Abstract:
A downshift control method during a coasting drive condition includes computing changes in output torque and input speed from initial output torque and initial input speed; reducing offgoing element pressure, provided a change in input speed exceeds a reference input speed change; using closed loop control based on output torque and a change in measured output torque to adjust oncoming element pressure such that output torque remains between predetermined maximum and minimum torques; and fully engaging the oncoming element.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to controlling an automatic transmission downshift during a coasting drive condition when a tie-up or neutral case occurs as evidenced by measured output torque. 
     2. Description of the Prior Art 
     Friction elements, i.e., the clutches and brakes, are utilized to alter a torque path within the gear sets to establish desired torque and speed ratios between input and output of an automatic transmission system. During a vehicle operation, a combination of engaged friction control elements determines a specific gear position in which an automatic transmission operates. When the vehicle is in a coasting condition wherein the vehicle continues to move forward while both acceleration pedal and brake pedal are released, a transmission controller may initiate the so called coasting downshift as the vehicle slows down. During the downshift process, some friction elements, referred to as off-going elements, are released while others, referred to as on-coming elements, are engaged, altering torque and speed relationship between input and output of the transmission. 
     A neutral state, i.e., a state in which no torque path is dominantly established within the gear sets, is caused by premature release of an offgoing control element or lack of oncoming control element torque capacity. When the neutral state occurs during a coasting downshift, a driver may perceive a shift shock or a momentary loss of drive torque. Additionally, if the driver steps in an acceleration pedal during the neutral state, engine speed may surge, followed by a shift shock due to a sudden engagement of on-coming element. The neutral state is not generally detectable by means of the sensors, such as transmission speed sensors, that are commonly available in volume production vehicles. 
     A transmission tie-up at a level perceptible to the driver may be caused by delayed release of the offgoing control element or torsional overcapacity of the oncoming control element, causing the gear sets to be over-constrained. When the tie-up state occurs during the coasting downshift, a driver may perceive a shift shock or a sudden loss of drive torque. A severe tie-up state may be detected by observing a measurable drop of transmission input speed signals in volume production vehicles. When the severe tie-up is detected, a controller may quickly reduce off-going clutch pressure to resolve the condition. 
     According to prior art coasting shift control methodologies, control pressure of off-going element is reduced through an open-loop approach while control pressure of on-coming clutch is raised through an open-loop approach. Open-loop pressure controls for both off-going clutch and on-coming clutch may be adaptively adjusted. That is, after completing each shift event, a transmission controller may adjust open-loop pressure profile for off-going element by a pre-determined amount to delay its release timing while prescribing faster pressure rise for on-coming clutch element by a pre-determined amount. This adjustment brings the coasting shift control toward tie-up state. When the tie-up is detected through speed measurements, the controller may reduce off-going element control pressure for its immediate release within the given shift. The detection of the tie-up state enables the controller to adaptively prescribe the control pressure profiles for avoiding both neutral and tie-up states in the subsequent coasting down-shift events. 
     The prior art methodologies that primarily rely on speed measurements can neither explicitly detect nor correct the neutral state within the current coasting downshift event. The prior art methodologies that primarily rely on speed sensors may detect the tie-up state, but does not provide means to control the amount of dropped drive torque level during the coasting downshift event. The adaptive open-loop pressure adjustments may not work well because off-going and on-coming clutch friction torques may unpredictably vary under different operating conditions even if control pressure profiles remain unchanged. The prior art adaptive pressure adjustments may not be able to account for changing hydraulic control system variability. Accordingly, even if optimal pressure profiles are adaptively identified, they may not be optimal for the subsequent shifts and neutral or tie-up state may still occur. 
     SUMMARY OF THE INVENTION 
     A downshift control method, during a coasting drive condition where a vehicle continues to move forward while both acceleration pedal and brake pedal are released, includes computing changes in output torque and input speed from initial output torque and initial input speed; reducing offgoing element pressure, provided a change in input speed exceeds a reference input speed change; using closed loop control based on output torque and a change in measured output torque to adjust oncoming element pressure such that output torque remains between predetermined maximum and minimum torques; and fully engaging the oncoming element. 
     The control strategy relies on direct measurement of output torque, which indicates the level of discomfort (called shift feel) perceptible to the occupants of a motor vehicle. 
     A controller adjusts or accelerates the offgoing control element release process based on the level of turbine speed dip when a tie-up state is detected. 
     The controller may also use a dip in output torque to adjust the offgoing control element release pressure profile. 
     The controller adjusts or reduces oncoming clutch pressure based on the level of the output torque dip. 
     The controller may also use a dip in turbine speed to adjust the oncoming control element pressure profile. 
     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 a transmission; 
         FIG. 2  is chart showing the state of control elements corresponding to each gears produced by the transmission of  FIG. 1 ; 
         FIG. 3  illustrates an algorithm that controls a downshift; 
         FIG. 4  contains graphs of the turbine speed, output torque, and hydraulic pressure supplied to the servos that actuate the offgoing element and oncoming element; and 
         FIG. 5  is a graph showing the variation over time of driveline variable during a coasting downshift. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  illustrates gearing, clutches, brakes, shafts and other components of a multiple-speed automatic transaxle  10  capable of producing six forward speed ratios and reverse drive. 
     A torque converter  12  includes an impeller  14  driven by an engine, a turbine  16  hydrokinetically coupled to the impeller, and a stator  18  between the impeller and turbine. A transmission input shaft  20  is secured to the turbine  16 . The stator  18  is secured against rotation to a transmission case  26 . 
     A gearset  28  includes a sun gear  30 , secured to input shaft  20 ; a carrier  32 ; a ring gear  34 ; and planet pinions  38  supported on carrier  32  and meshing with sun gear  30  and ring gear  34 . 
     Rear gearset  40  and front gearset  42  are also simple planetary gearsets. Gearset  40  includes planet pinions  44  supported for rotation on a carrier  46  and meshing with both a sun gear  48  and a ring gear  50 . Gearset  42  includes planet pinions  52  supported for rotation on carrier  54  and meshing with both a sun gear  56  and ring gear  58 . 
     Overdrive clutch C456 is secured to carrier  46  and ring gear  34 . Clutch 35R is secured to sun gear  48 . Intermediate brake CB26 is 34 is secured to sun gear  48  and to transmission case  26 . Low reverse brake CBLR is secured to carrier  46 , and transmission case  26 . Forward brake CB1234 is secured to sun gear  56  and output shaft  60 . A one-way clutch OWC  62  is secured to carrier  46 , ring gear  34 , clutch CBLR and case  26 . The transmission is equipped with an input speed sensor  64  and driveline torque sensor  66 . 
     Although a downshift may occur between any of the forward gears during a coasting drive condition where engine torque is typically reduced to a low level, the downshift control method is described with reference to a 3-2 downshift. As  FIG. 2  shows, third gear is produced when clutch C35R and brake CB1234 are engaged concurrently and OWC  62  overruns. A downshift to second gear is produced when the offgoing control element (clutch C35R) is disengaged, the oncoming control element (brake CB26) is engaged, brake CB1234 remains engaged and OWC  62  continues to overrun. 
     A downshift may occur with a tie-up case, which occurs when the offgoing element C35R and oncoming element CB26 both have enough torque capacity such that they compete for the dominant torque path through the transaxle  10 , resulting in a reduction in output torque, carried on output shaft  60  and a reduction in turbine speed, i.e., the speed of input shaft  20 . While the transmission gears are in a tie-up situation, clutch C35R may be securely engaged or may start slipping while carrying torque capacity. 
     Alternatively a downshift may occur with a neutral case, which occurs when the offgoing element C35R and oncoming element CB26 both slip, due to their having insufficient torque capacity, and without being able to establish a dominant torque path. 
       FIG. 3  illustrates an algorithm  70 , which is repetitively executed at intervals of about 7 ms and controls a downshift with tie-up and neutral cases during a coasting drive condition. After starting the algorithm at step  72 , the magnitude of torque carried by output shaft  60  and the speed of input shaft  20  before the downshift occurs are measured using a torque sensor  64  and a speed sensor  66 , at step  74 . Torque sensor  64  produces an electronic signal representing T out  the magnitude of torque carried by output shaft  60 . Speed sensor  66  produces an electronic signal representing RPM turb , the speed of input shaft  20 . 
       FIG. 4  contains graphs of the turbine speed, output torque, and hydraulic pressure supplied to the servos that actuate the offgoing element Pogc C35R and the oncoming element Pocc CB26. 
     At step  76  a test is performed to determine whether a coasting downshift has been commanded by a transmission controller. If the result of test  76  is logically negative, control returns to step  74  and the index (i) is incremented. 
     If the result of test  76  is logically positive indicating that the downshift has begun, at step  78  the magnitude of the following variables are established: T init =T out  (t 1 ) and RPM init =RPM turb  (t i ), as represented in  FIG. 4 . 
     At step  80  the magnitude of the following reference output torques and input speed, as represented in  FIG. 4 , are established and stored in electronic memory accessible to the controller that control transmission gear shifts: T max , T min  and ΔRPM thres  with T min  preferably set equal to 0 if T init  is sufficiently larger than zero. When T init  is close to zero or below zero, T min  may be set to a negative value. 
     The index (i) is incremented and control advances to step  82  where T out  (ti) and RPM turb  (ti) are measured again. 
     At step  84  the following computations are performed:
 
Δ T   out ( ti )= T   init   −T   out ( ti )
 
ΔRPM turb ( ti )=RPM init −RPM turb ( ti )
 
     At step  86  a test is performed to determine whether
 
ΔRPM turb ( ti )&gt;ΔRPM thres  
 
     If the result of test  86  is true, thereby indicating a tie-up is present, at step  88  the pressure Pogc (ti) of the offgoing element C35R is reduced based on the level of ΔRPM turb  and control advances to step  90 . 
     If the result of test  86  is false, control advances to step  90  where pressure Pocc (ti) of the oncoming element CB26 is adjusted through closed loop control based on T out  (ti) and ΔT out  (ti) to maintain T min &lt;T out &lt;T max . 
     At step  92  a test is performed to determine whether the coasting downshift is to be completed. If the result of test  92  is negative, control returns to step  82 . This loop continues as long as the transmission controller desires to maintain output torque level between T max  and T min , without completing downshift, while allowing both off-going clutch C35R and on-coming clutch CB26 to slip at non-zero torque capacity, respectively. Alternatively, torque capacity level of clutch C35R may be reduced to a non-significant level to allow clutch CB26 to dominantly affect an output torque level between T max  and T min  while it continues to slip without completing engagement. 
     If the result of test  92  is positive, that is, if the transmission controller determines that the coasting downshift is to be completed, at step  94  pressure Pocc (ti) of the oncoming element CB26 is increased to complete the downshift by fully engaging element CB26. 
     At step  96  execution of the downshift algorithm  70  is terminated. 
     The algorithm  70  in  FIG. 3  can be directly applied, excluding the step  88 , to on-coming clutch pressure control during a coasting down shift event for a transmission system wherein an off-going element is an over-running one-way-type torque coupling. 
       FIG. 5  is a graph showing the variation over time of driveline variables during a coasting downshift, wherein a lack of torque capacity, detected from the measured output torque, is compensated by increasing the oncoming control element pressure at  98  to reduce the output torque hole at  100  after an off-going clutch starts slipping, due to reduced torque capacity, to initiate turbine speed change. 
     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. For example, the elements of the invented method can be readily employed using output torque level inferred or indicated indirectly by sensory signals other than direct torque measurements.