1. Field of the Invention
The present invention relates to an apparatus for controlling the amount of slip of a lock-up clutch provided in a power transmitting system of a motor vehicle.
2. Discussion of the Related Art
In a motor vehicle having a fluid-filled power transmitting device equipped with a lock-up clutch such as a torque converter or fluid coupling incorporating such a lock-up clutch, it is proposed to control the lock-up clutch in a slip control mode (partially slipping or engaging mode) such that an actual amount of slip (slip speed) of the lock-up clutch, namely, a difference between the speeds of a pump impeller and a turbine impeller of the power transmitting device eventually coincides with a predetermined target slip speed, for the purpose of improving the fuel economy of the vehicle while minimizing the power loss due to slipping of the lock-up clutch during acceleration of the vehicle. The slip control mode is established when the running condition of the vehicle is in a predetermined slip control area which is intermediate between a fully releasing area in which the lock-up clutch should be held in a fully released state, and a fully engaging area in which the lock-up clutch should be held in a fully engaged state. These fully releasing, fully engaging and slip control areas are defined by parameters (e.g., throttle valve opening and vehicle running speed) indicative of the vehicle running condition. In a motor vehicle equipped with a fuel-cut device adapted to cut a supply of a fuel to the engine while the engine speed is higher than a predetermined fuel-cut speed, it is proposed to control the lock-up clutch in the slip control mode even during deceleration of the vehicle, in order to raise the engine speed and thereby increase the time duration during which the fuel supply to the engine is cut.
While the lock-up clutch is controlled in the slip control mode, the friction condition of the lock-up clutch may be unstable due to deterioration of a working oil or changing of the condition of the friction surface of the lock-up clutch. Consequently, the lock-up clutch may suffer from a phenomenon of so-called "judder" which involves a random or periodic variation or pulsation of the slip speed (amount of slip) of the lock-up clutch. It is desirable to terminate the slip control of the lock-up clutch in the event of detection of the "judder" phenomenon. The judder can be detected by monitoring a variation in the rotating speed of the output shaft of the fluid-filled power transmitting device, that is, a variation in the rotating speed of the turbine impeller. However, the rotating speed of the turbine impeller may have a periodic variation also when the vehicle is running on a bumpy road surface. It is therefore necessary to check if a detected variation in the turbine impeller speed really arises from the judder or a vehicle run on a bumpy road surface. To this end, it is proposed to interrupt the slip control of the lock-up clutch upon detection of an excessive variation in the turbine impeller speed, and check if the excessive variation continues even after the slip control of the lock-up clutch is interrupted. If the excessive variation continues after the interruption of the slip control, this indicates that the excessive variation in the turbine impeller speed is due to a vehicle run on a bumpy road surface. In this case, the slip control of the lock-up clutch is resumed. An example of a lock-up clutch control apparatus arranged as described above is disclosed in JP-A-4-224363.
Where an excessive variation in the turbine impeller speed is detected due to passage of the vehicle through a relatively narrow local raised or recessed area of a generally good road surface (e.g., paved road surface), the excessive variation does not continue for a long time, and the variation is reduced in a short time after the slip control of the lock-up clutch is interrupted. In this case, the lock-up clutch control apparatus indicated above erroneously determines that the detected excessive variation in the turbine impeller speed was due to the judder, even though the excessive variation was in fact due to the instantaneous passage of the vehicle through such narrow local raised or recessed area of the generally good road surface. In this case, the slip control of the lock-up clutch is not resumed, although it is desirable to resume the slip control because the detected excessive variation in the turbine impeller speed is not a result of a judder of the lock-up clutch.
Also proposed is a judder detecting device as disclosed in JP-A-5-172240, wherein judder determining means is inhibited from operating to determine the presence or absence of a judder of the lock-up clutch for if it is determined that the amplitude of vibrations received from the vehicle wheels through a suspension system is large enough to influence the determination by the judder determining means. In this judder detecting device, the judder determining means is less likely to make erroneous determination due to running of the vehicle on a bumpy road surface. However, the judder detecting device requires a vibration sensor for detecting the vibrations due to the bumpy road surface, and is accordingly complicated in construction. Further, the accuracy of determination as to whether the vibrations of the suspension or body of the vehicle will not influence the determination of the judder determining means and the accuracy of determination by the judder determining means are not necessarily satisfactory, since the detected vibrations of the vehicle suspension or body are considerably influenced by the weight and size of the vehicle body and the air pressure in the wheel tires.