Patent Publication Number: US-2023133525-A1

Title: Control system and method for follower e-pallet in leader-follower platoon arrangement

Description:
INTRODUCTION 
     Manufacturing plants and warehouse facilities require the coordinated movement of raw materials, subcomponents, and finished parts, often over considerable distances. Larger or relatively massive loads may be transported with the assistance of fork lifts, tractors, conveyor belts, and other power equipment. In contrast, smaller loads may be moved by hand or using manually-operated pallet trucks, wheeled dollies, or hand carts. Collectively, machine powered and manually operated lift assistance devices improve overall production efficiency, while at the same time greatly reducing load-related stresses and strains on human operators within the workplace. 
     Certain operations not requiring the assistance of heavy power equipment of the types described generally above may nevertheless not be suitable for manually-operated devices like hand carts and dollies. In such cases, a human operator may utilize one or more motorized electric pallets (“e-pallets”) each having a superstructure mounted on or integrally formed with a wheeled base platform. One or more electric traction motors provides a drive torque to driven road wheels of the e-pallet to help propel the e-pallet along a floor surface. 
     SUMMARY 
     The present disclosure pertains to methods and systems for controlling motor-driven electric pallets (“e-pallets”) that are lightly tethered together or in wireless communication with one another in a leader-follower platoon arrangement. As used herein, the contemplated platoon arrangement includes one lead e-pallet or another lead vehicle (“leader”) that is serially connected to or in wireless communication with one or more trailing (“follower”) e-pallets, such that the follower e-pallets are located aft of the lead e-pallet when the platoon is in forward motion. As used herein, “lightly tethered” refers to the possible serial linking together of one or more follower e-pallets via an intervening tether device, itself having integral length and angle sensors as set forth below. As appreciated in the art, an alternative wireless or “tether-less” solution may be envisioned to accomplish the same or similar tasks, including pallet pose estimation by calculation/measuring distance, azimuth, and yaw rate using, e.g., radar, ultrasonics, lidar, one or more cameras, ultra-wide band (UWB) communications, etc. Additionally, the platoon contemplated herein is characterized by an absence of vehicle-to-vehicle (V2V) communications between the various e-pallets, such that a given e-pallet does not offload data to another e-pallet in the platoon. The lack of a V2V communications capability gives rise to the local control strategy described herein. 
     For illustrative simplicity, the local control strategy for implementation aboard each respective of the follower e-pallets is described below with respect to a simplified two-member platoon embodiment, e.g., one in which a single follower is tethered to leader by the tether device to use a consistent non-limiting example configuration. The leader moves autonomously, is driven by an operator, or moves in response to a manual towing force imparted by the operator. Aboard the follower, a local controller optimizes locomotion of the follower using a variable target point (VTP), a variable distance setpoint (VDSP), and velocity estimate (Vest) of the leader. This collective set of information is used in lieu of a static tracking point on the leader so to prevent, among other things, instances of jackknifing, zig-zag turns requiring larger maneuver areas, an inability of the platoon to negotiate tight turns or circuitous hallways, poor tracking performance of the followers relative to the leader, and other possible stability and range of motion issues. 
     In terms of the VTP, the local controller is programmed to execute computer-readable instructions, with instruction execution causing the local controller to adaptively move the VTP within a given frame of reference, in real-time, to increase the overall stability of the platoon. The VTP is also used to set a desired distance between consecutive followers in embodiments in which more than one follower is used. 
     The VDSP for its part is also modified in real-time by the local controller based on an angle of articulation or azimuth angle between an axis of the leader, e.g., the tether device, and a leading edge of the follower. Modification of the VDSP is performed by the local controller to adapt to a limited range of motion of the tether device. The velocity of the leader in turn is estimated by the local controller and thereafter used to define a desired velocity of the follower. In this manner, the present local control strategy enables sharper cornering of the entire platoon without an accompanying loss of stability. 
     In a particular embodiment, a platoon of e-pallets includes a leader e-pallet and a follower e-pallet connected to or in wireless communication with the leader e-pallet to form the platoon of e-pallets, with the followers located aft of the leader. An axis of the leader, in this instance its longitudinal center axis, is arranged at an azimuth angle with respect to the follower. The platoon also includes a sensor suite, including a velocity sensor configured to measure a velocity of the follower, an angle sensor configured to measure the azimuth angle, and a length sensor configured to measure a distance from the follower to the leader. Such a distance could be a length of a tether device in the exemplary tethered embodiment. 
     The follower in this configuration includes a set of road wheels, an electric powertrain system connected to the set of road wheels and configured to provide an output torque thereto, and a local controller connected to the follower. The local controller is configured to adaptively move a VTP on the leader in response to the velocity, the azimuth angle, and the distance/length, and to thereafter control a dynamic output state of the electric powertrain system using the VTP. 
     The local controller may change a variable distance setpoint on the leader based on the azimuth angle to maintain a linear distance between the leader and follower. 
     In an aspect of the disclosure, the local controller may estimate a velocity of the leader as an estimated velocity, define a desired velocity of the follower using the estimated velocity, and thereafter control the dynamic output state of the electric powertrain system of the follower using the estimated velocity. 
     The length or distance sensor may include a string potentiometer or a wireless proximity sensor. 
     The leader in some implementations is configured to be towed by a human operator via another tether device, and the leader includes motorized drive wheels responsive to a towing force imparted by the human operator. 
     The set of road wheels may include a pair of front drive wheels. The electric powertrain system may include first and second electric motors respectively connected to a different one of the front drive wheels to provide the follower e-pallet with a differential steering capability. 
     In a possible embodiment, the local controller is configured to use a velocity term to provide a faster response at higher velocities of the platoon to enable the local controller, and to compensate for a relatively slow response of the optional tether device. 
     In the various embodiments set forth above, the follower may include a plurality of followers, i.e., one behind another. 
     In another aspect of the present disclosure, a method for controlling the platoon of e-pallets includes measuring, via a plurality of sensors of a sensor suite, a velocity of the follower e-pallet, an azimuth angle defined between a tether device and a leading edge of the follower e-pallet, and a length sensor configured to measure a length of the tether device. The method also includes adaptively moving a VTP on the leader pallet, via a local controller of the follower e-pallet, in response to the velocity, the azimuth angle, and the length. The method thereafter includes controlling a dynamic output state of the electric powertrain system of the follower e-pallet using the VTP. 
     In yet another embodiment, a follower e-pallet for use with a lead vehicle to which the follower e-pallet is connected in a platoon arrangement via a tether device includes an electric powertrain system connected to the set of road wheels and configured to provide an output torque thereto to propel the follower e-pallet, and a local controller connected to the follower e-pallet. The local controller is configured to receive, from a sensor suite, each of a measured length of the tether device, the azimuth angle, and a velocity of the follower e-pallet. The local controller is also configured to adaptively move the VTP on the leader e-pallet in response to the velocity, the azimuth angle, and the measured length, control a dynamic output state of the electric powertrain system using the VTP, and change a variable distance setpoint on the leader e-pallet to maintain a linear distance between the leader e-pallet and the follower e-pallet based on the azimuth angle. 
     The above features and advantages, and other features and attendant advantages of this disclosure, will be readily apparent from the following detailed description of illustrative examples and modes for carrying out the present disclosure when taken in connection with the accompanying drawings and the appended claims. Moreover, this disclosure expressly includes combinations and sub-combinations of the elements and features presented above and below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates motor-driven electric pallets (“e-pallets”) in a platoon arrangement in which one or more trailing e-pallets (“followers”) are tethered to or in wireless communication with a lead vehicle (“leader”) and locally controlled in accordance with the present disclosure. 
         FIG.  2    is a schematic illustration of an electrified powertrain system usable as part of the followers shown in  FIG.  1   . 
         FIGS.  3 A,  3 B, and  3 C  illustrate exemplary motion trajectories of a platoon of e-pallets. 
         FIG.  4    is a schematic kinematic diagram of a representative follower usable as part of the exemplary platoon shown in  FIG.  1   . 
         FIGS.  5 ,  6 , and  7    are schematic plan view illustrations of a platoon of e-pallets controller as set forth herein. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is susceptible of embodiment in many different forms. Representative examples of the disclosure are shown in the drawings and described herein in detail as non-limiting examples of the disclosed principles. To that end, elements and limitations described in the Abstract, Introduction, Summary, and Detailed Description sections, but not explicitly set forth in the claims, should not be incorporated into the claims, singly or collectively, by implication, inference, or otherwise. 
     For purposes of the present description, unless specifically disclaimed, use of the singular includes the plural and vice versa, the terms “and” and “or” shall be both conjunctive and disjunctive, “any” and “all” shall both mean “any and all”, and the words “including”, “containing”, “comprising”, “having”, and the like shall mean “including without limitation”. Moreover, words of approximation such as “about”, “almost”, “substantially”, “generally”, “approximately”, etc., may be used herein in the sense of “at, near, or nearly at”, or “within 0-5% of”, or “within acceptable manufacturing tolerances”, or logical combinations thereof. 
     Referring to the drawings, wherein like reference numbers refer to like features throughout the several views, and beginning with  FIG.  1   , a workspace  10  is shown in which a platoon  120  of follower electric pallets (“e-pallets”)  12 F trail a leader e-pallet  12 L. In the non-limiting embodiment of  FIG.  1   , the follower e-pallets  12 F are lightly tethered together and to the leader e-pallet  12 L via a respective tether device  25 . In various embodiments, the tether device  25  may be configured as a flexible device, or an extendable and contractable device such as a telescoping tube, or the tether device  25  can be made of strings. For simplicity, the e-pallets  12 L and  12 F are described below as “leader” and “follower”, respectively in such a tethered arrangement. However, those skilled in the art will appreciate that the present teachings may be used in platoons  120  in which the leader  12 L is configured differently, e.g., as an operator-driven or autonomously controlled vehicle or a towing robot, without limitation. Likewise, the tether device  25  may be thought of as part of the follower  12 F in some configurations, or the functions of the tether device  25  may be performed wirelessly, e.g., using radar systems, ultrasonics, one or more cameras, lidar systems, ultra-wide band (UWB) communications, etc. The platoon  120  is described below as utilizing the tether device  25  solely for illustrative consistency. 
     The platoon  120  of  FIG.  1    may be used in a wide range of facilities, such as but not limited to manufacturing plants, warehouses, supply depots, and schools, for the purpose of assisting a human operator  14  in transporting a load within the workspace  10 . Depending on the nature of the workspace  10  and of the various operations conducted therein, the load transported by the platoon  120  may be of various sizes, shapes, and constructions, e.g., products, cargo, raw materials, partially-assembled or fully-assembled parts or components, food, beverages, or other consumables, mail, packages, or other such items that may have to be moved within the workspace  10 . 
     To enable the platoon  120  to function in this manner, the leader  12 L and the follower(s)  12 F are respectively equipped with a local controller (C L )  500  and (C F )  50 . The local controller  500  receives input signals (arrow CC I *), such as when the operator  14  applies a towing force (arrow FT). In response, the local controller  500  transmits motor control signals (arrow CC O *) to one or more onboard electric traction motors to control propulsion functions of the leader  12 L. The local controller  500  may regulate operation of the leader  12 L using suitable closed-loop or open-loop dynamic control strategies informed by the input signals (arrow CC I *). For instance, the leader  12 L may be connected to the operator  14  by the flexible tether device  25 , which the operator  14  may grasp when towing the platoon  120  in the direction of arrow AA. Regardless of how the lead e-pallet  12 L is powered, however, the absence of vehicle-to-vehicle (V2V) communications within the platoon  120  of  FIG.  1    ensures that the leader  12 L cannot command actions of the follower(s)  12 F. 
     Instead, the followers  12 F act on their own, i.e., locally, doing so based on input signals (arrow CC I ) from a sensor suite  40 S into the respective local controllers  50 . The sensor suite  40 S or constituent sensors thereof may be considered as part of the follower  12 F or distinct therefrom in different embodiments. Aboard each one of the follower(s)  12 F, the local controller  50  is mounted on or housed within a superstructure  13 . The superstructure  13  may vary in its construction based on the transported load, but in general may embody a box-like container possibly including shelves, racks, bins, or other suitable structure for securely moving the load through the workspace  10 . The superstructure  13  in turn is connected to or formed integrally with a base platform  20 , e.g., a solid plate or planar surface of metal, plastic, and/or composite materials configured to support the collective weight of the local controller  50  and the above-described load. The base platform  20  in turn are connected to one or more road wheels  22 F and  22 R, e.g., via drive axles and a suspension system (not shown). 
     In the representative use case of  FIG.  1   , the operator  14  grasps the tether device  25  in the operator&#39;s hand  14 H, with the tether device  25  in turn being pivotably and/or rotatably connected to the superstructure  13  of the follower  12 F located immediately behind the leader  12 L. The operator  14  pulls or tows the leader  12 L in the general direction of arrow AA as the operator  14  walks through the workspace  10 . The towing forces (arrow FT) are thus imparted to the leader  12 L, thereby causing the leader  12 L to move relative to a floor surface  11 . 
     Depending on the relative velocities and ground speeds of the operator  14  and the leader  12 L with respect to the floor surface  11 , the tether device  25  extending to the leader  12 L and grasped by the operator  14 , as well as similar tether devices  25  connecting the follower(s)  12 F together or to the leader  12 L, may extend or contract in length, as indicated by double-headed arrow DD. At the same time, the local controller  50  of the follower(s)  12 F and the local controller  500  of the leader  12 L command a motor assist force (arrow FM), which is imparted by delivery of a motor drive torque to one or more of the road wheels  22 F and/or  22 R. Aboard the follower(s)  12 F, this action is performed in response to the input signals (arrow CC I ) by the transmission of motor control signals (arrow CC O ) from the local controller  50 , e.g., to corresponding motor control processors as appreciated in the art. The motor control signals (arrow CC O ) within the scope of the present disclosure may include a desired yaw rate (ω des ) and a desired velocity (V des ) of the follower(s)  12 F, for instance. 
     Referring briefly to  FIG.  2   , an electrified powertrain system  30  may be used to power the road wheels  22 F of the follower(s)  12 F of  FIG.  1    in some implementations. Similar structure may be used to power the leader  12 L, with possible differences in the composition of the input signals (arrow CC I *) relative to the set of input signals (arrow CC I ) used by the local controller  50 . The local controller  50  receives the input signals (arrow CC I ) from the sensor suite  40 S, which is inclusive of a length sensor  40 , an angle sensor  42 , and a velocity sensor  44  as described below. The input signals (arrow CC I ) may be provided to the local controller  50  over a suitable hardwired transfer conductors or a wireless connection, e.g., a short distance BLUETOOTH®, Wi-Fi, or near-field communication (NFC) link. 
     In order to perform the various motion control functions, the local controller  50  is programmed in software and equipped with application-specific amounts of volatile and non-volatile memory (M) and one or more processor(s) (P). The memory (M) includes or is configured as a non-transitory computer readable storage device(s) or media, and may include volatile and nonvolatile storage in read-only memory (ROM) and random-access memory (RAM), and possibly keep-alive memory (KAM) or other persistent or non-volatile memory for storing various operating parameters while the processor (P) is powered down. Other implementations of the memory (M) may include, e.g., flash memory, solid state memory, PROM (programmable read-only memory), EPROM (electrically PROM), and/or EEPROM (electrically erasable PROM), and other electric, magnetic, and/or optical memory devices capable of storing data, at least some of which is used in the performance of the present method. The processors (P) may include various microprocessors or central processing units, as well as associated hardware such as a digital clock or oscillator, input/output (I/O) circuitry, buffer circuitry, Application Specific Integrated Circuits (ASICs), systems-on-a-chip (SoCs), electronic circuits, and other requisite hardware needed to provide the programmed functionality. In the context of the present disclosure, the local controller  50  executes instructions via the processor(s) (P) to cause the local controller  50  to perform the present method. 
     Computer-readable non-transitory instructions or code embodying the method and executable by the local controller  50  may include one or more separate software programs, each of which may include an ordered listing of executable instructions for implementing the stated logical functions described below. Execution of the instructions by the processor (P) in the course of operating the followers  12 F causes the respective local controller(s)  50  to regulate motion of the followers  12 F. 
     The electrified powertrain system  30  of  FIG.  2    may include respective first and second road wheels  22 A and  22 B, e.g., arranged to function as oppositely-disposed front road wheels  22 F of the follower  12 F. The electrified powertrain system  30  in such a configuration may include a power supply  32 , e.g., a multi-cell battery pack, which in a representative configuration may be configured as a rechargeable multi-cell battery having a lithium-ion or other suitable battery chemistry. The electrified powertrain system  30  also includes first and second traction power inverter modules (TPIM A )  34 A and (TPIM B )  34 B respectively connected to first and second electric traction motors (MA)  36 A and (MB)  36 B. 
     When the electric traction motors  36 A and  36 B are embodied as alternating current (AC)/polyphase propulsion motors as shown, the TPIMs  34 A and  34 B are connected to the power supply  32  via a direct current voltage bus  33 . The TPIMs  34 A and  34 B are also connected to the electric motors  36 A and  36 B, respectively, via corresponding AC voltage busses  35 A and  35 B. Internal switching operations of the TPIMs  34 A and/or  34 B in this representative configuration is used to convert a DC voltage (VDC) present on the DC voltage bus  33  into an AC voltage (VAC) on the AC voltage bus  35 A and/or  35 B as needed in order to electrically energize one or both of the electric motors  36 A and  36 B. Embodiments may also be conceived of in which the electric motors  36 A and  36 B are DC motors, in which case one may omit the TPIMs  34 A and  34 B and associated power conversion circuitry. 
     With respect to locomotion of the follower  12 F, each road wheel  22 A and  22 B may be separately powered by a respective output torque, i.e., arrows TA and TB. In such a configuration, the follower  12 F may employ differential steering, which in turn is accomplished by rotating the road wheels  22 A and  22 B via corresponding output members  37 A and  37 B at different torques or speeds relative to one another. When executing a left-hand turn, for instance, the local controller  50  may command the output torque (TA) from the electric motor  36 A at a higher level or corresponding rotary speed than the output torque (TB) from traction motor  36 B. A similar steering effect may be enjoyed using a single electric traction motor  36 A or  36 B using an associated electronic differential, as will be appreciated by those skilled in the art, and therefore the configuration of  FIG.  2    is merely representative of one possible embodiment of the electrified powertrain system  30 . 
       FIGS.  3 A,  3 B, and  3 C  show exemplary motion trajectories for the platoon  120  using the tether device  25  to connect the leader  12 L to the follower  12 F. In contemplated embodiments, the tether device  25  may be a flexible telescoping mechanism that axially extends or compresses depending on the relative velocities of the leader and followers  12 L and  12 F, respectively. Additionally, the tether device  25  pivots relative to the particular surface to which the tether device  25  is attached. For example, in  FIG.  3 A  the tether device  25  is connected between a trailing edge  51  of the leader  12 L and a leading edge  53  of the follower  12 F. As an integral part of the tether device  25 , or alternatively as an add-on component, the length sensor  40  disposed on or within the tether device  25  measures and reports the deployed length of the tether device  25  for use by the local controller  50  of  FIG.  1    when controlling motion of the corresponding follower  12 F. Additionally, an angle sensor  42  is disposed on the follower  12 F and configured to measure an angle of articulation or azimuth angle, as part of the input signals (arrow CC I ), with the azimuth angle defined between the tether device  25  and the leading edge  53  of the follower  12 F. Other sensors aboard the follower  12 F may include one or more velocity sensors  44  and a yaw rate sensor  46 , e.g., an inertial measurement unit (IMU), such that the local controller  50  is aware of the yaw rate and velocity of the follower  12 F. 
       FIG.  3 A  represents simple straight-line motion of the platoon  120 . Within the scope of the disclosure, the local controller  50  places a variable target point (VTP)  80  at an optimal location on the leader  12 L. When the platoon  120  is traveling straight (arrow AA), the local controller  50  may place the VTP  80  on the leading edge  53  of the leader  12 L as shown. Such placement makes the follower  12 F less sensitive to the instantaneous readings from angle sensor  42 , and hence more stable. 
     When the leader  12 L begins to turn, however, as represented by arrow BB of  FIG.  3 B , the local controller  50  moves the VTP  80  toward the trailing edge  53  of the leader  12 L. Such placement allows the follower  12 F to more accurately track the motion of the leader  12 L while at the same time avoiding the need for wide turns. For instance, as shown in the turn trajectory CC of  FIG.  3 C , use of a fixed target point  800  on the leader  12 L leads to a higher turn radius relative to trajectory BB of  FIG.  3 B . As part of the disclosed strategy for control of the followers  12 F, therefore, the local controller  50  is configured to adaptively move the VTP  80  in a manner conducive to balancing stability and ensuring desired tracking performance. 
     E-PALLET KINEMATICS: referring briefly to  FIG.  4   , a kinematics diagram  48  illustrates relevant parameters for consideration by the local controller  50  when performing the present control strategy. The follower  12 F, shown in plan view in a nominal two-dimensional Cartesian xy coordinate frame and having a longitudinal centerline YY, includes the two powered road wheels  22 A and  22 B, which in turn are separated from each other by a distance (d). Thus, the distance between a given road wheel  22 A or  22 B and the centerline YY is d/2. Casters or other non-powered/passive wheels  22 C,  22 D may be used for the remaining road wheels  22 C and  22 D, as noted above, with corresponding velocity components V 22C  and V 22D . In the representative  FIG.  4    orientation, the road wheels  22 A and  22 B are respectively powered by a right motor and a left motor, i.e., the electric traction motors  36 A and  36 B of  FIG.  2   , with “right” and “left” being relative to a nominal forward-facing position of the operator  14 . The respective motor velocities are thus represented by arrows V LM  and V RM , which combine to produce a linear velocity (V F ) of the follower  12 F. The follower  12 F may also have a yaw rate (ω) about an instant center of rotation (ICR) as a point in free space, as understood in the art. 
     With continued reference to the representative diagram  48 , the velocity (V F ) of the follower  12 F may be expressed mathematically as V F =√{square root over ({dot over (x)} 2 +{dot over (y)} 2 )}, with the yaw rate (ω)={dot over (θ)}. Additionally: 
     
       
         
           
             
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     Ultimately, the velocities of the left and right motors, i.e., the electric traction motors  36 A and  36 B, are expressed as functions of the velocity (V F ), the yaw rate (ω), and the distance (d) between the road wheels  22 A and  22 B: 
     
       
         
           
             
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     The kinematics diagram  48  of  FIG.  4    therefore represents a particular configuration of the follower  12 F, one in which front steering is achieved in an exemplary instance via differential speeds of the road wheels  22 A and  22 B. Other embodiments may be contemplated within the scope of the present disclosure that use different versions of the kinematic diagram  48 , such as embodiments in which the road wheels  22 A and  22 B are steerable using a steering assembly, and therefore the representation of  FIG.  4    is intended to be illustrative of just one possible implementation. 
     Functions of the local controller  50  in the overall control of the follower  12 F will now be described with reference to  FIGS.  5 ,  6 , and  7   . As noted above with particular reference to  FIGS.  3 A,  3 B, and  3 C , the local controller  50  is embodied as an electronic control unit connected to the follower  12 F, e.g., mounted thereto or housed within the superstructure  13  shown in  FIG.  1   . As part of its programmed functionality, the local controller  50  is configured to adaptively move the VTP  80  of  FIGS.  3 A and  3 B  within a frame of reference on the leader  12 L, with the local controller  50  doing so in response to the estimated velocity of the leader  12 L, the azimuth angle between the follower  12 F and the tether device  25 , and the deployed length of the tether device  25  as measured by the length sensor  40 . The local controller  50  thereafter controls a dynamic output state of the electric powertrain system  30  of the follower  12 F using the VTP. 
       FIG.  5    schematically depicts the VTP  80  on the leader  12 L, with the follower  12 F moving with a velocity V F  and the leader  12 L moving with a velocity V L . The follower  12 F is connected to the leader  12 L by the above-described tether device  25 , which is omitted for clarity. The linear distance (d) between the follower  12 F and the leader  12 L along the longitudinal axis of the tether device  25  is measured and reported by the length sensor  40  of  FIGS.  2 - 3 C . Relevant parameters include a target length (L t ) between the VTP  80  and the trailing edge  51  of the leader  12 L, and an effective distance (d eff ) between the VTP  80  and the leading edge  53  of the follower  12 F. 
     The local controller  50  is thus configured to calculate the target length (L t ) between the particular point at which the tether device  25  is connected to the leader  12 L and the VTP  80 , e.g., as follows: 
         L   t   =L   t0 ·cos(α L )
 
     where α L  is the measured azimuth angle of the follower  12 F from the point of view of the leader  12 L, i.e., the angle between the tether device  25  and a connection point thereof on the trailing edge  51  of the leader  12 L, and L t0  is the nominal value of the L t  that represents the distance to the VTP  80  for straight ahead driving/motion. Knowledge of the target length (L t ) allows the local controller  50  to calculate the angle (γ) between the VTP  80  and the connection point of the tether device  25  on the follower  12 F, relative to the longitudinal axis of the leader  12 L, for instance as follows: 
     
       
         
           
             γ 
             = 
             
               a 
               ⁢ 
                  
               tan 
               ⁢ 
                  
               
                 ( 
                 
                   
                     d 
                     ⁢ 
                        
                     sin 
                     ⁢ 
                        
                     
                       ( 
                       
                         α 
                         L 
                       
                       ) 
                     
                   
                   
                     
                       d 
                       ⁢ 
                          
                       cos 
                       ⁢ 
                          
                       
                         ( 
                         
                           α 
                           L 
                         
                         ) 
                       
                     
                     + 
                     
                       L 
                       t 
                     
                   
                 
                 ) 
               
             
           
         
       
     
     The local controller  50  can thereafter derive the effective distance (d eff ) as a function of the above-described angle (γ), i.e.,: 
     
       
         
           
             
               d 
               eff 
             
             = 
             
               
                 
                   d 
                   ⁢ 
                      
                   cos 
                   ⁢ 
                      
                   
                     ( 
                     
                       α 
                       L 
                     
                     ) 
                   
                 
                 + 
                 
                   L 
                   t 
                 
               
               
                 cos 
                 ⁢ 
                    
                 
                   ( 
                   γ 
                   ) 
                 
               
             
           
         
       
     
     The corresponding azimuth angle (α) may then be calculated using the following equation: 
       α=α F +α L −γ
 
     The local controller  50  is also programmed to change a distance setpoint based on the angle (γ). This additional capability allows the local controller  50  to adapt in real time to the limited range of motion of the tether device  25 . By way of an illustration: 
       Distance setpoint=( r   c   +L   t )·(1− k   γ γ)
 
       Distance error= d   eff −distance setpoint
 
     where r c  is a nominal following distance setpoint and k γ  is a calibration value. The local controller  50  can then control operation of the follower  12 F in a closed loop to drive the distance error to zero. 
     Additional functionality of the local controller  50  includes the real-time estimation of the velocity of the leader  12 L, which the local controller  50  then uses to define the desired velocity of the follower  12 F. This allows the follower  12 F to move in unison with the leader  12 L, in the absence of a communication channel between the two: 
         V   L   =V   F  cos(α F +α L )+ {dot over (d)}  cos(α L )
 
     This capability allows the platoon  120  to corner sharply in tight spaces. 
     Referring now to  FIG.  6   , a representative corridor  100  is shown in which the platoon  120  negotiates a sharp turn in a hallway  85  demarcated by boundaries  90 , e.g., walls, barriers, or simple lane markers or lines on a plant floor. Using the present control strategy, the follower  12 F sets its desired velocity based on the estimated velocity of the leader  12 L and the azimuth (α) to ensure that, during such a tight turn, the velocity V F  of the follower  12 F does not overshoot. Avoidance of overshoot ensures that the follower  12 F is not pushed off track, which in turn optimizes cornering in tight spaces as exemplified by corridor  100 . 
     In terms of an associated velocity commands (V F_Cmd ) of the follower  12 F, and with reference to  FIG.  7   , the local controller  50  of  FIG.  2    calculates this value as a function of the above-described parameters: 
         V   F_cmd   =V   L ·cos(α L )=( V   F  cos(α F +α L )+ {dot over (d)}  cos(α L ))cos(α L ).
 
     Calculation of the velocity commands in this manner allows the follower  12 F to slow down or stop at intersections of the corridor  100  as needed to allow the leader  12 L to complete its cornering. This capability allows the leader and follower to form a 90° articulation at right angle corners. The relative yaw dependent speed command thus ensures the follower  12 F first corrects its own heading before speeding up. 
     The local controller  50  of  FIG.  2    may also use a velocity term V F   2  to provide a faster response at higher velocities of the platoon  120 , so as to better compensate for a relatively slow response of the tether device  25 . By way of an example calculation: 
     
       
      
       V 
       cmd 
       =k 
       d 
       ·d 
       error 
       +V 
       F_cmd  
      
     
         r   cmd =( k   α   +k   α,v   ·V   F   2 )α+ k   α,d   ·d ·sin(α)
 
     where k α , k α,v  and k α,d  are calibratable gain constants. The velocity term also helps prioritize lateral motion correction over longitudinal motion correction, thereby overcoming non-holonomic constraint of the follower  12 F. 
     The foregoing teachings may be implemented in method form, as will be appreciated by those skilled in the art. For instance, one may program the local controller  50  to execute instructions embodying a method for controlling the platoon  120  described in detail above. Such a method may include measuring, via the sensor suite  40 S of  FIGS.  1  and  2   , a velocity of the follower  12 F, an azimuth angle defined between the tether device  25  and the leading edge  53  of the follower  12 F, and the length of the tether device  25 . As part of such a method, the local controller  50  adaptively moves the VTP  80  on the leader  12 L, doing so in response to the velocity, the azimuth angle, and the length. The local controller  50  thereafter controls a dynamic output state of the electric powertrain system  30  ( FIG.  2   ) of the follower  12 F using the VTP  80 . 
     The solutions detailed above provide a number of controls and motion planning algorithms for control of the platoon  120  of lightly-tethered or wirelessly connected followers  12 F constructed as e-pallets as set forth above. The platoon  120  may operate in a variety of environments often having tight turns, e.g., intersecting perpendicular hallways such as the corridor  100  of  FIG.  6   , which the platoon  120  might not otherwise be able to negotiate absent the present teachings. Thus, the ability to control the spacing, azimuth, and velocity of a follower  12 F in an environment devoid of V2V communications, according to the foregoing strategy, greatly expands the utility of the platoon  120  and enables a more widespread adoption thereof in a host of industries. These and other attendant benefits will be readily appreciated by those skilled in the art in view of the foregoing disclosure. 
     The detailed description and the drawings or figures are supportive and descriptive of the present teachings, but the scope of the present teachings is defined solely by the claims. While some of the best modes and other embodiments for carrying out the present teachings have been described in detail, various alternative designs and embodiments exist for practicing the present teachings defined in the appended claims. Moreover, this disclosure expressly includes combinations and sub-combinations of the elements and features presented above and below.