Patent Publication Number: US-2003228954-A1

Title: Multi-stage skip downshift control for an automatic transmission

Description:
TECHNICAL FIELD  
       [0001] This invention relates to the control of skipped-ratio power-on downshifting in an automatic transmission that is optimized for sequential ratio shifting.  
       BACKGROUND OF THE INVENTION  
       [0002] In general, a motor vehicle automatic transmission includes a number of gear elements and selectively engageable friction elements (referred to herein as clutches) that are controlled to establish one of several forward speed ratios between the transmission input and output shafts. The input shaft is coupled to the vehicle engine through a fluid coupling such as a torque converter, and the output shaft is coupled to the vehicle drive wheels through a differential gearset. Shifting from a currently established speed ratio to a numerically adjacent speed ratio involves, in most cases, disengaging a clutch (off-going clutch) associated with the current speed ratio and engaging a clutch (on-coming clutch) associated with the new speed ratio.  
       [0003] Since most shifts involve numerically adjacent speed ratios (i.e., sequential shifting), the fluid control hardware can be designed to minimize the number of modulated pressure control valves, as disclosed for example, in the U.S. Pat. No. 5,601,506 to Long et al., issued on Feb. 11, 1997, and assigned to the assignee of the present invention. In Long et al., a set of relatively inexpensive on/off relay valves selectively couple the various transmission clutches to two modulated valves, such that a certain combination of clutches can only be coupled to a given modulated valve, and for any shift to a numerically adjacent speed ratio, one of the modulated valves is coupled to the on-coming clutch, and the other modulated valve is coupled to the off-going clutch. While such an arrangement can significantly simplify the control hardware and reduce manufacturing costs, it essentially rules out skip-shifting—that is, shifting to speed ratio other than a numerically adjacent speed ratio. Thus, if the transmission is operating in third gear, for example, and the engine load abruptly increases to a level for which first gear would be appropriate, the controller must successively perform sequential shifts from third-to-second, and from second-to-first, instead of skip-shifting from third-to-first. Accordingly, what is needed is a control methodology for performing skip-downshifts in a transmission where the control valve configuration is optimized for sequential shifting.  
       SUMMARY OF THE INVENTION  
       [0004] The present invention is directed to an improved downshift control for an automatic transmission optimized for sequential shifting, wherein skip-downshifting to a target gear is achieved by shifting the transmission to neutral and controlling the engine output torque while the transmission is reconfigured to establish the target speed ratio. The engine torque control is designed to allow the transmission input speed to reach the synchronization speed for the target speed ratio at a desired time, and the shift is completed when the transmission input speed actually reaches the synchronization speed. The engine torque control is predicated on a modeled engine torque parameter, and the torque control includes an adaptive parameter for adaptively adjusting the torque control to ensure that the input speed reaches the synchronization speed at the desired time.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0005]FIG. 1 is a diagram of an automatic transmission and microprocessor-based engine and transmission control units for carrying out the control of this invention.  
     [0006]FIG. 2 is a table indicating a relationship between transmission clutch activation and corresponding speed ratio.  
     [0007]FIG. 3 graphically depicts a single transition skip-downshift according to this invention as carried out by the engine and transmission control units of FIG. 1.  
     [0008]FIG. 4 is a flow diagram depicting a routine executed by the transmission control unit of FIG. 1 for adaptively adjusting a parameter of the shift depicted in FIG. 3.  
     [0009]FIG. 5 graphically depicts a double transition skip-downshift according to this invention as carried out by the engine and transmission control units of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0010] The control of this invention is described in the context of a multi-ratio power transmission having a planetary gearset of the type described in the U.S. Pat. No. 4,070,927 to Polak, and having an electro-hydraulic control of the type described in U.S. Pat. No. 5,601,506 to Long et al. Accordingly, the gearset and control elements shown in FIG. 1 hereof have been greatly simplified, it being understood that further detail regarding the fluid pressure routings and so forth may be found in the aforementioned patents.  
     [0011] Referring to FIG. 1, the reference numeral  10  generally designates a vehicle powertrain including engine  12 , transmission  14 , and a torque converter  16  providing a fluid coupling between engine  12  and transmission input shaft  18 . A torque converter clutch  19  is selectively engaged under certain conditions to provide a mechanical coupling between engine  12  and transmission input shaft  18 . The transmission output shaft  20  is coupled to the driving wheels of the vehicle in one of several conventional ways. The illustrated embodiment depicts a four-wheel-drive (FWD) application in which the output shaft  20  is connected to a transfer case  21  that is also coupled to a rear drive shaft R and a front drive shaft F. Typically, the transfer case  21  is manually shiftable to selectively establish one of several drive conditions, including various combinations of two-wheel-drive and four-wheel drive, and high or low speed range, with a neutral condition occurring intermediate the two and four wheel drive conditions.  
     [0012] The transmission  14  has three inter-connected planetary gearsets, designated generally by the reference numerals  23 ,  24  and  25 . The input shaft  18  continuously drives a sun gear  28  of gearset  23 , selectively drives the sun gears  30 ,  32  of gearsets  24 ,  25  via clutch C 1 , and selectively drives the carrier  34  of gearset  24  via clutch C 2 . The ring gears  36 ,  38 ,  40  of gearsets  23 ,  24 ,  25  are selectively connected to ground  42  via clutches C 3 , C 4  and C 5 , respectively.  
     [0013] As diagrammed in FIG. 2, the state of the clutches C 1 -C 5  (i.e., engaged or disengaged) can be controlled to provide six forward speed ratios (1, 2, 3, 4, 5, 6), a reverse speed ratio (R) or a neutral condition (N). For example, the first forward speed ratio is achieved by engaging clutches C 1  and C 5 . Shifting from one speed forward speed ratio to a numerically adjacent speed ratio is generally achieved by disengaging one clutch (referred to as the off-going clutch) while engaging another clutch (referred to as the on-coming clutch). For example the transmission  14  is shifted from first gear to second gear by disengaging clutch C 5  while engaging clutch C 4 .  
     [0014] The torque converter clutch  19  and the transmission clutches C 1 -C 5  are controlled by an electro-hydraulic control system, generally designated by the reference numeral  44 . The hydraulic portions of the control system  44  include a pump  46  which draws hydraulic fluid from a reservoir  48 , a pressure regulator  50  which returns a portion of the pump output to reservoir  48  to develop a regulated pressure in line  52 , a secondary pressure regulator valve  54 , a manual valve  56  manipulated by the driver of the vehicle and a number of solenoid operated fluid control valves  58 ,  60 ,  62 ,  64 .  
     [0015] The electronic portion of the control is primarily embodied in the engine control unit (ECU)  65  and the transmission control unit (TCU)  66 , illustrated in FIG. 1 as two separate modules. Both control units  65 ,  66  are microprocessor-based, and may be conventional in architecture. The ECU  65  controls the operation of engine functions such as fuel, spark timing, and so on depending on the control variables afforded by engine  12 , and the TCU  66  controls the solenoid operated fluid control valves  58 ,  60 ,  62 ,  64  based on a number of inputs to achieve a desired transmission speed ratio. The inputs to TCU  66  include signals representing the transmission input speed TIS, engine speed ES, a driver torque request TQ, and the transmission output speed TOS. Sensors for developing such signals may be conventional in nature, and have been omitted for simplicity. Additionally, ECU  65  supplies an engine output torque signal EOT to TCU  66 , and TCU  66  supplies a torque reduction command signal TQred to ECU  65 .  
     [0016] The control lever  82  of manual valve  56  is coupled to a sensor and display module  84  that produces an diagnostic signal on line  86  based on the control lever position; such signal is conventionally referred to as a PRNDL signal, since it indicates which of the transmission ranges (P, R, N, D or L) has been selected by the vehicle driver. Finally, the fluid control valves  60  are provided with pressure switches  74 ,  76 ,  78  for supplying diagnostic signals to TCU  66  on lines  80  based on the respective relay valve positions. The TCU  66 , in turn, monitors the various diagnostic signals for the purpose of electrically verifying proper operation of the controlled elements.  
     [0017] The solenoid operated fluid control valves  60 ,  62 ,  64  are generally characterized as being either of the on/off or modulated type. In general, modulated valves comprise a conventional pressure regulator valve biased by a variable pilot pressure that is developed by current controlled force motor, and a desired pressure is achieved by controlling the force motor current to a corresponding value. In the case of shifting, for example, TCU  66  determines pressure commands for smoothly engaging the oncoming clutch while smoothly disengaging the off-going clutch, develops corresponding force motor current commands for the respective modulated valves, and then supplies current to the force motors in accordance with the respective current commands. The converter clutch  19  is also controlled by a modulated valve  58 , which controls the fluid pressure in lines  70  and  72  for selectively engaging and disengaging the converter clutch  19 .  
     [0018] Since modulated valves are significantly more expensive to manufacture and control than simple on/off valves, and since the most commonly performed shifts (i.e., sequential shifts to numerically adjacent speed ratios) involve only one on-coming clutch and one off-going clutch, the control system  44  can be designed to minimize the number of modulated valves. In the illustrated embodiment, for example, the control system  44  includes just two modulated shift control valves  62 ,  64  combined with a set of on/off (relay) valves  60 . The relay valves  60  couple the modulated valves  62 ,  64  to the on-coming or off-going clutches, and couple the other clutches to either full line pressure or exhaust. Additionally, the design of the relay valves  60  can be simplified for sequential shifting if each of the modulated shift control valves  62 ,  64  is dedicated to non-sequential clutches. For example, modulated valve  62  may be dedicated to clutches C 1 , C 2  and C 4 , and modulated valve  64  may be dedicated to clutches C 3  and Cs. Referring to the table of FIG. 2, the relay valves  60  enable a sequential downshift from 5 th  to 4 th , for example, by coupling modulated valve  62  to on-coming clutch C 1  and coupling modulated valve  64  to off-going clutch C 3 . Similarly, the relay valves  60  enable a sequential downshift from 4 th  to 3 rd  by coupling modulated valve  62  to off-going clutch C 2  and coupling modulated valve  64  to on-coming clutch C 3 . Details of such a valve arrangement are set forth in the aforementioned U.S. Pat. No. 5,601,506 to Long et al., which is incorporated herein by reference.  
     [0019] While the illustrated valve arrangement can significantly simplify the control hardware and reduce manufacturing costs, it essentially rules out skip-shifting—that is, shifting to speed ratio other than a numerically adjacent speed ratio. For example, there is no provision for a skip-downshift from 3 rd  to 1 st  because both on-coming clutch C 5  and off-going clutch C 3  have to be controlled by the same modulated valve  64 . An even more difficult situation arises with so-called double-transition skip shifts in which two clutches are released and two clutches are engaged. This occurs, for example, with a skip-downshift from 5 th  to 2 nd , which requires engagement of clutches C 1  and C 4  and disengagement of clutches C 2  and C 3 ; here on-coming clutches C 1  and C 4  and off-going clutch C 2  all have to be controlled by the same modulated valve  62 .  
     [0020] While the above-described problem has previously been handled by performing sequential downshifts instead of a skip-downshift, the present invention provides a method of performing skip-downshifting, whether single transition or double transition. In either event, a commanded skip-downshift is recognized when two or more sequential downshift commands are generated in succession. At such point, TCU  66  aborts the usual downshift control logic, and executes the skip-downshift control logic of the present invention.  
     [0021] In the case of the 3-1 single transition skip-downshift, TCU  66  initially schedules a 3-2 sequential downshift when the driver suddenly depresses the accelerator pedal, and shortly thereafter schedules a 2-1 sequential downshift. The 3-2 downshift involves an off-going control phase during which clutch C 3  is gradually released to allow the engine  12  to accelerate toward the synchronous speed of 2 nd  gear, followed by an oncoming control during which clutch C 4  is engaged to complete the shift. However, if the 2-1 downshift is scheduled before the on-coming control phase of the 3-2 downshift is underway, TCU  66  recognizes that a skip-downshift is needed, and carries out the skip-downshift control of the present invention. In general, this involves a concurrent control of both engine torque and off-going clutch release. Releasing the off-going clutch C 3  effectively shifts the transmission  14  to neutral, and the engine torque command controls the rate at which the transmission input speed TIS increases toward the synchronous speed of the target gear (1 st ). Once the off-going clutch C 3  is released, the TCU  66  re-configures the relay valves  60  to couple the modulated valve  64  to the on-coming clutch C 5  for the target gear, and then activates the modulated valve  64  to fill and engage the on-coming clutch C 5 . The time required to re-configure the relay valve  60  and to engage the on-coming clutch C 5  can be easily estimated, and the engine torque command is calculated so that the transmission input speed TIS reaches the synchronous speed of the target gear (1 st ) some time after the estimated time has elapsed. The engine torque control also includes an adaptive parameter that is adjusted whenever significant timing errors are observed, so that in subsequent skip-downshifts the desired timing will be more nearly achieved.  
     [0022] The above-described skip-downshift is graphically depicted in FIG. 3, where Graph A depicts the engine throttle setting THR, Graph B depicts the transmission input speed TIS, Graph C depicts the engine output torque EOT, Graph D depicts the fluid pressure Pofg supplied to off-going clutch C 3 , and Graph E depicts the fluid pressure Ponc supplied to oncoming clutch C 5 . The throttle setting THR abruptly increases at time T0, and produces a corresponding increase in engine output torque EOT. The increased throttle setting also causes TCU  66  to command a sequential 3-2 downshift, which is initiated at time T1 by dropping the off-going clutch pressure Pofg to a calibrated value, and then decreasing Pofg at a calibrated ramp rate, as seen in Graph D; this allows the input speed TIS to increase toward the synchronization speed for 2 nd  gear, as seen in Graph B. However, by time T2, the throttle setting THR has increased to the point where TCU  66  commands a 2- 1  downshift. This initiates the skip-downshift routine of the present invention, which maintains the progressive reduction of Pofg, as seen in Graph D, and transmits a torque reduction command TQred to ECU  65  for producing a progressive reduction in engine output torque EOT, as seen in Graph C. When the off-going clutch C 3  begins to slip at time T3, Pofg is fully released as seen in Graph D, and the relay valve  60  is re-configured to couple modulated valve  64  to on-coming clutch C 5  instead of off-going clutch C 3 . Additionally, the torque reduction command TQred is adjusted to drop the engine output torque EOT to a value EOTdes that will allow the input speed TIS to reach the 1 st  gear synchronization speed Nsyn at desired time Tdes. As explained below, the torque value EOTdes can be computed based on the required change in input speed TIS, the time interval (Tdes) and an estimate of the engine inertia, and an adaptive term is used to trim the control over the course of one or more skip-shifts to help ensure that TIS actually reaches Nsyn at time (T3+Tdes). The desired time Tdes is determined relative to time T3, and must be at least as long as the time required to re-configure relay valve  60  and to at least partially engage the on-coming clutch C 5 . In the illustration of FIG. 3, the on-coming clutch C 5  is filled in preparation for engagement in the interval T4-T5, whereafter the on-coming pressure Ponc is reduced to a hold value Phold that maintains clutch C 5  in readiness for engagement. At time T6, a calibrated time before the completion of time Tdes, TQred is adjusted to drop the engine output torque EOT to a calibrated value EOTcal to prevent TIS from overshooting Nsyn, and the on-coming pressure Ponc is increased by a calibrated amount and then ramped upward to begin engagement of on-coming clutch C 5 . When TIS reaches Nsyn at time Tsyn (which coincides with time Tdes in the illustrated shift), TQred is adjusted to ramp EOT toward a value corresponding to the engine throttle setting THR, which is reached at time T8, after on-coming clutch C 5  is fully engaged at time T7.  
     [0023] As indicated above, the torque value EOTdes can be computed at time T3 according to the expression:  
       EOTdes=Ieng *[( Nsyn−TIS )/ Tdes]+TQad   (1)  
     [0024] where Ieng is an estimate of the engine inertia, (Nsyn−TIS) is the required change in input speed TIS, and Tad is an adaptive term is used to trim the control over the course of one or more skip-shifts to help ensure that TIS actually reaches Nsyn at time Tdes. In general, the adaptive term TQad can be updated upon completion of each such skip-downshift by executing a routine such as depicted in the flow diagram of FIG. 4. Referring to FIG. 4, the block  90  is first executed to determine if the interval (Tsyn-T3) is greater than Tlate or less than Tearly, where Tlate&gt;Tdes and Tearly&lt;Tdes. If block  90  is answered in the affirmative, there is significant control error, and the blocks  92 ,  94 ,  96  are executed to update the adaptive term TQad. The block  92  computes the actual and desired engine acceleration values ACCELact, ACCELdes according to the expressions:  
       ACCELact =( ESsyn−ES   3 )/( Tsyn−T   3 )  (2)  
       ACCELdes =( ESsyn−ES   3 )/( Tdes )  (3)  
     [0025] where ESsyn is the engine speed ES at time Tsyn, and ES3 is the engine speed ES at time T3. The block  94  then determines calculated and modeled engine torque values TQcalc, TQmod corresponding to the actual and desired engine acceleration values ACCELact, ACCELdes, as follows:  
       TQcalc=Ieng*ACCELact   (4)  
       TQmod=Ieng*ACCELdes   (5)  
     [0026] where Ieng is the engine inertia, as mentioned above. The block  94  then computes an engine torque model error TQerr according to the difference (TQcalc−TQmod), and finally, the block  96  updates the adaptive term Tad as follows:  
       Tad=Tad −( Ksf*TQerr )  (6)  
     [0027] where Ksf is a scale factor. In other words, the term TQad is adaptively updated to compensate for engine torque model errors that cause TIS to reach synchronization speed Nsyn too early or too late by converting the engine acceleration error to a torque error, and increasing or decreasing Tad by a specified percentage of such torque error. If TIS reaches Nsyn too early, the torque error TQerr is positive, and Tad is reduced by the product (Ksf*TQerr) so that in subsequent 3-1 skip-downshifts, EOTdes will be correspondingly reduced for lower engine acceleration in the interval (T3-Tsyn). Conversely, if TIS reaches Nsyn too late, the torque error TQerr is negative, and Tad is increased by the product (Ksf*TQerr) so that in subsequent 3-1 skip-downshifts, EOTdes will be correspondingly increased for higher engine acceleration in the interval (T3-Tsyn).  
     [0028] Double transition skip-downshifts are somewhat more complicated, as more clutches are involved. In the 5-2 skip-downshift, for example, clutches C 1  and C 4  need to be engaged and clutches C 2  and C 3  need to be disengaged. The difficulty is that on-coming clutches C 1  and C 4  both have to be controlled by the same modulated valve  62 . The off-going clutch C 2  is directly exhausted through the relay valves  60  when they are reconfigured to couple on-coming clutch C 4  to modulated valve  62 .  
     [0029] A 5-2 skip-downshift according to this invention is graphically illustrated in FIG. 5, where Graph A depicts the engine throttle setting THR, Graph B depicts the transmission input speed TIS, Graph C depicts the engine output torque EOT, Graph D depicts the fluid pressure Pofg1 supplied to off-going clutch C 3 , Graph E depicts the fluid pressure Pofg2 supplied to off-going clutch C 2 , Graph F depicts the fluid pressure Ponc1 supplied to on-coming clutch C 1 , and Graph G depicts the fluid pressure Ponc2 supplied to on-coming clutch C 4 . The throttle setting THR abruptly increases at time TO, causing TCU  66  to command a sequential 5-4 downshift, which is initiated at time T1 by dropping and then decreasing the pressure Pofg1 supplied to clutch C 3 , and initiating the fill period of oncoming clutch C 1 , as seen in Graphs D and F, respectively. However, by time T2, the throttle setting THR has increased to the point where TCU  66  commands a 4-3 downshift (and then a 3-2 downshift). This initiates the skip-downshift routine of the present invention, which transmits a torque reduction command TQred to ECU  65  for preventing the engine speed ES from exceeding the synchronous speed for 4 th  gear Nsyn4 until the off-going clutch C 3  begins to slip. Hence, when engine speed ES approaches Nsyn4 at time T3, the engine output torque EOT is reduced as required to prevent engine speed ES from overshooting Nsyn4. The off-going clutch C 3  also begins to slip at time T3, whereupon the pressure Pofg1 is progressively increased to help control the rate of increase in input speed TIS. Shortly thereafter at time T4, Ponc1 is increased to fully engage on-coming clutch C 1 . When the synchronous speed Nsyn4 for 4 th  gear is reached and oncoming clutch C 1  is fully engaged, off-going clutch C 3  is fully released as seen in Graph D, and the relay valve  60  is re-configured to couple modulated valve  62  to on-coming clutch C 4  and to couple off-going clutch C 2  (Pofg2) to exhaust. If the pressure switches  74 ,  76 ,  78  indicate that the relay valves  60  have not been reconfigured within a calibrated time, the skip-downshift is aborted, and the 5-4 sequential downshift is completed by releasing off-going clutch C 3  and engaging on-coming clutch C 1 . If the relay valves  60  are reconfigured within the calibrated time, the torque reduction command TQred is adjusted to drop the engine output torque EOT to a value EOTdes that will allow the input speed TIS to reach the 2 nd  gear synchronization speed Nsyn2 upon completion of desired time Tdes. The torque value EOTdes is calculated in the same way as in the single transition skip-downshift, only the synchronous speed Nsyn is the synchronous speed for 2 nd  gear, and Tdes is the desired time for achieving the 2 nd  gear synchronous speed. As also explained in relation to the single transition shifts, an adaptive term is used to trim the engine torque control over the course of one or more skip-shifts to help ensure that TIS actually reaches Nsyn2 at time (T5+Tdes). In the interval T6-T7, the on-coming clutch C 4  is filled in preparation for engagement, whereafter the on-coming pressure Ponc2 is reduced to a hold value Phold that maintains clutch C 4  in readiness for engagement. At time T8, a calibrated time before time (T5+Tdes), TQred is adjusted to drop the engine output torque EOT to a calibrated value EOTcal to prevent TIS from overshooting Nsyn, and the on-coming pressure Ponc2 is increased by a calibrated amount and then ramped upward to begin engagement of oncoming clutch C 4 . When TIS reaches Nsyn at time (T5+Tdes), TQred is adjusted to ramp EOT toward a value corresponding to the engine throttle setting THR, which is reached at time T10, after on-coming clutch C 4  is fully engaged at time T9.  
     [0030] Thus, double transition skip-downshifts are carried out in two stages: a first stage prior to reconfiguration of the relay valves  60  for allowing the input speed to accelerate to the synchronous speed of an intermediate gear, and a second stage following reconfiguration of the relay valves  60  in which the engine torque control of this invention is used to achieve the synchronous speed of the target gear. The transmission is essentially shifted to neutral once the synchronous speed of the intermediate gear is achieved, and the adaptive torque parameter TQad adaptively adjusts the torque control over time as described. However, in certain double transition shifts, it may be desirable to control engine torque in the first stage of the shift so that the synchronous speed of the intermediate gear is achieved at a desired time, instead of holding the engine speed ES at the intermediate synchronous speed.  
     [0031] Thus, the control of the present invention provides a methodology for performing skip-downshifts in a transmission where the control valve configuration is optimized for sequential shifting by allowing the transmission to shift to neutral and adaptively controlling the engine output torque while the transmission is reconfigured to establish the target speed ratio. While described in reference to the illustrated embodiment, it will be understood that various modifications in addition to those mentioned above will occur to those skilled in the art. Thus, it will be understood that control methods incorporating these and other modifications may fall within the scope of this invention, which is defined by the appended claims.