Abstract:
A control apparatus for an automatic transmission of a vehicle having a forward friction element for engaging or disengaging a turbine shaft with the automatic transmission in a forward running direction and a lock-up clutch for directly transmitting a rotation of an engine to the turbine shaft, comprises an abrupt deceleration control means for disengaging the forward engagement element and the lock-up clutch when an abrupt deceleration of the vehicle is detected, and a restoring means for canceling the abrupt deceleration control means and for restoring the forward friction element to an engagement state when an accelerator pedal is depressed for acceleration.

Description:
BACKGROUND OF THE INVENTION  
         [1]    1. 1. Field of the Invention  
           [2]    2. The present invention relates to a control apparatus for controlling an operation of an automatic transmission for a vehicle, and more particularly, to a control apparatus for disengaging a lock-up clutch when an abrupt brake is applied in order to prevent an engine stall.  
           [3]    3. 2. Background Art  
           [4]    4. Japanese Patent Application Laid-open No. Toku Kai-Hei 2-227342 discloses a drive system having a torque converter with a lock-up clutch, a planetary gear type forward and reverse changeover apparatus and a continuously variable transmission (CVT). The drive system is designed to disengage the lock-up clutch of the torque converter when an anti-lock brake (ABS) operates, that is, when the wheel slip is properly controlled, so that an engine stall is prevented.  
           [5]    5. However, particularly when a vehicle is abruptly braked on a road surface having low friction coefficient, even if the vehicle is equipped with ABS, depending upon braking conditions, there is a possibility that a wheel is locked to stop the rotation before the lock-up clutch is released and as a result an engine stall occurs.  
           [6]    6. In order to solve this problem, Japanese Patent Application Laid-open No. Toku-Kai-Hei 4-357357 proposes a drive system in which both forward clutch and reverse brake of the forward and reverse changeover apparatus are set to a disengagement condition. However, releasing both of the forward clutch and reverse brake may cause an overrun of the engine, in case where a driver depresses an accelerator pedal to accelerate the vehicle immediately after an abrupt braking.  
         SUMMARY OF THE INVENTION  
         [7]    7. It is an object of the present invention to provide a control apparatus of an automatic transmission capable of preventing an engine stall when an abrupt brake is applied and also capable of preventing an overrun of the engine when an accelerator pedal is depressed after the abrupt brake is applied. In order to attain the object, the control apparatus for an automatic transmission of a vehicle having a forward friction element for engaging or disengaging a turbine shaft with the automatic transmission in a forward running direction and a lock-up clutch for directly transmitting a rotation of an engine to the turbine shaft, comprises an abrupt deceleration control means for disengaging the forward engagement element and the lock-up clutch when an abrupt deceleration of the vehicle is detected, and a restoring means for canceling the abrupt deceleration control means and for restoring the forward friction element to an engagement state when an accelerator pedal is depressed for acceleration.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [8]    8.FIG. 1 is a skeleton diagram showing a drive system of an automatic transmission having a torque converter with a lock-up clutch;  
         [9]    9.FIG. 2 is a circuit diagram showing a hydraulic control circuit for controlling the drive system of FIG. 1;  
         [10]    10.FIG. 3 is a block diagram showing control processes in a CVT control unit and a clutch control unit; and  
         [11]    11.FIG. 4 is a flowchart showing a flow of control in a control apparatus of an automatic transmission.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [12]    12. Referring now to FIG. 1, reference numeral  1  denotes an engine of which a crankshaft  2  is connected with a converter case  4  of a torque converter  3  through a drive plate  5  and reference numeral  4   a  denotes a pump impeller provided in the converter case  4 . Reference numeral  6  denotes a turbine runner disposed opposite to the pump impeller  4   a  and connected with a turbine shaft  7 . A stator  8  is disposed between the pump impeller  4   a  and the turbine runner  6  and is supported by an one-way clutch  10  attached to a stator supporting shaft  9 . A lock-up clutch  11  mounted on the turbine shaft  7  is designed to engage or disengage with the drive plate  5 . The driving force of the engine  1  is transmitted to the turbine shaft  7  through the torque converter  3  or the lock-up clutch  11 .  
         [13]    13. An apply chamber  11   a  is provided on one side of the lock-up clutch  11  and a release chamber  11   b  is provided on the other side of the lock-up clutch  11 . The torque converter is operative when hydraulic pressure is supplied to the release chamber  11   b  and circulated through the apply chamber  11   a.  On the other hand, the lock-up clutch is engaged when hydraulic pressure is supplied to the apply chamber  11   a  and hydraulic pressure of the release chamber  11   b  is reduced. Further, it is possible to apply a slip control to the lock-up clutch  11  by regulating a pressure of the release chamber  11   a  so as to cause a slip in the lock-up clutch  11 .  
         [14]    14. The driving force of the turbine shaft  7  is transferred to a primary shaft  14  of a continuously variable transmission  13  through a forward and reverse changeover apparatus  12 . The primary shaft  14  is connected with a primary pulley  15  which comprises a fixed sheave  15   a  fixed to the primary shaft  14  and a moving sheave  15   b  provided opposite to the fixed sheave  15   a  and moving slidably in the axial direction on the primary shaft  14  through a ball spline so as to vary the groove width of the pulley  15 .  
         [15]    15. A secondary pulley  17  is mounted on a secondary shaft  16  provided in parallel with the primary pulley  14 . The secondary pulley  17  has a fixed sheave  17   a  fixed to the secondary shaft  16  and a moving sheave  17   b  moving slidably in the axial direction on the secondary shaft  16  so as to vary the groove width of the pulley  17 .  
         [16]    16. A drive belt  18  is looped over the primary and secondary pulleys  15 ,  17  so as to transmit the driving force from the primary pulley  15  to the secondary pulley  17 . The rotating speed of the secondary pulley  16  is continuously varied by means of changing the winding radius of the respective pulleys  15 ,  17  by changing the groove widths of both pulleys  15 ,  17 .  
         [17]    17. In order to vary the groove width of the primary pulley  15 , a cylinder  22  is mounted on the primary shaft  14  so that a primary oil chamber  21  is formed between the moving sheave  15   b  and the cylinder  22 , and in order to vary the groove width of the secondary pulley  17 , a plunger  24  is mounted on the secondary shaft  16  so that a secondary oil chamber  23  is formed between the moving sheave  17   b  and the plunger  24 .  
         [18]    18. The secondary shaft  16  is connected to an intermediate shaft  26   a  through gears  25   a,    25   b,  a gear  26   b  mounted on the intermediate shaft  26   a  meshes with a final reduction gear  28  of a differential  27  and the final reduction gear  28  drives wheels  30   a  and  30   b  through the differential  27  and axles  29   a  and  29   b.    
         [19]    19. Numeral  12  denotes a forward and reverse changeover apparatus, which comprises a sun gear  31  secured to the turbine shaft  7 , a carrier  33  connected with the primary shaft  14 , a pair of planetary pinions  34 ,  35  rotatably mounted on the carrier  33  and meshing with the sun gear  31  and a ring gear  32  provided around the planetary pinions  34 ,  35  and meshing therewith. There is provided a reverse brake  37  between a brake cylinder  36  fixed to a housing of the forward and reverse changeover apparatus  12  and the ring gear  32  and the reverse brake  37  is actuated by a hydraulic piston  38 .  
         [20]    20. The turbine shaft  7  is connected with a clutch drum  41  and a forward clutch  42  is provided between the clutch drum  41  and the carrier  33 . Further, a hydraulic piston  43  is slidably mounted on the clutch drum  41  to actuate the forward clutch  42 . When a hydraulic pressure is supplied to the hydraulic piston  43  to engage the forward clutch  42 , the rotation force of the turbine shaft  7  is transmitted to the primary shaft  14  through the carrier  33  to rotate the turbine shaft  14  in the same direction of the turbine shaft  7 . At this moment, since no hydraulic pressure is supplied to the hydraulic piston  38 , the reverse brake  37  is disengaged.  
         [21]    21. On the other hand, when a hydraulic pressure is supplied to the hydraulic piston  38  under the condition of the forward clutch  42  disengaged, the reverse brake  37  is engaged to restrict the rotation of the ring gear  32 . Therefore, the rotation of the turbine shaft  7  is transmitted to the carrier  33  through the pair of planetary pinions  34 ,  35 . Then, since the ring gear  32  is fixed, the carrier  33  and the primary shaft  14  connected therewith rotate in a reverse direction to that of the turbine shaft  7 .  
         [22]    22. Further, an oil pump  44  is driven by the converter case  4  to actuate hydraulic devices such as the primary pulley  15 , the secondary pulley  17 , the reverse brake  37 , the forward clutch  42  and the like.  
         [23]    23. Referring to FIG. 2, the oil pump  44  sucks oil from an oil pan  45  and discharges a hydraulic pressure from a discharge port. The discharge port is connected through a secondary pressure passage  46  to the secondary oil chamber  23  for actuating the moving sheave  17   b  of the secondary pulley  17  and is connected to a secondary pressure port of a line pressure control valve  47 . The line pressure control valve  47  regulates a secondary pressure Ps supplied to the secondary oil chamber  23  to a value corresponding to a driving force of the drive belt  18 . That is, when the engine output is large, for example when a vehicle travels on an uphill grade or makes a sharp acceleration, the secondary pressure P s  is raised to prevent a slip of the drive belt  18 . When the engine output is small, the secondary pressure P s  is reduced so as to save a loss of the oil pump  44 .  
         [24]    24. The secondary pressure passage  46  is connected with a secondary pressure port of a shift control valve  48 . A primary pressure passage  49  connected to a control pressure port of the shift control valve  48  is connected to the primary oil chamber  21  for actuating the moving sheave  15   b  of the primary pulley  15 . A primary pressure P p  regulated by the shift control valve  48  is supplied to the primary oil chamber  21 . Since the regulated primary pressure P p  is obtained by reducing the secondary pressure P s , it does not exceed the secondary pressure P s . However, since the pressure receiving area of the primary oil chamber  21  is designed to be larger than that of the secondary oil chamber  23 , the clamping force of the drive belt  18  is larger on the primary pulley  15  side than on the secondary pulley  17  side. Accordingly, the speed ratio can be varied continuously by changing the groove width of the primary pulley  15  by means of controlling the primary pressure so that the primary pressure becomes a value corresponding to a target speed ratio and a target shift speed.  
         [25]    25. A selector lever  50  provided in the passenger compartment is interconnected with a manual valve  51  and a reverse signal valve  52  to change over driving modes. A driver operates the selector lever  50  to select either of five ranges, “P” (parking) range, “R” (reverse) range, “N” (neutral) range, “D” (drive) range and “Ds” (sports drive). In coordination with the operation of the selector lever  50 , those valves  51 ,  52  take corresponding positions.  
         [26]    26. The secondary pressure passage  46  is connected to a clutch pressure passage  54  through a clutch pressure control valve  53 . When the selector lever  50  is positioned at either of “N”, “D” and “Ds” ranges, the clutch pressure passage  54  communicates with a pilot pressure passage  55  through the reverse signal valve  52 . Further, the pilot pressure passage  55  is connected to a pilot chamber  56   p  of a switch valve  56  through a branch pressure passage  55   a  and an electromagnetic valve  57  and the switch valve  56  is operated by energizing the electromagnetic valve  57 .  
         [27]    27. The switch valve  56  has a lock-up changeover section  56   a,  an oil cooler changeover section  56   b  and a lock-up release changeover section  56   c  and these are constructed so as to operate concurrently. FIG. 2 indicates a condition where no hydraulic pressure is supplied to the pilot chamber  56   p  of the switch valve  56 . When a hydraulic pressure is supplied to the pilot chamber  56   p,  the switch valve  56  is changed over to other positions.  
         [28]    28. The lock-up changeover section  56   a  has two positions, one for connecting an apply pressure passage  60  communicating with the apply chamber  11   a  with an oil cooler  59  through a cooling passage  58 , another for connecting the apply pressure passage  60  with the clutch pressure passage  54 . The oil cooler changeover section  56   b  has two positions, one for connecting the cooling passage  58  with the apply pressure passage  60 , another for connecting a lubrication oil pressure passage  61  communicating with a lubrication oil pressure port of the line pressure control valve  47  with the cooling passage  58 . The lock-up release changeover section  56   c  has two positions, one for connecting a release pressure passage  62  communicating with the release chamber  11   b  with the lubrication oil pressure passage  61 , another for connecting the release pressure passage  62  with the clutch pressure passage  54  through a slip pressure passage  63 . The forward and reverse changeover section  56   d  has two positions, one for connecting a changeover passage  64  with the slip pressure passage  63 , another for connecting the changeover pressure passage  64  with the clutch pressure passage  54 .  
         [29]    29. The hydraulic piston  38  of the reverse brake  37  is connected with a brake activation pressure passage  65  and the hydraulic piston  43  of the forward clutch  42  is connected with a clutch activation pressure passage  66 . The slip pressure passage  63  is provided with a slip pressure control valve  67  for regulating a slip pressure supplied to the slip pressure passage  63  to a desired pressure in accordance with an outside pilot pressure fed to an outside pilot chamber.  
         [30]    30. Further, in order to supply the outside pilot pressure to the slip pressure control valve  67 , there is provided a pilot pressure passage  68  between an pilot port of the slip pressure control valve  67  and the clutch pressure passage  54 . Further, there is provided an electromagnetic valve  69  in the pilot pressure passage  68  in order to control the pilot pressure. The electromagnetic valve  69  employs a duty solenoid valve in which the pilot pressure is adjusted by varying duty ratios of electric current supplied to a solenoid  69   a.  In this case, in place of the duty solenoid valve, a proportional type electromagnetic relief valve may be used.  
         [31]    31. The slip control of the lock-up clutch  11  is performed in the following manner:  
         [32]    32. When a signal is sent to the electromagnetic valve  57  to feed a hydraulic pressure to the pilot chamber  56   p  of the switch valve  56 , the release chamber  11   b  of the lock-up clutch  11  communicates with the slip pressure  63 . Then, the pressure of the release chamber  11   b  is adjusted through the slip pressure passage  63  by varying the duty ratio between 0% and 100%.  
         [33]    33. When the manual valve  51  is positioned at “D” range or “Ds” range by operating the selector lever  50 , the changeover pressure passage  64  communicates with the clutch activation pressure passage  66  and as a result the forward clutch  42  is engaged by a hydraulic pressure from the clutch pressure passage  54 . On the other hand, the manual valve is positioned at “R” range, the changeover pressure passage  64  communicates with the brake activation pressure passage  65  and as a result the reverse brake  37  is engaged by a hydraulic pressure from the clutch pressure passage  54 .  
         [34]    34. When the manual valve  51  is positioned at other ranges, both forward clutch  42  and reverse brake  37  are disengaged.  
         [35]    35. As shown in FIG. 2, since a clutch pressure P c  is supplied to the electromagnetic valve  57  at respective ranges “N”, “D” and “Ds”, under the ranges “D” and “Ds”, the switch valve  56  is changed over by sending a signal to the electromagnetic valve  57  and as a result the clutch pressure P c  is supplied to the apply chamber  11   a  of the torque converter. Further, a hydraulic pressure from the slip pressure control valve  67  communicates with the release pressure passage  62  and the slip pressure reduced by the electromagnetic valve  69  brings the lock-up clutch  11  into a lock-up condition. At this moment, the pilot pressure activates the clutch pressure control valve  53  through the pilot pressure passage  55  to generate a hydraulic pressure P c1  for activating the forward clutch  42 . This hydraulic pressure P c1  is supplied to the forward clutch  42  through the clutch activation pressure passage  66 .  
         [36]    36. At “R” or “P” range, no hydraulic pressure is supplied to the pilot pressure passage  55 . Under this condition, the clutch pressure control valve  53  generates a hydraulic pressure P c2  to be supplied to the reverse brake  37 . The hydraulic pressure P c2  is established at a higher pressure than the pressure P c1  to be supplied to the forward clutch  42  (P c1 &lt;P c2 ). Thus, at reverse range, a high brake pressure can be secured.  
         [37]    37. Reference numeral  71  denotes a CVT control unit for sending control signals to the line pressure control valve  47  and the shift control valve  48  and reference numeral  72  denotes a clutch control unit for sending control signals to the electromagnetic valves  57 ,  69 . These control units  71 ,  72  are included in a TCU (transmission control unit)  73  integrally.  
         [38]    38. Referring to FIG. 3, the CVT control unit  71  includes a micro-computer which is connected to miscellaneous sensors such as an engine speed sensor  74 , a primary pulley rotational speed sensor  75 , a secondary pulley rotational speed sensor  76  and a throttle opening angle sensor  77 . Signals from these sensors are sent to a continuously variable transmission control section  78  from which control signals are sent to respective solenoids of the line pressure control valve  47  and the shift control valve  48  through respective duty ratio establishing sections  79   a,    79   b.    
         [39]    39. The clutch control unit  72  includes a micro-computer which is connected with miscellaneous sensors, in addition to the aforesaid sensors  74  to  77 , such as a selector lever position sensor  81 , and a brake switch  82 . In a drive range judging section  83 , the current drive range is judged based on signals from the selector lever position sensor  81 . In a lock-up engagement judging section  84 , it is judged based on the signals from the aforesaid sensors  74  to  77  whether or not the lock-up clutch  11  should be engaged. Further, in a deceleration judging section  85 , it is judged based on the signals from the aforesaid sensors  74 ,  76 ,  77  and signals from the brake switch  82 , the front wheel speed sensor  91  and the rear wheel speed sensor  92 , whether or not the vehicle is in an abrupt deceleration.  
         [40]    40. Based on output signals from the drive range judging section  83 , the lock-up engagement judging section  84  and the deceleration judging section  85 , a forward clutch and lock-up clutch control section  86  outputs control signals to a switching section  87  and a duty ratio establishing section  88 . The switching section  87  outputs ON-OFF signals to the solenoid  57   a  of the electromagnetic valve  57  and the duty ratio establishing section  88  outputs duty signals to the solenoid  69   a  of the electromagnetic valve  69 .  
         [41]    41. That is, the clutch control unit  72  outputs ON-OFF signals to the electromagnetic valve  57  so as to engage or disengage the lock-up clutch  11  and also outputs duty signals to the electromagnetic valve  69  so as to control the hydraulic pressure for actuating the forward clutch  42 .  
         [42]    42. Thus, when the vehicle is abruptly decelerated, a signal for setting the pilot pressure of the pilot passage  68  at zero is outputted to the solenoid  69   a  of the electromagnetic valve  69 , and at the same time, an OFF signal for setting the pilot pressure of the pilot passage  55   a  at zero is outputted to the solenoid  57   a  of the electromagnetic valve  57 . As a result, the forward clutch  42  and the lock-up clutch  11  are disengaged, thereby an occurrence of the engine stall can be prevented. Further, when the accelerator pedal is depressed immediately after the abrupt acceleration, since the forward clutch  42  is engaged again, an overrun of the engine can be prevented.  
         [43]    43. In this embodiment, whether the vehicle is in an abrupt acceleration is judged based on a signal from the secondary pulley rotational speed sensor  76 , however the judgment of an abrupt acceleration may be performed by an ABS operation signal. In this case, as shown in FIG. 3, the deceleration judging section  85  receives signals from a front wheel speed sensor  91  and a rear wheel speed sensor  92 , respectively and judges an abrupt acceleration of the vehicle.  
         [44]    44. Next, an operation of the control apparatus will be described with reference to a flowchart in FIG. 4.  
         [45]    45. First, at a step S 1 , it is judged whether or not the vehicle is in a running mode, namely, the selector lever is positioned at “D” or “Ds” range. If it is judged that the vehicle is in a running mode, at a step S 2  a hydraulic pressure P c1  is supplied to the hydraulic piston  43  of the forward clutch  42  to engage the forward clutch  42 . The hydraulic pressure P c1  is controlled according to a duty ratio of electric current supplied from the clutch control unit  72  to the solenoid  69   a  of the electromagnetic valve  69 .  
         [46]    46. Then, at a step S 3 , it is judged whether or not a lock-up condition, that is, a running condition under which the lock-up clutch  11  is engaged, is satisfied. If the lock-up condition is satisfied, at a step S 4  the lock-up clutch  11  is engaged. Generally, the lock-up condition is satisfied when the vehicle speed is larger than a specified value and the changing rate of the engine speed is smaller than a specified value.  
         [47]    47. Thus, a signal is sent to the solenoid  57  of the electromagnetic valve  57  to change the switch valve  56  from a position indicated in FIG. 2 to another position. As a result, a hydraulic pressure P c  is fed to the apply chamber  11 . On the other hand, when the engagement condition is not satisfied, at a step S 5  the switch valve  56  is set at a position where the lock-up clutch  11  is disengaged, namely, is set in a released condition.  
         [48]    48. At a step S 6 , it is judged whether or not an abrupt deceleration control condition, that is, a running condition under which an abrupt deceleration control is operated, is satisfied. If the abrupt deceleration control condition is satisfied, at a step S 7  the forward clutch  42  is released and at a step S 8  the lock-up clutch  11  is also released.  
         [49]    49. The condition under which the abrupt deceleration control is operated is that the brake switch  82  is turned ON, the throttle opening angle sensor  77  detects a fully closed throttle and the deceleration of the vehicle is larger than a specified value V B .  
         [50]    50. In case of a vehicle employing the continuously variable transmission  13 , since the vehicle speed can be detected by a signal from the secondary pulley rotational speed sensor  76 , it can be detected whether the deceleration of the vehicle is larger than the specified value V B  or not, by detecting a revolution number N s  of the secondary pulley. For example, the abrupt deceleration condition can be judged by detecting that a differential of N s  with respect to time has exceeded the specified value V B , or a rate of change of the vehicle speed has exceeded a certain value.  
         [51]    51. Accordingly, when the brake is operated, a signal is sent from the brake switch  82  to the clutch control unit  72 , a deceleration (dN s /dt) of the vehicle is calculated based on a Ns signal from the secondary pulley rotational speed sensor  76 , and it is judged that the deceleration is larger than the specified value V B  (abrupt deceleration), the electromagnetic valve  57  is deenergized and at the same time a control signal of 100% duty ratio is sent to the electromagnetic valve  69  for the slip pressure control. Thus, the forward clutch  42  is released and further the hydraulic pressure in the apply chamber  11   a  of the lock-up clutch  11  is drained outside through the oil cooler  59 , thereby an occurrence of the engine stall can be prevented.  
         [52]    52. The condition for performing the abrupt deceleration control can be considered otherwise than described before. For example, in case where the decrement of the engine speed N e  becomes larger than a specified value N B  while the lock-up clutch  11  is in an engagement condition, that is, in case of dN e /dt&gt;N B , the sudden deceleration control may be performed. Further, in this case, the abrupt deceleration control may be suspended when the engine speed N e  is higher than a specified value in consideration of the effect of engine brake.  
         [53]    53. A step S 9  is for checking a restoring condition for canceling the abrupt deceleration control and for restoring the engagement of the forward clutch  42 . While the condition is not satisfied, both forward clutch and  42  and lock-up clutch  11  continue to be released to prevent an engine stall. If the condition is satisfied, the program returns to the step S 1  and when the accelerator pedal is depressed for acceleration immediately after the abrupt deceleration, the forward clutch  42  is engaged again to prevent an engine overrun.  
         [54]    54. The abrupt deceleration control is canceled, when either of the following conditions is satisfied; a case where the throttle opening angle sensor  77  detects a depression of the accelerator pedal, a case where the vehicle acceleration is larger than a specified value, a case where the engine speed is larger than a specified value, a case where a specified time (for example, 0.2 to 1 second) has elapsed after releasing the lock-up clutch  11  or a case where a difference between the engine speed N e  and the turbine speed N t  is larger than a specified value, for example N t /N p  is 60 to 80%.  
         [55]    55. The construction of the forward and reverse changeover apparatus  12  is not limited to the one shown in the embodiment of the present invention. Further, in this embodiment, the automatic transmission is formed by a continuously variable transmission but the control apparatus according to the present invention can be applied to other types of automatic transmissions.  
         [56]    56. While the presently preferred embodiment of the present invention has been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.