Vehicle-trailer backing-up control system with vehicle rear wheel steering

A method is disclosed for controlling a backing maneuver of an automotive vehicle and trailer combination in which the vehicle has operator-actuated front wheel steering and microprocessor-actuated, reversible electric motor driven rear wheel steering. For a given initial alignment of vehicle and trailer, the computer-executed method first determines whether the driver needs to pull forward before commencing the backing operation. The driver is then requested to turn the front wheels in a direction suitable for backing the vehicle without a trailer in the desired direction. The process then determines whether the driver needs to perform counter front wheel steering before backing. Then the process controls the steering of the rear wheels during the backing operation.

TECHNICAL FIELD
 This invention relates to steering control systems for automotive vehicles
 with front and rear wheel steering. More specifically, this invention
 relates to computer control systems for assisting the operator in
 backing-up a vehicle-trailer combination when the vehicle has
 driver-operated front wheel steering and on-board computer-controlled rear
 wheel steering.
 BACKGROUND OF THE INVENTION
 Automotive vehicles with coordinated front and rear wheel steering systems
 are known. In modern applications, such steering systems may be used,
 e.g., in relatively large sport utility vehicles or trucks. The operator
 retains control over the steering of the vehicle's front wheels, and a
 vehicle computer-based system controls steering of the rear wheels. For
 example, one rear wheel steering system includes an electric motor-driven,
 rack and pinion rear-wheel-steer actuator that, upon computer command,
 produces a desired rear-wheel-steer angle to enhance the handling and
 maneuverability of the vehicle.
 When the steerable rear wheels are set at an angle to the same side of the
 longitudinal axis of the vehicles as the front wheels, the system is
 considered to be providing "in-phase" rear wheel steering. "Out-of-phase"
 steering, conversely, is where the rear wheels are disposed to the
 opposite side of the vehicle longitudinal axis from the front wheels.
 Out-of-phase rear wheel steering markedly shortens the turning radius of a
 large vehicle. The on-board steering controller determines the
 rear-wheel-steer angle as a function of vehicle speed and the operator
 hand steering-wheel angle. The system, in general, will provide an
 out-of-phase steering angle at low vehicle speed to reduce the turning
 radius of a vehicle and in-phase steering at high vehicle speed to enhance
 directional stability.
 The steering controller, thus, continually monitors vehicle speed, forward
 or reverse, and the angles, right or left, of the steered wheels with
 respect to the longitudinal axis of the vehicle. The controller also
 determines the yaw rate (i.e., the turning motion) of the vehicle in
 setting the steering angle of the rear wheels. While the front wheels may
 be steered at angles, e.g., from +33.degree. (left) to -33.degree. (right)
 with respect to the vehicle axis, the steering range of the rear wheels is
 usually smaller, e.g., from +12.degree. to -12.degree.. In steering the
 front wheels, the operator typically can rotate the hand wheel up to about
 540.degree. in either direction from its wheel centered position.
 Larger automotive vehicles are often used to pull trailers, and sometimes
 it is necessary for the operator to back up the vehicle-trailer
 combination. Backing-up a front wheel steering vehicle is now within the
 experience and skill of vehicle operators who need to perform such jobs.
 But performing the same task with a vehicle-trailer combination is
 counterintuitive and not within common experience. It is an object of this
 invention to provide a computer-based, driver-interactive process to
 assist in backing-up a vehicle-trailer combination with the aid of rear
 wheel steering.
 SUMMARY OF THE INVENTION
 The invention is a driver interactive, driver advisor process that is
 performed largely on an on-the-vehicle microprocessor which generally
 would also be employed for control of the rear wheel steering.
 The invention is applicable to a vehicle having rear wheels that can be
 steered through a reversible electric motor, and rack and pinion gearing,
 commanded by a suitably programmed microprocessor (a controller). The
 controller utilizes a sensor of a known type to measure the rear wheel
 angle, .delta..sub.r, with respect to the vehicle axis. The vehicle has a
 hand steering wheel by which the operator controls the steering of the
 front wheels. The controller needs to know the angular position of the
 front wheels. This is suitably accomplished using a steering wheel
 position sensor and estimating the front wheel angle, .delta..sub.f, based
 on the input of the steering wheel sensor divided by the ratio of the
 steering gear. The vehicle also includes a trailer hitch for receiving the
 tongue of a trailer. The hitch includes a sensor for providing the angle
 of the trailer tongue or hitch to the longitudinal axis of the vehicle,
 .theta..sup.0, to the controller. The direction of the trailer is
 determined by the direction of its fixed tongue.
 As stated, the vehicle includes a microprocessor-based controller for using
 the appropriate sensor signals for controlling the electric motor-driven
 rear wheel steering and, additionally, performing the computations and
 algorithm steps for the control of vehicle-trailer backup. The controller
 needs to know vehicle travelling speed (forward or reverse), v.sub.x,
 which may be obtained from a special sensor or, as is often the case, from
 a brake system control module or by direct measurement of prop-shaft
 pulses.
 As will be described, the controller will also actuate the issuance of
 commands to the vehicle operator during the execution of the process of
 this invention. The unit broadcasting or issuing such commands will
 sometimes be referred to in this specification as "Driver Advisor."
 In the practice of the invention, the driver of a vehicle-trailer
 combination activates the controller when the driver wishes to receive aid
 in trailer backing or parking. Upon completion of the backing process,
 this aspect of the controller functions is shut off until needed on
 another trailer backing situation.
 When the process is initiated by the driver, the vehicle-trailer
 combination has current front and rear wheel angles and a hitch angle
 reported to the controller by the respective sensors. The vehicle is then
 stopped or backing slowly. The controller system then performs
 calculations to determine whether the current hitch angle, .theta..sup.0,
 exceeds the maximum permissible hitch angle, .theta..sup.0.sub.max, that
 can be overcome, while continuing to backup, utilizing the full capability
 of the front and rear steering. The maximum hitch angle,
 .theta..sup.0.sub.max, is a function of the maximum wheel angles, the
 wheel base and track of the vehicle, the distance from the vehicle rear
 axle to the hitch, and the trailer tongue length. This inquiry by the
 controller is referred to in the following specific embodiment section of
 this specification as Criterion 1. If the current hitch angle is larger
 than the maximum permissible hitch angle, the answer to the inquiry is
 "yes" and Criterion 1 is met. The process then requires a "pull forward"
 mode. The controller actuates the Driver Advisor to prompt the driver to
 pull the vehicle forward to reduce the hitch angle. The controller
 continues to receive sensor signals and assess the changing current hitch
 angle until this criterion is not met, and the "pull forward" mode is
 cleared.
 If Criterion 1 is not met or after a pull forward mode is cleared, the
 controller issues a command to the Driver Advisor to prompt the driver to
 first center the steering wheel and then steer such that the front wheel
 angle, .delta..sup.*.sub.f, is in the direction and amount where the
 driver would steer during backing toward the target position if without
 towing a trailer. Based on the driver's steering decision of the front
 wheel angle, .delta..sup.*.sub.f, the controller system determines a
 desired hitch angle, .theta..sup.*, that must be achieved in order to
 comply with the driver-determined front wheel angle.
 The controller system now calculates the maximum allowed front steering
 wheel angle, .delta..sub.f max, for achieving the desired hitch angle
 without counter steering of the front wheels. The controller calculates to
 determine this maximum value allowed for the driver's command of the front
 wheel angle such that with a potential available assist of a given maximum
 rear wheel steering angle, the trailer can be backed to where the driver
 wants to put it. The controller then determines whether the driver's front
 wheel angle command, .delta..sup.*.sub.f, (in road wheel angle) is beyond
 this maximum allowed front wheel angle, .delta..sub.f max. This is
 referred to here as Criterion 2. If the driver's command exceeds the
 maximum allowed angle, the answer is "yes" and Criterion 2 is met. If so,
 a "counter steer" mode is entered, and the Driver Advisor prompts the
 driver to counter steer, or turn the front wheels in the opposite
 direction, at the system-calculated amount, until Criterion 2 is not met
 and the "counter steering" mode is cleared. The vehicle-trailer
 combination is being slowly backed during this controller processing. If
 Criterion 2 is not met or after suitable counter-steered backing, the
 controller and process continue to the next stage of the vehicle-trailer
 backing process.
 The vehicle-trailer combination is still being backed slowly at the
 driver-set steering wheel position, .delta..sup.*.sub.f. The respective
 positions of the vehicle, trailer and steering wheel are such that the
 backing process can be successfully completed with the assistance of the
 programmed controller.
 The system correlates or maps the driver's command, .delta..sup.*.sub.f, to
 the desired hitch angle, .theta..sup.*, and continually calculates the
 difference, .DELTA..theta., between the desired and currently measured
 hitch angle to determine a desired incremental (positive or negative) rear
 wheel angle necessary to obtain the desired hitch angle. In a preferred
 embodiment, the rear wheel angle change calculation is based on a
 proportional-integral-derivative control algorithm that will be described
 in a following section of this specification. The incremental change in
 rear wheel angle is used to modify the rear wheel command in driving the
 rear wheel steering system and successfully completing the vehicle-trailer
 backing operation.
 In this automatic rear wheel control backing process, the system determines
 if driver's steering, the controlled rear wheel steering and the movement
 of the trailer are consistent with trailer movement and his/her steering
 as if backing a vehicle without towing a trailer. This determination is
 referred to here as Criterion 3. For example, Criterion 3 is met when the
 movement of the trailer, due to the combination of front and rear wheel
 steering, is different than the intuition of the driver. In the case of a
 "yes" response to Criterion 3, the system is set to an "assistance" or
 driver assurance mode, and the Driver Advisor will assure the driver that
 the vehicle/trailer is moving in the right direction. If Criterion 3 is
 not met, then Criterion 4 is met.
 When Criterion 4 is met, an "automatic" mode is set in the controller
 system, and the system enters automatic adjustment of the rear wheel angle
 without need of Driver Advisor assistance or notification. When the
 backing operation is complete, the backing subroutine of the controller is
 turned off.
 As indicated in the above summary, calculations are used to relate and
 determine several parameters such as present, desired or maximum front
 wheel, rear wheel and hitch angles. As will be described in more detail
 below, these involve complex trigonometric angular relationships and
 functions also including the wheelbase and track of the vehicle, the
 length of the fixed trailer tongue and the distance of the hitch from the
 rear axle of the vehicle. Equations have been developed to model the steps
 of this invention. As will be seen, since some of these parameters are
 fixed for a given vehicle and can be generalized for size classes of
 trailers, many of the process calculations can be done in advance for a
 vehicle design and the results stored as graphs or look-up tables in the
 computer memory of the controller.
 Thus, the subject process provides driver-initiated assistance in the
 backing of rear wheel-steerable vehicles that are hitched to a trailer.
 The process is efficient in terms of computer requirements and can
 initiate the broadcasting of instructions or advice to a vehicle operator
 during the stress of the backing operation. Other objects and advantages
 of this invention will become apparent from a detailed description of
 preferred embodiments of the invention which follow.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 The operation of a vehicle equipped for operator-controlled front wheel
 steering and computer-controlled rear wheel steering permits more flexible
 maneuvering of the vehicle, especially a large sport utility vehicle or
 truck, when moving in a forward direction. The driver soon adapts to the
 controller-assisted rear wheel steering and takes advantage of the
 improved maneuverability of the vehicle. However, when backing the vehicle
 with a hitched trailer, maneuvering becomes more complicated and
 counter-intuitive. FIGS. 1 and 2 illustrate a front and rear wheel
 steerable vehicle and an attached trailer. FIGS. 7-10 also show the same
 components in different backing-up positions to illustrate process
 parameters that are used in the practice of this invention.
 Vehicle 10 (shown in outline in FIG. 1) has steerable front wheels 12 and
 rear wheels 14. The directional heading of front wheels 12 is controlled
 through a driver-operated steering wheel 16 which is connected through a
 steering column 17 and associated steering linkage 18. Operator effort in
 steering front wheels 12 may be assisted by hydraulic or electrical
 mechanisms, not shown in these figures. The wheels 12 are turned together
 from positions straight ahead with a steering angle of zero with respect
 to the longitudinal axis 22 of the vehicle 10 to either the right or left
 of the axis 22. The maximum angular movement for a particular vehicle
 group is generally fixed and may be, for example, about +33.degree. (left)
 or -33.degree. (right).
 In the practice of this invention, it is necessary to know the present
 front wheel angle, .delta..sub.f, and the maximum fixed front wheel angle,
 .delta..sub.f max, (fixed by steering system design). The present or
 current wheel angle is suitably determined using a steering wheel position
 sensor 20 and estimating the front wheel angle, .delta..sub.f, based on
 the input of the steering wheel sensor divided by the known ratio (e.g.,
 16:1) of the driver steering wheel rotation to front wheel angular turning
 movement. The signal from sensor 20 is continually transmitted to steering
 controller 24.
 The rear wheels 14 of vehicle 10 are turned in unison using a two-way or
 reversible electric motor 26 and a rack and pinion steering mechanism 28.
 The turning of rear wheels 14 is actuated in a determined direction by a
 suitably programmed controller 24 in combination with a sensor 30 that
 measures the rear wheel angle, .delta..sub.r, with respect to the vehicle
 axis 22. The rear wheels by vehicle design also have a fixed maximum
 angle, .delta..sub.r max.
 The vehicle also includes a trailer hitch 32 for receiving a fixed tongue
 38, two wheel trailer 34. The hitch 32 includes a sensor 36 for providing
 the angle, .theta..sup.0, of the rigid trailer tongue 38 or hitch to the
 longitudinal axis 22 of the vehicle to the controller.
 As stated, controller 24 includes a microprocessor and associated computer
 input-output components for receiving and manipulating the appropriate
 sensor signals, e.g., 20 and 30 for controlling the rear wheel 14 steering
 and, additionally, performing the computations and algorithm steps for the
 control of vehicle-trailer backup. The controller 24 needs to know vehicle
 travelling speed, v.sub.x, in a fore-aft direction which may be obtained
 from a special sensor 40 or, as is often the case, from a brake system
 control module or by direct measurement of prop-shaft pulses. The
 controller will also obtain or determine the current yaw rate, v.sub.y, of
 the vehicle. In this preferred embodiment, controller 24 will also actuate
 the issuance of commands to the vehicle operator during the execution of
 the process of this invention. The unit broadcasting or issuing such
 commands 42 is referred to in this specification as "Driver Advisor."
 During much of the vehicle operating time, it is driving in a forward
 direction and the controller 24 is programmed to manage rear wheel
 steering in accordance with suitable algorithms. The assignee of this
 invention has rear wheel steering mechanism and control algorithm for
 quadra-wheel steering, known as QS4. At relatively low vehicle speeds, the
 rear wheels can be steered out of phase with the front wheels to shorten
 the turning radius of the vehicle. When the vehicle is backing, the rear
 wheel angle can often be used to quickly change the hitch angle.
 If the vehicle needs to be backed with a trailer in tow, the maneuvering of
 the vehicle 10 and trailer 34 can become quite complicated. Rear wheel
 steering controller 24 can be actuated by the vehicle operator to expand
 its steering managing scope to include the following associated or
 sub-process. In the description of the process, reference will be made to
 FIG. 3 which is a flow chart of the algorithm of the process. Reference
 will also be made to FIGS. 7-10 which illustrate the positions of the
 vehicle 10 and trailer 34 in various steps of the process.
 The entry to the subject trailer backing process is initiated by the driver
 of the vehicle, block 300 of FIG. 3. This may be done by signaling the
 controller by any suitable switching or other communication means that the
 driver wishes assistance in a vehicle-trailer backing maneuver. At the
 time this backing process is initiated, vehicle and trailer are in a
 current alignment and have a current velocity, v.sub.x, which may be zero
 or negative.
 In illustrating a preferred embodiment of the invention, certain design
 fixed parameters of the vehicle must be specified to perform the
 computer-based process. Here it is assumed that in the design of the
 vehicle, the maximum front wheel turning angle is 33.degree. (left) to
 -33.degree. (right) and, correspondingly, the maximum rear wheel turning
 angle is from 12.degree. to -12.degree. as illustrated in FIG. 7. The
 wheel base of the vehicle, L.sub.1, is 2.978 meters; the track, T, is 1.7
 m; the length of the trailer tongue, L.sub.2, is 5 m and the distance from
 the rear axle of the vehicle to the hitch, h, is 1.7 m. FIG. 7 also
 illustrates the trailer 34 at a maximum hitch angle,
 .theta..sup.0.sub.max, with respect to the axis 22 of the vehicle and
 hitch 32.
 In making determinations of wheel and hitch angles in accordance with the
 invention, a suitable approach is to use the equation below which
 describes the yaw rate relationship between vehicle and trailer under
 steady state:
EQU vehicle yaw rate=trailer yaw rate
 ##EQU1##
 where
 ##EQU2##
 is hitch velocity in y direction.
 ##EQU3##
 If .delta..sub.f &gt;.delta..sub.r ;y.sub.1 =T/2
EQU If .delta..sub.f &lt;.delta..sub.r ;y.sub.1 =-T/2
 .delta..sub.f, .delta..sub.r are front and rear wheel angles, and
 .delta..sub.f max, .delta..sub.r max are vehicle design maximum angles of
 front and rear wheels as, e.g., specified above, L.sub.1 is wheel base,
 L.sub.2 is tongue length, T is wheel track width and h is the vehicle rear
 axle to hitch distance. The values used to calculate the table data are:
 L.sub.1 =2.978 m; L.sub.2 =5 m; T=h=1.7 m.
 In the control process the following relationships are used.
 .theta..sup.0 : hitch angle information based on measurement from sensor
 .theta..sup.* : the desired hitch angle, determined based on driver's
 command of .delta..sup.*.sub.f and vehicle-trailer geometry information.
 See FIG. 4 for the sample graphical data based on the above-stated
 vehicle/trailer parameters.
 .theta..sup.0.sub.max : the maximum hitch angle at which the driver can
 overcome, while backing the vehicle, through fully utilizing maximum
 authority of the combination of front and rear wheel steering. The value
 is predetermined as a function of vehicle/trailer geometry (in present
 implementation, it is 60.0.degree.). .theta..sup.0.sub.max can be
 estimated by equation (2):
 ##EQU4##
 where:
EQU p=tan(.delta..sub.f max)-tan(.delta..sub.r max)
 ##EQU5##
 .DELTA..theta.: the difference between desired hitch angle .theta..sup.*
 and the measurement of the current position .theta..sup.0.
 .delta..sub.f : front wheel angle based on the measurement of steering
 wheel angle from sensor divided by gear ratio.
 .delta..sup.*.sub.f : driver's input as a command on front wheel, based on
 measurement of steering wheel angle from sensor when driver gives a
 steering command, divided by gear ratio.
 .delta..sub.f max : the maximum value allowed for driver's command of front
 wheel angle such that a given maximum rear wheel steering .delta..sub.r
 max can fulfill the requirement. This is determined based on equation (3),
 which is derived from equation (1). An example data is shown in FIG. 5 for
 .delta..sup.1.sub.f max at .delta..sub.r max =-12.degree. (to the right)
 and FIG. 6 for .delta..sup.2.sub.f max at .delta..sub.r max =12.degree.
 (left).
 ##EQU6##
 .DELTA..delta..sup.*.sub.r : the increment of rear wheel steering angle
 corresponding to .DELTA..theta. to modify rear wheel angle command,
 .delta..sup.*.sub.r =.delta..sup.*.sub.r0 +.DELTA..delta..sup.*.sub.r,
 based on a PID (Proportional-Integral-Derivative) control algorithm.
 ##EQU7##
 where .delta..sup.*.sub.r0 is the current rear wheel angle from
 measurement. The gains of P, I and D can be a set of
 experimentally-determined constants that result in fastest response with
 minimum overshoot of the rear wheel position.
 Continuing with the description of the FIG. 3 flowchart, upon activation of
 the process at block 300, the process proceeds to query block 302. In the
 step indicated by query block 302, the controller determines whether the
 present hitch angle, .theta..sup.0, as indicated by the hitch angle
 sensor, is greater than the maximum hitch angle, .theta..sup.0.sub.max, at
 which the driver can overcome, without pulling the vehicle forward,
 through fully utilizing maximum capability of the combination of front and
 rear wheel steering. The maximum hitch angle is estimated using equation
 (2). Thus, in block 302, the controller determines whether
 .vertline..theta..sup.0.vertline.&gt;.vertline..theta..sup.0.sub.
 max.vertline.; see illustration in FIG. 7. This is called Criterion 1 in
 block 302. In the example of .theta..sup.0.sub.max illustrated in FIG. 7,
 the front wheels are seen turned the maximum amount to the left and the
 rear wheels are turned their maximum amount to the right positioned to
 straighten out the trailer hitch angle from its present position. If the
 hitch angle were larger, the trailer could not be straightened without
 pulling the vehicle forward.
 If the answer to the block 302 query is "no," the process proceeds to block
 306.
 If the answer to the block 302 query is "yes," the process proceeds to
 block 304 where the process is in "pull forward" status and the Driver
 Advisor instructs the driver to pull forward. The driver straightens the
 front wheels and slowly pulls the vehicle forward, with the process
 cycling through blocks 302 and 304 until the hitch angle is suitably
 reduced, i.e., Criterion 1 is no longer met. Then "pull forward" status is
 cleared and a command is sent from the controller to the Driver Advisor to
 advise the driver to stop pulling forward and to shift into reverse and
 the process proceeds to block 306.
 In block 306, the controller has the Driver Advisor instruct the driver to
 backup and steer the front wheels to achieve a desired vehicle-trailer
 position as if no trailer was present. The driver turns his/her steering
 wheel for a front wheel angle position of best judgment,
 .delta..sup.*.sub.f, and commences backing slowly, and the controller
 determines the desired hitch angle, .theta.*, from the driver's input,
 .delta..sup.*.sub.f, block 308. The value of the desired hitch angle,
 .theta.*, may be determined experimentally, and preferably these
 relationships for a range of driver steering inputs and trailer sizes are
 made offline and stored in the memory of the controller. FIG. 4 presents
 the relationship between .delta..sup.*.sub.f and .theta.*. For example,
 referring to FIG. 4, for a driver input of .delta..sup.*.sub.f =10
 .degree., .theta.*=40.degree..
 The controller then determines the maximum front wheel angle, .delta..sub.f
 max, process block 310, the maximum value allowed for driver's command of
 front wheel angle such that a given maximum rear wheel steering
 .delta..sub.r max, can fulfill the requirement. This is determined based
 on equation (3), also stated above, which is derived from equation (1). An
 example of pre-calculated and graphed data is shown in FIG. 5 for
 .delta..sup.1.sub.f max at .delta..sub.r max =-12.degree. and FIG. 6 for
 .delta..sup.2.sub.f max at .delta..sub.r max =12.degree.. The negative
 sign (-) denotes that the wheels are turned right and the plus sign (+)
 denotes that the wheels are turned left.
 ##EQU8##
 In general, it is preferred that these values be precalculated and stored
 as look-up tables of data like that contained in the graphs of FIGS. 5 and
 6.
 The process proceeds to query block 312 for the following tests for
 Criterion 2:
 a. .theta..sup.0 &gt;.theta..sup.* and
 .delta..sup.*.sub.f.ltoreq..delta..sup.1.sub.f max ; or
 b. .theta..sup.0 &lt;.theta..sup.* and
 .delta..sup.*.sub.f.gtoreq..delta..sup.2.sub.f max
 The comparisons of the present hitch angle and desired hitch angle and the
 driver's input steering command and the maximum front wheel angles are for
 the purpose of determining whether the desired trailer position can be
 reached without the driver having to turn the front wheels in the opposite
 direction, i.e., counter steering. This is Criterion 2. And if either of
 the above conditions is met, the answer is "yes" and Criterion 2 is met.
 FIG. 8 illustrates a situation satisfying Criterion 2 and requiring
 counter steering. If Criterion 2 is met, the process moves to block 314,
 and otherwise to block 316.
 In block 314, "counter steer" status is set by the controller, and the
 Driver Advisor asks the driver to counter steer. As the driver turns
 his/her steering wheel in the opposite direction, the rear wheels are
 turned to their maximum angle in the direction opposite the new angle of
 the front wheels. The driver continues to back slowly and the process
 cycles between blocks 314, 310 and 312 until Criterion 2 is no longer met.
 The process then proceeds to block 316.
 In block 316, the Driver Advisor instructs the driver to restore his/her
 original steer angle, .delta..sup.*.sub.f, if the process has been in
 "counter steer" status. Otherwise, the driver maintains such front wheel
 steering angle. At this stage, the attitude of the vehicle and trailer
 permit the controller to manage the rear wheel steering angle so that the
 driver can back the trailer with normal adjustment of the front wheel
 direction. The process enters the backing "assistance" phase and proceeds
 to query box 318.
 At query box 318, the controller makes several angle comparisons to
 determine whether the controlled backing of the trailer may take it in a
 direction that causes the driver to think that he/she needs to change
 his/her front steering direction. In box 318, the controller determines
 which of Criterion 3 or 4 is applicable. FIGS. 9 and 10 illustrate
 examples of situations in which Criterion 3 and 4 are, respectively,
 applicable.
 Obviously, with many possible hitch angles, positive or negative, and the
 possibility of in-phase or out-of-phase rear wheel angles, the trailer can
 experience direction changes not anticipated by the driver. FIG. 9
 illustrates a Criterion 3 situation. The front wheels are turned to the
 left. The actual hitch angle is greater than the desired hitch angle. The
 rear wheels are turned to the left to reduce the hitch angle and it is
 seen that the trailer will move to its right. This is a situation that
 could confuse the driver and, accordingly, the controller causes the
 Driver Advisor to inform the driver that the trailer is moving in the
 correct direction, process block 320, before moving to calculation block
 322. If the vehicle-trailer attitude does not meet Criterion 3, Criterion
 4 applies. An example of the Criterion 4 situation is illustrated in FIG.
 10. As seen in that figure, the actual hitch angle is less than the
 desired hitch angle and with the wheel alignments the trailer will move to
 its left as the driver expects. Under Criterion 4, no notice is given to
 the driver, and the process moves directly to block 322.
 The controller's choice between Criterions 3 and 4 requires several
 comparisons as follows:
 The criteria 3:
 "assistance" status based on Criterion 3:
 .theta..sup.0 &gt;.theta..sup.* and .delta..sup.*.sub.f &gt;.delta..sup.1.sub.f
 max or .theta..sup.0 &lt;.theta..sup.* and .delta..sup.*.sub.f
 &lt;.delta..sup.2.sub.f max ; and
 .theta..sup.0..theta..sup.* &lt;0; or
 .theta..sup.0..theta..sup.* &gt;0 and
 .vertline..theta..sup.0.vertline.&gt;.vertline..theta..sup.*.vertline.
 .theta..sup.0 &gt;.theta..sup.* means in this case, the hitch angle must be
 reduced in order to reach the desired hitch angle;
 .delta..sup.*.sub.f &gt;.delta..sup.1.sub.f max means when .delta..sup.*.sub.f
 =.delta..sup.1.sub.f max at .delta..sub.r =-12.degree. (in order to
 minimize .theta.), hitch angle rate .theta.=0; in order to reduce hitch
 angle, it is necessary that .theta.&lt;0, that is .delta..sup.*.sub.f
 &gt;.delta..sup.1.sub.f max or:
 .theta..sup.0 &lt;.theta..sup.* means in this case, it is necessary to
 increase the hitch angle in order to reach the desired hitch angle;
 .delta..sup.*.sub.f &gt;.delta..sup.2.sub.f max means when .delta..sup.*.sub.f
 =.delta..sup.2.sub.f max at .delta..sub.r =12.degree. (in order to
 maximize .theta.), the hitch angle rate .theta.=0; in order to increase
 hitch angle, it is necessary that .theta.&lt;0, that is .delta..sup.*.sub.f
 &lt;.delta..sup.2.sub.f max
 .theta..sup.0..theta..sup.* &lt;0 means both hitch angles are in different
 sides (one positive and one negative), or if on the same side:
 .theta..sup.0..theta..sup.* &gt;0, then
 .vertline..theta..sup.0.vertline.&gt;.vertline..theta..sup.*.vertline. should
 be satisfied.
 Similarly for Criterion 4, the situation not satisfying Criterion 3 will be
 in Criterion 4.
 "automatic" status based on Criterion 4:
 Criterion 3:1, and
 .theta..sup.0..theta..sup.* &gt;0 and
 .vertline..theta..sup.0.vertline.&lt;.vertline..theta..sup.*.vertline.
 After checking both Criteria 3 and 4, block 318, the system will get into
 rear wheel steer control procedure. The system retrieves the current hitch
 angle, .theta..sup.0, and the desired hitch angle, .theta.*, and
 calculates the difference, .DELTA..theta.=.theta..sup.* -.theta..sup.0,
 block 322. Block 328 represents the continual stream of data processed by
 the respective, above-described sensors as the vehicle-trailer is backing.
 Such data includes vehicle speed, v.sub.x, front wheel angle,
 .delta..sub.f, rear wheel angle,.delta..sub.r, block 330, and hitch angle,
 .theta..sup.0.
 The value of .DELTA..theta., obviously, is a measure of how much the hitch
 angle or trailer direction must be changed to achieve the intentions of
 the vehicle operator. The change in direction is to be managed by
 controlling the direction of the rear wheels through controller 24 and the
 process of this invention. As stated, the controller is continually
 receiving information concerning the motion of the vehicle and trailer,
 blocks 328 and 330, and adjusts to the driver's steering of the front
 wheel to correct or minimize .DELTA..theta.. The correction step is
 indicated in block 324.
 In a preferred embodiment, the value of .DELTA..theta. is used in an
 equation like (4) to calculate a rear wheel steering correction to be
 determined as a desired incremental (positive or negative) rear wheel
 angle, .DELTA..delta..sup.*.sub.r.
 ##EQU9##
 In accordance with this relationship, the incremental rear wheel angle is
 based on a proportionality factor, P, of .DELTA..theta., an integral of
 .DELTA..theta. with time, t, multiplied by a constant I and a differential
 of .DELTA..theta. with time, t, multiplied by a constant D. Such factor
 and constants may be determined experimentally for a vehicle and trailer
 size class. Equation 4 is termed a PID (Proportional-Integral-Derivative)
 control algorithm. The resulting value of .DELTA..delta..sup.*.sub.r is
 then used to modify the rear wheel angle command .delta..sup.*.sub.r in
 the conventional rear wheel steering system, block 326. This correction is
 continually made until the backing process is completed. The signals of
 the controller 24 are used to continually actuate the electric motor 26
 controlling rear wheel angle. That is, the process cycles around blocks
 322-330 with possible continuous adjustments in rear wheel angle until the
 operator stops the vehicle and stops the process.
 In summary, upon activation of a suitable programmed controller, the
 subject process determines whether an arrangement of a front and rear
 wheel steering vehicle and trailer can (i) be backed up in its present
 orientation without first pulling forward to reduce the hitch angle and
 (ii) be backed in a direction indicated by a driver front wheel steering
 alignment without a counter front wheel steering maneuver by the driver.
 Upon making these corrections, if necessary, of the vehicle-trailer
 alignment, the process then manages the backing of the trailer by steering
 the rear wheels to reduce the difference between the continually-sensed
 hitch angle and the hitch angle calculated to move the trailer in the
 direction indicated by the operator. As an added feature, the process
 determines whether the backing of the trailer may take a path confusing to
 the operator and, in such case, assures the operator that the maneuver is
 proceeding properly.
 While the invention has been described in terms of certain preferred
 embodiments, it is apparent that other like practices could readily be
 adapted by one skilled in the art. Accordingly, the scope of the invention
 is to be considered limited only by the following claims.