Patent Publication Number: US-6340152-B1

Title: Seat suspension vibration damper

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a seat suspension vibration damper for dampening vibrations of a vehicle seat. 
     Numerous vehicle seat suspensions are known, including those having air bag or air spring suspensions for resiliently supporting a seat in a selected position. In such suspension systems, pressurized air is delivered to or exhausted from the air bag to adjust the elevation of the seat. The use of an air bag permits upward and downward vibrations of the seat. To counteract these vibrations, shock-absorbing cylinders have been used to dampen the seat vibrations. 
     In one known approach, as the elevation of the seat suspension is changed by inflating or deflating the air bag, the shock absorbing cylinder has a piston supporting rod which extends or retracts, depending upon the direction in which the seat elevation is changed. In this approach, the shock absorbing cylinder must be capable of extension and retraction throughout the entire range of seat elevation adjustment. In addition, these seat suspension systems are understood to use shock absorbing cylinders with pistons that apply a constant dampening force over the full stroke of the piston. If the dampening force were non-constant in such systems, problems would ensue. For example, in such systems a non-constant dampening force would mean that the ride provided by the seat would vary depending upon the seat elevation. 
     U.S. Pat. No. 3,951,373 illustrates one form of seat suspension utilizing a shock absorbing cylinder and an air bag or air spring. In this construction, the shock absorber is understood to have a stroke which is capable of extending and retracting throughout the full range of seat height adjustment. However, in this construction, a hand knob may be operated to adjust the throw of a shaft to thereby change the effective length of the shock absorber. 
     Although numerous seat suspension systems are known and a number of them have mechanisms for dampening seat vibrations, a need nevertheless exists for an improved vibration damper for a seat suspension system having new and non-obvious differences from vibration dampers used in known systems. 
     SUMMARY 
     A vibration damper is described for use in seat suspension systems such as those of the type which support a seat above the floor of a vehicle, the seat being raisable and lowerable to support the seat at various selected elevations relative to the floor of the vehicle, and wherein movement of the seat from a selected elevation in response to vibrations is permitted. The vibration damper is operable to dampen these seat vibrations when the vibration damper is operatively coupled to the seat suspension system. In one illustrated form, the vibration damper comprises a self-contained module which is convenient to install in a seat suspension system. Also, removal thereof, for example in the case of repair or replacement, is relatively convenient. 
     In one illustrated embodiment, the vibration damper includes a first member adapted for coupling to one of the vehicle and seat. The first member may comprise a housing or may take other configurations. The phrases “for coupling to” or “coupled to” one of the vehicle and seat includes direct and indirect connection to one of these components. For example, the first member may be connected to a base or other component of a seat support which is affixed to the vehicle. Alternatively, the first member may be directly or indirectly connected to the seat, for example to a platform or seat support upon which a seat is mounted or directly to the seat. In addition, the vibration damper of this embodiment includes a shock absorber adapted to be coupled to the other of the vehicle and seat, again, direct or indirect coupling is contemplated. Moreover, a latch is adapted to selectively couple the first member to the shock absorber such that when the shock absorber and first member are coupled together the shock absorber applies a dampening force to the seat. In addition, when the shock absorber and first member are decoupled from one another the shock absorber is relieved from applying a dampening force to the seat. 
     In accordance with a further aspect of an embodiment, the first member may comprise a housing with an interior and an exterior with the shock absorber being substantially positioned or disposed within the interior of the housing and thereby protected by the housing. In addition, the latch may also be carried by the housing and operated by a latch actuator to selectively couple and decouple the shock absorber to and from the housing to thereby selectively apply and relieve the application of the dampening force to the seat. The latch actuator may be a fluid actuator, such as a pneumatic actuator, an electrically operated actuator, such as a motor, for shifting the latch to couple and decouple the shock absorber in response to the actuator. Although less preferred, a mechanical actuator may be used of the type which is manually shifted to couple and decouple the shock absorber to and from the housing. 
     As another aspect of an embodiment, the shock absorber may include a cylinder or exterior housing with a latch gripping surface carried by the cylinder. The latch gripping surface may be a friction enhanced surface or may comprise a mechanism mounted to the cylinder such as a plurality of teeth. The shock absorber also may include a dampening piston within the cylinder and a piston rod coupled to the piston and having an end portion projecting outwardly from the piston for coupling to the other of the vehicle and seat (the other of the vehicle and seat in this case being the other of these components to which the first member or housing is not coupled). 
     The latch may include at least one latch arm with a latch surface which may be a friction enhanced surface. Like the latch gripping surface, the latch surface may comprise a plurality of teeth. The latch arm may be coupled to the first member or housing with the latch arm being movable between first and second positions. When the latch arm is in the first position, the latch surface of the latch arm engages the latch gripping surface carried by the cylinder to thereby couple the shock absorber to the first member or housing. When the latch arm is in the second position, the latch surface is disengaged from the latch gripping surface. The latch arm may be pivoted to the first member or housing, in the case wherein the first member takes the form of a housing, for pivoting movement between the first and second positions. In one specific approach, both the first member, such as the housing, and latch arm are pivotally coupled to said one of the vehicle and seat for pivoting about a common first pivot axis. 
     As another aspect of an illustrated embodiment, the shock absorber cylinder may be slidably coupled to the first member, or housing in the case the first member takes the form of a housing, for sliding movement relative to the first member. Thus, the first member may comprise a housing having a first side wall with an exterior surface at the exterior of the housing and an interior surface at the interior of the housing. The first side wall may include first and second side wall portions spaced apart from one another to define a guide slot therebetween. The slide element may be mounted to the cylinder and slidably coupled to the first and second side wall portions such that the slide element slides along the guide slot and guides the sliding motion of the cylinder and thereby the shock absorber relative to the housing. This sliding inter-connection of these elements may be independent of the operation of the latch. 
     In a more specific approach, the cylinder may be substantially disposed within the housing. In addition, the slide elements may include first and second inter-connected slide members which sandwich the respective first and second side wall portions therebetween. In this case, the first slide member may be positioned substantially within the housing and may include respective first and second teeth containing flange portions extending in a direction away from the first side wall of the housing, the first and second teeth containing flange portions being spaced apart from one another and positioned at opposite sides of the center of the cylinder from one another. Furthermore, the latch arm may have a generally U-shaped cross-section with a base and first and second leg portions. The first and second leg portions may each terminate in an elongated row of teeth and are aligned with a respective adjacent one of the first and second teeth supported flange portions of the first slide member. The teeth of the first and second leg portions engage the teeth of the respective adjacent flange portions when the latch arm is in the first position. 
     As a further aspect of an embodiment, the housing may include a second wall opposite to the first wall, the second wall including an arm flange receiving opening therein. The latch arm includes an arm flange projecting outwardly from the base toward the arm flange receiving opening. An actuator guide flange also projects outwardly from the second wall of the housing with the guide flange defining the actuator guide slot. A fluid actuator is provided for operating the latch, the actuator having an actuator cylinder which is pivoted to the housing, an actuator piston within the actuator cylinder, and an actuator piston rod. The actuator piston rod has an end portion projecting outwardly from the actuator cylinder. A link pivotally couples the end portion of the actuator piston rod to the latch arm flange. The end portion of the actuator piston rod is also coupled to the actuator guide flange such that the actuator guide slot guides the movement of the actuator piston rod during extension and retraction of the actuator piston rod. In this example, extension of the actuator piston rod shifts the latch arm to the first position and retraction of the actuator piston rod shifts the latch arm to the second position. 
     As yet another aspect of an embodiment, the shock absorber may be adapted to provide a non-linear dampening force to the seat to dampen seat vibrations. For example, the dampening force may be constant for a first range of movement of the seat in response to vibrations from a home position of the dampening piston and increasing for certain movements of the dampening piston in excess of the first range of movement. 
     In accordance with another embodiment, first and second latch arms are pivoted to the housing and are disposed at opposite sides of a cylinder disposed substantially within the housing. These latch arms each have a central portion and first and second end portions. The latch arms each pivot about an axis through a central portion of the latch arm. The first end portion of each latch arm includes a latch surface and the cylinder has an exterior with an elongated latch gripping surface. A fluid actuator is coupled to the second end of each of the arms by respective links. Extension and retraction of the fluid actuator, and more particularly of an actuator piston rod, is translated through the links into pivoting motion of the first and second latch arms between respective first positions in which latch surfaces of the latch arms engage the latch gripping surfaces of the cylinder and second positions in which the latch surface and latch gripping surfaces are disengaged. 
     The present invention is directed toward novel and non-obvious features of a vibration dampener, both individually and collectively, as set forth above and as additionally set forth in the drawings and description which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially broken away perspective view of one embodiment of a seat suspension system in accordance with the present invention in a partially elevated state. 
     FIG. 2 is a side elevation view of the seat suspension system of FIG. 1 in a lowered position relative to FIG.  1  and illustrating a portion of a seat supported by the seat suspension system. 
     FIG. 3 is an exploded view of the seat suspension system similar to the system of FIG.  1 . 
     FIG. 3A is a schematic illustration of one form of a seat support illustrating several specific pivot axis points for link members used in this support relative to an X-Y axis coordinate system. 
     FIG. 3B is a schematic illustration like that shown in FIG. 3A, but also showing a seat occupant and the path of travel of selected portions of the body of the seat occupant in response to vibrations, with the pivot axis points shown in FIG. 3B like those shown in FIG.  3 A. 
     FIG. 3C is similar to FIG. 3B except that the pivot axis points are located at different coordinates, this figure showing the effect of adjusting the coordinates on the motion of various body parts of the seat occupant. 
     FIG. 3D is like FIG. 3C with the pivot axis points at a location which defines a true parallelogram. 
     FIG. 4 is a perspective view of one form of vibration damper usable in the embodiment of FIG. 1 looking generally toward the front of the vibration damper. 
     FIGS. 5 and 6 are side elevation views of the vibration damper of FIG. 4, showing the vibration damper in respective latched and unlatched positions. 
     FIG. 7 is a top view of the vibration damper of FIG.  4 . 
     FIG. 8 is a rear elevation view of the vibration damper of FIG.  4 . 
     FIG. 9 is a partially exploded view of an alternative form of vibration damper usable in the embodiment of FIG.  1 . 
     FIG. 10 is a vertical sectional view through one form of vibration dampening cylinder for the vibration dampers of FIGS. 4 and 9. 
     FIG. 10A is a graph illustrating an example of the non-linear dampening force which may be applied by the dampening cylinder of FIG.  10 . 
     FIG. 10B is a graph illustrating an example of the dampening force versus displacement from a home position in response to shifting the piston of a shock absorbing damper of FIG. 10 at a velocity which varies sinusoidally. 
     FIG. 11 is a perspective view of an alternative embodiment of a seat suspension system in accordance with the invention. 
     FIG. 12 is a side elevation view, partially in section, of the seat suspension system of FIG. 10, shown with a seat in a partially elevated position and also showing a latch in a latched state. 
     FIG. 13 illustrates a portion of the seat suspension system of FIG. 11 with the latch shown in an unlatched state. 
     FIG. 14 illustrates an alternative form of vibration dampening cylinder for the seat suspension systems of FIGS. 1 and 11. 
     FIG. 15 is a side elevation view of a seat suspension system including an alternative form of latch. 
     FIG. 16 is an enlarged vertical sectional view of the latch used in the embodiment of FIG. 15 with the latch shown in a latched state. 
     FIG. 16A is a cross-sectional view of the latch of FIG. 16 taken in the direction of arrows  16   a — 16   a  and illustrating the operation of the latch to grip a rod passing through the latch. 
     FIG. 17 is a vertical sectional view through the latch of FIG. 15 showing the latch in an unlatched position to permit the passage of the rod through the latch. 
     FIG. 17A is a vertical sectional view through an alternative latch similar to that shown in FIGS. 16,  16 A and  17 . 
     FIG. 18 schematically illustrates an alternative embodiment of a seat suspension system in accordance with the present invention. 
     FIG. 19 schematically illustrates one form of control circuit for the illustrated seat suspension embodiments. 
     FIG. 20 illustrates an exemplary pneumatic circuit usable in the illustrated forms of seat suspension systems. 
     FIGS. 21A-21C schematically illustrates a valve which may be utilized to control both seat elevation adjustment and the operation of a latch. 
     FIGS. 22A,  22 B,  22 C,  22 D and  22 E illustrate in greater detail one suitable valve actuated by a single lever for simultaneously causing the unlatching of a latch and seat height adjustment. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1-3 illustrate one form of a seat suspension system  10  for supporting a seat  12  (a portion of which is shown in FIG. 2) for raising and lowering the seat in elevation relative to a floor  14  of a vehicle within which the seat suspension system is positioned. For example, the seat suspension system  10  may be mounted to the floor of a truck. In such a case, vibrations imparted to the truck during travel over a road surface can cause some vibration of the floor  14  and also of the supported seat  12 . Consequently, it is desirable in such applications to dampen the road vibrations. 
     The illustrated seat suspension system  10  includes a seat support, one form of which is generally indicated at  20 , which is raisable and lowerable to support the seat at various elevations relative to the floor  14  of the vehicle. The illustrated seat support includes a seat supporting member  22  to which the seat  12  is mounted (FIG.  2 ). First and second link elements  24 ,  26  are pivotally connected at their upper end portions to the support member  22  and at their lower end portions to a base member  30 . More specifically, link member  24  is pivoted at its upper end portion to support member  22  for pivoting about a first pivot axis  32  extending transversely through the seat support member. In addition, the lower end portion of link member  24  is pivoted to the base  30  for pivoting movement about a transverse axis  34  which is parallel to the axis  32 . In addition, link member  26 , which in the illustrated form is positioned below link member  24 , has an upper end portion pivoted to the seat support member  22  for pivoting about a transverse axis  36 . The link member  26  also has a lower end portion pivoted to the base  30  for pivotal movement about an axis  38 . Axes  32 ,  34 ,  36  and  38  in the illustrated form are parallel to one another. Consequently, a parallelogram type support is provided for the seat member  22 . The axes  32 - 36  may be positioned such that a line through axes  32 ,  34  is parallel to a line through axes  15   36 ,  38  in a true parallelogram support. However, the axes  32 - 38  may be positioned such that the line through axes  32 ,  34  is not parallel to the line through axes  36 , 38 ; 
     for example, to control seat motion as explained below in connection with FIGS. 3A-3D. 
     Base  30  is adapted for mounting to the floor  14  of the vehicle. For example, a plurality of fastener receiving openings, one being indicated at  40 , are provided for receiving fasteners which secure the base  30  in place. Alternatively, base  30  may be eliminated with link members  24 ,  26  being pivoted directly to the floor or other vehicle supports. Also, the base  30  may be adapted for mounting to a wall surface of a vehicle instead of the floor  14 . Nevertheless, the illustrated seat support is advantageous because the floor of the vehicle provides stable support for the seat support  20  and the seat support can be installed as a modular unit. 
     As best seen in FIG. 3, the illustrated base  30  includes first and second upright side elements  60 ,  62  interconnected by front and rear transverse cross-piece elements  64 ,  66 . Openings are provided in the respective side elements  60 ,  62  for receiving  30  pivot pins  70 ,  72  which pivot the lower end portions of the respective link members  24 ,  26  to the base  30  for pivoting about axes  34  and  38 . To reduce the weight of the overall construction, the upright elements  60 ,  62  may be of a generally hollow construction with a plurality of reinforcing ribs, some of which are indicated at  78 , and a perimeter flange  79  being provided for reinforcing these elements and also the openings through which the pins  70 ,  72  extend. The central region of base  30  may also be provided with a void  80  for further weight reduction purposes. In addition, as explained more fully below, the base  30  may include upright vibration damper supporting bracket elements  82 ,  84  to which a vibration damper  200  is interconnected, such as by pivot pins  86 ,  88 . The axis through pins  86  and  88  in the illustrated embodiment is parallel to the axes through the pins  70 ,  72 . The pins  70 ,  72  may, as shown in FIG. 1, comprise four short pins instead of longer pins transversing the width of the base 
     Although other materials may be used, typically the base  80  is of cast metal such as aluminum. Plastics and other durable materials may alternatively be used for the base. The base may also be of fabricated sheet steel. In addition, the base may take other forms. 
     The support member  22  shown in FIGS. 1-3 includes first and second upright side flanges  90 ,  92  having respective elongated spaced-apart planar seat mounting surfaces  94 ,  96 , which are generally parallel to one another. The mounting surfaces include plural fastener receiving openings, some being indicated at  98  in FIG. 1, for use in mounting the seat  12  to the seat support member  22 . Like base side components  60 ,  62 , the seat support side components  90 ,  92  may be generally hollow with a plurality of reinforcing ribs, some being indicated at  100  in FIG. 1, and a perimeter flange  101  or lip for reinforcing the side elements and the pivot axis defining openings. Pins  103 ,  105  (FIG. 3) pivot the respective upper end portions of link members  24 ,  26  to the seat support member  22 . As shown in FIG. 1, four such pins may be used, one at each corner of the interconnected seat support  20 . Also, front and rear cross components  102 ,  104  are provided to interconnect the side flanges  90 ,  92 . A central platform  108  may also be provided between these two flanges. Voids  110 ,  112  between these cross pieces further reduce the weight of the illustrated seat supporting member  22 . The cross pieces  102 ,  104  may include appropriate reinforcing ribs as shown. Like base  30 , the seat support member  22  may be of a durable material and may be cast of steel. Thus, base  30  and seat support member  22  may each be of a monolithic one-piece homogenous unitary cast structure. 
     The seat support member  22  may be eliminated with elements such as link members  24 ,  26  being connected directly to the seat. In such a case, the seat  12  would typically be rigidified at its base. However, by making seat support  20  a combination of base  30 , links  24  and  26  and an upper seat support member  22 , a modular construction results as the seat can be separately manufactured and installed to the seat support member  22  at a later time. 
     The link member  24  may include first and second side elements  130 ,  132  with transverse cross-piece portions  136  (one of which is shown in FIG.  1 ). These elements together form a platform-like, generally rectangular link member. For weight reduction purposes, the link member may be generally hollow with reinforcing ribs such as indicated at  138  in FIG.  1  and interior and exterior flanges  140 ,  141 . Flange  140  may bound an enlarged opening  143  through the link member  34  for accommodating the vibration dampener  200 , as explained below. The lower link member  26  may be similarly constructed with side elements  142 ,  144  and transverse cross-piece portions, one of which is shown at  146  in FIG.  1 . The link member  26  may also be generally hollow with reinforcing ribs (some being indicated at  147 ) extending between interior and exterior flanges  148 ,  149 . The central portion of link member  26  is generally hollow, although the illustrated link member  26  has a central cross-piece such as platform  150  for supporting an air spring as explained below. 
     As can be seen in FIG. 1, the base side members  60 ,  62  may be tapered with an increasing height from front to rear of the seat support  20 . Also, pivot axis  34  may be positioned rearwardly and above pivot axis  38 . Similarly, the side elements  90 ,  92  of seat support member  22  may be reduced in height from front to rear of the seat support  22  and may also accommodate a pivot axis  32  positioned above and rearwardly of the pivot axis  36 . Consequently, as the seat support is pivoted downwardly (shown moving from FIG. 1 to FIG.  2 ), the seat support member  22  shifts primarily downwardly with the surfaces  94 ,  96  remaining substantially level. Conversely, when the seat support is raised (moving from FIG. 2 to FIG.  1 ), the seat supporting surfaces  94 ,  96 , and thereby the seat  12 , moves primarily upwardly, with the seat remaining substantially level. 
     The link members  24 ,  26  may take different forms. For example, the link members  24  and  26  in FIG. 3 are of a somewhat different structure, with corresponding elements being assigned the same numbers., This illustrates the fact that the structure of link elements  24 ,  26  may be varied, with an alternative form being shown in FIG.  3 . The link members  24 ,  16  may also, for example, comprise individual spaced-apart arms at opposite sides of the seat. The seat support  20  may also take a variety of forms, although the form illustrated in FIGS. 1-3 offers a number of advantages. For example, a scissors-type seat support mechanism may be used, as well as other seat supports capable of raising and lowering the seat. 
     With reference to FIGS. 3A-3D, and with a number of the components of the seat support system, such as a seat height adjuster and latch (if used) eliminated for convenience, first and second link members  24 ,  26  are shown pivotally coupled at their lower end portions to base  30  and at their upper end portions to seat supporting member  22 . The centers of the pivots  38 ,  36  are indicated respectively by the numbers 1 and 2, and thus correspond to the pivot axes through pivots  38  and  36 . Similarly, the centers of the pivots  34 ,  32  are indicated respectively by the numbers 3 and 4, which thus correspond to the pivot axes through pivots  34 ,  32 . For convenience in this description, the first pivot axis thus corresponds to number 1, the second pivot axis thus corresponds to number 2, the third pivot axis thus corresponds to number 3, and the fourth pivot axis thus corresponds to number 4. 
     In these figures, an X-Y coordinate system is indicated, with the X axis being horizontal and in this case parallel to the illustrated floor  14 . In addition, the Y axis of this coordinate system is vertical. The coordinate system is located so that both the X and Y axes intersect first pivot 1 and thus first pivot 1 has coordinates (0, 0) in this X-Y coordinate system. 
     As the links  24 ,  26  are pivoted clockwise in FIG. 3A, the seat  12  is raised. Conversely, when the links are pivoted counter-clockwise in FIG. 3A, the seat  12  is lowered. In these figures, the seat support is shown with the line segment L extending from the first pivot 1 to the second pivot 2 and at an axle of θ relative to the X axis, with θ being 20.3 degrees. Obviously, the angle θ changes as the arms  24 ,  26  are pivoted to new locations. In FIG. 3A, the line segment R extends from the third pivot 3 to the fourth pivot 4, the line segment B extends from the first pivot 1 to the third pivot 3, and the line segment S extends from the second pivot 2 to the fourth pivot 4. 
     In the construction illustrated in FIG. 3A, the links  24 ,  26  are of unequal length. That is, the length or distance R between the third and fourth pivots 3, 4 is different from the length or distance L between the first and second pivots 1, 2. More specifically, the distance R is greater than the distance L. Consequently, the FIG. 3A construction comprises an unequal arm length parallelogram type support. In a specific embodiment, the distance R is from about twelve percent to about twenty percent greater than the distance L and assists in controlling the motion of the seat. Assume the pivot 3 is shifted to the location indicated by 3′ and thus segment R is shifted to R′. In this latter case, R′ is equal to L in length and R′ is parallel to L, providing a true parallelogram support. Although this can be done, as explained below the resulting motion of selected portions of a seat occupant&#39;s body have greater horizontal components of motion than if the illustrated FIG. 3A unequal parallel arm support is used. 
     Again referring to FIG. 3A, in this illustrated construction, the third pivot axis 3 is at an elevation which is above the elevation of first pivot axis 1. Also, in this illustrated construction, the fourth pivot axis 4 is positioned above and rearwardly of the second pivot axis 2. When the seat of this figure is in a raised position, the first and second link members  24 ,  26  are angled forwardly and upwardly relative to the floor of the vehicle. Furthermore, with the constructions shown in FIGS. 3B and 3C, the absolute value of the slope of line L relative to horizontal is greater than or equal to the absolute value of the slope of line R relative to horizontal when the seat is supported in various raised positions. Also, a plane extending upwardly through third and fourth pivots 3, 4 intersects a plane extending upwardly through first and second pivots 1,2 at a location which is above the floor of the vehicle when the seat is raised. 
     The pivot axes may be located relative to one another according to the formula L&lt;(S+R−B). In this formula, L is the distance between the first and second pivot axes, S is the distance between the second and fourth pivot axes, R is the distance between the third and fourth pivot axes, and B is the distance between the first and third pivot axes. 
     In the specific example shown in FIGS. 3A and 3B, and with θ at 20.3 degrees, the coordinates of pivot 1, as previously mentioned, are (0, 0) on the illustrated X-Y coordinate system; the coordinates of pivot 2 in millimeters are (−347.7, 128.5); the coordinates of pivot 3 are (259.6, 42.5); and the coordinates of pivot 4 are (−165.4, 177.8). 
     The location of these pivots in this illustrated embodiment may be selected such that the movement of a selected portion of a seat occupant&#39;s body in response to seat vibrations is confined to movement in a substantially vertical direction. For example, as best seen in FIG. 3B, with the specific coordinates set forth above, movement of various body points of a seat occupant  169  are shown. Specifically, with the seat at a selected elevation corresponding to the given θ and occupant  169  sitting on the seat and the seat being stationary, the position of the occupant&#39;s hip point  171 , shoulder point  173 , and eye point  175  are shown. For reference purposes, vertical line segments  177 ,  179  and  181  are shown passing through the respective hip point  171 , shoulder point  173 , and eye point  175 . In response to seat vibrations, as explained below, the seat may move both upwardly and downwardly from the static position. Correspondingly, the respective hip point  171 , shoulder point  173 , and eye point  175 , also move upwardly and downwardly. The X&#39;s shown adjacent line  177  indicate points through which the hip point  171  passes as it moves upwardly and downwardly in response to vibrations. Similarly, the X&#39;s adjacent to line  179  indicate the path followed by the shoulder point  173  as the seat moves upwardly and downwardly. Finally, the X&#39;s adjacent to line  181  indicate the path followed by eye point  175  as the seat moves in response to vibrations. 
     In FIG. 3B, the shoulder point  173  travels more closely along a vertical line than either the eye point or hip point. That is, the shoulder point is confined to move in a substantially vertical direction. 
     More specifically, assume that the total range of movement in response to vibration between the lower extreme and the upper extreme is called the suspension stroke. By substantially vertical motion, it is meant motion having a horizontal component which is no more than about seven percent of the length of the suspension stroke from vertical. For example, in FIG. 3B, with the pivots having the coordinates shown and with the suspension stroke (the total vertical motion of the seat in response to vibration) being 165 mm, the maximum deviation of hip point  173  from vertical line  179  is about 6.6 mm, or about four percent of the suspension stroke. 
     Since very little off-vertical motion occurs at this location, less undesirable rubbing of a three-point shoulder strap on the occupant&#39;s shoulder at this location takes place. Also, for occupant&#39;s prone to travel sickness, it is desirable to have less elliptical or off vertical movement of the stomach and inner ear of such occupants. By confining the shoulder to substantially vertical movement, a compromise is achieved. That is, the inner ear more closely moves in a vertical direction than would be the case if seat pivot locations were optimized to confine the motion of the stomach to a substantially vertical direction. In addition, the stomach moves more vertically than would be the case if the pivot locations were optimized to confine movement of the eye or inner ear of the occupant to substantially vertical movement. 
     FIG. 3C illustrates an example where the pivot axis points  1 - 4  are selected to minimize the horizontal component of movement of the eye point  175  of the seat occupant  169 . As can be seen in FIG. 3C, the shoulder point  173  has greater horizontal components of movement than the horizontal components of movement of shoulder point  173  in the case where the pivot axis points  1 - 4  are positioned as shown in FIG.  3 B. More specifically, in FIG. 3C, θ again is at 20.3 degrees. In addition, the coordinates of pivot 1 are (0, 0); the coordinates of pivot 2 are (−364.8, 35.9); the coordinates of pivot 3 are (259.6, 42.5); and the coordinates of pivot four are (182.3, 49.2). With these specific exemplary coordinates, the maximum deviation of eye point  175  from vertical line  181  is 10 mm, or about six percent of the entire 165 mm suspension stroke. 
     In these examples, the vertical lines  179 ,  181  are located to intersect the respective shoulder point  173  and eye point  175  when the seat is in the static position supporting an occupant with no seat vibration. Thus, the phrase “substantially vertical motion” is defined to mean motion of no more than about ten percent and most preferably no more than seven percent from vertical over the entire suspension stroke. 
     In comparison, FIG. 3D shows the seat support with the third pivot at 3′ to provide a true parallelogram support. In the FIG. 3D example, with θ again equal to 20.3 degrees, the coordinates of the pivots in millimeters are as follows: pivot 1 at (0, 0); pivot 2 at (−347.4, 128.5); the third pivot 3′ at (182.3, 49.2); and the fourth pivot at (−165.4, 177.8). In this case, with a suspension stroke of 165 mm of vertical motion, the shoulder point (as well as the hip and eye points) have a maximum deviation which is understood to be about 48.5 mm (about twenty-nine percent) from vertical. 
     Thus, the modified unequal parallelogram supports of FIGS. 3B and 3C are examples illustrating the selection of pivot axis locations to confine selected portions of an occupant&#39;s body to a more vertical direction. Moreover, it should be noted that the vertical motion achieved by the shoulder point in the FIG. 3B example does not deviate significantly from precise vertical motion of the shoulder achieved with some conventional scissors style seat supports. 
     It should be noted that the locations of pivots  32 - 36  may be varied. In addition, as previously mentioned, in some embodiments of a seat suspension system, the modified or true parallelogram type support may be replaced with scissors or other types of support mechanisms. 
     A seat height adjuster is used to raise and lower the seat  12  between various elevations relative to the floor  14  and to a selected elevation. In general, a mechanism may be employed which moves seat support  22  or either of the link members  24 ,  26  about their respective pivots. These mechanisms may, for example, comprise hydraulic or pneumatic activated screw jacks or other mechanisms. As a specific example, an air spring  170  (see FIGS. 1 and 3) may be used for this purpose. The illustrated air spring may, for example, be from the “Airide Springs” product line from Firestone of Carmel, Calif. The illustrated air spring  170  is supported on the platform  150  of link member  26  beneath the seat support member  22  and engages the undersurface of platform  108  of member  22 . Upon inflation of air spring  170 , the seat support member  22  and seat  12  is shifted upwardly. In contrast, upon deflation of air spring  170 , the seat support member  22 , and thereby the supported seat  12 , shifts downwardly. A seat height controller may be used to control the air pressure delivered to and removed from the air spring  170  to thereby adjust the height of the seat to a selected elevation. The seat height is adjustable by the seat height adjuster between an uppermost elevation and a lowermost elevation which is established by the mechanical limits of the seat suspension system  10 . For example, although variable, the seat may be adjustable in elevation from a lowest position which positions the hip point 437 mm above the floor  14  to a highest hip point position which is 537 mm above the floor, for a total elevation adjustment of 100 mm (about four inches. The term “hip point” refers to the location where a typical driver&#39;s hip joint is positioned when the driver is seated in the seat. The seat height adjuster, such as air spring  170 , allows the supported seat to move in response to vibrations, for example from the road surface. This provides for a more comfortable ride. 
     The vibration dampener  200  is provided to dampen the vibrations of the seat. In the form illustrated in FIGS. 1-3, the vibration dampener includes a shock absorber or dampening cylinder  201  with an external housing  202  and a dampening piston therein (one example being described below in connection with FIG.  10 ). The dampening piston is coupled to a piston rod  206 . The piston rod may be of circular cross-section or of any other cross-section. For example, FIGS. 3-9 show a shaft  206  having a square cross-section. The lower end portion of the vibration dampener  200  is, in the illustrated embodiment, pivoted to the respective elements  82 ,  84  of base  30  rearwardly of pivot  38 . The upper end portion of the vibration dampener in this form, and in particular the upper end of rod  206 , is pivoted at  210  (FIG. 1) rearwardly of pivot  32  and between respective ear flanges  212 ,  214  which project rearwardly from cross-piece portion  104  of the seat support member  22 . The dampening piston is biased towards a first or home position such that the vibration dampener, when engaged to the seat support  20 , dampens movements of the dampening piston away from the home position to thereby dampen corresponding vibrations of the seat  12 . In this specific embodiment, the dampening cylinder is supported for selective movement relative to the floor  14  of the vehicle. Consequently, the elevation of the first or home position is adjustable to correspond to adjustments in the selected elevation of the seat. More specifically, in this illustrated form, each time the seat height adjuster operates to adjust the height of the seat, the elevation of the dampening cylinder and thereby of the home position is adjusted. More specifically, the illustrated seat height adjuster and vibration damper cooperate to automatically and simultaneously adjust the elevation of the first position of the dampener with changes in the selected elevation of the seat. 
     The illustrated vibration dampener  200  includes a latch mechanism  220  which selectively engages and releases the dampening cylinder  201 . When released, the dampening cylinder  201  is free to move relative to the latch. That is, as the seat is raised, the dampening cylinder  201 , because it is coupled to seat member  22  at the upper end of rod  206 , is raised the same distance as the seat. Consequently, the home position of the dampening piston within the housing  202  remains at a constant location within the cylinder, yet the elevation of the home position changes with the changes in elevation of the cylinder  201 . 
     Once the seat height is at a desired elevation, the latch mechanism  220  may be operated to re-engage the dampening cylinder  201 . When re-engaged, the cylinder is in a fixed position relative to the seat support  20  and the dampening cylinder applies a dampening force to the seat. That is, vibrations in the seat cause corresponding vibrations in rod  206  which are dampened by the movement of the dampening piston within the cylinder  202 . Other ways of adjusting the elevation of the home position may also be used. For example, the cylinder may be supported by a jack or other mechanism which raises the cylinder an amount which corresponds to the change in seat elevation without decoupling the cylinder. Various forms of latch mechanisms  220  may also be used, some specific examples of which are explained in greater detail, below. 
     The latch  220  may be operated to release the dampening cylinder during the entire time the seat height is adjusted or only during portions of such time. The vibration dampener is thus adapted to selectively relieve and apply a dampening force in these embodiments in which the dampening cylinder is latched and released. Furthermore, with this approach, the position within the vibration dampening cylinder  201  need not extend and retract over the entire range of seat height adjustment controlled by the air spring,  170  or other seat height adjuster. In a specific example, the dampening cylinder  201  may be designed to allow movement of the dampening piston a total of about sixty-five millimeters (about 2.5 inches). Although this may be varied, this compares to a total seat height adjustment in this example of about 100 mm (about four inches). Furthermore, a vibration dampener may be used which applies a variable dampening force to seat vibrations, including a dampening force which varies non-linearly with the magnitude of seat movements in response to vibrations. For example, the applied dampening force may be higher for a more extreme movement in response to seat vibration than applied in the case of a lesser movement in response to seat vibration and may be varied non-linearly for such movements between such movements. More specifically, the applied dampening force may be constant for limited motions from a home position and increase with more extremes in motion from the home position. This is facilitated by a design in which the home position of the dampener is shifted with changes in elevation of the seat. 
     Assume the seat has been moved to an elevation which is three inches higher than the previous elevation to an elevation which is twelve inches above the floor of the vehicle. With the illustrated design, the home position of the dampening cylinder may also be shifted three inches. If the seat then moves downwardly a certain amount (for example, thirty millimeters), in response to a seat vibration, a first dampening force may be applied to such motion. If thereafter the seat moves downwardly thirty-five millimeters from the home position, a greater dampening force may be applied. The dampening force may be varied non-linearly with deviations from the home position. Upward changes in seat elevation in response to vibrations can be dampened in a similar manner. If the home position had not been changed, the seat ride would no longer be the same, at least for given movements in response to vibration if a non-linear dampening force were being applied. That is, in this specific example, if the home position remained fixed and the seat elevation had been raised three inches, a further upward movement (e.g. one-half inch) in elevation of the seat in response to a vibration would now make the seat three and one-half inches from the home position. If a non-linear dampening force were being applied, this force would differ from the force being applied if the seat had not been raised and the seat were influenced by the same vibration, as in this case the seat would only be one-half inch from the home position. Thus, the illustrated seat suspension system facilitates the use of a non-linear dampening force. Furthermore, this type of seat suspension  10  permits the application of substantially the same dampening force immediately after a height adjustment as the dampening force provided immediately before the height adjustment. 
     The seat suspension system  10  of FIGS. 1-3 also includes an optional seat leveling feature. In operation, if the load on the seat is varied, for example, by an occupant getting up from the seat, the air spring  170  will tend to expand in response to this reduced load. As a result, seat support  22  is raised and also the supported seat  12 . If the particular seat happens to be the driver&#39;s seat, the upper surface of the seat may engage or come very close to the undersurface of the vehicle steering wheel. When the driver returns and again sits on the seat, it can be difficult for the driver to fit his or her legs between the steering wheel and seat until after the driver&#39;s weight has been placed on the seat to again compress the air spring to move the seat back to its original position. In the illustrated embodiment, a seat position sensor may be used to detect motions of the seat which are outside the range of motions being dampened by the dampener. In response to detection of such out of range motion, the inflation of the air spring  170  is adjusted to return the seat toward the position it was in before the motion took place. In other words, when the driver leaves the seat and the seat rises, pressure on the air spring is relieved to bring the seat back toward the position it was in prior to the driver leaving the seat. Conversely, when the driver again sits on the seat and the seat tends to depress outside of normal dampening ranges, the air spring is inflated to again return the seat toward its home position. 
     In one illustrated form, the position sensor comprises a self-leveling valve  230  coupled by a link  232  to the vibration dampener  200 . More specifically, the link  232  is coupled to a bracket  234  connected to the dampening cylinder  202 . The link  232  is  10  slidably coupled to the valve stem of valve  230  to accommodate variations in the distance between the valve stem and bracket  234  during operation of the illustrated system. The illustrated valve  230  is a rotary leveling valve and has a dead zone corresponding to the movements by the seat in response to vibrations which do not result in self-leveling. For example, assume movements of twenty-five millimeters or less in extension and forty millimeters or less in compression of the dampening cylinder are dampened by the dampening cylinder. Most movements in response to vibration involve less than 10 mm in extension and 10 mm of compression. In some observations, over ninety percent (90%) of extensions and over ninety percent (90%) of compression were within this range. Thus, although variable, the dead zone may be set to permit movements of 10 mm extension and 10 mm compression. It is expected that movements in compression will deviate more from the home position. Thus, the dead zone in compression may be greater than the dead zone in extension, with 15 mm compression being a specific example. Once the dead zone motion is exceeded, this motion causes link  232  to operate the valve to commence inflation of the air spring  170  and raise the seat if the seat elevation has dropped below the dampening range. Conversely, the valve is operated to commence deflation of the air spring  170  to lower the seat in the event the seat elevation is raised beyond the range of motion being dampened. Although timers or other delays may be used to only respond to deviations of a significant duration, this option may be eliminated. Consequently, a momentary deviation outside the dead zone is minimized because little change in air spring inflation occurs during any such momentary deviation. As a specific example, valve  230  may be a Model 3107-1 leveling valve from GT Development of Seattle, Wash. Other position detection sensors such as from Wabco, Inc. or other sources may be used, although the illustrated approach is convenient and mechanically simple. When latch  220  is unlatched and the dampening cylinder is shifted from one position to the other in response to changes in the elevation of the seat, the link  232  shifts with the dampening cylinder and remains in the same relative position to the valve  230 . Consequently, the valve  230  in the illustrated approach does not operate when the seat elevation is deliberately being adjusted by the air spring  170 . 
     One form of suitable vibration dampening module  200  utilized in the embodiments of FIGS. 1-3 is illustrated and described below in connection with FIGS. 4-8. This form of vibration dampener  200  includes an outer housing  300  having pivot receiving openings  302 ,  304  through which pivot pins are inserted to pivot the housing to the respective ears  82 ,  84  of base  30  (FIG.  1 ). Consequently, the housing  300  (FIGS. 4-8) is free to pivot about an axis  306  and relative to the base member  30 . The housing  300  may include cut-outs, such as indicated at  308  (FIG. 4) for weight reduction purposes. In addition, the housing  300  may define a guide slot  310  extending along the full length of one side wall  312  of the housing. The side wall  312  of the illustrated housing includes respective flanges  314 ,  316  which extend inwardly toward one another and define the guide slot  310  therebetween. The illustrated housing is generally rectangular in cross-section with a side wall  320  opposing side wall  312  and side walls  322 ,  324  interconnecting the side walls  320  and  312 . The housing  300  may be stamped or otherwise formed from a durable material, with steel being a specific example. 
     A latch arm element  330  is pivoted to housing  300 , for example by the same pivots which couple the housing to the base  30 , so that the latch arm element  330  may pivot relative to the base. 
     As best seen in FIG. 7, latch arm element  330  may be generally U-shaped in cross-section, having a base portion  332  and first and second leg portions  334 ,  336 . A latch actuator engaging flange  338  extends rearwardly from base  332  and, in this case, through an opening  340  (see FIGS. 5 and 6) through the wall  320 . Each of the elements  334 ,  336  includes a respective latch surface  344 ,  346  which may comprise a friction enhanced surface. In the form shown in FIGS. 5 and 6, the latch surfaces comprise an elongated row of teeth  350 . The latch  330  is selectively operable to couple and decouple the dampening cylinder housing  202  to the housing  300  and thus the dampening cylinder  201  to the base  30  (FIG.  1 ). More specifically, the cylinder housing  202  (FIGS. 4-8) includes a latch gripping surface which may be a friction enhanced surface which is selectively engaged by the latch  330 . The latch gripping surface may be, as illustrated, provided by one or more members, such as a plate  360  (FIGS. 4-7) welded or otherwise secured to the exterior housing  202  of the cylinder  201 . The plate  360  (see FIG. 7) includes first and second outwardly projecting side legs  362 ,  364 . The leg  362  extends outwardly toward side wall  324  and turns inwardly along the interior of the wall  324  to extend toward wall  320 . The distal edge  366  of leg  360  comprises a friction enhanced latch gripping surface, such as an elongated row of teeth which are selectively engaged or disengaged by the corresponding teeth  350  of the latch  330 . Similarly, leg  364  extends outwardly toward the wall  322  and then turns to extend along the interior of wall  322  toward the wall  320 . The leg  364  terminates at its distal edge provided with a latch gripping surface  368  which may also comprise a row of gripping teeth. A backup plate  370  is connected to the cylinder  202  and, more specifically, this connection is made through the plate  360 . Backup plate  370  may include side flanges  372 ,  374  which are spaced from the respective legs  362 ,  364  as shown in FIG. 7 to receive the wall portions  314 ,  316  of the wall  312  therebetween. Consequently, when latch  330  is released to free gripping surfaces  366 ,  368  from surfaces  344 ,  346 , the cylinder may slide relative to housing  300 . In this case, the movement of the cylinder guided by flanges  314 ,  316 . 
     Referring to FIGS. 5 and 6, the illustrated vibration dampener  200  includes a latch actuator such as a pneumatic cylinder  390  having an internal piston (not shown) coupled to a piston rod  392 . 
     A link  400  is pivoted to the distal end  402  of the rod and also at  404  to the flange  238 . The distal end of the piston rod is also coupled to the housing  300  so that motion of the piston rod is guided. More specifically, as best seen in FIGS. 5 and 6, an elongated, generally upright slot  406  is provided in a flange  408  projecting rearwardly from housing  300 . The pin which couples link  400  to the distal end of rod  392  at location  402  may also extend through the slot  406  such that the slot guides the motion of the piston rod. 
     With reference to FIG. 5, the latch actuator may be biased to extend the cylinder as shown in this figure to latch the dampening cylinder  200  to the housing  300 . In operation of this embodiment, when the latch  330  is shown in the position of FIG. 5, the latch gripping teeth  344  engage the teeth  368  carried by the cylinder housing  202  and prevents changes in elevation of the cylinder housing  202  in relation to vibration dampener housing  300  and thus relative to the seat support  20 . When in this position, the latch arm  334  has been pivoted in the direction of arrow  410  (clockwise in this figure) to engage the latch and latch gripping surfaces. When latched as shown in FIG. 5, the vibration dampener is operable to dampen vibrations of the seat. Conversely, in the event the elevation of the seat is to be changed in this embodiment by operation of the air spring  170 , the latch  330  is unlatched. More specifically, fluid pressure is delivered to port  394  of actuator  390  causing the retraction of piston rod  392 . This motion, guided by the slot  406 , is coupled via the link  400  to the flange  338  and results in pivoting of the latch  330  in the direction of arrow  412  as shown in FIG.  6 . As a result, the latch surface  344  is disengaged from the latch gripping surface  368 . When in the disengaged state, the cylinder housing  202  raises or lowers as the seat height adjustment takes place. Consequently, dampening forces are not applied during seat height adjustment. Moreover, following seat height adjustment and re-engagement of the latch to the cylinder, in this example the same dampening force is applied to the seat as was applied immediately before the seat height adjustment took place. Although less advantageous, it is also possible to retain dampener cylinder  202  in a latched state during all or portions of the seat height adjustment (in which case the dampener would typically be operable over a broader range of motion). In this latter case, the dampener may be unlatched for shifting to the desired new position following all or portions of the seat height adjustment. However, by unlatching the dampener over the full seat height adjustment, the damper design can be optimized to dampen motion solely over a more limited range of motion rather than over the entire range of the seat height adjustment. As another alternative, although less preferred, the dampener cylinder  201  and/or the housing  300  may be carried by a mechanism which permits the entire assembly to move rather than coupling and decoupling the dampener cylinder. For example, the housing may be supported by a fluid actuated screw jack which adjusts the position of the housing and thereby the dampener to accommodate seat height adjustments. 
     An alternative form of vibration dampening module  430  is shown in FIG.  9 . Vibration dampener  430  includes a shock absorber or vibration dampener  432 , which may be like dampener  201  of FIG. 4, having an exterior housing  434  and a rod  436 . A piston (not shown) coupled to rod  436  dampens vibrations of the seat as the rod  436  moves. Vibration dampener  430  also includes an exterior housing, in this case formed of two generally U-shaped housing sections  442 ,  444 . The housing section  442  includes side flanges  446 ,  448  and a rear flange  450 . The housing section  444  includes side flanges  452  and  454 , and a base or back flange  456  which interconnects the flanges  452 ,  454 . When the housing is assembled, the housing section  444  nests within housing section  442  with the sets of leg flanges  446 ,  452  and  448 ,  454  abutting one another. When assembled, pivot receiving apertures  460 ,  462  through the respective flanges  446 ,  452  are aligned. In addition, pivot receiving openings  464 ,  466  of the respective flanges  448  and  454  are also aligned. A pivot axis  468  extends through these aligned openings. The housing is pivoted to the base  30  (FIG. 1) in the same manner as housing  300  with pivot axis  468  corresponding to apart the pivot axis  306  of the FIG. 4 embodiment. End flange  456  has a pair of spaced-apart openings  470 ,  472  which are aligned with a corresponding pair of openings (not shown) in the back wall  450  of housing section  442 . 
     In the FIG. 9 embodiment, the latch  480  includes first and second elongated pivot arms  482 ,  484 . Arm  482  includes a pivot pin portion  486  which is inserted through opening  470  when the module is assembled. An opposed pivot pin portion projects in the opposite direction through the corresponding opening in the back wall  450 . In the same manner, arm  484  includes a pivot pin portion  488  which is pivotally received within the opening  472  when the module is assembled. A pivot pin portion opposed to portion  488  extends in the opposite direction from the opposite side of arm  484  into a pivot pin receiving opening through back wall  450 . The arms  482 ,  484  are thus capable of rocking back and forth about the respective axes through pin portions  486 ,  488 . 
     The upper end portions  500 ,  502  of the respective arms  482 ,  484  are each provided with a respective latching surface  504 ,  506 . In the illustrated embodiment, the latching surfaces  504 ,  506  each face toward the cylinder  434 . A friction enhanced latching surface, which may be roughened or may comprise a plurality of transversely extending teeth on the respective surfaces  504 ,  506 , are provided for gripping the cylinder  434  when the cylinder is latched. The outer surface of the cylinder  434  includes or supports a friction enhanced latch gripping surface which may comprise a roughened surface. In the illustrated embodiment, the latch gripping surface comprises elongated rows of teeth extending in a direction parallel to the longitudinal axis of the cylinder as indicated at  512 ,  514  in FIG. 9, are welded or otherwise secured to the exterior of the cylinder  434 . When arm  482  is pivoted counter clockwise as indicated by arrow  516  in FIG.  9  and arm  484  is pivoted clockwise as indicated by arrow  518  in this figure, the latching surfaces  504 ,  506  of the arms  482 ,  484  engage the teeth  512 ,  514  carried by the cylinder housing. When latched, the dampener  432  operates in the same manner as the dampener  200  of FIG. 4 when it is in an engaged or latched condition. Conversely, when arms  482  and  484  are pivoted in respective directions opposite to arrows  516 ,  518 , surfaces  504 ,  506  become disengaged from the teeth  512 ,  514 . As a result, the dampener  432  is free to move with the seat as the seat height is adjusted by the air spring  170  or other seat height adjuster. 
     An actuator such as a pneumatic cylinder  520  is provided for selectively pivoting the arms  482 ,  484  during operation of the vibration dampening module  430 . Actuator  520  includes a piston rod  522  coupled to a U-shaped clevis  524 . A first link  526  is pivotally coupled at one end portion to clevis  524  for pivoting about a pivot axis  528 . The link  526  is also pivoted at its opposite end portion to a lower end portion  530  of arm  482  such that link  526  may pivot about a pivot axis  532 . Similarly, a link  534  is pivoted at one end portion to the clevis  524  for pivoting about the axis  528 . In addition, the opposite end portion of the link  534  is pivoted to a lower end portion  536  of arm  484  for pivoting about an axis  538 . When piston rod  532  is fully extended (and the rod may be biased in this position), the arms  482 ,  484  assume the position indicated in FIG. 9 to latch the dampener  432  to the housing sections  442 ,  444  and thereby to the base  30  (FIG. 1) of the seat support  20 . In contrast, the delivery of pressurized fluid to a port  540  of actuator  520  causes rod  522  to retract. In response, the links  526 ,  534  draw the lower end portions  530 ,  536  of arms  482 ,  484  inwardly. When this happens, the arms  482 ,  484  pivot in a direction opposite to arrows  516 ,  518  to release the surfaces  504 ,  512  and  506 ,  514  from one another so that the dampener  432  is free to move during seat height adjustment. The actuator  520  may be operated in the same manner as actuator  390  to accordingly control the latching and unlatching of vibration damper  430 . One suitable form of actuator  390 ,  520  is a spring biased pneumatic cylinder Model BLC 2T2 from Sprague Controls of Canby, Oreg. Also, the bracket  234  for receiving a link, like link  232  in FIG. 1, may be mounted to cylinder housing  434  for reasons explained above in connection with FIG.  1 . 
     The dampening shock absorbers  201  and  432  of FIGS. 4 and 9 may be conventional shock absorbers of the type which apply a linear dampening force in opposition to seat vibrations. Also, any suitable mechanism for dampening vibrations may be used in place of the piston containing dampening cylinders  201 ,  432 . However, the illustrated shock absorbers may be of the type which applies a non-linear dampening force to seat vibration. 
     An exemplary dampener of this type is illustrated in FIG.  10  and examples of its operation is indicated in FIGS. 10A and 10B. Non-linear dampeners are not known to have been heretofore used to dampen seat vibrations. However, non-linear dampening cylinders are available from companies such as Monroe Auto Equipment Company. 
     The specific shock absorber embodiment of FIG. 10 (with the components  360 ,  370  in FIG. 8 eliminated for convenience), includes interior and exterior chambers  600 ,  602 . Chamber  600  includes fluid charged regions  604 ,  606  above a fluid charged area  606 . Makeup fluid is stored in chamber  602  (e.g. below the dashed line  607  in this figure), with the flow of makeup fluid between the two chambers controlled by conventional valve  608 . Chamber  602  includes a gas charge (e.g. above the dashed line  607  in this figure). A piston  610  is mounted to the piston rod  206  for sliding within the chamber  600 . A biasing spring  612 , positioned between piston  610  and an interior stop  614 , urges the piston toward a home position indicated at  616  in FIG.  10 . The spring  612 , which may have a spring rate of about fifty Newtons per mm (although this is variable) may be a conical spring which collapses upon itself as it is compressed to reduce the amount of space required by the spring when fully compressed to thereby reduce the overall required operating length of the dampener  201 . The interior wall bounding chamber  600  may be provided with one or more longitudinally extending grooves (two being indicated at  620 ,  621 ). The grooves control the rate at which fluid bypasses the piston  610  as the rod  206  is extended and retracted to thereby control the applied dampening force. The cross-sectional area of the grooves may be greater at home position  616  than at areas further removed from the home position to vary the dampening force with the distance the piston travels from the home position. For example, the grooves may be tapered to reduce their cross-section, moving further away from the home position. Alternatively, more grooves may be provided closer to the home position than further away to decrease the resistance to lesser movements of the piston and rod away from the home position. Stepwise variations in the applied dampening force may also be provided, for example by including stepped differences in the cross-sectional area of the grooves at specific locations along the length of the cylinder. Also, the dampening force may be varied non-linearly with distances from the home position. For example, for greater movements of the piston from the home position, corresponding to greater vibrations of the seat, the dampening force may be increased non-linearly to a much higher level than applied to minor deviations or movements from the home position. Also, the dampening force may be constant for certain predetermined deviations from the home position and then increase as these predetermined deviations are achieved. For example, the bypass grooves may be of a constant cross-sectional dimension over these predetermined deviations and then decrease in cross-sectional dimension after the predetermined deviations from the home position are exceeded. As the piston approaches the design limits of its maximum compression and/or extension, bumpers  626  and  628  may be engaged and compressed to cushion the latter movements of the shock absorbing damper. 
     In addition, in the illustrated construction, differing forces may be provided in opposition to downward movements of the seat than in opposition to upward movements of the seat. In addition, greater travel of the dampening cylinder may be allowed in response to vibrations in one direction than another. More specifically, in an illustrated approach, lower dampening resistance is provided to counteract reductions in seat height (compression of cylinder  202 ) than to counteract increases in seat height (extension of cylinder  202 ). Also, further travel is allowed by the dampening cylinder  202  in response to compression (seat height reduction) than in extension. Typically, greater energy needs to be absorbed in compression (for example, if a truck hits a bump, the deck or floor of the truck tends to move upwardly toward the seat) than in extension. By providing a lesser dampening force in response to compression and permitting greater dampening movement of the cylinder in response to compression, a smoother ride is provided. 
     The profile of the dampening force applied in response to travel of the piston  610  may be varied. However, in one specific example, the dampening piston  610  is permitted to travel forty millimeters in compression (including bumper compression at the end of the dampener stroke) and twenty-five millimeters in extension (including bumper compression). 
     FIG. 10A (not to scale) illustrates the dampening applied in response to two velocities of vibration induced seat travel. Specifically, at a relatively low velocity of 4.7 inches per second, an exemplary dampening force profile in compression and extension is shown in dashed lines in FIG.  10 A. In addition, an exemplary dampening force profile at relatively high velocity of 26.7 inches per second is shown in the solid lines in FIG.  10 A. As can be seen in this figure, in this example, from a distance of about 15 mm in compression from the home position to a distance of about 10 mm in extension from the home position, the cross section at each of the grooves remains constant. Consequently, over this range the applied dampening force is constant. This dampening force resists the bulk of the vibrations, as most vibrations cause a relatively small range of seat motion. From about 10 mm of extension to about 12 mm of extension, in this example the cross-sectional dimension of the grooves is reduced (in this case, the grooves are tapered such that the resistance force increases generally linearly over this range). Again, stepwise or other variations can be provided. In this example, the resistance force varied non-linearly from the home position. If the variation were linear, the slope of the dampening force versus distance of travel from the home position would be constant. At about 12 mm of extension, the cylinder wall grooves have ended. The resistance against extension continues to increase, but at a different rate. When the bumper is engaged, the resistance increases more sharply and, in theory, goes to infinity after the bumper is fully compressed at the desired maximum extension, in this case, twenty-five millimeters. Conversely, looking at the compression side of the profile, from about 15 mm of compression to about 17 mm of compression, the cross-sectional dimension of the grooves is reduced so that the resistance against compression increases. Thus, the resistance against compression has also varied non-linearly from the home position. From about 17 mm of compression through the engagement of the bumper contact, the grooves in this specific example have ended. Resistance against compression continues to increase and increases more sharply approaching bumper engagement. Following full compression of the bumper, the resistance against compression, in theory, goes toward infinity. In this specific example, forty millimeters of maximum compression is permitted. As can be seen in FIG. 10A, in general, there is a hysteresis effect to the profile as indicated by the arrows shown in this graph. 
     Although variable, in a specific example, at point  650 , the end of the grooves with the piston extending at high velocity, a seven hundred pound dampening force is applied. In contrast, in this example, at high velocity and at the end of the grooves with the piston moving in compression, at point  652  the dampening force is sixty hundred fifty pounds. Also, at the low velocity of 4.7 inches per second, at point  654 , which corresponds to the end of the grooves when the piston is extending, a two hundred pound dampening force is applied. Conversely, at point  656 , corresponding to the end of the grooves with the piston moving at this low velocity in compression, a dampening force of one hundred points is applied. 
     FIG. 10B illustrates an exemplary response of the dampening cylinder  10  as it is extended and compressed at two sinusoidally speeds S   1   and S   2   . The speeds S   1   and S   2   are at a maximum at about the center of the range of motion with speed S   1   being greater than S   2   . Between locations  659  and  661  the bypass grooves are of a constant cross-sectional dimension and the applied resistance force is constant for a given speed S   1   , S   2   and in a given direction (extension or compression). As can be seen in this example, in general at a given displacement from the home position  663 , the magnitude of the resistance against compression (below the X axis) is lower than the magnitude of the resistance against extension (above the X axis). At location  661 , and over a distance in this case of about 2 mm (from 10 mm extension to 12 mm extension) the cross-sectional dimension of the bypass grooves taper. At location  667  engagement of the bumper commences. The maximum extension is indicated at  669 . In the other direction from the home position, from 15 mm to 17 mm, in this example, the bypass grooves taper (location  659  to location  671 ). At location  673 , the cushioning bumper is engaged with maximum compression occurring at location  675 . The dashed line  677  indicates the expected force/displacement curves at speed S   2   if no bypass grooves are provided. 
     FIGS. 11-13 illustrate an alternative form of seat suspension system with a latch external to a dampener and which is operable independently of the dampener. For convenience, components in FIG. 11 which correspond to similar components in FIGS. 1-3 have been identified with the same numbers with the superscript (′). Consequently, these corresponding components will not be discussed in detail, although they do illustrate some of the wide variations of configurations which may be used for these components. 
     The illustrated latch  220 ′ includes a sector plate  700  pivotally mounted to the base  30 ′ for pivoting about the axis  34 ′. The sector plate includes an arcuate slot  702 . A pin  704  extends from link member  24 ′ through the slot  704 . The sector plate is thus free to pivot relative to link member  24 ′ and about axis  34 ′ within the limits defined by the slot  702 . Sector plate  700  includes an arcuate outer edge portion  706  which comprises a latch engaging surface. The latch engaging surface may be friction enhanced and may include a plurality of teeth, some being indicated at  708  in these figures. 
     An upwardly extending bracket  720  having parallel spaced-apart legs  722 ,  724  may be included in the latch construction. The legs  722 ,  724  are mounted for pivoting about the axis  34 ′. An upper portion  726  of bracket  720  may be of an inverted, U-shaped construction, with side flanges  728  and  730  interconnected by a base or top portion  732 . The upper end of rod  206 ′ of the shock absorber  201  is pivoted to side flange  728  of the bracket  720  so as to be pivotal about an axis  734 . The leveling valve  230 ′ is supported by side flange  730 . A latch arm  740  is pivoted at  742  to the bracket side flange  728 . In addition, a latch arm extension  744  extends from an upper end portion of latch  740  and has an uppermost portion  746  which is pivoted for movement about the axis  734 . A lower portion of latch arm  740  is pivotally coupled to a link  752  for pivoting about an axis  754 . Link  752  supports a latch actuator, such as a pneumatic actuator  760 . In this case, the pneumatic actuator  760  is pivoted to link  752  for pivoting about an axis  762 . The illustrated actuator  760  may be the same as actuator  390  (FIG. 1) and, in this case, includes a piston coupled to a rod  764 . The rod  764  has its upper end portion  766  pivoted to side flange  722  for pivoting about a pivot axis  768 . The latch arm  740  includes a latch surface for engaging the latch gripping surface of the sector plate  700 . One form of latch gripping surface is indicated at  770  and comprises a plurality of teeth sized and positioned to engage the teeth  708  of the sector plate when the latch is in a latched condition, as shown in FIGS. 11 and 12. 
     To unlatch the latch, actuator  760  is activated to, in this case, extend the rod  764 , as shown in FIG.  13 . With the above-described linkage arrangement, when rod  764  extends, for example in respond to delivery of pressurized fluid to port  772  of the actuator  760 , the latch arm  740  pivots in the direction of arrow  782  in FIG. 13 10  (clockwise in this figure). This shifts the lower end portion  750  and the gripping surface  770  of latch arm  740  away from the teeth  708  of the sector plate  700 . When unlatched as shown in FIG. 13, links  24 ′ and  26 ′ may be moved by the air spring  270 ′ to adjust the seat elevation without changing the position of the shock absorbing damper  201 ′. That is, link  24 ′ may be moved relative to the sector plate  700 . After the seat has been adjusted to the desired elevation, piston rod  764  may be retracted to pivot arm  740  in a direction opposite to arrow  782  and into engagement with the teeth on the sector plate as shown on FIG.  12 . 
     The latch  220 ′ may be controlled in the same manner as the latch  220  of FIGS. 1-3. For example, the latch may be shifted to its unlatched position during the entire time the set height is being adjusted by air spring  270 ′ and then latched following seat height adjustment so that the same dampening force is applied immediately after seat height adjustment as was applied immediately before seat height adjustment. Alternative latch control approaches may also be used, as previously described. 
     In the construction of FIGS. 11-13, the leveling valve  230 ′ may operate in the same manner as leveling valve  230 . That is, with the latch in a latched condition and in the event the load on the seat is varied (for example, the occupant of the seat gets off the seat), the seat will tend to raise. When the seat raises beyond the upper limit established by leveling valve  230 , the leveling valve controls the exhaustion of gas from the air spring  270 ′ to readjust the position of the seat. In the illustrated construction, the seat position is readjusted toward its original home position. Conversely, when a load is reapplied to the seat and the seat lowers in elevation beyond a lower limit established by leveling valve  230 ′, the leveling valve  230 ′ operates to cause the delivery of additional air to air spring  270 ′ to again raise the seat toward the home position. When the latch is unlatched, the seat elevation may be freely adjusted as the leveling valve  230 ′ is decoupled and thus inoperative when the latch is unlatched during intentional seat height adjustment by the air spring  270 ′. 
     FIG. 14 illustrates an alternative form of shock absorber  800  which may be utilized in the disclosed embodiments of the seat suspension system. The illustrated FIG. 14 shock absorber includes a lower shock absorbing section  802  within which an internal piston may be provided and coupled to a piston rod  804 . A housing  806  defines a biasing spring receiving chamber  808  within which first and second biasing springs  810 ,  812  are provided. An internal spring separation plate  814  is mounted to the rod  804 . With this construction, spring  810  resists compression of the dampener and corresponding movement of the rod  804 . In contrast, the spring  812  resists extension of the rod  804 . The biasing forces exerted by the springs may be different and the distance of travel in compression and extension may be varied. As one specific example, spring  812  may provide twenty pounds force per inch resistance with a maximum extension of twenty-five millimeters, while spring  810  may provide forty pounds force per inch compression with a maximum compression distance of forty millimeters, the distances being from a home position indicated at  816  in FIG.  14 . At the home position in the illustrated form of shock absorber, the springs exert fifteen pounds force against one another. 
     Again, other vibration dampeners, including other forms of shock absorbers, may be utilized in the illustrated seat suspension systems. 
     FIGS. 15,  16 ,  16 A and  17  illustrate another form of latch mechanism which may be used in the illustrated seat suspension embodiments. Again, for purposes of convenience, components of the seat suspension of FIG.  15  and the related figures which correspond to components of FIG. 1 are illustrated with the same number as utilized in FIGS. 1-3, except with a double prime superscript (″). These corresponding components will not be described in any detail. 
     The vibration dampener  220 ″ comprises a rod gripping latching assembly  870  coupled at its upper end portion  872  to the seat support member  22 ″ at a location rearwardly of pivot  32 ″. The lower end portion of the vibration dampener  220 ″ is pivoted to base  30 ″ at a location forwardly of pivot  38 ″. More specifically, the illustrated rod gripping latch assembly  870  is pivoted to downwardly projecting ears, one being shown at  874  in FIG. 15, at the underside of seat supporting member  22 ″ for pivoting about a pivot axis  876 . The piston rod  206 ″ in this embodiment is of an extended length and passes through the rod gripping latch assembly  870 . When in a latched condition, the rod is gripped by the latch assembly  870  so that it does not move relative to seat support member  22 ″. Consequently, the dampener  200 ″ is operable to dampen seat vibrations. When unlatched, the rod  206 ″ is free to slide relative to the rod gripping latch assembly  870 . Thus, for example, the rod may be released during seat height adjustment by air spring  270 ″ so that the elevation of the seat may be adjusted to a new desired elevation. The latch may then be operated to re-engage the rod  206 ″ and again apply the dampening force to the seat support and the seat. Consequently, the dampening force applied to the seat may be the same immediately before and after a seat elevation adjustment by air spring  270 ″. The latching mechanism  220 ″ may be controlled in various manners, such as previously described in connection with the latching mechanism  220 . 
     An optional positioning sensor, such as leveling valve  230 ″, may be included in this system, as well. Leveling valve  230 ″ is coupled by links, including link  232 ″, to a collar  878  which is secured to rod  206 ″ for movement with the rod. Leveling valve  230 ″ may be operated in the same manner as leveling valve  230  and, hence, its operation will not be described in detail. 
     It should be noted that the construction of FIG. 15 is less compact than the construction of FIG. 1 due to the elongated nature of piston rod  206 ″. To reduce the overall length of the construction, latch mechanism  870  may be positioned adjacent to shock absorber  200 ″ on a separate rod or other element which is to be gripped. In this latter case, the shock absorber  201 ″ may be coupled to this alternative rod or element which is to be gripped. 
     With reference to FIGS. 16,  16 A and  17 , the illustrated FIG. 15 form of rod gripping latch assembly will next be described. More specifically, the illustrated latching assembly  870  includes an outer cylindrical housing  890  defining an upper chamber  892  and an enlarged lower chamber  894 . Upper and lower end caps  896 ,  898  are threaded or otherwise secured to housing  890  to enclose the respective ends of the latch mechanism  870 . A piston  900  is positioned within housing  890  and has an upper portion  902  and a lower enlarged portion  904 . Upper portion  902  supports an O-ring seal  906  which engages the interior wall of housing  890  in the region of upper chamber  892 . Lower enlarged portion  904  of piston  900  carries an O-ring  908  which sealingly engages the interior wall of housing  890  in the region of the lower chamber  894 . A biasing spring  910  is positioned between end cap  898  and piston  900 . 
     The assembly also includes at least one rod gripping element and, in the illustrated form, includes four such elements  912 ,  913 ,  914  and  915 . Each of the illustrated elements  912  through  915  is spring biased away from the rod  206 ″. For example, the rod gripping elements may be supported by a spring  916  having an enlarged upper end portion  918  secured between the cap  896  and housing  890 . The lower exterior surface of the rod gripping elements, such as indicated at  918  for element  912 , may taper downwardly into a wedge-shaped configuration. With piston  900  in the position shown in FIG. 16, an upper surface of piston  902  bears against the wedge surfaces  918  to urge the rod gripping elements  912 - 915  against the rod. The rod  206 ″ may include a friction enhanced outer surface to facilitate gripping of the rod by the rod gripping elements. In addition, the surface of gripping elements  912 ,  915  which engages the rod may also be friction enhanced. As a specific example, a plurality of concentric ridges, such as indicated at  920 , may be provided along the length of rod  206 ″. These ridges, although not apparent from the figures, may extend along a region of rod  206 ″ of a length corresponding to the full range of seat height adjustment so that, regardless of the height to which the seat has been adjusted, ridges  920  are in position for gripping by the elements  912 - 915  at all seat height adjustments. Corresponding grooves may be provided as indicated at  922  in the rod gripping surface of the elements  912 - 915 . In effect, teeth are thus provided on the surfaces  920  and  922  which mesh together to lock the rod  206 ″ to the latching assembly  870  when piston  900  is positioned as shown in FIG.  16 . 
     With reference to FIG. 17, to disengage the latch assembly  870  from the rod  206 ″, pressurized fluid such as air is delivered via line  924  to that portion of chamber  894  between the seals  906  and  908 . This shifts piston  900  downwardly as shown in FIG.  17 . Due to the spring bias on rod engaging elements  912 - 915 , these elements shift outwardly away from the rod (see arrows  925 ) so as to disengage the surfaces  920 ,  922  from one another. This permits sliding movement of the latch assembly  870  in either direction along the rod as indicated by double headed arrow  926  during seat height adjustment. Following seat height adjustment, pressure may be relieved on line  924 , permitting spring  910  to shift piston  900  upwardly to cause elements  912 - 915  to re-engage the rod  206 ″. 
     Thus, the latching mechanism  870  also allows the use of a dampener capable of applying a non-linear dampening force to the seat. During seat height adjustment in this example, however, the home position of the dampener remains at the same elevation. That is, in this case, the seat can be moved relative to the dampener rather than moving the dampener with the motion of the seat. Again, latching mechanism  870  may be controlled in the same manner as previously described for latching assembly  220 . 
     FIG. 17A illustrates another form of latch mechanism which is similar to that shown in FIGS. 15,  16 ,  16 A and  17  and which may be used in the illustrated seat suspension embodiments. For purposes of convenience, components of the seat suspension latch of FIG. 17A which correspond to components of the embodiment of FIGS. 15,  16 ,  16 A and  17  are illustrated with the same numbers. These corresponding components will not be described in any detail. The latch shown in FIG. 17A is longitudinally more compact than the latch shown in FIGS. 16 and 17 and thus may be included, for example, in applications where space limitations are greater. 
     With reference to FIG. 17A, although a cap such as  898  in FIG. 16 may be used, a mechanically simple spring retainer  899  is shown for retaining the biasing spring  910  in position. An annular notch  901  is provided around the interior perimeter of housing  890  and adjacent to the lower end of this housing. The spring retainer includes fingers  905  which fit within notch  901  when the spring retainer  899  is installed. These fingers are deflected during installation of the retainer  899  and then snap into the notch  901  when in position. An annular shelf  907  is provided near the lower end of housing  890  and above the notch  901 . A horizontal radially extending disk-like body portion  911  is included in the illustrated retainer  899 . The upper surface of the distal end of the body portion  911  engages the shelf  907 . Fingers  905  project from the body portion. A central portion  913  of body portion  911  is raised as shown to define a seat which is inserted into the interior of the spring  910 . A downwardly extending collar portion  915  of body  911  surrounds the shaft  206 ″. The retainer  899  may be stamped or otherwise formed from a single piece of sheet metal or other suitable material. 
     The locking elements  912 ,  914  (again, one or more such locking elements may be provided, with four such elements being included in the illustrated embodiment), each include a friction enhanced surface, in this illustrated case a toothed shaft gripping surface  922 . The surface  922  selectively engages a friction enhanced surface on shaft  206 ″, in this case toothed surface  920 , when the shaft is engaged by the latch. 
     The gripping elements  912 ,  914  are shaped and biased toward an unlatched state or condition in which surfaces  922  and  920  are disengaged from one another. When spring  910  urges piston  900  upwardly as shown in FIG. 17A, surfaces  918 ,  919  engage one another. As a result, the surfaces  922  and  920  are shifted into cooperative latching engagement. Conversely, as explained previously in connection with FIGS. 16 and 17, when pressurized air is delivered via air supply line  924  to chamber  894 , the piston  900  shifts downwardly from the position shown in FIG.  17 A. As a result, the surfaces  920 ,  922  disengage one another. When disengaged, because of the biasing on latching elements  912 ,  914  (and elements  913  and  915 , not shown in FIG.  17 A), the latch gripping elements are released from the shaft. 
     In the specific example shown, this biasing of elements  912 ,  914  is accomplished by an annular wave spring  925  which engages an undersurface of a radially outwardly projecting flange  923  of each of these latch gripping elements. A gap  927  is provided between the upper surface of flange  923  and the under surface of cap element  872 . Consequently, when piston  900  moves downwardly in FIG. 17A, wave spring  925  pivots the latch elements  912 ,  914  about a rocking or pivot point  917  with flange  923  moving toward the cap  872 . This in turn shifts the latch surfaces  922  away from the rod  206 ″, releasing the rod from the latch gripping elements. An inwardly projecting rib portion  921  of the latch elements  912 ,  914  is positioned within a radially inwardly extending annular notch  929  formed in cap piece  872 . Consequently, the latch elements  912 ,  914  do not shift longitudinally within the housing  890  when piston  900  is moved. 
     Again, the construction of FIG. 17A illustrates one variation of the latch of FIGS. 16 and 17 which is longitudinally more compact. Also, the latch of FIG. 17A may be used in the orientation shown in FIG. 17A or in any other orientation, such as in the opposite (e.g. rotated 180 degrees) position from the position shown in this figure. 
     FIG. 18 illustrates yet another embodiment of a seat suspension system having features in common with the previously described systems. Elements in common with those previously described have been given the same numbers, with a triple prime (′″) superscript and for this reason will not be described in detail. In the embodiment of FIG. 18, the seat support member  22 ′″ is raised and lowered by air spring  270 ′″ by respectively inflating and deflating the air spring. Vertical motion of the seat member  22 ′″ is guided by a rod  930  extending upwardly from the floor  14 ′″ of the vehicle. A slide  932  is slidably mounted to the rod  930 . A latch  934  selectively couples the slide  932  to the rod. During seat height adjustment, the latch  934  may be operated in the same manner as previously described for latch mechanism  220 . Thus, for example, the latch  234  may be unlatched to permit sliding of slide  232  upwardly and downwardly along rod  930  as the air spring  270 ′″ raises or lowers the seat. In addition, as an example of this operation, after the seat height has been adjusted, the latch  934  may be actuated to latch the slide  932  to the rod. A biasing spring  936  urges the slide  932  to a central or home position. Enlarged collars  938 ,  940  are provided at the respective upper and lower ends of the slide  932 . Collar  938  engages the bottom surface  942  of seat support member  22 ′″ to limit the upward motion of the seat in response to vibrations. Conversely, downward motion is limited by the extent to which collar  940  may travel in a downward direction before spring  936  is fully compressed. The slide  932  has a central portion which passes through an opening  948  through seat support member  22 ′″. The opening  948  is sized to prevent passage of the collars and spring  936  through the seat support member. A damper  200 ′″, which may be a simply shock absorber, engages the upper surface  946  of seat support member  22 ′″ and is coupled by links  950 ,  952 ,  954  and  956  to the slide  936  and the seat support member  22 ′″. When the seat vibrates, link  952  pivots about pivot a  958 , with this motion being dampened by shock absorber damper  200 ′″. Link  954  supports pivot  952  to permit this motion. 
     A seat position sensor, such as leveling valve  230 ′″, may be used to adjust the inflation of the air spring and the seat elevation upon changes in loading on the seat when the slide is latched to the rod  930 . Leveling valve  230 ′″ is coupled by a link  232 ′″ to the collar  938  for this purpose. 
     FIGS. 19-22 disclose one form of a suitable pneumatic circuit and valve for the illustrated seat suspension embodiments. It should be understood that the valve arrangement and pneumatic circuits may be varied. However, the circuitry described below utilizes a valve (or two separate valves) actuated by a single lever or single switch for simultaneously controlling both seat elevation adjustment and the latching and unlatching of a latch mechanism. Although this construction is advantageous, separately actuated valves may also be used for accomplishing these results. 
     FIG. 19 illustrates the seat  12  shown schematically on a seat support  20 . The seat may be raised and lowered as previously described, such as by the air spring  270 . In addition, the system may include a latch, such as latch  220 . Air spring  270  and latch  220  are shown schematically in FIG.  19 . In addition, the optional automatic leveling valve  230  is also shown in this figure coupled to the air spring  270 . In addition, a valve  960  is shown for controlling both the air spring  270  during seat height adjustment and the latch  220 . 
     With reference to FIG. 20, pressurized air is supplied along a line P to both the valve  960  and the leveling valve  230 . Exhaust lines E are also shown coupled to these valves. In the configuration shown in FIG. 20, no automatic leveling is occurring, the latch actuator  390  is in position to latch the latch mechanism  220 , and the seat height is not being adjusted. Thus, the air spring line AB is neither being supplied with pressurized air nor being exhausted through either of the valves  960  or  230 . In this situation, assume the seat height changes while latch  220  is latched. In such a case, the link  232  shifts valve  230  either upwardly or downwardly. If the seat has raised sufficiently to operate the valve  230 , the valve  230  shifts upwardly in FIG. 20, coupling air spring line AB through valve  230  to exhaust line E, resulting in deflation of the air bag until such time in this example as the seat has been lowered to the point where the auto leveling valve no longer operates. Conversely, in this situation, if a load is added to the seat, causing the seat to depress or lower, link  232  causes valve  230  to shift downwardly in FIG.  20 . If the seat lowers sufficiently to operate the valve  230 , pressure supply line P is coupled through valve  230  to the air spring line AB so as to inflate the air spring. The air spring will continue to inflate in this example until auto leveling valve  230  is no longer actuated. As previously described, for example in connection with FIG. 1, if the dampening valve is unlatched and the seat height is adjusted, link  232  does not operate the auto leveling valve  230 . 
     Next, assume the seat occupant desires to raise the elevation of the seat. In this case, a manual actuation lever  962  is shifted upwardly in FIG.  20 . This couples the pressure supply line P through valve  960  to the air spring line AB, causing the air spring to inflate and raise the seat. The valve  960  may be biased to the neutral position shown in FIG. 20 so that, upon releasing of the lever  962 , the valve returns to the position shown in FIG.  20  and further inflation of the air spring stops. In addition, in the circuit illustrated in FIG. 20, as the valve  960  shifts upwardly in this figure, the line P is also coupled through the valve to the latch actuator line AC. Consequently, pressurized fluid is delivered to the latch actuator  390 , causing the latch actuator to release the latch. This relieves the dampening force from being applied to the seat. After the seat reaches its desired elevation and valve  960  returns to the position shown in FIG. 20, line AC is again exhausted, causing latch actuator  390  to control the latch  220  to latch the dampener mechanism into its operative dampening force applying state. 
     If it is desired to lower the seat, lever  962  is moved downwardly. In this case, line AB is coupled through valve  960  to the exhaust line E, causing the deflation of the air bag. Simultaneously, pressurized air is delivered from line P to line AC, causing the latch actuator  390  to release the latch  220  as described above for the case when the seat was being raised. 
     FIGS. 21A-21C illustrate one form of valve  960  having a common housing  966  within which a slide plate  964  is positioned for accomplishing the dual functions of controlling the latch and seat height adjustment as described above in connection with FIG.  20 . In contrast, in FIG. 20 valve  960  had two valves, one for the seat height adjustment control and one for latch/unlatch control, which were controlled by a common actuator (e.g. lever  962 ). As yet another alternative, each of these two valves may have separate actuators which may be mechanically or electronically linked for simultaneous operation. FIG. 21A corresponds to the condition of valve  960  depicted in FIG. 20, with the latch actuator  390  latched (line AC being exhausted) and the seat in a constant position (line AB being neither supplied with pressurized air or being exhausted through the valve  960 ). 
     FIG. 21B illustrates the slide plate  964  shifted to a position whereby valve  960  controls the lowering of the seat and the unlatching of the latch  220 . That is, air spring supply line AB is shown exhausted, resulting in a deflation of the air bag. Simultaneously, the pressure line is coupled to line AC to cause the latch actuator to unlatch the latch. 
     In FIG. 21C, the valve slide plate  964  is shown in a position for raising the seat. In this case, pressure line P is coupled to line AB to inflate the air spring. Simultaneously, pressure line P is coupled to line AC to cause the latch actuator to unlatch the latch  220 . In FIGS. 21A,  21 B and  21 C, the slide plate  964  is shown in a common housing  966 . As previously described, additional control approaches may also be used in operating the latching mechanism  220  and seat height adjuster. Thus, although the above approach is advantageous, other approaches may be used by which these devices operate in other sequences (for example, the latch actuator being operated during only a portion of a time the seat height adjuster is operated, or in a sequential manner). 
     FIGS. 22A-22E illustrate a specific form of dual function valve  960 . Valve  960  includes a cover  970  overlaying housing  966 . The cover has a slot  972  through which the lever  962  projects. The lever  362  may be raised and lowered as indicated by the arrows in FIG. 22A from the centered position shown in this figure. FIG. 22B illustrates the connection of lever  962  to the slide plate  964  and shows the lever and slide plate comprised of a one-piece homogeneous unitary construction. These elements may, for example, be injected molded of plastic. A wave spring  974  biases the slide plate against O-ring seals surrounding ports through housing  966 . These ports are connected to the respective lines P, AB, AC and E. One of these O-rings is indicated at  975  in FIGS. 22A and 22B. A coil spring  976  is coupled to lever  962  and engages flanges projecting inwardly from cover  970  (one such flange being indicated at  978  in FIG.  22 B). Spring  976  biases the lever  962  to its centered position. Suitable flow paths are defined in slide plate  964  and correspond to the centered position (FIGS.  21 A and  22 C); the lower seat position (corresponding to FIGS.  21 B and  22 D); and the raised seat position (corresponding to FIGS.  21 C and  22 E). Again, other valve arrangements and controls may be used, for example, electronic controls. However, the specifically illustrated approach employs a single switch lever  962  for simultaneously controlling the raising or lowering of the seat by the seat height adjuster and also latching and relatching of a latch mechanism utilized in a number of the disclosed embodiments. 
     Having illustrated and described the principles of our invention with reference to several embodiments, it should be apparent to those of ordinary skill in the art that these embodiments may be modified in arrangement and detail without departing from these principles. We claim all such modifications as fall within the scope of the following claims.