Abstract:
A method of braking a vehicle which includes ground engaging wheels, and a braking system with ABS capability and including an operator actuated brake control, the ABS becoming operative in response to the operator actuating the brake control, and upon the braking system sensing the slippage or impending slippage of at least one of the ground wheels relative to the ground, to vary the braking force applied to the at least one of the wheels between, in alternative periods, an applied state in which the braking force is applied, and a released state in which the braking force is released, and characterised in that the method includes applying torque to assist acceleration of the wheel  16 - 19  at least during periods in which the braking force is released by the ABS.

Description:
BACKGROUND TO THE OF INVENTION 
       [0001]    This invention relates to a method of braking a vehicle in which a braking force is applied to ground wheels. 
         [0002]    More particularly but not exclusively the invention relates to a method of braking large wheels on vehicles, for examples, a tractor or loading and/or excavating machine. By “wheels” we include the wheel hub and tyre mounted on the hub. 
         [0003]    Such large wheel vehicles are being designed which are capable of high speed travel e.g. at speeds above 20 mph, on public roads. Accordingly the braking systems of such vehicles are required to comply with standards which apply to heavy road vehicles such as trucks. More particularly there is a requirement for the braking systems of such vehicles to have an ABS capability. 
         [0004]    ABS becomes operative in response to an operator actuating a brake control, and upon the braking system sensing the slippage or impending slippage of at least one of the ground wheels relative to the ground. The ABS varies the braking force applied to at least one of the wheels between, in alternative periods, an applied state in which the braking force is applied, and a released state in which the braking force is released. The lengths of the applied state and released state periods and the magnitude of the braking force applied during the applied state periods, are determined by an algorithm which determines a notional deceleration of the vehicle and constantly monitors the speed of the at least one wheel to test the validity of the determined notional deceleration. 
         [0005]    Thus for the ABS to operate optimally, it is essential that during the periods in which the braking force is released, the wheel accelerates due to its frictional contact with the ground, back towards a speed at which its angular velocity is close to the vehicle speed relative to the ground. 
         [0006]    In the case of vehicles having heavy wheels, for example large tractor wheels with an outside diameter 2.0 metres or more and perhaps an axial length of 0.6 metre or more, such wheels will inherently have large inertia. In the case of vehicles such as dumpers, the rolling circumference of a wheel could be about 6 metres. 
         [0007]    It has been found that during the periods in which the braking force is released, such wheels can fail to accelerate to anything like a speed at which their angular velocity is close to the vehicle speed due to their inertia. The problem is aggravated where there is a low coefficient of friction between the ground and the wheel, e.g. where the ground is icy, such that the wheels may slip relative to the ground during periods in which the braking force is released. 
         [0008]    The effect on the operation of the ABS is that in monitoring the speed of a wheel during the periods in which the braking force is released, the ABS could make an incorrect assumption about the vehicle speed relative to the ground, based on its monitoring of the wheel speed to test the validity of the determined notional vehicle deceleration. More particularly, the ABS could perceive that the vehicle is slowing down at a rate faster than actually it is, and as braking progresses the error will increase. Thus, as indicated by the graph shown in  FIG. 1 , there is the possibility of the ABS being misled into determining that the vehicle speed has been arrested, when in fact the vehicle is skidding along the ground, e.g. a road surface, with one or more locked wheels. 
         [0009]    In  FIG. 1 , the graph plots vehicle speed along the y-axis against time along the x-axis. The irregular curve indicated at  10  and the simplified line at  11 , is a plot of the notional vehicle speed, or a reference, determined by the ABS, e.g. by sensing the speed of the two front wheels of the vehicle, and determining the average. This is a standard function of an ABS achieved in different ways. 
         [0010]    The line  10  is indicative of a rate of deceleration of the vehicle the ABS perceives it is achieving, by applying the variable braking force according to an ABS algorithm. The curve indicated at  12  plots the actual vehicle speed relative to the ground as may be determined by an external sensing system. The shaded area of the graph between the two plots  11 ,  12  represents the error due to the inertia of the wheels, and the distance along the x-axis between where the two plots  11 ,  12  cross the x-axis indicates a period during which the wheel will be unwantedly and dangerously locked by the ABS. 
         [0011]    Thus vehicles with high inertia wheels present a particular problem where it is a requirement to provide a braking system with ABS capability. 
       DESCRIPTION OF THE PRIOR ART 
       [0012]    It is known for vehicles such as automobiles, particularly but not exclusively automobiles which have automatic transmissions, to provide so called “drag torque control”. In the event that the engine speed decreases rapidly, by for example a driver taking his foot off the accelerator pedal, particularly where the road speed is large and the coefficient of friction between the road and the wheel is low, there is a risk that the wheels will be locked up by the transmission, possibly leading to vehicle instability. Drag torque control operates by preventing the engine speed from reducing rapidly in order to avoid the possibility of wheels becoming locked by the transmission. The rate of engine speed reduction can be arranged to provide engine braking to the wheels, without permitting the wheels to lock up. 
       SUMMARY OF THE INVENTION 
       [0013]    According to one aspect of the present invention we provide a method of braking a vehicle which includes ground engaging wheels, and a braking system with ABS capability and including an operator actuated brake control, the ABS becoming operative in response to the operator actuating the brake control, and upon the braking system sensing the slippage or impending slippage of at least one of the ground wheels relative to the ground, to vary the braking force applied to the at least one of the wheels between, in alternative periods, an applied state in which the braking force is applied, and a released state in which the braking force is released, and characterised in that the method includes applying torque to assist acceleration of the at least one wheel at least during periods in which the braking force is released by the ABS. 
         [0014]    By virtue of the present invention, the inertia of the wheel, which may otherwise prevent the wheel being accelerated to bring its angular velocity close to the vehicle speed relative to the ground during periods in which the braking force is released by the ABS, is compensated for by the application of torque at least during those periods. Thus the risk of the ABS incorrectly determining the vehicle speed relative to the ground from its monitorings of wheel speed, is minimised, as the wheel will be accelerated by the applied torque to bring its angular velocity closer to the actual vehicle speed, and so the ABS will operate more effectively. 
         [0015]    The torque which is applied to the at least one wheel to accelerate the wheel may be applied constantly during ABS braking, but may, where suitable control can be achieved, be only applied during periods when the braking force is released. In each case, the applied torque should not provide any significant opposition to the braking force when applied. 
         [0016]    Preferably the torque which is applied is derived from a prime mover, such as an engine, which in normal use, acts, e.g. through a transmission, to drive the ground engaging wheels to provide the vehicle with forward or reverse motion. Thus the method may include, at least during those periods when the braking force is released and torque is applied to the at least one wheel to accelerate the wheel, maintaining a minimum engine speed so that torque is available for application to the at least one wheel to accelerate the wheel, as required. Practically, the engine speed may also be maintained during the alternative periods when the braking force is applied, but during those periods, the torque to accelerate the wheel is preferably not applied to the wheel. 
         [0017]    Thus the method may include providing a signal to an engine controller during braking, at least when the ABS is operated and it is desired to provide torque to the wheel, to maintain the engine speed at a minimum speed. This may involve not only preventing the engine speed falling below the minimum speed, but accelerating the engine speed if necessary up to a minimum speed if required. 
         [0018]    The method of the present invention may be applied not only to tractors, excavating/loading/shovelling and the like machines, but may be applied to any wheeled vehicle where it is desired to provide a braking system with ABS functionality, where the inertia of the wheels may potentially introduce errors into the ABS operation. 
         [0019]    However the invention has been developed specifically for vehicles which are tractors and working machines, in which case the vehicle may have a CVT to transmit power from the engine or other prime mover, to drive wheels of the vehicle. 
         [0020]    It will be appreciated that in a typical arrangement, when an operator driving a vehicle with a CVT in automatic mode, removes his foot from a foot operated control pedal, as the engine speed reduces in response, the CVT will reduce the transmission ratio between the engine and the wheels. 
         [0021]    Thus in performing the method of the present invention, during the periods in which the braking force is released by the ABS and torque is applied to accelerate the wheel, a signal may be sent to a CVT controller instead of or preferably in addition to a signal being sent to the engine controller, to require the CVT to select a transmission ratio appropriate for applying the required torque to the wheel during the periods in which the braking force is released. 
         [0022]    In the event that the transmission ratio is manually set, rather than automatically adjusted in response to an operator removing his foot from a foot operated engine speed control pedal, the method may include overriding such manual transmission ratio setting. 
         [0023]    It will be appreciated that the optimum durations of the periods in which the braking force is applied and released by the ABS when the ABS is operated, will depend on vehicle speed. For vehicles with lower inertia wheels, such as automobiles, the ABS may need to apply and release the braking force several times each second. For a tractor and the like vehicle, the ABS may only be required to apply and release the braking force say, once per second. 
         [0024]    Accordingly the engine speed controller and/or CVT controller and associated mechanical systems, must be able to respond when the ABS is operative, to apply the torque to the at least one wheel to accelerate the wheel to bring its angular velocity close to the vehicle speed relative to the road, and not to apply at least high driving torque during the periods when the braking force is being applied. 
         [0025]    The CVT may be controllable to reduce the transmission ratio to or towards zero during the periods while the braking force is applied, for example through hydrostatic pressure reduction where the CVT variator is a hydraulic motor, or the transmission may isolate the prime mover from the wheel e.g. by disengagement of a clutch. 
         [0026]    Where the invention is applied to a vehicle having an alternative kind of transmission, for example a manual transmission including a mechanical clutch, or a power-shift, automatic or automated-manual transmission, the method may include controlling the engine and/or transmission to provide a drive line for torque from the prime mover for application to the wheel at least in periods when the braking force is released by the ABS. 
         [0027]    It will be appreciated that it will only be a requirement to perform the method of the invention when imminent wheel slip is sensed typically in circumstances where the coefficient of friction between the wheel and the ground, is too low to overcome the inertia of the heavy wheel to allow the wheel to accelerate sufficiently during those periods when the braking force is released. Thus the method of the invention may include determining any discrepancy between sensed wheel angular velocity and the vehicle speed determined by the ABS, and applying torque to the wheel at least during periods in which the braking force is released, where any discrepancy is determined to be sufficiently great to indicate that the wheel is not accelerating to bring its angular velocity sufficiently towards the vehicle speed during those periods. 
         [0028]    Such an error may be determined by a torque controller which may analyse the actual wheel speed sensed, and compares it to a reference signal provided by the ABS which is indicative of vehicle deceleration determined by the ABS. If desired, there may be an auxiliary sensor such as an accelerometer to provide an input to the torque controller indicative of vehicle speed relative to the ground, independent of any wheel speed sensor, thus to enable the torque controller more accurately to determine any error between the vehicle speed determined by the ABS and actual vehicle speed relative to the ground. 
         [0029]    According to a second aspect of the invention we provide a vehicle which includes ground engaging wheels, and a braking system with ABS capability and including an operator actuated brake control, the ABS being operative upon the braking system sensing the slippage or impending slippage of at least one of the ground wheels relative to the ground, to vary the braking force applied to the at least one of the wheels between, in alternative periods, an applied state in which the braking force is applied, and a released state in which the braking force is released, and characterised in that the vehicle includes a torque controller which is operative to apply torque to assist acceleration of the wheel at least during periods in which the braking force is released by the ABS. 
         [0030]    The vehicle of the second aspect of the invention may have any of the features of a vehicle on which the method of the first aspect of the invention may be performed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    Embodiments of the invention will now be described with reference to the accompanying drawings in which: 
           [0032]      FIG. 1  is a graph plotting a perceived vehicle speed determined by an ABS against actual vehicle speed; 
           [0033]      FIG. 2  is a diagrammatic illustration of a vehicle to which the present invention may be applied; 
           [0034]      FIG. 3  is a diagrammatic illustration of the control system of the vehicle of  FIG. 2 ; 
           [0035]      FIG. 4  is a graph showing the vehicle deceleration expected by the ABS, the actual wheel speed and engine speed without the method of the present invention being performed; 
           [0036]      FIG. 5  is a graph similar to  FIG. 4 , when the method of the invention is performed. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0037]    Referring to  FIG. 2 , a vehicle  15  includes in this example, four ground wheels  16 - 19 , each of which are drivable from a prime mover  20 , which in this example is a conventional engine, via a transmission  22 . 
         [0038]    The engine  20  is controlled by an engine controller (such as is known as an engine management system)  24 , whilst the transmission  22 , in this case a CVT transmission  22 , is controlled by a transmission controller  25 . 
         [0039]    Each of the wheels  16 - 19  are braked wheels, there being a braking system  28  with ABS capability, with an ABS controller  30 , and in this example a mechanical brake control namely a foot pedal  31 . The braking system  28  could be an electrical braking system (EBS) if desired with a non-mechanical brake control, or may have electro-mechanical control, and where a mechanical brake control  31 , this could, if desired, be a hand operated control. 
         [0040]    In accordance with the present invention, the vehicle  15  includes a torque controller  35  which is connected to each of the braking system  28 , the engine controller  24  and a transmission controller  25  as hereinafter described. 
         [0041]    Referring to  FIG. 3 , the braking system  28  has inputs from not only the brake control  31 , but from wheel speed sensors  38  too. Each vehicle wheel  16 - 19  preferably has its own speed sensor, for example, typically provided by a toothed element which rotates with the wheel  16 - 19 , with there being an inductive speed pick up or other sensor, or alternatively an optical scanner which counts the teeth as they pass. Any other kind of wheel speed sensor could be provided. 
         [0042]    In another example, only the front wheel  16 ,  18  or only the rear wheels  17 ,  19  may have wheel speed sensors. 
         [0043]    In each case, the wheel speed sensors  38  sense the rotational speed of their respective wheels  16 - 19  which may be converted into an angular velocity in the sensor, or in the braking system  28  at large. 
         [0044]    An ABS controller  30  is shown included within the braking system  28 , and in the event that the braking system  28  determines that any wheel  16 - 19  is slipping or is about to slip relative to the ground, the ABS controller  30  varies the braking force applied to the respective wheel  16 - 19 , between, in alternative periods, an applied state in which the braking force is applied, and a released state in which the braking force is released. Otherwise the braking system  28  applies a constant braking force to all the wheels, depending on the input from the foot pedal  31 . 
         [0045]    In one example, the ABS may be proportional in that when the braking force is applied, the braking force is held for an “on” period before being released, or the braking force may be released almost immediately after application. 
         [0046]    To achieve safe braking, when the brakes are applied, a vehicle speed observer  40  of the braking system  28  determines the vehicle speed from the wheel speed sensor  38  input, and in the event that it is determined that a wheel is or is about to slip, thus provides an input indicative of vehicle speed, to the ABS controller  30  as indicated by the logic box  41  in  FIG. 3 . The vehicle speed observer  40 , by making comparisons between the sensed speeds of each of the wheels  16 - 19  or pairs of wheels  16 - 19 , is able to determine whether a wheel or wheels  16 - 19  is or is about to slip relative to the ground. In the event that no slippage or potential slippage is determined, the brakes will be applied to arrest the vehicle speed, conventionally with a braking force being continually applied. However, in the event that it is determined that a wheel is or may be about to slip, the ABS controller  30  applies a variable braking force to that wheel  16 - 19 . 
         [0047]    The magnitude of the braking force during the periods in which the variable braking force is applied, and the relative durations of the alternate applied and non-applied braking force periods, is determined by the ABS controller  30  to arrest the vehicle speed whilst maintaining steering ability, within a predetermined period. Whilst the variable braking force is being applied, the ABS controller  30  determines the vehicle speed from the wheel speed sensors  38  and continues to apply the variable braking force until either the vehicle speed is determined to be zero, or wheel slippage is no longer occurring, or the operator ceases braking. 
         [0048]    It will be understood that the angular velocity of the wheels  16 - 19 , at least during the periods in which the variable braking force is released, ought to equate to the actual vehicle speed. However as explained above, in the event that the wheels  16 - 19  are large and heavy and thus possess substantial inertia, and particularly in the case of a low coefficient of friction between the wheel  16 - 19  and the ground, a wheel  16 - 19  to which a variable braking force is applied by the ABS controller  30  may not accelerate sufficiently in periods when the braking force is released by the ABS to bring its angular velocity close to the vehicle speed. 
         [0049]    The effect of this is illustrated in  FIG. 1  as described above, and also in  FIG. 4 . In  FIG. 4 , there is a graph which illustrates at the plot indicated at  42 , the reference signal indicative of the estimated vehicle deceleration determined by the ABS controller  30 . The y-axis of the graph of  FIG. 4  thus plots vehicle speed against time. 
         [0050]    Plot  43  indicates the actual sensed wheel speed for a wheel  16 - 19  to which a variable braking force is being applied by the ABS controller  30 , the y-axis indicating the sensed wheel speed. 
         [0051]    The plot at  44  indicates the engine speed, which rapidly decreases from an engine speed I when the operator removes his foot from the vehicle accelerator to place his foot on the brake control  31 . Thus for all the plots, the x-axis plots time. 
         [0052]    To avoid the possibility of the wheel  16 - 19  to which the variable braking force is applied becoming locked before the vehicle speed has been arrested, at least during the periods in which the braking force is released, a torque is applied to the wheel  16 - 19  to assist the wheel accelerating to bring its angular velocity at least close to the actual vehicle ground speed. 
         [0053]    Referring again to  FIG. 3 , in the event of variable braking force being applied to any wheel  16 - 19 , an output from the braking system  28  is fed along line  45  and input to the torque controller  35 . The output from the braking system  28  includes information and data relating to the sensed wheel speed from the wheel speed sensors  38 , and may include the reference signal  42  (see  FIG. 4 ) giving the vehicle speed determined by the ABS controller  30 , and which may be lower than the true vehicle speed. However, if desired the torque controller  35  may make its own assessment of vehicle speed. 
         [0054]    In each case, the torque controller  35 , using an algorithm, compares the sensed wheel speed and the assessed vehicle speed  42  in order to establish if it is likely that the wheel  16 - 19  is, during the periods in which the braking force is released, accelerating to bring its angular velocity towards the vehicle speed relative to the ground. 
         [0055]    In  FIG. 4 , it can be seen that immediately the braking force is applied, at section a of the plot  43 , the wheel speed will decrease significantly, but that when the braking force is next applied, at section b of plot  43 , the wheel speed has not increased to a speed corresponding to that expected for the steady vehicle deceleration indicated by the plot  42  over the corresponding time period. In other words, the shape of the plot  43  over the sections a and b would appear to indicate that the there is wheel slippage relative to the ground during the variable braking force periods when the braking force is not being applied. 
         [0056]    To compensate, the torque controller  35  operates to apply some torque to the relevant wheel  16 - 19  at least during periods in which the braking force is released. 
         [0057]    This may be achieved in different ways. 
         [0058]    In the example particularly described, the transmission  22  is a CVT. Ordinarily with the CVT transmission  22  operating in automatic mode, when the operator removes his foot from the accelerator pedal, the engine  20  speed decreases as indicated by the plot  44  on the  FIG. 4  graph towards an idle speed. Also, the transmission controller  25  would respond by reducing the transmission ratio, as the engine  20  speed reduces. 
         [0059]    In accordance with the invention, the torque controller  35  acts both to prevent the transmission controller  25  reducing the transmission ratio at least towards or to zero, and preferably below a predetermined transmission ratio, and to maintain a minimum engine  20  speed, preferably controlling the rate of engine speed decrease, at least during the variable braking force periods in which the braking force is released by the ABS controller  30 . 
         [0060]    This is achieved by the torque controller  35  sending a signal along line  50  to the transmission controller  25 . Simultaneously, the torque controller  35  sends a signal along a line  51  to the engine controller  24 , to control the rate at which the engine  20  speed will reduce towards an idling speed, or if the engine  20  is revving at below a predetermined minimum speed, to cause the engine speed to accelerate to a minimum speed. Each of the engine speed controller  24  and transmission controller  25  may provide inputs to the torque controller  35  indicative of the state of the engine  20  and transmission  22 , which inputs may be used by the torque controller  35  carefully to control the torque applied to the wheel in accordance with the torque controller&#39;s  35  own algorithm. 
         [0061]    The torque controller  35  may be arranged to act to ensure that torque is applied to the slipping or potentially slipping wheel only during the periods in which the ABS controller  30  releases the braking force. 
         [0062]    Practically though, where the torque controller  35  is unable to process data fast enough, or data is not supplied fast enough, it may not be possible to ensure that torque is applied strictly only during the periods in which the braking force is released by the ABS. Accordingly in one proposal, an average torque is applied to the wheel during the entire time that the ABS controller  30  is applying, holding and releasing the braking force. The torque applied, although adequate to accelerate the wheel when the braking force is released, is not sufficient significantly detrimentally to effect braking when the braking force is applied and held. 
         [0063]    In one example, the transmission  22  may include a clutch  53  to which a signal may sent from the torque controller  35  along a line  54  to engage and disengage the clutch  53  and hence torque transmissed to a slipping wheel  16 - 19 , in concert with the ABS controller  30  releasing and applying and holding the braking force. However in another example, the torque controller  35  may apply the torque only during the periods in which the ABS releases the braking force, by increasing the transmission ratio of the CVT  22  from zero during these periods, e.g. through hydrostatic pressure increase where the CVT variater is a hydraulic motor, where the CVT  22  is able to respond quickly enough to signals from the torque controller  35 . 
         [0064]    Where the transmission  22  is not a CVT, the torque controller  35  may only operate a clutch  53  in order to achieve the application of torque to the braked wheel at least when the braking force is released by the ABS, but preferably simultaneously with controlling the engine  20  speed to ensure that the engine  20  is revving at a speed adequate for torque to be made available for application to the wheel. Where the transmission is a CVT the transmission ratio may be reduced during periods in which braking is applied, by e.g. hydrostatic pressure reduction where the variater is a hydraulic motor, from an otherwise higher level. 
         [0065]    In each case, the torque controller  35  will operate according to an algorithm which establishes the circumstances under which torque is applied to a braked wheel, and the amount of torque to be applied from the engine  20 , by controlling one or both of the engine  20  speed and transmission  22  ratio and/or clutch  53 , depending on the reference  42  or other vehicle speed estimation, and wheel speed signals received from the braking system  28 , and inputs from the engine speed controller  24  and transmission controller  25 . 
         [0066]    It will be appreciated that although using the ABS to obtain data for its operation, the torque controller  35  of the invention does not interfere with the normal operation of the braking system  28 , but is an enhancement which enables ABS to be used more effectively on a vehicle with high inertia wheels, on low friction ground surfaces. 
         [0067]    In  FIG. 5 , the graph indicates the effect of performing the present invention. The curve  42  again is a reference signal indicative of the rate of vehicle deceleration determined by the ABS as the ABS controller  30  applies the variable braking force to the wheel  16 - 19 . The curve  43  indicates that by applying torque to the wheel during the periods in which the braking force is released, the wheel accelerates during those periods in line with the vehicle deceleration shown by the curve  42 . Thus the braking system  28  is not misled into incorrectly determining the vehicle speed by merely using the signals from the wheel speed sensor  38 .  FIG. 5  also shows at curve  44 , the controlled rate of decrease of engine speed achieved by the torque controller  35  for a CVT transmission  22 . 
         [0068]    Various modifications may be made without departing from the scope of the invention. 
         [0069]    If desired, the torque controller  35  may be provided with data indicative of vehicle speed from a source independent of the ABS braking system  28 . For example, the torque controller  35  may receive an input from one or more accelerometers  49 , thus to improve the torque controller&#39;s  35  ability to assess when the sensed wheel speed and vehicle speed indicate slippage of the wheel when the ABS is operative, during the periods when the braking force is released, is occurring. 
         [0070]    In a vehicle without a CVT  22 , the torque controller  35  may only control the engine  20  speed and a clutch  53  to enable torque to be applied. 
         [0071]    If desired, although preferably the braking system  28  and especially the ABS controller  30  and torque converter  35  are operationally separate, if desired a single controller may be combined at least in hardware terms, to reduce components and system complexity. 
         [0072]    The various components of the control system described i.e. including the ABS braking system  28 , the torque controller  35 , wheel sensors  38 , engine controller  24  and transmission controller  25  where provided, and clutch  53  where provided, may be connected via a CAN bus, but may otherwise be connected, for example by means including wireless connections, as desired. 
         [0073]    Although the invention has particularly been described in relation to a vehicle such as a tractor or working machine, the invention may be applied to other vehicles where there may be a problem with wheel slippage in periods in which the ABS releases the braking force, due to high wheel inertia and low coefficient of friction between the wheel and the ground, or any other reason.