Motor vehicle steering box with Ackerman control

[0002] The present invention relates to steering systems for motor vehicles. More particularly, the present invention relates to a steering control system that permits achievement of virtually perfect Ackerman geometry. A further benefit to be derived from the system is greater freedom of placement of steering system components among other components of a vehicle.

[0003] Ackerman steering geometry can be explained by examining the relationships of components of a typical two-wheeled steering system situated on the front axle of a motor vehicle. As the vehicle negotiates a turn in a roadway, both the inside and the outside wheels must be turned to correspond to the turn. In doing so, both wheels take two distinct arc-like paths which ideally will have a common center point that is located at the point of intersection of a line extending inwardly from the rotational center of the rear axle of the vehicle and toward the central region of the turn being negotiated and two similar inwardly extending lines, one of which originates at the rotational center of the inside wheel of the front axle and the other of which has its starting point at the rotational center of the outside wheel of the front axle. For correct steering and to avoid undesirable scrubbing of rubber from the tread region of the tire on the outside wheel, the arc-like path taken by that wheel must have that same center point as the arcuate path taken by the inside wheel. However, because the inside and outside wheels are spaced apart from each other on the front axle, i.e., they have what is commonly called a “track width”, the radius of the curved path taken by the outside wheel must be larger than the radius of the path taken by inside wheel. As a consequence, the outside wheel must be turned inwardly at a turning angle that is less than the turning angle provided to the inside wheel. When the center points of inside and outside wheel paths are coincident, a condition known as perfect Ackerman exists. Such a state is desirable in a vehicle dynamics sense, because any deviation from it results in Ackerman error degradation of vehicle handling characteristics.

[0004] At the present time, the only means available for controlling Ackerman geometry is generally through the placement of various components of the steering system, and more specifically, through the positioning of the outer tie rod ball joints. Such positioning has often proved to be difficult or impossible, because movement of the outer tie rod ball joints in an outboard direction typically runs into physical constraints within the wheel rims. The present invention facilitates the placement process and helps to better cope with the indicated physical limitations by allowing greater freedom for placement of the tie rod ball joints and other components in a range of locations, which is advantageous for vehicle packaging issues.

[0005] It is therefore an advantage of the present invention to provide a motor vehicle steering box which can be used with light duty trucks and with other vehicles and which is provided with a means for specifically controlling Ackerman geometry regardless of the location of the outside tie rod ball joint and for independently controlling left and right tie rod links. This advantage is provided by a motor vehicle steering box comprising: a function generation means; a first connecting means for linking the function generation means to a steering column and steering wheel of the vehicle steering system; a second connecting means for linking the function generation means to a first tie rod of a linkage apparatus of the steering system; a third connecting means for linking the function generation means to a second tie rod of the linkage apparatus; and a means for ascertaining an angle of rotation of the steering wheel of the steering system when the vehicle makes a turn and for conveying the angle to the function generation means.

[0006] This advantage and other novel features of the present invention will become apparent in the following detailed description of the invention when considered in conjunction with the accompanying drawings.

[0007] A better understanding of the present invention will be had when reference is made to the accompanying drawings, wherein identical parts are identified by identical reference numerals and wherein:

[0008] FIG. 1 is a schematic top view of a four-wheeled, double axle motor vehicle with front wheels turned to left to illustrate Ackerman geometry;

[0009] FIG. 2 is a schematic top view of a steering system containing the present invention;

[0010] FIG. 3 is a schematic top view of a motor vehicle and steering system of the present invention, wherein typical dimensional data are provided;

[0011] FIG. 4 is a positional vector diagram in two dimensions for the configuration of the vehicle and steering system shown in FIG. 3;

[0012] FIG. 5 is a graph wherein changes in lengths of inner tie rod components of the steering system shown in FIG. 3, required to yield a condition of perfect Ackerman have been plotted;

[0013] FIG. 6 is a graph which, in addition to including the inner tie rod length changes plotted in FIG. 5, further includes length changes attainable through use of a first embodiment of the present invention; and

[0014] FIG. 7 is a graph which, in addition to including the inner tie rod length changes plotted in FIG. 5, further includes length changes attainable through use of a second embodiment of the present invention.

[0015] FIG. 1 provides a schematic illustration of the Ackerman geometry of a four-wheeled, double axle motor vehicle wherein the front wheels of the vehicle are turned to the left. (The direction in which the wheels are shown to be turned, i.e., to the left or to the right, is immaterial to the invention. The invention could have just as easily been described with the vehicle negotiating a right turn.) The vehicle, which is represented generally by the numeral 10, is shown to have a front axle 12, a rear axle 14, and a structural frame or chassis 16 that extends between and interconnects the front axle 12 and the rear axle 14. The front axle 12 is provided at its opposing ends with two conventional steerable tire/wheel assemblies 11 and 13 and the rear axle 14 is likewise outfitted with two conventional tire/wheel assemblies 15 and 17, both of which are non-steerable. As indicated by the orientation of assemblies 11 and 13 in FIG. 1, the vehicle 10 is making a left hand turn and traveling generally in a counter-clockwise direction designated by the arrow M. In making the turn, it will be observed that front wheels 11 and 13 take two distinct arc-like paths P_iand P_owhich ideally will have a common center point C which is located at the point of intersection of a line L₁, extending inwardly from the rotational center c_rof the rear axle 14 of the vehicle 10 and toward the central region of the turn being negotiated and two similar inwardly extending lines L₂and L₃, which originate respectively at the rotational center c_iof the inside wheel 11 of the front axle 12 and at the rotational center c_oof the outside wheel 13 of the front axle 12. For proper steering and to avoid undesirable scrubbing of rubber from the tread region of the tire on the outside wheel 13, the arc-like path P_otaken by the outside wheel 13 must have the same center point C as the arcuate path P_itaken by the inside wheel 11. However, because the inside and outside wheels 11 and 13 are spaced apart from each other on the front axle at a distance commonly called a track width T_w, the radius R_oof the curved path P_otaken by the outside wheel 13 must be larger than the radius R_iof the path taken by the inside wheel 11. As a consequence, the outside wheel 13 must be turned inwardly at a turning angle α_othat is less than the turning angle α_iprovided to the inside wheel. When the center points of inside and outside wheel paths are coincident at point C as shown in FIG. 1, a condition of perfect Ackerman exists. Such a state is desirable in a vehicle dynamics sense, because any deviation from it results in Ackerman error degradation of vehicle handling characteristics. As explained hereinbelow, the present invention provides a means for achieving virtually perfect Ackerman geometry.

[0016] The present invention is premised upon the notions that for any given steering system arrangement, including inner and outer tie rod locations, there is a set distance that the inner tie rods of the system must move in order to attain a desired wheel angle and that therefore, the typical rack and pinion steering system of a vehicle only needs to be replaced by a function generation device that provides the correct lengths on the inner tie rods.

[0017] FIG. 2 schematically shows a vehicle steering system that embodies the present invention. The steering system is generally identified by the reference numeral 20 and is comprised of the front axle 12, the front wheels 11 and 13, a mechanical function generator 21, left and right inner tie rods 22 and 23, left and right outer tie rods 24 and 25, left and right outer tie rod ball joints 26 and 27, and left and right inner tie rod ball joints 28 and 29. The steering system 20 also includes left and right steering arms 31 and 32 and king pin 33. Also as shown in FIG. 2, the length of the left inner tie rod 22 is shown by dimension L₁and the length of the right inner tie rod 23 is shown by dimension L₂.

[0018] For any given set of vehicle parameters, including wheel base, track width, steering arm locations and tie rod locations, it has been found that the length L₁of the left inner tie rod 22 that will result in a condition of perfect Ackerman geometry is mathematically related to the angle θ through which the steering wheel of the vehicle 10 is turned. The relationship may be expressed generally as L₁(θ). It also has been found that the length L₂of the right inner tie rod 23 required for perfect Ackerman geometry will similarly be mathematically related to the angle θ, however, by a different mathematical function that may be expressed generally as L₂(θ). The mathematical relationships L₁(θ) and L₂(θ) have been found to be mathematically associated with each other in accordance with the following formulae:

[0019] where:

[0020] θ is the angle through which the steering wheel of vehicle 10 is turned;

[0021] L₁is the length of the left inner tie rod 22;

[0022] L₂is the length of the right inner tie rod 23;

[0023] α_iis the turning angle of the inside wheel 11;

[0024] α_ois the turning angle of the outside wheel 13;

[0025] [G] is a transfer function that is determined purely by the layout of the steering system and that relates the length of the vehicle's rack and pinion assembly to the steering wheel angle θ; and

[0026] [T] is a transfer function that will generate the correct inside and outside link lengths, Link₁and Link₂, for any given rotational input (angle θ) of the steering wheel.

[0027] The foregoing formulae are employed to provide a function generation device 21 that is used in lieu of a conventional rack and pinion steering apparatus and to thereby generate the link lengths Link₁and Link₂that will result in virtually perfect Ackerman geometry for the rotational angle θ input from the steering wheel of vehicle 10. A variety of physical means may be utilized to provide the function generator with the capability to transform rotational motion of the steering wheel into the required linear motion of the inner tie rods 22 and 23. Such means include: circular gearing, elliptical gearing, cams, slotted cams, offset slider crank, four-bar linkage and five-bar linkage either individually or in combination with one or more of the others. Since any number of functions can be generated through use of the listed means, the physical placement of the outer tie rods 24 and 25 and the function generation device among other vehicle components situated in the front of vehicle 10 becomes immaterial and allows for additional freedom in determining the placement points.

[0028] Reference to FIGS. 3 and 4 will help to further illustrate the principles of the invention. FIG. 3 is a schematic diagram of the vehicle 10 and the steering system 20 thereof, which has been provided with dimensional data for the wheel base W (101.4 in.), the track width T_w(58 in.), the angle of turn α_i(9.090°=θ1) for the inside wheel 11, the distance between the center of the left outer tie rod ball joint 26 and the center of the left inner tie rod ball joint 28 (17.205 in.), the distance between the centerline of the function generator 21 and the center of the left inner tie rod ball joint 28 (8.779 in.), the distance between the center of the outer tie rod ball joint 26 and the center of the king pin 33 (6.020 in.) and the distance between the centerline of the function generator 21 and the left outer tie rod ball joint 26 (25.033 in.). The steering system 20 has a design steering ratio of 14.

[0029] FIG. 4 diagrammatically shows a positional vector drawing in two dimensions for the configuration of steering system 20 as provided in FIG. 3. The scalar values for the vectors R₁through R₅correspond, with one exception, directly to the dimensions shown in FIG. 3 for the left side of the steering system 20. By way of example, the scalar value for vector R₁is 6.020 in., the distance between the center of king pin 33 and the center of the outer tie rod ball joint 26. The scalar value for vector R₂is 17.205 in., the distance between left outer tie rod ball joint 26 and left inner tie rod ball joint 28, etc. The scalar value of vector R₄, the exception, will be determined trigonometrically by taking cos(9.090°) times 6.020, the distance between the king pin 33 and the center of the left outer tie rod ball joint 26.

[0030] In FIG. 4, two vector loops are shown for steering system 20. The first loop is comprised of vectors R₁, R₂, R₃, R₄and R₅, and the second loop includes R₆, R₇, R₈, R₉and R₄. Upon application of vector loop analysis to the two loops, it will be observed that:

[0031] {right arrow over (R₁+L )}+{right arrow over (R₂+L )}+{right arrow over (R₃+L )}+{right arrow over (R₄+L )}+{right arrow over (R₅+L )}=0  (eq. (1))

[0032] {right arrow over (R₆+L )}+{right arrow over (R₇+L )}+{right arrow over (R₈+L )}+{right arrow over (R₉+L )}+{right arrow over (R₄+L )}=0  (eq. (2))

[0033] R₃+R₉=K(K=Constant)  (eq. (3))

[0034] R₁cos(θ₁)+R₂cos(θ₂)+R₃cos(θ₃)+R₄cos(θ₄)+R₅cos(θ₅)=0  (eq. (4))

[0035] R₁sin(θ₁)+R₂sin(θ₂)+R₃sin(θ₃)+R₄sin(θ₄)+R₅sin(θ₅)=0  (eq. (5))

[0036] R₆Cos(θ₆)+R₇Cos(θ₇)+R₈cos(θ₈)+R₉cos(θ₉)+R₄cos(θ₄)=  (eq. (6))

[0037] R₆sin(θ₆)+R₇sin(θ₇)+R₈sin(θ₈)+R₉sin(θ₉)+R₄sin(θ₄)=0  (eq. (7))

[0038] Where:

[0039] R₁is the length from the left kingpin center to the left outer tie rod ball joint; θ₁is the angle from horizontal to vector made by the left kingpin center to the left outer tie rod ball joint

[0040] R₂is the length from the left outer tie rod ball joint to the left inner tie rod ball joint; θ₂is the angle from horizontal to vector made by the left outer tie rod ball joint to the left inner tie rod ball joint; and

[0041] R₃is the length from the left inner tie rod ball joint to the center of the function generator; θ₃is the angle from horizontal to vector made by the left inner tie rod ball joint to the center of the function generator; and

[0042] R₄is the length from the center of the function generator to lateral center point of the left and right kingpin centers; θ₄is the angle from horizontal to vector made by the center of the function generator to lateral center point of the left and right kingpin centers; and

[0043] R₅is the length from the lateral center point of the left and right kingpin centers to the left kingpin center; θ₅is the angle from horizontal to vector made by the lateral center point of the left and right kingpin centers and the left kingpin center; and

[0044] R₆is the length from the lateral center point of the left and right kingpin centers to the right kingpin center; θ₆is the angle from horizontal to vector made by the lateral center point of the left and right kingpin centers and the right kingpin center; and

[0045] R₇is the length from the right kingpin center to the right outer tie rod ball joint; θ₇is the angle from horizontal to vector made by the right kingpin center to the right outer tie rod ball joint; and

[0046] R₈is the length from the right outer tie rod ball joint to the right inner tie rod ball joint; θ₈is the angle from horizontal to vector made by the right outer tie rod ball joint to the right inner tie rod ball joint; and

[0047] R₉is the length from the right inner tie rod ball joint to the center of the function generator; θ₉is the angle from horizontal to vector made by the right inner tie rod ball joint to the center of the function generator.

[0048] Next simplifying the vector loops by dropping unnecessary trigonometric functions, the result is the following:

[0049] R₁cos(θ₁)+R₂cos(θ₂)+R₃−R₅=0  (eq. (8))

[0050] R₁sin(θ₁)+R₂sin(θ₂)−R₄=0  (eq (9))

[0051] R₆+R₇cos(θ₇)+R₈cos(θ₈)−R₉=0  (eq (10))

[0052] R₇sin(θ₇)+R₈sin(θ₈)−R₄=0  (eq. (11))

[0053] Rearranging the vector loops, θ₂and R₃can be solved algebraically as follows:

[0054] R₁cos(θ₁)+R₂cos(θ₂)+R₃−R₅=0  (eq (8))

[0055] R₁sin(θ₁)+R₂sin(θ₂)−R₄=0  (eq(9))

[0056] R₆+R₇cos(θ₇)+R₈cos(θ₈)−R₉=0  (eq. (10))

[0057] R₇sin(θ₇)+R₈sin(θ₈)−R₄=0  (eq (11)) $\begin{array}{cc}{\theta }_2={\sin }^{-1}\left|\frac{R_4-R_1\sin \left({\theta }_1\right)}{R_2}\right|& \left(\mathrm{eq}.\left(12\right)\right)\end{array}$

[0058] R₃=R₅−R₁cos(θ₁)−R₅cos(θ₂)  (eq. (13))

[0059] To solve for θ₇and θ₈, use of either the Newton Raphson Method or direct solving may be made. By direct solving, the result is:

[0060] R₇cos(θ₇)+R₈cos(θ₈)=(K−R₃)−R6  (eq. (14))

[0061] R₇sin(θ₇)+R₈sin(θ₈)=R₄  (eq. (15))

[0062] $\begin{array}{cc}{\theta }_8={\sin }^{-1}\left(\frac{R_4-R_7\sin \left({\theta }_7\right)}{R_8}\right)& \left(\mathrm{eq}.\left(16\right)\right)\\ {\theta }_7={\cos }^{-1}\left|\frac{K-R_3-R_6-R_8\cos \left[{\sin }^{-}1\left(\frac{R_4-R_7\sin \left({\theta }_7\right)}{R_8}\right)\right]}{R_7}\right|& \left(\mathrm{eq}.\left(17\right)\right)\end{array}$

[0063] Through the preceding mathematical analysis, the turning angle a, of the outside wheel 13, that is required to produce perfect Ackerman becomes known, since θ7=α_o, and the changes in length for the left inner tie rod 22 and the right inner tie rod 23 needed for perfect Ackerman may be determined by comparing the values R₃and R₉obtained through the analysis with the lengths of the inner tie rods 22 and 23 when vehicle 10 is traveling in a straight path and, wheels 11 and 13 are in an unturned position. In the case where α_i=9.090°, α_o≈8.263°, the change in length of the left inner tie rod 22 is an increase of approximately 0.920 in and the change of length of the right inner tie rod 23 is a decrease of approximately 0.864 in.

[0064] From the foregoing, it follows that the turning angle α_oof the wheel 13 and the changes in length for the left inner tie rod 22 and the right inner tie rod 23 needed for perfect Ackerman can be ascertained for any rotational angle θ of the steering wheel of vehicle 10 and inside wheel angle α_iof the wheel 11. A tabulation of such is provided in Table 1 below.

[0065] FIG. 5 provides a graph wherein the values for ΔL₁and ΔL₂corresponding to the values for α_ishown in Table 1 have been plotted. From an inspection of FIG. 5, it will be observed that in the example presented, i.e., the case where vehicle 10 executes a left hand turn, the change in lenght ΔL₁of the left inner tie rod 22 will increase in a non-linear fashion as α_iincreases and the change in length ΔL₂of the right inner tie rod 23 will decrease in a non-linear manner as α_iincreases. It will further be observed that for any given non-zero value of α_i, the absolute value of the change in length ΔL₁of the left inner tie rod 22 will be greater than the absolute value of the change in length ΔL₂of the right inner tie rod 23. Thus, the left inner tie rod 22 must experience a larger change in length than the right inner tie rod 23 in order that a condition of perfect Ackerman can be attained.

[0066] Referring back to Table 1, it will be noted that to the right of the column headed with ΔL₂there appear two additional columns headed ΔL₂′ and ΔL₂″. The numerical values presented in the additional columns have been generated by the application of least square approximation techniques. In the case of the column headed ΔL₂′, the linear equation

[0067] y=0.0816x

[0068] was applied, with R²=0.9848. The applied equation provides a linear approximation of the relationship that yielded the ΔL₂values shown in Table 1 and assumes that the mechanical function generator 21 of the steering system 20 is comprised of simple gearing means. FIG. 6 is a graph showing not only the values for ΔL₁and ΔL₂plotted previously in FIG. 5, but also the values of ΔL₂′ at 40 taken from Table 1. An examination of FIG. 6 will reveal that for values of α_iless than aproximately 30 degrees the decrease in the length ΔL₂′ of the outer tie rod 23 will be less than the ideal or requiired decrease in the length ΔL₂and for values of α_igreater than 30 degrees the decrease in the length ΔL₂′ will be more than ideal or required decrease in the length ΔL₂. Thus, with the mechanical function generator 21 being comprised of a relatively simple gearing means a condition approximating that which would provide perfect Ackerman may be achieved. A percent error analysis performed on the values of ΔL₂and ΔL₂′ plotted in FIG. 6 will show that as α_idecreases within a range from approximately 30 degrees to 1 degree, the percent error becomes increasingly positive, ranging from zero percent to nearly +26 percent, and that as a, increases within a range from approximately 30 degrees to 40 degrees, the percent error becomes increasingly negative, varying from zero percent to nearly −6.5 percent.

[0069] Referring back to the column headed ΔL₂″ in Table 1, it will be observed that the equation

[0070] y=−6e−6x³+0.0009x²−0.1037x

[0071] was used, with R²=1. This second mathematical function provides a non-linear approximation of the relationship yielding the ΔL₂values appearing in Table 1 and comtemplates use of a mechanical function generator 21 made up of a relatively complex mechanical arrangement. FIG. 7 provides a graphical plot of the values of ΔL₂″ at 42 taken from Table 1, along with the values of ΔL₁and ΔL₂as they were previously shown in FIG. 5. Upon inspection of FIG. 7, it will be noted that the curves plotted for ΔL₂and ΔL₂″ are virtually indistinguishable for all indicated values of α_i. Thus, a condition of nearly perfect Ackerman may be created by with the mechanical function generator 21 when it is constructed of more complex mechanical means which produces a change in length of the right inner tie rod 23 in accord with the above provided non-linear function. A percent error analysis serves to further illustrate the degree to which nearly perfect Ackerman may be attained. Analysis of the different values of ΔL₂and ΔL₂″ shown in Table 1 will reveal that for values of α_iwithin a range of 2 degrees to 5 degrees, the percent error is practically negligible, ranging from +0.01 percent to −0.01 percent. For values of α_ifalling within the range of 6 degrees to 40 degrees, the percent error will vary from −0.03 percent at α_i=6 degrees to −1.27 percent at α_i=40 degrees. At α_i=1 degree, percent error will be −0.03 percent.

[0072] Although the present invention has been described above in detail, the same is by way of illustration and example only and is not to be taken as a limitation on the present invention. Accordingly, the scope and content of the present invention are to be defined only by the terms of the appended claims.