Abstract:
A torque converter lockup clutch control system is provided for an associated automatic transmission. The system includes a torque converter having an impeller operatively coupled to an associated internal combustion engine and a turbine operatively coupled to an input shaft of the associated automatic transmission. A lockup clutch is disposed within the torque converter for selectively locking the impeller to the turbine. A pressure regulator is operatively coupled to the torque converter for adjusting the pressure of a working fluid to at least one shift clutch of the associated automatic transmission in response to a speed differential between the impeller and turbine of the torque converter. A shift clutch slip detector is provided for detecting a slip condition of the at least one shift clutch of the associated automatic transmission and outputs a slip value. A processing component is then provided for receiving the slip signal and prohibiting the lockup clutch from entering into an engaged state when the slip value exceeds a predetermined shift clutch slip threshold.

Description:
BACKGROUND 
   The present disclosure generally relates to a control strategy for a vehicle torque converter for an automatic transmission. More particularly, the torque converter is controlled in such a manner as to reduce or eliminate slip in the individual speed or shift clutches of the automatic transmission. 
   Conventional automatic transmissions rely on a torque converter for the primary purpose of transforming a high revolutions-per-minute (rpm) and low torque operating state of an internal combustion engine to a lower rpm and higher torque state. A prime example includes when the vehicle is at a stop, in a drive gear, and at or near idle. In this case, the torque converter transforms, via a hydraulic or viscous coupling, the idle speed of the engine (typically 300-900 rpm) to a speed of 0-100 rpm. At the same time, the speed reduction is accompanied by an increase in torque. This is beneficial since a large amount of starting torque is required to accelerate the vehicle from a stationary position. Conversely, when the vehicle is traveling at a higher rate of speed, the torque converter is no longer needed and to improve efficiency a lockup clutch is provided to directly couple the engine to the transmission. 
   In addition, the conventional automatic transmission uses a plurality of shift clutches to switch among various gear trains, each having a specific gear ratio or speed range. Under high load conditions a great deal of torque must be transmitted through the shift clutches. If the torque exceeds the design value for the clutches or if the clutches have become worn, shift clutch slip can occur that will lead to rapid wear-out of the shift clutch and or other related components. 
   To reduce or eliminate this possibility, other systems have been designed to monitor clutch slip and to increase the pressure to the slipping clutch. However, these systems are often complicated, require additional components and/or separate delivery systems. Other systems use a torque converter reaction torque to develop the necessary pressure to reduce shift clutch slip. However, these systems do not provide the necessary reaction torque when the torque converter lockup clutch is in the engaged state. Moreover, these systems do not restrict torque converter lockup in the event of a slipping shift clutch. 
   For these reasons, an improved torque converter lockup control system and strategy is needed to aid in reducing or eliminating shift clutch slip in an automatic transmission. 
   SUMMARY 
   According to one aspect, a torque converter lockup clutch system is provided for an associated automatic transmission. More particularly, in accordance with this aspect, the system includes a torque converter having an impeller operatively coupled to an associated internal combustion engine and a turbine operatively coupled to an input shaft of the associated automatic transmission. A lockup clutch is disposed within the torque converter for selectively locking the impeller to the turbine. The lockup clutch includes an engaged state and a disengaged state. A pressure regulator is operatively coupled to the torque converter for adjusting the pressure of a working fluid to at least one shift clutch of the associated automatic transmission in response to a speed differential between the impeller and turbine of the torque converter. A shift clutch slip detector is provided for detecting a slip condition of the at least one shift clutch of the associated automatic transmission and outputs a slip value. A processing component is provided for receiving the slip signal and prohibiting the lockup clutch from entering into the engaged state when the slip value exceeds a predetermined shift clutch slip threshold. 
   According to another aspect, a method for controlling a torque converter lockup clutch is provided for eliminating or reducing shift clutch slip in an automatic transmission. More particularly, in accordance with this aspect, the method includes determining if at least one shift clutch of the automatic transmission is slipping and outputting a corresponding slip value. The method also includes disengaging the torque converter lockup clutch if the lockup clutch is in an engaged state and the slip value exceeds a permitted shift clutch slip threshold. In addition, the method includes increasing a differential speed between an input portion and an output portion of the torque converter. Furthermore, the method includes generating a boost in hydraulic shift clutch pressure, via the increase in differential speed within the torque converter, for reducing slip in the at least one shift clutch. 
   According to yet another aspect, a torque converter lockup clutch system for an associated automatic transmission is provided. The system includes a torque converter including an impeller operatively coupled to an associated internal combustion engine, a turbine operatively coupled to an input shaft of the associated automatic transmission, and a stator disposed in a working fluid pathway between the impeller and the turbine. A lockup clutch is disposed within the torque converter for selectively locking the impeller to the turbine, the lockup clutch having an engaged state and a disengaged state. A reaction arm is attached to the stator of the torque converter for transmitting a reaction torque developed by the working fluid of the torque converter. A pressure regulator is operatively coupled to the reaction arm for adjusting the pressure of the working fluid to at least one shift clutch of the associated automatic transmission. A shift clutch slip detector is provided for detecting a slip condition of the at least one shift clutch, the slip detector outputting a slip signal. An engine control unit (ECU) is provided for receiving the slip signal and prohibiting the lockup clutch from entering into the engaged state, when the slip signal exceeds a predetermined shift clutch slip threshold. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of an engine and driveline, including a torque converter and automatic transmission, for a vehicle having an improved torque converter lockup clutch system. 
       FIG. 2  is an enlarged, more detailed schematic view of the torque converter and automatic transmission of the improved torque converter lockup clutch system of  FIG. 1 . 
       FIG. 3  is an end elevational view, partially in cross section, of the torque converter and the automatic transmission showing an interface between a stator reaction arm of the torque converter and a pressure regulator of the automatic transmission. 
       FIG. 4  is a block diagram illustrating a torque converter lockup clutch control method for permitting or prohibiting lockup clutch engagement based on shift clutch slip. 
       FIG. 5  is a timing chart illustrating a torque converter lockup clutch solenoid signal and shift clutch pressure in response to a slipping shift clutch. 
   

   DETAILED DESCRIPTION 
   Referring now to the drawings wherein the showings are for purposes of illustrating one or more exemplary embodiments,  FIG. 1  is a schematic view of a torque converter lockup clutch system  100  for a vehicle. Generally, the system  100  includes an internal combustion engine (ICE)  102  that is coupled to an automatic transmission  104  having a torque converter  106  and a gearbox  108 . The transmission  104  is then coupled to a driveline, differential, or series of shafts  110  which are then eventually coupled to one or more wheels  112  of a vehicle. In addition, an electronic processing unit (EPU)  114  is also used to monitor various parameters and to execute certain operations based on the operating conditions of the vehicle. As is known and understood by those skilled in the art, the EPU  114  can be a stand alone processing unit or may be combined with some other processing unit of the vehicle, such as an electronic control unit (ECU) of the engine  102 , for example. 
   With reference to  FIG. 2 , an enlarged, more detailed schematic view of the torque converter  106  and gearbox  108  of the automatic transmission  104  is shown. The torque converter  106  includes an impeller  116  which is directly coupled to and driven by the engine. As shown in  FIG. 2 , the impeller  116  also forms a portion of the outer housing of the torque converter  106 . The torque converter  106  further includes a turbine  118  that is typically fixed to an input shaft  130   a  of the gearbox  108 . The turbine  118  and the impeller  116  are hydraulically coupled via a working fluid, such as hydraulic grade oil. In addition, a stator  120  is disposed within a fluid flow pathway  122  of the working fluid between the impeller  116  and the turbine  118 . During operation, the fluid flow pathway  122  within the torque converter  106  resembles that of a toroid. Hydraulic fluid is drawn into the impeller  116  from the stator  120  where the fluid is accelerated and urged into the turbine  118 . In response, the turbine produces a torque which is then transferred to the input shaft  130   a . Due to this hydraulic coupling, the engine  102  is able to transmit substantial amounts of energy from an output or driveplate of the engine to the input  130   a  of the gearbox  108 . 
   With continued reference to  FIG. 2 , under certain circumstances it is preferable to directly or mechanically couple (as opposed to hydraulically couple) the output of the engine to the input of the gearbox  108  in order to improve efficiency and avoid fluid flow energy losses. This is accomplished through the use of a lockup clutch  124  that may be provided within the torque converter  106 . The lockup clutch  124  can be secured to the same housing or hub to which the turbine  118  is connected. Typically, the lockup clutch  124  includes an engaged state and a disengaged state. As shown in  FIG. 2 , the lockup clutch  124  is in the disengaged state and the turbine  118  is able to rotate somewhat independently of the impeller  116 . When the lockup clutch  124  is in the engaged state, a clutch surface of the lockup clutch  124  contacts an engaging portion  125  that is rigidly attached to or forms part of the impeller  116 . As such, the turbine and the impeller can rotate as an integrated unit. To move the lockup clutch  124  between the disengaged state and the engaged state, one or more of a hydraulic pressure, a biasing element, or an electromagnetic means may be used as is known and understood by those skilled in the art. 
   During operation, the impeller  116  is operating or rotating at the same speed as the engine, whereas the turbine  118  is operating at a speed consistent with a multiple of the wheel speed of the vehicle. When the differential speed between the impeller and turbine is great, a working fluid slip factor of the torque converter is high which results in a large reaction torque being developed by the turbine and the stator. However, when the differential speed between the impeller and turbine is low, the fluid slip factor is low and the reaction torque being generated by the turbine and the stator fluid flow is also low. With reference to  FIGS. 2 and 3 , the reaction torque produced from the impinging fluid flow on the stator  120  is transmitted via an annular stator shaft  126 , into the gearbox  108 , to a stator reaction arm  128 . The stator reaction arm  128  is then coupled to a pressure regulator  130  which adjusts the hydraulic pressure supplied to a plurality of shift clutches  132  relative to the reaction torque produced by the stator  120 . 
   With continued reference to  FIG. 2 , each shift clutch  132  is associated with a certain speed or gear ratio for the transmission  104 . For example, a first speed clutch  132   a  may selectively couple (via a first speed gear train  134   a ) the input shaft  130   a  to an intermediate shaft  130   b  thereby transmitting power to an output shaft  130   c  of the gearbox  108  or transmission  104 . As is commonly known and understood, the first or starting speed ratio of the transmission  104  (i.e. first gear or low) provides the greatest amount of torque to the drive line of the vehicle. Similarly, a second shift clutch  132   b  mechanically couples the input  130   a  of the gearbox  108  to the output  130   c  of the gearbox  108  via a second gear train  134   b  having a lower gear ratio than the first gear train  134   a  (otherwise known as second gear). Furthermore, a third speed shift clutch  132   c  may be provided for mechanically coupling the input  130   a  to the output  130   c  in both a forward direction and a reverse direction. In the forward direction, a third speed gear train  134   c  (having a lower gear ratio than the second gear train  134   b ) is used, whereas in the reverse direction, a reverse gear train  134   d  (having a reversing idler gear and a gear ratio similar to that of the first gear train  134   a ) is used. In order to select between the forward and the reverse directions, a slideably engaged coupler  137  may be used to selectively engage the third speed gear train  134   c  or the reverse gear train  134   d  to the intermediate output shaft  130   b . The coupler  137  may be urged between the third speed and the reverse positions via a mechanical linkage, electrical solenoid, and or a hydraulic piston as is known and understood by those skilled in the art. Though the illustrated transmission  104  is shown as a three-speed transmission with reverse, it is to be understood and appreciated by those skilled in the art that the transmission  104  could have fewer or more gears and need not be configured exactly as shown. 
   In addition, each of the plurality of shift clutches  132  includes a set of driving plates  135  and a set of driven plates  136 . The driving plates  135  serve as the input to the shift clutch and are secured to the input shaft  130   a  of the gearbox. Conversely, the driven plates  136  each serve as the output of the corresponding shift clutch and is secured to an output shaft or hub that only rotates when the driving plate engages or clamps onto the corresponding driven plate. The driving plates  135  and driven plates  136  will engage when supplied with adequate hydraulic fluid pressure as will be described in more detail below. Generally, the overall torque transfer ability of the clutch is limited by (1) the physical properties of the components and materials (e.g., yield strength, tensile strength, coefficient of friction, etc.) (2) the geometry of the clutch (size and or number of plates) and (3) the clamping force developed between the plates as a result of the actuating hydraulic fluid pressure. 
   With continued reference to  FIG. 2 , the shift clutches  132  may be individually actuated by a shift valve  138 . The shift valve  138  may be a hydraulic spool valve or other solenoid valve capable of diverting a source of pressurized hydraulic fluid to the appropriate shift clutch. As shown in  FIG. 2 , the shift valve  138  includes a first output port  138   a , a second output port  138   b , a third output port  138   c , and an input port  138   d . Each of the input ports  138   a ,  138   b ,  138   c  is connected to its respective shift clutch  132   a ,  132   b ,  132   c  via a series of hydraulic channels or tubing. A shift valve control line  139  is also provided that electromechanically and or hydraulically controls the shift valve  138  and the respective engagement of the shift clutches  132 . In addition, a hydraulic pump  140  generates a source of pressurized hydraulic fluid for the system. The pump  140  draws fluid from a reservoir  142  and delivers it to one or more system regulators  144  where various system pressures are set for the automatic transmission  104 . For example, the system regulator  144  may set the input pressure for the shift clutch pressure regulator  130  along the order of 200 psi (13.8 bar). In addition, the system regulator  144  may set the control pressure for the shift valve  138  or other clutch control devices (e.g, linear solenoids, spool valves, torque converter lockup solenoid, etc.) along the order of 85 psi (5.86 bar). The shift clutch pressure regulator  130  may then further reduce the input pressure provided by the system regulator  144  to a nominal shift clutch pressure on the order of 150 psi (10.3 bar). During periods of high torque transfer the output (or boost) pressure of the shift clutch regulator may even exceed 200 psi (13.8 bar). The output of the shift clutch pressure regulator  130  is then delivered to the shift clutches  132  via the shift clutch valve  138 . 
   Similar to the engagement of the shift clutches  132 , the lockup clutch  124  of the torque converter  106  may also be hydraulically actuated using the system pressure generated by the pump  140  and regulated by the system regulator  144 . The hydraulic fluid pressure necessary to engage the lockup clutch  124  may be delivered by a through bore along the input shaft  130   a  to a lockup clutch cavity  145 . To control the flow of the hydraulic or working fluid, a torque converter lockup clutch (TCLC) valve  146  is connected to the system pressure and may be hydraulically or electronically actuated via a control line  148 . By way of example, the control signal used to actuate the TCLC valve  146  may consist of a 5 v or 12 v DC signal. The DC signal is then used to energize a solenoid within the valve  146  to allow hydraulic fluid to travel into the input shaft  130   a  and into the torque converter  106 . The TCLC valve  146  may also be configured to prohibit fluid flow into the torque converter lockup clutch cavity when the control signal is at DC ground or 0 volts. When deactivated, the TCLC valve  146  would thus allow the pressure within the cavity to bleed down causing the lockup clutch  124  to disengage. It should be noted that the TCLC could operate in a variety of manners. For example, rather than using hydraulic fluid pressure to maintain the TCLC in the engaged state, the hydraulic fluid pressure could be used to maintain the TCLC in the disengaged state. 
   Now with reference to  FIG. 3 , a rear view of the torque converter  106  and engine  102  are shown in broken view through a vertical cross section of a front wall of the transmission housing  150 . As illustrated, a series of impeller blades  152  (shown in dashed lines) can be formed as part of the housing of the torque converter  106 . Also, a series of turbine blades  154  are shown disposed about the turbine  118 . As discussed previously, the stator  120  may be disposed between the impeller  116  and the turbine  118  for redirecting the flow into the impeller  116 . The reaction torque developed in the stator as a result of the impinging fluid flow (which is greater during periods of high fluid slip) is eventually transmitted to the stator arm  128 . The stator arm  128  contacts or is otherwise coupled to an adjusting member  128   a  of the shift clutch pressure regulator  130 . By way of example, the adjusting member can be a small diameter piston that includes a throttling portion. By displacing the piston or adjusting member  128   a , the regulator  130  increases or decreases the pressure to the clutch shift valve  138 . Thus, during periods of high fluid slip in the torque converter (and when high levels of torque are being transmitted to the shift clutches via the input shaft), the shift clutch pressure is also increased to prevent shift clutch slip. 
   With reference once again to  FIG. 2 , the gearbox  108  of the automatic transmission  104  may be outfitted with one or more sensors for determining an input speed and an output speed of the shift clutches  132 . For example, a first speed sensor  156  may be mounted proximal to the input shaft  130   a  of the gear box  108  for determining the input speed to the shift clutches  132 . A gear or other notched hub  158  may be provided allowing the sensor  156  to detect the individual notches of the hub  158  as they rotate past the sensor  156  to produce an electric pulse train. Knowing the spacing between the notches and maintaining a count of the notches over a given time can produce a reliable and accurate method of determining the speed of a shaft. Thus, a speed can be calculated for the driving plates  135  of the shift clutches  132 . Similarly, a second speed sensor  160  may be mounted proximal to an output shaft, such as the intermediate shaft  130   b  or the output shaft  130   c . As shown in  FIG. 2 , the second speed sensor  160  is mounted proximal to a notched hub  162  for providing a pulse train corresponding to the rotational speed of the intermediate output shaft  130   c . Knowing the gear ratios for each of the gear trains, the rotational speed of each driven plate  136  can be calculated based on the rotational speed of the intermediate or output shafts  130   b ,  130   c . Of course, a multitude of other techniques may be used to acquire or monitor the differential speed across the shift clutches. For example a magnetic pickup or interference device such as a Hall effect sensor may be used for producing the pulse train. Also, one or more inductive generating means may be used to provide an analog signal corresponding to the respective speeds of the input and output shafts. 
   Now with reference to  FIG. 4 , a method is illustrated for permitting or prohibiting torque converter lockup as a function of shift clutch slip. Generally, the first step (s 100 ) in the method involves determining which of the shift clutches  132   a ,  132   b ,  132   c  is active. In other words, the active shift clutch (ASC) is the shift clutch which is currently engaged or coupling the input shaft  130   a  to the output shaft  130   c  of the transmission  104 . The step of determining the ASC (s 100 ) can be accomplished by polling the shift valve control line  139  to determine the position of the shift valve  138  or by directly polling the controller or processing unit responsible for switching the shift valve  138 . In addition, the ASC could be determined by a series of calculations based on such inputs as throttle position, vehicle speed, road incline, etc. The next step (s 102 ) of the method includes obtaining the input and output speeds of the ASC. One way of acquiring the input speed of the ASC involves reading a speed signal from a first speed sensor (e.g., sensor  156 ) located proximal to the input shaft  130   a  (or the driving plates) of the shift clutches. To obtain the output speed of the ASC, a second speed sensor (e.g., sensor  160 ) can be mounted proximal to the driven plates of the ASC or to a shaft that is positively coupled to the driven plates of the ASC. Afterwards, the signals from the first and second speed sensors can be converted or manipulated in order to render a meaningful and relative speed value. The next step (s 104 ) involves calculating an ASC speed ratio (or gear ratio) based upon the acquired input and output speeds of the ASC. At the same time, a predetermined ASC speed ratio (or gear ratio) is retrieved by an electronic processing unit (e.g. an engine control unit (ECU)), such as EPU  114 . Since the gear ratios for all speeds of the transmission are factory set, these values can then be stored in a read only memory (ROM) bank of the EPU  114 . When appropriate, the predetermined gear ratio is then recalled from ROM for the gear train corresponding to the ASC. 
   Next, the method involves calculating an ASC slip value based on the calculated ASC speed ratio and the predetermined ASC speed ratio (s 106 ). Ideally, the calculated ASC speed ratio and the predetermined ASC speed ratio would always be identical. If a discrepancy exists then this implies that a shift clutch is slipping or that one of the gears in the associated gear train is defective. Of course, some slip or inaccuracy in the calculated ASC speed ratio will almost always be present and this unavoidable slip or inaccuracy would generally define a lower limit of an allowable ASC slip threshold. If the ASC slip value does not exceed the allowable ASC slip threshold (s 108 ), then the electronic processing unit, such as EPU  114 , will permit the torque converter lockup clutch  124  to be engaged under the typical operating conditions of the transmission  104  (s 110 ). If, however, the ASC slip value is above an allowable ASC slip threshold (s 108 ), then the electronic processing unit  114  determines if the torque converter lockup clutch  124  is engaged and may do so by monitoring the torque converter lockup clutch control line  148 . If the torque converter lockup clutch (TCLC) is engaged (s 112 ), then the electronic processing unit  114  will disengage the TCLC (s 114 ) and thereafter prohibit torque converter lockup (s 116 ). On the other hand, if the TCLC is not engaged (s 112 ), then the electronic processing unit directly prohibits torque converter lockup (s 116 ). In this manner, the method continuously operates in a loop type fashion permitting or prohibiting/disengaging torque converter lockup based primarily on if an ASC slip value has exceeded an allowable ASC slip threshold. 
   With reference to  FIG. 5 , a timing chart having three independent graphs illustrates one example of the method set forth above. A first graph (A) depicts shift clutch pressure present at the ASC, a second graph (B) depicts the state of the TCLC based on solenoid signal line voltage, and a third graph (C) depicts the relationship of calculated ASC speed ratio to the predetermined ASC speed ratio. The x axis generally represents time beginning at t=0. At t=0, it is assumed that the transmission is in a steady state, with the vehicle traveling at a steady speed, the torque converter lockup clutch (TCLC) engaged, and the ASC not slipping. With reference to graph (C) at t SLIP , calculated ASC speed ratio values begin to depart from the linear predetermined speed ratio values. This is most likely an indication that the ASC has started to slip. The departure between calculated and predetermined ASC speed ratio continues to grow until a maximum is reached at t TCLC DISENGAGED . At this point, the allowable ASC slip value has been exceeded and the electronic processing unit has disengaged the torque converter lockup clutch. Graph (B) illustrates that the TCLC solenoid control signal goes from a level of 5 volts just prior to t TCLC DISENGAGED  to 0 volts or DC ground at a point just after t TCLC DISENGAGED . This indicates that the electronic processing unit has switched off the valve or solenoid responsible for directing the hydraulic pressure and activating the torque converter lockup clutch into the engaged state. 
   As shown in graph (A) of  FIG. 5 , when the TCLC is disengaged, a rise in ASC pressure occurs since the impeller and turbine are now rotating at a larger differential speed (i.e. higher fluid slip) and generating a reaction torque at the stator and the stator reaction arm. As discussed previously, this causes the pressure regulator to boost the ASC pressure from a nominal 150 psi (10.3 bar) to a higher level of approximately 200 psi (13.8 bar). Referring once again to graph (C) and consistent with this rise in ASC pressure, the calculated ASC speed ratio gradually begins to return to the linear predetermined ASC speed ratio. This indicates that ASC slip is gradually reduced until eliminated because of the increase in ASC pressure. 
   It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.