The present invention relates to an arrangement for adapting the brake pressure in the brake system of a hauled vehicle, preferably a semitrailer or trailer, to the brake pressure in the brake system of a hauling vehicle, which arrangement comprises a valve which via a first pressure line is connected to a first driver- actuable control element for controllable pressure supply from a pressure source to brake elements on the vehicle wheels of the hauled vehicle, and an electronic control unit which, on the basis of detected vehicle parameters, emits an electrical signal for controlling the pressure supply to the brake elements on the vehicle wheels of the hauled vehicle.

In vehicle combinations consisting of a hauling truck with semitrailer or of a lorry with trailer, the brake system of the hauling truck and the brake system of the hauled semitrailer or trailer must be adapted to one another so that the brakes on each wheel axle in the vehicle combination respond for braking the load bearing on the respective wheel axle. This is considered neces¬ sary in order to prevent the brakes on one wheel axle from being overloaded and worn away abnormally quickly while the brakes on another wheel axle are underloaded, which can result in the surface layer of the brake lining on this wheel axle becoming hard and glassy and resulting in a braking effect which to a greater or lesser extent fails to materialise in an emergency situation in which a full braking effect is required.

However, in practice it is difficult to fully adapt the brake systems to one another; instead there is always a certain difference in the brake characteristic between the brake system of the hauling truck and the brake system of the semitrailer or trailer. The difference in

brake characteristic is most noticeable at low brake pressures and can result, at such brake pressures, in only the brakes of the hauling truck being applied during a brake sequence, and thereby having to brake the whole vehicle combination. On the other hand, it may happen that only the brakes of the semitrailer or trailer are applied during the brake sequence and have to brake the whole combination.

Since most decelerations take place at low or moderate brake pressures, the difference in brake characteristic is apparent to most drivers when they are driving the said vehicle combinations. The difference is often even more noticeable when older semitrailers or trailers are used, since the brakes on these often seize due to corrosion and inadequate maintenance.

There is at present no satisfactory solution to the problem described above. There are admittedly certain types of control valves on the market for the brakes of semitrailers or trailers, which allow the brake pressure fed to the brakes of the semitrailer or trailer to be adjusted manually within certain limits relative to the brake pressure fed to the brakes of the hauling vehicle. However, extensive measurement equipment is required in order for this adjustment to be carried out correctly, and even if the brake system of the hauling vehicle and the brake system of one semitrailer or trailer are adapted to one another, it is not certain that the same adjustment will be suitable for another semitrailer or trailer. The disadvantages of manual adjustment have in practice meant that the brake systems are adjusted once and for all for the most general driving condition and thereafter no further adjustment is carried out.

According to Swedish patent 431,184, in vehicle combina¬ tions with a light load the brakes of the lorry provide less brake power than the brakes of the trailer. In order to achieve this, the patent describes a brake system

comprising a relay valve which is actuated by a load- sensing valve in order to reduce the brake power in the back brakes of the lorry and to increase the brake power in the back brakes of the trailer relative to the brake power in the back brakes of the lorry at a reduced load on the lorry.

In the patented construction the brake pressure to a trailer is indeed controlled, but in this control method no account is taken of the difference in the brake characteristic between the brake systems of the lorry and the trailer, for which reason there is still a risk of the trailer or alternatively the hauling truck being forced to brake the whole vehicle combination.

In the case of a trailer, which unlike a semitrailer bears its entire load, it is known, for example in international patent WO 8705571, to arrange a force sensor which, during a brake sequence, determines the force acting on a draw bar between the lorry and the trailer. If a pulling force is acting on the draw bar, this is an indication of the fact that the brakes of the trailer are responsible for too much of the braking work. Correspondingly, a pressing force on the draw bar indi¬ cates that the brakes of the trailer are responsible for too little of the braking work. Depending on the indica- tions, a control valve is actuated in order to increase or reduce the brake pressure fed to the brakes of the trailer. However, this technique is not relevant to this invention.

Swedish patent application SE 8803179-4 describes an arrangement for brake adaptation between a hauling truck and a coupled semitrailer. However, the patent applica¬ tion is relatively cursory as regards the function of the valve controlling the brake pressure to the semitrailer, and it gives no directions on how the valve is to be connected to the compressed air system of the brakes.

The aim of the present invention is to provide an

arrangement for the brake system of a vehicle combina¬ tion, which arrangement makes it possible, more so than in the previously known solutions, to automatically adapt the brake characteristics of the individual vehicle units to each other in order to obtain reliable braking of the whole vehicle combination. To this end the invention is characterised in that the valve mentioned at the outset is connected via a second pressure line to a second control element, in that the second control element is connected to the electronic control unit which in turn is connected to sensors for determining vehicle parameters, and in that the second control element, on the basis of the said signal, supplies the valve with pressure for adapting the pressure supply to the brake elements of the hauled vehicle to the determined vehicle parameters.

With the invention, a good brake adaptation is automati¬ cally achieved between the brake systems of the hauling truck and of the semitrailer or trailer, especially at the relatively gentle decelerations of the vehicle combination which predominate in normal running. This leads to better use of the brake systems and to the brake linings being loaded and worn more evenly, thus reducing the risk of hardening of the surface layer of the brake linings. The brake adaptation takes place automatically in respect of the specific semitrailer or trailer which at the time is coupled to the hauling truck. Upon replac¬ ement of a semitrailer or trailer, the brake adaptation to this is made automatically.

Other features characterising in the invention will emerge from the subsequent patent claims and from the following description of an embodiment exemplifying the invention. In the description, reference is made to the attached figures, in which

Figure 1 shows a hauling truck with coupled semitrailer, the various components of their respective brake systems being shown diagrammatically.

Figure 2 shows a valve for adapting the brake system of

the semitrailer to the brake system of the hauling truck.

Figure 3 shows a diagram of individual deceleration of the hauling truck and the semitrailer at different brake pressures in a non-adapted condition, and Figure 4 shows a corresponding diagram after brake adaptation has been carried out.

The present invention can be applied both to a hauling truck with semitrailer and to a lorry with trailer. Despite this, the following description refers only to a hauling truck with semitrailer, but it is understood that the invention can equally be applied to a lorry with trailer.

Figure 1 shows diagrammatically a vehicle combination consisting of a hauling truck 1 and a semitrailer 2 coupled to the latter. The hauling truck 1 and the semi¬ trailer 2 are each equipped with conventional pneumatic brake systems for activating brake elements on each wheel. Since the design of such pneumatic brake systems is well known in heavy vehicles, the subsequent descrip¬ tion is confined to dealing only with those parts which are important to an understanding of the present inven¬ tion.

The brake system of the hauling truck 1 comprises a compressed air source in the form of a compressor (not shown) . The compressor acts continuously during running of the vehicle engine and supplies air to a pressure tank 5. The latter is connected via a supply line 6 on the one hand to a relay valve 7 at brake element 3 on the back wheel of the hauling truck and on the other hand to a valve 8.

The relay valve 7 and the valve 8 are connected to a driving brake valve 10 via an operating pressure line 9. This driving brake valve 10 is controlled by means of a

pedal 15 which can be actuated by the driver of the vehicle. The driving brake valve 10 is supplied via a supply line 22 with compressed air from the pressure tank 5. When the pedal 15 is actuated, compressed air enters into the operating pressure line 9. The brake pressure in the operating pressure line 9 thus corresponds to how much the pedal 15 is actuated.

The brake system of the semitrailer 2 also comprises a supply line 11 and an operating pressure line 12. Both lines 11, 12 are connected at their one end to a relay valve 13 at brake element 4 on the wheel of the semi¬ trailer 2. The supply line 11 is connected at its other end to the supply line 6 of the hauling truck 1, while the operating pressure line 12 is connected at its other end to the valve 8.

The valve 8 is designed to convey an operating pressure to the relay valve 13 of the semitrailer 2 depending on the brake pressure which the driving brake valve 10 supplies. This operating pressure can be higher than, lower than or equal to the operating pressure fed to the relay valve 7 of the hauling truck 1.

In order to connect the supply line 11 to the supply line 6 and the operating pressure line 12 to the valve 8, coupling pieces are used. The coupling pieces are shown diagrammatically in Figure 1 as a coupling block 14, which in practice is positioned in a suitable space between the hauling vehicle 1 and the semitrailer 2.

The arrangement according to the invention also comprises an electrical control unit 16 which is arranged in the brake system of the hauling truck 1 and is connected to a number of sensors which determine various vehicle parameters. In this example, the control unit 16 is connected to a pressure sensor 17, preferably electrical, in the operating pressure line 9 of the hauling truck 1. The control unit 16 is supplied through the pressure

sensor 17 with an electrical signal which represents the brake pressure in the hauling truck 1. Since the brake pressure depends on the force with which the driver of the vehicle acts on the pedal 15 of the driving brake valve 10, the signal can be considered to represent the brake capacity required by the driver of the vehicle.

The control unit 16 is also connected to a deceleration sensor 18 shown diagrammatically in Figure 1. In prac¬ tice, the deceleration sensor 18 can advantageously consist of an accelerometer. Tests which have been carried out have in fact shown that the size of the output signal from an accelerometer is independent of whether the vehicle is braked in order to obtain a constant speed on a downhill slope or is braked in order to reduce the speed on a horizontal course. The output signal is therefore correct in all braking situations.

In an alternative embodiment, the deceleration sensor can consist of a pendulum sensor. Tests which have been carried out have in fact shown that the size of the output signal from such a sensor depends on whether the vehicle is braked on a slope or on a horizontal course, but this "error" can be regarded as being negligible within the working range of the control unit 16.

A third sensor 19 is connected to the control unit. This sensor 19 detects the load on the rear axle of the hauling truck 1. If the hauling truck 1 is equipped with pneumatic suspension, the sensor 19 can consist of a pressure sensor integrated in the air bellows of the suspension, as shown in Figure 1. If the hauling truck is not equipped with air suspension, another conventional sensor can be arranged in order to detect the axle loading on the rear wheels of the hauling truck 1.

The three sensors 17, 18, 19 are thus each designed to detect vehicle parameters and to convey to the control unit 16 signals which represent these parameters. The

control unit comprises a memory unit 20 and a comparative circuit 21. Permanently and electronically stored in the memory unit 20 are different predetermined values for the anticipated deceleration of the hauling vehicle - which corresponds to the deceleration which is ideally desired of the whole vehicle combination - at different brake pressures and axle loading of the back wheels of the hauling truck.

The sensors 17, 19 which detect the brake pressure and axle- load are connected to the memory unit 20. On the basis of the detected brake pressure and axle load, the sensors 17, 19 emit signals to the memory unit 20. The signals are compared with the permanently stored values, after which the memory unit 20 emits a signal represent- ing the anticipated deceleration of the vehicle in the situation in question.

The signal emitted from the memory unit 20 is fed to the comparative circuit 21, to which there is also fed a signal from the deceleration sensor 18, which signal represents the actual deceleration. In the comparative circuit 21 the signal emitted from the memory unit 20 (anticipated deceleration) is compared with the signal from the deceleration sensor 18 (actual deceleration), after which the comparative circuit 21 emits an output signal representing the difference between the said signals. In an alternative embodiment, the control unit 16 can consist of a microprocessor in which a program processes the signals from the sensors 17, 18, 19 and, as a function thereof, emits an output signal to the control valve 23.

The output signal from the control unit 16 is fed through an electrical line to a control valve 23 which is ar¬ ranged in a control pressure line 24. One end of the control pressure line 24 is connected to the pressure tank 5 via supply line 22, while its other end is con¬ nected to the valve 8. The control valve 23 consists of

a so-called proportionality valve which is designed to convert the electrical input signal from the comparative circuit into a proportional pneumatic output signal which is fed to the valve 8 through the control pressure line.

Also arranged in the control pressure line 24 is a cut¬ off valve 25. The cut-off valve 25 is connected via an electrical line to a brake light switch 26 in direct contact with the driving brake valve 10. The brake light switch 26 is electrically connected to brake lights (not shown) at the rear parts of both the hauling truck 1 and the semitrailer 2. As emerges from Figure 2, the cut-off valve 25 consists of a pneumatic 3/2-way valve, previous¬ ly known per se, which is controlled by an electromagnet 50 which is connected to and is acted upon by the brake light switch 26 of the driving brake valve 10. The cut¬ off valve 25 is designed to open or close a pneumatic connection between the valve 23 and the valve 8 depending on whether the driving brake valve 10 and thus also the brake light switch 26 are actuated or not.

Figures 1 and 2 show that the valve 8 comprises a valve housing 30 in which there are arranged an inlet port 31, an outlet port 32, a run-off port 33 and a control pressure port 46. The inlet port 31 communicates with the pressure tank 5 via supply line 6, while the outlet port 32 communicates with the brake elements of the semi¬ trailer 2 via operating pressure line 12, the run-off port 33 communicates with the atmosphere, and the control pressure port 46 communicates with the driving brake valve 10 via the operating pressure line 9.

The valve 8 in Figure 2 comprises a tubular valve element 34 which is arranged sealed-off and slidable in the valve housing 30 and is pressed against a valve seat 35 by means of a spring 36. The upper part of the tubular valve element 34 is designed with a valve seat 37 for an outlet valve 47 formed on a first piston 38 which is arranged sealed-off and slidable in the valve housing and delimits

a chamber 48 which is connected to the operating pressure line 9 via the control pressure port 46.

The piston 38 is designed with an upwardly-directed rod 39 which extends sealed-off through a hole 40 in the valve housing 30 into another valve housing 41 where it is connected to a second piston 42. The second piston 42, which is pressed towards an upper end position by means of a spring 43, delimits a chamber 44 which communicates with the pressure tank 5 via an inlet port 49, the control pressure line 24 and the supply line 22. The underside of the piston 42 communicates with the atmo¬ sphere via a channel which is not shown.

The brake system essentially comprises two safety func¬ tions which function independently of the electrical components of the control unit 16 and which therefore guarantee a high degree of safety even if the control unit 16 is not functioning satisfactorily. The one safety function consists of the cut-off valve 25 in the control pressure line 24.

The cut-off valve 25 is designed to be actuated only when the pedal 15 is actuated. When the pedal 15 is not actuated, the cut-off valve 25 ensures that any leakage of air through the valve 23 is not conveyed to the chamber 44. Such a leakage of air there would otherwise be able to act on the second piston 42 in such a way that it opens the connection between the supply air inlet 31 and outlet 32 so that the vehicle combination begins to brake.

The other safety function consists of the fact that the valve 8 is designed in such a way that the difference between the brake pressure of the hauling truck 1 and semitrailer 2 can be at most approximately 2 bar. This prevents total locking of the brakes of the semitrailer 2 or total failure of the brakes of the semitrailer 2 if the electronics of the control unit 16 cease to function

during running of the vehicle combination.

The said 2 bar range is obtained by dimensioning the pistons 38, 42 and the springs 43 and 36 suitably in a manner known per se, given the knowledge of the relation- ship brake pressure = operating pressure x piston area.

In this exemplary embodiment the pistons 38, 42 are dimensioned in such a way that the projected area of the piston top of the second piston 42 is between 20 and 50 % of the corresponding area of the first piston 38, and at the same time the combined spring force of the springs 36, 43 is between 38 and 50 kp. In an advantageous embodiment, the projected area of the piston top of the second piston 42 is between 32 and 38 % of the correspon¬ ding area of the first piston 38, and at the same time the combined spring force is between 40 and 45 kp.

As was stated in the introduction, a hauling truck and a semitrailer usually have different brake characteristics on account of the fact that the brakes of the semitrailer often seize due to corrosion and inadequate maintenance. Figure 3 shows an example of the character of the in¬ dividual deceleration D of the hauling truck 1 and semitrailer 2, respectively, at different brake pres¬ sures.

Curve A represents the actual and anticipated decelera- tion of the hauling truck 1, which also corresponds to the desired deceleration for the whole vehicle combina¬ tion. Curve B represents the actual deceleration of the semitrailer 2 as a function of the brake pressure. A curve representing the actual deceleration of the whole vehicle combination as a function of the brake pressure will lie, depending on the weight of the vehicle parts 1, 2, somewhere between curves A and B, in a manner not shown here.

The function of the arrangement according to the

invention is as follows. The description of the function assumes that the brake systems of the hauling truck 1 and semitrailer 2 have not yet been adapted to one another. The description is divided into two parts. Part 1 and Part 2. Part 1 describes a braking sequence previously known per se, but also describes how the sensors included in the invention determine whether brake adaptation is required or not. Part 2 describes how the brake adapta¬ tion is effected.

Part 1

When the pedal 15 is actuated at the first deceleration after coupling, an electrical circuit (not shown) in the brake light switch 26 is closed, whereupon the brake light is lit and at the same time the electromagnet 50 is activated and opens a connection between the valve 23 and the valve 8. Air additionally flows from the pressure tank 5 via the driving brake valve 10 through the operat¬ ing pressure line 9 to the relay valve 7 and opens a passage for supply air which is supplied via the supply pressure line 6 to the brake element 3 of the hauling truck 1 and applies the brakes of the hauling truck 1.

When the pedal 15 is actuated, air also flows from the driving brake valve 10 via the operating pressure line 9 through the control pressure port 46 in the valve 8 to the chamber 48 above the piston 38. The piston 38 is then pressed downwards and the valve 47 bears against the valve seat 37 of the tubular valve element 34. The tubular valve element 34 is pressed downwards and com¬ presses the spring 36 and opens an air passage from the supply pressure line 6 via the inlet port 31 to the space under the piston 38 and through the outlet port 32 onwards through the operating pressure line 12 to the brake elements of the semitrailer 2. The air flow con¬ tinues until the pressure in the operating pressure line 12 is indentical to the pressure in the chamber 48. When the pressure is identical on both sides of the piston 38, the spring 36 presses the tubular valve element 34

against the valve seat 35 and closes the air passage from the supply pressure line 6 via the inlet port 31 to the space under the piston 38. The valve 8 is thus in a balance position corresponding to the force with which the driver actuates the pedal 15.

The sequence described now, in which the balance position is obtained, also takes place in the relay valve 7 of the hauling truck 1. However, the balance position is usually obtained at a lower brake pressure in the relay valve 7 of the hauling truck 1 than in the valve 8, since the brake system of a semitrailer often requires more force mechanically than the brake system of the hauling truck 1, especially in the case when (and this is not uncommon) it has received poor maintenance and/or has been exposed to corrosion.

The above is also shown in Figure 3 from which it can be seen that, in the case of a combination of hauling truck and semitrailer in which the brakes have not been adapt¬ ed, the brakes of the semitrailer 2 according to curve B begin to be applied at a considerably higher brake pressure, here referred to as Pb 2, than the brakes of the hauling truck 1 which, according to curve A, begin to be applied at a brake pressure here referred to as Pb 1. At a low brake pressure, that is to say at a brake pressure which is less than Pb 2 but greater than Pb 1, the semitrailer 2 on the whole performs no braking work, and instead all the braking work is effected by the hauling truck 1.

During the braking sequence the pressure sensor 17 on the operating pressure line 9 detects the brake pressure at the balance position, and at the same time the load sensor 19 detects the load of the hauling vehicle. The two sensors 17, 19 then emit signals which represent the said vehicle parameters.

The memory unit 20 of the control unit receives the

signals and, taking into account the brake pressure and load, emits a signal representing an anticipated deceler¬ ation of the hauling truck, which also corresponds to the desired deceleration of the whole vehicle combination. This signal is supplied to the comparative circuit 21 in which the signal is compared to a signal which represents the actual deceleration of the vehicle combination and which is supplied to the comparative circuit from the deceleration sensor 18.

As has been mentioned above with reference to Figure 3, in the case of a vehicle combination in which the brakes are not adapted, at a low brake pressure below or equal to Pb 2 the share of the braking work performed by the semitrailer 2 is if anything small, and the actual deceleration of the vehicle combination is therefore lower than the anticipated deceleration thereof. The comparative circuit 21 therefore emits to the valve 23 an output signal indicating that the operating pressure to the semitrailer 2 should be increased. The valve 23 converts the output signal to a pneumatic signal propor¬ tional thereto by means of opening to a greater or lesser extent the connection between the pressure tank 5 and the valve 8. The pneumatic signal controls the setting of the valve 8 in such a way that the operating pressure in the brake system of the semitrailer 2 increases relative to the operating pressure in the brake system of the hauling truck 1.

Part 2

The setting of the valve 8 is controlled by means of the fact that the pneumatic signal, that is to say the air which is supplied to the chamber 44 above the piston 42 through the control pressure line 24 and the inlet port 49, presses the piston 42 downwards. The piston rod 39 then alters, via the valve 47 of the piston 38, the position of the tubular valve element 34 bearing against the valve seat 35 in such a way that the air passage from the supply pressure line 6 via the inlet port 31 to the

space under the piston and through the outlet port 32 onwards through the operating pressure line 12 is once again opened. In this way the operating pressure to the brake elements of the semitrailer is increased relative to the operating pressure to the relay valve 7 of the hauling truck.

If the driver releases the pedal 15 slightly, the pres¬ sure in the chamber 48 decreases and the air pressure on the underside of the piston 38 presses the piston 38 upwards so that the valve 47 no longer bears against the valve seat 37. Air is then evacuated through the tubular valve element 34 and out through the outlet port 33 until the pressure in the operating pressure line 12 is once again identical to the pressure in the chamber 48 and a new balance position of the valve 8 is obtained. When the driver releases the pedal 15 completely, the brake light is extinguished and the electromagnet 50 is deactivated and thus closes the connection between the valve 23 and the valve 8 but opens a connection between the chamber 44 and the atmosphere, as in the position in which the valve 25 is shown in Figure 2. The air is evacuated from the chamber 44 via this connection.

When the pedal 15 is released and the air pressure in the chamber 48 is reduced, the piston 38 also moves, under the effect of the force from the spring 43, upwards in the valve housing 30 so that the valve 47 exposes the opening of the tubular valve element 34. The air which has acted on the brake element 3 of the semitrailer 2 is evacuated through the opening and the outlet port 33.

When the driver releases the pedal 15, air is of course also evacuated from the brake system of the hauling truck 1 in a manner known per se but not described in detail here.

The operating pressure of the semitrailer is adjusted after each braking sequence, and for this reason the

vehicle combination comprises a detection element (not shown) which detects whether the vehicle combination is braked or not. The detection element, which is connected to the valve 23, can advantageously be integrated in the pressure sensor 17 since one indication that a braking sequence is not being carried out is that the pressure in the brake system is zero bar.

The adjustment can be carried out per se for the whole of the detected discrepancy at one time, but preferably in a predetermined step, for example 0.1 bar after each deceleration if the discrepancy is equal to or exceeds 0.1 bar. Should the discrepancy be below the said pres¬ sure value, the adjustment is carried out for the whole discrepancy at one time.

Figure 4 shows an example of the character of the in¬ dividual deceleration of the hauling truck 1 and semi¬ trailer 2, respectively, after they have been adapted to one another and a number of initial brakings have been carried out. It should be noted here that the pressure Pb for the hauling truck (curve A) relates to its actual brake pressure, while that for the semitrailer (curve B) corresponds to an increased brake pressure regulated by the valve 8.

The deceleration of the hauling truck 1 which is shown by curve A has the same character as before the brake adaptation. Curve B, which represents the deceleration of the semitrailer 2 as a function of the brake pressure, has in contrast been displaced in parallel upwards relative to what is shown in Figure 3. This parallel displacement has taken place in steps of about 0.1 bar pressure change for each new braking with another brake pressure than the preceding one. During normal running about six to seven brakings are required before the curve

B has been moved so as to intersect the curve A at an intermediate level Pb 3 in a brake pressure range of between 0-2 bar, for example. The curve B is moved around

this intermediate level Pb 3, which is shown in Figure 4, in steps of 0.1 bar depending on whether the brake pressure becomes higher or lower relative to what was the case in the preceding braking. On the Y axis of the diagram in Figure 4 the curve B is curved downwards towards the origin of the co-ordinates, which entails a comparatively rapid brake application on the semitrailer 2 when the driver actuates the pedal 15. How rapid the brake application is can to a certain extent be con- trolled by the adjustment of the valves 8 and 23 by the control system. In this respect it is of course attempted to control the curve B as far as possible to agree with curve A in order for the hauling truck and semitrailer each to play their part in the braking work. However, the arrangement according to the invention in practice permits an adaptation of the brake characteristic curves of the hauling truck and semitrailer in a manner as shown in Figure 4 and the associated description.

If, during a subsequent deceleration, the driver of the vehicle actuates the pedal 15 with less force than in the preceding deceleration which gave the relative position of the curves A and B according to Figure 4, an actual deceleration of the vehicle combination is detected which is greater than the anticipated deceleration stored in the memory of the control unit. This is based on the fact that the control system in this braking sequence has controlled brake pressure Pb 5 to the semitrailer in accordance with curve B which obtained its position from the preceding braking sequence. The brakes of the semi- trailer 2 therefore carry out too much braking work compared to the hauling truck 1. The control unit 16 therefore actuates the valve 23 which controls the pneumatic signal to the valve 8, which then reduces the operating pressure to the semitrailer 2 relative to the operating pressure to the hauling truck 1. In this way the curve B is displaced in parallel downwards 0.1 bar to a position B ^" , which therefore constitutes the reference for the brake pressure to the semitrailer which the

control system uses in the subsequent braking sequence. Similarly, the curve B is displaced in parallel 0.1 bar or the like to an upper position B ^" if the deceleration following the braking sequence which resulted in the position according to curve B (Pb 6) takes place at a pressure which thus exceeds the preceding brake pressure Pb 3. In this way the control unit 16 continuously and automatically displaces the curve B in parallel and in steps upwards or downwards and thus adapts the brake systems of the hauling truck 1 and semitrailer 2 to each other. However, the control unit 16 can only move the curve B in parallel upwards to an upper limit which is indicated by a dot-and-dash line in Figure 4 and which is determined by the chosen dimensions of the pistons 38, 42 and springs 43, 36.

The described brake adaptation between the hauling truck 1 and semitrailer 2 is most urgent at low brake pres¬ sures, since most decelerations take place at a relative¬ ly low brake pressure. The brake adaptation is primarily intended to take place when the brake pressure falls below a predetermined pressure, preferably about 2 bar. The brake adaptation arrangement according to the inven¬ tion which has been described thus allows the brake characteristics of the hauling truck and semitrailer to come closer to one another. This means that the gentle decelerations usually occurring particularly in normal driving can take place with both the hauling truck and semitrailer actively participating in the braking work, in contrast to what is the case in a vehicle combination in which the brakes are not adapted. However, the inven¬ tion also permits a more effective braking at higher brake pressure, since the semitrailer at such powerful decelerations participates much more in the braking work than if no brake adaptation according to the invention had been carried out between the hauling truck and semitrailer. This is clear from the fact that the dif¬ ference in deceleration level between curves A and B for one and the same high brake pressure is significantly

less in the system with brake adaptation as in Figure 1 than in the system with no brake adaptation as in Figure

3.

In one embodiment of the invention, the control unit detects whether the motor brake function is being used or not. If motor brake function is being used, the drive gears in fact perform a certain braking work. This must be taken into account in order to prevent any inaccurate information from being supplied to the control unit 16 regarding the braking work which is actually being carried out by the brake elements on the wheels of the hauling truck 1 and semitrailer 2.

The same applies if the vehicle is equipped with an additional brake, a so-called retarder, or with an automatic brake control system (ABS), in which case the control unit 16 can be supplied with signals which indicate when these are activated. The signals are used in this case to modify the setting of the valve 8 by the control unit 16. In this context it should be pointed out that the invention is particularly advantageous in a vehicle equipped with an automatic brake control system (ABS) . Such a system ensures that none of the vehicle wheels are locked, which means that the brake adaptation can be carried out without taking into consideration the risk of wheel-locking.

The invention should not be considered to be limited by the embodiment described, but instead can be modified in a number of alternative embodiments within the scope of the following patent claims. The invention is not limited to pneumatic brake systems, but can also be applied, for example, to electro-pneumatic brake systems. In such a brake system, the control element of the control valve can consist of an electrical circuit which effects the control by means of suitable signal processing.