Abstract:
A vehicle roll control system, comprising a front torsion bar, a front hydraulic actuator ( 34 ) attached to the front torsion bar; a rear torsion bar, a rear hydraulic actuator ( 34 ) attached to the rear torsion bar; and control means ( 70 - 99 ) connected to the front and rear hydraulic actuators and controlling the operation thereof on detection of a predetermined vehicle condition; wherein each front and rear hydraulic actuator comprises a housing, a piston ( 62,62 ) making a sealing sliding fit inside the housing to define a first fluid chamber ( 58,58 ) and a second fluid chamber ( 60,60 ), and a piston rod ( 64,64 ) connected to the piston and extending through the second fluid chamber and out of the housing; wherein the control means acts on detection of the predetermined vehicle condition to apply a fluid pressure to the first fluid chamber of the front hydraulic actuator which is different from the fluid pressure applied to the first fluid chamber of the rear hydraulic actuator and/or apply a fluid pressure to the second fluid chamber of the front hydraulic actuator which is different from the fluid pressure applied to the second fluid chamber of the rear hydraulic actuator, and wherein the control means comprises a source of fluid pressure ( 80 ), a fluid reservoir ( 81 ), a pressure control valve ( 99 ) fluidly connected between the pressure source and the reservoir, a directional valve ( 82 ) fluidly connected between the pressure control valve and the hydraulic actuators, and a pressure relief valve ( 83,84 ) fluidly connecting the directional valve to the pressure source or the reservoir; wherein the pressure relief valve is actuated to create the pressure differential between the first fluid chambers and/or to create the pressure differential between the second fluid chambers.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a roll control system for a motor vehicle. 
       BACKGROUND OF THE INVENTION 
       [0002]    EP-A-1103395 discloses a vehicle roll control system in which a pair of directional valves and a pressure control valve are used to control the movement of the piston of hydraulic actuators associated with the front and rear axles of a motor vehicle. WO-A-03/093041 discloses a vehicle roll control system in which a pair of pressure control valves and a directional valve are used to control the movement of the piston of hydraulic actuators associated with the front and rear axles of a motor vehicle. In both cases, each hydraulic actuator has a first fluid chamber positioned on one side of the piston, and a second fluid chamber positioned on the other side of the piston. The first fluid chambers of the front and rear hydraulic actuators receive hydraulic fluid at substantially the same pressure; and the second fluid chambers of the front and rear hydraulic actuators receive hydraulic fluid at substantially the same pressure. 
       SUMMARY OF THE INVENTION 
       [0003]    The aim of the present invention is to provide a roll control system which is an improvement to the above mentioned arrangements. 
         [0004]    A vehicle roll control system in accordance with the present invention is characterised by the features specified in claim  1 . 
         [0005]    In the present invention, the control means for the hydraulic circuit is capable of providing fluid pressure to the first fluid chamber of the front hydraulic actuator which is different from the fluid pressure provided to the first fluid chamber of the rear hydraulic actuator; and/or is capable of providing fluid pressure to the second fluid chamber of the front hydraulic actuator which is different from the fluid pressure provided to second fluid chamber of the rear hydraulic actuator. 
         [0006]    The present invention provides a system which allows an aggressive roll control strategy and balance strategy which leads to improvements in motion, turning, and stability (braking in turn at high speed). The present invention also provides continuous control between right turn and left turn. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0008]      FIG. 1  is a schematic presentation of a vehicle incorporating a vehicle roll control system in accordance with the present invention; 
           [0009]      FIG. 2  is an enlarged view of the front and rear portions of the vehicle roll control system shown in  FIG. 1 ; 
           [0010]      FIG. 3  is a side view of the first arm of the vehicle roll control system shown in  FIG. 2 ; 
           [0011]      FIG. 4  is a side view of the second arm, hydraulic actuator (shown in cross-section) and lever arm of the vehicle roll control system shown in  FIG. 2 ; 
           [0012]      FIG. 5  is a schematic diagram of the hydraulic and electrical control circuit of the vehicle roll control system shown in  FIG. 1  when the directional valve and pressure relief valves are de-actuated or in their fail-safe mode; 
           [0013]      FIG. 6  is a schematic diagram of a first alternative hydraulic and electrical control circuit of the vehicle roll control system shown in  FIG. 1  when the directional valve and the pressure relief valves are de-actuated or in their fail-safe mode; 
           [0014]      FIG. 7  is a schematic diagram of a second alternative hydraulic and electrical control circuit of the vehicle roll control system shown in  FIG. 1  when the directional valve and the pressure relief valves are de-actuated or in their fail-safe mode; 
           [0015]      FIG. 7A  is a schematic diagram of a third alternative hydraulic and electrical control circuit of the vehicle roll control system shown in  FIG. 1  when the directional valve and the pressure relief valves are de-actuated or in their fail-safe mode; 
           [0016]      FIG. 8  is a schematic diagram of a fourth alternative hydraulic and electrical control circuit of the vehicle roll control system shown in  FIG. 1  when the directional valve and the pressure relief valves are de-actuated or in their fail-safe mode; 
           [0017]      FIG. 8A  is a schematic diagram of a fifth alternative hydraulic and electrical control circuit of the vehicle roll control system shown in  FIG. 1  when the directional valve and the pressure relief valves are de-actuated or in their fail-safe mode; 
           [0018]      FIG. 9  is a schematic diagram of a sixth alternative hydraulic and electrical control circuit of the vehicle roll control system shown in  FIG. 1  when the directional valve and the pressure relief valves are de-actuated or in their fail-safe mode; 
           [0019]      FIG. 9A  is a schematic diagram of a seventh alternative hydraulic and electrical control circuit of the vehicle roll control system shown in  FIG. 1  when the directional valve and the pressure relief valves are de-actuated or in their fail-safe mode; 
           [0020]      FIG. 10  is a schematic diagram of an eighth alternative hydraulic and electrical control circuit of the vehicle roll control system shown in  FIG. 1  when the directional valves and the pressure relief valve are de-actuated or in their fail safe mode; 
           [0021]      FIG. 11  is a schematic diagram of a ninth alternative hydraulic and electrical control circuit of the vehicle roll control system shown in  FIG. 1  when the directional valves and the pressure relief valve are de-actuated or in their fail safe mode; 
           [0022]      FIG. 12  is a view of a portion of a vehicle roll control system in accordance with a second embodiment of the present invention; 
           [0023]      FIG. 13  is a view of a portion of a vehicle roll control system in accordance with a third embodiment of the present invention; 
           [0024]      FIG. 14  is a cross-section view of the hydraulic actuator of the vehicle roll control system of  FIG. 13 ; 
           [0025]      FIG. 15  is a cross-sectional view of an alternative embodiment of hydraulic actuator for the vehicle roll control system of  FIG. 13 ; and 
           [0026]      FIG. 16  is a cross-sectional view of a further alternative embodiment of hydraulic actuator for the vehicle roll control system of  FIG. 13 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0027]    Referring to  FIG. 1 , a vehicle  10  is shown schematically and comprises a pair of front wheels  12  each rotatably mounted on an axle  14 , a pair of rear wheels  16  each rotatably mounted on an axle  18 , and a shock absorbing system  20  associated with each wheel. A portion  22  of a vehicle roll control system in accordance with the present invention is associated with the front wheels  12 , and a portion  24  of the vehicle roll control system in accordance with the present invention is associated with the rear wheels  16 . The portions  22 ,  24  are substantially the same but with modifications made solely to allow fitting to the vehicle  10 . 
         [0028]    Referring in more detail to  FIGS. 2 to 4 , the portion  22  of the vehicle roll control system for the front of the vehicle comprises a torsion bar  26 , a first arm  28 , a second arm  30 , a lever arm  32 , and a hydraulic actuator  34 . The torsion bar  26  is mounted on the vehicle by a pair of resilient mounts  36  in conventional manner to extend longitudinally between the wheels  12 . The first arm  28  ( FIG. 3 ) is fixed at one end  38  by a splined connection  40  to the torsion bar  26 . The other end  42  of the first arm  28  is connected to the axle  14  of one of the front wheels  12  by a tie rod  43 . The second arm  30  ( FIG. 4 ) is rotatably mounted at one end  44  on the torsion bar  26  by way of a bearing  46 . The other end  48  of the second arm  30  is connected to the axle  14  of the other front wheel  12  by a tie rod  49 . The first and second arms  28 , 30  extend substantially parallel to one another when the vehicle is stationary, and substantially perpendicular to the torsion bar  26 . 
         [0029]    The lever arm  32  ( FIG. 4 ) is fixed at one end  50  to the torsion bar  26  by a splined connection  52  substantially adjacent the one end  44  of the second arm  30  and the bearing  46 . The lever arm  32  extends substantially perpendicular to the torsion bar  26  to a free end  54 . The front hydraulic actuator  34  ( FIG. 4 ) extends between, and is connected to, the free end  54  of the lever arm  32  and the other end  48  of the second arm  30 . The front hydraulic actuator  34  comprises a housing  56  which defines first and second fluid chambers  58 , 60  separated by a piston  62  which makes a sealing sliding fit with the housing. As shown in  FIG. 4 , the housing  56  is connected to the other end  48  of the second arm  30 , and the piston  62  is connected to the free end  54  of the lever arm  32  by a piston rod  64  which extends through the second fluid chamber  60 . It will be appreciated that these connections may be reversed. The fluid chambers  58 , 60  contain hydraulic fluid and are fluidly connected to fluid lines  66 ,  68  respectively. The portion  24  of the vehicle roll control for the rear of the vehicle is substantially the same, but with the components (which are primed) having a different layout. The rear hydraulic actuator  34 ′ is substantially the same as the front hydraulic actuator  34 . 
         [0030]    The hydraulic and electrical control circuit of the vehicle roll control system of  FIGS. 1 to 4  is shown in  FIG. 5 . The hydraulic circuit includes a fluid pump  80 , a fluid reservoir  81 , a directional valve  82 , a first pressure relief valve  83 , a second pressure relief valve  84 , and a pressure control valve  99 . The directional valve  82  has eight ports  85 - 92 . The first pressure relief valve  83  has three ports  93 - 95 . The second pressure relief valve  84  has three ports  96 - 98 . The pressure control valve  99  is fluidly connected between the pump  80  and the reservoir  81 . Fluid filters may be positioned after the pump  80  and/or before the reservoir  81 . 
         [0031]    The directional valve  82  has a first port  85  fluidly connected to the fluid pump  80 ; a second port  86  fluidly connected to the first port  93  of the first pressure relief valve  83 ; a third port  87  fluidly connected to the fluid pump  80 ; a fourth port  88  fluidly connected to the first port  96  of the second pressure relief valve  84 ; a fifth port  89  fluidly connected to the first chamber  58 ′ of the rear actuator  34 ′ by way of fluid line  66 ′; a sixth port  90  fluidly connected to the second chamber  60 ′ of the rear actuator  34 ′ by way of fluid line  68 ′; a seventh port  91  fluidly connected to the first chamber  58  of the front actuator  34  by way of fluid line  66 ; and an eighth port  92  fluidly connected to the second chamber  60  of the front actuator  34  by way of fluid line  68 . The directional valve  82  is solenoid actuated, and has a de-actuated state (shown in  FIG. 5 ) in which the first and second ports  85 , 86  are fluidly connected; the third and fourth ports  87 , 88  are fluidly connected; the fifth and seventh ports  89 , 91  are fluidly connected; and the sixth and eighth ports  90 , 92  are fluidly connected. In the actuated state of the directional valve  82 , the first and eighth ports  85 , 92  are fluidly connected; the second and seventh ports  86 , 91  are fluidly connected; the third and sixth ports  87 , 90  are fluidly connected; and the fourth and fifth ports  88 , 89  are fluid connected. In an alternative arrangement, the directional valve  82  may be hydraulically actuated by first and second pilot (on/off) valves (not shown). 
         [0032]    The second port  94  of the first pressure relief valve  83  is fluidly connected to the pump  80 . The third port  95  of the first pressure relief valve  83  is fluidly connected to the reservoir  81 . In the de-actuated state of the first pressure relief valve  83  (shown in  FIG. 5 ), the first port  93  is fluidly connected to the third port  95 , and the second port  94  is fluidly isolated. In the actuated state of the first pressure relief valve  83 , the first port  93  is fluidly connected to the second port  94 , and the third port  95  is fluidly isolated. 
         [0033]    The second port  97  of the second pressure relief valve  84  is fluidly connected to the pump  80 . The third port  98  of the second pressure relief valve  84  is fluidly connected to the reservoir  81 . In the de-actuated state of the second pressure relief valve  84  (shown in  FIG. 5 ), the first port  96  is fluidly connected to the third port  98 , and the second port  97  is fluidly isolated. In the actuated state of the second pressure relief valve  84 , the first port  96  is fluidly connected to the second port  97 , and the third port  98  is fluidly isolated. 
         [0034]    The first and second pressure relief valves  83 , 84  are preferably solenoid actuated as shown in  FIG. 5 . Alternatively, the pressure relief valves  83 , 84  may be hydraulically actuated by first and second pilot (on/off) valves (not shown). 
         [0035]    The pump  80  may be driven by the vehicle engine and hence continuously actuated. Alternatively, the pump  80  may be driven by an electric motor or any other suitable means, either continuously, or variably. The pressure control valve  99  is actuated to adjust the fluid pressure in the hydraulic system between a predetermined minimum pressure and a predetermined maximum pressure. The pressure control valve  99  is also actuated to adjust the pressure differential between the first and second chambers  58 ,  58 ′, 60 ,  60 ′ of the hydraulic actuators  34 , 34 ′ respectively (when the directional valve  82  and pressure relief valves  83 , 84  are also actuated as required). 
         [0036]    The electrical control circuit includes an electronic and/or computerised control module  70 . The control module  70  operates the fluid pump  80 , the directional valve  82 , the pressure control valve  99 , and the pressure relief valves  83 , 84 , when required. The control module  70  actuates the valves  82 - 84 , 99  dependent on predetermined vehicle conditions which are determined by signals from one or more sensors, such as a first pressure sensor  76  (which detects the fluid pressure associated with the first chamber  58  of the front hydraulic actuator  34 ), a second pressure sensor  77  (which detects the fluid pressure associated with the first chamber  58 ′ of the rear hydraulic actuator  34 ′), a third pressure sensor  75  (which detects the fluid pressure associated with the second chambers  60 , 60 ′ of the actuators  34 , 34 ′), a lateral g sensor  74  (which monitors the sideways acceleration of the vehicle), a steering sensor  72  (which monitors the steering angle of the front wheels  12 ), a vehicle speed sensor  78 , and/or any other relevant parameter. 
         [0037]    If the control module  70  detects that roll control is required (due, for example, to cornering of the motor vehicle  10 ), the control module determines if the module has to generate a force F, F′ which acts on the piston rods  64 , 64 ′ respectively to extend the front and/or rear actuators  34 , 34 ′, or to compress the front and/or rear actuators, in an axial direction. In the present invention, the force F on the front actuator  34  may be different from the force F′ on the rear actuator  34 ′ dependent on the actuation of the pressure relief valves  83 , 84 ; and the value of the pressure differential is set by the pressure control valve  99 . 
         [0038]    In this embodiment, the roll control system can be operated in four different modes when the directional valve  82  is actuated and the pressure control valve  99  is actuated. In a first mode, when the first pressure relief valve  83  is actuated and the second pressure relief valve  84  is de-actuated, the second fluid chambers  60 ,  60 ′ of the front and rear hydraulic actuators  34 ,  34 ′ are at substantially the same pressure, the first fluid chamber  58  of the front hydraulic actuator is at a pressure which is substantially equal to or less than the pressure in the second chambers dependent on the pressure relief valve  83 , and the first fluid chamber  58 ′ of the rear hydraulic actuator is at a different pressure. In a second mode, when the first pressure relief valve  83  is de-actuated and the second pressure relief valve  84  is actuated, the second fluid chambers  60 ,  60 ′ of the front and rear hydraulic actuators  34 ,  34 ′ are at substantially the same pressure, the first fluid chamber  58 ′ of the rear hydraulic actuator is a pressure which is substantially equal to or less than the pressure in the second chambers dependent on the pressure relief valve  84 , and the first fluid chamber  58  of the front hydraulic actuator is at a different pressure. In a third mode, when the pressure relief valves  83 ,  84  are de-actuated, the first fluid chambers  58 ,  58 ′ of the hydraulic actuators  34 ,  34 ′ are at substantially the same pressure, and the second fluid chambers  60 ,  60 ′ of the hydraulic actuators are at substantially the same pressure but at a different pressure to the first fluid chambers. In a fourth mode, when the pressure relief valves  83 ,  84  are actuated, the second fluid chambers  60 , 60 ′ of the front and rear hydraulic actuators  34 ,  34 ′ are at substantially the same pressure, the first fluid chamber  58  of the front hydraulic actuator is at a pressure which is substantially equal to or less than the pressure in the second chambers dependent on the pressure relief valve  83 , and the first fluid chamber  58 ′ of the rear hydraulic actuator is at a pressure which is substantially equal to or less than the pressure in the second chambers dependent on the pressure relief valve  84 . Also, in this fourth mode, the pressures in the first chambers  58 ,  58 ′ may be different from one another dependent on the pressure relief valves  83 ,  84 . In all of the above modes, the value of any pressure differential is control by the pressure control valve  99  and the pressure relief valves  83 ,  84 . This arrangement provides improvement management of the compression or expansion of the hydraulic actuators, and hence provides improved roll control of the vehicle. 
         [0039]      FIG. 6  illustrates a first alternative of the hydraulic and electrical control circuit of the vehicle roll control system shown in  FIGS. 1 to 4 .  FIG. 6  is a modification of the hydraulic circuit shown in  FIG. 5 , in which changes have been made to the directional valve  82 . In this alternative, in the de-actuated state of the directional valve  82 , the fifth, sixth, seventh and eighth ports  89 - 92  are fluidly isolated from one another and from the other ports  85 - 88 . The operation of this first alternative is substantially the same as the operation of the arrangement shown in  FIG. 5 . 
         [0040]      FIG. 7  illustrates a second alternative of the hydraulic and electrical control circuit of the vehicle roll control system shown in  FIGS. 1 to 4 .  FIG. 7  is a modification of the hydraulic circuit shown in  FIG. 5 , in which changes have been made to the directional valve  482 . In this alternative, the directional valve  482  comprises six ports, a first port  485 , a second port  486 , a third port  487 , a fourth port  488 , a fifth port  489  and a sixth port  490 . The first port  485  is fluidly connected to the pump  80 . The second port  486  is fluidly connected to the first port  93  of the first pressure relief valve  83 . The third port  487  is fluidly connected to the first port  96  of the second pressure relief valve  84 . The fourth port  488  is fluidly connected to the first fluid chamber  58 ′ of the rear actuator  34 ′. The fifth port  489  is fluidly connected to the first fluid chamber  58  of the front actuator  34 . The sixth port  490  is fluidly connected to the second fluid chambers  60 ,  60 ′ of the front and rear actuators  34 ,  34 ′. In the de-actuated state of the directional valve  482 , the first, second, third and sixth ports  485 - 487 ,  490  are fluidly isolated from one another and from the other ports  488 ,  489  (which are fluidly connected). The operation of this second alternative is substantially the same as the operation of the arrangement shown in  FIG. 5 . 
         [0041]      FIG. 7A  illustrates a third alternative of the hydraulic and electrical control circuit of the vehicle roll control system shown in  FIGS. 1 to 4 .  FIG. 7A  is a modification of the hydraulic and electrical circuit shown in  FIG. 7 , in which changes have been made to the operation of the directional valve  482 ′. In this alternative, the directional valve  482 ′ is hydraulically actuated (rather than solenoid actuated) by first and second pilot (on/off) valves  491 ,  492 . The pilot valve  491 ,  492  are fluidly connected in series between the pump  80  and the reservoir  81 . The actuator  493  for the directional valve  482 ′ is fluidly connected to the flow path between the pilot valves  491 ,  492 . The operation of the pilot valves  491 ,  492  is controlled by the control module  70 . Other than the actuation of the directional valve  482 ′ by the pilot valves  491 ,  492 , the operation of this third alternative is substantially the same as the operation of the arrangement shown in  FIG. 5 . 
         [0042]      FIG. 8  illustrates a fourth alternative of the hydraulic and electrical control circuit of the vehicle roll control system shown in  FIGS. 1 to 4 .  FIG. 8  is a modification of the hydraulic circuit shown in  FIG. 7 , in which changes have been made to the directional valve  482 . In this alternative, in the de-actuated state of the directional valve  482 , the first, second and third ports  485 - 487  are fluidly connected to one another. The operation of this fourth alternative is substantially the same as the operation of the arrangement shown in  FIG. 5 . 
         [0043]      FIG. 8A  illustrates a fifth alternative of the hydraulic and electrical control circuit of the vehicle roll control system shown in  FIGS. 1 to 4 .  FIG. 8A  is a modification of the hydraulic and electrical circuit shown in  FIG. 8 , in which changes have been made to the operation of the directional valve  482 ′. In this alternative, the directional valve  482 ′ is hydraulically actuated (rather than solenoid actuated) by first and second pilot (on/off) valves  491 ,  492 . The pilot valve  491 ,  492  are fluidly connected in series between the pump  80  and the reservoir  81 . The actuator  493  for the directional valve  482 ′ is fluidly connected to the flow path between the pilot valves  491 ,  492 . The operation of the pilot valves  491 ,  492  is controlled by the control module  70 . Other than the actuation of the directional valve  482 ′ by the pilot valves  491 ,  492 , the operation of this fifth alternative is substantially the same as the operation of the arrangement shown in  FIG. 5 . 
         [0044]      FIG. 9  illustrates a sixth alternative of the hydraulic and electrical control circuit of the vehicle roll control system shown in  FIGS. 1 to 4 .  FIG. 9  is a modification of the hydraulic circuit shown in  FIG. 7 , in which changes have been made to the directional valve  482 . In this alternative, in the de-actuated state of the directional valve  482 , the second and third ports  485 - 487  are fluidly connected to one another, whilst the first and sixth ports  485 ,  490  are fluidly isolated. The operation of this sixth alternative is substantially the same as the operation of the arrangement shown in  FIG. 5 . 
         [0045]      FIG. 9A  illustrates a seventh alternative of the hydraulic and electrical control circuit of the vehicle roll control system shown in  FIGS. 1 to 4 .  FIG. 9A  is a modification of the hydraulic and electrical circuit shown in  FIG. 9 , in which changes have been made to the operation of the directional valve  482 ′. In this alternative, the directional valve  482 ′ is hydraulically actuated (rather than solenoid actuated) by first and second pilot (on/off) valves  491 ,  492 . The pilot valve  491 ,  492  are fluidly connected in series between the pump  80  and the reservoir  81 . The actuator  493  for the directional valve  482 ′ is fluidly connected to the flow path between the pilot valves  491 ,  492 . The operation of the pilot valves  491 ,  492  is controlled by the control module  70 . Other than the actuation of the directional valve  482 ′ by the pilot valves  491 ,  492 , the operation of this seventh alternative is substantially the same as the operation of the arrangement shown in  FIG. 5 . 
         [0046]    The above-described embodiments all operate in substantially the same way, but provide different hydraulic circuit arrangements for their respective fail-safe modes, as illustrated in the drawings. Also, the selection is dependent on the type of hydraulic actuator that is used. 
         [0047]      FIG. 10  illustrates an eighth alternative of the hydraulic and electrical control circuit of the vehicle roll control system shown in  FIGS. 1 to 4 .  FIG. 10  is a modification of the hydraulic circuit shown in  FIG. 5 , in which the second pressure relief valve has been removed, and a second directional valve  184  is positioned between the first directional valve  82  and the pump  80  and reservoir  81 . 
         [0048]    The second directional valve  184  has a first port  185  fluidly connected to the fluid pump  80 ; a second port  186  fluidly connected to the reservoir  81 ; a third port  187  fluidly connected to the first port  93  of the pressure relief valve  83 ; a fourth port  188  fluidly connected to the reservoir  81 ; a fifth port  189  fluidly connected to the fourth port  88  of the first directional valve  82 ; a sixth port  190  fluidly connected to the third port  87  of the first directional valve  82 ; a seventh port  191  fluidly connected to the second port  86  of the first directional valve  82 ; and an eighth port  192  fluidly connected to the first port  85  of the first directional valve  82 . The second directional valve  184  is solenoid actuated, and has a de-actuated state (shown in  FIG. 10 ) in which the first and eighth ports  185 , 192  are fluidly connected; the second and seventh ports  186 , 191  are fluidly connected; the third and sixth ports  187 , 190  are fluidly connected; and the fourth and fifth ports  188 , 189  are fluidly connected. In the actuated state of the second directional valve  184 , the first, seventh and eighth ports  185 , 191 , 192  are fluidly connected; the third, fifth and sixth ports  187 , 189 , 190  are fluidly connected; and the second and fourth ports  186 , 188  are fluidly isolated. 
         [0049]    Also, in this eighth alternative, there are only two pressure sensors  76 ′, 77 ′. The first pressure sensor  76 ′ detects the fluid pressure. associated with the front hydraulic actuator  34 , and a second pressure sensor  77 ′ detects the fluid pressure associated with the rear hydraulic actuator  34 ′. 
         [0050]    In this embodiment, the roll control system can be operated in four different modes when the directional valve  82  is actuated. In a first mode, when the first pressure relief valve  83  is de-actuated and the second directional valve  184  is de-actuated, the first and second fluid chambers  58 ′,  60 ′ of the rear hydraulic actuator  34 ′ and the first fluid chamber  58  of the front hydraulic actuator  34  are at substantially the same pressure, and the second fluid chamber  60  of the front hydraulic actuator is at a different pressure. In a second mode, when the first pressure relief valve  83  is actuated and the second differential valve  184  is de-actuated, the first fluid chambers  58 ,  58 ′ of the front and rear hydraulic actuators  34 ,  34 ′ are at substantially the same pressure, the second fluid chamber  60  of the front hydraulic actuator is at a different pressure to the first fluid chambers, and the second fluid chamber  60 ′ of the rear hydraulic actuator is a pressure which is substantially equal to or less than the pressure in the second chamber of the front hydraulic actuator dependent on the pressure relief valve  83 . In a third mode, when the pressure relief valve  83  is de-actuated and the second direction valve  184  is actuated, the first and second fluid chambers  58 ,  60  of the front hydraulic actuator  34  are at substantially the same pressure, and the first and second fluid chambers  58 ′,  60 ′ of the rear hydraulic actuator  34 ′ are at substantially the same pressure but at a different pressure to the fluid chambers of the front actuator. In a fourth mode, when the pressure relief valve  83  and the second directional valve  184  are actuated, the first and second fluid chambers  58 ,  60  of the front hydraulic actuator  34  are at substantially the same pressure, and the first and second fluid chambers  58 ′, 60 ′ of the rear hydraulic actuator  34 ′ are at substantially the same pressure which is substantially the same as or less than the pressure in the fluid chambers of the front actuator dependent on the pressure relief valve  83 . In the above modes, the value of any pressure differential is control by the pressure control valve  99  and the pressure relief valve  83 . This arrangement provides improvement management of the compression or expansion of the hydraulic actuators, and hence provides improved roll control of the vehicle. 
         [0051]    Although the directional valves  82 ,  184  are shown as solenoid actuated, these valves may, as an alternative, be hydraulically actuated by pilot (on/off) valves. 
         [0052]      FIG. 11  illustrates a ninth alternative of the hydraulic and electrical control circuit of the vehicle roll control system shown in  FIGS. 1 to 4 .  FIG. 11  is a modification of the hydraulic circuit shown in  FIG. 11 , in which changes have been made to the first directional valve  82 . In this alternative, in the de-actuated state of the first directional valve  82 , the fifth, sixth, seventh and eighth ports  89 - 92  are fluidly isolated from one another and from the other ports  85 - 88 . The operation of this ninth alternative is substantially the same as the operation of the arrangement shown in  FIG. 10 . 
         [0053]    The above-described embodiments of  FIGS. 10 and 11  both operate in substantially the same way, but provide different hydraulic circuit arrangements for their respective fail-safe modes, as illustrated in the drawings. Also, the selection is dependent on the type of hydraulic actuator that is used. 
         [0054]    In the present invention, in all of the above embodiments, the valves of the hydraulic circuit are actuable to provide fluid pressure to the first fluid chamber of the front hydraulic actuator which is different from the fluid pressure provided to the first fluid chamber of the rear hydraulic actuator; and/or actuable to provide fluid pressure to the second fluid chamber of the front hydraulic actuator which is different from the fluid pressure provided to second fluid chamber of the rear hydraulic actuator. 
         [0055]    The present invention is also applicable for use with a vehicle roll control system, the front portion  122  of which is as shown in  FIG. 12  and the rear portion of which is substantially identical to the front portion. In this embodiment in accordance with the present invention, the front portion  122  comprises a torsion bar  126 , a first arm  128 , and a hydraulic actuator  134 . The first arm  128  is fixed at one end  138  to one end  140  of the torsion bar  126 . The other end  142  of the first arm  128  is connected to one of the shock absorbers  120 . The hydraulic actuator  134  has a piston rod  164  which is fixed to the other end  187  of the torsion bar  126 . The housing  156  of the actuator  134  is connected to the other shock absorber  120 . The hydraulic actuator  134  is substantially the same as the actuator  34  described above with reference to  FIGS. 1 to 5 , and has a fluid line  166  connected to a first fluid chamber inside the housing, and another fluid line  168  connected to a second fluid chamber inside the housing. The first and second fluid chambers inside the housing  156  are separated by a piston secured to the piston rod  164 . The fluid lines  166 , 168  for each hydraulic actuator are connected to a hydraulic circuit as shown in  FIG. 5 , which is controlled by a control circuit as shown in  FIG. 5 , or any one of the arrangements shown in  FIGS. 6 to 11 . The roll control system is operated in substantially the same manner as that described above with reference to  FIGS. 1 to 5 , or any one of  FIGS. 6 to 11 . 
         [0056]    The present invention is also applicable for use with a vehicle roll control system as shown in  FIG. 13 . In this third embodiment in accordance with the present invention, the front portion  222  of the system comprises a torsion bar  226 , a first arm  228 , a second arm  228 ′, and a hydraulic actuator  234 . The rear portion of the system is substantially identical. The first arm  228  is fixed at one end  238  to one end  240  of the torsion bar  226 . The other end  242  of the first arm  228  is connected to one of the shock absorbers  220 . The second arm  228 ′ is fixed at one end  238 ′ to the other end  287  of the torsion bar  226 . The other end  242 ′ of the second arm  228 ′ is connected to the other shock absorber  220 ′. The torsion bar  226  is split into first and second parts  290 , 292 , respectively. The first and second parts  290 , 292  of the torsion bar  226  have portions  294 , 296 , respectively, which are axially aligned. The axially aligned portions  294 , 296  are connected by a hydraulic actuator  234 . 
         [0057]    The hydraulic actuator  234 , as shown in  FIG. 14 , comprises a cylindrical housing  256  which is connected at one end  239  to the portion  294  of the first part  290  of the torsion bar  226 . The actuator  234  further comprises a rod  241  positioned inside the housing  256 , extending out of the other end  243  of the housing, and connectable to the portion  296  of the second part  292  of the torsion bar  226 . The rod  241  has an external screw thread  249  adjacent the housing  256 . Balls  251  are rotatably positioned in hemispherical indentations  253  in the inner surface  255  of the housing  256  adjacent the screw thread  249 . The balls  251  extend into the screw thread  249 . The rod  241  is slidably and rotatably mounted in the housing  256  at the other end  243  by way of a bearing  259  positioned in the other end  243 . This arrangement allows the rod  241  to rotate about its longitudinal axis relative to the housing  256 , and to slide in an axial direction A relative to the housing. A piston chamber  261  is defined inside the housing  256 . The rod  241  sealing extends into the piston chamber  261  to define a piston rod  264 , and a piston  262  is secured to the end of the piston rod inside the piston chamber. The piston  262  makes a sealing sliding fit with the housing  256  and divides the chamber  261  into a first fluid chamber  258  and a second fluid chamber  260 . The first fluid chamber  258  is fluidly connected to fluid line  266 , and the second fluid chamber  260  is fluidly connected to fluid line  268 . 
         [0058]    The fluid lines  266 , 268  are connected to a hydraulic circuit as shown in  FIG. 5 , which is controlled by a control circuit as shown in  FIG. 5 , or any one of the arrangements shown in  FIGS. 6 to 11 . The roll control system  222  is operated in substantially the same manner as that described above with reference to  FIGS. 1 to 5 , or any one of  FIGS. 6 to 11 
   An alternative arrangement for the hydraulic actuator of  FIG. 14  is shown in  FIG. 15 . In this alternative embodiment, the actuator  334  comprises a cylindrical housing  356  which is connected at one end  339  to the portion  294  of the first part  290  of the torsion bar  226 . The actuator  334  further comprises a rod  341  positioned inside the housing  356 , extending out of the other end  343  of the housing, and connectable to the portion  296  of the second part  292  of the torsion bar  226 . The rod  341  has an external screw thread  349  adjacent the housing  356 . Balls  351  are rotatably positioned in hemispherical indentations  353  in the inner surface  355  of the housing  356  adjacent the screw thread  349 . The balls  351  extend into the screw thread  349 . The rod  341  is sidably and rotatably mounted in the housing  356  at the other end  343  of the housing by way of a bearing  359  positioned in the other end. The rod  341  makes a sliding guiding fit with the inner surface  355  of the housing  356  at its end  341 ′ remote from the second part  292  of the torsion bar  226 . This arrangement allows the rod  341  to rotate about its longitudinal axis relative to the housing  356 , and to slide in an axial direction A relative to the housing. First and second fluid chambers  358 , 360  are defined inside the housing  356 . The rod  341  makes a sealing fit with the inner surface  355  of the housing  356  by way of seal  371  to define a piston  362 . The first fluid chamber  358  is positioned on one side of the piston  362 , and the second fluid chamber  360  is positioned on the other side of the piston. A seal  369  is positioned adjacent the bearing  359 . A portion  364  of the rod  341  defines a piston rod which extends through the second fluid chamber  360 . The first fluid chamber  358  is fluidly connected to fluid line  366 , and the second fluid chamber  360  is fluidly connected to fluid line  368 . The fluid lines  366 , 368  are fluidly connected with one of the hydraulic circuits shown in  FIGS. 5 to 11  to actuate the actuator  334 .   
 
         [0060]    A further alternative arrangement of hydraulic actuator  334 ′ is shown in  FIG. 16 . In this further alternative embodiment, the actuator  334 ′ is substantially the same as the actuator  334  shown in  FIG. 15 , but without the sliding guiding fit of the free end  341 ′ of the rod  341  with the housing  356 . 
         [0061]    In a preferred arrangement, the cross-sectional area of the first fluid chamber of each hydraulic actuator described above is substantially double the cross-sectional area of the piston rod of the hydraulic actuator, when considered on a radial basis. Such an arrangement provides the same output force from the hydraulic actuator in either direction, using the same fluid pressure. 
         [0062]    In the preferred arrangement described above, a hydraulic actuator is provided for both the front of the vehicle and the rear of the vehicle, and these hydraulic actuators are substantially the same. In an alternative arrangement, the hydraulic actuator for the front of the vehicle may be a different type to the hydraulic actuator for the rear of the vehicle. 
         [0063]    In any of the roll control systems described above, the hydraulic actuator may include a check valve (not shown, but preferably mounted in the piston) which allows flow of hydraulic fluid from the first fluid chamber to the second fluid chamber only when the fluid pressure in the first fluid chamber is greater than the fluid pressure in the second fluid chamber. With such an arrangement, the second fluid chamber can be connected to a reservoir during servicing of the actuator to bleed air from the hydraulic fluid. Also, the presence of the check valve reduces the risk of air being sucked into the second fluid chamber should the fluid pressure in the second fluid chamber fall below the fluid pressure in the first fluid chamber, and provides further improvements in ride comfort.