Abstract:
Shock absorber containing a flow channel ( 16 ), provided with a valve assembly, made in the piston section. This valve assembly is designed to be actuated hydraulically. To this end there is an auxiliary chamber ( 22, 42 ) which on one side is eliminated by a movable valve body ( 18 ). This valve body is provided with an inlet for oil from the flow channel to the auxiliary chamber. There is an outlet ( 21, 41 ) (that can be closed off). The surface area of the valve body acting on the valve seat ( 19 ) is less than the surface area of the valve body in the auxiliary chamber, so that fluid flowing through the inlet of the valve body results in an increase in the volume of the auxiliary chamber and thus in closing movement of the valve body. Because filing of the auxiliary chamber takes some time, a frequenncy-dependend closing characteristic of the flow channel is obtained.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a shock absorber comprising a cylinder section and a piston section that van be moved therein, the piston section and cylinder section each being provided with fixings for connection with the parts that can be moved with respect to one another and said piston section being sealed with respect to said cylinder section in order to delimit chambers on either side thereof, a firsts chamber close to the fixing of said piston section and a second chamber close to the fixing of said cylinder section, a connecting channel being arranged between said chambers for connecting said chambers, said connecting channel containing a valve assembly for influencing the fluid steam through said connecting channel, comprising a valve seat and a part that can move with respect thereto under liquid pressure and seals off the valve seat, said movable part of the valve assembly forms a boundary wall of an auxiliary chamber, said auxiliary chamber being provided with an inlet which, viewed in the direction of the movement of the fluids opens upstream of said valve assembly. 
   Shock absorbers of this type are generally known in the art. These are used in a wide variety of vehicles, the aim being to adapt the damping characteristics to the driving conditions. Auxiliary flow channels and main flow channels are employed. En the case of minor liquid movement damping can take place in the auxiliary flow channel, whilst in the case of more substantial liquid movements in the shock absorber the main flow channel with its own damping characteristics is also effective. These constructions react directly to the liquid movement, that is to say corresponding to the speed of movement of piston and cylinder section with respect to one another, the various valves are opened and closed. A frequency-dependent control by means of retarding the damping characteristics with respect to the mutual movement of piston/cylinder by hydraulic means is not known. 
   Electronic controls have been proposed in the art for providing an electronic delay in the build-up of force. That is to say, in the case of a large number of movements in rapid succession the damping is restricted, whilst more damping occurs in the case of a movement of longer duration. Although they are effective, electronic controls of this type are of particularly complex construction. Particularly rapid sensors, computation units and actuators are needed to enable the valves present to react sufficiently quickly. 
   A device with which a ball can be moved in an auxiliary chamber is described in U.S. Pat. No. 4,660,686. In the rest position this ball assumes a position such that the volume of the auxiliary chamber is maximum. This ball beans on a spring plate, which sprig plate is able to close off an opening located beneath it. When pressure is applied in the direction in which the spring plate opens, pressure is also applied to the rear of the ball. Because the diameter of the ball is greater than the diameter of the opening close to the valve plate, with identical pressure the force of the ball on the valve plate will be greater and the valve plate will closed. A relief valve is present in the liquid feed channel to the rear of the ball and when a specific value is exceeded this relief valve will open, as a result of which the pressure on the rear of the ball decreases and the valve plate will thus be opened to a greater or lesser extent. That is to say, depending on the pressure in the system, the valve plate will be open to a greater or lesser extent. This reaction is immediate, that is to say without any delay. 
   In U.S. Pat. No. 5,392,883 a device is described consisting of an auxiliary chamber in which there is an opening with a valve under slight spring pressure which operates in one direction, which valve is also provided with an opening, which, however, has a smaller passageway. In the rest position the volume of the auxiliary chamber is minimum. Here again, adjustment of the flow opening takes place immediately (delay of at most a few milliseconds) in response to the pressure conditions. 
   SUMMARY OF THE INVENTION 
   The aim of the invention is to provide a shock absorber with which frequency-dependent damping can be obtained by hydraulic means, that is to say a construction with which a greater or lesser resistance to flow is created depending on the duration of the mutual movement of piston/cylinder and not on the pressure prevailing therein. 
   This aim is realised with a shock absorber as described above, in that in the non-loaded state the volume of said auxiliary chamber can be increased with respect to the initial or rest volume under the influence of the liquid issuing through said inlet. 
   According to the present invention, an auxiliary chamber of variable volume is provided. The liquid flowing into this auxiliary chamber will increase the volume of the auxiliary chamber and move the movable part of the valve assembly towards the stationary part of the valve assembly, as a result of which the throttling effect is obtained. The volume of the auxiliary chamber is minimal in the rest position and the volume thereof is increased only by filling with damping liquid. 
   Because it takes some time (tens of hundreds of milliseconds) to fill the auxiliary chamber sufficiently and to build up pressure to provide (further closing) movement of the valve body, a hydraulic frequency-dependent control can be provided. 
   In order to be able to control the characteristics of the additional damping obtained in this way in an optimum manner, that is to say gradually to provide further throttling action, according to an advantageous embodiment of the invention the effective surface area of the boundary wall that is subjected to the pressure of the liquid which flows into the auxiliary chamber decreases as the volume of the auxiliary chamber increases. That is to say, the greater the extent to which the movable part of the valve assembly bears against the stationary part of the valve assembly (seat), the smaller is the surface area that is under the influence of the pressure in the auxiliary chamber, as a result of which the closing movement is retarded or even terminates before complete closing. A throttling characteristic which builds up gradually is obtained by this means. As described above, this effect arises with a time delay. 
   With this arrangement the connecting channel with associated valve assembly can be arranged either in the shock absorber or outside it. If the connecting channel is arranged in the shock absorber it is preferably made in the piston section. However, it is also possible to arrange the various parts outside the shock absorber. The valve assembly can be connected to both the fat and the second chamber with the aid of pipes or channels. External influencing of the behaviour of the valve assembly can optionally be achieved with a construction of this type. 
   In all cases the structural variants described below can be used. For instance, the auxiliary chamber can be provided with an outlet to allow the liquid to flow away. 
   It is possible to achieve the increase in volume of the auxiliary chamber essentially by means of the movable part of the valve assembly itself. That is to say this movable part or the valve body executes an appreciable movement during which increasing pretension on the valve plate is obtained. According to another variant a diaphragm can be present in the auxiliary chamber. This diaphragm can, moreover, operate the movable part of the valve assembly. 
   The frequency characteristics of the absorber obtained in this way can be adapted as desired in a wide variety of ways. Possibilities are the selection of the bore of the inlet of the auxiliary chamber, selection of the surface area of the valve body in the auxiliary chamber, selection of the surface area of the valve body compared with the valve seat and selection of the volume of the valve body. Moreover, the pressure build-up in the auxiliary chamber can be influenced by influencing the outlet of the auxiliary chamber. This can be effected both by means of selection of the size of the outlet and by means of fitting a (spring-loaded) shut-off valve therein. Further variants will be clear to those skilled in the art. 
   The invention can be employed both in the main flow channel and in the auxiliary flow channel of an absorber. 
   According to an advantageous embodiment of the invention, the closing movement of the valve body is a movement along the longitudinal axis of the shock absorber oriented towards the first chamber. 
   A restriction can be made in the connecting channel to provide initial throttling. Optionally, it is possible to be able to influence this restriction from the outside using means that are known in the art. According to an advantageous embodiment, a bore is made in the valve body in the inlet for the auxiliary chamber. As a result a complicated bypass is not needed, but a direct connection is produced between the connecting channel and the auxiliary chamber. 
   The valve body can be constructed as a piston, optionally in combination with a diaphragm and spring plate. 
   Furthermore, it is possible to construct the valve seat/valve body with a variable first surface. That is to say the effective closing surface area of the valve body can be relatively large during a first portion of the closing movement and smaller during a second portion, as a result of which the second portion of the closing movement is executed more rapidly because the second surface of the valve body remains constant. The valve body can be so sized that the effective rigidity of the closing surface increases during closure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail below with reference to illustrative embodiments shown in the drawing. 
     In the drawing: 
       FIG. 1  shows, highly diagrammatically, a general view of a shock absorber; 
       FIG. 2  shows a detail of a first embodiment of the piston section according to the invention; 
       FIG. 2   a  shows a detail according to  FIG. 2  in a first position; 
       FIG. 2   b  shows a detail according to  FIG. 2  in a second position; 
       FIG. 3  shows a detail of a second embodiment of the piston section according to the present invention; 
       FIG. 3   a  shows a detail according to  FIG. 3  in a first position; 
       FIG. 3   b  shows a detail according to  FIG. 3  in a second position; 
       FIG. 4  shows a detail of a third embodiment of the piston section according to the resent invention; 
       FIG. 4   a  shoes a detail according to  FIG. 4  in a first position; 
       FIG. 4   b  shows a detail according to  FIG. 4  in a second position; and 
       FIG. 5  shows the force/speed characteristic of a shock absorber according to the invention at different frequencies. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1  a shock absorber is indicated in its entirety by  1 . This comprises a cylinder section  2  and a piston section  3 . The piston section  3  comprises a piston  4  which delimits two chambers located opposite one another inside the cylinder section. A first chamber is indicated by  8  and a second chamber by  9 . The piston section  3  can be fixed to, for example, vehicle bodywork through an eye  10 . Eye  11  serves, for example, for fixing to part of the suspension system of a vehicle. The shock absorber shown here is of the type where the cylinder section  2  contains an annular channel  7  for storing absorber fluid and where there is a bottom valve  6  which connects second chamber  9  to this fluid reservoir  7 . It must be understood that the invention can be employed with any type of absorber, in which content them may or may not be an external reservoir which can be constructed in some way known in the art. 
   The construction of the piston section  3  in interaction with the cylinder section  2  is of importance for the present invention. More particularly, the present invention relates to a valve construction with which the behaviour of the liquid stream from the first chamber  8  to the second chamber  9 , that is to say when the connecting eyes  10 ,  11  move apart, can be influenced as a function of the duration of such a movement. 
   A detail of the piston section  3  is shown in  FIG. 2 . The direction of movement described above is indicated by arrow  26 . Piston section  3  consists of two parts, a main part  4  and an auxiliary part  5 . In the main section  4  on the one hand, a valve construction is fitted which influences the flow from the second chamber  9  to the first chamber  8 , that is to say in the case of an absorber movement in the direction opposed to arrow  26 . Because such a valve system is not of importance for understanding of the present invention and for the scope of protection, this will not be discussed further. 
   Furthermore, there are two possible connections between first chamber  8  and second chamber  9  in the case of movement in the direction of the arrow  26 . 
   A first main passageway is formed by a number of main flow channels  12 . These can be closed off by a (polygonal) plate valve  13 , which is driven into the closed position with the aid of spring  30 . The present invention can also be used with other bypass constructions known from the state of the art. On movement in the direction of arrow  26 , the liquid pressure built up as a result will, in the case of sufficiently rapid movement, overcome the force of spring  30  and a connection between first chamber  8  and second chamber  9  will be produced via main flow channels  12 . 
   Piston section  3  furthermore has an auxiliary part  5 . It can be seen from  FIG. 2  that this is not provided with a seal with resect to cylinder section  2 . There is an auxiliary flow channel  16  in piston section  3 . A restriction  17  has been made in this auxiliary flow channel. Auxiliary flow channel  16  terminates in a valve seat  19 . A valve body  18 , on which valve  14  bears, is fitted interacting with valve seat  19 . This valve  14  optionally closes against valve set  19 . On the opposite side the valve body  18  and more particularly the diaphragm  24  joined thereto has a second surface that is indicated by  15 . The second surface  15  has a larger area than the first surface. An inlet  20  extends through the valve body. This is the inlet of an auxiliary chamber  22  that on one side is delimited by the valve body/diaphragm  24  described above and on the other side by closing wall  25  that is joined firmly (and in a sealed manner) to auxiliary part  5 . An outlet  21  has been made this closing wall  25 . The outlet of the auxiliary flow channel  16  is indicated by  23 . 
   A detail of  FIG. 2  is shown in  FIGS. 2   a  and  b .  FIG. 2   a  shows the rest position. 
   When the piston section  3  moves in the direction of arrow  26  (see also  FIG. 2   b ) liquid will be moved through auxiliary flow channel  16 , a first regulation of the damping characteristics taking place with the aid of restriction  17 . The mayor proportion of the liquid will move away through outlet  23  into second chamber  9 . A proportion thereof will issue through inlet  20  into apiary chamber  22 . Because the outlet  21  has a flow resistance, the pressure will rise in chamber  22 . As a result the valve body  18  will move towards the valve seat  19 , that is to say the volume of the chamber  22  will become greater. Valve body  18  moves, inter alia, towards valve seat  19  because the area below diaphragm  24  is larger than the first surface  14 . Theoretically, the surface area could be essentially the same. When the valve body  18  moves towards valve seat  19  the liquid stream between valve  14  and valve seat  19  skill be throttled, as a result of which less liquid is able to move away via the outlet from the auxiliary flow channel  23  and damping occurs. 
   Because the filling of chamber  22  takes some time—after all the volume of liquid has to be moved—it Ill be understood that the closure of valve  14  is time-dependent. The closing movement is also pressure-dependent. Closure will take place more rapidly with a higher liquid flow trough auxiliary flow channel  16 . However, the time characteristic is the most important. In  FIG. 2   b  it is shown that when valve body  18  moves upwards the contact surface with diaphragm  24  is smaller because this separates from valve body  18  near the edges. As a result the effective surface area of valve body  18  on which the effective pressure within chamber  22  acts decreases. Sizing is such that when the pressure in chamber  22  increases the effective surface area on which the pressure within chamber  22  acts on the valve body  18  decreases in such a way that a gradual closing effect at valve seat  19  is obtained In the position shown in  FIG. 2   a  valve  14  bears with relatively low force on seat  19 , as a result of which liquid is able to flow past relatively easily. That is to say, in the case of a short, rapid movement plate  19  opens easily and the damping will be softer. 
   In the position shown in  FIG. 2   b  plate  19  is under appreciable pretension. That is to say, in the case of a longer rapid movement plate  19  will open to liquid less easily and the damping will be harder. The hardening of the damping over time (from position in  FIG. 2   a  to position in  FIG. 2   b ) can, for example, be proportional or logarithmic. 
   A frequency-dependent characteristic of a damping can be achieved with a construction of this type. It must be understood that in the example according to  FIG. 2  this damping relates only to the auxiliary flow channel. That is to say, in the case of more rapid movements the effect will be appreciably restricted by the presence of the main flow channels  12 . However, it is possible to omit such main flow channels or appreciably to restrict the cross-sectional dimension thereof. 
   A second variant of the invention is shown in  FIG. 3 . In this embodiment as well the piston section  33  consists of a main section  34  and an auxiliary section  35 . Main section  34  corresponds to the a section  4  except for the restriction  17 . That is to say, with this embodiment as well there are a number of main flow channels, as well as a connecting channel  16 . 
   The auxiliary section  35  has been modified in this embodiment. As in the case of the embodiment according to  FIG. 2 , here again there is no seal between the side wall of the auxiliary section  35  and the cylinder section. An auxiliary chamber is indicated by  42 . As in the case of the embodiment according to  FIG. 2  this is provided with an inlet  40 . Restricted discharge of liquid is also possible via the outlet or slit  50  made in the spacer ring, as a result of which this liquid is moved via outlet  43  to chamber  9 . Auxiliary chamber  42  is delimited by a valve body  44  and by end wall  45  as well as a part of auxiliary section  35 . Support ring  38  is fitted in a fixed position. Valve body  44  consists of a deformable plate  51  which constitutes a limit of the auxiliary chamber  42 . Part  51  presses against a rigid part  52  that is joined via an intermediate rig to closing plate  46 . This can interact with a valve set  39 . There is a restriction  37  in the auxiliary flow channel  16  and this has the same effect as restriction  17 . A diaphragm  44  is arranged below closing plate  46 . 
   The pack  44 - 46  is held together by a fixing such as a rivet in which the inlet  40  has been made. 
   Outlet orifice  41  can be closed off by a resilient plate pack  47 . 
   The construction according to  FIG. 3  functions as follows when it moves in the direction of arrow  26 , starting from  FIG. 3   a , without the main flow channels  12  becoming effective. Oil flows through auxiliary flow channel  16  throttled by restriction  37  and past valve  46  towards outlet  43 . Some of the liquid will flow through auxiliary bore  40  into chamber  42 . This liquid is not able to exit via outlet  41  because of the effect of resilient plate pack  47  that closes off this opening. As a result the volume of between the auxiliary chamber will be increased by deformation of the diaphragm  44 . This will result in closing plate  46  being pressed further onto the seat, as a result of which the oil flow between valve seat  39  and closing plate  46  is throttled. The pressure in chamber  42  is prevented from rising too high through the opening of outlet  41  as a result of resilient plate  47  moving away. Because plate  52  is rigid, on movement in the direction of arrow  26  plate  51  will press against plate  52  over a smaller surface area, as a result of which the effective surface area of plate  52  that is subjected to the pressure in the auxiliary chamber decreases. This is illustrated in  FIG. 3   b.    
   However, with this embodiment as well it takes some time to fill chamber  42  with liquid to such an extent that closure by the valve body takes place. 
   A further variant of the present invention is shown in  FIG. 4 . As before,  FIG. 4   a  shows the rest position and  FIG. 4   b  the position after movement in the direction of arrow  26 . The variant shown in  FIG. 4  has a piston section  53  consisting of a main section  54  and an auxiliary section  55 . Main section  54  corresponds to main section  34 , leaf springs being used instead of a helical spring. The auxiliary section has been modified somewhat compared with the variant shown in  FIGS. 3   a  and  b . The valve body is indicated by  64 . This consists of a rigid plate  72  and a sealing diaphragm plate  71 . There is also a closing plate  66 . In contrast to the embodiment shown in  FIG. 3 , there is no resilient plate pack  47 . Furthermore, there is an auxiliary spring  67  underneath diaphragm  71 . The valve seat with which valve body  64  interacts is indicated by  59 . 
   On movement in the direction of arrow  26  the auxiliary chamber  62  will be filled with liquid. At the same time, if the movement of the piston rod relative to the cylinder is sufficiently rapid, liquid will flow past the plate  66  loaded under low pension by spring  67 . When the pressure in auxiliary chamber  62  rises, both plate  73  (compensation plate) will move outwards and diaphragm  71  will become curved. On further movement in the direction of the arrow  26  the area of the diaphragm that is in contact with part  72  will decrease. As a result of the presence of spring  67 , the contact force of plate  66  on the seat can be determined very accurately. That is to say it will be possible in particular accurately to control the damping during brief violent shocks (soft damping). 
   The variants described above describe a frequency-dependent damping. The frequency at which the valve body  18 ,  46  starts to close and completely closes is selected depending on the parameters of the vehicle concerned. In general this frequency is between 1 and 10 Hertz. 
   The force/speed characteristic is shown diagrammatically in  FIG. 5  as an example. The central continuous line shows a conventional optimised damping characteristic. Variants of the invention are indicated by a broken line to the left and right of this continuous line. The left-hand broken line represents damping at a relatively low frequency of approximately 1 Hertz and the right-hand line represents damping at a relatively high frequency such as 10 Hertz. It can clearly be seen that the damping characteristics can be adapted to the driving conditions. 
   After reading the above description, further variants which fall within the scope of the appended claims will be immediately apparent to those skilled in the art.