Automatic clutches have been used for quite some time in the power trains of various motor vehicle types. In particular, a main or startup clutch located between a propulsion engine and a transmission and normally configured as a dry friction clutch, is used to shiftably transfer the torque produced by the propulsion engine to the input shaft of the transmission and enable the startup, particularly with transmissions, with discrete gear ratios. Clutches of this type are referred to as startup clutches.
Since internal combustion engines can only provide a torque, sufficient for the startup, above a certain engine rotational speed; a startup with a slipping clutch is necessary even when starting in the lowest gear of the transmission after the vehicle has been stationary. In fully manually shifted transmissions, the driver, based on his experience, can dose the engine rotational speed and the engine torque and the clutch slip and the torque transmitted by the clutch by actuating the accelerator pedal and the clutch pedal at the same time in such a way that with a slipping clutch initially a driving speed of the vehicle is achieved, which allows the startup clutch to be fully engaged at the selected gear ratio of the transmission. The driver must dynamically adjust the positions of the two pedals in relation to each other, particularly depending upon the driving resistance (startup resistance). With high load and startup processes on steep grades, the driver must considerably depress the accelerator pedal in order to generate a sufficient engine torque for startup on that grade. This engine torque is available at a comparatively high engine rotational speed. The clutch must be engaged slowly to prevent too much torque from being withdrawn from the engine as a result of premature excessive engagement which, due to the inert weight of the vehicle and the very low driving speed of the same, would cause a rapid drop in the engine rotational speed and stalling of the engine.
Semi-automatic, automatic and automated transmissions with corresponding automatically operable or automated startup clutches were developed to relieve the driver from having to perform these complicated control tasks. These are generally designed in such a way that a startup operation is possible with a relatively low clutch load so that overall lower clutch slip during the startup process and relatively early complete engagement of the automated clutch are possible.
Two different operating modes of the motor vehicle should be taken into consideration.
In the startup mode, as explained above, it is desirable to allow the clutch to slip only as long as necessary and reach a driving state with a fully engaged clutch as smoothly as possible.
In the maneuvering mode, the vehicle is supposed to be moved at a very low speed. For this reason, it is generally necessary to allow the clutch to slip for a longer period due to the speed-torque curve of the propulsion engine and the predefined lowest gear ratio. In this case, the startup clutch is fully engaged clearly much more slowly and with considerably greater accelerator pedal deflections than in the startup mode.
With automated clutches, however, it is difficult to determine the driver's true request and adjust the clutch accordingly, simply based on the accelerator pedal position and/or the movement data of the accelerator pedal. Without further provisions, the maneuvering mode range is typically limited to small gears and relatively small accelerator pedal deflections, which make careful and precise maneuvering extremely difficult, particularly when a certain level of pedal deflection of the accelerator pedal is already required for stopping the vehicle on an incline.
In order to increase the deflection range of the accelerator pedal available for maneuvering, it is known to provide a specific increased pedal path of the accelerator pedal for a maneuvering mode, particularly in the lowest forward gear and the reverse gear or the reverse gear with the lowest gear ratio. During a deflection of the accelerator pedal of up to 75% of the maximum deflection, the pressure force of the startup clutch is considerably reduced in these gears in comparison with the startup mode. However, if the driver achieves an accelerator pedal deflection of 75%, as in this case, the startup mode is automatically activated. When the 75% threshold is exceeded in the maneuvering mode, which is frequently critical for safety, this may result in a vehicle behavior that is unexpected for the driver. This is particularly true if, due to unusually high driving resistance as a result of a high load and a steep gradient, no vehicle speed or only a very low vehicle speed was reached by the time this threshold value for the accelerator pedal deflection was reached. In this case, if the driver further deflects the accelerator pedal to end rolling back as quickly as possible, there is an acute risk that as a result he will exceed the threshold for a change into the startup mode and the vehicle will move forward unexpectedly at a comparatively high speed.
A known solution is to explicitly predefine the operating mode of the motor vehicle with the help of input provided by the driver. For this purpose, special shifting elements may be provided in the vehicle. A particularly advantageous solution is known from the previously unpublished application DE 10 2005 021 414 of the Applicant, wherein a sensor element enables the driver to request a maneuvering mode. Other known solutions provide switches with two switch positions for this purpose. However, particularly when maneuvering on gradients, there is a risk that the vehicle will accidentally roll down the gradient or will at least react differently than the driver is accustomed to from standard maneuvering operations on level ground, due to the intentionally reduced pressure force of the startup clutch.
In order to enable a fast actuation of a brake due to the frequently higher risk potential in maneuvering mode, the Applicant discloses a method in EP 1 268 231 B1, wherein a controlled inching procedure of the vehicle is automatically initiated while the gas pedal is not activated following the actuation of a control device. The creeping speed can be predetermine depending upon the activation time or the number of times that the control device was activated.
These solutions make a considerable contribution for simplification of the operation of the vehicle and for an improvement of the safety. However, it is necessary for the driver to remember to actuate the shifting element to set the creeping gear. Furthermore, fixing a preselected speed does not always meet practical requirements in an ideal manner, wherein the speed is increasingly reduced only when approaching a loading ramp or the like. Even if the vehicle speed can be adjusted with the help of the operating element, this process does not correspond to the intuitive speed adjustment, via the accelerator pedal, to which the driver is accustomed.
Overall, it would be desirable if the vehicle behavior, in response to a pedal deflection brought about by the driver, could be decoupled to a greater extent from the factors influencing the driving resistance, which act on the vehicle at that time, particularly the gradient under the vehicle and the load. In this way, a substantially uniform vehicle behavior could be achieved for different operating conditions, thus creating considerably improved comfort and safety for less experienced drivers, particularly during maneuvering procedures.
Under these circumstances, it is the object of the invention to provide a method for controlling or regulating a clutch torque of a startup clutch of a motor vehicle which, first of all, reliably prevents unintentional rolling of the motor vehicle down a grade and, second, enables a comfortable maneuvering mode, which a separate actuation of an operating element is not required, in order to activate the maneuvering mode, and also enables a precisely dosed speed definition when maneuvering on grades, where the resulting vehicle speed is supposed to be substantially independent from the gradient and load.