DE102008029895A1 - Brake system for vehicle i.e. lorry, has brake controller determining braking torque-error characteristic value, and calculating operating brake-braking demand and/or permanent brake-braking demand based on characteristic value - Google Patents

Abstract

The system has a brake controller (26) connected with braking torque detecting devices (36, 38). The devices measure a braking torque characteristic value that characterizes a real braking torque of a vehicle (10). The controller performs a time-dependent detection of the braking torque characteristic value, and determination of braking torque-error characteristic value that characterizes a deviation between the real braking torque and a total braking torque. The controller calculates operating brake-braking demand and/or permanent brake-braking demand based on the error characteristic value. An independent claim is also included for a method for controlling a brake system.

Description

The The invention relates to a braking system for a vehicle having (a) at least a service brake, (b) at least one retarder, and (c) a brake control, which is set up to perform a method comprising the steps of (i) detecting a vehicle deceleration arrangement, the one to reach delay of the vehicle, (ii) calculating a retarder brake request, the one applied by the retarder retarder braking torque encoded from the vehicle deceleration request, (iii) sending the retarder brake request to the retarder for setting a retarder braking torque, (iv) detecting a Continuous brake status message from the sustained brake via its adjusted retarder braking torque, (v) calculating a service brake request, the one applied by the service brake service brake braking torque encoded based on the vehicle deceleration request and the retarder status message, and (vi) transmitting the service brake request to the service brake for setting a total braking torque from retarder braking torque and service brake braking torque, that of the vehicle deceleration request equivalent.

According to one second aspect, the invention relates to a method for controlling such a brake system.

Such Vehicles are for example commercial vehicles. To the wear of the service brakes too reduce, continuous brakes are provided which generate a braking torque can and show a significantly lower wear. Compared to the Service brakes, which are usually friction brakes, show the Retarder usually a delayed Ansprechverhal th, so that they are not used for the sole braking of the vehicle can.

It is therefore known to control the retarder so that they a generate predetermined retarder braking torque. In time, in this retarder braking torque is built up, take over the service brakes provide the missing brake torque.

To the Determine how big that from the service brakes applied service brake torque must be, for example, by the driver using the brake pedal requested vehicle delay is reached, sends the retarder, in particular via a Data bus, a signal that she uses the applied by her continuous braking brake torque indicates. The brake control calculates from this information that of brake torque to be applied to the service brakes, the vehicle deceleration request to meet.

adversely known braking systems is that they are an undesirable Fluctuation of the braking deceleration tend. Leading for example, that the driver the brake pedal on a constant Holding position, However, the brake system a time-varying delay of Vehicle generated. That's disturbing and thus undesirable.

Of the Invention is based on the object, a brake system and a method to propose to operate such a braking system, in which at vehicle delay request the deceleration caused by the braking system is more even.

The Invention solves the problem of a generic brake system, in which the Brake control with a brake torque detection device, the is arranged to measure a braking torque characteristic, the one real braking torque of the vehicle is characterized, connected, and is set up to perform a method comprising the steps of (i) time-dependent detection of the braking torque characteristic value, (i) determining at least one brake torque error characteristic that is a deviation between the real braking torque and the total braking torque, the the vehicle deceleration request corresponds, describes and (ii) calculating a service brake request and / or the retarder brake request based on the at least a brake torque error characteristic. According to a second aspect, the invention solves the problem by a method according to the independent method claim.

Advantageous At the invention is that the vote between the retarder and the service brake can be significantly improved. It has namely proved that the retarder status messages sent by the persistent brake in some cases not exactly with the actual from the retarder applied, real retarder braking torque matches. The Brake control then determines a faulty service brake braking torque and controls the service brakes accordingly incorrectly. It has It also pointed out that the deviations between those in the Continuous Brake Status Message encoded retarder brake torque and the real applied by the retarder braking torque systematic Are nature so that they are amenable to correction. By the invention a self-calibration is possible.

Another advantage is that the invention with simple means can be implemented. Thus, the sensors and sensing devices necessary to detect the parameters necessary to the process are typically present in the vehicles. It is therefore only necessary to modify the brake control so that it performs the specified steps in the operation of the brake system. This is easy and inexpensive.

One Another advantage is that the braking torque error characteristic simultaneously to diagnose any malfunction of the service brakes or the duration of braking can be used. Bring the brakes for example, constantly one outside a predetermined tolerance interval lying on braking torque, so this indicates a malfunction that makes a repair necessary. damage on the retarder or the service brake can be so fast and with less effort be recognized.

in the The scope of the present description is under a service brake in particular a friction brake understood in the two friction partners be brought into frictional contact with each other, so that a braking torque is built.

Under a retarder is in particular a primary acting retarder (primary retarder) or a secondary acting retarder (secondary retarder) Understood. A primary retarder is arranged in the power flow between the engine and transmission. Examples for primary retarders are engine brakes as exhaust damper brakes or as a valve-controlling continuous brakes. secondary retarder are arranged in the power flow behind the transmission, for example on the propshaft and include, for example, hydraulically acting Retarder or eddy current brakes.

Under the brake control is understood in particular to be any device which is arranged and adapted to the service brake and the brakes head for. It does not have to be an independent component act, it is also possible that the brake control is an integral part of another controller is, for example, a central control unit.

Under detecting a vehicle deceleration arrangement In particular, every process is understood in which an information is detected, due to the deceleration of the brake control Vehicle is to initiate. It is possible, but not necessary, that the vehicle deceleration request by a driver, for example by brake pedal, the driving tool is triggered. For example, it is also possible that the vehicle deceleration request from a driver assistance system, an anti-lock braking system or another auxiliary system of the brake system or the vehicle is sent.

at calculating the retarder brake request from the vehicle deceleration request It does not have to be an invoice in the strict mathematical sense act, it is sufficient if, for example, a database is read in the for predetermined vehicle deceleration requirements appropriate retarder braking requirements are stored.

Under In particular, each of the Continuous brake generated message, with this information about it how big that is from it applied retarder braking torque is. The in the Durability Status message encoded slow brake torque is in particular based on Operating parameters of the retarder determined, for example a Coil current and a shaft speed in the case of an eddy current brake.

If parked in the context of the present description on the braking torque is, it is not necessary that this braking torque explicitly represented in the form of a size is. As a rule, braking torques are processed during processing represents the brake control by numbers that have a unique Allow the final encoded braking torque. So it is also possible, that instead of a braking torque detects the respectively associated delay is to represent the corresponding braking torque.

Under In particular, the braking torque detecting device becomes each device understood, which is adapted to capture a value from the can be closed on the real applied braking torque. For example The brake torque detection device comprises a wheel sensor, with a rotational movement of a wheel of the vehicle is detected on which the retarder and / or the service brake act. At known extent can from the signal of the wheel sensor then to the speed of the vehicle and by deriving time on the acceleration getting closed. With known mass of the vehicle, which is on im The prior art can be calculated, then becomes from the delay closed on the concern of the braking moments. In the same way is determined by the known mass of the vehicle from the vehicle deceleration request calculated the necessary braking torque. Such a method belongs to the state of the technique.

Under the braking torque error characteristic is in particular a numerical value, a size or understood a set of numbers or sizes that describes deviations between the braking torque characteristic and the real braking torque.

According to one preferred embodiment describes the at least one braking torque error characteristic at least one time independent relative Deviation between the relative braking torque and the total braking torque, wherein the brake control is arranged so that the calculating the service brake request based on the brake torque error characteristic adjusting the service brake request so that reduces the deviation between the real braking torque and the total braking torque becomes.

In In other words, the brake controller is configured to provide a relative error between the retarder braking torque, the retarding brake in its continuous brake status message, on the one hand, and the real retarder braking torque applied by the retarder on the other hand, and then its retarder brake request adapts accordingly. Provides, for example, the retarder always 5% too low, the brake control increases the Service brake request brake so that this shortfall is balanced becomes. Alternatively increased the brake control the retarder braking torque request this 5%, so that the retarder in the end the correct retarder braking torque applies.

Prefers describes the at least one brake torque error characteristic at least also a time dependent relative deviation between the real braking torque and the total braking torque, wherein the brake control is configured so that the calculating the service brake request based on the brake torque error characteristic adjusting the service brake request so that the mean time-dependent Deviation between the real braking torque and the total braking torque is reduced. This is to be understood in particular that not only time-independent, but also time-dependent deviations be detected alternatively or additively, so that the brake control at any time so corrected service brake request sends the service brakes exactly the service brake braking torque apply that to the desired delay of the vehicle leads.

According to one preferred embodiment the at least one braking torque error characteristic at least one Dead time of the endurance brakes. It has been shown that a faulty dead time especially noticeable Deviations between the vehicle deceleration request and the indeed from the vehicle shown delay leads.

Prefers the brake control is set up so that the calculation of the Service brake request based on the brake torque error characteristic only for a subsequent braking operation takes place. This will avoid that for the Calculation necessary calculation time the reaction speed of the Brake system impaired.

In a preferred embodiment is the brake torque error characteristic over a predetermined number at braking determined. As stated above, has been found to be faulty retarder status messages usually based on systematic errors. It exists however also a stochastic part. The more braking operations used will become smaller to calculate the brake torque error characteristic the variance of the determined brake torque error characteristic value.

Prefers the brake controller is configured to enter a service brake diagnostic mode by performing the following steps: (i) Sending a retarder brake request to the retarders, which encodes a retarder brake torque of zero, and outputting from a message when the brake torque error characteristic is a predetermined one Threshold exceeds. In this case, the service brakes have a malfunction that to correct. Spending the message may cause a message perceivable by the driver is displayed, but that is unnecessary. So it is sufficient that in one place the Information is deposited that exceeded the predetermined threshold is. For example, this message can be transmitted to a fleet management system become.

Preferably, the brake controller is further configured to switch to a sustained brake diagnostic mode in which the following steps are performed: (i) at a first brake, sending a retarder brake request to the retarder wherein the retarder brake request is non-zero retarder (Ii) at a second brake, transmitting a retarder brake request to the retarder that encodes a retarder brake torque of zero, and (iii) outputting a message when the brake torque error score at the first brake exceeds a predetermined threshold and at the second braking a predetermined threshold value falls below. In this case, high true probability that the brakes are faulty.

In addition, according to the invention a vehicle with a braking system described above. In addition, according to the invention Method in which steps are performed, including the service brake control is set up as described above.

in the Below, the invention will be explained in more detail with reference to the accompanying drawings. there shows

1 an inventive vehicle with a brake system according to the invention.

1 schematically shows a vehicle 10 in the form of a truck, a motor 12 and one via a cardan shaft 14 driven drive axle 16 having. The drive axle 16 and a front axle 18 are assigned by associated service brakes 20.1 . 20.2 , which are schematically drawn, braked.

The vehicle 10 also includes a retarder acting as a secondary retarder 22 in the form of a to the cardan shaft 14 engaging electric eddy current brake. The service brakes 20.1 . 20.2 and the retarder 22 are available via a CAN bus 24 in conjunction with a brake control 26 , which in turn is in connection with a not shown central control unit.

In a driver's cabin 28 of the vehicle 10 is a brake pedal 30 via which a driver issues a vehicle deceleration request to the brake controller 26 can deliver. The stronger the brake pedal 30 tilted from a rest position, the greater the vehicle deceleration request.

At a wheel 32 the drive axle 16 and at a wheel 34 the front axle 18 are respective wheel sensors 36 respectively. 38 arranged, the rotational speed ω ₃₂ and ω _{34 of} the wheels 32 respectively. 34 to the brake control 26 to transfer. The brake control 26 calculated from the rotational speeds ω ₃₂ , ω ₃₄ by deriving after the time t a rotational delay ω. _real . In the brake control 26 is also a mass m of the vehicle 10 saved and the brake control 26 is designed to from the rotational delay ω. _real and the mass m a delay a _real and in addition from the radii of the wheels 32 . 34 To calculate a total braking torque M _total , that about the drive axle 16 and the front axle 18 is applied.

The brake control 26 is set up to perform the procedure described below. First, the brake control detects 26 over the CAN bus 24 a vehicle deceleration request a _desired from the brake pedal 30 , From the vehicle deceleration request a _set the brake control calculates 26 a retarder brake request in the form of a via the CAN bus 24 signal to be sent, the one from the retarder 22 applied retarder braking torque M _22, encoded _target . The retarder 22 then increases a coil current in the eddy current brake so that a braking torque M ₂₂ on the cardan shaft 14 builds. The retarder 22 then sends a retarder status message, which encodes an abutment of the retarder brake torque M _{22, status} , to the brake control 26 ,

The brake control 26 calculated on the basis of the vehicle deceleration requirement a _target and the mass m of the vehicle on the one hand and the duration of the permanent brake 22 notified retarder braking torque M _{22, status} a service brake request brake, a service brake braking torque M _20, encoded _target and sends it to the service brake 20 ,

The service brake 20 then brakes the wheels of the drive axle 16 and the front axle 18 in accordance with this service brake braking torque M _{20, target} . Unless the retarder 22 in their retarder _status message the correct retarder braking torque M _22, transmitted _status , add the braking torque to the correct total braking torque M _total .

The wheel sensors 36 . 38 Constantly detect, for example, every 3 milliseconds, the rotational speeds ω ₃₂ , ω _{34 of} the wheels of the vehicle. The brake control 26 calculates a real braking torque M _{real of} the vehicle 10 , The wheel sensors 36 . 38 Together with any existing evaluation logic for the measured values, they are therefore part of a braking torque detection device.

The total braking torque is the sum of the retarder braking torque M _{22, status} and the service brake braking torque M _{20, desired} , which has been sent to the service brakes. The following applies: M _total = M _{22, status} + M _{20, setpoint} . The brake control 26 calculated from the real braking torque M _real and the total braking torque M _total deviation ΔM.

Ideally, the deviation ΔM is always zero. In the real case, the deviation is time-dependent and the time-dependent deviation ΔM (t) is almost always different from zero. So that the time deviation .DELTA.M (t) represents a braking torque-error characteristic value that describes a difference between the actual braking torque M and the _real total braking torque M _total time dependent.

From this deviation ΔM (t) calculates the brake control 26 the parameters described below and thus corrects the service brake request, so that the deviation ΔM (t) is smaller in the time average. Alternatively or additionally, it may be provided that a deviation downwards, in which the vehicle 10 less delayed than the vehicle deceleration request, are weighted more heavily, so that any under braking is reduced.

berechnet. Die Zeitpunkte t₁ und t₂ bezeichnen dabei beispielsweise den Beginn und das Ende eines Bremsvorgangs. Stellt sich dabei heraus, dass δ_rel anzeigt, dass die Dauerbremse 22 stets einen beispielsweise um 5% zu hohen Wert in ihrer Statusmeldung übermittelt, so berechnet die Bremsensteuerung 26 beim nächsten Bremsvorgang die Betriebsbremsen-Bremsanforderung anhand der um diese 5% korrigierten, aus dem Dauerbremsen-Statusnachricht entnommenen Dauerbremsen-Bremsmoment M_22,Status.For this purpose, for example, the time-independent relative deviation δ _rel as

calculated. The times t ₁ and t ₂ indicate, for example, the beginning and end of a braking process. Turns out that δ _rel indicates that the retarder 22 always transmits a value that is too high by 5% in its status message, for example, the brake control calculates 26 at the next braking operation, the service brake request brake on the basis of this 5% corrected, taken from the endurance brake status message retarder braking torque M _{22, status} .

The brake control 26 is also set up to be requested by a driver of the vehicle 10 or switches after a predetermined time interval or after a predetermined route in a service brake diagnostic mode. The brake control sends 26 when the brake pedal 30 is pressed, a retarder brake request to the endurance brakes 22 , which encodes a retarder braking torque M _{22, setpoint} = 0.

Thereafter, as described above, the deviation ΔM (t) is calculated and, in turn, a relative deviation δ _{rel, 20 is} determined therefrom. The thus determined relative deviation δ _{rel, 20} is compared with a threshold δ _{rel, 20, threshold} and a warning message is issued to the driver when this threshold is exceeded. This threshold may be, for example, 0.1. The brake control 26 stores the value δ _{rel, 20} , which has the relative deviation between the service brake request M _{20, target} and the actual of the service brakes then applied braking torque M _{20, real} . This measurement is repeated during a large number of braking operations and an average is calculated from this.

In, for example, predetermined time intervals, the brake control also switches to a continuous brake diagnostic mode in which the relative deviation δ _{rel, 20 is} determined either as described above. Alternatively, the stored value is read out. Alternatively, it will then be deduced from this value to a relative deviation δ _{rel, 22 of} the duration brakes, by simply considering the corresponding differences.

By subtracting from the service brakes actually applied service brake torque M _{20, real} , which can be calculated by the method described above by means of the relative deviations δ _{rel, 20 of} the real total braking torque M _total and by correlation calculation between the braking request and the real Total braking torque also a dead time t _tot the endurance brakes 22 be calculated. This dead time t _tot is from the brake control 26 stored and in subsequent braking operations, the service brake request brake is based on this dead time of the endurance brakes 22 calculated.

It can also be a time-dependent relative deviation and to correct the service brake request for braking be used. One more way is a use of a neural network that uses the backpropagation algorithm corrects the service brake request.

Claims (17)

Braking system for a vehicle ( 10 ) with (a) at least one service brake ( 20 ), (b) at least one continuous brake ( 22 ) and (c) a brake control ( 26 ) arranged to perform a method comprising the steps of (i) detecting a vehicle deceleration request (a _target ) indicative of a deceleration of the vehicle ( 10 ) (ii) calculating a retarder brake request that is one of the retarder ( 22 ) applied to the continuous braking braking torque (M _{12, target} ), from the vehicle deceleration request (a _target ), (iii) sending the retarder braking request to the retarder ( 22 ) for setting the retarder brake torque (M _{22, Soll} ), (iv) detecting a retarder status message from the retarder ( 22 ) via its retarder braking torque (M _{22, status} ), (v) calculating a service brake request that is one of the service brake ( 20 ) applied service brake braking torque (M _{20, target} ), based on the vehicle deceleration request (a _target ) and the retarder status message, and (vi) sending the service brake request to the service brake brake ( 20 ) for setting a total braking torque (M _total ) from the endurance braking torque (M _{22, status} ) and service brake sen braking torque (M _{20, setpoint} ), which corresponds to the vehicle deceleration request (a _target ), characterized in that the brake control ( 26 ) (d) with a braking torque detecting device ( 36 . 38 ) arranged to measure a brake torque characteristic (M _real ) representing a real braking torque (M _real ) of the vehicle ( 10 ), and (e) is arranged to perform a method comprising the steps of: (i) detecting the braking torque characteristic (M _real ) over time, (ii) determining at least one brake torque failure characteristic (ΔM (t), δ _rel , t _tot ) describing a deviation between the real braking torque (M _real ) and the total braking torque (M _total ), and (iii) calculating the service brake request and / or the retarder request based on the at least one brake torque error value (ΔM (t), δ _rel , t _tot ). Braking system according to claim 1, characterized in that - the at least one braking torque error characteristic value (ΔM (t), δ _rel , t _tot ) at least also describes a time-independent relative deviation between the real braking torque (M _real ) and the total braking torque, the brake control ( 26 ) is configured so that calculating the service brake request based on the brake torque error characteristic (ΔM (t), δ _rel ), an adjustment of the service brake request, - so that the deviation between the real braking torque (M _real ) and the Total braking torque (M _total ) is reduced. Brake system according to one of the preceding claims, characterized in that - the at least one braking torque error characteristic value (ΔM (t), δ _rel , t _tot ) at least also a time-dependent relative deviation between the real braking torque (M _real ) and the total braking torque ( M _total ), where - the brake control ( 26 ) is configured such that calculating the service brake request based on the brake torque error characteristic (ΔM (t), δ _rel , t _tot ) comprises adjusting the service brake request, so that the mean time-dependent deviation between the real brake torque (M _reak ) and the total braking torque (M _total ) is reduced. Brake system according to one of the preceding claims, characterized in that the at least one braking torque error characteristic value (ΔM (t), δ _rel , t _tot ) at least also a dead time (t _tot ) of the continuous brakes ( 22 ) describes. Braking system according to one of the preceding claims, characterized in that the calculation of the service brake request for braking on the basis of the braking torque error characteristic value (ΔM (t), δ _rel , t _tot ) takes place only for a subsequent braking operation. Braking system according to claim 5, characterized in that the braking torque error characteristic value (ΔM (t), δ _rel , t _tot ) is determined over a predetermined number of braking operations. Brake system according to one of the preceding claims, characterized in that the brake control ( 26 ) is arranged to switch to a service brake diagnostic mode in which the following steps are performed - sending a retarder brake request to the endurance brakes ( 22 ) encoding a retarder brake torque (M _{22, setpoint} ) of zero, and outputting a message when the brake torque error characteristic value (δ _{rel, 20} ) exceeds a predetermined threshold. Brake system according to one of the preceding claims, characterized in that the brake control ( 26 ) is arranged to switch to a continuous brake diagnostic mode in which the following steps are performed: - at a first braking transmission of a retarder braking request to the retarder ( 22 ), which encodes a non-zero retarder braking torque (M _{22, Soll} ), - in a second braking transmission of a retarder braking request to the retarder ( 22 ) outputting a retarder brake torque (M _{22, setpoint} ) of zero, and outputting a message when the brake torque error characteristic (δ _{rel, 22} ) exceeds a predetermined threshold during the first braking and a predetermined threshold during the second braking below. Vehicle with a braking system according to one of the preceding Claims. Method for controlling a braking system with - a motor ( 12 ), - at least one service brake ( 20 ) and - at least one continuous brake ( 22 ) comprising the steps of: (a) detecting a vehicle deceleration request (a _target ) indicative of a deceleration of the vehicle ( 10 (b) calculating a retarder brake request that is one of the retarder ( 22 ) applied brakes braking torque (M _{22, desired} ) encoded from the vehicle deceleration request (a _desired ), (c) transmitting the retarder brake request to the retarder ( 22 ) for setting the retarder braking torque (M _{22, Soll} ), (d) detecting a retarder status message from the retarder ( 22 via its retarder braking torque (M _{22, status} ), (e) calculating a service brake request for braking, which is one of the service brake ( 20 ) applied service braking brake torque (M _{20, setpoint} ) based on the vehicle deceleration request (a _target ) and the retarder status message and (f) sending the service brake request (M _total ) to the service brake for adjusting a total braking torque Retarder braking torque (M _{22, status} ) and service brake braking torque (M _{20, desired} ) that corresponds to the vehicle deceleration request (a _desired ), characterized by the steps (g) time-dependent detection of a braking torque characteristic that a real braking torque ( M _real ) of the vehicle ( 10 ), (h) determining at least one brake torque error characteristic (ΔM (t), δ _rel , t _tot ) describing a deviation between the real braking torque (M _real ) and the total braking torque (M _total ), and (i ) Calculating a service brake request based on the at least one brake torque error characteristic (ΔM (t), δ _rel , t _tot ). A method according to claim 10, characterized in that - the at least one braking torque error characteristic value (ΔM (t), δ _rel , t _tot ) at least also a time-independent relative deviation between the real braking torque (M _real ) and the total braking torque (M _total ), wherein calculating the service brake request based on the brake torque error characteristic (ΔM (t), δ _rel , t _tot ) comprises adjusting the service brake request, so that the deviation between the actual brake torque (M _real ) and the total braking torque (M _total ) is reduced. Method according to one of claims 10 or 11, characterized in that - the at least one braking torque error characteristic value (ΔM (t), δ _rel , t _tot ) at least also a time-dependent relative deviation between the real braking torque (M _real ) and the total Describes brake torque (M _total ), - wherein calculating the service brake request brake based on the brake torque error characteristic (.DELTA.M (t), δ _rel , t _tot ) adjusting the service brake request brake, - so that the average time-dependent deviation between the real Braking torque (M _real ) and the total braking torque (M _total ) is reduced. Method according to one of claims 10 to 12, characterized in that the at least one braking torque error characteristic at least also a dead time (t _tot ) of the endurance brakes ( 22 ) describes. Method according to one of claims 10 to 13, characterized in that the calculation of the service brake request brake based on the brake torque error characteristic value (.DELTA.M (t), δ _rel , t _tot ) takes place only for a subsequent braking operation. A method according to claim 14, characterized in that the braking torque error characteristic value (ΔM (t), δ _rel , t _tot ) is determined over a predetermined number of braking operations. A method according to any one of the preceding claims, characterized by the step of switching to a service brake diagnostic mode, wherein the following steps are performed - transmitting a retarder brake request to the end brakes that encodes a retarder brake torque (M _{22, status} ) of zero Outputting a message if the brake torque error characteristic value (ΔM (t), δ _rel , t _tot ) exceeds a predetermined threshold value. Method according to one of claims 10 to 16, characterized by the step of switching to a continuous brake diagnostic mode in which the following steps are carried out: - at a first braking transmission of a retarder brake request to the retarder, which is a non-zero retarder Braking torque (M _{22, setpoint} ) coded, - in the case of a second braking transmission of a retarder brake request to the sustained brake, which encodes a retarder braking torque (M _{22, Soll} ) of zero, issuing a message if the braking torque fault characteristic value in the first braking exceeds a predetermined threshold and at the second braking below a predetermined threshold.