Abstract:
This invention relates generally to systems used to steer trailing sections of articulated vehicles and trailers. The preferred embodiment of this invention is a steerable dolly that can be manually and/or automatically switched among multiple modes. The dolly receives its primary input from the angle of the rear of the front trailer (or other towing vehicle). There are at least two modes. In one mode the wheels of the dolly are steered in the opposite direction as the rear trailer, providing more maneuverability. In the other mode the wheels are steered in the same direction, providing a more stable mode. It is envisioned that the more stable mode would be used at high speeds while the more maneuverable mode would be used at lower speeds. There is also disclosed an alternate steering algorithm, which provides for additional maneuverability beyond that of the mode in which the wheels are steered in an opposite direction.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates generally to systems used to steer subsections of articulated vehicles and trailers and specifically to steering systems controlled by a microcomputer or a programmable controller to follow a lead subsection. The invention generally relates to articulated vehicles and trailers with three or more subsections. The invention also includes provisions for a plurality of operator selectable steering modes as well as a plurality of automatic steering modes.  
           [0002]    The preferred embodiment of the invention demonstrates a way of applying the principles of the invention to dollies used in over-the-road tractor-trailer combinations. The desired steering axle angle is primarily a function of the angle between the steered section (the dolly) and the section in front of the steered section. Additional data and additional algorithms can be used by the software of the programmable controller to provide a refinement of the steering response.  
         BACKGROUND OF THE INVENTION  
         [0003]    Over-the-road transport companies find it difficult at times to compete with other freight haulers because of weight limits on the roads and bridges. Multi-trailer arrangements are a possible solution to some of these problems because they spread the load over a longer stretch of pavement and reduce the columnar loading on bridges. These arrangements generally involve a semi-trailer carried by the tractor with one or more full trailers composed of semi-trailers carried by “dollies”.  
           [0004]    The most common and widely used dolly is the standard Type A dolly, which hitches to the towing vehicle or first trailer using a single point hitch. The standard Type A dolly provides “wagon tongue” steering for the trailer it is carrying by allowing the entire dolly to steer relative to its semi-trailer about the fifth wheel vertical axis on the dolly as well as relative to the towing trailer about the single point hitch vertical axis. The dolly tires do not, however, steer relative to the dolly frame.  
           [0005]    Commercial vehicles of either truck and full trailer or multi-trailer configurations which employ the standard Type A dollies generally possess undesirable characteristics such as limited maneuverability and instabilities caused by rearward amplification. Rearward amplification, sometimes described as a “crack-the-whip” phenomenon, implies that in rapid evasive maneuvers such as emergency lane changes, the rearward elements of the vehicle train such as the dolly and the trailer carried by the dolly experience motions which are substantially amplified compared to the motions of the towing tractor and first trailer. Rearward amplification is known to be the basic cause of many accidents in which roll over of the last trailer or second trailer occurs while the remaining elements of the vehicle remain unscathed.  
           [0006]    A second general class of dollies known as Type B dollies represent an improvement over standard Type A dollies. Type B dollies are generally characterized by a double tow bar arrangement which eliminates steering of the dolly with respect to the towing vehicle, most commonly the first trailer. The Type B dollies have been effective to a degree against some of the instability problems and are slightly more maneuverable than the standard Type A dollies. However, they cause other problems such as introducing other types of instabilities, causing stresses on the rear of the forward trailer, and increasing unloading delays due to difficulty in accessing the back of the forward trailer for some configurations.  
           [0007]    Lengthy trailer combinations have always been plagued by these same two problems of instability and lack of maneuverability. Many transport companies have looked toward steerable dollies as a possible solution to these problems. Steering systems for dollies generally make use of Ackerman geometry and generally derive from modifications to the standard Type A dolly. With Ackerman geometry all tires of the dolly and the tires on the rear axles of both the first and any subsequent trailers are aligned tangent to circular paths which all have the same turn center. Ackerman geometry is the most desirable steering configuration for low speed maneuvering since it minimizes tire scuffing, wear and structural stress. With some modifications, Ackerman geometry can be adapted to provide stability when traveling at higher speeds, but this stability comes at the expense of maneuverability. An example of Ackerman geometry as applied to the steering of the rear tandem wheels of a semi-trailer is disclosed in U.S. Pat. No. 2,342,697.  
           [0008]    The prior art teaches ratio steering systems for dollies with two principal steering modes. When the steering axle of the dolly is steered in a direction to make it initially more nearly parallel to the back of the trailer it is following, the behavior of the dolly and of the system as a whole tends to greater stability. This mode of steering could be called the stability type of ratio steering. This type of steering behavior is particularly desirable at higher speeds on the open road. When the steering axle of the dolly is steered in the opposite direction to the above, the dolly and the trailer that it carries are caused to swing wider around the corner, producing better maneuverability characteristics for the system as a whole. This mode of steering could be called the cornering type of ratio steering. This type of steering is particularly useful when maneuvering among obstacles or along curved streets at lower speeds.  
           [0009]    These methods for automatically steering rear sections of articulated vehicles have all been based on a single basic algorithm: a certain angle change of the section in front of the steered section results in a certain change in the angle of the steering axle. This type of steering can be called “ratio steering”. In the mode in which the wheels are steered in the opposite direction as the section in front, a certain change in angle results in a change in the opposite direction, roughly corresponding to a ratio of 1:−1 (one-to-minus-one). In general, as the ratio proceeds in the negative direction, i.e. 1:−5, 1:−10, 1:−50, the turning radius is decreased. A type B dolly has a ratio corresponding to 1:negative infinity, causing an immediate extreme correction in the direction of rotation. As the ratio becomes more positive, the system becomes generally less maneuverable, with an increasing turning radius. A ratio of 1:0 corresponds to a standard a type A dolly. As the ratio increases toward 1:1, the system emulates a trailer with an increasingly long wheelbase. A mode in which the wheels turn in the same direction as the front section and have a ratio of 1:1 induces a crab type motion in which the entire system translates horizontally, but is incapable of changing direction. As the ration becomes greater, the trailer becomes unusable, with the rear trailer turning more than the front. Several patents have explored some of the possibilities for this type of ratio steering.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention makes several contributions and extensions to the art of ratio steering for articulated vehicles, and particularly for dollies used in over-the-road freight transport. First, the present invention provides a method whereby the ratio to be used can be selected automatically by the software in the controlling microprocessor based on parameters such as vehicle turning angle or vehicle speed. At high speeds on relatively straight roads, the dolly operates in modes with positive steering ratios that are characterized by stability with rearward amplification substantially eliminated. When negotiating sharper turns at lower speeds, the dolly automatically switches to the cornering modes (negative steering ratios) that cause the dolly to swing wider around corners. These modes allow the dolly to maximize its maneuverability at low speeds. At intermediate speeds, a steering ratio will be chosen that is the best compromise of stability and maneuverability at that speed.  
           [0011]    Second, the present invention provides for one or more additional corrections to the steering at a point later in the turn when the cornering type of ratio steering is ineffective. The overarching purpose here is to “correct” the path of the steered dolly so that it follows as close as possible the path taken by the tractor as it turned the corner. If the tongue of the dolly is long enough, these “oversteer corrections” will indeed allow the steering axle of the dolly to approximate a path that a tractor driver might take as he oversteers around the corner. In this case, then, the trailer carried by the dolly rounds the corner on a path that is reasonably close to the path taken by the first trailer. With this correction, the dolly that is an embodiment of the present invention substantially exceeds the cornering capabilities of the Type B dollies while maintaining superior stability when operating at higher speed on the open road.  
           [0012]    All steering modes used by the present invention conform to a type of modified Ackerman geometry. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a diagrammatic view of an embodiment of a Variable Steering Ratio Digital Dolly with Oversteer Towed behind a Tractor-Trailer Combination Rig  
         [0014]    [0014]FIG. 2 is a diagrammatic perspective plan view of a Variable Steering Ratio Digital Dolly with Oversteer  
         [0015]    [0015]FIG. 3 is a diagrammatic back plan view of a Variable Steering Ratio Digital Dolly with Oversteer  
         [0016]    [0016]FIG. 4 is a diagrammatic bottom view taken along lines  4 - 4  of FIG. 3 of a of a Variable Steering Ratio Digital Dolly with Oversteer  
         [0017]    [0017]FIG. 5 is a diagrammatic top view of a Variable Steering Ratio Digital Dolly with Oversteer Utilizing a Controller for Steering the Dolly  
         [0018]    [0018]FIG. 6 is a diagrammatic view of a Variable Steering Ratio Digital Dolly without the Oversteer Correction  
         [0019]    [0019]FIG. 7 is a diagrammatic back plan view of Switchable Steering Ratio Geared Dolly with Oversteer is a top plan view of a of Switchable Steering Ratio Geared Dolly with Oversteer  
         [0020]    [0020]FIG. 8 is a diagrammatic top plan view taken along lines  8 - 8  of FIG. 7 of a of Switchable Steering Ratio Geared Dolly with Oversteer  
         [0021]    [0021]FIG. 9 is a diagrammatic top plan view of Switchable Steering Ratio Geared Dolly with Oversteer showing upper partial-circular track  
         [0022]    [0022]FIG. 10 is a diagrammatic schematic drawing of an Oversteer Gearbox  
         [0023]    [0023]FIG. 11 is a diagrammatic top view of the Switchable Steering Ratio Geared Dolly without the Oversteer Correction  
         [0024]    [0024]FIG. 12 is a diagram of a travel distance sensor  
         [0025]    [0025]FIG. 13 is a diagram of the sensors and controller for the Variable Steering Ratio Digital Dolly with Oversteer Correction 
     
    
       [0026]    List of Reference Numbers  
         [0027]    [0027] 30  tractor  
         [0028]    [0028] 36  fifth wheel  
         [0029]    [0029] 40  front trailer  
         [0030]    [0030] 44  sensor THETA_D1  
         [0031]    [0031] 47  attachment assembly  
         [0032]    [0032] 49  controller  
         [0033]    [0033] 50  dolly  
         [0034]    [0034] 53  sensor THETA_S1  
         [0035]    [0035] 55  tongue  
         [0036]    [0036] 58  axle central pivot support  
         [0037]    [0037] 60  steering axle assembly  
         [0038]    [0038] 63  L, R hydraulic tanks  
         [0039]    [0039] 64  circular bearing plates  
         [0040]    [0040] 65  trailer mounting bar pivot  
         [0041]    [0041] 66  trailer mounting bar  
         [0042]    [0042] 67  fifth wheel  
         [0043]    [0043] 68  hydraulic motor  
         [0044]    [0044] 69  chain  
         [0045]    [0045] 70  L, R running wheels  
         [0046]    [0046] 71  L, R running wheels  
         [0047]    [0047] 72  F, B transverse axles  
         [0048]    [0048] 73  track attachment plate and assembly  
         [0049]    [0049] 74  steering sprocket  
         [0050]    [0050] 75  rear lower partial-circular track  
         [0051]    [0051] 76  power output sprocket  
         [0052]    [0052] 77  sprocket  
         [0053]    [0053] 80  rear trailer  
         [0054]    [0054] 81  sensor THETA_R1  
         [0055]    [0055] 97  shaft  
         [0056]    [0056] 98  shaft  
         [0057]    [0057] 100  forward partial-circular track  
         [0058]    [0058] 104  roller  
         [0059]    [0059] 105  roller  
         [0060]    [0060] 106  L, R hinge assemblies  
         [0061]    [0061] 107  bar  
         [0062]    [0062] 108  ball and socket attachment  
         [0063]    [0063] 110  roller  
         [0064]    [0064] 111  gear  
         [0065]    [0065] 112  angle gear  
         [0066]    [0066] 113  angle gear  
         [0067]    [0067] 114  shaft  
         [0068]    [0068] 115  main gearbox  
         [0069]    [0069] 118  gear  
         [0070]    [0070] 119  angle gear  
         [0071]    [0071] 120  angle gear  
         [0072]    [0072] 121  roller  
         [0073]    [0073] 122  shaft  
         [0074]    [0074] 123  roller  
         [0075]    [0075] 124  gear  
         [0076]    [0076] 125  neutral lock gearbox  
         [0077]    [0077] 126  trailer orientation shaft  
         [0078]    [0078] 127  angle gear  
         [0079]    [0079] 128  angle gear  
         [0080]    [0080] 129  shaft  
         [0081]    [0081] 130  oversteer gearbox  
         [0082]    [0082] 131  square hitch shaft  
         [0083]    [0083] 132  forward enabling switch  
         [0084]    [0084] 133  bearing plates  
         [0085]    [0085] 134  bearing plates  
         [0086]    [0086] 140  rear upper partial-circular track  
         [0087]    [0087] 144  joint  
         [0088]    [0088] 146  pin and lock set  
         [0089]    [0089] 148  pin and lock set  
         [0090]    [0090] 150  rear enabling switch  
         [0091]    [0091] 151  air valve  
         [0092]    [0092] 152  control box  
         [0093]    [0093] 153  indicator lights  
         [0094]    [0094] 154  stability line  
         [0095]    [0095] 155  chain  
         [0096]    [0096] 156  cornering air line  
         [0097]    [0097] 159  roller  
         [0098]    [0098] 160  bracket  
         [0099]    [0099] 161  drive axle  
         [0100]    [0100] 560  gear  
         [0101]    [0101] 562  gear  
         [0102]    [0102] 564  gear  
         [0103]    [0103] 568  shell  
         [0104]    [0104] 570  gearbox  
         [0105]    [0105] 878  encoders on dolly wheels  
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0106]    The following description will disclose four distinct preferred embodiments of the invention. These four distinct embodiments are as follows:  
         [0107]    1. A digital embodiment with an oversteer correction capability, as seen in FIGS. 1, 2,  3 ,  4 ,  5 .  
         [0108]    The name we will use for this embodiment is the Variable-Steering-Ratio Digital Dolly with Oversteer. We have called it Variable Steering Ratio because it varies in a continuous manner among a wide variety of distinct steering ratios as a continuous function of vehicle speed. It can be designed to vary among distinct steering ratios as a function of any available parameter such as turning angle, speed, etc. if desired. This embodiment of the invention also provides the capability for oversteer correction.  
         [0109]    2. A digital embodiment without an oversteer correction capability, as seen in FIG. 6.  
         [0110]    The name we will use for this embodiment is the Variable Steering Ratio Digital Dolly. This embodiment of the invention is the same as embodiment three, except no oversteer correction is available on this embodiment of the invention.  
         [0111]    3. A mechanical embodiment with an oversteer correction capability, as seen in FIGS. 7, 8,  9 .  
         [0112]    The name we will use for this embodiment is the Switchable-Steering-Ratio Geared Dolly with Oversteer. We have called it Switchable Steering Geared Ratio because it switches between at least two distinct steering ratios using a mechanical gearbox without stopping the vehicle. It can be designed to switch among any number of distinct steering ratios if desired. Oversteer is the additional correction added to the steering method that, for negative steering ratios, enables the dolly to swing wider, later in the turn than any type of ratio-type steering can produce. This oversteer correction will be particularly useful for the longer dollies that are needed to spread heavier loads over longer stretches of pavement and to reduce the columnar loading on bridges.  
         [0113]    4. A mechanical embodiment without an oversteer correction capability, as seen in FIG. 11.  
         [0114]    The name for this embodiment is the Switchable Steering Ratio Geared Dolly. This embodiment of the invention is the same as embodiment one except it has no capability for oversteer correction.  
         [0115]    [0115]FIG. 1—Variable-Steering-Ratio Digital Dolly with Oversteer Towed Behind a Tractor-Trailer Combination Rig  
         [0116]    [0116]FIG. 1 illustrates a typical application of a Variable-Steering-Ratio Digital Dolly with Oversteer  50  that is one of the preferred embodiments of the invention. A tractor  30  of a tractor-trailer combination has a forward trailer  40  coupled thereto via a fifth wheel  36 , while a second, rear trailer  80  is coupled to the forward trailer  40  via the dolly  50  we will describe below. The rear trailer  80  is carried via a fifth wheel  67  mounted on the dolly  50 .  
       Overview of Variable Steering Ratio Digital Dolly with Oversteer  
       [0117]    This preferred computer-controlled embodiment of the invention as seen in FIGS. 1, 2,  3 ,  4 ,  5  consists of three pivotally connected sections, and has an equivalent function to the first, mechanical embodiment. The first section, the tongue  55 , connects the dolly  50  to the towing vehicle  40 . Mounted above the tongue  55  is a fifth wheel  67  with a trailer orientation sensor THETA_R1  81  for determining the angle between the dolly tongue  55  and the rear trailer  80 . The steering axle central pivot support  58  extends up through the tongue  55  from below, and is attached firmly at the bottom to the steering axle assembly  60 . The steering axle assembly  60  and the central pivot support pivot  58  with respect to the dolly tongue  55 . There is an axle orientation sensor  53  to determine the angle between the tongue  55  and the steering assembly  60 , as well as a hydraulic motor  68  able to change the angle between the steering assembly  60  and the dolly tongue  55 . At the front of the dolly tongue  55  there is a front pivot orientation sensor  44  which determines the angle between the dolly tongue  55  and the front trailer  40 .  
         [0118]    [0118]FIG. 2—Variable Steering Ratio Digital Dolly with Oversteer in Perspective  
         [0119]    [0119]FIG. 2 shows the Variable-Steering-Ratio Digital Dolly with Oversteer  50  in a perspective view. Referring to this figure, the dolly  50  has a rigid dolly tongue  55 , which in this embodiment will be coupled to the front end of the rear trailer  80  (FIG. 1) via a fifth wheel  67 . The fifth wheel  67  is mounted on a trailer mounting bar  66  that is fixed solidly onto the top of the axle assembly central pivot support  58  (FIG. 3) that extends up from below the dolly tongue. A rear trailer orientation sensor, THETA_R1  81 , mounted on the dolly tongue just forward of the fifth wheel  67 , measures the movement of the rear trailer as it rotates around the fifth wheel latch  67 . This sensor is mounted on a roller  159  that rolls along the bottom of the trailer (circling around the kingpin of the trailer) as the trailer rotates. A bracket  160  supports the sensor  81  and the roller  159  and presses them tightly upward against the bottom of the trailer. An air cylinder connected between the dolly tongue and the bracket maintains this upward pressure whenever a kingpin is latched in the fifth wheel latch, but allows the bracket to relax down against the body of the dolly tongue at other times to avoid damage during hitching operations.  
         [0120]    The dolly tongue  55  comprises the central rigid structural member below the trailer mounting bar  66  and pivots around the axle assembly central pivot support  58  (FIG. 3) as torque is applied by the hydraulic steering motor  68  at the direction of the controller  49 . The hydraulic steering motor  68  receives high pressure hydraulic fluid from the high pressure hydraulic tank  63 L in which hydraulic fluid is separated from highly compressed gasses by a diaphragm of some sort. The hydraulic fluid is discharged to the low pressure hydraulic tank  63 R. An air motor turns a hydraulic pump  64  to replenish the high pressure hydraulic tank  63 L as needed.  
         [0121]    Underneath the back section of the dolly  50  is a transverse steering axle assembly  60 , on which is solidly mounted the axle assembly central pivot support  58  (FIG. 3) that supports the upper portions of the dolly. Two spaced pairs of running wheels  70  L, R and  71  L, R are mounted on each of the transverse axles  72 F (Front) and  72 B (Back) by any conventional suspension system. In this embodiment the suspension system is omitted for clarity of illustration since it is a standard assembly. The hydraulic motor  68  that controls the steering of the transverse axles  72 F and  72 B, is mounted on the right side of the dolly tongue  55  and somewhat in front of the steering axle assembly  60 .  
         [0122]    The hitch pivot attachment assembly  47  with its front pivot orientation sensor THETA_D1  44  is located near the front of the dolly tongue  55 .  
         [0123]    [0123]FIG. 3—View of Variable-Steering-Ratio Digital Dolly with Oversteer from the Back Looking Forward  
         [0124]    [0124]FIG. 3 shows a view of the back of the Variable-Steering-Ratio Digital Dolly with Oversteer. The heavy axle central pivot support  58  is mounted solidly on the steering axle assembly  60  and extends upward through the center of the dolly tongue  55 . The dolly tongue  55  pivots freely around this axle central pivot support  58  and is stabilized there by bearing plates  134 . Thus, the steering axle assembly  60  including the transverse axles  72  F,  72  B are allowed to pivot with respect to the dolly tongue  55  in response to the torque applied by the hydraulic steering motor and the steering chain (FIG. 4). The top section of the axle central pivot support  58  extends upward through the center of the dolly tongue  55 . A heavy trailer mounting bar  66  is solidly attached to this axle central pivot support  58 , and supports the fifth wheel  67 . The kingpin of the rear trailer  80  (FIG. 1) is allowed to pivot above the fifth wheel  67  with respect to the dolly tongue  55  as the rear trailer  80  (FIG. 1) swings from side to side with respect to the dolly  50 , but this orientation of the trailer  80  (FIG. 1) with respect to the dolly  50  is accurately measured by the trailer orientation sensor assembly which measures the rotation of the kingpin of the rear trailer  80  inside the fifth wheel latch assembly.  
         [0125]    [0125]FIG. 4—Variable-Steering-Ratio Digital Dolly with Oversteer Viewed Looking Upward from a Cut Just Below the Tongue  
         [0126]    The view of the Variable-Steering-Ratio Digital Dolly with Oversteer shown in FIG. 4 shows more clearly the arrangement of the hydraulic steering motor  68  and the axle orientation sensor assembly  53 . The hydraulic steering motor  68  is mounted solidly on the tongue  55  of the dolly. A heavy steering chain  69  extends between the power output sprocket  76  of the hydraulic steering motor assembly  68  and a steering sprocket  74  that is attached solidly to the steering axle central pivot support  58  (shown here as a cut). The rotation of the power output sprocket  76  and the steering chain  69  then forces the rotation of the steering axle assembly (not shown—located in 3-D space between the viewer and the diagram) with respect to the dolly tongue  55 .  
         [0127]    A second, smaller sprocket is attached to the steering axle central pivot support  58  for determining orientation data. A smaller chain  155  extends from this sprocket  58  to the sprocket  77  on the axle orientation sensor  53 . Any movement of the steering axle assembly  60  (FIG. 3) with respect to the dolly tongue  55  is transferred to and accurately measured by this axle orientation sensor  53 , and this orientation data is then transferred to the controller  49  for use in steering the dolly.  
         [0128]    [0128]FIG. 5—Variable-Steering-Ratio Digital Dolly with Oversteer Viewed from Above  
         [0129]    The Variable-Steering-Ratio Digital Dolly with Oversteer shown in FIG. 5 is a preferred embodiment of the invention. It is similar to the Switchable-Steering-Ratio Geared Dolly with Oversteer except that controller  49  is used to transfer information. The steering information and the oversteer information are transferred from the steering input section, the axle orientation system and the trailer orientation system, respectively, to the powered steering system of the dolly  50  by controller  49 . An additional source of oversteer information will also be available in the form of a measurement of the differential rotation of the dolly wheels  71  L, R as measured by the travel distance encoders  878   a, b . If this difference is accumulated and then decayed at a prescribed rate per linear foot of travel, it can be used along with or instead of the oversteer information received from the trailer orientation system. Pressurized hydraulic fluid is utilized as an energy source. The software in controller  49  will be able to adjust the amount of oversteer as needed, and will also vary the steering ratio continuously as the speed of the dolly changes so that no gearbox will be required. Full redundancy is recommended for all the electronic components to minimize the consequences of failures, but full redundancy is not included in this embodiment of the invention.  
         [0130]    At the front of the dolly  50 , an optical pulse rotation encoder, front pivot orientation sensor THETA_D1  44 , (or some other such sensor) will record the rotation of the dolly tongue  55  about the forward pivot point and transfer this information via pulse counting circuit to the controller  49 . At the rear of the dolly  50  another optical pulse rotation encoder, axle orientation sensor THETA_S1  53  (FIG. 4), (or some other such sensor) will record the rotation of the steering axle assembly  60 . A third optical pulse rotation encoder (or some other such sensor), rear trailer orientation sensor, THETA_R1  81 , mounted on the dolly tongue just forward of the fifth wheel  67 , measures the movement of the trailer  80  carried by the dolly as it rotates around the fifth wheel latch  67 , to provide input to the oversteer correction algorithms. This sensor is mounted on a roller  159  that rolls along the bottom of the trailer (circling around the kingpin of the trailer) as the trailer rotates. A bracket  160  supports the sensor  81  and the roller  159  and presses them tightly upward against the bottom of the trailer. An air cylinder connected between the dolly tongue and the bracket maintains this upward pressure whenever a kingpin is latched in the fifth wheel latch, but allows the bracket to relax down against the body of the dolly tongue at other times to avoid damage during hitching operations.  
         [0131]    Some appropriate method must be provided for scaling the readings of these sensors into degrees of rotation and to calibrate them periodically to assure that any shifting is taken into account.  
         [0132]    A reversible hydraulic motor  68  geared down to a moderate speed will provide the energy for turning the axles  72  F,  72  B when the software detects that movement is required. This hydraulic motor  68  is provided with an automatic braking mechanism that locks the gear train into position at times when no action is required of the hydraulic motor  68 . Loss of air pressure will also activate the braking mechanism.  
         [0133]    An optical rotation encoder  878  (or other such sensor) will record the rotation of the drive shaft for each front dolly wheel  71  L, R. The software in the controller  49  will use this information in two ways. The sum of the counts will be scaled to yield the distance traveled by the dolly in any given time interval. The difference between the counts for the two wheels  71  L, R will be scaled to yield a measure of the amount of cornering that the dolly wheels  71  L, R are undergoing.  
       Overview of the Variable-Steering-Ratio Digital Dolly without Oversteer  
       [0134]    In this alternate embodiment of the invention, as seen in FIG. 6, a controller  49  controls the switching among a multitude of steering ratios. The dolly consists of two sections that pivot in relation to one another. The upper section, the dolly tongue  55 , is connected at the front to the front trailer  40  by an attachment assembly  47 , and at the back to the rear trailer  80  via a fifth wheel  67 . Below the dolly tongue  50  is a steering axle assembly  66 , which pivots with respect to the dolly tongue  55 . There is a front pivot orientation sensor THETA_D1  44  between the dolly tongue  55  and the front trailer  40  that detects the angle between the centerline of the front trailer  40  and the center line of the dolly tongue  50 . Another sensor, the axle orientation sensor  53 , is located between the dolly tongue  55  and the steering axle assembly  60 , which detects the angle between the centerline of the dolly tongue  55  and the centerline of the steering axle assembly  60 . The controller  49  reads the input from the front sensor and generates an appropriate steering angle for the steering axle assembly  60 . A hydraulic motor  68  is mounted so that it can rotate the steering axle assembly  60  to the desired steering angle.  
         [0135]    [0135]FIG. 6: The Variable-Steering-Ratio Digital Dolly without the Oversteer Correction  
         [0136]    This alternate embodiment of the invention is physically identical to the Variable-Steering-Ratio Digital Dolly with oversteer if we omit the sections relating to the measurement of the trailer orientation, the sections relating to the measurement of the rotation of the dolly wheels, and the sections relating to the oversteer algorithms in the controller. There will be no sensor to measure the rotation of the trailer kingpin inside the fifth wheel latch. There will, of course, be no sensors to measure the rotation of the dolly wheels. There will be no algorithms in the computer software to apply any oversteer correction. The remainder of the dolly will be unchanged.  
         [0137]    For a detailed description of this alternate embodiment of the invention, simply refer to the detailed description of the Variable-Steering-Ratio Digital Dolly with oversteer, and make allowance for the above differences.  
       Overview of Switchable-Steering-Ratio Geared Dolly with Oversteer  
     Best Seen in FIGS.  7 ,  8 ,  9 , and  10   
       [0138]    This preferred mechanical embodiment of the invention consists of three pivotally connected sections. At the front of the dolly tongue  55  there is a front partial-circular track  75 , which is connected at the sides to a hitch bar  131  that pivots with the front trailer  40  (FIG. 1). As the front trailer  40  (FIG. 1) rotates with regard to the dolly tongue  55 , the front partial-circular track  100  passes between the roller  110  and the gear  111 . The outside of the front partial-circular track  100  has teeth sized to mesh with the gear  111 , therefore as the front partial-circular track  100  rotates the gear  111  also rotates. Mounted below the gear  111  is an angle gear  112 , which meshes with another angle gear  113  to transfer the rotation to a horizontal plane. The angle gear  113  is connected to a shaft  114  that carries the rotation to a gearbox  115 . There is a roller  104  mounted on the front partial-circular track  100  which activates an enabling air valve  132  when the front trailer  40  (FIG. 1) and dolly tongue  55  are aligned. The enabling air valve  132  sends a signal to the gear box  115  indicating that the sections are aligned.  
         [0139]    There are two partial-circular tracks at the rear of the dolly tongue, one above the other. The lower rear partial-circular track  75 , best seen in FIG. 8, is connected to the steering axle assembly  60  via the track attachment plate and assembly  73 , and the steering axle central pivot support  58 , and has a gear  118  (hidden) and roller  121  (hidden) like the front partial-circular track  75 . Above the gear  118  (hidden) is an angle gear  120  that meshes with an angle gear  119  connected to a shaft  122 , which goes into the neutral-lock gearbox  125 . There is a roller, as in the front track, with an enabling air valve that signals the neutral-lock gearbox  125  that the dolly tongue  55  and the axle assembly  60  are aligned.  
         [0140]    The rear trailer is not mounted on a single fifth wheel, but on a double fifth wheel  67  L, R on a trailer mounting bar  66  that pivots above the dolly tongue  55 . Attached to the trailer mounting bar  66  is an upper rear partial-circular track  140  which rotates in an equivalent assembly, consisting of a roller  123  (hidden by angle gear  127 , gear  124  (hidden by angle gear  127 ), angle gear  127 , angle gear  128  and shaft  126  passing above the other two gearboxes  125 ,  115 , leading to the oversteer gearbox  130 .  
         [0141]    [0141]FIG. 7—View of Switchable-Steering-Ratio Geared Dolly with Oversteer from the Back Looking Forward  
         [0142]    [0142]FIG. 7 shows a view of the back of the Switchable-Steering-Ratio Geared Dolly with Oversteer. The heavy axle central pivot support  58  is mounted solidly on the steering axle assembly  60  and extends upward through the center of the dolly tongue  55 . The dolly tongue  55  pivots freely around this axle central pivot support  58  and is stabilized there by bearing plates  134 . Thus, the steering axle assembly  60  including the transverse axles  72  F,  72  B are allowed to pivot with respect to the dolly tongue  55  in response to the torque applied by the lower rear partial-circular track  75  (FIG. 8).  
         [0143]    The top section of the axle central pivot support  58  extends upward through the center of the dolly tongue  55 . A heavy trailer mounting bar central pivot  65  that is attached to this axle central pivot support  58  supports and allows pivoting of the trailer mounting bar  66  with the two fifth wheels  67  L, R. Circular bearing plates  134  provide stability for this trailer mounting bar central pivot  65 . The rear upper partial-circular track  140  (FIG. 9) attaches solidly to the trailer mounting bar  66  so that as the trailer mounting bar  66  rotates with the rotation of the rear trailer  80 , the upper partial-circular track  140  is also forced to rotate. The rotation of the upper partial-circular track  140  causes the gears in the trailer rotation gear train to rotate, providing the oversteer input for the oversteer gearbox  130  (FIG. 8). Thus, the trailer mounting bar  66  and the rear trailer  80  (FIG. 1) are allowed to pivot above the dolly tongue  55  as the rear trailer  80  (FIG. 1) swings from side to side with respect to the dolly  50 , but this orientation of the trailer  80  (FIG. 1) with respect to the dolly  50  is accurately measured by the upper partial-circular track  140  and its associated gearing.  
         [0144]    [0144]FIG. 8—View of Switchable-Steering-Ratio Geared Dolly with Oversteer from Top Looking Down  
         [0145]    [0145]FIG. 8 shows a view from above of the primary sections of the Switchable-Steering-Ratio Geared Dolly with Oversteer with the upper rear partial-circular track  140  and associated fifth wheel  67  and trailer mounting bar  66  removed. This same figure is shown in its entirety in FIG. 9. The steering gear ratio used by this dolly  50  can be switched from the driver&#39;s cab without stopping the vehicle. The switchability of the steering system of this dolly  50  allows it to be operated in a geared stability mode at higher speeds on the open road, then shifted into a different mode to operate as a geared cornering mode dolly  50  for better cornering ability at lower speeds. In the geared cornering mode, an oversteer correction is used to assist in swinging wide around corners.  
         [0146]    As seen in FIG. 7, the axle central pivot support  58  extends upward through the dolly tongue  55  and, at a point just above the dolly tongue  55 , has an attachment plate extending outward. This attachment plate is connected via track attachment assemblies track attachment plate and assembly  73  to the extremities of a large lower rear partial-circular track  75 . The lower partial-circular track  75  must be somewhat longer than a semicircle, since turns of more than 90 degrees will require more than a full 180 degrees of rotation. The bottom of the lower rear partial-circular track  75  is in the same plane with the top of the dolly tongue  55 . The gear teeth on the front of the lower rear partial-circular track  75  are sized to mesh with the teeth of a small gear  118  (hidden) mounted on the dolly tongue  55 . This lower rear partial-circular track  75  passes between a roller  121  (hidden) and the small gear  118  (hidden) on the top of the main axial member of the dolly tongue  55 . The roller  121  (hidden) and the gear  118  (hidden) are positioned to press tightly against the sides of the lower rear partial-circular track  75  so that as the gear  118  (hidden) rotates, it causes the lower rear partial-circular track  75  to move between the gear  118  (hidden) and the roller  121  (hidden). This in turn will cause the axles  72  F,  72  B to rotate about a vertical axis, changing the orientation of the dolly running wheels  70  L, R, and  71  L, R. The small gear  118  (hidden) is rigidly attached to a large 90-degree gear  120  above it, both of which rotate about the same axis. The large 90-degree gear  120  is mounted high enough to easily stay clear of the lower rear partial-circular track  75  as it moves. The large 90-degree gear  120  has 45 degree teeth along its outer lower edge designed to mesh with a smaller 90-degree gear  119  rotating at a 90-degree angle to it and located directly below its front edge. This smaller 90-degree gear  119  which is rotating around an axis parallel to the main axial member of the dolly tongue  55  is mounted on a shaft  122 .  
         [0147]    The shaft  122  enters a neutral-lock gearbox  125  through the back wall of the neutral-lock gearbox  125 . This neutral-lock gearbox  125  performs its functions at the beginning and at the end of each shifting sequence. It starts each mode shifting sequence by disconnecting all steering gears in front of the neutral-lock gearbox  125  from all steering gears behind it and then locking the gears behind it into a static position. Then after all other shifting operations are completed, and when a forward enabling air valve  132  indicates that the forward section is aligned, the neutral-lock gearbox  125  completes the sequence by unlocking and reconnecting the gears behind it to the gears in front of it. Since no shifting sequence can begin unless a rear enabling air valve  150  has indicated that the rear section is in alignment, this method assures that at the completion of each shifting sequence, all sections are properly aligned and centered. The operation of this rear enabling air valve  150  will be dealt with more fully later on in this section. In practice, of course, all these events may take place in a very short interval of time, since all the actions are automatically controlled by air pressure.  
         [0148]    A shaft  129  coming out through the front wall of the neutral-lock gearbox  125  goes through another wall and into the main gearbox  115 . The purpose of this main gearbox  115  is to select the dolly operating mode by changing the ratio and/or the direction of the rotational input from a front shaft  97  to a new output rotation of the rear shaft  129 . A detailed discussion of this operation will be presented in the operation section, however we will summarize the specifications here which would be needed when ordering this gearbox from a manufacturer.  
         [0149]    When ordering the gearbox, the following requirements will need to be specified. All gear shifting should be performed by high-pressure air. All gear positions should be stable; i. e. no changes in gear position can occur if no high-pressure air is applied to the system. The input rotation enters the front of the box, and two gear ratios must be available to the shaft  129  coming out the back. The gear shifting should be performed by only two high-pressure airlines. Pressure on the first air line, which we will call the stability air line  154 , should cause the output rotation to be shifted to straight or forward, with the magnitude of the gear ratio being equal to the value calculated in the theory section for Stability mode. This gear ratio will depend on the relative lengths of the dolly  50  and the rear trailer  80  (FIG. 1), but will in general be around 0.75. Pressure on the second airline, which we will call the cornering airline  156 , should cause the output rotation to be shifted to reversed with a gear ratio of −1 (−1 rotation out to the back/one rotation in from the front).  
         [0150]    In addition to the gearing requirements, the main gearbox  115  will provide some control and information functions. The main gearbox  115  must activate switches when in a particular mode which will show the main gearbox  115  status to the driver using indicator lights  153  on a control box  152  in the driver&#39;s cab.  
         [0151]    When the rear enabling air valve  150  detects alignment, the air pressure is passed on to the neutral-lock gearbox  125  and the mode switching operation is initiated. When the forward enabling air valve  132  detects alignment, the air pressure is passed on to the main gearbox  115  to allow completion of the mode switching operation.  
         [0152]    The control box  152  will be located in the driver&#39;s cab. The face of the control box  152  will have two indicator lights  153 , one for each mode. The control box  152  will have an air valve  151  that will be used by the driver to turn on high-pressure air to either the stability airline  154  or the cornering air line  156 , but not to both, with the other line in each case dumped to atmosphere. Two high-pressure airlines  154 ,  156  will be routed between the control box  152  and the dolly  50 .  
         [0153]    A front shaft  97  comes out the front of the main gearbox  115  and enters through the back wall of the oversteer gearbox  130 . This oversteer gearbox  130  also receives rotational input from the trailer orientation shaft  126  (removed in this figure, but shown in the upper view of this figure, FIG. 9). The rotation of the trailer orientation shaft  126  (FIG. 9) is caused by, and is an indicator of, the rotation of the trailer mounting bar  66  and thus of the trailer  80  (FIG. 1) itself around the trailer mounting bar central pivot  65 . The rotation of this trailer orientation shaft  126  (FIG. 9) will be added to the rotation coming into the front of the oversteer gearbox  130  and the combined rotations will be output through the back shaft  97  to the main gearbox  115 . A detail of this operation will be shown in FIG. 10. A shaft  98   b  coming out the front of the oversteer gearbox  130  is the outer section of a splined shaft  98   a  having splines on the inside. The inner section of the forward shaft  98   a  having splines on the outside, slides inside the outer splined front shaft  98   b . These splined shafts  98   a ,  98   b  are designed to allow the length of the dolly tongue  55  to be adjusted as needed for different rear trailer  80  (FIG. 1) lengths. Similarly, at a joint  144 , a smaller dolly tongue section  55   b  slides into a larger dolly tongue section  55   a , allowing the main frame to be easily adjusted. Two pin and lock sets  146  and  148  secure this attachment to prevent slippage or movement during operation. The front end of the splined shaft  98   a  connects to a 90-degree gear  113 . The 90-degree gear  113  connects to a larger 90-degree gear  112  in a manner that is similar to the connections to the lower rear partial-circular track  75  except that gear  113  is above gear  112 . A smaller gear  111  above 90-degree gear  112  is rigidly attached to 90-degree gear  112  so that its axis coincides with the axis of 90-degree gear  112 . This smaller gear  111  then meshes and presses tightly against the back of a forward partial-circular track  100  while a roller  110  roils tightly against the front or inside of the forward partial-circular track  100 . As the forward trailer  40  (FIG. 1) turns, the forward partial-circular track  100  is forced to move between the roller  110  and the gear  111 , causing the 90-degree gear  112 , and thus the attached linkages to rotate.  
         [0154]    A roller  104  is mounted near the forward partial-circular track  100  on a mounting brace. Its mounting brace is attached solidly to the dolly tongue  55  and the roller  104  is positioned to roll along the top of the forward partial-circular track  100  as it moves. When the forward trailer  40  is aligned with the centerline of the dolly, a bump on the top of the forward partial-circular track  100  pushes the roller  104  up, causing the forward enabling air valve  132 , which is also mounted on the mounting brace, to be activated. This enabling air valve  132  then passes the air pressure on to the main gearbox  115 , enabling the completion of a mode shifting sequence when the driver has signaled for a mode change.  
         [0155]    The forward partial-circular track  100  is attached to a hitch assembly  47  at its extremities via some sort of hinge-type attachment that allows some up and down hinging action while preventing vertical or horizontal movement at the point of attachment. In this embodiment, we will use hinge assemblies  106 L and  106 R to represent the attachment arrangements for the forward partial-circular track  100 . The heavy central member of the dolly tongue  55  attaches to a larger attachment point using a larger device that will be represented by ball-and-socket attachment assembly  108 . This ball-and-socket attachment assembly  108  must bear the full weight and traction forces of the dolly  50  and the rear trailer  80  while allowing free pivoting around a vertical axis as well as limited movement about horizontal axes. A solidly built square hitch shaft  131  extending forward from the attachment assembly  47  will slide into a receiving hole in the hitching apparatus of the forward trailer  40 . The forward trailer  40  (FIG. 1) must be modified to have a receiving hole that is compatible with the dolly hitch shaft  131  and a way of latching the hitch shaft  131  firmly into its receiving hole. Note that the forward partial-circular track  100  is not solidly attached to the dolly tongue  55 , but travels across it, in contact with it, during turns. Note also that the ball-and-socket attachment assembly  108  for the main dolly attachment must be positioned in the center of the forward partial-circular track  100  during operation.  
         [0156]    Moving toward the back of the dolly, a roller  105  is mounted in a manner similar to the front roller  104 . Its mounting brace is attached solidly to the dolly tongue  55  and the roller  105  is positioned to roll along the top of the lower rear partial-circular track  75  as it moves. When the axles  72  F,  72  B are perpendicular to the dolly tongue  55 , a bump on the top of the lower rear partial-circular track  75  pushes the roller  105  up, causing the rear enabling air valve  150 , which is also mounted on the mounting brace, to be activated. This rear enabling air valve  150  then passes the air pressure on to the neutral-lock gearbox  125 , enabling the initiation of a mode shifting sequence when the driver has signaled for a mode change.  
         [0157]    [0157]FIG. 9—Preferred Embodiment of Invention Using a Switchable-Steering-Ratio Geared Dolly with Oversteer (Upper Partial-Circular Track)  
         [0158]    The rear trailer  80  is rigidly connected by the two fifth wheels  67 L, R to the trailer mounting bar  66 . To measure the angle of the rear trailer  80  (FIG. 1) in relation to the centerline of the dolly tongue  55 , the ends of the rear upper partial-circular track  140  attach to the trailer mounting bar  66 . This rear upper partial-circular track  140  is mounted sufficiently above the lower rear partial-circular track  75  to easily clear it during operation and to allow unobstructed operation of both rotational systems and is also of somewhat greater diameter than the diameter of the lower rear partial-circular track  75 . This rear upper partial-circular track  140  passes between a roller  123  and the small gear  124  directly above the main axial member of the dolly tongue  55  and somewhat above the gear  120 . The roller  123  and the small gear  124  are positioned to press tightly against the sides of the rear upper partial-circular track  140  so that as the rear upper partial-circular track  140  moves between the gear  123  and the roller  124 , the small gear  123  is caused to rotate about its vertical axis. The small gear  123  is rigidly attached to a large 90-degree gear  127  above it, both of which rotate about the same axis. The large 90-degree gear  127  is mounted high enough to easily stay clear of the rear upper partial-circular track  140  as it moves. The large 90-degree gear  127  has 45 degree teeth along its outer lower edge designed to mesh with a smaller 90-degree gear  128  rotating at a 90-degree angle to it and located directly below its front edge. This smaller 90-degree gear  128  which is rotating around an axis parallel to the main axial member of the dolly tongue  55  is mounted on a shaft  126  which passes forward above the neutral-lock gearbox  125  and the main gearbox  115  and into the oversteer gearbox  130 . Here its rotation will be combined with the rotational input from the front steering section of the dolly  50  to control the steering of the dolly wheels  71  L, R,  72  L, R.  
         [0159]    [0159]FIG. 10—Oversteer Gearbox  130   
         [0160]    [0160]FIG. 10 is a detail of the oversteer gearbox  130  (FIG. 9) containing an inner gearbox  570  and a set of planetary gears  560 ,  562 ,  564 . The oversteer gearbox  130  (FIG. 9) combines two rotational inputs. The first is from the steering input system at the front of the dolly, which is received from shaft  98   b . The second rotational input is from the back of the dolly from trailer orientation shaft  126 . When the shaft trailer orientation shaft  126  enters the oversteer gearbox  130  (FIG. 9), it enters the inner gearbox  570 . Then the rotational input is geared up or down and sent out from inner gearbox  570  to shaft  558 . The planetary gears  511  and shell  568  add this rotation to the rotation received from the front steering input section. The oversteer gearbox  130  then sends this combined rotational output along a shaft  97  into the front of the main gearbox  115  (FIG. 9) directly behind it.  
         [0161]    In inner gearbox  570  the rotational input from the trailer orientation system in the back is first geared down to provide the desired level of oversteer compensation. More oversteer will cause the dolly to swing wider when turning a corner, and less oversteer will cause the dolly to not swing so wide. The rotational input is then combined with the steering input from the front of the dolly using a system of planetary gears  511 . Inside the system of planetary gears  511 , four smaller 90-degree gears  560   a, b, c, d  revolve around two central larger 90-degree gears  562 ,  564  that carry the rotation in from the front and out to the main gearbox  115  (FIG. 9) in the back respectively. The four smaller 90-degree gears  560   a, b, c, d  are mounted on shafts  561   a, b, c, d  which attach solidly to the shell  568  and which form a rigid “X” shape between the two central larger 90-degree gears  562 ,  564 . The rotational input from the trailer orientation system is placed by a system of gears onto the outer planetary “shell”  568 . If the planetary shell  568  is held stationary, rotation is simply passed straight through from the front to the back, reversing the direction of rotation but with no change in the magnitude of the rotation, as the planetary gears  560   a, b, c, d  maintain their position and transfer the rotation between the two central large 90-degree gears  564 ,  566 . If the trailer  80  (FIG. 1) rotates with respect to the dolly, this rotation causes the planetary shell  568  to move around its central axis, and this rotation is added to or subtracted from the rotation coming into the front. Note that the direction of rotation of the shaft leaving the back of the oversteer gearbox  130  must be again reversed to maintain the original rotation information received from the front.  
       Overview of the Switchable-Steering-Ratio Geared Dolly without Oversteer  
       [0162]    This alternate embodiment of the invention in FIG. 11 is identical to the Switchable-Steering-Ratio Geared Dolly in FIGS. 7, 8,  9 ,  10  with oversteer except that it demonstrates the possibility of constructing a Switchable model without the use of the oversteer correction. At the front of the dolly tongue  55  there is a forward partial-circular track  100 , which is connected at the sides to a bar  107  that pivots with the front trailer  40 . As the front trailer  40  rotates with regard to the dolly tongue  55 , the forward partial-circular track  100  passes between the roller  110  and the gear  112 . The forward partial-circular track  100  is toothed on its outer edge, and runs between the roller  110  and the gear  111  mounted on the dolly tongue  55 . The roller  110  and gear  111  press tightly against the forward partial-circular track  100 , keeping it from slipping. Below the gear  111  is an angle gear  112  that meshes with an angle gear  113 , converting the rotation to horizontal. Connected to the angle gear  113  is a shaft  98 , which runs to the gearbox  130 . To determine if the front trailer  40  is aligned with the dolly tongue  55 , there is a roller  104  that when the dolly tongue  55  and the centerline of the front trailer  40  are aligned, activates an enabling air valve  132  that transmits a signal to the main gearbox  115 . At the rear of the dolly tongue  55  there is a lower rear partial-circular track  75  which is connected at the ends to a track attachment plate and assembly  73  that in turn attaches solidly to the steering axle central pivot support  58  just above the tongue of the dolly. Like the front partial-circular track  100 , it runs between a roller  121  and a gear  118 . Above the gear  118  is an angle gear 120 , which, with the other angle gear  119 , converts the rotation to a horizontal axis. Connected to the angle gear  119  is a shaft  122 , which runs to the neutral lock gear box  125 . As in the front, there is a roller  105  that activates an enabling air valve  150  when the centerline of the dolly  55  and the centerline of the steering axle assembly  60  are aligned, transmitting a signal to the neutral lock gear box  125 .  
         [0163]    [0163]FIG. 11: The Switchable-Steering-Ratio Geared Dolly without the Oversteer Correction  
         [0164]    This alternate embodiment of the invention is physically identical to the Switchable-Steering-Ratio Geared Dolly with oversteer if we omit the sections relating to the measurement of the trailer orientation and the sections relating to the oversteer gearbox  130  (FIG. 9). There will be no upper rear partial-circular track  140  and none of the gearing between the upper rear partial-circular track  140  and the oversteer gearbox  130 . There will, of course, be no oversteer gearbox  130  at all. The trailer mounting bar  66  will be mounted solidly on the top of the axle central pivot support  58  (FIG. 7), instead of having the ability to swivel. The remainder of the dolly will be unchanged.  
         [0165]    For a detailed description of this alternate embodiment of the invention, simply refer to the detailed description of the Switchable-Steering-Ratio Geared Dolly with oversteer.  
         [0166]    [0166]FIG. 12—Dolly Travel Distance Sensors  
         [0167]    [0167]FIG. 12 shows a view of the dolly travel distance sensors  878   a, b  (optical rotation encoders or some other such sensors) that measure the rotation of the dolly drive axles  161  inside the axle housings of the front two sets of dolly running wheels  71  L, R. The drive axles  161  are connected to the running wheels  71  L, R in a manner that is similar to the way the drive axles of a tractor are attached to the drive wheels of the tractor. The rotation measured by these sensors is used by the controller  49  to determine the timing of data acquisition from the sensor array and also as a second source for the oversteer correction data and make allowance for the above differences.  
         [0168]    Operations  
         [0169]    A Dolly Using Gears and a Gearbox for Switching Between Steering Ratios and Oversteer for Assistance in Turning Corners  
         [0170]    Introduction  
         [0171]    The primary features of interest in this alternative embodiment of the invention is its switchability between at least two steering ratios without stopping the vehicle and its use of oversteer for turning corners. At least one of these steering ratios must be designed to provide more stability at higher speeds, and at least one ratio must be designed for better cornering ability and maneuverability at lower speeds. In this alternative embodiment the stability mode is the mode designed to provide stability at higher speeds. It corresponds roughly to the mode of steering used in the Steerable Type A dolly of the prior art. In this alternative embodiment the cornering mode is the mode designed to provide more maneuverability. This mode corresponds to a mode of steering that would be produced by crossed steering arms. In the cornering mode, an oversteer correction is used to enable a wider swing around corners than would be possible with the cornering mode alone.  
         [0172]    Input to the Steering System  
         [0173]    In overview, the input to the steering system of the Switchable-Steering-Ratio Geared Dolly with oversteer is derived from the angle between the forward trailer  40  and the dolly  50 . This input will be picked up by the forward partial-circular track  100  and transferred via the forward part of the geartrain into the oversteer gearbox  130 . Inside this oversteer gearbox  130 , the rotations from the front will be combined with the rotations from the trailer orientation system using a planetary system of gears. The combined rotations will then be sent on back into main gearbox  115 . The main gearbox  115  chooses the ratio, which will determine many of the characteristics of the dolly&#39;s steering. Then the output from the main gearbox  115  is transferred via the neutral lock gearbox  125  and the back part of the geartrain to the lower rear partial-circular track  75 . The back of the lower rear partial-circular track  75  is attached to a track attachment plate and assembly  73  that is in turn solidly attached to the steering axle central pivot support at a point just above the tongue of the dolly, and causes the axles  72 F and  72 B to rotate about their pivot points, the axle central pivot support  58 , in response to the original input from the front of the dolly as processed by the various gearboxes.  
         [0174]    As we mentioned above, the angle between the forward trailer  40  and the dolly  50  provides the primary input for our steering system. As this angle varies during a turning operation, we see from FIG. 8 that the forward partial-circular track  100  moves between the roller  110  and the small gear  111 . These two rotary members are pressed tightly against the two sides of the forward partial-circular track  100  to prevent slippage of the gear  111 , so that the gear  111  is forced to rotate by the movement of the forward partial-circular track  100 . This rotational movement is ratioed up by 90-degree gear  112  and converted to rotation about an axis parallel to the main axial member of the dolly tongue  55  by the 90-degree gear  113 . The shaft  98  then carries this rotational movement back into the oversteer gearbox  130 . This oversteer gearbox  130  also receives rotational input from the trailer orientation shaft  126 . The rotation of the trailer orientation shaft  126  is caused by, and is an indicator of, the rotation of the trailer mounting bar  66  and thus of the trailer  80  (FIG. 1) itself around the trailer mounting bar central pivot  66 . The rotation of this trailer orientation shaft  126  will be added to the rotation of the shaft  118  coming into the front of the oversteer gearbox  130 , and the combined rotations will be output through the back shaft  97  to the main gearbox  115 . A detail of this operation is shown in FIG. 10.  
         [0175]    Operation of the Gearbox  
         [0176]    The primary purpose of this main gearbox  115  is to select the dolly operating mode by changing the ratio and/or the direction of the rotational input from the front shaft  97  to a new output rotation of the rear shaft  129 . Although any number of different steering ratios could be easily provided, only two operating modes are enabled in this embodiment. We will assume for our purposes here that the forward partial-circular track  100  and the lower rear partial-circular track  75  have the same diameter and that corresponding gears in front of the main gearbox  115  are the same size as their corresponding gear behind the main gearbox  115 . If the direction of the input from the front is unchanged by the main gearbox  115  and the gear ratio is equal to the value calculated in the theory section below, the dolly  50  will operate in the most stable mode. If the direction of the input is reversed but the gear ratio is equal to −1 (−1 revolution out to the back)/(1 revolution in at the front), the dolly  50  will operate in the cornering mode. These modes will be selectable by the driver from the cab without stopping the vehicle. Actual shifting will not begin, however, until the dolly  50  is lined up straight forward as sensed by the rear enabling switch  150 . This prevents the off centering and skewing that would occur if shifting could be initiated at any position. In this embodiment, shifting is initiated by activating the valve on the control box  152  in the driver&#39;s cab to place air pressure on either the stability air line  154  or the cornering air line  156 . Note that a substantial interval of time may elapse before shifting is completed, since the shifting will not be initiated in the main gearbox  115  until the rear section of the dolly  50  is in alignment as signaled by the rear enabling switch  150 . Air pressure in the stability air line  154  will shift the dolly  50  into the stability mode by shifting the main gearbox  115  to provide straight or forward rotation at a gear ratio as calculated in the theory section below. This gear ratio will depend on the relative lengths of the dolly  50  and the rear trailer  80 , but will in general be around 0.6. Air pressure in the cornering air line  156  will shift the main gearbox  115  to provide reversed rotation at the output with a gear ratio of −1 (−1 rotation out to the back/one rotation in from the front). Switches inside the main gearbox  115  will inform the driver as to which mode is currently in force by activating indicator lights  153  on the dashboard. All mode switch actuators in main gearbox  115  are stable in position so that loss of air will not cause any mode switch. In this embodiment, then, two control air lines  154 ,  156  and two switch indicator lights  153  on the control box  152  will comprise the communication network between the drivers cab and the Switchable-Steering-Ratio Geared Dolly with oversteer which is an alternative embodiment of this invention.  
         [0177]    The specifications for ordering the main gearbox  115  were given in the description section, including that all gear shifting should be performed by high-pressure air. All gear positions should be stable; i. e. no changes in gear position can occur if no high-pressure air is applied to the system. The input rotation enters the front of the main gearbox  115 , and two gear ratios must be available to the shaft  129  coming out the back. The gear shifting should be performed by only two high-pressure airlines  154 ,  156 .  
         [0178]    Pressure on the first air line, which we will call the stability air line  154 , should cause the output rotation to be shifted to straight or forward, with the magnitude of the gear ratio being equal to the value calculated in the theory section for stability mode. This gear ratio will depend on the relative lengths of the dolly  50  and the rear trailer  80 , but will in general be around 0.6. Pressure on the second air line, which we will call the cornering air line  156 , should cause the output rotation to be shifted to reversed with a gear ratio of −1 (−1 rotation out to the back/one rotation in from the front).  
         [0179]    In addition to the gearing requirements, the main gearbox  115  will provide some control and information functions. The main gearbox  115  must activate switches inside the main gearbox  115  when in a particular mode which will show the main gearbox  115  status to the driver using indicator lights  153  on the control panel  152  in the drivers cab.  
         [0180]    At this point we will also note that the two high pressure air lines  154 ,  156  (FIG. 8) used to control the ratio shifting must be routed from the control panel  152  in the driver&#39;s cab to the forward enabling switch  132  and the rear enabling switch  150 . When the rear-enabling switch  150  detects alignment, the air pressure is passed on to the neutral lock gearbox  125  and the ratio switching operation is initiated. When the forward enabling switch  132  detects alignment, the air pressure is passed on to the main gearbox  115  to allow completion of the ratio switching operation.  
         [0181]    The control box  152  will be located in the driver&#39;s cab. The face of the control box  152  will have two indicator lights  153 , one for each mode. The control box  152  will have an air valve  151  that will turn on high pressure air to either the stability air line  154  or the cornering air line  156 , but not to both, with the other line in each case dumped to atmosphere. In review, two high-pressure airlines  154 ,  156  will be routed between the control box  152  and the dolly  50 .  
         [0182]    Operation of the Neutral Lock Gearbox  
         [0183]    The shaft  129  coming out through the back wall of the main gearbox  115  goes through another wall and into the neutral lock gearbox  125 . The neutral lock gearbox  125  performs its functions at the beginning and at the end of each shifting sequence. When the driver has applied pressure to one of the control air lines  154 ,  156 , and when the rear enabling switch  150  has permitted that pressure to be transferred to the main gearbox  115 , the neutral lock gearbox  125  starts a ratio shifting sequence by disconnecting all steering gears in front of the neutral lock gearbox  125  from all steering gears behind it and then locking the steering gears behind it into a static position. Then after all other shifting operations are completed, and when the forward enabling switch  132  indicates that the forward section is aligned, the neutral lock gearbox  125  completes the sequence by unlocking and reconnecting the gears behind it to the gears in front of it. Since no shifting sequence can begin unless the back enabling switch  150  has indicated that the rear section is in alignment and no shifting sequence can terminate unless the forward enabling switch  132  has indicated that the forward section is in alignment, this method assures that at the completion of each shifting sequence all sections are properly aligned and centered. In practice, of course, all these events may take place in a very short interval of time if the vehicles are traveling in a straight line, since all the actions are automatically controlled by air pressure. It is worth noting here that while the neutral lock gearbox  125  has the back section locked, the dolly  50  will be operating in the standard non-steerable A mode. This mode could thus be easily made available if desired, but it would have few advantages over the other two modes that are available.  
         [0184]    Output from the Gearbox to Steer the Dolly Axle  
         [0185]    In FIG. 8 the shaft  122  carries the output rotational movement from the neutral lock gearbox  125  to the gear  119 . The gear  119  then picks up this movement, ratios it back down, and converts it back to rotation about a vertical axis. Gear  118 , with the help of roller  120  then converts this rotational movement into movement of the lower rear partial-circular track  75  which then causes the transverse axles  72 F and  72 B to rotate about their axle central pivot support point  58 , steering the dolly  50 .  
         [0186]    Summary and Miscellaneous for Switchable-Steering-Ratio Geared Dolly with Oversteer  
         [0187]    In summary, the input to the steering system of the dolly  50  is the angle between the back of the forward trailer  40  and the dolly  50 . The output from the system is the orientation of the transverse axles  72 F and  72 B, and thus of the running wheels  70  R,  71  R and  70 L,  71  R of the dolly  50 . The manipulation of the input by the main gearbox  115  and by the oversteer gearbox  130  is the key to the steering characteristics of the dolly  50  in this alternative embodiment of the invention. When the main gearbox  115  is in the stability mode, the operation of the dolly  50  at higher speeds will be more stable. When the main gearbox  115  is in the cornering mode, the rear trailer  80  will be more maneuverable and will have less of a tendency to cut the corners during turning operations. When a substantial oversteer component is input, the dolly will oversteer in order to avoid cutting the corner with the trailer  80  (FIG. 1) it is carrying.  
         [0188]    The behavior of the Switchable-Steering-Ratio Geared Dolly with oversteer during backing operations is of particular interest. Normally a “double” is almost impossible to back, but if the dolly is shifted into stability mode, this section will behave much like a single-axle trailer with a very long wheelbase. The string will then become only slightly harder to back than a single trailer.  
         [0189]    A Dolly That Uses Controllers and Hydraulic Motors for Steering, That Continuously Varies the Steering Ratio Using the Software in the Controllers, and Which Uses Oversteer to Assist in Steering Around Corners  
         [0190]    The Variable-Steering-Ratio Digital Dolly with oversteer  50  shown in FIGS. 1, 2,  3 ,  4 ,  5  is a preferred embodiment of the invention. It is similar to the Switchable-Steering-Ratio Geared Dolly with oversteer except that the ratio steering information and the oversteer information are transferred from the front steering input sensor, THETA_D 1  44 , the axle orientation sensor THETA_S1  53 , and the trailer orientation sensor THETA_R1  81  to the powered steering system of the dolly  50  via the controller  49 . An additional source of oversteer information will be available in the form of an accumulated measurement of the difference between the rotation of the dolly wheel  71  L and the dolly wheel  71  R that is then decayed at a prescribed rate per linear foot of travel. Pressurized hydraulic fluid is utilized as an energy source. The software in controller  49  will be able to adjust the amount of oversteer as needed, and will also continuously vary the steering ratio as the dolly speed varies so that no gearbox will be required. Preferably, this steering controller  49  will be implemented in software and executed on a processor based system (not shown) that includes random access memory (not shown) and other types of memory (not shown). A diagram of the information flow for this embodiment of the invention is shown in FIG. 13. The controller electronics package includes the steering controller  49  itself, the pulse counting circuits, the analog-to-digital converters, and any other electronic equipment that may be needed to support the controller along with its input and output peripheral devices and circuits.  
         [0191]    At the front of the dolly  50 , an optical pulse rotation encoder (or some other such sensor)  856  will record the rotation about the pivot point near the dolly hitch and transfer this information via pulse counting circuit  858  to the controller  49 . At the rear of the dolly  50  another optical pulse rotation encoder (or some other such sensor)  864  will record the rotation of the steering axle assembly as it changes orientation. A third optical pulse rotation encoder (or some other such sensor)  865  will record the rotation of the rear trailer as it moves from side to side with respect to the dolly tongue to provide input to the oversteer algorithms.  
         [0192]    A reversible hydraulic motor  866  geared down to a moderate speed will provide the energy for changing the orientation of the axles  72 F and  72 B when the software detects that movement is required. This hydraulic motor  866  is provided with an automatic braking mechanism that locks the gear train into position at times when no action is required of the hydraulic motor  866 . Low air pressure and/or low hydraulic pressure will also cause the hydraulic motor  866  to move to its center position and then activate the braking mechanism for the steering control. An air motor operates a hydraulic pump to keep the hydraulic fluid in one reservoir pressurized, while the return fluid is stored in an open tank.  
         [0193]    The software in the controller  49 will compare the number of degrees of rotation input from the front sensor to the number of degrees of rotation input from the steering axle orientation sensor. For a 1-to-1 reverse ratio without oversteer input, the software will control the hydraulic motor  866  to maintain exactly the same number of negative degrees of rotation of the steering axle assembly as it senses of positive degrees of rotation from the front input sensor (see definition of positive angles below for sign convention). Other steering ratios for other modes would be handled by simple mathematical manipulation of the input from front input sensor and from the axle orientation sensor. The details of this operation will be shown below.  
         [0194]    The data from the trailer orientation sensor and/or the delayed difference between the rotation information received from the two dolly wheels  71  L, R will be multiplied by an appropriate oversteer factor and combined with the input from the front input sensor to add oversteer steering behavior to the dolly. The algorithms add the oversteer data to the information obtained from the front input sensor before any adjustments are made for the various steering ratios. Thus the effect of the oversteer will be reduced along with the reduction in maneuverability as the steering ratio [(positive degrees of rotation of the steering axle assembly)/(positive degrees of rotation of the front input sensor)] moves from a more negative ratio toward a ratio of zero. Since the stability modes (steering ratios greater than zero) minimize steering behavior that causes movement of the dolly tongue with respect to the trailer carried by the dolly, and since the stability modes will not be needed when turning sharp corners at lower speeds, oversteer behavior should not produce significant effects when these steering modes are selected.  
         [0195]    As mentioned above, an optical rotation encoder  878  will record the rotation of the drive shaft for each front dolly wheel  71  L, R. The software in the controller  49  will use this information in two ways. The sum of the counts will be scaled to yield the distance traveled by the dolly in any given time interval. The difference between the counts for the two front wheels  71  L, R will be scaled to yield a measure of the amount of cornering that the dolly wheels  71  L, R are undergoing. If this difference is accumulated for each interval of travel and then decayed at a prescribed rate per linear foot of travel, it can be used along with or instead of the input from the trailer orientation sensors as input to the oversteer algorithms.  
         [0196]    Details of Software Algorithms  
         [0197]    At this point we will attempt to describe the details of the algorithms and the physical basis for the algorithms that will control the steering behavior of the dolly. The controller  49  will complete one full cycle of calculations each time a distance has been traveled that corresponds to one “travel interval”. The pulse counters (inside the electronics package with the controller) then, will accumulate counts until they detect a “travel interrupt” from a circuit that is counting and scaling the pulses from the wheel encoders. When the travel interrupt is received, each pulse counter (inside the electronics package with the controller) will transfer its counts to the controller  49  in the form of a digital number.  
         [0198]    Note that the optical rotation encoders, steering input sensor THETA_D1  44 , axle orientation sensor THETA_S1  53 , trailer orientation sensor THETA_R1  81 , and travel distance encoders  878   a, b , are designed so that negative rotation will be detected by the pulse counters (inside the electronics package with the controller) and subtracted off the accumulated totals.  
         [0199]    For each travel interval, the controller  49  now has a number for each encoder that represents the movement of that encoder during that travel interval. The remainder of the processing will take the form of mathematical manipulation of these numbers. The controller  49  will maintain running totals of the counts input from the encoder on the front input section of the dolly steering system (front steering input sensor THETA_D1), the encoder on the trailer orientation system (trailer orientation sensor THETA_R1  81 ), and the encoder measuring the rotation of the dolly steering axle assembly(axle orientation sensor THETA_S1). At the completion of each travel interval, the controller  49  will apply a calibration adjustment and a scaling factor to the above running totals to convert them into calibrated degrees of positive rotation. The angle of the front steering input sensor, THETA_D1  44 , will be positive when the first trailer is rotated clockwise of the straight ahead position with respect to the tongue of the dolly. THETA_D1 will be negative when the first trailer is rotated counterclockwise of the straight ahead position. The angle of the trailer orientation section, THETA_R1  81 , will be positive when the tongue of the dolly is rotated clockwise of the straight ahead position with respect to the trailer carried by the first dolly. THETA_R1 will be negative when the tongue of the dolly is rotated counterclockwise of the straight ahead position. The angle of the axle orientation section, THETA_S1  53 , will be positive when the steering axle assembly is rotated clockwise of the straight ahead position with respect to the tongue of the dolly. THETA_S1 will be negative when the steering axle assembly is rotated counterclockwise of the straight ahead position.  
         [0200]    The pulses from the travel distance encoders  878   a, b  on the right and the left front dolly wheels  71  R and  71  L will each go to two different counters. Both sets of pulses will be input to a circuit that will accumulate counts until it reaches a predetermined point where the average of the distance traveled by the two wheels is one “travel interval”. At this point, this circuit generates a “travel interrupt” that causes the controller  49  to acquire the accumulated counts and/or data from each counter and/or analog sensor. The counters for the wheel encoders and for all the data encoders then reset to zero counts and begin accumulating counts again. Since the Digital Dolly that does not use oversteer does not have travel distance encoders, it will acquire data based on fixed time intervals rather than on the basis of fixed travel intervals.  
         [0201]    The pulses from the travel distance encoders  878   a, b  on the dolly wheels also go to counters that are queried by the controller  49  at the end of each travel interval. At the completion of each travel interval, the processor will apply a calibration adjustment and a scaling factor to these counts to convert them to calibrated feet of linear travel during the travel interval. The numbers obtained from this operation will, again, be used for two distinctly different purposes.  
         [0202]    First, two decayed running totals of the difference between the travel of the left wheel and the travel of the right wheel will be maintained by the controller  49 .  
           DELT 2= DELT 2+( DELT   —   L−DELT   —   R )−DECREMENT  
         [0203]    And  
           DELT 1= DELT 1+( DELT   —   R−DELT   —   L )−DECREMENT  
         [0204]    Where DELT2 is the decayed running total of the difference between the travel of the left wheel  71  L minus the travel of the right wheel  71  R, and DELT1 is the decayed running total of the difference between the travel of the right wheel  71  R minus the travel of the left wheel  71  L. Also, DELT_L is the travel of the left wheel  71  L in the latest travel interval and DELT_R is the travel of the right wheel  71  R in the latest travel interval. The number DECREMENT represents the amount of decay in each travel interval and can be adjusted as needed to change the oversteer characteristics of the system. Generally any accumulation in the delayed running totals DELT1 and DELT2 should decay within less than 100 feet or so to zero. At the end of any travel interval in which DELT1 is less than zero, we will set DELT1=0. At the end of any travel interval in which DELT2 is less than zero, we will set DELT2=0.  
         [0205]    At the completion of each travel interval, the processor will also use the distances traveled during the interval by the left and right wheels, DELT_L and DELT_R to complete the following calculation:  
           SPD=[ 2* AV*SPD*DELT _TIME+ DELT   —   L+DELT   —   R ]/[(2* AV+ 2)* DELT _TIME)] 
         [0206]    Where SPD is the average running speed, DELT_R and DELT_L are the distances traveled during the latest interval of the right and left front wheels  71  L, R respectively, and DELT_TIME is the number of seconds of time since the last travel interrupt. The number AV is representative of the number of intervals over which the average speed is calculated. A larger AV will produce a SPD that varies more slowly with momentary velocity changes.  
         [0207]    The steering ratio could be varied as a function of turning angle, speed, or any other such variable, but in this preferred embodiment of the invention, the steering ratio will be varied continuously by the processors as the speed of the dolly changes. At higher speeds, the controller  49  will automatically control the dolly in a manner that is more stable (a more positive steering ratio), and at lower speeds, the processors will automatically control the dolly in a manner that has better cornering ability (a more negative steering ratio). In order to accomplish this we will choose a correction factor, CORR, that is dependent upon the average speed of the dolly. From the above discussion related to geared steering control, a steering ratio of −4 produced very responsive steering and a steering ratio of about +0.6 (depending upon the ratio of the dolly length to the length of the dolly and the rear trailer  80  together) produced very stable steering. If we wanted to vary the correction factor CORR linearly between −4 and +0.6 as the speed increased from 8 ft/sec to 30 ft/sec, we could write:  
         
       CORR=M*SPD+B  
     
         [0208]    Where M is the slope of the line and B is the intercept. Then if we substitute into this equation at the two chosen points, we have:  
           CORR=− 4= M* 8+ B    
         [0209]    And  
           CORR=+ 0.6= M* 30+ B    
         [0210]    Solving these two equations for the two unknowns, we get:  
         M=0.2091  
         [0211]    And  
         B=−5.673  
         [0212]    Therefore, for the above requirements we have:  
           CORR= 0.2091* SPD− 5.673 whenever 8&lt; SPD&lt; 30 ft/sec.  
         [0213]    If SPD is less than 8, then we will set:  
           CORR=− 4 for  SPD&lt; 8 ft/sec.  
         [0214]    And if SPD is greater than 30, we will set:  
           CORR= 0.6 for  SPD&gt; 30 ft/sec.  
         [0215]    The processor will then determine the steering necessary at each travel interval by the following calculation:  
         DELTA=[THETA —   D 1+ FAC 1*THETA —   R 1+ FAC 2*( DELT 1− DELT 2)]* CORR− THETA —   S 1  
         [0216]    where FAC1 and FAC2 are the oversteer factors for the trailer orientation system and the accumulated dolly wheel delayed difference system respectively, and DELTA is the amount of movement needed by the axle steering system. A positive DELTA will cause the hydraulic motor to move the dolly axles  72 F and  72 B more to the right and a negative DELTA will cause the hydraulic motor  68  to move the dolly axles  72 F and  72 B more to the left. If the absolute value of DELTA is larger, the hydraulic motor  68  will move the axles  72 F and  72 B more quickly. During operation, the hydraulic motor  68  should act to maintain DELTA near zero.  
         [0217]    More Detailed and/or Theoretical Information  
         [0218]    The above discussion contains all the information that is necessary to understand the parts of the ratio type steering and/or of oversteer which are relevant to what is claimed by this patent, but a little more detail might help the reader to understand some of the less obvious points. The following presentation is believed to be correct, but in any case does not affect the validity or value of a trailer system having steering ratios that can be switched without stopping the vehicle and/or using oversteer to assist in turning corners.  
         [0219]    Conditions Necessary for “Maximum Stability” 
         [0220]    As mentioned above, the stability modes of the Variable-Steering-Ratio Digital Dolly are roughly equivalent to the operation of the Steerable Type A dolly of the prior art. When the forward trailer  40  turns to the right, the controller  49  (or the gearbox  115 ) causes a rotation of the dolly axles  72 F and  72 B about a vertical axis so that the back of the dolly  50  also swings to the right, cutting across the corner as the turn is completed. If the steering ratio is just right, the dolly  50  will stay almost exactly between the center hitchpoint of the forward trailer and the center of the rear axle of the trailer carried by the first dolly. In this configuration, the rear trailer  80  and the dolly  50  act much like a single unit and handle in a manner similar to the way a single axle trailer with a very long wheel base would handle.  
         [0221]    In general, for the dolly  50  to remain directly aligned with the centerline of the rear trailer  80  without sideways scrubbing of the tires the axle must be oriented according to the following formula:  
         tangent (THETA —   S 1)=[(LENGTH —   R 1)/(LENGTH —   R 1+LENGTH —   D 1)]*tangent(THETA —   D 1)  
         [0222]    where THETA_S1 is the angle between the centerline of dolly  50  and the perpendicular to the dolly axles  72 F and  72 B, THETA_D1 is the angle between the centerline of the forward trailer and the centerline of the dolly  50 , LENGTH_R1 is the distance from the kingpin where the rear trailer  80  attaches to the dolly  50  to the center of the back axle of the rear trailer  80 , and LENGTH_D1 is the length of the dolly  50 , from the attachment point at the front of the dolly  50  to the point where the kingpin of the rear trailer  80  attaches to the dolly  50 .  
         [0223]    If only small turning angles are considered then THETA_S1 is approximately equal to tangent THETA_S 1, and THETA_D1 is approximately equal to tangent THETA_D1. The above formula then reduces to:  
         THETA —   S 1=(LENGTH —   R 1/LENGTH —   R 1+LENGTH —   D 1)*THETA —   D 1  
         [0224]    If the length of the rear trailer  80  is 30′ and the length of the entire vehicle assembly is 45′, the controller  49  must rotate the dolly axle 2 degrees for every 3 degrees of movement between the centerline of the forward trailer  40  and the dolly  50  centerline. If the steering ratio [(positive degrees of rotation in from the steering axle orientation sensor)/(positive degrees of rotation in from the front steering input sensor)] approaches zero, the maneuverability of the linked vehicles is improved at the expense of stability as the dolly  50  approaches the configuration of the standard Type A dolly.  
         [0225]    In the cornering modes, the Variable-Steering-Ratio Digital Dolly behaves as if it has steering arms that are crossed. When the forward trailer  40  turns to the right, this dolly  50  turns its steering axle to the left to swing wide around the corner. A negative steering ratio [(negative degrees of rotation produced in the steering axle orientation sensor when positive degrees of rotation are input from the front steering input sensor)] for these modes is not as critical as for the stability modes. It will be clear, however, that negative steering ratios that approach zero will produce less pronounced cornering capabilities but better stability as the mode again approaches the behavior of the standard Type A dolly. Steerable Type B behavior is produced if we let the steering ratio of the Variable-Steering-Ratio Digital Dolly approach negative infinity (infinite negative degrees of rotation in from the steering axle orientation sensor for one positive degree of rotation in from the front steering input sensor), that is, even the slightest turn causes a large correction and the dolly swings instantly into line behind the forward trailer. For this embodiment, some kind of device or mechanism must be used to force the dolly to move in the direction that its wheels are pointing because the required movement is so strongly against the natural tendency of the system.  
       Generality of Concepts  
       [0226]    The concepts involved in this invention are most easily explained by describing specific devices that “embody” or exemplify these concepts. An expert in the field will quickly see that, in almost all cases, the invention could easily be constructed using any device that performs the desired function. The description of any particular embodiment of the invention is not intended in any way to limit the invention to some particular embodiment, but only to assist the reader in understanding the concepts involved in this invention.