Abstract:
The invention generally relates to an improvement of conventional Hydraulically Power Assisted Steering system (HPAS-system) arranged to supply a steering assist force to the steering assembly of a vehicle as a response to a torque applied by a driver to the steering wheel. In such HPAS-systems a certain drivers torque always results in a certain assist force. The invention therefore discloses a valve that can be actuated to dynamically alter the steering assist force produced by the HPAS-system. This makes it possible to dynamically adjust the assist force so that an appropriate force may be delivered to fit the specific driving scenario.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a Hydraulically Power Assisted Steering system (HPAS-system) arranged to supply a steering assist force to the steering assembly of an automobile or a vehicle. In particular the invention relates to an HPAS-system including a rotary valve unit, which may be actuated to dynamically alter the steering assist force produced by the HPAS-system. 
     DESCRIPTION OF RELATED ART 
     Various steering arrangements for assisting a driver in steering an automobile or a vehicle are well known in the art. Especially it is well known that the turning direction of a vehicle can be maneuvered by a steering wheel that is mechanically connected to the road wheels through a steering assembly. In assisting the driver it is common to use an auxiliary system to generate an additional steering force, which is applied to the steering assembly of the vehicle. The additional steering force is suitably adapted to reduce the effort required by the driver in changing the direction of the road wheels. 
     Traditionally, various Hydraulic Power Assisted Steering (HPAS) systems have been used to add a certain amount of assist force to the steering assembly of a vehicle. These traditional HPAS-systems are typically based on an assist characteristic, a so-called boost-curve. The shape of a boost-curve is typically and essentially determined by the design of the valve and the pump of the HPAS-system. The boost-curve in a traditional HPAS-system is therefore static. 
     According to the function of a traditional boost-curve a certain torque applied by the driver to the steering wheel result in a certain predetermined assist force supplied by the HPAS-system to the steering assembly of the vehicle. This predetermined assist force increases as the driver applies more torque to the steering wheel, and decreases as the driver applies less torque to the steering wheel. The use of a static boost-curve gives a static relation between a steering effort required from the driver and a corresponding assist force supplied by the HPAS-system, i.e. the relation follows a static and predetermined curve. 
     Nevertheless, the amount of steering effort required from the driver and the appropriate assist force that should be supplied by the HPAS-system may vary depending on various external circumstances and especially dependent upon the specific driving scenario, e.g. dependent upon the vehicle speed, the vehicle turning angle etc. Future customer functions such as my-split braking aid or Lane Keeping Aid demand more flexible solutions in terms of steering wheel assist force. 
     From the steering gears perspective this presupposes a more dynamic change in the relation between assist force and drivers torque. This can be achieved with an Electric Power Assisted Steering (EPAS) using an electric motor to supply an assist torque to the vehicle steering assembly. Consequently, the introduction of new steering related customer functions in passenger cars essentially depends of the implementation of EPAS-systems. 
     However, some HPAS-system has been developed to achieve a more dynamic change in the relation between assist force and drivers torque. 
     U.S. Pat. No. 5,593,002 (Okada et al.) shows a HPAS-system comprising a rotary valve unit actuated according to a twisting angle provided in a torsion bar connected between an input shaft and a pinion shaft and a pinion, and a conversion mechanism which can change the condition of the rotary valve unit for a given twisting angle of the torsion bar. It should be noted that the flow of oil through the rotary valve in Okada is not directly affected by said change of valve condition. However, when a torque is applied to the steering wheel more or less oil may flow through the valve depending on the valve operative condition, see e.g. col. 5 line 41-col. 6 line 63. The change of condition in Okada may be seen as a multiplicative or lever system affected by the speed of the vehicle. 
     U.S. Pat. No. 5,513,720 (Yamamoto et al.) shows a HPAS-system that comprises a steering mechanism having a torsion bar, a rotary valve connected to an oil pump and disposed between an input shaft and an output shaft, a valve driving mechanism having a pressed portion projected on either the input shaft or the output shaft and a plunger on the one shaft of the input shaft or output shaft on which the pressed portion is not projected for pressing the pressed portion, setting a target assist force of an assist force obtained by rotating the rotary valve in the torsion direction of the torsion bar and an assist force obtained by rotating the rotary valve in the reverse direction to the torsion direction, a plunger driving mechanism for driving the plunger so that the preset assist force is obtained, controlling the pressure itself of the rotary valve to the operation angle of the rotary valve. This arrangement may be used to achieve a more dynamic change in the relation between assist force and driver torque. However, it should be noted that the rotary valve is rotated by generating in the torsion bar  9  a torsional moment in the forward direction (right direction) or in the reverse direction (left direction), see e.g. col. 8 lines 33-49. This means that the torsion bar is exposed to extra tensional strain, which reduces its deflecting response to driver-applied torque and which may reduce the useful life of the torsion bar. Moreover, the force needed to deflect the torsion bar is fairly large, with a bulkier and heavier construction as a consequence. 
     To summarise, the prior art cited above may in some respect offer a solution to achieve a more dynamic change in the relation between assist force and drivers torque in an HPAS-system. However, the prior art have several drawbacks. 
     SUMMARY OF THE INVENTION 
     The invention offers a simple solution to enable a dynamic change in the relation between assist force and drivers torque in an HPAS-system. In particular, the invention offers a solution that may be implemented by simple modifications of conventional HPAS-systems, comprising a rotary valve actuated according to a twisting angle provided in a torsion bar or a similar deflecting device or turnable device connected between a steering shaft attached to a steering wheel and a pinion shaft or similar attached to the steering rack or similar, where the actuation of the rotary valve determines the assist force F ass  that is supplied by the HPAS-system to the steering assembly of the vehicle. 
     Such conventional HPAS-systems may be understood as a servo system having a controller that tries to minimize or reduce the angular difference α Δ  between the turning angle α sw  of the steering wheel and the turning angle α ps  of the pinion shaft. In other words these conventional HPAS-systems may be understood as a servo system that tries to reduce or minimize any twisting of the torsion bar. 
     However, the invention is not limited to conventional HPAS-systems and it should be understood that the torsion bar and other parts of the vehicle steering assembly may be substituted for other parts having the same or similar function, provided that the rotary valve may be actuated to reflect a larger angular difference α Δ  when the driver applies more torque to the steering wheel and actuated to reflect a smaller angular difference α Δ  when the driver applies less torque to the steering wheel. 
     As previously stated in the background of the invention the use of a conventional HPAS-system having a static boost-curve gives a static relation between the steering effort required from the driver and the corresponding assist force supplied by the HPAS-system. In other words, the relation between drivers torque and assist force follows a static and predetermined curve, whereby a specific α Δ  results in a specific assist force F ass . Obviously there is a need for a more flexible solution than the one offered by the static solution in conventional HPAS-systems. 
     The invention therefore discloses an arrangement and a method that i.a. enables a varying offset angle α off  to be more or less dynamically added to or subtracted from the angular difference α Δ  between the steering wheel turning angle α sw  and the turning angle of the pinion shaft α ps , i.e. α Δ ±α off . 
     This may be accomplished by arranging one part of the rotary valve to be non-rotatably supported on the vehicle steering shaft, while another part of the rotary valve, e.g. the valve house, may be supported on the pinion shaft so that it may be displaced in relation to the pinion shaft, preferably rotatably displaceable a small angle α off  with respect to the pinion shaft. The valve house may then rotate together with the supporting pinion shaft, however displaced by a small angle α off  with respect to the pinion shaft. The same applies mutatis mutandis if the valve house or similar part is alternatively supported on the steering shaft. 
     This means that an angular difference α Δ  reflected by the rotary valve may be increased by an small offset angle +α off , which further opens the rotary valve to increase the hydraulic pressure in a hydraulic piston or similar for supplying an increased steering assist force F Δass  to the steering rack, resulting in a total amount of steering assist force F ass +F Δass . The angular difference α Δ  may conversely be decreased by a small offset angle −α off , which slightly closes the rotary valve to decrease the hydraulic pressure for supplying an decreased steering assist force F ass −F Δass . 
     By arranging the valve house or similar part of the rotary valve to be dynamically actuated a small offset angle α off  in relation to the supporting pinion shaft it is possible to dynamically adjust the assist force F ass  corresponding to an angle α Δ , with a certain amount of assist force ±F Δass , corresponding to an offset angle ±α off , so that an appropriate assist force F ass ±F Δass  is delivered by the HPAS-system to fit the specific driving scenario, where a control mechanism determines the offset angle α off  depending on at least one external or internal vehicle input parameter. 
     The forces needed to obtain a displacement or an offset—e.g. an offset angle α off —by directly actuating a part of a rotary valve are fairly low, mainly comprising flow forces created within the valve and friction forces emanating from the actuated valve part. 
    
    
     
       DESCRIPTION OF THE FIGURES 
       The following figures form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these figures in combination with the detailed description of specific embodiments presented herein. 
         FIG. 1  shows a diagrammatic view of an HPAS-system for a wheeled vehicle according to the present invention. 
         FIG. 2  shows a cross-section of the interconnection assembly  130 . 
         FIG. 3  shows a perspective view of the first and second cylindrical valve member  305 ,  310 . 
         FIG. 4  shows a cross section of the interconnection assembly  130  cut through the line A-A in  FIG. 2 . 
         FIG. 5  shows a second embodiment of the present invention. 
         FIG. 6  shows a cross section of a part of the second embodiment, cut through the line B-B in  FIG. 5 . 
         FIG. 7  shows a third embodiment of the present invention. 
         FIG. 8  shows a diagrammatic view of the third embodiment. 
     
    
    
     Reference signs in the Figures are as follows: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Ref. 
                 Feature 
               
               
                   
                   
               
             
             
               
                   
                 100 
                 Hydraulic Power Assisted Steering System (HPAS-system) 
               
               
                   
                 120 
                 Steering Wheel 
               
               
                   
                 121 
                 Steering Shaft 
               
               
                   
                 122 
                 Pinion Shaft 
               
               
                   
                 123 
                 Pinion Gear Assembly 
               
               
                   
                 124 
                 Rack 
               
               
                   
                 125 
                 Tie Rods 
               
               
                   
                 126 
                 Connector Rods 
               
               
                   
                 127 
                 Road Wheels 
               
               
                   
                 128 
                 Servo Pump 
               
               
                   
                 129 
                 Servo-Motor 
               
               
                   
                 130 
                 Interconnecting Assembly 
               
               
                   
                 135 
                 Lid 
               
               
                   
                 140 
                 Treaded Bolts 
               
               
                   
                 210 
                 Torsion Bar 
               
               
                   
                 220 
                 Serrated Coupling 
               
               
                   
                 225 
                 Coupling Arrangement 
               
               
                   
                 300 
                 Rotary Valve 
               
               
                   
                 305 
                 First Cylindrical Valve Member 
               
               
                   
                 310 
                 Second Cylindrical Valve Member 
               
               
                   
                 315 
                 Inlet Through-Hole 
               
               
                   
                 320 
                 First Chamber 
               
               
                   
                 325 
                 First Outer Through-Hole 
               
               
                   
                 330 
                 First Inner Through-Hole 
               
               
                   
                 335 
                 Second Chamber 
               
               
                   
                 340 
                 Second Inner Through-Hole 
               
               
                   
                 345 
                 Second Outer Through-Hole 
               
               
                   
                 350 
                 Third Chamber 
               
               
                   
                 360 
                 Outlet Through-Hole 
               
               
                   
                 370 
                 Flange Portion 
               
               
                   
                 371 
                 Recess Chamber 
               
               
                   
                 372 
                 Inlet-Outlet Port 
               
               
                   
                 373 
                 Inlet-Outlet Port 
               
               
                   
                 374 
                 Wall Portion 
               
               
                   
                 400 
                 Cog Wheel 
               
               
                   
                 405 
                 Cog Wheel Shaft 
               
               
                   
                 410 
                 Electric Stepping Motor 
               
               
                   
                 415 
                 Cogged Ring 
               
               
                   
                 500 
                 Electric Stepping Motor 
               
               
                   
                 505 
                 Eccentric Axis 
               
               
                   
                 510 
                 Diagonal Slot 
               
               
                   
                 511 
                 Rivet 
               
               
                   
                 515 
                 Cylindrical Flange Portion 
               
               
                   
                 520 
                 Valve House 
               
               
                   
                 525 
                 Empty Space 
               
               
                   
                 530 
                 Guide Flange 
               
               
                   
                   
               
             
          
         
       
     
     DETAILED DESCRIPTION OF THE INVENTION 
     The HPAS-System 
     Referring to  FIG. 1 , a Hydraulic Power Assisted Steering system  100  (hereinafter denoted HPAS-system) is illustrated. The HPAS-system  100  is preferably a system for use for steering the road wheels of an automobile or a vehicle, which system  100  is equipped with a rotary valve  130  according to the present invention. Although the HPAS-system  100  is described in connection with a hydraulic power assisted steering of road wheels of an automobile, it should be appreciated that the HPAS-system  100  according to the present invention may be employed to steer any number of front and/or rear wheels or other propulsion equipment of a steered vehicle. 
     The HPAS-System Steering Assembly 
     The HPAS-system  100  shown in  FIG. 1  comprises a steering assembly, i.a. comprising a steering wheel  120 . The steering wheel  120  is generally disposed in the vehicle passenger compartment and manually operated by the driver of the vehicle to steer the road wheels  127 . Further, the steering assembly includes a steering shaft  121 , operatively coupled to the steering wheel  120 . Said steering shaft  121  rotates in synchronization with the steering wheel  120  and is preferably directly attached to the steering wheel  120 . The steering assembly also employs a pinion shaft  122 , operatively engaged with steering shaft  121 . The steering shaft  121  and the pinion shaft  122  are interconnected via an interconnecting assembly  130 . Said pinion shaft  122  is preferably coupled at one end to a pinion gear assembly  123  for converting angular rotation of the pinion shaft  122  to linear movement of a rack  124 , where the rack  124  is coupled on opposite ends to tie rods  125  and connector rods  126 , which are movable to control left and right rotation of the road wheels  127 . 
     It should be appreciated that the steering wheel  120 , the steering shaft  121 , the pinion shaft  122 , the pinion gear assembly  123 , the rack  124 , the tie rods  125 , the connector rods  126  and the road wheels  127  as shown in  FIG. 1  only illustrates one of several suitable steering assemblies known to the person skilled in the art. It follows that the invention is not limited to the steering assembly shown in  FIG. 1 . 
     Moreover, as will be further explained below the interconnecting assembly  130  of the steering assembly i.a. comprises a torsion bar  210  and a rotary valve  300  connected to a servo pump  128  (schematically indicated in  FIG. 1 ) for the supply of pressurized servo fluid, as is well known in the vehicle steering art. The rotary valve  300  is further connected to a hydraulic servo-motor  129  (schematically indicated in  FIG. 1 ) that is arranged for supplying a steering assist force to the steering assembly, so as to reduce the steering effort required by the driver in changing the direction of the road wheels  127 , as is also well known in the vehicle steering art. The servo-motor  129  may e.g. be a piston arrangement for supplying an assist force to the rod  124  or a rotating arrangement for supplying an assist torque to the pinion shaft  122 , or some other suitable hydraulic arrangement. 
     The Interconnecting Assembly and the Rotary Valve 
     The interconnecting assembly schematically  130  indicated in  FIG. 1  is further illustrated in  FIG. 2 . As is shown in  FIG. 2  one end of the torsion bar  210  is operatively connected to the pinion shaft  122  via a serrated coupling  220 , whereas the other end of the torsion bar  210  is operatively coupled to the steering shaft  121  via a coupling arrangement  225 , e.g. a suitable toothed coupling or a claw coupling. 
     Further, the interconnecting assembly  130  comprises a rotary valve  300 . As illustrated in  FIG. 2  the rotary valve  300  may be enclosed inside an extension of the pinion shaft  122  sealed e.g. by a lid  135  and threaded bolts  140 . Alternatively and conversely, the rotary valve  300  may be arranged on the outside of the shafts so as to enclose and surround a part of the pinion shaft  122 , a part of the steering shaft  121 , and a part of the torsion bar  210 , and possibly also other components of an interconnecting assembly  130 . 
     The rotary valve  300  in  FIG. 2  illustrates the principles of an exemplary rotary valve. Possible sealing arrangements and other details well known by a person skilled in the art to be a part of a rotary valve has been omitted for the sake of clarity. 
     The rotary valve  300  illustrated in  FIG. 2  comprises a first cylindrical valve member  305  and a second cylindrical valve member  310 , adapted to regulate the flow of a hydraulic fluid such as oil or similar. The first cylindrical valve member  305  may in a preferred embodiment constitute a part of the valve house. The second cylindrical valve member  310  is drive or press fitted on the steering shaft  121  and consequently arranged to rotate together with the steering shaft  121 , whereas the first valve member  305  is rotatably or turnable connected to the pinion shaft  210 , such that the first valve member  305  may rotate together with the pinion shaft  122  at an offset angle α off  with respect to the pinion shaft  122 . 
     The right side structure of the rotary valve  300  in  FIG. 2  will now be described with reference to numbered parts in  FIGS. 2 ,  3  and  4 . 
     The extension of the pinion shaft  122  comprising the rotary valve  300  in  FIG. 2  has an inlet through-hole  315  for receiving a pressurized hydraulic fluid from the servo pump  128 , and a first chamber  320  communicating with two first outer through-holes  325  arranged in the first cylindrical valve member  305 , where the first outer through-holes  325  are adapted to dynamically communicate with a corresponding pair of first inner through-holes  330  arranged in the second cylindrical valve member  310  where the first inner through-holes  330  communicate with a second chamber  335  arranged inside the second cylindrical valve member  305 . 
     Said second chamber  335  communicates with a second pair of inner through-holes  340  arranged in the second cylindrical valve member  310 , where the second pair of inner through-holes  340  are adapted to dynamically communicate with a second pair of outer through-holes  345  arranged in the first cylindrical valve member  305 , where the second outer through-holes  345  communicate with a third chamber  350 , which in turn communicates with an outlet through-hole  360  for an outlet of the received pressurized hydraulic fluid, where both the third chamber  350  and the outlet through-hole  360  are arranged in the extension of the pinion shaft  122  for supplying pressurized hydraulic fluid to the servo-motor  129 . 
     The first cylindrical valve member  305  and the second cylindrical valve member  310  of the rotary valve  300  are further illustrated in  FIG. 3 , showing a perspective view of the members  305 ,  310 , where the member  310  has been lifted from the member  305 . The members  305 ,  310  are illustrated with the through-holes  330 , of member  310  in a position where they partly coincide with the through-holes  325 ,  345  of member  305  if the two members  305 ,  310  had been put together in an operative position, i.e. if the members  305 ,  310  had been operatively arranged in a rotary valve  300  as shown in  FIG. 2 . Such an operative position of the through-holes  330 ,  340 ,  325 ,  345  indicates that there is an angular difference α Δ  between the turning angle α sw  of the steering wheel and the turning angle α ps  of the pinion shaft, e.g. caused by a driver turning the steering wheel  120 . When the through-holes  330 ,  340  in an operative position coincide with the through-holes  325 ,  345  a flow of pressurized hydraulic fluid passes from the servo-pump  128  through the rotary valve  300  and to the servo-motor  129  as described above, whereupon the servo-motor  129  may deliver an assist force F ass  to reduce the steering effort required by the driver in changing the direction of the road wheels  127 . 
     It should be added that the valve members  305 ,  310  may preferably be operatively arranged to vary the position of the through-holes  330 ,  340  and  325 ,  345  from a complete overlap, corresponding to a large α Δ , to a gradual decrease of the overlap, corresponding to a decrease in α Δ , where no overlap at all corresponds to α Δ =0. Where there is no overlap at all there is consequently no flow of hydraulic fluid to the servo-motor and there is consequently no assist force F ass  delivered from the servo-motor. 
     The rotary valve  300  may also comprise a flange portion  370  formed as a protrusion of the first cylindrical valve member  305  and arranged at the lower end of said member  305 . The flange portion  370  protrudes into a recess chamber  371  arranged in the extended portion of the pinion shaft  122 , as can be seen, in  FIG. 2 . The flange portion  370  and the recess chamber  371  are further illustrated in  FIG. 4 , showing a cross section of the interconnection assembly  130  cut through the line A-A in  FIG. 2 . As can be seen in  FIG. 4  the recess chamber has a first inlet-outlet port  372  arranged to the right and a second inlet-outlet port  373  arranged to the left, arranged to receive and expel hydraulic fluid. The flange portion  370  is arranged to rotate an offset angle ±α off  together with the first cylindrical valve member  305 , as illustrated by the two opposite arrows in  FIG. 4 . The maximum rotation angle α max  in this embodiment is determined by the size of the recess chamber  317 , extending as a cut ring-shaped circle-segment along the wall of the pinion shaft  122 . It is further preferred that the flange portion  370  is tightly arranged towards a wall portion  374  of the recess chamber  371 , formed by the outer wall periphery of the pinion shaft  122 , so as to cut the recess chamber  371  into a left and a right hydraulic chamber. In this way the first cylindrical valve member  305  may be rotated clockwise a small offset angle α off  by increasing the hydraulic pressure in the right hydraulic chamber and decreasing the hydraulic pressure in the left chamber, whereas the first cylindrical valve member  305  may be rotated counter clockwise a small offset angle α off  by increasing the hydraulic pressure in the left hydraulic chamber and decreasing the hydraulic pressure in the right chamber. Hence, once a certain hydraulic pressure has been established in said chambers the first cylindrical valve member  305  will rotate together with the pinion shaft  122 , however possibly displaced by an small angle α off  with respect to the pinion shaft. 
     By a dynamic change of the hydraulic pressure in said right and left chamber it is possible to dynamically adjust the assist force F ass , corresponding to an angle α Δ , with a certain amount of assist force ±F Δass , corresponding to an offset angle ±α off , so that an appropriate assist force F ass ±F Δass  is delivered by the servo-motor  129  to fit the specific driving scenario, where a control mechanism determines the offset angle α off  depending on at least one external or internal vehicle input parameter, e.g. vehicle speed, vehicle acceleration, vehicle turning angle etc. The choice of control mechanism is not important to the present invention and it may e.g. be any suitably programmed computer system. 
     The right side structure of the rotary valve  300  has now been described with numbered references to the different parts in  FIGS. 2 ,  3  and  4 . A corresponding left side structure of the rotary valve  300  is also illustrated in  FIG. 2 . The left side structure has the same function and the same parts as the right side structure and the left side structure is therefore not described in any detail. However, it should be noted that the right side and the left side are preferably separated for supplying a left chamber and a right chamber respectively in a piston arrangement that supplies an assist force to the rod  124 . 
     The invention is not limited to the rotary valve  300  illustrated in  FIGS. 2 ,  3  and  4 . On the contrary, a rotary valve according to the present invention embodiments may e.g. have only one inlet through-hole  315  and one outlet through-hole  360 , in which case there may be only one first, second and third chamber  315 ,  335  and  350  and such an embodiment may only have the through-holes  325 ,  330 ,  340 ,  345 , where said chambers and said through-holes may extend a full circle or nearly a full circle around the steering shaft  121 . Moreover, some embodiments may have only one first through-hole  325 ,  330  and/or only one second through-hole  340 ,  345 , whereas other embodiments may have three or more such through-holes. In addition, the through-holes  325 ,  330 ,  340 ,  345  in the first and second cylindrical valve member  305 ,  310  are not limited to any specific shape. On the contrary, they may have any suitable shape, e.g. rounded, elongated and/or angular. In addition, the through-holes  325 ,  330 ,  340 ,  345  may be arranged in any suitable direction, e.g. more or less in the axial direction with respect to the shafts  121 ,  122  and/or more or less in the rotational direction of the shafts  121 ,  122 . 
     In brief, the present invention may generally be implemented in a vast variety of rotary valves that is well known to a person skilled in the art. 
     Other Embodiments 
     In a second embodiment of the present invention the rotary valve  300  as illustrated in  FIGS. 2 ,  3  and  4  may be adapted to have the first cylindrical valve member  305  rotated an offset angle α off  by a cog wheel or a similar toothed device. This may be accomplished by the arrangement illustrated in  FIG. 5  showing a cog wheel  400  operatively connected to a cog wheel shaft  405  that is operatively connected to an electric stepping motor  410  or a piezoelectric or magnetostrictive motor or similar, where the electric motor  410  in turn is attached to the pinion shaft  122  for rotating together with the shaft  122 . 
     According to this embodiment the first cylindrical valve member,  305  is adapted so as to be provided with a cogged ring  415  or a similar toothed device for interaction with the cog wheel  400  or similar. The flange portion  370  has consequently been omitted in this second embodiment. The arrangement in  FIG. 5  is further illustrated in  FIG. 6 , showing a cross section of the cog wheel  400  and the cogged ring  415  cut through the line B-B in  FIG. 5 . Hence, once the cog wheel  400  has rotated the first cylindrical valve member  305  an offset angle α off  the first cylindrical valve member  305  will rotate together with the pinion shaft  122 , however possibly displaced by an small angle α off  with respect to the pinion shaft  122 . 
     The second embodiment makes it possible to have the first cylindrical valve member  305  rotated an offset angle α off  by commanding the electric motor  410  to rotate the cog wheel  400  an appropriate angle α cog . Consequently, by commanding the motor  410  to dynamically change the rotation angle α cog  of the cog wheel  400  it is possible to dynamically adjust the assist force F ass , corresponding to an angle α Δ , with a certain amount of assist force ±F Δass , corresponding to an offset angle ±α off , so that an appropriate assist force F ass ±F Δass  is delivered by the servo-motor  129  to fit the specific driving scenario, where a control mechanism determines the offset angle α off  depending on at least one external or internal vehicle input parameter, e.g. vehicle speed, vehicle acceleration, vehicle turning angle etc. The choice of control mechanism is not important to the present invention and it may e.g. be any suitably programmed computer system. 
     It should be noted that the second embodiment is essentially similar to the first embodiment as described above, except for the adaptations now mentioned. 
     In a third embodiment of the present invention a rotary valve  300  as illustrated in  FIG. 2-4  may be adapted to have the first cylindrical valve member  305  rotated an offset angle α off  by moving the valve member  305  up and/or down. 
     This may be accomplished by non-rotatably attaching the first cylindrical valve member  305  to a valve house  520  that i.a. encases the first and second cylindrical valve members  305 ,  310  as shown in  FIG. 7 . The valve house  520  may somewhat be similar to the extension of the pinion shaft  122  shown in  FIG. 2 , which i.a. encases the first and second cylindrical valve members  305 ,  310 . However, the valve house  520  is arranged so that it may freely rotate a small offset angle α off  with respect to the pinion shaft  122 . This is illustrated in  FIG. 7  by the small empty space  525  that cylindrically surrounds the top of the pinion shaft  122  and which consequently separates the lower end of the valve house  520  from the pinion shaft  122 . 
     The valve house  520  in this third embodiment may be rotated a small offset angle α off  with respect to the pinion shaft  122  according to the arrangement illustrated in  FIG. 8 . The arrangement comprises an electric stepping motor  500  or a piezoelectric or magnetostrictive motor or similar that may be arranged in a position that is separated from the steering assembly. The electric motor  500  may rotate an eccentric axis  505 , e.g. an oval axis or rectangular axis or similar, that engages a guide flange  530  or similar that is arranged on the outer periphery of the valve house  520 . Since the axis  505  is eccentric a rotation α ecc  of the axis  505  in the guide flange  530  will cause the valve house  520  to move up and/or down. 
     The rotational movement of the valve house  520  is then preferably obtained by a diagonal track or slot  510 , e.g. arranged as a cylindrical flange portion  515  that is arranged to extend axially downward from the lower part of the valve house  520 , where the slot  510  is guided by a rivet  511  or some other suitable guiding device that is arranged on the pinion shaft  122 . Hence, when the valve house  520  and the cylindrical flange  515  firmly attached thereto are move up or down by a slight rotation of the eccentric axis  505  that is actuated by the motor  500  this will cause the valve house  520  to rotate as the diagonal slot  510  moves guided by the rivet  511 . The valve house  520  and the cylindrical flange  515  may be dynamically moved up and down by the eccentric axis  505  actuated by the electric motor  500  so that the diagonal slot  510  may take any position between position A and position B, as indicated in  FIG. 8 . 
     A small movement of the slot  510  guided by the rivet  511  will cause the valve house  520  rotate a small offset angle α off  with respect to the pinion shaft  122 , where a movement of the slot  510  from position A to position B corresponds to the maximum rotation angle α max  of the valve house  520  in this embodiment. This maximum rotation angle α max  is similar to the α max  previously discussed in connection with the first embodiment and  FIG. 4 . 
     Hence, once the eccentric axis  505  has been rotated an angle α ecc  by the electric motor  500  to displace the valve house  520  and the first cylindrical valve member  305  attached thereto an offset angle α off  the first cylindrical valve member  305  will rotate together with the pinion shaft  122 , however possibly displaced by an small angle α off  with respect to the pinion shaft  122 . 
     The third embodiment makes it possible to have the first cylindrical valve member  305  rotated an offset angle α off  by commanding the electric motor  500  to rotate the eccentric axis  505  an appropriate angle α ecc . Consequently, by commanding the motor  500  to dynamically change the rotation angle α ecc  of the eccentric axis  505  it is possible to dynamically adjust the assist force F ass , corresponding to an angle α Δ , with a certain amount of assist force ±F Δass , corresponding to an offset angle ±α off , so that an appropriate assist force F ass ±F Δass  is delivered by the servo-motor  129  to fit the specific driving scenario, where a control mechanism determines the offset angle α off  depending on at least one external or internal vehicle input parameter, e.g. vehicle speed, vehicle acceleration, vehicle turning angle etc. The choice of control mechanism is not important to the present invention and it may e.g. be any suitably programmed computer system. 
     It should be noted that the third embodiment is essentially similar to the first embodiment as described above, except for the adaptations now mentioned. 
     All of the processes and/or apparatus disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the processes and/or apparatus of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the apparatus and/or processes and in the steps or in the sequence of steps of the processes described herein without departing from the concept and scope of the invention. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the scope and concept of the invention.