Patent Publication Number: US-11035389-B2

Title: Drift compensation system for drift related to damping of mass-induced vibration in machines

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of PCT/US2018/029392, filed on Apr. 25, 2018, which claims the benefit of U.S. Patent Application Ser. No. 62/491,889, filed on Apr. 28, 2017, and claims the benefit of U.S. Patent Application Ser. No. 62/535,524, filed on Jul. 21, 2017, the disclosures of which are incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of hydraulic systems and, more particularly, to systems for damping mass-induced vibration in machines. 
     BACKGROUND 
     Many of today&#39;s mobile and stationary machines include long booms or elongate members that may be extended, telescoped, raised, lowered, rotated, or otherwise moved through the operation of hydraulic systems. Examples of such machines include, but are not limited to: concrete pump trucks having articulated multi-segment booms; fire ladder trucks having extendable or telescoping multi-section ladders; fire snorkel trucks having aerial platforms attached at the ends of articulated multi-segment booms; utility company trucks having aerial work platforms connected to extendable and/or articulated multi-segment booms; and, cranes having elongate booms or extendable multi-segment booms. The hydraulic systems generally comprise a hydraulic pump, one or more linear or rotary hydraulic actuators, and a hydraulic control system including hydraulic control valves to control the flow of hydraulic fluid to and from the hydraulic actuators. 
     The long booms and elongate members of such machines are, typically, manufactured from high-strength materials such as steel, but often flex somewhat due at least in part to their length and being mounted in a cantilever manner. In addition, the long booms and elongate members have mass and may enter undesirable, mass-induced vibration modes in response to movement during use or external disturbances such as wind or applied loads. Various systems have been developed to reduce or eliminate the mass-induced vibration. However, while reducing or eliminating the mass-induced vibration, such systems may cause slight undesirable movement of a hydraulic actuator (referred to herein as “drift” or “drifting”) connected to a boom or elongate member, thereby causing the boom or elongate member to correspondingly move and no longer be positioned as needed. Such drifting may be substantial enough in some cases to necessitate re-positioning of the boom or elongate member by a machine operator. 
     Therefore, there is a need in the industry for a system, including apparatuses and methods, for compensating for drift in the position of a hydraulic actuator of a machine with which damping of mass-induced vibration is used, and that addresses this and other problems, issues, deficiencies, or shortcomings. 
     SUMMARY 
     Broadly described, the present invention comprises a system, including apparatuses and methods, for compensating for movement or drift of the piston of a hydraulic actuator of a machine (and corresponding movement or drift of a machine member whose position is controlled by such hydraulic actuator) resulting, at least in part, from damping of mass-induced vibration. In one inventive aspect, the system compensates for drift due to damping of mass-induced vibration by reducing or eliminating additional volume present in a load holding chamber of a hydraulic actuator as a result of such damping. In another inventive aspect, the system determines a flow rate of hydraulic fluid to compensate for drift based on the additional volume of a chamber of a hydraulic actuator due to damping of mass-induced vibration. In still another inventive aspect, the system determines a flow rate of hydraulic fluid to be supplied to a hydraulic actuator appropriate to cause desired movement of the hydraulic actuator and compensation for drift caused by damping of mass-induced vibration. In another inventive aspect, the system determines a flow rate of hydraulic fluid to compensate for drift due to damping of mass-induced vibration based on the difference of a measured position of a portion of a machine compared to a desired position of the portion of a machine. In another inventive aspect, the system determines a flow rate of hydraulic fluid to compensate for drift due to damping of mass-induced vibration based on the difference of a measured pressure of a chamber of a hydraulic actuator compared to a desired pressure of the chamber of a hydraulic actuator. 
     Other inventive aspects, advantages and benefits of the present invention may become apparent upon reading and understanding the present specification when taken in conjunction with the appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  displays a pictorial view of a mobile machine in the form of concrete pump truck configured with a drift compensation system in accordance with an example embodiment of the present invention and with a damping system for reducing mass-induced vibration. 
         FIG. 2  displays a schematic representation of the relationship between control valves of the drift compensation system, a control manifold of the damping system for reducing mass-induced vibration, and a hydraulic actuator of the mobile machine of  FIG. 1 . 
         FIG. 3  displays a block diagram representation of the drift compensation system in accordance with the example embodiment of the present invention. 
         FIG. 4  displays a control diagram representation of the control methodology used by the drift compensation system in accordance with the example embodiment of the present invention. 
         FIG. 5  displays a flowchart representation of a method for compensating for drift in accordance with the example embodiment of the present invention. 
         FIG. 6  displays a control diagram representation of the control methodology used by another drift compensation system in accordance with the example embodiment of the present invention. 
         FIG. 7  displays a flowchart representation of another method for compensating for drift in accordance with the example embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF AN EXAMPLE EMBODIMENT 
     Referring now to the drawings in which like elements are identified by like numerals throughout the several views,  FIG. 1  displays a machine  100  configured with a drift compensation system  200 , in accordance with an example embodiment of the present invention, for compensating for movement or drift in the position of a hydraulic actuator&#39;s piston  114  resulting from damping to reduce or eliminate mass-induced vibration. More specifically, in  FIG. 1 , the machine  100  comprises a concrete pump truck having an articulated, multi-segment boom  102  that is connected to the remainder of the concrete pump truck by a skewing mechanism  104  that enables rotation of the boom  102  about a vertical axis relative to the remainder of the concrete pump truck. The boom  102  comprises a plurality of elongate boom segments  106  that are pivotally connected by pivot pins  108  in an end-to-end manner. The machine  100  also comprises a plurality of hydraulic actuators  110  that are attached to and between each pair of pivotally connected boom segments  106 . The hydraulic actuators  110  generally comprise linear hydraulic actuators operable to extend and contract, thereby causing respective pairs of pivotally connected boom segments  106  to rotate relative to one another about the pivot pin  108  coupling the boom segments  106  together. In some examples, sensors  105  (e.g., inclinometers, position sensors, angular position sensors, gyroscopes, pressure sensors, etc.) can be used to track the position of the boom  102 . 
     Each hydraulic actuator  110  has a cylinder  112  and a piston  114  located within the cylinder  112  (see  FIGS. 1 and 2 ). The piston  114  slides within the cylinder  112  and, with the cylinder  112 , defines a plurality of chambers  116  for receiving pressurized hydraulic fluid. A rod  118  attached to the piston  114  extends through one the chambers  116 , through a wall of the cylinder  112 , and is connected to a boom segment  106  to exert forces on the boom segment  106  causing movement of the boom segment  106 . A first chamber  116   a  (also sometimes referred to herein as the “non-load holding chamber  116   a ”) of the plurality of chambers  116  is located on the rod side of the actuator&#39;s piston  114  and a second chamber  116   b  (also sometimes referred to herein as the “load holding chamber  116   b ”) of the plurality of chambers  116  is located on the opposite side of the actuator&#39;s piston  114 . 
     Before proceeding further, it should be noted that while the drift compensation system  200  (sometimes referred to herein as the “system  200 ”) is illustrated and described herein with reference to a machine  100  comprising a concrete pump truck having an articulated, multi-segment boom  102 , the drift compensation system  200  may be applied to and used in connection with any machine  100  having long booms, elongate members, or other components the movement of which may induce vibration therein. It should also be noted that the drift compensation system  200  may be applied to and used in connection with mobile or stationary machines having long booms, elongate members, or other components in which mass-induced vibration may be introduced by their movement. Additionally, as used herein, the term “hydraulic system” means and includes any system commonly referred to as a hydraulic or pneumatic system, while the term “hydraulic fluid” means and includes any incompressible or compressible fluid that may be used as a working fluid in such a hydraulic or pneumatic system. 
     Referring back to  FIGS. 1 and 2 , the machine  100  additionally comprises a plurality of control valves  120  that supply hydraulic fluid to the hydraulic actuators  110 . According to the example embodiment, the control valves  120  comprise solenoid-actuated, metering valves having independently operable control valve spools  122   a ,  122   b  (also sometimes referred to herein as “valve spools  122   a ,  122   b ” or “spools  122   a ,  122   b ”) movable to fully open, fully closed, and intermediate positions between the fully open and fully closed positions. It should be appreciated and understood, however, that in other example embodiments, the control valves  120  may comprise other types of valves having similar capabilities and functionality. 
     The control valves  120  are generally arranged such that each control valve  120  is associated and operable with a particular hydraulic actuator  110 . In such arrangement, the first control valve spool  122   a  of the control valve  120  supplies hydraulic fluid to the actuator&#39;s non-load holding chamber  116   a  and the second control valve spool  122   b  of the control valve  120  supplies hydraulic fluid to the actuator&#39;s load holding chamber  116   b . The control valve spools  122   a ,  122   b  are operable to supply hydraulic fluid to each of the actuator&#39;s chambers  116   a ,  116   b  at a flow rate, Q cmd , needed to cause operation of the hydraulic actuator  110  and movement of an associated boom segment  106  or elongate member in response to receiving a command based on the particular movement of the beam segment  106  or elongate member desired by the machine&#39;s operator. The control valve spools  122   a ,  122   b  are further operable to independently adjust the flow rate of hydraulic fluid supplied to each chamber  116   a ,  116   b  of the hydraulic actuator  110  in accordance with commands, signals, or other direction received from a damping system  124  (described below) that dampens mass-induced vibration. 
     When the machine&#39;s boom  102  is rotated by the skewing mechanism  104  or when connected boom segments  106  are rotated relative to one another about a respective pivot pin  108 , vibration is induced in the boom  102  and boom segments  106  because the boom  102  and its boom segments  106  have mass and are being moved relative to the remainder of concrete pump truck or relative to one another. In order to dampen such mass-induced vibration, the machine  100  further includes a damping system  124  having a plurality of control manifolds  126  operable to dampen mass-induced vibration. The damping system  124  may comprise a system that reduces or eliminates mass-induced vibration detected and measured by motion sensors mounted to the machine&#39;s boom  102  or elongate member, by pressure sensors that measure the pressure of hydraulic fluid in a hydraulic actuator&#39;s chambers  116   a ,  116   b , or by use of other devices and methods. 
     As illustrated in  FIG. 2 , each control manifold  126  is fluidically located and connected between a control valve  120  and a hydraulic actuator  110 . Generally, a control manifold  126  and a hydraulic actuator  110  are associated in one-to-one correspondence such that the control manifold  126  participates in controlling the flow of pressurized hydraulic fluid delivered from a spool  122   a ,  122   b  of a control valve  120  to a chamber  116   a ,  116   b  of the hydraulic actuator  110 . The control manifold  126  associated with a particular hydraulic actuator  110  is, typically, mounted near the hydraulic actuator  110  (see  FIG. 1 ). 
     More particularly, each control manifold  126  is connected to the non-load holding chamber  116   a  of hydraulic cylinder  110  for the flow of hydraulic fluid therebetween by hose  128   a , and is connected to the load holding chamber  116   b  of hydraulic cylinder  110  for the flow of hydraulic fluid therebetween by a hose  128   b . Additionally, each control manifold  126  is connected to a control valve spool  122   a  for the flow of hydraulic fluid therebetween by hose  130   a , and is connected to a control valve spool  122   b  for the flow of hydraulic fluid therebetween by hose  130   b . In addition, the control manifold  126  is fluidically connected to a hydraulic fluid tank or reservoir (not shown) by a hose  132  for the flow of hydraulic fluid from the control manifold  126  to the hydraulic fluid tank. It should be appreciated and understood that although hoses  128 ,  130 ,  132  are used to connect the control manifold  126  respectively to hydraulic cylinder  110 , control valves  120 , and a hydraulic fluid tank or reservoir in the example embodiment described herein, the hoses  128 ,  130 ,  132  may be replaced in other example embodiments by tubes, conduits, or other apparatuses suitable for conveying or distributing hydraulic fluid. 
     One example of the drift compensation system  200  is illustrated in block diagram form in  FIG. 3 . Briefly described above, the system  200  is operable to compensate for drift in the position of a hydraulic actuator&#39;s piston  114  (and, hence, in the position of a boom  102 , boom segment  106 , or elongate member controlled by the hydraulic actuator  110 ) due to adjustments in the flow rate of hydraulic fluid delivered to the hydraulic actuator  110  made by a co-present damping system  124  to dampen mass-induced vibration. At a high level, the system  200  provides such compensation by determining a bias volume present within the hydraulic actuator&#39;s cylinder  112  resulting from drift of the actuator&#39;s piston  114  due to dampening of mass-induced vibration by the damping system  124 , calculating a flow rate of hydraulic fluid necessary to eliminate the bias volume, and adding the calculated flow rate to the flow rate of hydraulic fluid required to operate the hydraulic actuator  110  as commanded by the machine&#39;s operator. In some examples, the system  200  provides such compensation for drift in the position of a hydraulic actuator&#39;s piston  114  without the use of or need for cylinder position sensors. In other examples, drift compensation system provides such compensation for drift in the position of a hydraulic actuator&#39;s piston  114  using position sensors  105 , as described below. 
     The system  200  comprises a processing unit  202  operable to execute a plurality of software instructions that, when executed by the processing unit  202 , cause the system  200  to implement the methods and otherwise operate and have functionality as described herein. The processing unit  202  may comprise a device commonly referred to as a microprocessor, central processing unit (CPU), digital signal processor (DSP), or other similar device and may be embodied as a standalone unit or as a device shared with components of the hydraulic system with which the system  200  is employed. The processing unit  202  may include memory for storing the software instructions or the system  200  may further comprise a separate memory device for storing the software instructions that is electrically connected to the processing unit  202  for the bi-directional communication of the instructions, data, and signals therebetween. 
     Also, the drift compensation system  200  comprises a plurality of control valves  204  that are operable to control the flow of pressurized hydraulic fluid to control manifolds  126  and, hence, to their respective connected hydraulic actuators  110  in order to cause the hydraulic actuators  110  to extend or contract. According to the example embodiment described herein, the system&#39;s control valves  204  comprise the same control valves  120  described above such that the control valves  120  are, in a sense, shared and a part of the machine&#39;s conventional control system used to move the boom  102  or elongate member, the damping system  124  used to dampen mass-induced vibration, and the drift compensation system  200  that reduces or eliminates drift caused by operation of the damping system  124 . Thus, each control valve  204  of the system  200  includes control valve spools  206   a ,  206   b  corresponding to control valve spools  122   a ,  122   b  described above. 
     The control valves  204  are electrically connected to processing unit  202  by respective communication links  208  for receiving control signals from processing unit  202  causing the valves&#39; solenoids to energize or de-energize, thereby correspondingly moving the valves&#39; spools  206   a ,  206   b  to allow full flow of hydraulic fluid through the control valves  204 , no flow of hydraulic fluid through the control valves  204 , or partial flow of hydraulic fluid through the control valves  204 . Stated slightly differently, the flow rate of hydraulic fluid from a control valve  204  is determined at least in part by signals, data, or instructions received from processing unit  202  via communication links  208 . 
     The drift compensation system  200  additionally comprises a plurality of control valve sensors  210  that measure various parameters related to or indicative of the operation of respective control valves  204 . Such parameters include, but are not limited to, hydraulic fluid supply pressure (P s ), hydraulic fluid tank pressure (P t ), hydraulic fluid delivery pressure (P a , P b ), hydraulic fluid temperature (T), and control valve spool displacement (x a , x b ), where subscripts “a” and “b” correspond to actuator chambers  116   a ,  116   b  and to the first and second control valve spools  206   a ,  206   b  of a control valve  204 . The control valve sensors  210  are generally attached to or at locations near respective control valves  204  as appropriate to obtain measurements of the above-identified parameters. The control valve sensors  210  are operable to obtain such measurements and to produce and output signals representative of such measurements. Communication links  212  connect the control valve sensors  210  to processing unit  202  for the communication of such output signals to processing unit  202 , and may utilize wired and/or wireless communication devices and methods for such communication. 
     According to an example embodiment, the control valves  204 , control valve sensors  210 , and processing unit  202  are co-located in a single, integral unit. However, it should be appreciated and understood that, in other example embodiments, the control valves  204 , control valve sensors  210 , and processing unit  202  may be located in different units or locations. It should also be appreciated and understood that, in other example embodiments, the control valves  204  may comprise independent metering valves not a part of the system  200 . 
     During operation of the drift compensation system  200  and as illustrated in the control diagram of  FIG. 4 , the control valve sensors  210  produce electrical signals or data representative of the hydraulic fluid supply pressure (P s ) to control valve spools  206   a ,  206   b , hydraulic fluid tank pressure (P t ), hydraulic fluid delivery pressure (P a , P b ) at the work ports of control valve spools  206   a ,  206   b , hydraulic fluid temperature (T), and the spool displacement (x a , x b ) of the control valve spools  206   a ,  206   b . The processing unit  202  receives the signals or data from control valve sensors  210  via communication links  212 . Operating under the control of stored software instructions and based on the received input signals or data, the processing unit  202  generates output signals or data for delivery to the control valves  204  via communication links  208 . More particularly, the processing unit  202  produces separate actuation signals or data to cause the operation of control valves  204  and spools  206   a ,  206   b  in accordance with the method described below. 
     The system  200  operates in accordance with a method  300  illustrated in  FIG. 5  to compensate for drift due to damping of mass-induced vibration. Operation according to method  300  starts at step  302  and proceeds to step  304  where the processing unit  202  uses signals, data, or information (including, but not limited to, hydraulic fluid temperature (T), hydraulic fluid supply pressure (P s ) to control valve spools  206   a ,  206   b , hydraulic fluid delivery pressure (P b ) at the work port of control valve spool  206   b , and the spool displacement (x b ) of control valve spool  206   b ) received from valve sensors  210  to determine the flow rate (Q b ) of hydraulic fluid through control valve spool  206   b  that is associated only with damping of mass-induced vibration. It should be noted that the flow rate (Q b ) of hydraulic fluid includes no portion associated with any purpose other than damping and does not include, for example and not limitation, a portion associated with or resulting from an operator&#39;s command to move the boom  102 , boom segment  106 , or elongate member controlled by the connected hydraulic actuator  110 . 
     Next, at step  306 , the processing unit  202  calculates the bias volume (V drift ) of the load holding chamber  116   b  of the connected hydraulic actuator  110  which results from damping of mass-induced vibration. The bias volume (V drift ) is related to the flow rate (Q b ) of hydraulic fluid through control valve spool  206   b  associated solely with damping by:
 
 V   drift ∫ t_on   t_off   Q   b   dt.  
 
     Continuing at step  308  of method  300 , the processing unit  202  determines the drift compensation flow rate (Q driftComp ) required to cancel out the bias volume. The drift compensation flow rate (Q driftComp ) is given by:
 
 Q   driftComp   =−k   drift   ·V   drift  
 
where: k drift  is a constant; and
 
     V drift  is the bias volume. 
     It should be appreciated and understood that in other example embodiments, the drift compensation flow rate (Q driftComp ) required to cancel out the bias volume may be determined using other methods such as, but not limited to, proportional-integral (PI) control. 
     Subsequently, at step  310 , the drift compensation flow rate (Q driftComp ) is added to the flow rate (Q cmd ) required to cause movement of the hydraulic actuator  110  in response to input received from the machine&#39;s operator via a joystick or other input device. The resulting flow rate (Q Total ) comprises the flow rate that control valve spool  206   b  must supply to hydraulic actuator  110  to cause movement of the machine&#39;s boom  102  or a boom segment  106  as desired by the machine&#39;s operator and to reduce or eliminate drift. Then, at step  312 , signals or data representative of the resulting flow rate (Q Total ) are communicated to control valve spool  206   b , causing the spool  206   b  to adjust and supply hydraulic fluid to hydraulic actuator  110  at a flow rate appropriate to cause desired movement of the machine&#39;s boom  102  or boom segment  106  while also reducing or eliminating drift. After communication of the resulting flow rate and adjustment of control valve spool  206   b  such that drift is substantially reduced or eliminated, the method  300  ends at step  314 . 
     Another example of a drift compensation system  400  is illustrated schematically in  FIG. 6 . Similar to the system  200  described above, system  400  is operable to compensate for drift in the position of a hydraulic actuator&#39;s piston  114  (and, hence, in the position of a boom  102 , boom segment  106 , or elongate member controlled by the hydraulic actuator  110 ) due to adjustments in the flow rate of hydraulic fluid delivered to the hydraulic actuator  110  made by a co-present damping system  124  to dampen mass-induced vibration. At a high level, in some examples, the system  400  provides such compensation by determining the position of a segment(s)  106  of the boom  102  using an external sensor(s)  105 , calculating a flow rate of hydraulic fluid necessary to move the actuator  110  to eliminate the offset positioning from a predetermined position, and adding the calculated flow rate to the flow rate of hydraulic fluid required to operate the hydraulic actuator  110  as commanded by the machine&#39;s operator. In other examples, the system  400  provides such compensation by determining the difference (error) in measured hydraulic fluid pressure (P a  or P b  corresponding to actuator chambers  116   a ,  116   b ) and a predetermined desired pressure, calculating a flow rate of hydraulic fluid necessary to move the actuator  110  to eliminate the error in pressure values compared to the predetermined value, and adding the calculated flow rate to the flow rate of hydraulic fluid required to operate the hydraulic actuator  110  as commanded by the machine&#39;s operator. 
     In some examples, the system  400  is configured to use data from sensors  105  positioned on downstream boom segments  106  (i.e., toward the free end of the boom  102 ) to correct for motion of the upstream segment  106 . In other examples, the system  400  is configured to use data from sensors  105  located on the segment  106  having the actuator  110  attached thereto (e.g., angular position sensor, gyroscope, actuator cylinder position sensor, etc.). In other examples, the system  400  is configured to use data from sensors positioned on the actuator  110  and in communication with the actuator chambers  116   a ,  116   b  (e.g., pressure sensors). Alternatively, the pressure sensors  105  may be embedded in the control valves  120 . 
       FIG. 7  shows a method  402  of operating the system  400 . The method  402  starts at step  404  and proceeds to step  406  where the processing unit  202  receives signals, data, or information (including, but not limited to, linear position data, angular position data, inclinometer position data, and hydraulic fluid pressure (P a , P b ) data) that is representative of actuator drift. Next, at step  306 , the processing unit  202  determines the drift compensation flow rate (Q driftComp ) required to cancel out the positional drift of the actuator  110 . 
     In some examples, the drift compensation flow rate (Q driftComp ) is given by:
 
 Q   driftComp   =P   PropGAIN ( x   desired   −x   measured )
 
where: P PropGAIN  is a constant;
 
     x measured  is a measured position by a sensor  105 ; and 
     x desired  is a predetermined desired position value set within the processor unit  202 . 
     P PropGAIN  can be a preset constant value assigned to compensate for drift. In some examples, P PropGAIN  can be altered over time. In other examples, the P PropGAIN  can be altered based on specific conditions or operation of the machine  100 . In some examples, x desired  is a measured value and can be obtained by recording a position when the damping system  124  is activated. In some examples, the x desired  can be altered based on the preference of the operator. 
     In some examples, the drift compensation flow rate (Q driftComp ) is given by:
 
 Q   driftComp   =P   PropGAIN ( P   desired   ·P   measured )
 
where: P PropGAIN  is a constant;
 
     P measured  is a measured pressure in at least one actuator chamber  116   a ,  116   b ; and 
     P desired  is a predetermined desired pressure value set within the chosen pressure chamber  116   a ,  116   b  set within the processor unit  202 . 
     Like x desired  above, P desired  can be a measured value and can be obtained by recording a pressure in a chamber  116   a ,  116   b  when the damping system  124  is activated. In some examples, the P desired  can be altered based on the preference of the operator. 
     In other examples still, a proportional-integral-derivative (PID) type controller can be used in replacement to, or in conjunction with, the processing unit  202 , to calculate the drift compensation flow rate (Q driftComp ). In such examples, the PID controller can calculate an error value as the difference between a measured position or pressure and a set desired position or pressure value. Once the error value is calculated, the PID controller can provide a drift compensation flow rate (Q driftComp ) based on proportional, integral, and derivative terms. When using a PID controller, the (Q driftComp ) can then be given as:
 
( Q   driftComp )= P+I+D  
 
     The proportional term (P) which can account for present measured errors (i.e. current drift values), can be given as:
 
 P=P   PropGAIN ( x   desired   −x   measured ); or
 
 P=P   PropGAIN ( P   desired   −P   measured )
 
     The integral term (I), which can account for past errors (i.e. past drift values) over time, can be given as:
 
 I=I   IntGAIN ∫ t_on   t_off ( x   desired   −x   measured ) dt ; or
 
 I=I   IntGAIN ∫ t_on   t_off ( P   desired   −P   measured ) dt  
 
where: I IntGAIN  is a constant.
 
     The derivative term(D), which can account for future errors (i.e. future drift values), with respect to time, can be given as:
 
 D=D   derivGAIN ( x   desired   −x   measured ) d ( t )/ dt ; or
 
 D=D   derivGAIN ( P   desired   −P   measured ) d ( t )/ dt  
 
where: D derivGAIN  is a constant.
 
     In some examples, P PropGAIN , I IntGAIN , and D derivGAIN  are all different predetermined values. In other examples, at least one constant can be equal to another constant. 
     In some examples, when utilizing a pressure difference, a filter can be applied to the P measured  values before using the values to calculate the error. In some examples, the filter can filter out high frequency feedback. 
     Subsequently, at step  412 , the drift compensation flow rate (Q driftComp ) is added to the flow rate (Q cmd ) required to cause movement of the hydraulic actuator  110  in response to input received from the machine&#39;s operator via a joystick or other input device (also shown in  FIG. 6 ). The resulting flow rate (Q Total ) comprises the flow rate that the control valve spool  206   b  must supply to hydraulic actuator  110  to cause movement of the machine&#39;s boom  102  or a boom segment  106  as desired by the machine&#39;s operator and to reduce or eliminate drift. Then, at step  412 , signals or data representative of the resulting flow rate (Q Total ) are communicated to control valve spool  206   b , causing the spool  206   b  to adjust and supply hydraulic fluid to the hydraulic actuator  110  at a flow rate appropriate to cause desired movement of the machine&#39;s boom  102  or boom segment  106  while also reducing or eliminating drift. After communication of the resulting flow rate and adjustment of control valve spool  206   b  such that drift is substantially reduced or eliminated, the method  402  ends at step  414 . 
     In some examples, the systems  200  and  400  can be disabled when the absolute value of the drift error is below a certain predefined threshold to prevent conflict with the damping system  124 . 
     Whereas the present invention has been described in detail above with respect to example embodiments thereof, it should be appreciated that variations and modifications might be effected within the spirit and scope of the present invention. 
     EXAMPLES 
     Illustrative examples of the system disclosed herein are provided below. An example of the system may include any one or more, and any combination of, the examples described below. 
     Example 1 
     In combination with, or independent thereof, any example disclosed herein, a system for compensating for drift of a hydraulic actuator connected to an elongate member of a machine, the drift resulting from damping of mass-induced vibration produced by movement of the elongate member, the system includes a control valve that is operable to control the delivery of hydraulic fluid to the hydraulic actuator. The system includes a plurality of sensors that are operable to measure one or more properties related to the flow of hydraulic fluid through the control valve and to output signals corresponding to measurements of the one or more properties. The system includes a processing unit that is operable to receive the output signals and to cause the control valve to adjust the flow rate of hydraulic fluid from the control valve to the hydraulic actuator by an amount to compensate for drift of the hydraulic actuator. 
     Example 2 
     In combination with, or independent thereof, any example disclosed herein, the processing unit is further operable to calculate the amount of additional volume in a chamber of the hydraulic actuator due to drift of the hydraulic actuator. 
     Example 3 
     In combination with, or independent thereof, any example disclosed herein, the processing unit is further operable to calculate a flow rate of hydraulic fluid that reduces the additional volume. 
     Example 4 
     In combination with, or independent thereof, any example disclosed herein, the processing unit is further operable to calculate a flow rate of hydraulic fluid that reduces the additional volume and supplies an amount of hydraulic fluid to the hydraulic actuator sufficient to cause the hydraulic actuator to operate in response to machine operator input. 
     Example 5 
     In combination with, or independent thereof, any example disclosed herein, at least one sensor of the plurality of sensors is embedded in the control valve. 
     Example 6 
     In combination with, or independent thereof, any example disclosed herein, the plurality of sensors comprise at least one pressure sensor operable to measure the pressure of hydraulic fluid. 
     Example 7 
     In combination with, or independent thereof, any example disclosed herein, the plurality of sensors comprise at least one flow rate sensor operable to measure the flow rate of hydraulic fluid. 
     Example 8 
     In combination with, or independent thereof, any example disclosed herein, the plurality of sensors comprise at least one spool displacement sensor operable to measure displacement of a spool of the control valve. 
     Example 9 
     In combination with, or independent thereof, any example disclosed herein, the control valve comprises a metering valve. 
     Example 10 
     In combination with, or independent thereof, any example disclosed herein, a method for compensating for drift of a hydraulic actuator operable to move an elongate member of a machine, the drift resulting from damping of mass-induced vibration produced by movement of the elongate member, the method including collecting data representative of properties of hydraulic fluid delivered by a control valve to the hydraulic actuator. The method includes calculating a flow rate of hydraulic fluid from the control valve to reduce drift of the hydraulic actuator. The method includes adjusting the control valve to deliver the calculated flow rate of hydraulic fluid to the hydraulic actuator. 
     Example 11 
     In combination with, or independent thereof, any example disclosed herein, the step of calculating includes a step of determining the volume within a load holding chamber of the hydraulic actuator resulting from drift due at least in part to damping of mass-induced vibration. 
     Example 12 
     In combination with, or independent thereof, any example disclosed herein, the step of determining includes a step of calculating the flow rate of hydraulic fluid from the control valve based at least in part on data representative of a property of the hydraulic fluid. 
     Example 13 
     In combination with, or independent thereof, any example disclosed herein, the property comprises the pressure of the hydraulic fluid supplied to the hydraulic actuator. 
     Example 14 
     In combination with, or independent thereof, any example disclosed herein, the property comprises the pressure of the hydraulic fluid supplied to the control valve. 
     Example 15 
     In combination with, or independent thereof, any example disclosed herein, the property comprises the temperature of the hydraulic fluid supplied to the control valve. 
     Example 16 
     In combination with, or independent thereof, any example disclosed herein, the property comprises the displacement of a spool of the control valve. 
     Example 17 
     In combination with, or independent thereof, any example disclosed herein, the method further includes a step of combining the calculated flow rate with a flow rate of hydraulic fluid sufficient to cause movement of the hydraulic actuator in response to input received from a machine operator. 
     Example 18 
     In combination with, or independent thereof, any example disclosed herein, a system for compensating for drift of a machine, the drift resulting from damping of mass-induced vibration produced by movement of an elongate member, the system including a hydraulic actuator connected to an elongate member and a control valve operable to control the delivery of hydraulic fluid to the hydraulic actuator. The system includes at least one sensor operable to measure one or more properties related to a position of at least one of the hydraulic actuator and elongate member. The sensor is operable to output signals corresponding to measurements of the one or more properties. The system includes a processing unit operable to receive the output signals and to cause the control valve to adjust the flow rate of hydraulic fluid from the control valve to the hydraulic actuator by an amount to compensate for drift of the hydraulic actuator. 
     Example 19 
     In combination with, or independent thereof, any example disclosed herein, the processing unit is a proportional-integral-derivative processing unit. 
     Example 20 
     In combination with, or independent thereof, any example disclosed herein, the sensor is at least one of an inclinometer, linear position sensor, angular position sensor, and gyroscope. 
     Example 21 
     In combination with, or independent thereof, any example disclosed herein, the sensor is a pressure sensor in communication with the hydraulic actuator.