WO2008110797A1

WO2008110797A1 - Trailer braking systems

Info

Publication number: WO2008110797A1
Application number: PCT/GB2008/000866
Authority: WO
Inventors: Colin Ross
Original assignee: Knorr-Bremse Systems For Commercial Vehicles Ltd
Priority date: 2007-03-12
Filing date: 2008-03-12
Publication date: 2008-09-18
Also published as: EP2121396A1; GB2449216A; GB0704746D0

Abstract

A braking system for a trailer for a motor vehicle, the braking system comprising brake cylinders (18-21) for the front axle and a rear axle trailer, brake pressure into the brake cylinders being controllable by a brake pressure control module (13). The system further comprises transducers to measure the brake pressure on the front and the rear axles, such that, in use, the brake pressure control module (13) is adapted to decrease the brake pressure on the front axle and increase the pressure on the rear such that the sum of the brake forces on the front and rear axles remains substantially constant.

Description

Trailer Braking Systems

The invention relates to a braking system for trailers of commercial or utility vehicles.

GB 707372 discloses a braking system for a road vehicle in which on initial application has a higher brake pressure applied to the rear axle than the front axle. Once the deceleration reaches a pre-determined value of about 0.25g, the pressures at the front and rear axles are equalised. On harder braking with a deceleration of greater than 0.5g, the pressure at the front axle will start to increase over the pressure at the rear axle.

EPl 414684 discloses a trailer braking system for a trailer having steerable wheels, which has a single common ABS valve to control the brake pressure to the wheels of the front axle. The ABS valve is controlled by an electronic braking module associated with the rear axle of the trailer in dependence on the differential slip between the front and rear axles.

Trailers of this design suffer from the problem that due to load transfer the brake force generated on the front axle is higher than on the rear axle as it is necessary to install larger brake chambers on the front axle to minimise stopping distance at high brake demands. This in turn leads to a further problem, hi most driving situations, braking is carried out in the low pressure range but the differential brake actuation leads to differential brake lining wear, which ultimately can cause overheating of the front axle brakes. The standard approach to deal with this problem is to install special valves in the case of trailers with a conventional braking system or in the case of electronic control utilise a specific control algorithm. This is usually referred to as brake force equalisation.

The present invention seeks to provide a braking system for a trailer having different brake pressure modulators for the front and rear axles.

According to the invention there is provided a braking system for a trailer for a motor vehicle, the braking system comprising brake cylinders for the front axle and a rear axle trailer, brake pressure into the brake cylinders being controllable by a brake pressure control means, the system further comprising brake pressure sensing means adapted to measure the brake pressure on the front and the rear axles, wherein, in use, the brake pressure control means is adapted to control the front and rear braking pressures so that the braking forces generated on the front and rear axles remains substantially equal.

Preferably, the brake pressure on the front axle is reduced and the brake pressure on the rear axle is increased to compensate for the different size of brake chambers. Preferably, the brake pressures are manipulated until the brake demand pressure reaches a predetermined pressure. Preferably, the predetermined pressure is in the range up to 2.5 bar. Preferably, a transducer is provided to determine the load on the front axle and the brake pressure on the front axle is adjusted in dependence on the output of the transducer. Alternatively, the load on the rear axle may be determined, wherein when a brake demand is made, the brake pressure on the rear axle is adjusted in dependence on the load and generates a wheel slip, wherein the slip on the front and rear axles is adjusted so that the slip is substantially the same such that the difference in braking force at the front and rear axle can be determined, which braking force can then be adjusted.

Therefore in contrast to known systems which should be properly categorised as brake lining wear equalisation systems, the invention provides for brake force equalization for a given demand and load condition.

Exemplary embodiments of the invention will now be described in greater detail with reference to the drawings in which:

Fig. 1 shows a trailer braking system;

Fig. 2 shows a graph of brake force distribution v brake pressure;

Fig. 3 shows characteristics of the brake force;

Fig. 4 shows a graph of the control logic

Figure 1 shows a trailer electronic braking system in which the utility vehicle trailer has a steerable front axle with front wheels 1, 2 and a rear axle with rear wheels 3, 4. Rotational wheel speed sensors 5 8 are in each case assigned to the front wheels 1 , 2 and the rear wheels 3, 4, and are connected by way of electric lines 9 12 with an electropneumatic brake pressure control module 13 (EBS module) which is primarily assigned to the rear axle brakes. One brake 14-17 is in each case assigned to the front wheels 1, 2 and the rear wheels 3, 4, which brake 14-17 can be applied by means of brake cylinders 18, 19 of the front axle or spring-loaded brake cylinders 20, 21 of the rear axle.

The braking system of the trailer vehicle can be connected by way of three connections, specifically a pneumatic supply line connection 22, a pneumatic control line connection 23 and an electric control connection 24, with the braking system of a tractor or a further trailer.

The supply line connection 22 is connected by way filter 25 and a parking valve 26 with an air brake reservoir 27. From the air brake reservoir 27, a pneumatic line 28 leads to a supply input of the pressure control module 13. hi addition, a pneumatic line 29 branches off the parking valve 26 to the pressure control module 13. A pneumatic line 30 extending between the parking valve 26 and the air brake reservoir 27 is connected with a supply input 31 of an pressure control module 32.

The pressure control module 32 is assigned jointly to both brake cylinders 18, 19 of the front axle and is connected with the brake cylinder 18 by way of a pneumatic line 33 and with the brake cylinder 19 by way of a pneumatic line 34. The pressure control module 32 has an ECU 50 which is connected by way of electric line 35 with the pressure control module 13.

Furthermore, the pressure control module 32 has a pneumatic control input 36 which is connected by way of a filter 37 with the pneumatic control connection 23. The pneumatic control input 36 is also connected by way of a pneumatic control line 38 with a pneumatic control input of the pressure control module 13. The pressure control module 13 has an integrated pressure sensor (not shown) which measures the pressure in the pneumatic control line 38, that is, the control pressure present at the pneumatic control input 36 of the pressure modulating valve. The pressure control module ECU is provided with an integrated pressure sensor so that the actual brake pressure in the brake cylinders 18, 19 is known. The pressure control module 13 has pneumatic outputs 39 42 which are connected by way of assigned pneumatic lines with the spring brake cylinders 20 or 21.

Furthermore, air springs 43 44 are provided at the rear axle and permit a determination of the axle load, particularly of the dynamic axle load during braking and starting by means of a pressure transducer. One or more of the air springs 43, 44 is connected by way of a pneumatic line with the pressure control module 13 which is shown here only as an example by means of the pneumatic line 55. A pressure transducer being optionally integrated into the pressure control module 13 as a means of transposing the air spring pressure into a load measurement. Correspondingly air springs 45, 46 are provided at the front axle, a pressure transducer 47 measures the pressure and transmits the value via electric line 48 to the pressure modulating module 32. This information is transmitted to the pressure control module 13 via electric line 35.

During a braking operation, the driver defines a braking demand signal by way of the brake pedal and the pneumatic control line as well as the electrical control line. The brake pressure control module 13 controls pressures to the brake cylinders 20, 21 and subsequently monitors the speed of the wheels 3, 4.. A braking demand is transmitted via the electric line 35 to the pressure control module 32 which subsequently controls the pressure at the brake cylinders 18, 19. Wheel speeds from wheels 1, 2 are transmitted from the pressure control module via electric line 35 to the pressure control module 13. Relative front to rear pressure control can then be realised from predefined load to pressure parameters or via the respective wheel slip generated by the respective braking forces.

In a first embodiment according to the invention, the load on the front axle is measured by a transducer 47 adapted to measure the output of the air springs 45, 46 at the front axle and pass this information to the pressure control module 13 by way of the communication means 35. The air springs 43,44 for the rear axle provide information on the loading of the rear axle. Figure 2 shows the brake force distribution without any equalization where the load on the front axle is initially the same as on the rear axle. Due to load transfer to the front axle the brake force generated at the front axle is higher than on the rear axle, which is generally acceptable for high decelerations to minimise stopping distance. However for 99% of service braking, the brake pressure will be below 2 bar, and generally in the region of 1.2-1.4 bar. As Figure 2 illustrates, this will lead to a non symmetrical brake distribution leading to uneven brake lining wear which can then lead to overheating.

In accordance with the invention the pressure control module 13 is adapted to control the brake pressure at the front and rear axles so that whilst the overall braking force remains the same, the brake force is equalised between the front and rear axles in particular at the pressures used for normal servicing braking. Therefore at pressures up to about 2.5 bar, the pressure control module 13 will increase the brake pressure at the rear axle and correspondingly decrease the brake pressure at the front axle so that the resultant braking forces are substantially equal but the combined braking forces remains the same. This function is illustrated in Figure 3.

Once the brake pressure demand increases above 2.5 bar, the pressure control module allows the pressure at the front axle to be greater than the pressure at the rear axle as pressures above 2.5 bar are typically associated with high decelerations where the instantaneous lining wear is of less importance as this condition does not occur often.

As Figure 3 shows, the system permits two performance lines that diverge as brake demand increases and the sum of the two give the combined trailer braking force, which remains substantially constant irrespective of the manipulation of the pressure on the front and rear axles.

In a second alternative embodiment, slip control is used to obviate the need for an additional transducer 47 and electric line 48 at the front axle as this does not require any load information from the front axle.

In this embodiment the load characteristics of the rear axle are defined and used as a datum for any given demand and load condition. When the driver makes a brake demand the rear axle generates a brake chamber pressure associated with the predefined pressure load relationship which in turn generates a wheel slip. The brake pressure on the front axle is then increased by the pressure control module 13 so that the speed of the front wheels 1,2 is the same as the rear wheels 3,4 thereby equalising the braking rate.

Under certain load conditions, equal braking rate will not lead to equal brake force. However, once the equal braking rate is established, the pressure control module 13 can determine the brake pressure at the brake valve 32. This information can then be used to determine the difference in braking pressure between the front and rear axles and then based on the known front to rear braking ratio the pressure at the front axle can then be decreased if necessary and the pressure at the rear axle increased if necessary to ensure that the braking force is substantially equal at both axles for service braking. For higher brake pressure such as in emergency braking the pressure at the front axle can be allowed to be higher than at the rear axle as shown in Figure 3.

Figure 4 shows a graph of the control logic of the invention, the front and rear brakes are applied simultaneously by having fixed parameters based on the pressure required for the brakes to generate a braking force and this is likely to be different between front and rear axles. In the initial condition (zone 1) the axle loading is equal. Load transfer during braking increases the load on the front axle.

This graph illustrates a condition where axle loading are initially equal and load transfer during braking increases the load on the front axle, hi Zone 1 the front and rear braking forces are equal, hi Zone 2 there is a transition between equal braking forces and back to uncontrolled brake force distribution with Zone 3 being the braking forces without any form of brake force distribution.

If the front axle load increases then the uncontrolled front and rear braking forces diverge whereas if the rear axle load is higher than the front the uncontrolled front and rear braking forces converge where it is possible for the rear brake force to be above the front. However the brake force equalisation will function in the same way so that in Zone 1 equal braking forces are produced.

Claims

1. A braking system for a trailer for a motor vehicle, the braking system comprising brake cylinders for the front axle and a rear axle trailer, brake pressure into the brake cylinders being controllable by a brake pressure control means, characterised in that the system further comprising brake pressure sensing means adapted to measure the brake pressure on the front and the rear axles, wherein, in use, the brake pressure control means is adapted to decrease the brake pressure on the front axle and increase the pressure on the rear such that the sum of the brake forces on the front and rear axles remains substantially constant.

2. A braking system according to Claim 1, wherein the brake pressure on the front axle is increased and the brake pressure on the rear axle is reduced until the front and rear braking forces are substantially equal.

3. A braking system according to Claim 1 or Claim 2, wherein the brake pressures are manipulated until the brake demand pressure reaches a predetermined pressure.

4. A brake system according to Claim 3, wherein the predetermined pressure is in the range up to 2.5 bar.

5. A brake system according to any one of Claims 1 to 4, wherein a transducer is provided to determine the load on the front axle and the brake pressure on the front axle is adjusted in dependence on the output of the transducer.

6. A brake system according to any one of Claims 1 to 4, wherein the load on the rear axle is determined, wherein when a brake demand is made, the brake pressure on the rear axle is adjusted in dependence on the load and generates a wheel slip, wherein the slip on the front and rear axles is adjusted so that the slip is substantially the same such that the difference in braking force at the front and rear axle can be determined, which braking force can then be adjusted.