Abstract:
Systems, methods, and apparatus for performing automatic positioning of a haulage vehicle. One method includes receiving, with a controller, an input representing selection of a selection mechanism. The method also includes, in response to receiving the input, determining, with the controller, a current position of a bucket of the haulage vehicle, and retrieving, with the controller, a predetermined carry position from a memory. The method also includes comparing, with the controller, the current position of the bucket to the predetermined carry position, and, when the current position of the bucket differs from the predetermined carry position, automatically controlling, with the controller, an actuator to move the bucket to the predetermined carry position.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Nos. 62/167,808 and 62/167,814 both filed on May 28, 2015. The entire content of both provisional applications is incorporated by reference herein. 
     
    
     BACKGROUND 
       [0002]    Embodiments of the invention provide mining machines including an energy storage device, such as a flywheel. In particular, some embodiments of the invention provide use of a flywheel energy storage system on a rubber tired, articulated front end loading machine with a switched reluctance drive system. 
       SUMMARY 
       [0003]    Mining equipment commonly works in highly cyclical applications, where direction changes and routine start and stop activities are frequent. These cyclical actions may be used to dig, load, move, and dispatch minerals. 
         [0004]    For rubber tired loaders or trucks these cycles may occur over a period from approximately 30 seconds up to and exceeding approximately 3 to 4 minutes depending on the application. The variations between cycle periods of different applications may be attributed to the length of the haul (the distance the machine traverses between the point where the machine collects the material and the point where the machine dumps the material). 
         [0005]    For example, for a surface front end loader loading trucks in an open cut mine, the length of the haul may be approximately 30 meters. Accordingly, if the front end loader has a machine speed of less than approximately 15 kilometers per hour (“kph”), the front end loader may complete a cycle in less than 30 seconds. However, for an underground loader operating in a block or panel cave, the length of the haul may exceed approximately 300 meters. Thus, if the underground loader has a machine speed of approximately 20 kph, the underground loader may complete a cycle in approximately 4 minutes. 
         [0006]    Similarly, haulage equipment, such as shuttle cars, repeatedly complete the task of retrieving material from a mining machine, hauling the material to a crushing or material handling solution, such as a conveyor, and then returning to the mining machine to gather another load. 
         [0007]    Large shovels and draglines also operate in a cyclical manner. For example, shovels and draglines dig and dump in a cyclical motion where the direction of machine swing is reversed to return to a start position while accelerating and decelerating a large vehicle mass. 
         [0008]    Accordingly, there are opportunities to improve the efficiency of cyclical operation of mining equipment through the use of energy storage. One opportunity includes capturing the kinetic energy in the movement of the machine, storing the energy, and using the stored energy for the next movement phase of the cycle. Another opportunity includes smoothing the peak power load of a power source by storing energy from the power source at times of low load and using the stored energy to assist the power source to drive the peak load. This functionality allows the power source, which may be a diesel engine, a transformer, or a trail cable, to be downsized reducing installation and maintenance costs. An opportunity also exists, through the same efficiency gain, to improve the overall performance of a machine type, for a given energy consumption. 
         [0009]    Accordingly, embodiments of the invention use an energy storage device that includes a flywheel or another form of kinetic energy storage system (“KESS”). The KESS may be used with switched reluctance (“SR”) technology to store energy in a kinetic form for later use. Thus, embodiments of the invention incorporate one or more KESSs into a high power, mining traction application, which may be used on surface machines and underground machine incorporating SR technology. 
         [0010]    In some embodiments, machines incorporating a KESS as described herein may include a diesel engine as the primary power source. In this embodiment, the KESS performs a power averaging and boost function using both braking energy and energy from the diesel engine output shaft. However, it should be understood that the KESS may also be used with other (non-diesel) power sources. As described in more detail below, the KESS may assist the engine during load peaks and may draw from the engine during load dips. Accordingly, with a properly-sized KESS, the KESS may be used to achieve full power averaging, wherein an engine runs continuously at a near constant load (for example, without variation). Using the power averaging provided by the KESS allows the engine to be downsized. Similarly, power averaging may extend engine life and maximize fuel savings by running the engine in a constant output state. 
         [0011]    Furthermore, in some embodiments, the diesel engine may be replaced with a different power source, such as a battery. In particular, the full power averaging provided by a traction system with KESS (as developed with a diesel engine) may optimize a battery solution for some machines, such as a load haul dump (“LHD”) or a shuttle car. It should be understood that other power sources, such as fuel cells could also be used as an alternative to a diesel engine (for example, due to the power density of liquid fuel storage over batteries). 
         [0012]    For example, some embodiments provide a haulage vehicle including a bi-directional electrical bus, a power source, a motor, a kinetic energy storage system, and a controller. The power source is coupled to the bi-directional electrical bus through a first power converter. The motor is coupled to the bi-directional electrical bus through a second power converter. The motor is powered by energy available on the bi-directional electrical bus and operates a drive mechanism included in the haulage vehicle. The kinetic energy storage system is coupled to the bi-directional electrical bus through a third power converter and includes a flywheel and a switched reluctance motor. The controller is configured to communicate with the kinetic energy storage system and the power source. The controller is also configured to operate the kinetic energy storage system as a primary power source for the bi-directional electrical bus and to operate the power source as a secondary power source for the bi-directional electrical bus when the kinetic energy storage system cannot satisfy an energy demand on the bi-directional electrical bus. 
         [0013]    Other embodiments provide a method of operating a haulage vehicle. The method includes determining, with a controller configured to communicate with a kinetic energy storage system and a power source included in the haulage vehicle, an energy demand on a bi-directional electrical bus included in the haulage vehicle and determining, with the controller, energy available through the kinetic energy storage system. The method also includes operating, with the controller, the kinetic energy storage system as a primary power source for the bi-directional electrical bus when the energy available through the kinetic energy storage system satisfies the energy demand and operating, with the controller, the power source as a secondary power source for the bi-directional electrical bus when the energy available through the kinetic energy storage system cannot satisfy the energy demand. 
         [0014]    Additional embodiments provide a haulage vehicle that includes a bucket moveable in at least one direction, an actuator for moving the bucket in the at least one direction, an operator control including a selection mechanism, and a controller. The controller is configured to receive an input representing selection of the selection mechanism. In response to the input, the controller is configured to determine a current position of the bucket, retrieve a predetermined carry position from a memory, compare the current position of the bucket to the predetermined carry position, and, when the current position of the bucket differs from the predetermined carry position, automatically operate the actuator to move the bucket to the predetermined carry position. 
         [0015]    Further embodiments provide a method of automatically operating a haulage vehicle. The method includes receiving, with a controller, an input representing selection of a selection mechanism. The method also includes, in response to receiving the input, determining, with the controller, a current position of a bucket of the haulage vehicle, and retrieving, with the controller, a predetermined carry position from a memory. The method also includes comparing, with the controller, the current position of the bucket to the predetermined carry position, and, when the current position of the bucket differs from the predetermined carry position, automatically controlling, with the controller, an actuator to move the bucket to the predetermined carry position. 
         [0016]    Other aspects of the invention will become apparent by consideration of the detailed description, accompanying drawings, and accompanying appendix. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  illustrates a power curve for a mechanical drive system. 
           [0018]      FIG. 2  illustrates a power curve for a switched reluctance (“SR”) drive system. 
           [0019]      FIG. 3  schematically illustrates system architecture for a diesel-hybrid SR surface loader. 
           [0020]      FIG. 4  is a graph of a SR machine efficiency curve. 
           [0021]      FIGS. 5 and 6  illustrate power curves for an SR drive system with a kinetic energy storage system (“KESS”). 
           [0022]      FIG. 7  illustrates a power curve for a SR drive system with a KESS and a battery or fuel cell. 
           [0023]      FIG. 8  schematically illustrates system architecture for a SR drive system with a KESS. 
           [0024]      FIG. 9  illustrates a control curve for a KESS. 
           [0025]      FIG. 10  is a perspective view of mining equipment, specifically, a front-end loader. 
           [0026]      FIG. 11  schematically illustrates functional elements of the mining equipment of  FIG. 10 . 
           [0027]      FIG. 12  schematically illustrates a controller included in the mining equipment of  FIG. 10 . 
           [0028]      FIG. 13  schematically illustrates potential power flow within the equipment of  FIG. 10 . 
           [0029]      FIG. 14  schematically illustrates power flow within the equipment of  FIG. 10  for charging the kinetic energy storage system. 
           [0030]      FIG. 15  schematically illustrates power flow in the equipment of  FIG. 10  for performing propulsion using the kinetic energy storage system. 
           [0031]      FIG. 16  schematically illustrates power flow in the equipment of  FIG. 10  for performing propulsion without using the kinetic energy storage system. 
           [0032]      FIG. 17  schematically illustrates power flow in the equipment of  FIG. 10  for performing light braking. 
           [0033]      FIG. 18  schematically illustrates power flow in the equipment of  FIG. 10  for performing heavy braking and charging the kinetic energy storage system. 
           [0034]      FIG. 19  schematically illustrates power flow in the equipment of  FIG. 10  for performing heavy breaking without charging the kinetic energy storage system. 
           [0035]      FIG. 20  schematically illustrates a mining machine including multiple kinetic energy storage systems. 
           [0036]      FIG. 21  illustrates a load haul dump (“LHD”) with a bucket positioned in a dump position. 
           [0037]      FIG. 22  illustrates the LHD of  FIG. 20  with the bucket positioned in a dig position. 
           [0038]      FIG. 23  illustrates the LHD of  FIG. 20  with the bucket positioned in a carry position. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. 
         [0040]    In addition, it should be understood that embodiments of the invention may include hardware, software, and electronic components or modules that, for purposes of discussion, may be illustrated and described as if the majority of the components were implemented solely in hardware. However, one of ordinary skill in the art, and based on a reading of this detailed description, would recognize that, in at least one embodiment, aspects of the invention may be implemented in software (for example, stored on non-transitory computer-readable medium) executable by one or more processing units, such as a microprocessor, an application specific integrated circuits (“ASICs”), or another electronic device. As such, it should be noted that a plurality of hardware and software based devices, as well as a plurality of different structural components may be utilized to implement the invention. For example, “controllers” described in the specification may include one or more electronic processors or processing units, one or more computer-readable medium modules, one or more input/output interfaces, and various connections (for example, a system bus) connecting the components. 
         [0041]    As noted above, embodiments of the invention incorporate one or more kinetic energy storage systems (“KESSs”) into a machine traction drive train (for example, high power), which may be used on mining machines (for example, surface and underground mining machines) incorporating SR technology. Accordingly, embodiments of the invention may use KESSs with an electric drive system. Electrical drive systems may burn 30% to 40% less fuel than a mechanical drive equivalent. These savings in fuel may be achieved through differences of the respective equipment drive trains and the relative efficiencies. In particular, mechanical drive systems currently used in surface mining applications employ a conventional mechanical drivetrain, with a torque converter, semi-automatic or automatic transmission/transfer case, and differentials. Mechanical drive systems, however, may be inefficient due to operation of the torque converter and may require a large engine for supplying high power output even though the engine may not consistently operate at a peak output level. For example,  FIG. 1  illustrates a power curve for a mechanical drive system. 
         [0042]    Switched reluctance electric drive systems provide further efficiency advantages over mechanical drive transmission systems. For example, a switched reluctance drive system may allow the engine to be downsized due to the ability of the system to maintain the engine speed at a peak output level. For example,  FIG. 2  illustrates a power curve for a switched reluctance drive system. Furthermore,  FIG. 3  schematically illustrates a surface loader with a diesel-hybrid SR drive. As described above, surface loaders operate or perform substantially cyclical operations. For example, a surface loader cycle of operation may include approximately four machine direction changes during one cycle that may last approximately 40 seconds. 
         [0043]    In particular, as illustrated in  FIG. 3 , the surface loader includes an engine  10  combined with a motor/generator  12  (for example, an SR motor/generator) and a traction system  13 . The traction system  13  illustrated in  FIG. 1  includes four SR motors  14 . Each SR motor  14  may supply electrical power to one wheel of the loader. The SR motors  14  and the motor/generator  12  are connected by an electrical bus  16  (for example, a direct current (“DC”) bus). One or more converters  18  connect the motor/generator  12  to the electrical bus  16 . Similarly, one or more converters  18  connect the SR motors  14  to the electrical bus  16 . The converters  18  may convert energy supplied by the motor/generator  12  into power supplied over the electrical bus  16 . Similarly, the converters  18  may convert energy supplied over the electrical bus  16  into energy usable by the SR motors  14 . 
         [0044]    In the system illustrated in  FIG. 3  the revolutions per minute (“RPM”) of the engine  10  is independent of the traction motor speed provided through the SR motors  14 . In other words, each SR motor  14  may draw or provide rotational energy to the engine drive line at any speed with little penalty in terms of efficiency loss. In some embodiments, the speed of the engine  10  may be set to run at the lowest RPM at which maximum engine horsepower is available. 
         [0045]    The speed setting of the engine  10  (at the peak of the power curve) facilitates the opportunity to increase the speed of the engine  10  above the governor set speed (over speed the engine), which causes the fuel injectors to stop supplying fuel to the engine  10  and allows the drive line to be used as a flywheel to store braking energy. Mechanical drive systems are inefficient in passing energy from the engine driveshaft to the wheels, particularly when there is a high differential in speed (for example, due to torque converter operation in high differential speed conditions). The engine in mechanical drive machines will commonly be under high load at speeds below the peak horsepower curve; meaning that they are burning fuel at less than maximum engine efficiency. Because mechanical drive systems typically require high power at non-optimal engine RPM&#39;s, engines may be oversized in relative terms, where the nameplate rating of the engine is similar but a bigger engine volumetric capacity is required. Bigger engines lead to a machine design that has higher operating and rebuild costs in addition to greater frictional losses. 
         [0046]      FIG. 4  illustrates an SR machine efficiency curve. An SR system may provide full torque to the wheels during a stall while consuming only approximately 10% of engine horsepower. This may occur due to the low reactive losses of the SR system. For example, the only significant losses may be the copper losses brought about by the internal resistance of the motor coils and the current passing through them. Accordingly, the SR machine (motor or generator) may have an almost flat efficiency curve across its speed range as illustrated in  FIG. 4  above. 
         [0047]    A mechanical drive train by comparison is typically at full horsepower output during a stall. The torque converter requires this power to produce torque. Most of this horsepower is lost as heat, which is a byproduct of the torque production process. Furthermore, a torque converter is inefficient whenever significant slip or speed differential is present between the input and output shafts. 
         [0048]    Effectively both systems are at zero percent efficiency during a stall because the power output of a stationary shaft is zero. In this condition, transmission efficiency could be measured as a function of output torque against power consumption. However, in this scenario, the SR drive system is more efficient at producing torque per unit of power consumed as compared to the typical mechanical driveline. 
         [0049]    Also, on a mechanical drive machine, conventional brakes are used. These brakes are typically multi-pack wet disc brakes. Like all mechanical brakes, these devices convert kinetic energy into heat. The heat on a multi disc brake is transferred to hydraulic oil and is dissipated by way of a radiator cooling system. 
         [0050]    In SR drive machines, such as a surface loader, braking energy is diverted back to the engine drive line. In some embodiments, this braking energy is used as described below. In particular, the braking energy may first supply the parasitic losses around the machine. These include but are not limited to the engine fan and other cooling fans, air conditioner and battery charging alternator. These systems are low power in comparison to the braking energy that is being diverted so there is significant energy remaining to be dealt with. 
         [0051]    Next, the working hydraulic systems loads may be supplied with energy. This includes the hoist, bucket, and steer hydraulic functions. Any remaining energy may be used to contribute energy to the drive train. For example, the SR generator, now acting as a motor, contributes motive force to the drive train to a point where the engine&#39;s governor is able to reduce or cut off fuel supply to the injectors. At this point the engine may not be consuming any fuel and frictional and windage losses of the engine are being compensated for by the SR generator. In these embodiments, the engine speed may be increased up to the engine&#39;s mechanical limit, at which point the engine becomes an energy storage device (a flywheel), albeit with poor efficiency due to the engine&#39;s friction and windage. The overhead of engine speed (for example, approximately 300 RPM) above the governor cutoff point may be used on the next propulsion phase to boost available power to the traction system above that of the nameplate rating of the engine. Using the driveline as an energy storage device as described above provides an energy storage option when the cycle speed of the machine is fast (for example, less than approximately 50 seconds) and the energy storage capacity is low as the energy stored on the driveline may be reused by the traction system before it is consumed by the engine&#39;s friction and windage losses. Accordingly, this energy storage option may be used on surface loaders at high altitude where less overall oxygen is available for engine combustion. For example, at high altitude larger diameter turbo charges are often required to supply air to the engine. These turbo charges take a long time to spin up to a working speed due to the larger inertial mass. This time constraint effects the response time of the engine. Therefore, a KESS may supplement the traction system power needs while turbo charges get up to a working speed. 
         [0052]    However, for underground mining, as an alternative to or in combination with storing energy on the engine drive line, a KESS may be used to store braking energy. The KESS provides gains in fuel efficiency and, consequently, reduces emissions. In particular, the KESS provides a longer duration, higher capacity, and higher efficiency storage solution than the driveline storage solution used on surface loaders as described in the previous paragraph. For example,  FIG. 5  illustrates a power curve for a switched reluctance drive system including a KESS. As illustrated in  FIG. 5 , the KESS may provide a power boost to supplement output from the engine, may allow the engine to be downsized, or a combination thereof. For example,  FIG. 6  illustrates a power curve for a switched reluctance drive system including a KESS larger than the KESS represented in  FIG. 5 . As illustrated in  FIG. 6 , the larger KESS may provide maximum power averaging of the engine while providing high peak power to the traction system. Furthermore,  FIG. 7  illustrates a power curve for a switched reluctance drive system including a KESS and a battery (for example, a sodium battery), a fuel cell, or both. As illustrated in  FIG. 7 , with an averaged power source, alternative energy supply technologies may be employed, such as fuel cell technologies and battery technologies. 
         [0053]    The operational profiles of an underground mining machine different significantly from operational profiles of a surface mining machine, such as a loader, shovel, and the like. For example, a surface operational profile is commonly short, where the machine encounters four direction changes in a 40 second cycle period and spends approximately 8 to 10 seconds at stall filling the bucket. In contrast, there are two main modes of operation in the underground environment: (1) development and (2) production work. Both modes of operations differ from surface operation in terms of haul distance and resultant cycle time. For example, an underground machine may haul materials over distances up to 200 meters in mine development and over 350 meters in production and these distances result in cycle times that vary from approximately 2 to approximately 3 minutes. 
         [0054]    Also, in underground mining environments, the production environment is predominantly flat. For example, the maximum grades seen in this operation are around 1 in 50. As described above, underground machines may haul material over distances exceeding 350 meters. Also, in the production cycle, the machines will typically complete two forward trams and two reverse trams. In addition, in many mines, a production loader may visit many extraction points at varying distances from the crusher to collect ore. Accordingly, the nature of this cycle may be dependent on mine layout and distance of the ore to the crushing plant loading hopper. 
         [0055]    Based on this type of environment, one opportunity to store energy in the production cycle is during braking events. To maximize productivity, the underground machine should be able to accelerate and decelerate quickly. Accordingly, during deceleration, energy taken from the traction motors may be captured for later reuse by a KESS. Additionally, when the engine is in a low demand situation, some of its available power could be used to supply energy to KESS. As noted above, using stored energy in this manner allows the diesel engine to be downsized by averaging engine output power over a cycle. In addition to downsizing the engine, which reduces costs, the magnitude of the downsize may also, in some embodiments, result in a smaller block size engine being used, which provides additional performance gains as the friction and windage losses of the engine are further reduced. 
         [0056]    The KESS used in these situations may be capable of storing the energy of one or two braking events (for example, approximately 1.2 mega joules (“MJ”) per event) with high power capacity (for example, approximately 500 kilowatts (“KW”)) to allow the KESS to be filled or emptied in a matter of seconds. The KESS may also be configured to provide efficient uptake and release of energy and retain stored energy with minimal loss over time. 
         [0057]    With regard to the development environment, a larger percentage of the development work occurs around the mine entry road or decline. These declines are typically at a slope of approximately 1 in 6.5. When working in the development environment, the underground machine digs out of the bottom of the decline, where the road is being extended, through drill and blast techniques. The machine then trams up the incline haul between approximately 25 to approximately 200 meters where the machine dumps the material or loads the material into a truck. The underground machine then returns to the dig face, which involves driving down the approximately 200 meter slope while braking to manage speed. 
         [0058]    The up slope haul is engine power intensive and impacts transmission life while the down slope return typically places a large strain on brakes. A KESS that stores the braking energy generated on the down slope run to the dig face (for example, yielding up to 10 MJ) may provide a significant boost to the engine on the up slope run. 
         [0059]      FIG. 8  illustrates a SR drive system with a KESS  30 . The KESS  30  includes an SR drive motor  30   a  and a flywheel  30   b.  In the configuration illustrated in  FIG. 8 , the KESS  30  may be configured to store braking energy as the machine slows (the speed of a drive mechanism decreases) as per operator command. The energy may be held in the KESS  30  for several minutes. When the operator commands the machine to accelerate, the KESS  30  releases energy to the traction system, supplementing the energy supplied from an engine (for example, a diesel engine) via a motor/generator. In some embodiments, this release of energy from the KESS  30  allows the machine to have a peak horsepower available of approximately double the output of the engine alone. 
         [0060]    There may be periods during the operating cycle when the engine is not operating at full load. During these periods the engine power may be used to “top up” the KESS  30 . This functionality may ensure that the KESS  30  is charged or full prior to an acceleration event. 
         [0061]    In some embodiments, the speed of the KESS  30  may be loosely tied to machine speed. For example, as the machine speeds up (the speed of a drive mechanism increases), the KESS  30  may slow (the rotational speed of the flywheel  122  may decrease) as a function of the release of energy from the KESS  30 . Conversely, as the machine slows (the speed of a drive mechanism decreases), the KESS  30  may be charged and will correspondingly speed up (the rotational speed of the flywheel  122  increases). One advantage of this operation of the KESS  30  is that the gyroscopic forces of the KESS  30  will be lowest when the machine is at high speed and rapid movement or contact with the wall could result in a significant bearing or housing overload. In some embodiments, a target machine speed may be received from an operator control. 
         [0062]    For example, in some embodiments, the speed of rotation of the KESS  30  (the rotational speed of the flywheel  30   b ), and, therefore, the energy stored within the KESS  30 , is controlled as a function of machine speed. For example,  FIG. 9  illustrates a control curve comparing a speed of the machine to a rotational speed of the KESS  30 . The line  90  indicates a target speed of the KESS  30  for a given machine speed and the area  92  surrounding the line  90  indicates an allowable range of variation around the target speed. The relationship illustrated in  FIG. 9  may be employed to provide management of the gyroscopic forces of the KESS  30 , which may be very high when high machine angular velocity (rate of direction change) coincides with high speed of rotation of the KESS  30 . The shape of the curve also takes into account the energy required to accelerate and decelerate the machine and may be defined for the specific equipment and application. 
         [0063]    As described in more detail below, in some embodiments, as the speed of the machine increases (during acceleration) energy is taken from the KESS  30  and provided to traction motors by placing the energy onto a bi-directional bus (for example, a DC bus) powering the traction motors. This supply of energy reduces the speed of the rotation of the KESS  30 . When the traction system requires more power than the KESS  30  is providing, a diesel engine may provide supplemental energy. Similarly, when the KESS  30  provides more energy than is required by the traction motors, excess energy may be dissipated across braking grids. 
         [0064]    Likewise, as the speed of the machine decrease (during deceleration), the KESS  30  is commanded to increase speed, and the energy required to increase the KESS  30  speed is taken from the bi-directional bus. This energy is supplied by the traction motors operated in a braking mode of operation. In some embodiments, when the KESS  30  does not receive sufficient energy from the traction motors during the braking mode of operation to meet the requirements of the speed curve, energy may be accepted from the diesel engine via a generator. Similarly, when the KESS  30  is receiving an excess of energy, energy may be routed to the engine driveline via the generator to overcome any driveline losses and defuel the engine. Any additional excess energy may be dissipated across the braking grids as heat. 
         [0065]    Accordingly, as described above, the KESS  30  may supply or harvest energy from the bi-directional bus as determined by the control curve illustrated in  FIG. 9 . The engine, through the functionality of the generator, may supply energy only when there is a shortfall between the energy supplied by the KESS  30  and the energy demanded by the traction motors. Variation between supply and demand is a function of the operating conditions in which the machine is situated. For example, the grade or slope and rolling resistance of the roadway upon which the machine is operating may alter the supply and demand balance between the KESS  30  and the traction motors both when the traction motors are operating in a propulsion mode and a braking mode, which alerts the amount of energy demanded or supplied. Accordingly, in rudimentary terms, the KESS  30  may be the primary power source for the bi-directional bus and the engine may be a secondary power source for the bi-directional bus, such as when the KESS  30  cannot satisfy the an energy demand on the bi-directional electrical bus. 
         [0066]    Accordingly, in the underground mining space, one benefit of the KESS  30  is that peak engine horsepower of machine operating in that environment may be reduced. This may be an important factor as engine horsepower may be a determining factor in ventilation requirements of an underground mine, which is a significant capital spend for the customer. For example, many jurisdictions use nameplate engine horsepower as the basis for ventilation air flow in compliance standards. 
         [0067]    For surface machines, a KESS provides benefits in high altitude situations where engine response is diminished due to thinner air (less overall oxygen is available for engine combustion). For example, to overcome the thin air issue, engine manufacturers typically increase the diameter of the turbo chargers. This increased diameter increases the inertia of the turbo chargers resulting in longer turbo lag (wait time for the turbo charger to build speed and boost). The KESS may be used to provide a supplemental energy source to the machine while engine horsepower output is increasing. For example, the KESS may be used to smoothly load the engine to provide driveline response and hence better operational performance, provide additional power boost from braking energy that would otherwise be dissipated as heat, or a combination thereof. 
         [0068]    It should be understood that the size of the KESS (for example, energy capacity and power rating) is based on the requirements of the application. For example, some applications may use a KESS solution as a low capacity and high power solution or other combinations of capacity and power depending on the operational needs of the machine. For example, when a machine is providing maximum power for extended periods, the machine may be equipped with a KESS that provides high energy storage capability and a high power rating. 
         [0069]    For example,  FIG. 10  illustrates mining equipment  100  according to one embodiment of the invention. The mining equipment  100  may be an underground mining machine (for example, a continuous miner, a haulage system, a longwall shearer, a loader, and the like) or a surface mining machine (for example, a wheel loader, a hybrid shovel, a dragline miner, and the like). The mining equipment  100  may include a chassis  101  and a traction system  102 , such as a plurality of wheels rotatably coupled to the chassis  105 . The mining equipment  100  may also include other movable systems and components, such as a cable reel or a swing system. In the embodiment illustrated in  FIG. 10 , the mining equipment  100  is a load, haul, dump (“LHD”) commonly used in underground mining environments. 
         [0070]    As illustrated in  FIG. 11 , the mining equipment  100  includes a generator/engine  103 . The generator/engine  103  may include a diesel engine that outputs mechanical energy and a generator that converts mechanical energy output by the engine into electrical energy. In some embodiments, the generator includes a SR generator. In some embodiments, the generator may be used as a motor that increases the speed of the engine (for example, to use the engine as an energy storage device used separately or in combination with the kinetic energy storage system described below). It should be understood that in some embodiments, the mining equipment  100  includes one or more generators powered by one or more engines. 
         [0071]    The generator/engine  103  provides mechanical power (shown in dashed lines in  FIG. 11 ) to hydraulic pumps  104 , which may drive working hydraulics and cooling fans and parasitics  107  using hydraulic energy (shown in dot-dashed lines in  FIG. 11 ). In particular, rotational energy is passed through the generator and is provided to the hydraulic pumps  104  through a mechanical connection between the hydraulic pumps  104  and the generator/engine  103 . The generator/engine  103  also provides electrical power (shown in solid lines in  FIG. 11 ) to a bi-directional electrical bus  106  (for example, a capacitive direct current (“DC”) bus). The bi-directional electrical bus  106  supplies electrical power to one or more traction motors  108  (for example, SR motors). For example, as illustrated in  FIG. 11 , the mining equipment  100  includes a front left traction motor  108 A, a front right traction motor  108 B, a rear left traction motor  108 C, and a rear right traction motor  108 D. Each traction motor  108  powers a wheel or other drive mechanism included in the traction system  102 . In particular, each traction motor  108  converts electrical power received over the bi-directional electrical bus  106  into rotational energy for driving a drive mechanism. In some embodiments, one or more of the traction motors  108  include SR motors. 
         [0072]    In some embodiments, the bi-directional electrical bus  106  is in communication with one or more converters  110 . The converters  110  may be configured to transmit energy through the bi-directional electrical bus  106  or to receive power from the bi-directional electrical bus  106  (for example, to use the bi-directional electrical bus  106  as a bi-directional bus). Each converter  110  may be used as a DC-to-DC converter, a DC-to-AC inverter, an AC-to-DC rectifier, or another type of power converter. Alternatively or in addition, a converter  110  may be used as a motor controller for a traction motor  108 . For example, the converter  110  may be configured to sense characteristics of a traction motor  108  and respond to the sensed characteristics. In some embodiments, one or more of the converters  110  use insulated-gate bipolar transistor (“IGBT”) electrical switching devices. In some embodiments, a plurality of (for example, parallel) converters may be used for a component coupled to the bi-directional electrical bus  106 . For example, the KESS  120  may be associated with one or more parallel converters that govern energy into the KESS  120  or out of the KESS  120 . Also, in some embodiments, the KESS  120  may be associated with one or more parallel converters governing energy into the KESS  120  and parallel converters governing energy out of the KESS  120 . The use of a plurality of parallel converters may impact the performance of the KESS  120  (for example, faster charging, faster discharging, increased charging potential, increased discharge potential, or a combination thereof). 
         [0073]    As illustrated in  FIG. 11 , each traction motor  108  is associated with a braking grid  112 . The braking grid  112  converts kinetic energy of the traction motor into thermal energy (heat) during braking of the mining equipment  100 . 
         [0074]    The mining equipment  100  also includes a kinetic energy storage system (“KESS”)  120 . The KESS  120  may include a flywheel  122  and a motor/generator  124 . In some embodiments, the motor/generator  124  includes a variable speed motor, such as a variable speed SR motor/generator. For example, the act of storing and recovering energy from a KESS is associated with speeding up and slowing down the rotating mass. Accordingly, the wide constant speed and power range of an SR motor is well suited for the KESS. The flywheel  122  is mechanically coupled to the motor/generator  124 . The motor/generator  124  is configured to receive electrical energy from the bi-directional electrical bus  106  and output rotational energy to the flywheel  122 , and, alternatively, to receive rotational energy from the flywheel  122  and output electrical energy to the bi-directional electrical bus  106 . Accordingly, upon receiving electrical energy, the motor/generator  124  rotates the flywheel  122  to store kinetic energy. Stored energy may be harvested from the KESS  120  by using rotational energy from the flywheel  122  to rotate a rotor included in the motor/generator  124 , which converts the rotational energy into electrical energy that may be supplied to the bi-directional electrical bus  106 . In some embodiments, the flywheel  122  included in the KESS  120  has a rotational speed from approximately 0 to approximately 6500 RPM, which allows the KESS  120  to provide energy output up to approximately 4000 horsepower (“hp”) per second (approximately 3 MJ). In other embodiments, the flywheel  112  has a rotational speed from approximately 3000 RPM to approximately 10000 RPM or from approximately 5000 RPM to approximately 8000 RPM. Similarly, in some embodiments, the KESS  120  provides energy output from approximately 1 MJ to approximately 15 MJ or from approximately 2 MJ to approximately 7 MJ. As noted above, the energy output of the KESS  120  may depend on the configuration of the one or more converters coupling the KESS  120  to the bi-directional electrical bus  106 . 
         [0075]    Although not illustrated in  FIG. 11 , the mining equipment  100  also includes one or more controllers that manage operation of the generator/engine  103  and the KESS  120 . In particular, the mining equipment  100  may include a controller that issues commands to the KESS  120 , including commands relating to torque on the motor/generator  124  to store energy to or harvest energy from the KESS  120 . Similarly, the equipment may include a controller that issues commands to the generator/engine  103  relating to output levels of the engine, the generator, or both. Furthermore, the mining equipment  100  may include a controller that issues commands to the traction motors  108  driving the traction system  102 . It should be understood that this functionality may be performed by a single controller or a plurality of controllers. Also, in some embodiments, the functionality or a portion thereof may be performed by one or more controllers located remote from the mining equipment  100 , such as in a remote control station for the mining equipment  100 . In some embodiments, in some embodiments, functionality performed by the controller described here may be included in another component. For example, the controller may be included in the KESS  120  (for example, within a common housing). 
         [0076]    In some embodiments, as described above with respect to  FIG. 9 , the mining equipment  100  may include a controller that issues commands to the KESS  120  and the generator/engine  103  to supply or harvest energy based on the speed of the mining equipment  100 . In particular, as described in more detail below, the controller may issue commands to the KESS  120  and the generator/engine  103  to use the KESS  120  as a primary power source for the bi-directional electrical bus  106 . 
         [0077]      FIG. 12  illustrates one example of a controller  150  included in the mining equipment  100 . As illustrated in  FIG. 12 , the controller  150  includes an electronic processor  152  (for example, one or more microprocessors, application specific integrated circuits (“ASICs”), or other electronic devices), a computer-readable, non-transitory memory  154 , and an input/output interface  156 . It should be understood that the controller  150  may include additional components than those illustrated in  FIG. 12  and the configuration of components illustrated in  FIG. 12  are provided as only one example. The memory  154  stores instructions executable by the electronic processor  152  to issue commands as noted above (for example, through the input/output interface  156 ). For example, the controller  150  may issue commands to control the power flows described below with respect to  FIGS. 13-19 . The controller  150  may also use the input/output interface  158  to receive information (for example, operating parameters, such as machine speed, steering direction, bus voltage, engine speed sensors, engine load, traction system load or command functions, hydraulic system load or command functions, and the like) that the controller  150  may use to determine when and what type of commands to issue. For example, in some embodiments, the controller  150  controls the KESS  120  based on one or more signals measured, received, or calculated for the mining equipment  100 . It should be understood that the input/output interface  156  may communicate with components external to the controller  150  (for example, the KESS  120 , the generator/engine  103 , an engine controller, and the like) over a wired or wireless connection, including local area networks and controller area networks. 
         [0078]      FIG. 13  illustrates the potential power flows within the mining equipment  100 . In particular, as illustrated in  FIG. 13 , the hydraulic pumps  104  consume energy provided by the generator/engine  103 . However, the generator/engine  103  may also receive energy from the bi-directional electrical bus  106  (for example, during braking events). Furthermore, each traction motor  108  may receive energy from the bi-directional electrical bus  106  and supply energy to the bi-directional electrical bus  106 . Similarly, the KESS  120  may receive energy from the bi-directional electrical bus  106  and supply energy to the bi-directional electrical bus  106 . In contrast, the braking grids  112  only consume energy from the bi-directional electrical bus  106 . 
         [0079]      FIG. 14  illustrates power flow within the mining equipment  100  for charging the KESS  120 . In particular, as illustrated in  FIG. 14 , power supplied by the generator/engine  103  is provided to the bi-directional electrical bus  106 , which supplies power for charging the KESS  120 . In some embodiments, the KESS  120  is charged during start-up of the mining equipment  100 . However, in other embodiments, the KESS  120  may be charged during times of low load on the generator/engine  103 . 
         [0080]      FIG. 15  illustrates power flow in the mining equipment  100  for performing propulsion using the KESS  120 . In particular, after the KESS  120  is charged, the KESS  120  may supply power to the bi-directional electrical bus  106 . The power is consumed by the traction motors  108 . In some embodiments, the KESS  120  acts as the primary or master power source for the traction motors  108 . If the KESS  120  cannot fully supply the traction motors  108  with needed power, the traction motors  108  may receive power from the generator/engine  103 , which, as illustrated in  FIG. 15 , also supplies power to the bi-directional electrical bus  106 . Accordingly, in this arrangement the KESS  120  is the primary provider of energy to the traction system  102  with the generator/engine  103  providing backup supply. The KESS  120  is a more responsive power source than the generator/engine  103 . Accordingly, by using the more responsive power source first, the traction system  102  may increase speed faster than a conventional drive system would allow. Furthermore, using the KESS  120  as the primary provider of energy may reduce the need to operate the generator/engine  103  at full capacity. In particular, as described above, using the KESS  120  as the primary power source to the traction system  102  may allow the generator/engine  103  to operate at a steadier output, which saves fuel and lowers engine output requirements. 
         [0081]    Accordingly, during operation of the mining equipment  100 , the controller  150  may be configured to determine an energy demand on the bi-directional electrical bus  106  and determine energy available through the KESS  120 . When the energy available through the KESS  120  satisfies the energy demand, the controller  150  may be configured to operate the KESS  120  as a primary power source for the bi-directional electrical bus  106  (e.g., controlling a rotational speed of the flywheel  122  included in the KESS  120 ). However, when the energy available through the KESS  120  cannot satisfy the energy demand, the controller  150  may operate the generator/engine  103  as a secondary power source (e.g., with any available energy from the KESS  120 ) for the bi-directional electrical bus  106  to satisfy the energy demand. 
         [0082]      FIG. 16  illustrates power flow in the mining equipment  100  for performing propulsion without using the KESS  120 . In this situation, the traction motors  108  consume energy from the bi-directional electrical bus  106 , which is supplied solely by the generator/engine  103 . This situation may be used when the KESS  120  is not charged, is malfunctioning, or is not present. 
         [0083]      FIG. 17  illustrates power flow in the mining equipment  100  for performing light braking. As illustrated in  FIG. 17 , during braking of the traction system  102 , the traction motors  108  act as generators and supply electrical energy to the bi-directional electrical bus  106 . In the situation illustrated in  FIG. 17  (light braking), the energy supplied by the traction motors  108  may be supplied to the generator included in the generator/engine  103 . The generator may use the received energy to speed up the drive line between the generator/engine  103  and the hydraulic pumps  104  (for example, speed up the engine to a set speed point where fuel injectors are commanded to cease delivering fuel to the engine). In some situations, when the drive line is being motivated by the generator included in the generator/engine  103 , the generator/engine  103  reduces fuel consumption (for example, to operate at a zero fuel level). 
         [0084]    Similarly,  FIG. 18  illustrates power flow in the mining equipment  100  for performing heavy braking and charging the KESS  120 . As illustrated in  FIG. 18 , in these situations, the traction motors  108  act as generators and supply electrical power to the bi-directional electrical bus  106 . In the situation illustrated in  FIG. 18  (heavy braking), the energy generator by the traction motors  108  and supplied to the bi-directional electrical bus  106  may be supplied to the generator included in the generator/engine  103  and to the KESS  120 . 
         [0085]      FIG. 19  illustrates power flow in the mining equipment  100  for performing heavy breaking without charging the KESS  120  (for example, the KESS  120  is full, malfunctioning, or not represent). As illustrated in  FIG. 19 , in these situations, the traction motors  108  act has generators and supply electrical power to the bi-directional electrical bus  106 . Some of the supplied power is provided to the generator included in the generator/engine  103 . However, some of the supplied power is also supplied to one or more of the braking grids  112 , which convert the energy into heat. 
         [0086]    It should be understood that other modes of operation may be used with the KESS  120 . For example, in some embodiments, the generator/engine  103  may be used as the primary power source of the traction system  102  and the KESS  120  may provide backup power supply. In this configuration, a controller may be configured to issue commands to the KESS  120  that may be based on the operating speed of the traction system  102 . 
         [0087]    Also, in some embodiments, a user interface is provided for the mining equipment  100  that allows an operator to configure the KESS  120 . In some embodiments, the user interface may also display (for example, textually or graphically) the current amount of energy stored in the KESS  120 . 
         [0088]    It should also be understood that more than one KESS  120  may be used for a particular mining machine depending on the energy needs of the machine and the characteristics of the KESS  120 . Also, in some embodiments, multiple KESSs  120  may be used to reduce gyroscopic effects associated with a KESS (the rotation of the flywheel). For example, two separate KESSs  120  (a first KESS  120  and a second KESS  120 ) may be contained within a single housing with the flywheels  122  counter-rotating reduce the gyroscopic effects on the machine. For example, a first KESS  120  may include a first flywheel  122  that rotates in a first direction, and a second KESS  120  may include a second flywheel  122  that rotates in a second direction opposite the first direction. Similarly, four KESSs  120  (a first KESS  120 , a second KESS  120 , a third KESS  120 , and a fourth KESS  120 ) may be positioned at four cardinal directions along a plane to reduce gyroscopic effects. For example, as illustrated in  FIG. 20 , the first KESS  120  may be is positioned at a first cardinal direction along a plane, the second KESS  120  may be positioned at a second cardinal direction along the plane, the third KESS  120  may be positioned at a third cardinal direction along the plane, and the fourth KESS  120  may be positioned at a fourth cardinal direction along a plane. 
         [0089]    As noted above, the mining equipment  100  may include a haulage vehicle, such as an LHD commonly used in underground mining environments. As illustrated in  FIG. 20 , an LHD  200  includes a bucket  202  supported by one or more arms  204 , wherein the bucket  202  is movable in at least one direction (for example, a horizontal height, an angle from a horizontal position, or the combination thereof). The bucket  202  may be moved using one or more actuators (changing the position of the bucket  202 , the arms  204 , or both), such as one or more hydraulic actuators, rams, and the like, included in the LHD  200 . The bucket  202  may be moved based on input received from an operator control, such as a joystick, lever, button, touch screen, and the like included in the LHD  200 . A controller, such as the controller  150  described above or a separate, similar controller, included in the LHD  200  may receive the input and control the one or more actuators according (for example, by issuing commands to the one or more actuators). In some embodiments, the controller is also configured to provide an automatic return-to-dig functionality. 
         [0090]    For example, when the bucket  202  of the LHD  200  is in a non-dig position (for example, a dump position as illustrated in  FIG. 21 ), an operator operating the LHD  200  may press a selection mechanism (for example, “return to dig” selection mechanism), such as a button, positioned on an operator control included in the LHD  200  (for example, a right or left hand joystick of the LHD  200 , a touchscreen, and the like) or at a remote control station of the LHD  200 . When the operator selects this selection mechanism, the controller  150  receives a signal from the selection mechanism (for example, directly or over one or more networks) and, in response, automatically controls the one or more actuators associated with the bucket  202  to reposition the bucket  202  to a predetermined dig position (for example, a predetermined height, a predetermined angle, or the combination thereof) (see, for example,  FIG. 22 ). As illustrated in  FIG. 22 , the return-to-dig position may be defined as the bucket  202  being approximately horizontal with the ground or the material being dug. 
         [0091]    For example, the controller  150  may access the predetermined dig position from a memory (such as the memory  154  included in the controller  150 ) and compare the stored predetermined dig position to a current position of the bucket  202 . As described below, the controller  150  may use data collected by one or more sensors to determine the current position of the bucket  202 . When the positions differ, the controller  150  may control the one or more actuators to change the current position of the bucket  202  to match the stored predetermined dig position. For example, when the current height of the bucket  202  is greater than the height included in the predetermined dig position, the controller  150  may control the one or more actuators to lower the bucket  202 . Similarly, when the current angle of the bucket  202  is greater than the angle included in the predetermined dig position, the controller  150  may control the one or more actuators to decrease the angle of the bucket  202 . 
         [0092]    In some embodiments, the controller  150  may repeatedly compare a current position of the bucket  202  to the stored predetermined dig position while moving the bucket  202  until the positions align. Alternatively or in addition, the controller  150  may initially compare a current position of the bucket  202  to the stored predetermined dig position and determine an amount of movement necessary to bring the bucket  202  in align with the stored predetermined dig position. The controller  150  may then command movement of the bucket  202  based on the determined distance. Accordingly, in either configuration, the controller  150  translates a difference between the current position and the stored position into one or a series of commands to the one or more actuators simulating commands received from an operator control. Accordingly, using the selection mechanism allows the operator to concentrate on driving the LHD  200  without having to also perform multiple joystick movements to return the bucket  202  to a dig position. 
         [0093]    In some embodiments, an operator may manually adjust the predetermined dig position (for example, the predetermined height, the predetermined angle, or the combination thereof) to suit the operator&#39;s preferences or the operating environment. For example, the operator may be able to signal when the bucket  202  is in a desired dig position (for example, by selecting a selection mechanism or operating an operator control). The controller  150  receives the operator input and saves the current position of the bucket  202  (for example, the current height, the current angle, or the combination thereof). The controller  150  may determine the current position based on data collected by one or more sensors communicating with the controller  150  (for example, a pressure sensor, an encoder, an inclinometer, and the like). The stored positional information may be recalled and applied when the operator subsequently selects the “return to dig” selection mechanism. In some embodiments, the modified predetermined dig position may be stored as an absolute position (for example, a height and an angle). However, alternatively or in addition, the modified predetermined dig position may be stored as an offset to the default predetermined dig position (for example, a height offset and an angle offset). In some embodiments, the modified dig position may be reset to the default predetermined dig position after the LHD  200  is shut down and restarted. In other embodiments, the modified dig position may be rest to the default predetermined dig position manually (for example, in response to selection of a “reset to default” selection mechanism). 
         [0094]    Alternatively or in addition, the controller  150  included in the LHD  200  may provide automatic return-to-carry functionality. For example, when the bucket  202  of the LHD  200  is in a non-dig position (for example, a dump position as illustrated in  FIG. 21 ), an operator operating the LHD  200  may press a selection mechanism (for example, “return to carry” selection mechanism), such as a button, positioned on an operator control included in the LHD  200  (for example, a right or left hand joystick of the LHD  200 , a touchscreen, and the like) or a remote control station for the LHD  200 . When the operator selects this selection mechanism, the controller  150  receives a signal from the selection mechanism (for example, directly or over one or more networks) and, in response, automatically controls the one or more actuators associated with the bucket  202  to reposition the bucket  202  to a predetermined carry position (for example, a predetermined height, a predetermined angle, or the combination thereof) (see, for example,  FIG. 23 ). 
         [0095]    For example, the controller  150  may access the predetermined carry position from a memory (such as the memory  154  included in the controller  150 ) and compare the stored predetermined carry position to a current position of the bucket  202 . As described above, the controller  150  may use data collected by one or more sensors to determine the current position of the bucket  202 . When the positions differ, the controller  150  may control the one or more actuators to change the current position of the bucket  202  to match the stored predetermined carry position. For example, when the current height of the bucket  202  is less than the height included in the predetermined carry position, the controller  150  may control the one or more actuators to raise the bucket  202 . Similarly, when the current angle of the bucket  202  is less than the angle included in the predetermined carry position, the controller  150  may control the one or more actuators to increase the angle of the bucket  202 . 
         [0096]    In some embodiments, the controller  150  may repeatedly compare a current position of the bucket  202  to the stored predetermined carry position while moving the bucket  202  until the positions align. Alternatively or in addition, the controller  150  may initially compare a current position of the bucket  202  to the stored predetermined carry position and determine an amount of movement necessary to bring the bucket  202  in align with the stored predetermined carry position. The controller  150  may then command movement of the bucket  202  based on the determined distance. Accordingly, in either configuration, the controller  150  translates a difference between the current position and the stored position into one or a series of commands to the one or more actuators simulating commands received from an operator control. Accordingly, using the selection mechanism allows the operator to concentrate on driving the LHD  200  without having to also perform multiple joystick movements to return the bucket  202  to a carry position. 
         [0097]    In some embodiments, an operator may manually adjust the predetermined carry position (for example, the predetermined height, the predetermined angle, or the combination thereof) to suit the operator&#39;s preferences or the operating environment. For example, the operator may be able to signal when the bucket  202  is in a desired carry position (for example, by selecting a selection mechanism or operating an operator control). The controller  150  receives the operator input and saves the current position of the bucket  202  (for example, the current height, the current angle, or the combination thereof). The controller  150  may determine the current position based on data collected by one or more sensors communicating with the controller  150  (for example, a pressure sensor, an encoder, an inclinometer, and the like). The stored positional information may be recalled and applied when the operator subsequently selects the “return to carry” selection mechanism. In some embodiments, the modified predetermined carry position may be stored as an absolute position (for example, a height and an angle). However, alternatively or in addition, the modified predetermined carry position may be stored as an offset to the default predetermined carry position (for example, a height offset and an angle offset). In some embodiments, the modified carry position may be reset to the default predetermined carry position after the LHD  200  is shut down and restarted. In other embodiments, the modified carry position may be rest to the default predetermined carry position manually (for example, in response to selection of a “reset to default” selection mechanism). 
         [0098]    As illustrated in  FIG. 23 , the carry position may be defined as the bucket  202  being rolled back and the arms  204  being low (the bucket  202  is low and tucked in to place the bucket  202  in a very stable position so that the machine may be driven over long distances commonly performed using LHDs). In particular, the carry position and, subsequently, the automatic return-to-carry functionality may provide benefits in situations where, once the operator fills the bucket  202  or dumps the bucket  202 , the operator has to drive the LHD  200  over a great distance (for example, greater than approximately 500 feet). For example, surface wheel loaders typically travel less than 300 feet during a round trip between a haul truck and a dig face. This distance generally does not warrant placing the bucket into a carry position. Rather, while traveling this distance, the surface loader arms may be used to fully raise the bucket or drop the bucket back into a dig position. In contrast, LHD return distances are typically 1000 feet or greater. Accordingly, the automatic return-to-carry functionality provides benefits for LHDs driven a long distance where it not desirable (for example, for stability purposes) to drive with the bucket  202  fully raised. 
         [0099]    Thus, embodiments of the invention provide, among other things, a kinetic energy storage system for a mining machine. The kinetic energy storage system may be used to power a traction system of the mining machine using energy stored during engine start-up, low engine load, and braking events. 
         [0100]    Various features and advantages of the invention are set forth in the following claims.