Abstract:
A subassembly comprising a hydraulic power steering valve ( 5 ) and an electrical actuator ( 32 ), which can twist a component part ( 28 ) of the power steering valve, thus generating steering wheel torque superimposition.

Description:
[0001]     The present invention. relates to hydraulic power steering systems for motor vehicles and, in particular, to power steering valves that are used in power steering systems of this type.  
       BACKGROUND OF THE INVENTION  
       [0002]     In road tests, it has been shown that in certain driving situations it is easier for the driver to track the vehicle if there is an active influencing of the steering wheel torque as it is haptically experienced by the driver on the steering wheel. The steering wheel torque as it is experienced by the driver can be actively influenced in an especially simple manner by using an electronically controlled electrical power steering system, because, in this case, it is simple to realize steering wheel torque. superimposition using electronic control signals. Due to the limited availability of electrical energy from the  12 -volt vehicle electrical systems that are customary today, vehicles having high front-axle loads are almost exclusively equipped with hydraulic power-assisted steering systems.  
         [0003]     The present invention is based on the objective, for vehicles having a hydraulic power steering system, of realizing cost-effective, electronically controlled steering wheel torque superimposition.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0004]     For this purpose, according to the present invention, a subassembly is provided that is made up of a hydraulic power steering valve and an electrical actuator, which can twist a component part of the power steering valve, thus generating steering wheel torque superimposition. In this manner, depending on the direction of the superimposition, there is an increase or decrease in the steering force support, steering wheel torque remaining constant, i.e., a decrease or an increase in the necessary steering wheel torque, given a constant overall steering force (the sum of the hand power of the driver and the steering force support provided by the power-assisted steering).  
         [0005]     According to the preferred embodiment of the present invention, the power steering valve has an input shaft, an output shaft, and a valve sleeve, which is accommodated in a housing, the actuator being able to twist the valve sleeve relative to the housing. Therefore, the desired steering wheel torque superimposition can be realized by an electronically controlled offset between the input shaft and the valve sleeve.  
         [0006]     Advantageous embodiments of the present invention will be reflected in the subclaims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The present invention is described below on the basis of a preferred embodiment, which is depicted in the attached drawings. In the latter:  
         [0008]      FIG. 1  depicts a schematic cutaway view of a subassembly according to the present invention; and  
         [0009]      FIG. 2  schematically depicts a cutaway view along the plane II-II in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]     In  FIG. 1 , a power steering valve  5  is shown, which has a housing  10 . In the housing, an input shaft  12  is rotatably supported and is provided with control grooves  14 . Non-rotatably attached to input shaft  12  is one end of a torsion bar  16 , whose other end is non-rotatably connected to output shaft  18 . Output shaft  18  is provided with a pinion  20 , which can engage in a toothed rack (not shown). The toothed rack is connected to the steerable vehicle wheels.  
         [0011]     The segment of input shaft  12  provided with control grooves  14  is arranged in a valve sleeve  22 , which, in power steering valves according to the related art, is non-rotatably coupled to output shaft  18 . Due to the relative rotation between control grooves  14  of input shaft  12  and valve housing  22 , a hydraulic flow provided by a pump (not shown) is controlled, such that one or the other chamber of a hydraulic cylinder is acted upon with the oil flow as a function of the relative rotation between input shaft  12  and valve sleeve  22 , which is converted into the steering wheel support force. This is well known from the related art, so it is not necessary to discuss it further.  
         [0012]     In contrast to the related art, in the power steering valve according to the present invention, valve sleeve  22  is not rigidly coupled to output shaft  18 , but rather is connected to it by a gear unit  24 . Gear unit  24  is formed by two planetary gears arranged next to each other, of which one is assigned to valve housing  22  and the other is assigned to output shaft  18 .  
         [0013]     A sun wheel  26  is non-rotatably connected to valve housing  22 . Sun wheel  26  is surrounded by a ring gear  28 , which is supported in housing  10  so as to be able to twist about a small angle. Ring gear  28  is provided with an arm  30 , which an actuator  32  engages. The latter is designed as an electrical actuator, which can carry out a stroke motion. In this manner, ring gear  28  can be twisted in housing  10  by arm  30 , the rotational axis of ring gear  28  coinciding with the longitudinal axis of input shaft  12  and of output shaft  18 .  
         [0014]     Arranged immediately adjoining the first planetary gear that is formed by sun wheel  26  and ring gear  28  is a second planetary gear. The latter has a sun wheel  34  that is non-rotatably connected to input shaft  18  as well as a ring gear  36  that is non-rotatably accommodated in housing  10 .  
         [0015]     In the first planetary gear, a plurality of planetary pinions  38  is arranged, and in the second planetary gear, a plurality of planetary pinions  40  is arranged. Planetary pinions  38  of the first planetary gear and planetary pinions  40  of the second planetary gear are both supported on a common axle  42 . In addition, because the number of teeth and the diameter of the ring gear, the sun wheel, and the planetary pinions of the first planetary gear coincide with those of the ring gear, sun wheel, and planetary pinions of the second planetary gear, the circumferential velocity of planetary pinions  38  corresponds exactly to that of planetary pinions  40 , due to the coupling by axle  42 .  
         [0016]     The planetary gear operates in the following manner: when actuator  32  arrests ring gear  28 , valve sleeve  22  is non-rotatably connected to output shaft  18 . Any rotation of output shaft  18  is transmitted in a conformal manner to valve sleeve  22  by sun wheel  34 , planetary pinion  40 , axle  42 , planetary pinion  38 , and sun wheel  26 . The relative rotation between the input shaft and the valve sleeve of the power steering valve, and therefore the quantity and direction of the steering wheel support that is made available is a function above all of the steering wheel torque of the driver, that is transmitted via torsion bar  16 . The power steering valve operates in the usual manner, which is familiar from the related art.  
         [0017]     If an electronic control unit detects that steering wheel torque superimposition is desirable in order to assist the driver in tracking the vehicle, ring gear  28  of the first planetary gear is displaced by actuator  32  in the one or the other direction. In this way, a relative rotation inevitably results between sun wheel  26  of the first planetary gear and sun wheel  34  of the second planetary gear, because the motion of planetary pinions  38 ,  40 , is only determined by the relative rotation between ring gear  36  of the second planetary gear and sun wheel  34  of the second planetary gear, which is fixedly connected to the valve housing. The relative rotation between both sun wheels  34 ,  26 , leads to a twisting of valve housing  22  relative to input shaft  12 , thus generating a change in the metering of the oil flows to the chambers of the hydraulic cylinder. Superimposed on the steering support determined by the deflection of torsion bar  16  is a further steering support, which is represented either as an increase or decrease in the steering force support, at a constant steering wheel torque, or as a decrease or increase of the necessary steering wheel torque, at a constant total steering force.  
         [0018]     In contrast to the depicted embodiment, it is also conceivable to twist the other ring gear or one of the two sun wheels, using the actuator. In this manner as well, the desired offset between the input shaft and the valve sleeve can be achieved.  
         [0019]     According to one undepicted refinement of the present invention, a centering device is provided which urges ring gear  28  into a neutral position, which is adopted when actuator  32  fails.