Abstract:
A method for determining the return spring pressure of a clutch in a vehicle transmission includes using proportion-integral-derivative (PID) control logic of a controller to introduce a calibrated error into a pressure command of a holding clutch during a coast-down maneuver of the vehicle. The calibrated error causes a slip level to occur across the holding clutch. The method includes maintaining the slip level using the PID control logic and separately commanding engine torque at different threshold low values during the coast-down maneuver. The pressure/input torque relationship for the holding clutch is recorded for both threshold low levels as the controller continues to maintain the slip. The return spring pressure is calculated using the recorded pressure/input torque relationship. The holding clutch may be controlled a subsequent shift maneuver using the learned return spring pressure. A vehicle is also disclosed having a controller configured to execute steps of the method.

Description:
TECHNICAL FIELD 
     The present disclosure relates to a method and system for learning the return spring pressure of a clutch in an automatic transmission using proportional-integral-derivative (PID) control. 
     BACKGROUND 
     An automatic transmission generally includes a number of gear elements and clutches that couple a transmission input shaft with a transmission output shaft. The various clutches are selectively engaged to establish a desired speed ratio. Clutch engagement is typically achieved by moving a clutch piston from an initial position into engagement with a friction clutch pack. Shifting from one speed ratio to another is performed automatically by a transmission controller which applies a clutch associated with the current speed ratio, i.e., the off-going clutch, and releases a clutch associated with a desired new speed ratio, i.e., the on-coming clutch. Upon release, a clutch return spring gently returns the apply piston to its initial position. 
     SUMMARY 
     A method is disclosed herein for accurately learning a return spring pressure of a clutch used in a vehicle transmission. Knowledge of the return spring pressure is a useful clutch control variable that can be relatively difficult to determine imperceptibly to a driver of the vehicle. The present method is executed during a coast-down maneuver to minimize driver disturbances, for instance when the vehicle is traveling on an extended downgrade and input torque to the transmission reaches a threshold level and remains sufficiently stable. 
     Under low engine torque conditions, a pressure command delivered to a given torque holding clutch in the transmission is held to a relatively low feed-forward critical capacity. At the same time, a controller having proportional-integral-derivative (PID) control logic introduces a small error in the pressure command to another holding clutch via the PID control logic. The controller maintains this slip. A controlled flare in turbine speed ensures and is held steady. 
     Once the PID response to the error is stabilized, e.g., within a calibrated +/− pressure window over a calibrated duration, the controller records a clutch input pressure/input torque relationship for the particular holding clutch whose pressure command has temporarily dropped due to the introduced error. The controller continues to hold the controlled slip during the flare. Thereafter, input torque from the engine may be adjusted downward, e.g., by requesting increased spark retardation from an engine control unit, and the entire sequence may be repeated at this lower level. Using the two recorded clutch input pressure/torque points, the controller can then calculate the return spring pressure by extrapolation or using other means, thereafter updating any required gain values used in determining clutch pressure for subsequent shift events. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a vehicle having an automatic transmission and a controller which learns a clutch return spring pressure during a coast-down maneuver as set forth herein. 
         FIG. 2  is a lever diagram for an example transmission whose clutch return spring pressures may be evaluated according to the present approach. 
         FIG. 3  is a lever diagram for an alternative example transmission to the transmission shown in  FIG. 2 . 
         FIG. 4  is a flow chart describing an example method for learning a clutch return spring pressure during a coast-down maneuver. 
         FIG. 5  is a time plot of clutch pressure, turbine speed, and a PID control signal. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, wherein like reference numbers correspond to like or similar components throughout the several figures, and beginning with  FIG. 1 , a vehicle  10  includes a controller  26 . The controller  26  is configured, via execution of the present method  100 , to learn a return spring pressure of a clutch during a coast-down maneuver as set forth herein with reference to  FIGS. 4 and 5 . Any of the clutches used aboard the vehicle  10  may be evaluated using the present method  100  provided the clutch holds torque during the coast-down maneuver, i.e., acts as a holding clutch. 
     The vehicle  10  includes an internal combustion engine  12  that is coupled to an automatic transmission  14  via a hydrodynamic torque converter  16 . The engine  12  delivers engine torque (arrow T E ) via an engine output shaft  13  which rotates at engine speed (arrow N E ). The transmission  14  includes a transmission input shaft  15  which rotates at an input speed (arrow N T ). Transfer of input torque (arrow T I ) to the transmission  14  occurs through the torque converter  16 , as is understood in the art and described below. 
     The transmission  14  also has an output shaft  18  that ultimately conveys a transmission output torque (arrow T O ) transmitted from various clutch and gear sets  17  of the transmission  14 . The transmission output torque (arrow T O ) is ultimately delivered to a set of drive wheels  24 . The clutch and gear sets  17  can be selectively actuated via electro-hydraulic controls (not shown) powered by fluid that is delivered under pressure from a fluid pump  33 . The pump  33  is configured to draw fluid  37  from a transmission sump  35 . 
     The transmission  14  of  FIG. 1  may be configured as any multi-speed transmission, e.g., a 6-speed or an 8-speed transmission, possible embodiments for which are described herein with reference to  FIGS. 2 and 3 . Thus, the clutches of the clutch and gear sets  17  may be selectively engaged and disengaged as needed to establish the desired speed ratios. In steady state, at least one of the clutches of the clutch and gear sets  17  holds input torque and is therefore referred to herein as a holding clutch. 
     The controller  26  selectively executes the present method  100 , e.g., by executing computer code or instructions recorded on tangible, non-transitory memory  95 , during a coast-down maneuver of the vehicle  10  to thereby learn the return spring pressure of the various holding clutches in the clutch and gear sets  17 . An example embodiment of method  100  is disclosed below with reference to  FIG. 4 . Example holding clutches are described with reference to  FIGS. 2 and 3 . 
     The controller  26  may be be configured as a microprocessor-based device having such common elements as a microprocessor or CPU, and/or read only memory (ROM), random access memory (RAM), electrically-programmable read-only memory (EPROM), etc., some of which may be designated as the memory  95  noted above. The controller  26  also includes logic circuitry including but not limited to proportional-integral-derivative (PID) control logic  90 , a high-speed clock (not shown), analog-to-digital (A/D) circuitry, digital-to-analog (D/A) circuitry, a digital signal processor or DSP, and the necessary input/output (I/O) devices and other signal conditioning and/or buffer circuitry. 
     As is understood in the art, PID refers to a control loop feedback mechanism and associated logic which uses three terms, i.e., the proportion (P), integral (I), and derivative (D) terms, with each representing the respective present, past, and future error values. The logic generating the I term is referred to as the integrator herein, with injection or introduction of a PID error via the integrator being a step in the present method  100 . The present controller  26  thus uses PID logic to calculate an error value in a given process variable as a difference between a measured value and a desired or calibrated value, and controls the process inputs as a function of the three control terms. 
     An engine control unit (ECU)  29  could also be used either as a separate device as shown or integrated with the controller  26 . If separate, the controller  26  is in communication with the ECU  29  as indicated by double-headed arrow  21 . The controller  26  may request a specific level of managed engine torque (arrow  11 ) from the ECU  29  during the coast-down maneuver as part of the execution of the present method  100 , with the ECU  29  responding via any suitable means of reducing engine torque (arrow T E ), e.g., spark retarding or the like. 
     The torque converter  16  shown in  FIG. 1  has a stator  30  positioned between a pump  32  and a turbine  34 . A torque converter clutch  31  may also be used to selectively lock the pump  32  to the turbine  34  above a threshold lockup speed, as will be understood by those of ordinary skill in the art. The pump  32  may be coupled to the output shaft  13  to thereby rotate at engine speed (arrow N E ). Within the torque converter  16 , the turbine  34  is driven by fluid  37 , with the turbine  34  in turn connected to the input shaft  15  of the transmission  14 . Thus, rotation of the turbine  34  ultimately rotates the input shaft  15  at a turbine speed (arrow N T ) that is less than or equal to the engine speed (arrow N E ), with viscous drag or friction losses within the transmission  14  tending to reduce the turbine speed (arrow N T ) to a level somewhat less than engine speed (arrow N E ), as will be readily understood by those of ordinary skill in the art. 
     Referring to  FIG. 2 , in a non-limiting example embodiment the transmission  14  of  FIG. 1  may be configured as an 8-speed transmission having a plurality of gear sets and clutches, i.e., the clutches and gears  17  of  FIG. 1 . In particular, the transmission  14  may include a braking clutch CB1278R, i.e., clutch  36 . The nomenclature CB1278R represents that this particular device is a braking clutch (CB), and is engaged in each of 1 st , 2 nd , 7 th , 8 th , and reverse (R) gears. The transmission  14  also includes another braking clutch CB12345R, or clutch  41 , which selectively connects an element of a first gear set  40  to a stationary member  28  when engaged. Clutches  36  and  41  are connected to respective nodes  42  and  46  of first gear set  40 . In one embodiment, node  42  can be a sun gear (S 4 ) of the gear set  40 , while node  46  may be a ring gear (R 4 ) of the same gear set. Gear set  40  also includes a node  44 , which may be a carrier member (PC 4 ) in the embodiment shown. 
     Node  42  is also connected to a node  52  of a second gear set  50 . Node  54  of gear set  50  is connected to an input side of a rotating clutch C13567, i.e., clutch  38 , as is the transmission input shaft  15  with input torque (arrow T I ). Node  56  is connected to a third gear set  60  as explained below. In one embodiment, gear set  50  may be a planetary gear set wherein nodes  52 ,  54 , and  56  are a sun gear (S 1 ), a carrier member (PC 1 ), and a ring gear (R 1 ), respectively. 
     The third gear set  60  includes nodes  62 ,  64 , and  66 , which in one embodiment may be ring gear (R 2 ), carrier member (PC 2 ), and sun gear (S 2 ), respectively. A rotating clutch C23468, i.e., clutch  58 , may be connected between the output of clutch  38  and node  66 , and between node  56  of gear set  50  and node  66  of gear set  60 . Node  62  may be connected to a fourth gear set  70  having nodes  72 ,  74 , and  76 . Nodes  72 ,  74 , and  76  may be a sun gear (S 3 ), carrier member (PC 3 ) and ring gear (R 3 ), respectively. In particular, node  62  may be connected to node  72  via a rotating clutch C45678R, i.e., clutch  48 . Node  64  of gear set  60  may be directly connected to node  74  of gear set  70 , which in turn may be connected to the transmission output shaft  18  (also see  FIG. 1 ). Nodes  76  and  44  and nodes  74  and  64  may be continuously connected via a respective interconnecting member  45  and  47 . 
     Referring to  FIG. 3 , the transmission  14  of  FIG. 2  may be alternatively embodied as a transmission  114  having a 6-speed configuration. In this embodiment, the transmission input shaft  15  may be connected to a first gear set  140  having nodes  142 ,  144 , and  146 , which may be embodied as a ring gear (R 3 ), carrier member (PC 3 ), and sun gear (S 3 ) as shown. The input shaft  15  may be directly connected to node  142 , and to a clutch C456, i.e., clutch  51 . Node  144  is connected to a clutch C1234, i.e., the clutch  138 , and to an input side of a rotating clutch C35R, i.e., clutch  53 . Node  146  is grounded to the stationary member  28 . 
     A second gear set  150  includes nodes  152 ,  154 ,  156 , and  158 , which may be embodied as a sun gear (S 1 ), ring gear (R 1 ), carrier gear (PC 1 ), and another sun gear (S 2 ), respectively. A braking clutch CB26, i.e., clutch  43 , may selectively connect node  158  to the stationary member  28 . Node  154  is directly connected to the transmission output shaft  18 . Node  156  is connected to a braking clutch CBR 1 , i.e., clutch  136 , which is also connected to a stationary member  28 . 
     Depending on the operating gear, the identity of the specific holding clutches will vary. The present method  100  may be used to learn the return spring pressure of a holding clutch. For instance, clutches  48  and  58  of  FIG. 2  may act as holding clutches while coasting in 4 th  gear. As all clutches in the transmission  14  apply via fluid pressure and release via a return spring, as is known in the art, knowledge of the return spring pressure may be used by the controller  26  to fine tune the overall control of that particular clutch. 
     Referring to  FIG. 4 , the present method  100  for learning the return spring of a clutch in a transmission such as the transmission  14  of  FIG. 2  or the transmission  114  of  FIG. 3  commences at step  102 . In this initial step, the controller  26  of  FIG. 1  determines the engine torque (arrow T E ) from the engine  12 , e.g., via communication with the ECU  29 , and determines whether this input torque is sufficiently stable or unchanging. Step  102  entails determining whether the vehicle  10  of  FIG. 1  is traveling on an extended downgrade of a sufficient length for conducting the subsequent control steps. Step  102  may include initiating a timer of the controller  26  and counting through a calibrated duration to determine if the grade is merely transient or is in fact sustained. In an example embodiment, a stable engine input torque (arrow T E ) may be present at about levels of about 20 Nm+/−5 Nm. 
     At step  104 , the controller  26  of  FIG. 1  requests management of the engine torque (arrow T E ) at a first level, for instance by requesting active torque management from the ECU  29  which is then accomplished via spark retardation or other means. The first level may be a low threshold torque, e.g., about 25 Nm to about 15 Nm in one possible embodiment. The ECU  29  thereafter locks engine torque (arrow T E ) at this requested level. 
     At step  106 , the controller  26  of  FIG. 1  decreases the pressure command to both holding clutches in an embodiment in which only two clutches of the transmission  14  hold torque during the coast-down maneuver, from a level of their calibrated maximum pressure. As is understood in the art, a clutch pressure command may be determined as a function of transmission input torque (T I ) and a calibrated gain K, i.e., P=ƒ(T IN ·K). The gain K can be modified over time as a result of the method  100  as noted below. Step  106  entails leaving one of the two holding clutches at a slightly higher pressure than the other, e.g., 5 to 10 kPA higher in an example embodiment. 
     Referring to  FIG. 5  in conjunction with  FIG. 4 , a set of curves  80  may be used to demonstrate the present approach. The first holding clutch may have a pressure (trace  82 ) that is initially at a pressure level of P 1 , and dropping at step  106  at about t 0  to a lower pressure level of P 1N  corresponding to that clutch&#39;s critical feed-forward pressure. The second holding clutch, with corresponding pressure trace  84 , is dropped to a slightly lower level as noted above. 
     At step  108  of  FIG. 4 , the controller  26  of  FIG. 1  then introduces a calibrated error  85  via a PID command (trace  88 ) in the integrator or I term at approximately t 1 . This calibrated error  85  causes a slip to occur across the second holding clutch, and thus a small flare  87  to occur in turbine speed (trace  86 ), e.g., approximately 20 RPM to 30 RPM above the baseline level of the turbine speed before the flare  87  is introduced. The clutch pressure changes in response to the error  85 , as indicated by arrow  83  in trace  84  of  FIG. 5 . The pressure (trace  84 ) should be given a calibrated amount of time to stabilize to within an allowable pressure window and within a calibrated duration. The error  85  should also be large enough to force a corrective action to occur in response to the error via the PID logic  90  of the controller  26 , but yet small enough that the flare  52  is not perceptible to a driver of the vehicle  10  of  FIG. 1 . The particular value of the error  85  may be expected to vary with the design of the transmission  14 . The controller  26  thereafter maintains the flare  52  by maintaining the slip across the second holding clutch. 
     At step  110 , after the clutch pressure (trace  84 ) is sufficiently stable, the controller  26  records the relationship between the pressure (trace  84 ) to the second holding clutch after the error  85  is introduced at about t I , and the input torque to the holding clutch. Step  112  may entail recording the pressure command and input torque as corresponding values in a lookup table. 
     At step  112  the controller  26  may request active torque management from the ECU  29  at a lower level than that requested at step  104 , e.g., about 15 Nm to about 5 Nm. The ECU  29  thereafter locks the input torque from the engine  12  at this lower level. 
     At step  114 , the controller  26  of  FIG. 1  again records the relationship between the pressure command on the second holding clutch and the input torque to that clutch, this time as a second data point. As with step  110 , step  114  may entail recording the pressure command and input torque as corresponding values in a lookup table. 
     At step  116 , the controller  26  uses the two recorded data points to extrapolate the relationship between the pressure command on the second holding clutch and the input torque at 0 Nm of input torque, i.e., the return spring pressure. That is, knowing the relationship at, e.g., 15 Nm and 5 Nm, the controller  26  can estimate the relationship at 0 Nm. 
     At step  118 , the controller  26  determines whether all holding clutches have been evaluated during this particular maneuver. If so, the method  100  is finished. Otherwise, the method  100  proceeds to step  120 . 
     At step  120  the controller  26  may increase the pressure on the second holding clutch and decrease the pressure on the first holding clutch, i.e., swap traces  82  and  84  of  FIG. 5 . Thereafter, the controller  26  can repeat steps  108 - 116  for the first holding clutch to learn the return spring pressure of the first holding clutch. In all embodiments, the above method  100  proceeds only so long as the vehicle  10  of  FIG. 1  remains in the coast-down maneuver. That is, step  102  may operate in a continuous loop evaluating whether steady state coasting conditions remain present, and can smoothly exit the method  100  when, for instance, a driver requests increased engine torque and thereby exits the coast-down maneuver. If the present method  100  has not finished executing at that point the controller  26  may disregard the incomplete results and start anew with the new coast-down maneuver. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.