Brake control arrangement and method for controlling the reference speed thereof

TECHNICAL FI ELD

The present invention relates to a brake control arrangement for a motor vehicle according to the preamble of claim 13. The invention also relates to a method for controlling the reference speed of such a brake control arrangement. Furthermore, the invention relates to a computer program comprising computer program code for implementing a method according to the invention, a computer program product comprising a data storage medium readable by an electronic control unit and having said computer program stored thereon, and an electronic control unit.

BACKGROUND ART

Heavy motor vehicles, such as lorries, towing vehicles and buses, are often provided with a brake control arrangement which can be enabled by the driver of the vehicle in order to automatically control one or several brakes of the vehicle so as to restrict the speed of the vehicle when the vehicle descends a downhill slope. A target speed for the brake control arrangement is settable by the driver. In this description and the subsequent claims, this target speed for the brake control arrangement is referred to as "reference speed". In dependence on information as to the set reference speed and the actual speed of the vehicle, the brake control arrangement will endeavour to restrict the speed of the vehicle to the reference speed. Such a brake control arrangement is normally adapted to control one or several auxiliary brakes of the vehicle. An auxiliary brake is a supplement to the ordinary wheel brakes of the vehicle and is configured to produce a retarding torque for decreasing the rotational speed of the engine crankshaft. An auxiliary brake may for instance be an engine brake, an exhaust brake, an electromagnetic retarder or a hydraulic retarder. The auxiliary brakes are with advantage used for constant speed keeping purposes in downhill slopes and for

moderate decelerations, whereas the vehicle's ordinary wheel brakes are mainly used for powerful and sudden braking operations. With the use of auxiliary brakes, the ordinary wheel brakes, which normally constitute friction brakes of disc brake or drum brake type, are prevented from overheating with the associated risk of brake failure and the wear thereof is reduced.

Brake control arrangements of the above-mentioned type are for instance known from EP 0 336 913 A2 and EP 0 825 933 A1 .

US 5 634 446 A discloses a cruise control system provided with brake control means which controls auxiliary brakes of a vehicle in order to restrict the speed of the vehicle to a given reference speed when the vehicle accelerates under the effect of gravity in a downhill slope to a speed above the ordinary target speed set for the cruise control system.

DISCLOSURE OF THE INVENTION

The object of the present invention is to propose a new and advantageous manner of regulating the reference speed of a brake control arrangement of a motor vehicle.

According to the invention, this object is achieved by a method having the features defined in claim 1 and a brake control arrangement having the features defined in claim 13.

According to the invention, the reference speed, which is set to a basic value that is adjustable by the driver of the vehicle, is automatically and temporarily increased from the basic value to a modified value when it is established that one or several given conditions are fulfilled so as to thereby allow the vehicle to be accelerated under the effect of gravity in a downhill slope to a speed above the speed corresponding to the basic value. Hereby, the vehicle may under certain conditions accelerate to and temporarily maintain a speed above the speed corresponding to said basic value so as to take additional advantage of the ki-

netic energy provided "for free" by the acceleration of the vehicle due to gravity in a downhill slope descended by the vehicle. This will create opportunities for improved fuel economy and a higher mean speed of the vehicle. Said increase of the reference speed should be kept so small that it won't jeopardize the driving safety and won't feel inconvenient to the driver, but large enough to be capable of giving a beneficial contribution to the fuel economy and/or mean speed of the vehicle. The increase of the reference speed from the basic value to the modified value suitably corresponds to a speed increase in the order of 1 -5 km/h.

According to an embodiment of the invention:

- a braking power value representing the magnitude of the braking power ordered by the brake control arrangement is repeatedly or continuously established during a braking operation controlled by the brake control arrangement;

- the largest braking power value established during an ongoing braking operation is stored in a memory; and

- the reference speed is increased from the basic value to the modified value when it is established that the braking power value has dropped during an ongoing braking operation to a given level below the largest braking power value stored during the ongoing braking operation.

This embodiment is based on the fact that the braking power ordered by the brake control arrangement will be gradually reduced as the declivity of a downhill slope descended by the vehicle is reduced. The fact that the braking power value has dropped during an ongoing braking operation to a given level below the largest braking power value stored during the ongoing braking operation is in this case taken as an indication that the vehicle is approaching the end of a downhill slope, and the speed of the vehicle is then allowed to be boosted to a certain extent by the increase of the reference speed at the presumed end of the downhill slope so as to thereby take additional advantage of the acceleration of the vehicle due to gravity and allow the vehicle to leave the downhill slope rolling at an increased speed somewhat higher than the reference speed. Hereby, the vehicle is allowed

to roll a longer distance after the downhill slope before it is necessary to throttle up in order to maintain the vehicle at a desired minimum speed, as compared to the case when the vehicle leaves the downhill slope rolling at the reference speed.

According to an alternative embodiment:

- a declivity value representing the declivity of a downhill slope presently descended by the vehicle is repeatedly or continuously established during a braking operation controlled by the brake control arrangement; and

- the reference speed is increased from the basic value to the modified value when it is established that the declivity value has dropped during an ongoing braking operation below a given threshold value from a value above this threshold value.

The fact that the declivity value has dropped during an ongoing braking operation below a given threshold value is in this case taken as an indication that the vehicle is approaching the end of a downhill slope, and the speed of the vehicle is then allowed to be boosted to a certain extent by the increase of the reference speed at the presumed end of the downhill slope.

According to another alternative embodiment:

- a driving resistance value representing the driving resistance of the vehicle is repeatedly or continuously established during a braking operation controlled by the brake control arrangement; and

- the reference speed is increased from the basic value to the modified value when it is established that the driving resistance value has increased during an ongoing braking operation above a given threshold value from a value below this threshold value. The fact that the driving resistance value has increased during an ongoing braking operation above a given threshold value is in this case taken as an indication that the vehicle is approaching the end of a downhill slope, and the speed of the vehicle is then allowed to be boosted to a certain extent by the increase of the reference speed at the presumed end of the downhill slope.

According to another embodiment of the invention, the reference speed is increased from the basic value to the modified value when it is established that the following conditions are fulfilled:

- the output torque of the vehicle engine has been decreased to zero,

- the speed of the vehicle was above a given threshold speed when the engine output torque reached zero, and

- the vehicle has accelerated to reach the reference speed or a speed at a given level below the reference speed within a given time limit from the moment when the engine output torque reached zero.

Hereby, it will be possible to achieve a certain delay of the initiation of a braking operation by the brake control arrangement in a situation when the vehicle, for instance due to an acceleration in a previous downhill slope, enters a downhill slope at such a high speed that the vehicle very soon reaches the set reference speed for the brake control arrangement.

Further advantageous features of the method and the brake control arrangement according to the invention are indicated in the dependent claims and the following description.

The invention also relates to a computer program having the features defined in claim 18, a computer program product having the features defined in claim 19 and an electronic control unit having the features defined in claim 20.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, a specific description of embodiments of the invention cited as examples follows below. In the drawings:

Fig 1 is a schematic diagram of a combustion engine with an associated driveline, illustrating an embodiment of a brake control arrangement according to the present invention,

Fig 2 is a schematic outline diagram of an electronic control unit for implementing a method according to the invention,

Fig 3 is a flow diagram illustrating a method according to an embodiment of the invention, and

Fig 4 is a flow diagram illustrating a method according to another embodiment of the invention.

MODES FOR CARRYING OUT THE INVENTION

A part of a drive line 1 of a motor vehicle 40 is schematically illustrated in Fig 1 . The drive line 1 comprises a vehicle engine 2, for instance in the form of a conventional combustion engine, which is connected to driving wheels (not shown) of the vehicle via a clutch 3 and a gearbox 4. The output shaft 5 of the engine 1 is connected to the input shaft 6a of the gearbox via said clutch 3, which is arranged to transfer the torque exerted by the engine to the input shaft 6a of the gearbox.

The vehicle is provided with a brake control arrangement 20, which comprises brake control means 21 adapted to control the braking power of one or several brakes of the vehicle in dependence on information as to a reference speed S _ref and the actual speed of the vehicle so as to automatically endeavour to restrict the speed of the vehicle to the reference speed S _ref when the vehicle descends a downhill slope with the brake control arrangement enabled. If the driving resistance of the vehicle is negative, the brake control means 21 will control said brake or brakes so as to strive to restrict the speed of the vehicle to the reference speed S _rβf . Thus, the brake control means 21 will increase the braking power when the speed of the vehicle increases above the reference speed S _ref and reduce the braking power when the speed of the vehicle falls below the reference speed.

The brake control arrangement 20 further comprises speed setting means 22, by means of which the driver of the vehicle may set the reference speed S _ref to a desired basic value V _bas . The brake control arrangement 20 further comprises processing means 23 adapted to automatically and temporarily increase the reference speed S _ref from the basic value V _bas to a modified value V _mod when it has established that one or several given conditions are fulfilled so as to thereby allow the vehicle to be accelerated under the effect of gravity in a downhill slope to a speed above the speed corresponding to the basic value V _bas .

The brake control means 21 may be implemented by an electronic control unit 25. The processing means 23 may be integrated in one and the same unit 25 as the brake control means 21 , as illustrated in Fig 1 , but may alternatively be arranged in a separate electronic control unit which is connected to the brake control means 21 so as to send information thereto as to the value of the reference speed S _ref .

The brake control means 21 may be adapted to control one or several auxiliary brakes of the vehicle, such as an engine brake and/or an exhaust brake and/or an electromagnetic retarder and/or a hydraulic retarder.

The brake control arrangement 20 may be adapted to operate independently when enabled by the driver, in which case the speed setting means 22 may be configured to allow the driver to directly set the magnitude of the basic value V _bas for the reference speed S _ref . However, the brake control arrangement 20 may also be adapted to operate under the supervision of a cruise control system which, when enabled by the driver, controls the operation of the vehicle engine 2 so as to endeavour to keep the speed of the vehicle essentially equal to a target speed set by the driver. If the vehicle descends a downhill slope with the cruise control system and the brake control arrangement 20 enabled, i.e. with the cruise control system and the brake control arrangement in operational state, the brake control means 21 will initiate a brak-

ing operation so as to endeavour to restrict the speed of the vehicle to the reference speed S _ref when the vehicle under the effect of gravity has been accelerated above the target speed and reached the reference speed S _rβf . When the brake control arrangement 20 operates under the supervision of a cruise control system, the speed setting means 22 may be configured to allow the driver to set the magnitude of the basic value V _bas for the reference speed S _ref as a desired offset from the target speed for the cruise control system.

The increase of the reference speed S _ref from the basic value V _bas to the modified value V _mod suitably corresponds to a speed increase in the order of 1 -5 km/h, which implies that the modified value V _mod is to correspond to a vehicle speed which is approximately 1 -5 km/h higher than the vehicle speed corresponding to the basic value V _bas .

If the driver sets a new basic value V _bas for the reference speed S _ref in a situation when the reference speed S _ref has been increased above the previously set basic value V _bas , the reference speed S _ref is to be reset to the new basic value V _bas .

When the brake control arrangement 20 operates under the supervision of a cruise control system and the target speed set for the cruise control system by the driver is below a certain speed, for instance about 60 km/h, the reference speed S _ref is with advantage restricted to the basic value V _bas , which implies that no increase of the reference speed S _ref from the basic value V _bas to the modified value V _mOd is allowed in this situation.

In the illustrated example, the brake control means 21 is adapted to control an auxiliary brake 10 in the form of a retarder, which acts on the output shaft 6b of the gearbox 4.

The actual speed of the vehicle, i.e. the prevailing speed at which the vehicle is running, may be measured or estimated in any suitable manner. The speed of the vehicle may for instance

be estimated based on measuring values of the rotational speed of some part of the vehicle's drive shaft, such as the output shaft 6b of the gearbox 4. In the example illustrated in Fig 1 , the actual vehicle speed is established by means of a measuring member 8 arranged to generate a value representing the rotational speed of the gearbox output shaft 6b. The brake control arrangement 20 is arranged to receive information from this measuring member 8.

Different alternative examples of conditions for the above-mentioned increase of the reference speed S _ref will be described hereinafter.

According to a first alternative:

- a braking power value V _bp representing the magnitude of the braking power ordered by the brake control arrangement 20 is repeatedly or continuously established during a braking operation controlled by the brake control arrangement;

- the largest braking power value V _bp?max established during an ongoing braking operation is stored in a memory 24 of the brake control arrangement; and

- the processing means 23 is adapted to increase the reference speed S _ref from the basic value V _bas to the modified value V _mod when it is established that the braking power value V _bp has dropped during an ongoing braking operation to a given level below the largest braking power value V _{bp max} stored during the ongoing braking operation.

In this case, the processing means 23 is suitably adapted to increase the reference speed S _ref from the basic value V _bas to the modified value V _mod when it is established that the braking power value V _bp has dropped during an ongoing braking operation below a threshold value T _bp defined as a function of the largest braking power value V _{bp max} stored during the ongoing braking operation. This threshold value T _bp may for instance constitute a predefined fraction, for instance 50%, of the largest braking power value V _bp?max stored during the ongoing braking operation.

According to another alternative:

- a declivity value V _d representing the declivity of a downhill slope presently descended by the vehicle is repeatedly or continuously established during a braking operation controlled by the brake control arrangement 20; and

- the processing means 23 is adapted to increase the reference speed S _ref from the basic value V _bas to the modified value V _moc ι when it is established that the declivity value V _d has dropped during an ongoing braking operation below a given threshold value T _d from a value above this threshold value.

The declivity value V _d may be established based on measuring values from an inclination sensor or by means of a suitable calculation model based on different parameters of the vehicle, such as engine load, engine rotational speed and vehicle speed, in a manner well-known to a person skilled in the art.

According to a further alternative:

- a driving resistance value V _dr representing the driving resistance of the vehicle is repeatedly or continuously established during a braking operation controlled by the brake control arrangement 20; and

- the processing means 23 is adapted to increase the reference speed S _ref from the basic value V _bas to the modified value V _mod when it is established that the driving resistance value V _dr has increased during an ongoing braking operation above a given threshold value T _dr from a value below this threshold value.

The driving resistance value V _dr may be established by means of a suitable calculation model based on different parameters of the vehicle, such as engine load, engine rotational speed and vehicle speed, in a manner well-known to a person skilled in the art.

The aim of the conditions according to the above-mentioned alternatives is to allow the reference speed S _ref to be increased to a certain extent when the vehicle approaches the end of a down- slope and are based on the fact that the declivity of a down-

slope normally decreases near the end thereof. Thus, these conditions are used in order to detect that the vehicle is about to reach the end of a downhill slope. When the respective condition is fulfilled, it is presumed that the vehicle is about to reach the end of a downhill slope and the reference speed S _ref is increased so as to allow the vehicle to leave the downhill slope at a speed somewhat higher than the speed corresponding to the basic value V _bas set by the driver for the reference speed S _rβf .

After an increase of the reference speed S _ref to the modified value V _mod at a presumed end of a downhill slope, the processing means 23 may be adapted to:

- reset the reference speed S _ref to the basic value V _bas when it is established that the vehicle has decelerated to a speed below a given threshold speed T ₃ ; and/or

- decrease the reference speed S _ref towards the basic value V _bas when it is established that the vehicle has accelerated to a speed above the increased reference speed, i.e. to a speed above the speed corresponding to the modified value V _mod .

Said threshold speed T ₃ may for instance be about 2 km/h lower than the speed of the vehicle at the moment when the condition for an increase of the reference speed S _ref was fulfilled for the first time during the vehicle's descending of the downhill slope in question.

Said decrease of the reference speed S _ref from the modified value V _mod towards the basic value V _bas is with advantage performed gradually, for instance with steps of a given magnitude at certain time intervals. The magnitude of said steps may for instance be about 0.1 km/h and the time interval between each step may for instance be about 0.5 seconds. This decrease of the reference speed S _ref towards the basic value V _bas is interrupted when the reference speed S _ref has reached the basic value V _bas and may also be interrupted when the vehicle has decelerated to a speed below the above-mentioned threshold speed T ₃ .

According to another embodiment of the invention, the processing means 23 is adapted to increase the reference speed S _ref from the basic value V _bas to the modified value V _moc ι when it is established that the following conditions are fulfilled:

- the output torque of the vehicle engine 2 has been decreased to zero,

- the speed of the vehicle was above a given threshold speed T _v0 when the engine output torque reached zero, and

- the vehicle has accelerated to reach the reference speed S _ref or a speed at a given level below the reference speed within a given time limit T _time from the moment when the engine output torque reached zero.

The aim of the conditions according to the last mentioned alternative is to allow the reference speed S _ref to be increased to a certain extent when the vehicle enters a downhill slope at such a high speed that the vehicle very soon reaches a speed corresponding to the basic value V _bas set for the reference speed S _rβf . Hereby, a retardation of the vehicle under the effect of an automatic braking operation initiated by the brake control arrangement 20 is delayed. The time limit T _time may for instance be about 3-10 seconds, suitably about 5 seconds.

Said threshold speed T _v0 is with advantage defined as a function of the basic value V _bas set for the reference speed S _ref and is of course to be lower than the speed corresponding to the basic value V _bas . When the brake control arrangement 20 operates under the supervision of a cruise control system, the threshold speed T _v0 is also to be higher than the target speed for the cruise control system, i.e. in this case the threshold speed T _v0 is to be located between said target speed and the reference speed S _rβf .

That the vehicle has accelerated to reach the reference speed S _ref or a speed at a given level below the reference speed may of course be established based on information as to the prevailing speed of the vehicle. Said given level may for instance be about 0.1 -0.5 km/h below the reference speed S _rβf . That the vehicle has

accelerated to reach the reference speed S _ref may alternatively be established by detecting that an automatic braking operation has been initiated by the brake control arrangement 20, i.e. that the brake control arrangement has ordered a braking power of a value above zero.

After an increase of the reference speed S _ref to the modified value V _mod at the beginning of a downhill slope, the processing means 23 is adapted to decrease the reference speed S _ref towards the basic value V _bas when it is established that the vehicle has reached the increased reference speed, i.e. a speed corresponding to the modified value V _mod . This decrease of the reference speed S _ref from the modified value V _moc ι towards the basic value V _bas is with advantage performed gradually, for instance with steps of a given magnitude at certain time intervals. The magnitude of said steps may for instance be about 0.1 km/h and the time interval between each step may for instance be about 0.5 seconds. This decrease of the reference speed S _ref towards the basic value V _bas is interrupted when the reference speed S _ref has reached the basic value V _bas .

A flow diagram illustrating an embodiment of an inventive method for controlling the reference speed S _ref of a brake control arrangement 20 of a motor vehicle is shown in Fig 3. In a first step S1 , a basic value V _bas for the reference speed S _ref is determined or received. In a second step S2, the reference speed S _ref is set to the basic value V _bas . In a third step S3, a braking power value V _bp representing the magnitude of the braking power ordered by the brake control arrangement 20 is established. If this constitutes the first braking power value V _bp established during a braking operation initiated by the brake control arrangement 20, this braking power value V _bp is stored in a memory 24 as the largest braking power value V _{bp max} established during the ongoing braking operation, and a new braking power value V _bp is established. In a fourth step S4, each new braking power value V _bp is compared with the stored braking power value V _bp?max representing the largest braking power value established during the

ongoing braking operation. If this comparison indicates that the new braking power value V _bp is higher than the stored braking power value V _{bp max} , the new braking power value is stored in the memory 24 as the largest braking power value V _bp?max established during the ongoing braking operation and steps S3 and S4 are repeated. If the comparison indicates that the new braking power value V _bp is lower than the stored braking power value V _{bp max} , it is established in step S6 whether or not the new braking power value V _bp has dropped to a given level below the stored braking power value V _bp?max . If this is the case, the reference speed S _ref is increased from the basic value V _bas to the modified value V _moc ι in step S7. If this is not the case, steps S3 and S4 are repeated.

A flow diagram illustrating another embodiment of an inventive method for controlling the reference speed S _ref of a brake control arrangement 20 of a motor vehicle is shown in Fig 4. In a first step S1 ', a basic value V _bas for the reference speed S _ref is determined or received. In a second step S2', the reference speed S _ref is set to the basic value V _bas . In a third step S3', it is established whether or not the output torque of the vehicle engine 2 has been decreased to zero. If this is not the case, step S3' is repeated after a certain time interval. If this is the case, it is established in step S4' whether or not the speed of the vehicle was above a given threshold speed T _v0 when the engine output torque reached zero. If this is not the case, the procedure proceeds to step S7' and the reference speed S _ref is kept equal to the basic value V _bas . If it is established in step S4' that the speed of the vehicle was above the given threshold speed T _v0 when the engine output torque reached zero, is established in step S5' whether or not the vehicle has accelerated to reach the reference speed S _rβf , or alternatively a speed at a given level below the reference speed, within a given time limit T _time from the moment when the engine output torque reached zero. If this is the case, the reference speed S _ref is increased from the basic value V _bas to the modified value V _mod in step S6'. If this is not the case, the procedure proceeds to step S7' and the reference speed S _ref is kept equal to the basic value V _bas .

Computer program code for implementing a method according to the invention is suitably included in a computer program, which is loadable into the internal memory of a computer, such as the internal memory of an electronic control unit of a motor vehicle. Such a computer program is suitably provided via a computer program product comprising a data storage medium readable by an electronic control unit, which data storage medium has the computer program stored thereon.

A computer program according to an embodiment of the invention comprises computer program code for causing a computer:

- to determine or receive a basic value V _bas for a reference speed

S ref. j

- to determine or receive a value representing the actual speed of a vehicle;

- to control one or several brakes of the vehicle in dependence on the reference speed S _ref and the actual speed of the vehicle so as to endeavour to restrict the speed of the vehicle to the reference speed S _ret ; and

- to automatically and temporarily increase the reference speed S _ref from the basic value V _bas to a modified value V _mod when it is established that one or several given conditions are fulfilled so as to thereby allow the vehicle to be accelerated under the effect of gravity in a downhill slope to a speed above the speed corresponding to the basic value V _bas .

Fig 2 very schematically illustrates an electronic control unit 30 comprising an execution means 31 , such as a central processing unit (CPU), for executing computer software. The execution means 31 communicates with a memory 33, for instance of the type RAM, via a data bus 32. The control unit 30 also comprises data storage medium 34, for instance in the form of a hard disc, a Flash memory or a memory of the type ROM, PROM, EPROM or EEPROM. The execution means 31 communicates with the data storage medium 34 via the data bus 32. A computer program comprising computer program code for implementing a

method according to the invention is stored on the data storage medium 34.

The invention is of course not in any way restricted to the embodiments described above. On the contrary, many possibilities to modifications thereof will be apparent to a person with ordinary skill in the art without departing from the basic idea of the invention as defined in the appended claims.