Abstract:
A method for controlling a transmission includes applying a reference stroke pressure to an oncoming control element while executing a downshift to a target gear, determining a stroke pressure adjustment in response to a turbine speed flare during the downshift, and re-executing the downshift while applying to the oncoming element an adapted stroke pressure that is a sum of the stroke pressure adjustment and the reference stroke pressure.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to automatic transmissions for automotive vehicles, in particular to gear ratio changes produced in such transmissions by control elements such as friction clutches and brakes. 
     2. Description of the Prior Art 
     It is difficult to calibrate excellent shift quality under all circumstances for coasting or power-off downshifts in synchronous automatic transmissions. Such calibrations are especially difficult under low-speed conditions where vehicle noise levels are low and gear ratio steps are largest. Coasting downshifts present the largest inhibitor to a fully synchronous automatic transmission design. 
     Control strategies require calibration flexibility to ensure consistent, smooth downshifts under all operating conditions, but conventional gear shift control strategies are insufficient to meet the current requirements for coasting shift quality and cost. 
     Incorrect clutch stroke pressure can result in shifts that tie-up or have excessive flare. Such shifts are very noticeable to the vehicle operator. Shifts that repeatedly tie-up or flare can also result in damage to the control elements. 
     Conventional stroke pressure adaptation methods rely on analyzing the behavior of the clutch controlling the ratio change of an automatic transmission. They adjust the stroke pressure based on the amount of time it takes to reach a predetermined percent of shift completion. Such methods work in most cases for the measured events, but they sometimes lead to incorrect adaptation when other events use the same information. 
     A need exists in the industry for a technique that evaluates a shift event, and adapts pressure applied to the non-controlling element, which previously had no evaluation for stroke pressure and could therefore not adapt out of a bad shift. 
     SUMMARY OF THE INVENTION 
     A method for controlling a transmission includes applying a reference stroke pressure to an oncoming control element while executing a downshift to a target gear, determining a stroke pressure adjustment in response to a turbine speed flare during the downshift, and re-executing the downshift while applying to the oncoming element an adapted stroke pressure that is a sum of the stroke pressure adjustment and the reference stroke pressure. 
     The method may further include determining a maximum delta between a maximum and a minimum rate of change of a speed of an output of the transmission, if the delta is greater than a reference delta, determining a stroke pressure adjustment in response to the delta, and re-executing the downshift while applying to the oncoming element an adapted stroke pressure that is a sum of the second stroke pressure adjustment and the reference stroke pressure. 
     This flare based algorithm is robust since it operates under near steady state, light torque, closed pedal conditions and is based on the quantity of flare that occurs. The algorithm evaluates the shift event to adapt the non-controlling element. 
     The control measures the flare and then accordingly adjusts the stroke pressure on the oncoming clutch to prevent any future shift quality degradation due to the flare. The control also prevents any over-learning of stroke pressure, which can lead to a tie-up and shift quality degradation. 
     The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram showing the kinematic arrangement of an automatic transmission; 
         FIG. 2  is a table showing the engaged and disengaged state of friction elements that control the transmission of  FIG. 1 ; 
         FIG. 3  is system to which the control strategy can be applied; 
         FIG. 4  is a series of graphs showing the change of several variables during a downshift in which stroke pressure adjustment is being determined; and 
         FIG. 5  is logic flow diagram of an algorithm in which the magnitude of control element stroke pressure is updated for later gear changes. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, there is illustrated in  FIG. 1  an engine crankshaft  12  secured to a hydraulic torque converter  14 , which includes a bladed impeller  16 , a bladed turbine  18 , a stator  20 , an overrunning or one-way clutch  22 , a lock-up clutch  24 , and a torsional vibration damper  26 . Hydraulic fluid exiting the impeller  16  hydrokinetically drives turbine  18  when clutch  24  is open. The engine shaft  12  drives turbine  18  directly when clutch  24  is engaged. The turbine  18  drives the input shaft  28  of an automatic transmission  30 , whose input shaft  28  and output shaft  30  are aligned. 
     A simple planetary gearset  32  includes a ring gear  34  secured to the input shaft  28 , a planet carrier  34  connected to a first control clutch  36  and to a second control clutch  38 , planet pinions  40 , and a sun gear  42 , fixed relative to the casing  44 . The input shaft  28  directly drives a third control clutch  46 . 
     A double planetary gearset  50 , of the Ravigneaux type, includes sun gears  52 ,  54 , planet carrier  56 , planet pinions  58  supported on carrier  56  and meshing with planet pinions  60  and with sun gear  54 . The ring gear  24  meshes with planet pinions  58 . 
     Friction brake  62  secures carrier  56  against rotation on casing  44  and releases that connection alternately. Friction brake  64  secures sun gear  52  and clutch  38  against rotation on casing  44  and releases that connection alternately. 
       FIG. 2  shows a table of the engaged and disengaged states of the friction control elements  36 ,  38 ,  46 ,  62 ,  64  that control operation of the transmission  30  in seven modes, six forward speeds and reverse drive. The seven modes correspond to seven combinations of pairs of the three clutches  36 ,  38 ,  46  and two brakes  62 ,  64 . The change from each combination to the next, or to the one thereafter, is achieved throughout the whole range by changing only one of the two friction elements engaged, i.e. exclusively by single transition shifts. 
     As illustrated in  FIG. 2 , the second forward gear is produced when clutch  36  and brake  64  are engaged. A downshift to first gear is produce by maintaining clutch  36  engaged, disengaging brake  64  and engaging brake  62 . When a 2-1 downshift is executed, the off-going control element is brake  64  and the oncoming control element is brake  62 . When a 3-2 downshift is executed, the off-going control element is clutch  38  and the oncoming control element is brake  64 . 
     Referring now to  FIG. 3 , an electronic controller  70  includes a central processing unit CPU  71 , which communicates with various input signals, and electronic memory  72  containing control algorithms stored there in coded form readable by the CPU. The memory for each control element, i.e., clutches  36 ,  38 ,  46  and brakes  62 ,  64 , contains a base stroke pressure  108 . A KAM cell  73  for each control element is updated with a stroke pressure adjustment in response to the result of a previous shift, as described below with reference to  FIG. 5 . The contents of KAM cell  73  is added to the reference or base stroke pressure of the control element for the respective shift to produce an adapted stroke pressure that is used in each shift following the shift in which the most recent stroke pressure adjustment is determined. 
     The input signals include a signal  74  produced by accelerator pedal sensor  76 , which produces a signal representing the desired wheel torque in response to the vehicle operator&#39;s manual control of the accelerator pedal  78 . A sensor  80  produces a signal  82  representing the magnitude of torque produced by turbine  18  and transmitted by input shaft  28 . Input signal  84  represents the vehicle speed. Input signal  86  represents the engine throttle position, i.e., the extent to which the engine throttle  88  is open. Input signal  90  represents the transmission oil temperature TOT. Input signal  92  represents the engine coolant temperature. Input signal  73  represents the position of the PRNDL  94 . Input signal  95  represents the speed of output shaft  30 . 
     Controller  70  produces and sends output signals to solenoids, which control the magnitude of hydraulic pressure, which alternately applies, releases and strokes brakes  62 ,  62 , respectively. Clutches  36 ,  38 ,  46  are similarly actuated by controller  70  in response to shift control algorithms stored in electronic memory  72 . 
     Any speed flare or tie-up across the turbine  18  is very strongly dependent on the stroke pressure of the on-coming control element. Stroke pressure is the pressure of hydraulic fluid in the oncoming control element that is sufficient to fill the servo cylinder that actuates the control element, to overcome the force of the servo return spring, and to take up clearances in the servo and control element, but is insufficient to transmit torque through the oncoming control element. 
     Although the adaptive control of a 2-1 downshift is described here, the control is applicable to any downshift having nearly steady state conditions and a closed or released accelerator pedal  78 , preferably with the speed of the turbine  18  being less than the engine speed. 
     In  FIG. 4  at  100 , controller  70  issues a command to execute a 2-1 downshift from the current gear to a target gear. The filtered turbine speed (NTBART)  102  after initially decreasing increases toward the synchronous speed of the target gear, i.e., first gear. Synchronous speed is the speed that the input shaft  28  and turbine  18  would have at the current vehicle speed with the transmission operating in the target gear. 
     The speed of input  28  and turbine  18  is seen to flare above or overshoot  106  the synchronous speed  104  because the base or reference stroke pressure  108  of the oncoming element (brake  62 ) was too low. If the speed of input  28  and turbine  18  were less than the synchronous speed  104 , the transmission becomes susceptible to a tie-up. 
     Controller  70  initially boosts pressure in brake  62  to  110 , after which that pressure decreases to the reference or base stroke pressure  108  and thereafter increases along a ramp  112  to a pressure at which brake  62  is closed and transmits torque. 
     The filtered speed of output  20  (NOBART  114 ) decreases during the downshift. During the downshift, controller  70  repetitively monitors NOBART  114  and the rate of change  116  (dot_noflt) of NOBART  114 . Controller  70  determines repetitively the maximum  118  and minimum  120  rate of change  116  of NOBART  114  and the maximum difference or delta  122  between the maximum  118  and minimum  120  rate of change  116  of NOBART  114  during the downshift. 
     Referring now to the algorithm of  FIG. 5 , at step  130  controller  70  commands a power-on, low torque downshift, such as a 2-1 or 3-2 downshift. Before step  130  is performed the oncoming control element  62  must be calibrated to use the slip adapt function; the transmission temperature  90  must be between upper and lower reference limits; engine coolant temperature  92  must be between upper and lower reference limits; the powertrain must be producing positive drive, i.e., torque is transmitted from the engine to the vehicle wheels; and a code indicating the current downshift must be established. If each of these criteria is satisfied, step  130  is executed. 
     At step  132  a test is made to determine whether any prohibition to executing the adapt stroke pressure control algorithm is present. The prohibitions include: pedal position  74  greater than a maximum reference position, a rate of change of pedal position  74  above a maximum reference rate of change, the speed  95  of output shaft  30  greater than a maximum reference speed or less than a minimum reference speed, the rate of change of the speed of output shaft  30  greater than a maximum reference rate of change, turbine torque greater than a maximum torque or less than an minimum reference torque, a rate of change of turbine torque above a maximum reference rate of change, and the PRNDL  94  not in the DRIVE position. If the result of test  132  is logically positive, the control is exited at step  134 . 
     If the result of test  132  is negative, control advances to step  136  where controller  70  stores the maximum  118  and minimum  120  of the derivatives of output shaft speed  114  that occur after command  100  and before turbine speed  102  reaches the synchronous speed  104 . 
     At step  138 , controller  70  waits for turbine speed  102  to reach the synchronous speed  104  of the target gear and determines the difference between the maximum  118  and minimum  120  of the derivatives of output shaft speed  114 . 
     At step  140 , controller  70  performs a test to determine whether the delta or difference  122  between in the maximum  118  and minimum  120  derivatives of output shaft speed  114  is greater than a calibratable reference delta value. 
     If the result of test  140  is positive, at step  142  controller  70  either decreases or does not change the stroke pressure adjustment in KAM cell  73  because a tie-up has been indicated. When the subject downshift is executed subsequently, at step  152  controller  70  retrieves the stroke pressure adjustment from KAM cell  73  using the indexing parameters: oncoming control element  62  and the subject downshift. Controller  70  then controls an adapted stroke pressure for the subject downshift by adding the stroke pressure adjustment to the reference or base stroke pressure  108  and applies the adapted stroke pressure to the oncoming control element  62  instead of the reference stroke pressure  108  during the subsequent downshift. 
     If the result of test  140  is negative, at step  144  controller  70  records in memory  72  the maximum flare  106  of turbine speed above the synchronous speed  104  that occurs during the end of the shift phase. 
     At step  146  a test is made to determine whether the flare  106  is greater than a calibratable reference flare. Test  146  is performed to avoid inaccuracies resulting from noise in the acquired data. 
     If the result of test  146  is negative, control advances to step  148 , where no change in the stroke pressure adjustment contained in the KAM cell  73  of the oncoming control element  62  is made. 
     If the result of test  146  is positive, a sufficiently large flare has occurred to update the stroke pressure of the oncoming element  62 . At step  150 , a calibratable stroke pressure adjustment commensurate with the magnitude of flare  106  is placed in the KAM cell  73  of the oncoming control element  62  for the subject downshift. 
     When the subject downshift is executed subsequently, at step  152  controller  70  retrieves the stroke pressure adjustment from KAM cell  73  using the indexing parameters: oncoming control element  62  and the subject 2-1 downshift. Controller  70  produces an adapted stroke pressure for the downshift by adding the stroke pressure adjustment to the reference or base stroke pressure and applies the adjusted stroke pressure to the oncoming control element  62  instead of the reference stroke pressure  108  during the subsequent downshift. 
     The algorithm learns the correct stroke pressure based on analysis of prior shift events. Flare is used as a basis to detect an under-stroke pressure condition and to increase stroke pressure. Tie-up detection is used to detect an over-stroke pressure condition and to decrease, or not increase stroke pressure. 
     In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.