BACKGROUND In traditional brake systems for a utility vehicle such as a truck, truck with trailer or tractor unit with semitrailer, the wheel brakes are controlled by pressure regulation while the vehicle is in motion. This means that both in pneumatic and in hydraulic brake systems there is a certain brake pressure which corresponds to each position of the foot brake pedal operated by the driver. Wheel brakes have a limited service life during which their efficiency gradually decreases. This decrease in efficiency may be due to diminishing friction between their rotating part and their movement-inhibiting part, a phenomenon known as"glassing". The cause of glassing is often low-pressure braking over relatively long periods. Diminishing efficiency may also be due to brake mechanism jamming. Reduced efficiency can be counteracted by applying greater brake pressure in order to achieve the same retardation of the vehicle as when the efficiency of its wheel brakes was undiminished. This is usually done by the driver increasing the deflection of the foot brake pedal. This increase in deflection of the foot brake pedal also warns the driver that the efficiency of the wheel brakes has decreased.

In some electronic brake systems the wheel brakes are retardation-regulated. This means that for each position of the foot brake pedal there is a corresponding retardation of the vehicle. An electronic control unit receiving information from wheel speed sensors regulates the brake pressure so as to provide a retardation which corresponds to the retardation"ordered"by the pedal deflection. A disadvantage of this type of electronically controlled brake system is that the pedal deflection does not warn the driver

of reduced efficiency of the wheel brakes, since a given pedal deflection always produces the same retardation.

In modern systems for setting wheel brake pressures on a semitrailer for a tractor unit or on a trailer for a truck, the brake pressure on the semitrailer or trailer is adapted so as to reduce the loads in the coupling between the tractor unit or truck and the semitrailer or trailer respectively when braking while the vehicle is in motion. However, this does not cater for any possible reduction in efficiency of the wheel brakes on the tractor unit or truck.

OBJECT OF THE INVENTION One object of the present invention is to improve the monitoring of the state of retardation-regulated wheel brakes of utility vehicles.

DESCRIPTION OF THE INVENTION The object stated above is achieved by a method and a system with the characterising features in patent claims 1 and 7 respectively.

Determining the efficiency of a towing vehicle's wheel brakes on the basis of comparing an expected towing vehicle retardation with an actual towing vehicle retardation makes it possible to obtain information about the state of the brakes, information which can be used for settings and activation of corrective or remedial measures.

When the towing vehicle is travelling with a trailer, an expected and an actual combination retardation for the combination of the vehicle and the trailer, and a trailer wheel brake efficiency are preferably determined and a brake pressure is set on the trailer.

This reduces loads on a coupling between towing vehicle and trailer during braking.

Information means are preferably activated to warn the driver of the towing vehicle that its wheel brake efficiency is below a predetermined value. Thus a driver who in the case of retardation-control brakes is not warned by the brake pedal reflection that the efficiency of the wheel brakes has decreased nevertheless receives information in this respect.

A wheel brake conditioning function of the utility vehicle is preferably activated if the towing vehicle wheel brake efficiency is below a predetermined value. A diagnosed reduction in brake efficiency thus results in immediate action to improve same.

DESCRIPTION OF THE DRAWINGS The invention is described below in more detail with reference to the accompanying drawings, in which Fig. 1 is a diagram schematically illustrating a brake system according to one embodiment of the invention, Fig. 2 depicts a flowchart illustrating a method according to one embodiment of the invention, and Fig. 3 depicts a flowchart illustrating a method according to another embodiment of the invention.

DESCRIPTION OF EMBODIMENTS In the ensuing, the term"towing vehicle"covers both trucks and tractor units. Tractor units are not arranged to carry directly any payload but to haul a so-called semitrailer.

Trucks are arranged to carry payload and may (but need not necessarily) be arranged to haul a trailer. In the ensuing, the term"trailer"covers both trailers arranged to be hauled by a truck and semitrailers arranged to be hauled by a tractor unit.

Referring to Fig. 1, the driver of a towing vehicle (such as a truck) which is intended to haul a trailer can, while the towing vehicle is in motion, operate a brake control 1 in its driving cab. The towing vehicle's wheel brakes are retardation-regulated, which means that for a certain retardation of it there is a corresponding specific position for the brake control 1. In Fig. 1, the position of the brake control 1 is measured by the distance 8.

A brake control sensor 2 is arranged to detect the distance S which corresponds to the position of the brake control 1 and send signals corresponding to the distance 8 to an electronic control unit (ECU) for the brake system. The ECU is arranged to send control signals to each of at least one brake operating device 3. Fig. 1 depicts schematically a compressed air brake system in which each brake operating device is connected to a compressed air reservoir 3'. Each brake operating device 3 is connected to the brake 4 on at least one of the towing vehicle's wheels 5 by means of at least one brake line 6.

Instead of being pneumatic, the brake system may be hydraulic, in which case each brake line is filled with brake fluid in the form of hydraulic fluid instead of air.

The ECU is also arranged to receive from pressure sensors signals corresponding to the pressure in the brake lines 6. The ECU can thus receive separate signals about the brake pressure on each wheel axle or each wheel. The ECU may also receive information about the wheel brake force as a function of the brake pressure on the assumption of undiminished efficiency of the wheel brakes. This information may refer to each axle or wheel. Information received about the wheel brake force may for example take the form of a wheel brake factor for each axle, the unit of measure of which may be kN/bar. A typical value for the wheel brake factor per axle of a towing vehicle is 10 kN/bar.

Information about the wheel brake force as a function of brake pressure when the efficiency of the wheel brakes is undiminished can be stored in a memory which is accessible to the ECU.

A wheel speed sensor 7 on at least one of the vehicle's wheels is arranged to send signals to the ECU corresponding to the speed of the respective wheel. At least one torque

sensor TS is arranged to detect the engine torque on the basis of known technology. The vehicle's weight can be calculated on the basis of signals from the wheel speed sensor during a vehicle acceleration period and information about the engine torque over the same period. Weight information may also be derived from a memory in which it is stored.

Fig. 2 depicts a flowchart implementing a method according to one embodiment of the invention. The first step is the driver operating the brake control 1, thereby changing its position. One object of the method described in Fig. 2 is to determine the efficiency of the towing vehicle's wheel brakes. To this end, the method in Fig. 2 has to be applied when the towing vehicle is travelling without a trailer. The brake control sensor 2 detects the distance 8, which corresponds to a position of the brake control 1, and sends to the ECU a position signal LS corresponding to the distance 8.

An expected retardation Refis calculated on the basis of information about the respective brake pressure per axle, the wheel brake factor per axle for brakes with undiminished efficiency, and the vehicle's weight. The ECU sends to each of the brake operating devices 3 a signal to exert a pressure P in the respective brake line 6 so that the respective brake 4 is activated.

An actual retardation Rb,, can be calculated by means of signals received by the ECU from the wheel speed sensor 7 over at least one predetermined period of time. A towing vehicle wheel brake efficiency nb is calculated, corresponding to the ratio between the actual towing vehicle retardation Rb, and the expected towing vehicle retardation Rbf.

Information on the towing vehicle wheel brake efficiency (nb) is stored to enable it to be used, for example, for settings or remedial measures.

As illustrated in Fig. 1, the ECU is arranged to send a signal when necessary in order to light up a warning lamp 8 on an information display in the driving cab. If the wheel brake efficiency nb is below a predetermined value (e. g. 0.75), a signal to light up the

warning lamp 8 is sent. Alternatively, or in addition, an acoustic signal corresponding to a signal from the ECU may be activated in the driving cab when necessary. The warning lamp 8 lighting up informs the driver of reduced efficiency of the brakes, and appropriate measures such as servicing or repairs can be adopted.

Reverting to the flowchart in Fig. 2, an iterative process is conducted for adjusting the brake pressure of the towing vehicle's wheel brakes until the actual retardation Rb, corresponds to the expected retardation Rbf. If the actual retardation Rb"is less than the expected retardation Rbf, the ECU sends a signal to each of the brake operating devices 3 to increase the pressure in the respective brake line 6 by a predetermined step value i, whereupon the braking action of the respective brake 4 increases. This is followed by calculation of a new value of the actual retardation Rb, by means of signals received by the ECU from the wheel speed sensor 7. If the new actual retardation Ru, value is also less than the expected retardation Rbf the ECU again sends a signal to each of the brake operating devices 3 to increase the pressure further in the respective brake line 6 by a predetermined step value i. This iterative process is repeated until the actual retardation Rb, corresponds to the expected retardation Rbf.

Fig. 3 depicts a method according to another embodiment of the invention. The object here is to set the brake pressure of the wheel brakes on a trailer which is coupled to a towing vehicle, in order to minimise the loads in the coupling between the towing vehicle and the trailer during braking when the vehicle is in motion.

The first step is to determine the efficiency of the towing vehicle's wheel brakes. This is done in the same way as in the method described above in order to warn the driver that their efficiency has decreased below a certain level, i. e. when the towing vehicle is in motion without a trailer.

Referring to Fig. 1, the driver operates the brake control 1 when the towing vehicle is travelling without a trailer. The ECU provides information about the distance 6, which

corresponds to the position of the brake control 1. The towing vehicle's expected retardation Rbf is calculated from its weight (which may be stored in a memory in the ECU), its wheel brake factor per axle for brakes with undiminished efficiency, and its brake pressure per axle. The ECU sends a signal to each of the brake operating devices 3, whereupon a pressure P is exerted in the respective brake line 6. The towing vehicle's actual retardation Rb, is calculated by means of the wheel speed sensor 7. A towing vehicle wheel brake efficiency nb is determined, corresponding to the ratio between the actual towing vehicle retardation RbV and the expected towing vehicle retardation Rbf.

When the towing vehicle is provided with a trailer, the above steps are repeated, i. e. the driver operates the brake control 1, and the ECU provides information about the distance 8 corresponding to a position of the brake control 1. An expected combination retardation Rkf for the combination of towing vehicle and trailer is determined on the basis of the brake pressure per axle, the wheel brake factor per axle for brakes with undiminished efficiency, and the weight mk for the combination of towing vehicle and trailer. The weight is determined from information about the speed of at least one wheel and the engine torque.

An actual combination retardation Rk, for the combination is determined using information about the speed of at least one of the wheels. A trailer wheel brake efficiency vs is determined by using information about the expected combination retardation Rkf and the actual combination retardation Rk,. The trailer wheel brake efficiency nS can be determined on the basis of the formula nx = mS/(mk * Rkf/Rkv-mb/n), in which ms is the trailer's weight, which is determined as the difference between the combination's weight m and the towing vehicle's weight mb.

The brake pressure Ps of the trailer brakes is set on the basis of the trailer wheel brake efficiency nS. The trailer brake pressure Ps can then be adjusted by a factor which corresponds to the inverted value of the trailer wheel brake efficiency nS.

The vehicle may be equipped with a wheel brake conditioning function whereby the wheel brakes are conditioned to counteract a reduced efficiency of them caused by, for example,"glassing". This function reduces the pressure of certain brakes or puts them out of action, while the pressure of other brakes is increased.

According to a further embodiment of the present invention it is possible, if the vehicle wheel brake efficiency nb is below a predetermined value, for information about this to be sent to an electronic monitoring system in the vehicle. This information may be used for activating said wheel brake conditioning function. The information may also be used for activating other functions in the vehicle. Another such function may be to increase at the time of braking the action of a retarder brake and/or an exhaust brake, in order to increase the total braking power.

Information that the towing vehicle's wheel brake efficiency nb is below a predetermined value may also be used for activating a speed limiting and/or engine speed limiting function so that the vehicle does not travel above a certain speed during reduced wheel brake efficiency.