AN ARRANGEMENT AND A METHOD TO DISTRIBUTE BRAKE FORCE IN A VEHICLE

Abstract: The invention relates to a device and a procedure that distribute a braking effect in a vehicle. The vehicle contains a main brake (1a, b) that allows braking of the driven (3) and undriven wheels (2) of the vehicle, an auxiliary brake (4, 8) that allows braking of the driven wheels (3) of the vehicle, and a control unit (13) that is arranged to distribute the requested braking effect so that the auxiliary brake (4, 8) is activated in the first instance to provide the requested braking effect. When the braking effect of the auxiliary brake (4, 8) is insufficient, the control unit (13) activates the main brake (1a, b) to provide a supplementary braking effect. The control unit (13) is arranged to apply the braking effect of the main brake (1a) to the vehicle's undriven (2) wheels in the first instance and when a braking effect of a decided value (v) has been applied to the undriven wheels (2) the control unit (13) is arranged to apply the braking effect also from the main brake (1b) to the vehicle's driven wheels (3).

Description: An arrangement and a method to distribute brake force in a vehicle BACKGROUND OF INVENTION AND STATE OF THE ART The invention relates to a device and a procedure for distributing a braking effect in a vehicle according to patent claims 1 and 1 1 . Besides the main brake, heavier vehicles often have an auxiliary brake that can be a retarder and/or an exhaust brake or a compression brake. An auxiliary brake of this type transfers the braking effect only to the driven wheels of the vehicle. The auxiliary brake is used primarily to reduce the wear on the vehicle's main brake. The main brake on a vehicle normally comprises a pneumatic system that is controlled by an electric control unit. The pneumatic system is arranged to supply a brake pressure to the brake cylinders located at each of the vehicle's wheels. The brake cylinders are connected to the friction pads that can be pressed against the brake devices, such as brake discs or brake drums. The auxiliary brake functions thus only on the vehicle's driven wheels. When the vehicle requires a braking effect that is greater than the auxiliary brake can provide, the main brake is activated to give the supplementary braking effect. This supplementary braking effect is applied to the driven and undriven wheels of the vehicle. A greater braking effect is thus applied to the driven wheels than to the undriven wheels. This is disadvantageous particularly on slippery roads and the driven wheels reach relatively quickly a braking level when they start to slip on the road surface. Due to this the auxiliary braking effect on the driven wheels must be reduced so that the main brake can apply further braking effect to the vehicle's wheels. For this reason the total braking effect of the auxiliary brake cannot be fully utilised when the road is slippery. A separated braking effect between the vehicle's driven and undriven wheels means that it is difficult to give the vehicle an optimal stability during braking. Furthermore, the difference in tyre wear, already caused by the driving effect between the driven and undriven wheels, increases even more with the braking distribution mentioned above. As the main brake is only applied with a supplementary braking effect that is normally distributed equally among the driven and undriven wheels, a relatively long time can elapse between the occasions when a powerful braking is applied to the individual brake pads of the wheels brakes. Therefore, a film can form on the contact surfaces of the brake pads that decreases the friction between the brake pad and the braking device, at least during the initial stages of brake application. SUMMARY OF INVENTION The object of the invention is to provide a device and a procedure for braking a vehicle with a main brake and an auxiliary brake, where the capacity of the auxiliary brake can be utilised fully even when the road is slippery and where the total braking effect of the vehicle is distributed more evenly between the driven and undriven wheels of the vehicle compared to a conventional brake device. This object is achieved using the device and procedure of the type mentioned in the introduction, which are characterised by the distinctive features named in the characterised parts to the patent claims 1 and 1 1. As the main brake initially applies a braking effect only to the vehicle's undriven wheels, a gradual equalisation of the braking effect between the driven and undriven wheels is achieved with a requested increase in braking effect. The advantage of such an equalisation of braking effect is that the stability of the vehicle increases when braking and that tyre wear is distributed evenly between the driven and undriven wheels. As the braking effect of the main brake is initially applied only to the vehicle's undriven wheels, the braking load on the brake components of the driven wheels reaches a high level not too early, a level that causes a risk of the driven wheels slipping on the road surface. Since the main brake does not apply a braking effect to the brake components of the driven wheels before the driven wheels have been subjected to a braking effect of the value mentioned, the brake components of the driven wheels, such as brake disks, are subjected to a relatively low load and can thus be dimensioned relatively modestly. According to a preferred embodiment of the invention, the value of braking effect mentioned has a certain relationship to a maximum braking effect that the auxiliary brake is capable of applying to the driven wheels. By maximum braking effect that the auxiliary brake is capable of, is meant the maximum braking effect that the auxiliary brake can apply to the driven wheels without any slipping on the road surface. For dry road conditions the maximum braking effect mentioned is limited essentially only by the auxiliary brake's own capacity but for slippery road conditions the wheels can start to slip on the road before the full capacity of the auxiliary brake has been utilised. The maximum braking effect of the auxiliary brake is also influenced by the vehicle's load since a greater vehicle load results in an increase in wheel grip on the road surface. The relationship mentioned above can be decided by various factors and is dependent on, for example, the type of vehicle and its use. However, it is often advantageous if the vehicle's total braking effect can be distributed essentially equally to the driven and undriven wheels. The above mentioned relationship can then include the ratio between the number of driven and undriven wheels. In other words, if the vehicle has more undriven than driven wheels, the main brake must apply a larger braking effect to the undriven wheels than the auxiliary brake applies to the driven wheels, to be able to distribute the braking effect essentially equally to the individual driven and undriven wheels. Another factor of importance in giving the vehicle an optimum braking effect is the normal forces that act on the respective wheels. The control unit can thus be arranged in such a way that it receives information about the normal forces that act on at least one driven wheel and one undriven wheel and that the above mentioned relationship is decided also by the ratio between the normal forces acting on the driven and undriven wheels. If the control unit receives information about how the normal forces are distributed between the driven and undriven wheels, it can then control the distribution of the braking effect so that the total braking effect is essentially distributed in relation to the normal forces that act on the driven and undriven wheels when the mentioned value is reached. According to another preferred embodiment of the invention, the control unit is arranged to receive information about the normal forces acting on at least two undriven wheels and to distribute the braking effect of the main brake between each of the undriven wheels essentially depending on the normal forces acting on the wheel. Alternatively the braking effect of the main brake can be initially distributed essentially equally and with an increase in the requested braking effect depending on the normal forces acting on the wheel. When a large braking effect is requested and the above mentioned value is exceeded, the control unit can distribute the braking effect of the main brake between the driven and undriven wheels so that each of the driven and undriven wheels receive a braking effect that is in proportion to the normal forces acting on the wheel in question. The braking effect on the driven wheels is here the sum of the braking effects applied by the auxiliary brake and the main brake whereas the braking effect on the undriven wheels is applied only by the main brake. According to another preferred embodiment of the invention, the main brake comprises wheel brakes with brake pads which use a pneumatic system to press against a brake device that rotates together with the respective wheels. If such a main brake is to be able to provide a maximum braking effect, it is essential that the brake pads at least with a certain regularity are pressed hard against the brake devices, which to advantage are brake discs, in order to prevent a film forming on the contact surfaces of the brake pads. Such a film decreases the friction between the brake pads and the brake devices. According to the invention, by concentrating the braking effect applied by the main brake to the undriven vehicle wheels, the brake pads of the undriven wheels are subjected more frequently to a heavy brake application that counteracts the forming of such a film. According to another preferred embodiment of the invention, the auxiliary brake comprises a retarder. A retarder is an auxiliary brake that is used on most modern heavy vehicles. The auxiliary brake can alternatively to or in combination with the retarder comprise an exhaust brake or a compression brake. According to another preferred embodiment of the invention, the vehicle includes one driven pair of wheels and two undriven pairs of wheels. This type of heavy vehicle usually includes a front pair of steering wheels, a rear pair of driven wheels, and a liftable rear pair of undriven wheels. BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention are described below by way of examples with reference to the attached drawings, in which: Fig. 1 depicts schematically the available brakes in a heavy vehicle. Fig. 2 depicts schematically an embodiment of an arrangement according to the present invention, Fig. 3 depicts schematically as a histogram how the braking effect can be distributed according to the present invention. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Fig. 1 depicts schematically the location of available brakes on a heavy vehicle. The vehicle includes a main brake in the form of pneumatically applied wheel brakes 1 a for the undriven vehicle wheels 2 and wheel brakes 1 b for the driven vehicle wheels 3. The wheel brakes 1 a, b include conventional brake cylinders, but are not shown in the figures, with friction pads which are arranged to press against brake discs to brake the vehicle. A supplementary brake in the form of an exhaust brake 4 is arranged in connection with the vehicle engine. An alternative auxiliary brake to an exhaust brake 4 is a compression brake, which however is not shown in Fig. 1 . An internal combustion engine 5 in the vehicle is connected to a gearbox 6 from which an output drive shaft 7 extends. A further auxiliary brake in the form of a retarder 8 is connected to the drive shaft 7 in order to brake the drive shaft 7 and thus brake the vehicle's driven wheels 3. Fig. 2 depicts schematically a braking device that includes a brake pedal 9 with which the driver requests a braking effect on the vehicle. The value of the requested braking effect stands in relationship to the degree of depression of the brake pedal 9. The driver's depression movement of the brake pedal is transferred via a link system to a rod 10 that slides in a foot valve 11 . A sensor 12 is arranged to sense the position of the rod 10 in the foot valve 1 1. The sensor 12 sends essentially continuously signals about the position of the rod 10 in the foot valve 1 1 to a control unit 13. The value of the signals thus corresponds to the degree of depression of the brake pedal 9 and the braking effect on the vehicle requested by the driver. When the brake pedal 9 is depressed, the control unit 13 is arranged to activate in the first instance the vehicle's auxiliary brake, which is a retarder 8 in Fig. 2. When activated the retarder 8 provides a braking effect on the vehicle's driven wheels 3. When the requested braking effect exceeds the maximum braking effect a max that the retarder 8 can give, the control unit 13 also activates the main brake in the form of the wheel brakes 1 a, b to give the vehicle a supplementary braking effect. Using a pneumatic system 14, the control unit 13 can convey an essentially arbitrary brake pressure to each of the wheel brakes 1 a, b and can in this way distribute the braking effect essentially arbitrarily between the individual wheels 2, 3. To allow the control unit 13 to be able to distribute the requested braking effect in an optimum way, a sensor 15 is arranged to sense the value of the normal forces that act on each one of the vehicle wheels. Such a sensor is depicted schematically in Fig. 2. The value of the normal forces can be determined by the sensor 15, which senses the spring pressure of the respective wheels 2, 3. The sensor 15 can essentially continuously transmit signals, concerning the normal forces acting on the respective wheels 2, 3, to the control unit 13. Fig. 3 depicts three bars i , I I , in that schematically show how a braking effect, requested from the brake pedal 9, can be distributed. The value of the requested braking effect, that stands in relationship to the degree of depression of the pedal 9 and a value t of the requested brake torque, is represented by the height of the bars. The first bar I depicts a braking effect requested by the vehicle driver that is relatively small and has a value less than the maximum braking effect a max . of the retarder. Here the control unit 13 activates the retarder 8, which applies the whole of the requested braking effect. Bar i comprises a first area 16 which symbolises the braking effect that the retarder 8 applies to the vehicle's driven wheels 3. Since the activation time of the retarder 8 is longer than that of the wheel brakes 1 a, b, there is a possibility that the wheel brakes 1 a, b are activated for a shorter period in the first area 16, mentioned above, before the retarder 8 can apply the requested braking effect. The brake device can therefore always provide the requested braking effect with a very short time delay. The second bar II depicts a braking effect, requested by the vehicle driver, which has a value that exceeds the maximum braking effect a max the retarder 8 can apply. The first area 16 of the bar is here a maximum and the retarder 8 applies its maximum braking effect a max , which is distributed to the vehicle's driven wheels 3. The bar I I also depicts a second area 17, above the first area 16, that symbolises the supplementary braking effect applied by the wheel brakes 1 a to the undriven wheels 2 of the vehicle. Since the braking effect to the driven wheels 3 is constant in an area that stretches from a max to a marginal value I, a growing requested braking effect in the named area causes an equalisation of the difference between the applied braking effect to the undriven 2 and the driven wheels 3. There are many advantages with such an equalisation, e.g. more even tyre wear between the driven 3 and undriven wheels 2, the vehicle's stability improves during braking and a greater braking effect can be applied to the vehicle, above all when the road is slippery, before the driven wheels 3 start to slide on the road surface. Furthermore, the brake pads on the undriven wheels 2 can be applied with a greater force and more frequently, which counteracts the formation of a film on the contact surfaces of the brake pads that reduces friction. The third bar i n depicts a relatively large braking effect on the vehicle requested by the vehicle driver. To ensure that the vehicle receives the requested braking effect, the retarder 8 is activated at maximum effect, which is depicted by the first area 16. Moreover, the wheel brakes 1 a are activated so that they apply a braking effect to the undriven wheels 2 that extends beyond the marginal value I mentioned above, which is depicted by the second area 17. The marginal limit I designates the limit at which the control unit 13 starts to distribute the braking effect of the wheel brakes 1 a,b, to both the undriven 2 and the driven wheels 3. A third area 18 depicts here the braking effect that the wheel brakes 1 b apply to the driven wheels 3 after the marginal value I has been passed. The marginal value I is the sum of the retarder's maximum braking effect a max and a decided value v, which is the braking effect that the wheel brakes 1 a apply to the undriven wheels 3 before the wheel brakes 1 a, b start to distribute their braking effect to both the undriven 2 and driven wheels 3. The value v stands in a relationship to the retarder's 8 maximum braking effect a max _. The named relationship comprises the ratio between the number of undriven 2 and driven wheels 3, so that the braking effect applied by the retarder 8 and the wheel brakes 1 a, b can be distributed essentially equally between the undriven 2 and the driven wheels 3 when the named marginal value I is reached. When the named marginal value I has been passed, the wheel brakes 1 a, b apply a braking effect to both the undriven 2 and driven wheels 3. The retarder 8 and the wheel brakes 1 a, b can together provide a maximum braking effect to the vehicle equivalent to a maximum braking torque t max . Since the sensor 15 can sense the normal forces acting on the respective wheels 2, 3, the control unit 13 is allowed to apply a suitable braking pressure to the respective wheel brakes 1 a, b so that each of the driven 2 and the undriven wheels 3 receives a braking effect in proportion to the normal forces acting on the respective wheels 2, 3. The invention is in no way limited to the embodiments described in the drawings but may be modified freely within the scope of the patent claims.

Claims: Patent claims 1. Arrangement for distributing the braking effect in a vehicle, whereby the arrangement comprises a main brake (1 a, b) which allows braking of the driven (3) and undriven wheels (2) of the vehicle, an auxiliary brake (4, 8) which allows braking of the driven wheels (3) of the vehicle, and a control unit (13) that is arranged to distribute the requested braking effect so that the auxiliary brake (4, 8) is activated in the first instance to provide the requested braking effect and when the braking effect of the auxiliary brake (4, 8) is insufficient to activate the main brake (1 a, b) to provide a supplementary braking effect, characterised in that the control unit (13) is arranged in the first instance to apply the braking effect of the main brake (1 a) to the vehicle's undriven (2) wheels and when the undriven wheels (2) have received a braking effect of a decided value (v) the control unit (13) is arranged to apply also the braking effect of the main brake (1 b) to the vehicle's driven wheels (3). 2. Arrangement according to claim 1 , characterised in that the named value (v) stands in relationship to a maximum braking effect (a max ) that the auxiliary brake (4, 8) is capable of applying to the driven wheels (3). 3. Arrangement according to claim 2, characterised in that the named relationship involves the ratio between the driven and undriven wheels. 4. Arrangement according to claim 3, characterised in that the control unit (13) is arranged to receive information about the normal forces that act on at least one driven wheel (3) and one undriven (2) and that the named relationship also decides the normal forces which act on the driven (3) and the undriven wheels (2). 5. Arrangement according to any one of the foregoing claims, characterised in that the control unit (13), via a sensor (15), is arranged to receive information about the normal forces that act on at least two undriven wheels and distribute the braking effect of the main brake between each one of the undriven wheels essentially dependent on the value of the normal forces that act on the respective wheels. 6. Arrangement according to claim 4, characterised in that when the named value (v) is exceeded, the control unit (13) is arranged to distribute the braking effect of the main brake (1 a, b) between the driven (3) and undriven wheels (2) so that each one of the driven (3) and undriven wheels (2) receives a braking effect that is proportional to the normal forces acting on the wheel (2, 3) in question. 7. Arrangement according to any one of the foregoing claims, characterised in that the main brake comprises wheel brakes (1 a, b) with brake pads that using a pneumatic system are pressed against the brake device which is arranged to rotate at the respective wheel. 8. Arrangement according to any one of the foregoing claims, characterised in that the auxiliary brake comprises a retarder (8). 9. Arrangement according to any one of the foregoing claims, characterised in that the auxiliary brake comprises an exhaust brake (4) or compression brake. 10. Arrangement according to any one of the foregoing claims, characterised in that the vehicle includes one driven pair of wheels (3) and two undriven pairs of wheels (2). 1 1 . Arrangement to distribute a braking effect in a vehicle, in that the vehicle contains a main brake (1 a, b) which allows braking of the driven (3) and undriven wheels (2) of the vehicle, an auxiliary brake (4, 8) which allows braking of the driven wheels (3) of the vehicle, and a control unit (13) which distributes the requested braking effect so that the auxiliary brake (4, 8) is activated in the first instance to provide the requested braking effect and when the braking effect is insufficient to activate the main brake (1a, b) to provide a supplementary braking effect, characterised in that the control unit (13) in the first instance applies the braking effect of the main brake (1 a) to the vehicle's undriven (2) wheels and when the undriven wheels (2) have received a braking effect of a certain value (v) the control unit (13) applies even the braking effect of the main brake (1 b) to the vehicle's driven wheels (3).

Inventors: VAAGSTEDT NILS-GUNNAR, VAAGSTEDT, NILS-GUNNAR
Applicants: SCANIA CV ABP, VAAGSTEDT NILS GUNNAR, SCANIA CV AKTIEBOLAG (PUBL), VAAGSTEDT, NILS-GUNNAR
Application year: 2000
Main classification: B60T8
IPC codes: B60T7, B60T8, B60T13