WO2013112544A1

WO2013112544A1 - Variable stiffness suspension mechanism

Info

Publication number: WO2013112544A1
Application number: PCT/US2013/022691
Authority: WO
Inventors: Olugbenga Moses ANUBI; Carl David CRANE, III; Shannon C. RIDGEWAY
Original assignee: University Of Florida Research Foundation, Inc.
Priority date: 2012-01-23
Filing date: 2013-01-23
Publication date: 2013-08-01

Abstract

A variable stiffness mechanism (301, 801, 1301, 1801) includes a cantilever arm (310) having a first end (313) rotatably attachable to a sprung mass (304), and having a second end (314) attachable to an unsprung mass (303); a variable strut (320) having a top (321) slidably attached to a bracket (330) and having a bottom (322) slidably attached the cantilever arm, wherein the bracket is attachable to the sprung mass; and at least one device (340) for relocating orientation of the variable strut. The variable stiffness mechanism has an effective spring constant between the sprung mass and the unsprung mass, and wherein the effective spring constant depends on a frequency of excitation of the unsprung mass. A suspension system (300, 800, 1300, 1800) includes the variable stiffness mechanism in a cooperative arrangement with a conventional spring (302).

Description

VARIABLE STIFFNESS SUSPENSION MECHANISM

Statement Regarding Federally Sponsored Research

This invention was made with Government support under Contract No.: DE-FG04-86NE37967. The Government may have certain rights in this invention. Cross-Reference to Related Applications

This application is based upon and claims priority to U.S. Provisional Patent Application Serial No. 61/589,624, entitled "VARIABLE STIFFNESS MECHANISM", filed January 23, 2012, the disclosure of which is hereby incorporated by reference in its entirety.

Field of the Invention The present invention generally relates to the field of suspension systems, and more particularly relates to variable geometry suspension systems.

Background of the Invention

Springs are used in many common applications such as in automotive suspension systems. Typically, a spring has an unchanging spring constant, which is a ratio of an applied external force to a resulting change in length of the spring.

Attempts at improving passive suspension designs have utilized one of three techniques:

adaptive, semi-active, and fully active suspension. An adaptive suspension utilizes a passive spring and an adjustable damper with slow response to improve the control of ride comfort and road holding. A semi-active suspension is similar, except that an adjustable damper has a faster response and a damping force is controlled in real-time. A fully active suspension replaces the adjustable damper with active elements, such as hydraulic, pneumatic and electromagnetic damper control, which can achieve optimum vehicle control, but at a high cost due to design complexity. As a result of research in semi-active suspension systems, a gap in capabilities between semi-active and fully active suspension systems has been narrowed. Semi-active suspension systems are widely used in the automobile industry due to their small weight and volume, as well as low energy consumption compared to fully active suspension systems. A semi-active suspension systems may include one or both of a fast response magneto-rheological (MR) damper and a fast response electro-rheological (ER) damper. ER and MR fluids are composed of a suspension of polarized solid particles dispersed in a non-conducting liquid. In known passive suspension systems, both a viscous damping coefficient and a spring rate of suspension elements are usually used as optimization arguments.

Theoretically, a semi-active suspension system involves both damping modulation and stiffness modulation. However, most practical implementations of semi-active suspension systems only control a viscous damping coefficient of a shock absorber while keeping stiffness constant. This is partly due to a relatively lower energy requirement for damping modulation. Another reason is due to unavailability of pragmatic low-power stiffness modulation methods. Furthermore, it has been shown that a combined variation of stiffness and damping achieves a better performance than a variation of damping or stiffness alone.

Semi-active suspension systems fall into a general class of variable damper, variable lever arm, and variable stiffness. Variable damper semi-active suspension systems are capable of varying the damping coefficients across their terminals. Initial practical implementations were achieved using a variable orifice viscous damper. By closing or opening the variable orifice, damping characteristics change from soft to hard and vice versa. With time, use of ER and MR fluids replaced the use of variable orifices. When an electric field (or a magnetic field for MR) is imposed, the polarized solid particles align along a direction of the electrical field. When this happens, a yield stress of the ER or MR fluid changes, hence a damping effect occurs.

Controllable rheological properties make ER and MR fluids suitable for use as smart materials for active devices, transforming electrical energy to mechanical energy.

Variable lever arm semi-active suspension systems conserve energy between the suspension and spring storage. Such suspension systems are characterized by controlled force variation which consumes minimal power. The main idea behind their operation is a variation of a force transfer ratio, which is achieved by moving a point of force application. If the point of force application moves orthogonally to an acting force, then, theoretically, no mechanical work is involved in the control. This phenomenon is called reciprocal actuation.

The variable stiffness feature of known semi-active suspension systems is achieved either by changing a free length of a spring or by a mechanism that changes a stiffness of the suspension system using one or more moving parts. In one known variable stiffness semi-active suspension system, a hydro-pneumatic spring with a variable stiffness characteristic is used. In another known variable stiffness semi-active suspension system, a desired stiffness variation is achieved by augmenting a variable lever arm type system with a traditional, or conventional, passive suspension system. The control of semi-active suspension systems has been a subject of much research. An initial aim of a controlled suspension was solely centered on ride comfort. One of the initial control concepts developed is a sky-hook damper. The sky-hook damper is a fictitious damper between a sprung mass and an inertial frame (fixed in the sky). A damping force of the sky-hook damper reduces vibration of the sprung mass. A similar concept, called ground-hook, has also been developed for road-friendly suspension systems. These control concepts have also been applied to semi-active suspensions. Other control concepts that have been applied to semi-active and fully active suspension systems include: optimal control, robust control, and robust optimal control. A known variable geometry actuator that reduces power consumption of active suspension elements is a 3D concept called a Delft Active Suspension (DAS). The DAS concept includes a wishbone that can be rotated over an angle and is linked to a pretensioned spring at a variable length. The spring pretension generates an effective actuator force at an end of the wishbone. A force is controlled by varying a position of the pretensioned spring via an electric motor. While the DAS has inherent variation in stiffness, it is disadvantageous because it may be a source of discomfort to a passenger in a vehicle and/or may lead to instability in an absence of a secondary spring. Also, there is a possibility of fail-safe issues as with most active suspension systems. A modified fixed spring design with optimized geometry, called electromechanical low-power active suspension (eLPAS), may overcome some challenges posed by the DAS concept, in that a moving mass is reduced. Packaging and stiffness variation issues were overcome by optimized geometry. However, ensuring a good fail-safe behavior remains an open problem.

A "suspension system" is a collective term given to system of springs, damper and linkages that isolates a vehicle body (sprung mass) from a wheel assembly (unsprung mass).

An "independent suspension system" is a suspension system that has no linkages between two hubs of a same axle.

Summary of the Invention

In one embodiment, a variable stiffness mechanism is disclosed. The variable stiffness mechanism comprises a cantilever arm having a first end rotatably attachable to a sprung mass, and having a second end attachable to an unsprung mass; a variable strut having a top slidably attached to a bracket and having a bottom slidably attached the cantilever arm. The bracket is attachable to the sprung mass. The variable stiffness mechanism also comprises a first device for relocating the top of the variable strut along a horizontal slot on the bracket, thereby causing a location of engagement of the variable strut with the bracket to relocate. The variable stiffness mechanism also comprises a second device for relocating the bottom of the variable strut along a longitudinal slot on the cantilever arm, thereby causing a location of engagement of the variable strut with the cantilever arm to relocate. The variable stiffness mechanism has an effective spring constant between the sprung mass and the unsprung mass. The effective spring constant depends at least on one or both of: the location of engagement of the variable strut with the bracket and the location of engagement of the variable strut with the cantilever arm. In another embodiment, a variable stiffness mechanism is disclosed. The variable stiffness mechanism comprises a cantilever arm having a first end rotatably attachable to a sprung mass, and having a second end attachable to an unsprung mass; a variable strut having a top attachable to the sprung mass and having a bottom slidably attached the cantilever arm; and a device for relocating the bottom of the variable strut along a longitudinal slot on the cantilever arm, thereby causing a location of engagement of the variable strut with the cantilever arm to relocate. The location of engagement of the variable strut with the cantilever arm depends on a frequency of excitation of the unsprung mass.

In yet another embodiment, a variable stiffness mechanism is disclosed. The variable stiffness mechanism comprises a cantilever arm having a first end rotatably attachable to a sprung mass, and having a second end attachable to an unsprung mass; a variable strut having a top slidably attached to a bracket and having a bottom attached the cantilever arm. The bracket is attachable to the sprung mass. The variable stiffness mechanism also comprises a device for relocating the top of the variable strut along a horizontal slot on the bracket, thereby causing a location of engagement of the variable strut with the bracket to relocate. The variable stiffness mechanism has an effective spring constant between the sprung mass and the unsprung mass. The effective spring constant depends on at least the location of engagement of the variable strut with the bracket.

In still another embodiment, a variable stiffness mechanism is disclosed. The variable stiffness mechanism comprises a cantilever arm having a first end rotatably attachable to a sprung mass, and having a second end attachable to an unsprung mass; a variable strut having a top slidably attached to a bracket and having a bottom slidably attached the cantilever arm; and a device for relocating the variable strut along a horizontal slot on the bracket, thereby causing a location of engagement of the variable strut with the bracket to relocate and causing a location of engagement of the variable strut with the cantilever arm to relocate. The bracket is attachable to the sprung mass. The location of engagement of the variable strut with the bracket and the location of engagement of the variable strut with the cantilever arm depend on a frequency of excitation of the unsprung mass. In a further embodiment, a suspension system is disclosed. The suspension system comprises a conventional strut having a top coupled to a sprung mass and having a bottom coupled to an unsprung mass. The suspension system also comprises a variable stiffness mechanism that cooperates with the conventional strut. The variable stiffness mechanism includes a cantilever arm with a first end rotatably attached to the sprung mass and with a second end coupled to the unsprung mass; a variable strut with an top slideably attached to the sprung mass and a bottom that engages the cantilever arm at a location of engagement; and a device for relocating the variable strut, thereby causing the location of engagement to relocate. The stiffness of the variable stiffness mechanism between the sprung mass and the unsprung mass depends on at least a distance between the location of engagement and the first end of the cantilever arm. The distance between the location of engagement and the first end of the cantilever arm depends on a frequency of excitation of the unsprung mass.

Brief Description of the Drawings

The accompanying Figures, in which like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention, in which:

FIG. 1 is a schematic of a lever arm. FIG. 2 is a schematic of an idealized quarter car model of a suspension system.

FIG. 3 is a perspective view of a suspension system in accordance with one embodiment of the invention including a variable stiffness mechanism in accordance with the invention, and a conventional spring. FIG. 4 is a front view of the suspension system of FIG. 3, with the variable stiffness mechanism at high stiffness.

FIG. 5 is a front view of the suspension system of FIG. 3, with the variable stiffness mechanism at low stiffness. FIG. 6 is a side view of the suspension system of FIG. 3, showing cutline 7-7. FIG. 7 is a cut view through cutline 7-7 of FIG. 6.

FIG. 8 is a perspective view of a suspension system in accordance with another embodiment of the invention including a variable stiffness mechanism in accordance with the invention, and a conventional spring. FIG. 9 is a front view of the suspension system of FIG. 8, with the variable stiffness mechanism at high stiffness.

FIG. 10 is a front view of the suspension system of FIG. 8, with the variable stiffness mechanism at low stiffness.

FIG. 11 is a side view of the suspension system of FIG. 8, showing cutline 12-12. FIG. 12 is a cut view through cutline 12-12 of FIG. 11.

FIG. 13 is a perspective view of a suspension system in accordance with yet another embodiment of the invention including a variable stiffness mechanism in accordance with the invention, and a conventional spring.

FIG. 14 is a front view of the suspension system of FIG. 13, with the variable stiffness mechanism at high stiffness.

FIG. 15 is a front view of the suspension system of FIG. 13, with the variable stiffness mechanism at low stiffness.

FIG. 16 is a side view of the suspension system of FIG. 13, showing cutline 17-17.

FIG. 17 is a cut view through cutline 17-17 of FIG. 16. FIG. 18 is a perspective view of a suspension system in accordance with still another embodiment of the invention including a variable stiffness mechanism in accordance with the invention, and a conventional spring. FIG. 19 is a front view of the suspension system of FIG. 18, with the variable stiffness mechanism at high stiffness.

FIG. 20 is a front view of the suspension system of FIG. 18, with the variable stiffness mechanism at low stiffness. FIG. 21 is a side view of the suspension system of FIG. 18, showing cutline 22-22.

FIG. 22 is a cut view through cutline 22-22 of FIG. 21.

FIG. 23 is a graph of magnitude versus frequency illustrating car body acceleration. FIG. 24 is a graph of magnitude versus frequency illustrating suspension deflection. FIG. 25 is a graph of magnitude versus frequency illustrating dynamic tire force. FIG. 26 is a graph of acceleration versus distance illustrating car body acceleration. FIG. 27 is a graph of deflection versus distance illustrating suspension deflection. FIG. 28 is a graph of force versus distance illustrating dynamic tire force.

Detailed Description As required, detailed embodiments of the present invention are disclosed herein. However, it is to be understood that the disclosed embodiments are merely examples of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure and function. Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the invention.

The terms "a" or "an", as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language). The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically. The variable stiffness suspension system in accordance with the invention improves suspension performance by modulating suspension force. An variable stiffness suspension system comprises, in one embodiment, a conventional MacPherson-type passive suspension system augmented, in parallel, with a variable stiffness mechanism. The variable stiffness suspension system includes reciprocal actuation and has stiffness variation control that is orthogonal to gravity. The variable stiffness suspension system uses spring and damper forces that are in linear regions of their operating ranges. Best performance may be achieved by optimally varying both the stiffness and damping (lower performance index) rather than by varying either the stiffness or damping alone. Therefore, a semi-active version of the variable stiffness suspension system varies both the stiffness and damping of the suspension element.

The variable stiffness suspension system improves suspension performance by varying stiffness in response to road disturbance. By "stiffness" it is meant an effective stiffness between a sprung mass and an unsprung mass between which is coupled the variable stiffness suspension system. The variable stiffness suspension system was analyzed using a quarter car model that incorporates kinematic details of the mechanical design. A passive system shows superior performance over a conventional counterpart. Passive refers to a case where only pure mechanical elements such as springs and dampers are used. In other words, no active force generating element is used or any control algorithm implemented. As a result, the problem of ensuring a good fail-safe behavior is addressed. The following is a basic forward analysis of the variable stiffness. In a published paper entitled "Design and Analysis of a Variable Stiffness Mechanism", by Anubi et al., which is hereby fully incorporated herein, a new variable stiffness mechanism was presented. The variable stiffness mechanism is a simple arrangement of two springs, a lever arm and a pivot bar. The stiffness is varied by changing a horizontal position d of the pivot while keeping a point of application of an external load constant. It was shown in the aforesaid published paper that the effective stiffness is a rational function of d. FIG. 1 illustrates a variable stiffness mechanism (hereinafter "mechanism") 100 that is a modification of the variable stiffness mechanism described in the aforesaid published paper. Lever arm PAC is pinned to, and free to rotate about, a fixed point C. Spring AB is constrained to remain vertical and can translate horizontally as indicated by the double-headed arrow. Point A slides inside a lever arm while point B remains at ground level. Without loss of generality, an external force F is assumed to act vertically downwards at point P on the lever which has a horizontal distance L from C. Θ is a clockwise inclination of a lever arm from a horizontal plane. H is a height of a fixed point C from the ground, d is a length of segment AC of the lever arm. A goal is to vary an overall stiffness of the mechanism 100 by adjusting d accordingly. This is achieved through a control force u. Let k and 1₀ be a spring constant and a free length, respectively, of the spring AB, and let δ be vertical deflection of point P. An overall free length δο of the mechanism 100 is defined as vertical displacement of point P when no external force is acting on the mechanism. Summing moments about C and equating to zero yields

Hence, a force equation of the system becomes

where

and

are the overall stiffness and the overall free length of the mechanism 100, respectively. In other words, the whole system can be represented as a single spring whose spring constant is determined by d according to Equation (3). When spring AB is on a left side of force F, a resultant spring is stiff er than the spring AB. When the spring AB is on a right side of the force F, the resultant spring is softer. Softness or stiffness of the resultant spring depends on how far right/left of force F is the spring AB. Hence, by using the control force u to drive d, the overall stiffness of the mechanism 100 can be varied dynamically.

Referring now to FIG. 2. The stiffness characteristics of a variable stiffness suspension system (hereinafter "suspension system") 200 are examined, as follows. The sprung and unsprung masses of the suspension system 200 are isolated by a parallel combination of a variable stiffness mechanism and a MacPherson strut. An idealized quarter car model shown in FIG. 2 comprises a quarter car body (sprung mass), the MacPherson strut, a variable strut, a wishbone and a wheel assembly (unsprung mass). The term "strut" means a combination of a spring and a damper or shock absorber. The sprung mass is suspended from the unsprung mass by the MacPherson strut and the variable strut. The wishbone is free to rotate about point O. Points A and B are fixed on a vehicle, or car, body. Points D and F are such that line DF always remains perpendicular to line CE. Point D can slide along line CE. Generalized coordinates are chosen as vertical displacement y_s of the sprung mass, horizontal displacement d of the variable strut and rotation angle Θ of the wishbone. Assuming that the MacPherson strut is near vertical, the stiffness between the sprung and unsprung masses is approximately

where k_s and k are the spring constants of the MacPherson strut and the variable strut, respectively, and 1 is a length of the wishbone. k_eff(d) varies from a minimum value of k_s when d = 0 to a maximum value of k_s + ksec²9 as d approaches 1 . This shows that the suspension system 200 can continuously vary the stiffness between the sprung and unsprung mass in the range [k_s, k_s + 2k], θ ε [-π/2, π/2] in response to road disturbance. The following is an explanation of nonlinear dynamics modeling and linearization about an equilibrium configuration. A detailed dynamic modeling of the system using the Lagrange equation of motion was performed. The assumptions adopted in FIG. 2 are summarized as follows: (1) Horizontal movement of the sprung mass is neglected, i.e., only vertical displacement y_s is considered. (2) Sprung and unsprung masses are lumped at some points, hence are assumed to be particles. (3) Coil spring deflection, tire deflection, deflection force and damping force are in linear regions of their operating ranges.

Let:

y_u vertical displacement of the unsprung mass (wheel assembly),

lu distance between points X and X',

1₃ distance between F and a center of mass of a variable strut along DF,

h_u distance between points G and X',

Is, l's equilibrium and non-equilibrium lengths of the MacPherson strut, respectively,

1₃, l's equilibrium and non-equilibrium lengths of the variable strut, respectively,

coordinates of point B measured from a reference frame at point O on a chassis,

1 length of a wishbone,

I length of the segment OF of the wishbone,

k_c, b_c spring constant and damping coefficient of a control strut,

k_t, b_t tire spring constant and damping coefficient,

b_s, b₃ damping coefficients of the MacPherson and variable struts, respectively, m_s, m_u sprung and unsprung masses,

m₃ mass of the variable strut,

I moment of inertia of the wishbone,

e_F unit vector from O to F along the wishbone. Position vectors, P_B, P_D, P_F, and Ρ_χ of the points B, D, F and X are given by:

where

is the position vector of point O and the unit vector ê_F is given by

The change in length, Δ1₃, of the variable strut is given by

Similarly, the change in length, Δ1₈, of the MacPherson strut is given by

where

and the operator " || || " denotes the Euclidean norm operator which is defined for the vector

as

The respective position vectors of the centers of mass of the variable strut and the unsprung mass are

Differentiating Equations (12), (13), (14) and (15) yields

Let the generalized coordinate vector be defined as

and be the kinetic energy, potential energy and Rayleigh dissipation

functions of the system respectively, then

where

Substituting Equations (16) through (18) into Equations (20), (24) and (25) yields

and

Define the Lagrangian as

then, the Lagrange equation of motion is given by

which implies that

Where the inertia matrix M(q) is given by

the Coriolis/centripetal matrix is given by

the damping matrix B(q) is given by

where

the stiffness factor K(q) is given by

the gravity factor G(q) is given by

and the input vector W(q) is given by

Performance of a passive system was evaluated by comparing frequency response of the suspension system 200 with a conventional MacPherson counterpart. The performance of suspension systems are usually evaluated using the following measures: ride comfort, suspension deflection, and road holding.

Ride comfort is the amount of vibration experienced by a passenger on a vehicle. It is quantified using the force transferred to a chassis. Because force is proportional to acceleration, ride comfort is given in terms of chassis acceleration,

Suspension deflection characterizes an amount of deflection undergone by the suspension element. Suspension deflection provides a metric to determine a tendency of the suspension element to exceed its physical deflection limit. Suspension deflection is given in terms of an angular displacement Θ of the wishbone.

Road holding is an extent to which the road-tire contact is maintained. Road holding is characterized by deflection undergone by a tire. Hence, road holding or dynamic tire force is given in terms of a deflection y_u - r undergone by the tire.

The suspension system 200 shows improved performance in terms of ride comfort, suspension deflection and road holding. FIGS. 23-25 show the frequency responses for the suspension system 200 and for a conventional MacPherson suspension system. An L₂-gain analysis-based feedback controller and a state feedback control were designed. The L2-gain analysis-based feedback controller was designed using the concept of energy dissipation. Feedback control gains were obtained by solving a system of linear matrix inequalities which resulted from the L₂-gain analysis. An L₂-gain analysis-based feedback controller further improves the performance of the system by compensating for unattainable stiffness and damping in a passive case of the suspension system 200. The passive case of the suspension system 200 shows better performance over the conventional counterpart which implies a good fail-safe behavior.

In a time domain simulation, performance of a closed loop system of the passive case of the suspension system 200 and performance of a conventional MacPherson suspension were compared. In the time domain simulation, a vehicle traveling at a steady horizontal speed of 40mph was subjected to a road bump of height 10cm. Responses (chassis acceleration, suspension deflection and tire deflection) were compared for the conventional MacPherson suspension, and for the passive and active cases of the suspension system 200. FIGS. 26, 27 and 28 show car body acceleration, suspension deflection, and dynamic tire force responses, respectively. In the time domain simulation, the suspension system 200 provided globally better performance than the conventional MacPherson suspension in terms of ride comfort, suspension deflection and road holding.

FIG. 3 is a perspective view of a variable stiffness suspension system (hereinafter "suspension system") 300 in accordance with one embodiment of the invention. The suspension system 300 comprises a variable stiffness mechanism (hereinafter "mechanism") 301 in accordance with the invention and a conventional MacPherson strut 302. The suspension system 300 and the conventional MacPherson strut 302 act in cooperation with each other in that each is coupled to a same sprung mass and to a same unsprung mass. The suspension system 300 and the conventional MacPherson strut 302 are arranged in a parallel configuration.

The mechanism 301 includes a cantilever arm, or wishbone, 310 having arms 311 and 312, and a control strut 352 disposed between the arms. The wishbone 310 includes a longitudinal slot 317. The wishbone 310 is coupled to a sprung mass by a revolute joint 315. The sprung mass is not shown except for a small portion 304. In one embodiment, the sprung mass is an automobile. In such embodiment, a first end 313 of the wishbone 310 is coupled to the frame or the body of the automobile. In one embodiment, the unsprung mass 303 is a wheel assembly which may include a hub, hub extensions, a wheel, a tire, brake components, etc. In such embodiment, a second end 314 of the wishbone 310 is coupled to the wheel assembly by a ball joint 318. The mechanism 301 also includes a variable strut (hereinafter "strut") 320 having a top 321 and a bottom 322. The strut 320 includes a spring 324 and a damper 326. In one embodiment, the spring 324 is a linear spring. In one embodiment, the spring 324 is a coil spring. In other embodiments, the spring 324 is another type of spring, such as a gas spring. The spring 324 has a spring constant k and a free length 1. The mechanism 301 has an effective spring constant, or stiffness, k_eff that is advantageously variable. In general, k_eff of the mechanism 301 is less or equal to k of the spring 324. In one embodiment, the damper 326 is a shock absorber. The mechanism 301 further includes a bracket 330 attachable to the sprung mass by an attachment member 333 and by an attachment member 334. There is no relative movement between the bracket 330 and the sprung mass because of the attachment member 333 and the attachment member 334. The bracket 330 includes a substantially horizontal slot 331. The top 321 of the strut 320 is slidably attached to the bracket 330. The bottom 322 of the strut 320 is slidably attached the wishbone 310.

The mechanism 301 includes a first device 340 that can relocate the top 321 of the strut 320 along the horizontal slot 331 on the bracket 330, thereby causing a location of engagement 345 of the strut with the bracket to relocate. The first device 340 relocates the top 321 of the strut 320 along the horizontal slot 331 on the bracket 330, depending on a frequency of excitation of the unsprung mass. As result of the top 321 of the strut 320 being relocated along the horizontal slot 331 on the bracket 330, a location of engagement 345 of the strut with the bracket relocates. In one embodiment, the first device 340 includes an upper control strut 342 having one end 341 coupled to the bracket 330 and having another end 343 coupled to the top 321 of the strut 320 by a revolute joint 327. In one embodiment, the upper control strut 342 comprises a linear coil spring 344 and a damper 346. The linear coil spring 344 experiences some compression due the weight of the vehicle. The mechanism includes a second device 350 that can relocate the bottom 322 of the strut 320 along the longitudinal slot 317 on the wishbone 310, thereby causing a location of engagement 355 of the strut with the wishbone 310 to relocate. The second device 350 relocates the bottom 322 of the strut 320 along the longitudinal slot 317 on the wishbone 310, depending on a frequency of excitation of the unsprung mass. As result of the bottom 322 of the strut 320 being relocated along the longitudinal slot 317 on the wishbone 310, a location of engagement 355 of the strut with the wishbone relocates. In one embodiment, the second device 350 is disposed between the arms 311, 312 of the wishbone 310. In one embodiment, the second device 350 includes a lower control strut 352 having one end 351 coupled to the wishbone 310 and having another end 353 coupled to the bottom of the strut 320 by a revolute joint 328. In one embodiment, the lower control strut 352 comprises a linear coil spring 354 and a damper 356. The linear coil spring 354 experiences some compression due the weight of the vehicle. FIG. 3 shows the mechanism 301 in an intermediate state of stiffness.

The mechanism 301 has an effective spring constant, or stiffness, between the sprung mass 304 and the unsprung mass that depends on an orientation of the strut 320 relative to the other portions of the mechanism. More specifically, the stiffness of the mechanism 301 between the sprung mass and the unsprung mass depends at least on one or both of: the location of engagement 345 of the strut 320 with the bracket 330, and the location of engagement 355 of the strut with the wishbone 310. The location of engagement 345 of the strut 320 with the bracket 330 and the location of engagement 355 of the strut with the wishbone 310 depend on a frequency of excitation of the unsprung mass.

FIG. 4 is a front view of the suspension system 300 with the mechanism 301 at high stiffness.

For example, a distance from the sprung mass of the location of engagement of the strut 320 with the bracket 330 increases as the frequency of excitation of the unsprung mass decreases. As a result, the orientation of the strut 320 becomes more nearly vertical. The stiffness of the mechanism 301 increases as the strut 320 becomes more nearly vertical. Therefore, the stiffness of the mechanism 301 increases as the frequency of excitation of the unsprung mass decreases.

A smooth road tends to create lower frequency, rather than higher frequency, excitations.

Consequently, the stiffness of the mechanism 301 advantageously increases when the vehicle is on a smooth road.

FIG. 5 is a front view of the suspension system 300 with the mechanism 301 at low stiffness. For example, a distance from the sprung mass of the location of engagement of the strut 320 with the bracket 330 decreases as the frequency of excitation of the unsprung mass increases. As a result, the orientation of the strut 320 becomes farther from vertical. The stiffness of the mechanism 301 decreases as the strut becomes farther from vertical. Therefore, the stiffness of the mechanism 301 decreases as the frequency of excitation of the unsprung mass increases. A bumpy road tends to create higher frequency, rather than lower frequency, excitations.

Consequently, the stiffness of the mechanism 301 advantageously decreases when the vehicle is on a bumpy road. FIG. 6 is a side view of the suspension system 300, showing cutline 7-7.

FIG. 7 is a cut view through cutline 7-7 of FIG. 6. FIG. 7 shows the mechanism 301 at a high stiffness. The suspension system 300 shown in FIGS. 3-7 is a suspension system for a land vehicle, and more specifically, a suspension system for a front wheel of an automobile. The suspension system 300 shown in FIGS. 3-7 is configured as an independent suspension system. More specifically, the suspension system 300 shown in FIGS. 3-7 is configured as a single wishbone, independent suspension system. Other configurations of the suspension system 300 are foreseeable.

FIG. 8 is a perspective view of a suspension system 800 in accordance with another embodiment of the invention including a variable stiffness mechanism (hereinafter "mechanism") 801 in accordance with the invention, and the conventional spring 302. The mechanism 801 comprises the wishbone 310 having a first end 313 rotatably attachable to a sprung mass, and having a second end 314 attachable to an unsprung mass 303. The strut 320 has a top 321 that is attachable to the sprung mass and has a bottom 322 that is slidably attached the wishbone. The mechanism 801 also comprises a device 850 for relocating the bottom of the strut along a longitudinal slot 317 on the wishbone 310, thereby causing a location of engagement 855 of the strut 320 with the wishbone 310 to relocate. The location of engagement 855 of the strut 320 with the wishbone 310 depends on a frequency of excitation of the unsprung mass. The top of the strut 320 is coupled to a fixed location 848 on the sprung mass by a revolute joint 827. In one embodiment, such as shown in FIG. 8, the top of the strut 320 is coupled to a fixed location 848 on the bracket 330 by a revolute joint 827. FIG. 9 is a front view of the suspension system 800 with the mechanism 801 at high stiffness. The bottom 322 of the strut 320 is toward the left side of FIG. 10, which means that the location of engagement 855 of the strut with the wishbone 310 is nearer to the left side of FIG. 9.

FIG. 10 is a front view of the suspension system 800 with the mechanism 801 at low stiffness. The bottom 322 of the strut 320 is toward the right side of FIG. 10, which means that the location of engagement 855 of the strut with the wishbone 310 is nearer to the right side of FIG. 9.

FIG. 11 is a side view of the suspension system 800, showing cutline 12-12.

FIG. 12 is a cut view through cutline 12-12 of FIG. 11. FIG. 12 shows the mechanism 801 at a high stiffness. The portions of the suspension system 800 shown in FIGS. 8-12 that also appear in the suspension system 300 shown in FIGS. 2-7 have analogous function, and, therefore, are not described in detail again. FIG. 13 is a perspective view of a suspension system 1300 in accordance with yet another embodiment of the invention including a variable stiffness mechanism (hereinafter

"mechanism") 1301 in accordance with the invention, and a conventional spring. The mechanism 1301 comprises the wishbone 310 having a first end 313 rotatably attachable to a sprung mass a second end 314 attachable to an unsprung mass 303. The mechanism 1301 comprises the strut 320 having a top 321 slidably attached to the bracket 330 and having a bottom 322 attached the wishbone. The bracket 330 is attachable to the sprung mass. The mechanism 1301 also comprises a device 1340 for relocating the top of the strut 320 along the horizontal slot 331 on the bracket 330, thereby causing the location of engagement 345 of the strut with the bracket to relocate. The bottom 322 of the strut 320 is coupled to a fixed location 1358 on the wishbone 310 by the revolute joint 328.

FIG. 14 is a front view of the suspension system 1300, with the mechanism 1301 at high stiffness. The top 321 of the strut 320 is toward the left side of FIG. 14, which means that the location of engagement 345 of the strut with the bracket 330 is nearer to the left side of FIG. 14. FIG. 15 is a front view of the suspension system 1300, with the mechanism 1301 at low stiffness. The top 321 of the strut 320 is toward the right side of FIG. 14, which means that the location of engagement 345 of the strut with the bracket 330 is nearer to the right side of FIG. 14.

FIG. 16 is a side view of the suspension system 1300, showing cutline 17-17. FIG. 17 is a cut view through cutline 17-17 of FIG. 16. FIG. 17 shows the mechanism 1301 at a high stiffness. The portions of the suspension system 1300 shown in FIGS. 13-17 that also appear in the suspension system 300 shown in FIGS. 2-7 have analogous function, and, therefore, are not described in detail again.

FIG. 18 is a perspective view of a suspension system 1800 in accordance with still another embodiment of the invention including a variable stiffness mechanism (hereinafter

"mechanism") 1801 in accordance with the invention, and a conventional spring. The mechanism 1801 comprises the wishbone 310 having a first end 313 rotatably attachable to a sprung mass and a second end 314 attachable to an unsprung mass 303. The mechanism 1801 also comprises the strut 320 having a top 321 slidably attached to the bracket 330 and having a bottom 322 slidably attached the wishbone 310, wherein the bracket is attachable to the sprung mass. The mechanism 1801 further comprises a device 1840 for relocating the strut 320 along the horizontal slot 331 on the bracket 330, thereby causing the location of engagement 1845 of the strut with the bracket to relocate and causing the location of engagement 1855 of the strut with the wishbone to relocate. The location of engagement 1845 of the strut 320 with the bracket 330 and the location of engagement 1855 of the strut with the wishbone 310 depend on a frequency of excitation of the unsprung mass. FIG. 19 is a front view of the suspension system 1800, with the mechanism 1801 at high stiffness. The strut 320 is toward the left side of FIG. 19, which means that the location of engagement 345 of the strut with the bracket 330 and the location of engagement 1855 of the strut with the wishbone 310 are nearer to the left side of FIG. 14.

FIG. 20 is a front view of the suspension system 1800, with the mechanism 1801 at low stiffness. The strut 320 is toward the right side of FIG. 19, which means that the location of engagement 345 of the strut with the bracket 330 and the location of engagement 1855 of the strut with the wishbone 310 are nearer to the right side of FIG. 14.

FIG. 21 is a side view of the suspension system 1800, showing cutline 22-22.

FIG. 22 is a cut view through cutline 22-22 of FIG. 21. FIG. 21 shows the mechanism 1801 at a high stiffness. The portions of the suspension system 1800 shown in FIGS. 18-22 that also appear in the suspension system 300 shown in FIGS. 2-7 have analogous function, and, therefore, are not described in detail again.

FIG. 23 is a graph of magnitude versus frequency illustrating car body acceleration.

FIG. 24 is a graph of magnitude versus frequency illustrating suspension deflection. FIG. 25 is a graph of magnitude versus frequency illustrating dynamic tire force.

FIG. 26 is a graph of acceleration versus distance illustrating car body acceleration.

FIG. 27 is a graph of deflection versus distance illustrating suspension deflection.

FIG. 28 is a graph of force versus distance illustrating dynamic tire force.

An advantage of the mechanism 301 is power saving. The direction of travel of the control struts is advantageously not against gravity. In other embodiments, instead of the control struts, a type of active actuator can be used, such as hydraulic or pneumatic. In yet other embodiments, a semi-active (MR, ER, etc.) system can be used instead of the control struts. The mechanism 301 is, in one embodiment, a passive system in which no sensors, actuators, or computer control elements are needed or used.

It should be clear by now that the variable stiffness mechanism 301 has an effective spring constant between the sprung mass and the unsprung mass that depends on orientation of the strut 320.

Low frequency excitation occurs when a vehicle is traveling on a smooth road or when road height changes slowly. Typically, low frequency means 4-10MHz. At lower frequency excitation, there is minimal transfer of energy from the near- vertical strut 320 to the horizontal control struts 342, 352. High frequency excitation occurs when a vehicle goes over a bump, or when the road height changes rapidly. Typically, high frequency means higher than 10MHz. At higher frequency excitation, there is maximum transfer of energy from the near- vertical strut 320 to the horizontal control strut 342, 352. Referring now to FIG. 5, this transfer of energy causes the top 321 of the strut 320 to move to the right, which causes the control struts 342, 352 to become more compressed. As the upper control strut 342 compresses, the top 321 of the strut 320 moves to the right, as shown in FIG. 5. As the top 321 of the strut 320 moves to the right in FIG. 5, the stiffness becomes less because the strut 320 moves farther from the near- vertical position. The farther that the top 321 of the strut 320 moves to the left, the less stiff is the suspension because the strut moves farther from a vertical position. The closer the strut 320 is to the vertical position, the greater is the stiffness provided by the strut.

At low frequencies (frequencies below the natural frequency of the equivalent single spring system), the spring in the strut 320 responds faster than the road excitation. As a result, all the input energy gets transferred through the strut 320 to the car body, then to a passenger. Because this energy is low, the resultant vibration felt by the passenger is low. Also, because all the input energy is absorbed by the spring in the strut 320, the bottom 322 of the strut does not move much, thereby causing the deflection of the control strut, from the near- vertical position, to be minimal.

However, at high frequencies (frequencies above the natural frequency of an equivalent single spring system), the spring 324 in the strut 320 responds slower than the road excitation. As a result, only part of the input energy is transferred through the strut. The rest of the input energy is transferred to the control strut 342, 352. This causes the strut 320 to deflect farther from the near- vertical position. The sprung mass and the unsprung mass vibrate at different frequencies because road input energy is partially absorbed by the suspension system 300. The frequency of vibration of the unsprung mass is closer to the road excitation frequency which determines (and advantageously varies) the stiffness of the suspension system 300. It is the control strut(s) 342, 352 of the suspension system 300 that partial absorbs the road input energy.

It should now be clear that the control strut advantageously becomes maximally compressed when subjected to high frequencies excitations and becomes minimally compressed when subjected to low frequencies excitations. It is a combination of the mechanism design

(kinematic arrangement) of the mechanism 301 and the conservative nature of springs which cause this behavior.

A conventional suspension lacks a control strut. In a conventional suspension, the bottom of its suspension strut is fixed which means that the conventional suspension lacks any means for facilitating partial absorption of the road input energy by the suspension strut. As a result, all the input energy is transferred to the passenger through the suspension strut. Only the energy dump by the shock absorber is available. This is a reason that the variable stiffness suspension system achieves a significant improvement in performance over the conventional system at high frequencies.

Similar results can be obtained for low frequency excitations by adding a linear actuator to the control strut. A carefully designed control law can modulate transfer of energy even at low frequency, thereby ensuring a significant improvement across the entire frequency spectrum.

Pure linear springs are conservative, which means they absorb energy during a half-cycle and supply the absorbed energy during the remaining half of the cycle. Overall, net energy absorbed/supply by linear springs over a complete cycle is zero. Therefore, having a system purely composed of springs does not dissipate energy. All the absorbed energy will be given back to the system over the next half-cycle. This is where dampers come into play. By their nature, dampers "dump" energy. This means that they dissipate, in the form of heat, mechanical energy. By adding dampers (shock absorbers) to the upper control strut 342 and to the lower control strut 352, the energy absorbed by them over a half-cycle is dissipated and are not available to the system over the next half-cycle. In this way, the input energy is attenuated. Effect of amplitude of excitations on the stiffness is only significant at high frequencies.

However, the amplitudes at high frequencies are assumed to be within stability margins of the system. High frequency amplitudes correspond to the depths of pot-holes, speed humps, bumps and the general roughness of the road. Unnecessarily high amplitude at high frequencies will result in instability, the effect of which can cause the vehicle to overturn, for example.

However, this is more of a property of the road and the speed of the vehicle than the suspension system. The direction and amount that control strut moves left or right depends on several factors such as the frequency of excitation, the stiffness of all the springs, the values of the damping coefficient of the shock absorbers, and the masses and the inertias of the rigid bodies. The drawings show the arrangement of the mechanical elements, not the actual behavior.

The mechanism 301, 801, 1301 and 1801 does not include the unsprung mass 303 or the sprung mass. Rather, the variable suspension mechanism 301, 801, 1301 and 1801 can be attached to the unsprung mass 303 and/or to the sprung mass (or the unsprung mass 303 and/or the sprung mass can be attached to the mechanism 301, 801, 1301 and 1801).

The mechanism 301, 801, 1301 and 1801 is more fully described in the published paper entitled, "Variable Stiffness Suspension Mechanism: Design, Analysis and ControV , by Anubi, et al., which is hereby incorporated by reference in its entirety.

The terms "upper", "lower", "top", "bottom", "left", "right" and the like, in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein.

Unless stated otherwise, terms such as "first" and "second" are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements.

Although specific embodiments of the invention have been disclosed, those having ordinary skill in the art will understand that changes can be made to the specific embodiments without departing from the spirit and scope of the invention. Another application of the suspension system 300, 800, 1300 and 1800 is for use in active vibration control for an earthquake resistant building. Another application of the suspension system is for a hand-held video camera mount whose purpose is to dampen the shaking motion of a camera. The scope of the invention is not to be restricted, therefore, to the specific embodiments, and it is intended that the appended claims cover any and all such applications, modifications, and embodiments within the scope of the present invention. What is claimed is:

Claims

1. A variable stiffness mechanism (301), comprising:

a cantilever arm (310) having a first end (313) rotatably attachable to a sprung mass (304), and having a second end (314) attachable to an unsprung mass (302);

a variable strut (320) having a top (321) slidably attached to a bracket (330) and having a bottom (322) slidably attached the cantilever arm, wherein the bracket is attachable to the sprung mass;

a first device (340) for relocating the top of the variable strut along a horizontal slot (331) on the bracket (330), thereby causing a location of engagement (345) of the variable strut with the bracket to relocate; and

a second device (350) for relocating the bottom of the variable strut along a longitudinal slot (317) on the cantilever arm, thereby causing a location of engagement (355) of the variable strut with the cantilever arm to relocate,

wherein the variable stiffness mechanism (301) has an effective spring constant between the sprung mass and the unsprung mass, and wherein the effective spring constant depends at least on one or both of: the location of engagement (345) of the variable strut with the bracket and the location of engagement (355) of the variable strut with the cantilever arm.

2. The variable stiffness mechanism of claim 1, in which one or both of: the location of engagement (345) of the variable strut with the bracket and the location of engagement (355) of the variable strut with the cantilever arm, depends on a frequency of excitation of the unsprung mass.

3. The variable stiffness mechanism of claim 1, in which the variable strut (320) comprises a spring (324) and a damper (326).

4. The variable stiffness mechanism of claim 3, in which the spring (324) is a linear coil spring and the damper (326) is a shock absorber.

5. The variable stiffness mechanism of claim 2, in which the first device (340) includes an upper control strut (342) having one end (341) coupled to the bracket and another end (343) coupled to the top of the variable strut.

6. The variable stiffness mechanism of claim 5, in which the upper control strut comprises a coil spring (344) and a damper (346).

7. The variable stiffness mechanism of claim 5, in which the first device (340) is coupled to the top of the variable strut by a re volute joint (327).

8. The variable stiffness mechanism of claim 2, in which the second device (350) includes a lower control strut (352) having one end (351) coupled to the cantilever arm and another end (353) coupled to the bottom of the variable strut.

9. The variable stiffness mechanism of claim 8, in which the lower control strut comprises a coil spring (354) and a damper (356).

10. The variable stiffness mechanism of claim 8, in which the second device (350) is coupled to the bottom of the variable strut by a revolute joint (324).

11. The variable stiffness mechanism of claim 1, in which the sprung mass is a land vehicle and the unsprung mass is a wheel assembly.

12. The variable stiffness mechanism of claim 11, in which the cantilever arm is a wishbone of a suspension system of the land vehicle.

13. The variable stiffness mechanism of claim 12, including a conventional strut (302) having one end coupled to the unsprung mass (303) and another end coupled to the sprung mass.

14. A variable stiffness mechanism (801), comprising:

a cantilever arm (310) having a first end (313) rotatably attachable to a sprung mass, and having a second end (314) attachable to an unsprung mass (303);

a variable strut (320) having a top (321) attachable to the sprung mass and having a bottom (322) slidably attached the cantilever arm; and

a device (850) for relocating the bottom of the variable strut along a longitudinal slot (851) on the cantilever arm, thereby causing a location of engagement (855) of the variable strut with the cantilever arm to relocate,

wherein the location of engagement (855) of the variable strut with the cantilever arm depends on a frequency of excitation of the unsprung mass.

15. The variable stiffness mechanism of claim 14, in which the top of the variable strut is coupled to a fixed location (848) on the sprung mass by a revolute joint (827).

16. The variable stiffness mechanism of claim 14, in which the variable stiffness mechanism (801) has an effective spring constant between the sprung mass and the unsprung mass, and wherein the effective spring constant depends on at least the location of engagement (855) of the variable strut with the cantilever arm.

17. The variable stiffness mechanism of claim 14, in which the effective spring constant increases as the frequency of excitation of the unsprung mass decreases, and in which the effective spring constant decreases as the frequency of excitation of the unsprung mass increases.

18. The variable stiffness mechanism of claim 14, in which the variable strut (320) comprises a spring (324) and a damper (326).

19. The variable stiffness mechanism of claim 18, in which the spring (324) is a coil spring and the damper (326) is a shock absorber.

20. The variable stiffness mechanism of claim 15, in which the device (850) includes a control strut (852) having one end (851) coupled to the cantilever arm and another end (853) coupled to the bottom of the variable strut.

21. The variable stiffness mechanism of claim 20, in which the control strut comprises a coil spring (344) and a damper (346).

22. The variable stiffness mechanism of claim 20, in which the control strut is coupled to the bottom of the variable strut by a revolute joint (828).

23. The variable stiffness mechanism of claim 14, in which the sprung mass is a land vehicle and the unsprung mass is a wheel assembly.

24. The variable stiffness mechanism of claim 23, in which the cantilever arm is a wishbone of a suspension system of the land vehicle.

25. A variable stiffness mechanism (1301), comprising:

a variable strut (320) having a top (321) slidably attached to a bracket (330) and having a bottom (322) attached the cantilever arm, wherein the bracket is attachable to the sprung mass; and

a device (1340) for relocating the top of the variable strut along a horizontal slot (331) on the bracket (330), thereby causing a location of engagement (345) of the variable strut with the bracket to relocate,

wherein the variable stiffness mechanism has an effective spring constant between the sprung mass and the unsprung mass, and wherein the effective spring constant depends on at least the location of engagement (345) of the variable strut with the bracket.

26. The variable stiffness mechanism of claim 25, in which the location of engagement (345) of the variable strut with the bracket depends on a frequency of excitation of the unsprung mass.

27. The variable stiffness mechanism of claim 26, in which a distance between the location of engagement and the first end of the cantilever arm increases as the frequency of excitation of the unsprung mass decreases, and in which a distance between the location of engagement and the first end of the cantilever arm decreases as the frequency of excitation of the unsprung mass increases.

28. The variable stiffness mechanism of claim 25, in which the bottom of the variable strut is coupled to a fixed location (1358) on the cantilever arm by a revolute joint (328).

29. The variable stiffness mechanism of claim 25, in which the variable strut (320) comprises a spring (324) and a damper (326).

30. The variable stiffness mechanism of claim 29, in which the spring (324) is a coil spring and the damper (326) is a shock absorber.

31. The variable stiffness mechanism of claim 25, in which the device (1340) includes a control strut (342) having one end coupled to the bracket and another end coupled to the top of the variable strut.

32. The variable stiffness mechanism of claim 31, in which the control strut comprises a coil spring (344) and a damper (346).

33. The variable stiffness mechanism of claim 31, in which the control strut is coupled to the top of the variable strut by a revolute joint (327).

34. The variable stiffness mechanism of claim 25, in which the sprung mass is a land vehicle and the unsprung mass is a wheel assembly.

35. The variable stiffness mechanism of claim 34, in which the cantilever arm is a wishbone of a suspension system of the land vehicle.

36. The variable stiffness mechanism of claim 35, including a conventional strut (302) having one end coupled to the unsprung mass (303) and another end coupled to the sprung mass.

37. A variable stiffness mechanism (1800), comprising:

a variable strut (320) having a top (321) slidably attached to a bracket (330) and having a bottom (322) slidably attached the cantilever arm, wherein the bracket is attachable to the sprung mass; and

a device (1840) for relocating the variable strut along a horizontal slot (331) on the bracket (330), thereby causing a location of engagement (1845) of the variable strut with the bracket to relocate and causing a location of engagement (1855) of the variable strut with the cantilever arm to relocate,

wherein the location of engagement (1845) of the variable strut with the bracket and the location of engagement (1855) of the variable strut with the cantilever arm depend on a frequency of excitation of the unsprung mass.

38. The variable stiffness mechanism of claim 37, in which the variable stiffness mechanism (1801) has an effective spring constant between the sprung mass and the unsprung mass, and wherein the effective spring constant depends the location of engagement (1845) of the variable strut with the bracket and the location of engagement (1855) of the variable strut with the cantilever arm.

39. The variable stiffness mechanism of claim 37, in which the variable strut (320) comprises a spring (324) and a damper (326).

40. The variable stiffness mechanism of claim 39, in which the spring (324) is a coil spring and the damper (326) is a shock absorber.

41. The variable stiffness mechanism of claim 37, in which the device (1840) includes a control strut (1842) having one end coupled to the bracket and another end coupled to the top of the variable strut.

42. The variable stiffness mechanism of claim 41, in which the control strut comprises a coil spring (344) and a damper (346).

43. The variable stiffness mechanism of claim 41, in which the variable strut remains in a substantially vertical position as the control strut relocates the variable strut along the horizontal slot on the bracket.

44. The variable stiffness mechanism of claim 37, in which the bottom of the variable strut is coupled to the cantilever arm by a prismatic joint (1829) and a slider mechanism (1847) that engages and slides along a longitudinal slot in the cantilever arm.

45. The variable stiffness mechanism of claim 37, in which the sprung mass is a land vehicle and the unsprung mass is a wheel assembly.

46. The variable stiffness mechanism of claim 45, in which the cantilever arm is a wishbone of a suspension system of the land vehicle.

47. The variable stiffness mechanism of claim 46, including a conventional strut (302) having one end coupled to the unsprung mass (303) and another end coupled to the sprung mass.

48. A suspension system (300, 800, 1300, 1800), comprising:

a conventional strut (302) having a top coupled to a sprung mass and having a bottom coupled to an unsprung mass (303); and

a variable stiffness mechanism (301, 801, 1301, 1801), cooperating with the conventional strut, the variable stiffness mechanism including:

a cantilever arm with a first end rotatably attached to the sprung mass and with a second end coupled to the unsprung mass,

a variable strut with an top slideably attached to the sprung mass and a bottom that engages the cantilever arm at a location of engagement, and

a device for relocating the variable strut, thereby causing the location of engagement to relocate,

wherein the stiffness of the variable stiffness mechanism between the sprung mass and the unsprung mass depends on at least a distance between the location of engagement and the first end of the cantilever arm, and

wherein the distance between the location of engagement and the first end of the cantilever arm depends on a frequency of excitation of the unsprung mass.

49. The suspension system of claim 48, in which a distance between the location of engagement and the first end of the cantilever arm increases as the frequency of excitation of the unsprung mass decreases.

50. The suspension system of claim 48, in which a distance between the location of engagement and the first end of the cantilever arm decreases as the frequency of excitation of the unsprung mass increases.

51. The suspension system of claim 48, in which the conventional strut and the variable stiffness mechanism are arranged in a parallel configuration.

52. The suspension system of claim 48, in which the variable strut comprises a spring and a damper.

53. The suspension system of claim 52, in which the spring is a linear coil spring.

54. The suspension system of claim 48, in which the bottom of the variable strut is coupled to the cantilever arm by a revolute joint and a slider mechanism, the slider mechanism for allowing the bottom of the variable strut to slide along a length of the cantilever arm while the variable strut rotates relative to the cantilever arm.

55. The suspension system of claim 48, in which the device for relocating the variable strut includes one of: an active actuator, a semi-active actuator and a passive actuator.

56. The suspension system of claim 55, in which the suspension system is an active suspension system and in which the device for relocating the variable strut includes a linear actuator.

57. The suspension system of claim 55, in which the device for relocating the variable strut is coupled to the top of the variable strut.

58. The suspension system of claim 55, in which the device for relocating the variable strut is coupled the bottom of the variable strut.

59. The suspension system of claim 48, in which the device for relocating the variable strut comprises two portions:

a first portion coupled to the top of the variable strut, and

a second portion coupled to the bottom of the variable strut.

60. The suspension system of claim 59, in which the suspension system is an active suspension system, in which the first portion includes a first linear actuator, and in which the second portion includes a second linear actuator.

61. The suspension system of claim 48, in which the conventional strut is coupled to the variable stiffness mechanism in a parallel configuration, and a total spring constant of the suspension system is a sum of a spring constant of the conventional strut and the effective spring constant of the variable stiffness mechanism.

62. The suspension system of claim 48, in which the effective spring constant also depends on at least one of the following:

a spring constant of the variable strut, and

a free length of the variable strut.

63. The suspension system of claim 48, in which the sprung mass is a land vehicle and in which an axis of a wheel is coupled to the second end of the cantilever arm.

64. The suspension system of claim 63, in which the cantilever arm is a wishbone of a single wishbone independent suspension system of the land vehicle, in which the device for relocating the variable strut is coupled to the bottom of the variable strut, and in which the device for relocating the variable strut is disposed between arms of the wishbone.

65. The suspension system of claim 64, in which the conventional strut is a

MacPherson strut (302).