Patent Publication Number: US-2009218867-A1

Title: Plant Suspension System with Weight Compensation

Description:
FIELD OF INVENTION 
     This disclosure relates to plant suspension systems supporting loads of varying weight. 
     BACKGROUND 
     Plant suspension systems have long been employed in various vehicles to isolate cargo and/or personnel from jolts encountered by vehicles during travel across roads, across or through water, through air, etc. Such plant suspension systems may take any of a variety of forms, including suspended platforms atop which cargo is set and suspended seats in which personnel sit. Some of such plant suspension systems are passive suspension systems that employ springs of one or more types (e.g., mechanical springs, gas springs, etc.). Other plant suspension systems are active suspension systems that employ actuators of one or more types (e.g., hydraulic rams, linear motors, etc.). Still other plant suspension systems incorporate differing combinations of passive and active suspension elements. 
     In plant suspension systems in vehicles, especially vehicles traveling over land, it is commonplace to provide isolation from jolts directed in a generally vertical direction (i.e., along a substantially vertical axis). A subset of those plant suspension systems, also provide isolation from jolts directed in at least one generally horizontal direction (i.e., along at least one substantially horizontal axis, such as a “fore-aft” axis or a lateral axis). Unfortunately, a number of these plant suspension systems suffer from allowing the suspended plant to move along that at least one horizontal axis in a manner that changes depending on the weight of the load supported by the plant. In other words, a number of these plant suspension systems employ a design that attempts to achieve a balance between isolating a load from a horizontal jolt encountered by a vehicle and controlling the manner in which a load is permitted move horizontally as part of that isolation effort, but with the limitation that the balance is optimal only for what is deemed to be an average load weight. 
     Optimizing only for an average load weight means that a load having a weight less than that average load weight will have an inertia in any horizontal movement arising from a horizontal jolt that is all too easily overcome by the horizontal suspension elements such that the load is less effectively isolated from a horizontal jolt. In essence, such lighter loads are more readily subjected to a horizontal jolt than would a load having a load weight that matches the average load weight. 
     Further, optimizing for an average load weight also means that a load having a weight greater than that average load weight will have an inertia in any horizontal movement arising from a horizontal jolt that cannot be effectively overcome by the horizontal suspension elements as needed such that the load is less effectively isolated from a horizontal jolt. Indeed, a load having a weight greater than that average load weight may acquire enough inertia in a horizontal movement as to utterly overcome the horizontal suspension elements with the result that the load is subjected to “secondary” jolts arising from the plant reaching bump stops or other physical barriers that define a physical limit to a range of travel allowed for by the plant suspension system in a given horizontal direction. Beyond the concern of the load being repeatedly subjected to these secondary jolts, damage to the suspension system may result over time from repeated impacts between components of the suspension system occurring each time that end of that range of travel is reached. 
     It is unlikely that any plant suspension system will be used to support only loads having a weight that matches what is deemed to be the average load weight for which that plant suspension system is optimized, and indeed, it is far more likely that the majority of loads supported by any plant suspension system will have weights that are either less than or greater than (but not equal to) that average load weight. One solution that has been previously implemented is to incorporate the ability to manually adjust one or more suspension elements of plant suspension systems to accommodate different load weights so that the plant suspension system continues to behave optimally. However, where loads may be frequently changed or may even fluctuate in weight (such as where the load is a person), such manual adjustment becomes cumbersome, since such loads require frequent weighing to determine how their weight has changed in preparation for making such manual adjustments. 
     SUMMARY 
     An apparatus meant to be incorporated into a suspension system suspending a plant (i.e., an overall plant that includes a physical plant and possibly a load that the physical plant supports) of a vehicle acts to alter the spring constant of at least one spring of the suspension system in response to changes in the weight of the plant so that a resonant frequency of the at least one spring isolating the plant from a jolt encountered by the vehicle during travel remains substantially unchanged despite changes to the weight of the plant. 
     In one aspect, an apparatus comprising a suspension system coupled to a portion of a vehicle and isolating a plant of the vehicle from a jolt encountered by the vehicle during travel, and a first spring incorporated into the suspension system to isolate the plant from at least a portion of the jolt along a substantially horizontal axis through movement along the substantially horizontal axis at a predetermined resonant frequency, wherein the first spring has a variable spring constant that changes in response to changes in weight of the plant to substantially maintain the predetermined resonant frequency. 
     Implementations may include, and are not limited to, one or more of the following features. The plant may include a seat. The first spring may be a gas spring, a hydraulic spring, a spring employing a combination of gas and fluid, or a mechanical spring. The first spring may be linked to a second spring that transfers a change in a pressure of a gas, fluid or combination of gas and fluid to the first spring to adjust the spring constant of the first spring. The first spring may be linked to a second spring that transfers a change in a torque to the first spring to adjust the spring constant of the first spring. Also, a third spring acting at least partially in opposition to the first spring may also be linked to the second spring, such that the spring constant of the third spring is also adjusted so that the first and third springs cooperate to substantially maintain the predetermined resonant frequency. Further, these linkages may each incorporate a damper acting as a low pass filter that substantially permits the second spring to change the spring constant of the first spring (and/or of the third spring, if present) in response to a change in weight of the plant, but which substantially prevents the second spring from changing the spring constant of the first spring (and/or of the third spring, if present) in response to the second spring operating to isolate the plant from the jolt. 
     In one aspect, a method comprising adjusting a spring constant of a first spring of a plant suspension system of a vehicle encountering a jolt during travel in response to changes in weight of the plant to substantially maintain a predetermined resonant frequency of movement in isolating the plant from at least a portion of the jolt along a substantially horizontal axis. 
     Implementations may include, and are not limited to, one or more of the following features. Transferring a change in a pressure of a gas, fluid or combination of gas and fluid from a second spring to the first spring to adjust the spring constant of the first spring. Transferring a change in a torque from a second spring to the first spring to adjust the spring constant of the first spring. Adjusting a spring constant of a third spring acting at least partially in opposition to the first spring so that the first and third springs cooperate to substantially maintain the predetermined resonant frequency. Dampening a transfer of a pressure or a torque between the second spring and the first spring (and/or the third spring, if present) in a manner that serves as a low pass filter that substantially permits the second spring to change the spring constant of the first spring (and/or of the third spring, if present) in response to a change in weight of the plant, but which substantially prevents the second spring from changing the spring constant of the first spring (and/or of the third spring, if present) in response to the second spring operating to isolate the plant from the jolt. 
     Other features and advantages of the invention will be apparent from the description and claims that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a plant suspension system. 
         FIG. 2  depicts another plant suspension system. 
     
    
    
     DETAILED DESCRIPTION 
     It should be noted that although the following discussion and accompanying figures center on implementations of a plant suspension system in which the overall plant includes a physical plant implemented as a seat in which a person sits, what is disclosed in that discussion is also applicable to other implementations of plant suspension systems. Other possible forms of suspended plant include, and are not limited to, a suspended trailer floor of a tractor trailer truck, a suspended cabinet in a recreational vehicle, a suspended personnel cabin on board an airplane, a suspended pool table on board a sea vessel, and a whole suspended room on board a sea vessel. Still other possible implementations of suspended plant to which what is disclosed herein is applicable will be clear to those skilled in the art. 
     It should also be noted that although this discussion centers on suspension systems addressing jolts along substantially horizontal and/or vertical axes and/or planes, this should not be construed as a directional limitation. What is disclosed and claimed herein may be applied to suspension systems configured to address jolts occurring in any given direction, including in rotational directions, and may be applied regardless of how directions of movement are described (e.g., with reference to Cartesian, polar or other coordinate systems). Further, directional terms such as “horizontal” and “vertical” are meant to provide a form of shorthand description for structures that are substantially horizontal or vertical at a time when a vehicle is substantially level with the Earth or substantially plumb, and should not be taken as imposing a requirement of being precisely horizontal or vertical. As those skilled in the art will readily recognize, it is not uncommon for portions of a vehicle that are oriented substantially horizontally or vertically while the vehicle is substantially level with the Earth or substantially plumb (and therefore are referred to as “horizontal” or “vertical” for ease of discussion) to cease to actually be substantially horizontal or vertical as the vehicle is operated to climb or descend inclines, or to be otherwise positioned so as to no longer be level with the Earth or substantially plumb. This same understanding also applies to other directional terms such as “upward,” “downward,” “forwardly” and “rearwardly.” 
       FIG. 1  shows a form of plant suspension system  1000  isolating a load that it supports from jolts arising from forces acting on a vehicle during vehicular travel. The plant suspension system  1000  incorporates a physical plant  110 , a vertical suspension system  130 , and a horizontal suspension system  150 . The vertical suspension system  130  isolates the physical plant  110  from jolts occurring along a substantially vertical axis, and the horizontal suspension system  150  isolates the physical plant  110  from jolts occurring along at least one substantially horizontal axis. It should be noted that although the plant suspension system  1000  is depicted as being in the form of a seat suspended in relation to a vehicle floor  190 , as has already been discussed, the plant suspension system  1000  may take any of a variety of forms, and further, those skilled in the art will readily recognize that the physical plant  110  may be suspended in relation to any of a variety of other portions of a vehicle into which the plant suspension system  1000  is installed. 
     As will be explained in greater detail, the plant suspension system  1000  incorporates a load weight compensation capability in which the resonant frequency of the movement along at least one axis of movement of the horizontal suspension system  150  remains substantially constant despite changes in overall plant weight arising from changes in the weight of a load (not shown) supported by the physical plant  110 . In embodiments where the physical plant  110  is a seat, it is preferred that this resonant frequency be maintained close to or within the range of 1 Hz to 2 Hz for personnel comfort. Discomfort arising from induced movement of organs within the human body, vibration of skeletal structures and/or other physical effects have been observed in suspension systems where a resonant frequency well outside this range has been permitted. In other embodiments where the physical plant  110  is not meant to support people, a different resonant frequency (or range of resonant frequencies) may be chosen that is deemed more appropriate to the characteristics of whatever load is expected to be supported. 
     As will be explained in greater detail, a transfer of gaseous and/or hydraulic pressure from the vertical suspension system  130  to the horizontal suspension system  150  is employed to adjust a spring constant of one or more suspension elements of the horizontal suspension system  150  to accommodate loads of differing weights supported by the physical plant  110  and maintain a substantially constant resonant frequency. However, it should also be noted that although the plant suspension system  1000  is depicted and discussed herein as having suspension elements operating along substantially horizontal and vertical axes, as has already been discussed, alternate forms of the plant suspension system  1000  may employ suspension elements operating along axes of any orientation. Therefore, this discussion of interaction between horizontal and vertical suspension elements should be taken as being but an example, and not meant to be so limiting. 
     As depicted, the horizontal suspension system  150  is a passive suspension system incorporating a pair of suspension elements  155  and  156  implemented as gas and/or hydraulic springs acting in opposition to each other. In essence, the suspension elements  155  and  156  allow the physical plant  110  to move along that substantially horizontal axis to isolate a load supported by the physical plant  110  from jolts acting on a vehicle along that axis. However, as those skilled in the art will readily recognize, this depicted quantity, configuration and type of technology of suspension elements is but one example of a wide variety of possible quantities, configurations and types of technologies that may be employed in any given implementation of the horizontal suspension system  150 . For example, a hybrid of hydraulic and gas-based operation may be employed. Further, possible implementations of the horizontal suspension system  150  may be partially active suspension systems incorporating one or more actuators (in addition to the passive suspension elements  155  and  156 ) that actively move the physical plant  110  along at least one axis in a substantially horizontal plane under the control of a controller (not shown) that responds to indications of horizontal accelerations of a vehicle. These other possible implementations may employ the suspension elements  155  and  156  to assist such actuators making up an active portion of the horizontal suspension system  150 , and/or to take over for such actuators in the event of a malfunction of such an active portion of the horizontal suspension system  150 . 
     As is further depicted, the vertical suspension system  130  incorporates a single suspension element  135  implemented as a gas and/or hydraulic spring acting generally in opposition to the force of gravity which tends to pull the physical plant  110  downwards towards the Earth. In essence, the suspension element  135  allows the physical plant  110  to move along a substantially vertical axis to isolate a load supported by the physical plant  110  from jolts acting on a vehicle along that axis. However, as is the case with the horizontal suspension system  150 , those skilled in the art will readily recognize that the depicted quantity, configuration and type of technology of suspension elements of the vertical suspension system  130  is but one example of a wide variety of possible quantities, configurations and types of technologies that may be employed in any given implementation of the vertical suspension system  130 . For example, a hybrid of hydraulic and gas-based operation may be employed. Further, possible implementations of the vertical suspension system  130  may be partially active suspension systems incorporating one or more actuators in addition to the suspension element  135 . As is the case with the horizontal suspension system  150 , these other possible implementations may employ the suspension element  135  to assist such actuators making up an active portion of the vertical suspension system  130 , and/or to take over for such actuators in the event of a malfunction of such an active portion of the vertical suspension system  130 . 
     The suspension element  135  of the vertical suspension system  130  is coupled to each of the suspension elements  155  and  156  of the horizontal suspension system  150  through a pair of linkages  142  through which gas and/or liquid is able to flow between the suspension element  135  and each of the suspension elements  155  and  156 . In this way, gas and/or liquid is employed to transfer pressure through the linkages  142  such that the linkages  142  are correctly characterized as being gas-based linkages and/or hydraulic linkages, respectively. More specifically, gas and/or liquid pressure arising within the suspension element  135  as a result of the weight of a load and the physical plant  110  (i.e., the overall plant weight) are transferred to the suspension elements  155  and  156  through the linkages  142 . This transfer of pressure has the effect of altering the gas and/or liquid pressures within each of the suspension elements  155  and  156  to thereby alter the spring constants of each of the suspension elements  155  and  156 . This allows the spring constants of each of the suspension elements  155  and  156  to be automatically adjusted in response to the different weights of different loads (presuming that the weight of the physical plant  110  does not change), such that the spring behavior of the suspension elements  155  and  156  is caused to become stiffer in response to heavier loads and to become less stiff in response to lighter loads. 
     It may be deemed desirable for the plant suspension system  1000  to substantially maintain a selected resonant frequency (or a resonant frequency within a selected range of resonant frequencies) of movement of the physical plant  110  in counteracting jolts. This resonant frequency or range of resonant frequencies may be selected based on various characteristics of the load expected to be supported by the physical plant  110 . By way of example, where the load is expected to include personnel, then as previously mentioned, it may be desirable for the plant suspension system  1000  to maintain a resonant frequency within the range of 1 Hz to 2 Hz for the comfort of those personnel. By way of another example, where the load is expected to include a volume of liquid, it may be desirable for the plant suspension system  1000  to maintain a resonant frequency calculated to minimize sloshing based on viscosity or another characteristics of that liquid. Other types of loads may have any of a number of characteristics making a different resonant frequency or range of resonant frequencies more desirable. To accomplish this, various characteristics of the suspension elements  135 ,  155  and  156 , and/or various gas and/or liquid characteristics may be chosen to cause horizontal movement under the control of the horizontal suspension system  150  to substantially maintain a selected resonant frequency independent of the weights of various loads supported by the physical plant  110 . More precisely, one or more of these various characteristics may be chosen to ensure that the manner in which the spring constants of the suspension elements  155  and  156  are altered in response to the weight of different loads causes the suspension elements  155  and  156  to substantially maintain a selected resonant frequency (or a resonant frequency within a selected range of resonant frequencies) for movement arising from the horizontal suspension system  150  responding to jolts along a substantially horizontal axis. 
     Each of these linkages  142  incorporates corresponding dampers  145  and  146  to control the rates of flow of gas and/or liquid between the suspension element  135  and each of the suspension elements  155  and  156 . The dampers  145  and  146  prevent the spring-like nature of the suspension elements  155  and  156  from being substantially defeated as a result of gas and/or liquid being allowed to flow all too freely between suspension elements  155  and  156 . The dampers  145  and  146  also substantially prevent all too brief changes in pressure of gas and/or liquid within the suspension element  135  arising from jolts along a substantially vertical axis from being transferred to either of the suspension element  155  and  156 . To put this another way, the dampers  145  and  146  serve as low pass filters to allow only relatively low frequency changes in pressure of gas and/or liquid within the suspension element  135  to be transmitted through the linkages  142  to the suspension elements  155  and  156 . Therefore, relatively short duration changes in pressure within the suspension element  135 , such as might occur due to a vertical jolt arising during vehicle travel, will be substantially isolated from the suspension elements  155  and  156 . In contrast, relatively long duration changes in pressure with the suspension element  135 , such as might occur due to the loading or unloading of a load supported by the physical plant  110 , will be conveyed to the suspension elements  155  and  156  through the linkages  142 . In this way, the pressure within the suspension element  135 , which bears a relationship to the weight of the load supported by the physical plant  110  (again, presuming that the weight of the physical plant  110  does not change, so that the weight of the load is the only portion of the plant weight that varies), is used to adjust the spring constants of each of the suspension elements  155  and  156  for that weight. By way of example, where the load is expected to include personnel, a time constant of 5 seconds to 10 seconds may be selected. 
     Although the coupling of the suspension element  135  to each of the suspension elements  155  and  156  is depicted as being implemented with entirely separate ones of the linkages  142 , those skilled in the art will readily recognize that the linkages  142  may be implemented in a wide variety of other configurations of tubing and/or piping. Further, although the linkages  142  are depicted and described as directly conveying gas and/or liquid between the suspension element  135  and each of the suspension elements  155  and  156 , those skilled in the art will readily recognize that the linkages  142  may be implemented in a more indirect form incorporating one or more gas-based and/or hydraulic relays and/or other devices providing an indirect transfer of pressure. 
     Although the linkages  142  are depicted as incorporating physically distinct dampers  145  and  146  positioned amidst each of the linkages  142 , those skilled in the art will readily recognize that the dampers  145  and/or  146  may take any of a wide variety of forms, and may be positioned at the point of connection between the linkages  142  and one or more of the suspension elements  135 ,  155  and  156 . By way of example, at least a portion of the very tubing and/or piping of which the linkages  142  are formed may have an internal diameter chosen to be small enough to serve as a damper. By way of another example, each of the dampers  145  and  146  may be implemented as a sintered metal plug having dimensions and a porosity chosen to achieve a selected rate at which gas and/or liquid flows therethrough. Further, where the linkages  142  incorporate one or more relays and/or other devices to provide an indirect transfer of pressure, such relays and/or other devices may also serve as one or both of the dampers  145  and  146 . 
     In addition to the transfer of gas and/or liquid between the suspension element  135  and each of the suspension elements  155  and  156  through the linkages  142 , the suspension elements  155  and  156  may be more directly coupled through another linkage  143  to more directly permit gas and/or liquid to be transferred between them in some embodiments. In a manner not unlike the linkages  142 , the linkage  143  incorporates another damper  149 , again to prevent the spring behavior of the suspension elements  155  and  156  from being substantially defeated. The linkage  143  may be provided where it is deemed desirable to permit a flow of gas and/or liquid between the suspension elements  155  and  156  at rate greater than what is possible indirectly through the linkages  142  and both of the dampers  145  and  146 . 
     It should be noted that although the horizontal suspension system  150  has been depicted as implemented with the opposing pair of separate and distinct suspension elements  155  and  156 , those skilled in the art will readily recognize that the suspension elements  155  and  156  may alternatively be physically combined into a single dual-chamber suspension element in which each chamber is coupled to the suspension element  135  through the dampers  145  and  146 . Further, where such a dual-chamber suspension element is employed, and where it is desired to have a more direct flow of gas and/or liquid between those two chambers (such as what is provided through the linkage  143  incorporating the damper  149 ), such a coupling may be made via a passage formed directly between the two chambers with the passage being formed to have dimensions chosen to allow the passage to serve as an implementation of the linkage  143  incorporating the damper  149 . 
     Given that the suspension element  135  is a gas and/or hydraulic spring, as has been discussed, the suspension element  135  may be coupled to one or both of a supply valve  132  to add gas and/or fluid to the suspension element  135  and a bleed valve  133  to release gas and/or fluid from the suspension element  135 . In some embodiments, the supply valve  132  and the bleed valve  133  are employed to allow the distance of the physical plant  110  from the vehicle floor  190  to be adjusted. Where the physical plant  110  is a seat, such a distance may be made adjustable for the comfort of personnel sitting in it. In other embodiments, the supply valve  132  and the bleed valve  133  may be operable by a controller (not shown) to actively move the physical plant  110  closer to and further away from the vehicle floor  190  to counteract jolts, such that the vertical suspension system  130  thereby becomes (at least partially) an active suspension system. 
       FIG. 2  shows another form of plant suspension system  2000  also isolating a load that it supports from jolts arising from forces acting on a vehicle as a result of vehicular travel. It should be noted that due to a number of substantial similarities between the plant suspension system  1000  of  FIG. 1  and the plant suspension system  2000  of  FIG. 2 , corresponding elements have been designated with identical numerical labels. Like the plant suspension system  1000  of  FIG. 1 , the plant suspension system  2000  of  FIG. 2  incorporates a physical plant  110 , a vertical suspension system  130  isolating the physical plant  110  from jolts occurring along a substantially vertical axis, and a horizontal suspension system  150  isolating the physical plant  110  from jolts occurring along at least one substantially horizontal axis. The plant suspension system  2000  also incorporates a load weight compensation capability in which the resonant frequency of the movement along at least one axis of movement of the horizontal suspension system  150  remains substantially constant despite changes in load weight. Further, although the plant suspension system  2000  is depicted as being in the form of a seat suspended in relation to a vehicle floor  190 , in other embodiments, the physical plant  110  may be suspended in relation to any of a variety of other portions of a vehicle into which the plant suspension system  2000  is installed. 
     Like the horizontal suspension system  150  of the plant suspension system  1000 , the horizontal suspension system  150  of the plant suspension system  2000  is depicted as being a passive suspension system incorporating a pair of suspension elements  155  and  156  acting in opposition to each other. However, unlike horizontal suspension system  150  of the plant suspension system  1000 , the suspension elements  155  and  156  in the plant suspension system  2000  are implemented as coiled mechanical springs. Further, like the suspension elements  155  and  156  of the plant suspension system  1000 , the suspension elements  155  and  156  of the plant suspension system  2000  allow the physical plant  110  to move along a substantially horizontal axis to isolate a load supported by the physical plant  110  from jolts acting along that axis. As those skilled in the art will readily recognize, this depicted quantity, configuration and type of technology of suspension elements is but one example of a wide variety of possible quantities, configurations and types of technologies that may be employed in any given implementation of the horizontal suspension system  150 . Further, possible implementations of the horizontal suspension system  150  may be partially active suspension systems incorporating one or more actuators in addition to the passive suspension elements  155  and  156 . These other possible implementations may employ the suspension elements  155  and  156  to assist such actuators, and/or to take over for such actuators in the event of their malfunction. 
     Like the vertical suspension system  130  of the plant suspension system  1000 , the vertical suspension system of  130  of the plant suspension system  2000  is depicted as incorporating a single suspension element  135  acting generally in opposition to the force of gravity which tends to pull the physical plant  110  downwards towards the Earth. However, unlike the vertical suspension system  130  of the plant suspension system  1000 , the suspension element  135  of the plant suspension system  2000  is implemented as a coiled mechanical spring. Further, like the suspension element  135  of the plant suspension system  1000 , the suspension element  135  of the plant suspension system  2000  allows the physical plant  110  to move along a substantially vertical axis to isolate a load supported by the physical plant  110  from jolts acting along that axis. As is the case with the horizontal suspension system  150 , those skilled in the art will readily recognize that the depicted quantity, configuration and type of technology of suspension elements of the vertical suspension system  130  is but one example of a wide variety of possible quantities, configurations and types of technologies that may be employed in any given implementation of the vertical suspension system  130 . Further, possible implementations of the vertical suspension system  130  may be partially active suspension systems incorporating one or more actuators in addition to the suspension element  135 . As is the case with the horizontal suspension system  150 , these other possible implementations may employ the suspension element  135  to assist such actuators, and/or to take over for such actuators in the event of their malfunction. 
     As is depicted, one end of the coil of the suspension element  135  of the vertical suspension system  130  is coupled to one end of the coils of each of the suspension elements  155  and  156  of the horizontal suspension system  150  through a triplet of intermeshed beveled toothed gears of a linkage  147  to transfer torque among the suspension elements  135 ,  155  and  156 . In contrast to the linkages  142  and  143  of the plant suspension system  1000  being gas-based and/or hydraulic in nature, the transfer of torque through the linkage  147  of the plant suspension system  2000  results in the linkage  147  that is correctly characterized as a mechanical linkage. The other ends of the coils of each of these suspension elements is fixed in a manner that does not allow those ends to rotate relative to the rest of the plant suspension system  2000 . A shaft of the linkage  147  that couples one end of the coil of the suspension element  135  to its corresponding one of the triplet of gears of the linkage  147  extends through a drag brake  148  incorporated into the linkage  147  to introduce a predetermined amount of friction acting against the transfer of torque among these three suspension elements, thereby acting as a damper to control the rate at which torque is transferred. 
     As those familiar with coil springs will readily recognize, as a coil spring is compressed such that the ends of a coil are moved towards each other along the axis of the coil, the diameter of the coil tends to increase (i.e., the coil tends to expand radially) and/or one end of the coil tends to rotate relative to the other end in a rotational direction that tends to increase the quantity of windings in the coil. Therefore, as load weight and the weight of the physical plant  110  (i.e., the weight of the overall plant) bear down on the suspension element  135 , the resulting compression of the coil of the suspension element  135  causes the end of that coil that is coupled to one of the triplet of gears of the linkage  147  to rotate in a direction that corresponds to an increase in the number of windings in that coil. As that rotation occurs, corresponding ends of the coils of the suspension elements  155  and  156  that are also coupled to corresponding ones of the triplet of gears of the linkage  147  are rotated in a direction that actually tends to decrease the number of windings in those coils. By rotating the coils of the suspension elements  155  and  156  in a direction that tends to decrease the number of windings, a greater resistance against being compressed is introduced into the coils of each of the suspension elements  155  and  156 , thereby increasing the stiffness of their spring behavior. The degree of rotation of the rotatable ends of the coils of all three of these suspension elements is increased with any increase in the weight of the load supported by the physical plant  110 , and correspondingly, the stiffness of the spring behavior of each of the suspension elements  155  and  156  is also increased with any increase in the weight of the load (again, presuming that the weight of the physical plant  110  is unchanging). In this way, and not unlike the suspension elements  155  and  156  of the plant suspension system  1000 , the spring constants of each of the suspension elements  155  and  156  of the plant suspension system  2000  are increased as the weight of the load increases. 
     In a manner somewhat like the dampers  145  and  146  of the plant suspension system  1000 , the drag brake  148  of the plant suspension system  2000  serves as a damper to substantially prevent spurious alterations in the spring constants of each of the suspension elements  155  and  156  arising from the suspension element  135  counteracting jolts along a substantially vertical axis. The friction introduced by the drag brake  148  against rotational movement prevents the triplet of gears of the linkage  147  from rotating quickly enough to transfer spurious rotations between the coils of these three suspension elements. To put this another way, the drag brake  148  in its role as a damper functions as a low pass filter allowing only relatively low frequency rotations in the coil of the suspension element  135  to be transmitted to the coils of the suspension elements  155  and  156 . In this way the spring constants of the suspension elements  155  and  156  are altered to compensate for the weight of the physical plant  110  and the load (i.e., the weight of the overall plant), but not altered in response to the suspension element  135  serving to isolate the physical plant  110  and the load from jolts along a substantially vertical axis. 
     In the plant suspension system  2000 , one or more physical characteristics of the coils (e.g., dimensions, choice of material, etc.) of the suspension elements  155  and  156  are sized and/or selected relative to those same characteristics of the coil of the suspension element  135  to ensure that the resonant frequency of movement of the physical plant  110  along the substantially horizontal axis of the horizontal suspension system  150  is substantially maintained independent of variations in the weight of the load. 
     Other implementations are within the scope of the following claims and other claims to which the applicant may be entitled.