Process and Device for Controlling Locking Differentials

The invention relates to a process and device for continuously controlling the degree of locking of externally controlled locking differentials in a driven axle of a multi-axle vehicle. Externally controlled locking differentials, in this context, are in contrast to standard torque or speed sensing locking differentials.

Locking differentials with external control means usually comprise plate couplings by means of which the open differential may be locked. By using suitable adjusting members it is possible to continuously vary by hydraulic means the locking torque transmitted by the plate coupling. This is important in order to make it possible, largely by way of electronic control, to adapt the locking torque in respect of traction and driving stability to the different slip and friction conditions between the tires and the road surface. With the help of speed sensors at the driven and non-driven axle it is possible to identify the slip conditions of the driving wheels and to prevent wheel spinning by controlling the locking differential.

There are prior art regulating processes which, as a function of the driving situation determined by the wheel speed sensors, set a certain locking effect, and for different driving situations there exist a multitude of different reactions demonstrated by the locking coupling. In view of the different relationships between interfering factors, this leads to complex and therefore intricate and expensive control strategies.

With prior art differentials of the above type, a high degree of slip at the driven axle leads to the locking coupling being completely closed so that with a 100% locking effect both wheels of the driven axle rotate at exactly the same speed. In such a case, evaluation of the speed no longer allows it to be determined whether there is still a need for actuating the locking coupling. Therefore, changes in gripping conditions, wheel load, steering angle or driving torque (interfering factors) can no longer be taken into account for adapting and setting the correct locking torque. Temporarily eliminating the locking effect in order to use the behavior of the wheel speeds for calculating the actual or corrected locking effect for the next time interval would be disadvantageous for driving stability because of a locking torque which increases and decreases at intervals. Actuating a controlled locking differential as described above is particularly disadvantageous for front wheel drive vehicles because of negative steering effects.

Accordingly, it is an object of the present invention to provide a process for continuously controlling the degree of locking of externally controlled locking differentials which reacts quickly when slip conditions at the driven axle change as a result of changes in the interfering factors such as the gripping nature of the road surface, wheel loads and other wheel forces.

Pursuant to this object, and others which will become apparent hereafter, one aspect of the present invention resides in that a nominal value .DELTA..sub.v soll of the differential speed for the driven wheels is determined as a function of the vehicle speed v.sub.F determined at the non-driven wheels. The wheel speed differential .DELTA..sub.v ist between the driven wheels is recorded and readjusted for offsetting the influence of the interfering factors by varying the locking effect on the nominal value .DELTA..sub.v soll of the differential speed always deviating from zero, preferably as long as the wheel speed differential is greater than the nominal value of the differential speed. Since the locking differential is never completely locked, the two speed sensors at the wheels of the driven axle are always able to detect different adhesion locking conditions, the basic principle of the control strategy in accordance with the invention being that the locking coupling of the differential gear should never have a 100% locking effect, but that a limit to the wheel speed differential should always be built in. As long as this limit is below the nominal value of the differential speed, the control circuit is not actuated. By means of such closed control loop control means for the locking torque it is possible to take into account the effect of further interfering factors in addition to changes in the friction valves, for instance changes in the wheel load, the steering angle and the driving torque, indirectly via recording the speed. In a preferred embodiment, especially the nominal value of the differential speed is influenced as a function of the steering angle .delta..sub.L determined especially via the speed recorded at the steered wheels.

In a further embodiment, the process is carried out in such a way that the associated nominal value of the differential speed is low and constant at low driving speeds and increases, especially linearly, at higher driving speeds.

The nominal value of the differential speed assumed at low driving speeds and during starting is selected so as to be as low as possible so that even with the slightest spinning of a driving wheel the locking effect occurs. With an increasing driving speed and an increasing steering angle, the nominal value of the differential speed increases. During braking or when actuating the brake pedal, the invention proposes that the locking effect should be eliminated as quickly as possible so that a locking effect of the driven axle influencing the braking operation is prevented. This is particularly important for the functioning of the anti-blocking system.

A device in the form of a closed loop circuit suitable for carrying out the process is insensitive to interfering signals and even with an actuated locking coupling of the locking differential, it is capable of continuously readjusting the coupling torque in accordance with adhesion locking conditions by evaluating the individual wheel speeds.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

FIG. 1 shows a diagram explaining the influencing factors;

FIG. 2 shows a diagram of a first embodiment of the control circuit in accordance with the invention; and

FIG. 3 shows a diagram of a second embodiment of the control circuit in accordance with the invention.

FIG. 1 shows the non-driven front wheels V.sub.L, V.sub.R, the rear wheels H.sub.L, H.sub.R connected via a differential gear 1 to a locking coupling 2, an internal combustion engine 3 and an electronic control system 4--all forming part of the schematic vehicle drive. Apart from indicating the speed sensing of all four wheels, the diagram indicates by arrows the interrelationships between the electronic control system and the engine control system, the steering system, the automatic transmission and active wheel suspension, whereas the locking coupling is directly connected to the electronic control system 4.

FIGS. 2 and 3 again indicate the non-driven front wheels V.sub.L, V.sub.R, the driven rear wheels H.sub.L, H.sub.R, the differential gear 1, the locking coupling 2 and the internal combustion engine 3. The electronic control unit consists of several units, i.e. a unit 4.sub.1 for calculating the actual speed differential value .DELTA..sub.v ist from the wheel speed at the driven wheels, a unit 4.sub.2 for calculating the driving speed v.sub.F and the steering angle .delta..sub.L at the non-driven wheels, a unit 4.sub.3 for calculating the nominal value of the differential speed as a function of the vehicle speed v.sub.F and the steering angle .delta..sub.L, an adding member for adding the values of the nominal value .DELTA..sub.v ist of the wheel speed and of the nominal value .DELTA..sub.v soll of the differential speed, a PID control unit 4.sub.4 for controlling the locking effect of the locking coupling (FIG. 2) and a signal limiting device 4.sub.5 which outputs no signal if the values added up are positive, as well as a PID control unit 4.sub.6 (FIG. 3).

While the invention has been illustrated and described as embodied in a process and device for controlling locking differentials, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.