Abstract:
Methods are disclosed for regulating the braking torque generated by the braking system of a vehicle which include a friction lining, a brake surface, an actuator for displacing the friction lining by means of a medium towards he brake surface with a predetermined brake pressure in order to generate the brake torque, the method including restricting the predetermined brake pressure below a maximum brake pressure for the speed of the vehicle only when the actuator is actuated when the vehicle is traveling at a speed below a predetermined speed. Apparatus is also disclosed for carrying out the method.

Description:
TECHNICAL FIELD 
     The present invention relates to a method for regulating braking torque generated by a braking system in a wheeled vehicle according to the preamble of claim  1 , and to an arrangement for regulating braking torque generated by a braking system in a wheeled vehicle according to the preamble of claim  11 . The invention further relates to the use of pressure regulating means in a braking system of a wheeled vehicle to restrict brake torque generated by said braking system. 
     The invention is primarily intended for, but not restricted solely to, applications in braking systems of commercial vehicles such as trucks and lorries. 
     BACKGROUND OF THE INVENTION 
     Electronic control is being employed to an ever increasing extent in vehicles to ensure optimal efficiency and reliability of various operational systems. A recent development in the truck industry is the availability of electronic braking systems (EBS). Although compressed air is still used to apply the brakes in EBS, electronics are used to command pressure-regulating modules to pressure the brake cylinders with compressed air to thereby engage the brakes. At least theoretically, by being able to monitor various forces, load changes and individual braking rates, EBS can continuously modify the braking on each wheel to thereby provide optimal distribution of the braking force. Anti-lock braking (ABS) and traction-control (ASR) functions can also be integrated in the EBS. 
     As is described in WO 91/09758, EBS can be employed to adapt the brake pressure supplied to the brakes of a trailer to that supplied to the brakes of the hauling truck. 
     Irrespective of whether a truck is fitted with EBS, during an emergency stop the brake torque generated by the brakes rapidly rises to a short duration peak value before leveling off to a lower value. This effect is illustrated in FIG. 1 for a drum brake system in which brake torque is plotted as a function of time for emergency braking from three different speeds. Thus, curve A represents emergency braking from 20 km/h, curve B from 60 km/h and curve C from 100 km/h. It is apparent that the maximum value of peak brake torque arises when emergency braking from relatively low speed. 
     The use of disc brakes on commercial vehicles is becoming more widespread. Generally, disc brakes are lighter than corresponding drum brakes, thereby contributing to reduced unsprung weight. Furthermore, they have the ability to dissipate heat more effectively than drum brakes, which implies that they are often less prone to brake fade from high speeds. In order to slow down a heavily laden truck from high speeds, the disc brake actuators must be capable of exerting high pressure on the brake pads. As is apparent from FIG. 2, this high braking pressure results in even greater peak values for the brake torque than those attained with a drum brake system when emergency braking from corresponding speeds. 
     The high peak values of brake torque when emergency braking from low speed, and as represented by curve D in FIG. 2, create significant problems for the truck manufacturer. Firstly, the vehicle suspension components and axles, particularly at the front of the vehicle, must be dimensioned to be able to withstand these peak braking torques. This implies a substantial increase in weight and manufacturing costs. Secondly, the peak braking torque may be sufficient to activate the ABS system even if the truck is travelling on a high-friction surface such as dry asphalt. On such surfaces and at speeds below say 25 km/h, a shorter braking distance is in fact attained if the ABS does not cut in. Thirdly, the severe jolt which is transmitted through the truck upon emergency application of the brakes at low speeds implies that the cargo has to be more securely tied down than would be the case if such peak brake torques did not arise. This means of course that extra time and expense is involved in equipping the truck with adequate cargo-retaining means. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method and an arrangement for overcoming the problems associated with peak brake torque arising during emergency braking from low speeds. 
     This object is achieved by the method according to claim  1  and by the arrangement according to claim  11 . 
     Preferred embodiments of the method and arrangement are detailed in the respective dependent claims. 
     The invention further provides for the use of pressure regulating means in a braking system of a wheeled vehicle to restrict brake torque generated by said braking system to a predetermined value below a maximum value when said vehicle is travelling below a predetermined speed when said braking system is initially actuated. 
     In accordance with the invention, when emergency braking from a speed below a predetermined value, braking pressure is restricted to a predetermined value which lies below the maximum value of pressure which is otherwise attainable above the predetermined value of speed. In this manner, the peak brake torque value when emergency braking from low speeds will be relatively lower without compromising desirable high braking pressure when emergency braking from higher speeds. This implies that even more effective brakes can be fitted to a vehicle without having to strengthen existing suspension components and mountings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects and advantages of the present invention will become apparent from the subsequent detailed description of preferred embodiments in which reference is made to the attached drawings, of which: 
     FIG. 1 shows the relationship between brake torque and time for a vehicle fitted with a conventional drum brake system being emergency braked from various speeds; 
     FIG. 2 corresponds to FIG. 1, though for a vehicle fitted with a conventional disc brake system; 
     FIG. 3 is a schematic representation of a vehicle brake system provided with an arrangement according to the present invention; and 
     FIG. 4 shows the relationship between pressure and speed in a brake system operated according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As previously mentioned, FIG. 1 is a graph of brake torque against time for emergency braking from three different speeds for a typical drum brake system fitted to a commercial vehicle. By the expression “emergency braking” it is hereby meant that the driver of the vehicle attempts to reduce the speed of the vehicle as quickly as possible. Curve A represents emergency braking from 20 km/h, curve B from 60 km/h and curve C from 100 km/h. In all cases, when the brakes are initially applied, a peak braking torque is attained for a short duration. This peak is most noticeable when emergency braking from low speed. 
     In a similar manner, FIG. 2 shows corresponding curves D, E and F for a commercial vehicle fitted with a disc brake system. As clearly illustrated by curve D, the peak value of the braking torque when emergency braking from low speed is significantly increased when compared to both braking from higher speeds and the peak torque which is attained with the drum brake system. 
     The curve G illustrated by a dashed line in FIG. 2 represents a limited peak brake torque value which is attainable in accordance with the method and arrangement of the present invention. It is to be noted that the reduction of the peak brake torque value affects only the braking curve from low speed. The brake torque values are not affected for emergency braking from higher speeds. Nevertheless, due to the inherent flexibility of the present system achieved thanks to its compatibility with EBS, the manufacturer and/or workshop can easily reprogram the system so that peak brake torque can be restricted during emergency braking from above any chosen speed. 
     FIG. 3 schematically illustrates a braking system, generally denoted by reference numeral  10 , of the EBS type incorporating the arrangement according to the present invention which permits generation of the curve G of FIG.  2 . The preferred embodiment of FIG. 3 is a compressed air braking system, with compressed air conduits being depicted by thicker lines and electrical leads being depicted by thinner lines. It is to be understood, however, that the invention can be practised on a braking system having an operating medium other than compressed air, for example a hydraulic system. 
     The system  10  of FIG. 3 comprises a brake pedal  12  connected to a brake valve  14 . In a manner which will be explained in the following, depression of the brake pedal  12  causes an actuator  16  associated with each wheel of the vehicle (four in the shown embodiment) to effect displacement of a friction lining  18  towards a brake surface  20  on, in the present case, a brake disc  22 . 
     Compressed air for the braking system  10  is stored in rechargeable reservoirs  24 ,  26 ,  28 . One reservoir  24  supplies compressed air to the rear brakes of the vehicle, whilst another reservoir  26  supplies compressed air to the front brakes. In a typical brake system, the pressure of the compressed air in the reservoirs  24 ,  26  is between 8.2 and 12 bar. Should the pressured drop below 8.2 bar, a compressor (not shown) cuts in to recharge the reservoirs. The third reservoir  28  supplies compressed air at a pressure of between 8.2 and 8.5 bar to a parking brake. The three reservoirs also supply compressed air to a trailer modulator valve  29  to which air lines to a trailer (not shown) may be connected to thereby permit braking of the trailer. A drainage tank  30  may be included in the braking system for the collection of water from the compressed air. 
     The respective reservoirs  24  and  26  supply compressed air to a pressure modulator  32  associated with each wheel. In the shown embodiment, the rear wheels are served by a tandem pressure modulator. In a manner which will be described in the following, each pressure modulator  32  is controlled by signals sent from a central electronic processing unit (CPU)  34  to thereby regulate the supply of compressed air to its respective actuator  16 . 
     The CPU receives signals from a number of sensors and, dependent on the information content of such signals, instructs each pressure modulator  32  to supply a particular pressure of compressed air to its respective actuator  16 . Accordingly, the brake valve  14  is provided with a sensor  36  to detect displacement of the brake pedal  12  which is an indication of the degree of braking effort which is desired by the driver. In the shown embodiment, the vehicle is provided with rear air suspension  38 . Thus, pressure sensors  40  relay information to the CPU  34  so that an indication of the load on the rear suspension can be obtained. The system further includes at each wheel a wheel speed sensor  42  and a friction lining wear sensor  44 . The information relayed by these sensors is used i.a. to modulate the ABS function of the braking system. 
     By taking into account e.g. the rear axle load, the speed of rotation of each wheel, the degree of wear of each friction lining and the degree of braking effort which is desired by the driver, the CPU can apportion pressure between the actuators  16  so that the most advantageous braking effect is attained. 
     In accordance with the present invention, the CPU is provided with means in the form of a comparator for comparing the speed at which the vehicle is travelling when, in the event of an emergency braking, the brake pedal  12  is initially depressed to a predetermined speed value stored in a memory of the CPU. If the comparator determines that the vehicle speed is less than the predetermined speed value, the CPU signals the pressure modulators  32  to restrict the pressure to the actuators  16  to a first predetermined value of pressure which is below the normal maximum value. Since the pressure to the actuators is restricted, the braking pressure with which the friction linings  18  act on the brake surfaces  20  will be correspondingly lower than the otherwise attainable maximum value. As a consequence, the peak braking torque will be lower than that which would otherwise have been attained if the CPU had not instructed the pressure modulators  32  to restrict the pressure. 
     Conversely, should the comparator determine that the vehicle speed is higher than the predetermined speed value stored in the CPU, the CPU will permit the pressure modulators  32  to allow maximum pressure to be transmitted to the actuators  16 . Thus, above the predetermined speed value, the actuators  16  are able to cause the friction linings  18  to exert a maximum braking pressure on the braking surfaces  20 . 
     With reference to FIG. 2, if the predetermined speed value stored in the CPU is chosen to be, say, 40 km/h, emergency braking from 20 km/h will generate a brake torque/time curve corresponding to curve G. In other words, the peak brake torque will be significantly lower than if the CPU had not caused the pressure modulators to restrict the pressure to the first predetermined value of pressure. In the event of emergency braking from 60 km/h and 100 km/h, curves corresponding to curves E and F respectively will be obtained. 
     The actual value of the predetermined speed value is not critical. Preferably, it should be below about 60 km/h so that braking performance above this speed is not compromised. Advantageously, the predetermined speed value can be below about 50 km/h and most preferably at about 40 km/h. 
     Similarly, the actual degree of restriction of the pressure of the compressed air acting on the actuators is not critical. Nevertheless, it is advantageous if the maximum attainable pressure is at least 10% greater, preferably at least 20% greater, than the first predetermined value of pressure. 
     In a preferred embodiment of the invention, a second predetermined speed value is stored in the CPU, the second predetermined speed value being at least 10% higher than the first predetermined speed value. Thus, the pressure of the compressed air acting on the actuators is limited to a first predetermined value of pressure in the event of emergency braking when the vehicle is travelling at a speed below the first predetermined speed value and is permitted to be a maximum value when the vehicle is subjected to emergency braking from a speed above the second predetermined speed value. Should the vehicle be travelling at a speed between the first predetermined speed value and the second predetermined speed value when emergency braking is instigated, the pressure modulators  32  are instructed to provide pressure to the actuators  16  at a pressure value which lies between the first predetermined value of pressure and the maximum pressure. This situation is illustrated in FIG. 4 in which the first predetermined speed value is denoted by v 1  and the second predetermined speed value by v 2 . 
     In a preferred embodiment of the invention, the second predetermined speed value v 2  is between 30% and 60% higher than the first predetermined speed value v 1 . Advantageously, the first predetermined value of speed v 1  lies between about 10 km/h and 40 km/h and the second predetermined value of speed v 2  lies between about 20 km/h and about 60 km/h. The first predetermined value of pressure P 1  may typically be about 7.5 bar and the maximum pressure P m  about 9 bar+/−0.5 bar. In the event of a fault in the arrangement according to the invention, and to prevent possible damage being inflicted on the vehicle due to excessively high peak brake torques, the brake system may be designed to operate at the first predetermined value of pressure throughout the entire speed range of the vehicle. Naturally, the vehicle driver can be informed of the fault via a warning signal in the vehicle cab. It is to be appreciated that the vehicle is capable of meeting legislated braking distances even if the brake system is operated at the first predetermined value of pressure throughout the entire speed range of the vehicle. 
     It will be apparent from FIG. 4 that the value of pressure between the first predetermined value of pressure P 1  and maximum pressure P m  varies in a substantially linear manner. It is to be understood, however, that the invention may also make use of a non-linear pressure variation between P 1  and P m . 
     The invention is not restricted to the embodiments described above and shown in the drawings, but may be varied within the scope of the appended claims. For example, The second predetermined speed value may correspond to the maximum speed of the vehicle such that the pressure value acting on the actuators increases gradually from the first predetermined value of pressure below the first predetermined speed value to maximum pressure at the maximum speed of the vehicle. Alternatively, the pressure value acting on the actuators may be reduced to a value below the first predetermined value of pressure the slower the vehicle is travelling when emergency braking is initiated.