Abstract:
In a magneto-rheological variable damper ( 6 ), the piston ( 16 ) comprises an inner yoke ( 32 ), an outer yoke ( 31 ) concentrically surrounding the inner yoke and held in position by pair of end plates ( 33, 34 ) made of non-magnetic material and disposed on either axial end surfaces of the piston. The piston may be circumferentially surrounded by a piston cover ( 30 ) that may be made of non-magnetic material and serves a slide member that engages the inner circumferential surface of a cylinder ( 12 ). Alternatively, the end plates may serve as the slide member. In either case, the durability of the damper can be improved by using a wear resistant material for the slide member. In particular, the outer yoke may be made of material such as Permendur which has a high saturation magnetic flux density but a poor mechanical property. Thereby, the range of the damping force can be expanded. The slide member may be made of non-magnetic material having a favorable wear resistance such as austenite stainless steel and aluminum alloy. It is also possible to apply a plating or other surface processing to the sliding surface of the slide member to improve the wear resistance thereof.

Description:
CROSS REFERENCE TO COPENDING APPLICATION(S) 
     U.S. patent application Ser. No. 11/954,292 filed Dec. 12, 2007 
     TECHNICAL FIELD 
     The present invention relates to a variable shock absorber or damper that can produce a variable damping force for use in an automotive wheel suspension system, and in particular to a variable damper that is highly durable in use and can vary the damping force over a wide range. 
     BACKGROUND OF THE INVENTION 
     Various forms of variable dampers have been proposed for use in wheel suspension systems for the purposes of improving the ride quality and achieving a motion stability of the vehicle. In a common conventional variable damper, a rotary valve is incorporated in the piston for varying an effective area of an orifice that communicates the two chambers on either side of the piston with each other, and such a rotary valve is typically actuated mechanically by using a suitable actuator. More recently, it has become more common to use magneto-rheological fluid for the actuating fluid of the damper, and control the viscosity of the fluid by supplying corresponding electric current to a magnetic coil which is incorporated in the piston. According to such an arrangement, the overall structure can be simplified, and the response property of the damper can be improved. See U.S. Pat. No. 6,260,675, for instance. 
     The piston of the damper disclosed in U.S. Pat. No. 6,260,675 comprises a cylindrical inner yoke, a coil wound around the outer periphery of the inner yoke, a pair of end plates placed on either axial end of the inner yoke, and a cylindrical outer yoke coaxially surrounding the inner yoke and end plates. The inner yoke and outer yoke are both made of magnetic material, and are retained in a spaced apart relationship by the end plates so as to define an annular flow passage between them. The end plates typically consist of disks made of non-magnetic material, and are each provided with a plurality of arcuate slots communicating with the annular passage, an annular recess for engaging a projection on the corresponding axial end of the inner yoke and an annular groove for engaging a ring that secures the inner end of the piston rod to the piston. The inner yoke, end plates and outer yoke are securely attached to one another by crimping each axial end of the outer yoke against the peripheral edge of the corresponding end plate. 
     In such a damper, it has been a common practice to use carbon steel as the material for the outer yoke because carbon steel is a soft magnetic material and has a favorable mechanical property. However, the saturation magnetic flux density of carbon steel is not very high so that the variable range of the damping force cannot be made so wide as desired. The inventors experimented the use of soft magnetic materials including iron-cobalt alloy (such as Permendur) having high saturation magnetic flux densities, but such materials do not have a high hardness, and have relatively poor elongation and drawing properties. Therefore, such materials were not found to be suitable for use in the outer yoke of conventional magneto-rheological dampers. 
     More specifically, if material such as Permendur is used for the outer yoke which forms the outermost shell of the piston in the damper disclosed in U.S. Pat. No. 6,260,675, because the outer circumferential surface of the outer yoke constantly slides over the inner circumferential surface of the cylinder during use, the outer yoke wears out very rapidly. Wear in the outer yoke creates a play between the piston and cylinder in time, and this play not only causes noises but also reduces the damping force due to leakage of magnetic fluid through the play between the piston and cylinder. 
     Also, in the damper disclosed in U.S. Pat. No. 6,260,675, the axial ends of the outer yoke are crimped onto the axial end surfaces of the end plates so as to integrally hold the end plates, inner yoke and outer yoke together. However, material such as Permendur having a poor drawing property is not suitable for crimping. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of such problems of the prior art, a primary object of the present invention is to provide a variable damper which can be made highly compact without impairing the performance of the damper. 
     A second object of the present invention is to provide a variable damper which can vary the damping force thereof over a wide range. 
     A third object of the present invention is to provide a variable damper which is durable in use. 
     According to the present invention, at least one of such objects can be accomplished by providing a variable damper, comprising: a cylinder filled with magnetic fluid or magneto-rheological fluid therein and having an end connected to one of a vehicle side member and a wheel side member; a piston slidably received in the cylinder to separate an interior of the cylinder into two chambers and provided with a flow passage communicating the two chambers; and a piston rod extending out of an end of the cylinder remote from the one end thereof, and having an outer end connected to the other of the vehicle side member and the wheel side member and an inner end connected to the piston; the piston comprising a cylindrical outer yoke defining an outer peripheral part of the piston, an inner yoke coaxially received in the outer yoke at a prescribed gap and a coil retained in the inner yoke to provide a magnetic flux extending across the gap, the gap providing at least a part of the flow passage; the piston further comprising a slide member defining a space between an outer circumferential surface of the outer yoke and an inner circumferential surface of the cylinder. 
     Thus, the durability of the damper can be improved by using a wear resistant material for the slide member. In particular, the outer yoke may be made of material such as Permendur which has a high saturation magnetic flux density but a poor mechanical property. Thereby, the range of the damping force can be expanded. 
     The slide member may be made of non-magnetic material having a favorable wear resistance such as austenite stainless steel and aluminum alloy. It is also possible to apply a plating or other surface processing to the sliding surface of the slide member to improve the wear resistance thereof. 
     According to a preferred embodiment of the present invention, the slide member comprises a piston cover that covers an outer circumferential surface of the outer yoke. Thus, the leakage of magnetic flux from the outer yoke is reduced so that the damping force can be increased for a given drive current. The piston cover may comprise a pair of axial ends crimped onto corresponding axial ends of the piston. Thereby, the number of components can be reduced and the assembly process can be simplified. According to a particularly preferred embodiment of the present invention, the piston further comprises a pair of end plates disposed on either axial end of the inner yoke, and held in position by the axial ends of the piston cover crimped thereon, each end plate being made of non-magnetic material and provided with openings along an outer periphery thereof so as to form a part of the flow passage. 
     According to a certain aspect of the present invention, the piston further comprises a pair of end plates each fixedly secured to either axial end of the inner yoke and provided with an annular shoulder for fixedly clamping the outer yoke in a coaxial and spaced relationship to the inner yoke. Also, each axial end of the inner yoke and an opposing face of the corresponding end plate may be provided with mutually corresponding annular shoulders to hold the end plate in a coaxial relationship to the inner yoke. Thereby, the leakage of the magnetic flux in the axial direction can be minimized, and the damping force can be maximized for a given drive current. Also, the various components can be integrally held together in a coaxial relationship at a high precision in a simple manner. 
     According to another aspect of the present invention, the inner yoke is provided with a central bore having a first end receiving an inner end of the piston rod and a second end closed by a liquid tight plug, and the piston rod is provided with a central bore for passing a lead wire electrically connecting the coil to an external circuit, a seal member being interposed between the piston rod and a wall of the central bore of the inner yoke to seal off the magnetic fluid from the central bore of the piston rod. Thus, the lead wire for the coil can be conveniently led out from the piston rod to an external circuit, and is favorably sealed off from the magnetic fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Now the present invention is described in the following with reference to the appended drawings, in which: 
         FIG. 1  is a perspective view of a rear wheel suspension system to which the present invention is applied; 
         FIG. 2  is a vertical sectional view of a first embodiment of the variable damper according to the present invention; 
         FIG. 3  is an enlarged view of a part of  FIG. 2  indicated by a circle III; 
         FIG. 4  is an exploded perspective view of the piston assembly partly in section; 
         FIG. 5  is a graph showing the relationship between the stoke speed and damping force under a zero drive current condition and a maximum drive current condition; 
         FIG. 6  is a view similar to  FIG. 3  showing a second embodiment of the present invention; and 
         FIG. 7  is a view similar to  FIG. 3  showing a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a rear wheel suspension system  1  consisting of a H-shaped torsion beam suspension system to which the present invention is applied. This suspension system comprises a pair of trailing arms  2  and  3 , a torsion beam  4  connecting intermediate parts of the trailing arms  2  and  3  with each other, a pair of suspension springs  5  consisting of coil springs for the corresponding trailing arms, respectively, and a pair of dampers  6  for the corresponding trailing arms. Each damper  6  consists of a variable damping force damper using MRF (Magneto-Rheological Fluid), and is configured to vary the damping force thereof under the control of an ECU  9  mounted in a car trunk or the like. 
     As shown in  FIG. 2 , the damper  6  of the illustrated embodiment consists of a monotube type comprising a cylindrical cylinder  12  filled with MRF, a piston rod  13  extending out of the cylinder  12  in a slidable manner, a piston  12  attached to the inner end of the piston rod  13  and separates the interior of the cylinder  12  into an upper chamber  14  and a lower chamber  15 , a free piston  18  defining a high pressure gas chamber  17  in a lower part of the cylinder  12 , a cylindrical cover  19  having a larger inner diameter than the outer diameter of the cylinder  12  and attached to the piston rod  13  in a coaxial relationship to the cylinder  12  to protect the piston rod  13  from contamination, and a bump stopper  20  attached to the piston rod  13  to define the limit of movement of the damper  6  at the time of full bound in a resilient manner. 
     The cylinder  12  is connected to a bracket formed in the upper surface of the corresponding trailing arm  2  via a bolt  21  passed through the bracket and an eyepiece  12   a  formed in the lower end of the cylinder  12 . The upper end of the piston rod  13  is provided with a threaded portion  13   a  which is connected to a damper base  24  (formed in an upper part of a wheel house) vcia a pair of rubber bushes  23  interposing a damper base member and a nut  23  threaded onto the piston rod  13 . 
     The piston  16  is incorporated with a magnetizable liquid valve (MLV) as will be described hereinafter. As shown in  FIGS. 3 and 4 , the piston  16  comprises a piston cover  30  covering the outer periphery of the piston  16  and having an outer circumferential surface slidably engaging the inner circumferential surface of the cylinder  12 , an outer yoke  31  formed as a cylindrical thin shell member and retained immediately inside the piston cover  30 , an inner yoke  32  formed as a cylindrical member coaxially disposed inside the outer yoke  31 , a pair of end plates  33  and  34  consisting of disk members mounted on either axial end of the inner yoke  32  in a coaxial relationship, an MLV coil  35  received in an annular groove formed around an axially central part of the outer periphery of the inner yoke  32  and molded therein with resin and an engagement ring  36  holding the piston rod  13  to the piston  16 . The upper end plate  33  is provided with a central opening  33   d  for receiving the piston rod  13 , and the inner yoke  32  is provided with a central bore including a section of a relatively large diameter at an upper end thereof so as to receive the inner end of the piston rod  13 . 
     The outer yoke  31 , inner yoke  32  and end plates  33  and  34  are integrally held together by crimping the axial ends of the piston cover  30  onto the outer axial end surfaces of the corresponding end plates  33  and  34 . The end plates  33  and  34  are provided with axial flanges  33   a ,  34   a  directed toward each other along the outer peripheries thereof (thereby defining annular shoulders), and the outer yoke  31  is provided with an annular shoulder  31   a ,  31   b  on an outer periphery of each axial end thereof. Each axial end of the outer yoke  31  fits into the interior of the corresponding axial flange  33   a ,  34   a , and the axial end of the axial flange  33   a ,  34   a  rests upon the corresponding annular shoulder  31   a ,  31   b . The circumferential surfaces of the end plates  33  and  34  and the outer yoke  31  jointly define a flush cylindrical surface which is in close contact with the inner surface of the piston cover  30 . 
     The engagement ring  36  consists of a wire member made of spring steel formed into the shape of letter C. The piston rod  13  is provided with an annular groove  13   b , and a corresponding annular groove is formed in an axial bore of the piston  16  for receiving the piston rod  13  at an interface between the upper end plate  33  and inner yoke  32 . When assembling the piston  16 , the engagement ring  36  is initially pushed into the annular groove  13   b  of the piston rod  13 , and once the corresponding end of the piston rod  13  is introduced into the corresponding axial bore of the piston  16 , the engagement ring  36  is allowed to expand into the annular groove formed in the bore. Because the annular groove formed in the bore of the piston is not deep enough to entirely receive the engagement ring  36 , the piston rod  13  is joined to the piston  16  by the engagement ring  36 . The bore in the piston  16  is passed entirely axially through the inner yoke  32 , but is closed by a plug  37  at the opposite axial end having a relatively smaller inner diameter, and an O-ring  38  is interposed between the piston rod  13  and bore to achieve a required sealing of the lead wires for the MLV coil  35 . 
     The piston cover  30  of the illustrated embodiment is made of non magnetic material such as austenite stainless steel which may be SUS304 or SUS316 (JIS), and the outer yoke  31  is made of soft magnetic material such as Permendur (trade name: high magnetic permeability alloy containing iron an cobalt by about a same amount). However, it is also possible that the piston cover  30  is made of other non-magnetic material such as aluminum alloy or magnetic material such as carbon steel. Likewise, the outer yoke  31  may be made of other suitable soft magnetic material having a relatively high saturation magnetic flux density. The outer circumferential surface of the outer yoke  31  is spaced (by a gap substantially equal to the thickness of the piston cover  30 ) from the inner circumferential surface of the cylinder  12 . 
     The inner yoke  32  is a solid integral member made of magnetic material such as S25C and other carbon steels, and is provided with axial flanges  32   a  and  32   b  on the end surfaces thereof at the outer periphery thereof. Each end plate  33 ,  34  is provided with a central projection  33   b ,  34   b  that fits into the interior of the corresponding axial flange  32   a ,  32   b . The inner circumferential surface of the outer yoke  31  opposes the outer circumferential surface of the inner yoke  32  with a certain annular gap defined therebetween. The end plates  33  and  34  are made of non-magnetic material such as aluminum alloy that may be Duralumin (or other durable material such as austenite stainless steel), and each provided with four arcuate through holes  33   c ,  34   c  along an outer periphery thereof. Thereby, an annular passage  39  extending axially across the piston  16  is defined by the through holes  33   c  and  34   c  of the end plates  33  and  34  and the annular gap between the inner yoke  32  and outer yoke  31 . 
     When the vehicle is in motion, the ECU  9  determines a target damping force for each of the wheels according to the accelerations of the vehicle obtained from a fore-and-aft G sensor, a lateral G sensor and a vertical G sensor, the vehicle speed obtained from a vehicle speed sensor, the rotational speed of each wheel and other data, and supplies a corresponding electric current to each MLV coil  35 . The electric current causes a change in the viscosity of the MRF flowing through the annular passage  39 , and this causes a corresponding increase or decrease of the damping force of the damper  6 . 
     In the disclosed embodiment, because the outer yoke  31  is made of Permendur having a high magnetic permeability and a high saturation magnetic flux density, although it has a relatively small thickness, a strong magnetic field is produced in the piston  16 . Therefore, combined with the contribution of the piston cover  30  made of non-magnetic material in controlling the leakage of magnetic flux, a relatively large damping force or a large dynamic range of the damping force can be achieved. 
       FIG. 5  is a graph showing the relationship between the damping force and stroke speed when a drive current is supplied to the MLV coil  35  and no drive current is supplied to the MLV coil  35 . The illustrated embodiment is compared with an example which is identically constructed and identically dimensioned except for that the outer yoke is made of carbon steel (S25C). When no drive current is supplied, the illustrated embodiment and example for comparison demonstrate an identical relationship between the stroke speed and damping force. When a maximum drive current is supplied to the MLV coil  35  ( 5  A in the illustrated embodiment), the damping force of the illustrated embodiment (solid line) is significantly greater than that of the example for comparison (dotted line) over the entire range of the stroke speed. 
     The piston cover  30  is made of stainless steel that is known to have a high wear resistance so that the damper  6  is highly durable in use. Also, austenite stainless steel demonstrates a favorable extension and drawing capability so that the crimping work can be performed with ease, and this improves the efficiency of the assembly work. 
       FIG. 6  shows a second embodiment of the present invention. The parts corresponding to those of the previous embodiment are denoted with like numerals without repeating the description of such parts to avoid redundancy. 
     In the second embodiment, no piston cover is used. The end plates  33  and  34 , inner yoke  32  and outer yoke  31  are held integrally together by a plurality of threaded bolts  40  passed axially through the end plates  33  and  34  and inner yoke  32 . In this embodiment also, the outer yoke  31  is made of Permendur, and the outer circumferential thereof opposes the inner circumferential surface of the cylinder  12  with an annular gap defined therebetween owing to the end plates  33  and  34  that clamp the inner yoke  32  between them. The end plates  33  and  34  have a larger outer diameter than the outer yoke  31  so that the outer circumferential surfaces of the end plates  33  and  34  are in sliding engagement with the inner circumferential surface of the cylinder  12  while the outer circumferential surface of the outer yoke  31  is spaced from the inner circumferential surface of the cylinder  12  by a small gap. If desired, the end plates  33  and  34  may be made of any non-magnetic wear resistant material and/or may be surface coated or processed at the outer circumferential surfaces thereof so that a desired durability may be achieved. 
       FIG. 7  shows a third embodiment of the present invention. The piston  16  comprises a substantially solid cylindrical inner yoke  126  made of magnetic material, and an end of the piston rod  13  is received in a hole  135  formed centrally in an axial end of the inner yoke  126 . The inner yoke  126  consists of axially separated two parts  126   a  and  126   b  which are integrally and axially joined to each other by four threaded bolts  127  passed into the inner yoke  126  from the axial end remote from the piston rod  113 . 
     The outer circumferential surface of the inner yoke  126  is formed with an annular groove  128  in an axially central part thereof, and a coil  130  is received therein via a coil bovine  129 . In this case also, the coil  130  is wound in the circumferential direction, and molded in resin. The outer peripheral part of the annular groove  128  is provided with axial extensions  131  formed by extending parts of the inner yoke  126  from the both axial ends into the peripheral part of the coil  130 . In the illustrated embodiment, the axial extensions  131  extend from the both axial ends in a symmetric manner. The outer circumferential surface of the inner yoke  128  generally defines a cylindrical surface having a fixed radius, and the parts thereof located on either axial side of the annular groove  128  form pole pieces  32   a  and  32   b.    
     The parting plane between the two parts of the inner yoke  126  is located on the same plane as a side surface of the annular groove  128  remote from the piston rod  13 . An end plate  134  ( 134   a ,  134   b ) made of non-magnetic material is placed on each end surface of the inner yoke  126 , and the piston rod  13  is passed through a central opening of the end plate  134   a  facing the piston rod  13 . Each end plate  134  is formed with four arcuate slots  136  extending along a common concentric circle adjacent to the outer peripheral edge thereof at a regular angular interval. 
     A cylindrical flux ring or outer yoke  140  is interposed between the peripheral edges of the two ends plates  134  so that the inner circumferential surface of the outer yoke  140  opposes the outer circumferential surface of the inner yoke  126  at a prescribed gap  141  in a concentric relationship. The outer yoke  140  is made of Permndur or other material having a favorable magnetic property. The outer circumferential surface jointly defined by the outer yoke  140  and the two end plates  134  and having a fixed radius is covered by a cylindrical piston cover  143  (which may be made of non-magnetic material such as austenite stainless steel, aluminum alloy or other wear resistant material), and each axial end  143   a ,  143   b  thereof is crimped onto the outer peripheral part of the corresponding end plate  134  so that the two end plates  134 , inner yoke  26  and outer yoke  140  are held in a fixed manner. If desired, the piston cover  143  may also be made of magnetic wear resistant material. 
     The peripheral part of the central opening of the end plate  134  is formed with a beveled portion  137  that faces inwardly toward the center of the piston  16  and the corresponding outer circumferential surface of the piston rod  13  is formed with an annular groove  136 , and a C ring  138  is received in an annular recess jointly formed by the beveled portion  137  and annular groove  136  so as to secure the piston rod  13  with respect to the piston  16 . 
     The piston rod  13  is internally provided with an axial bore  146  which coaxially aligns with a similar bore formed in the inner yoke  126 . The end surface of the inner yoke part  126   a  facing the other inner yoke part  126  is formed with a central recess  147  that receives a hermetic seal member  145  therein. Lead wires  144  of the coil  130  are passed through the axial bore  146  of the piston rod  13  and the central bore of the inner yoke  126 , and is then sealably passed through the hermetic seal member  145 . The lead wires  144  are then passed through a groove (not shown in the drawing) extending radially along the end surface (parting plane  133 ) of either one of the inner yoke parts  126   a  and  126   b , and reach the coil  130 . An O ring is provided in the bottom part of the hole  135  to seal off the central bore  144  of the piston rod  13  from the MRF. The peripheral part of the hermetic seal member  145  is welded to the corresponding part of the inner yoke  126  to achieve a liquid tight seal for a similar purpose. 
     The mode of operation of this damper is described in the following. When the corresponding wheel moves relatively to the vehicle body owing to the movement of the vehicle, this displacement is transmitted to the piston  16  via the piston rod  13 , and causes a relative displacement between the piston  16  and cylinder  12 . As a result, the volumes of the two chambers  14  and  15  change, and the MRF is forced to flow through the flow passage formed by the arcuate slots  136  of one of the end plates  134   a , the gap  141  between the inner yoke and outer yoke and the arcuate slots  136  of the other end plate  134   b . When the coil  130  is not energized, the MRF is allowed to flow without encountering any significant resistance, and produces a relatively low damping force which is substantially proportional to the relative speed between the piston  16  and cylinder  12 . When the coil  130  is energized, the magnetic field produced in the gap  141  applies a relatively strong flow resistance to the MRF that flows through the gap  141 , and this produces a relatively high damping force which is substantially proportional to the relative speed between the piston  16  and cylinder  12 . Thus, by supplying controlled electric current to the coil  130 , a desired damping control can be achieved. 
     Thus, even though Permndur or material having a favorable magnetic property (high saturation magnetic flux density) but a poor mechanical property is used for the outer yoke  140 , a high durability and reliability can be achieved. 
     The dampers of the illustrated embodiments were for use in rear wheel suspension systems of four wheel motor vehicles, but may also be used for front wheel suspension systems of four wheel motor vehicles. Likewise, the damper of the present invention can be used for motorcycles and three-wheel motor vehicles as well. 
     Although the present invention has been described in terms of preferred embodiments thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims. 
     The contents of the original Japanese patent application on which the Paris Convention priority claim is made for the present application as well as any prior art mentioned therein are incorporated in this application by reference.