then slip speed will reduce. If the torque resisting the slip is less than the torque being transmitted through the clutch, then the slip speed will increase. Therefore, a method to recover a locked state in a slipping clutch is to increase clamping force to increase torque resisting the slip. Many methods in control logic are envisioned to boost clamping force. One exemplary method to accomplish this boost is to sum a pressure required to meet a clutch reactive torque requirement (PCL—REQ) and an offset pressure (POFFSET) calibrated to control the slip. Methods described herein discuss adjustments to hydraulic line pressure in terms of adjustments to POFFSETfor convenience. However, it will be appreciated that POFFSETis simply an efficient means to discuss adjusting a minimum hydraulic line pressure taking clutch reactive torque into account. In broader terms, the disclosure could equivalently discuss maintaining a minimum hydraulic line pressure in the context of adjustments to PCL REQinstead of increasing POFFSET, but POFFSET, as an exemplary embodiment, provides a clear illustration of utilization of incremental increases.

FIG. 3is a graphical representation of an exemplary slip event and corresponding clutch slip recovery event in accordance with the present disclosure. Two graphs are depicted sharing a common timescale. The top portion of the graph depicts slip speed or the relative angular velocity between connective surfaces of a clutch. Some embodiments of synchronous clutches can sustain low levels of slip or brief occurrences of slip without significant degradation to performance or risk of damage. A method is depicted to classify a significant slip event as an occurrence of slip above some threshold slip magnitude for more than a threshold duration of time. Slip initially increases from zero in excess of the threshold slip magnitude. If slip reduces below the threshold quickly, in a time less than the threshold slip duration, then the slip event can be disregarded as not significant. However, if slip in excess of the threshold slip magnitude is detected for longer than the threshold slip duration, then the slip event can be classified as significant, and a slip remediation action can be initiated. As described above, an increase in clamping force applied to the clutch connective surfaces increases the torque within the clutch resisting slip. POFFSETas an exemplary method to control an increase in clamping force is depicted in the bottom portion of the graph.

Hydraulic line pressure is effected by a hydraulic pump. One known configuration of a powertrain includes a main hydraulic pump mechanically driven by an engine, wherein engine rotation directly drives the hydraulic pump and supplies hydraulic pressure to the hydraulic control system. In hybrid powertrain applications or application utilizing an engine start/stop strategy, deactivation or idling of the engine can reduce or remove hydraulic pressure available from the main hydraulic pump. An auxiliary hydraulic pump is known to be used, utilizing for example an electric motor, to provide alternative or additional hydraulic pressure. In particular in association with an auxiliary pump, hydraulic pressure is managed, where the auxiliary pump is not operated at full capacity when the maximum hydraulic pressure is not needed. However, in association with a perceived demand for a maximum available hydraulic line pressure (PMAX) the auxiliary pump can be controlled to provide additional pressure. Control of the auxiliary hydraulic pump is known to be accomplished through an auxiliary hydraulic pump control module processing inputs and issuing auxiliary pump speed commands. Hydraulic pressure can additionally be managed in relation to a main hydraulic pump or an auxiliary hydraulic pump by managing consumption of hydraulic flow in functions consuming pressurized hydraulic fluid.

Returning toFIG. 3, POFFSETdescribes an increase to commanded hydraulic line pressure in order to remediate the detected slip event. Upon initiation of slip remediation, POFFSETis commanded to PMAXin order to provide the most rapid and effective means to control slip possible. Once slip speed is reduced below the threshold slip magnitude, POFFSETcan be reduced below PMAXand restored to a normal level. Restoration of POFFSETcan be accomplished in a step pattern, immediately restoring POFFSETto zero. However, in order to avoid reoccurrence of slip, a method stepping down POFFSETincrementally has been shown to be beneficial. The number of increments, the incremental decreases in POFFSET, and the hold times for each incremental decrease before the next decrease may be developed experimentally, empirically, predictively, through modeling or other techniques adequate to accurately predict clutch and hydraulic control system operation, and a multitude of POFFSETreduction strategies might be used by the same powertrain for different clutches and for different conditions or operating ranges. The threshold slip magnitude and the threshold slip duration as described in the above method can be a simple value or may be variable depending upon vehicle conditions. These values can be developed experimentally, empirically, predictively, through modeling or other techniques adequate to accurately predict clutch operation.

The method described inFIG. 3remediates significant slip events by increasing clamping force through a slip event. However, each slip event is treated in isolation, and no action is taken to account for changing behavior in the clutch. As described above, wear in a clutch tends to increase the clamping force required to achieve a given clutch torque capacity.FIG. 4is a graphical representation of a slip event and a corresponding clutch slip recovery event, including a method to gradually, incrementally increase POFFSETin order to adjust clutch torque capacity, in accordance with the present disclosure. Similarly toFIG. 3,FIG. 4includes a graph on a top portion depicting slip speed and, on a common timescale, a graph on a bottom portion depicting POFFSETas an exemplary method to control a slip remediation event. As described above, in the event that slip in excess of a threshold slip magnitude is detected for longer than a threshold slip duration, slip remediation is initiated, wherein POFFSETis increased to PMAX. Once slip speed is reduced below the threshold slip magnitude, POFFSETis reduced in increments as described inFIG. 3. However, instead of returning to the initial POFFSETlevel, zero in the exemplary data, POFFSETis reduced to some increased minimum POFFSET. This increased POFFSETcreates an incrementally increased clamping force upon the clutch. By increasing the clamping force upon the clutch, the clutch can exert an increased clutch torque capacity than was exerted before the minimum POFFSETwas increased. This incremental increase in clutch torque capacity in response to a slip event serves to offset gradual degradation in the clutch.

Other methods to increase POFFSETor otherwise increase applied force to a clutch are envisioned. For example, a look-up table or a time or usage based clutch wear estimate function can be utilized to gradually implement an increased clamping force to compensate for clutch wear. However, lookup tables and functional estimates are prone to error. Such error can lead to unnecessarily high hydraulic line pressures, creating inefficient requirements upon the auxiliary hydraulic pump. An adjustment to clamping force based upon a feedback signal such as a slip event indication provides for necessary increases to clamping force based upon an indication that clutch torque capacity has been reduced below expected levels for a given clamping force. Other methods to control increases to clamping force envisioned include applying statistical analysis to a set of slip events in order to evaluate and predict a clutch wear rate. Such a predicted clutch wear rate, if determined to meet some minimum level of confidence, can be used to anticipatorily increase clamping force through the life of the clutch. Additionally, it will be appreciated by one having ordinary skill in the art that clutch slip in the above described method is being used as a means to evaluate wear on the clutch. Any method to evaluate wear on the clutch, for example, running the clutch in a test mode upon vehicle start-up, with an asynchronous spin, touching connective surfaces clutch state evaluation of the clutch, a clutch wear health estimate could be used in place of the significant clutch slip event indicated above. The disclosure envisions many methods to evaluate wear upon the clutch and is not intended to be limited to the specific exemplary embodiments described herein.

FIG. 5graphically depicts incremental increases to minimum POFFSETover a number of slip events, in accordance with the present disclosure. POFFSETis depicted through a number of slip events, each slip event initiating a corresponding slip remediation. With each remediation, as described above, POFFSETreturns to an incrementally increased minimum value. The magnitude of each incremental increase in minimum POFFSETcan be a set increment, can be based upon a logarithmic or scaled function, or the magnitude can change according to some monitored variable such as the magnitude of the subject slip event or the maintenance status of the clutch.

FIG. 6graphically illustrates a series of incremental increases in minimum POFFSETover a number of slip events and corresponding exemplary clutch torque capacity data, in accordance with the present disclosure. Minimum POFFSETdepicts increases to a minimum POFFSETterm as described above. Clutch torque capacity results from a line pressure applying a clamping force to clutch connective surfaces, as described above. Without factoring resulting clutch torque capacity for wear, clutch torque capacity can be expressed as a function of line pressure applied to the clutch. In order to account for wear, a modification to the function relating clutch pressure to clutch torque capacity can be programmed, including a modulation factor such as POFFSETto account for the effects of wear. However, retaining the calculation of clutch torque capacity as a simple equation based upon line pressure applied to the clutch is beneficial due to resulting simplified calculations. Clutch torque capacity is used in many modules and calculations throughout the powertrain, and simplified calculation of clutch torque capacity benefits each of these downstream uses. A preferred method is disclosed wherein line pressure to the clutch is modulated by the POFFSETterm, and clutch torque capacity calculations assume zero wear upon the clutch connective surfaces. InFIG. 6, a reference clutch torque capacity is depicted, demonstrating a clutch torque capacity that can be estimated for a given line pressure applied to a clutch. An unmodified clutch torque capacity is depicted illustrating exemplary behavior of a clutch for the given line pressure and experiencing normal wear through the life of the clutch wherein no increase to minimum POFFSETis utilized. As described above, wear gradually reduces clutch torque capacity resulting from application of a particular clamping force resulting from the line pressure. Because, in the exemplary data, no offset is utilized to increase clamping force, the unmodified clutch torque capacity for a given reactive torque input decreases over time. An incrementally increased clutch torque capacity is depicted, illustrating periodic increases to clutch torque capacity for a given reactive torque input, corresponding to increases in POFFSET. While wear continues to degrade the ability of the clutch to transmit reactive torque through the life of the clutch, as depicted by the general downward trend in the data, increases to POFFSETsustains clutch torque capacity for a given input.

The method above describes a remediation response to a slip event, including a boost of POFFSETto a PMAXand then a gradual reduction in POFFSETto an increase minimum POFFSETafter the slip event has been contained. However, selection of the incremental increase in minimum POFFSET, depending upon how small the increment is, can fail to resist slip proximately in time to the remediation event.FIG. 7is a graphical representation of a slip event and a corresponding clutch slip recovery event, including a method to set POFFSETin response to a reoccurrence of slip, in accordance with the present disclosure. As described above, a significant slip event is detected and a remediation event is initiated. After the slip event is initially contained, POFFSETis reduced in increments toward a predetermined incrementally increased minimum POFFSETexpected to subsequently deter slip in the foreseeable future. However, before the new POFFSETis reached in accordance with the expected increase, slip in excess of the threshold slip magnitude occurs. A number of reactions, comprising a slip reoccurrence recovery cycle, are contemplated in response to reoccurrence of slip. One exemplary method is to return POFFSETimmediately to PMAXand treat the new slip as a new slip event. In another exemplary method to react to the reoccurrence of slip, POFFSETat the level where slip occurred is boosted by a recovery POFFSETincrease. Slip is monitored through a threshold recovery duration to evaluate whether the recovery POFFSETincrease is effective to contain the second slip event. Two curves are depicted inFIG. 7, one wherein the second slip event is contained and another wherein the second slip event is not contained in the threshold recovery duration. In a case wherein after the threshold recovery duration, slip is reduced to or below the threshold slip magnitude, the recovery POFFSETincrease can be determined to have remediated the second slip event. In this case, the new minimum POFFSETcan be set to the level set by the recovery POFFSETincrease. In a case wherein after the threshold recovery duration, slip is not reduced to or below the threshold slip magnitude, the recovery POFFSETincrease can be determined to have not remediated the second slip event. In this case, the second slip event can be treated as a new case, POFFSETcan be increased to PMAX, and methods described herein, such as the method described in relation toFIG. 4, can be employed to remediate the slip event. A new POFFSETterm can subsequently be selected in excess of the level indicated by the recovery POFFSETincrease that failed to recover the second slip event.

FIG. 8schematically illustrates an exemplary process to calculate and combine clutch pressure requirements, incorporating a pressure required to meet clutch reactive torque requirements and POFFSET, in accordance with the present disclosure. Process300comprises clutch reactive torque pressure module310, offset pressure module320, and summation block330. As described above, powertrain control mechanisms, am HCP for example, determine how much torque is to be applied to each clutch in the powertrain. Clutch reactive torque pressure module310inputs requested clutch reactive torque and applies programming, including factors based upon clutch specifications, to determine clutch pressure required to achieve a clutch torque capacity capable of transmitting the requested clutch reactive torque. Clutch reactive torque pressure module310outputs PCL—REQ. Offset pressure module320inputs clutch slip data in accordance with methods disclosed herein and outputs POFFSET. Summation block330sums PCL—REQand POFFSETand outputs a minimum effective pressure (PMIN—EFF) required to satisfy the required clutch torque capacity to transmit the request clutch reactive torque and compensate the clutch torque capacity for clutch capacity due to clutch degradation.

FIG. 9depicts an exemplary process flow, wherein a pressure offset term is managed in a cycle, in accordance with the present disclosure. Exemplary process400starts at step410and proceeds initially to an adapted state at step420. The process waits at step420until a significant slip event is detected. Upon detection of a significant slip event, the process proceeds to step430wherein POFFSETis stepped up to PMAXin accordance with methods described herein in order to contain the slip event. At step440, after the slip event has been contained, the process adapts POFFSETthrough a series of wait and step down actions, gradually decreasing POFFSET. If no reoccurrence of slip is detected through step440, then in step450a new minimum POFFSETis defined based on methods described herein, and the process reiterates to step420. If a reoccurrence of slip is detected in step440, then the process reiterates to step430, wherein POFFSETis increased to PMAXand the containment process is restarted.

It is understood that modifications are allowable within the scope of the disclosure. The disclosure has been described with specific reference to the preferred embodiments and modifications thereto. Further modifications and alterations may occur to others upon reading and understanding the specification. It is intended to include all such modifications and alterations insofar as they come within the scope of the disclosure.