Patent Publication Number: US-2023163526-A1

Title: System and method for positioning a conductive rod powering a work machine

Description:
TECHNICAL FIELD 
     The present disclosure relates to a system and method for axially positioning a conductive rod that conveys electrical power to a moving vehicle. More specifically, the present disclosure relates to a conductive rod for an electrically powered work machine, a process for pneumatically extending and retracting the conductive rod to maintain connection with a roadside power source, and the work machine using the conductive rod. 
     BACKGROUND 
     Heavy work machines, such as earth-moving vehicles or hauling trucks, require significant power to carry out their functions. The machines themselves can be of substantial weight, and their loads require large amounts of power to move. Diesel engines typically provide that power, but they can have disadvantages. For instance, in some implementations, heavy work machines may need to travel large distances through rugged terrain. At a remote mining site, for example, groups of these machines are often employed to ferry extreme loads along roadways, or haul routes, extending between various locations within the mining site. Supplies of diesel fuel may be far away from such locations or not easily delivered to such locations. In addition, the groups of diesel machines can generate significant pollution. 
     Electrical power has been used to supplement these diesel engines while the work machines move. In some environments, the electrical power is delivered from wires over the haul route to a pantograph on the work machine as the machine travels the haul route, as in a cable car. But overhead wires cannot reliably provide sufficient electrical energy to power a heavy work machine during long movements. Nor can the overhead delivery provide enough current to charge backup batteries for an electric machine at the same time. In addition, connection with overhead wires tolerates only small lateral movements by the vehicle before arcing or disconnection occurs. As a result, electrical power provided through overhead wires typically supplements, rather than replaces, power generated by diesel engines in heavy work machines. 
     In other environments, on the other hand, a power rail based on the ground may provide electrical power to heavy work machines. Establishing an electrical connection with a power rail can require precise movements of a conductor from a heavy work machine, however, and maintaining an electrical connection with a power rail while a heavy work machine moves can be particularly challenging. In locations such as a mining site, the haul route may be uneven, hilly, and pocked. These variations may lead to irregular movements by the machine or unexpected changes in position by the power rail, causing the machine to disconnect from the rail. Steering deviations for the heavy work machine could also disrupt the connection of the machine with the power rail, detracting from the value of rail-based delivery of electrical power. 
     One approach for providing electrical power to a vehicle through a rigid conductor is described in U.S. Patent App. Pub. No. 2017/0106767A1 (“the &#39;767 application”). The &#39;767 application describes a vehicle power supply method in which a charging arm of fixed length is deployed transversely from a vehicle to contact a power supplying apparatus. To reach the power supplying apparatus with the arm, a device is caused to slide along the side of the vehicle, which forces a damper unit to push the arm around an axis of rotation and displace its end outwardly from the vehicle. A rotating arm of fixed length, as described in the &#39;767 application, however, has a limited reach from the side of the vehicle, which limits the lateral movement available in steering. Moreover, support for the arm is essentially limited to one end of the arm at the axis of rotation, which may be inadequate to support a long arm of substantial weight. Such a mechanism would also be prone to failure when used in harsh conditions such as mine sites, paving sites, and construction sites. As a result, the system described in the &#39;767 application is not desirable for heavy work machines having high electrical power loads or operating in environments in which the machines may substantially deviate along their paths when moving. 
     Examples of the present disclosure are directed to overcoming deficiencies of such systems. 
     SUMMARY 
     In an aspect of the present disclosure, a work machine includes an electric engine, an air compressor, and a conductor rod extending along a longitudinal axis from a first end proximate the work machine to a second end spaced laterally from the work machine. The conductor rod has a central passageway extending circumferentially around the longitudinal axis pneumatically coupling the first end to the second end. The conductor rod has a first cylindrical shell extending from the first end toward the second end and having a first diameter, a first stop attached to the first cylindrical shell, and a first sequence of first conductive tubes and first tubular cavities concentrically positioned alternatingly around the central passageway with the first conductive tubes extending from the first end. The conductor rod further has a second cylindrical shell extending from the second end toward the first end and having a second diameter different from the first diameter, a second stop attached to the second cylindrical shell, and a second sequence of second conductive tubes and second tubular cavities concentrically positioned alternatingly around the central passageway with the second conductive tubes extending from the second end. The first cylindrical shell and the second cylindrical shell are slidably mated along the longitudinal axis, where the first conductive tubes are slidable into the second tubular cavities, and the second conductive tubes are slidable into the first tubular cavities. A retraction cavity is positioned between the first cylindrical shell and the second cylindrical shell and between the first stop and the second stop, and a channel connects the air compressor to at least the retraction cavity. The work machine also includes an open-loop directional control valve configured, in a default state, to couple a first cavity of the first tubular cavities to atmosphere and, in an active state, to couple the first cavity to the channel. 
     In another aspect of the present disclosure, an apparatus for conducting electrical power to a work machine includes a conductor rod having a central passageway around a longitudinal axis that pneumatically connects a base to a tip of the rod. A pneumatic cylinder extends from the base toward the tip and terminates at a cylinder end. The pneumatic cylinder includes cylinder tubes, made of conductive material concentrically positioned around the central passageway and extending from the base to proximate the cylinder end, and cylinder cavities between the cylinder tubes. A piston extends from the tip toward the base and terminates at a piston end. The piston includes piston tubes, made of conductive material concentrically positioned around the central passageway and extending from the tip to proximate the piston end, and piston cavities between the piston tubes. The cylinder tubes are radially offset from the piston tubes, and the piston is slidably mated within the pneumatic cylinder. The conductor rod further includes a retraction cavity enclosed between an inner surface of the pneumatic cylinder and an outer surface of the piston. The apparatus also includes an open-loop directional control valve configured to couple at least one of the cylinder cavities to a first inlet or couple the at least one of the cylinder cavities to atmosphere, as well as a channel coupled to the first inlet, to the retraction cavity, and to the central passageway. 
     In yet another aspect of the present disclosure, a method includes providing pressurized air from a compressor within a work machine to a conductor rod on the work machine and causing a piston, slidingly engaged within a pneumatic cylinder of the conductor rod, to extend along a longitudinal axis from the pneumatic cylinder in an open-loop mode. The pneumatic cylinder has cylinder tubes concentrically positioned around a central passageway, and the piston has piston tubes concentrically positioned around the central passageway and radially offset from the cylinder tubes. Causing the piston to extend includes feeding the pressurized air into an annular retraction cavity of the conductor rod positioned between an inner wall of the pneumatic cylinder and an outer wall of the piston and pneumatically coupling an annular cylinder cavity to the pressurized air. The annular cylinder cavity is positioned near a base of the conductive rod proximate the work machine and is bounded in the pneumatic cylinder by successive cylinder tubes and a piston tube slidably mated between the successive cylinder tubes. An extension area around a first radial surface in the annular cylinder cavity is larger than a retraction area around a second radial surface in the annular retraction cavity. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic illustration of an electrically powered work machine coupled to a roadside power source via a conductive rod, connector, and trailing arms in accordance with an example of the present disclosure. 
         FIG.  2    is a longitudinal section of a rear view of the conductive rod of  FIG.  1    when retracted in accordance with an example of the present disclosure. 
         FIG.  3    is a schematic diagram of a pneumatic control circuit for causing axial movement of the conductive rod of  FIG.  2    in accordance with an example of the present disclosure. 
         FIG.  4    is a schematic diagram of another pneumatic control circuit for causing axial movement of an alternative conductive rod in accordance with an example of the present disclosure 
         FIG.  5    is a flow chart depicting a method of axially moving the conductive rod as in  FIG.  3    in accordance with an example of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Wherever possible, the same reference numbers will be used throughout the drawings to refer to same or like parts.  FIG.  1    illustrates an isometric view of a work machine  100  within an XYZ coordinate system as one example suitable for carrying out the principles discussed in the present disclosure. Exemplary work machine  100  travels parallel to the X axis along a roadway, also termed a haul route  101 , typically from a source to a destination within a worksite. In one implementation as illustrated, work machine  100  is a hauling machine that hauls a load within or from a worksite within a mining operation. For instance, work machine  100  may haul excavated ore or other earthen materials from an excavation area along haul route  101  to dump sites and then return to the excavation area. In this arrangement, work machine  100  may be one of many similar machines configured to ferry earthen material in a trolley arrangement. While a large mining truck in this instance, work machine  100  may be any machine that carries a load between different locations within a worksite, examples of which include an articulated truck, an off-highway truck, an on-highway dump truck, a wheel tractor scraper, or any other similar machine. Alternatively, work machine  100  may be an off-highway truck, on-highway truck, a dump truck, an articulated truck, a loader, an excavator, a pipe layer, or a motor grader. In other implementations, work machine  100  need not haul a load and may be any machine associated with various industrial applications including, but not limited to, mining, agriculture, forestry, construction, and other industrial applications. 
     Referring to  FIG.  1   , and relevant to the present disclosure, an example work machine  100  includes a frame  103  powered by electric engine  102  to cause rotation of traction devices  104 . Traction devices  104  are typically four or more wheels with tires, although tracks or other mechanisms for engagement with the ground along haul route  101  are possible. Electric engine  102  functions to provide mechanical energy to work machine  100  based on an external electrical power source, such as described in further detail below. An example of mechanical energy provided by electric engine  102  includes propelling traction devices  104  to cause movement of work machine  100  along haul route  101 , but electric engine  102  also includes components sufficient to power other affiliated operations within work machine  100 . For instance, in some implementations, electric engine  102  includes equipment for converting electrical energy to provide pneumatic or hydraulic actions within work machine  100 . While electric engine  102  is configured to operate from an external electrical power source, electric engine  102  typically includes one or more batteries for storing electrical energy for auxiliary or backup operations. 
     In accordance with the principles of the present disclosure, work machine  100  further includes a conductor rod  106  configured to receive electrical power from a power rail  108 . In some examples, power rail  108  is one or more beams of metal arranged substantially parallel to and a distance above the ground. In  FIG.  1   , power rail  108  is positioned to be substantially parallel to the X axis and the direction of travel of work machine  100 . Support mechanisms hold power rail  108  in place along a distance at the side of haul route  101  for work machine  100  to traverse. The support mechanisms and power rail  108  may be modular in construction, enabling their disassembly and reassembly at different locations or their repositioning along the existing haul route  101 . In many examples, such as within a mining site, power rail  108  will not be configured continuously at a fixed distance along a side of haul route  101  and at a fixed height above the ground due, at least in part, to the variation of the terrain. Therefore, it is expected that the vertical, horizontal, and angular positions of the surface of power rail  108  in the XYZ planes will vary along haul route  101 . Moreover, while shown in  FIG.  1    to the left of work machine  100  as work machine  100  travels in the direction of the X axis, power rail  108  may be installed to the right of work machine  100  or in other locations suitable to the particular implementation. 
     Power rail  108  provides a source of electrical power for work machine  100  as either AC or DC. In some examples, power rail  108  has two or more conductors, each providing voltage and current at a different electrical pole. In one implementation (e.g., an implementation in which the power rail  108  includes three conductors), one conductor provides positive DC voltage, a second conductor provides negative DC voltage, and a third conductor provides 0 volts relative to the other two conductors. The two powered conductors within power rail  108  provide +1500 VDC and −1500 VDC. These values are exemplary, and other physical and electrical configurations for power rail  108  are available and within the knowledge of those of ordinary skill in the art. 
     Conductor rod  106  enables electrical connection between work machine  100  and power rail  108 , including during movement of work machine  100  along haul route  101 . In the example shown in  FIG.  1   , conductor rod  106  is an elongated arm resembling a pole.  FIG.  1    shows conductor rod  106  positioned along a front side of work machine  100 , with respect to the direction of travel of work machine  100  in the direction of the X axis. In this arrangement, conductor rod  106  is located in  FIG.  1    in the Y-Z plane essentially along the Y axis with a first end near a right side of work machine  100  and a second end at a left side of work machine  100 . Conductor rod  106  may be attached to any convenient location within work machine  100 , such as to frame  103 , in a manner to couple conductor rod  106  to power rail  108 . Shown in  FIG.  1    as extending to a left side of work machine  100  toward power rail  108 , conductor rod  106  may alternatively be arranged to extend to a right side and at any desired angle from work machine  100  such that conductor rod  106  may be coupled to power rail  108  for obtaining electrical power. 
     As embodied in  FIG.  1   , conductor rod  106  includes a barrel  109  mounted to frame  103  of work machine  100 . Barrel  109  has a hollow interior and may be a conductive metal having suitable mechanical strength and resiliency, such as aluminum. Within barrel  109 , an arm  110  is retained. Arm  110  is slidably engaged within conductor rod  106  such that it may be extended or retracted axially, i.e., along the Y axis in  FIG.  1   , to adjust the reach of conductor rod  106 . Specifically, in a retracted position, arm  110  is caused to slide within barrel  109  of conductor rod  106  such that a length of conductor rod  106  roughly spans the width of work machine  100 . A junction  112  serves as the junction or interface between arm  110  and barrel  109 , which is the main body of conductor rod  106 . When arm  110  is fully retracted or collapsed into barrel  109 , junction  112  essentially becomes the left edge of conductor rod  106 . On the other hand, when arm  110  is extended from barrel  109  of conductor rod  106 , arm  110  may reach from work machine  100  to proximate power rail  108  on the side of haul route  101 . 
     Within, and possibly including barrel  109 , conductor rod  106  includes a series of electrical conductors passing longitudinally, at least from a head  122  at a proximal end to a tip  124  at a distal end. Typically, the conductors within conductor rod  106  are formed of a metallic material and are rigid. In some examples, the conductors are concentric tubes, or hollow cylinders, of solid metal such as aluminum nested together and sized to provide electrical capacity sufficient for powering work machine  100 . Tubular conductors within arm  110  slidably engage with corresponding tubular conductors within barrel  109  to maintain electrical continuity as arm  110  is extended or retracted. 
     At a distal end of work machine  100  at tip  124 , a connector assembly  114  provides an interface to power rail  108  via trailing arms  116  and contactor  118 . Power rail  108  is typically arranged along a side of haul route  101 , and work machine  100  is steered so that it traverses haul route  101  substantially in parallel with power rail  108 . Thus, in reference to  FIG.  1   , power rail  108  and a travel path for work machine  100  are substantially in parallel with each other and with the X axis. Contactor  118  is configured to maintain an electrical connection with power rail  108  while sliding along its surface in the direction of the X axis as work machine  100  moves. In some examples, trailing arms  116  are conductors coupled to contactor  118 , each conducting voltage and current at a different electrical pole and corresponding to the conductors within conductor rod  106 . In operation, electrical power is accessed from power rail  108  via contactor  118 , which remain in contact during movement of work machine  100 , and the electrical power is conducted through trailing arms  116  into connector assembly  114 . 
     From connector assembly  114 , the electrical power is conveyed at tip  124  through the nested tubular conductors within arm  110  and barrel  109  to head  122  of conductor rod  106  and through a head-end interface  120  to work machine  100 . Head-end interface  120  provides at least an electrical connection between conductor rod  106  and work machine  100  for powering electric engine  102  and otherwise enabling operations within work machine  100 . In some examples, head-end interface  120  may also provide an interface for inputs to control mechanical operation of conductor rod  106 , such as passageways for pressurized air of a pneumatic control system to extend and retract arm  110  or signaling for electronic controls. 
     Focusing more on conductor rod  106  from  FIG.  1   ,  FIG.  2    illustrates a longitudinal section of conductor rod  106  when arm  110  is retracted, or collapsed, into barrel  109 . More specifically,  FIG.  2    depicts a longitudinal section of conductor rod  106  between head-end interface  120  and connector assembly  114 , from head  122  to tip  124 , when viewed facing in the direction of travel for work machine  100 , i.e., in the direction of the X axis. Thus, conductor rod  106  lies in the Y-Z plane, as indicated in  FIG.  2   . 
     Referring to the right side of  FIG.  2   , barrel  109  contains an arrangement of concentric conductors of tubular shape, i.e., as hollow cylinders. In this example, from the axial center outward, first cylinder conductor  202  is positioned at a center of barrel  109  and is a tubular conductor made of aluminum or a similar metal with high electrical conductivity and high mechanical strength. For instance, an aluminum alloy such as 6061-T6 may be used for first cylinder conductor  202  and other conductive tubes in conductor rod  106 . In some examples, first cylinder conductor  202  has an outer diameter of approximately 3.5 inches. First cylinder conductor  202  begins at head  122  and extends axially along conductor rod  106  around a longitudinal Y axis to a barrel end  205 . Barrel end  205  in this example of  FIG.  2    is approximately radial to junction  112 . As a tube, first cylinder conductor  202  defines first cylinder cavity  204  within its inner surface. If arm  110  were removed from barrel  109  in  FIG.  2   , first cylinder cavity  204  would be an open space within first cylinder conductor  202  traveling the length of conductor rod  106  from head  122  to barrel end  205 . In one example, first cylinder cavity  204  has a diameter of about 2.5 to 3 inches. 
     A second cylinder conductor  206  concentrically surrounds first cylinder conductor  202 . As with first cylinder conductor  202 , second cylinder conductor  206  is a tubular conductor made of aluminum or a similar metal with high electrical conductivity and high mechanical strength. Second cylinder conductor  206  is similarly positioned around a Y axis within  FIG.  2    and spans a distance from head  122  to barrel end  205 . In one example, second cylinder conductor  206  has an outer diameter of about 5 to 5.5 inches. These dimensions, as well as other dimensions discussed below, are merely examples and could be greater or lesser than the stated values. Being arranged concentrically around and, by definition, having a larger diameter than first cylinder conductor  202 , second cylinder conductor  206  forms a radial gap between it and first cylinder conductor  202 . In the example of  FIG.  2   , that gap is filled by second cylinder insulation  208 , which is a closed cell polyurethane foam. Other types of materials for second cylinder insulation  208  that provide electrical insulation and lightweight support within conductor rod  106  will be available and apparent to those of ordinary skill in the field. In some examples, second cylinder insulation  208  has a thickness of about 0.75 inches. 
     Moving farther out radially on the right side of  FIG.  2   , third cylinder conductor  210  concentrically surrounds second cylinder conductor  206  and first cylinder conductor  202 . Third cylinder conductor  210  is a tubular conductor made of aluminum or a similar metal with high electrical conductivity and high mechanical strength. As with the other tubes discussed, third cylinder conductor  210  extends from head  122  to barrel end  205  within conductor rod  106 . In one example, third cylinder conductor  210  has an outer diameter of about 8 to 9 inches. A third cylinder cavity  212  between second cylinder conductor  206  and third cylinder conductor  210  is an open space, which, if arm  110  were removed from barrel  109  in  FIG.  2   , would form a tubular cavity extending from head  122  to barrel end  205 . 
     Concentrically around third cylinder conductor  210  and the other tubular conductors, fourth cylinder conductor  214  forms an outer conductive path from head  122  to barrel end  205 . Similarly, fourth cylinder conductor  214  is a tubular conductor made of an aluminum alloy or a similar metal with high electrical conductivity and high mechanical strength. In one example, fourth cylinder conductor  214  has an outer diameter of about 14 inches. A gap between an outer diameter of third cylinder conductor  210  and an inner diameter of fourth cylinder conductor  214 , in some examples, is about 0.75 inches and is filled with fourth cylinder insulation  216 , which is a closed cell polyurethane foam or similar substance. 
     Radially beyond fourth cylinder conductor  214 , a covering or barrel shell  218  encases conductor rod  106 . Barrel shell  218  is typically a metal or similar substance providing structural integrity to conductor rod  106 . Barrel shell  218  has an inner diameter in excess of an outer diameter of fourth cylinder conductor  214 . As a result, a retraction cavity  220  of a tubular shape is formed between fourth cylinder conductor  214  and barrel shell  218  that extends from head  122  to barrel end  205 . A stop  222 , which is part of a housing for conductor rod  106  at junction  112 , defines a longitudinal end for retraction cavity  220  away from head  122 . Stop  222  generally aligns radially with barrel end  205  and junction  112 . 
     The various annular or tubular cavities within barrel  109 , namely, first cylinder cavity  204 , third cylinder cavity  212 , and the head end of retraction cavity  220  (barrel shell cavity  242 , described below), are sealed or capped by the attachment of head-end interface  120  to their ends at head  122 . The attachment of head-end interface  120  is such as to provide an airtight seal within these cavities, for purposes to be understood further below. 
     Viewing  FIGS.  1  and  2    together, arm  110  is a substantially cylindrical body having a smaller outer diameter than an inner diameter of barrel shell  218  and that mates and slides into barrel  109 . As well as providing a longitudinal end for retraction cavity  220 , stop  222  also defines an inner diameter through which arm  110  slides, as shown to the left of  FIG.  2   . By sliding, it is meant that arm  110  may move longitudinally along the Y axis in connection with one or more surfaces within barrel  109  as arm  110  is moved axially with respect to conductor rod  106 , from left to right in  FIG.  2    for retraction and from right to left in  FIG.  2    for extension. The result of the sliding is the increase or decrease in the overall length of conductor rod  106  via arm  110 , as illustrated in  FIG.  1   . 
     Referring now to the left side of  FIG.  2   , arm  110  also contains a series of concentric conductors of cylindrical or tubular shape. In this example, from the axial center outward, first piston conductor  224  is positioned at a center of arm  110  and is, as with the other tubular conductors of arm  110 , made of a metal such as aluminum 6061-T6 or similar substance having high electrical conductivity and high mechanical strength. First piston conductor  224  extends from tip  124  to an arm end  225 , shown at the right side of  FIG.  2   . Being tubular, first piston conductor  224  has a first piston cavity  226  within its inner diameter that is filled with air or another gas. A second piston conductor  228  concentrically surrounds first piston conductor  224  and extends from tip  124  to arm end  225 . Second piston conductor  228  is made of a conductive material, and in some examples has an inner diameter of between about 5 and 6 inches. A space defined as second piston cavity  230  is formed between the inner diameter of second piston conductor  228  and the outer diameter of first piston conductor  224 , which is left unfilled other than with air or a similar gas. 
     Moving radially outward from second piston conductor  228 , a third piston conductor  232  axially centered on the Y axis concentrically surrounds second piston conductor  228 . Similarly made of a conductive material, third piston conductor  232  is set off radially from second piston conductor  228  a distance of less than 1 inch, which is filled with a third piston insulation  234 . As with second cylinder insulation  208  and fourth cylinder insulation  216 , third piston insulation  234  can be a closed cell polyurethane foam or comparable substance providing electrical insulation and lightweight stability. Finally, an arm shell  236  of conductive material such as metal concentrically surrounds third piston conductor  232  from tip  124  to about arm end  225 . In some examples, arm shell  236  has an outer diameter of about 11.625 inches. Within an inner diameter of arm shell  236 , an arm shell cavity  238  of free space exists between arm shell  236  and third piston conductor  232 . 
     In some examples, the outer surface of arm shell  236  includes gasket  240 , which serves to stably set apart arm shell  236 , and arm  110  generally, from barrel shell  218 . As illustrated in  FIG.  2   , as arm  110  is retracted or extended within barrel  109 , gasket  240  separates retraction cavity  220  from a barrel shell cavity  242 . As well, gasket  240  can help retain arm  110  within conductor rod  106  in a state of maximum extension by butting against stop  222 . Additionally, in a manner discussed below, a radial wall of gasket  240  provides leverage for pneumatic pressure applied within retraction cavity  220  to cause arm  110  to be retracted. 
     As illustrated,  FIG.  2    represents an arrangement in which conductor rod  106  essentially has two longitudinal halves. A first half, barrel  109 , on the right side of  FIG.  2   , includes barrel shell  218  enclosing a series of tubular cylinder conductors aligned along the Y axis. Those cylinder conductors, viewed radially from the center axis, are first cylinder conductor  202 , second cylinder conductor  206 , third cylinder conductor  210 , and fourth cylinder conductor  214 . Within that concentric arrangement, tubular regions of open space exist within first cylinder cavity  204  and third cylinder cavity  212 . Further, barrel shell  218  encases barrel  109  and forms an open space within retraction cavity  220  and barrel shell cavity  242 . On the left side of  FIG.  2   , arm  110  includes arm shell  236  enclosing a series of tubular piston conductors also aligned along the longitudinal axis of conductor rod  106 . Those piston conductors, viewed radially from the center axis, are first piston conductor  224 , second piston conductor  228 , and third piston conductor  232 . Within that concentric arrangement, tubular regions of open space exist within first piston cavity  226  and second piston cavity  230 . Further arm shell  236  encases arm  110  and forms an open space within arm shell cavity  238 . 
     In an operating state for conductor rod  106 , arm  110  is inserted into barrel  109  to form a nested configuration of the piston conductors and the cylinder conductors. The radial sequence of tubular conductors within barrel  109  are the inverse of, and complementary to, the radial sequence of tubular conductors within arm  110 . For example, when arm  110  is inserted into barrel  109 , the outer diameter of first piston conductor  224  fits within the inner diameter of first cylinder conductor  202 , and the empty space within first cylinder cavity  204  enables first piston conductor  224  to slide forward into barrel  109 . During and after the sliding, first piston conductor  224  maintains electrical contact with first cylinder conductor  202 , permitting electrical conductivity between those tubular conductors. When first piston conductor  224  is mated within first cylinder conductor  202 , first piston cavity  226  and first cylinder cavity  204  together form a central passageway axially through conductor rod  106  at least from head  122  to tip  124 . 
     Similarly, when the combination of second piston conductor  228 , third piston conductor  232 , and interposed third piston insulation  234  are slid as part of arm  110  into barrel  109 , the outer diameter of third piston conductor  232  fits within the inner diameter of third cylinder conductor  210 , and the inner diameter of second piston conductor  228  fits over the outer diameter of second cylinder conductor  206 . As a result, the sandwich of second piston conductor  228 , third piston conductor  232 , and third piston insulation  234  slide into the empty space defined by third cylinder cavity  212 . In doing so, third piston conductor  232  slides against and electrically contacts third cylinder conductor  210 , and second piston conductor  228  slides against and electrically contacts second cylinder conductor  206 . In some examples, and as shown similarly in  FIG.  2   , when conductor rod  106  is fully collapsed, at least some volume of empty space will remain within third cylinder cavity  212 , which will have an annular or tubular shape and be defined radially by portions of second cylinder conductor  206  and third cylinder conductor  210 . 
     Conversely, when arm  110  is inserted into barrel  109 , the cylinder conductors will slide into cavities within the piston from left to right in  FIG.  2   , and the cylinder conductors will become nested with the piston conductors. For example, the combination of first cylinder conductor  202 , second cylinder conductor  206 , and second cylinder insulation  208  will slide into the open space defined by second piston cavity  230  within arm  110 , during which, as mentioned, first cylinder conductor  202  electrically contacts first piston conductor  224  and second cylinder conductor  206  electrically contacts second piston conductor  228 . Likewise, in the illustrated example, the sandwich of third cylinder conductor  210 , fourth cylinder conductor  214 , and fourth cylinder insulation  216  will slide into the open space defined by arm shell cavity  238  within arm  110 . Third cylinder conductor  210  will then slidingly contact third piston conductor  232 , and fourth cylinder conductor  214  will do the same against arm shell  236 . 
     While  FIG.  2    illustrates a longitudinal section of an exemplary conductor rod  106  in which tubular conductors may be slidably nested together,  FIG.  3    is a pneumatic control system  300  of a representative arrangement for causing arm  110  to move axially with respect to barrel  109 . Namely, pneumatic controls provide a select flow of pressurized air into various cavities of conductor rod  106  to create axial forces on arm  110  so that tip  124  may be stably positioned over power rail  108  before and during movement of work machine  100 . The pneumatic controls enable operation in an open-loop control mode and in a closed-loop control mode. 
     As shown at its center,  FIG.  3    schematically illustrates conductor rod  106  from  FIG.  2   . For simplicity, only cavities are labeled within conductor rod  106 . To the right and below conductor rod  106 ,  FIG.  3    shows components related to a pneumatic control circuit configured to cause arm  110  to either retract or extend from within barrel  109  in an open-loop mode. The left of  FIG.  3    shows components related to the pneumatic control circuit configured to cause arm  110  to either retract or extend from within barrel  109  in a closed-loop mode based on relative movement between tip  124  and contactor  118 , in a manner described below. 
     For both operational modes, a compressor  302  provides a source of pressurized air for use in pneumatic control. Typically, compressor  302  is mounted within work machine  100 , and draws in ambient air from air breather  314  through an air path  312 . In one example, compressor  302  is selected with the capability to provide an air flow rate of about 18 CFM. Coupled to the output of compressor  302 , a condenser  304 , which is also installed on work machine  100 , cools and drains condensate from the pressurized air. From condenser  304 , the pressurized air is typically fed through hoses or other passageways, such as air path  306 , into conductor rod  106 , such as through head-end interface  120 . A regulator  315  positioned between air path  312  and air path  306  helps maintain nominal air pressure on air path  306 . 
     Pneumatic control system  300  includes the functionality for an operator to select either an open-loop or closed-loop mode of operation for conductor rod  106 . In  FIG.  3   , actuation of a switch, such as a solenoid associated with a pneumatic control valve  308 , determines the mode of operation. In some examples, pneumatic control valve  308  is a standard three-port, two-position (3/2) normally closed directional control valve, which is readily available to one of ordinary skill in the art. The pressurized air within air path  306  is fed to an inlet of pneumatic control valve  308 . First cylinder cavity  204  of the central passageway of conductor rod  106  is coupled via air path  310  to an outlet of pneumatic control valve  308 . An exhaust of pneumatic control valve  308  is connected to air path  312 , which leads to atmosphere by way of air breather  314 . Pneumatic control valve  308  therefore functions as a switch to connect first cylinder cavity  204  and the central passageway of conductor rod  106  to the atmosphere or to pressurized air. In its default state, pneumatic control valve  308  connects air path  310  to air path  312 , exposing the central passageway to the atmosphere, setting pneumatic control system  300  to operate in an open-loop or manual mode of operation dictated largely by the control circuit at the right side of pneumatic control system  300 . In its activated state, pneumatic control valve  308  is switched to cause pressurized air within air path  306  to flow into air path  310  and then into first cylinder cavity  204 , energizing the left side of pneumatic control system  300  and activating a closed-loop or feedback mode of operation at the distal end of conductor rod  106 . 
     For either mode of operation, the extension or retraction of arm  110  in the example of  FIG.  3    is governed largely by the radial surface areas within the annular or tubular-shaped volumes of retraction cavity  220 , third cylinder cavity  212 , and second piston cavity  230 . For retraction, pressurized gas within retraction cavity  220  from air path  306  causes a force to act on the radial surface of gasket  240 , pushing arm  110  axially into barrel  109 , i.e., in the direction of the −Y axis. While not shown in  FIG.  2   , third cylinder cavity  212  and second piston cavity  230  are pneumatically connected by way of passage  329  within second cylinder insulation  208  ( FIG.  3   ). As a result, pressurized gas within either of these cavities flows into the other and causes axial forces to press on the radial surfaces of third cylinder cavity  212  and second piston cavity  230 . These forces tend to push arm  110  axially out from barrel  109 , i.e., in the direction of the +Y axis. The balance or difference between the retraction forces in retraction cavity  220  and the extension forces within third cylinder cavity  212  and second piston cavity  230  lead to the direction of axial movement for arm  110 . In some examples, the radial surface area of third cylinder cavity  212  and second piston cavity  230 , which are each annular in shape, is selected to be about twice that of the radial surface area of retraction cavity  220 . 
     Referring first to the open-loop mode, pneumatic control valve  318  and pneumatic control valve  324  serve as gates to open third cylinder cavity  212  and second piston cavity  230  as possible extension cavities. Pneumatic control valve  318  and pneumatic control valve  324  are standard two-port, two-position (2/2) pressure control valves readily available in the market. Pneumatic control valve  318  is normally closed, and when air path  310  is not under pressure, air path  322  at the inlet of pneumatic control valve  318  is connected to air path  320  at its outlet. Conversely, pneumatic control valve  324  is normally open. Therefore, when pneumatic control valve  324  is not under pressure, as when pneumatic control valve  308  indicates an open-loop mode by connecting air path  310  to air path  312 , pneumatic control valve  324  prevents the passage of gas between its outlet at air path  328  and its inlet at air path  340 . Accordingly, for the example of  FIG.  3   , when an open-loop mode is selected at pneumatic control valve  308 , second piston cavity  230  is sealed by pneumatic control valve  324  and connected by passage  329  to third cylinder cavity  212 , and third cylinder cavity  212  is unsealed by pneumatic control valve  318 . 
     Extension and retraction of arm  110  in an open-loop mode is directed by pneumatic control valve  316 , which is a standard three-port, two-position (3/2) proportional directional control valve that is normally closed. Without any interaction from an operator, pneumatic control valve  316  will connect air path  322  at its output to air path  312  at its exhaust and allow air from third cylinder cavity  212  and second piston cavity  230  to pass to the atmosphere via air breather  314 . As such, the cavities within conductor rod  106  will not provide substantial forces from air pressure to resist the retraction forces applied within retraction cavity  220 . Consequently, arm  110  will move axially into barrel  109 . Thus, pneumatic control system  300  by default includes a fail-safe feature by causing arm  110  to retract within barrel  109  due to the air pressure in retraction cavity  220  causing a force against gasket  240  that urges arm  110  axially into barrel  109 , i.e., to retract within work machine  100 . 
     In the example illustrated, an operator can manually control extension and retraction of arm  110  by activating the proportional solenoid on pneumatic control valve  316 . With that activation, pneumatic control valve  316  exposes air path  322  to air path  306 , passing amounts of pressurized gas proportional to activation of pneumatic control valve  316  through pneumatic control valve  318  and into third cylinder cavity  212  and second piston cavity  230 . With minimal activation, the amount of pressurized gas provided to third cylinder cavity  212  and second piston cavity  230  will not generate axial forces in those cavities sufficient to offset the axial forces within retraction cavity  220 , such that arm  110  will retract as it would in a default situation. Increasing the actuator position for pneumatic control valve  316  will increase the amount of pressurized gas provided to third cylinder cavity  212  and second piston cavity  230 , and retraction of arm  110  will slow to a point, when the actuator is near its median position in some examples, that the forces of retraction and extension will equal each other and the axial movement of arm  110  will stop. As the actuator of pneumatic control valve  316  is adjusted beyond its median position in some examples, the air flow into third cylinder cavity  212  and second piston cavity  230  will increase and the pressure in third cylinder cavity  212  and second piston cavity  230  will become sufficient that the axial forces in those cavities will overcome the opposing axial forces in retraction cavity  220  and extension of arm  110  outward from conductor rod  106  will begin. As extension occurs, air will be pushed out of retraction cavity  220  and combined in air path  306  with air provided from compressor  302 , resulting in regenerative feedback from retraction cavity  220 . 
     An operator may proportionally adjust the actuation of pneumatic control valve  316  to cause controlled extension or retraction of arm  110  until tip  124  is located laterally away from work machine  100  as desired by the operator. At this point, the air flow through pneumatic control valve  316  may be adjusted until the forces sourced by air path  320  axially expanding second piston cavity  230  and third cylinder cavity  212  to cause extension are balanced by the forces sourced by air path  306  axially expanding retraction cavity  220  to cause retraction. Arm  110  will then be in a steady state and fixed axial position. An example of this steady state position could be to locate tip  124  and contactor  118  over power rail  108  for connection of work machine  100  to receive electrical power. 
     After adjusting the position of arm  110  in an open-loop mode, an operator can turn on a closed-loop, or automatic, mode of operation by activating a solenoid on pneumatic control valve  308 . In some examples, the closed-loop mode of operation involves maintaining an axial position of arm  110  with respect to an external reference point, such as with respect to contactor  118  on power rail  108  as work machine  100  moves along haul route  101 . For instance, referring to  FIG.  1    and the left side of  FIG.  3   , trailing arms  116  are connected to arm  110  at one end and connected to contactor  118  at an opposite end. A pivot  330  enables trailing arms  116  to swing laterally, i.e., along the Y axis, as a relative position between contactor  118  and arm  110  changes along the Y axis. If work machine  100  veers a lateral distance away from power rail  108 , i.e., along the −Y axis, contactor  118  will “move” a proportional distance in the opposite direction, i.e., along the +Y axis. That relative movement may present a risk that contactor  118  will become detached from power rail  108 . In the closed-loop mode of operation, pneumatic control system  300  can compensate for the changes in lateral position by automatically extending or retracting arm  110  an opposite amount. 
     In particular, for the example of  FIG.  3   , a linkage  332  in pneumatic control system  300  is coupled to trailing arms  116  and is configured to move horizontally in proportion to the lateral movement of contactor  118  and trailing arms  116 . Linkage  332 , which includes a double-wedge cam  334 , can be used with pneumatic control valve  336  and pneumatic control valve  338  to compensate for changes in lateral movement of the reference point represented by contactor  118 . Pneumatic control valve  336  and pneumatic control valve  338  are standard two-port, two-position (2/2) proportional pressure control valves that are normally closed. Each of pneumatic control valve  336  and pneumatic control valve  338  is mechanically actuated by a lever that follows a respective angled face of double-wedge cam  334 , as illustrated in  FIG.  3   . As pneumatic control valve  336  and pneumatic control valve  338  are normally closed, second piston cavity  230  and third cylinder cavity  212  remain isolated. However, if trailing arms  116  and linkage  332  move to the left in  FIG.  3   , pneumatic control valve  336  becomes activated proportional to the movement and exposes pressurized air on air path  326  to pass to air path  340  and into second piston cavity  230  and third cylinder cavity  212 . This addition of pressurized air will change the balance between extension forces in second piston cavity  230  and third cylinder cavity  212  on the hand, and retraction forces in retraction cavity  220  on the other hand, leading to gradual extension of arm  110 . As arm  110  is extended, trailing arms  116  will be pivoted back to the right in  FIG.  3    to compensate for its previous leftward movement. When contactor  118  returns to a position aligned with tip  124 , linkage  332  will be in a horizontal position such that double-wedge cam  334  will have moved rightward and again closed pneumatic control valve  336 . The feedback system enables the automatic adjustment of the length for conductor rod  106  to compensate relative movements between contactor  118  and work machine  100 . 
     Similarly, if after contactor  118  is connected with power rail  108 , work machine  100  moves closer to power rail  108 , i.e., to the left in  FIG.  3   , then contactor  118  and trailing arms  116  will swing to the right. As linkage  332  likewise moves to the right, double-wedge cam  334  will activate pneumatic control valve  338 , which will proportionally exhaust some of the pressurized air in second piston cavity  230  to the atmosphere through air path  342  and air breather  344 . The loss of air from second piston cavity  230  will change the balance of axial forces within the cavities of conductor rod  106  such that the retraction forces in retraction cavity  220  will cause arm  110  to begin retracting. As arm  110  is retracted, trailing arms  116  will be pivoted back to the left in  FIG.  3    to compensate for its previous rightward movement. When contactor  118  returns to a position aligned with tip  124 , linkage  332  will be in a horizontal position such that double-wedge cam  334  will have moved leftward and again caused pneumatic control valve  338  to close. 
     The use of two components in pneumatic control valve  336  and pneumatic control valve  338  for following two faces of double-wedge cam  334  gives more fidelity for mechanically tuning the feedback system compared with using a single valve. While the faces of double-wedge cam  334  are shown as straight angles, the surfaces may be arcuate or have other shapes to achieve a desired feedback response. A mechanical linkage as shown in  FIG.  3    provides rapid assessment and correction of positional changes for contactor  118 , but alternative sensors could also be employed. For instance, electronic devices may be used to determine relative movement of a reference point such as contactor  118  and to compensate for the movement, although their response time may be longer. 
     In addition, the use of pneumatic control valve  318  and air path  326 , together with pneumatic control valve  308 , provide a safety feature for conductor rod  106  and work machine  100 . In particular, the use of pneumatic control valve  308  to switch from an open-loop mode to a closed-loop mode, causes pneumatic control valve  318  to isolate air path  320  from air path  322 . As a result, if a change is made to pneumatic control valve  316  accidentally during closed-loop mode, movement of arm  110  in the open-loop mode will not occur. 
       FIG.  4    illustrates another example of a pneumatic control system  400  in a representative arrangement for causing arm  110  to move axially within barrel  109 .  FIG.  4    depicts an alternate arrangement of tubular conductors and cavities within barrel  109  and arm  110  for conductor rod  106  compared with the longitudinal section in  FIG.  2    and the schematic diagram of  FIG.  3   . In essence, the tubular conductors within barrel  109  and arm  110  are inverted or switched from each other in  FIG.  4    compared to the example in  FIGS.  2  and  3   . For instance, while  FIGS.  2  and  3    depict two tubes of insulation at second cylinder insulation  208  and fourth cylinder insulation  216  within barrel  109 ,  FIG.  4    shows that arm  110  has the two tubes of insulation at second piston insulation  410  and piston shell insulation  416 . Similarly, in  FIGS.  2  and  3   , arm  110  has one insulation layer at third piston insulation  234 , while in  FIG.  4   , barrel  109  has the one layer of insulation at third cylinder insulation  406 . Consequently, with respect to cavities available for forcing extension of barrel  109  pneumatically, barrel  109  in  FIG.  4    has a central passageway formed by first cylinder cavity  204  and first piston cavity  226  as well as second cylinder cavity  404  and barrel shell cavity  408 , while arm  110  has third piston cavity  412 . 
     While having generally the same principles of operation as pneumatic control system  300  in  FIG.  3   , pneumatic control system  400  has fewer components and operates its open-loop and closed-loop modes simultaneously. For instance, to the right and below conductor rod  106  in  FIG.  3   , components related to a pneumatic control circuit configured to cause arm  110  to either retract or extend from within barrel  109  in an open-loop mode are shown. Compressor  302 , condenser  304 , and air breather  314  operate similarly to  FIG.  4   , resulting in pressurized air being fed through air path  306  to retraction cavity  220 . As such, the pressurized air into retraction cavity  220  causes retraction as a fail-safe and default configuration. Pneumatic control system  400  in some examples also feeds pressurized air via air path  306  in the central passageway of conductor rod  106  formed by first cylinder cavity  204  and first piston cavity  226 . As a result, the arrangement in pneumatic control system  400  provides pneumatic energy to the left of conductor rod  106  in  FIG.  4    for use remotely from work machine  100  at tip  124 , such as for a closed-loop mode of operation described below. 
     In some examples, barrel shell cavity  408  and third piston cavity  412  cooperate to control axial movement of arm  110  with respect to barrel  109 . Barrel shell cavity  408  is an annular or tubular-shaped space with a volume related to the distance of insertion of arm  110  into barrel  109 . Pneumatic control valve  418 , which is readily available to one of ordinary skill in the art, is a standard three-port, two position (3/2) proportional directional control valve that is normally closed. In the arrangement illustrated, pneumatic control valve  418  controls access of barrel shell cavity  408  to either atmospheric air or pressurized air. Specifically, in its default condition, pneumatic control valve  418  has its outlet connected to its exhaust, which couples air path  420  and barrel shell cavity  408  to atmospheric pressure via air path  312  and air breather  314 . In this condition, the axial forces generated by pressurized air within retraction cavity  220  will cause retraction of arm  110 . An actuator for the solenoid associated with pneumatic control valve  418  may be modified to proportionally adjust the exposure of the pressurized air in air path  306  at the inlet of pneumatic control valve  318  to air path  420  at the outlet. As pressurized air is provided to barrel shell cavity  408  circumferentially around barrel  109 , axial forces within barrel shell cavity  408  may eventually equal the axial forces acting in retraction cavity  220 . In that situation, conductor rod  106  will be held in a stationary position axially. If adjustment of pneumatic control valve  418  leads to the axial forces within barrel shell cavity  408  exceeding the axial forces within retraction cavity  220 , arm  110  will move axially outward from barrel  109 . Variation of the air flow through pneumatic control valve  418  and the axial movement of arm  110  can be used to adjust the position of arm  110 , such as tip  124 , laterally from work machine  100 . The adjustment can be done, for example, to align contactor  118  for connection to power rail  108  to provide electrical power to work machine  100 . 
     As mentioned and shown, air path  306  delivers pressurized air through first cylinder cavity  204  and first piston cavity  226 , i.e., through a central passageway in conductor rod  106 , to air path  430 . At this location, the pressurized air delivered through conductor rod  106  may be used for variety of purposes, such as providing pneumatic controls to adjust trailing arms  116  or contactor  118 . As well, the pressurized air in air path  430 , together with pneumatic control valve  424 , third piston cavity  412 , and second cylinder cavity  404  may be used to adjust the axial position of arm  110  in a closed-loop mode of operation. Third piston cavity  412  and second cylinder cavity  404  are pneumatically connected through passage  414  that extends between them within third cylinder insulation  406 . The radial areas within third piston cavity  412  and second cylinder cavity  404  exceed the radial areas within retraction cavity  220 , such that axial forces in the combined volumes of third piston cavity  412  and second cylinder cavity  404  can be made larger than the axial forces in retraction cavity  220  to cause extension. Pneumatic control valve  424  a standard three-port, two position (3/2) proportional directional control valve that is normally closed. In its default condition, pneumatic control valve  424  couples air path  422 , along with third piston cavity  412  and second cylinder cavity  404 , to the atmosphere by way of air path  426  and air breather  428 . In its activated condition, pneumatic control valve  424  will couple third piston cavity  412  and second cylinder cavity  404  to pressurized air within air path  430 . As a proportional device, pneumatic control valve  424  may provide a gradual adjustment or mixing between air path  430 , air path  426 , and air path  422 . 
     As discussed above with respect to  FIG.  3   , a linkage  332  in  FIG.  4    may be coupled to trailing arms  116  and configured to move horizontally in proportion to the lateral movement of contactor  118  and trailing arms  116 . Linkage  332 , which includes a wedge-shaped cam  432 , can be used with pneumatic control valve  424  to compensate for changes in lateral movement of the reference point represented by contactor  118 . In some examples, pneumatic control valve  424  is mechanically actuated by a lever that follows an angled face of wedge-shaped cam  432 . If trailing arms  116  and linkage  332  move to the left in  FIG.  4   , i.e., conductor rod  106  moves along the Y axis in  FIG.  1   , pneumatic control valve  424  becomes activated proportional to the movement and exposes pressurized air on air path  430  into air path  422 , and thus into third piston cavity  412  and second cylinder cavity  404 . This addition of pressurized air will change the balance between extension forces in third piston cavity  412  and second cylinder cavity  404  and retraction forces in retraction cavity  220 , leading to gradual extension of arm  110 . As arm  110  is extended, trailing arms  116  will be pivoted back to the right in  FIG.  4    to compensate for its previous leftward movement. When contactor  118  returns to a position aligned with tip  124 , linkage  332  will be in a horizontal position such that wedge-shaped cam  432  will have moved rightward and decreased the opening of pneumatic control valve  424  with respect to air path  430 . The feedback system enables the automatic adjustment of the length for conductor rod  106  to compensate relative movements between contactor  118  and work machine  100 . 
     Similarly, if after contactor  118  is connected with power rail  108 , work machine  100  moves closer to power rail  108 , i.e., to the left in  FIG.  4   , then contactor  118  and trailing arms  116  will swing to the right. As wedge-shaped cam  432  likewise moves to the right, wedge-shaped cam  432  will cause pneumatic control valve  424  to proportionally exhaust some of the pressurized air in third piston cavity  412  to the atmosphere through air path  422  and air path  426 . The loss of air from third piston cavity  412  will change the balance of axial forces within the cavities of conductor rod  106  such that the retraction forces in retraction cavity  220  will cause arm  110  to begin retracting. As arm  110  is retracted, trailing arms  116  will be pivoted back to the left in  FIG.  4    to compensate for its previous rightward movement. When contactor  118  returns to a position aligned with tip  124 , linkage  332  will be in a horizontal position such that wedge-shaped cam  432  will have moved leftward and again caused pneumatic control valve  424  to adjust the flow of air between air path  430 , air path  422 , and air path  426 . 
     Accordingly, pneumatic control system  400  enables pneumatic control of axial movement for arm  110  within barrel  109  for both open-loop and closed-loop modes of operation. With a device such as pneumatic control valve  418 , an operator can move tip  124  of arm  110  into position in open-loop mode for connection of contactor  118  to power rail  108 . After electrical power flows and work machine  100  moves, pneumatic control system  400  provides closed-loop control over the axial position of arm  110  to help maintain connection between contactor  118  and power rail  108 . Variations to pneumatic control system  400  consistent with the disclosed principles will be apparent to those skill in the art. For instance, the cavities selected for open-loop extension (barrel shell cavity  408 ) and for closed-loop extension (third piston cavity  412  and second cylinder cavity  404 ) could be changed. Moreover, the pneumatic controls may be altered or added to for different performance characteristics. As well, pressurized air flowing from head  122  to tip  124  within conductor rod  106  could be used at the distal end of conductor rod  106  to energize functions other than closed-loop axial movement of arm  110 . In addition, more or fewer conductors and cavities between them may be selected based on the particular implementation. 
     Turning from pneumatic control systems for conductor rod  106  as illustrated in  FIGS.  3  and  4    to methods for operating those systems,  FIG.  5    is a flowchart of representative steps for controlling axial movement of conductor rod  106  consistent with the present disclosure. Generally embodied as  500  in  FIG.  5   , the method begins with a step  502  of providing pressurized air from a compressor within a work machine to a conductor rod on the work machine. The work machine is to be energized by electrical power, and the conductor rod is configured to provide the electrical power from outside the work machine. Thereafter, method  500  involves causing a piston that is slidingly engaged within a pneumatic cylinder of the conductor rod to retract along a longitudinal axis into the pneumatic cylinder in an open-loop mode. The pneumatic cylinder includes cylinder tubes concentrically positioned around a central passageway, and the piston includes piston tubes concentrically positioned around the central passageway and radially offset from the cylinder tubes. 
     In more particular steps of method  500 , the pressurized air is fed (step  504 ) into an annular retraction cavity on a cylinder end of the conductor rod and an annular cylinder cavity is pneumatically coupled (step  506 ) to the pressurized air. As explained in more detail for examples above, the annular retraction cavity is positioned between an inner wall of the pneumatic cylinder and an outer wall of the piston. In addition, the annular cylinder cavity is positioned near a base of the conductive rod proximate the work machine and is bounded in the pneumatic cylinder by successive cylinder tubes and a piston tube slidably mated between the successive cylinder tubes. 
     Further steps of method  500  involve feeding pressurized air into a central passageway of the cylinder end of the conductor rod (step  508 ) and extracting the pressurized air from the central passageway of the piston end of the conductor rod (step  510 ). Accordingly, pressurized air is provided at a distal end of the conductor rod, possibly at a side of haul route  101  when arm  110  is extended. 
     After work machine  100  receives electrical power through the conductor rod, the method may continue in receiving an indication from a sensor at the piston side of the conductor rod of a change in position of a reference point. Typically, the reference point is relative to a tip and remote from the conductor rod (step  512 ). Finally, method  500  entails a step  514  where, in response to the indication from the sensor, an annular piston cavity is selectively coupled to the pressurized air extracted from the central passageway or to the atmosphere. Accordingly, the conductive rod is caused to change in length in a closed-loop mode, and a position of a tip of the conductive rod is controlled to continue a secure contact with a source of electrical power for the work machine. 
     It will be understood that descriptions of an operator activating one or more solenoids associated with directional control valves for causing extension or retraction of arm  110  with respect to barrel  109  will include several possible implementations. For instance, the operator or similar personnel could directly adjust one or more of the solenoids manually. In other examples, an electronic controller, processor, or similar device provides electronic signals for causing adjustment to the one or more solenoids. The controller may be an electronic control module (not shown) that executes a plurality of electronic control functions within work machine  100  either automatically under control of software instructions or on demand in response to electronic input from an operator or other individual associated with work machine  100 . 
     Those of ordinary skill in the field will appreciate that the principles of this disclosure are not limited to the specific examples discussed or illustrated in the figures. For example, while the examples illustrate pressurized air supplied to certain cavities within barrel  109  and arm  110 , the pressurized air could be supplied to different ones of the cavities within those sections. Similarly, while angular surfaces on double-wedge cam  334  and wedge-shaped cam  432  are illustrated, curved surfaces or other configurations for those cams to optimize feedback from trailing arms  116  could be used. Likewise, the particular components for the open-loop and closed-loop controls are within the knowledge and selection of those of ordinary skill in the art working from the present disclosure. In addition, the principles disclosed are not limited to implementation on a work machine. Any moving vehicle deriving electrical power from a ground-based conductor rail could benefit from the examples and techniques disclosed and claimed. 
     Industrial Applicability 
     The present disclosure provides a work machine powered electrically by a conductor rod and a pneumatic control system for moving the conductor rod axially for contact with power rails along the side of a haul route. The conductor rod has a barrel with an extendable arm axially moveable within the barrel. Both the barrel and the arm have tubular-shaped concentric conductors that slide against each other as the arm moves axially and maintain electrical conductivity for the electrical power from the power rails to the work machine. An axial passageway of open space passes through the centermost tubular-shaped conductors. Voids between selected conductors in the barrel and in the arm are filled with insulation so that a radial interface between the arm and the barrel is substantially solid, but for the axial passageway. Arrangements of pneumatic control valves provide pressurized air to selected cavities formed at ends or sides of the tubular-shaped conductors, causing axial forces that are balanced to effect retraction and extension of the arm. 
     As noted above with respect to  FIGS.  1 - 5   , an example work machine that is electrically powered generally includes an electric engine, an air compressor, and a conductor rod  106 . The conductor rod  106  has a central passageway along a longitudinal axis pneumatically coupling a head  122  to a tip  124 . A barrel  109  extending from head  122  toward tip  124  has cylinder tubes concentrically positioned around the central passageway and cylinder cavities between the cylinder tubes. An arm  110  extending from tip  124  toward head  122  has piston tubes concentrically positioned around the central passageway and piston cavities, where the cylinder tubes are radially offset from the piston tubes and arm  110  is slidably mated with barrel  109 . Pressurized air is fed to a retraction cavity  220  enclosed between an inner surface of arm  110  and an outer surface of barrel  109 . One or more directional control valves, such as pneumatic control valve  316  are configured, in a default state, to couple one of the cylinder cavities to atmosphere and, in an active state, to couple the one of the cylinder cavities to pressurized air for extending the arm  110 . 
     In the examples of the present disclosure, pneumatic control system  300  and pneumatic control system  400  enable axial movement of arm  110  with respect to barrel  109 . In an open-loop mode, the pneumatic controls provide pressurized air to retraction cavity  220  and then permit an operator to introduce pressurized air to one or more expansion cavities, such as third cylinder cavity  212  and barrel shell cavity  408 . If the axial forces in the expansion cavities exceed the opposing axial forces in retraction cavity  220 , arm  110  will extend, and a tip  124  can be positioned for connecting contactor  118  to power rail  108  to obtain electrical power for work machine  100 . In a closed-loop mode, pneumatic control system  300  and pneumatic control system  400  accept mechanical feedback regarding lateral movement of contactor  118  relative to arm  110  and pneumatically adjust the axial position of conductor rod  106  to compensate for the movement. Accordingly, the pneumatic control system enables accurate attachment of conductor rod  106 , which may be several meters in length, to power rail  108  and ensures continued connection of work machine  100  to its source of electrical power. In particular, as work machine  100  may veer from power rail  108  along haul route  101 , the pneumatic control systems automatically compensate by adjusting the axial length of conductor rod  106 . As a result, supply of electrical power is maintained to work machine  100  despite deviations in lateral positioning caused by steering, road conditions, or positions of power rail  108 . In addition, various fail-safe features within pneumatic control system  300  and pneumatic control system  400  ensure that arm  110  safely retracts into barrel  109  and that inadvertent axial movements of barrel  109  are avoided while work machine  100  is moving. 
     Unless explicitly excluded, the use of the singular to describe a component, structure, or operation does not exclude the use of plural such components, structures, or operations or their equivalents. As used herein, the word “or” refers to any possible permutation of a set of items. For example, the phrase “A, B, or C” refers to at least one of A, B, C, or any combination thereof, such as any of: A; B; C; A and B; A and C; B and C; A, B, and C; or multiple of any item such as A and A; B, B, and C; A, A, B, C, and C; etc. 
     While aspects of the present disclosure have been particularly shown and described with reference to the embodiments above, it will be understood by those skilled in the art that various additional embodiments may be contemplated by the modification of the disclosed machines, systems and methods without departing from the spirit and scope of what is disclosed. Such embodiments should be understood to fall within the scope of the present disclosure as determined based upon the claims and any equivalents thereof.