Abstract:
A brake control system for a motor vehicle having a plurality of wheels, brakes for applying a braking effort to one or more of the wheels, and movement sensing means for detecting movement of the vehicle. The system comprises: brake actuation means for actuating the brakes to supply a braking effort; and brake control means for controlling the brake actuation means, wherein the brake control means is arranged to determine an acceleration of the vehicle based on movement detected by the movement sensing means and to ensure that the brake actuation means supplies a braking effort if the determined acceleration exceeds a set acceleration limit.

Description:
TECHNICAL FIELD 
     The present invention relates to a brake control system for a motor vehicle. In particular, though not exclusively, it relates to a brake control system for facilitating the control of a motor vehicle on a slope. Aspects of the invention relate to a system, to a method and to a vehicle. 
     BACKGROUND 
     There are a number of existing brake control systems for facilitating the control of motor vehicles on slopes. 
     EP 0 784 551 B1 discloses a brake control system comprising a hill descent mode in particular for vehicles used in off-road conditions. The brake control system comprises an electronic control unit and a brake control system to control brakes associated with each wheel of the vehicle. A driver interface in the form of a switch is used to cause the control unit to enter a hill descent mode when the vehicle is also detected to be in gear. In this mode the control unit controls the speed of the vehicle by braking to slow the vehicle when a detected vehicle speed is above a preset target speed. 
     When driving a motor vehicle off road it might be necessary to come to a standstill on a slope. In this context, EP 1 777 133 A1 discloses a brake control system in which a brake control means is arranged to detect when the vehicle is stationary and is held on a slope by a braking effort supplied by brakes in accordance with the operation of a brake command means (such as a brake pedal). The release of the braking effort is controlled as a function of the steepness of the slope following operation of the brake command means to release the brakes. 
     Whilst the brake control system of EP 1 777 133 A1 helps to smooth transition from a stationary position to hill descent to a degree, it does not exercise control over the vehicle once it is in hill descent. The system of EP 1 777 133 A1 may of course be used in conjunction with the system of EP 0 784 551 B1, but this can still lead to jerky (and therefore, to a degree, uncontrolled) hill descent in some circumstances, for example if the vehicle gathers speed following the release of the brakes by the system of EP 1 777 133 A1 and then has to be braked heavily when the target speed of the system of EP 0 784 551 B1 is exceeded. 
     It is an aim of this invention to provide an improved brake control system that overcomes or minimises the above problem. Other aims and advantages of the invention will become apparent from the following description, claims and drawings. 
     SUMMARY OF THE INVENTION 
     Aspects of the invention therefore provide a system, a method and a vehicle as claimed in the appended claims. 
     According to another aspect of the invention for which protection is sought, there is provided a brake control system for a motor vehicle having a plurality of wheels, brakes for applying a braking effort to one or more of the wheels, and movement sensing means for detecting movement of the vehicle, the system comprising: brake actuation means for actuating the brakes to supply a braking effort; and brake control means for controlling the brake actuation means, wherein the brake control means is arranged to determine an acceleration of the vehicle based on movement detected by the movement sensing means and to ensure that the brake actuation means supplies a braking effort if the determined acceleration exceeds a set acceleration limit. 
     By ensuring that the brake actuation means supplies a braking effort when the acceleration of the vehicle exceeds a set acceleration limit, the system of the invention enables smooth hill descent throughout, thus enhancing control over the vehicle. 
     Advantageously, the brake control means may be arranged to determine the acceleration of the vehicle based on movement of the vehicle detected during a time cycle, preferably of 100 milliseconds or less. Of course the acceleration of the vehicle may conveniently be determined continuously whilst the system is activated, for example based on a plurality of time cycles each preferably of 100 milliseconds or less, more preferably 50 milliseconds or less, most preferably 20 milliseconds or less. Shorter time cycles lead to more accurate and immediate determination of acceleration and hence a more accurate and immediate braking response, which in turn ensures even smoother hill descent and even better control over the vehicle. 
     It will be appreciated that the determination of the acceleration of the vehicle may occur within the movement sensing means itself. Thus the brake control system, and in particular the brake control means, may function integrally with and/or include the movement sensing means. More generally the brake control means may comprise a single electronic control unit or one or more control units. 
     The brake control means may be configured (or arranged) to set the acceleration limit as a fixed value, e.g. a value below 1 ms −2 . The fixed acceleration limit may be based on a user input, e.g. via an interface. Alternatively, and more preferably, the brake control means may be arranged to set the acceleration limit dynamically based on one or more vehicle conditions or driver inputs. 
     In an embodiment, the brake control means may be arranged to measure a desired drive torque and to set the acceleration limit dynamically based on the desired drive torque. The brake control means may measure the desired drive torque form the signal of an accelerator pedal position sensor of the vehicle, optionally further taking into account a signal from a gear selection sensor. An accelerator pedal position sensor and/or a gear selection sensor may thus function integrally with and/or form part of the brake control system. 
     Setting the acceleration limit dynamically based on a desired drive torque enables particularly smooth hill descent of the vehicle since it ensures that the acceleration of the vehicle conforms with the intention of the driver. In this regard it is advantageous that the brake control means be configured to increase the set acceleration limit with increasing desired torque. 
     Determining the desired drive torque based on the signal of the gear selection sensor may involve multiplying the signal of the accelerator pedal position sensor by a factor associated with a selected gear. Advantageously, the brake control means may be arranged to set the acceleration limit independently of the signal of the accelerator pedal position sensor if no gear is selected (i.e. the vehicle is in neutral). 
     For maximum conformity with driver intentions, the brake control means may be arranged to determine a desired acceleration associated with the desired drive torque and to set the acceleration limit at a threshold equal to or above the desired acceleration. The desired acceleration may conveniently correspond to an estimate of the acceleration that would result from the desired drive torque in a zero gradient state of the vehicle (i.e. on level ground). Advantageously, the brake control means may be arranged to set the acceleration limit at a value in the range of from 0 to 1 ms −2  above the desired acceleration. 
     The brake control means may also be arranged to set the acceleration limit in dependence on the gradient of the vehicle. The gradient of the vehicle may be detected by a gradient sensing means of the vehicle, such as an inclinometer, which may function integrally with and/or form part of the brake control system. The brake control means may be arranged to lower the acceleration limit in response to an increased gradient of the vehicle. 
     Advantageously, the direction of travel of the vehicle may be monitored by the brake control means (by communication with the movement sensing means). The brake control means may advantageously be arranged to determine whether the vehicle is travelling in an intended direction or an unintended direction with reference to a selected gear. 
     Advantageously, the brake control means may be arranged to set the acceleration limit in dependence on the direction of travel, with the acceleration limit optionally being set lower when the vehicle is moving in an unintended direction than when the vehicle is moving in an intended direction 
     The brake control system may optionally have activated and deactivated states, which may be selectable via a user interface. The brake control means may deactivate the system either conventionally, by placing the system in standby, or, conveniently, by setting the acceleration limit to a high level that cannot be achieved by the vehicle. 
     The system may comprise a driver brake pedal and a brake pedal position sensor in communication with the brake control means. In some cases driver input will obviate the need for additional braking effort in which case the brake control means simply monitors the braking effort. Thus the brake control means may preferably only add to the supply of braking effort when the relevant braking effort is not already being provided by input from a driver, e.g. though a brake pedal. In the present invention braking effort may advantageously be supplied as needed to supplement driver input. Braking effort may be supplied to all wheels of the vehicle to maximise traction. 
     The brake control system may be integral or function in conjunction with other brake control systems, for example such as those described in EP 0 784 551 B1 and/or EP 1 777 133 A1. 
     According to a further aspect of the invention for which protection is sought, there is provided a method for controlling the brakes of a motor vehicle, the method comprising: measuring a desired drive torque, setting an acceleration limit of the vehicle based on the desired drive torque, determining an acceleration of the vehicle and, if the determined acceleration exceeds the set acceleration limit, ensuring that the brakes supply a braking effort. 
     According to a still further aspect of the invention for which protection is sought, there is provided a motor vehicle comprising a brake control system according to one aspect of the invention or adapted to use a method according to another aspect of the invention. 
     Within the scope of this application it is envisaged that the various aspects, embodiments, examples, features and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings may be taken independently or in any combination. Features and advantages recited in respect of the one aspect of the invention apply mutatis mutandis to any other aspect of the invention, except where there is incompatibility of features. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described by way of example with reference to the accompanying drawings of which: 
         FIG. 1  is a schematic drawing of a brake control system for a motor vehicle embodying one form of the present invention, and 
         FIG. 2  is a flowchart summarising the working of the brake control system of  FIG. 1  in an operational mode. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , there is shown a motor vehicle  10  in a dashed outline with a brake control system having a hill descent mode, according to a first embodiment of the invention. 
     When the hill descent mode is activated, the brake control system is in an activated state for the purpose of the invention. By contrast, when the hill descent mode is deactivated, the brake control system is deactivated for the purpose of the invention. 
     The brake control system, and in particular the hill descent mode, may additionally perform other brake functions not related to the invention, such those described in EP 0 784 551 B1 or EP 1 777 133 A1. 
     The brake control system includes in a conventional manner a braking arrangement with a disc brake  11  on each of four wheels  12 . Each disc brake  11  is hydraulically operated through a hydraulic line  13  by a brake hydraulic control (HC) unit  14  which functions as a brake actuation means. The HC unit  14  is controlled by an electronic control (EC) unit  15 , which functions as a brake control means.  FIG. 1  is only a diagram of the most important functional components of the brake control system in accordance with the invention. Further details of the brake control system are described in EP 0 784 551 B1 and are included in the description by reference and thus will not be further described. 
     The EC unit  15  receives a vehicle speed signal from movement sensing means in the form of a vehicle speed sensor  16 , an individual wheel speed signal from a wheel speed sensor  17  on each wheel  12  and a driver demand signal from an accelerator (or throttle) pedal  18 , which incorporates an accelerator pedal position sensor  19 . The EC unit  15  also receives a hill descent signal from a driver hill control switch  20 , a gear signal from a drive gear sensor  21  and an inclination angle signal from a gradient sensing means, in this example an inclinometer  22 . The EC unit  15  receives a pedal braking signal from a brake light switch  9  on a brake pedal  24 . 
     The brake pedal  24  is also associated with a master cylinder  8 , which is connected to a brake pressure sensor  23  forming part of the HC unit  14 . The brake pressure sensor  23  is arranged to send a driver braking signal from the brake pressure sensor  23  to the EC unit  15 . 
     The EC unit  15  is active to control the HC unit  14  whenever the hill control switch  20  is switched on by the driver to select hill descent mode. The EC unit can operate in conjunction with (i.e. to supplement) operator inputs from the brake and accelerator pedals  24 ,  18 . 
     When driving the motor vehicle  10  it may be desired to descend a slope. To assist drivers with descending slopes in a controlled manner, the EC unit  15  is configured (arranged), in the activated state of the system, to ensure that the brake actuation means supplies a braking effort when the movement sensing means determines that the acceleration of the vehicle exceeds a set acceleration limit. 
     In a first operational mode of the system, the acceleration limit is set at a fixed value of 0.5 ms −2 . The EC unit  15  monitors the movement of the vehicle, as detected by the vehicle speed sensor  16  and determines the acceleration of the vehicle in cycles of 10 milliseconds. If the acceleration is found to exceed the acceleration limit of 0.5 ms −2  then the EC unit  15  checks the signal from the brake pressure sensor  23 , which is also continuously monitored by the EC unit  15 , to determine if a suitable braking effort is already being applied by the driver via the brake pedal  24 . In the event that insufficient (if any) braking effort is being applied via the brake pedal, the EC unit  15  issues a command to the HC unit  14  to apply and maintain a braking effort to bring the acceleration of the vehicle within the limit of 0.5 ms −2 . 
     A second operational mode of the system will now be described with reference to  FIG. 2 . In this mode the acceleration limit is set at a fixed value of 0.5 ms −2  only if the vehicle is not in gear or is travelling in an unintended direction. If on the other hand the vehicle is travelling in an intended direction associated with a selected gear, the acceleration limit is set dynamically based on a desired drive torque. 
     In the second operational mode, once the hill descent mode is activated, the EC unit  15  checks, via the gear sensor  21  whether a gear is selected. If no gear is selected, or if the EC unit determines through the vehicle speed sensor  16  that the vehicle is travelling in an unintended direction (based on a selected gear), the EC unit  15  sets a fixed acceleration limit of 0.5 ms −2 . 
     However, if at any stage the EC unit  15  is notified by the gear sensor  21  that a gear is selected and the vehicle is moving in the intended direction of travel associated with the selected gear, the EC unit proceeds to set the acceleration limit dynamically based on a desired drive torque indicated by the accelerator pedal position sensor  19 . 
     Specifically, the EC unit dynamically determines (or looks up in a table or map) a desired acceleration associated with the accelerator pedal position and the selected gear. The desired acceleration corresponds to an estimation of the acceleration that would be expected if the vehicle were in a zero gradient state, i.e. on level ground, and is thus based on known information about the vehicle such as weight and drag. 
     The desired acceleration, which is determined in cycles of 10 milliseconds, serves as the basis for setting the acceleration limit: the acceleration limit is set at 0.5 ms −2  above the desired acceleration. In a variant of the second operational mode (not shown in  FIG. 2 ), the EC unit  15  checks the signal from the inclinometer  22 , adjusts the acceleration limit to be equal to the desired acceleration if the vehicle is detected to be on a gradient greater than 20%. 
     Once the acceleration limit has been set, either as a fixed value (when no gear is selected or the vehicle is moving in an unintended direction) or as a dynamic value (when the vehicle is moving in an intended direction, the EC unit  15  proceeds to monitor the movement of the vehicle, as detected by the vehicle speed sensor  16 , and determines the acceleration of the vehicle continuously, in cycles of 10 milliseconds. If the acceleration is found to exceed the acceleration limit then the EC unit  15  checks the signal from the brake pressure sensor  23 , which is also continuously monitored by the EC unit  15 , to determine if a suitable braking effort is already being applied by the driver via the brake pedal  24 . In the event that insufficient (if any) braking effort is being applied via the brake pedal, the EC unit  15  issues a command to the HC unit  14  to apply and maintain a braking effort to bring the acceleration of the vehicle within the limit. 
     The following operational examples are provided to illustrate the second mode of operation of the brake control system according to the first embodiment of the invention: 
     The vehicle  10  is driven down a steep slope in a downhill gear. Vehicle acceleration exceeds desired acceleration plus 0.5 ms −2 . The EC unit  15  sends a command to the HC unit  14  to generate a braking effort and limits the vehicle acceleration to desired acceleration plus 0.5 ms −2 . 
     The vehicle  10  is driven down a steep slope in a downhill gear. The driver changes gear and the new gear results in a vehicle acceleration. The acceleration exceeds desired acceleration plus 0.5 ms −2 . The EC unit  15  sends a command to the HC unit  14  to generate a braking effort and limits the vehicle acceleration to desired acceleration plus 0.5 ms −2 . 
     It is possible to modify the second operational mode such that the acceleration limit associated with movement an unintended direction is set based on desired torque, as described in respect of movement in an intended direction. 
     More generally, it will be appreciated that a number of modifications can be made to the brake control system of the first embodiment without departing from the scope of the invention. For example, it is not necessary for the activation of the brake control system to be dependent on the activation of a hill descent mode. Thus, the brake control system of the first embodiment can, according to a second embodiment of the invention, be modified to remain active whenever the vehicle is operated.