Abstract:
A method for modeling a coil spring on a suspension system in terms of derived torque and force characteristics of the spring comprising the steps of providing a force field generator for simulating the spring, securing the force field generator to the suspension system, activating the force field generator to produce forces for characterizing the spring, measuring the forces; and deriving a spring design based upon the measured forces.

Description:
RELATED APPLICATION  
       [0001]    This application is related to Provisional Application Serial No. 60/341,681, filed Dec. 18, 2001, the teachings of which are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The invention pertains to a method and apparatus for modeling coil springs and in particular to a method employing a force field generator on a suspension system to simulate the actual coil spring behavior so that the spring and suspension system may be tested without making actual colt springs.  
           [0003]    Traditionally, coil springs are used for applications to exert a one-dimensional force along a given coil spring axis. However, in recent years, there has been an increasing trend in which coil springs are designed to provide forces in multi-dimensional space. Such forces may be developed by means of a pitch control spring or an offset type suspension.  
           [0004]    [0004]FIG. 1 is a schematic illustration of an automobile suspension  10  employing a McPherson strut  12 . The McPherson strut is a well known device commonly used in modern automobile suspensions which employs a coil spring  14  and a damper  16 . Typically, the spring and damper have coaxial central displacement axes A. As a result of the geometry of the suspension, the damper  16  receives a bending moment  18  which is transmitted by the tire  20  to the lower end of the strut through the suspension linkage  22 , as shown. Bending moment  18  produces a side load  26  on the damper transverse to the strut displacement axis A, which results in a source of extra friction in the telescopic joint  28  of the damper  16 . This results in diminished damper operation and riding discomfort. The coil spring may be designed to exert forces in directions parallel to as well as normal to the strut displacement axis (directions  28  and  30 , respectively). In the design of the spring, the normal component of the spring force  30  may be tailored to reduce the side load  26  on the strut and thereby improve performance.  
           [0005]    Finite element analysis (FEA) (sometimes referred to as Finite Element Modeling (FEM)) is a well-known tool for designing coil springs of the type referred to hereinabove. However, modern springs have specification requirements which tend to be more and more complicated. Accordingly, efforts are needed to develop new types of tools to supplement FEA or to provide new design development capability that cannot be accomplished by FEA.  
           [0006]    A coil spring may be modeled as a mechanical device that produces force and torque between two planes between which the flat opposite ends of the spring are mounted. Hereinafter, the two planes are referred to as the lower and upper spring planes. In static and quasi-static force-torque analysis, each coil spring may be designed to have its own force and torque characteristics, which may be observed at a given spring plane after the kinematics relationship between the planes is established. In other words, the force torques and geometry of the model characterizes the spring.  
           [0007]    A coil spring designer must often evaluate the performance of a spring developed by FEM within an integrated mechanical system environment containing the spring. This type of evaluation is usually performed through kinematics and dynamics computer simulation software packages. ADAMS and WORKING MODEL are two known examples. However, exporting a spring model developed by FEA into third party kinematics and dynamics simulation software packages is not always a smooth and convenient process. The FEA file must first be converted into a specific file format required by the particular simulation package to be used. This type of conversion is not always available. Further, even if a finite element analysis file is successfully exported, it may significantly increase the computational load of the simulation package.  
           [0008]    A newly designed spring must often be tested not only through simulations but also by experiments. Building a physical prototype of a newly designed spring is costly and time consuming as well.  
           [0009]    It would therefore be desirable to provide a model which would enable a designer to simulate spring characteristics without using an FEA feature. It is particularly desirable to employ such a model in an automobile suspension.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention is based upon the discovery that a force field generator may be employed to model a coil spring to realize the spring force and torque characteristics. In an exemplary embodiment, a parallel mechanism comprising lower and upper platforms and a plurality of linkages, linking the platforms with a six degree freedom of mobility, in an automobile suspension, is employed to model the spring.  
           [0011]    The method allows a designer to simulate spring behavior and test a suspension using such a spring without using finite element analysis techniques and without having to make a spring in order to perform the tests.  
           [0012]    The method also permits the designer to physically realize the performance of a newly designed spring with its integrated mechanical system. For example, if a physical model is available, and if the characteristics of the new spring are realized, it is possible to test the performance of the new spring integrated in a mechanical suspension system without a physical prototype of the spring. In other words, it is possible to test the performance of the spring without making a spring.  
           [0013]    In addition, the model may be employed to perform more complex and active experiments. For example, the model may be used to discover or characterize any desirable spring force and torque characteristics by generating various force-torque patterns in the model.  
           [0014]    In an exemplary embodiment, the model employs a Stewart platform to produce an artificial force field of torques and forces for characterizing the spring and for manipulating the model. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a schematic illustration of a known McPherson-type strut in an automobile suspension;  
         [0016]    [0016]FIG. 2 is an exemplary force field generator employed as a spring model, according to the present invention, coupled to an automobile suspension;  
         [0017]    [0017]FIG. 3 is a schematic illustration of the geometric relationship between upper and lower plates of the parallel mechanism shown in FIG. 2;  
         [0018]    [0018]FIG. 4 a  and  4   a  are schematic side sectional elevations of positions of the mechanism shown in FIG. 2 in extended and compressed states, respectively;  
         [0019]    [0019]FIG. 5 is a schematic of a hydraulic circuit for controlling the hydraulic cylinders generating the forces and torques in the model;  
         [0020]    [0020]FIG. 6 is a schematic block diagram illustrating the control loop for operating the hydraulic circuit of FIG. 5; and  
         [0021]    [0021]FIG. 7 is a comparison of the side force in a normal spring and in a pitch controlled spring designed using the model of FIG. 2. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0022]    A suspension  40 , similar to the suspension  10  shown in FIG. 1, and wherein similar elements have the same reference numbers, is shown in FIG. 2. The suspension  40  employs a force field generator  42  for carrying out the method according to the invention. The mechanism  42 , known as a Stewart Platform, employs a lower platform  44  having a central axis A secured to the lower suspension linkage  22  and fire  20  as shown; an upper platform  46  secured to the vehicle (not shown) via bushing  24  and a set of six linkages  48 - 1  . . .  48 - 6  linking the platforms with a six degree of freedom of mobility. Each linkage  48 - 1  . . .  48 - 6  has a corresponding central axis Ac- 1  . . . Ac- 6 , and includes an upper end joint  50 , a lower end joint  52  and an intermediate telescopic joint  34 . Each upper and joint  50 - 1  . . .  50 - 6  may comprise a conventional universal joint secured to the upper platform  46  for rotation about corresponding orthogonal axes lying in a plane P 1 - . . . P 6  for each corresponding linkage. Each plane P 1  . . . P 6  is normal to the corresponding central axis Ac- 1  . . . Ac- 6  of the associated link  48 - 1  . . .  48 - 6 . The arrangement allows the linkage  48  to move relative to the upper platform  46  but not to rotate about the corresponding central axis AC for each link.  
         [0023]    The lower link  52  may be connected to the lower platform  42  by a spherical or ball joint which has similar freedom as the upper joint. It is unnecessary to constrain the axial rotation of the lower spherical end as this is accomplished by means of the limited degree of freedom afforded by the universal joint employed in the upper end joint as discussed above. It is also possible to use spherical joints at either end of the links if an appropriate constraint is employed to avoid rotation of the links about their respective axes.  
         [0024]    The intermediate link comprises a force actuator, such as a hydraulic cylinder, secured at opposite ends to the upper and lower joints as shown.  
         [0025]    [0025]FIG. 3 illustrates the kinematics of the force field generator  42  employed in the present invention. The fixed location of the lower joint  52  with respect to the lower platform  42  is indicated by the vector: 
           A P Ai =[ A x Ai ,  A y Ai , 0, 1] T   
         [0026]    The upper-left superscript of the vector indicates the frame of reference.  
         [0027]    The fixed location of the upper joint  50  of the same leg with respect to the upper frame  42  is indicated by the vector: 
           B P Bi =[ B x Bi ,  B y Bi , 0, 1] T   
         [0028]    Let  A   B T be the transformation matrix to represent the location and orientation of the lower frame  42  with respect to the upper frame  46 .  
         [0029]    Then the vector: 
           B P Ai =[ B x Ai ,  B y Ai , 0, 1] T   
         [0030]    which specifies the location of the lower joint of the i th  leg with respect to the upper frame  46 , is given by 
           B P Ai = A    B T A P Ai   (1) 
         [0031]    Let B u     i    be the unit vector representing the direction of the i th  leg from lower Joint  52  to upper joint  50  with respect to the upper frame  46 . This unit vector is calculated as  
               u   i               B     =       (         x   Bi               B     -     x   Ai               B       ,       y   Bi               B     -     y   Ai               B       ,     -     z   Ai               B         )             (       x   Bi               B     -     x   Ai               B       )     2       +       (       y   Bi               B     -     y   Ai               B       )     2     +       z   Ai         2           B                 (   2   )                               
 
         [0032]    Let F and M be the external force and torque vectors acting on the origin of frame  46 . Let f i  be the magnitude of the force along the i th  leg. Neglecting the gravitational forces of all components of the mechanism  42 , the force equilibrium based upon the quasi-static force analysis is given by 
         [0033]    [0033]                   ∑     i   =   1     6                       f   i          u   i               B         +   F     =   0           (   3   )                                 
         [0034]    The moment of equilibrium given by  
                   ∑       P   Bi               B     ×     f   i          u   i               B         +   M     =   0       i   =   1     6           (   4   )                               
 
         [0035]    Because equations (3) and (4) contain three components, there are a total of six equations to solve for six unknown forces f 1 , f 2 , . . . f 6 .  
         [0036]    [0036]FIGS. 4 a  and  4   b  illustrate an exemplary force field generator which is similar to the arrangement illustrated in FIG. 2. In FIG. 4 a , the force field generator  60  is fully extended and in FIG. 4 b , the force field generator is shown fully compressed. The exemplary force field generator  60  includes a damper  62  having a housing  64  and a rod  66  telescopically mounted therein. A lower plate  68  is secured to the housing  64  and upper plate  70  is secured to a free end  72  of the shaft  66 . A plurality of hydraulic cylinders  74  are secured between the lower and upper plates  68  and  70  by corresponding lower and upper spherical joints  76  and  78 , respectively. A schematic illustration only two hydraulic cylinders  74  are shown for clarity. It should be understand that, in an exemplary embodiment, six hydraulic cylinders are employed in an arrangement similar to that illustrated in FIG. 2.  
         [0037]    The hydraulic cylinder  74  includes a housing portion  80  and a shaft portion  82  telescopically secured therein. A force sensor  84  may be located on the shaft  82 . The force sensor may be a strain gauge which senses the force exerted by the hydraulic cylinder between the lower and upper plates  68  and  70 .  
         [0038]    [0038]FIG. 5 illustrates an exemplary hydraulic circuit for operating the hydraulic cylinders shown in FIG. 4 a  and  4   b . The hydraulic circuit  90  includes a hydraulic pump  92  which is coupled to hydraulic cylinder  94  through a control valve  96 , as shown. The cylinder  94  is connected to a load  98  which may be represented by the opposed upper and lower plates, and a force sensor  100  is secured between the cylinder  94  and the load  98 . The force sensor produces an output which is coupled to an amplifier  102  which provides a feedback signal as an input to a PC having an input/output board  104 . The output of the I/O board  104  is coupled to an amplifier circuit  106 , which provides feedback to a proportional pressure reducing valve  108  which is coupled in the fluid circuit between the hydraulic pump  92  and the cylinder  94 . The arrangement in FIG. 5 may be used to control the forces on the load exerted by each of the cylinders and in this way a spring may be modeled or characterized by the forces produced by the hydraulic cylinders.  
         [0039]    [0039]FIG. 6 illustrates the feedback circuit in further detail. A computer  110  produces a digital output which is converted in D/A converter  112  to an analog signal which in turn is coupled to hydraulic controller  114 . The hydraulic converter  114  controls the force field generator  116 . Force sensors  118  in the force field generator  116  produce output signals which are coupled to the computer via D/A converter  120  as illustrated. The computer may be programmed to control the hydraulic controller  114  in order to produce selected forces in the cylinders.  
         [0040]    The force field generator described herein may be employed to realize spring characteristics by adjusting the axial forces on the cylinders.  
         [0041]    Referring to FIGS. 2 and 7, the side force  26  is that force which results from the moment produced by the suspension system. The side force in a conventional spring is illustrated in FIG. 7, which is a plot of the side force versus the spring height. In accordance with the invention, a spring may be modeled to produce a desired side force. If the spring is produced with a uniform pitch, the spring characteristic may be exemplified by the corresponding curve in FIG. 7. If the spring is designed with a non-uniform pitch, the side force may be modified and in fact lowered in accordance with the curve illustrated in FIG. 7 and labeled pitch control spring. In accordance with the invention, the pitch control spring may be designed to further reduce the side force and thereby improve the performance of the suspension.  
         [0042]    In accordance with the invention, two kinds of spring shape may be prepared for finite element modeling in accordance with a given specification for the spring. For example, the spring may have a free height of 400 mm, a coil diameter of 150 mm, a wire diameter of 13 mm and 45.5 of turns. Using this specification, a reaction force vector for each spring may be computed using a MARC program. The reaction force vector is then converted into the six axial forces of the force generator by solving equations 3 and 4 above. These are then implemented in an ADAMS model with 3D spline interpolation.  
         [0043]    When the side force acts on the damper, the pressure is concentrated on the sealed portion of the damper. This causes an increase in friction. Therefore, it is important to measure the side force at the sealed location which is defined as the inlet of the piston to the cylindrical tube. The simulation is carried out by moving the tire  20  upward which the degrees of freedom of the force field generator  42  are constrained. When all the forces on the cylinders are sensed, the side force is calculated. The side force may be that associated with a normal spring or a pitch control spring, the latter reducing the side force due to the countervailing moment produced by such a spring. As illustrated, the pitch control spring cancels a portion of the bending moment acting on the damper and as a result, the magnitude of the side vector  22  is also smaller than one with a normal spring. By suitable manipulation of the variables, the side force produced by the normal spring in the pitch control spring may be optimized for the system.  
         [0044]    A similar characterization of the spring may be developed in which the axis of the spring and the axis of the damper are separately controlled. Such an arrangement is illustrated in an article entitled “Development of L-shaped Coil Spring to Reduce Friction on the McPherson Strut Suspension System” published in 2000 by Hamano et al., a copy of which is attached hereto and is incorporated by reference.  
         [0045]    While there has been described what are presently considered to be the exemplary embodiments of the invention, it will be apparent to those skilled in the art that various modifications may be made therein without departing from the invention and it is intended in the appended claims to cover such changes and modifications that fall within the spirit and scope of the invention.