Heavy motor vehicles are increasingly being equipped with auxiliary brakes, such as retarders, exhaust brakes and compression brakes. Such auxiliary brakes have in the course of time reached an ever higher level of braking performance. Auxiliary brakes are dimensioned to be able to brake vehicles when they are heavily loaded. A heavily loaded vehicle usually results in good friction between braked wheels and the running surface during a braking process. An auxiliary brake can therefore apply a powerful braking effect without risk of the braked wheels beginning to skid relative to the running surface.

When the vehicle driver calls for a corresponding braking effect when the vehicle is travelling unloaded, the grip of the braked wheels on the running surface is significantly inferior, particularly if the running surface is slippery. This results in risk of the braked wheels beginning to skid so much relative to the running surface that the vehicle's ABS system will be activated and disable the functioning of the auxiliary brake. In such situations the vehicle driver has to activate the ordinary brakes, since it takes an auxiliary brake such as a hydraulic retarder about 4 to 5 seconds to be able to resume applying the required braking action after being emptied of hydraulic oil.

Auxiliary brakes such as hydraulic retarders contain circulating hydraulic oil which substantially absorbs the heat energy generated during a braking process. The hydraulic oil is usually cooled by the motor vehicle's ordinary cooling system before the hydraulic oil can be used in the retarder again. A known way of preventing coolant boiling in the cooling system is to use information about the coolant temperature as a basis for limiting the braking effect which the driver calls for from auxiliary brakes.

SUMMARY OF THE INVENTION The object of the present invention is to provide an arrangement and a method for controlling the braking of an auxiliary brake in a vehicle in such a way as to allow substantially optimum adaptation of the auxiliary brake's braking effect to the vehicle's load and the running surface without causing too much skidding.

This object is achieved with the arrangement and method mentioned in the introduction which are characterised by what is indicated in the characterising parts of patent claims 1 and 11. In this way it is usually possible to prevent excessive skidding by appropriate limitation of the value of the braking effect called for, relative to the amount of skidding of the braked wheel on a running surface. The necessary limitation value usually increases with increasing skidding of the braked wheel. The parameter may be a measured and/or calculated percentage of wheel skidding, but the parameter may also be a speed difference between the braked wheel and an unbraked wheel of the vehicle. Such a speed difference is also proportional to the degree of wheel skidding relative to the running surface. Other parameters related to the degree of braked wheel skidding may also be used.

According to a preferred embodiment of the invention, the control unit is designed to limit also on the basis of information about the rate of increase of the parameter the braking effect called for. By taking into account both the parameter value and the rate of increase in the parameter value, the control unit can do a further refined determination of the limiting value of the braking effect called for. This means that the same degree of skidding of a wheel in different situations may provide different values for limiting the braking effect called for, depending on the rate of increase of the degree of skidding. On a graph of parameter value as a function of time, the rate of increase of the parameter at a point in time can be estimated as a derivative at that time. The derivative is estimated with advantage as the difference between the parameter value at a particular time and a previous parameter value, divided by the time difference between the parameter values.

With advantage, the control unit is designed to limit the braking effect called for, at a value calculated according to a mathematical algorithm. Such a mathematical algorithm may be so designed as to suitably weight the value and rate of increase of the parameter

with appropriate factors to make it possible to calculate for the braking effect a limiting value which applies a minimum limitation to the braking effect called for, without the degree of wheel skidding becoming too great.

According to another preferred embodiment of the invention, the control unit is designed to estimate the parameter on the basis of information about the speed of the braked wheel and an unbraked wheel of the vehicle. To this end, suitable speed-detecting sensors may be used to measure the respective wheel speeds and send measurement signals related to the measured speeds to a unit which calculates a parameter value such as the difference in rotation speed between the wheels. A parameter value corresponding to a percentage degree of skidding may also be calculated on the basis of the difference in rotation speed between the wheels. If two or more of the vehicle's wheels are braked by the auxiliary brake, the control unit is with advantage designed to relate the parameter to the braked wheel which exhibits the greatest degree of skidding. The braked wheel most at risk is thus prevented from reaching too great a degree of skidding. With advantage, the control unit is designed to limit the auxiliary brake's braking effect when the parameter exceeds a first limit value. The braking effect called for therefore only begins to be limited when the parameter exceeds said first limit value. Unnecessary limitations of braking effect when the wheel exhibits small degrees of skidding are thus avoided. A suitable such first limit value may be a degree of skidding of 4 to 5%.

According to another preferred embodiment of the invention, the auxiliary brake incorporates a retarder. Hydraulic retarders are the commonest form of auxiliary brake on heavy motor vehicles. Heavy motor vehicles often also incorporate an auxiliary brake in the form of an exhaust brake. Another type of auxiliary brake is a compression brake.

Such auxiliary brakes may be used individually or in combination in a motor vehicle.

According to another preferred embodiment of the invention, the vehicle incorporates an ABS system designed to completely disable the auxiliary brake's braking effect when the parameter exceeds a second limit value. It is above all such a total disablement of the auxiliary brake's braking action that is as far as possible to be prevented by the present invention. The ABS system is normally activated and the auxiliary brake disabled if a braked wheel exceeds a degree of skidding of about 10%. An auxiliary brake in the form

of a hydraulic retarder therefore takes about 4 to 5 seconds to resume being able to apply a necessary braking action after a rapid emptying of hydraulic oil from the retarder. If the ABS system disables the auxiliary brake, the vehicle driver has to depress the brake pedal so that the vehicle's ordinary brakes are activated.

BRIEF DESCRIPTION OF THE DRAWING A preferred embodiment of the invention is described below by way of example with reference to the attached drawing in which: Fig. 1 depicts schematically an embodiment of an arrangement according to the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Fig. 1 depicts schematically a motor vehicle with selected components. The motor vehicle incorporates non-driving wheels 1 and driving wheels 2 which are in contact with a running surface 3. The vehicle incorporates not only ordinary brakes (not depicted in Fig. 1) but also at least one auxiliary brake in the form of a retarder 4. The retarder 4 is usually hydraulic and incorporates a stator and a rotor which form a toroidal space. This toroidal space is designed to be filled with hydraulic oil when it is desired to apply a braking action to the vehicle. The retarder 4 is fitted adjacent to an output shaft from a gearbox 5 which is connected to the vehicle's engine 6. The rotor of the retarder 4 is provided with driving power by the vehicle's driveline, which includes inter alia a propeller shaft 7 for transmission of driving torque to the vehicle's driving wheels 2. An auxiliary brake 1 in the form of a retarder 4 thus only applies braking action to the vehicle's driving wheels 2. A first brake control in the form of a brake pedal 8 and a second brake control in the form of a hand control 9 for activating the retarder 4 are so situated as to be easy for the vehicle driver to operate. The brake controls 8,9 are settable so that they initiate a variable braking effect called for from the retarder 4. The braking effect called for from the retarder 4 is transmitted in the form of a signal to a control unit I I via a control line 10. Using information about such a braking effect called for, the control unit I I is designed to send a control signal to the retarder 4 via a signal line 12. A

first sensor 13 is designed to measure the speed of at least one non-driving wheel 1 and send a measurement signal related to the wheel speed to an ABS unit 15 via a signal line 14. A second sensor 16 is designed to measure the speed of at least one driving wheel 2 and send a measurement signal related to the wheel speed to the ABS unit 15 via a signal line 17. The ABS unit 15 is designed to calculate at short intervals of time the difference between the speed of the non-driving wheel 1 and the driving wheel 2. The calculated speed difference is a parameter which varies with the degree of skidding of the wheel 2 relative to the running surface 3. The speed difference can also be converted to a percentage degree of skidding of the braked wheel 2. A percentage degree of skidding is thus an alternative parameter related to the degree of skidding of the wheel 2 relative to the running surface. If the difference in speed between the non-driving wheel I and the driving wheel 2 exceeds a second limit value which corresponds to a degree of skidding of about 10%, the ABS unit 15 sends a control signal via a signal line 18 to the control unit 11, which initiates immediate emptying of hydraulic fluid from the retarder 4 so that the braking action of the retarder 4 momentarily ceases completely. Via a signal line 19 the control unit 11 receives information about the temperature of the coolant in the vehicle's cooling system and possibly also the temperature of the hydraulic oil. If the coolant or the hydraulic oil exceeds a certain temperature, the control unit 11 is designed to variably adjust the retarder's function downwards within a temperature range.

The technique described above is used in many heavy motor vehicles, but such motor vehicles are increasingly being equipped with auxiliary brakes such as retarders 4 with ever higher levels of braking performance. The retarder 4 is dimensioned to be able to brake the vehicle when it is heavily loaded. When the vehicle is heavily loaded, good friction is usually obtained between the braked wheel 2 and the running surface 3, and a large braking effect can be applied without risk of the braked wheel 2 beginning to skid too much relative to the running surface 3. When the vehicle driver uses a brake control 8,9 to call for a corresponding braking effect when the vehicle is travelling unloaded on a slippery running surface, there is risk of the wheel 2 beginning to skid so much relative to the running surface that the vehicle's ABS system 15 will be activated and the braking function of the retarder 4 be disabled.

When an arrangement and method according to the present invention are used in a motor vehicle as above, sensors 13 and 16 measure at short intervals of time the respective speeds of the non-driving wheel 1 and the driving wheel 2 during at least one braking process. This information is sent to the ABS unit 15, which calculates the speed difference between the wheels I and 2. The control unit l l receives information concerning the calculated speed difference between the wheels 1, 2. If the wheel speed is measured on two or more driving wheels 2, the control unit 11 is designed to estimate the speed difference for the braked wheel 2 which exhibits the greatest degree of skidding relative to the running surface. So long as the degree of skidding during a braking process is below a certain first limit value which may correspond to a percentage degree of skidding of 4%, the control unit 11 will not reduce the braking effect called for by the driver. But if the control unit 11 receives a speed difference which exceeds the first limit value, the control unit 11 is designed to limit the braking effect called for, at a value calculated according to a mathematical algorithm. The mathematical algorithm incorporates a parameter related to the degree of skidding of the braked wheel 2 relative to the running surface 3. Such a parameter may thus be the speed difference between the non-driving wheel 1 and the driving wheel 2. The mathematical algorithm also incorporates an estimate of the rate of increase of the parameter, i. e. in this case the rate of increase of the speed difference. The rate of increase of the speed difference is calculated as the difference between the calculated speed difference and at least one previously calculated speed difference, divided by the time difference between said values. The previously calculated speed difference may be the latest previous calculated value or a value calculated further back in time, e. g. the tenth latest calculated value.

Such a mathematical algorithm advantageously also incorporates suitable constants and control factors. The purpose of the mathematical algorithm is to calculate instantaneously a substantially minimum limiting value of the braking effect called for, which results in maximum probability of the wheel 2 being prevented from exceeding the second limit value when the ABS unit 15 initiates emptying of the retarder 4 and disabling of the latter's braking function. The algorithm thus takes into account both the speed difference and the rate of increase of the speed difference.

When it is desired to brake a motor vehicle by means of an auxiliary brake 4, the driver uses at least of the brake controls 8, 9 to call for a desired braking effect. A control signal

related to the braking effect called for is sent to the control unit 1I via the signal line 10.

At the same time, the control unit 11 receives information at short intervals of time from the ABS unit 15 via the signal line 18 about the speed difference between the non-driving wheel 1 and the driving wheel 2. The control unit 11 assesses initially whether the speed difference exceeds a first limit value which corresponds to a degree of skidding of 4%. If the degree of skidding does not exceed the first limit value, the control unit 11 passes on via the signal line 12 a control signal with an unreduced value relative to the braking effect called for from the retarder 4. If, on the other hand, the speed difference exceeds the limit value, the mathematical algorithm is calculated. The mathematical algorithm calculates a minimum limitation value of the braking effect called for, at which there is great probability that the wheel 2 will be prevented from exceeding the second limit value. Thereafter the control unit 11 sends a control signal to the retarder 4 which initiates a retarder braking effect which is the difference between the braking effect called for and the limiting braking effect calculated by the algorithm. New measured values for the speed difference of the wheels 1,2 are received by the control unit 11 at short intervals of time. Each value received is stored for at least a short time in the control unit 11. For each new value received which exceeds the first limit value, the control unit calculates an appropriate limitation value of the braking effect called for from the retarder 4 at the time. The result is rapid and exact control of the braking effect applied by the retarder 4 to the driving wheel 2 during a braking process. Such control does in most cases prevent the wheel 2 from exceeding a skid value corresponding to the second limit value.

The invention is in no way limited to the embodiment described but may be varied freely within the scopes of the patent claims. For example, substantially any desired parameter related to the degree of skidding of the braked wheel relative to the running surface may be used. The auxiliary brake need not be a retarder but may for example be an exhaust brake or a compression brake. A combination of auxiliary brakes may also be used.