Abstract:
A steering system and method are capable of steering a plurality of vehicles arranged in a train with adjacent vehicles pivotally connected to each other for movement about a vertical axis. Each vehicle has a pair of steerable wheels with one pair at one end of the train being a selected leading pair having its steering angle determined by an operator. An electrical control system automatically steers all of the wheels trailing behind the leading pair. Vehicle angle sensors measure intercar angles between adjacent vehicles and provide this information to the control system. An indicator provides the controller with the current distance traveled by the train. Wheel angle sensors provide signals indicative of the current steering angle for each wheel pair. The controller adjusts the actual steering angle for each trailing pair to a desired angle by calculating adjustments based on the measurement inputs.

Description:
This application is a continuation of International application no. PCT/CA2008/000242 filed on Feb. 7, 2008, and claims priority of Canadian Patent Application No. 2,588,161 filed on May 9, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to steering systems and methods for a train of vehicles, such as a train of mobile conveyor machines. 
     Belt conveyors are well known and are efficient means for moving large quantities of materials such as ore, coal and granular stone over a predetermined distance extending either horizontally, vertically or both. One form of conveyor system known for mining applications is a system involving a series of conveyors mounted on wheels so as to make the system easily movable. Because of the manner in which mines are developed and extended, it may be necessary for a relatively long conveyor system to be moved along a substantially curved or zig zag course. Under such circumstances, it can be difficult and time consuming to move the conveyor system when required. It will also be appreciated that it may be necessary to move the conveyor system and to make adjustments to the system fairly frequently as the mining machine advances in a mine. 
     U.S. Pat. No. 5,366,059 issued Nov. 22, 1994 to Prairie Machine &amp; Parts Mfg. Ltd. describes and illustrates a conveyor system comprising a plurality of conveyor vehicles connected together in the form of a train and also describes a steering system for steering this train of vehicles. All but one of the vehicles in the train has a single pair of steerable wheels with the vehicle at the outby end of the train (that is the end to which the mined material is being delivered) having two pairs of steerable wheels. Hydraulic cylinders are used to steer each of the pairs of steerable wheels and there is a control mechanism for controlling and coordinating these cylinders in order to set the steering angles of the pairs of wheels. 
     The aforementioned known steering system uses a control system that has sensors for determining the current steering angle for a selected pair of wheels and generating an electrical signal indicative thereof and an electronic memory for storing a series of these electrical signals as the train is travelling. There is also a mechanism for determining the distance the wheels on the train have been travelling. The control system sets the steering angle for each pair of wheels other than the selected pair at substantially the same steering angle that the selected pair had when they were at the location where the respective further pair is located. 
     One difficulty with this known system is that all of the axles in the train except for first and last axles must be pivotally connected to adjoining vehicles by front and rear pivot devices, each providing a vertical pivot axis. The front pivot device is located forwardly of its respective axle and the rear pivot device is located rearwardly of its respective axle. Furthermore, this steering system requires a mechanism for locking each of the axles (except for the first and last axles) in a position at a right angle to the longitudinal centre line of either the vehicle immediately in front of the respective axle or the vehicle immediately to the rear thereof. Needless to say, this type of pivoting axle system adds substantially to the cost of these conveyor vehicles. Moreover, although it is desirable to provide a conveyor vehicle system which does not require a high mine ceiling in which to operate, this known steering system which requires the use of a series of axles on which to mount the wheels is not particularly desirable from the standpoint of reducing the height of the conveyor system. 
     There is a need in the mobile belt conveyor industry to provide an improved train of conveyor vehicles which can be easily and reliably steered automatically by an operator, for example an operator located at the leading end of the train of vehicles who is steering a leading set of wheels, that is the pair of wheels at the end of the train towards which the train is moving. 
     In addition, there is a perceived need to provide a steering system that can be used to steer a plurality of vehicles arranged end-to-end in a train, this train of conveyors being suitable for use in low mining seams, and in particular a steering system for such a train of vehicles that can be used without the need to mount each pair of wheels on a single axle that can be pivoted about a vertical pivot axis. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, a steering system capable of steering a plurality of vehicles in a train includes a selected pair of transversely aligned propelling devices, each including a steerable ground engaging wheel and adapted for mounting on opposite sides of one of the vehicles. The system also has a first power actuator mechanism for steering the selected pair of propelling devices as the train moves over the ground and at least two further pairs of transversely aligned propelling devices, each pair including two steerable ground engaging wheels. Each of the further pairs is connected to one or more further vehicles and supports same. Each propelling device of each further pair is adapted for mounting on a respective one of two opposite sides of its respective further vehicle. A second power actuator mechanism is provided for steering each of the further pairs of propelling devices and at least one angle sensor is provided for measuring a selected intercar angle between the or each pair of adjacent vehicles and generating a first electrical signal indicative thereof. The system further includes a controller for controlling each second power actuator mechanism in order to set a steering angle of each of the further pairs of propelling devices. This controller includes a device for indicating distances that the wheels in the train have been travelling from a selected location and mechanisms for determining the current steering angle of each of the selected pair and the at least the two further pairs of propelling devices and generating respective second electrical signals indicative thereof. The controller also has means for storing a series of the steering angles of the selected pair measured by the determining mechanisms as the train of vehicles is travelling on a support surface and a system for sending electrical steering signals to the second power actuator mechanisms in order to operate the second power actuator mechanisms and to thereby steer the further pairs of propelling devices. The controller, during use of the steering system, adjusts the current steering angle of each further pair to a desired steering angle on the basis of the distance traveled by the train from the selected location, the determined current steering angle of the respective pair, and the measured intercar angle for the respective pair. The intercar angle for each pair of adjacent vehicles is defined by central longitudinal axes of the respective pair and a pivot point about which one of the respective pair can pivot relative to the other vehicle of the respective pair in a generally horizontal plane. 
     In an exemplary embodiment of this system, each of the propelling devices includes a hydraulic motor for driving its respective wheel and a non-rotating wheel support structure for detachably connecting the propelling device to a main frame of the respective vehicle, which is a conveyor vehicle. 
     According to another aspect of the invention, a steering system is provided for at least three vehicles connected together to form a train. Adjacent vehicles in the train are pivotally connected to each other for pivotable movement about a substantially vertical axis and each vehicle has a pair of steerable wheels mounted thereon and supporting the vehicle. These pairs of steerable wheels include a selected leading pair mounted on one of the vehicles located at one end of the train and this leading pair has its steering angle determined by an operator controlling a steering unit of a leading pair. Each pair has an actuator mechanism for steering its respective pair of wheels which are mounted on opposite sides of the vehicle. The steering system includes an electrical controller for automatically steering all of the pairs of wheels trailing behind the leading pair, this controller including a memory unit for storing sensed data. There are also vehicle angle sensors for measuring selected intercar angles between adjacent vehicles, generating first electrical signals indicative thereof, and transmitting these first electrical signals to the controller system. The intercar angle is an angle defined by central longitudinal axes of the respective pair of adjacent vehicles and the substantially vertical axis about which one of the respective pair can pivot relative to the other vehicle of the pair. This system also has a first wheel angle sensor for providing an electrical signal indicative of the current steering angle of the selected leading pair of wheels to the controller. The memory unit is adapted to store a series of the sensed steering angles for the selected leading pair. Each of the stored steering angles corresponds to the steering angle of the leading pair at a series of locations along a path of travel of the train. Additional wheel angle sensors can sense actual steering angles of the pairs of wheels trailing behind the leading pair and provide electrical signals indicative thereof to the controller. The controller during use of the steering system adjusts the actual steering angle for each trailing pair of wheels to a desired steering angle. The controller calculates an adjustment amount for each trailing pair on the basis of the following parameters: 
     (i) the current distance traveled by the train; 
     (ii) a respective one of the stored series of sensed steering angles, the controller choosing the respective one of the steering angles representing the steering angle of the leading pair of wheels when the leading pair was at approximately the current location of the respective trailing pair, and 
     (iii) the current intercar angle associated with the respective trailing pair, this associated intercar angle having its defining vertical axis close to the respective trailing pair. 
     In a particular exemplary embodiment, the steering angle of a respective one of the trailing pairs is only adjusted if the desired steering angle is either greater than a small positive predetermined amount or less than a small negative amount equal to the small positive predetermined amount multiplied by −1. 
     According to a further aspect of the invention, a method of steering a train of at least three vehicles connected together includes providing a train of at least three vehicles comprising first and second end vehicles and at least one intermediate vehicle, with each vehicle having at least one pair of propelling devices mounted thereon and supporting the vehicle. Each propelling device includes a steerable wheel pivotable about a substantially vertical axis in order to steer the vehicle. The propelling devices of each pair are mounted on opposite sides of the respective vehicle. Each vehicle further includes a power steering mechanism for pivoting the wheels of the pair about their respective vertical axes and a controller for operating the power steering system in order to steer the vehicle. Adjacent vehicles of the train are pivotally connected to each other for pivotal movement about a substantially vertical vehicle pivot axis. The method includes causing the train to move over ground in a desired direction towards one of the end vehicles and steering a leading pair of the wheels on the one end vehicle to a desired steering angle. This desired steering angle is sensed on a continual or frequent periodic basis and first signals are provided which are indicative of these steering angles to the controller. A series of these desired steering angles is stored as the train is moved over the ground. The current steering angle for each pair of wheels trailing the leading pair of wheels relative to the direction of travel of the train is also sensed on a continual or frequent periodic basis. An indication of the distance traveled by the train from a selected location is provided to the controller. Also readable indications of intercar angles between pairs of adjacent vehicles are provided to the controller. Each intercar angle is defined by central longitudinal axes of a respective pair of the vehicles and the vehicle pivot axis of the respective pair. Steering angle corrections are calculated for trailing pairs of the wheels by means of the controller as the train is moving, these steering angle corrections being a function of the following: 
     (i) the distance traveled by the train of vehicles; 
     (ii) the stored desired steering angles of the leading pair of vehicles with the controller selecting a stored steering angle for each trailing pair that represents the desired steering angle when the leading pair of wheels was at about the same location where the respective trailing pair is currently located; 
     (iii) the current intercar angle associated with the respective trailing pair, this associated intercar angle having its defining vertical axis located between the respective trailing pair; and 
     (iv) the current steering angle of each pair of wheels trailing the leading pair of wheels. 
     In an exemplary version of this method, the controller includes a programmable logic controller (PLC) on each of the vehicles, these PLCs including a master PLC and a plurality of intelligent slave PLCs. 
     These and other aspects of the disclosed steering system and steering method for steering a plurality of vehicles in a train, and in particular a train of conveyor machines will become more readily apparent to those having ordinary skill in the art from the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, 
         FIG. 1  is a top view of an exemplary embodiment of an intermediate mobile conveyor machine or vehicle steerable with the steering system of the present invention; 
         FIG. 2  is a side elevation of the intermediate conveyor vehicle of  FIG. 1 ; 
         FIG. 3  is a bottom view of the conveyor machine of  FIGS. 1 and 2 ; 
         FIG. 4  is a sectional elevation taken along the line IV-IV of  FIG. 2 ; 
         FIG. 5  is a perspective view taken from above and from the tail pulley end of the conveyor machine, this view showing an end section of the machine including its two wheels; 
         FIG. 6  is a sectional elevation taken along the line VI-VI of  FIG. 1 , this view showing details of the power steering arrangement for each wheel; 
         FIG. 7  is a detail top view of a left hand wheel unit assembly of the vehicle of  FIGS. 1 and 2 , this view omitting the wheel itself for sake of illustration; 
         FIG. 8  is a detail sectional elevation taken along the line VIII-VIII of  FIG. 7 ; 
         FIG. 9  is a detail end view of the wheel unit assembly of  FIG. 7 , this view being taken from the left side of  FIG. 7  and showing the wheel mounted on the assembly; 
         FIG. 10  is a detail sectional elevation taken along the line X-X of  FIG. 9 ; 
         FIG. 11  is a perspective detail view of a mounting plate weldment used to support each wheel and its hydraulic motor; 
         FIG. 12  is a detail perspective view illustrating a pivotable motor support member mounted adjacent each wheel; 
         FIG. 13  is a vertical cross-section taken along the line XIII-XIII of  FIG. 15  illustrating how the rollers of the hitch unit engage the curved track; 
         FIG. 14  is a detail view showing the transverse cross-section of the curved track according to an exemplary embodiment; 
         FIG. 15  is a bottom view showing a portion of the curved track and the hitch unit mounted for rolling movement on the track; 
         FIG. 16  is a vertical cross-sectional detail taken along the line XVI-XVI of  FIG. 15 ; 
         FIG. 17  is a top view of a roller mounting support body which is part of the hitch unit; 
         FIG. 18  is a vertical cross-section of the roller mounting support body taken along the line XVIII-XVIII of  FIG. 17 ; 
         FIG. 19  is a perspective view of the roller mounting support body of  FIG. 17  taken from above and from its pivot pin end; 
         FIG. 20  is perspective view of a pivoting hitch frame which is pivotably connected to the support body of  FIG. 17 , this hitch frame being shown from above and from an inner side thereof; 
         FIG. 21  is an inner side view of the hitch frame of  FIG. 20 ; 
         FIG. 22  is a vertical cross-section of the hitch frame taken along the line XXII-XXII of  FIG. 21 ; 
         FIG. 23  is a perspective view of an intercar angle sensor assembly mountable on the hitching apparatus; 
         FIG. 24  is a bottom view of the sensor assembly of  FIG. 23 ; 
         FIG. 25  is an axial cross-section of the sensor assembly taken along the line XXV-XXV of  FIG. 24 ; 
         FIG. 26  is a partial bottom view of the conveyor machine showing the end where the tail pulley is mounted and the hitching apparatus; 
         FIG. 27  is a detail plan view of the hitching apparatus mounted on one end of a mobile conveyor machine and showing an adjacent end section of an adjacent conveyor machine connected thereto; 
         FIG. 28  is a vertical cross-section taken along the line XXVIII-XXVIII of  FIG. 27  showing one of the two pivot pin connections joining the two conveyor machines; 
         FIG. 29  is a side elevation of a loading car vehicle which can be provided at a loading end of a train of conveyor vehicles; 
         FIG. 30  is a side elevation of a discharge car vehicle which can be provided at an unloading end of a train of conveyor vehicles; 
         FIG. 31  is a top view of the discharge car vehicle of  FIG. 30 ; 
         FIG. 32  is a schematic illustration of a network of programmable logic controllers (PLCs) for a train of conveyor vehicles; 
         FIG. 33  is an electrical circuit diagram illustrating the circuits connected to each PLC to steer each conveyor vehicle and to cause this vehicle to turn in a desired direction; 
         FIGS. 34A and 34B  are two parts of a flow chart illustrating the steering logic used for steering each pair of wheels trailing a leading pair of wheels; and 
         FIGS. 35A and 35B  are left and right portions of a hydraulic schematic illustrating the hydraulic systems used to drive and to steer each conveyor vehicle. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Major components of an intermediate conveyor vehicle, which can be steered with the present steering system and method along with other vehicles connected thereto, are illustrated in  FIGS. 1 to 5  of the drawings. The illustrated, low profile intermediate conveyor vehicle  10  has been shown without the usual flexible conveyor belt, the location of which is only indicated in chain-link lines in  FIG. 1  for sake of illustration. This conveyor belt  12  is an endless conveyor belt and can be of standard construction depending upon the type of material being conveyed by the conveyor system. The illustrated exemplary vehicle is intended for use as an intermediate conveyor car of which there may be five, ten or more in a train of conveyor vehicles similar to the train illustrated and described in U.S. Pat. No. 5,366,059. It will be understood that in addition to a plurality of intermediate conveyor vehicles pivotably connected end-to-end, there can also be a loading conveyor vehicle  540  shown in  FIG. 29  which is located at the end of the train adjacent the mining machine, and a discharge conveyor vehicle  542  shown in  FIGS. 30 and 31  located at the opposite end of the train which is referred to as the outby end, that is, the end to which the train of vehicles delivers the material. The loading car assembly  540  can be constructed in a similar manner to the illustrated intermediate car assembly  10 , except that it need not be provided with a hitch mechanism at its inby or hopper end, since there is no need to attach this end to another conveyor vehicle. Also the loading car normally has a larger hopper  544  to receive the ore from the mining machine. As for the discharge conveyor vehicle  542 , it is provided with two pairs of transversely aligned wheel units  546 ,  548  rather than a single pair of these wheel units described hereinafter. However, the wheel units on the discharge car can be constructed in the same manner as described hereinafter, including their steering mechanism and their hydraulic drive mechanism. The discharge car is also provided with a pivotable cross-conveyor  550  for discharging the material onto a permanent or fixed conveyor in the mine. A cross-conveyor and its use is described and illustrated in U.S. Pat. No. 5,366,059. A detailed description of its cross-conveyor herein is deemed unnecessary as cross-conveyor systems are well known in the mobile conveyor industry. 
     Turning now to the intermediate conveyor vehicle  10 , this vehicle has a conveyor mechanism  14  that includes an elongate, substantially horizontal frame  16  and a series of spaced apart conveyor roller devices  18  mounted on the horizontal frame  16  and adapted to support rotatably an upper run of the continuous conveyor belt  12  extending between opposite end sections of the vehicle. The roller devices  18  can be of standard construction available from conveyor parts suppliers. Each illustrated roller device comprises three metal rollers  20  which are pivotably connected together in an end-to-end fashion by their central shafts. The outer end of each outer roller is connected by a chain  22  (see  FIG. 5 ) to a vertical support post  24  mounted on a main, longitudinally extending frame member of the main frame  16 . The height of each pair of posts  24  varies as shown to gradually increase the height of the roller devices. In addition to the cylindrical, rotatable metal rollers  20 , there can also be provided impact rollers  26  of known construction positioned below a U-shaped hopper member  28 . It will be understood that the impact roller helps to absorb the impact of material dropping onto the conveyor belt at this location. 
     The conveyor mechanism  14  further includes a tail pulley unit mounted adjacent one end of the conveyor mechanism on the horizontal frame  16  and having a rotatable tail pulley indicated at  32 . Further details of the construction of the tail pulley unit are provided hereinafter with reference to  FIGS. 5 and 26 . The conveyor mechanism  14  further includes a head pulley unit  34  mounted adjacent the second end of the conveyor mechanism opposite the first end where the tail pulley is located. The head pulley unit includes a rotatable head pulley  36  which, in a known manner, can be provided with a gripping cylindrical surface which enables the head pulley unit to drive the conveyor belt  12 . There is also an electric motor mechanism  38  which can be considered part of the head pulley unit since it rotates the head pulley  36  to move the conveyor belt and thus to transport material from the tail pulley to the head pulley. A belt scraper  40  of known construction can be mounted adjacent to the head pulley to help keep the conveying surface of the belt clean. Mounted adjacent to the head pulley at the outby end of the vehicle is a material hopper  42  which helps direct the material onto the conveyor belt of the next conveyor vehicle of the train. In order to provide a conveyor vehicle  10  having a low profile, there is provided a pivoting hitch mechanism  44  at the inby end of the vehicle. This hitch mechanism includes a curved steel track and a rolling hitch device  48  having two sets of grooved rollers located at  50  and  52  on two opposite V-shaped sides of the track  46 . Two car hitch pins  54  are located on opposite sides of the hitch device  48  which is able to pivot about a central longitudinal axis of the vehicle by mean of central pivot pin  56  (see  FIG. 13 ). Located near the opposite head pulley end of the car are two hitch pin holders  60 , one on each side of the frame  16 . 
     The conveyor vehicle  10  has a pair of transversely aligned wheel units indicated generally at  62  for supporting and moving the conveyor vehicle. Each of these wheel units is separately connected to the horizontal frame  16  including any extension thereof. In particular, each wheel unit is connected to a respective longitudinally extending side of the frame. Each wheel unit has its own solid wheel with the wheel on the left side indicated at  64  and the wheel on the right hand side indicated at  66 . As explained more fully hereinafter, each wheel  64 ,  66  is mounted for pivotable movement about a substantially vertical pivot axis for steering purposes, that is, to steer the vehicle  10 . In an exemplary embodiment of the conveyor vehicle, each wheel unit includes a standard hydraulic motor  68  shown clearly in  FIG. 8 . This motor is used to rotate or drive the wheel of the respective wheel unit. Also, each wheel unit includes a non-rotating wheel support structure indicated generally at  70  for detachably connecting the wheel unit to the horizontal frame, including any extension of this frame. The left hand wheel unit  62 , with its wheel removed, is illustrated in  FIG. 7  and is illustrated with its wheel in  FIGS. 8 and 9 .  FIG. 7  also shows a power steering mechanism or power steering means  72  for steering the wheel of this wheel unit. The illustrated power steering mechanism includes a hydraulic linear actuator having a hydraulic cylinder  74  and an actuator rod  76  slidable in the cylinder. A steering arm  77  having a L-shape is rigidly attached at one end to an upper section of a motor support member  144  (see  FIG. 12 ) and is pivotably connected at its other end to the rod  76  by means of a bolt and nut  80  (see  FIG. 9 ). The closed end of cylinder  74  is pivotably mounted by means of lugs  82  to an end of a horizontally extending, elongate arm section  84  which is part of a wheel unit mounting plate  86 . A nut and bolt combination  88  pivotably connects a short connecting plate  90  that is rigidly attached to the end of the cylinder to the lugs  82 . The actuator rod  76  can be provided with a spherical bearing  92  that is connected by threads to the outer end of the rod. This bearing is connected to the steering arm  77  by the nut and bolt  80 . Each hydraulic linear actuator  72  in an exemplary embodiment includes means for determining the current steering angle of the pair of wheels and for providing an electrical signal indicative thereof to the PLC for the vehicle. In one particular embodiment, the steering angle sensor is a linear position sensor sold under the trade name Positek, this sensor being indicated at  75  in  FIG. 33 . This sensor is mounted in the hydraulic cylinder  74  and the size of the signal generated is dependent on the amount of extension of the rod  76 . 
     In addition to the arm section  84 , the flat mounting plate  86  includes a main plate portion  96  shown in  FIG. 6 . This main plate portion has a generally rectangular shape except for cut-off bottom corners  98 . The arm section  84  extends horizontally from an upper corner of the main plate portion  96 . An advantage provided by the arm section  84  is that the hydraulic cylinder can then be pivotably mounted to the same mounting plate  86  as the wheel and its hydraulic motor  68 . As can be seen from  FIGS. 6 and 11 , each wheel unit  62  and, in particular its mounting plate  86  (which is part of the wheel support structure), is formed with a plurality of apertures or holes indicated generally by reference  100  which are provided to receive fasteners, preferably bolts, used to attach the respective wheel unit to the frame  16 , including any extension thereof. A plurality of fasteners  102  for this purpose are insertable through spaced-apart apertures  104 , four of which can be seen in  FIG. 6 . There are a plurality of the apertures  104  formed in each longitudinally extending side of the frame  16  and optionally additional apertures can be provided in extension plates attachable to the main frame members. The apertures  100  formed in the mounting plate  86  are located in opposite end sections of the mounting plate as clearly shown in  FIG. 11 . The illustrated aperture arrangement permits the height of each wheel unit relative to the horizontal frame  16  to be adjusted between either one of two possible positions, but it will be appreciated by those skilled in the art that by providing further apertures  104 , for example, on each longitudinal frame member or an extension plate, it is possible to provide for more than two possible height positions for each wheel unit. In the position of the wheel unit illustrated in  FIG. 6 , the wheel unit  62  is at its maximum height relative to the frame  16 . In this position, the overall height of the conveyor vehicle will be a minimum height which, in an exemplary embodiment, is only four feet or forty-eight inches as compared to earlier conveyor vehicles such as those described and illustrated in U.S. Pat. No. 5,366,059 which had an overall height of six feet or seventy-two inches. In this position of the wheel units in the exemplary embodiment, the ground clearance provided under the vehicle is six inches. However, in the event that mining conditions require greater ground clearance and provided the mine area has an adequate ceiling or working height for the conveyor system, the wheel units can be moved to the second position which can provide an additional four inches of ground clearance for a total of ten inches. In this case, the overall height of the conveyor vehicle is fifty-two inches. 
     Turning now to the wheel support structure  70 , shown in  FIGS. 8 and 11 , the wheel support structure includes upper and lower, horizontally extending wheel supporting arms  120 ,  122 , both with rounded distal ends. The upper arm  120  can be formed from a single steel plate welded to the top of mounting plate  86  and is formed with a round hole  122  to receive an upper pivot pin member. As illustrated, the lower support arm  122  can be constructed of two short plate members  124 ,  126  which are welded together at  128  and which extend at an obtuse angle to one another as shown in  FIG. 8 . The strength and rigidity of the connection between the sloping plate  124  and plate  86  can be strengthened by two vertically extending gussets  130  which are welded to these plates. A top pivot pin  132  is mounted in the hole  122  and is connected to the upper arm  120  by six screws  134 . The pivot pin can be provided with a central passageway (not shown) that extends downwardly from grease zerk  136 . There is also a bottom pivot pin  138  having a reduced top end extending into a circular recess  140  formed in the rounded end section of the lower support arm  122 . 
     In order to pivotably support the wheel and its hydraulic motor  68 , there is provided a substantially annular motor support member  144  shown in  FIG. 12 . This support member has a circular recess  146  formed on its top side and into this recess a reduced bottom end section of the top pivot pin  132  extends. Mounted in this recess is a spherical angular contact bearing  148  which, in one embodiment, has a bore measuring 1¾″and has an outside diameter of 2 13/16 inch. Protecting this bearing and extending around the top edge of the bearing is a suitable seal such as a Chesterton Super Wiper seal  150 . Similarly, extending around a reduced upper portion of the bottom pivot pin is a spherical angular contact bearing  152  which is sealed by means of a Chesterton super wiper seal  154 . The bottom pivot pin can be greased through grease zerk  156 . 
     Returning to  FIG. 12 , it will be seen that the motor support member  144  has a bottom extension  160  which is welded to the annular portion of the support member  144  and which has a circular hole  162 . The bottom pivot pin projects through the hole  162  from the bottom and is detachably connected to the extension  160  by six screws  164  which extend through a flange extending around the bottom of this pivot pin. Formed between the extension  160  and the annular portion of support member  144  is a cavity  166  which receives the rounded end portion of the horizontal plate  126 . In this way, the support member  144  is pivotably supported from below. 
     It can also be seen from  FIG. 12  that the support member  144  has a radially inwardly extending connecting flange  170 , this flange being formed with a series of fastener holes  172 . As shown in  FIG. 10 , six screws  174  can be used to attach the hydraulic motor  68  to the flange  170  along with its associated planetary gear box  176 . The planetary gear box has an annular rotating flange  178  which is attached by nine hex nuts  180  to a circular plate  182  forming a central portion of the hub of the wheel. The nuts are threaded onto studs  184  visible in  FIG. 7 , these studs extending through the rotating flange on the gear box. It is understood that the left and right wheels  64 ,  66  are solid rubber wheels and, in one embodiment, each wheel measures  10 ″×24″ in diameter. The left and right wheels  64 ,  66  are connected by a steering tie rod  190  shown in  FIG. 4  which ensures that the wheels pivot in the same way at the same time. It is connected at each end to the tie rod arm  78  of the respective wheel by means of a bolt with a nylon insert lock nut  192 . 
     It will be understood that the hydraulic motor for each wheel unit is provided with pressurized hydraulic fluid through hydraulic lines and fittings of standard construction which are readily available and well known in the art. Most of these lines are not shown for ease of illustration. Some of these lines are indicated at  194  in  FIG. 10 . Connecting fittings for these lines can be supported by a small bracket  196  shown in  FIG. 12 . It will be understood that the hydraulic motor itself and its gear box are of standard construction and accordingly a detailed description herein is deemed unnecessary. 
     Various other features in the illustrated exemplary low profile conveyor vehicle that are shown in  FIGS. 1 to 3  include a plastic energy chain  200  through which electrical cables and wires are fed for the operation of the vehicle and an energy chain guide  202  which helps to support the movement of the energy chain. Mounted to the frame on the left side is an electrical power box  204  of standard construction, this box having an access door  206 . Mounted to the same side of the frame is an electric motor  208  which powers first and second hydraulic pumps  210  and  212 , the first pump  210  being used to drive the hydraulic motors for the wheels and the second motor  212  being used to power other hydraulic components on the vehicle. Two standard filters for the hydraulic system are provided at  214  on the right side of the vehicle. Mounted above these filters is a junction box  216 . A third hydraulic filter can be provided at  218  adjacent the pump  212 . On or between the two longitudinal main frames of the frame  16  and adjacent one of the cross-frames  220  is a hydraulic fluid reservoir  222 . Another junction box for electrical components including connectors is provided on the right side at  224 . The side mounted electrical motor  38  for the head pulley is connected to a conveyor gear box  226  which has an output shaft connected to the shaft of the head pulley. In one embodiment, the motor  38  is a 7.5 kwatt or 10 hp motor. Also on the right side of the vehicle there is mounted to the longitudinal frame member a hydraulic assembly manifold  230  which is protected by a shroud or guard  232 . On the same side of the frame near the motor  38  is a control box containing a programmable logic controller for controlling the operation and steering of the vehicle, the box indicated at  234 . In a known manner, the vehicle  10  can also be provided with water sprayers, two of which are indicated at  240 ,  241 . Water hoses (not shown) are connected to the sprayers to reduce dust levels generated by the conveyor system. 
       FIGS. 13 and 15  illustrate the pivot mechanism or pivoting hitch mechanism  44  for pivotably connecting the mobile conveyor machine of  FIGS. 1 and 2  at its inby end (also sometimes referred to herein as its first end) to an adjacent end section of another mobile conveyor machine which can be constructed in the same manner as the machine or vehicle of  FIGS. 1 and 2 . As indicated above, this pivot mechanism includes the curved track  46  which can be of uniform transverse cross-section and, in an exemplary version, has the cross-section illustrated in  FIG. 14 . The curved track is bent in a horizontal circular arc as clearly shown in  FIG. 5 , for example, and this arc has a center of curvature located midway between the propelling devices, that is the wheels  64 ,  66 . This center of curvature is indicated at C in  FIG. 3 . The center of curvature is on a common axis of rotation for the two wheels when these two wheels are positioned to move the conveyor mechanism in a straightforwards direction. This axis of rotation is indicated at A in  FIG. 26 . The track is rigidly mounted on the supporting frame  16  which includes a curved bumper frame  350  having a rectangular transverse cross-section, this frame extending the length of the track. The track, which is preferably made of machined solid steel, can be welded to the bumper frame. Each end of the track can be fitted with a rectangular stop plate  352  secured in place by screws (for example, three screws) threaded into holes formed in each end of the track. This plate  352  can be provided with an additional hole (not shown) to secure its respective end of a roller chain  374  (described below). An exemplary form of the track has a cross-section such as that shown in  FIG. 14 . The track has two opposite roller engaging sides  354 ,  356 , with the side  354  being on the inner side of the track and forming a concave curve and the side  356  being on the outer side and forming a convex curve. Each of these sides in the exemplary illustrated version engages three rollers with one of the rollers engaging the side  356  being shown in cross-section at  358  in  FIG. 13 . Another roller  360  is shown in part in  FIG. 13  and this is one of the three rollers engaging the side  354 . The three rollers engaging the side  356  form a first set of rollers and the three rollers engaging the side  354  form a second set of rollers. It will be seen that the track  46  is captured and held between the first and second set of rollers. The rollers of both sets have V-grooves  362  formed about their circumferences. The use of three rollers in each set helps keep the hitch unit correctly oriented on the track at all times. 
     Turning now to the exemplary cross-section illustrated in  FIG. 14 , the outer convex side  356  which faces towards an adjacent end of the mobile conveyor machine has an upper sloping surface  364  which extends at a 45° angle to the vertical centerline Z of the track. The outer surface also has a lower sloping surface  366  which extends at a 45° angle to the axis Z and there can be a short vertical surface provided at  368 . The inner roller engaging side  354  is similarly shaped with 45° sloping surfaces at  370  and  371 . These surfaces can be formed by a standard machining process. Also formed in the track member is a rectangular groove  372  which can extend the length of the track member on the side  356 . The purpose of this groove is to accommodate a length of roller chain  374  used in conjunction with an angle sensor described hereinafter. 
     Turning now to the construction of a roller mounting support body  376  illustrated in  FIGS. 17 to 19 , this body is used to rotatably support the aforementioned two sets of rollers which engage the track  46 . This body includes a horizontally extending support plate  378  which can have a generally trapezoidal shape and is formed with six circular holes  380  which accommodate upwardly extending shafts  381  of the rollers. If desired, a shallow circular recess  382  can be formed around each hole to partially accommodate a nut  382  which is shown in  FIG. 13  and threaded onto the roller shaft by suitable threads (not illustrated). Each roller unit is a standard roller and therefore has not been shown in detail. The support body also has a vertical pivot pin support plate  384  which is fixedly connected to an edge of the roller support plate  378 . The two plates can be welded together at  386 . The horizontally extending, central pivot pin  56  is mounted in a circular hole formed centrally in the support plate  384 . The pin  56  is formed with a circumferential flange  388  near its inner end, this flange resting against the support plate  384 . The inner end of the pivot pin can be welded to the plate  384 . To strengthen the support body  376  two rectangular side plates can be welded thereto at  390 ,  392 . A support plate  394  can be welded to the top edge of the plate  384  and is shaped to form an obtuse angle. Four fastener holes  396  can be formed in the outer end of this support plate. The arm  394  is used to detachably connect one end of the aforementioned energy chain  200 . If desired, a cover plate  395  (see  FIG. 26 ) can be attached to the bottom of the support body  376  by means of screws  400  inserted through the cover plate and threaded into holes  402 . It will be understood that each roller is provided with internal bearings (not shown) of standard construction which allow the roller to rotate freely about its shaft. 
     Turning now to the construction of the pivoting hitch mechanism or hitch frame  44  illustrated separately in  FIGS. 20 to 22 , this frame is pivotably connected to the support body  376  by means of the pivot pin  56 . The pivot pin extends into a pin passageway which is formed in a transverse center of the frame. It will be understood that the passageway  404  which has a circular cross-section extends in a radial direction relative to the radius of the track  46 . The hitch frame includes a central block  406  in which the passageway is formed, two tubular arm sections  408 ,  410  and two end sections  412 ,  414  located on opposite sides of the pivot pin and spaced therefrom, these end sections being adapted for a pivot connection to an adjacent end section of a second or another mobile machine (similar to or the same as the illustrated machine of  FIGS. 1 and 2 ) during use of the hitch apparatus. Each arm section  408 ,  410  can be formed from a horizontal top plate  416 , a similar, horizontal bottom plate  418 , an inner rectangular plate  420  and a rectangular, vertical outer plate  422  (see  FIG. 5 ). These plates can be made of ¾ inch steel plate and can be rigidly connected by welding. Each end section  412 ,  414  can be formed from a bent steel plate forming an obtuse angle as shown in  FIG. 20 . The plate used can be one inch steel plate and its connection to its arm section can be strengthened by a triangular brace or gusset  424 . A circular hole  426  is formed in the rounded end of each end section to receive a respective one of the car hitch pins  56  shown in  FIGS. 1 ,  2  and  5 . Thus, the hitch apparatus of this invention can be pivotably connected to an adjacent second mobile machine by means of these hitch pins which permit relative pivotable movement about a horizontal axis between the two mobile machines or mobile conveyors. 
     As shown in  FIGS. 13 and 22 , a grease passageway  430  can be formed in the top of the block  406  and a grease fitting or grease zerk is mounted in the block at the outer end of this passageway. Extending around the pivot pin are fiberglass bushings  432  with one located adjacent the flange  388  and the other located adjacent the outer end of pin passageway  404 . Also, a fiberglass thrust bearing  434  can be sandwiched between the inner end of the block  406  in a shallow, circular recess  436  and the flange  388 . Hitch mechanism  44  is retained on the central pivot pin  56  by means of a donut-shaped retainer plate  435  which can be ⅞ th  inch plate having a central hole measuring 1 13/16 th  inch. Both the plate  436  and the pivot pin are formed with aligned holes to receive a dowel pin  438  which acts to prevent rotation of the plate relative to the pin. The plate  436  is held in place by 3½ inch long screw  440  which extends into a threaded hole formed in the center of the pivot pin. It will thus be seen that the hitch mechanism  44  is free to pivot about the horizontal pivot axis formed by the pivot pin thereby allowing relative movement about this pivot axis between the adjacent connected mobile conveyor vehicles. 
     For use with an automatic steering system for a train of these mobile conveyor machines of the type described above, it can be desirable for the steering system to know the intercar angle between adjacent cars in the train. Due to the fact that the present mobile conveyor machine has no pivot joint located at the pivot axis between adjacent cars (in other words, there is only a virtual pivot point midway between the two wheels of the machine described herein), a special intercar angle sensor can be provided in conjunction with the pivot mechanism of the present invention so that an electrical signal indicative of the intercar angle can be provided to the steering control for the conveyor train. An exemplary form of such a sensor is illustrated in  FIGS. 15 ,  16  and  23  to  25 . This sensor indicated generally by reference  450  is able to determine the angle between the central longitudinal axis of the illustrated mobile conveyor machine  10  and a central longitudinal axis of an adjacent mobile conveyor machine which can be constructed in the same or a similar manner as the illustrated machine. A complete steering system for a train of such vehicles or machines is provided with one of these angle sensors between each pair of adjacent vehicles. Although not shown in  FIG. 23 , the sensor includes the aforementioned tensioned roller chain  374  mounted on the track  46 . It will be appreciated that this chain forms a series of sprocket engaging recesses formed along one side of the track for at least most of the length of the track. These recesses are formed by recess forming members (ie. the pins of the chain). The recesses could also be formed by machining them into the track member itself. The other major component of the sensor is a rotational position transducer  452  which is mounted on the hitch mechanism  44  and, in particular, on the roller support body  376 . The sensor has a sensing sprocket  454  which drives a potentiometer to measure the intercar angle. The transducer  452  can, in one embodiment, send electrical signals on a continual or frequent periodic basis to a Siemens programmable logic controller (PLC) used to steer the train of vehicles. This transducer, which can be of standard construction, can have a signal output ranging between 4-20 milliamps with the output depending upon the sensed intercar angle. 
     With reference to  FIGS. 24 and 25 , in addition to the transducer, there is shown a mounting arm  456  which includes an annular end section  458  on which the transducer can be mounted. The arm  456  is attached by a pivot pin  457  to the bottom edge of the side plate  392 , this pivot pin extending through hole  458 . There can be mounted in this hole two Oilite friction bearings  460 , one at each end. Rotatably mounting the shaft for the sprocket  454  are two deep groove ball bearings  462  arranged next to one another. The bearings can be held in an opening by means of a retaining ring  464 . On the transducer side of the bearings there can be a further external retaining ring  466 . Welded to the arm on the side opposite the sprocket is a transducer mount  468  which extends through an arc of more than 270°. Attached to this mount by four screws  470  is a transducer mounting plate  472 . The central shaft of the transducer extends through this plate and is received within a central passageway formed in sprocket shaft  474  and is secured thereto (for example by a set screw) for rotation therewith. The transducer is detachably mounted to the plate  472  by four screws  476 . The end of an electrical control cable operatively connected to the transducer is indicated at  478 . 
     It will be seen from the above description that the transducer and its mounting are pivotably mounted to the plate  392  of the hitch mechanism. An elongate coil spring  480  (see  FIG. 15 ) is then provided to bias the sensor and in particular its sprocket  454  into engagement with the roller chain  374 . The reason for this spring mounting is to provide some flexibility to the sensor mount, thereby reducing the possibility of damage, for example, if something such as dirt or a stone should become lodged in the sprocket or the roller chain. It should also be noted that the roller chain is kept under tension itself by means of an adjustable tension rod  482  at one or both ends of the chain. 
       FIGS. 27 and 28  illustrate an actual connection between the inby end of one mobile conveyor machine constructed according to the invention with the outby end of another mobile machine  500 , only an end section of which is shown. Two car hitch pins  54  are used to attach the rolling hitch device  48  to the two ends of the frame  16 . Each hitch pin can be held in place by a washer plate  502  and a screw  504  that extends through the plate  502  and into a threaded hole in the end of the hitch pin. 
     Turning now to the mounting mechanism for the tail pulley  32 , this mounting system as seen most clearly in  FIG. 26  includes two parallel links or swing arms  330  and  332  which are pivotably mounted on pivot pin sleeves  334  fixedly mounted on the inside of the frame  16 . The inner ends of the links are located along the longitudinal centreline of the car and are pivotably connected to central mounting frame  336  which provides support for a substantially vertically extending pivot pin  338 . Pivotably connected to this pin is a belt control arm  240  which in turn is pivotably connected to the actuator rod of a belt training hydraulic cylinder  242 . The closed end of this cylinder is pivotably connected to an adjustable horizontal support plate  244 . 
     The tail pulley itself comprises two rotatable pulley sections  246  and  248  which rotate about a non-rotating central support shaft (not shown) located along the centerline of the pulley indicated at  250 . The shaft extends from opposite sides of a central, circular support block  252  rigidly connected to one end of the control arm  240 . Mounted on opposite sides of the support block are two central bearings located at  254 , each rotatably supporting a respective one of the pulley sections  246 ,  248 . In a known manner, the exterior of these pulley sections comprises a series of parallel, spaced-apart metal slats, the inner ends of which are mounted on an outer annular support member which contains the central bearing. An outer bearing located at  256  is mounted on the outer end of each section of the shaft  50  to support the outer end of the respective pulley section. Horizontally extending frame members  260 ,  262  are fixedly connected to the central frame  336  and are also connected to the plate  244 . These frame members are used to apply force to the tail pulley in order to tension same. 
     The position of the tail pulley can be adjusted for belt training purposes using the hydraulic cylinder  242 . In order to provide an automatic system for correcting the position of the conveyor belt, a photosensor system can be provided at each end of the tail pulley. As illustrated, there are two photoemitters  266  mounted on the curved track  46 . For each of these photoemitters there is a photoreceiver  268  which can be seen in  FIG. 26 . As long as the conveyor belt is properly centered on the tail pulley, pulses of a light beam can travel from each photoemitter  266  (through the gaps in the adjacent pulley section) to its respective photoreceiver which is mounted on the inside of one of the longitudinal frame members forming the frame  16 . However, if the belt moves transversely on the tail pulley so as to block entirely one of the light beams, this provides a signal to a programmable logic controller which causes retraction or extension of the actuator rod of the hydraulic cylinder  242 . The actuator rod will move in a direction so as to cause the central shaft of the tail pulley to be pivoted in a horizontal plane so as to tighten the belt on the side to which the belt has moved. This will tend to cause the belt to move back towards its center position. 
     As illustrated, each photosensor is aligned with the end section of the tail pulley so that the light beam is regularly broken by the parallel slats on the exterior of the tail pulley. Because of this arrangement, each photoreceiver sends a pulse signal to the programmable logic controller when the belt is not entirely blocking the light beam. Thus, if the belt is properly centered, pulse signals are being sent to the controller by both photoreceivers  268 . When a pulse signal is not being emitted by one of the light receivers, then this indicates that the belt has moved too much in the direction of this particular receiver and the control system will take steps to re-center the belt. 
     Extending from a small winch  270  is a two inch wide nylon strap  272 . The winch and strap are positioned above one photoreceiver  268  and are mounted on the inside of the main frame  16  of the vehicle. The strap extends to a metal hook which extends through a hole formed in the end of a vertical connecting plate  276 . The plate  276  is rigidly connected to one edge of the horizontal plate  244 . It will be appreciated that once the conveyor belt is mounted in place and extends around the tail pulley (as well as the head pulley) the conveyor belt can be tensioned properly by pulling on the strap  272  which in turn will cause the frame members  260 ,  262  and the attached central frame  336  to move in a direction towards the tail pulley end of the vehicle. 
       FIG. 32  illustrates an exemplary network of programmable logic controllers for a train of mobile conveyor vehicles as described above. This control system operates by means of a master PLC indicated at  600  which can be located on the discharge car which, as indicated, is preferably equipped with two PLCs, one for each of its two pairs of wheels since the PLCs can also be used for steering purposes. There are fifteen intelligent slave PLCs in this particular system which can be numbered from one to fifteen and which, as shown in the drawings, have Profibus addresses numbered from 21 to 35. In a preferred embodiment, the PLCs are operated by wireless radio commands using two receiving radios, one receiving radio located on the discharge car and the second receiving radio located at the load car. The load car receiving radio is indicated at  602  while the discharge car receiving radio is indicated at  604 . In this way, a train of conveyor vehicles can be operated from either end of the train by a radio transmitter unit  603 , which preferably is a portable unit carried by the operator. All commands come from the master PLC and go to the others by the Profibus system. The same computer code can be used in each of the slave PLCs so that the conveyor cars are interchangeable for any particular job. It will be understood that the load car radio will be used by the load car operator to operate the conveyor system when a mining operation is underway. The discharge car radio can be used by the discharge car operator who may also be operating the cross-conveyor on the discharge car. The radio control system is interlocked so as to prevent conflicting signals, with the switch to determine which radio is operable being located on the load car radio. As illustrated in  FIG. 32 , the discharge receiving radio can be assigned Profibus address  40  while the load car receiving radio can be assigned Profibus address  41 . A manually operated joystick control  601  can be provided on the transmitter unit  603 . This joystick control is used to steer the leading pair of wheels on the train through the PLC on the leading vehicle. The joystick control is of standard construction and accordingly a detailed description herein is deemed unnecessary. 
       FIG. 33  illustrates those portions of the electrical circuit provided on each conveyor vehicle in order to steer the vehicle and to cause the vehicle to tram in a desired direction. This electrical circuit is connected to the programmable logic controller (PLC) for the respective vehicle. For safety in a mine environment, the circuit includes an intrinsically safe barrier  555 . Intrinsically safe barriers will prevent a strong enough spark or thermal effect that could cause ignition of combustible material in the air. This barrier is connected to the input of the PLC for the vehicle at  556 . The PLC which could be a slave PLC indicated at  610  together with the barrier  555  are housed in a flame proof enclosure indicated partially by the chain link line  558 . The barrier includes a barrier rack in which a plurality of intrinsically safe barrier components  562  are plugged, these barrier components being well known in the mining equipment art for the purpose of preventing short circuiting. Also shown in  FIG. 33  are a first solenoid valve  164  for steering the wheels of the vehicle to the right and second solenoid valve  566  for steering the wheels to the left. These valves can be of identical construction. These valves control the flow of hydraulic fluid into or out of the hydraulic cylinders  74  of the linear actuators that pivot the wheels of the vehicle about their respective vertical axes. There are also connected by safe barrier components two further solenoid valves  568 ,  570 . Again, these two valves can be of identical construction and suitable valves are available from Bosch Rexroth of Germany. These valves can also be of the same type as the valves  564 ,  566  used for steering. The valve  568  is used to direct hydraulic fluid to the two hydraulic motors for the two wheels  64 ,  66  in order to tram or drive the vehicle in the outby direction. Similarly the solenoid valve  570  is used to direct hydraulic fluid to the same two motors to tram or drive the vehicle in the inby direction. Also connected to one of the barrier components  562  is the rotational transducer which is used to measure the intercar angle for the vehicle. In one exemplary embodiment, this transducer is one sold by Celesco. The aforementioned steering angle sensor  75  is electrically connected to its own safe barrier  572 . In one exemplary embodiment, each barrier component  562  is a Phoenix contact and the safety barrier  572  is one sold by Pepperl and Fuchs. 
     Turning now to the software flow chart of  FIGS. 34A and 34B , it should be recognized that this software program is carried out by a plurality of PLCs, one for each pair of wheels in a train of conveyor vehicles. As explained below, these include a master PLC  600  and a number of intelligent slave PLCs, with all of the PLCs being linked and coordinating their steering operation as the train moves in either the reverse or forward direction. All commands come from the master PLC  600 . It will be understood that the terms “controller”, “controller system”, and “control means” as used herein can include a plurality of PLCs working as one overall steering control system. It will be further understood that the leading pair of wheels in the train as determined by the direction of movement of the train is steered manually that is by human operator at the leading end of the train using a separate steering control for this specific purpose, for example the joystick control  601 . The objective of the automatic steering system described hereinafter is to have each pair of trailing wheels follow the same path of movement as the leading pair of wheels. 
     Once the train of conveyor vehicles has commenced movement at a fixed, predetermined rate of travel, for example by pushing an appropriate tramming ON button and provided the steering system is set to automatic steering, the master PLC first determines the direction of travel of the train at step  576 , that is, is the train moving in the outby direction or in the inby direction. The sequence of subsequent software steps taken will depend to a certain extent on this direction of travel and the steps illustrated in  FIGS. 34A and 34B  assume an inby direction of travel. The computer program for steering is quite similar for an outby direction of travel with any significant difference being noted in the following description of the program for steering in the inby direction. The illustrated series of program steps apply generally to each pair of trailing wheels in the train of vehicles with a sample pair of wheels being identified by the letter Y in  FIG. 34A . For each set of trailing wheels, the Profibus system on an ongoing and continual basis (or frequent periodic basis) provides a setpoint angle for the wheels Y. This setpoint (SP) angle is taken from the memory of the master PLC and it is the desired steering angle for the wheels Y, this angle being the angle that the leading pair of wheels had when the leading wheels were in the same location as the current location of the wheels Y. After obtaining the setpoint angle in step  578 , the master PLC then calculates a broad deadband range by calculating a maximum angle (SP+) of this range and a minimum angle (SP−) for the range. In one exemplary embodiment, the broad deadband range R is 6 degrees but this range can vary to some extent depending upon the particular steering system and its characteristics. Accordingly, SP+=SP+3 degrees, while SP−=SP −3 degrees. These two calculations are indicated by steps  580 ,  582 . 
     The next program step indicated at  581  is to calculate a modified intercar angle (MIA) by multiplying the measured intercar angle at the wheels Y by −1. This program step is only required for movement in the inby direction and is not required for movement in the outby direction, which is a more stable direction of movement of this steering system. The next two steps represented by  582  and  584  represent two calculations required to determine the desired steering angle range for the wheels Y. This calculates a setpoint negative (SPN) by adding the modified intercar angle (MIA) to SP−. Also the program calculates setpoint plus (SPP) by adding the modified intercar angle (or if moving in the outby direction, simply the intercar angle) to SP+. In step  586 , the program determines if the setpoint (SP) for the wheels Y is greater than a predetermined small angle which in one exemplary version of this system is 2 degrees. As indicated in step  588 , if the setpoint for wheels Y is greater than 2 degrees, then the program will multiply the measured wheel angle position, that is the actual existing position of the wheels Y, by a predetermined factor X to obtain an adjusted steering position (ASP). The reason for this is to adjust for drift in the position of the wheels Y as the train of vehicles is moving in the inby direction. The amount of this drift will vary because it depends on such factors as the length of the conveyor train and other factors. The amount of the factor X for each conveyor system can be calculated by means of trial runs of the particular conveyor system when it has been built. In one exemplary embodiment for such a system, the factor X is 0.5. Note also that the factor X can change or be different for each pair of wheels in the train of conveyor vehicles. If it is determined that the SP for wheels Y is not greater than 2 degrees (for example), the program in step  590  then asks if the setpoint for the wheels is greater than −2. If the answer is no, then the program proceeds to step  588  to multiply the measured wheel angle position by factor X. However, if the answer is yes, then step  592  applies and the measured wheel angle position is multiplied by one to obtain the adjusted steering position (ASP). In other words, there will be no adjustment to the setpoint if the desired steering angle is less than a small predetermined angle (either positive or negative) since under these circumstances drift of the wheels is not a significant problem. With respect to these drift adjustment steps, if the train is moving in the outby direction, the factor X is different for the outby direction of movement than it is for the inby direction. Again, the amount is determined by testing and steering a particular train of vehicles. In one version of a vehicle train, the factor X for movement in the outby direction was 1.2. 
     In the next step  594 , a narrow deadband range is determined, firstly by calculating a maximum steering angle (SA+) by adding one-half of the narrow deadband range to the setpoint (SP). In one steering system, the narrow deadband range was set at 1 degree so that one half of the narrow deadband range was ½ degree. In a similar manner, in step  596  the program calculates the minimum steering angle (SA−) for this narrow deadband range by subtracting ½ degree from the setpoint SP. 
     Turning now to the additional program steps illustrated in  FIG. 34B , in step  598  the program determines if the adjusted steering position (ASP) is greater than the maximum steering angle (SA+). If the answer is yes, the program turns on an error plus inhibit (EPI) signal in the PLC (step  610 ). If the answer is no, the program in step  612  then determines if the ASP is less than SA−. If the answer is yes, the program turns on an error minus inhibit (EMI) signal in step  614 . Although not indicated in the flow chart, it will be understood that if ASP should be less than SA+ and more than SA−, then neither the EPI signal nor the EMI signal is turned on for the remaining calculations. 
     The distance traveled by the train is determined by the number of pulses generated by the controller, this number being dependent directly on the amount of time the train has been tramming from a selected location, for example the point at which the train was last stopped. With the illustrated train of conveyor vehicles, the steering program is set up so that the first pair of trailing wheels is adjusted to a desired steering angle which is based on the steering angle of the leading pair of wheels with a delay factor of one half car or vehicle length in time. In other words, the desired steering angle for the first pair of trailing wheels is calculated on the basis of the actual steering angle of the leading set of wheels which existed before the train moved one-half car length to its current position. In the case of the remaining trailing wheels, there is a full car length delay in the application of the steering angle of the leading pair of wheels between a respective pair of trailing wheels and the pair of wheels immediately in front of this pair. In program step  618 , the program determines if the adjusted steering position for the wheels is greater than the maximum angle (SP+) of the deadband range. If the answer is yes, then the solenoid valve  566  is energized to turn the wheels Y to the left as indicated in step  620 . Overshoot is avoided or trimmed by the use of the error plus inhibit (EPI) signal which together with an error timer reads the actual angle of the wheels every 250 milliseconds. If the measured actual wheel angle is in the narrow deadband range, then the solenoid valve  566  is turned off, preventing further turning movement to the left. 
     The additional steps  624 ,  626  and  628  are the program steps for turning the wheels Y to the right when moving in the inby direction. The program determines if the adjusted steering position (ASP) is less than the minimum angle (SP−) of the deadband range. If it is, then turning of the wheels to the right is required and in step  626 , the solenoid valve  564  for the wheels is energized to cause the hydraulic cylinders for the wheels Y to turn the wheels right. Again overshoot is prevented by using the aforementioned error minus inhibit signal (EMI) on a continual or frequent basis during the turning movement using an error timer. The actual angle of the wheels can be measured every 250 milliseconds and once the actual measured angle is within the narrow deadband range, the solenoid is de-energized. 
     Turning now to the hydraulic system used to tram the conveyor train and to provide hydraulic power to steer the wheels,  FIGS. 35A and 35B  are left and right portions of a hydraulic schematic illustrating the hydraulic systems on one of the conveyor vehicles. It will be understood that the intake or loading conveyor vehicle and the intermediate conveyor vehicles have identical hydraulic systems, an exemplary version of which is shown in  FIGS. 35A and 35B . Also, the discharge car includes similar hydraulic systems as those shown in these figures but it includes as well an additional conveyor lift and swing hydraulic system, which is not illustrated as it does not form part of the present invention and is a standard hydraulic system construction. 
     Turning first to the hydraulic system components illustrated in  FIG. 35A , indicated in the bottom right corner is the hydraulic fluid reservoir  222  which can have a capacity of 60 liters of hydraulic oil. The reservoir has 2 outgoing hydraulic lines  630 ,  632 . The line  630  leads to the auxiliary hydraulic pump  212 , which is used to provide hydraulic fluid to the auxiliaries of the vehicle, including the steering cylinders. A suitable pump for this purpose is that built by Bosch Rexroth (hereinafter referred to as “Rexroth), model number A10VS010DFR-52R-PUC64N00. 
     As explained above, this hydraulic pump is driven by the electric motor  208  as is another hydraulic pump  210 . The motor can be an explosion proof 10 horsepower motor operating at 1,470 rpm rotating in the clockwise direction. The pump  210  which is used to provide hydraulic fluid to the hydraulic motors for the two wheels of the vehicle in one embodiment is a pump sold by Rexroth under model number AA10VG18DGM1/1DR-NSC66F0145. Connected to pump unit  210  and controlling its operation is a manifold assembly  634  which has 3 parts including sub-plate  636 , sandwich plate  638  having shuttle cartridge at  640 , and solenoid valve  642 . The solenoid valve is an electrically operated open-close valve which is operated by the PLC of the vehicle. In one version of the vehicle, the manifold assembly is one made by Rexroth, model number G341/12-A1-PM-114. Hydraulically connected to the shuttle cartridge  640  is a flow control delay brake  644  which can be a flow control made by Hydac, model number DRV-8-1.1/12. Connected to this delay brake is a brake check valve  646  which can be a ball valve made by DMIC, model number BV3D-0250SA-111. 
     The outlet port of pump unit  210  is connected by hydraulic line  648  to the two hydraulic motors  68 , which drive the 2 wheels  64 ,  66 . As indicated above, each hydraulic motor is operably connected to a planetary gearbox  176 , which is connected to the respective wheel in order to drive same. In an exemplary embodiment of this drive system, the gearbox  176  is a two-stage planetary gearbox with a reduction ratio of 43:1. A parking brake  650  can be provided for each wheel. The wheel drive unit for each wheel outlined by chain link line  652  can be a wheel drive sold by Rexroth, model number GFT7T2.4042. A second hydraulic line  654  also extends from a port of hydraulic pump unit  210  to the two hydraulic motors  68 . Thus hydraulic fluid can flow in either direction through lines  648  and  654 , the direction depending on the direction of travel of the vehicle required. In one embodiment, the flow rate through these lines is 7 gallons per minute. Connected to the line  654  is a hot oil shuttle  656  which can be a unit sold under the name Command, model number HOSV-10-N-C-6TS. The line  648  is also operatively connected to the shuttle  656 . The shuttle is used to direct returning hydraulic oil to the reservoir  222  through hydraulic return line  658  shown in  FIG. 35B . Line  658  is connected to the reservoir via return filter  214 . This filter can be one sold under the name Western Filter, model number E0211B2R10. There is another filter  214 , which can be referred to as a suction filter, connected in the hydraulic line  632  between the reservoir and pump unit  210 . 
     The auxiliary pump unit  212  is operated by and controlled by the valve unit at  660 , which in turn is controlled by the vehicle&#39;s PLC. The outlet port of the pump unit  212  is connected to hydraulic line  662 , which branches into two lines, one of which extends through filter  664 , which can be a 10 um filter made by Hydac, model number DFBH/HC30G10B1.1/12. This filter is connected via hydraulic line  666  to a manifold assembly  668 . In one exemplary embodiment, this manifold assembly unit is made by Rexroth, model number VB8.037.A2. Connected to this manifold assembly are two similar solenoid valves  670  and  672 , which are controlled by the programmable logic controller of the vehicle. These solenoid valves can be valves sold by Rexroth, model number 4WE6J5X/BG12-19NXHCKL. The solenoid valve  670  is used to control the belt training hydraulic linear actuator  242 , the operation of which has been described above. The solenoid valve  670  is mounted on a reducing valve  674  which can be one made by Rexroth, model number ZDR6DP2-4X/75YM/12. The line  666  is connected by a passageway in the manifold assembly to the inlet port  676  of solenoid valve  672 . The outlet port of the manifold valve is connected by another passageway  680  in the manifold to hydraulic return line  658 . Depending on the desired direction for turning the two wheels of the vehicle, the solenoid valve  672  can direct hydraulic fluid to either end of the two hydraulic linear actuators, which are used to steer the two wheels of the vehicle. Hydraulic oil in the cylinder is allowed to flow out of the cylinder from the end opposite the end of the cylinder into which the hydraulic fluid is being fed by solenoid valve  672 . 
     Turning to the components which make up the manifold assembly  668  which has a load sensing capability, these include relief valve  682 , which can be a valve sold under the name Command, model number RVDA-08-N-S-0-30/28. This relief valve is connected to line  680  and to the test point at  684 . Also mounted in the manifold are two shuttle cartridges  686 ,  687 , which can be of the same construction. These cartridges provide hydraulic feedback for the associated hydraulic cylinder to the auxiliary pump and they are part of the load sensing system. They ensure that the pump provides enough hydraulic pressure to enable the cylinder to operate as intended. In one embodiment, these cartridges are sold by Sun, model CDAB-XBN. Mounted in each of the hydraulic lines extending from the solenoid valves  670 ,  672  are flow control units  690 , which can be of similar construction, for example the unit sold by Command, model number FCVL-08-N-S-0-FF. The units  690  are used to set the speed at which the hydraulic cylinders move in order to get the correct response in the hydraulic system. Also mounted in each hydraulic line extending from solenoid valve  670 ,  672  are cartridges  692 , which are counter balance valves that are factory set at 3,000 PSI. A cartridge that can be used for this purpose is Command cartridge, model number CBPA-08-N-S-0-30 CB. Each cartridge  692  functions like a pilot operated relief valve so that when the maximum set pressure (which is adjustable) is achieved, the cartridge will dump the oil for safety reasons. 
     As indicated above, the discharge car vehicle illustrated in  FIGS. 30 and 31  has two pairs of steerable wheels. Because it is necessary to steer both sets of wheels on this car, the hydraulic systems illustrated in  FIGS. 35A and 35B  are duplicated on the discharge car vehicle, which is also provided with an additional manifold assembly and two solenoid valves for the conveyor lift hydraulic cylinder and the conveyor swing hydraulic cylinder used to maneuver the cross conveyor on the discharge vehicle. 
     While the present invention has been illustrated and described as embodied in an exemplary embodiment, ie. an embodiment having particularly utility for use as a steering system and method for mobile conveyor vehicles, it is to be understood that the present invention is not limited to the details shown herein, since it will be understood that various omissions, modifications, substitutions and changes in the forms and details of the disclosed conveyor vehicle and its method of operation may be made by those skilled in the art without departing in any way from the spirit and scope of the present invention. For example, those of ordinary skill in the conveyor art will readily adapt the present disclosure for various other steering applications involving a train of vehicles without departing from the spirit and scope of the present invention.