Abstract:
A method for reducing steering instability in a hydraulic power steering system for a vehicle having a pair of cylinder lines through which hydraulic fluid is conveyed in assisting turning a vehicle wheel, including the steps of determining which of said pair of cylinder lines results in less steering instability when the vehicle wheel is turned; calculating the fluid inertance of the hydraulic fluid conveyed through the cylinder line that results in less steering instability; and altering at least one dimension of the cylinder line that results in greater steering instability when the vehicle wheel is turned to substantially match the fluid inertance thereof to the calculated fluid inertance.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and all benefit of U.S. Provisional Patent Application Ser. No. 61/093,774 filed Sep. 3, 2008, the complete disclosure of which is expressly incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The subject invention relates generally to methods for reducing steering instability in hydraulic power steering systems. 
         [0004]    2. Description of the Related Art 
         [0005]    Instability in hydraulic power steering systems (“steering instability”) is a common problem that can develop when integrating a hydraulic power steering system into the architecture of a vehicle. During a steering event, vehicles having steering instability may exhibit excessive noise and/or vibration when the steering system goes unstable. Steering instability is the result of undamped eigenvalues, i.e., resonances, in the open loop control response of the vehicle steering system. Steering instability typically manifests itself in either a left or a right turn. Less frequently, steering instability occurs during turns in both directions. Either way, steering instability will usually develop into a warranty issue or customer complaint if not resolved. 
         [0006]    There have previously been multiple attempts to resolve the issue of steering instability. However, these attempts are typically vehicle specific and can have significant impact on other ride and handling attributes. These attempts have included tuning return lines, tuning supply lines, changing the steering valve gain, and vehicle changes including changes to chassis stiffness, chassis mass, tire spring rates, suspension bushings, . . . etc. . . . Typically, such vehicle changes are beyond the control of steering system suppliers, and require the involvement of the OEM customer and/or chassis component or system suppliers. 
         [0007]    Most prior attempts at resolving steering instability issues have been developed around the steering system itself. Within the steering system, return and supply line tuning have previously provided reliable solution methods for reducing steering instability for most situations. However, such tuning can adversely affect other steering system attributes. Tuning a supply line to reduce steering instability typically conflicts with tuning intended to address other steering system-related objectives like reducing pump noise or improving steering feel, whereas tuning a return line to reduce steering instability typically results in system packaging and/or cooling issues because the amount of steel required in the return line to adequately dissipate heat usually conflicts with the amount required to substantially reduce steering instability. 
         [0008]    An improved method for addressing issues of steering instability while avoiding the above conflicts, and without requiring vehicle changes is desirable. 
       SUMMARY OF THE INVENTION 
       [0009]    It is to be understood that the detailed description and specific examples discussed, while indicating preferred embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
         [0010]    The present invention provides a method for reducing steering instability in a hydraulic power steering system for a vehicle having a pair of cylinder lines through which hydraulic fluid is conveyed in assisting turning a vehicle wheel, including the steps of determining which of said pair of cylinder lines results in less steering instability when the vehicle wheel is turned; calculating the fluid inertance of the hydraulic fluid conveyed through the cylinder line that results in less steering instability; and altering at least one dimension of the cylinder line that results in greater steering instability when the vehicle wheel is turned to substantially match the fluid inertance thereof to the calculated fluid inertance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
           [0012]      FIG. 1  is partially schematic view of a hydraulic power steering system for a vehicle, including a partially sectioned view of its steering gear assembly including a pair of left and right hand turn-assisting cylinder lines of differing lengths, each respectively associated with hydraulically assisting turning in one of the two opposite wheel turning directions; 
           [0013]      FIG. 2A  is schematic view of an exemplified pair of prior cylinder lines included in the steering system of  FIG. 1 , the cylinder lines both being in their original states (OS) and having a common diameter, the shorter, right hand cylinder line here being the one associated with the turning direction in which relatively greater steering instability results or is exhibited, and thus the longer, left hand cylinder line is designated the first cylinder line, to be emulated, and the shorter, right hand cylinder line is designated the second cylinder line, to be modified; 
           [0014]      FIG. 2B  is similar to  FIG. 2A , except that here the longer, left hand cylinder line is the one associated with the turning direction in which relatively greater steering instability results or is exhibited, and thus the shorter, right hand cylinder line is designated the first cylinder line, to be emulated, and the longer, left hand cylinder line is designated the second cylinder line, to be modified; 
           [0015]      FIG. 3  is a schematic view of a first embodiment of an altered pair of cylinder lines including, from  FIG. 2A , the exemplified first cylinder line in its original state (OS) and the second cylinder line in a modified state (MS) resulting from application of the inventive method; 
           [0016]      FIG. 4  is a schematic view of a second embodiment of an altered pair of cylinder lines including, from  FIG. 2B , the exemplified first cylinder line in its original state (OS) and the second cylinder line in a modified state (MS) resulting from application of the inventive method; 
           [0017]      FIG. 5  is a schematic view of a third embodiment of an altered pair of cylinder lines including, from  FIG. 2A , the exemplified first cylinder line in its original state (OS) and the second cylinder line in a modified state (MS) resulting from application of the inventive method; 
           [0018]      FIG. 6  is a schematic view of a fourth embodiment of an altered pair of cylinder lines including, from  FIG. 2A , the exemplified first cylinder line in its original state (OS) and the second cylinder line in a modified state (MS) resulting from application of the inventive method; and 
           [0019]      FIG. 7  is a schematic view of a fifth embodiment of an altered pair of cylinder lines including, from  FIG. 2B , the exemplified first cylinder line in its original state (OS) and the second cylinder line in a modified state (MS) resulting from application of the inventive method. 
       
    
    
       [0020]    Corresponding reference characters indicate corresponding parts throughout the several views. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and may herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
         [0021]    Moreover, it is to be noted that the Figures are not necessarily drawn to scale and are necessarily not drawn to the same scale. In particular, the scale of some of the elements of the Figures is greatly exaggerated to emphasize characteristics of the elements. Elements shown in more than one Figure that may be similarly configured have been indicated using the same reference numerals. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    The following description of preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention or its uses. 
         [0023]    Referring to  FIG. 1 , a hydraulic power steering system for a vehicle is shown at  10 , and includes steering gear assembly  12 . Gear assembly  12  includes a cylinder  14  formed in housing  16  and in which is disposed a rack  18 . Pinion  20  is meshed with rack  18 , which is operatively connected to a vehicle wheel. In particular, the opposite ends  22 ,  24  of rack  18  are each operatively connected to a respective road wheel  26 ,  28  to effect their turning left and right with corresponding motion of rack  18  in opposite linear directions, as is well known in the art. 
         [0024]    Housing  16  of gear assembly  12  accommodates hydraulic steering valve  30  through which extends pinion shaft  31  rotatably fixed to pinion gear  20 , with steering valve  30  actuated through rotation of the pinion. Pinion shaft  31  is rotatably fixed to one end of intermediate shaft  32 , the other end of which is rotatably fixed to the rotatable shaft of steering column  34 . Pivoting links may be provided at the ends of intermediate shaft  32 , the length of which may be telescopically varied during operation to accommodate relative changes in position between gear assembly  12  and steering column  34 . The rearward end of the shaft of steering column  34  is rotatably fixed to vehicle steering wheel  36 , which is a hand wheel. 
         [0025]    Hydraulic power steering system  10  further includes reservoir  38  and pump  40  which are in fluid communication, with substantially incompressible hydraulic power steering fluid at low pressure being provided from the outlet of reservoir  38  to the inlet of pump  40 . Pump  40  is typically included in the engine&#39;s accessory drive (not shown), and reservoir  38  may be an integrated component of pump  40 , or connected thereto via feed line  41  as shown. Supply line  42  carries power steering fluid at high pressure from the discharge of pump  40  to supply fitting  44  of hydraulic steering valve  30 . Return line  46  extends from return fitting  48  of valve  30  to the inlet of reservoir  38 . Typically, hydraulic fluid is continually circulated through the above-described circuit during engine and pump operation. 
         [0026]    Attached to valve  30  are left hand cylinder line  50  and right hand cylinder line  52  respectively in fluid communication with left hand fluid chamber  54  and right hand fluid chamber  56  in cylinder  14 . Together, the pair of cylinder lines  50 ,  52  is designated by reference numeral  57 . High pressure fluid received by valve  30  through supply line  42  is alternatively directed to one of left and right hand lines  50 ,  52  as vehicle steering wheel  36  is turned in left/counterclockwise or right/clockwise direction. As used herein, the terms left hand cylinder line and left hand fluid chamber are those which high pressure fluid is conveyed through or introduced into in assisting a left hand turn of the vehicle, and conversely the right hand cylinder line and right hand fluid chamber are those which high pressure fluid is conveyed through or introduced into in assisting a right hand turn of the vehicle. 
         [0027]    Within cylinder  14 , expandable and contractible fluid chambers  54  and  56  are separated and defined by intermediate piston  58  fixed to linearly moveable rack  18 . In assisting a left hand turn, for instance, fluid under pressure is supplied from valve  30  to left hand fluid chamber  54  through left hand cylinder line  50 , which assists in causing chamber  54  to expand by urging piston  58 , and thus rack  18 , rightward as viewed in  FIG. 1 , with rightward movement of rack  18  causing road wheels  26 ,  28  to correspondingly turn to the left. Simultaneously, fluid in contracting right hand fluid chamber  56 , the volume of which is made smaller by the rightward movement of piston  58 , is received in right hand cylinder line  52  and returned to valve  30 , from which it is returned to reservoir  38 . 
         [0028]    Conversely, in assisting a right hand turn, fluid under pressure is supplied from valve  30  to right hand fluid chamber  56  through right hand cylinder line  52 , which assists in causing chamber  56  to expand by urging piston  58  and rack  18  leftward as viewed in  FIG. 1 , with leftward movement of rack  18  causing road wheels  26 ,  28  to correspondingly turn to the right. Simultaneously, fluid expelled from contracting left hand fluid chamber  54  is returned to valve  30  via left hand cylinder line  50 , and returned to reservoir  38  via return line  46 . Thus, such hydraulic power steering systems assist turning hand and/or road wheels. 
         [0029]    Hydraulic power steering systems as so far described are well known to and understood by those of ordinary skill in the art and not further described herein. Examples of prior hydraulic power steering systems are generally disclosed in U.S. Pat. No. 5,495,711 to Kalkman et al., and U.S. Pat. No. 4,828,068 to Wendler et al., the entire disclosures of which are expressly incorporated herein by reference. 
         [0030]    The cylinder lines  50 ,  52  used to direct fluid between chambers  54 ,  56  and valve  30  during a steering event have also been shown to impact instability. The length and cross sectional flow area of each cylinder line  50 ,  52  control a parameter called fluid inertance (or fluid inductance). 
         [0031]    Fluid inertance I is defined formulaically as 
         [0000]        I=ρ*L/A    (1) 
         [0000]    where ρ=density of the fluid, L=length of the cylinder line, and A=cross-sectional flow area of the cylinder line. 
         [0032]    The influence of fluid inertance is a key factor in the performance of hydraulic power steering system  10  and therefore can have a significant effect on the steering instability. Prior cylinder lines  50 ,  52  are typically made of a common material and have a common cross-sectional flow area or internal diameter, and by design their lengths are typically asymmetrical because of the different distances between their connections to chambers  54 ,  56  and steering valve  30  (see  FIG. 1 ). This results in prior cylinder lines  50 ,  52  producing different fluid inertances. This, in part, is a contributing factor into why some vehicles demonstrate steering instability in either right or left turns, but not both. To reduce steering instability in a hydraulic steering system  10 , the present invention provides methods to substantially match the fluid inertance between the first and second cylinder lines  50 ,  52 . Specifically, steering instability may be reduced by matching or closely matching the fluid inertance of the cylinder line associated with the objectionable steering instability to the fluid inertance of the cylinder line not associated with the objectionable steering instability. 
         [0033]    It is to be understood that the cylinder line  50 ,  52  associated with the relatively greater amount of resulting or exhibited steering instability could be either a left hand cylinder line  50  or a right hand cylinder line  52 . Thus, of the pair of cylinder lines, the “stable” cylinder line whose fluid inertance is to be substantially matched in the other, “unstable” cylinder line may be either the longer cylinder line (for example, left hand cylinder line  50  in  FIGS. 1 and 2A ), or the shorter cylinder line (for example, right hand cylinder line  52  in  FIGS. 1 and 2B ). 
         [0034]    Regardless of whether a left hand cylinder line  50  or a right hand cylinder line  52 , the “stable” cylinder line that results in or exhibits a relatively smaller degree of steering instability, and whose fluid inertance is to be substantially matched according to the inventive method, is referred to herein as first cylinder line  68  or  68 ′ depending, respectively, on whether it is the longer or shorter of the pair of cylinder lines in its original design or state (irrespective of whether a right or left hand cylinder line). The “unstable” cylinder line that results in or exhibits a relatively greater degree of steering instability, and which is to be modified to substantially match the fluid inertance of first cylinder line  68 ,  68 ′ is referred to herein as the second cylinder line  70  or  70 ′ depending, respectively, on whether it is the shorter or longer of the pair of cylinder lines in its original design or state (again irrespective of whether a right or left hand cylinder line). Primed reference numerals herein generally relate to cases in which the first cylinder line is the shorter and the second cylinder line is the longer of the pair. 
         [0035]    In performing the inventive method, it is preferable that the replicated first cylinder line  68 ,  68 ′ be left unaltered from its original design or state, and that the resulting cylinder line modifications are made to second cylinder line  70 ,  70 ′ which in its original state is associated with the relatively greater level of resulting or exhibited steering instability. In  FIGS. 2-7 , each cylinder line that is depicted in its respective original, unaltered state is labeled OS, and each second cylinder line modified as a result of applying the inventive method is respectively labeled MS to indicate that, relative to its original configuration, it is shown in a modified state that substantially matches the fluid inertance of its respectively paired first cylinder line. In each of  FIGS. 3-7 , the paired cylinder lines produce substantially equal fluid inertances. Notably,  FIGS. 2-7  show the cylinder lines schematically as being straight. It is to be understood, however, that each cylinder line may be configured to include bends or curves along its length as packaging considerations warrant, and that dimension L is associated with distance along the length of the cylinder line however configured, rather than a distance extending directly between two points. It is envisioned that the altered pair of cylinder lines may be included in steering gear assembly  12  supplied by a steering system vendor to its OEM customer. 
         [0036]    One embodiment of the inventive method involves equalizing the fluid inertances of the cylinder lines by modifying the length of the cylinder line  50 ,  52  through which high pressure fluid is conveyed from valve  30  to its respective cylinder fluid chamber  54 ,  56  in assisting a left or right hand turn and is associated with the steering direction that results in or exhibits a relatively greater level of steering instability (the “unstable” line), to match the length of the cylinder line  50 ,  52  associated with the other steering direction that results in or exhibits relatively less steering instability (the “stable” line). This length-altering approach, in which the cross sectional flow area of the cylinder lines remain substantially unaltered, is preferably applied to a pair of cylinder lines in which the shorter original state cylinder line is the “unstable” or second cylinder line associated with the steering direction resulting in or exhibiting a relatively greater degree of steering instability. 
         [0037]    Referring to  FIGS. 1 and 2 , right hand cylinder line  52  has overall length LR that is shorter than overall length LL of left hand cylinder line  50 . That is, length LR of right hand cylinder line  52  is less than length LL of left hand cylinder line  50 . Ordinarily, cylinder lines  50  and  52  are made of identical tubing material, and along their entire lengths have an identical cross-sectional shape and size, such as having a common first flow area or first inner diameter D 1  as shown. 
         [0038]    With reference to  FIG. 2A , in the case where the steering instability is found in the steering direction associated with the shorter, right hand cylinder line  52 ,  70 , second line length L 2  of the second or right hand cylinder line  52 ,  70  is extended to match length L 1  (which may be overall length LL) of the first or left hand cylinder line  50 ,  68 , their common first flow areas or inner diameters D 1  remaining unaltered from their original states, as shown in  FIG. 3 , wherein the resulting first embodiment altered pair of cylinder lines  157  includes modified second cylinder line  170  which produces substantially the same fluid inertance as that of its paired first cylinder line  68 . 
         [0039]    With reference to  FIG. 2B , however, in the case where second cylinder line  70 ′ is longer, left hand cylinder line  50 , it may not be possible to reduce its overall, second line length L 2 ′, LL to match shorter overall length L 1 ′, LR of first, right hand cylinder line  52 ,  68 ′. Therefore, application of the inventive method in such a situation may involve instead changing the cross-sectional flow area of one or both of the cylinder lines  50 ,  52  as described further below. 
         [0040]    Increasing the cross-sectional flow area will decrease the fluid inertance of a given line, and decreasing its cross-sectional flow area will increase its fluid inertance. As shown in  FIG. 2B , the respective lengths L 1 ′ and L 2 ′ of first and second cylinder lines  68 ′ and  70 ′ are different, but through application of the inventive method the fluid inertances of cylinder lines  68 ′,  70 ′ are made substantially equivalent by increasing the inner diameter of longer second line  70 ′ from original, first flow area or first diameter D 1  to larger, second flow area or second diameter D 2  to yield modified second cylinder line  270 ′ of  FIG. 4 , which depicts second embodiment altered pair of cylinder lines  257  resulting from application of the inventive method. 
         [0041]    Should the “stable” first cylinder line  68  be the longer, and the “unstable” second cylinder line  70  be the shorter of the pair of cylinder lines, as illustrated in  FIG. 2A , and it not be possible or desirable to lengthen second cylinder line  70  from length L 2  to match length L 1  as in first embodiment altered cylinder line pair  157  shown in  FIG. 3 , then the inner diameter of shorter, second line  70  may be reduced from its original first flow area or first diameter D 1  to a smaller, third flow area or third diameter D 3  as shown in  FIG. 5 , which illustrates third embodiment pair of altered cylinder lines  357  resulting from application of the inventive method, which includes original first cylinder line  68  and modified second cylinder line  370 . 
         [0042]    A changed flow area of the second cylinder line  70 ,  70 ′ need not be constant or over its entire length in order to substantially match its fluid inertance to that of first cylinder line  68 ,  68 ′. Referring to  FIG. 6 , there is shown fourth embodiment altered pair of cylinder lines  457  resulting from the inventive method applied to a pair of lines wherein first line  68  is the longer of the two, and modified second line  470  is the shorter, i.e., their original states may be as lines  68 ,  70  illustrated in  FIG. 2A . In  FIG. 6 , the second line length L 2  of modified second cylinder line  470  includes a modified middle segment or portion LA disposed between opposite end segments or portions LB and LC. The overall length L 2  of modified second line  470  shown in  FIG. 6  may be substantially unchanged from that of original state second line  70  shown in  FIG. 2A . In modified second line  470 , the flow area or inner diameter of middle portion LA has been reduced from original first flow area or diameter D 1  to smaller third flow area or diameter D 3 . In cylinder line pair  457 , first flow area or first diameter D 1  of portions LB and LC of modified second line  470 , and of first line  68 , remains unaltered, and in second line  470  the length and flow area of segment LA may be proportionally varied in substantially matching the fluid inertance of modified second line  470  to that of first line  68 . Additionally, the lengths of end segments LB and LC maintained at original first flow area or first diameter D 1  may be varied and their fluid inertances summed with that of middle segment LA to substantially match the total fluid inertance of modified second cylinder line  470  to that of first cylinder line  68 . 
         [0043]    Referring now to  FIG. 7 , there is shown a fifth embodiment altered pair of cylinder lines  557  resulting from application of the inventive method. Of cylinder line pair  557 , first line  68 ′ is the shorter and modified second line  570 ′ is the longer, i.e., their original states may be as lines  68 ′,  70 ′ illustrated in  FIG. 2B . In  FIG. 7 , the second line length L 2 ′ of second cylinder line  570 ′ includes modified middle segment or portion LA′ disposed between opposite end segments or portions LB and LC. The overall length L 2 ′ of modified second line  570 ′ may be substantially unchanged from that of second line  70 ′ in its original state (OS) shown in  FIG. 2B . The flow area or inner diameter of modified middle portion LA′ has been increased from original first flow area or first diameter D 1  to larger second flow area or diameter D 2 . In cylinder line pair  557 , the first flow area or first inner diameter D 1  of portions LB′ and LC′ of modified second line  570 ′, and of first line  68 ′, remains unaltered, and in second line  570 ′ the length and flow area of segment LA′ may be proportionally varied in substantially matching the fluid inertance of modified second line  570 ′ to that of first line  68 ′. Additionally, the lengths of end segments LB′ and LC′ maintained at original first diameter D 1  may be varied and their fluid inertances summed with that of middle segment LA′ to substantially match the total fluid inertance of modified second cylinder line  570 ′ to that of first cylinder line  68 ′. 
         [0044]    The altered pairs of cylinder lines shown in  FIGS. 6 and 7 , wherein the first diameters D 1  of end segments LB, LC, LB′, LC′ of modified second lines  470  and  570 ′ remain unchanged from original state second lines  70 ,  70 ′, and which include original state first lines  68 ,  68 ′ also of first diameter D 1 , eliminate the need for port size changes at the connections to cylinder  14  or valve  30 , which could impact the manufacture of a steering gear. For instance, maintaining original, first diameter D 1  at the opposite ends of lines  68 ,  68 ′ and  470 ,  570 ′ facilitates their common use of existing tube fittings  60 ,  62 ,  64 ,  66  and their mating receiving ports provided in cylinder  14  and valve  30 . 
         [0045]    Lastly, if instability is present in both turning directions, the designer need only find a stable frequency or cylinder line configuration for system  10 , and then adjust the dimensions of right hand and/or left hand cylinder line  50 ,  52 , and thus their resulting fluid inertances, until steering stability is achieved. 
         [0046]    While the invention has been described with reference to exemplary embodiments or alternatives, it will be understood by those skilled in the art that various changes can be made and equivalents can be substituted for elements or steps thereof without departing from the scope of the invention. In addition, many modifications can be made to adapt a particular situation, method or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments or practices disclosed for carrying out this invention, but that the invention will include all embodiments or alternatives falling within the scope of the appended claims.