Patent Publication Number: US-2021171098-A1

Title: Steering Mechanism and System for Backing a Trailer

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 15/839,794 filed Dec. 12, 2017, which is a continuation-in-part of U.S. Ser. No. 14/686,744 filed Apr. 14, 2015, which is a continuation-in-part of U.S. Ser. No. 13/628,261 filed Sep. 27, 2012, now U.S. Pat. No. 9,004,519. U.S. Ser. No. 13/628,261 claims the benefit of U.S. provisional application No. 61/626,961 filed Sep. 28, 2011. The applications and patent listed above are incorporated herein by reference in their entirety. 
    
    
     STATEMENT REGARDING FEDERALLY FUNDED DEVELOPMENT 
     This invention was not made with any Federal or State government support. 
     FIELD OF THE INVENTION 
     This application relates to steering systems for trailers that use a towbar that pivots horizontally about a pivot point for steering wheels of the trailer via one or more steering rods, and particularly to steering systems such that, when backing a trailer, the steered wheels are moved oppositely with respect to lateral movement of the towbar. 
     BACKGROUND OF THE INVENTION 
     Steerable trailers or wagons are characterized by a tow bar (tongue) connection between a trailer and a hitch on a tow vehicle. The tow bar usually comprises a first horizontal pivot and a first vertical pivot at the hitch and a second vertical pivot and second horizontal pivot at the front of the trailer chassis. The vertical pivots decouple the vertical trailer loads on the rear suspension of the tow vehicle and provide for improved handling by eliminating trailer weight on the hitch. The horizontal pivots at the hitch and front of the trailer chassis allow steering of the front wheels of the trailer via steering rods attached between the front wheels and the towbar such that horizontal displacement of the towbar steers the wheels. This configuration, however, causes steering of the trailer to be difficult when backing up. This difficulty during a backing operation involving two horizontal pivots has been compared to “pushing a rope”. 
     BRIEF SUMMARY OF THE INVENTION 
     The present disclosure includes and improves upon existing methods and apparatus for backing a trailer by providing methods, apparatus, and systems for automated backing of a wagon or trailer with steered wheels on the front axle. The methods, apparatus, and systems allow for manufacture of partially or fully automatically steered front axle wagon archetypes, and for retrofitting existing wagons or trailers to be partially or fully automatically steered. 
     The present invention involves steering systems that, when backing a trailer, the steered wheels are moved oppositely with respect to lateral movement of the towbar by steering rods that are mounted to a sliding carriage on the towbar. The carriage moves the steering rods forward or backward with respect to a pivot point such that the steering direction relative to the horizontal angle of the towbar is reversed as the steering rods pass from proximity of one side of the pivot point to the other. Automated steering control involves sensors providing feedback that may include the angle of the towbar with respect to the trailer chassis and relative turning velocities of trailer wheels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The elements of the drawings are not necessarily to scale relative to each other, usually in order to enhance clarity, with emphasis placed instead upon clearly illustrating the principles of the disclosure. Like reference numerals designate corresponding parts throughout the several views of the drawings in which: 
         FIG. 1  is a diagram with terminology and concepts for describing of a steered wheel trailer; 
         FIG. 2  is a top view of a first embodiment of a wagon comprising a steering mechanism; 
         FIG. 3  is a top view of a first embodiment of a wagon comprising a steering mechanism; 
         FIG. 4  is a perspective view of a bracket and steering mechanism assembly; 
         FIG. 5  is a side cross section view of a bracket and steering mechanism assembly; 
         FIG. 6  is a cross sectional view diagram showing construction of the assembly in  FIG. 4 . 
         FIG. 7  is a perspective view showing details of the carriage assembly shown in  FIG. 6 ; 
         FIG. 8  is a perspective view of a front axle assembly for retrofitting a conventional steered wheel wagon or trailer; 
         FIG. 9  is a flowchart illustrating a first embodiment of control logic for a general process of controlling a steering mechanism; 
         FIG. 10  is a flowchart illustrating a second embodiment of a control system logic for steering a steered-wheel type wagon while backing; 
         FIG. 11  is a flowchart illustrating a third embodiment of a control system logic for steering a steered-wheel type wagon while backing; and 
         FIG. 12  is a flowchart illustrating a fourth embodiment of a control system logic for steering a steered-wheel type wagon while backing. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The terms “wagon” and “trailer” are used interchangeably with their commonly understood meanings in the context of a towed vehicle comprising a towbar/tongue. The term “microcontroller” is used to indicate a microprocessor or computer used to control a process such as the steering of a trailer, including controllable components. 
       FIG. 1  is a diagram illustrating terminology and concepts useful for a description of methods and apparatus related to steering control for a steered wheel trailer. For the sake of familiarity, the vehicle  15  shown may be thought of as an automobile backing up in the direction of the arrow running from A to B. The same principles apply if the vehicle  15  is a forward moving forklift with rear wheel steering or a steered wheel trailer or wagon being pushed by a tow vehicle that is backing up. The yaw rotation rate θ w  of the vehicle  15 , that is the rate at which the yaw of the vehicle is changing, is proportional to the velocity of steered wheel  11  v w  and the sine of steering angle, δ, and inversely proportional to the wheel base L measured between front to rear axles A and B. The distance from the measured wheel to the instantaneous center of rotation of the chassis is indicated by p. Combining these with the geometry of the diagram leads to a formula that is useful for controlling the steering of a backing wagon or trailer: 
       θ w   =v   w   /ρ=v   w  sin ϕ/ L   (Eq. 1).
 
       FIGS. 2 and 3  are top views of a first embodiment of a trailer or wagon  10  comprising a towbar  6  with a horizontal pivot point  5  and equipped with a steering control mechanism  12 . The steering control mechanism  12  comprises a carriage  2  that is axially movable in or on a carriage slide  3  in or on the tow bar  6 . Three of the many possible positions for carriage  2  are shown: a neutral position N (solid lines), a forward position F (dashed lines), and a rear position R (dashed lines). A steering rod  14  is connected to a steerable wheel  11  at a lateral end and to the carriage  2  at a medial end. When the carriage  2  is positioned such that the medial end of the steering rod  14  is vertically aligned with the horizontal pivot point  5  at the N position, moving the towbar horizontally has no steering effect on the wheels  11 . When the carriage  2  is positioned forward of the horizontal pivot point  5 , e.g. at the F position, horizontal movement of the towbar causes the wheels  11  to be steered in the same direction as the end of the towbar  6  as shown in  FIG. 3 . When the carriage  2  is positioned rearward with respect to the horizontal pivot point  5 , e.g. at the R position, horizontal movement of the towbar causes the wheels  11  to be steered in the opposite direction from the end of the towbar  6  ( FIG. 3 ). As carriage  2  moves further from the horizontal pivot point  5 , the steering response of the control system is increased, as steering leverage is increased relative to the length of the steering arm  4  ( FIG. 2 ). 
     Referring to  FIG. 3 , the steering angle, δ, of wagon  10  is proportional to tow bar angle ϕ relative to the wagon chassis with δ=G*ϕ, where G is a controlled input between 1 and −1, representing position of carriage  2  on slide  3  relative to the horizontal pivot axis  5 , with associated steering leverage provided by the tow bar rotation and relative distance between the carriage  2  and the horizontal pivot  5  relative to steering arm  4  length. When the carriage  2  is forward of the horizontal pivot axis  5  by a distance equal to the length of steering arm  4 , G is equal to 1. G is equal to 0 when located over pivot axis of the tow bar, as there is exactly no steering leverage as tow bar  1  is rotated, and G is equal to −1 when the carriage is behind the horizontal pivot axis  5  at a distance equal to the length of steering arm  4 . If the steering center is perpendicular to wagon chassis  1 , front to back, divergent hitch motion from the steering center warrants forward motion of carriage  2 , whereas convergent hitch motion from the steering center warrants movement aft or rear of carriage  2 , behind pivot  5 . 
     In the unique towing configuration when the steered wheels  11  are aligned in the forward direction and the carriage  2  is in the N position, the steered wheels  11  track straight forward when pulled straight forward and track straight backward when the trailer is pushed straight backward independent of the towbar angle. To affect automatic steering while backing and as towbar  6  is moved left or right of center, carriage  2  is automatically moved forward and aft of the N position by a steering control system in accordance with a steering control logic executed by a steering control software or program. 
     The steering control system comprises a microcontroller  9  with control software operationally coupled to and controlling a steering control mechanism  12  ( FIG. 3 ). The steering control mechanism  12  comprises an actuator  58  for moving the carriage  2  in response to signals from the microcontroller  9 . The actuator  58  and and/or the microcontroller may be powered by a battery or other power storage device  8  or by power from a tow vehicle connected to the trailer  10 . The microcontroller  9  may be any suitable computing device capable of receiving the required input, executing software, and controlling the actuator  58 . Examples of a suitable microcontroller include a PIO controller such as a programmable ARDUINO® MEGA® computing platform available from Arduino®, and motor controllers such as a SYREN 50® available from Dimension Engineering, Inc. Programming of the ARDUINO® board, for example, is accomplished by connecting a personal computer to the board and uploading programs via an ARDUINO® interface. These and similar microcontrollers are also applicable to more advanced control systems comprising video, optical, infrared, magnetic Hall-effect, and ultrasonic sensors, which may be used to improve real-time characterization of tow-vehicle path, tow bar angle and rotation rate, and for collision avoidance and lane-drifting indication. 
     The steering control system comprises one or more sensors  75 ,  77 ,  95 ,  97  for gathering information such as towbar angle, wheel speed, carriage position, and geographical location of the trailer. Examples of encoding sensors  77  may include counting registers, relative NB encoders and absolute encoders. Positional sensors  75  may include rotation and position sensors such as mechanical limit switches, optical switches, and Hall-effect proximal sensors. Additional circuitry may include signal and power filters to ensure proper operation, along with shielding of analog higher power cables and other automotive workmanship standards. 
     Additionally or alternatively, to positional and encoding sensors  75 ,  77 , the sensors may include one or more of visual, optical, or laser range finders  95  and sonar sensors  97  to provide continuous feedback on the relative positions and angles of one or more of the trailer chassis  13 , the towbar  6 , the hitch  7 , the steered wheel(s)  11 , and the tow vehicle. The visual, optical, or laser range finders  95  and sonar sensors  97  are positioned on the front axle A of the chassis  13  of an axle assembly  94  in  FIG. 8  but these sensors may individually or collectively be located at other positions on the chassis  13  of a trailer  10 , on the towbar  6  or the towing vehicle in accordance with the distance(s) and or angle(s) being measured and the configuration of the paired trailer and towing vehicle. 
     A feedback control component of the steering software is preferably stored in the microcontroller  9  on the chassis  13  or body or frame of the trailer but may be housed on a separate microprocessor or computer communicating with the microcontroller  9 . The microcontroller receives information in the form of sensor data from the sensors  75 ,  77 ,  95 ,  97  and controls the actuator  58  that moves the carriage  2  according to calculations performed by the microcontroller  9 . Alternatively, the steering system may comprise a first microprocessor located on the towing vehicle and a second microprocessor located on the trailer  10  with the second microprocessor acting as a microcontroller for controlling the actuator  58  and the first microprocessor collecting information from sensors, performing calculations, and sending instructions to the second microprocessor. 
     The steering control system may employ existing left and right-hand turn signals as command input to the microprocessor  9  for amplified or attenuated steering response, for backing or towing around corners. The steering control system may further enhance performance by using video imagery guidance to determine relative road and curb locations to provide further guidance to the steering system control system. 
     To maintain stable control when backing, carriage  2  is adjusted relative to the horizontal pivot point  5  to develop sufficient steering leverage on towbar  6  for stable control of autonomous steering. When the towbar angle is changing during backing, as when backing a wagon around an arc and making corrections, carriage  2  is constantly changing as corrections change the towbar angle. When backing the trailer around an arc without making corrections, carriage  2  is initially positioned to correspond to the arc and held steady to correspond to the unchanging towbar angle. To an operator, manually steering the tow vehicle while backing is similar to backing with no wagon attached because the automatic steering system automatically steers the wagon along a common trajectory. 
     The trailer  10  shown in  FIGS. 2 and 3  comprises a drag link connecting steering arm  4  of wheel  11 . It should be apparent that the steering mechanism  12  is equally applicable to other types of steered wheel trailers, for example comprising two steering arms, with similar results. 
     A steering system comprising the steering mechanism  12  may be incorporated into a trailer during manufacture or a conventional steered wheel trailer may be retrofitted to comprise the steering mechanism  12  as part of a steering system.  FIGS. 4 and 5  are perspective and side cross sectional views of one embodiment of a bracket and steering mechanism assembly  50  that can replace conventional bracket types connecting steered wheel trailers to their respective towbars during manufacture or after manufacture to convert or retrofit a conventionally steered trailer or wagon. The bracket  55  comprises a horizontal clevis pin or tube  52  that is configured to be fitted into a towbar clevis for vertical pivoting of the towbar and a vertical clevis pin or tube  54  that is configured to be fitted into a towbar clevis for horizontal pivoting of the towbar. In this embodiment, a steering control mechanism assembly  56  is mounted to and below the bracket portion. The steering control mechanism assembly  56  comprises a control actuator  58 , which may be an electric motor, such as a stepper motor or a servo motor, or a linear actuator that may be manually, electrically, hydraulically or pneumatically powered. Where an electric motor is used, a gear reduction set may be incorporated with the motor for additional mechanical advantage. A robust structural enclosure  59  houses internal components of the steering control assembly  56 . The actuator  58  is functionally connected to move a carriage  2  within the enclosure  59  to any position to one side or another of the axis  62  of vertical pivot pin or tube  54 . 
     For embodiments comprising an electric motor actuator  58 , such as a geared servomotor shown in  FIG. 5 , the gear motor shaft may be connected to a lead screw  70 . Threads  71  on the lead screw engage threads  72  incorporated in carriage  2  to drive carriage  2  to any position on the lead screw. The lead screw is supported at each end in enclosure  59  by appropriate structural bearings. In addition to the lead screw, carriage  2  is further supported in sliding relation by structural members, such as rods  74  ( FIGS. 5 and 7 ) that are, in turn, supported at each end at the end walls of enclosure  59 . Rods  74  extend through openings  76  of the carriage  2  to allow the carriage to slide along rods  74  when motor  58  rotates lead screw  70 . Appropriate bearing materials are used between the rods and openings of carriage  2 , preferably with appropriate lubricants on the rods and lead screw. The steering tie rods extend outward from enclosure  59  through a longitudinal slot corresponding to all possible positions of carriage  2  along lead screw  70 . Flaps of a resilient material, brushes or a sliding window structure, may be used to cover the slots to generally prevent entry of water, dirt and other contaminants into the interior of enclosure  59 . Since enclosure  59 , carriage  2 , rods  74 , lead screw  70 , and associated components must bear steering loads associated with towing and backing the wagon or trailer, they are preferably fabricated of hardened, robust materials, and sealed against harsh environmental conditions. 
       FIG. 6  shows steering tie rods  60 ,  64  connected to carriage  2  by knuckle joints  66 ,  68  respectively, that are connected to conical openings  69  in carriage  2  so that ends of the steering tie rods are moved with the carriage to any position on one side or the other of axis  62  ( FIG. 4 ). The offset arrangement connecting the steering knuckles  66 ,  68  allows for a flat profile, which provides better ground clearance for embodiments of the steering mechanism  12  positioned beneath the towbar  6 , and maintains an Ackerman steering geometry. 
     The embodiment shown in  FIG. 5  comprises one or more sensors  75 ,  77  representative of simple feedback information for microprocessor  9 . The sensors sense at least and position or location of carriage  2  and one or more of the towbar angle or the rate of change of the towbar angle with respect to the trailer chassis  13  or stationary front axle. Rate of change is used, for example, when PIO-type controllers are used to control steering. Here, carriage position sensor  75 , which may be a shaft encoder or any other appropriate encoder, is mounted to sense rotation and rotation angles of lead screw  70 . 
     The microcontroller  9  in this embodiment counts and tracks the of number and direction of rotations of the lead screw to keep track of location or position of carriage  2 . Sensor  77 , which may be another encoder such as an absolute encoder or any other appropriate encoder for providing angular information, has a body fixed on or near vertical pivot tube  54 , with its shaft rotatably connected to a pin or the like fixed to the chassis  13  or front axle as by belts and pulleys, sprockets, drive chains or meshing gears. As the towbar is rotated about axis  62 , encoder  77  provides a signal representative of towbar angle with respect to fixed front axle A or chassis of the trailer. Limit switches (not shown) may be provided at each end of travel of the carriage  2  within enclosure  59 , with the one limit switch, for example, mounted to an inner wall of the distal end of enclosure  59  that supports bearing  73  and sensor  75 . A second limit switch may be mounted, for example, to an inner wall of the proximal end of enclosure  59  that supports actuator  58 . The first limit switch at the distal end of lead screw  70  may be activated by contact with the carriage  2 , and prevents damage to the enclosure, lead screw and carriage by providing a signal that stops rotation of the lead screw to stop the carriage before it contacts the distal end wall of enclosure  59  or runs out of thread on the lead screw. The second limit switch at the proximal end of the lead screw near actuator  58  may also be activated by contact with the carriage, and similarly prevents damage to the carriage, lead screw and enclosure by providing a signal that stops rotation of the lead screw. The position of the first limit switch may be used to set a reference point for towing wherein the steered front wheels are steered directly with sideways towbar displacement. In other words, the carriage  2  is positioned at a towing position where the front wheels are steered to accurately track wheels of a tow vehicle while towing as determined by sideways towbar displacement after the carriage contacts the second limit switch. 
     A signal provided by this first limit switch may be used to reference sensor  75  to a reference position when the carriage contacts the first limit switch, and then activate servo motor  58  a predetermined number of turns of lead screw  74  to drive carriage  2  to the towing position. In other embodiments, the towing position may be the position of the carriage  2  when it activates the first limit switch. Where the carriage  2  is driven to a towing position a short distance away from the first limit switch after activating the first limit switch, power to motor  58  may be interrupted so that the wagon may be safely towed without any chance of the carriage being moved. Since it would be very difficult for steering loads to backdrive the lead screw to shift the position of carriage  2 , no movement of the carriage would occur during towing with motor  58  deenergized. However, encoder  75  could still be used to monitor position of carriage  2 , and reenergize motor  58  to correct the position of the carriage if necessary, or provide a signal to an operator that the carriage position has shifted and recalibration of the carriage position is necessary. In other embodiments, a positive lock, such as a solenoid-driven pin on the carriage driven through an opening in the enclosure, may be used to lock the carriage in place. This would also relieve steering stresses on the leadscrew and rods that the carriage rides along. Typically, the towing position of carriage  2  on lead screw  70  would be only a short distance from the distal wall of enclosure  59 , such as an inch or so, in order to allow extreme backing corrections that may move the carriage past the towing position toward the distal wall of enclosure  59 . In addition, positioning the carriage  2  at a towing position a short distance from the first limit switch eliminates excessive wear and tear on the limit switch. 
     One method for retrofitting a conventional steered wheel trailer to comprise a steering system and steering mechanism  12  comprises removing the front axle of the trailer and replacing it with an axle assembly  94  comprising a bracket and steering mechanism assembly  50  as shown in  FIG. 8 . The axle assembly  94  may be fixedly welded or attached by fasteners to the front chassis of trailer or wagon in the manner of a conventional trailer or wagon. Steering tie rods  66 ,  64  are mounted to knuckle joints on the carriage  2  to allow for movement of the steering tie rods as carriage  2  is moved. 
       FIG. 9  is a flowchart illustrating a simple embodiment of control logic for a general process of controlling a steering mechanism  12 . At box  20 , the microprocessor/microcontroller  9  receives a signal from the tow vehicle that the tow vehicle is about to commence backing. The signal may be taken from a back-up light of the tow vehicle when the tow vehicle is put in reverse, or by other indications such as a radio command from the tow vehicle to a receiver on the wagon (not shown). When the back-up signal is received by the microcontroller  9 , the microcontroller signals the actuator/motor  58  to drive carriage  2  to a calibration position at box  172 . This sets a reference point from which the carriage position while backing may be referenced from, such as a binary count of 0, or any other convenient binary count from an encoder  75 . Such a calibration may not be needed every time the wagon is backed and nonvolatile RAM may be used to store a last position of carriage  2  as determined, for example, by counting a number of turns a lead screw  70 . After calibration at box  22 , actuator/motor  58  is commanded to drive carriage  2  to a nominal backing position, which may be a null or neutral position where the medial end of the steering tie rod  14  at the carriage is above the horizontal pivot point  5  for the towbar, or a point near the pivot point of the towbar. As the tow vehicle commences to back the trailer, the query is posed at box  174  as to whether the towbar angle is changing. If the answer is YES, then the process flows to box  176  where the carriage is driven toward a position to reduce the change of the towbar angle. In other words, if the towbar angle is increasing, then the carriage  2  is driven to steer the wheels to minimize the rate increase. 
     The process loops back to box  174  until the carriage is at a position such that no change of towbar angle occurs, which backs the wagon in an arc determined by the tow vehicle backing along the same arc. When the tow vehicle changes direction, causing a change of towbar angle, then the answer at box  174  is NO, and the process flows to box  178  where the query is posed as to whether the towbar is centered for backing the wagon straight back. If the towbar is centered, then the answer is YES and the process flows back to box  174  and repeats. If the answer at box  178  is NO, then the carriage is moved toward centering the towbar at box  180  and the process loops back to repeat at box  178 . This loop causes the trailer to be backed straight when the towbar is centered. The processes are usually occurring and repeating with a frequency of around 30 Hz so dithering of the carriage may occur around the optimal positions of the carriage for backing straight back and around an arc as minor backing corrections are made. Such dithering may cause small jerking motions of the carriage that may cause undue wear or other undesirable effects, so a controller for backing may use a proportional-integral differential (PIO) algorithm, or any combination thereof, to smooth the carriage motions and achieve a faster response. 
       FIG. 10  shows one embodiment of a control system logic for steering a trailer while backing. Microcontroller  9  receives a backup signal  20  to initiate process steps for steering the trailer into the measured velocity path of the hitch  7  ( FIG. 3 ) relative to wagon chassis  13 . Microcontroller  9  receives information  21  on the current position of the carriage  2  and optionally other information such as towbar angle and wheel velocity. Microcontroller sends a command  22  to a motor N to move the carriage into position for backing. Tow bar  6  yaw, or rotation angle, is measured and compared to pre-set limits for rotation angle and angular rate  23 . If tow bar rotation is within pre-set limits, steering mechanism actuation is inhibited. If tow bar  6  rotation exceeds preset limits, the steering control software determines the tow bar  6  angular position and rate in step  24  and, from that, wagon chassis  13  angular rotation rate in step  25  (e.g. using equation 1), enabling determination of relative motion of tow vehicle hitch  7  to wagon chassis  13  independent of wagon rotation in step  26 . 
     Given a longitudinal line of symmetry N from front to back and tow bar  6  rotation relative to the line of symmetry, a yaw rotational rate ϕ of tow bar  6  about horizontal pivot  5  can be measured with a rotation sensor connected between tow bar  6  and wagon chassis  13 . In this steering control system embodiment, step  27  determines if tow vehicle hitch  7  is converging or diverging from the line of symmetry N. If hitch  7  is converging toward the line of symmetry or diverging away from the line of symmetry, steering control system response in step  29  is to command the steering mechanism  12  to steer in the same direction as tow bar  6 . If hitch  7  is diverging away from the line of symmetry, the steering control system response is to command steering mechanism  12  to steer in the opposite direction of tow bar  6 . 
       FIG. 11  shows an embodiment of a steering control system for a steered-wheel type wagon for the steering mechanism  12 . Steering in the same direction as tow bar  6  is accomplished by driving the carriage  2  forward of the horizontal pivot  5 , as shown in block  31 , whereas steering opposite of tow bar  6  is accomplished by driving the carriage  2  rearward, or aft of the horizontal pivot  5  as shown in block  30 . As the carriage  2  moves further from horizontal pivot  5 , the steering response of the control system is increased, as steering leverage is increased relative to the length of the steering arm  4  ( FIG. 2 ). 
       FIG. 12  shows an embodiment of a steering control system for a steered-wheel type wagon for the steering mechanism  12 . This embodiment of the steering mechanism  12  and control logic comprises automated or semiautomated steering assist technology. For example, video imagery guidance may be used to determine relative road and curb locations and provide further guidance to the steering system control system as shown in block  34 . The steering control system  12  may incorporate one or more of forward facing video, optical sensors, and infrared sensors in addition or as an alternative to the sensors already described to determine the orientation of the tow vehicle as shown in block  32  for enabling determination of tow vehicle path as shown in block  33 . It is also possible for the sensors to be distributed between the tow vehicle and the trailer or wagon and for the sensor data from each be combined. 
     The steering system may additionally or alternatively be used with a trailer or wagon that is being towed by trailer or wagon that is itself being towed by a tow vehicle for application in multi-trailer towing applications. For example, a tow dolly for towing a second trailer may be equipped with a steering control system in which the towbar angle sensor is replaced by an optical sensor. In such an embodiment, the optical sensor is positioned to allow a view of the front of the trailer attached to a fifth wheel of the tow dolly. Reference markers may be placed at front corners of the trailer within a field of view of the optical sensor to be used by a microprocessor/microcontroller to calculate an angle of the trailer. In another embodiment, an optical sensor may be located just in front of the fifth wheel with a field of view including an underside of the front of the trailer. A strip of material including reference lines viewable by the optical sensor is positioned underneath the front of the trailer in view of the sensor, for example by magnetic strip or other temporary attachment means. In operation, as the trailer angle changes, the reference lines correspondingly move and are detected by the optical sensor. 
     Inputs to the microcontroller  9  for controlling the steering mechanism  12  may be signals from a digital encoder  75  representative of carriage  2  position and signals from a digital encoder  77  representative of angle of the towbar  6  with respect to the front axle, or wagon chassis. Encoder  77  may also provide a rate of change, or frequency, signal of the towbar angle with respect to the front axle, as derived from a changing digital bit rate corresponding to how fast the towbar angle is changing. Where the towbar angle is constant, as when backing around an arc with no steering corrections, encoder  77  may indicate towbar angle relative to a straight ahead position of the towbar. When a steering correction is made, the towbar angle will change depending on whether the towbar angle is increasing or decreasing from a centered position and a rate of change of towbar angle will also be provided. The rate of change may be used to determine a speed in driving carriage  2  faster or slower in order to accommodate the new, changing towbar angles. In other embodiments the frequency, or rate of change of towbar angle may not be needed, with a microprocessor calculating where the carriage should be in almost instantaneous increments. By way of example, where encoder  75  reads 360 degrees for each rotation of lead screw  70 , then calculations may be made by a microprocessor of the microcontroller  9  in one degree increments of towbar angle so that carriage  2  can be driven a predetermined number of degrees or rotations for each degree of towbar angle as the towbar angle changes. The number of degrees or rotations of the leadscrew per degree of towbar angle may be determined by thread pitch of leadscrew  70 . The only requirement of simply calculating carriage position from each degree of changing towbar angle is that motor  58  must be sufficiently fast and powerful to drive the lead screw at a speed sufficient to keep up with changing towbar angles (referred to as phase margin). It has been found that a calculation rate of 30 calculations per second for determining carriage position with respect to towbar angle is well within the range of any anticipated rate of towbar angle change, and well within speed of most microprocessors. 30 calculations per second for carriage position corresponds to a fastest towbar swinging rate, for instance, from a straight ahead position to a 30 degree position, in one second. This would be an extremely fast rate when backing a wagon. Similarly, motors of sufficient capacity to drive leadscrew  70  in order to keep up with the microprocessor calculations, i.e. change of carriage position with towbar angle, exist, the selection of which would be apparent to one of ordinary skill in the art. A slower rate of calculation may be used with a correspondingly slower, less powerful motor, such as a calculation rate of perhaps 15-20 calculations per second corresponding to a rate of towbar change of 15-20 degrees in one second. Likewise, calculations may be made for greater than one degree increments of towbar change, such as a calculation of carriage position for every two or three degrees of towbar angle change, or even every 5 degrees, depending on the pitch of leadscrew  70 . From this it should be apparent that the process of constantly adjusting position of carriage  2  with changing towbar angle causes the carriage to be dithered about the nominal or neutral backing position as minor corrections in backing are made in order to cause the tow vehicle and wagon to be backed as though no wagon was attached to the tow vehicle.