Abstract:
A shock absorber has a compression valve assembly that provides a high damping load during a compression stroke and an extension valve assembly that provides a high damping load during an extension stroke. One or more digital valve assemblies is positioned to work in parallel with the compression valve assembly and the extension valve assembly to provide a lower damping load. The lowering of the damping load is based upon the cross sectional area of flow passages provided by the one or more digital valve assemblies.

Description:
FIELD 
       [0001]    The present disclosure relates generally to hydraulic dampers or shock absorbers for use in a suspension system such as a suspension system used for automotive vehicles. More particularly, the present disclosure relates to a digital damper valve which is combined with the conventional passive valve systems to determine the damping characteristics of the hydraulic damper. 
       BACKGROUND 
       [0002]    This section provides background information related to the present disclosure which is not necessarily prior art. 
         [0003]    Shock absorbers are used in conjunction with automotive suspension systems to absorb unwanted vibrations which occur during driving. To absorb the unwanted vibrations, shock absorbers are generally connected between the sprung portion (body) and the unsprung portion (suspension) of the automobile. A piston is located within a pressure tube of the shock absorber and the pressure tube is connected to the unsprung portion of the vehicle. The piston is connected to the sprung portion of the automobile through a piston rod which extends through the pressure tube. The piston divides the pressure tube into an upper working chamber and a lower working chamber both of which are filled with hydraulic fluid. Because the piston is able, through valving, to limit the flow of the hydraulic fluid between the upper and the lower working chambers when the shock absorber is compressed or extended, the shock absorber is able to produce a damping force which counteracts the vibration which would otherwise be transmitted from the unsprung portion to the sprung portion of the vehicle. In a dual-tube shock absorber, a fluid reservoir or reserve chamber is defined between the pressure tube and a reserve tube. A base valve is located between the lower working chamber and the reserve chamber to also produce a damping force which counteracts the vibrations which would otherwise be transmitted from the unsprung portion of the vehicle to the sprung portion of the automobile. 
         [0004]    As described above, for a dual-tube shock absorber, the valving on the piston limits the flow of damping fluid between the upper and lower working chambers when the shock absorber is extended to produce a damping load. The valving on the base valve limits the flow of damping fluid between the lower working chamber and the reserve chamber when the shock absorber is compressed to produce a damping load. For a mono-tube shock absorber, the valving on the piston limits the flow of damping fluid between the upper and lower working chambers when the shock absorber is extended or compressed to produce a damping load. During driving, the suspension system moves in jounce (compression) and rebound (extension). During jounce movements, the shock absorber is compressed causing damping fluid to move through the base valve in a dual-tube shock absorber or through the piston valve in a mono-tube shock absorber. A damping valve located on the base valve or the piston controls the flow of damping fluid and thus the damping force created. During rebound movements, the shock absorber is extended causing damping fluid to move through the piston in both the dual-tube shock absorber and the mono-tube shock absorber. A damping valve located on the piston controls the flow of damping fluid and thus the damping force created. 
         [0005]    In a dual-tube shock absorber, the piston and the base valve normally include a plurality of compression passages and a plurality of extension passages. During jounce or compression movements in a dual-tube shock absorber, the damping valve or the base valve opens the compression passages in the base valve to control fluid flow and produce a damping load. A check valve on the piston opens the compression passages in the piston to replace damping fluid in the upper working chamber but this check valve does not contribute to the damping load. The damping valve on the piston closes the extension passages of the piston and a check valve on the base valve closes the extension passages of the base valve during a compression movement. During rebound or extension movements in a dual-tube shock absorber, the damping valve on the piston opens the extension passages in the piston to control fluid flow and produce a damping load. A check valve on the base valve opens the extension passages in the base valve to replace damping fluid in the lower working chamber but this check valve does not contribute to the damping load. 
         [0006]    In a mono-tube shock absorber, the piston normally includes a plurality of compression passages and a plurality of extension passages. The shock absorber will also include means for compensating for the rod volume flow of fluid as is well known in the art. During jounce or compression movements in a mono-tube shock absorber, the compression damping valve on the piston opens the compression passages in the piston to control fluid flow and produce a damping load. The extension damping valve on the piston closes the extension passages of the piston during a jounce movement. During rebound or extension movements in a mono-tube shock absorber, the extension damping valve on the piston opens the extension passages in the piston to control fluid flow and produce a damping load. The compression damping valve on the piston closes the compression passages of the piston during a rebound movement. 
         [0007]    For most dampers, the damping valves are designed as a normal close/open valve even though some valves may include a bleed flow of damping fluid. Because of this close/open design, these passive valve systems are limited in their ability to adjust the generated damping load in response to various operating conditions of the vehicle. 
       SUMMARY 
       [0008]    This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
         [0009]    A valve assembly for a shock absorber includes a digital valve assembly which is used in conjunction with the typical passive valve assembly. When the digital valve assembly is closed, a firm or high damping load is generated. Softer or lower damping loads are achieved through various combinations of the digital valve assembly working in conjunction with the passive valve assembly. 
         [0010]    Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0011]    The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
           [0012]      FIG. 1  is an illustration of an automobile having shock absorbers which incorporate the valve design in accordance with the present disclosure; 
           [0013]      FIG. 2  is a side view, partially in cross-section of a dual-tube shock absorber from  FIG. 1  which incorporates the valve design in accordance with the present disclosure; 
           [0014]      FIG. 3  is an enlarged side view, partially in cross-section, of the piston assembly from the shock absorber illustrated in  FIG. 2 ; 
           [0015]      FIG. 4  is an enlarged side view, partially in cross-section of the base valve assembly from the shock absorber illustrated in  FIG. 2 ; 
           [0016]      FIG. 5  is an enlarged side view, partially in cross-section of the digital valve assembly from the shock absorber illustrated in  FIG. 2 ; 
           [0017]      FIG. 6  is an enlarged cross-sectional perspective view of the digital valve assembly illustrated in  FIGS. 2 and 5 ; 
           [0018]      FIG. 7  is a graph of force vs. velocity for the shock absorber illustrated in  FIGS. 2-6 ; 
           [0019]      FIG. 8  is a side view, partially in cross-section, of a mono-tube shock absorber which incorporates the valve design in accordance with the present disclosure; 
           [0020]      FIG. 9  is an enlarged side view, partially in cross-section of the piston assembly shown in  FIG. 8 ; 
           [0021]      FIG. 10  is an enlarged cross-sectional perspective view of the digital valve assembly illustrated in  FIGS. 8 and 9 ; 
           [0022]      FIG. 11  is an enlarged cross-sectional view of a shock absorber and rod guide assembly in accordance with another embodiment of the present disclosure; 
           [0023]      FIG. 12  is an enlarged cross-sectional view of the digital valve assembly illustrated in  FIG. 11 ; 
           [0024]      FIG. 13  is an enlarged cross-sectional view of a piston rod assembly in accordance with another embodiment of the present disclosure; 
           [0025]      FIG. 14  is an enlarged cross-sectional view of the digital valve assembly illustrated in  FIG. 13 ; 
           [0026]      FIG. 15  is a cross-sectional side view of a shock absorber assembly in accordance with another embodiment of the present disclosure; 
           [0027]      FIG. 16  is an enlarged cross-sectional view of the digital valve assemblies illustrated in  FIG. 15 ; 
           [0028]      FIG. 17  is an enlarged cross-sectional perspective view of the base valve assembly illustrated in  FIGS. 15 and 16 ; 
           [0029]      FIG. 18  is a cross-sectional view of a base valve assembly in accordance with another embodiment of the present disclosure; 
           [0030]      FIG. 19  is an enlarged cross-sectional perspective view of the base valve assembly illustrated in  FIG. 18 ; 
           [0031]      FIG. 20  is a cross-sectional view of a base valve assembly in accordance with another embodiment of the present disclosure; and 
           [0032]      FIG. 21  is an enlarged cross-sectional perspective view of the base valve assembly illustrated in  FIG. 20 . 
       
    
    
       [0033]    Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
       DETAILED DESCRIPTION 
       [0034]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. There is shown in  FIG. 1  a vehicle incorporating a suspension system having shock absorbers, each of which incorporates a valve assembly in accordance with the present invention, and which is designated generally by the reference numeral  10 . Vehicle  10  includes a rear suspension  12 , a front suspension  14  and a body  16 . Rear suspension  12  has a transversely extending rear axle assembly (not shown) adapted to operatively support a pair of rear wheels  18 . The rear axle is attached to body  16  by means of a pair of shock absorbers  20  and by a pair of springs  22 . Similarly, front suspension  14  includes a transversely extending front axle assembly (not shown) to operatively support a pair of front wheels  24 . The front axle assembly is attached to body  16  by means of a pair of shock absorbers  26  and by a pair of springs  28 . Shock absorbers  20  and  26  serve to dampen the relative motion of the unsprung portion (i.e., front and rear suspensions  12 ,  14 ) with respect to the sprung portion (i.e., body  16 ) of vehicle  10 . While vehicle  10  has been depicted as a passenger car having front and rear axle assemblies, shock absorbers  20  and  26  may be used with other types of vehicles or in other types of applications including, but not limited to, vehicles incorporating non-independent front and/or non-independent rear suspensions, vehicles incorporating independent front and/or independent rear suspensions or other suspension systems known in the art. Further, the term “shock absorber” as used herein is meant to refer to dampers in general and thus will include McPherson struts and other damper designs known in the art. 
         [0035]    Referring now to  FIG. 2 , shock absorber  20  is shown in greater detail. While  FIG. 2  illustrates only shock absorber  20 , it is to be understood that shock absorber  26  also includes the valve assembly design described below for shock absorber  20 . Shock absorber  26  only differs from shock absorber  20  in the manner in which it is adapted to be connected to the sprung and unsprung masses of vehicle  10 . Shock absorber  20  comprises a pressure tube  30 , a piston assembly  32 , a piston rod  34 , a reserve tube  36  and a base valve assembly  38 . 
         [0036]    Pressure tube  30  defines a working chamber  42 . Piston assembly  32  is slidably disposed within pressure tube  30  and divides working chamber  42  into an upper working chamber  44  and a lower working chamber  46 . A seal  48  is disposed between piston assembly  32  and pressure tube  30  to permit sliding movement of piston assembly  32  with respect to pressure tube  30  without generating undue frictional forces as well as sealing upper working chamber  44  from lower working chamber  46 . Piston rod  34  is attached to piston assembly  32  and extends through upper working chamber  44  and through a rod guide assembly  50  which closes the upper end of pressure tube  30 . The end of piston rod  34  opposite to piston assembly  32  is adapted to be secured to the sprung mass of vehicle  10 . Valving within piston assembly  32  controls the movement of fluid between upper working chamber  44  and lower working chamber  46  during movement of piston assembly  32  within pressure tube  30 . Because piston rod  34  extends only through upper working chamber  44  and not lower working chamber  46 , movement of piston assembly  32  with respect to pressure tube  30  causes a difference in the amount of fluid displaced in upper working chamber  44  and the amount of fluid displaced in lower working chamber  46 . The difference in the amount of fluid displaced is known as the “rod volume” and it flows through base valve assembly  38 . 
         [0037]    Reserve tube  36  surrounds pressure tube  30  to define a fluid reservoir chamber  52  located between tubes  30  and  36 . The bottom end of reserve tube  36  is closed by a base cup  54  which is adapted to be connected to the unsprung mass of vehicle  10 . The upper end of reserve tube  36  is attached to rod guide assembly  50 . Base valve assembly  38  is disposed between lower working chamber  46  and reservoir chamber  52  to control the flow of fluid between chambers  46  and  52 . When shock absorber  20  extends in length, an additional volume of fluid is needed in lower working chamber  46  due to the “rod volume” concept. Thus, fluid will flow from reservoir chamber  52  to lower working chamber  46  through base valve assembly  38  as detailed below. When shock absorber  20  compresses in length, an excess of fluid must be removed from lower working chamber  46  due to the “rod volume” concept. Thus, fluid will flow from lower working chamber  46  to reservoir chamber  52  through base valve assembly  38  as detailed below. 
         [0038]    Referring now to  FIG. 3 , piston assembly  32  comprises a piston body  60 , a compression valve assembly  62  and a rebound valve assembly  64 . Compression valve assembly  62  is assembled against a shoulder  66  on piston rod  34 . Piston body  60  is assembled against compression valve assembly  62  and rebound valve assembly  64  is assembled against piston body  60 . A nut  68  secures these components to piston rod  34 . 
         [0039]    Piston body  60  defines a plurality of compression passages  70  and a plurality of rebound passages  72 . Seal  48  includes a plurality of ribs  74  which mate with a plurality of annular grooves  76  to retain seal  48  during sliding movement of piston assembly  32 . 
         [0040]    Compression valve assembly  62  comprises a retainer  78 , a valve disc  80  and a spring  82 . Retainer  78  abuts shoulder  66  on one end and piston body  60  on the other end. Valve disc  80  abuts piston body  60  and closes compression passages  70  while leaving rebound passages  72  open. Spring  82  is disposed between retainer  78  and valve disc  80  to bias valve disc  80  against piston body  60 . During a compression stroke, fluid in lower working chamber  46  is pressurized causing fluid pressure to react against valve disc  80 . When the fluid pressure against valve disc  80  overcomes the biasing load of spring  82 , valve disc  80  separates from piston body  60  to open compression passages  70  and allow fluid flow from lower working chamber  46  to upper working chamber  44 . Typically spring  82  only exerts a light load on valve disc  80  and compression valve assembly  62  acts as a check valve between chambers  46  and  44 . The damping characteristics for shock absorber  20  during a compression stroke are controlled in part by base valve assembly  38  which accommodates the flow of fluid from lower working chamber  46  to reservoir chamber  52  due to the “rod volume” concept. During a rebound stroke, compression passages  70  are closed by valve disc  80 . 
         [0041]    Rebound valve assembly  64  is termed a passive valve assembly which comprises a spacer  84 , a plurality of valve discs  86 , a retainer  88  and a spring  90 . Spacer  84  is threadingly received on piston rod  34  and is disposed between piston body  60  and nut  68 . Spacer  84  retains piston body  60  and compression valve assembly  62  while permitting the tightening of nut  68  without compressing either valve disc  80  or valve discs  86 . Retainer  78 , piston body  60  and spacer  84  provide a continuous solid connection between shoulder  66  and nut  68  to facilitate the tightening and securing of nut  68  to spacer  84  and thus to piston rod  34 . Valve discs  86  are slidingly received on spacer  84  and abut piston body  60  to close rebound passages  72  while leaving compression passages  70  open. Retainer  88  is also slidingly received on spacer  84  and it abuts valve discs  86 . Spring  90  is assembled over spacer  84  and is disposed between retainer  88  and nut  68  which is threadingly received on spacer  84 . Spring  90  biases retainer  88  against valve discs  86  and valve discs  86  against piston body  60 . When fluid pressure is applied to valve discs  86 , they will elastically deflect at the outer peripheral edge to open rebound valve assembly  64 . A shim is located between nut  68  and spring  90  to control the preload for spring  90  and thus the blow off pressure as described below. Thus, the calibration for the blow off feature of rebound valve assembly  64  is separate from the calibration for compression valve assembly  62 . 
         [0042]    During a rebound stroke, fluid in upper working chamber  44  is pressurized causing fluid pressure to react against valve discs  86 . Prior to the deflecting of valve discs  86 , a bleed flow of fluid flows through a bleed passage defined between valve discs  86  and piston body  60 . When the fluid pressure reacting against valve discs  86  overcomes the bending load for valve discs  86 , valve discs  86  elastically deflect opening rebound passages  72  allowing fluid flow from upper working chamber  44  to lower working chamber  46 . The strength of valve discs  86  and the size of rebound passages will determine the damping characteristics for shock absorber  20  in rebound. When the fluid pressure within upper working chamber  44  reaches a predetermined level, the fluid pressure will overcome the biasing load of spring  90  causing axial movement of retainer  88  and the plurality of valve discs  86 . The axial movement of retainer  88  and valve discs  86  fully opens rebound passages  72  thus allowing the passage of a significant amount of damping fluid creating a blowing off of the fluid pressure which is required to prevent damage to shock absorber  20  and/or vehicle  10 . 
         [0043]    Referring to  FIG. 4 , base valve assembly  38  comprises a valve body  92 , a compression valve assembly  94  and a rebound valve assembly  96 . Compression valve assembly  94  and rebound valve assembly  96  are attached to valve body  92  using a bolt  98  and a nut  100 . The tightening of nut  100  biases compression valve assembly  94  towards valve body  92 . Valve body  92  defines a plurality of compression passages  102  and a plurality of rebound passages  104 . 
         [0044]    Compression valve assembly  94  is termed a passive valve assembly which comprises a plurality of valve discs  106  that are biased against valve body  92  by bolt  98  and nut  100 . During a compression stroke, fluid in lower working chamber  46  is pressurized and the fluid pressure within compression passages  102  reacts against valve discs  106 . Prior to the deflection of valve discs  106 , a bleed flow of fluid will flow through a bleed passage defined between valve discs  106  and valve body  92 . The fluid pressure reacting against valve discs  106  will eventually open compression valve assembly  94  by deflecting valve discs  106  in a manner similar to that described above for rebound valve assembly  64 . Compression valve assembly  62  will allow fluid flow from lower working chamber  46  to upper working chamber  44  and only the “rod volume” will flow through compression valve assembly  94 . The damping characteristics for shock absorber  20  are determined in part by the design of compression valve assembly  94  of base valve assembly  38 . 
         [0045]    Rebound valve assembly  96  comprises a valve disc  108  and a valve spring  110 . Valve disc  108  abuts valve body  92  and closes rebound passages  104 . Valve spring  110  is disposed between nut  100  and valve disc  80  to bias valve disc  108  against valve body  92 . During a rebound stroke, fluid in lower working chamber  46  is reduced in pressure causing fluid pressure in reservoir chamber  52  to react against valve disc  108 . When the fluid pressure against valve disc  108  overcomes the biasing load of valve spring  110 , valve disc  108  separates from valve body  92  to open rebound passages  104  and allow fluid flow from reservoir chamber  52  to lower working chamber  46 . Typically valve spring  110  exerts only a light load on valve disc  108  and compression valve assembly  94  acts as a check valve between reservoir chamber  52  and lower working chamber  46 . The damping characteristics for a rebound stroke are controlled in part by rebound valve assembly  64  as detailed above. 
         [0046]    Referring now to  FIGS. 5 and 6 , rod guide assembly  50  is illustrated in greater detail. Rod guide assembly  50  comprises a rod guide housing  120 , a seal assembly  122 , a retainer  124  and a digital valve assembly  126 . 
         [0047]    Rod guide housing  120  is assembled into pressure tube  30  and into reserve tube  36 . Seal assembly  122  and retainer  124  are assembled to rod guide housing  120  and reserve tube  36  is rolled or formed over as illustrated at  128  to retain rod guide assembly  50 . A bushing  130  assembled into rod guide housing  120  accommodates for the sliding motion of piston rod  34  while also providing for a seal for piston rod  34 . A fluid passage  132  extends through rod guide housing  120  to allow fluid communication between upper working chamber  44  and digital valve assembly  126  as discussed below. 
         [0048]    Digital valve assembly  126  is a two position valve assembly which has a different flow area in each of the two positions. Digital valve assembly  126  comprises a valve housing  140 , a sleeve  142 , a spool  144 , a spring  146  and a coil assembly  148 . Valve housing  140  defines a valve inlet  150  which is in communication with upper working chamber  44  through fluid passage  132  and a valve outlet  152  which is in fluid communication with reservoir chamber  52 . While this embodiment and other embodiments described later include spring  146  in the digital valve assemblies, it is within the scope of the present disclosure to use digital valve assemblies that do not include spring  146 . Digital valve assemblies that do not include spring  146  are moved between their two positions by reversing the current or reversing the polarity of the power provided to the digital valve assembly. 
         [0049]    Sleeve  142  is disposed within valve housing  140 . Sleeve  142  defines an annular inlet chamber  154  which is in communication with valve inlet  150  and a pair of annular outlet chambers  156  and  158  which are in communication with valve outlet  152 . 
         [0050]    Spool  144  is slidingly received within sleeve  142  and axially travels within sleeve  142  between coil assembly  148  and a stop puck  160  disposed within sleeve  142 . Spring  146  biases spool  144  away from coil assembly  148  and towards stop puck  160 . A shim  162  is disposed between coil assembly  148  and sleeve  142  to control the amount of axial motion for spool  144 . A first O-ring seals the interface between stop puck  160 , sleeve  142  and valve housing  140 . A second O-ring seals the interface between coil assembly  148 , sleeve  142  and rod guide housing  120 . 
         [0051]    Spool  144  defines a first flange  164  which controls fluid flow between annular inlet chamber  154  and annular outlet chamber  156  and a second flange  166  that controls fluid flow between annular inlet chamber  154  and annular outlet chamber  158 . Flanges  164  and  166  thus control fluid flow from upper working chamber  44  to reservoir chamber  52 . 
         [0052]    Coil assembly  148  is disposed within sleeve  142  to control the axial movement of spool  144 . The wiring connections for coil assembly  148  can extend through rod guide housing  120 , through sleeve  142 , through valve housing  140  and/or through reserve tube  36 . When there is no power provided to coil assembly  148 , the damping characteristics will be defined by the flow area of digital valve assembly  126  in its first position, piston assembly  32  and base valve assembly  38 . The movement of spool  144  is controlled by supplying power to coil assembly  148  to move digital valve assembly to its second position. Digital valve assembly  126  can be kept in its second position by continuing to supply power to coil assembly  148  or by providing means for retaining digital valve assembly  126  in its second position and discontinuing the supply of power to coil assembly  148 . The means for retaining digital valve assembly  126  in its second position can include mechanical means, magnetic means or other means known in the art. Once in its second position, movement to the first position can be accomplished by terminating power to coil assembly  148  or by reversing the current or reversing the polarity of the power supplied to coil assembly  148  to overcome the retaining means. The amount of flow through digital valve assembly  126  has discrete settings for flow control in both the first position and the second position. While the present disclosure is described using only one digital valve assembly  126 , it is within the scope of the disclosure to use a plurality of digital valve assemblies  126 . When multiple digital valve assemblies  126  are used, the total flow area through the plurality of digital valve assemblies  126  can be set at a specific number of total flow areas depending on the position of each individual digital valve assemblies  126 . The specific number of total flow areas can be defined as being 2 n  flow areas where n is the number of digital valve assemblies  126 . For example, if four digital valve assemblies  126 , the number of total flow areas available would be 2 4  or sixteen flow areas. 
         [0053]      FIG. 7  discloses a force vs. velocity curve for shock absorber  20 . Line A represents the bleed flow and the firm setting when digital valve assembly  126  is closed. Line B represents the bleed flow and the combination of the passive valving in piston assembly  32  or base valve assembly  38  in combination with a first opening degree of digital valve assembly  126 . Line C represents the bleed flow and the combination of the passive valving in piston assembly  32  or base valve assembly  38  in combination with a second opening degree of digital valve assembly  126  greater than the first opening degree. Line D represents the bleed flow and the combination of the passive valving in piston assembly  32  or base valve assembly  38  in combination with a fully opened digital valve assembly  126 . 
         [0054]    Fluid will flow through digital valve assembly  126  will occur both during a rebound or extension stroke and during a compression stroke. During a rebound or extension stroke, fluid in upper working chamber  44  is pressurized which then forces fluid flow through digital valve assembly  126  when it is opened. During a compression stroke, fluid flows from lower working chamber  46  to upper working chamber  44  through piston assembly  32  due to the “rod volume” concept. When digital valve assembly  126  is opened, an open flow path between upper working chamber  44  and reservoir chamber  52  is created. Additional fluid flow will flow through piston assembly  32  and through digital valve assembly  126  because this open flow path creates the path of least resistance to reservoir chamber  52  in comparison to flow through base valve assembly  38 . 
         [0055]    Referring now to  FIG. 8-10 , a mono-tube shock absorber  220  in accordance with the present invention is illustrated. Shock absorber  220  can replace either shock absorber  20  or shock absorber  26  by modifying the way it is adapted to be connected to the sprung mass and/or the unsprung mass of the vehicle. Shock absorber  220  comprises a pressure tube  230 , a piston assembly  232  and a piston rod assembly  234 . 
         [0056]    Pressure tube  230  defines a working chamber  242 . Piston assembly  232  is slidably disposed within pressure tube  230  and divides working chamber  242  into an upper working chamber  244  and a lower working chamber  246 . A seal  248  is disposed between piston assembly  232  and pressure tube  230  to permit sliding movement of piston assembly  232  with respect to pressure tube  230  without generating undue frictional forces as well as sealing upper working chamber  244  from lower working chamber  246 . Piston rod assembly  234  is attached to piston assembly  232  and it extends through upper working chamber  244  and through an upper end cap or rod guide  250  which closes the upper end of pressure tube  230 . A sealing system seals the interface between rod guide  250 , pressure tube  230  and piston rod assembly  234 . The end of piston rod assembly  234  opposite to piston assembly  232  is adapted to be secured to the sprung mass of vehicle  10 . The end of pressure tube  230  opposite to rod guide  250  is closed by a base cup  254  which is adapted to be connected to the unsprung mass of vehicle  10 . 
         [0057]    A compression valve assembly  256  associated with piston assembly  232  is termed a passive valve assembly which controls movement of fluid between lower working chamber  246  and upper working chamber  244  during compression movement of piston assembly  232  within pressure tube  230 . The design for compression valve assembly  256  controls in part the damping characteristics for shock absorber  220  during a compression stroke. An extension valve assembly  258  associated with piston assembly  232  is termed a pressure valve assembly which controls movement of fluid between upper working chamber  244  and lower working chamber  246  during extension or rebound movement of piston assembly  232  within pressure tube  230 . The design for extension valve assembly  258  controls in part the damping characteristics for shock absorber  220  during an extension or rebound stroke. 
         [0058]    Because piston rod assembly  234  extends only through upper working chamber  244  and not lower working chamber  246 , movement of piston assembly  232  with respect to pressure tube  230  causes a difference in the amount of fluid displaced in upper working chamber  244  and the amount of fluid displaced in lower working chamber  246 . The difference in the amount of fluid displaced is known as the “rod volume” and compensation for this fluid is accommodated by a piston slidably disposed within pressure tube  230  and located between lower working chamber  246  and a compensation chamber  260 . Typically compensation chamber  260  is filled with a pressurized gas and the piston moves within pressure tube  230  to compensate for the “rod volume” concept. 
         [0059]    Referring now to  FIG. 9 , piston assembly  232  comprises a piston body  262 , compression valve assembly  256  and extension valve assembly  258 . Compression valve assembly  256  is assembled against a shoulder  266  on piston rod assembly  234 . Piston body  262  is assembled against compression valve assembly  256  and extension valve assembly  258  is assembled against piston body  262 . A nut  268  secures these components to piston rod assembly  234 . 
         [0060]    Piston body  262  defines a plurality of compression passages  270  and a plurality of rebound passages  272 . Seal  248  includes a plurality of ribs  274  which mate with a plurality of annular grooves  276  to retain seal  248  during sliding movement of piston assembly  232 . 
         [0061]    Compression valve assembly  256  is termed a passive valve assembly which comprises a retainer  278 , a valve disc  280  and a spring  282 . Retainer  278  abuts shoulder  266  on one end and piston body  262  on the other end. Valve disc  280  abuts piston body  262  and closes compression passages  270  while leaving rebound passages  272  open. Spring  282  is disposed between retainer  278  and valve disc  280  to bias valve disc  280  against piston body  262 . During a compression stroke, fluid in lower working chamber  246  is pressurized causing fluid pressure to react against valve disc  280 . Prior to the opening of valve disc  280 , a bleed flow of fluid will flow through a bleed passage defined by valve disc  280  and piston body  262 . When the fluid pressure against valve disc  280  overcomes the biasing load of spring  282 , valve disc  280  separates from piston body  262  to open compression passages  270  and allow fluid flow from lower working chamber  246  to upper working chamber  244 . The damping characteristics for shock absorber  220  during a compression stroke are controlled by compression valve assembly  256 . During a rebound stroke, compression passages  270  are closed by valve disc  280 . 
         [0062]    Extension valve assembly  258  is termed a passive valve assembly which comprises a spacer  284 , a plurality of valve discs  286 , a retainer  288  and a spring  290 . Spacer  284  is threadingly received on piston rod assembly  234  and is disposed between piston body  262  and nut  268 . Spacer  284  retains piston body  262  and compression valve assembly  256  while permitting the tightening of nut  268  without compressing either valve disc  280  or valve discs  286 . Retainer  278 , piston body  262  and spacer  284  provide a continuous solid connection between shoulder  266  and nut  268  to facilitate the tightening and securing of nut  268  to spacer  284  and thus to piston rod assembly  234 . Valve discs  286  are slidingly received on spacer  284  and abut piston body  262  to close rebound passages  272  while leaving compression passages  270  open. Retainer  288  is also slidingly received on spacer  284  and it abuts valve discs  286 . Spring  290  is assembled over spacer  284  and is disposed between retainer  288  and nut  268  which is threadingly received on spacer  284 . Spring  290  biases retainer  288  against valve discs  286  and valve discs  286  against piston body  262 . When fluid pressure is applied to valve discs  286 , they will elastically deflect at the outer peripheral edge to open extension valve assembly  258 . A shim  296  is located between nut  268  and spring  290  to control the preload for spring  290  and thus the blow off pressure as described below. Thus, the calibration for the blow off feature of extension valve assembly  258  is separate from the calibration for compression valve assembly  256 . 
         [0063]    During a rebound stroke, fluid in upper working chamber  244  is pressurized causing fluid pressure to react against valve discs  286 . Prior to the deflection of valve discs  286 , a bleed flow of fluid will flow through a bleed passage defined by valve discs  286  and piston body  262 . When the fluid pressure reacting against valve discs  286  overcomes the bending load for valve discs  286 , valve discs  286  elastically deflect opening rebound passages  272  allowing fluid flow from upper working chamber  244  to lower working chamber  246 . The strength of valve discs  286  and the size of rebound passages will determine the damping characteristics for shock absorber  220  in rebound. When the fluid pressure within upper working chamber  244  reaches a predetermined level, the fluid pressure will overcome the biasing load of spring  290  causing axial movement of retainer  288  and the plurality of valve discs  286 . The axial movement of retainer  288  and valve discs  286  fully opens rebound passages  272  thus allowing the passage of a significant amount of damping fluid creating a blowing off of the fluid pressure which is required to prevent damage to shock absorber  220  and/or vehicle  10 . 
         [0064]    Referring now to  FIG. 10 , piston rod assembly  234  is illustrated in greater detail. Piston rod assembly  234  comprises a piston rod  298  and a digital valve assembly  300 . Piston rod  298  is a hollow piston rod that defines an internal bore  302  within which digital valve assembly  300  is located. An inlet passage  304  extends through the lower post portion of piston rod  298  to allow communication between lower working chamber  246  and internal bore  302 . One or more outlet passages  306  extend through piston rod  298  to allow communication between upper working chamber  244  and internal bore  302 . 
         [0065]    Digital valve assembly  300  is a two position valve assembly which has a different flow area in each of the two positions. Digital valve assembly  300  comprises a sleeve  312 , a plurality of spools  144 , a plurality of springs  146 , a plurality of coil assemblies  148  and a circuit board  314 . Sleeve  312  defines a valve inlet  320  which is in communication with lower working chamber  246  through inlet passage  304 ; a valve outlet  322  which is in communication with upper working chamber  244  through outlet passages  306 ; a plurality of annular inlet chambers  324  each of which is in communication valve inlet  320 ; and a pair of annular outlet chamber  326 ,  328  associated with each inlet chamber  324  and each of which is in communication with valve outlet  322 . 
         [0066]    Each spool  144  is slidingly received within sleeve  312  and axially travels within sleeve  312  between a respective coil assembly  148  and a respective stop puck  160  disposed within sleeve  312 . Each spring  146  biases a respective spool  144  away from coil assembly  148  and towards stop puck  160 . A respective shim  162  is disposed between each coil assembly  148  and each spool  144  to control the amount of axial motion for spool  144 . A first O-ring seals the interface between stop puck  160 , sleeve  142  and piston rod  298 . A second O-ring seals the interface between coil assembly  148 , sleeve  142  and circuit board  314 . 
         [0067]    Spool  144  defines first flange  164  which controls fluid flow between a respective annular inlet chamber  324  and a respective annular outlet chamber  326  and second flange  166  that controls fluid flow between the respective annular inlet chamber  324  and a respective annular outlet chamber  328 . Flanges  164  and  166  thus control fluid flow between upper working chamber  244  and lower working chamber  246 . 
         [0068]    Each coil assembly  148  is disposed within sleeve  312  to control the axial movement of a respective spool  144 . The wiring connections for coil assemblies  148  extend to circuit board  314  and then through internal bore  302  of piston rod  298 . Circuit board  314  is disposed in internal bore  302  immediately above sleeve  312 . An O-ring seals the interface between circuit board  314  and piston rod  298 . While circuit board  314  is illustrated as being in internal bore  302 , it is within the scope of the present disclosure to locate circuit board  314  external to shock absorber  220 . When there is no power provided to coil assemblies  148 , the damping characteristics will be defined by the flow area of each digital valve assembly  300  in its first position and piston assembly  232 . The movement of each spool  144  is controlled by supplying power provided to each coil assembly  148  to move the respective digital valve assembly to its second position. Digital valve assemblies  300  can be kept in the second position by continuing to supply power to each coil assembly  148  or by providing means for retaining digital valve assemblies  300  in the second position and discontinuing the supply of power to each coil assembly  148 . The means for retaining each digital valve assembly  300  in its second position can include mechanical means, magnetic means or other means known in the art. Once in its second position, movement to the first position can be accomplished by terminating power to each coil assembly  148  or by reversing the current or reversing the polarity of the power supplied to each coil assembly  148  to overcome the retaining means. The amount of flow through each digital valve assembly  300  has discrete settings for flow control in both the first position and the second position. While the present disclosure is described using multiple digital valve assemblies  300 , it is within the scope of the disclosure to use one digital valve assembly  300 . When multiple digital valve assemblies  300  are used, the total flow area through the plurality of digital valve assemblies  300  can be set at a specific number of total flow areas depending on the position of each individual digital valve assemblies  300 . The specific number of total flow areas can be defined as being 2 n  flow areas where n is the number of digital valve assemblies  300 . For example, if four digital valve assemblies  300 , the number of total flow areas available would be 2 4  or sixteen flow areas. 
         [0069]    The force vs. velocity curve for shock absorber  20  illustrated in  FIG. 7  is applicable to shock absorber  220 . The curves A, B, C and D illustrated in  FIG. 7  are achieved using digital valve assembly  300 . 
         [0070]    Referring now to  FIGS. 11-12 , a rod guide assembly  400  in accordance with the present disclosure is illustrated. Rod guide assembly  400  can be used in place of rod guide assembly  50 . Rod guide assembly  400  comprises a rod guide housing  420 , a seal assembly  422 , and a plurality of digital valve assemblies  426 . 
         [0071]    Rod guide housing  420  is assembled into pressure tube  30  and into reserve tube  36 . Seal assembly  422  is assembled to rod guide housing  420  and reserve tube  36  is rolled or formed over as illustrated at  428  to retain rod guide assembly  400 . One or more bushings  430  assembled into rod guide housing  420  accommodates for the sliding motion of piston rod  34  while also providing for a seal for piston rod  34 . A fluid passage  432  extends through rod guide housing  420  to allow fluid communication between upper working chamber  44  and digital valve assembly  426  as discussed below. A fluid passage  434  extends through rod guide housing  420  to allow fluid communication between digital valve assembly  426  and reservoir chamber  52 . A plurality of seal ports  436  extend through rod guide housing  420  to accommodate the flow of fluid between piston rod  34  and bushings  430 . 
         [0072]    Each digital valve assembly  426  is identical and thus only one digital valve assembly  426  will be described. It is to be understood that the description below applies to all digital valve assemblies used in rod guide assembly  400 . Digital valve assembly  426  is a two position valve assembly which has a different flow area in each of the two positions. Digital valve assembly  426  comprises a sleeve  442 , spool  144 , spring  146  and coil assembly  148 . Sleeve  442  is disposed within a valve port  450  defined by rod guide housing  420 . Sleeve  442  defines an annular inlet chamber  454  which is in communication with fluid passage  432  and a pair of annular outlet chambers  456  and  458  which are in communication with fluid passage  434 . 
         [0073]    Spool  144  is slidingly received within sleeve  442  and axially travels within sleeve  442  between coil assembly  148  and stop puck  160  disposed within sleeve  442 . Spring  146  biases spool  144  away from coil assembly  148  and towards stop puck  160 . Shim  162  is disposed between coil assembly  148  and spool  144  to control the amount of axial motion for spool  144 . A first O-ring seals the interface between stop puck  160  and a retainer  460  secured to sleeve  442 . A second O-ring seals the interface between coil assembly  148  and a retainer  462  secured to sleeve  442 . 
         [0074]    Spool  144  defines first flange  164  which controls fluid flow between annular inlet chamber  454  and annular outlet chamber  456  and second flange  166  that controls fluid flow between annular inlet chamber  454  and annular outlet chamber  458 . Flanges  164  and  166  thus control fluid flow from upper working chamber  44  to reservoir chamber  52 . 
         [0075]    Coil assembly  148  is disposed within sleeve  442  to control the axial movement of spool  144 . The wiring connections for coil assembly  148  can extend through rod guide housing  420 , through sleeve  442  and/or through reserve tube  36 . When there is no power provided to coil assembly  148 , the damping characteristics will be defined by the flow area of digital valve assembly  426  in its first position, piston assembly  32  and base valve assembly  38 . The movement of spool  144  is controlled by supplying power to coil assembly  148  to move digital valve assembly to its second position. Digital valve assembly  426  can be kept in its second position by continuing to supply power to coil assembly  148  or by providing means for retaining digital valve assembly  426  in its second position and discontinuing the supply of power to coil assembly  148 . The means for retaining digital valve assembly  426  in its second position can include mechanical means, magnetic means or other means known in the art. Once in its second position, movement to the first position can be accomplished by terminating power to coil assembly  148  or by reversing the current or reversing the polarity of the power supplied to coil assembly  148  to overcome the retaining means. The amount of flow through digital valve assembly  426  has discrete settings for flow control in both the first position and the second position. While the present disclosure is described using a plurality of digital valve assemblies  426 , it is within the scope of the disclosure to use a single digital valve assembly  426 . Similar to rod guide assembly  50 , digital valve assemblies  426  control damping loads in both extension and compression strokes for shock absorber  20 . When multiple digital valve assemblies  426  are used, the total flow area through the plurality of digital valve assemblies  426  can be set at a specific number of total flow areas depending on the position of each individual digital valve assemblies  426 . The specific number of total flow areas can be defined as being 2 n  flow areas where n is the number of digital valve assemblies  426 . For example, if four digital valve assemblies  426 , the number of total flow areas available would be 2 4  or sixteen flow areas. 
         [0076]    The force vs. velocity curve for shock absorber  20  illustrated in  FIG. 7  is applicable to shock absorber  20  when it incorporates rod guide assembly  400  in place of rod guide assembly  50 . The curves A, B, C and D illustrated in  FIG. 7  are achieved using digital valve assemblies  426 . 
         [0077]    Seal assembly  422  includes a check seal  470  which allows fluid to flow from the interface between piston rod  34  and bushings  430  to reservoir chamber  52  through seal ports  436  and fluid passage  434  but prohibit fluid flow from reservoir chamber  52  or fluid passage  434  through seal ports  436  to the interface between piston rod  34  and bushings  430 . The upper portion of sleeve  442 , above retainer  462  defines a flow passage  472  to allow fluid flow from seal ports  436  to reach fluid passage  434  and thus reservoir chamber  52 . 
         [0078]    Referring now to  FIGS. 13 and 14 , a piston rod assembly  500  in accordance with the present disclosure is illustrated. Piston rod assembly  500  can be used in place of piston rod assembly  234 . Piston rod assembly  500  comprises a piston rod  508  and a plurality of digital valve assemblies  510 . Piston rod  508  is a hollow piston rod that defines an internal bore  512  within which the plurality of digital valve assemblies  510  are located. An inlet passage  514  extends through the lower post portion of piston rod  508  to allow communication between lower working chamber  246  and internal bore  512 . One or more outlet passages  516  extend through piston rod  508  to allow communication between upper working chamber  244  and internal bore  512 . 
         [0079]    As illustrated in  FIG. 13 , the plurality of digital valve assemblies  510  are stacked atop each other within internal bore  512 . Each digital valve assembly  510  is identical and thus, only one digital valve assembly will be described. It is to be understood that the description below applies to all digital valve assemblies  510  used in piston rod assembly  500 . 
         [0080]    Digital valve assembly  510  is a two position valve assembly which has a different flow area in each of the two positions. Digital valve assembly  510  comprises a sleeve  522 , spool  144 , spring  146  and coil assembly  148 . A single circuit board  524  is utilized for the plurality of digital valve assemblies  510 . Sleeve  522  defines a valve inlet  530  which is in communication with lower working chamber  246  through inlet passage  514 ; a valve outlet  532  which is in communication with upper working chamber  244  through outlet passages  516 ; an annular inlet chamber  534  each of which is in communication valve inlet  530 ; and a pair of annular outlet chamber  536 ,  538  associated with inlet chamber  534  and each of which is in communication with valve outlet  532 . 
         [0081]    Each spool  144  is slidingly received within sleeve  522  and axially travels within sleeve  522  between coil assembly  148  and stop puck  160  disposed within sleeve  522 . Spring  146  biases spool  144  away from coil assembly  148  and towards stop puck  160 . Shim  162  is disposed between coil assembly  148  and sleeve  522  to control the amount of axial motion for spool  144 . A first O-ring seals the interface between stop puck  160  and a washer  540  attached to sleeve  522 . A second O-ring seals the interface between coil assembly  148  and a washer  542  attached to sleeve  522 . 
         [0082]    Spool  144  defines first flange  164  which controls fluid flow between annular inlet chamber  534  and annular outlet chamber  536  and second flange  166  that controls fluid flow between annular inlet chamber  534  and annular outlet chamber  538 . Flanges  164  and  166  thus control fluid flow between upper working chamber  244  and lower working chamber  246 . 
         [0083]    Coil assembly  148  is disposed within sleeve  522  to control the axial movement of spool  144 . The wiring connections for coil assembly  148  extend to circuit board  524  and then through internal bore  512  of piston rod  508 . Circuit board  524  is disposed in internal bore  302  immediately above the plurality of digital valve assemblies  510 . An O-ring seals the interface between circuit board  524  and piston rod  508 . While circuit board  524  is illustrated as being in internal bore  512 , it is within the scope of the present disclosure to locate circuit board  524  external to shock absorber  220 . 
         [0084]    When there is no power provided to coil assemblies  148 , the damping characteristics will be defined by the flow area of digital valve assemblies  510  in the first position and piston assembly  232 . The movement of each spool  144  is controlled by supplying power to each coil assembly  148  to move digital valve assemblies  510  to the second position. Digital valve assemblies  510  can be kept in the second position by continuing to supply power to each coil assembly  148  or by providing means for retaining digital valve assemblies  510  in the second position and discontinuing the supply of power to coil assemblies  148 . The means for retaining digital valve assemblies  510  in the second position can include mechanical means, magnetic means or other means known in the art. Once in the second position, movement to the first position can be accomplished by terminating power to each coil assembly  148  or by reversing the current or reversing the polarity of the power supplied to each coil assembly  148  to overcome the retaining means. The amount of flow through each digital valve assembly  510  has discrete settings for flow control in both the first position and the second position. While the present disclosure is described using multiple digital valve assemblies  510 , it is within the scope of the disclosure to use one digital valve assembly  510 . When multiple digital valve assemblies  510  are used, the total flow area through the plurality of digital valve assemblies  510  can be set at a specific number of total flow areas depending on the position of each individual digital valve assemblies  510 . The specific number of total flow areas can be defined as being 2 n  flow areas where n is the number of digital valve assemblies  510 . For example, if four digital valve assemblies  510 , the number of total flow areas available would be 2 4  or sixteen flow areas. 
         [0085]    The force vs. velocity curve for shock absorber  20  illustrated in  FIG. 7  is applicable to shock absorber  220  in cooperation with the plurality of digital valve assemblies  510 . The curves A, B, C and D illustrated in  FIG. 7  are achieved using digital valve assemblies  510 . 
         [0086]    Referring now to  FIGS. 15 and 16 , a shock absorber  620  in accordance with another embodiment of the present disclosure is illustrated. Shock absorber  620  can replace shock absorber  20  or  220 . Shock absorber  620  comprises a pressure tube  630 , piston assembly  32 , piston rod  34 , a reserve tube  636 , a base valve assembly  638 , an intermediate tube  640  and a plurality of digital valve assemblies  642 . While shock absorber  620  is illustrated having a plurality of digital valve assemblies  642 , it is within the scope of the present disclosure to utilize a single digital valve assembly  642 . 
         [0087]    Pressure tube  630  defines a working chamber  644 . Piston assembly  32  is slidably disposed within pressure tube  630  and divides working chamber  644  into an upper working chamber  646  and a lower working chamber  648 . A seal is disposed between piston assembly  32  and pressure tube  630  to permit sliding movement of piston assembly  32  with respect to pressure tube  630  without generating undue frictional forces as well as sealing upper working chamber  646  from lower working chamber  648 . Piston rod  34  is attached to piston assembly  32  and extends through upper working chamber  646  and through an upper rod guide assembly  650  which closes the upper end of pressure tube  630 . A sealing system seals the interface between upper rod guide assembly  650 , reserve tube  636  and piston rod  34 . The end of piston rod  34  opposite to piston assembly  32  is adapted to be secured to the sprung mass of vehicle  10 . Because piston rod  34  extends only through upper working chamber  646  and not lower working chamber  648 , extension and compression movements of piston assembly  32  with respect to pressure tube  630  causes a difference in the amount of fluid displaced in upper working chamber  646  and the amount of fluid displaced in lower working chamber  648 . The difference in the amount of fluid displaced is known as the “rod volume” and during extension movements it flows through base valve assembly  638 . During a compression movement of piston assembly  32  with respect to pressure tube  630 , valving within piston assembly  32  allow fluid flow from lower working chamber  648  to upper working chamber  646  and the “rod volume” of fluid flow flows through digital valve assemblies  642  and/or fluid flow will flow through base valve assembly  638  as described below. 
         [0088]    Reserve tube  636  surrounds pressure tube  630  to define a fluid reservoir chamber  652  located between tubes  640  and  636 . The bottom end of reserve tube  636  is closed by a base cup  654  which, with the lower portion of shock absorber  620 , is adapted to be connected to the unsprung mass of vehicle  10 . The upper end of reserve tube  636  is attached to intermediate tube  640  but it could extend up to upper rod guide assembly  650 . Base valve assembly  638  is disposed between lower working chamber  648  and reservoir chamber  652  to control the flow of fluid from reservoir chamber  652  to lower working chamber  648 . When shock absorber  620  extends in length, an additional volume of fluid is needed in lower working chamber  648  due to the “rod volume” concept. Thus, fluid will flow from reservoir chamber  652  to lower working chamber  648  through base valve assembly  638  as detailed below. When shock absorber  620  compresses in length, an excess of fluid must be removed from lower working chamber  648  due to the “rod volume” concept. Thus, fluid will flow from lower working chamber  648  to reservoir chamber  652  through digital valve assemblies  642  and/or through base valve assembly  438  as detailed below. 
         [0089]    Piston assembly  32  is described above for shock absorber  20  and the description of that embodiment applies here also. 
         [0090]    Base valve assembly  638  is the same as base valve assembly  38  described above except that valve body  92  in base valve assembly  38  is replaced by valve body  692  for base valve assembly  638 . Valve body  692  is the same as valve body  92  in relation to compression valve assembly  94  and rebound valve assembly  96 . Valve body  692  is different from valve body  92  in that valve body  692  defines a plurality of cylinder end ports  694  each of which accepts a respective digital valve assembly  642  as described below. 
         [0091]    Intermediate tube  640  engages upper rod guide assembly  650  on an upper end and it engages valve body  692  at its opposite end. An intermediate chamber  696  is defined between intermediate tube  640  and pressure tube  630 . A passage  698  is formed in upper rod guide assembly  650  for fluidly connecting upper working chamber  646  and intermediate chamber  696 . 
         [0092]    Referring to  FIGS. 16 and 17 , the operation of shock absorber  620  will be described when digital valve assemblies  642  contribute to the damping characteristics for shock absorber  620 . As discussed above, when no power is provided to digital valve assemblies  642 , the damping characteristics are provided by piston assembly  32  during an extension stroke and base valve assembly  638  during a compression stroke. During a rebound or extension stroke, compression valve assembly  62  closes the plurality of compression passages  70  and fluid pressure within upper working chamber  646  increases. Fluid is forced from upper working chamber  646 , through passage  698 , into intermediate chamber  696  to reach digital valve assemblies  642 . 
         [0093]    During a compression stroke, compression valve assembly  62  will open to allow fluid flow from lower working chamber  648  to upper working chamber  646 . Due to the “rod volume” concept, fluid in upper working chamber  646  will flow from upper working chamber  646 , through passage  698 , into intermediate chamber  696  to reach digital valve assemblies  642 . 
         [0094]    The plurality of digital valve assemblies  642  are the same and only one digital valve assembly  642  will be described. It is to be understood that the description below applies to all of digital valve assemblies  642 . Digital valve assembly  642  is a two position valve assembly which has a different flow area in each of the two positions. Digital valve assembly  642  comprises a sleeve  742 , spool  144 , a spring  146  and coil assembly  148 . Sleeve  742  defines a valve inlet  750  which is in communication with intermediate chamber  696  and a valve outlet  752  which is in fluid communication with reservoir chamber  652 . 
         [0095]    Sleeve  742  is disposed within cylinder end port  694  of valve body  692 . Sleeve  742  defines an annular inlet chamber  754  which is in communication with valve inlet  750  and a pair of annular outlet chambers  756  and  758  which are in communication with valve outlet  752 . 
         [0096]    Spool  144  is slidingly received within sleeve  742  and axially travels within sleeve  742  between coil assembly  148  and a stop puck  760  disposed within sleeve  742 . Spring  146  biases spool  144  away from coil assembly  148  and towards stop puck  760 . A shim  762  is disposed between coil assembly  148  and sleeve  742  to control the amount of axial motion for spool  144 . A first O-ring seals the interface between stop puck  760 , sleeve  742  and a first retainer  764  attached to sleeve  742 . A second O-ring seals the interface between coil assembly  148 , sleeve  742  and a second retainer  766  attached to sleeve  742 . 
         [0097]    Spool  144  defines first flange  164  which controls fluid flow between annular inlet chamber  754  and annular outlet chamber  756  and second flange  166  that controls fluid flow between annular inlet chamber  754  and annular outlet chamber  758 . Flanges  164  and  166  thus control fluid flow from intermediate chamber  696  to reservoir chamber  652 . 
         [0098]    Coil assembly  148  is disposed within sleeve  742  to control the axial movement of spool  144 . The wiring connections for coil assembly  148  can extend through valve body  692 , through sleeve  742 , through base cup  654  and/or through reserve tube  636 . When there is no power provided to coil assembly  148 , the damping characteristics will be defined by the flow area of digital valve assembly  642  in its first position, piston assembly  32  and base valve assembly  638 . The movement of spool  144  is controlled by supplying power to coil assembly  148  to move digital valve assembly  642  to its second position. Digital valve assembly  642  can be kept in its second position by continuing to supply power to coil assembly  148  or by providing means for retaining digital valve assembly  642  in its second position and discontinuing the supply of power to coil assembly  148 . The means for retaining digital valve assembly  642  in its second position can include mechanical means, magnetic means or other means known in the art. Once in its second position, movement to the first position can be accomplished by terminating power to coil assembly  148  or by reversing the current or reversing the polarity of the power supplied to coil assembly  148  to overcome the retaining means. The amount of flow through digital valve assembly  642  has discrete settings for flow control in both the first position and the second position. While the present disclosure is described using multiple digital valve assemblies  642 , it is within the scope of the disclosure to use one digital valve assembly  642 . When multiple digital valve assemblies  642  are used, the total flow area through the plurality of digital valve assemblies  642  can be set at a specific number of total flow areas depending on the position of each individual digital valve assemblies  642 . The specific number of total flow areas can be defined as being 2 n  flow areas where n is the number of digital valve assemblies  642 . For example, if four digital valve assemblies  642 , the number of total flow areas available would be 2 4  or sixteen flow areas. 
         [0099]    The force vs. velocity curve for shock absorber  20  illustrated in  FIG. 7  is applicable to shock absorber  620  in cooperation with the plurality of digital valve assemblies  642 . The curves A, B, C and D illustrated in  FIG. 7  are achieved using digital valve assemblies  642 . 
         [0100]    Referring now to  FIGS. 18 and 19 , a base valve assembly  838  in accordance with another embodiment of the present disclosure is illustrated. Base valve assembly  838  is a replacement for base valve assembly  638 . Base valve assembly  838  is the same as base valve assembly  838  except for valve body  692 . Valve body  692  in base valve assembly  638  has been replaced with valve body  844  in base valve assembly  838 . Valve body  844  defines a plurality of cylinder end ports  846  each of which accepts a respective digital valve assembly  642 . The operation and function of base valve assembly  838  is the same as that described above for base valve assembly  638 . 
         [0101]    Referring now to  FIGS. 20 and 21 , a base valve assembly  938  in accordance with another embodiment of the present disclosure is illustrated. Base valve assembly  938  is a replacement for base valve assembly  638 . Base valve assembly  938  is the same as base valve assembly  638  except for valve body  692  and digital valve assembly  642 . Valve body  692  in base valve assembly  638  has been replaced with valve body  944  in base valve assembly  938  and digital valve assembly  642  has been replaced with a digital valve assembly  948 . Valve body  944  defines a plurality of cylinder end ports  946  each of which accepts a respective digital valve assembly  948 . Digital valve assembly  948  is the same as digital valve assembly  642  except that sleeve  742  is replaced by sleeve  950 . Sleeve  950  is the same as sleeve  742  except that valve outlet  752  of sleeve  742  is replaced by valve outlet  952  of sleeve  950 . Valve outlet  752  of sleeve  742  is open along the entire axial length of sleeve  742 . Outlet  952  of sleeve  950  is open only at the bottom surface of sleeve  950 . 
         [0102]    Digital valve assembly  948  is disposed within intermediate chamber  696  as illustrated in  FIG. 20 . Intermediate tube  640  is enlarged as shown at  960  to accommodate digital valve assembly  948 . The operation and function of base valve assembly  938  is the same as that described above for base valve assembly  638 . 
         [0103]    The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the invention, and all such modifications are intended to be included within the scope of the invention.