Proportional solenoid valve opening control system

Abstract

This proportional solenoid valve opening control system is for making it possible to obtain a constant flow rate characteristic against an increase in valve differential pressure by correcting the position of a movable iron core by electrical current correction control, and comprises a current control circuit for controlling a current supplied to a solenoid through a switching element that is ON and OFF-controlled by a PWM signal. The current control circuit further includes: an amplitude detection unit that detects an amplitude value of a sawtooth-shaped current waveform formed by a current rise during the ON period and a current drop during the OFF period in one pulse period, from a detected current value of the solenoid; and a current correction amount calculation unit that calculates a correction current amount corresponding to a current amount for offsetting a displacement amount between the current position of the movable iron core and a movable iron core position at which a target command flow rate is obtained, on the basis of the detected amplitude value, and adds the correction current amount to a command signal.

Description

Proportional solenoid valve opening control system

[0001] The present invention relates to a proportional solenoid valve opening control system that performs current correction control based on the movable core position estimated from the detected current amplitude of the solenoid, in order to improve the flow rate reduction characteristics of the proportional solenoid valve that accompany changes in the valve differential pressure.

[0002] In a typical hydraulic electromagnetic valve (solenoid valve), when the solenoid coil is not energized, the valve spool is positioned in a closed position by the biasing force of a spring. When the solenoid coil is energized, the electromagnetic attraction force generated attracts the movable iron core to the fixed iron core, thereby displacing the valve spool in the valve opening direction.

[0003] In particular, proportional solenoid valves attract a moving iron core with electromagnetic force proportional to the current flowing through the solenoid coil, which in turn displaces the valve spool against the spring force, thereby achieving a predetermined valve opening (opening area) and the desired flow rate of working fluid. Therefore, in proportional solenoid valves, the position of the moving iron core and valve spool is controlled to achieve a desired valve opening by controlling the current to the solenoid. In proportional solenoids, a thrust proportional to the current flowing through the coil is obtained as the output of the solenoid, but in reality, the position of the moving iron core is controlled by balancing the thrust of the solenoid with the resistance of the spring, which acts as a load.

[0004] As described above, proportional solenoid valves can arbitrarily control the valve opening between closed and fully open by controlling the current flowing through the solenoid, and can adjust the flow rate as desired, making them effective for highly accurate speed control in that they allow continuous flow rate adjustment that is not possible with ordinary on / off solenoid valves that switch the valve between two states, open and closed.For this reason, proportional solenoid valves are widely used to control a variety of hydraulic drive systems in various construction and civil engineering machinery, including heavy machinery such as cranes and excavators, agricultural machinery, and fishing machinery such as machines for hoisting fishing nets.

[0005] Furthermore, current control for proportional solenoids generally uses PWM (Pulse Width Modulation) control, which controls the ratio of ON time to OFF time within one cycle, i.e., modulates the pulse width, to obtain an output current with a desired waveform.

[0006] Pulse width modulation is performed by controlling the timing of ON-OFF switching of current in a switching element. A transistor such as an FET (field effect transistor) is used as the switching element. Therefore, in reality, a pulse-width-modulated square wave voltage is applied to the solenoid coil, but the current rises during the ON period according to the square wave and falls during the OFF period, so the control current to the solenoid flows as a sawtooth current waveform.

[0007] In essence, the duty ratio can be increased by increasing the pulse width of the ON time in one cycle, and decreased by decreasing the pulse width of the ON time. By adjusting this ON-OFF ratio, any output between 0% and 100% duty ratio can be obtained.

[0008] 9, in a proportional solenoid valve, a command current value Oc is converted into a digital signal by an analog / digital converter 5, and a PWM converter 51 modulates the pulse width of a command signal Os in a current control circuit 50 to generate a predetermined PWM signal Ps according to the level of the command signal Os, and outputs the PWM signal Ps as a switching control signal. Then, a switching element 3 (FET) interposed between the solenoid 2 and the power source is controlled to switch ON / OFF in accordance with this PWM signal Ps, thereby obtaining a desired current waveform and supplying a control current corresponding to the command current value Oc to the solenoid 2.

[0009] In addition, a commutation diode is essential for such PWM control, and as shown in Figure 9, by connecting a commutation diode 4 to both ends of the solenoid 2, the current is commutated when the solenoid is off, preventing the generation of back electromotive force.

[0010] The current control circuit 50 can be configured with a CPU processing unit of a one-chip microcomputer. A one-chip microcomputer is a microprocessor that incorporates a CPU, memory devices (ROM and RAM), various input / output devices, a timer, an analog-to-digital converter, and other components all mounted on a single integrated circuit (IC chip). A specific function can be processed with just one small IC chip like this, and with recent trends toward lower prices, lower power consumption, higher performance, and smaller size, one-chip microcomputers are not only used to control proportional solenoid valves like those mentioned above, but are also commonly used to control automated devices and electrical equipment in many fields.

[0011] As described above, a control current is supplied to the solenoid 2 by PWM control based on the command signal Os, the movable iron core and the valve spool are positioned by an attractive force corresponding to the control current, and the target flow rate corresponding to the command current value Oc is obtained at a predetermined valve opening.

[0012] However, changes in the power supply voltage and load resistance cause changes in the control current flowing through the solenoid 2. To suppress such changes, as shown in Figure 9, a current control circuit 50 generally uses a negative feedback circuit 52 to correct the command current value by negative feedback based on the detected current value from the current detector 6 on the output side of the solenoid 2, thereby improving the stability of the control current.

[0013] Such PWM-controlled proportional solenoid valves are used for continuous position control of actuators in various hydraulic systems, as disclosed in, for example, Patent Documents 1 and 2.

[0014] Utility Model Registration No. 2531827 Japanese Patent Laid-Open No. 8-303628

[0015] In addition, proportional solenoid valves require not only control of the valve spool drive position, but also control to maintain the target flow rate.In other words, to keep the flow rate constant even if there is a change in the valve differential pressure between the inlet and outlet sides, the thrust generated at any position of the valve spool / movable core, i.e., the attractive force of the coil, is controlled.

[0016] Normally, the fluid force that increases with an increase in the valve differential pressure acts on the valve spool in the valve closing direction. Therefore, by adjusting the spool shape and the spring force, the amount of displacement of the valve spool due to the fluid force (flow path opening area) and the flow rate that changes with the valve differential pressure can be balanced to obtain a differential pressure-flow rate characteristic in which the flow rate remains constant despite changes in the valve differential pressure.

[0017] However, it has been difficult to maintain a constant flow rate across all flow rate and differential pressure ranges in response to changes in valve differential pressure simply by adjusting the spool shape and spring biasing force. Generally, a design (spool design) that prioritizes maximum flow rate by increasing the maximum value of flow rate relative to valve differential pressure can maintain a constant flow rate in the flow rate versus valve differential pressure (differential pressure-flow characteristics) in a relatively small flow rate range. However, in a relatively large flow rate range, the fluid force is too large relative to the solenoid suction force, resulting in a characteristic in which the flow rate decreases as the valve differential pressure increases, as shown in Figure 10. In other words, in a proportional solenoid valve that only counters the biasing force of the spring, as shown in Figure 11, the current flowing through the proportional solenoid is constant, so the fluid force causes the armature and valve spool to move in the valve closing direction, exceeding the increase in flow rate due to an increase in valve differential pressure. As a result, the flow rate decreases as the differential pressure increases.

[0018] Furthermore, when the spring's biasing force against fluid force is set to a large value, the amount of displacement of the movable iron core and valve spool that can be changed by the attractive force of the proportional solenoid becomes smaller, reducing the maximum flow rate that the valve can actually discharge.On the other hand, in the case of valves with a valve spool position detection sensor, spool positioning control is performed with an emphasis on repeatability, and by setting the gain high, control is performed in a direction that prevents spool displacement due to fluid force, and it has not been considered that the flow rate can be kept constant by correcting the valve opening.

[0019] In proportional solenoid valves, it is desirable to improve the characteristic of the flow rate decrease that occurs with an increase in the valve differential pressure as described above, but because the valve has a sensorless configuration that does not have a position detection sensor for the valve spool, it has not been possible to keep the flow rate constant by electrically controlling the movable iron core and valve spool position to correct the valve opening.

[0020] In view of the above problems, an object of the present invention is to provide a proportional solenoid valve position control system that can correct the position of the movable iron core and valve spool by electrical current correction control, even in a sensorless proportional solenoid valve, without causing any adverse effects such as reducing the maximum flow rate of the valve, and can obtain constant flow rate characteristics even when the valve differential pressure increases.

[0021] In order to achieve the above object, a proportional solenoid valve position control system according to the invention of claim 1 is a proportional solenoid valve in which a valve spool is displaced together with a movable iron core by an electromagnetic attractive force proportional to the current passed through the solenoid, and a flow rate of a working fluid is determined by a valve position corresponding to said displacement, and the proportional solenoid valve position control system achieves a target valve position by controlling the current passed through the solenoid from a power source, the proportional solenoid valve position control system comprising: a switching element that controls the current supplied to the solenoid by turning on and off the current from the power source; a current control circuit that outputs a switching control signal that commands the switching element to switch between ON and OFF based on a command signal that indicates a command current value corresponding to a target flow rate; and a memory unit that stores instructions for processing to be executed by the current control circuit, wherein the current control circuit generates, from the command signal, a PWM conversion unit that adjusts a PWM signal to a duty ratio such that an ON-OFF pulse width in one pulse period becomes a duty ratio corresponding to the command current value, and outputs the PWM signal to the switching element; and a negative feedback circuit that performs negative feedback on a detected current value from the output side of the solenoid and adds a deviation from the command current value to the command signal for correction, and further comprises a current correction circuit that calculates a correction current amount to offset a change in flow rate accompanying a change in valve differential pressure of the proportional solenoid valve and corrects the current to the solenoid, wherein the current correction circuit has an amplitude detection unit that detects the amplitude value of a sawtooth-shaped current waveform formed by a current rise during the ON period and a current drop during the OFF period in one pulse cycle from the detected current value of the solenoid, and a current correction amount calculation unit that calculates and outputs a correction current amount equivalent to a current amount that offsets the amount of displacement between the current position of the movable iron core and the movable iron core position at which a target command flow rate is obtained, based on the detected amplitude value detected by the amplitude detection unit, and adds the correction current amount to the command signal.

[0022] A proportional solenoid valve position control system according to a second aspect of the present invention is the proportional solenoid valve position control system according to the first aspect, characterized in that the current correction amount calculation unit applies to the detected amplitude value a characteristic function that is set in advance based on the correlation between the amplitude value of a current waveform based on a PWM signal of the solenoid and the distance of the movable iron core from a position where the solenoid is attracted to a fixed iron core, and that is stored in the memory unit, to estimate the current position of the movable iron core and identify a valve differential pressure at the current position, identify a current value at a movable iron core position where a target command flow rate is obtained at the valve differential pressure, and output the difference between that current value and the current value at the current position as the current correction amount.

[0023] A proportional solenoid valve position control system according to the invention of claim 3 is the proportional solenoid valve position control system of claim 1, characterized in that the current correction amount calculation unit outputs the difference between the detected amplitude value and a theoretical amplitude value at no flow rate for the command current value as the correction current amount, and the current correction circuit further comprises a weighting unit that multiplies the current correction amount by a weighting coefficient that is selectably set in advance in accordance with a change in the rate of flow rate reduction accompanying an increase in valve differential pressure and is stored in the memory unit.

[0024] The proportional solenoid valve position control system according to the invention of claim 4 is the proportional solenoid valve position control system of claim 1, characterized in that the current control circuit is composed of a CPU calculation unit mounted on a one-chip microcomputer, and the memory unit is a storage device mounted on the one-chip microcomputer.

[0025] The proportional solenoid valve position control system according to claim 5 is the proportional solenoid valve position control system according to claim 1, characterized in that the amplitude detection unit obtains an AC component from which a DC component has been removed by differentiating a detected current signal obtained from an analog / digital converter based on a detected current value from the current detection unit, and smooths the pulse wave converted to DC by full-wave rectifying the AC component to output an amplitude signal as a gently shaped waveform.

[0026] The proportional solenoid valve position control system of claim 6 is the proportional solenoid valve position control system of claim 1, characterized in that the amplitude detection unit acquires from the PWM conversion unit a pulse one-cycle signal indicating one pulse cycle during which ON-OFF control of the switching element is performed, and identifies the maximum current value and minimum current value in the one pulse cycle section identified by the pulse one-cycle signal from the detected current value of the solenoid acquired by the current detection unit, and calculates the difference between the maximum and minimum current values ​​as the current amplitude of the PWM cycle.

[0027] According to the proportional solenoid valve position control system of the present invention, the current control circuit which controls the current supplied to the solenoid of the proportional solenoid valve by PWM control further comprises a current correction circuit which calculates a current correction amount to offset the amount of displacement between the current position of the movable iron core and the movable iron core position at which the target command flow rate is obtained, based on the amplitude value of the sawtooth current waveform detected from the detected current value of the solenoid, and adds this to the command signal to correct the current to be supplied to the solenoid, so that by correcting the current to the solenoid, it is possible to obtain the effect that by this current correction to the solenoid, it is possible to suppress increases and decreases in flow rate that occur with increases and decreases in valve differential pressure, and to improve the flow rate characteristics relative to valve differential pressure to a constant flow rate.

[0028] In addition, because this type of current correction control can provide constant flow characteristics relative to the valve differential pressure, it is possible to simplify the valve spool structure and design the valve to increase the maximum flow rate by increasing the fluid force. Furthermore, because it can respond to load behavior that occurs due to load fluctuations at the site of use, it is expected that the proportional solenoid valve of the present invention will have a wider range of application than conventional ones.

[0029] 1 is a circuit block diagram showing a schematic configuration of a proportional solenoid valve position control system according to one embodiment of the present invention. It is a graph showing flow characteristics as a result of current correction control performed by the proportional solenoid valve position control system of FIG. 1, with corrected flow rate (L / min) plotted on the vertical axis against valve differential pressure (MPa) on the horizontal axis. It is a graph showing the current correction amount relative to the pre-correction current value as a result of current correction control performed by the proportional solenoid valve position control system of FIG. 2, with corrected current (A) plotted on the vertical axis against valve differential pressure (MPa) on the horizontal axis. It is a graph showing the relationship between the PWM-controlled current amplitude value and the distance from the attracting position of the movable iron core to the fixed iron core of the solenoid. It is a graph showing the correlation between the distance (mm) from the attracting position of the fixed iron core of the movable iron core on the horizontal axis and the PWM-controlled current amplitude effective value (A) on the vertical axis, using actual measurement data for each command current value of 0.8 A, 1 A, 1.2 A, and 1.4 A. 1 is an explanatory diagram showing the operation of amplitude detection by a first amplitude detection unit in a current correction circuit of the proportional solenoid valve position control system of the present invention. 2 is a schematic flow chart showing the operation of amplitude detection by a second amplitude detection unit in a current correction circuit of the proportional solenoid valve position control system of the present invention. 3 is an explanatory diagram showing a process of determining a weighting coefficient used in a weighting unit in the current correction circuit of the proportional solenoid valve position control system of the present invention. 4 is a circuit block diagram showing the schematic configuration of a current control circuit of a conventional proportional solenoid valve. 5 is a graph showing the change characteristics of flow rate (vertical axis) with respect to change in valve differential pressure (horizontal axis) of a conventional proportional solenoid valve. 6 is a graph showing the change characteristics of flow rate and valve spool movable core position (vertical axis) with respect to change in valve differential pressure (horizontal axis) at a constant current of a conventional proportional solenoid valve.

[0030] The proportional solenoid valve position control system according to the present invention is a PWM-controlled proportional solenoid valve in which the current control circuit that controls the current passed through the solenoid to achieve the target flow rate of the working fluid further comprises, in addition to a conventional negative feedback circuit, a current correction circuit that corrects the current in accordance with the amount of displacement of the movable iron core due to an increase or decrease in the valve differential pressure that causes a change in the flow rate of the proportional solenoid valve.

[0031] The current correction circuit of the present invention comprises an amplitude detection unit that detects the amplitude of a sawtooth current waveform based on a PWM signal from the detected current value of the solenoid, and a current correction amount calculation unit that calculates and outputs a current correction amount that offsets the amount of displacement between the current position of the movable iron core and the movable iron core position at which a target command flow rate is obtained, based on the detected amplitude value detected by the amplitude detection unit, and adds the output current correction amount to the command signal. This current correction circuit makes it possible to suppress changes in flow rate that accompany increases and decreases in valve differential pressure, and to improve the flow rate characteristics relative to the valve differential pressure to maintain a constant flow rate.

[0032] The current supplied to the solenoid of a proportional solenoid valve has a sawtooth-shaped current waveform formed by the current rising during the ON period and the current falling during the OFF period due to PWM control. The inventors discovered that there is a correlation between the amplitude of this sawtooth-shaped current waveform and the position of the movable iron core, and that this relationship between the current amplitude and the movable iron core position does not change depending on the valve differential pressure. Therefore, the current position of the movable iron core can be estimated by detecting the sawtooth-shaped current amplitude from the current value detected from the output side of the solenoid. They then came up with the idea that the control current to the solenoid can be corrected by obtaining a current correction amount that offsets the amount of displacement between this current position and the movable iron core position at which the desired command flow rate is obtained, and thus arrived at the present invention.

[0033] That is, the amplitude of the sawtooth current waveform is determined by the solenoid inductance, and therefore changes with changes in the magnetic path of the solenoid. As shown in Figure 4, the current amplitude increases as the movable core approaches the position where the solenoid attracts the fixed core. Based on this characteristic, a characteristic function can be obtained in advance that correlates the current amplitude with the distance from the fixed core's position where the movable core is attracted. Therefore, by using this characteristic function, the current position of the movable core at that point can be estimated from the detected amplitude of the sawtooth current waveform detected from the current value detected from the output side of the solenoid.

[0034] On the other hand, not only the relationship between the current amplitude and the movable core position, but also the relationship characteristics between the flow rate and the valve differential pressure with respect to the movable core position using the current in the solenoid valve as a parameter can be determined by experimental measurements in advance. Therefore, if the current position of the movable core is identified based on these relationship characteristics, the valve differential pressure at that current position can also be identified, and further, the movable core position at which the target command flow rate is obtained at that valve differential pressure can also be identified.

[0035] Therefore, by acquiring a current amount that offsets the amount of displacement between the movable core position at which the target command flow rate is obtained and the current position as a correction amount, and adding this current correction amount to the command current value, the flow rate can be controlled to a constant value. In this case, in reality, the current value at the current position and the current value at the movable core position at which the target command flow rate is obtained can be specified based on the relationship characteristics known in advance for the solenoid valve as described above, and the current correction amount can be obtained by calculating the difference between the two current values.

[0036] The correlation between the amplitude of the sawtooth-shaped PWM-controlled current of the solenoid and the distance of the movable iron core from the fixed iron core attraction position is shown in the graph of Figure 5 using actual measurement data for different command current values ​​of 0.8 A, 1 A, 1.2 A, and 1.4 A. In this graph, the effective value of the PWM current amplitude (A) is plotted on the vertical axis, while the distance (mm) of the movable iron core from the fixed iron core attraction position is plotted on the horizontal axis, with the flow rate decreasing as the distance increases.

[0037] From the graph in Figure 5, it can be seen that the correlation between the effective current amplitude and the distance of the movable core from the fixed core attraction position has a nearly similar characteristic function (relationship between current amplitude and position) for each current value. Therefore, if such a characteristic function is set in advance and stored in the memory unit, it can be used in actual current correction control. In addition, the memory unit can also store flow rate characteristics and movable core position characteristics relative to valve differential pressure that have been experimentally measured in advance for the solenoid valve, and these relationship characteristics can also be used in the current correction amount calculation unit.

[0038] Furthermore, in the present invention, it is possible to simplify the process for obtaining the current correction amount and obtain a rough current correction amount. In this case, the no-flow movable core position when there is no flow at the command current value of the solenoid valve, i.e., when the valve opening is determined only by the solenoid attractive force and the opposing spring force, is considered to be the movable core position at which the target command flow rate is obtained at the valve differential pressure at the current position of the movable core. The difference in current values ​​at both positions can be considered to be the difference in amplitude values ​​(current values) corresponding to both positions. Therefore, the current correction amount calculation unit of the present invention can be configured to calculate the difference between the detected amplitude value (current value) corresponding to the current position of the movable core and the theoretical amplitude value (current value) at the no-flow time as the current correction amount.

[0039] The current correction amount calculated as described above is weighted according to the rate of decrease in the flow rate that decreases with an increase in the valve differential pressure, and the weighted amount is added to the command signal as the final correction amount. This makes the current correction amount more appropriate, thereby optimizing the correction control.

[0040] This is because proportional solenoid valves may have flow characteristics in which the displacement rate of the movable iron core with an increase in valve differential pressure, i.e., the rate at which the flow rate decreases, is not constant. Therefore, if weighting coefficients corresponding to different rates of decrease in flow rate are set in advance from a weighting function corresponding to this flow rate characteristic, the correction amount finally added to the command signal can be adjusted by multiplying the current correction amount by an appropriate weighting coefficient, enabling more appropriate and efficient current correction control.

[0041] 8, it can be seen that no correction is necessary during the period X from when the valve differential pressure is zero until it reaches the pressure required to achieve the target flow rate. Therefore, while the movable iron core moves from the valve open position in the closing direction until the period X has elapsed, the weighting coefficient by which the current correction amount is multiplied can be set to "0."

[0042] Then, after the target flow rate is reached and the period X has elapsed, as described above, the flow rate decreases with an increase in the valve differential pressure, but correction control is performed to keep the flow rate constant by adding a current correction amount to compensate for this decrease, and after the target flow rate is reached, the control current to the solenoid is increased by correction, so that the displacement rate of the movable iron core and the decrease in flow rate that accompanies the increase in the valve differential pressure are suppressed after the period Y has elapsed. Therefore, if the weighting coefficient corresponding to the period Y is set to "1," and the weighting coefficient corresponding to the period Z in which the displacement rate of the movable iron core and the decrease rate of flow rate become smaller after the period Y has elapsed are set to a value greater than the period Y, for example "1.5," these weighting coefficients can be appropriately used in actual current correction control.

[0043] By appropriately selecting and using the weighting coefficients as described above in accordance with changes in the rate of flow rate reduction, as shown as the predicted flow rate characteristics after correction in FIG. 8, the flow rate reduction accompanying the increase in valve differential pressure over periods Y and Z after the target flow rate has been achieved is suppressed to an appropriate correction amount, and the flow rate characteristics shown before correction are improved to become approximately constant flow rate characteristics.

[0044] The amplitude detector in the current correction circuit of the present invention is of the following two types depending on the method of detecting the current amplitude value.

[0045] The first amplitude detection unit, as shown in Figure 6, acquires from the PWM conversion unit a pulse one-period signal indicating one pulse period T during which the switching element is turned on and off, and also acquires the detected current value of the solenoid from the current detection unit, stores the maximum current value M and minimum current value n in the section of one period T from the detected current value, and obtains the difference between the two as the current amplitude of the PWM period.

[0046] As shown in FIG. 7, the second amplitude detection unit obtains the detected current value of the solenoid from the current detection unit as a detected current signal Cs via an analog / digital converter, differentiates this detected current signal Cs (S1) to remove the DC component, and full-wave rectifies the AC component ACc (S2) to obtain a pulse wave Pc converted to DC, smooths this pulse wave Pc (S3), and outputs it as an amplitude signal Ams.

[0047] The proportional solenoid valve position control system according to the present invention, which has the above-described configuration, can also be configured with a one-chip microcomputer, just like conventional proportional solenoid valve current control systems. In this case, the current control circuit of the present invention can be configured in the CPU processing unit of the one-chip microcomputer, and the storage device (ROM / RAM) of the one-chip microcomputer can be used as the storage unit for storing the above-described characteristic functions, weighting coefficients, etc. Therefore, since the hardware configuration is essentially the same as conventional systems except for the CPU processing unit, there is almost no increase in cost compared to conventional systems.

[0048] As an embodiment of the present invention, the schematic configuration of a proportional solenoid valve position control system 1 configured with a one-chip microcomputer is shown in the circuit block diagram of Figure 1. Note that this embodiment takes as an example a case in which the proportional solenoid valve is a spring-balanced, direct acting spool type with three positions and four ports (pressure port P, cylinder ports A and B, return port T). With this proportional solenoid, the control direction is selected by energizing one of the two solenoids, and in this embodiment, the control is performed in the flow direction of P → B and A → T.

[0049] The proportional solenoid valve position control system 1 of this embodiment includes a switching element 3 that controls the current supplied to the solenoid 2 by turning on and off the power supply, and a current control circuit 10 that outputs a switching control signal that instructs the switching element 3 on the ON / OFF switching timing based on a command signal Os that is a command current value Oc corresponding to a target flow rate that has been converted into a digital signal via an analog / digital converter 5.

[0050] In this embodiment, the current control circuit 10 is composed of a CPU processing unit of a one-chip microcomputer, and a commutation diode 4 is connected across the solenoid 2 to commutate the current when the solenoid is off and prevent the generation of back electromotive force. Also, a FET (field effect transistor) is used for the switching element 2.

[0051] The memory device (ROM / RAM) 30 of the one-chip microcomputer stores instructions for the processes executed by the current control circuit 10, and can store not only a characteristic function previously set based on the correlation between the amplitude of the sawtooth current waveform based on the PWM signal Ps of the solenoid 2 and the distance of the movable iron core from the position where the solenoid attracts the fixed iron core, as shown in FIG. 5, but also characteristics showing the relationship between the flow rate and the valve differential pressure relative to the movable iron core position, using the current in the solenoid valve as a parameter, which have been experimentally measured in advance.

[0052] In the current control circuit 10, the command signal Os is input to the PWM conversion unit 11 via the amplifier G, and a PWM signal Ps adjusted to a duty ratio such that the ON-OFF pulse width in one pulse period becomes a duty ratio equivalent to the command current value Oc is generated and output as the switching control signal.

[0053] A current detector 6 is provided on the output side of the solenoid 2 to detect the current value, and a current control circuit 10 is provided with a negative feedback circuit 12. This negative feedback circuit 12 smooths and negatively feeds back a detected current signal Cs, which is a digital signal converted from the detected current value from the current detector 6 via an analog / digital converter 7, and adds the deviation from the command signal Os to the command signal Os. This provides a control current for the solenoid 2 that suppresses changes in the energizing current due to fluctuations in the power supply voltage and changes in the load resistance.

[0054] In this embodiment, the current control circuit 10 further includes a current correction circuit 20 that corrects the amount of current supplied to the solenoid 2 so as to offset the change in flow rate that accompanies the change in valve differential pressure.

[0055] The current correction circuit 20 includes an amplitude detection unit 21 that detects the amplitude value of a sawtooth current waveform based on the PMN signal from the detected current signal Cs and outputs it as a detected amplitude signal Ams, a theoretical amplitude calculation unit 22 that outputs the theoretical current amplitude value at no flow rate at the command current value based on the command signal Os as a theoretical amplitude signal Tms, and a current correction amount calculation unit 23 that compares the detected amplitude signal Ams with the theoretical amplitude signal Tms and outputs the difference obtained as a current correction amount Ca.

[0056] In this embodiment, the amplitude detection unit 21 employs the detection method shown in Fig. 7. That is, the amplitude detection unit 21 differentiates the detected current signal Cs (S1) to remove the DC component to obtain an AC component ACc, full-wave rectifies the AC component ACc (S2) to convert it into a DC pulse wave Pc, and then smooths the pulse wave Pc (S3) to obtain an amplitude signal Ams with a gently shaped waveform.

[0057] The current correction circuit 20 of this embodiment further includes a weighting unit 24 that applies a weighting corresponding to the rate of change of flow rate to the current correction amount Ca to obtain an appropriate correction amount CA. The weighting unit 24 uses three weighting factors, "0," "1," and "1.5," which are set in advance in correspondence with the flow rate characteristics shown in FIG. 8 and stored in the storage device 30.

[0058] The results of current correction control in the proportional solenoid valve position control system 1 according to this embodiment having the above configuration are shown in the graphs of Figures 2 and 3. Figure 2 plots the corrected flow rate (L / min) on the vertical axis against the valve differential pressure (MPa) on the horizontal axis, showing the flow rate characteristics before correction, while Figure 3 plots the corrected current (A) on the vertical axis against the valve differential pressure (MPa) on the horizontal axis, showing the amount of current correction relative to the current value before correction.

[0059] As can be seen from the results shown in FIGS. 2 and 3, the current correction control of this embodiment resulted in an improvement in the valve differential pressure-flow rate constant characteristic, which was almost consistent with the prediction shown in FIG.

[0060] 1: Proportional solenoid valve opening control system 2: Solenoid 3: Switching element 4: Commutation diode 5, 7: Analog / digital converter G: Amplifier 6: Current detection unit 10, 50: Current control circuit 11, 51: PWM conversion unit 12, 52: Negative feedback circuit 20: Current correction circuit 21: Amplitude detection unit 22: Theoretical amplitude calculation unit 23: Current correction amount calculation unit 24: Weighting unit

Claims

1. A proportional solenoid valve opening control system for a proportional solenoid valve in which a valve spool is displaced together with a movable iron core by an electromagnetic attractive force proportional to the current passed through the solenoid, and the flow rate of a working fluid is determined by a valve opening corresponding to said displacement, controls the current passed from a power source to the solenoid to achieve a desired valve opening, the system comprising: a switching element that controls the current supplied to the solenoid by turning on and off the current from the power source; a current control circuit that outputs a switching control signal that commands the switching element to switch on and off based on a command signal indicating a command current value corresponding to a desired flow rate; and a memory unit that stores instructions for processing to be executed by the current control circuit, wherein the current control circuit generates a PWM signal as the switching control signal, the PWM signal being adjusted from the command signal to a duty ratio such that the ON-OFF pulse width in one pulse period is a duty ratio corresponding to the command current value; and a negative feedback circuit that performs negative feedback on a detected current value from the output side of the solenoid and adds a deviation from the command current value to the command signal for correction, and further comprises a current correction circuit that corrects the current to the solenoid by calculating a correction current amount that offsets a change in flow rate accompanying a change in valve differential pressure of the proportional solenoid valve, wherein the current correction circuit has: an amplitude detection unit that detects the amplitude value of a sawtooth-shaped current waveform formed by a current rise during the ON period and a current drop during the OFF period in one pulse cycle from the detected current value of the solenoid; and a current correction amount calculation unit that calculates and outputs a correction current amount equivalent to a current amount that offsets the amount of displacement between the current position of the movable iron core and the position of the movable iron core at which a target command flow rate is obtained, based on the detected amplitude value detected by the amplitude detection unit, and adds the correction current amount to the command signal.

2. The proportional solenoid valve opening control system according to claim 1, characterized in that the current correction amount calculation unit applies a characteristic function to the detected amplitude value, which is set in advance based on the correlation between the amplitude value of the current waveform based on the PWM signal of the solenoid and the distance of the movable iron core from the solenoid's fixed iron core attraction position and is stored in the memory unit, to estimate the current position of the movable iron core and identify the valve differential pressure at the current position, calculate the current value at the movable iron core position where the target command flow rate is obtained at the valve differential pressure, and output the difference between this current value and the current value at the current position as the current correction amount.

3. The proportional solenoid valve opening control system according to claim 1, characterized in that the current correction amount calculation unit outputs the difference between the detected amplitude value and the theoretical amplitude value at no flow rate for the command current value as the correction current amount, and the current correction circuit further comprises a weighting unit that multiplies the current correction amount by a weighting coefficient that is selectably set in advance in accordance with changes in the rate of flow rate reduction accompanying an increase in valve differential pressure and is stored in the memory unit.

4. The proportional solenoid valve opening control system according to claim 1, characterized in that the current control circuit is composed of a CPU calculation unit mounted on a one-chip microcomputer, and the memory unit is a memory device mounted on the one-chip microcomputer.

5. The proportional solenoid valve opening control system according to claim 1, characterized in that the amplitude detection unit obtains an AC component by removing a DC component from a detected current signal obtained from an analog / digital converter based on the current value detected by the current detection unit, and smooths the pulse wave converted to DC by full-wave rectifying the AC component to output an amplitude signal as a gently shaped waveform.

6. The proportional solenoid valve opening control system according to claim 1, characterized in that the amplitude detection unit acquires from the PWM conversion unit a pulse one-cycle signal indicating one pulse cycle during which the switching element is controlled to be on or off, and identifies the maximum current value and minimum current value in the one pulse cycle section identified by the pulse one-cycle signal from the detected current value of the solenoid acquired by the current detection unit, and calculates the difference between the maximum and minimum current values ​​as the current amplitude of the PWM cycle.

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