Abstract:
A method for operating an electrohydraulic valve initially derives a characterization value that denotes how magnetic hysteresis affects valve operation. Upon receiving a command that designates a desired magnitude of electric current to be applied to the electrohydraulic valve, that command is modified based on the characterization value to compensate for the magnetic hysteresis. The modified command then is employed to apply electric current to the electrohydraulic valve.

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 power systems with electrically operated control valves, and more particularly to electrical circuits that control the application of electricity to such valves. 
     2. Description of the Related Art 
     A wide variety of machines have movable members which are driven by a hydraulic actuator, such as a cylinder and piston arrangement, that is controlled by a hydraulic valve. For example, backhoes have a tractor on which is mounted a boom, arm and bucket assembly with each of those components being driven by one of more cylinder-piston arrangements. The flow of fluid to and from each hydraulic actuator is controlled by a hydraulic valve that traditionally was manually operated by the machine operator. 
     There is a present trend away from manually operated hydraulic valves toward electrical controls and the use of solenoid valves. This type of control simplifies the hydraulic plumbing, as the control valves do not have to be located near an operator station, but can be located adjacent the hydraulic actuator being driven by the fluid. This change in technology also facilitates computerized control of the machine functions. 
     Application of pressurized fluid from a pump to the hydraulic actuator is controlled by a set of electrohydraulic proportional pilot-operated valves. These valves employ a solenoid coil which generates a magnetic field that moves an armature in one direction to open a valve. The armature acts on a valve element which opens and closes a pilot passage that in turn causes a main valve poppet to move with respect to a primary valve seat located between the inlet and outlet of the valve. The amount that the valve opens is directly related to the magnitude of electric current applied to the solenoid coil, the electric current produces a variable magnetic field that moves the armature to open the pilot poppet to varying degrees, thereby enabling proportional control of the hydraulic fluid flow. Either the armature or another component is spring loaded to close the valve when electric current is removed from the solenoid coil. 
     Magnetic hysteresis is the retention of magnetism induced in ferromagnetic materials and affects the operation of the valve as the applied electric current changes. For example, as the electric current decreases to close the valve the residual magnetism tends to keep the valve open slowing the response of the valve to the change in the electric current level. This phenomenon causes a difference between the flow of fluid through the valve that is desired and the actual flow. 
     Precise control of the electric current that is applied to the solenoid valve is essential for accurate control of the machine motion. However, the magnetic hysteresis adversely affects the precision of that control. 
     SUMMARY OF THE INVENTION 
     A control circuit alters the level of electric current applied to operate an electrohydraulic valve so as to compensate for the effects of magnetic hysteresis on valve operation. 
     The control circuit implements a method that determines an amount of magnetic hysteresis affecting operation of the electrohydraulic valve. Thereafter when a command is produced that designates a desired magnitude of electric current to be applied to the electrohydraulic valve, the command is adjusted for the effects of the magnetic hysteresis to produce a compensated command. Electric current then is applied to the electrohydraulic valve in response to the compensated command. 
     In a preferred embodiment of the control method, the amount of magnetic hysteresis is determined by varying the magnitude of electric current while sensing a parameter that indicates an amount that the electromagnetically operated valve is open. That parameter could be the position of a valve element, position of a solenoid that operates the valve, or a force in the valve, for example, A first set of data is produced indicating a relationship between the magnitude of electric current and the position of the valve while opening, and a second set of data is produced indicating that relationship while that valve is closing. Additional sets of data are acquired by opening and closing the valve to different positions. The acquired sets of opening and closing data are analyzed to derive a value that characterizes the magnetic hysteresis of the electrohydraulic valve. 
     In a preferred embodiment, the electric current command is adjusted during valve closure by reducing the desired magnitude of electric current so that the valve has similar responses during opening and closing. The adjustment of the electric current command involves calculating a difference between the desired magnitude of electric current designated by that command and the magnitude of electric current designated by a previous electric current command. That difference is multiplied by the previously derived magnetic hysteresis characterization value. The product of that multiplication is added to a previous compensation value to produce a new compensation value that is employed to adjust the current command. The process also may include limiting the new compensation value to a predefined range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a hydraulic system that incorporates the present invention for operating valves that control a hydraulic actuator; 
         FIG. 2  is a graph of the relationship between electric current applied to operate a valve and the position of the valve during opening and closing; 
         FIG. 3  graphically illustrates a step in the process for characterizing magnetic hysteresis of a valve; and 
         FIG. 4  is a control diagram depicting a magnetic hysteresis compensation algorithm employed by the system controller to operate a valve in the hydraulic system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With initial reference to  FIG. 1 , a machine such as an agricultural or construction vehicle has mechanical members that are operated by a hydraulic system. The hydraulic system  10  includes a variable displacement pump  12  that is driven by a motor or engine (not shown) to draw hydraulic fluid from a tank  15  and furnish the hydraulic fluid under pressure into a supply line  14 . 
     The supply line  14  is connected to a valve assembly  20  comprising four electrohydraulic proportional (EHP) valves  21 ,  22 ,  23  and  24 , that control the flow of hydraulic fluid to and from a hydraulic actuator, such as cylinder  28 , in response to electrical signals from a system controller  16 . The first EHP valve  21  governs the flow of fluid from the supply line  14  to a first conduit  34  connected to the head chamber  26  of the cylinder  28 . The second EHP valve  22  selectively couples the supply line  14  to a second conduit  32  which leads to the rod chamber  25  of the cylinder  28 . The third EHP valve  23  is connected between the first conduit  34  and a return line  30  to the system tank  15 . The fourth EHP valve  24  controls flow of fluid between the second conduit  32  and the return line  30 . Each of the four EHP valves  21 - 24  may be a pilot operated valve that is driven by a solenoid, such as the valve described in U.S. Pat. No. 6,328,275, for example. The flow of fluid through this type of valve is proportionally controlled by varying the magnitude of electric current applied to the coil of the solenoid. 
     The valve assembly  20  and the cylinder  28  form a hydraulic function  35  for operating a component of the machine. Additional hydraulic functions can be connected to the supply and return lines  14  and  30  and operated by the system controller  16 . 
     The system controller  16  receives signals from a user input device, such as joystick  18  or the like, and from a number of pressure sensors. One pair of pressure sensors  36  and  38  detect the pressure within the cylinder rod and head chambers  25  and  26 , respectively. Another pressure sensor  40  is placed in the supply line  14  near the outlet of the pump  12 , while pressure senor  42  is located in the tank return line  30 , to provide pressure measurement signals. The system controller  16  executes a software program that responds to these input signals by producing output signals which control the variable displacement pump  12  and the four EHP valves  21 - 24 . 
     With continuing reference to  FIG. 1 , the system controller  16  includes a microcomputer  50  which is connected by a conventional set of signal busses  52  to a memory  54  in which the software programs and data used by the microcomputer are stored. The set of signal busses  52  also connects input circuits  55  and output circuits  56  to the microcomputer  50 . The input circuits  55  interface the joystick  18  and the pressure sensors to the system controller and the output circuits  56  provide signals to devices that indicate the status of the hydraulic system  10  and the functions being controlled. 
     A set of valve drivers  58  in the system controller  16  responds to commands from the microcomputer by generating pulse width modulated (PWM) signals that are applied to the solenoid coils of the EHP valves  21 - 24 . Each PWM signal is generated in a conventional manner by switching a DC voltage at a given frequency. When the hydraulic system is on a vehicle, such as an agricultural tractor, the DC voltage is supplied from a battery and an alternator. By controlling the duty cycle of the PWM signal, the magnitude of electric current applied to the solenoid coil of a given valve can be varied, thus altering the degree to which that valve opens. 
     In order to extend the rod  46  from the cylinder  28 , the operator moves the joystick  18  in the appropriate direction to send an electrical signal to the system controller that indicates the desired velocity for the associated machine member. The system controller  16  responds to the joystick signal by generating electric current commands designating electric current magnitudes for driving the solenoid coils of selected EHP valves in order to produce the motion indicated by the machine operator. 
     If the operator desires to extend the rod  46  from the cylinder  28 , the generated electric current commands activate the first and fourth EHP valves  21  and  24 . Opening the first valve  21  sends pressurized hydraulic fluid from the supply line  14  through the into the head chamber  26  of cylinder  28  and the fluid from the rod chamber  25  flows through the fourth EHP valve  24  to the tank  15 . The system controller  16  monitors the pressure in the various hydraulic lines to ensure that proper motion occurs. To retract the rod  46  into the cylinder  28 , the system controller  16  opens the second and third EHP valves  22  and  23 , which sends pressurized hydraulic fluid from the supply line  14  into the cylinder&#39;s rod chamber  25  and exhausts fluid from the head chamber  26  to tank  15 . 
     Typical control of the machine involves the human operator manipulating the joystick  18  to extend and retract the piston rod  46  with respect to the cylinder  28  which produces bidirectional motion of the machine components connected to the piston rod. Thus, the hydraulic valves in assembly  20  are opened and closed to various degrees by correspondingly varying the electric currents applied to those valves. The response of a given hydraulic valve to changes in the electric current applied to its solenoid coil is affected by magnetic hysteresis caused by the residual magnetism of the ferromagnetic materials in the valve. For example, while electric current applied to a valve increases as represented by curve  60  in  FIG. 2 , the position of the valve, or more precisely a flow control element (a poppet or spool) within the valve, changes until reaching a fully open position at a maximum electric current level (I MAX ). When the valve then is closed by reducing the electric current, the position of the valve changes according to a second curve  62 . Because of the magnetic hysteresis the electric current to valve position relationship is different during opening and closing the valve. Note that the valve reaches a given position at a lower electric current level while closing than when the valve was opening. The two curves  60  and  62  depict a conventional hysteresis function. 
     If the valve is only partially opened before the operator commands closure, a slightly different hysteresis function occurs. For example, if the valve is opened to an intermediate position indicated by point  64  in  FIG. 2  and then commanded to close, the relationship of the closure electric current to valve position follows the dashed line  66 . As a consequence, there is not a fixed relationship between the magnitude of the electric current applied to the solenoid coil and the position of the valve, as well as the amount of fluid flow through the valve. The present invention compensates the electric current command sent to the valve drivers  58  in order to account for the magnetic hysteresis and thus more precisely control the position of the valve and the fluid flow there through. 
     The present compensation technique accounts for the amount that the closing curve  62  differs from the opening curve  60 . Specifically, when the valve is closing the command from the microcomputer  50  designating the amount of electric current to be applied to a given valve, is adjusted by subtracting a compensation factor. For example, as graphically shown in  FIG. 2 , a command designating an electric current level A opens the valve to a position at point  67  when the valve is opening, but the same electric current command results in a different valve position at point  68  when the valve closes. As a result, in order that the command designating electric current level A places the valve into the same position during opening and closing, the current command during closure must be adjusted to designate a lower electric current level B, as designated at point  69 . Thus, the difference between electric current levels A and B (e.g. 30 ma) is defined as the magnetic hysteresis for the full cycle of the valve and at that point must be subtracted from the electric current command during closure to compensate for the magnetic hysteresis. 
     However, that current level difference is not constant during the entire closure process. Note that during the initial part of the motion from the fully open position, for example a point  61 , a smaller current level difference is present than when the valve has closed farther such as at points  67  and  69 . This initial part of the motion also shifts depending upon the position to which the valve is opened before closure commences. For example, if the valve is opened only to point  64  in  FIG. 2 , the closure produces a resultant relationship between electric current and valve position designated by the dashed line  66  which deviates from the closing curve  62  that occurs during valve closure from the full open position. Therefore, in order to accurately compensate for magnetic hysteresis, this variation must be taken into account. 
     As a consequence, the magnetic hysteresis compensation technique employs several variables defining the operating characteristic of a particular valve or particular valve model. Although, it is desirable for optimum compensation to characterize the operation of each specific electrical operator, significant compensation can be achieved by classifying the characteristics of a particular design of the valve and its electrical operator (e.g. a solenoid) which then are used for all valves of that type. The characterization process involves operating the valve in a cycle between open and closed position. This is accomplished by increasing the level of electric current applied to the valve from zero to a level at which the valve is fully open, and then decreasing the current until returning to the fully closed position. At various increments during this electric current cycle, the position of the valve is measured to provide data similar to that denoted by curves  60  and  62  in  FIG. 2 . The position of the valve can be measured directly or indirectly by measuring a related parameter, such as the position of the solenoid. Then, a similar set of small current cycles are performed by opening the valve to less than fully open, for example, 0% to 20% of full open, 0% to 40%, 20% to 60%, etc. The resultant data compiled by the small cycles is then compared to the data from the full valve cycle. The rate at which the small cycles data approaches the full cycles data is calculated. 
     Specifically, the magnetic hysteresis characterization determines the amount that the closing curves (e.g.  62  and  66 ) deviate from the opening curve  60 . Therefore, data points defining the opening curve  60  are considered to have a zero percent error, whereas the data points on the closing curve  62  are considered as a 100 percent error. Similarly an error percentage is calculated for the data from a partially opened valve, that is the percentage the each data point of the small valve operating cycle deviates from the full cycle.  FIG. 3  is an exemplary graph of such error percentages. The percent error data are examined to determine the rate at which it makes the transition from point  64  to point  65  where the small cycle curve  66  joins the full cycle closing curve  62 . As seen from the plot of the exemplary data, the small cycle data approaches the full cycle data (100% error) at a rate of 0.3% per milliamp. This small cycle transition gain (0.3% per milliamp) is multiplied by the magnetic hysteresis for the full cycle (e.g. 30 ma) to produce a value (e.g. 9% or 0.09) for a variable designated rHYSTERESIS which characterizes the magnetic hysteresis of this particular valve. 
     The magnetic hysteresis characterization variable rHYSTERESIS is used by the electric current command compensation algorithm that is independently executed by the microcomputer  50  for each of the valves  21 - 24  in assembly  20 . The compensation algorithm  70  depicted in  FIG. 4  commences upon the receipt of a new electric current command (I CMD ) which is produced by the microcomputer  50  in response to the signal from joystick  18 . The electric current command is produced by any conventional technique, such as the one described in U.S. Pat. No. 6,775,974, for example. The new electric current command is stored temporarily, as denoted by function  72  that has an output at which the value of the previous electric current command (I CMD OLD ) is provided. The previous electric current command is subtracted from the new electric current command (I CMD ) at a first function  74  to produce the difference, designated by an intermediate value ΔI CMD . The intermediate value, or command difference, ΔI CMD  then is multiplied at a second function  76  by the magnetic hysteresis characterization value rHYSTERESIS, which for the exemplary system was determined to be 0.09. The resultant product is added to the previous magnetic hysteresis compensation value IHYSTERESIS OLD  at summation function  78  to produce a preliminary compensation factor (I COMP ). 
     In the exemplary hydraulic system, magnetic hysteresis compensation is active only when the associated valve is closing so that the valve position to electric current relationship during closure will be similar to that when the value is opening. Therefore, by definition the hysteresis compensation value IHYSTERESIS must be zero while the electric current command difference ΔI CMD  is positive, as occurs during valve opening. In addition, the hysteresis compensation value may not exceed a level equal to or slightly smaller than the magnitude of the full cycle magnetic hysteresis (e.g. 30 ma), as that corresponds to the maximum amount of hysteresis requiring compensation. These minimum and maximum compensation limits are respectively defined by two variables IHYSTERESIS MIN  and IHYSTERESIS MAX , stored in the memory  54  of the system controller  16  to define the range of values that may be subtracted from the current command during valve closure. For the exemplary hydraulic system, IHYSTERESIS MIN  equals −30 ma and IHYSTERESIS MAX  equals 0.0 ma. 
     Limiting the magnetic hysteresis compensation value to this range of values is achieved by applying the preliminary compensation factor (I COMP ) to a first limit function  80  which restricts the compensation value IHYSTERESIS to a negative number that is no more negative than the maximum amount that the full sweep hysteresis curves  60  and  62  deviate from each other. The first limit function  80  for the exemplary hydraulic system restricts the magnetic hysteresis compensation value IHYSTERESIS to between −30 ma and 0.0 ma. Thus when the valve is opening and the preliminary compensation factor (I COMP ) is positive (the commanded current is increasing), the value of IHYSTERESIS at the output of the first limit function  80  will be zero. It is only upon valve closure that the magnetic hysteresis compensation value IHYSTERESIS has a non-zero value and that value may not adjust the current command more than the full cycle magnetic hysteresis. 
     The magnetic hysteresis compensation value IHYSTERESIS is applied to an output summation function  82  where it is combined with the present electric current command I CMD . Because IHYSTERESIS has a negative number during valve closure, the output summation function  82  reduces the current command (I CMD ) by the amount of the compensation value to produce the compensated electric current command (I CMD COMP ). The compensated electric current command is transmitted to the valve driver  58  associated with the particular valve and used to control the duty cycle of the PWM signal that drives that valve. 
     The new value of the magnetic hysteresis compensation value IHYSTERESIS also is stored temporarily in the memory of the system controller  16  as denoted by function  84 , to provide the previous compensation value IHYSTERESIS OLD  each time the compensation algorithm is executed. That previous compensation value is fed back and added at summation function  78  to the produce a preliminary compensation factor (I COMP ). This loop provides an accumulation of the error due to the hysteresis. A second limit function  86  sets the previous compensation value to zero, if the incoming electric current command (I CMD ) is zero thereby clearing the accumulated hysteresis error for the next operation of the valve. 
     In the exemplary hydraulic system, the magnetic hysteresis compensation was employed during valve closure by subtracting a compensation value IHYSTERESIS from the electric current command (I CMD ) so that the electric current to valve position responses are similar during opening and closing. However, the magnetic hysteresis compensation could have been applied during valve opening by adding a hysteresis compensation value to the electric current command to adjust the valve response while opening to approximate the response that occurs during closing. 
     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.