Abstract:
Operation of an electrically operated valve is calibrated by applying a gradually increasing electric current to the valve. While that is occurring pressure at either the inlet or outlet of the valve is measured to detect when the valve opens. When the valve opens the level of the electric current then being applied to the valve is employed to determine an initial current level to use subsequently whenever the valve is to be opened.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to pilot operated proportional hydraulic valves which are electrically controlled, and particularly to calibrating the control of such valves. 
     The application of hydraulic fluid to an actuator, such as a cylinder and piston arrangement, can be controlled by a set of solenoid operated pilot valves. A pump supplies hydraulic fluid under pressure to an electro-hydraulic valve (EHV) assembly, such as the one described in U.S. Pat. No. 5,878,647. The EHV assembly includes a fluid distribution block on which four solenoid valves are mounted to control the flow of fluid to and from chambers of a hydraulic cylinder connected to the fluid distribution block. A first pair of the solenoid valves governs the fluid flow to and from the piston chamber of the cylinder, and a second pair of the solenoid valves controls the fluid flow to and from the rod chamber. By sending pressurized fluid into one cylinder chamber and draining fluid from the other chamber, the piston can be moved in one of two directions. The rate of flow into a chamber of the cylinder is varied by controlling the degree to which the associated supply valve is opened, which results in the piston moving at proportionally different speeds. 
     Solenoid operated pilot valves are well known for controlling the flow of hydraulic fluid and employ an electromagnetic coil which moves an armature in one direction to open a valve. The armature acts on a pilot poppet that controls the flow of fluid through a pilot passage in a main valve poppet. The amount that the valve opens is directly related to the magnitude of electric current applied to the electromagnetic coil, thereby enabling proportional control of the hydraulic fluid flow. A spring acts on the armature to close the valve when electric current is removed from the solenoid coil. An example of a solenoid operated pilot valve of this type is described in the aforementioned U.S. Patent. 
     Such proportional solenoid valves usually have a spring preload force that acts on the pilot poppet. As a consequence a substantial current level is required to produce an electromagnetic force that overcomes the spring force and produces opening movement of the pilot poppet. If the control circuit commences applying current to the valve from zero when the operator first moves a manual control device, that device must be moved a certain amount before sufficient current is applied to the electromagnetic coil to open the valve. This produces a dead band of wasted motion of the manual control device. 
     To overcome this dead band problem, control circuits have been designed to apply a predefined current level above zero upon initial movement of the control device. In other words as shown in FIG. 1, the current applied to the electromagnetic coil jumps from zero to that predefined initial current level I INT  when the operator initially moves the control device from the off position. The predefined initial current level is set to produce a force on the armature of the solenoid that is slightly less than the spring preload force. Thus the valve does not open immediately when the control device is moved from the off position. As the control device continues to be moved the coil current increases causing pilot valve to open thereby producing a small flow through the valve. Eventually the coil current increases to a level I O  at which the main valve poppet opens. This operation virtually eliminates the dead band of wasted operator motion. The difference between the initial current level I INT  and the current level I O  at which the main valve poppet opens is referred to an the “margin”. 
     A problem in this operation arises due to relaxation of the spring preload force with age which results in the valve opening at a significantly lesser force produced by the electromagnetic coil, thus decreasing the margin. Such relaxation can result from fatigue of the valve spring, deformation of the pilot poppet-seat interface, or deformation of the main poppet-seat interface. In pressure compensated solenoid valves, changes in the compensation mechanism with age also produces relaxation of the spring preload force. When significant relaxation occurs, the valve may jump from a closed position to a substantial flow position when the initial current level is applied to the valve. This inhibits control at low flow rates. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for calibrating control of a fluid valve having an inlet, an outlet and an electrically operated actuator. When the fluid valve is to be opened, a predefined initial level of electric current is applied initially to the electrically operated actuator. The calibration involves applying pressurized fluid to the inlet of the electrically operated valve and applying an electric current at varying levels to the electrically operated actuator. The pressure at one of the inlet and the outlet is measured, thereby producing a pressure measurement which is employed to determine when the fluid valve opens. For example, opening of the valve is indicated when the rate of change of the measured pressure changes more than a given amount. 
     A difference between the electric current level which was being applied when the fluid valve opened and the predefined initial level then is calculated. The predefined initial level is changed in response to that difference. In the preferred embodiment of the invention, the predefined initial level is set to a fixed amount less than the level of the electric current which was being applied when the fluid valve opened. This calibration ensures that the initial level of current applied to open the valve will be a desired amount less that the current level at with the valve begins to open. Thus uniform operation of the valve occurs, even as the valve ages. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing the relationship between electric current applied to a proportional solenoid valve and fluid flow; 
     FIG. 2 is schematic diagram of a hydraulic system that incorporates the present invention; and 
     FIG. 3 is a flowchart of a software routine that is executed by a controller to recalibrate electrical operation of the proportional solenoid valve. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 2, electro-hydraulic valves are utilized in a hydraulic system  10  to control bidirectional movement of an actuator  11 . The actuator  11  may comprise a piston  12  within a cylinder  13  thereby defining a piston chamber  14  and a rod chamber  15  on opposite sides of the piston. Application of pressurized fluid to one or the other of those chambers  14  or  15  produces movement of the piston  12  within the cylinder. Such pressurized fluid is produced by a variable displacement pump  16  having an output connected to pump supply line  18 . 
     The pump supply line  18  is coupled to the cylinder chambers  14  and  15  by a pair of inlet valves  20  and  22 . Each inlet valve  20  and  22  is a solenoid operated, proportional valve and preferably has a pilot poppet, such as the type described in U.S. Pat. No. 5,878,647, the description of which is incorporated herein by reference. The output of the first inlet valve  20  is applied to the piston chamber  14  of the actuator  11 . Similarly, the output of the second inlet valve  22  is applied to the rod chamber  15  of the actuator  11 . 
     The variable displacement pump  16  is controlled by a signal at a control input  24 . This signal is produced in response to the greatest load pressure from the cylinder chambers  14  and  15 . For that purpose, each of the chambers  14  and  15  is connected by a separate check valve  26  and  27 , respectively, to a load sense line  28 , which at any given point in time carries a pressure signal corresponding to the greatest pressure in those cylinder chambers. That pressure signal is applied to a load sense circuit  30  that responds by producing the control signal at the control input  24  of the variable displacement pump  16 . Alternatively, the check valve  26  and  27  and the load sense line  28  can be replaced by an electrical load sensing mechanism. 
     A first pressure sensor  31  is connected to the pump supply line  18  and provides a signal indicating the pressure in that line to a controller  25 . The supply line from the inlet valves  20  and  22  to the cylinder chambers  14  and  15  also have separate pressure sensors  32  and  33 , which send signals to the controller  25 . Pressure sensors  32  and  33  provide input signals that respectively indicate the pressures in the piston and rod chambers  14  and  15 . 
     The chambers  14  and  15  of actuator  11  are connected by third and fourth outlet valves  34  and  36  to a fluid reservoir, or tank  38 , for the hydraulic system  10 . Each outlet valve  34  and  36  is a solenoid operated, proportional valve of the same type as the inlet valves  20  and  22 . 
     All the inlet and outlet valves are controlled by electrical signals from the controller  25  that are produced in response to the operator activating a manual control device, such as joystick  45 . Depending upon the amount to which the operator moves the joystick  45 , the controller  25  varies the magnitude of current applied to the respective valves which determines the degree to which the valve opens and thus the rate of fluid flow through the valves. The controller  25  is a microcomputer based device that executes a software program which governs the operation of the hydraulic system  10 . 
     A fourth pressure sensor  40  provides an input signal to the controller  25  which indicates the pressure in a line  42  leading from the first and second outlet valves  34  and  36  to the fluid reservoir  38 . 
     Periodically, the controller  25  calibrates the inlet and outlet valves  20 ,  22 ,  34  and  36  to ensure that the margin between the initial coil current and the current level at which the each valve opens remains at the desired value. Prior to initiating the calibration procedure, the operator places the member of the machine, which is controlled by the actuator  11 , into a non-load bearing position. On a lift truck for example, the mast would be lowered completely in order to calibrate the hydraulic valves for the mast actuator. 
     With the actuator  11  in the non-load bearing position, the operator activates a calibration switch  44  which sends a signal to the controller  25 . In response to that calibration signal, the controller commences executing a software routine which implements the calibration procedure  50  depicted in FIG.  3 . Calibration also can be activated automatically upon equipment shutdown when the actuators typically are placed into a non-load bearing position. 
     At the first step  52  of the calibration procedure  50 , the controller  25  opens the outlet valves  34  and  36  for a predefined interval of time. That interval has a sufficient duration so that any fluid pressure trapped within the chambers  14  and  15  of the actuator  11  will be released by draining the hydraulic fluid to the system tank  38 . The software execution then advances to step  54  where the controller  25  issues a command to the load sense circuit  30  to raise the output pressure of pump  16  to a predefined level. Then the electric current I C  that is applied by the controller  25  to the electromagnetic coil of the first input valve  20  is set to the first current level at step  56 . The first current level is less than the initial current level I INT  in the graph of FIG.  1 . 
     Referring again to FIGS. 2 and 3, the input pressure to the associated chamber  14  of the actuator  11  then is measured by the controller  25  reading the output signal from the pressure sensor  32  at step  58 . At step  60  if there was a previous pressure measurement, the two measurements are utilized to calculate the rate of rise in pressure in the cylinder chamber  14 . Because the pressure is measured at fixed time intervals, that rate of rise can be determined merely by calculating the difference between the most recent pressure measurement and the previous pressure measurement. The controller  25  then determines at step  62 , whether the rate of pressure rise exceeds a given threshold amount which indicates that the main poppet of the first inlet valve  20  has opened. If that threshold has not been exceeded, indicating that the first inlet valve  20  remains closed, the program execution branches to step  64 , where the coil current I C  applied to the first inlet valve  20  is increased by a fixed amount. If the desired current margin between levels I INT  and I O  in FIG. 1 is 0.1 amps, for example, the coil current may be increased by 0.01 amps. That new current level that is applied to the electromagnetic coil of the first input valve  20  and steps  58 - 64  are repeated until the rate of pressure rise exceeds a predefined threshold value X at step  62 . 
     When this occurs, the existing margin is calculated by the controller at step  66 . Specifically, the margin is the coil current level I O  at which the valve opened minus the level of the initial current I INT . Then a determination is made at step  68  whether the existing margin differs from the desired margin by more than a given amount Y. This indicates that the actual margin has decreased significantly below the desired margin value. If such a decrease has occurred, the program execution advances to step  70  where the initial current level I INT  is set equal to the present current level I C , at which the valve opened, minus the desired margin. This new value for the initial current level I INT  is stored in the memory of the controller  25 , thereby recalibrating the operation for this first input valve  20 . 
     A determination then is made at step  72  whether there is an additional inlet valve (e.g.  22 ) to calibrate. If so, that valve is selected and the process returns to step  56  where the process repeats for that other valve. When all of the valves have been calibrated the procedure  50  terminates. 
     A similar procedure can be utilized to calibrate the outlet valves  34  and  36 . In this case, the inlet valves  20  and  22  are both opened and so as to apply pressure from the pump  18  through the chambers  14  and  15  of the actuator  11  to the inlets of both outlet valves  34  and  36 . The inlet valves  20  and  22  are then closed to trap the pressure in the cylinder chambers. Next, the controller  25  applies current to the electromagnetic coil of the selected outlet valve and gradually increases that current while monitoring the pressure in the corresponding chamber  14  or  15  of the actuator  11 . That pressure is indicated by the pressure sensor  32  or  33  associated with that cylinder chamber. 
     When the selected output valve  34  or  36  opens the associated pressure drops significantly. When that occurs the current I C  that is being applied to the electromagnetic coil of the valve corresponds to the current level I O  at which the valve opens. That current level I C  along with the initial current I INT  for the outlet valve then are used as previously described to determine whether the current margin should be reset.