Abstract:
A vehicular damper includes a cylinder, a piston rod in the cylinder having a hollow end defining an axial rod passage and a cylindrical piston affixed to the end of the rod dividing the cylinder into a compression chamber and a rebound chamber. A valve plate on the rod adjacent the piston has a plurality of soft channels extending from an outer end at the rebound chamber to an inner end at the axial rod passage and a solenoid actuated cylinder in the rod passage is movable to open and close the inner ends of the soft channels. A bi-directional working disc contacts with the soft channels&#39; outer ends to provide damping in rebound and compression when the solenoid valve is open. In contrast, the piston has a plurality of firm rebound channels and a like plurality of firm compression channels extending therethrough with a uni-directional rebound working disc in valve contact with the outlet of the rebound chambers and a uni-directional compression working disc in valve contact with the outlet of the compression chambers. The damper provides valved parallel flow between compression and rebound chambers through the soft channels and through the firm channels with independent spring rate control through the firm rebound and compression channels.

Description:
TECHNICAL FIELD 
     This invention relates generally to fluid dampers for vehicles and more particularly, to dampers of the type which are known as “active”. 
     The invention is specifically applicable to and will be described with particular reference to a mono-tube shock absorber or strut which is electrically controlled to function with two different damping characteristics. However, those skilled in the art will recognize that the invention is also applicable to twin tube shock absorbers and struts and, in theory, may be applicable to dampers having variable damping rates. 
     BACKGROUND OF THE INVENTION 
     The typical fluid dampers used in vehicle suspensions, such as hydraulic shock absorbers and struts, filter out road inputs from being transferred to the vehicle&#39;s body and associated passenger compartment by dissipating energy. Two common types of vehicle fluid dampers, each having a cylinder and piston, are mono-tube and twin tube shock absorbers. The preferred embodiment of this invention is directed to mono-tube shock absorbers and struts. 
     As is well known, a mono-tube shock absorber essentially comprises a cylinder or tube filled with hydraulic fluid and into which extends a piston rod having a piston fixed to its end. Generally, the upper end of the piston rod extending out of the tube is adapted for connection to the sprung mass (body) of a motor vehicle and the lower end of the tube or cylinder is connected to the unsprung mass (wheel assembly) of the vehicle. Relative movement of the sprung and unsprung masses of the vehicle produces relative axial movement of the piston which is in sealing sliding engagement with the tube walls and divides the tube into two chambers, conventionally referred to as a rebound chamber on one side of the piston and a compression chamber on the opposite side. 
     Relative movement of the piston within the cylinder is provided by valving that controls fluid flow from the pressurized chamber past or through the piston to the unpressurized chamber. Two of the more common passive types of valves used in fluid dampers are deflected disc type valves (digressive valves) and blow-off type valves. With a deflected disc valve, a disc stack is positioned as an obstruction in a fluid flow path. During piston movement, once sufficient pressure is developed, the disc stack is deflected to provide an increased flow area. The extent to which the disc stack resists deflection principally determines the damping characteristics of the fluid damper. In a blow-off valve, a single valve disc is generally biased by a spring to normally close-off a fluid flow passage. Sufficient fluid pressure causes the valve to lift, compressing the spring and providing an increased fluid flow area. Different rate springs and preloads allow the valve to blow-off at different pressures thereby regulating damping loads. This invention relates to disc-type valves. 
     The ride handling characteristics of a damper for a motor vehicle (load vs velocity performance curve) is determined by the rebound and compression characteristics of the piston valve in a passive mono-tube application. However, it is desirable to have at least a two stage damper for both rebound and compression. For example, during vehicle cornering maneuvers in which the piston undergoes low speed compression, it is desirable for the vehicle to have stiff or “firm” ride handling characteristics. Conversely, when the vehicle travels over pot holes at relatively high vehicle speeds in which the piston undergoes high speed compression, it is desirable to have “soft” ride handling characteristics. Different vehicles require different handling characteristics. Conventional mono-tube shock absorbers with passive valves can only affect a compromise. 
     The prior art has long recognized this problem and has developed designs in which the valve orifice, which controls the damping forces, is electrically adjustable. Conceptually, sensors determine the operating condition of the vehicle and algorithms determine a desired orifice size based on the operating conditions. Electronics generate an orifice size signal inputted to electrical apparatus which mechanically adjusts the orifice size. While conceptually sound, there are problems in the commercial implementation of this concept. 
     The prior art has recognized such problems and has developed a solenoid actuated, shuttle type, shut-off valve. More particularly, an economical solenoid can be designed to fit into a piston rod and develop sufficient force to move a spring biased shuttle valve from an unenergized position to an energized position. A mono-tube piston can be equipped with two passive valves, each controlling rebound and compression, with one of the two valves selectively cut in or out of operation by the solenoid shuttle valve. For example, both valves operate to provide the soft suspension for highway cruising while only one valve operates to provide the stiff compression for cornering. The solenoid is actuated automatically by sensors sensing or predicting the operating conditions of the vehicle. Additionally, the vehicular operator can be provided with a manual override control that forces the solenoid into an energized or de-energized condition. This invention is applicable to this type of active damper and uses a solenoid to selectively cut in and out a passive valve. 
     Two-stage, solenoid operated active dampers are described in detail in U.S. Pat. No. 5,690,195 to Kruckemeyer et al., issued Nov. 25, 1997 and U.S. Pat. No. 5,706,919 to Kruckemeyer et al., issued Jan. 13, 1998. The &#39;195 patent illustrates an arrangement where the hydraulic fluid passes in parallel to different valves and the &#39;919 patent illustrates an arrangement where the hydraulic fluid passes serially through the two valves. This invention is an improvement over the &#39;919 and &#39;195 patents, specifically, the &#39;195 parallel flow patent. The &#39;195 patent is incorporated by reference herein, specifically for its disclosure of the solenoid, the solenoid actuated shuttle valve and the digressive disc stack valve working in conjunction with the solenoid which are substantially the same as that disclosed herein. 
     The &#39;195 patent uses a bi-directional digressive disc stack valve for the passive valve which is always on and typically provides the firm ride handling characteristics of the vehicle. As is well known, bi-directional, digressive disc stack valves cannot provide independently set flow rates for both rebound and compression. Because this valve is normally used to provide the firm handling characteristics of the vehicle, it is highly desirable for the manufacturers, especially those manufacturing “performance” vehicles, to be able to independently set or tune the rebound and compression spring rates of this valve. This is not possible in the &#39;195 patent. 
     A more subtle point is that a valve for the firm mode requires relatively high, unimpeded flow rates through the piston. That is, a valve always provides the flow restriction in a passage, i.e., the orifice. However, the passage upstream or downstream of the orifice affects flow through the orifice, i.e., a backpressure at certain flow rates can affect flow through the orifices. Again, the function of the firm valve is to assure high flow rates. Because bi-directional disc stack valves require flow in one direction to unseat the outer edge of the disc and opposite flow to unseat the inner edge of the disc, a serpentine flow path to the orifice often occurs. Such a flow path could adversely affect performance of the valve at certain conditions, i.e., high flow producing turbulence. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an important aspect of the invention to provide an improved, parallel flow active damper, especially suited for mono-tube applications, in which at least one of the parallel valves can independently control the flow rate through the damper for both rebound and compression damper modes. 
     In accordance with one aspect of the invention, a vehicular damper is provided which includes a cylinder and a piston rod in the cylinder having a hollow end defining an axial rod passage and the rod or cylinder moves relative to the other. A cylindrical piston is affixed to the end of the rod and divides the cylinder into a compression chamber and a rebound chamber. The piston has a plurality of firm rebound channels and a plurality of firm compression channels extending therethrough. A uni-directional rebound working disc is in valved contact with the outlet of the rebound channels and a uni-directional compression working disc is in valved contact with the outlet of the compression channels. A valve plate is provided on the rod adjacent the piston and has soft channels extending from an outer end at the rebound chamber to an inner end at the axial rod passage. A solenoid actuated cylinder is movable in the axial rod passage to open and close the inner ends of the soft channels. A bi-directional working disc is provided in valved contact with the soft channels&#39; outer ends whereby valved parallel flow through the valve plate and piston occurs when the solenoid actuated cylinder opens the soft channels inner ends while the uni-directional discs independently control flow at all times through the firm rebound and compression channels. 
     In accordance with another aspect of the invention, the piston has a rebound face surface on one side and a compression face surface on the opposite side with a cylindrical rod opening extending therethrough. The rebound face surface has an annular compression valve seat extending therefrom and the compression face surface has an annular rebound valve seat extending therefrom. The rebound disc stack digressive valve includes at least an annular rebound uni-directional working disc and an annular rebound spacer disc with the rebound working disc having an outside diameter greater than the diameter of the rebound valve seat. Similarly, the compression disc stack digressive valve includes at least an annular compression uni-directional working disc and an annular compression spacer disc with the annular compression working disc having an outside diameter greater than the diameter of the compression valve seat. All the discs have substantially circular inside and outside diameters whereby the discs can be mounted at any angular orientation relative to the piston rod for simple assembly. In the preferred embodiment, the digressive valve disc stacks include rebound and compression orifice discs having annular bleed slots extending radially inward from the peripheral edge of the orifice disc. In addition, the disc stacks include annular preload/adjust rebound and compression discs. The orifice disc is between the working disc and valve seat and the preload/adjust disc is between the orifice disc and valve seat. The orifice disc and preload/adjust disc as well as any additional working disc and spacer disc have substantially circular outside and inside diameters void of any aligning notches or protrusions. 
     In accordance with another aspect of the invention, the piston is an assembly of first and second substantially identical, sintered metal cylindrical halves with each half having on one side a half face surface which is either the piston&#39;s rebound or compression face surface and an interior match face surface on its opposite side with the match surfaces in mating contact with one another to form the piston. Each half has i) on its half face surface, an annular valve seat protruding therefrom which is one of the firm rebound or compression valve seats, ii) a plurality of circumferentially spaced outer channels having outer end openings in the face surface spaced radially outward from the annular valve seat with each outer channel axially tapering in an “L” shaped configuration from the outer end face opening to a trapezoidal inner end opening in the match surface (the inner end opening being substantially larger than the outer end opening and radially extending from a position adjacent the rod opening to a position beyond the diameter of the annular valve seat) and iii) a plurality of circumferentially spaced inner channels having face end openings adjacent the half face surface spaced radially inward from the annular valve seat with each inner channel axially tapering in an “L” shaped configuration to a trapezoidal match end opening in the match surface. The inner channel match end opening is substantially larger than the inner channel face end opening and (like the outer channel&#39;s inner end opening) radially extends from a position adjacent the rod opening to a position beyond the diameter of the annular valve seat whereby the inner channels of one half mate with the outer channels of the opposite half to form one of the rebound and compression channels while the outer channels of the one half mate with the inner channels of the opposite half to form the other one of the rebound and compression channels. Importantly, the rebound and compression channels have increasing cross-sectional flow areas from the end openings of each channel minimizing any tendency towards turbulent flow or excessive backpressures at high flow rates through the piston. At the same time, the inner and outer channels can be easily formed as straight through passages when the metal (powder metal in the preferred embodiment) is sintered into final half piston form, a material known to those skilled in the art as not conducive to machining operations. 
     Another aspect of the invention is that an improved damper results with less cost than otherwise required to produce the damper. Specifically, the firm digressive rebound and compression disc stacks allow, as noted, independent control of damper flow in rebound and compression modes to improve damper performance while the rebound and compression disc stacks are free of any aligning notches or protuberances making assembly easier and reducing the time thereof. Additionally, the configuration of the firm rebound and compression channels is relatively easy to form in a sintered part (reducing the cost thereof while enhancing the characteristics of the fluid flow through the channel thereby improving valve performance. 
     These and other objects, features and advantages of the present invention will become apparent from the following Detailed Description of the Invention taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take physical form in certain parts and arrangement of parts, a preferred embodiment of which will be described in detail herein and illustrated in the accompanying drawings which form a part hereof and wherein: 
     FIG. 1 is a partial, longitudinally extending, cross-sectioned view of a mono-tube fluid vehicle damper according to the present invention; 
     FIG. 2 is an exploded view of the damper valving shown in FIG. 1; 
     FIG. 3 is a pictorial view of the interior surface of a piston half; 
     FIG. 4 is a pictorial view of the outside or face surface of a piston half; and, 
     FIG. 5 is a view identical to FIG. 1 but marked to show fluid flow paths through the damper. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings wherein the showings are for the purpose of illustrating a preferred embodiment of the invention only and not for the purpose of limiting the same, there is shown in cross sectional view in FIG. 1 the valving portion of a hydraulic damper for a vehicle suspension, preferably a mono tube suspension damper embodied as a mono tube shock absorber  10 . Shock absorber  10  includes a single cylinder or tube  11  which has a closed lower end (not shown) and an upper end closed by a conventional rod guide (also not illustrated). Within tube  11  is a piston rod  12 , the upper end of which (not shown) extends in a sliding, sealing manner through the rod guide in a conventional manner. The bottom end of piston rod  12  contains a valve assembly comprising several valves which will be discussed below and the valve assembly is held in a tight assembled manner by piston nut  13  at the end of piston rod  12 . 
     As is conventional (and thus not shown or described in detail), the upper end of piston rod  12  is adapted to be connected to the sprung mass (body) of a motor vehicle in a conventional manner. Similar means of attachment is provided at the lower end of tube  11  for attachment to the unsprung mass (wheel assembly) of the vehicle, also in a conventional manner. Relative movement between the sprung and unsprung masses of the vehicle, to which shock absorber  10  is connected, produces relative axial sliding movement of the piston valve assembly within tube  11 . 
     The valve assembly includes a cylindrical piston  15  which in the preferred embodiment is an assembly comprised of two identical piston halves  15 A,  15 B. About the circumference of piston  15  is a seal  16  which maintains sealing contact with the inside of tube  11  as piston  15  slides relative to tube  11 . Seal  16  divides the interior of tube  11  into a compression chamber  17  on one side of seal  16  and a rebound chamber  18  on the opposite side of seal  16 . 
     The piston valve assembly, in addition to piston  15 , includes a valve plate  20  which abuts, at one end, a flux plate  21 . 
     Flux plate  21  has a bottom tubular portion  22  defining an open ended axial rod passage  23 . Tubular portion  22  extends from an annular flux base  25  which in turn is crimped in a sealed manner to a solenoid cup  26 . Solenoid cup  26  in turn is fixed in an immovable, sealed manner to a solenoid nut  27  which in turn threadingly engages the actual threaded end of piston rod  12  in a sealed manner i.e. O-rings. Tubular portion  22  of flux plate  21  can thus be viewed as an extension of the end of piston rod  12  and provides a hollow or axial rod passage  23 , which as will be explained below, provides a fluid communication path between compression and rebound chambers  17 ,  18 . For purposes of this invention axial rod passage  23  is to be viewed as either integrally formed within the end of piston rod  12  or, as shown in the preferred embodiment of FIG. 1, as an extension of piston rod  12  which extension is then the end of piston rod  12 . Within rod passage  23  is a shuttle or solenoid cylinder valve  29 . 
     Solenoid valve  29  is axially movable in rod passage  23  between a first position when solenoid coil  30  is not energized and a second position when solenoid coil  30  is energized. As will be explained below, when solenoid valve is at one of its positions fluid flow within rod passage  23  can occur (i.e., the preferred embodiment) and when solenoid valve  29  is at its other position, fluid flow cannot occur. In accordance with the invention the closed position of solenoid valve  29  can occur when solenoid coil  30  is activated or alternatively, the closed position can occur when solenoid coil  30  is not activated. An on/off solenoid actuated valve arrangement is used because, among other reasons, high pressures and fluid viscosity considerations require the solenoid to develop a significant force to move solenoid valve  29 . Cost and size considerations in view of the current state of the solenoid art preclude commercial utilization of coils that can develop precise, progressive flux patterns that can variably position solenoid valve  29  in rod passage  23 . If electrical coils, which can precisely position solenoid valve  29 , become commercially available, the present invention would still be practiced and would not be obsolete because of its accurate control for firm damping. 
     Reference to Kruckemeyer et al., U.S. Pat. No. 5,690,195 can be had for a more detailed explanation of the operation of the solenoid than that presented herein. The solenoid arrangement illustrated in FIG. 1 is conceptually identical to that disclosed in the &#39;195 patent. Generally, an insulated electrical lead  32  extends from outside shock absorber  10  through piston rod  12  and communicates with a contact  33  carried to coil  30  which has coil turns wound on a bobbin  34 . The magnetic circuit includes a pole piece  35  and an air gap  36  between pole piece  35  and solenoid valve  29 . Magnetic flux from coil  30  passes between pole piece  35  and solenoid valve  29  and the end of the solenoid valve is conical to establish a gradient reduction in axial magnetic flux as solenoid valve  29  moves away from pole piece  35 . The on/off signal transmitted by electrical lead  32  can be generated either by an electronic control in a conventionally known manner or in response to manual selection by the operator as noted in the Background. Again, this aspect of the invention is conventional. 
     Tubular portion  22  of flux plate  21  has a hub section  40  adjacent annular flux space  25  and hub section  40  has a plurality of radial passages  41  circumferentially spaced thereabout as best shown in FIG.  2 . Cylindrical valve plate  20  which in an assembled condition abuts annular flux base  25  has a plurality of soft channels  42  extending there through which in number equal radial passages  41 . Each soft channel has a first end opening confronting or opening to rebound chamber  18  and a second end opening  44  which is adjacent to a corresponding radial passage  41 . To assure alignment of soft channels  42  with radial passages  41 , a spline  45  is provided on flux plate tubular portion  22  and spline  45  fits within one of several circumferentially spaced spline recesses  46  provided in the hub section opening of valve plate  20  as best shown in FIG.  2 . 
     On the side of valve plate  20  which faces rebound chamber  18  and adjacent first opening of soft channels  42  is a circular soft valve seat  48  and adjacent the central opening in valve plate  20  is a cylindrical hub seat  49 . A soft digressive valve disc stack  50  seats against soft valve seat  48  and followed by a similarly sized centering disc  52 . After centering disc  52  is hub seat  49  to provide control valving of fluid flow through soft channels  42  either from rebound chamber  18  to compression chamber  17  or vice versa. In the preferred embodiment soft valve stack  50  as best shown in FIG. 2 includes a preload/adjust disc  51 , indirectly against a mono tube working disc  53  in turn followed by an orifice disc  54  which in turn is followed by a clamp spacer disc  55  which abuts a retainer disc  56 . Valve plate  20  is tooled or machined so that soft valve seat  48  extends axially beyond hub seat  49  (or hub seat  49  is recessed relative to soft valve seat  48 ) by a prescribed distance (preload amount) and preload/adjust disc  51  is provided to change or eliminate the amount of offset i.e. preload. It is to be appreciated that when soft valve stack  50  is assembled the discs abut each other from hub seat  49  so that the outer portion of mono tube working disc  53  deflects relative to its inner portion by a preset distance, i.e., the axial recess. The preload distance thus changes the opening pressure of mono tube working disc  53  and acts like a compressed coil spring so that the higher the preload, the higher the pressure has to be for the disc to open. The preload can be changed by changing the thickness and/or the number of preload/adjust discs and/or the diameter of the preload/adjust disc. It should be recognized that such changes also effects the rate of soft valve stack  50  and therefore effects the performance of soft valve stack  50  after the stack has been opened. As best shown in FIG. 2 preload/adjust disc  51  and centering disc  52  have circumferentially spaced centering tangs radially extending from the outer periphery of preload/adjust and centering disc  51 ,  52 . Mono tube working disc  53  has an O.D. (outside diameter) which extends beyond outside soft valve seat  48  and an I.D. (inner diameter) which is slightly larger than the outside diameter of tangs  58  on centering disc  52 . Orifice disc  54  has bleed channels  59  at its outer periphery which extend radially inward a distance equal to or greater than the I.D. of mono tube working disc  53 . The surface of retainer disc  56  facing mono tube working disc  53  functions as a stop limiting deflection of mono tube working disc  53 . Soft digressive valve stack  50  operates in a conventional manner. When the shock absorber moves in compression the outside diameter of mono tube working disc  53  deflects downwardly when viewed in FIG.  1 . When the shock absorber moves in rebound the inside diameter of mono tube working disc  53  deflects upwardly from orifice disc  54  and clamp disc  55  when viewed in FIG.  1 . Soft digressive valve stack  50  is conventional and reference to the &#39;195 patent can be had for additional description. Note that all the discs in soft valve disc stack  50 , with the exception of mono tube working disc  53 , have splined recesses for receiving spline  45  and fixing the angular position of the disc. 
     Piston  15  is a cylinder having at one side a rebound face surface  60  defining a portion of a compression chamber  17  and on its opposite side a compression face surface  61  defining a portion of rebound chamber  18 . A firm circular compression valve seat  63  is formed in compression face surface  61  and a similar firm circular rebound valve seat  64  is formed in rebound face surface  60 . A plurality of circumferentially spaced rebound channels  66  extend through piston  15  and a like plurality of circumferentially spaced compression channels  67  likewise extend through piston  15  with compression channels  67  alternately spaced between rebound channels  66 . Each compression channel  67  has an end opening adjacent compression chamber  17  and an opposite end opening adjacent compression valve seat  63 . Similarly, each recess channel  66  has an end opening adjacent rebound chamber  18  and an opposite end opening adjacent rebound valve seat  64 . 
     A firm digressive compression disc valve stack  70  is in valve contact with firm compression seat  63 . In the preferred embodiment and as best shown in FIG. 2 firm digression compression valve stack  70  includes a compression preload/adjust disc  71  seated against compression face surface  61  followed by a compression orifice disc  72  which in turn is followed by compression working disc  73  followed by compression spacer disc  74  which abuts disc retainer  56 . Compression preload/adjust disc  71  is sized and functions in the manner stated above. Compression working disc  73  has an outside diameter extending beyond the diameter of compression valve seat  63 . Compression orifice disc  72  has approximately the same outside diameter as the O.D. of compression working disc  73  and includes bleed channels  76  which extend radially inward from the periphery of compression orifice disc  72  to a distance inside compression valve seat  63 . When the shock absorber is in compression, compression working disc  73  and compression orifice disc  72  deflect upwardly off compression valve seat  63  when viewed in FIG.  1 . Firm compression disc stack  70  does not deflect when shock absorber  10  is in rebound. 
     Adjacent rebound face surface  60  in contact with rebound valve seat  64  is a firm digressive rebound disc stack  80 , best shown in FIG.  2 . Rebound stack  80  includes a rebound preload/adjust disc  81  in contact with the hub portion of piston  15 . In contact with rebound preload/adjust disc  81  is a larger rebound orifice disc  82  and in contact with rebound orifice disc  82  is a similarly sized rebound working disc  83 . There are additional rebound working discs designated by reference numerals  84 ,  85  and  86 . The additional working discs have varying O.D.s and thicknesses to establish a preset spring rate. Adjacent last additional working disc  86  is a rebound clamp/spacer disc  87  which in turn abuts a rebound retainer/stop disc  88  which in turn is held by piston rod nut  13 . Rebound orifice disc  82  has at least one bleed channel  89  extending radially inward from its peripheral edge to a position inward of rebound valve seat  64 . When shock absorber  10  is in rebound, rebound orifice disc  82  and rebound working discs  83 - 86  deflect downward when viewed in FIG.  1 . 
     It is appreciated that when shock absorber  10  is assembled valve plate  20 , soft valve disc stack  50 , firm compression disc stack  70 , piston  15  and firm rebound stack  80  are assembled onto the axial end of piston rod  12 , shown in the preferred embodiment as tubular portion  22 , and held in place as an assembly by piston rod nut  13 . The number, size (O.D. and I.D.) and thickness of the working disc and preload/adjust disc as well as the size of the bleed passages in the orifice disc can and will vary depending on the specific performance characteristics desired by the end customer for the shock absorber. 
     It is well known that the piston of a damper is to be constructed of a material that undergoes minimal thermal expansion and contraction. The damper essentially operates by dissipating the energy absorbed from road vibrations by heat generated when the hydraulic fluid in the damper is transferred between compression and rebound chambers  17  and  18  vis-a-vis the valve stacks. Additionally, the wide operating temperature ranges that an automotive vehicle is exposed to significantly affects the viscosity of the hydraulic fluid in turn subjecting the piston and its internal passages to varying pressures and forces. Accordingly, from metallurgical considerations, a tough, hard material exhibiting minimal deflection and minimal thermal expansion and contraction is required. It has been found that powder metals formed into a piston configuration such as by a press and sintered in a furnace provide a good material for the piston. When used herein the expression “sintered metal” piston includes not only a piston formed of powder metals but also metal composites composed of filings or metal particles fused or sintered together into a desired shape. Further “sintered metal” can include such metal compositions that also have minor percentages of non-metallic rigidizing materials such as carbon or graphite fibers or even silicon with carbon or graphite. 
     Because of the difficulty in machining sintered metal and because the piston is essentially stamped in a die, it has been known to make the piston into two axial halves. The halves can then be pressed together and sintered into a unitary piston or the halves can be individually sintered and glued together or the halves can simply be assembled like a disc stack and held in place as a piston by piston nut  13 . This invention uses a sintered metal piston formed of two axially extending halves shown as  15 A and  15 B in FIG.  1 . 
     This invention uses two identical piston halves  15 A,  15 B assembled in any manner as described above to form a piston assembly  15 , the outer periphery of which receives seal  16 . Because the piston halves are identical, only one piston half will be described and new reference numerals will be used in describing the configuration and shape of a piston half  90 . 
     Each piston half  90  has an interior or match face surface shown by reference numeral  91  in FIG. 3 which is a pictorial view of match face surface  91 . Each piston also has an exterior face surface designated by reference numeral  92  and best shown by the pictorial representation of this surface in FIG.  4 . Exterior face surface  92  is either rebound face surface  60  or compression face surface  61  of piston  15  depending on which side a specific piston half  90  is positioned when assembled onto piston rod  12 . Extending axially from exterior face surface  92  is an annular valve seat  93  radially spaced outward from a central hub  94  having a central hub seat  95  which in turn is axially spaced inward from annular valve seat  93 . As best shown in FIG. 3 a plurality of ribs  96  extending radially outward from hub  94  form a plurality of circumferentially spaced channels described above as rebound and compression channels  66 ,  67  respectively. For describing the piston halves (which are identical) the channels will be referred to as outer and inner channels. 
     Each outer channel extends from an outer surface end opening  97  in exterior face surface  92  which is spaced radially outwardly from valve seat  93 . In the preferred embodiment there are six outer channels having outer end openings designated  97 A,  97 B,  97 C through  97 F. Outer end openings  97  taper radially inwardly between adjacent ribs  96  to an inner end opening  98  which is trapezoidal in configuration as best shown in FIG.  3 . The outer channels in cross section can be viewed as being “L” shaped and an inverted “L” is drawn in FIG. 3 for the outer channel extending between openings  97 D and  98 D. With respect to the sizing of the channel and the channel openings (for both inner and outer channels) and while generally speaking patent drawings are schematic depictions of the invention, the cross-sectioned valving illustrated in FIG. 1 is generally proportionally correct and generally proportionately to scale. FIG. 1 shows that the channel area in the radial direction i.e. the direction of the “L” shaped configuration, dramatically increases from outer end opening  97  to inner end opening  98  and it is to be recognized that when the two halves are assembled together the inner end opening  98  represents the center of the outer channel. For dimensional considerations, the cross-sectional area of inner end opening  98  is at least twice as large as the cross-sectional area of outer end opening  97 . 
     Inner channels commencing radially inward of valve seat  93  are also formed in each piston half  90 . Each inner channel has a face end opening  99  in exterior face surface  92  adjacent to and extending radially inwardly from valve seat  93  and commencing at the base of valve seat  93 . In the preferred embodiment the inner channels are six in number and alternate between the outer channels so that there are six face end openings designated  99 A,  99 B,  99 C through  99 F as shown in FIG.  3 . The inner channels expand radially outward to form match end openings  100  which are trapezoidal in configuration and in fact identical to inner end openings  98  of the outside channels. As with the outer channels, the cross sectional configuration of the inner channels can be viewed as being “L” shaped with an “L” being drawn for a channel having face end opening  99 C and match end opening  100 C in FIG.  3 . When the piston halves are assembled by mating match face surfaces  91  of one piston half with the other, the inner channels of one piston half mate with the outer channels of the opposite piston half to form rebound and compression channels  66 ,  67  respectively. Specifically, an inside end opening  98  of an inner channel matches with a match end opening  100  of an outer channel to form either a rebound or a compression channel. Importantly, the configuration of each rebound and compression channel is such that from each end opening of each channel the cross sectional area increases in a radially tapering direction to a maximum cross sectional area at the axial center of each channel that is significantly larger than the cross sectional areas of the end openings of each rebound and compression channel. 
     Test results have indicated that the rebound and compression channel configurations with their associated disc stacks have exhibited extremely good response that correlates well with the design rates at all flows. While not wishing to be necessarily bound by any specific theory that accounts for this performance, it is believed that the firm channel configurations minimize or tend to minimize turbulent flow through the channel and minimize back pressure through the rebound and compression channels. It is believed such variables can exist in other flow passage designs present in conventional pistons which otherwise provide a more torturous or serpentine path. The effect of opening up the channels to provide a free flow path minimizing turbulent flow and back pressure reduces a variable in the design of the shock absorber which is otherwise difficult to account for. That is, fluid flow through piston  15  is now principally accounted for by the spring rates established by the firm compression disc stack  70  and the firm rebound disc stack  80 , especially at high flow conditions. 
     Referring now to FIG. 5 there is shown the fluid flow paths through the disc stack valves in rebound and compression. When shock absorber  10  is in a compression mode, piston  15  moves (relative to tube  11 ) towards the right as viewed in FIG. 5 towards compression chamber  17  and fluid flows through the valve stacks from compression chamber  17  into rebound chamber  18 . Provided that solenoid cylinder valve  29  is in its open position as shown in FIG. 5, fluid will flow from compression chamber  17  through axial rod passage  23 , past radial passages  41  and into soft channels  42 . When the pressure of the fluid in soft channels  42  exceeds a preset amount soft disc stack  50  will deflect from soft valve seat  48  into rebound chamber  18  and the fluid flow path will be shown by the dashed arrow line designated by reference numeral  110 . At the same time or in parallel with soft channel flow  110 , fluid in compression channels  67  will pass through the compression valve seat  63  by deflecting compression disc stack  70  when the pressure exceeds a set amount and the path will follow that shown by the solid arrow in FIG. 5 designated by reference numeral  113 . When solenoid cylindrical valve  29  is in a closed position fluid can only flow in the path indicated by compression flow arrow  113 . 
     When shock absorber  10  is in a rebound mode, piston  15  moves (relative to tube  11 ) towards the left in FIG.  5  and fluid must flow from rebound chamber  18  through the valving arrangement into compression chamber  17 . With solenoid cylinder valve  29  in the open position as shown in FIG. 5, fluid flows into soft channels  42  by causing deflection of the inner portion of mono tube working disc  53  (to the left as viewed in FIG. 5) and from there fluid travels through axial rod passage  23  into compression chamber  17  along the path shown by dashed line arrow designated by reference numeral  112 . At the same time or in parallel, fluid enters rebound channel  66  and passes past rebound valve seat  64  when the pressure in rebound channel  66  is great enough to cause a deflection of firm rebound disc stack  80  towards the right when viewed in FIG.  5  and the rebound flow path will follow that shown by the solid arrow designated by reference numeral  115 . When solenoid cylindrical valve  29  is in a closed position fluid can only flow in the path indicated by rebound flow arrow  115 . Note that both rebound and compression flow paths  115 ,  113  have only one undulation of a sine wave. The typical “S” shape is not present. 
     It should be apparent that mono tube working disc in the soft disc stack  50  is controlling both the compression flow when it deflects at its O.D. and rebound when it deflects at its I.D. Thus the spring rates for compression and rebound through soft disc stack  50  are dependent on one another. They cannot be set independently. By providing separate disc stacks  70  and  80  in piston  15  the rebound and compression rates can be individually tuned because each disc stack operates independently of the other. 
     In the preferred embodiment and with solenoid cylinder valve  29  in its open position, soft disc stack  50  is set to deflect with less pressure than that required for firm compression disc stack  70  and firm rebound disc stack  80 . Thus normal undulations in the road surface are damped through soft disc stack whereas rapid changes in the road surface or hard cornering result in relative movement of piston activating firm compression and rebound disc stacks. In this condition the rapid flow of fluid through all the channels occurs and the open channels provided in piston  15  produce a more responsive valve than other parallel flow arrangements. 
     Preferably the firm valving is through the piston vis-a-vis the rebound and compression channels described and the soft valving is through the center of the piston rod. However, these functions can be reversed so that soft valving is through the rebound and compression channels and firm valving is occurring through the axial rod passage. Also preferably the invention is used for a mono tube shock absorber or a strut arrangement. However, the invention can be used in a twin tube arrangement providing additional ranges of control for a twin tube damper. Further, the solenoid coil  30  is described as a two position, on-off arrangement. However, the arrangement will function with its firm mode advantage if solenoid coil  30  is a multi-position solenoid. 
     The invention has been described with reference to a preferred embodiment. Obviously alterations and modifications will occur to those skilled in the art upon reading and understanding the detailed description of the invention. It is intended to cover all such modifications and alterations insofar as they come within the scope of the invention.