Abstract:
A control device for an automatic transmission for a vehicle including a torque converter interposed between an output shaft of an engine and an input shaft of the automatic transmission, the torque converter having a lock-up clutch for mechanically connecting the output shaft and the input shaft in a direct manner, and a lock-up clutch engagement control unit for engaging the lock-up clutch by a predetermined engagement force in a predetermined operational region determined by a throttle angle and a vehicle speed. The control device further includes a shift map having a slip region for the lock-up clutch set in relation to a plurality of shift characteristics preliminarily set according to vehicle speeds, the slip region being defined by a downshift line and a slip start line deviated from the downshift line by a predetermined range of throttle angle toward lower throttle angles. When the throttle angle falls within the slip region before kickdown, the slip control of the lock-up clutch is performed by the lock-up clutch engagement control unit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a control device for an automatic transmission for a vehicle, and more particularly to a control device for an automatic transmission for a vehicle for performing the slip control of a lock-up clutch at a predetermined gear position before kickdown. 
     2. Description of the Related Art 
     In general, an automatic transmission having a torque converter includes a lock-up clutch for directly connecting an output shaft of an engine and an input shaft of the automatic transmission in the condition where the gear position of the automatic transmission is a predetermined gear position and the rotational speed of the engine is greater than or equal to a predetermined rotational speed, in order to improve the fuel economy. A lock-up clutch engagement characteristic line is set in a shift map, and the lock-up clutch is controlled to be engaged at a vehicle speed higher than that corresponding to the lock-up clutch engagement characteristic line. In the case of kickdown, the lock-up clutch is controlled to be disengaged at the time the accelerator pedal angle becomes larger than that corresponding to the lock-up clutch engagement characteristic line. Further, in the case of running on an uphill road, the lock-up clutch engagement characteristic line is shifted toward higher vehicle speeds to limit the engagement of the lock-up clutch in a high vehicle speed region. 
     As mentioned above, the lock-up clutch engagement characteristic line is fixedly set in the shift map in the prior art. Accordingly, in the case that the lock-up clutch engagement characteristic line is shifted toward higher vehicle speeds to reduce the range of the lock-up region, smooth running can be attained. However, the fuel consumption is increased. Conversely, in the case that the range of the lock-up region is set wide, the fuel consumption can be reduced. However, smooth running becomes difficult to attain in this case. Thus, it is difficult to attain both the reduction in fuel consumption and the improvement in drivability. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a control device for an automatic transmission for a vehicle which can attain smooth running and low fuel consumption by performing the slip control of a lock-up clutch before kickdown. 
     In accordance with an aspect of the present invention, there is provided a control device for an automatic transmission for a vehicle, including a torque converter interposed between an output shaft of an engine and an input shaft of the automatic transmission, the torque converter having a lock-up clutch for mechanically connecting the output shaft and the input shaft in a direct manner; lock-up clutch engagement control means for engaging the lock-up clutch by a predetermined engagement force in a predetermined operational region determined by a throttle angle and a vehicle speed; and a shift map having a slip region for the lock-up clutch set in relation to a plurality of shift characteristics preliminarily set according to vehicle speeds, the slip region being defined by a downshift line and a slip start line deviated from the downshift line by a predetermined range of throttle angle toward lower throttle angles; wherein when the throttle angle falls within the slip region before kickdown, the slip control of the lock-up clutch is performed by the lock-up clutch engagement control means. 
     With this configuration, the lock-up clutch can be slipped at an optimum throttle angle before kickdown. Accordingly, smooth running and low fuel consumption can be attained. 
     Preferably, when downshift is not performed within a predetermined period of time from the start of the slip control of the lock-up clutch, the slip control of the lock-up clutch is canceled to engage the lock-up clutch. By canceling the slip control of the lock-up clutch to engage the lock-up clutch in such a case, the fuel economy can be improved. 
     Preferably, the predetermined range of throttle angle is set wider on an uphill road than on a level road. By changing the range of the slip control of the lock-up clutch according to the slope of a road surface, the drivability both on a level road and on an uphill road cab be improved. 
     The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic plan view showing a general configuration of a vehicle including a slip control device for a lock-up clutch according to a preferred embodiment of the present invention; 
         FIG. 2  is a block diagram of an electronic control unit; 
         FIG. 3  is a hydraulic circuit diagram of a torque converter in disengaging the lock-up clutch; 
         FIG. 4  is a hydraulic circuit diagram of the torque converter in engaging the lock-up clutch; 
         FIG. 5  is a flowchart showing the control sequence of the slip control of the lock-up clutch before kickdown according to the preferred embodiment of the present invention; 
         FIG. 6  is a table showing a pre-kickdown lock-up clutch slip determination ΔAP amount according to a vehicle speed and a road surface slope in the case of a fifth gear position; 
         FIG. 7  is a table similar to  FIG. 6  in the case of a fourth gear position; 
         FIG. 8  is a graph showing a lock-up clutch slip control region on a shift map for a level road; 
         FIG. 9  is a graph showing a lock-up clutch slip control region on a shift map for a heavy uphill road; and 
         FIG. 10  is a time chart showing the pre-kickdown lock-up clutch slip control according to the preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a schematic plan view showing a general configuration of a vehicle including the control device according to the present invention. This vehicle is a front-wheel drive vehicle, which includes a pair of left and right drive wheels  6   a  and  6   b  to which the torque of an engine  2  is transmitted through an automatic transmission  4  and a pair of left and right driven wheels  8   a  and  8   b  rotating with running of the vehicle. A known torque converter  14  is interposed between a crankshaft  10  of the engine  2  and a main shaft  12  of the automatic transmission  4 . Referring to  FIG. 3 , there is shown a hydraulic circuit diagram of the torque converter  14  in disengaging a lock-up clutch.  FIG. 4  is a hydraulic circuit diagram of the torque converter  14  in engaging the lock-up clutch. 
     As shown in  FIG. 3 , the torque converter  14  includes a pump impeller  16  connected to the crankshaft  10 , a turbine runner  18  connected to the main shaft  12 , a stator  20  supported through a one-way clutch  22  to a fixed portion, and a lock-up clutch  24  adapted to connect the pump impeller  16  and the turbine runner  18 . The lock-up clutch  24  includes a clutch piston  28  adapted to abut against the inner surface of a torque converter cover  26 . A first oil chamber  30  and a second oil chamber  32  are formed on both sides of the clutch piston  28 . 
     When pressurized oil is supplied to the first oil chamber  30  to bring the clutch piston  28  into abutment against the torque converter cover  26 , the lock-up clutch  24  is engaged to thereby transmit the torque of the crankshaft  10  directly to the main shaft  12 . Conversely, when pressurized oil is supplied to the second oil chamber  32  to bring the clutch piston  28  into separation from the torque converter cover  26 , the lock-up clutch  24  is disengaged to thereby break the mechanical connection between the crankshaft  10  and the main shaft  12 . 
     The hydraulic circuit of the torque converter  14  includes an oil pump  36  for pumping up a hydraulic fluid from an oil tank  34  and a regulator valve  38  for regulating the pressure of the hydraulic fluid from the oil pump  36  to a predetermined regulator pressure. A lock-up shift valve  40  functions to transmit the regulator pressure to the second oil chamber  32  of the torque converter  14  and to connect the first oil chamber  30  to the oil tank  34  when the lock-up clutch  24  is disengaged, whereas functions to transmit the regulator pressure to the first oil chamber  30  of the torque converter  14  and to connect the second oil chamber  32  to a lock-up control valve  42  which will be hereinafter described when the lock-up clutch  24  is engaged. 
     The lock-up control valve  42  functions to relieve the pressure of the hydraulic fluid supplied from the second oil chamber  32  through the lock-up shift valve  40  and to thereby regulate the pressure in the second oil chamber  32 , thereby controlling an engagement force of the lock-up clutch  24 . A lock-up timing valve  44  is operated by a throttle pressure at a high vehicle speed to thereby operate the lock-up control valve  42 , thereby making the second oil chamber  32  open to the atmosphere to fully engage the lock-up clutch  24 . 
     A first solenoid valve  46  is an on/off controlled valve. When the first solenoid valve  46  is turned off, a modulator pressure is transmitted to the left end of the lock-up shift valve  40  to rightward move the spool of the lock-up shift valve  40 , so that the regulator pressure is transmitted to the second oil chamber  32  of the torque converter  14 , and the first oil chamber  30  is connected to the oil tank  34 , thereby disengaging the lock-up clutch  24 . When the first solenoid valve  46  is turned on, the modulator pressure is relieved to leftward move the spool of the lock-up shift valve  40 , so that the regulator pressure is transmitted to the first oil chamber  30  of the torque converter  14 , and the second oil chamber  32  is connected to the lock-up control valve  42 , thereby engaging the lock-up clutch  24 . 
     A second solenoid valve  48  is a linear solenoid valve. When the second solenoid valve  48  is turned off, the modulator pressure is transmitted to the lock-up control valve  42  and the lock-up timing valve  44  to rightward bias the spool of the lock-up control valve  42  and the spool of the lock-up timing valve  44 . When the second solenoid valve  48  is turned on, the modulator pressure is relieved to cancel the above-mentioned biasing force. The degree of opening of the lock-up control valve  42  can be steplessly controlled by changing the value of a current supplied to the second solenoid valve  48 . When the degree of opening of the lock-up control valve  42  is increased, the back pressure in the second oil chamber  32  of the torque converter  14  is decreased to thereby increase the engagement force of the lock-up clutch  24 . Conversely, when the degree of opening of the lock-up control valve  42  is decreased, the back pressure in the second oil chamber  32  of the torque converter  14  is increased to thereby decrease the engagement force of the lock-up clutch  24 . 
     Referring again to  FIG. 1 , the engine  2  is provided with engine speed detecting means  50  for detecting an engine speed Ne, and the automatic transmission  4  is provided with main shaft speed detecting means  52  for detecting a main shaft speed Nm and shift position detecting means  54  for detecting a shift position P. A throttle valve  58  is provided in an intake passage  56 . The throttle valve  58  is provided with throttle angle detecting means  60  for detecting a throttle angle θ TH . Further, each of the rear wheels  8   a  and  8   b  as the driven wheels is provided with vehicle speed detecting means  62  for detecting a vehicle speed V. Reference numeral  53  denotes slope detecting means for detecting the slope of a road surface on which the vehicle is running. In this preferred embodiment, a G sensor for computing the slope from a longitudinal acceleration G of the vehicle is used as the slope detecting means  53 . As a modification, the slope detecting means  53  may be provided by means for directly detecting the angle of inclination of a vehicle body with respect to a horizontal plane. 
       FIG. 2  shows an electronic control unit (ECU)  64  for performing computations on output signals from the various detecting means mentioned above according to a control program and driving the first and second solenoid valves  46  and  48  to control the speed ratio of the torque converter  14 . The electronic control unit  64  includes a central processing unit (CPU)  66  for performing the computations, a read only memory (ROM)  68  preliminarily storing the control program and data such as various tables, and a random access memory (RAM)  70  for temporarily storing the output signals from the various detecting means and the results of the computations. 
     The electronic control unit  64  further includes an input circuit  72  to which the engine speed detecting means  50 , the main shaft speed detecting means  52 , the slope detecting means  53 , the shift position detecting means  54 , the throttle angle detecting means  60 , and the vehicle speed detecting means  62  are connected, and an output circuit  74  to which the first solenoid valve  46  and the second solenoid valve  48  are connected. Thus, the CPU  66  in the electronic control unit  64  performs computations on the various signals input through the input circuit  72  and on the data stored in the ROM  68  according to the control program to be hereinafter described, and controls the values of currents supplied through the output circuit  74  to the first and second solenoid valves  46  and  48 . Accordingly, the engagement force of the lock-up clutch  24  can be changed to control the speed ratio of the torque converter  14 . 
     The slip control of the lock-up clutch before kickdown according to the preferred embodiment of the present invention will now be described in detail with reference to the flowchart shown in  FIG. 5 . In step S 10 , it is determined whether or not the lock-up clutch (LC) is under the slip control, i.e., whether or not a pre-kickdown LC slip control flag F_LCOFPKD is on. If the answer in step S 10  is negative, the program proceeds to step S 11  to retrieve a pre-kickdown LC slip determination ΔAP amount DAPPKD. 
     For example, when the present gear position is a fifth gear position, DAPPKD is set according to the slope of a road surface as shown in  FIG. 6 , whereas when the present gear position is a fourth gear position, DAPPKD is set according to the slope of a road surface as shown in  FIG. 7 . In  FIGS. 6 and 7 , N is a level road, L is a light uphill road, M is a medium uphill road, H is a heavy uphill road, and H2 is a double heavy uphill road. In  FIG. 6 , for example, 0.2 indicates that the throttle angle is 0.2/8, and 0.5 indicates that the throttle angle is 0.5/8. 
     Referring again to the flowchart shown in  FIG. 5 , the program proceeds to step S 12  after retrieving DAPPKD in step S 11 . In step S 12 , it is determined whether or not DAPPKD is 0. If the answer in step S 12  is negative, the program proceeds to step S 13  to determine whether or not downshift is to be performed at the present accelerator pedal angle (AP angle)+DAPPKD. If the answer in step S 13  is affirmative, the program proceeds to step S 14  to start the slip control of the lock-up clutch that has been engaged. Further, the pre-kickdown LC slip control flag F_LCOFPKD is set, and a timer A is set to a predetermined time. 
     Further, the accelerator pedal angle is set to a reference accelerator pedal angle, i.e., a pre-kickdown LC slip control starting accelerator pedal angle APPKDS. By starting the slip control of the lock-up clutch in step S 14 , the engine speed is increased, so that a reduction in linearity due to the engagement of the lock-up clutch can be prevented. Accordingly, smooth running and low fuel consumption can be both attained. If the answer in step S 12  is affirmative, i.e., if DAPPKD=0, it is unnecessary to perform the slip control of the lock-up clutch, and the program is therefore ended. Further, if the answer in step S 13  is negative, the program is ended. 
     After starting the slip control of the lock-up clutch in step S 14 , the determination in step S 10  with the next timing is that the lock-up clutch is under the slip control. Accordingly, the program proceeds to step S 15  to determine whether or not kickdown (KD) has been performed. If the answer in step S 15  is affirmative, the program proceeds to step S 16  to finish the slip control according to the present invention. In other words, the lock-up clutch is reengaged and the pre-kickdown LC slip control flag F_LCOFPKD is reset to 0. 
     If the answer in step S 15  is negative, the program proceeds to step S 17  to determine whether or not the predetermined time set in the timer A has elapsed. If the answer in step S 17  is affirmative, the program proceeds to step S 16  to finish the slip control according to the present invention. If the answer in step S 17  is negative, the program proceeds to step S 18  to determine whether or not the accelerator pedal has been returned, i.e., whether or not AP angle&lt;APPKDS−DAPPKDF, where DAPPKDF stands for a pre-kickdown LC slip control finishing ΔAP amount. If the result in step S 18  is affirmative, the program proceeds to step S 16  to finish the slip control according to the present invention, to reengage the lock-up clutch, and to reset the pre-kickdown LC slip control flag F_LCOFPKD. 
     Referring to  FIG. 8 , there is shown a pre-kickdown LC slip control region on a shift map for a level road according to the preferred embodiment of the present invention. In  FIG. 8 , each heavy line shows an LC slip control start line. In this preferred embodiment, the pre-kickdown LC slip control region on the level road shift map includes an LC slip control region  76  before kickdown from the sixth gear position to the fifth gear position, an LC slip control region  78  before kickdown from the fifth gear position to the fourth gear position, and an LC slip control region  80  before kickdown from the second gear position to the first gear position. 
     As apparent from this shift map, in the condition where the vehicle can be accelerated without slipping the lock-up clutch, the slip control of the lock-up clutch is not performed. In other words, in this condition, each downshift line coincides with the corresponding LC slip control start line. In contrast, in the LC slip control regions  76 ,  78 , and  80 , the lock-up clutch is slipped before reaching the respective downshift lines. Accordingly, the engine speed can be increased to thereby increase the drive force, so that a reduction in linearity due to the engagement of the lock-up clutch can be prevented. 
     Referring to  FIG. 9 , there is shown a pre-kickdown LC slip control region on a shift map for a steep upward slope (heavy uphill road) according to the preferred embodiment of the present invention. In this preferred embodiment, the pre-kickdown LC slip control region on the heavy uphill road shift map includes an LC slip control region  82  before kickdown from the fifth gear position to the fourth gear position, an LC slip control region  84  before kickdown from the fourth gear position to the third gear position, an LC slip control region  86  before kickdown from the third gear position to the second gear position, and an LC slip control region  88  before kickdown from the second gear position to the first gear position. 
     In  FIG. 9 , each heavy line shows an LC slip control start line similar to that shown in  FIG. 8 . However, as apparent from the comparison between  FIG. 8  and  FIG. 9 , the slip control of the lock-up clutch on an uphill road is started at an accelerator pedal angle smaller than that on a level road. In other words, the range of the LC slip control region on an uphill road is set wider than that on a level road. 
     The pre-kickdown LC slip control according to the preferred embodiment of the present invention will now be described more specifically with reference to the time chart shown in  FIG. 10 . In  FIG. 10 , SH stands for a gear position. At the time t 1 , it is determined that downshift is to be performed at AP angle+DAPPKD. Accordingly, the slip control of the lock-up clutch is started with ΔAP=DAPPKD before kickdown. At the time t 2 , kickdown is performed, and the slip control of the lock-up clutch is therefore finished to reengage the lock-up clutch. At the time t 3 , the downshift line is crossed to pass through the upshift line, so that the gear position is upshifted by one. 
     At the time t 4 , the AP angle becomes the pre-kickdown LC slip control starting accelerator pedal angle APPKDS, so that the slip control of the lock-up clutch is started. When the accelerator pedal is returned as shown by a broken line  90 , the change in accelerator pedal angle from the time t 4  to the time t 5  becomes the pre-kickdown LC slip control finishing ΔAP amount=DAPPKDF. Accordingly, at the time t 5 , the slip control of the lock-up clutch is canceled to reengage the lock-up clutch as shown by a broken line  92 . Also when the predetermined time set in the timer A as shown by an arrow  94  has elapsed at the time t 6 , the slip control of the lock-up clutch is canceled to reengage the lock-up clutch. 
     The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.