Patent Publication Number: US-2023140923-A1

Title: Bogie systems for autonomous and remote-piloted vehicles

Description:
INCORPORATION BY REFERENCE 
     An Application Data Sheet (ADS) is filed concurrently with this specification as part of the present application. Each application that the present application claims benefit of or priority to as identified in the concurrently filed ADS is incorporated by reference herein in its entirety and for all purposes. 
     BACKGROUND 
     Disclosed herein are various improvements to bogie systems for autonomous and remote-piloted vehicles, particularly four-wheeled vehicles that utilize two sets of two-wheeled bogies. A bogie is a structure that supports a set of two or more wheels. 
     SUMMARY 
     Details of one or more implementations of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages will become apparent from the description, the drawings, and the claims. 
     The concepts presented herein were developed in the course of designing autonomous wheeled delivery robots that feature a chassis that includes, among other things, a cargo storage area, power source (battery or batteries), controller, and sensors. The chassis of such robots is connected with two bogies, each of which supports a pair of wheels. The wheels are powered by hub motors within the wheels, although alternative approaches may place the drive motors for the wheels in the bogies themselves. The bogies are able to move relative to the chassis in order to facilitate travel by the robot over uneven terrain. While the concepts discussed herein were developed for autonomous delivery robots of a generally small size, e.g., similar in size to a shopping cart or baby carriage, it will be understood that such concepts may also be generally applicable to other apparatuses sharing a similar architecture, e.g., bogie-equipped apparatuses. Such alternative apparatuses do not necessarily need to be autonomous and can include remote-piloted vehicles and/or manned vehicles, including wheelchairs (which may be piloted by the occupant, remotely piloted, autonomously piloted, or operable under multiple such control schemes) and lawnmowers (which may be similarly pilotable). 
     The present disclosure is directed to a number of different enhancements to bogie structures for autonomous, remote-piloted, and live-piloted vehicles. Such a vehicle may have a pair of opposing bogies that may each support a pair of the vehicle&#39;s wheels. In some such vehicles, the bogies may be rotatably coupled with a chassis of the vehicle through a differential mechanism of some sort such that when one of the bogies rotates relative to the chassis, the other bogie is caused by the differential mechanism to rotate in the opposite direction (and vice versa). 
     In some such implementations, the differential mechanism may be an active mechanism, i.e., there may be one or more actuators that may be controlled so as to actively cause the two bogies to rotate in opposite directions responsive to receipt of one or more control signals. Such systems may, for example, allow the bogies to be actively controlled so as to allow the vehicle to raise one wheel up off the ground so as to allow the vehicle to mount an obstacle, such as a curb or step. 
     In some such systems with active differential mechanisms, the differential mechanism may be equipped with a slip mechanism that may allow for some limited amount of actuation of the differential actuator without any corresponding movement of the bogies before the slip mechanism engages and causes the torque or forces generated by the differential actuator to actually be transferred to the bogies, thereby causing their rotation in opposite directions. Such implementations allow for small amounts of movement of the bogies to be passively driven by the terrain that the bogies may traverse, e.g., small displacements due to surface roughness, small obstacles, etc., but also allow for the differential actuator to be selectively engaged when greater amounts of displacement are needed in one of the wheels in order to surmount an obstacle such as a curb. 
     In another version of such systems, the slip mechanism may be replaced by a specialized control algorithm that may cause a motor that is used as the differential actuator to be powered to a point where the differential does not move due to the motive force provided by the motor, but also where the motor provides no or almost no resistance to movement of the differential due to the application of forces or torques external to the motor. 
     In some implementations, the bogies of such vehicles may be equipped with steerable front and/or rear wheels, each such steerable wheel being caused to change its steering angle responsive to input provided by a corresponding steering actuator. The steering angle, as the term is used herein, refers to the angle by which a wheel is generally rotated from a neutral position in order to negotiate a turn, where the neutral position is the position the wheel would be in when the vehicle is travelling straight. Since each bogie has its own steering actuator(s) for its steerable wheel(s), this allows each steerable wheel to be independently controlled by a controller of the vehicle. This, in turn, allows for such vehicles to implement a variety of steering strategies that may provide performance benefits in the form of better handling, cornering, and high- and low-speed stability. 
     For example, in some implementations, the controller of the vehicle may be configured to cause the steering actuators of the bogies to control the steering angles of the steerable wheels in a turn such that the steerable wheels change their steering angles consistent with Ackermann steering geometry, e.g., such that the steerable wheels of the vehicle are each rotated to a steering angle that causes the rotational axes of the wheels of the vehicle to all meet at a single common point (when viewed from above). 
     In some additional such implementations, the controller may implement such Ackermann steering behavior in a closed-loop fashion that monitors the rotational state of each steerable wheel (with respect to the steering angle thereof) so that the steerable wheels maintain Ackermann steering geometry throughout the range of their travel. Such implementations may help ensure that the Ackermann steering geometry is maintained throughout a turn even if there are issues with the one or more of the steering actuators that cause one or more of the steerable wheels to not change its steering angle as expected. 
     In some implementations, the steering angles of the steerable wheels of such a vehicle may be controlled differently depending on the speed of the vehicle. For example, in some implementations, such a vehicle may switch between, for example, Ackermann steering geometry and one or more other types of steering geometry (parallel, reverse Ackermann, or a steering geometry in between any of those types of geometry). Such systems may allow such vehicles to change from Ackermann steering, which may be very suitable for low-speed maneuvering, at low speeds to parallel or reverse Ackermann steering, which may be more suitable than Ackermann steering at higher speeds, at higher speeds. 
     In some implementations, the steering actuators that are used to change the steerable wheels&#39; steering angles may, for example, be provided by rotational motors that drive a worm gear that engages with a pinion gear that is fixed with respect to an upright that supports a steerable wheel. Such implementations allow such vehicles to make very fine steering angle adjustments that may allow for more precise maneuvering, but also allow the steerable wheels to be “parked” in a particular position that makes it impossible for the vehicle to roll away without having at least one of the wheels skid across the ground. For example, such steering actuators may be actuated so as to cause two opposing steerable wheels to both be turned inward by the same angular amount, effectively making the vehicle pigeon-toed when parked. If worm gears are used in the steering actuator drivetrain, this makes it impossible to back-drive the steering actuator, making it impossible for the parked steerable wheels to accidentally straighten absent active participation of the controller and steering actuators—even when power to the vehicle is completely lost. This may further enhance the safety and security of such vehicles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various implementations disclosed herein are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings, in which like reference numerals refer to similar elements. 
         FIG.  1    depicts a rendering of an example autonomous vehicle that may be improved through implementation of one or more of the concepts discussed herein. 
         FIG.  2    depicts an isometric view of an example apparatus equipped with bogies 
         FIG.  3    shows the example apparatus of  FIG.  2    but partially exploded and from an opposite side. 
         FIGS.  4  and  5    show front and side views of the example apparatus of  FIGS.  2  and  3   . 
         FIGS.  6  through  8    depict various views of an example apparatus with a differential mechanism being actively controlled by a differential actuator so as to cause bogies of the apparatus to actively counter-rotate. 
         FIGS.  9  through  17    depict views of an apparatus equipped with twin bogies, similar to that discussed above with respect to  FIGS.  2  through  8   , during various stages of mounting an elevated feature, e.g., a curb. 
         FIG.  18    depicts a detail view of an example differential mechanism and an example slip mechanism. 
         FIG.  19    depicts a top view of the example differential mechanism and example slip mechanism of  FIG.  18   . 
         FIG.  20    depicts an alternate differential actuator arrangement. 
         FIG.  21    depicts an example of a worm-gear-driven steering system. 
         FIG.  22    depicts a flow chart for a technique for entering a park mode. 
         FIG.  23    depicts a flow chart for a technique for parking a vehicle when a battery charge level is low. 
         FIG.  24    depicts various steering angles in various parking configurations. 
         FIGS.  25  through  27    depict various example steering geometries. 
         FIG.  28    depicts a flow chart for a technique for performing speed-dependent steering mode selection. 
         FIG.  29    depicts a flow chart for a closed-loop steering control technique. 
         FIGS.  30 A and  30 B  depict a flow chart for a closed-loop wheel speed control technique. 
     
    
    
     The above-listed Figures are provided by way of example only and are not intended to be limiting. Other implementations of the concepts described herein and shown in the Figures will be evident to those of ordinary skill in the art reading this disclosure and are also considered to be within the scope of this disclosure. 
     DETAILED DESCRIPTION 
     Importantly, the concepts discussed herein are not limited to any single aspect or implementation discussed herein, nor to any combinations and/or permutations of such aspects and/or implementations. Moreover, each of the aspects of the present invention, and/or implementations thereof, may be employed alone or in combination with one or more of the other aspects and/or implementations thereof. For the sake of brevity, many of those permutations and combinations will not be discussed and/or illustrated separately herein. 
       FIG.  1    depicts a rendering of an example autonomous vehicle that may be improved through implementation of the one or more of the concepts discussed herein. As can be seen, the vehicle in question includes a chassis  128  that is connected with two bogies  102  that are each connected with two wheels  112 . The chassis  128  includes a cargo compartment  170  and a battery compartment  174 , and supports a plurality of sensors  172  that may include imaging sensors, lidar and/or radar sensors, proximity sensors, etc. that may provide data to an onboard controller that controls various systems of the vehicle, e.g., drive motors for the wheels  112 , steering actuators, differential actuators (if used), etc. The remaining Figures in this disclosure generally show the chassis as a relatively small housing that acts as a bridging structure between the two bogies, but it will be appreciated that the chassis may, as shown in  FIG.  1   , be much larger than the structure that is shown in the remaining Figures and may support or include various additional components, including, but not limited to, one or more of the specific example components or structures discussed above. 
       FIG.  2    depicts an isometric view of an apparatus  200  equipped with bogies;  FIG.  3    shows the same apparatus but partially exploded and from an opposite side. The apparatus  200  includes a chassis  228  that includes a differential mechanism  232  that has two rotatable parts  234 , each of which is rotatable relative to the chassis  228 . The differential mechanism  232  is configured such that rotational movement of one rotatable part  234  relative to the chassis  228  and about a common axis  240  causes the other rotatable part  234  to rotate relative to the chassis in the opposite direction and about the common axis  240 . 
     Each rotatable part  234  is connected with a corresponding bogie  202 , e.g., to a bogie structure  204  of the corresponding bogie  202 . The bogie structures  202  may, for example, be shallow V-shaped structures in which the rotatable part  234  connected therewith connects with the bogie structure  204  near the inflection point of the V, while wheels  212  may be connected with the ends of the V. Other types and shapes of bogie structure  204  may also be used in place of the depicted example. For example, while the depicted bogie structure  204  is shown as a single, rigid piece, other implementations may utilize bogie structures  204  that include multiple components or that incorporate some amount of articulation that allows the angle formed between the bogie structure  204  arms to change, e.g., to provide for vertical wheel travel or to raise or lower the center of gravity of the apparatus. 
     As can be seen in  FIG.  3   , two wheels  212  may be rotatably connected with each bogie  202 . For example, each bogie may have a first axle  208  and a second axle  210 . The first axle may be a steerable axle and may be connected with an upright  218  that may be caused to pivot about a steering axis through actuation of a corresponding steering actuator  220 . The opposing bogie  202  may have a similar configuration with a counterpart upright  218  and steering actuator  220 . It will also be understood that while the depicted implementation does not include such features, some vehicles according to the present disclosure may be configured such that all four wheels are steerable, e.g., the two bogies  202  may be equipped with uprights  218  and steering actuators  220  on both ends thereof, thereby allowing all four wheels to be steered. 
     The wheels  212  that are rotatably connected with each bogie  202  may each be rotatably connected with that bogie  202  by way of the first axle  208  and the second axle  210  thereof. In the depicted implementation, the first axle  208  and the second axle  210  are both fixed with respect to the upright  218  and the bogie structure  204 , respectively, and the wheels  212  each contain a drive motor  216  that may be actuated to cause that wheel  212  to rotate about a corresponding wheel axis  214  relative to the bogies  202 . Such drive motors may, for example, be hub motors. In other implementations, one or both of the first axle  208  and the second axle  210  may be rotatable about the corresponding wheel axis  214  relative to the bogies, and the drive motors  216  therefore may instead be located within the bogie structures  202 . 
     The differential mechanism  232  that is depicted is an active differential mechanism that is coupled with a differential actuator  242  by way of a differential worm gear  244  that drives a differential worm pinion gear  246 . The differential worm pinion gear  246  is rotatably coupled, in this case by a slip mechanism (discussed later below), with the differential mechanism  232 . In some implementations, other types of differential actuators  242  may be used, e.g., an electric motor, such as a stepper motor or a brushless direct-current (BLDC) motor, that may be directly coupled to the differential mechanism  232  (without the use of a worm gear, or with the use of a back-drivable gear), a linear actuator that may actuate a crank that is rotatably coupled with a gear in the differential mechanism so as to cause it to rotate, etc. It will be understood that in some implementations, a passive differential mechanism may be used. In such implementations, the differential actuator  242  may be omitted and the side gears and pinion gear of the differential mechanism (discussed further below) may not be actively driven at all. 
     The chassis  228  that is shown is a simplified representation; in actual practice, the chassis  228  may be much larger or attached to a much larger structure, e.g., a vehicle body containing a power source (such as a battery), the controller, various sensors used for navigation, one or more cargo compartments, etc. 
       FIGS.  4  and  5    show front and side views of the apparatus of  FIGS.  2  and  3   .  FIG.  4    shows the differential mechanism  232  in more detail. The depicted differential includes two side gears  236  that are each fixed with respect to a corresponding one of the rotatable parts  234 . A pinion gear  238  is rotatably mounted with respect to the chassis  228  so as to mesh with both side gears  236 ; when any of the two side gears  236  and the pinion gear  238  is caused to rotate relative to the chassis, the other two remaining gears will also rotate, thereby causing the two rotatable parts  234  to rotate in opposite directions. The side gears  236  and the pinion gear  238  may, as shown, be bevel gears, although other types of gearing that provide equivalent movement may be used instead if desired. 
       FIG.  5    shows the apparatus  200  in a “neutral” position, i.e., with both bogies  202 /bogie structures  204  positioned such that there is no angular displacement between the two bogies  202  (e.g., with the bogie structures aligned so that the wheel axes  214  of the wheels  212  connected with the first axles  208  are coaxial (assuming the steering angle of the steerable wheels is at zero degrees) and so that the wheel axes  214  of the wheels  212  connected with the second axles  208  are coaxial. This configuration generally represents the position of the bogies  202  when the apparatus is resting on level, flat ground. The chassis  228  in this configuration will generally be in a desired orientation, e.g., parallel (or nearly parallel) to the ground. 
       FIGS.  6  through  8    depict various views of the apparatus  200  with the differential mechanism being actively controlled by the differential actuator  242  so as to cause the bogies  202  to actively counter-rotate. The neutral positions of the bogies  202  are shown by dotted outlines  206 . Also shown in  FIGS.  6  through  8    is a surface  258 , e.g., a street, parking lot, sidewalk, floor, or other nominally flat structure.  FIG.  6    also depicts an elevated feature  260 , which may be a curb, step, other feature that is elevated with respect to the surface  258 . 
     As is evident from  FIGS.  6  through  8   , by causing the differential mechanism  232  to actuate so as to introduce a deliberate amount of counter-rotation in the bogies  202 , the bogies  202  may be caused to enter a state in which only three of the four wheels  212  are able to remain in contact with the surface  254 . The fourth wheel  212  is caused to be raised up off of the surface  258 . 
     It will be apparent that the center of gravity of such an apparatus, as well as the slope of the surface  258  on which the apparatus rests when approaching the elevated feature  260 , may affect which wheel  212  lifts up off of the surface  258 —for example, if the elevated feature  260  is at the bottom of a significant down slope, i.e., the surface  258  slopes down towards the elevated feature  260 , the wheel  212  that is lifted off of the surface  258  when the bogies are differentially actuated may actually be the wheel that is furthest from the elevated feature  260 , thereby preventing the apparatus from mounting the elevated feature. However, in most practical use cases, the surface  258  may generally be close enough to horizontal that such issues do not develop. For example, an apparatus such as is shown in  FIG.  1    may be able to operate on inclines of as much as 20° during normal rolling movement. However, the added tilt of the apparatus that may occur during elevated feature mounting by the apparatus may cause the apparatus to, if elevated feature mounting is attempted at the extreme limit of the normal operational incline range, either tip over or be unable to lift the correct wheel in order to attempt a mounting operation. However, the apparatus will still be able to mount elevated features from surfaces that are in lower incline ranges and may be able to mount an elevated feature, e.g., a curb, from a surface that is, for example, at ± ˜ 10° from horizontal. This discussion is provided simply as an example and is not intended to be viewed as limiting the disclosed bogie-equipped systems discussed herein to the particular performance characteristics discussed above. 
       FIGS.  9  through  17    depict views of an apparatus equipped with twin bogies, similar to that discussed above with respect to  FIGS.  2  through  8   , during various stages of mounting an elevated feature, e.g., a curb. 
     In  FIG.  9   , the apparatus has been caused, e.g., by a controller, to approach an elevated feature (a curb). The apparatus  900  has a chassis  928  that incorporates an active differential as discussed above. The controller of the apparatus has caused the apparatus to approach the elevated feature  960  such that the fore-aft centerline of the chassis  928  is at an oblique angle to the elevated feature  960 , e.g., at an oblique angle to the edge of the curb. The angle may generally be selected so as to be at least large enough that once the wheel  912  that is closest to the elevated feature  960  is caused to be raised up by the active differential, the apparatus  900  may be caused by the controller thereof to move closer to the elevated feature  960  such that the raised wheel  912  is able to be placed (or at least partially placed) over the elevated feature  960  before the next-closest wheel to the elevated feature  960  contacts the elevated feature  960 . Ideally, the angle may be such that the center of the raised wheel  912  is able to be moved over the elevated feature  960  before the other wheel  912  associated with that same end of the chassis contacts the elevated feature  960 . For a vehicle having proportions similar to that shown, such an angle may be selected to be approximately within 15° to 40° of an axis that is parallel to the surface  958  and nominally perpendicular to the elevated feature  960  (curb). However, such an approach angle may change depending on the particular design of the apparatus. For example, an apparatus with a larger track (transverse spacing between wheels  912 ) may be able to accommodate a much larger angular range for the approach angle. 
     It will be understood that the apparatus  900  may be caused to approach the elevated feature  960  such that either the left wheel  912  or the right wheel  912  associated with a first end  930  of the apparatus, e.g., the front end (although in some implementations, the first end may be the back end of the apparatus), is the closest wheel  912  to the elevated feature. The bogie that supports the wheel  912  that is closest to the elevated feature may be referred to herein as the “leading bogie,” while the other bogie may be referred to as the “trailing bogie.” It will be understood that the leading and trailing bogies may be selected to be different bogies by the controller depending on what approach angle is adopted by the apparatus when approaching the elevated feature  960 . Generally speaking, the controller may be caused to select one of the two bogies to be the leading bogie and the other of the two bogies to be the trailing bogie based on which bogie has (or will have) a wheel  912  that is closest to the elevated feature  960  once the apparatus is positioned in front of the elevated feature  960 . It will also be understood that each bogie may have a “leading” wheel and a “trailing wheel”; the leading wheel of each bogie is the wheel of that bogie that will be the first of the two wheels of that bogie that mount the elevated feature, while the trailing wheel of each bogie is the wheel of that bogie that is the last of the two wheels of that bogie that mount the elevated feature. 
     In  FIG.  10   , the differential actuator has been caused by the controller to actuate so as to cause the leading bogie to rotate such that the wheel  912  associated with the first end  930  that is closest to the elevated feature  960  (the left front wheel in this example) is caused to move closer to the chassis  928  and such that the trailing bogie is caused to rotate in the opposite direction such that the other wheel  912  associated with the first end moves away from the chassis  928 . This causes the left front wheel to raise up off the surface  958 , leaving the apparatus solely supported by the right front wheel  912  and the two rear wheels  912 . The raised wheel  912  has been caused to be raised high enough that the underside of the raised wheel  912  is higher than the upper edge of the elevated feature  960 . However, in some instances, the raised wheel may be at a somewhat lower elevation such that the lowest part of the raised wheel  912  is lower than the upper edge of the elevated feature—the raised wheel may, in at least some such cases, still be high enough for the apparatus to eventually mount the elevated feature  960 . 
     In  FIG.  11   , the controller has caused the apparatus  900  to move closer to the elevated feature  960 . In doing so, the raised wheel  912  of the leading bogie is caused to advance onto or over the elevated feature  960 . The apparatus may, for example, be caused to move towards the elevated feature  660  until the opposing wheel  912  on the trailing bogie (or both the opposing wheel  912  on the trailing bogie and the trailing wheel on the leading bogie) contacts or becomes proximate to the side of the elevated feature  960 , as shown in  FIG.  11   . 
     In  FIG.  12   , the controller has caused the bogies to be actuated in the reverse direction, e.g., so as to cause the left front wheel  912  to move away from the chassis  928  (and towards the elevated feature  960 ) and the right front wheel  912  to move towards the chassis  928 . In some cases, the actuation of the differential shown in  FIG.  12    may be performed while the right front wheel  912  is being actively pushed into contact with the elevated feature  960  by the drive motors of the wheels, thereby causing the right front wheel to “drive up” the side of the elevated feature  960  (assisted by the actuation of the differential by the differential actuator). 
     In  FIG.  13   , the controller has caused the apparatus  900  to move even closer to the elevated feature  960 . In doing so, the raised leading wheel  912  of the trailing bogie is caused to advance onto or over the elevated feature  960 . The apparatus may, for example, be caused to move towards the elevated feature  660  until the trailing wheel  912  on the leading bogie contacts or becomes proximate to the side of the elevated feature  960 , as shown in  FIG.  13   . 
     In  FIG.  14   , the controller has caused the bogies to be further actuated in the reverse direction, e.g., so as to cause the left front wheel  912  to move further away from the chassis  928  (and towards the elevated feature  960 ) and the right front wheel  912  to move more towards the chassis  928 , thereby causing the trailing wheel  912  on the leading bogie to rise up off the surface  958  to an elevated position in preparation for mounting the elevated feature  960  while the two leading wheels  912  and the trailing wheel  912  of the trailing bogie support the apparatus  900 . 
     In instances in which the trailing wheel  912  of the leading bogie and the leading wheel  912  of the trailing bogie both contact the elevated feature  960  at the same time, the apparatus may be caused to drive into the elevated feature  960  while actuating the differential so as to perform both rotations of the bogies shown in  FIGS.  12  and  14    in a generally continuous fashion, thereby causing both the trailing wheel  912  of the leading bogie and the leading wheel  912  of the trailing bogie to simultaneously climb up the side of the elevated feature  960  and both generally mount the elevated feature simultaneously. 
     In  FIG.  15   , the controller has again caused the apparatus  900  to move even closer to the elevated feature  960 . In doing so, the raised trailing wheel  912  of the leading bogie is caused to advance onto or over the elevated feature  960 . The apparatus may, for example, be caused to move towards the elevated feature  660  until the trailing wheel  912  on the trailing bogie contacts or becomes proximate to the side of the elevated feature  960 , as shown in  FIG.  15   . 
     In  FIG.  16   , the controller has caused the bogies to be actuated in the original direction (e.g., similar to the rotation shown in  FIG.  10   ), e.g., so as to cause the left front wheel  912  to move closer to the chassis  928  and the right front wheel  912  to move away from the chassis  928 , thereby causing the trailing wheel  912  on the trailing bogie to rise up off the surface  958  to an elevated position in preparation for mounting the elevated feature  960  while the two leading wheels  912  and the trailing wheel  912  of the leading bogie support the apparatus  900 . 
     In  FIG.  17   , the controller has again caused the apparatus  900  to move fully onto the elevated feature  960 . Having successfully mounted the elevated feature  960 , the apparatus may now proceed to continue on to its destination. 
     In implementations that feature an actively controllable differential for the bogies, it may be desirable to allow for some amount of “passively driven” bogie movement to occur without necessarily causing deliberate contra-rotation of the bogies. For example, a passive differential mechanism may be used in bogie-equipped apparatuses to allow such apparatuses to drive more smoothly across bumpy ground (e.g., gravel, cracks, pavement seams), small vertical features (such as where a sloping curb cut may not join with the street in a perfectly flush manner), etc. The passive counter-rotation of the bogies acts to cause the wheels of the apparatus to remain more fully engaged with the ground even when one of the wheels is, for example, kicked up due to running over a pebble or other small obstacle. The differential mechanism thus acts as a sort of mechanical smoothing filter that mitigates the effects that may be caused when the apparatus traverses small discontinuities or surface imperfections that might otherwise cause the apparatus to bounce or skip in a manner that may make the apparatus more difficult to control (or which may be more harmful to contents of the apparatus being transported). 
     Apparatuses with actively controllable differentials may be designed so as to still provide the features of a passive differential mechanism, thereby allowing the apparatus to enjoy the benefits of a passive differential mechanism while also enjoying the benefits of an active differential mechanism, e.g., such as being able to surmount elevated features such as curbs. 
     Two example systems or approaches for providing hybrid active/passive differential systems are discussed below—one is a passive mechanical system and the other uses an algorithm to control the differential actuator so as to cause the differential mechanism to behave in a manner similar to a passive differential mechanism. 
     The passive mechanical system that may be used to enable such a hybrid approach may be implemented through the use of a slip mechanism, which was briefly mentioned earlier.  FIG.  18    depicts a detail view of the portion of the apparatus  200  located within the dashed circle in  FIG.  2    but with portions of the chassis  228  cut away to reveal various internal features not visible in  FIG.  2   . More clearly visible in  FIG.  18    is the slip mechanism  248 . The slip mechanism  248  includes a positive stop  250  that protrudes from a shaft  254  that is driven by the differential actuator  242 . The pinion gear  238 , in this case, includes a cylindrical body that encircles the shaft  254  directly behind the gear teeth. The cylindrical body has a missing arcuate sector that is large enough that the positive stop  250  is able to rotate for some distance before it encounters either radial wall of the arcuate sector, which provide stop surfaces  252  (only one is visible, but another exists on the opposite side of the missing arcuate sector). 
       FIG.  19    depicts a top view of the differential mechanism of  FIG.  18    with only the pinion gear  238 , side gears  236 , stop structure  250 , and the shaft  254  shown. As can be seen, there are actually two diametrically opposed stop structures  250  shown—the second stop structure  250  is a duplicate of the first, although this may not necessarily always be the case. The pinion gear  238  has raised stop features  256  that each extend around the shaft  254 . The raised stop features  256  are both angular sector shapes that extend through approximately  ˜ 150° of arc in this case. This allows, in this particular case, for approximately a maximum of 30° of rotation (θ 1 +θ 2 ) to occur between the shaft  254  and the pinion gear  238  before the stop structures  250  come into contact with any of the stop surfaces  252 . 
     When an actuated differential such as the one shown in  FIG.  19    is used in a bogie-equipped vehicle such as those discussed herein, the differential actuator may be caused to rotate such that each positive stop  250  is located midway between the two stop surfaces  252  when the bogies are in the “neutral” position. This allows for a maximum amount of travel to occur in both rotational directions before the stop structure  250  engages with either of the stop surfaces  252 . Thus, the differential mechanism shown will act as a passive differential mechanism up until the point where the positive stop(s)  250  engage with the stop surface(s)  252 . However, the range of motion permitted by the slip mechanism may be quite large, thereby allowing for any realistic amount of passive contra-rotation of the bogies to occur during normal terrain traversal. 
     When it is desired to engage in abnormal terrain traversal, e.g., such as requiring the apparatus to mount an elevated feature that it cannot simply or safely drive over, the differential actuator may be actuated so as to cause the slip mechanism to travel to the limit of its “slip range” such that the positive stop(s) engage with the stop surface(s). Further actuation of the differential actuator will then cause the differential mechanism to actively cause the two bogies to contra-rotate relative to each other (and the chassis), thereby allowing for movements such as those discussed above with respect to  FIGS.  9  through  17   . 
     It will be understood that other types of slip mechanisms  248  may be used as well, including ones in which the features discussed above are reversed (the positive stop(s)  250  are fixed with respect to the pinion gear  238  and the stop surfaces  252  are fixed with respect to the shaft  254 ), ones in which the positive stops  250  extend in a direction parallel to the rotational axis of the shaft  254  (instead of radially with respect to the shaft  254 ). 
     It will also be acknowledged that while the slip mechanism  248  and the differential actuator  242  are shown as being configured to provide motive input to the pinion gear  238 , other configurations may be arranged so as to provide a motive input to one of the side gears  236 . As a first one of the side gears  236 , the pinion gear  238 , and the other one of the side gears  236  are all meshed in series to provide an intermeshed gear train that causes all of the gears to turn in unison, the motive input that is used to actuate the differential may be applied to any of the side gears  236  or the pinion gear  238  (or to any of the components that are fixed with respect to one of those gears) to equal effect. Similarly, the slip mechanism  248 , if used, may be implemented so as to allow such motive input to be communicated to whatever part is to be rotationally driven while still allowing the driven part to remain stationary for some amount of motive input. 
     The slip mechanism shown in  FIGS.  18  and  19    is configured such that it is impossible to back-drive the differential actuator through rotation of the bogies relative to the chassis. The use of a worm gear on the output of the differential actuator prevents rotation of the pinion gear  236  from being transmitted to the differential actuator. Such a mechanism, however, may be very well-suited to use in an active differential since worm gears provide a very high torque multiplier effect that may allow a relatively small differential actuator to cause the bogies to contra-rotate (which will also involve lifting the weight of the apparatus somewhat). 
       FIG.  20    depicts an alternate differential actuator arrangement in which a differential actuator  2042  has an output shaft  2054  that is fixed with respect to a pinion gear  2038  that drives (or is driven by) two side gears  2036 . Each side gear is fixed with respect to a corresponding rotatable part  2034 . The rotatable parts  2034  and the shaft  2054  may all be rotatable with respect to a chassis  2028 . The depicted differential mechanism of  FIG.  20    is back-drivable, i.e., if sufficient torque is applied to either rotatable part  2034  relative to the chassis  2028 , the torqued rotatable part  2034  will rotate and cause the side gears  2036  and the meshed pinion gear  2038  to rotate, thus causing the differential actuator to be back-driven (caused to move through the application of external torque as opposed to through the application of electrical power). 
     Back-drivable differential mechanisms such as that shown in  FIG.  20    may be used to provide a virtual slip mechanism that uses a motor control algorithm to provide performance that simulates a passive differential mechanism despite the pinion gear  2038  being fixedly mounted to the output shaft of the differential actuator (which may be an electric motor, e.g., a stepper motor, BLDC motor, or servo motor). 
     For example, a controller of an apparatus that features a differential mechanism such as that shown in  FIG.  20    may be configured to cause the differential mechanism to operate in a mode in which the differential mechanism is to operate in a manner similar to a passive differential mechanism. When operating in such a mode, the controller may monitor information received from one or more sensors, e.g., rotational encoders or other similar sensors, that may provide information on the rotational state of one or both rotatable parts (and thus the bogies) and, based on such information, cause the differential actuator to be placed in one of at least two states. 
     In a first state, a sinusoidal electrical current is supplied to the differential actuator at an amplitude that is just slightly below the current level needed to actually cause the differential actuator to start moving. This current level may be thought of as the “starting current” that is necessary to a) overcome whatever frictional loads may exist in the various rotational interfaces that exist in the differential gear train and the rotatable parts and b) cause the bogies to rotate so as to cause at least some of the load on one of the wheels of the apparatus to be decreased. By supplying a sinusoidal current that has an amplitude just below this current level, the differential mechanism is, in effect, primed so that any small change in torque (in either direction) that is applied to the rotatable parts will cause the differential actuator to move (the sinusoidal application of current ensures that regardless of direction, there will generally always be a half-cycle of the sinusoidal current in which the direction of the torque applied by the differential actuator lines up with the direction of the externally applied torque—when this happens, the differential actuator will start to rotate). Such sinusoidal currents may be applied, for example, at a frequency that is based on the current apparatus speed or a potential maximum speed of the apparatus. For example, a frequency of  ˜ 100 Hz or more, or  ˜ 400 Hz or more, may be used for apparatuses that are traveling at or have a maximum speed of 3 m/s or less. In many implementations, such sinusoidal currents may have an amplitude that is less than the starting current but within 90% or 95% of the starting current. 
     In a second state, the current that is caused to be supplied to the differential actuator is changed to a non-sinusoidal, e.g., unidirectional, current that is set to be slightly less than the amount of current needed to keep the differential actuator in motion responsive to the application of a small external torque to one of the rotatable parts. This may, for example, be a current level that changes depending on the degree to which the bogies have been actuated. The direction of the current may also be selected so as to cause motive torque from the differential actuator to be applied to the rotatable parts in the same direction as the direction of rotation that the external torque is urging. 
     In the first state, the output of the differential actuator is thus caused to generally remain (barely) stationary unless an external torque is delivered to it, e.g., by one of the bogies being jostled due to encountering an obstacle. In the second state, the output of the differential actuator is caused to apply torque to the rotatable parts that is just below the amount of torque needed to keep or maintain the rotatable parts in motion. The amount by which the current that is supplied to the differential actuator is caused to undershoot the different target current ceilings (e.g., the “starting” current and the “in-motion” current) may depend on various factors, although in some implementations, setting the amplitude or magnitude of the current to be less than 100% but greater than or equal to 90% or 95% of the relevant target current ceiling may provide such an effect. Thus, in both states, the differential actuator is caused to remain in a state where it provides almost no resistance to external torques applied to the differential, e.g., from rotation of the bogies relative to the chassis. This, in effect, causes the differential mechanism to generally act as if the differential actuator were not present when the passive differential mode is engaged. 
     In practice, when the controller is operating the differential actuator in passive differential mode, the controller may cause the differential actuator to switch between the first state and the second state responsive to various conditions. 
     For example, in some implementations, the controller may initially place the differential actuator in the first state, e.g., by causing a sinusoidal current to be provided to the differential actuator. The controller may then monitor the information regarding the rotational state of the rotatable parts that are connected with the differential mechanism to determine by how much the rotatable parts have rotated from their rotational positions when in the neutral position. When the rotatable parts (and thus the bogies connected therewith) have rotated from the neutral position by an amount that exceeds a first threshold amount (in either direction from the neutral position), then the controller may place the differential actuator in the second state, e.g., by causing a non-sinusoidal current to be provided to the differential actuator so as to cause rotation of the differential actuator in the same direction as the rotation of the bogies is causing the differential actuator to rotate in. The controller may further monitor the rotational displacement of the rotatable parts from their neutral positions and may in some implementations, for example, cause the differential actuator to re-enter the first state when the measured rotational displacements show that the rotatable parts are engaged in motion that would return them to the neutral position. For example, when the bogies are caused to rotate by external forces applied to the wheels by imperfections in the surface on which the apparatus is driving, the bogies may be first caused to rotate away from the neutral position in opposite directions. However, at some point the external forces may be removed (e.g., after the apparatus has driven over the imperfections), thereby removing the forces that keep the bogies in the angularly displaced state. The bogies may then start to return to the neutral state due to gravitational loading—when this occurs, the rotational displacements of the rotatable parts relative to their neutral position orientations will decrease, which will be observable from the information regarding the rotational state. During the rotational motion of the bogies that causes them to return to their neutral state, the controller may cause the differential actuator to enter the first state. Alternatively, the controller may cause the differential actuator to remain in the second state during such restorative motion, but the direction in which the controller causes the differential actuator to actuate may be reversed so as to somewhat urge the bogies to return to their neutral positions. 
     In another implementation, the controller may use the information regarding the rotational state of the rotatable parts to determine a rotational velocity or rate of the rotatable parts. When the rotational velocity or rate of the rotatable parts is below some threshold amount, then the controller may cause the differential actuator to operate in the first state. However, when the rotational velocity or rate of the rotatable parts is above the threshold amount, then the controller may cause the differential actuator to operate in the second state. 
     In view of the above discussion, it will be understood that actively controlled differential mechanisms, if used, may be equipped with either mechanical slip mechanisms or software-based electromechanical slip mechanisms (in the form of a back-drivable motor that is controlled as discussed above). Either slip mechanism may be used to provide an actively controllable differential that may be switched between a mode in which the differential mechanism acts as a “passive” differential mechanism (responsive only to inputs received by way of rotations of the bogies caused by externally applied forces) and a mode in which the differential mechanism is an “active” system that can be actuated so as to deliberately cause a certain amount of contra-rotation in the bogies. If an actively controlled differential mechanism is used that utilizes the control scheme discussed above to cause the differential mechanism to behave as a “passive differential mechanism,” the current that is supplied to the differential actuator when the differential mechanism is to be operated as an “active differential mechanism” may be greater than the starting current and sufficient to overcome the torque exerted on the bogies, for example, due to the weight and center of gravity of the apparatus. 
     The discussion above has largely focused on differential mechanisms and control schemes therefor for bogie-equipped apparatuses. However, as discussed above, bogie-equipped apparatuses may also feature independently controllable steering actuators that may be used to provide various steering enhancements that are not possible with traditional steering systems (in which left and right steerable wheels have steering angles controlled by a single steering input). 
     As discussed earlier with respect to  FIG.  3   , some bogie-equipped apparatuses may have steering systems in which a linear actuator (such as steering actuator  220  in  FIG.  3   ) is controlled so as to extend or retract from a neutral position so as to push or pull on a crank arm is connected with (or part of) an upright (such as upright  218 ), thereby causing the upright, which may be pivotally mounted, to rotate about a steering axis. Each upright may have a first axle that is connected therewith such that when the upright is rotated, the first axle connected therewith rotates as well. Such an arrangement is somewhat similar to that used in a traditional vehicle steering system, e.g., where left-right linear translation of a steering rack is transferred to the crank arms of uprights on both sides of the vehicle so as to cause similar rotation of the uprights. However, in the traditional steering system, the rotation of both uprights is linked to the common movement of the steering rack. In the system discussed above, however, each steering actuator may be individually controlled, thereby allowing the steering angles of the steerable wheels to be independently controlled. 
     Another example steering actuator system is shown in  FIG.  21   . In the example of  FIG.  21   , the steering actuator  2120  includes a steering drive motor  2122  and a steering worm gear  2124  that are housed within a bogie structure  2104 . Instead of having a crank arm, the upright  2118  may have a steering pinion gear  2126  that is fixed with respect to the upright  2118 . A wheel  2112  may be rotatably coupled to the upright  2118 , e.g., by way of a hub motor  2116 . The use of a worm gear arrangement in the depicted steering system is somewhat counterintuitive, however, as worm gears are generally not back-drivable and also generally have slower response times due to their low gear ratios. Because of this, the use of a worm gear in a steering mechanism would typically be avoided in vehicles since this could result in the steering angles of the wheels becoming locked in a particular orientation should the steering actuator lose power. In a manned vehicle, this could have catastrophic consequences for the occupant(s) since the vehicle may be unable to steer to avoid an obstacle or another oncoming vehicle. Additionally, the low gear ratio of worm gear drive systems would require electronic control of the worm gear drive that would, in effect, multiply a single turn of the steering wheel into a much larger number of worm gear rotations, e.g.,  10  to  20 , for example. Any failure of the electronic control that implements this multiplication effect would necessarily render the steering system useless since the driver would be unable to control the steering system. In the vast majority of manned vehicles, the steering mechanism is mechanically coupled to the steering input (steering wheel) such that rotation of the steering wheel directly causes, even if there is no electrical power available, the steering system to actuate—this allows such vehicles to be steered even in the event of complete electrical power failure. 
     In the context of unmanned autonomous vehicles, however, the use of worm gear-driven steering actuators may present less of a risk since there are no occupants in the vehicle. Moreover, many such vehicles may also generally be much smaller than a manned vehicle and operated at generally lower speeds, e.g., 1 m/s to 3 m/s (e.g., on the order of a pedestrian&#39;s walking or running speed), thus presenting a lower risk to other vehicles and/or pedestrians should there be a steering system failure. The use of a worm gear drive in the steering system also allows such autonomous vehicles to use lower-power steering actuators (thus extending battery life) and to implement unique steering system actuation paradigms that may be implemented in order to place such apparatuses in a “park” mode that may be more effective at preventing unintended movement of the apparatus. 
       FIG.  22    depicts a flow chart of a parking technique that may be used, for example, with the worm-gear driven steering system discussed above. In block  2202 , a signal may be received by a controller of a vehicle having independently steerable wheels that indicates that the vehicle is to enter a “park” mode. Once such a signal is received, the technique may proceed to block  2204 , in which a determination may be made as to whether or not the vehicle (or apparatus) is in motion. 
     If the apparatus is determined in block  2204  to not be in motion, then the technique may proceed to block  2206 , in which the controller may cause the steering actuators to actuate so as to cause two opposing steerable wheels to enter into a “toe-in” or “toe-out” state. The toe-in and toe-out states are actually similar states, but viewed from ends of the apparatus. In either state, at least one of the steerable wheels is steered to a non-zero steering angle such that the rotation axes of the steerable wheels, when viewed from above, cross each other at a location that is inboard of the steerable wheels. For example, one steerable wheel may actually be kept straight (a steering angle of 0°) and the other steerable wheel steered to any non-zero angle (although preferably to a steering angle of at least 5 degrees and more preferably to whatever the maximum steering angle limit is, or close to it, e.g., within 5 degrees of the maximum steering angle possible given the rotational constraints of the steering mechanism). In most cases, however, both opposing steerable wheels may be steered so as to have non-zero steering angles.  FIG.  24    shows six examples of different toe-in and toe-out steering configurations—examples (A) through (C) are examples of (A) a toe-in state in which only one steerable wheel is steered to a non-zero steering angle, (B) a toe-in state in which both steerable wheels are steered to non-zero steering angles, and (C) a toe-out state in which both steerable wheels are steered to non-zero steering angles. As can be seen, the black dots located at the intersections of the dash-dot-dash lines in each example show where the rotational axes (represented by the dash-dot-dash lines) of the steerable wheels cross each other when viewed from above. Examples (D) through (F) are similar examples of such steering configurations but implemented in an apparatus that has four-wheel steering. Thus, example (D) features two wheels that have been steered to non-zero steering angles and two wheels that have been kept stationary at 0° steering angle, example (E) features both fore and aft wheels that have been steered so as to be toe-in, and example (F) features both fore and aft wheels that have been steered so as to be toe-out. Other permutations of such steering configurations may be used as well. It will also be appreciated that the steering angles used for each wheel in examples (B), (C), (E), and (F), while shown as being equal in magnitude, may also be different for two or more of the wheels, e.g., 15° for one wheel and 30° for the opposing wheel. As can be seen, the locations where the wheel axles cross one another when viewed from above are, in each depicted example, inboard of the steerable wheels. 
     If it is determined in block  2204  that the apparatus is in motion, then the technique may instead proceed to block  2208 , in which the controller may control the drive motor(s) to stop providing power to one or more of the wheels (if such power is being provided) and/or cause a braking system of the apparatus to engage, thereby causing the apparatus to stop. In some implementations, the drive motors may also serve as the braking system, e.g., power supplied to the drive motors may be switched off and a resistive load may instead be applied across the terminals of the drive motor that previously were used to provide electrical power to the drive motor, thereby turning the drive motor into a generator and causing the kinetic energy of the apparatus to be converted into electrical power that is dissipated through the resistive load. The resistive load may, in some implementations, be provided by way of a battery charging circuit such that such kinetic energy may be at least partially recaptured to help recharge the battery or batteries. Regardless, the controller may cause the apparatus to stop in block  2208  if the apparatus is determined to be in motion in block  2204 . 
     If the above-discussed technique is implemented in an apparatus featuring a worm-gear driven steering system, such as that discussed above with respect to  FIG.  21   , the worm gear may, in effect, act to lock the wheels in the park configuration, making it impossible to straighten the wheels out unless the controller causes the steering actuator(s) thereof to actuate them out of the toe-in or toe-out configuration. This may be particularly advantageous in the context of an autonomous vehicle, as such a system may be used to park the apparatus and keep it stationary without needing to consume any electrical power to maintain the parked state. For example, there may be no need to continue providing power to the drive motors to keep them locked in a particular state. Additionally, the self-locking nature of a worm-gear-driven steering actuator also allows the parked state to be maintained even when all power to the apparatus is lost. Such a feature may prevent unintended unpowered movement of the apparatus that may present a hazard to pedestrians or property. 
       FIG.  23    depicts an example flow chart for a technique for engaging parking mode in an apparatus responsive to a low battery event. In block  2302 , the controller of such an apparatus may receive battery charge level information (or battery health information—generally any information that may be used by the controller to determine if a loss of power may be imminent). The information that may be received in block  2302  may also or alternatively be any information that may indicate a potential fault state or system or component failure that may present a safety issue if the apparatus continues to attempt to operate in a normal fashion despite the fault state or system failure. For example, information indicative of a battery failure (short circuit, thermal runaway, low charge level, etc.), sensor failure (e.g., indicating a non-responsive sensor, a saturated sensor (one that is at the limit of its sensing capability), a sensor that is reporting values that appear to be erroneous or inaccurate given the context that the apparatus is in, etc.—such can be with respect to tilt sensors, accelerometers, gyroscopic sensors, GPS sensors, lidar sensors, radar sensors, proximity sensors, light sensors, imaging sensors, etc.), motor failure (a motor that does not appear to be responding or responding slower than expected, a motor that is exerting an unexpected amount of torque (either above or below an expected torque range), etc.—such can be with respect to drive motors for the wheels, differential actuator motors, steering motors, etc.), processor or memory failures, software failures, etc. may be used to trigger engagement of a parking mode. In block  2304 , a determination may be made as to whether the battery charge level is below a threshold level (or if the information obtained in block  2302  indicates that the apparatus should or should not be entered into a park mode). If not, then the technique may return to block  2302  and another indication of the battery charge level may be obtained. However, if the battery charge level is below the threshold level, then the technique may proceed to block  2306 , in which a signal may be generated to engage park mode, e.g., a signal such as is received in block  2202  of  FIG.  22   . The technique of  FIG.  22    may then be implemented to park the apparatus. 
     The parking technique discussed above may also be practiced with non-worm-gear-driven steering actuation systems, although such implementations may not offer as many advantages as may be obtained with practicing such techniques using worm-gear-driven steering systems. For example, such alternative actuation systems may be back-drivable, making them less able to maintain the parking state when power to the steering actuation system is removed. 
     The use of independent steering actuation systems for each steerable wheel also permits particular steering techniques to be implemented in bogie-equipped apparatuses. For example, in some implementations featuring steerable wheels with independently controllable steering actuators, such actuators may be controlled differently based on the speed of the vehicle. 
     In a traditional steering mechanism, the steering angles adopted by two opposing steerable wheels are linked such that when either steerable wheel is steered to a particular steering angle, the other steerable wheel will always be caused to steer into a single, corresponding steering angle (which may or may not be the same steering angle). Thus, each steerable wheel&#39;s steering angle is a function of the other steerable wheel&#39;s steering angle. Depending on the particular steering geometry used in such traditional steering systems, the steering angles produced by a traditional steering mechanism may fall into several categories.  FIGS.  25  through  27    depict various examples of different categories of steering angle paradigms that may be implemented in steering systems. 
       FIG.  25    depicts an example of what are commonly described as Ackermann steering angles. In an Ackermann steering geometry system, the steering linkage is designed so that when one of a pair of steerable wheels is turned to a particular steering angle, the other wheel of the pair of steerable wheels is caused to turn by a corresponding steering angle such that the rotational axes of the two steered wheels cross each other, when viewed from above, at a location that—in the ideal case—lies along the rotational axes of the non-steerable wheels (when viewed from above). Thus, all four wheels in an Ackermann steering geometry system will be rolling along concentric circular paths that are all centered on the point where the rotational axes of the wheels cross when viewed from above. Such an arrangement generally allows each wheel to roll without slipping during a turn, although dynamic loading during higher-speed turns may cause Ackermann steering geometry to be less effective. The steering geometry shown in  FIG.  25    is representative of “true” Ackermann steering, but any steering geometry in which the steering angle of the “outer” steerable wheel (furthest from the center of the turn—angle α in  FIG.  25   ) is less than the steering angle of the “inner” steerable wheel (angle β in  FIG.  25   ) may be referred to as an Ackermann steering geometry. 
       FIG.  26    depicts an example of what is commonly described as a parallel steering arrangement in which two opposing steerable wheels are caused to steer by the same amount and in the same direction (e.g., angle α in  FIG.  26    equals angle β in  FIG.  26   ). Thus, the rotational axes of the steerable wheels will generally remain parallel for all steering angles of the steerable wheels. 
       FIG.  27    depicts an example of what is commonly called reverse Ackermann steering. In this steering geometry, the steering mechanism is configured such that the steering angle of the “outside” wheel (angle α in  FIG.  27   ) is greater than that of the “inside” wheel (angle β in  FIG.  27   ). Such a steering geometry may be more effective in steering at higher speeds due to speed-dependent dynamic loading on the vehicle wheels. 
     In traditional steering mechanisms, the particular steering geometry that is implemented by a steering mechanism is generally fixed by the structure of the steering mechanism. Thus, for example, a steering mechanism that is set up to provide Ackermann steering angles will not be capable of providing parallel steering or reverse Ackermann steering geometries. Moreover, different steering geometries have different advantages and disadvantages under different circumstances, e.g., at different speeds and/or turn radii. Since the steering geometry that is provided by a particular traditional steering mechanism is essentially built-in to that steering mechanism, such steering mechanisms typically have particular “sweet spots” where their steering performance is at a maximum and their steering performance outside of those sweet spots may be reduced. In systems such as those discussed herein in which the steering angle of each steerable wheel may be separately controlled, the steering geometries that the controller causes to be adopted by the steerable wheels may be dynamically changed based on, for example, the vehicle&#39;s speed, thereby allowing a steering geometry to be selected and used that is most well-suited to the speed at which the vehicle is traveling. 
       FIG.  28    depicts a flow chart of a technique for implementing dynamic steering geometry selection in an apparatus such as those discussed herein. In block  2802 , a speed of the apparatus may be determined, e.g., based on wheel speed or other sensors (such as radar, imaging sensors, light detection and ranging (LIDAR), etc.). In block  2804 , a determination may be made as to whether the speed of the apparatus from block  2802  is within a first speed range. If it is determined in block  2804  that the speed of the apparatus is within the first speed range, then the technique may proceed to block  2806 , in which the controller may cause the steering actuators to be controlled according to a first steering mode before returning to block  2802 . If it is determined in block  2804  that the speed of the apparatus is not within the first speed range, then the technique may proceed to block  2808 , in which a determination may be made as to whether the speed of the speed of the apparatus from block  2802  is within a second speed range. If it is determined in block  2808  that the speed of the apparatus is within the second speed range, then the technique may proceed to block  2810 , in which the controller may cause the steering actuators to be controlled according to a second steering mode before returning to block  2802 . 
     It will be understood that the technique may involve any number of decision/speed evaluation blocks such as blocks  2804  or  2808 , with each block comparing the measured speed against a predetermined speed range and, if the speed falls within the associated speed range, causing the controller to control the steering actuators according to a particular steering mode that corresponds with that speed range. Ultimately, if the speed does not fall within any of the specific ranges evaluated, the controller may cause the steering actuators to be controlled according to a default steering mode or, alternatively, the ranges may be selected so as to cover all potential speeds of the vehicle such that there is an explicitly defined steering mode associated with each possible speed (as opposed to there being a “default” steering mode that is applied if the speed falls within none of the evaluated speed ranges). 
     There may, as shown in  FIG.  28   , be three different steering modes (where N=3), as few as two steering modes (in which case blocks  2808  and  2810  may be omitted), or four or more steering modes. Each of the steering modes, when active, may cause the controller to actuate the steering actuators to produce a different type of corresponding steering geometry. For example, if the first speed range is between 0 and 10 mph, the first steering mode may cause the controller to actuate the steering actuators so as to produce steering angles consistent with an Ackermann steering geometry when the vehicle speed is between 0 and 10 mph and then cause the controller to actuate the steering actuators to produce steering angles consistent with a negative Ackermann steering geometry when the vehicle speed is greater than 10 mph. 
     It will be understood that there may be any number of steering modes that the controller may be caused to engage depending on the speed of the apparatus. For example, there may be a plurality of different steering modes that each cause the steering angles for the steerable wheels to be controlled according to different Ackermann steering geometries, e.g., one steering mode in which the steering actuators are controlled to produce true Ackermann steering angles, one or more additional steering modes in which the steering actuators are controlled to produce steering angles in which the difference between the steering angles is, for each such additional steering mode, a different percentage of the difference between the true Ackermann steering angles for those wheels for a given vehicle heading. Such a system thus allows for the steering system to gradually change the steering geometries used based on speed of the apparatus, allowing for potentially smoother transitions between steering geometries. For example, instead of suddenly switching from a steering mode in which true Ackermann steering angles are used for speeds between 0 and 10 mph to a steering mode in which reverse Ackermann steering angles are used for speeds above 10 mph, such a system may instead be configured to use the true Ackermann steering mode for speeds between 0 and 7 mph but then sequentially apply multiple steering modes at speeds above 7 mph and up to 10 mph that are each configured to cause the steering actuators to generate steering angles that are increasingly further and further from true Ackermann steering angles (and closer and closer to parallel steering angles or reverse Ackermann steering angles) as the speed approaches 10 mph. It will also be understood that the example speeds discussed above are simply examples and that the various speed ranges associated with each steering mode may be selected according to the speeds at which the steering angles associated with that steering mode are, for example, most effective. This may be highly dependent on the particular geometry of the vehicle, e.g., wheelbase and track, and is thus not limited to a particular value or values that is or are generally applicable across all vehicle implementations. 
     Another system that may be used in apparatuses with independently steerable wheels is a closed-loop control system. In traditional steering systems in which a pair of steerable wheels are steered in tandem by a common mechanical steering system, the steering angles of the two wheels are linked together in a way that makes it impossible—aside from minor variations that may occur due to tolerances or mechanical slop (or larger variations that may arise due to component failure that may result in one or both of the steerable wheels no longer being controllable at all via the steering system)—for the steering angles of the steerable wheels to adopt any other relative positioning for a given actuation state of the steering system. In contrast, systems such as those discussed herein that feature an independently controllable steering actuator for each steerable wheel can experience situations in which one or both of the steerable wheels actually deviate from the desired angular positions for those wheels during a turn. For example, if one steering actuator experiences exhibits slower-than-expected response time, the steerable wheel steered by that actuator may have a steering angle that lags the steering angle of the other steerable wheel—in such an instance, the steady state steering angles that are ultimately adopted by the two steerable wheels (assuming that a steady state steering angle is being used) may be the desired steering angles, e.g., true Ackermann steering angles, but the steering angles of the two steerable wheels at any given point in time during the transition to those desired steering angles may not be true Ackermann steering angles. 
     A closed-loop control algorithm may be used to actively compensate for such situations during actuation of a steering system featuring a pair of independently steerable wheels.  FIG.  29    depicts a flow diagram for an example of such a control technique, although it will be appreciated that other control algorithms may be used to similar effect, and that this disclosure is not limited to the particular technique illustrated in  FIG.  29   . 
     In such a system, a desired heading angle for the apparatus may be determined, e.g., based on a determination from a navigation system, in block  2902 . The desired heading angle may represent a vector that is tangent to a desired path that the apparatus is to follow for a given time interval. The closed-loop control algorithm (or a controller that implements, in hardware and/or software, such an algorithm) may determine, in block  2904 , a commanded heading that the apparatus should be caused to adopt in order to facilitate reaching the desired heading from the current heading. For example, if the desired heading angle is 10° and the current heading is 0°, the control algorithm may interpolate the heading change into smaller commanded heading increments, causing the heading to gradually change over a given time interval. For example, a navigation system may be configured to provide desired heading updates at a frequency of 10 Hz to the closed-loop control algorithm. The controller that implements the control algorithm may, in turn, operate at a frequency of 100 Hz and may, in the example of a heading of 10° noted above, determine that such a heading is to be attained by changing the heading in 1° increments during each operational cycle of the controller, e.g., via commanded headings of +1° per cycle. 
     Once the commanded heading is obtained, the technique may proceed to blocks  2906   a  and  2906   b , in which the controller may calculate the steering angles that the steerable wheels will theoretically need to be steered to in order to cause the apparatus to adopt the current commanded heading. Such steering angle calculations may be determined according to the particular steering mode that is being used, e.g., the particular steering geometry that the actuators are being controlled to adopt (e.g., true Ackermann steering, parallel steering, reverse Ackermann steering, etc.). Thus, for a given commanded heading, two different (or, for parallel steering, identical) steering angles may be determined, one for each steerable wheel. Blocks  2906  through  2912  are each performed for each wheel, with one set of blocks having an “a” suffix to indicate that they are performed for the first wheel, and the other set of blocks having a “b” suffix to indicate that they are performed for the second wheel. It will be appreciated that some or all of these blocks may be performed in parallel or series with their counterpart blocks having the other suffix, if desired. 
     In blocks  2908 , the controller may cause the first and second steering actuators to actuate so as to cause the respective steerable wheels that are steered thereby to rotate to the calculated steering angle (or, more specifically, send signals that should cause those steering actuators to rotate to the respective steering angles for those respective wheels). For example, the first steering actuator may be caused to rotate the wheel connected therewith to the first calculated steering angle, and the second steering actuator may be caused to rotate the wheel connected therewith to the second calculated steering angle. 
     In blocks  2910 , the controller may obtain the actual steering angles of both steerable wheels, e.g., from a rotational encoder or other angular position-sensing sensor that is able to determine what the actual steering angle α respective one of the steering wheels is in. It is important to realize that the actual steering angle that a steerable wheel is in may not match the steering angle that the steerable wheel is supposed to be in based on the inputs provided by the associated steering actuator. For example, the steering actuator may be provided with an input that should cause the steering actuator to turn the wheel by 1°, but due to wear and tear on the steering actuator, or perhaps increased loading on the wheel, the amount that the wheel actually steers by is actually 0.9°. The controller may then determine separate theoretical headings for the apparatus based on the current steering mode and the actual steering angles of each wheel. 
     For example, if the current steering mode in a bogie-equipped vehicle having a track to wheelbase ratio of 3:4 is true Ackermann steering and a first wheel of the steerable wheels has a steering angle of 40° (the first wheel in this example being the inner wheel during a turn), then the second wheel of the steerable wheels should necessarily have a steering angle of 27.3° under that steering mode assuming that the second wheel actually moved consistent with that steering mode. Furthermore, the theoretical apparatus heading (which is generally evaluated with respect to a particular point in space relative to the apparatus—in this example, the headings are evaluated at a location that is in the center of the track/wheelbase of the apparatus) that would result from actual true Ackermann steering based on the actual steering angle of the first wheel should be 17.7°. 
     If the second wheel were to have an actual steering angle of 27.3° in such an example, then the steerable wheels would actually be engaging in actual true Ackermann steering. However, for various reasons, the two steering angles of the steerable wheels may not always be consistent with the motion characteristics of the current steering mode. For example, if the actual steering angle of the second wheel is 20° instead of 27.3° in the example above, then the first wheel should, assuming that the current steering mode is being adhered to, have a steering angle of 26.6° and the theoretical heading based on the current steering mode and the current steering mode should be 11.9°. 
     In blocks  2912 , the headings that are calculated with respect to the actual steering angles of both steerable wheels are compared against the current commanded heading in order to determine respective angles between the calculated headings for each of the two steerable wheels and the current commanded heading. Thus, there will be a first angle that represents the angular distance between the current commanded heading and the first heading based on the first wheel position, and a second angle that represents the angular distance between the current commanded heading and the second heading based on the second wheel position. 
     In block  2914 , the first angle and the second angle may be compared to one another to see whether the two angles are within a predetermined tolerance of one another, e.g., within 5% of each other. If so, then this may be interpreted by the controller as indicating that the two steerable wheels are actually demonstrating steering performance that is consistent with the current steering mode and that no left/right correction to steering angle is needed. In this case, the technique may proceed to block  2916 , in which the next commanded heading may be determined based on, for example and at least in part, either or both of the first heading and the second heading (for example, since both are generally the same, either may be viewed as being representative of the other—alternatively, the first and second headings may be averaged and the average heading then used, at least in part, to determine the next commanded heading). Once the next commanded heading is determined, the technique may return to block  2904  and be repeated for the next time step. 
     If it is determined in block  2914  that the first angle and the second angle are outside of the predetermined tolerance from one another, then this may be interpreted by the controller as being indicative of a situation in which one of the two steerable wheels is lagging or leading the other with respect to where they should be positioned with respect to steering angle given the current steering mode. In such situations, the technique may proceed to block  2916 , in which a determination may be made as to whether the first angle is smaller than the second angle. If it is, then the technique may proceed to block  2918   a  in which the next commanded heading may be determined based at least in part on the second heading. This generally represents the case in which the second wheel is lagging the first wheel with respect to the steering angles that each is supposed to be at under the constraints of the current steering mode. In such situations, it is generally not possible to cause the second wheel to make up the difference since whatever is causing the lag will likely prevent the even quicker or larger movements that would be needed to “catch up” the second wheel to where it is supposed to be. Accordingly, the controller may instead cause the next commanded heading to be determined based at least in part on the second heading, which may allow the first and second wheels to be steered in a way that is consistent with the current steering mode. 
     If it is determined in block  2916  that the first angle is greater than the second angle, then the technique may proceed to block  2918   b , in which the next commanded heading may be determined based at least in part on the second heading (similar to what was done in block  2918   a , but for the other heading). In such situations, it is generally not possible to cause the first wheel to make up the difference since whatever is causing the lag will likely prevent the even quicker or larger movements that would be needed to “catch up” the first wheel to where it is supposed to be. Accordingly, the controller may instead cause the next commanded heading to be determined based at least in part on the first heading, which may allow the first and second wheels to be steered in a way that is consistent with the current steering mode. 
     It will be understood that the above closed-loop control algorithm may be used with a variety of different steering modes, not necessarily just an Ackermann steering mode as discussed above. The technique may be used to provide for more reliable steering performance in a steering system in which the steering angles of the steerable wheels are not physically constrained to one another by virtue of a common mechanical actuation system. 
     Another closed-loop control algorithm that may be used in bogie-equipped vehicle systems is an algorithm that controls the wheel speeds of the wheels of the vehicle according to, for example, the steering mode currently in use (and the current steering angles in use). 
       FIG.  30    (which is split into  FIGS.  30 A and  30 B ) depicts a flow diagram for a wheel speed closed-loop control technique that may be practiced in bogie-equipped vehicles having a separately controllable drive motor for each of four wheels. In block  3002 , a desired velocity for the apparatus may be determined. In block  3004 , the controller may determine a current commanded velocity for the apparatus for the current execution cycle of the technique, e.g., similar to what was discussed earlier with respect to the commanded heading. 
     In blocks  3006 , separate determinations of what the different wheel speeds should be, and thus what the rotational rates for each wheel should be, may be made according to the current commanded velocity and the current steering mode. For example, the outer wheels in a turn will roll along paths that have larger effective radii than the paths along which the inner wheels roll along. As a result, the outer wheels will—assuming no wheel slip occurs—need to travel further for a given portion of a turn than the inner wheels. There may also be fore/aft speed differences, e.g., the front wheels may (at least in front-steer, two-wheel steering systems) follow paths that have larger effective radii than the paths followed by their rear wheel counter parts (for example, the front inner wheel will follow a path having a larger effective radius than the rear inner wheel). Accordingly, each wheel of the vehicle may, for a given turn, experience different amounts of rotation during the turn. Correspondingly, each wheel should, assuming no wheel slip, also experience proportionately different velocities (and thus wheel rotation rates) during the turn. These wheel rotation rates may be calculated based on the current commanded velocity for the vehicle, the heading, and the steering mode that is currently in use. 
     In blocks  3008 , the controller may cause each drive motor for each wheel to be actuated at the rotational rate corresponding to that wheel. Thus, the first drive motor may be caused to actuate at the first rotational rate, the second drive motor at the second rotational rate, and so forth. 
     In blocks  3010 , the controller may determine, based on wheel speed sensors for each of the wheels, a difference between the actual rotational rate of each wheel and the rotational rate that was calculated in blocks  3006 . For example, a first delta may be determined in block  3010   a  for the first wheel by subtracting the actual rotational rate of the first wheel from the calculated first rotational rate of block  3006   a . Similarly, a second delta may be determined in block  3010   b  for the second wheel by subtracting the actual rotational rate of the second wheel from the calculated second rotational rate of block  3006   b.    
     In block  3012 , a determination may be made as to whether all four of the deltas determined in blocks  3010  are within some tolerance of one another, e.g., all within 5% of one another. If so, then the technique may proceed to block  3014 , in which the next commanded velocity may be determined based on, at least in part, one or more of the four actual wheel speeds (for example, an average velocity for the apparatus may be determined based on the four actual wheel speeds). The technique may then return to block  3004  and be repeated for the next control cycle. 
     If it is determined in block  3012  that at least two of the deltas are not within the predetermined tolerance of each other, the technique may proceed to blocks  3016 - 3024 , in which the deltas may be evaluated to see which has the largest value. Generally speaking, the largest delta will indicate the wheel that is lagging the most behind the other wheels. The technique may then proceed to blocks  3018 - 3028 , as appropriate, in which the next commanded velocity may be determined based, at least in part, on the current actual rotation rate of the wheel that is lagging the most behind the other wheels. This has the effect of generally scaling down the velocities in question such that the slowest wheel sets the pace for the remaining wheels, thereby avoiding situations in which one or more wheels may skid or freewheel, which may compromise steering stability. 
     It will be appreciated that the various techniques discussed herein may be implemented in a variety of ways, including via software, hardware, application-specific integrated circuits, analogue circuits, combinations of two or more thereof, or other systems capable of being used to perform the technique discussed above. 
     It will also be appreciated that the techniques, systems, and apparatuses discussed herein may be implemented separately in various vehicles or in various combinations or sub-combinations in vehicles. 
     The term “between,” as used herein and when used with a range of values, is to be understood, unless otherwise indicated, as being inclusive of the start and end values of that range. For example, between 1 and 5 is to be understood to be inclusive of the numbers 1, 2, 3, 4, and 5, not just the numbers 2, 3, and 4. 
     It is to be understood that the phrases “for each &lt;item&gt; of the one or more &lt;items&gt;,” “each &lt;item&gt; of the one or more &lt;items&gt;,” or the like, if used herein, are inclusive of both a single-item group and multiple-item groups, i.e., the phrase “for . . . each” is used in the sense that it is used in programming languages to refer to each item of whatever population of items is referenced. For example, if the population of items referenced is a single item, then “each” would refer to only that single item (despite the fact that dictionary definitions of “each” frequently define the term to refer to “every one of two or more things”) and would not imply that there must be at least two of those items.