Patent Publication Number: US-2011056192-A1

Title: Technique for controlling pumps in a hydraulic system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     Not Applicable 
     STATEMENT REGARDING FEDERALLY  SPONSORED RESEARCH OR DEVELOPMENT  
     Not Applicable 
     BACKGROUND OF THE INVENTION  
     1. Field of the Invention 
     The present invention relates to hydraulic systems for excavators; and more particularly to controlling a plurality of pumps used in such hydraulic systems. 
     2. Description of the Related Art 
     Large excavators, such as power shovels, have a crawler truck on which the cab of the excavator is mounted. A boom is connected to the cab by a pivot joint that enables the boom to move up and down. The boom has a remote end to which one end of an arm is pivotally connected and a bucket is pivotally attached to the other end of the arm in turn has its own remote end to which. The bucket may be a clam-type having two pieces which open and close like a clam shell. The boom, the arm and the bucket are moved with respect to each other by separate hydraulic actuators in the form of cylinder and piston assemblies. 
     Large excavators have a hydraulic system with multiple pumps that can be selectively activated based on the demand for hydraulic fluid by the actuators. When deactivated, a fixed displacement pump continued was hydraulically “unloaded” by a valve that was opened to route the pump&#39;s output flow directly to the fluid reservoir. Alternatively, a variable displacement pumps was deactivated by destroking it. With those deactivation methods, however the pump still contributed to the parasitic losses as it was driven by the prime mover even when unloaded. 
     The multiple pump systems also typically activated and deactivated the pumps in a fixed order so that one pump always was utilized when hydraulic fluid was needed and the remaining pumps were activated in the same order as the demand for hydraulic fluid rose. Similarly as that demand decreased, the pumps were deactivated in the reverse order. As a result, the pumps were exposed to different amounts of use and thus required maintenance and replacement at different intervals. 
     Certain types of excavators, such as those used in mining operations, are operated continuously, 24 hours a day, and thus have to be taken out of service in order for maintenance to be performed. As a consequence, it is desirable to minimize the number of times that the excavator is removed from service. 
     SUMMARY OF THE INVENTION  
     A hydraulic system includes plurality of pumps that provide pressurized fluid to a hydraulic actuator. The plurality of pumps are controlled by a method that measures how much each of the plurality of pumps has been used. For example, that amount of use of a given pump may be determined by measuring an amount of time that the pump operates or by measuring the aggregate amount of work that the performs. When the pump is driven by an electric motor, the amount of work is derived from the voltage and current applied to the electric motor, for example. 
     The demand for fluid to operate the hydraulic actuator is determined and a number of the plurality of pumps are selectively activated to supply enough fluid to meet that demand. The pumps are selectively activated in sequential order from the pump with a least amount of use to the pump with a greatest amounts of use. That activation tends to operate the pumps that have been used the least so that all the pumps will have approximately the same amount of usage and tend to require maintenance and replacement at about the same time. 
     Another aspect of the present invention involves a hydraulic system that has a plurality of pumps which provide pressurized fluid to a plurality of hydraulic actuators. With this system, a usage value is produced for each pump indicating an amount that the respective pump has been used. For each of the plurality of hydraulic actuators, one of the pumps is assigned to each hydraulic actuator in response to the usage values for the plurality of pumps. The pumps with lower usage values are assigned to hydraulic actuators which work more, so as to equalize the use of each pump. The assignment of pumps to hydraulic actuators changes with changes in the usage values for the plurality of pumps. When a given one of the plurality of hydraulic actuators is to operate, hydraulic fluid is routed from the assigned pump to that hydraulic actuator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a side view of an excavator which incorporates the present invention; 
         FIG. 2  is a schematic diagram of the hydraulic system for the excavator which has a plurality of pumps driven by electric motors; 
         FIG. 3  is a flowchart of a software routine executed by a supervisory controller in  FIG. 2  to measure the wear of the motors and pumps in the hydraulic system; 
         FIG. 4  is a software routine executed by the supervisory controller to vary the assignment of the different pumps to the various hydraulic actuators; and 
         FIGS. 5 and 6  are two tables depicting different assignments of the pumps to hydraulic functions on the excavator. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     With initial reference to  FIG. 1 , an excavator, such as a front power shovel  10 , has a crawler assembly  12  for moving the shovel across the ground. A cab  14  is pivotally mounted on the crawler tractor so as to swing in left and right. A boom  16  is pivotally mounted to the front of the cab  14  and can be raised and lowered by a boom hydraulic actuator  22  in the form of a first double-acting cylinder-piston assembly. An arm  18  is pivotally attached to the end of the boom  16  that is remote from the cab  14  and can be pivoted with respect to the boom by an arm hydraulic actuator  23  in the form of a second double-acting cylinder-piston assembly. At the remote end of the arm  18  from the boom is attached to a work tool, such as a bucket  20 , that faces forward from the cab  14 , hence this type of excavator is referred to as a front power shovel. The bucket  20  is pivoted or “curled” about the end of the arm  18  by a curl hydraulic actuator  24 , in the form of a third double-acting cylinder-piston assembly. The bucket  20  is made up of two sections which can be opened and closed like a clam shell by a clam hydraulic actuator  25  ( FIG. 2 ). The two bucket sections are held closed together during a digging operation and are separated in order to dump material into a truck or onto a pile. 
     With reference to  FIG. 2 , the hydraulic system  30  for operating the power shovel comprises a set of four pumps  31 ,  32 ,  33 , and  34  which draw fluid from a reservoir or tank  71 . Each pump  31 ,  32 ,  33 , and  34  has a supply outlet that is connected to a separate primary supply lines  45 ,  46 ,  47 , and  48 . The pressurized fluid from the supply outlet of the first pump  31  is fed into a first primary supply line  45 , the second pump  32  feeds a second primary supply line  46 , the third pump  33  feeds a third primary supply line  47 , and the fourth pump  34  feeds a fourth primary supply line  48 . The pumps  31 - 34  have fixed displacements so that the amount of fluid that is pumped is directly proportional to the speed at which the pump is driven. Each of the four pumps  31 ,  32 ,  33 , and  34  is driven by a separate electric motor  41 ,  42 ,  43  and  44  respectively. Each motor  41 ,  42 ,  43  and  44  is operated by a variable speed drive  57 ,  58 ,  59 , and  60  which vary the frequency of the alternating current applied to the respective motor in order to operate the motor at a desired speed. Any of several well known variable speed drives can be utilized, such as the one described in U.S. Pat. No. 4,263,535, which description is incorporated herein by reference. Each combination of a pump, motor and variable speed drive forms a drive-motor-pump assembly (DMP)  26 ,  27 ,  28 , and  29 . It should be understood that a hydraulic system that employs the present invention may have a greater or lesser number of DMP&#39;s. 
     Each pump  31 - 34  has a case drain through which fluid leakage flows from the pump to the reservoir  71 , as is well known. Each of those case drains is coupled to a reservoir return line  72  by a separate flow meter  35 ,  36 ,  37  and  38  connected to the respective variable speed drive  57 ,  58 ,  59 , and  60 . A separate temperature sensor  61 ,  62 ,  63  and  64  is mounted on each of the motors  41 ,  42 ,  43 , and  44  respectively, to sense the temperature and provide a signal back to the associated variable speed drive  57 ,  58 ,  59 , and  60 . Thus in addition to controlling the speed of the associated motor, each variable speed drive also gathers data about the motor temperature and the pump drain flow. 
     The DMP&#39;s  26 ,  27 ,  28 , and  29  and specifically the variable speed drives  57 ,  58 ,  59 , and  60  are controlled by a supervisory controller  50  which is a microcomputer based device that responds to control signals from the human operator of the power shovel and other signals to control the hydraulic actuators  22 ,  23 ,  24 , and  25  to operate the shovel as desired. Those signals are received by the supervisory controller  50  over a conventional control network  51 . The supervisory controller responds to those signals by determining the amount of hydraulic fluid necessary to be produced by each pump  31 ,  32 ,  33 , and  34  and accordingly controls the motor  41 ,  42 ,  43 , and  44  that drives the respective pump is a manner well known in the art. 
     The four primary supply lines  45 ,  46 ,  47 , and  48  feed into a distribution manifold  52  which selectively directs the fluid flow from each pump to different ones of the four hydraulic actuators  22 ,  23 ,  24 , and  25 . Specifically, the manifold  52  has a first actuator supply line  66  which feeds a solenoid operated first control valve  80  for the boom hydraulic actuator  22 . The first control valve  80  is a three-position, four-way valve which directs fluid from the first actuator supply line  66  to one of the chambers of the cylinder of the boom hydraulic actuator  22  and drains fluid from the other cylinder chamber into the reservoir return line  72  that leads to the reservoir  71 . Depending upon the position of the first control valve  80 , the first hydraulic actuator  22  is driven in either of two directions to thereby raise or lower the boom  16 . Similarly, the second, third, and fourth actuator supply lines  67 ,  68 , and  69  from the distribution manifold  52  are connected by similar second, third, and fourth control valves  81 ,  82 , and  83  to the arm hydraulic actuator  23 , the curl hydraulic actuator  24 , and the clam hydraulic actuator  25 , respectively. The four actuator control valves  80 - 83  are independently operated by separate signals from the supervisory controller  50 . Although the present hydraulic system  30  utilizes control valves  80 - 83  between the distribution manifold  52  and the hydraulic actuators  22 - 25 , the control valves could be eliminated by incorporating their functionality into additional valves in the distribution manifold to control flow to and from each cylinder chamber. 
     The present distribution manifold  52  has a matrix of sixteen distribution valves  84 - 99 . Each distribution valve couples one of the primary supply lines  45 ,  46 ,  47 , or  48  to one of the actuator supply lines  66 ,  67 ,  68 , or  69 . Therefore, when a given distribution valve  84 - 99  is electrically operated by a signal from the supervisory controller  50 , a path is opened between the associated primary supply line and actuator supply line, thereby applying pressurized fluid from the pump connected to that primary supply line to the control valve  80 ,  81 ,  82 , or  83  connected to that actuator supply line. For example, when distribution valve  85  is activated fluid from the first pump  31  flows through the first primary supply line  45  into the second actuator supply line  67  and onward to the second control valve  81 . By selectively operating one or more of the distribution valves  84 - 99 , the output from each pump  31 - 34  can be used to operate each of the four hydraulic actuators  22 ,  23 ,  24 , or  25 . This results is a given pump being assigned to a hydraulic actuator. It should be understood that on a particular power shovel, there may be a greater or lesser number of pumps and a greater or lesser number of hydraulic actuators; in which case the distribution manifold  52  will be configured with a corresponding different number of distribution valves. For example, hydraulic motors may independently drive the left and right tracks of the crawler assembly  12  to propel the power shovel. 
     It also should be understood that the output from two or more pumps can be combined to supply the same hydraulic actuator  22 - 25 . For example, if only the arm hydraulic actuator  23  is active, the output from multiple pumps can be combined so that the arm is driven to dig into the earth with maximum speed and force. When another shovel function is to operate simultaneously with the arm, one or more of the pumps previously connected to the arm function is reassigned to provide fluid to that other shovel function by redirecting the flow through the distribution manifold  52 . Also should a DMP  26 - 29  fail, it is deactivated by shutting off the associated variable speed drive and disconnecting the associated pump by closing all the valves in the distribution manifold  52  that are connected to the respective primary supply line. In this case, fluid from the remaining pumps supplied through the distribution manifold to operate the hydraulic actuators. If, however, the output of a particular pump is not required at a given point in time, its variable speed drive is deactivated so that the motor and thus that pump do not operate. 
     For very large power shovels, relatively large forces encountered by the arm hydraulic actuator  23  and curl hydraulic actuator  24  during a digging operation. In addition, the arm and curl hydraulic actuators  23  and  24  tend to be operated for longer periods of time than the other hydraulic actuators. The claim hydraulic actuator  25  associated with the bucket  20  typically is significantly smaller and consumes far less hydraulic fluid. In previous power shovels, a given pump often was dedicated to supplying fluid to one of the hydraulic actuators and thus the motor-pumps combinations performed different levels of work. In other words, because the pumps and motors for the arm and the bucket curl functions perform considerably more work than other pumps and motors in the hydraulic system, those heavily worked components tended to require more maintenance and more frequent replacement than the other motors and pumps. Therefore, the different motor/pump combinations required servicing at different times at during which the entire power shovel had to be taken out of service. The resultant downtime adversely affected the power shovel&#39;s overall productivity and economy of operation. 
     The present invention overcomes the problems with such previous systems by dynamically changing the assignment of the DMP&#39;s to the hydraulic actuators so that each motor/pump combination is exposed to substantially the same amount of use and work. As a consequence, all the DMP&#39;s will require maintenance and possible replacement at about the same point in time. Thus, the service and replacement intervals for the DMP&#39;s are synchronized so that the maintenance intervals, mean time to repair, and mean time between failure are optimized and provide a longer mean time between failure for the entire hydraulic system. This reduces the number of service down periods over the life of the excavator and thereby increases productivity. 
     In order to determine the usage of the DMP&#39;s, the supervisory controller  50  gathers data regarding the operation of their motors and pumps, such as electric current and voltage applied to the motor, motor temperature, speed, torque, aggregate operating time, and amount of pump drain flow. The accumulated data is utilized to determine the relative amount of work performed by each DMP  26 ,  27 ,  28 , and  29 . To this end the supervisory controller  50  executes different software routines that gather and analyze the pump and motor data to estimate the remaining anticipated life of those components and the aggregate amount of use that they have provided. The term DMP is being used to refer to performance of the motor/pump combination as well as performance of the individual motor and pump therein. 
     With reference to  FIG. 3 , a DMP life routine  100  is executed periodically on a timed-interrupt basis by the supervisory controller  50 . This software routine commences at step  102  where a finding is made whether at least one actuator  22 - 25  of the power shovel  10  is currently being operated. The execution of the routine loops through this step until one of the hydraulic actuators  22 - 25  begins operating, at which time the process advances to step  104 . At this juncture, the supervisory controller  50  obtains data indicating the magnitudes of the electric current and voltage that each variable speed drive  57 - 60  is applying to its associated motor  41 - 44 . Each variable speed drive contains circuitry for measuring the magnitude of the voltage and current and converting those measurements into digital data for transmission to the supervisory controller  50  as is well known. Next, the recorded electrical data are used at step  106  to compute the average RMS power consumed by each motor during a predefined measurement time period. At step  108 , the newly computed RMS power values are compared to the rated value for each respective motor, as specified by the motor manufacturer to determine whether the operation exceeds the rated power for that motor. If so, for each motor the magnitudes that its rated power value is exceeded are integrated at step  110  to derive a value indicative of the aggregate excessive use of the motor. Those excessive use values then are used at step  112  to calculate the life expectancy of each motor  41 - 44 . For example, the greater the amount of time that the rated power is exceeded and the aggregate magnitude of that excess decreases the life of the motor from the nominal life expectancy specified by the motor manufacturer. The nominal life expectancy is based on the rated power level not being exceeded. An empirically derived relationship for the particular type of motor is used to calculate a how much the motor life expectancy has decreased due to the actual duration of excessive power operation and the aggregate magnitude of that excessive power. The duration of excessive power operation is based on the sampling period for the motor electrical values. The decrease in the expected motor life and the nominal life expectancy are used to project a life expectancy for each motor  41 - 43 . That information is then stored in a table within the supervisory controller  50 . 
     Thereafter at step  114 , the DMP life routine  100  enters a section at step  116  in which the present life expectancy of each pump  31 - 34  is estimated. The supervisory controller  50  initially records the speed and torque of the motors  41 - 43 , which information is derived from the electric voltage and current levels applied by the variable speed drives  57 - 60 . Alternatively, the speed and torque data can be measured by sensors attached to the drive shaft linking a motor to a pump. The supervisory controller  50  also obtains the amounts of fluid flow exhausting from the pump case drains. Those flow rates are sensed by the flow meters  35 ,  36 ,  37 , and  38  connected to circuitry in the variable speed drives  57 ,  58 ,  59 , and  60  which relay the case drain flow data to the supervisory controller. Then at step  118 , the amounts of fluid flow and pressure at the supply outlet of each pump  31 - 34  are derived from the respective speed and torque values. Specifically, the flow is the product of the speed and the fixed pump displacement. The torque correlates directly with the pump supply outlet pressure. Alternatively the fluid flow and pressure can be measured directly by sensors at the supply outlet of each pump  31 - 34 . 
     At step  120 , the values for the amounts of supply outlet fluid flow, pump pressure, and the case drain flow are compared with data provided by the manufacturer of the pumps to determine the present point on the life cycle for each pump. Specifically, the leakage of the pump represented by the flow from the pump case drain increases as a pump ages. In other words, the older the pump, the greater the case drain flow, however, the actual case drain flow at any point in time also is a function of the fluid flow and pressure produced at the supply outlet by the pump. That is, the case drain flow increases as the flow and pressure produced by the pump increase. A typical pump manufacturer has correlated the expected pump case drain flow for various pressure and flow amounts at different times during the life cycle of the pump. By comparing the actual fluid flow, pressure, and pump case drain flow to manufacturer specification data, the supervisory controller  50  is able to determine the remaining life of each of the pumps  31 - 34 , at step  122 . This determination is stored within the memory of the supervisory controller  50  for display to the pump operator and service personnel, as well as for determining the trends of the pump life cycle to estimate when pump maintenance and replacement will be required. 
     With reference to  FIG. 4 , the supervisory controller  50  also executes a software DMP assignment routine  130 , that allocates the output of each pump  31 - 34  to one of the hydraulic actuators  22 - 25  based on the accumulated amount of use of each DMP  26 - 29 . As noted previously, the arm and bucket curl hydraulic actuators  23  and  24  operate more frequently and demand a greater amount of force from the hydraulic system than the boom and bucket clam hydraulic actuators  24  and  25 . Therefore, the DMP&#39;s that supply fluid to the arm and bucket curl hydraulic work more intensely than other DMP&#39;s. The DMP assignment routine  130  determines the aggregate amount of work that each motor/pump combination has performed and adjusts the assignment of the DMP&#39;s  26 - 29  to the various hydraulic actuators  22 - 25  to approximately equalize the work being performed. This results in all the motor/pump combinations incurring essentially the same amount of wear so that they should require maintenance and ultimately replacement at the approximately same time. 
     The DMP assignment routine  130  commences at step  132  where a finding is made whether the hydraulic system  30  is currently operating at least one actuator, if so, the routine advances to step  134 . At that point, the present assignments of the four DMP&#39;s  26 ,  27 ,  28  and  29  to the different hydraulic actuators  22 ,  23 ,  24 , and  25  is recorded as a table in the memory of the supervisory controller  50 .  FIG. 5  depicts an exemplary table in which for each hydraulic function one of the DMP&#39;s is designated. That table also is used by the supervisory controller  50  in opening and closing the distribution valves  84 - 99  in the distribution manifold  52  to direct fluid from each pump to the designated hydraulic actuator. For the exemplary table, the supervisory controller  50  would open distribution valve  96  to direct the fluid from the fourth pump  34  to the boom supply line  66 , and open distribution valve  85  to direct the fluid from the first pump  31  to the arm supply line  67 . Similarly distribution valve  94  is opened to direct the fluid from the third pump  33  to the curl supply line  68  and distribution valve  91  is opened to direct the fluid from the second pump  32  to the clam supply line  69 . 
     Returning to the DMP assignment routine  130  in  FIG. 4 , the total amount of time that each DMP  26 - 29  has operated when assigned to each hydraulic actuator is determined at step  136 . For each DMP, the supervisory controller  50  implements a separate timer in software that runs whenever the respective DMP is operating. This provides a cumulative record of the total time that each motor  41 - 44  and each pump  31 - 34  has operated. 
     At step  138  the magnitudes of electric voltage and current that the respective variable speed drive  57 ,  58 ,  59 , and  60  applies to the associated motor  41 ,  42 ,  43  and  44  are read by the supervisory controller  50 . Each variable speed drive  57 ,  58 ,  59 , and  60  stores a digitized temperature value resulting from a signal produced by the temperature sensor  61 ,  62 ,  63  or  64  attached to the associated motor  41 ,  42 ,  43 , or  44 , respectively. The temperature values also are read from the variable speed drives and stored within the memory of the supervisory controller  50  at step  140 . 
     At step  142 , the electrical values read for each motor  41 - 44  are used to determine the amount of work that the respective DMP performed. Specifically, the current and voltage levels for a particular motor are multiplied to produce a value denoting the amount of electrical power consumed during the time interval between measurements. Not all consumed input electrical power is converted into mechanical power for driving the pump, because energy is lost as heat produced in the motor. The measured temperature of the respective motor is used to calculate the amount of the electrical power that was consumed in heating that motor, i.e., the heat power loss. Therefore, the mechanical power provided by the associated pump  31 - 34  is calculated by subtracting the heat power loss from the amount of electrical power consumed. The resultant mechanical power value then is integrated over the measurement interval to derive the amount of work that the pump performed. The new amount of work then is added to a sum of similar amount of work calculated previously to provide a measurement of the aggregate amount of work that the pump has performed since its installation. This work computation is performed individually for each of the pumps  31 - 34  and the resultant aggregate amounts of work are stored in the supervisory controller  50 . At step  144 , the DMP&#39;s  26 - 29  are ranked in order of the aggregate amount of work that each has performed. 
     As noted previously, the DMP&#39;s supplying the arm and curl hydraulic actuators  23  and  24  perform a greater amount of work over time than the boom and claim hydraulic actuators  22  and  25 . Thus the DMP&#39;s that control the flow of fluid to the arm and curl hydraulic actuators corresponding perform a greater amount of work. The purpose of the DMP assignment routine  130  is to equalize the aggregate amounts of work that the motor/pump combinations perform so that they are subjected to substantially equal amount of wear and therefore require maintenance and ultimately replacement at approximately the same time. Doing so reduces how often the power shovel  10  must be taken out of operation. 
     In a standard configuration of the distribution manifold  52 , a separate pump  31 - 34  is connected to feed fluid to a different hydraulic actuator  22 - 25 . Which pump is connected to which hydraulic actuator is determined dynamically in response to the ranking of the DMP&#39;s based on the aggregate amount of work that each performed. The DMP to hydraulic actuator assignments are recorded as a table in the memory of the supervisory controller  50  and  FIG. 5  depicts as exemplary set of those assignments. Therefore at step  146 , the DMP work rankings are inspected to ensure that the DMP&#39;s with the least aggregate amounts of work are assigned to the arm and curl hydraulic actuators  23  and  24 . Assume for example that upon entering step  146 , the DMP to hydraulic actuator assignments are as depicted in  FIG. 5 , the second DMP  27  now has the greatest aggregate amount of work, and the fourth DMP  29  has the least aggregate amount of work. The supervisory controller  50  in this case will reassign the second DMP  27  to the bucket claim hydraulic actuator  25 , and the fourth DMP  29  to the arm hydraulic actuator  25  as depicted in  FIG. 6 . The rearrangement of the DMP to hydraulic actuator assignments causes the supervisory controller  50  two change the configuration of open and closed distribution valves  86 - 97  to connected the pumps  31 - 34  in each DMP to the hydraulic actuator  22 - 25  designated in the assignment table. 
     For machines in which the different hydraulic actuators are subjected to substantially equal forces, the assignment of DMP&#39;s can be based on operating time. For example, the DMP that with the lowest aggregate amount of work is assigned to the hydraulic actuator that operates most often. Similarly the DMP that with the greatest aggregate amount of work is assigned to the hydraulic actuator that operates least often. In another variation of the present control technique, when a hydraulic actuator is operate, the inactive DMP with the lowest aggregate amount of work is assigned to provide fluid that actuator. 
     In another situation, a given hydraulic actuator may have a varying demand for hydraulic fluid depending on the force acting on that actuator. One DMP alone may not be able to meet all demand levels. Therefore at higher demand levels, multiple pumps are used to provide fluid to that given hydraulic actuator. Here the DMP&#39;s are assigned to the given hydraulic actuator in order from the DMP with the lowest aggregate amount of work to the DMP with the greatest aggregate amount of work. Thereafter, when the demand for hydraulic fluid from a hydraulic actuator decreases, the DMP&#39;s are unassigned in the reverse order. Specifically, the DMP with the greatest aggregate amount of work is disconnected first and the DMP with the lowest aggregate amount of work remains connected until fluid not longer is needed. 
     The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.