Patent Publication Number: US-9421984-B2

Title: Sand monitoring and control system for a machine

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to a sand monitoring and control system and, more particularly, to a sand monitoring and control system for a machine. 
     BACKGROUND 
     Railroad locomotives may experience wheel slip during rainy or icy weather conditions. For example, wheels of a locomotive may slip on a wet or icy railroad track when the locomotive attempts to start pulling stationary railroad cars. The wheels may also slip, for example, when a locomotive operator applies the brakes to stop a fast moving locomotive on a slippery railroad track. Locomotives typically include a sand dispensing system, which dispenses sand near the wheels of the locomotive. The sand comes between the wheels of the locomotive and the railroad track, increasing friction between the contacting surfaces and providing improved traction. 
     A locomotive operator may detect wheel slip based on signals from a wheel slip sensor. The operator may then push a button or engage a lever to dispense sand from sand boxes located on the locomotive. The operator may, however, be unaware of the amount of sand remaining in the sand boxes or of the flow-rate at which sand is being dispensed. Moreover, when the sand boxes are empty, do not have a sufficient amount of sand, or when the flow-rate of sand is too low, the operator may find it difficult to control the wheel slip. Because of safety restrictions on many railroads, a manual inspection of the sand boxes or the valves, which control the sand flow-rate, before or during operation of the locomotive, is difficult and inaccurate. 
     One attempt to address some of the problems described above is disclosed in U.S. Pat. No. 8,397,560 of De Sanzo et al. that issued on Mar. 19, 2013 (“the &#39;560 patent”). In particular, the &#39;560 patent discloses a system for monitoring a sand reservoir including at least one sand level indicator. The sand level indicator of the &#39;560 patent provides a visual display external to the sand reservoir to indicate the quantity of sand within the reservoir. The &#39;560 patent further discloses that the sand level indicator can transmit a signal, which indicates the presence or absence of sand in the sand reservoir, to a remote station. In addition, the &#39;560 patent discloses that the sand level and related data may be used, among other things, to avoid an “out of sand” condition, to detect excess sand usage, or to trigger an alert if the sand level has not decreased over a predefined amount of time. 
     Although the &#39;560 patent discloses a system for monitoring a sand reservoir, the disclosed system may still be inadequate. For example, the system of the &#39;560 patent does not detect the flow-rate of sand from the sand boxes. Thus, the system of the &#39;560 patent may not allow the operator to adjust the flow-rate at which sand is dispensed to adequately respond to a detected wheel slip condition. Moreover, although the system of the &#39;560 patent may detect an “out of sand” condition, it does not provide the operator with any alternative method of providing sand to the wheels of the locomotive. 
     The sand monitoring and control system of the present disclosure solves one or more of the problems set forth above and/or other problems in the art. 
     SUMMARY 
     In one aspect, the present disclosure is directed to a sand monitoring and control system for a machine. The sand monitoring and control system may include a sand box configured to hold sand. The sand monitoring and control system may further include a duct connected to the sand box. The duct may be configured to dispense sand from the sand box to a wheel of the machine. The sand monitoring and control system may also include a valve connected to the duct. In addition, the sand monitoring and control system may include a controller in communication with the valve. The controller may be configured to adjust the valve to control a flow-rate of sand through the duct. 
     In another aspect, the present disclosure is directed to a method of traction control for a machine. The method may include detecting wheel slip using a slip sensor. The method may also include opening a valve to allow sand to flow from a sand box through a duct when wheel slip has been detected. The method may further include dispensing the sand to a wheel of the machine. The method may include determining a flow-rate of the sand flowing through the duct. In addition, the method may include adjusting the valve to control the flow-rate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a pictorial illustration of an exemplary disclosed machine; 
         FIG. 2  is a schematic of an exemplary disclosed sand monitoring and control system for the machine of  FIG. 1 ; 
         FIG. 3  is a flow chart illustrating an exemplary disclosed method of traction control performed by the sand monitoring and control system of  FIG. 2 ; and 
         FIG. 4  is a flow chart illustrating another exemplary disclosed method of traction control performed by the sand monitoring and control system of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary embodiment of a machine  10 . For example, as shown in  FIG. 1 , machine  10  may be a locomotive designed to pull rolling stock. Machine  10  may have a platform  12 . A plurality of wheels  14 ,  16  may be configured to support platform  12 . Wheels  14 ,  16  may also be configured to engage track  18 . Although  FIG. 1  illustrates wheels  14  as located nearer a front end of machine  10  and wheels  16  located nearer a rear end of machine  10 , it is contemplated that wheels  14 ,  16  may be located in any wheel position on machine  10 . Wheels  14 ,  16  may have traction motors (not shown) associated with them, which may drive wheels  14 ,  16  to propel machine  10  in a forward or rearward direction. 
     Machine  10  may have an engine  20  mounted on platform  12 . Engine  20  may be configured to drive one or more generators  22 , which may generate power to drive the traction motors. The one or more generators  22  may also be mounted on platform  12  of machine  10 . Although  FIG. 1  depicts one engine  20 , it is contemplated that machine  10  may have more than one engine  20 , which may drive the one or more generators  22 . In an exemplary embodiment, as shown in  FIG. 1 , engine  20  may be lengthwise aligned on platform  12  along a travel direction of machine  10 . One skilled in the art will recognize, however, that engine  20  may be located in tandem, transversally, or in any other orientation on platform  12 . 
     Machine  10  may include one or more sand boxes  24 ,  52 . Ducts  26  may be connected at one end to sand boxes  24 ,  52 . Distal ends of ducts  26  may be disposed near wheels  14 ,  16 . Each duct  26  may allow sand from a sand box  24  or  52  to be dispensed near a wheel  14  or  16 , respectively. Each duct  26  may also be equipped with a nozzle  28  to direct sand from sand box  24  or  52  to wheel  14  or  16 , respectively, so that sand may be crushed between wheels  14 ,  16  and track  18  to provide improved traction to wheels  14 ,  16 . In one exemplary embodiment, as shown in  FIG. 1 , duct  27  may allow sand from a sand box  24  or  54  to be dispensed near a wheel on a side opposite to a side on which duct  26  dispenses sand. Such a configuration may allow sanding operations to be performed regardless of a travel direction of machine  10 . Although  FIG. 1  shows only one wheel  14  and one wheel  16  having both ducts  26  and  27 , one skilled in the art would recognize that ducts  26  and  27  may dispense sand from first or second sand boxes  24 ,  52  on both sides of only some wheels  14 ,  16  or all wheels  14 ,  16  of machine  10 . 
     In another exemplary embodiment, as shown in  FIG. 1 , each wheel  14  or  16  may have its own dedicated sand box  24  or  52 , respectively. It is contemplated, however, that more than one sand boxes  24 ,  52  and/or ducts  26 ,  27  may supply sand to wheels  14 ,  16 . It is also contemplated that one sand box  24  or  52  may supply sand to more than one wheel  14  or  16  using one or more ducts  26 ,  27 . Sand boxes  24 ,  52  may be fixedly attached to machine  10  or may be removable. In an exemplary embodiment, as shown in  FIG. 1 , sand boxes  24 ,  52  may be located on platform  12  near wheels  14 ,  16 , respectively. One skilled in the art will recognize, however, that sand boxes  24 ,  52  may be located anywhere on machine  10 . 
       FIG. 2  illustrates an exemplary disclosed sand monitoring and control system  40  for machine  10 . As shown in the figure, sand monitoring and control system  40  may include sand dispensing system  50 , instrumentation system  70 , and remote monitoring system  90 . Sand dispensing system  50  may include first sand box  24 , second sand box  52 , first valve  54 , second valve  56 , first sensor  58 , second sensor  60 , slip sensor  62 , and controller  64 . First and second sand boxes  24 ,  52 , may be configured to hold sand for use in fraction control operations for machine  10 . Although  FIG. 2  illustrates only two sand boxes, namely first and second sand box  24  and  52 , it is contemplated that sand monitoring and control system  40  may include any number of first and second sand boxes  24  and/or  52 . It is also contemplated that first and second sand boxes  24 ,  52  may be located on the same machine  10  or on different machines  10 . One end of ducts  26  and/or  27  may be connected to each of first and second sand boxes  24 ,  52 . Distal ends of ducts  26 ,  27  may be disposed near wheels  14 ,  16 . It is contemplated that more than one duct  26  and/or  27  may be connected to each of first and second sand boxes  24 ,  52  to allow sand to be dispensed from each of first and second sand boxes  24 ,  52  to more than one wheel  14 ,  16 . It is also contemplated that ducts  26 ,  27  may be connected to first and second sand boxes  24 ,  52  to permit sand to be dispensed from more than one first and/or second sand box  24 ,  52  to a single wheel  14  or  16 . 
     First valve  54  may be connected to duct  26  of first sand box  24 . First valve  54  may be selectively adjustable to control a first flow-rate of sand from first sand box  24  to first wheel  14 . Second control valve  56  may be connected to duct  26  of second sand box  52 . Second control valve  56  may be selectively adjustable to control a second flow-rate of sand from second sand box  52  to second wheel  16 . 
     First valve  54  may be a two position or proportional type valve having a valve element movable to allow sand to flow from first sand box  24  through duct  26  to first wheel  14 . The valve element in first valve  54  may be hydraulic or pneumatic and may be operable to move between a flow-passing position and a flow-blocking position. It is contemplated that the valve element in first valve  54  may be solenoid-operable, mechanically-operable, electrically-operable, or operable in any other manner known in the art. In the flow-passing position, first valve  54  may permit sand to flow from first sand box  24  through duct  26  to first wheel  14 , causing improved traction between first wheel  14  and track  18 . In contrast, in the flow-blocking position, first valve  54  may completely block sand from flowing through duct  26 . Second valve  56  may have a structure and function similar to that of first valve  54 . It is contemplated that in one exemplary embodiment first and/or second valves  54 ,  56  may be fixed-flow valves in which the valve element may have only two positions such that firs and/or second valves  54 ,  56  may either be fully open or fully closed. 
     First sensor  58  may be attached to or mounted near first sand box  24 . First sensor  58  may be configured to determine a level of sand in first sand box  24 . As used in this disclosure, level of sand refers to a height of sand as measured from a bottom surface or a reference location near the bottom surface of first or second sand box  24 ,  52 . As used in this disclosure, level of sand may also refer to a depth of the surface of the sand as measured from a reference location above the surface of the sand. It is also contemplated that first sensor  58  may be configured to determine an amount of sand in first sand box  24 . As used in this disclosure, amount of sand refers to the volume, mass, or weight of the sand in first or second sand box  24 ,  52 . 
     First sensor  58  may be configured to determine a level or amount of sand in first sand box  24  at different times. It is also contemplated that first sensor  58  may be configured to determine the level or amount of sand in first sand box  24  at a time specified by controller  64  or by an operator of machine  10 . First sensor  58  may be an acoustic sensor, an ultrasonic sensor, an infra-red sensor, an optical sensor, a load cell, a pressure sensor, or any other type of sensor known in the art for the measurement of height, depth, volume, mass, or weight. 
     First sensor  58  may be configured to communicate information regarding the level or amount of sand in first sand box  24  to controller  64 . First sensor  58  may communicate information to controller  64  wirelessly or through wires or cables connecting first sensor  58  to controller  64 . Second sensor  60  may be configured to determine a level of sand or an amount of sand in second sand box  52 . Second sensor  60  may have a structure and function similar to that of first sensor  58 . Although,  FIG. 2  illustrates an embodiment in which only one sensor is associated with each of first and second sand boxes  24 ,  52 , one skilled in the art would recognize that more than one first and second sensors  58 ,  60  may be attached to or mounted near first and second sand boxes  24 ,  52 , respectively, to determine the level or amount of sand in first and second sand boxes  24 ,  52 . 
     Slip sensor  62  may be configured to determine whether first wheel  14  or second wheel  16  may be slipping on track  18 . Slip sensor may determine a wheel slip condition based on a speed of machine  10 , a position or distance of travel of machine  10 , a rotational speed of first or second wheel  14  or  16 , and a dimension of first or second wheel  14  or  16 . As used in this disclosure, rotational speed may be measured in terms of revolutions per unit time or in terms of an angular speed. In one exemplary embodiment, slip sensor  62  may detect a wheel slip condition by comparing the distance travelled by machine  10  in a given time period with the linear distance travelled by a center of first or second wheel  14  or  16  in the same time period. The distance travelled by machine  10  may be determined based on the speed of machine  10  and the distance travelled by the center of first or second wheel  14  or  16  may be determined based on the angular speed of first or second wheel  14  or  16 , respectively. Slip sensor  62  may communicate information regarding wheel slip to controller  64  wirelessly or through wires or cables connecting slip sensor  62  to controller  64 . It is also contemplated that slip sensor  62  may communicate information regarding an amount or magnitude of wheel slip to controller  64 . 
     Although the above disclosure describes detecting wheel slip using slip sensor  62 , one skilled in the art would recognize that wheel slip may be detected in many other ways. For example, signals from an on-board or off-board radar system, or an on-board GPS system may be used by controller  64  to detect wheel slip. It is also contemplated that measurements of current flow to the traction motors associated with wheels  14 ,  16 , signals from encoders associated with wheels  14 ,  16 , and/or signals from generators  22  may be used by controller  64  to detect wheel slip. 
     Controller  64  may be in communication with first and second valves  54 ,  56 , first and second sensors  58 ,  60 , slip sensor  62 , and instrumentation system  70 . Controller  64  may be configured to monitor a first flow-rate of sand flowing from first sand box  24  and a second flow-rate of sand flowing from second sand box  52 . Controller  64  may determine the first flow-rate based on signals and/or information communicated by first sensor  58  to controller  64 . It is contemplated that controller  64  may receive information regarding an amount of sand in first sand box  24  from first sensor  58 . It is also contemplated that controller  64  may receive information regarding a level of sand in first sand box  24  from first sensor  58  and may determine an amount of sand in first sand box  24  based on the level information. It is further contemplated that controller  64  may determine the first flow-rate based on characteristics of first valve  54  and an amount of opening of first valve  54 . For example, controller  64  may determine the first flow-rate as half of a maximum flow-rate allowed by first valve  54  when the amount of opening of first valve  54  is half of a maximum amount of opening. It is also contemplated that controller  64  may determine the first flow-rate based on the detected amount or magnitude of wheel slip detected by slip sensor  62 . Controller  64  may determine the second flow-rate based on signals, information regarding a level or amount of sand in second sand box  52  received from second sensor  60 , opening of second valve  56 , and/or the amount or magnitude of wheel slip detected by slip sensor  62  in a similar manner. 
     Controller  64  may be configured to selectively adjust first valve  54  to increase or decrease the first flow-rate of sand. For example, controller  64  may be configured to increase the first flow-rate by moving a valve element in first valve  54  to increase a flow area of sand through first valve  54 . Similarly, controller  64  may decrease the first flow-rate by decreasing the flow area in first valve  54 . Controller  64  may be configured to monitor and control the second flow-rate by adjusting second valve  56  in a manner similar to the adjustments of first valve  54 . 
     It is contemplated that in an exemplary embodiment in which first and/or second valves  54 ,  56  is a fixed-flow valve, controller  64  may control an amount of sand delivered to wheels  14 ,  16  by allowing first and/or second valves  54 ,  56  to remain open for a longer or shorter time period, respectively. One skilled in the art would recognize that the first and/or second flow-rate from first and/or second valves  54 ,  56  may be lower than the target flow-rate because first and/or second valves  54 ,  56  may be malfunctioning or because there may be some blockage in duct  26 . For example, when controller  64  detects that first flow-rate is less than the target flow-rate, controller  64  may keep first valve  54  open for a longer period of time. 
     Controller  64  may embody a single microprocessor or multiple microprocessors, field programmable gate arrays (FPGAs), digital signal processors (DSPs), etc. Controller  64  may be configured to control operations of sand monitoring and control system  40 . It is contemplated that controller  64  may be configured to control operations of machine  10 . Additionally or alternatively, controller  64  may be configured to communicate with another controller (not shown), which may be configured to control operations of machine  10 . Various other known circuits may be associated with controller  64 , including power supply circuitry, signal-conditioning circuitry, actuator driver circuitry (i.e., circuitry powering solenoids, motors, or piezo actuators), communication circuitry, and other appropriate circuitry. 
     Instrumentation system  70  may include button  72 , display  74 , alarm  76 , communications module  78  and antenna  80 . Button  72  may be configured to permit an operator of machine  10  to control dispensing of sand from first and/or second sand boxes  24 ,  52 . Button  72  may be configured to communicate a signal to controller  64 , which may selectively open or close first and second valves  54 ,  56 , to dispense sand from first and second sand boxes  24 ,  52 , respectively. It is contemplated, however, that button  72  may be configured to directly communicate with first and second valves  54 ,  56  to selectively open or close first and second valves  54 ,  56  to dispense sand from first and second sand boxes  24 ,  52 , respectively. Although  FIG. 2  illustrates only one button  72 , it is contemplated that instrumentation system  70  may be equipped with more than one button  72  to initiate and control the flow of sand from the one or more first and second sand boxes  24 ,  52 . It is also contemplated that separate buttons  72  may be used to initiate and control the flow of sand from first sand box  24  and second sand box  52 . It is further contemplated that buttons  72  may be activated by touching, pressing, rotating, and/or moving buttons  72 . It is also contemplated that buttons  72  may take the form of levers, wheels, touch control widgets, or any other structure known in the art for adjusting first and second valves  54 ,  56 . 
     Instrumentation system  70  may include display  74 , which may be configured to display information received from controller  64 . Display  74  may be monochromatic or may be capable of displaying a multitude of colors. Display  74  may be a liquid crystal display, a cathode ray tube display, a touch screen display, a plasma display, a light-emitting-diode display, or any other type of display known in the art for displaying information to an operator of machine  10 . Display  74  may also be configured to display widgets and/or other graphics, which may be activated using touch controls by an operator of machine  10  to control or monitor sand monitoring and control system  40 . 
     Instrumentation system  70  may include an alarm  76 . Alternatively or additionally, alarm  76  may be located within a control cabin of machine  10 , within remote monitoring system  90 , or at a central location for monitoring the status of one or more machines  10  and one or more sand monitoring and control systems  40 , for example, in a central control room or maintenance department. Alarm  76  may be audible, visual, or both. In one exemplary embodiment, alarm  76  may be included in display  74 . Alarm  76  may be triggered by controller  64 , when controller  64  determines that the first or second flow-rate differs from a target flow-rate or when first or second sand boxes  24 ,  52  run out of sand. 
     Instrumentation system  70  may include a communications module  78 , which may be configured to communicate information and data received from display  74  or from controller  64  to remote monitoring system  90 . Communications module  78  may communicate wirelessly with remote monitoring system  90 . Communications module  78  may be equipped with an antenna  80  to transmit or receive signals to and from server  92 . Although  FIG. 2  depicts communications module  78  as transmitting and receiving signals wirelessly via antenna  80 , it is contemplated that communications module  78  may receive signals via other methods known in the art. For example, communications module  78  may receive signals from other communications devices (not shown) or from remote monitoring system  90  via a wired connection, a network connection, a cellular connection, a satellite connection, or by any other means of communication known in the art. 
     Remote monitoring system  90  may include server  92  and antenna  94 . Server  92  may include one or more servers configured to interact with one or more communications modules  78  or controllers  64 . Server  92  may be a desktop computer or a server computer. Server  92  may be implemented as a server, a server system comprising a plurality of servers, or a server farm comprising a load balancing system and a plurality of servers. Alternatively, server  92  may be a portable computer, for example, a laptop computer, a tablet computer, or another mobile device known in the art. Server  92  may include a number of components, such as one or more processor(s), memory device(s) and other storage devices for storing instructions executed by the processor(s) and/or for storing for electronic communications and other data. Examples of memory devices and other storage devices include hard drives, NOR, NAND, ROM devices, etc. Server  92  may also include a display device for displaying data and information. Server  92  may be equipped with input devices, which may include physical keyboards, virtual touch-screen keyboards, mice, joysticks, styluses, etc. In one exemplary embodiment, server  92  may also be capable of receiving input through a microphone using voice recognition applications. Server  92  may be equipped with antenna  94  to wirelessly communicate with communications module  78  or controller  64 . It is contemplated however that server  92  may transmit or receive signals from communications modules  78  or controllers  64  via a wired connection, a network connection, a cellular connection, a satellite connection, or by any other means of communication known in the art. 
     Server  92  may be configured to receive information and data from communications module  78 . Server  92  may use the information and data to determine a first flow-rate and a second flow-rate of sand flowing from first and second sand box  24 ,  52 , respectively. Additionally or alternatively, server  92  may also determine an amount of sand remaining in first and second sand boxes  24 ,  52 . Server  92  may compare the first or second flow-rate to a target flow-rate and communicate results of the comparison to communications module  78 . It is also contemplated that server  92  may direct controller  64  to adjust first and/or second valves  54 ,  56  to control first and/or second flow-rates of sand from first and second sand boxes  24 ,  52 , respectively. In one exemplary embodiment, server  92  may be configured to trigger alarm  76  when first or second sand box  24 ,  52  runs out of sand, or when the first or second flow-rate differs from the target flow-rate. Server  92  may use information regarding sand usage and amounts obtained from one or more communications modules  78  or controllers  64  in one or more machines  10  to perform safety or supply audits and monitor the availability of sand throughout the railroad system. One skilled in the art would recognize that all the functions of server  92  described above may be performed by controller  64  or vice-versa. 
     Controller  64  and/or server  92  may also be configured to log failures in a maintenance archive, which may be stored on machine  10  or at a remote location. For example, when controller  64  or server  92  detects that the first or second flow-rate differs from the target flow-rate, controller  64  or server  92  may log the detected condition in a maintenance archive. Controller  64  and server  92  may also log trends in the first and/or second flow-rates of sand in the maintenance archive. The maintenance archive may be used to schedule maintenance for sand monitoring and control system  40 . Additionally or alternatively, controller  64  and/or server  92  may use the information in the maintenance archive to control first and/or second valves  54 ,  56  to adjust the first and/or second flow-rates of sand. 
     An exemplary traction control operation of sand monitoring and control system  40  will be described next. 
     INDUSTRIAL APPLICABILITY 
     The disclosed sand monitoring and control system may be used in any machine or power system application where it is beneficial to improve traction control by using sand to increase the friction between wheels of the machine and a surface (e.g. track) in contact with the wheel. The disclosed sand monitoring and control system may find particular applicability with mobile machines such as locomotives during rainy or wintry weather conditions. The disclosed sand monitoring and control system may provide an improved method for controlling the traction of the machine in such adverse weather conditions by monitoring and controlling a flow-rate of sand from a sand box. For example, the disclosed sand monitoring and control system may provide an improved method for dispensing sand based on a controlled flow-rate of sand exiting a sand box. The disclosed sand monitoring and control system may also provide an improved method for dispensing sand from a second sand box when a first flow-rate of sand from a first sand box is too low or when the first-sand box has run out of sand. Operation of sand monitoring and control system  40  will now be described. 
       FIG. 3  illustrates an exemplary method  100 , which may be performed by sand monitoring and control system  40 . Controller  64  may monitor signals from a slip sensor  62  (Step  102 ). Controller  64  may determine whether wheel slip has been detected (Step  104 ). Alternatively, controller  64  may determine whether the operator has activated button  72 . When controller  64  determines that a wheel slip has been detected (Step  104 , Yes) or when the operator has activated button  72 , controller  64  may open first valve  54  (Step  106 ) to dispense sand from first sand box  24 . When controller  64  determines, however, that no wheel slip has been detected (Step  104 , No), controller  64  may return to step  102  and continue to monitor signals from slip sensor  62 . 
     After opening first valve  54  in step  106 , controller  64  may determine first flow-rate (Step  108 ) of sand flowing from first sand box  24 . Controller  64  may determine the first flow-rate in many ways. For example, controller  64  may receive information from first sensor  58  about a level of sand in first sand box  24  at two different times. Controller may use information regarding the dimensions of first sand box  24  to determine an amount of sand in first sand box  24  based on the level at the two different times. Controller  64  may determine the first flow-rate based on the amounts of sand in first sand box  24  at the two different times and the elapsed time. In an exemplary embodiment, controller may receive information regarding the amounts of sand remaining in first sand box  24  at two different times directly from first sensor  56 . Controller  64  may use this information to determine the first flow-rate of sand. For example, if the amounts of sand in first sand box  24  are determined to be Q1 and Q2 at times t1 and t2, respectively, controller  64  may determine the first flow-rate as a ratio of the amount of sand dispensed (Q1−Q2) and the time elapsed (t2−t1). It is contemplated that controller  64  may also determine the first flow-rate, for example, based on an amount by which first valve  54  may be open. For example, if first valve  54  is half-way open, controller  64  may determine the first flow-rate as half the maximum flow-rate allowed by first valve  54 . It is further contemplated that controller  64  may determine the first flow-rate based on an amount or magnitude of wheel slip detected by slip sensor  62 . For example, controller  64  may determine the first flow-rate of sand based on an amount of frictional force required to eliminate or reduce the detected amount of wheel slip. After determining the first flow-rate in step  108 , controller  64  may proceed to step  110 . 
     In step  110 , controller  64  may determine whether the first-flow rate is &lt;a target flow-rate. Controller  64  may receive information regarding the target flow-rate from server  92  via communications module  78 . Server  92  may determine the target flow-rate based on historical data on sand usage from first and second sand boxes  24 ,  52  on one or more machines  10 . Alternatively, server  92  may determine the target flow-rate based on a speed of machine  10 . For example, server  92  may determine an amount of frictional force needed to reduce the speed of machine  10  from a current speed to a lower speed to prevent wheel slip. In an exemplary embodiment, server  92  may determine the target flow-rate based on characteristics of first valve  54 . For example, server  92  may estimate a target flow-rate based on a maximum flow-rate of first valve  54 . Server  92  may determine the maximum flow-rate based on the maximum amount of sand that may be dispensed from first sand box  24  through duct  26  in a given time period when first valve  54  is in a fully open position. Server  92  may communicate the target flow-rate to controller  64  directly or through communications module  78 . Although in the above description, server  92  has been described as determining the target flow-rate, it is contemplated that controller  64  may determine the target-flow rate in a manner similar to that described for server  92 . 
     When controller  64  determines that the first flow-rate is &lt;a target flow-rate (Step  110 , Yes), controller  64  may proceed to step  112 . First flow-rate may be less than the target flow-rate for many reasons. For example, first flow-rate may be less than the target flow-rate because first sand box  24  may have run out of sand, because first valve  54  may be functioning improperly, or because of an undetected blockage in duct  26 . As described below, method  100  may permit dispensing of sand from second sand box  52  in such cases to provide the operator of machine  10  an alternative option of performing traction control operations to reduce or eliminate a detected wheel slip condition. 
     When controller  64  determines, however, that the first flow-rate is not &lt;the target flow-rate (Step  110 , No), controller  64  may determine whether the first flow-rate&gt;the target flow-rate (Step  114 ). When controller  64  determines that the first flow-rate is greater than the target flow-rate (Step  114 , Yes), controller  64  may adjust first valve  54  to decrease the first flow-rate so that the first flow-rate is about equal to the target flow-rate (Step  116 ). When controller  64  determines, however, that the first flow-rate is not greater than the target flow-rate (Step  114 , No), controller  64  may proceed to step  126 . One of ordinary skill in the art would recognize that when the first flow-rate is neither less than the target flow-rate (Step  110 , No) nor greater than the target flow-rate (Step  114 , No), then first flow-rate would be equal to the target flow-rate. 
     Returning to step  112 , controller  64  may determine whether first valve  54  is fully open (Step  112 ). When controller  64  determines that first valve  54  is fully open (Step  112 , Yes), controller  64  may open second valve  56  (Step  118 ) to dispense sand from second sand box  52 . When controller  64  determines, however, that first valve  54  is not fully open (Step  112 , No), controller  64  may adjust first valve  54  to increase first flow-rate (Step  120 ). Thus when the first flow rate is less than the target flow-rate, controller  64  may adjust first valve  54  to increase the first flow-rate so that the first flow-rate is about equal to the target flow-rate. After adjusting first valve  54  in step  120 , controller  64  may return to step  108  to determine first flow-rate again. Thus, by adjusting the first flow-rate to a target value, controller  64  may allow the operator of machine  10  to provide an adequate amount of sand to control a detected wheel slip condition on machine  10 . 
     Continuing from step  118 , controller  64  may determine second flow-rate (Step  122 ). Controller  64  may determine second flow-rate in a manner similar to that by which controller  64  determined first flow-rate in Step  108 . Alternatively, server  92  may determine second flow-rate in a manner similar to its determination of first flow-rate described above and communicate the second flow-rate to controller  64  directly or via communications module  78 . After determining second flow-rate, controller  64  may adjust second valve  56  so that total flow-rate≈(i.e. about equal to) target flow-rate (Step  124 ). Thus, by allowing sand from second sand box  52  to be dispensed when the first flow-rate of sand from first sand box  24  is below the target flow-rate, method  100  provides an improved way of performing traction control. Controller  64  may determine total flow-rate as a sum of the first and second flow-rates. Controller  64  may adjust second valve  56  by performing actions similar to those described above for first valve  54  in steps  114  and  116 . After adjusting second valve  56 , controller  64  may continue to step  126  to continue sand dispensing operations. 
     Controller  54  may end the sand dispensing operations by turning off first and/or second valves  54 ,  56  after a specified amount of time or when wheel slip has been eliminated or reduced to an acceptable amount. The specified amount of time may be determined by controller  64 , server  92 , or may be specified by an operator of machine  10 . Controller  64  or server  92  may determine when wheel slip has been eliminated or reduced to an acceptable amount by monitoring signals from slip sensor  62 . 
       FIG. 4  illustrates another exemplary method  200 , which may be performed by sand monitoring and control system  40  when first and/or second valves  54 ,  56  are fixed-flow valves. Controller  64  may monitor signals from a slip sensor  62  (Step  202 ). Controller  64  may determine whether wheel slip has been detected (Step  204 ). Alternatively, controller  64  may determine whether the operator has activated button  72 . When controller  64  determines that a wheel slip has been detected (Step  204 , Yes) or when the operator has activated button  72 , controller  64  may open first valve  54  (Step  206 ) to dispense sand from first sand box  24 . When controller  64  determines, however, that no wheel slip has been detected (Step  204 , No), controller  64  may return to step  202  and continue to monitor signals from slip sensor  62 . 
     After opening first valve  54  in step  206 , controller  64  may determine a first flow-rate (Step  208 ) of sand flowing from first sand box  24 . Controller  64  may determine the first flow-rate using methods similar to those described above with regard to step  108  of method  100 . After determining the first flow-rate in step  208 , controller  64  may determine whether the first-flow rate is ≈(about equal to) a target flow-rate (Step  210 ). Controller  64  may determine or receive information regarding the target flow-rate in a manner similar to that described for method  100 . When controller  64  determines that the first flow-rate is ≈a target flow-rate (Step  210 , Yes), controller may keep first valve  54  open for a first amount of time Δt1 (Step  212 ). After the first amount of time Δt1 has elapsed, controller  64  may proceed to step  220 . When controller  64  determines that the first flow-rate is not equal to the target flow-rate (Step  210 , No), controller  64  may proceed to step  214 . 
     Controller  64  may determine whether the first-flow rate is &lt;the target flow-rate (Step  214 ). When controller  64  determines that the first flow-rate is &lt;the target flow-rate (Step  214 , Yes), controller  64  may keep first valve  54  open for a second amount of time Δt2&gt;Δt1 (Step  216 ). By keeping first valve  54  open for a longer period of time compared to Δt1, controller  64  may ensure that sufficient sand may be dispensed to wheels  14 ,  16  to reduce or eliminate the detected wheel slip when first flow-rate is less than the target flow-rate. After time Δt2 has elapsed, controller  64  may proceed to step  220 . 
     When controller  64  determines that the first flow-rate is not less than the target flow-rate (Step  214 , No), controller  64  may keep first valve  54  open for a third amount of time Δt3&lt;Δt1 (Step  218 ). By keeping first valve  54  open for a shorter period of time compared to Δt1, controller  64  may ensure that too much sand is not dispensed to wheels  14 ,  16  when the first flow-rate is greater than the target flow-rate. After time Δt3 has elapsed, controller  64  may proceed to step  220 . Controller  64  may close first valve  54  (Step  220 ) to end sanding operations. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed sand monitoring and control system without departing from the scope of the disclosure. Other embodiments of the sand monitoring and control system will be apparent to those skilled in the art from consideration of the specification and practice of the sand monitoring and control system disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.