Independent rear wheel toe-in control in a vehicle four wheel steering system

The rear wheels of a motor vehicle are independently steered in an inward (toe-in) direction to improve yaw stability and on-center response. In moderate-to severe turning maneuvers, the rear wheel on the outside of the turn is controlled. In low-to-moderate turning maneuvers, the rear wheel on the inside of the turn is controlled.

This invention is directed to steering the rear wheels of a four wheel 
steer motor vehicle, and more particularly, to a control method in which 
the toe-in angle of each rear wheel is independently controlled. 
BACKGROUND OF THE INVENTION 
Various four wheel vehicle steering systems have been proposed for the dual 
purpose of improving the vehicle stability and maneuverability during 
turning maneuvers, and improving the on-center responsiveness, as compared 
to a two wheel steering system. The stability improvement, achieved by 
steering both rear wheels at a relatively small angle in-phase with the 
front wheels, is implemented primarily at moderate-to-high vehicle speeds. 
The on-center response improvement, achieved by steering both rear wheels 
at a relatively small angle out-of-phase with the front wheels, is also 
implemented primarily at moderate-to-high vehicle speeds. The 
maneuverability improvement, achieved by steering both rear wheels at a 
relatively large angle out-of-phase with the front wheels, is implemented 
primarily at low vehicle speeds. 
Since the above-described controls steer both rear wheels in unison, the 
rear wheels each experience toe-in and toe-out steering movements. The toe 
angle is defined as the steering angle of the wheel relative to the 
longitudinal or roll axis of the vehicle. Toe-in refers to a toe angle 
which points the wheel toward the forward longitudinal axis of the 
vehicle, and toe-out refers to a toe angle which points the wheel away 
from the forward longitudinal axis. 
The above-described characteristic of conventional rear wheel steering 
systems causes several problems. Perhaps most importantly, the vehicle 
must be designed to accommodate relatively large peak-to-peak movement of 
the rear wheels. This, of course, impacts styling, tire and wheel 
selection, trunk capacity, etc. Furthermore, special toe angle alignment 
procedures and equipment must be used. Also, failure analysis and 
detection becomes very complicated, especially if electrical actuators are 
employed. 
SUMMARY OF THE PRESENT INVENTION 
The present invention is directed to an improved steering system which 
achieves the moderate-to-high speed advantages of conventional four wheel 
steering systems through independent toe-in control of each rear wheel. 
In the basic configuration, the control improves vehicle yaw stability in 
moderate-to-high speed turning maneuvers through toe-in control of the 
rear wheel on the outside of the turn. As the vehicle enters the turn and 
begins to roll about its longitudinal axis, the normal force on the rear 
wheels shifts toward the outside of the turn. This increases the lateral 
force produced by the outside rear wheel while decreasing the lateral 
force produced by the inside rear wheel, thereby producing a progressively 
increasing rear cornering force in the same direction (in-phase) as the 
front wheel cornering force. In the illustrated embodiment, further 
improvements in on-center response and yaw initiation are achieved through 
toe-in control of the rear wheel on the inside of the turn. 
Many of the disadvantages of conventional four wheel steering systems are 
inherently overcome with the above-described system. For example, vehicle 
packaging concerns are simplified since the rear wheels are never steered 
in a toe-out direction. Alignment procedures are similarly simplified 
through the provision of an adjustable mechanical stop at the point of 
zero steering angle. On a system level, actuator travel requirements are 
reduced and failure mode concerns associated with unintentional toe-out 
steering are eliminated. As an additional advantage, the peak and 
steady-state power requirements are reduced since in most maneuvers only 
one rear wheel is steered at any point in time.

DETAILED DESCRIPTION OF THE INVENTION 
Referring particularly to FIG. 1, the reference numeral 10 generally 
designates a motor vehicle having four steerable wheels. The front wheels 
12, 14 are steered together in response to rotation of a driver operated 
hand wheel 16. The hand wheel 16 is mechanically connected via steering 
column 18 to a pinion gear 20, which is maintained in meshing engagement 
with teeth formed on a front rack 22 which, in turn, is connected to front 
wheel tie rods (not shown). 
A position transducer (T) responsive to the rotary position of steering 
column 18 provides a handwheel position signal as an input to a 
computer-based control unit 24 via line 28. For control purposes, the 
signal represents the steering angle Df of the front wheels as positioned 
by the vehicle operator via rotation of the hand wheel 16. The vehicle 
speed Nv is detected by a suitable speed pickup, possibly via an 
electronic engine control module, providing a vehicle speed signal Nv to 
control unit 24 via line 30. 
The left (L) and right (R) rear wheels 32L, 32R are provided with steering 
knuckles 34L, 34R and linear electric actuators 36L, 36R for effecting 
limited independent toe-in steering. The actuators 36L, 36R are mounted on 
lower control arms 60L, 60R of the vehicle suspension, and are coupled to 
the steering knuckles 34L, 34R via output links 50L, 50R. The actuators 
36L, 36R are controlled by control unit 24 via the conductor pairs 62, 64, 
respectively. 
Referring to the partially sectioned view of right actuator 36R in FIG. 5a 
, it will be seen that each actuator 36L, 36R comprises an electric motor 
40 having an armature 42 connected to drive a sector gear 46 via an 
integral worm gear 48, an axle pin 52 and eccentric journal 54 for 
connecting the sector gear 46 to the respective output link 50R, and a 
torsional spring 56 for biasing the motor armature 42 to a zero steering 
angle position. 
In the illustrated embodiment, motor 40 is mechanized with a permanent 
magnet DC machine, electric power being supplied to the rotor windings via 
a conventional commutator and brush assembly 58. The output links are 
positioned to effect a desired rear steering angle by energizing the motor 
40 at a controlled current level. Motor output torque is developed in 
known relation to the motor current, and the output torque is opposed by 
the restoring or centering force of torsional spring 56. This results in a 
characteristic motor current vs. desired position relationship as depicted 
by the graph of FIG. 5b. This means that the control unit 24 can 
independently position the rear wheels 32L, 32R by scheduling suitable 
motor current values for the actuator motors 40L, 40R. 
The primary control according to this invention is a variable toe-in of the 
rear wheel on the outside of the turn (outside wheel), as schematically 
depicted in FIG. 2a, the specific steering angle being determined as a 
function of the front wheel steering angle Df and vehicle speed Nv. The 
control is employed primarily at moderate-to-high vehicle speeds and 
off-center front steering angles, and has the effect of improving vehicle 
yaw stability in slippery road conditions and in severe steering 
maneuvers. FIG. 2b graphically depicts representative outside wheel 
steering commands Dro as a function of front steering angle Df for various 
increasing vehicle speeds V1, V2 and V3. As indicated by the graph, the 
outside rear steering angle Dro generally increases with both front 
steering angle Df and vehicle speed Nv. 
At the initiation of a steering maneuver, the normal force is approximately 
equal on both left and right rear wheels. Thus, the initial in-phase 
steering of left rear wheel 32L produces a lateral force of moderate 
magnitude. However, as the body of the vehicle begins to roll about its 
longitudinal axis, the normal force on the left rear wheel 32L increases 
and the normal force on the right rear wheel decreases. This increases the 
lateral force produced by the left rear wheel 32L, thereby producing a 
progressive net increase in the rear cornering force which is in-phase 
with the front wheel cornering force. Since typical evasive maneuvers 
include both initiation and recovery phases, it will be understood, of 
course, that the right rear wheel 32R becomes the "inside wheel" during 
the unshown recovery phase of the example of FIG. 2a. In such a maneuver, 
the control of this invention operates to reduce the phase delay between 
the consequent front and rear wheel force reversals, thereby increasing 
yaw plane stability. 
The preferred embodiment of this invention also includes a secondary 
variable toe-in control of the rear wheel on the inside of the turn 
(inside wheel), as schematically depicted in FIG. 3a, the specific 
steering angle again being determined as a function of the front wheel 
steering angle Df and vehicle speed Nv. This control is employed primarily 
at moderate vehicle speeds near on-center front steering angles, and has 
the dual effect of improving on-center responsiveness and increasing yaw 
initiation at the beginning of a steering maneuver. 
FIG. 3b graphically depicts representative inside wheel steering commands 
Dri as a function of front steering angle Df for various decreasing 
vehicle speeds V1, V2 and V3. As indicated by the graph, the inside rear 
steering angle Dri generally decreases with vehicle speed Nv; the angle 
Dri initially increases with increasing front wheel steering angle Df, but 
is progressively removed with further increasing front wheel steering 
angle Df. In some applications, it may be desirable to schedule the inside 
rear steering angle Dri as a function of the rate of change in front wheel 
steering angle (d(Df)/dt) instead of, or in addition to, front wheel 
steering angle Df and vehicle speed Nv. 
Again, the normal force is approximately equal on both left and right rear 
wheels at the initiation of the steering maneuver, and the initial toe-in 
of right rear wheel 32R produces a lateral force of moderate magnitude. 
However, since the rear lateral force is now out-of-phase with the front 
wheels 12, 14, it tends to augment the on-center responsiveness of the 
vehicle when no significant steering maneuvers are intended. If the 
operator continues to increase the front wheel steering angle Df, as in a 
significant steering maneuver, the inside rear wheel steering angle Dri 
rapidly diminishes, as seen in the graph of FIG. 3b, but serves to 
initiate yawing of the vehicle for improved maneuverability and control. 
In significant steering maneuvers, there is also a natural reduction in 
the lateral force produced by the inside wheel toe-in as the body of the 
vehicle begins to roll about its longitudinal axis, reducing the normal 
force on the inside rear wheel 32R. 
The graph of FIG. 4 overlays representative inner and outer rear wheel 
steering angle commands Dri, Dro as a function of front wheel steering 
angle Df for a given vehicle speed Nv of 40 MPH in a system incorporating 
both inside wheel and outside wheel controls. As indicated, the inside 
wheel control is generally dominant at relatively small front wheel 
steering angles where the normal force is substantially equalized, while 
the outside wheel control is generally dominant at moderate-to-relatively 
high steering angles. 
FIGS. 6 and 7 illustrate the control of this invention from an electrical 
standpoint. FIG. 6 is a circuit diagram of the system of FIG. 1, including 
the computer-based control unit 24, and FIG. 7 depicts a flow diagram 
representative of computer program instructions executed by the control 
unit 24 in carrying out the control of this invention. 
Referring to FIG. 6, the reference numerals used in previous figures are 
repeated where appropriate. Thus, the control unit 24 is connected to 
energize the actuator motors 40L and 40R via lines 62 and 64 in response 
to front steering angle and vehicle speed input signals Df, Nv on lines 28 
and 30, respectively. Electrical power is supplied to the various 
components of control unit 24 by the vehicle storage battery 70 via 
ignition switch 71 and lines 72 and 73. The battery 70 is connected to 
motors 40L, 40R via line 74 and normally open switch arm 78 of relay 76; a 
motor current return path is provided by line 79. 
The control unit 24 includes a microcomputer (uC) 80, left and right PWM 
Driver circuits 82L, 82R, left and right power FETs 84L, 84R, left and 
right resistive shunts 86L, 86R, and relay driver RD. The microcomputer, 
which may be a Motorola MC68HC11 Microcontroller or equivalent, develops 
and outputs left and right actuator current commands Ia1 and Ia2 to PWM 
Driver circuits 82L, 82R for effecting desired left and right rear 
steering angles Dr1 and Dr2. As indicated above, the motor current 
produces motor torque which opposes the centering torque of torsional 
spring 56, resulting in a current-related actuator position. The PWM 
Driver circuits, which may be Motorola TL494 PWM Control Circuits or 
equivalent, modulate the conduction of power FETs 84L and 84R via lines 
90L and 90R to maintain the commanded actuator currents Ia1 and Ia2, 
respectively. Analog voltages indicative of the actual motor current 
levels are supplied as inputs to the PWM Driver Circuits 82L and 82R via 
lines 92L an 92R, respectively. 
Microcomputer 80 also controls the energization of relay coil 94 via relay 
driver RD. The relay coil 94 is connected to the battery 70 at one end via 
ignition switch 71, and at the other end via relay driver RD. In normal 
operation of the vehicle, the microcomputer 80 controls the relay driver 
RD to energize relay coil 94, closing the switch arm 78 to establish a 
battery current path for motors 40L, 40R. 
Referring to the flow diagram of FIG. 7, the block 100 is executed at the 
initiation of each period of vehicle operation to initialize the various 
steering parameters and memory registers. Thereafter, the blocks 102-114 
are repeatedly executed, as indicated by the flow diagram lines. Other 
related functions not directly related to the present invention, such as 
diagnostic error checking and instrumentation updating, are also 
performed. 
The blocks 102 and 104 are first executed to read the front steering angle 
and vehicle speed input signals Df and Nv, and to determine the desired 
inside and outside rear wheel steering angles Dri and Dro. As indicated 
above, with respect to the graphs of FIGS. 3b and 2b, the steering angles 
Dri and Dro are determined as a combined function of the front wheel 
steering angle Df and the vehicle speed Nv. To this end, data points 
defining the traces depicted in those graphs are stored in nonvolatile 
memory of microcomputer 80 in three-dimensional look-up table arrays. 
If the front steering angle Df indicates that the vehicle is being steered 
to the right, as determined at block 106, the block 108 is executed to 
assign the outside wheel steering angle Dro to the left actuator control 
variable Drl and the inside wheel steering angle Dri to the right actuator 
control variable Dr2. If the vehicle is being steered to the left, the 
block 110 is executed to assign the outside wheel steering angle Dro to 
the right actuator control variable Dr2 and the inside wheel steering 
angle Dri to the left actuator control variable Drl. 
Blocks 112 and 114 are then executed to determine and output the left and 
right actuator current commands Ia1, Ia2 to PWM Driver Circuits 82L, 82R, 
completing the routine. As indicated above, in reference to the graph of 
FIG. 5b, the current commands Ia1 and Ia2 are determined as a function of 
the desired left and right steering angle commands Dr1 and Dr2, 
respectively. To this end, data points defining the trace of FIG. 5b are 
stored in nonvolatile memory of microcomputer 80 in a two-dimensional 
look-up table array. Since the actuators 36L and 36R have essentially 
equivalent performance characteristics, the same tables are used for both. 
While this invention has been described in reference to the illustrated 
embodiment, it will be understood that various modifications will occur to 
those skilled in the art, and that this invention is not limited thereto. 
For example, various supportive control loops, such as rear wheel position 
feedback controls, may be added. Steering systems incorporating such 
modifications may thus fall within the scope of this invention which is 
defined by the appended claims.