WO2013025510A2

WO2013025510A2 - Variable stiffness mechanism

Info

Publication number: WO2013025510A2
Application number: PCT/US2012/050355
Authority: WO
Inventors: Olugbenga Moses ANUBI; Carl David CRANE, III; Shannon C. RIDGEWAY
Original assignee: University Of Florida Research Foundation, Inc.
Priority date: 2011-08-15
Filing date: 2012-08-10
Publication date: 2013-02-21
Also published as: WO2013025510A3

Abstract

A variable suspension mechanism (102) includes an arrangement of two springs (121 and 122), a lever (130) having a receiving location (134) for receiving an external force, and a pivot element (140) slideably coupled to the lever. The variable suspension mechanism includes an actuator (150) for relocating the pivot element causes a fulcrum (136) of the lever to relocate. An effective stiffness of the variable suspension mechanism relative to the external force applied at the receiving location is a rational function that depends on at least spring constants of the two springs and a location of the pivot element relative to the two springs. A suspension system (100) includes the variable suspension mechanism coupled to a conventional spring (106) in a series arrangement.

Description

VARIABLE STIFFNESS 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. 61/523,624, filed August 15, 2011, 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. A typical spring has an unchanging spring constant, which is a ratio of an applied external force to a resulting change in length of the spring.

Improvement of passive suspension designs is an area of much research. The past approaches utilize one of three techniques: adaptive, semi-active, or fully active suspension. An adaptive suspension system 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 system is similar, except that the adjustable damper has a faster response and the damping force is controlled in real-time. A fully active suspension system replaces the damper with a hydraulic actuator or another force-generating device, which can achieve optimum vehicle control, but at a high cost due to design complexity, use of an expensive hydraulic actuator, etc. The fully active suspension system is also not fail-safe in the sense that performance degradation results whenever the control fails, which may be due to either mechanical, electrical or software damage. Research in semi-active suspension systems has continued to advance with respect to their capabilities, thereby narrowing the gap between semi-active and fully active suspension systems. Today, semi-active suspension systems, e.g., using a magneto-rheological (MR) or an electro-rheological (ER) fluid, 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. However, most semi-active suspension systems can only change the viscous damping coefficient of a shock absorber while keeping the stiffness constant. In passive suspension systems, both the damping coefficient and the spring rate of the suspension elements are usually used as optimization arguments. Therefore, a semi-active suspension system that varies both the stiffness and damping of the suspension element could provide more flexibility in balancing competing design objectives.

Theoretically, the concept of 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. This is partly due to the relatively lower energy requirement for damping modulation. Another reason is due to the unavailability of pragmatic low-power stiffness modulation methods. Meanwhile, 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 orifice, the damping characteristics change from soft to hard and vice versa. With time, use of electro-rheological (ER) and magneto-rheological (MR) fluids replaced the use of variable orifices. ER and MR fluids are composed of a suspension of polarized solid particles dispersed in a non-conducting liquid. When an electric field (or a magnetic field for MR) is imposed, the particles become aligned along the direction of the imposed field. When this happens, the yield stress of the fluid changes, hence the 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. They are characterized by controlled force variation which consumes minimal power. The main idea behind their operation is the variation of the force transfer ratio which is achieved by moving the point of force application. If this point moves orthogonally to the acting force, then, theoretically, no mechanical work is involved in the control. This phenomenon is called "reciprocal actuation".

Variable stiffness semi-active suspension systems exhibit a variable stiffness feature. This is achieved either by changing a free length of a spring or by a mechanism that changes the effective 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 passive suspension system.

The control of semi-active suspension systems has gained much research interest over the years. 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 the sprung mass vibration. 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. Summary of the Invention

In one embodiment, a variable stiffness mechanism is disclosed. The variable stiffness mechanism comprises a lever that includes a receiving location on the lever for receiving an external force; a first spring with one end attached to a first point on a body and another end attached to an end of the lever, the first spring having a first spring constant; a second spring with one end attached to a second point on the body and another end attached to another end of the lever, the second spring having a second spring constant; a pivot element, disposed between the first spring and the second spring, the pivot element having one end slideably attached to a line on the body and another end that engages the lever at a fulcrum of the lever, wherein the line on the body extends between the first point and the second point on the body; and an actuator for relocating the pivot element, thereby causing the fulcrum of the lever to relocate. The variable stiffness mechanism has an effective stiffness relative to the external force applied to the receiving location on the lever. The effective stiffness depends on at least: a spring constant of the first spring, a spring constant of the second spring and a location of the pivot element relative to the first spring. In another embodiment, a suspension system for a land vehicle is disclosed. The suspension system for a land vehicle comprises a variable stiffness mechanism coupled to a wheel assembly, and a conventional spring with one end coupled to a land vehicle and another end coupled to the variable stiffness mechanism. The variable stiffness mechanism, includes a lever that includes a receiving location on the lever for receiving an external force; a wishbone with one end rotatably attached to the land vehicle and another end attached to the wheel assembly; a first spring with one end slideably attached to a line on the wishbone and another end attached to an end of the lever, the first spring having a first spring constant; a second spring with one end slideably attached to the line on the wishbone and another end attached to another end of the lever, the second spring having a second spring constant; a pivot element, disposed between the first spring and the second spring, the pivot element having one end slideably attached to the line on the wishbone and another end that engages the lever at a fulcrum of the lever; and an actuator for relocating the pivot element relative to the first spring, thereby causing the fulcrum of the lever to relocate. The conventional spring is attached to the lever at a receiving location between the fulcrum of the lever and one of a left end and a right end of the lever. The variable stiffness mechanism has an effective stiffness with respect to a force applied to the other end of the wishbone. The effective stiffness depends on at least: a spring constant of the first spring, a spring constant of the second spring and a location of the pivot element relative to the first spring.

In still another embodiment, a suspension system is disclosed. The suspension system comprises a variable stiffness mechanism coupleable to a sprung mass, and a conventional spring with a top end attached to the variable stiffness mechanism and a bottom end coupleable to an unsprung mass. The variable stiffness mechanism includes: a lever, including a receiving location; a first spring with one end attached to a first fixed reference point on a sprung mass and another end attached to an end of the lever, the first spring having a first spring constant; a second spring with one end attached to a second fixed reference point on the sprung mass and another end attached to another end of the lever, the second spring having a second spring constant; a pivot element, disposed between the first spring and the second spring, the pivot element having one end slideably attached to a fixed reference line on the sprung mass and another end that engages the lever at a fulcrum of the lever; and an actuator for relocating the pivot element relative to the first spring, thereby causing the fulcrum of the lever to relocate. The variable stiffness mechanism has an effective stiffness for an external force applied to the receiving location on the lever. The effective stiffness depends on at least: a spring constant of the first spring, a spring constant of the second spring and a location of the pivot element relative to the first spring. 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 perspective view of one embodiment of a suspension system in accordance with the invention including a variable stiffness mechanism in accordance with the invention and a conventional spring.

FIG. 2 is another perspective view of the one embodiment of the suspension system.

FIG. 3 is a side view of the one embodiment of the suspension system with the variable stiffness mechanism at low stiffness.

FIG. 4 is a side view of the one embodiment of the suspension system with the variable stiffness mechanism at high stiffness.

FIG. 5 is another side view of the one embodiment of the suspension system with cut line 6-6 indicated.

FIG. 6 is a cut view of the one embodiment of the suspension system through cut line 6-6 of FIG. 5.

FIG. 7 is a schematic a system representing the variable stiffness mechanism, including a pivot element.

FIG. 8 is a schematic of the suspension system of FIG. 1, in which the conventional spring is above the variable stiffness mechanism.

FIG. 9 is a schematic of another suspension system, in which the conventional spring is below the variable stiffness mechanism.

FIG. 10 is a plot of effective stiffness K of the variable stiffness mechanism versus distance d from the pivot element of application of force.

FIG. 11 is a plot of the effective stiffness K of the variable stiffness mechanism versus 1/d. FIG. 12 is a plot of effective free length 1₀ of the variable stiffness mechanism versus d. 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. FIG. 1 is a perspective view of one embodiment of a variable stiffness suspension system

(hereinafter "suspension system") 100 in accordance with the invention. The suspension system 100 comprises a variable stiffness mechanism 102 in accordance with the invention and a conventional spring 104. The conventional spring 104 and the variable stiffness mechanism 102 are coupled in a series configuration. In the one embodiment of a suspension system 100 shown in FIG. 1, the conventional spring 104 is above the variable stiffness mechanism 102, and the conventional spring 104 is coupled to a sprung mass. In another embodiment of a suspension system (shown only in the schematic of FIG. 9), a conventional spring 105 is below the variable stiffness mechanism 102 and the conventional spring 105 is coupled to an unsprung mass.

The suspension system 100 shown in FIGS. 1-6 is a suspension system for a land vehicle, and more specifically, a suspension system for a front wheel of an automobile. The suspension system 100 shown in FIGS. 1-6 is configured as a double wishbone suspension system which includes an upper wishbone 106 and a lower wishbone 108. Other configurations of the suspension system 100 are foreseeable. In the suspension system 100 shown in FIG. 1 , the lower wishbone includes a rod 202 (see FIG. 2). One end (a left end as shown in FIG. 1) of each wishbone 106 and 108 is coupled to an automobile (not shown in FIGS. 1-6) by revolute joints 111-114. In one embodiment, the sprung mass is an automobile, and the one end of each wishbone 106 and 108 is coupled to the frame or the body of the automobile. Another end (a right end as shown in FIG. 1) of each wishbone 106 and 108 is coupled to an unsprung mass 115. In one embodiment, the unsprung mass 115 is a wheel assembly which may include a hub, hub extensions, a wheel, a tire, brake components, etc. The other end of each wishbone 106 and 108 is coupled to the wheel assembly by a ball joint 116 and 117, respectively.

The one embodiment of the variable stiffness mechanism 102 comprises a first spring 121 and a second spring 122. In one embodiment, the first spring 121 and the second spring 122 are linear springs. In one embodiment, the first spring 121 and the second spring 122 have a same spring constant. In one embodiment, the first spring 121 and the second spring 122 have a same free length. In the one embodiment shown in FIGS 1-6, the first spring 121 and the second spring 122 are coil springs. In another embodiment (not shown), the first spring 121 and the second spring 122 are gas springs, or yet another type of spring. In the one embodiment shown in

FIGS. 1-6, one end (a lower end as shown in FIGS. 1-6) of the first spring 121 is coupled to the rod 202 by a prismatic joint 601 (see FIG. 6). One end (a lower end as shown in FIGS. 1-6) of the second spring 122 is coupled to the rod 202 by a prismatic joint 602 (see FIG. 6). The variable stiffness mechanism 102 includes a lever 130. The lever has a receiving location 134. In the one embodiment shown in FIGS. 1-6, one end (a top end as shown in FIGS. 1-6) of the conventional spring 104 is coupled to the sprung mass, and another end (a bottom end as shown in FIGS. 1-6) of the conventional spring is coupled to the receiving location 134. Another end (an upper end as shown in FIGS. 1-6) of the first spring 121 is coupled to an end (a left end as shown in FIG. 1) of the lever 130 by a revolute joint 611 (see FIG. 6). Another end (an upper end as shown in FIGS. 1-6) of the second spring 122 is coupled to another end (a right end as shown in FIG. 1) of the lever 130 by a revolute joint 612 (see FIG. 6). The lever 130 includes a longitudinal slot 135. The variable stiffness mechanism 102 includes a pivot element 140. One end (a top end as shown in FIGS. 1-6) of the pivot element 140 is coupled to the lever 130, within the longitudinal slot 135, by a revolute joint 613 (see FIG. 6). Another end (a bottom end as shown in FIGS. 1-6) of the pivot element 140 is coupled to the rod 202 by a prismatic joint 623 (see FIG. 6).

In the one embodiment shown in FIGS. 1-6, the variable stiffness mechanism 102 includes a first shock absorber 131 within the first spring 121 and a second shock absorber 132 within the second spring 122. In the one embodiment shown in FIGS. 1-6, the variable stiffness mechanism 102 includes an actuator 150. The actuator 150 is coupled to the pivot element 140 by a revolute joint 653 (see FIG. 6) near a mid-point (between the top end and the bottom end) of the pivot element. The actuator 150 is coupled to the sprung mass by brackets 151 and 152. The actuator 150 shown in FIGS. 1-6 is active electrical linear actuator. Any desired behavior can be forced on the variable stiffness mechanism 102 using a properly designed control algorithm for the actuator 150. An advantage of the variable stiffness mechanism 102 is power saving. The direction of action of the actuator 150 is advantageously not against gravity as with the direction of travel of known active actuators. As a result, no gravity compensation is involved. This greatly reduces the amount of power required by the actuator 150. In other embodiments, another type of active actuator can be used, such as hydraulic or pneumatic. In yet other embodiments, a semi-active (MR, ER, etc.) system is used instead of the actuator 150. In still other embodiments, a purely passive spring-damper system can be used instead of the actuator 150. FIG. 2 is another perspective view of the one embodiment of the suspension system 100.

FIG. 3 is a side view of the one embodiment of the suspension system 100 with the variable stiffness mechanism 102 at low stiffness.

FIG. 4 is a side view of the one embodiment of the suspension system 100 with the variable stiffness mechanism 102 at high stiffness. A comparison of FIGS. 3 and 4 shows that a fulcrum 136 of the lever 130 changes as the pivot element 140 translates (horizontally in FIGS. 1-6).

FIG. 5 is another side view of the one embodiment of the suspension system 100 with cut line 6-6 indicated.

FIG. 6 is a cut view of the one embodiment of the suspension system 100 through cut line 6-6 of FIG. 5. Advantageously, an effective stiffness K of the variable stiffness mechanism 102 can be varied. This is accomplished by translating a location of the pivot element 140. In the one embodiment shown in FIGS. 1-6, the pivot element 140 is translated by the actuator 150. The actuator 150 moves the pivot element 140 in an approximately horizontal direction, while the pivot element maintains a substantially vertical orientation, thereby changing a location of the fulcrum 136 of the lever 130. The extent to which the actuator 150 moves the pivot element 140 is limited by the length of the slot 135 in the lever 130. The effective stiffness K of the variable stiffness mechanism 102 can be varied between a minimal value and infinity (a stiff, rigid spring).

The effective stiffness K can be actively controlled or passively allowed to vary. Simulation results have been obtained for both cases as applied to a vehicular suspension system. It has been shown that the passive variable stiffness mechanism 102 reduces vertical acceleration felt by a passenger in a vehicle by approximately 40% as the vehicle goes over a 10cm tall bump. It has been shown that an actively controlled variable stiffness mechanism 102 reduces the acceleration by approximately 60%.

FIG. 7 is a simplified and idealized schematic of a system representing the variable stiffness mechanism 102. In FIG. 7, an external force F is indicated by a vertical arrow. The external force F is shown being applied to the receiving location 134 of the lever 130. The external force F is constrained to move vertically (up and down in FIG. 7), and the pivot element 140 is constrained to move horizontally (left and right in FIG. 7). The first spring 121 (on the left side of FIG. 7) and the second spring 122 (on the right side of FIG. 7) have spring constants ki and k₂, respectively, and have free lengths loi and I₀₂, respectively, and the springs are constrained so that they can only be deflected vertically. Li is a distance between the first spring 121 and a point P of application of a vertical force. Li is a distance between the second spring 122 and the point P of application of the vertical force. The effective stiffness K of the variable stiffness mechanism 102 is varied by changing d, a distance of the pivot point O from the point P of application of the vertical force, while keeping the point of application of an external load constant. The variable suspension mechanism 102 has an effective stiffness K that is a rational function of a horizontal position d of the pivot element 140.

FIG. 8 is a schematic of the suspension system 100 shown in FIGS. 1-6, in which the conventional spring 104 is above the variable stiffness mechanism 102.

FIG. 9 is a schematic of another suspension system (not shown), in which the conventional spring 105 is below the variable stiffness mechanism 102.

Given parameters, ki, k₂, Li, L₂, loi, I₀₂, the external force F and the horizontal distance d of the pivot element 140 from the point of application of F, an expression for the effective stiffness K and the effective free length 1₀ of the of the variable stiffness mechanism 102 is derived as follows. Let Fi and F₂ be the spring forces acting on the lever at points A and B with heights xi and x₂ from the ground respectively (see FIG. 7). Let the function Δ(χ, l_b, l_c) and the function Δ (x, l_b, l_c) be defined as b ' Equation (1)

b ' Equation (2)

Thus, Fi and F₂ can be written as

Fi = Δ (χι, l_bi, l_ci)(xi - l₀i)ki + A(x l_bl, k Pi Equation (3)

F₂ = Δ (x₂, l_b2, Ic2)( ₂ - lo₂)k₂ + Δ(χ₂, l_b2, l_c2)P₂ Equation (4) where l_bi and l_b2 are the block lengths of the left and right springs, respectively; where l_ci and l_c2 are the open lengths of the left and right springs, respectively; and where Pi and P₂ are the pure reaction forces of the blocked springs.

Equations (3) and (4) capture the cases when the springs behave as rigid bars (blocked or open) or as compliant members (xi e (l_bi l_ci), x₂ e (l_b2 l_c2)). Taking moments about point O and dividing by d gives

L_[ + d

F K Equation (5) with

L_[ + d

χ_λ = H δ

L₂ + d

x₂ = H δ Equation (6) where Li and L₂ are the horizontal distances of the vertical springs, ki and k₂, respectively, from the center of the lever, and H is the height of the pivot element 140. Substituting Equations (3), (4) and (6) into Equation (5), yields

F = Κδ - C Equation (7) where δ is vertical displacement of the point of application of the force F, and where

(L_L + d)² _ (L₂ - df

K = k_y A_l + k₂ A₂ Equation (8)

d² d² (H - l_0l )( £, + . ) (H - l_0l )( L₂ - d ) Ly + d L₂ —d

C = k_x A_x - + k₂ A₂ ^■ + A₁ P, ^■ + A₂ P₂

Equation (9) where

Consider when the left spring is blocked, i.e., xi = 1M, and A_l = 0— > A_l = 1 , the system of FIG.

7 becomes statically indeterminate and rigid provided that X2 < 1_C2- However, any decrease in F causes the system to revert to the state where both springs are neither blocked nor open. A similar situation exists for the case where the left spring is open instead, and also for when the right spring is open or blocked. Thus Equation (8) becomes

K = ^xi ^e ( hi hi )' ^xi ^e ( hi hi ) Equation (10)

otherwise

Equation (10) is the expression for the overall stiffness of the system of FIG. 7 from which it is easily seen that the system is rigid when either or both the left and right springs become blocked or open, or d = 0. It is, however, possible in design to restrict xi and X2 in the range where K never goes unbounded except in the neighborhood of d = 0. This is done by using springs of zero free length and by also satisfying the condition

< Equation (11)

The ratio L₂/Li is termed the aspect ratio, and the space {(l_bi l_ei) (1M l_ei) } \ {d = 0} is termed the useful space of the mechanism. Now, consider the system of FIG. 7 restricted to the useful space and whose aspect ratio is such that the condition of Equation (11) is satisfied. FIG. 10 is a plot of effective stiffness K of the system shown in FIG. 7 versus distance d from the pivot element 140 of application of force. From FIG. 10, it can be easily seen that the minimum stiffness occurs at the boundary of the parameter d, and is given by: k₂ ( r + 1 )² , r < l

4min ( k₁ , k₂ ), r = l

( + i , r > \

Equation (12) where r = L₂/Li is the aspect ratio of the mechanism. FIG. 11 is a plot of the effective stiffness K of the system shown in FIG. 7 versus 1/d. From FIG. 11, the behavior of the system as d→∞ can be easily visualized. Let 1₀ be the effective free length of the system, then

1ο = Η - δ₀ Equation (13) where δο is the deflection when F = 0 which is given

C

K

Thus, the effective free length 1₀ becomes

H - — Equation (14)

N₂ d² + N₁ d + N_c

Equation (15)

D₂ d² + D₁ d + D_c where

No = H(kiLi² + k₂L₂ ²)

Ni = kiLi(H + loi) - k₂L2(H + lo2)

N₂ = kiloi + k₂l₀₂

D₀ = k₁L₁ ² + k₂L₂ ²

Di = 2(kiLi - k2L₂)

FIG. 12 is a plot of effective free length 1₀ of the system shown in FIG. 7 versus d. FIG. 12 shows the variation of the effective free length lo with respect to d. The maximum free length occurs when d = d_max, where d_max is the solution to

A d² _m^ + A A _^ max + A A

max = 0 Equation (16)

#1 N₂ N₂ #1

However, from a practical point of view, it might be desired to keep constant the effective free length 1₀ of the system for all values of d. In that case, an additional constraint on Equation (14) can be written as

N₂ d² + N_! d + N₀ = Mp₂ d² + D₁ d + D₀ ), A e ^i or

(N₂ - XD₂ ) d² + {N, - XD_X ) d + (N₀ - D₀ ) = 0 which implies that k_t +

Equation (17)

Any combination of the system parameters ki, k₂, 1₀ι, I02, L_b L₂ and Η that satisfies Equation (17) results in a constant effective free length 1₀ of the variable stiffness mechanism 102 for all values of d. One example of such a case is given as l_m = l_m = H — L = H, /d The suspension system 100 comprises a traditional passive suspension system augmented with the variable stiffness mechanism 102. Such a traditional passive suspension system typically comprises a conventional coil spring. A goal is to improve performance of a suspension system by varying stiffness in response to road disturbance. The suspension system 100 was analyzed using a quarter-car model. The passive case shows much better performance in ride comfort over known traditional suspension systems.

Analysis of the invariant equation shows that the car body acceleration transfer function magnitude can be reduced at both the tire -hop and rattle space frequencies using the lever displacement transfer function thereby resulting in a better performance over known traditional passive suspension systems. An H∞ controller is designed to correct for the performance degradation in the rattle space thereby providing the best trade-off between the ride comfort, suspension deflection and road holding.

The variable stiffness mechanism 102 is, in one embodiment, a passive system in which no sensors, actuators, or computer control elements are needed. By "passive", it is meant that the variable stiffness mechanism 102 comprises only purely mechanical elements. In another embodiment, the actuator 150 is one of active and semi-active.

The variable stiffness mechanism 102 is more fully described in the published article entitled, "Design and Analysis of a Variable Stiffness Mechanism", by Anubi, et al., which is hereby incorporated by reference in its entirety. The variable stiffness mechanism 102 is also more fully described in the published article entitled, "Semi-global Output Feedback Asymptotic Tracking for an Under-actuated Variable Stiffness Mechanism" , by Anubi, et al., which is hereby incorporated by reference in its entirety. The suspension system 100 is more fully described in the published article entitled, "A New Variable Stiffness Suspension Mechanism" , by Anubi, et al., which is hereby incorporated by reference in its entirety.

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 100 is for use in active vibration control for an earthquake resistant building. Another application of the suspension system 100 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.

Claims

What is claimed is: CLAIMS

1. A variable stiffness mechanism (102), comprising:

a lever (130), including a receiving location (134) on the lever for receiving an external force;

a first spring (121) with one end attached to a first point on a body and another end attached to an end of the lever, the first spring having a first spring constant;

a second spring (122) with one end attached to a second point on the body and another end attached to another end of the lever, the second spring having a second spring constant; a pivot element (140), disposed between the first spring and the second spring, the pivot element having one end slideably attached to a line on the body and another end that engages the lever at a fulcrum (136) of the lever, wherein the line on the body extends between the first point and the second point on the body; and

an actuator (150) for relocating the pivot element, thereby causing the fulcrum of the lever to relocate,

wherein the variable stiffness mechanism has an effective stiffness relative to the external force applied to the receiving location on the lever, the effective stiffness depending on at least: a spring constant of the first spring, a spring constant of the second spring and a location of the pivot element relative to the first spring.

2. The variable stiffness mechanism of claim 1 , in which the spring constant of the first spring is equal to the spring constant of the second spring.

3. The variable stiffness mechanism of claim 2, in which the first spring and the second spring have a same free length.

4. The variable stiffness mechanism of claim 2, in which the first spring and the second spring have different free lengths.

5. The variable stiffness mechanism of claim 1, in which the spring constant of the first spring is not equal to the spring constant of the second spring.

6. The variable stiffness mechanism of claim 5, in which the first spring and the second spring have a same free length.

7. The variable stiffness mechanism of claim 5, in which the first spring and the second spring have different free lengths.

8. The variable stiffness mechanism of claim 1, in which the first spring and the second spring are linear springs.

9. The variable stiffness mechanism of claim 1, in which the actuator for relocating the pivot element relative to the first spring is one of: an active actuator, a semi-active actuator, and a passive actuator.

10. The variable stiffness mechanism of claim 9, in which the actuator for relocating the pivot element relative to the first spring is another spring.

11. The variable stiffness mechanism of claim 1 , in which the pivot element is rigid.

12. The variable stiffness mechanism of claim 1, in which the pivot element is elastic.

13. A suspension system (100) for a land vehicle, comprising:

a variable stiffness mechanism (102) coupled to a wheel assembly (115); and a conventional spring (104) with one end coupled to a land vehicle and another end coupled to the variable stiffness mechanism,

wherein the variable stiffness mechanism, includes:

a lever (130), including a receiving location (134) on the lever for receiving an external force,

a wishbone (108) with one end rotatably attached to the land vehicle and another end attached to the wheel assembly,

a first spring (121) with one end slideably attached to a line on the wishbone and another end attached to an end of the lever, the first spring having a first spring constant, a second spring (122) with one end slideably attached to the line on the wishbone and another end attached to another end of the lever, the second spring having a second spring constant,

a pivot element (140), disposed between the first spring and the second spring, the pivot element having one end slideably attached to the line on the wishbone and another end that engages the lever at a fulcrum (136) of the lever, and

an actuator (150) for relocating the pivot element relative to the first spring, thereby causing the fulcrum of the lever to relocate,

wherein the conventional spring is attached to the lever at a receiving location between the fulcrum of the lever and one of a left end and a right end of the lever, and wherein the variable stiffness mechanism has an effective stiffness with respect to a force applied to the other end of the wishbone, the effective stiffness depending on at least: a spring constant of the first spring, a spring constant of the second spring and a location of the pivot element relative to the first spring.

14. The suspension system for a land vehicle of claim 13, in which the first spring and the second spring have a same spring constant and a same free length.

15. The suspension system for a land vehicle of claim 13, in which the actuator for relocating the pivot element relative to the first spring is one of: an active actuator, a semi-active actuator, and a passive actuator.

16. A suspension system (100), comprising:

a variable stiffness mechanism (102) coupleable to a sprung mass; and

a conventional spring (104) with a top end attached to the variable stiffness mechanism and with a bottom end coupleable to an unsprung mass (115),

wherein the variable stiffness mechanism, includes:

a lever (130), including a receiving location (134);

a first spring (121) with one end attached to a first fixed reference point on a sprung mass and another end attached to an end of the lever, the first spring having a first spring constant;

a second spring (122) with one end attached to a second fixed reference point on the sprung mass and another end attached to another end of the lever, the second spring having a second spring constant;

a pivot element (140), disposed between the first spring and the second spring, the pivot element having one end slideably attached to a fixed reference line on the sprung mass and another end that engages the lever at a fulcrum (136) of the lever; and an actuator (150) for relocating the pivot element relative to the first spring, thereby causing the fulcrum of the lever to relocate,

wherein the variable stiffness mechanism has an effective stiffness for an external force applied to the receiving location on the lever, the effective stiffness depending on at least: a spring constant of the first spring, a spring constant of the second spring and a location of the pivot element relative to the first spring.

17. The suspension system of claim 16, in which the top end of the conventional spring is coupled to the lever at the receiving location.

18. The suspension system of claim 16, in which the suspension system is a suspension system for a land vehicle, in which the sprung mass is the land vehicle, and in which the unsprung mass is a wheel assembly of the land vehicle.

19. The suspension system of claim 16, in which the first spring and the second spring have a same spring constant and a same free length.

20. The suspension system of claim 16, in which in which the actuator for relocating the pivot element relative to the first spring is one of: an active actuator, a semi-active actuator, and a passive actuator.