Abstract:
An apparatus for controlling the stability of a vehicle including a controller which is adapted to detect when conditions exist under which vehicle rollover is likely and, when such conditions exist, to trigger an alarm signal or initiate a control intervention by means of which the vehicle travel is controlled to reduce the likelihood of vehicle rollover, wherein the apparatus further includes an accelerometer, the accelerometer being configured to measure the acceleration of the vehicle along two non-parallel axes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority of United Kingdom patent application No. 0721464.6 filed on Nov. 1, 2007, the content of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a vehicle stability control system, particularly, but not exclusively to a stability control system for a heavy goods vehicle including a tractor and a trailer or a semi-trailer. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is known to provide a heavy goods vehicle including a tractor and a trailer or semi-trailer with a stability control system which is configured to detect when there is a risk that the vehicle will rollover, and to initiate a stability control intervention, which typically includes the automatic application of the vehicle brakes, in order to reduce the vehicle speed sufficiently to minimise the risk of rollover occurring. Such a stability control system is disclosed in U.S. Pat. No. 6,176,555, and in this system, if the lateral acceleration of the vehicle exceeds a predetermined amount, a test braking pulse is applied to the wheels of the vehicle and the vehicle ABS system monitored. If application of the test braking pulse causes an ABS control intervention to be initiated, it is deduced that one or more of the vehicle wheels has lifted off the ground, and therefore rollover is likely. A stability control intervention, such as automatic actuation of the vehicle brakes, is then initiated. It will be appreciated, therefore, that in order to determine when to apply a test braking pulse, the vehicle must be provided with a sensor capable of determining the lateral acceleration of the vehicle. 
       SUMMARY OF THE INVENTION 
       [0004]    According to a first aspect of the invention we provide an apparatus for controlling the travel of a vehicle including a controller which is adapted to detect when conditions exist under which vehicle stability is compromised and, when such conditions exist, to trigger an alarm signal or initiate a control intervention by means of which the vehicle travel is controlled to improve the vehicle stability, wherein the apparatus further includes accelerometer means, the accelerometer means being configured to measure the acceleration of the vehicle along two non-parallel axes. 
         [0005]    In prior art systems, the lateral acceleration of the vehicle is measured using a single axis accelerometer, and, as such, it is necessary to mount the accelerometer in a particular, known, orientation relative to the vehicle, typically generally perpendicular to the longitudinal axis of the vehicle. The accelerometer is typically mounted inside an electronic control unit (ECU) but the orientation in which the ECU is mounted varies from vehicle to vehicle. This means that the orientation of the accelerometer within the ECU has to be set according to the vehicle to which the ECU will be fitted. 
         [0006]    By virtue of providing accelerometer means which can measure the acceleration of the vehicle along two non-parallel axes, the lateral acceleration of the vehicle can be determined accurately without the need to position the accelerometer in a particular known orientation relative to the vehicle. This means that a standard ECU and accelerometer combination can be fitted to a wide range of vehicles. 
         [0007]    Preferably the accelerometer means is configured to measure the acceleration of the vehicle along two substantially perpendicular axes. The accelerometer may be configured to measure the acceleration of the vehicle along three substantially perpendicular axes. 
         [0008]    Advantageously, the system also includes a programmable electronic controller which has a first input for receiving a signal from the accelerometer means, the signal representing the vehicle acceleration along each of the two non-parallel axes, and a second input for receiving a signal representing the speed of the vehicle in a known direction relative to the vehicle, wherein the controller is programmed to determine the orientation of the accelerometer means relative to the vehicle by comparing the signal at the first input with the signal at the second input. Alternatively or additionally, the programmable electronic controller has further inputs for receiving signals representing the speed of at least one wheel on a first side of the vehicle and the speed of at least one wheel on a second, opposite side of the vehicle, wherein the controller is programmed to determine the orientation of the accelerometer means relative to the vehicle by comparing the signal at the first input with the signal at the further inputs. 
         [0009]    The controller is advantageously programmed to process the signals at the first input to determine the lateral acceleration of the vehicle, once the orientation of the accelerometer means relative to the vehicle has been determined. The controller may also be programmed to initiate a stability control test braking intervention or a stability control intervention if it determines that the lateral acceleration of the vehicle exceeds a predetermined amount. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0010]    An embodiment of the invention will now be described, by way of example only, with reference to the accompany drawing of which 
           [0011]      FIG. 1  shows a schematic of a vehicle fitted with an apparatus in accordance with a first aspect of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    Referring now to the Figure, there is shown a vehicle  10  comprising a tractor  12  and a trailer  14 . The trailer has wheels  16   a,    16   b,  and is fitted with an electronic braking system (EBS) apparatus for controlling the stability of the vehicle, in particular for reducing the risk of vehicle rollover. The wheels  16   a ,  16   b  of the trailer  14  are each fitted with a brake (not shown) which is operated by a brake actuator  18   a ,  18   b . The brake actuators  18   a ,  18   b  in this example are operated pneumatically, but it will be appreciated they could equally be actuated by hydraulic fluid pressure. 
         [0013]    A compressed air braking demand signal is supplied to the brake actuators  18  by one of two modulators  20   a ,  20   b . In this example, the first modulator  20   a  is connected to the actuators  18   a  on a first side of the trailer  14 , and the second modulator  20   b  is connected to the actuators  18   b  on the second side of the trailer  14 . The actuators  18   a ,  18   b  could, however, all be connected to a single modulator, or more than one modulator could be provided for the actuators  18  on a particular side of the trailer  14 . 
         [0014]    The braking system further includes an electronic control unit (ECU)  22  which is electrically connected to the modulators  20   a ,  20   b  and to a brake pedal (not shown) provided in a driver&#39;s cab of the tractor  12 . When operated, the brake pedal generates an electrical braking demand signal which is transmitted to the ECU  22 . The ECU  22  processes the braking demand signal, and transmits an appropriate electrical brake operating signal to the modulators  20   a ,  20   b.    
         [0015]    The braking system also includes a plurality of wheel speed sensors, one being provided to measure the rotational speed of each of the trailer wheels  16 . The ECU  22  provides the braking system with anti-lock braking control (ABS) and is connected to the wheel speed sensors so as to receive from each wheel speed sensor a signal representing the rotational speed of the respective wheel. The ECU  22  is programmed with conventional ABS algorithms to modify the braking force applied to the wheels  16  if wheel lock is detected. 
         [0016]    In addition to providing electronic braking control and anti-lock braking control, the ECU  22  is also programmed to detect when vehicle rollover is likely, and, if required, initiate a stability control intervention, which in this example, includes applying the vehicle brakes so as to reduce the vehicle speed. There are many known methods of detecting when vehicle rollover is likely, and these use signal outputs from a variety of sources such as wheel stroke sensors, wheel speed sensors, or a lateral accelerometer. Whilst one particular methodology of detecting vehicle rollover is described below, it will be appreciated that the present invention is applicable to any configuration of stability control system in which a lateral accelerometer is employed in the determination of whether vehicle rollover is likely. 
         [0017]    In this example, the ECU  22  receives a signal from an accelerometer from which it determines the lateral acceleration of the trailer  12 . If the lateral acceleration of the trailer  12  exceeds a predetermined amount, the ECU  22  determines in which direction the vehicle is turning, and sends a brake operating signal to the modulator  20   a  associated with the wheels  16   a  on the inside of the vehicle&#39;s turning curve, so as to apply a low level test braking pulse to these wheels  16   a . The ECU  22  then monitors the signals from the wheel speed sensors associated with the wheels to which the test braking pulse was applied, and if the slip of any of these wheels  16   a  exceeds a predetermined amount, initiates a stability control braking intervention. In this example, a stability control braking intervention comprises sending a braking demand signal to the modulator  20   b  associated with the wheels  16   b  on the outside of the vehicle&#39;s turning curve, so that a braking force is applied to these wheels  16   b , and the vehicle speed reduced. It will be appreciated, however, that the control intervention could comprise reducing the vehicle speed by throttling the engine, or that, instead of initiating an automatic control intervention, ECU  22  could be programmed to initiate a rollover alarm signal on the detection of wheel lift. 
         [0018]    In this example, the ECU  22  is provided with a dual axis accelerometer, i.e. a sensor which is capable of providing two signal outputs representing the acceleration of the vehicle along two perpendicular axes. The accelerometer is of standard construction, and includes two pairs of parts, in this example, micro-machined plates, one of which is fixed relative to the ECU  22  and one of which is permitted to move relative to the other under the force generated by the acceleration of the vehicle. In this example, means is provided to measure the electrical capacitance between the plates, which varies in accordance with the plate separation. The accelerometer provides a positive acceleration signal when the plate separation increases, and a negative acceleration signal when the plate separation decreases. 
         [0019]    One pair of plates is oriented perpendicular to the first accelerometer axis, and provides a measure of vehicle acceleration parallel to this axis, and the other pair of plates is oriented perpendicular to the second accelerometer axis, and provides a measure of vehicle acceleration parallel to the second axis. It will be appreciated that any configuration of accelerometer which provides a measure of acceleration along two generally parallel axes may be employed instead, however. 
         [0020]    The ECU  22  is adapted to be fitted to an appropriate mounting on the vehicle in an orientation such that a first one of the accelerometer axes is generally parallel to the longitudinal axis of the trailer  12 , whilst a second one of the accelerometer axes extends generally perpendicular to the longitudinal axis A of the trailer  12 . It will therefore be appreciated that the signal representing the acceleration along the first accelerometer axis corresponds to the longitudinal acceleration of the trailer  12 , whilst the signal representing the acceleration along the second accelerometer axis corresponds to the lateral acceleration of the trailer  12 , the latter being used by the ECU  22  in determining whether a test braking pulse is required. 
         [0021]    The mounting for the ECU  22  varies from vehicle to vehicle, and is such that the ECU  22  may be mounted with either one of the accelerometer axis parallel to the longitudinal axis A of the trailer  12 . Whilst it would be possible for a user to programme the ECU  22  to tell it which accelerometer signal represents the lateral acceleration of the trailer  12 , the ECU  22  is, instead, programmed automatically to work this out for itself, thus removing the need for any user intervention. 
         [0022]    To do this, the ECU  22  uses the inputs from the wheel speed sensors to calculate the speed of the trailer  12  parallel to its longitudinal axis A (the vehicle reference speed) using algorithms employed in standard ABS control systems. Each time the vehicle is started, the ECU  22  uses this vehicle reference speed to calculate the trailer acceleration parallel to the longitudinal axis A of the trailer  12 . It then compares the acceleration calculated from the speed signal, to the accelerometer readings, and deduces that the accelerometer axis for which the acceleration reading most closely matches the acceleration values calculated from the speed signal is generally parallel to the longitudinal axis A of the trailer  12 , and therefore that the signal representing the trailer acceleration along the other accelerometer axis corresponds to the lateral acceleration of the trailer  12 . 
         [0023]    As will be appreciated, although it is known which of the accelerometer axes A is parallel to the longitudinal axis of the trailer  12  and which is perpendicular to the longitudinal axis A of the trailer  12 , the accelerometer can still be oriented in one of two ways. If desired, the ECU  22  can also be programmed to determine in which of these two possible orientations, the accelerometer lies. 
         [0024]    As mentioned above, the signal representing the acceleration is positive if the plate separation increases and negative if the plate separation decreases. The vehicle cannot travel above a certain speed in reverse gear, and therefore if the vehicle reference speed exceeds that speed, it can be deduced that the vehicle is accelerating in the forward direction. This could, alternatively, be deduced if directional wheel speed sensors are provided. If, when the vehicle is travelling in the forward direction and is accelerating, the longitudinal acceleration signal from the accelerometer is negative, it can be deduced that the accelerometer is oriented with the fixed plate of the pair perpendicular to the longitudinal axis A of the trailer  12  towards the rear of the vehicle relative to the floating plate, whilst if the signal from the accelerometer is positive, it can be deduced that the accelerometer is oriented with the fixed plate of the pair perpendicular to the longitudinal axis A of the trailer  12  towards the front of the vehicle relative to the floating plate. 
         [0025]    If the ECU  22  has a non-volatile memory, it could be programmed to store this information in its memory, so that it does not need to make this determination every time the vehicle  10  is started up. It is, however, advantageous to repeat this process every time the vehicle is started, at least to verify that the information stored in the memory is correct, in case the ECU  22  has been transplanted into a different vehicle and mounted in a different orientation. 
         [0026]    The distance over which the accelerometer orientation determination can be made will vary—if the vehicle accelerates and decelerates rapidly shortly after start-up, the determination can be made sooner than otherwise. Typically, the determination can be made within the first few hundred meters of travel. The roll stability detection system is, of course, disabled until a determination of the lateral acceleration of the trailer  12  has been made. 
         [0027]    In order to speed up the determination of the accelerometer orientation, the ECU  22  can also be programmed to use a comparison of wheel speed measurements on opposite sides of the trailer  12 . When the vehicle is turning, the wheels on the outside of the turn travel faster than the wheels on the inside of the turn. Thus by comparing the speeds of wheels on opposite sides of the trailer  12 , the ECU  22  can determine if the vehicle is turning, and in which direction the vehicle is turning. 
         [0028]    Whilst the vehicle is turning, the accelerometer will detect acceleration both parallel and perpendicular to the longitudinal axis A, and the accelerometer readings will be both positive, both negative, or one negative and the other positive, depending on the configuration of the accelerometer, the direction the vehicle is turning, whether the vehicle speed is increasing or decreasing as the vehicle turns, and the orientation of the accelerometer relative to the trailer  12 . The configuration of the accelerometer, i.e. the arrangement of the fixed plates relative to the floating plates, is known, as are the direction of the turn, and vehicle speed. It therefore possible to use the sign of the two accelerometer readings to deduce the orientation of the accelerometer relative to the trailer  12 , and hence determine which reading corresponds to the lateral acceleration of the trailer  12 . 
         [0029]    Again the distance over which the accelerometer orientation determination can be made in this way will vary—if the vehicle turns to a significant degree shortly after start-up, the determination can be made sooner than otherwise. Similarly, if the vehicle travels only in a straight line after start-up, this method of determining the accelerometer orientation cannot be used, but, if the lateral acceleration is required only in detection of roll instability, as roll stability detection is only required when the vehicle is negotiating a turn, this is not a particular problem. 
         [0030]    The accuracy of this method of determining the accelerometer orientation is, however, compromised if there is vehicle braking, as, if braking occurs, if the wheels  16   a  on one side of the trailer  12  are travelling on a different surface to the wheels  16   b  on the other side of the trailer  12 , the wheels may slip to differing degrees. Consideration of the wheel speed differential at such a time would give rise to a false indication that the vehicle is turning. Consequently, in this embodiment of the invention, the ECU  22  is programmed not to use the wheel speed differential in determining the accelerometer orientation whilst the vehicle is braking. This need not be the case, however, and any inaccuracies in the determination of the accelerometer orientation due to vehicle braking could be avoided by processing the wheel speed information and accelerometer signals over a longer period of time, thus improving the confidence level of the orientation determination. 
         [0031]    By using both the vehicle reference speed and the wheel speed differential, the time taken to determine the accelerometer orientation can be minimised. It will be appreciated that either method could be used alone, however. 
         [0032]    If the ECU  22  has a non-volatile memory, it could be programmed to store the accelerometer orientation in its memory, so that it does not need to make this determination every time the vehicle  10  is started up. It is, however, advantageous to repeat this process every time the vehicle is started, at least to verify that the information stored in the memory is correct, in case the ECU  22  has been transplanted into a different vehicle and mounted in a different orientation. 
         [0033]    Once the orientation of the accelerometer has been determined, the longitudinal accelerometer reading is not required for the stability control system described above. It can, however, be used for other purposes, such as to provide an indication of the longitudinal inclination of the vehicle, when moving or stationery. 
         [0034]    The system described above is a relatively simple embodiment of the invention. In more sophisticated systems according to the invention, the ECU  22  can be programmed to determine the lateral acceleration of the trailer  12  whatever the orientation of the accelerometer. This can be achieved by using directional wheel speed sensors to provide a signal representing the vehicle velocity, and standard mathematical vector resolution techniques in the comparison of the velocity signal derived longitudinal acceleration with the accelerometer derived accelerations to determine the orientation of the accelerometer axes relative to the longitudinal axis A of the trailer  12 , and then using this information in resolving the accelerometer readings to provide a value for the transverse acceleration of the trailer  12 . It will be appreciated that, this can only be achieved using a dual axis accelerometer if the accelerometer orientation is constrained to the extent that the accelerometer axes can extend in any direction in a plane parallel to the longitudinal axis A of the trailer  12 . 
         [0035]    If the accelerometer orientation is completely unconstrained, determination of the lateral acceleration of the trailer  12  using the method described above is possible, providing a three axis accelerometer, i.e. an accelerometer which provides a measure of acceleration along three non-parallel axes. 
         [0036]    Where dual or three axis accelerometers are used as described above, preferably the axes are orthogonal. This need not be the case, however, as the standard mathematical vector resolution techniques can equally be applied using non-orthogonal but non-parallel axes. 
         [0037]    When using a three axis accelerometer, the measurement of the vehicle acceleration in a generally vertical direction can be stored by the ECU  22  in order to provide an indication of the harshness of the operating conditions of the vehicle over time. It will be appreciated that the vertical vibrations to which the trailer  12  is subjected during use can cause damage to the trailer  12 , and therefore this information can be used to determine when the trailer  12  may need to be serviced, repaired or replaced. 
         [0038]    When used in this specification and claims, the terms “comprises” and “comprising” and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.