Abstract:
A servomechanism includes a controller which dynamically estimates the resistance of the solenoid coil in an electrohydraulic valve as part of determining a level of electric voltage to apply to open the valve. The servomechanism receives a current setpoint designating a desired electric current level and senses the actual level of current flowing through the coil. A proportional term is derived from the current setpoint and the actual level of current. Creation of a derivative term is based on the difference between the current setpoint and the actual level of current. A feedforward term is produced by estimating the resistance of the electrohydraulic valve and limiting the feedforward term to a predefined range of acceptable values. The proportional term, derivative term, and the feedforward term are summed to define a desired voltage level, and a PWM signal for driving the electrohydraulic valve is generated based on the desired voltage level.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims benefit of U.S. Provisional Patent Application No. 60/556,115 filed Mar. 25, 2004. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable  
       BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     The present invention relates to hydraulic power systems with electrically operated control valves, and more particularly to electrical servomechanisms that control the application of electricity to such valves.  
         [0005]     2. Description of the Related Art  
         [0006]     A wide variety of machines have moveable members which are driven by an 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 has been manually operated by the machine operator.  
         [0007]     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.  
         [0008]     Application of pressurized fluid from a pump to the hydraulic actuator can be controlled by a set of proportional, solenoid operated pilot 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 pilot poppet 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, 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.  
         [0009]     When an operator desires to move a member on the machine, an input device, such as a joystick, is manipulated to produce an electrical signal that indicates the desired motion. This signal is received by a controller which responds by applying electric current to the solenoid valves connected to the hydraulic actuator associated with the machine member. To drive a cylinder-piston type hydraulic actuator, one solenoid valve is opened to supply pressurized fluid to a cylinder chamber on one side of the piston and another solenoid valve opens to drain fluid from the opposite cylinder chamber. Varying the magnitude of electric current applied to the solenoid valves alters the amount of fluid flow to the hydraulic actuator, thereby moving the machine member at proportionally different speeds.  
         [0010]     Therefore, precise control of the electric current that is applied to the solenoid valve is essential for accurate control of the machine motion. However, it is difficult to precisely control the electric current. For one thing, the resistance of the solenoid coil changes significantly with temperature, which may vary from below −20° C. to over 100° C. for hydraulic equipment used outdoors. As a result, a given voltage level applied to the valve can produce different steady state electric current levels depending upon the temperature of the solenoid coil. Another factor affecting control accuracy is the back electromotive force (emf) that is generated as the solenoid armature moves. The back emf affects the net magnitude of electric current flowing through the solenoid coil. In addition, operation of the solenoid and the valve elements tend to be non-linear which makes their modeling difficult for control purposes.  
         [0011]     Therefore, it is desirable to account for variation of the solenoid coil resistance when determining the magnitude of electric voltage to apply to open the hydraulic valve the desired amount.  
       SUMMARY OF THE INVENTION  
       [0012]     A servomechanism that operates an electrohydraulic valve includes a stage that estimates the resistance of a coil in the valve and adjusts the level of electric voltage applied to the valve in response to changes of that resistance estimate.  
         [0013]     The servomechanism receives a current setpoint which designates a desired level of electric current to be applied to a coil of the electrohydraulic valve and also senses an actual level of electric current flowing through the coil. A proportional term is produced based on a current error obtained from current setpoint and the actual level of electric current. The actual level of electric current also is used to produce a coil resistance value. The coil resistance value and the proportional term are employed to derive a desired voltage level for the coil which then is used to generate a PWM signal for driving the coil.  
         [0014]     Preferably, the coil resistance value is derived by determining an estimated resistance based on the actual level of electric current and a previous desired voltage level. A resistance error then is determined as the difference between the estimated resistance and a previous coil resistance value. The resistance error is multiplied by an observer gain value and the result is added to the previous coil resistance value to produce a new coil resistance value.  
         [0015]     In a preferred embodiment, a desired voltage level is derived by multiplying the new coil resistance value by the current setpoint and adding the product to the proportional term. A derivative term also may be added to produce the desired voltage level. That derivative term is obtained by multiplying a derivative gain value by a derivative of the current error. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a schematic diagram of a hydraulic system which employs the present invention;  
         [0017]      FIG. 2  is a block diagram of the system controller for the hydraulic system; and  
         [0018]      FIG. 3  is a control diagram depicting the operation of a current servomechanism that is implemented by the system controller. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     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 .  
         [0020]     The supply line  14  is connected to a valve assembly  20  comprising four proportional electrohydraulic (EH) 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 electricity from a system controller  16 . The first EH valve  21  governs the flow of fluid from the supply line  14  to a first conduit  30  connected to the head chamber  26  of the cylinder  28 . The second EH 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 EH valve  23  is connected between the first conduit  30  and a return line  34  that leads to the system tank  15 . The fourth EH valve  24  controls flow of fluid between the second conduit  32  and the return line  34 . Each of the four EH 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.  
         [0021]     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  34 , to provide pressure measurement signals to the system controller  16 . 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 EH valves  21 - 24 .  
         [0022]     With reference to  FIG. 2 , 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  54 . The input circuits  55  interface the user input device 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.  
         [0023]     A set of valve drivers  58  responds to the microcomputer commands by generating pulse width modulated (PWM) signals that are applied to the solenoid coils of the EH 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 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. Each valve driver  58  is associated with a current sensor  59  that produces a signal indicating the magnitude of electric current flowing to the respective EH valve  21 - 24 . A value indicating the sensed current magnitude is available for reading by the microcomputer  50 . For each EH valve  21 - 24 , the microcomputer synchronously averages an integral number of current samples acquired during each period of the PWM signal to derive a value for the actual coil current of that valve.  
         [0024]     In order to extend the rod  46  from the cylinder  28 , the operator moves the joystick  18  in the appropriate direction to indicate the desired movement to the system controller  16 . This motion of the joystick sends an electrical signal to the system controller to indicate the desired velocity for the associated machine member. The system controller  16  responds to the joystick signal by generating a current setpoint designating a desired electric current magnitude for driving the solenoid coils of selected EH valves in order to produce the motion designated by the machine operator.  
         [0025]     In the case of extending the rod  46 , the generated current setpoints activate the first and fourth EH valves  21  and  24  thereby sending pressurized hydraulic fluid from the supply line  14  through the first EH valve into the head chamber  26  of cylinder  28 . The application of that fluid causes movement of the piston  44  which extends the piston rod  46 . That piston motion also forces fluid from the rod chamber  25  through the fourth EH valve  24  to the tank  15 . The system controller  16  monitors the pressure in the various hydraulic lines to ensure that proper motion occurs.  
         [0026]     To retract the rod  46  into the cylinder  28 , the system controller  16  opens the second and third EH 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 .  
         [0027]     To obtain the commanded motion, the controller  16  executes a software program that responds to the signals from the joystick  18  and the pressure sensors by producing current setpoint values which define levels of electric current to open selected ones of the EH valves  21 - 24  the desired amounts. As noted previously, the actual degree to which a given electric voltage opens the EH valve is a function of the resistance of the valve&#39;s solenoid coil and that resistance varies with temperature and other factors. Nevertheless, the dynamically varying coil resistance is difficult to measure and has not been measured or estimated previously by the valve control circuits. However, with an ever increasing desire for greater accuracy in machine control and for more automatic control with reduced operator intervention, compensation for variation of a valve&#39;s performance due to temperature changes is highly desirable.  
         [0028]     The valve control software program includes a routine that implements a control function which adjusts the voltage setpoint (i.e. the PWM duty cycle) for each EH valve  21 - 24  to compensate for changes in the solenoid coil resistance. The current compensation function  100 , depicted for one of the valves by the control diagram in  FIG. 3 , utilizes a set of defined constants which are stored in the memory  54  of the controller  16  and are represented by boxes  102 - 112  in the upper left corner of the control diagram. A constant nominal coil resistance value  106  specifies the ideal resistance at a nominal operating temperature for the solenoid coil in the type of valve being controlled. A similar nominal resistance value  104  for the valve driver  58  also is provided. These two nominal resistance values are added together at a first summing node  114  to produce a nominal resistance value R nom .  
         [0029]     This nominal resistance value R nom  then is used to define upper and lower limits for a resistance error signal that is produced in another section of function  100 , as will be described. These limits prevent the resistance error signal from drifting beyond a range of acceptable values, and prevent small errors at very low current setpoint values from producing a significant change in the resistance estimate. Specifically, a lower resistance error limit (R error min ) is produced at the output of a first multiplier  116  which receives the nominal resistance value from summing node  114  and a constant minimum resistance error percentage value  102  (e.g. −10%). A similar upper resistance error limit (R error max ) is produced at the output of a second multiplier  118  as the product of the nominal resistance value and a constant maximum resistance error percentage  108  (e.g. +10%).  
         [0030]     Similarly, the nominal resistance value is used to define another pair of limits which specify maximum and minimum values of the resistance estimate derived by the current compensation function  100 . Specifically, the nominal resistance value R nom  is multiplied by a constant minimum percentage  110  (e.g. 67%) in multiplier  119  to produce the minimum coil resistance value R min . A similar derivation of the upper resistance limit R max  is produced at the output of a fourth multiplier  120  that combines a constant maximum percentage  112  (e.g. 166%) with the nominal resistance value R nom . These defined limits are employed elsewhere by the control function  100 , as will be described.  
         [0031]     The current compensation function  100  comprises a first processing stage  121  that produces a proportional term, a second processing stage  122  that produces an feedforward term, and a third processing stage  123  that produces a derivative term. The current compensation function  100  receives the current setpoint, which specifies the level of electric current to open the associated hydraulic valve the amount necessary to create the fluid flow needed to produce the desired machine motion. A value corresponding to the actual current flowing through the valve, as indicated by the respective current sensor  59 , is subtracted from the current setpoint to produce an current error value I error  at a second summing node  124  in the first processing stage  121 . The current error value then is multiplied by a constant proportional gain  126  at fifth multiplier  128  to produce a proportional term that is applied to one input of a third summing node  130 .  
         [0032]     In the second processing stage  122 , the actual coil current value also is applied to the divisor input of a first divider  132 , which has a dividend input that receives a previous value of a desired voltage level for the PWM signal. That previous value was produced during the prior computation cycle of the compensation function  100 , as will be described, and corresponds to the present voltage applied across the solenoid coil of the respective valve. The resultant value produced at the output of the first divider  132  is an estimate of the resistance, R est . This resistance estimate is applied to one input of a fourth summing node  134  that subtracts a previous resistance value R previous , that was produced during the previous computation cycle and fed back to the fourth summing node. The output of the fourth summing node  134  is a resistance error R error  corresponding to the difference between the newly estimated resistance and the previous resistance value. The resistance error R error  then is limited by a first limiter  136  to the range of values as set by the upper and lower error limits R error max  and R error min . The output of the first limiter  136  is multiplied by a constant observer gain  140  at a sixth multiplier  138  and the product is applied to a fifth summing node  142  where it is added to the previous resistance value R previous . The result is restricted by a second limiter  144  to values between the upper and lower resistance limits R max  and R min , thereby producing a resistance value R.  
         [0033]     The resistance value R then is processed by a first unit delay  145  to produce the previous resistance value R previous  that is fed back to the fourth and fifth summing nodes  134  and  142  during the next computation cycle of the compensation function  100 . A seventh multiplier  146  multiplies the resistance value R by the current setpoint to produce a feedforward term that is applied to another input of the third summing node  130 .  
         [0034]     The third summing node  130  also receives a derivative term produced by the third processing stage  123 . In that third processing stage  123 , a second unit delay  152  provides an output value that corresponds to the previous current error from the prior computation cycle of the compensation function  100 . The previous current error is subtracted from the present current error I error  at a sixth summing node  150  to produce a current error difference. The current error difference then is multiplied at an eighth multiplier  154  by a constant derivative gain  156  to produce the derivative term that is applied to the third summing node  130 . The output of the third summing node  130  is a desired voltage level for driving the valve&#39;s solenoid coil.  
         [0035]     That desired voltage level from the third summing node  130  is applied to one input of a minimum value selector circuit  157  which has another input that receives a battery terminal voltage value. The battery terminal voltage is measured and low pass filtered by the controller at process block  162  to produce the value that is applied to the minimum value selector circuit  157 . The lesser value of the battery terminal voltage and the desired voltage level from the minimum value selector circuit  157  is processed by a third unit delay  158  to produce the previous selected value which is fed back to the dividend input of the first divider  132 .  
         [0036]     The desired voltage level from the third summing node  130  also is translated into a fraction of the voltage level available from the battery or other electrical power source of the machine. To do so, the desired voltage value is applied to the dividend input of a second divider  160 , the output of which is multiplied in a ninth multiplier  164  by a value corresponding to the 100 percent duty cycle of the PWM signal for driving the valve&#39;s solenoid coil. This produces a product that defines a percentage of the full PWM duty cycle which is adjusted by an offset value  166  in the seventh summing node  168  to produce a value defining the duty cycle for the PWM valve driver  58 . The duty cycle value is limited at a third limiter  170  to a range between 0% and 100%. This limiting is necessary as the desired voltage level may exceed the maximum voltage available across the battery terminals or other power supply and thus otherwise could be unattainable. The limited PWM output value indicates the duty cycle for the pulse width modulation circuit in the valve drivers  58  and is sent to those drivers.  
         [0037]     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.