Abstract:
A method for controlling a coast down downshift that is produced in an automatic transmission by disengaging an off-going control element and engaging an oncoming control element, including the steps of determining a first desired pressure magnitude of the off-going control element and a first desired pressure magnitude of the oncoming control element, executing the current downshift using said first desired pressure magnitudes, determining during execution of the current downshift corrections of said first desired pressure magnitudes that occur during the current downshift, determining a second desired pressure magnitude of the off-going control element and a second desired pressure magnitude of the oncoming control element, using said corrections and the second desired pressure magnitude to determine a subsequent desired pressure magnitude of the off-going control element and a subsequent desired pressure magnitude of the oncoming control element, and executing a downshift using said subsequent desired pressure magnitudes.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a multiple speed step-ratio automatic transmission for a motor vehicle and, in particular, to the control of a coasting downshift performed by the transmission. 
         [0003]    2. Description of the Prior Art 
         [0004]    During a coasting downshift of an automatic transmission in which shifts occur between individual gear ratios, a lash transition can occur when torque converter turbine speed and engine speed (or torque converter impeller speed) cross at too high of a rate. This lash transition excites components in the transmission and driveline system, which can result in an audible clunk disturbance to the vehicle operator and passengers. Under certain conditions, the lash can cross and cross back, and the customer may experience both an excessively long gear shift and a double clunk. 
         [0005]    The technical reason for the clunk during the coasting downshift sequence is the rate at which the on-coming synchronous element, such as a brake comprising a band and drum, gains torque capacity relative to the off-going synchronous element losing torque capacity. The relative element capacities are determined by estimated engine torque, engine inertia, and vehicle deceleration rate, which are translated into hydraulic control pressures and apply forces acting on the elements. This translation is further impacted by a rise in the interface temperature of the on-coming band and drum, which increases the likelihood of a torque reversal and subsequent lash transition during the gear shift. 
         [0006]    The apply force on the band is proportional to torque for both an on-coming friction control element and an off-going control element. The torque reversal causes a crossing of driveline lash, which creates a torque disturbance and the audible clunk. 
         [0007]    A need exists in the automotive industry for a technique to adjust the relative pressures of the on-coming and off-going control elements during a coasting downshift to reduce lash and therefore minimize the rate at which that lash is crossed based on system level inputs. 
       SUMMARY OF THE INVENTION 
       [0008]    A method for controlling a coast down downshift that is produced in an automatic transmission by disengaging an off-going control element and engaging an on-coming control element, including the steps of determining a first desired pressure magnitude of the off-going control element and a first desired pressure magnitude of the on-coming control element, executing the current downshift using said first desired pressure magnitudes, determining during execution of the current downshift corrections of said first desired pressure magnitudes that occur during the current downshift, determining a second desired pressure magnitude of the off-going control element and a second desired pressure magnitude of the on-coming control element, using said corrections and the second desired pressure magnitude to determine a subsequent desired pressure magnitude of the off-going control element and a subsequent desired pressure magnitude of the on-coming control element, and executing a downshift using said subsequent desired pressure magnitudes. 
         [0009]    In order to reduce the likelihood of a torque reversal a “controlled tie-up” of the coasting synchronous downshift is produced. Due to low operating torques and pressure variability, the pressures are adapted as a function of temperature and vehicle deceleration rate. The controlled tie-up creates a controlled torque reversal rate between the off-going and on-coming friction elements during the synchronous coasting downshift. 
         [0010]    The controlled tie-up is accomplished by raising the starting off-going element pressure to a value higher than that required based on system level inputs, thereby allowing the synchronous shift hand-off from off-going element to on-coming element to occur in a torque and pressure region where the rate of the hand-off of the shift can be managed. 
         [0011]    The two starting servo pressure magnitudes of the on-coming element and off-going element are further adjusted until the desired hand off rate is obtained. 
         [0012]    In addition to producing a “controlled tie-up”, and in order to provide for the range of operating conditions and unit-to-unit variability, the servo pressures of the on-coming element  74  and off-going element  70  ( FIG. 1 ) are adapted for changing conditions. To accomplish this adaptation, an algorithm of prioritized shift performance metrics determines the magnitudes of various desired corrective metrics, stores the metric values in memory, iteratively revises the stored metric values, and recalls them for use in controlling subsequent coasting downshifts. 
         [0013]    The algorithm learns and stores in memory corrective adjustments of the synchronous pressures to avoid large levels of lash based on system level inputs. 
         [0014]    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 
         [0015]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0016]      FIG. 1  is a schematic diagram showing the kinematic arrangement of an automatic transmission operating in a forward gear; 
           [0017]      FIG. 2  is a schematic diagram showing the showing the kinematic arrangement of the transmission of  FIG. 1  operating in a lower forward gear; and 
           [0018]      FIG. 3  is a logic flow diagram representing an algorithm for controlling successive coasting downshifts. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]    Referring now to the drawings, there is illustrated in  FIG. 1  the kinematic arrangement of an automatic transmission  10 . A torque converter includes a bladed impeller wheel  12  connected to the crankshaft  14  of an internal combustion engine, a bladed turbine wheel  16 , and a bladed stator wheel  18 . The impeller, stator and turbine wheels define a toroidal fluid flow circuit, whereby the impeller  12  is hydrokinetically connected to the turbine  16 . The stator  18  is supported rotatably on a stationary stator sleeve shaft  20 , and an overrunning brake  22  anchors the stator to the shaft  20  to prevent rotation of the stator in a direction opposite the direction of rotation of the impeller, although free-wheeling motion in the opposite direction is permitted. 
         [0020]    The torque converter includes a bypass clutch  24  located within the torque converter  26 . When clutch  24  is engaged, the turbine and impeller are mechanically connected to a transmission input shaft  28 ; when clutch  24  is disengaged, the turbine and impeller are hydrokinetically connected and mechanically disconnected. Fluid contained in the torque converter is supplied to the torque converter from the output of an oil pump assembly and is returned to an oil sump, to which an inlet of the pump is connected hydraulically. 
         [0021]    A planetary gear system includes first, second, and third gear units  30 ,  32 ,  34 . Gear unit  30  includes a first sun gear  31 , a ring gear  35 , a carrier  36  secured to input  28 , and planet pinions  38  supported on carrier  36  and meshing with sun gear  32  and ring gear  34 . An overrunning coupling  40  includes an inner race  42  connect to input  28  and carrier  36 , an outer race  44  connected to ring gear  34 , and a set of sprags for alternately driveably connecting and releasing the races  42 ,  44 . 
         [0022]    Gear unit  32  includes a sun gear  44 , a ring gear  46 , a carrier  48  secured to an output shaft  50 , and planet pinions  52  supported on carrier  48  and meshing with sun gear  44  and ring gear  46 . 
         [0023]    Gear unit  34  includes a sun gear  54  secure to sun gear  44 , a ring gear  56 , a carrier  58  secured to output shaft  50 , and planet pinions  60  supported on carrier  58  and meshing with sun gear  54  and ring gear  56 . An overrunning coupling  62  includes an inner race  64  held against rotation of the transmission case  66 , an outer race  68  connected to carrier  58 , and a set of sprags for alternately driveably connecting and releasing the races  64 ,  68 . 
         [0024]    A high clutch  70 , a hydraulically-actuated friction control element, alternately connects an intermediate shaft  72  to sun gears  44 ,  54  when clutch  70  is engaged, and releases that connection when clutch  70  is disengaged. An intermediate brake band  74 , a second hydraulically-actuated friction control element, alternately connects sun gears  44 ,  54  and case  66  when brake band  74  frictionally engages a brake drum  76 , and releases that connection when brake band  74  is disengaged. A reverse brake band  77 , a third hydraulically-actuated friction control element, alternately connects carrier  58  and outer race  68  when brake band  77  frictionally engages a brake drum  78 , and releases that connection when brake band  77  is disengaged. A forward high clutch  80 , another hydraulically-actuated friction control element, alternately connects intermediate shaft  72  to ring gear  46  when clutch  80  is engaged, and releases that connection when clutch  80  is disengaged. 
         [0025]    Transmission  10  operates in fourth gear when clutches  70  and  80  are engaged and the other control elements are disengaged. With the transmission so disposed, sun gears  44  and  54  are driven through clutch  80  at the speed of shaft  72 , ring gear  46  is driven through clutch  70  at the speed of shaft  72 , and carrier  48  and output shaft  50  rotate at the speed of intermediate shaft  72 . 
         [0026]    A coasting downshift to third gear from fourth gear occurs when high clutch  70  is disengaged synchronously with engagement of intermediate brake  74 , forward clutch remains engaged, and the other friction elements remain disengaged. With the transmission so disposed, ring gear  46  is driven through clutch  80  at the speed of shaft  72 , sun gear  44  is held against rotation on case  66  due to the engagement of brake  74  with brake drum  76 , and carrier  48  and output shaft  50  are underdriven relative to the speed of intermediate shaft  72 . 
         [0027]    Each of control elements  70 ,  74 ,  77  and  80  are actuated to engage and disengage in response to a variable magnitude of pressure present within a hydraulic cylinder of the respective servo associated with each of the control elements. In order to reduce the likelihood of a torque reversal that could produce a clunk during a power-off or coasting downshift, a need exists for a “controlled tie-up” of the coasting synchronous downshift. Due to the low operating torque capacities of the control elements and the variability of the servo pressures, a need exists also to adapt the actuating servo pressures as a function of temperature and vehicle deceleration rate. 
         [0028]    Delivering a “controlled tie-up” creates a controlled torque reversal rate between the off-going friction element  70  and on-coming friction element  74  during the synchronous coasting downshift. The “controlled tie-up” is accomplished by raising the starting servo pressure of off-going element  70  to a value higher than that required based on system level inputs, thereby allowing the synchronous shift to occur in a torque and pressure region where the rate of the torque transfer from the off-going element  70  to the on-coming element  74  during the downshift can be managed. 
         [0029]    Referring to  FIG. 3 , the algorithm, initialized and started at step  100 , identifies at step  102  that a synchronous coasting downshift of transmission  10  is required. Generally the need for the downshift is determined by a transmission controller from electronic signals representing the vehicle speed and the updated position of an accelerator pedal, i.e., the extent to which the pedal is depressed by the vehicle operator. These vehicle parameters together determine the desired gear in which the transmission should be operating according to a function stored in electronic memory, accessible to the microprocessor, and indexed by current vehicle speed and current accelerator pedal position. When these parameters indicate that a downshift is required to produce the desired gear, the algorithm detects at step  102  that the downshift is required and about to occur. 
         [0030]    The vehicle deceleration rate and the temperature of the automatic transmission fluid (ATF) in the transmission oil sump are determined at step  104 . 
         [0031]    At step  106 , a desired pressure in the servo of the on-coming friction element and a desired pressure in the servo of the off-going friction element are determined as a function of engine combustion torque, engine inertia torque, transmission oil temperature and vehicle speed. 
         [0032]    At step  108 , corrective values acquired during previous coasting downshifts between the subject gears and stored in electronic memory, preferably keep alive memory (KAM), are used to change the values determined from step  106  to produce the desired pressure in the servo of the on-coming friction element and the desired pressure in the servo of the off-going friction element for the current downshift. 
         [0033]    At step  110 , the coasting downshift is executed, and variables such as delay, duration, and lash during the current downshift are determined and stored in electronic memory. 
         [0034]    Delay is the length of a period that begins at a reference event, such as the start of the downshift, and ends at a reference percentage of completion of the current downshift, such as five percent completion. The algorithm compares the length of delay during the current downshift to a reference range of delay. Duration is the length of a period that begins at a reference percentage of completion of the current downshift event, such five percent completion, and ends at a second reference percentage of completion of the current downshift, such as eighty-five percent completion. The algorithm compares duration of the current downshift to a reference duration. 
         [0035]    Lash, the difference between the speed of turbine  16  and the speed of the engine crankshaft  14 , is a measured parameter determined from signals representing these speeds that are produced by speed sensors. The algorithm determines the magnitude of the maximum lash that occurs after turbine speed begins to exceed engine speed and compares it to a reference lash range. 
         [0036]    At step  112 , a check is made to determine whether the current delay is within the reference delay range. If the result of step  112  is logically true, control passes to step  114 . If the result of step  112  is logically false, control passes to step  116  where a correction of the off-going friction element pressure is determined from a schedule of off-going friction element pressure corrections that varies inversely with the magnitude of the current delay. For example, if the current delay is short relative to the reference range of delay, the off-going friction element pressure is increased and control passes to step  118 . If the current delay is long relative to the reference range of delay, the off-going friction element pressure correction is decreased and control passes to step  118 . 
         [0037]    At step  118 , a check is made to determine whether the current lash relative to the measured delay is with the reference lash range. If the result of step  118  is logically true, indicating that the current lash is within the lash range, control passes to step  120  where the off-going friction element pressure correction due to delay alone with acceptable lash is recorded in KAM. 
         [0038]    If the result of step  118  is logically false, control passes to step  122  where a correction of the off-going friction element pressure, determined on the basis of delay, is further adjusted on the basis of lash. If the current magnitude of lash is high relative to the reference lash range and delay is short relative to the reference range, the off-going friction element pressure due to lash may be increased further. If the current magnitude of lash is high relative to the reference lash range and delay is long relative to the reference range, the off-going friction element pressure due to lash may be decreased further. If the current magnitude of lash is low relative to the reference lash range and delay is long relative to the reference range, the off-going friction element pressure due to lash may be decreased further. If the current magnitude of lash is low relative to the reference lash range and delay is short relative to the reference range, the off-going friction element pressure due to lash may be decreased further. Then control passes to step  120 , where the off-going friction element pressure corrections due to delay and lash are recorded in KAM for use during execution of a subsequent coasting downshifts between the subject gears. 
         [0039]    If the result of step  114  is logically true, indicating that duration is within the reference duration range, control passes to step  124  where no correction of the on-coming pressure element pressure is made and execution of the algorithm for the current downshift ends. 
         [0040]    If the result of step  114  is logically false, control passes to step  126 , where a correction of the on-coming friction element pressure is determined from a schedule of such pressure corrections that varies inversely with the magnitude of the current duration. For example, if the difference between the current duration and the reference duration range is zero, the on-coming friction element pressure correction may be −1.5 psi. If the difference between the current duration and the reference duration range is greater than zero, the off-going friction element pressure correction may be −0.50 psi. 
         [0041]    Then control passes to step  128  a check is made to determine whether the current delay is within the reference delay range. If the result of step  128  is logically true, indicating that the current delay is within the reference delay range, control passes to step  130  where the on-coming friction element pressure correction due to duration alone is recorded in KAM. 
         [0042]    If the result of step  128  is logically false, indicating that the current delay is without the reference delay range, control passes to step  132  where a correction of the on-coming friction element pressure due to delay is determined as described with reference to step  116 . Then, at step  130 , the on-coming friction element pressure corrections due to duration and delay are recorded in KAM for use during execution of subsequent coasting downshifts between the subject gears. 
         [0043]    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.