JP2009282819A - Flow control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform good control by automatically updating proportional gains in performing proportional control action compatible with changes of state such as pressure occurring in a line. <P>SOLUTION: Inverse numbers of the opening and the flow gradient of a control valve 2, a normalized proportionality factor Kz obtained by dividing the gradient by the maximum flow rate of the control valve 2 at its maximum opening, and the characteristic resistance R0 of the control valve 2 at the maximum opening are preset as initial values in an off-line state. A plurality of data consisting of a first pressure loss ΔP between the entrance 4 and the exit 5 of the control valve 2 and a flow rate F measured by a flowmeter 3 are measured as necessary in an on-line state. Based on the first pressure loss ΔP and the flow rate F, a second pressure loss ΔE occurring between the control valve 2 and the flowmeter 3 and total resistance (r) at the maximum opening of the control valve 2 are computed, and a new proportionality gradient Ke is calculated from them and the preset normalized proportionality factor Kz. An appropriate proportionality gain (1/Ke) is calculated from its inverse number and used in proportionality control action. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a flow rate control method for controlling the flow rate of a fluid.

  Conventional flow control systems use a number of methods that use a proportional control action to control the flow rate of the fluid. In the flow control method using these proportional control operations, when the flow control system is shipped from the factory or installed on the pipe in the field, the proportional gain that seems to be optimal is fixedly set in the controller, and the flow meter The control valve is driven by feedback so that there is no deviation between the target flow rate and the actual flow rate obtained from the flow meter based on the flow rate information from.

  However, the proportional gain described above is optimized from the relationship between the opening and flow rate of the control valve when the pressure on the inlet side of the control valve used in the control system is constant. As a result, the optimization condition is not satisfied. As a result, the optimal control with the proportional gain that is fixed at the initial stage cannot be guaranteed, and it may be difficult to implement the flow rate control with a fast response.

  In addition, in the flow control method using a control valve, the rotary motion of the stepping motor is converted into a slide motion to drive a flow control section such as a diaphragm or a needle to adjust the cross-sectional area through which the fluid passes through the control valve. To be done. That is, it is considered that the opening of the control valve can be adjusted by the number of steps input in pulses because the passage cross-sectional area of the fluid is proportional to the distance that the needle or the like slides.

  However, when the drive pulse and the control valve movement are actually linked, a step-out phenomenon in which the control valve does not drive according to the drive pulse often occurs, especially when operating at a high speed, depending on the number of pulses applied. As a result, the opening of the control valve cannot be obtained, and the optimum opening of the control valve cannot be determined uniquely.

  As described above, in the conventional flow rate control method, when the environmental conditions such as the pressure, flow rate, and temperature in the pipe, particularly the pressure on the inlet side of the control valve 2 described above changes greatly, the initially set proportional gain is not necessarily optimal. Disappear. Therefore, even if the opening of the control valve is adjusted, particularly in the case of a small amount of fluid control, problems such as overshoot and undershoot occur, and the responsiveness is lowered, so that stable flow rate control becomes impossible. In addition, in order to accurately respond to environmental conditions, fine pressure measurement is required, and the control system becomes complicated.

  An object of the present invention is to provide a simple control system that automatically updates an optimal proportional gain when performing proportional control operation so as to solve the above-described problems and cope with a change in state such as pressure in a pipe. It is an object to provide a flow rate control method that performs good control using the.

  In order to achieve the above object, the flow control method according to the present invention measures the flow rate of a fluid with a flow meter and drives the control valve by a proportional control operation using a proportional gain to perform flow control beforehand. In the flow control method in which the control process conditions are obtained in advance, the proportional gain is updated online using these conditions, and the flow control is performed using the updated proportional gain. The pressure on the inlet side of the valve was fixed, and the control valve was driven between the fully closed state and the maximum opening, and was obtained from the correspondence between the opening of the control valve and the flow rate obtained from the flow meter. The reciprocal of the gradient, the normalized proportional constant obtained by dividing the gradient by the flow rate at the maximum opening of the control valve, and the specific resistance of the control valve at the maximum opening of the control valve are obtained in advance. A second pressure loss that occurs between the flow meter outlet from the control valve inlet through the flow meter and the control valve, piping, and flow rate when the control valve is set to the maximum opening. A preliminary step of previously inputting the initial value of the total resistance generated in the meter to the arithmetic and control unit, and in the online mode, a first differential pressure before and after the control valve is generated by the control valve in operation. Based on a plurality of data consisting of a first step of obtaining the pressure loss of the pressure and the flow rate at that time by the flow meter, and the first pressure loss and the flow rate obtained in the first step. In addition, a second pressure loss caused by the pipe including the control valve and the flow meter and a total resistance by the pipe including the control valve and the flow meter at the maximum opening of the control valve are obtained by calculation. And the step determined in the second step 3 based on the pressure loss of 2 and the total resistance, the third step of calculating the flow rate at the maximum opening of the control valve, and the maximum opening of the control valve determined in the third step And calculating the gradient based on the normalized proportionality constant set off-line and obtaining the new proportional gain from the reciprocal thereof and setting it in the arithmetic and control unit; and the flowmeter And a fifth step of driving the control valve by a proportional control operation using the proportional gain while measuring the flow rate to feedback control the flow rate, and repeating the first to fifth steps. And

  The flow control method according to the present invention automatically obtains an optimal proportional gain using a simple control system and performs good flow control.

  The present invention will be described in detail based on the illustrated embodiment.

  FIG. 1 is a configuration diagram of a flow rate control system. A control valve 2 for adjusting the flow rate by adjusting the opening degree and a flow meter 3 for measuring the flow rate are arranged in series in the pipe 1 through which the fluid flows. A detection end of a differential pressure sensor 6 for measuring a pressure loss between the inlet 4 and the outlet 5 of the control valve 2 is connected. The output of the differential pressure sensor 6 and the output of the flow meter 3 are sent to the arithmetic control device 7. The output of the arithmetic control device 7 is connected to the control valve 2.

  The fluid flows from the pipe inlet side of the pipe 1, and the pressure loss between the inlet 4 and the outlet 5 of the control valve 2 is measured by the differential pressure sensor 6. The fluid whose flow rate is adjusted by the control valve 2 is measured by the flow meter 3 and reaches the flow meter outlet 8.

  The first pressure loss that is the pressure difference between the inlet 4 and the outlet 5 of the control valve 2 measured by the differential pressure sensor 6 and the flow rate value measured by the flow meter 3 are sequentially sent to the arithmetic and control unit 7. These pieces of information are arithmetically processed in the arithmetic and control unit 7 as will be described later, and are output as signals for controlling the opening degree of the control valve 2.

  Various types of valves can be used as the control valve 2 for controlling the flow rate. In this embodiment, a needle valve is used. Various types of flowmeters 3 can also be used, but an ultrasonic flowmeter is used in this embodiment.

  The eye of this embodiment is to appropriately update the proportional gain of the proportional control operation (P operation) that has deviated from the optimum value due to the pressure change at the inlet 4 of the control valve 2 online. In the proportional control operation, the control valve 2 is fully opened based on the first pressure loss generated between the inlet 4 and the outlet 5 during the operation of the control valve 2 and the flow rate obtained by the flow meter 3. An appropriate proportional gain is obtained by estimating the flow rate when the valve is released to the degree, and feedback control is performed using this proportional gain.

  FIG. 2 is a graph showing the flow rate F obtained by changing the opening degree d of the control valve 2 under the pressure P at the inlet 4 of the control valve 2. Thus, the gradient K from the initial rising portion and the flow rate Fp when the control valve 2 is opened from the fully closed state to the maximum opening degree dp are defined.

  FIG. 3 is a graph when the pressure at the inlet 4 is changed from Pa to Pd. From FIG. 3, the gradients Ka, Kb, Kc, Kd at the respective pressures and the flow rates Fap, Fbp, Fcp, Fdp at the maximum opening degree dp are obtained.

  FIG. 4 is a graph showing the flow rate values normalized by dividing the flow rate value at each opening by the flow rates Fap, Fbp, Fcp, and Fdp at the maximum opening dp of each control valve 2. As shown in FIG. 4, the normalized flow rate is substantially equal to the degree of opening of the control valve 2 even when the pressure at the inlet 4 of the control valve 2 changes from Pa to Pd. From the linear portion of the rising edge of this curve, a normalized proportionality constant Kz that does not depend on the pressure at the inlet 4 is defined. Since this normalized proportionality constant Kz does not depend on the pressure, it is obtained by dividing the gradient of the linear portion of the opening and the flow rate obtained with the pressure of the inlet 4 being constant by the flow rate Fp at the maximum opening dp. Can do.

The normalized proportionality constant Kz corresponds to the ratio between the gradient (Ka to Kd) corresponding to each pressure of the inlet 4 shown in FIG. It can be expressed by the following equation (1).
Kz = Ka / Fap = Kb / Fbp = Kc / Fcp = Kd / Fdp (1)

As can be seen from FIG. 3, if the environmental conditions at the time of flow rate control change and the pressure at the inlet 4 becomes, for example, Pe (not shown), the gradient Ke at that time and the proportional gain (1 / Ke) that is the reciprocal thereof. Should also change. At this time, the gradient Ke can be expressed as an equation (2) obtained by converting the equation (1).
Ke = Kz × Fep (2)

  Equation (2) indicates that the gradient Ke to be obtained is obtained by multiplying the normalized proportionality constant Kz by the flow rate Fep of the maximum opening dp. At this time, the normalized proportional constant Kz may be obtained from the gradient at a certain pressure at the inlet 4 and the flow rate at the maximum opening dp according to the above-described method, and stored in the arithmetic and control unit 7 in advance. Regarding the flow rate at the maximum opening dp when the pressure of 4 changes, it is virtually impossible to measure this every time during the control operation.

  In order to solve this problem, in the present embodiment, the flow rate Fp at the maximum opening dp of the control valve 2 is automatically calculated from the first pressure loss before and after the control valve 2 and the flow rate measured by the flow meter 3. Estimated.

  FIG. 5 is an equivalent circuit that models the above system. Symbols of the equivalent circuit are defined as follows.

ΔP: a first pressure loss measured by the differential pressure sensor 6 between the inlet 4 and the outlet 5 of the control valve 2.
ΔE: an estimated second pressure loss that occurs between the inlet 4 of the control valve 2, the flowmeter 3, and the flowmeter outlet 8.
F: Flow rate actually measured by the flow meter 3.
r ′: resistance in the flow meter 3 and the pipe 1 excluding the control valve 2 between the inlet 4 and the flow meter outlet 8.
r: Estimated total resistance (r = R0 + r ′) generated in the control valve 2, the pipe 1, and the flow meter 3 when the control valve 2 is at the maximum opening dp.
R0: Specific resistance of the control valve 2 at the maximum opening dp.
R: resistance of the control valve 2 depending on the opening degree of the control valve 2, which is the resistance of the control valve 2 added to the specific resistance R0 and decreases as the opening degree increases, and at the maximum opening degree dp, R = 0.

From the equivalent circuit of this system shown in FIG. 5, the following equations (3) and (4) are derived for the first and second pressure losses ΔP and ΔE.
ΔP = (R + R0) × F (3)
ΔP = ΔE−r ′ × F (4)

When the expressions (3) and (4) are expanded, the following expression is obtained.
(R + R0) × F = ΔE−r ′ × F
(R + R0 + r ′) × F = ΔE

From this equation, the relationship F = ΔE / (R + R0 + r ′) is obtained. Here, it can be assumed that when the maximum opening degree dp is R = 0, and the total resistance r at the maximum opening degree dp of the control valve 2 is r = R0 + r ′ as described above. The flow rate Fp at the degree dp is obtained by the following equation (5).
Fp = ΔE / r (5)

  Thus, the flow rate Fp at the maximum opening dp of the control valve 2 is obtained by calculation from the second pressure loss ΔE in the pipe 1 and the total resistance r at the maximum opening dp of the control valve 2.

  Based on the above concept, the maximum flow rate Fp is obtained according to the equation (6), the proportional gain (1 / Ke) is obtained from the proportional gradient Ke obtained by returning to the equation (2), and is automatically updated. For this purpose, the control process conditions are obtained offline in advance, and the proportional gain is updated online using these conditions.

  FIG. 6 is a flowchart of the operation of the first embodiment. In step S1 which is offline by the program built in the arithmetic and control unit 7, as a preliminary process, the reciprocal of the gradient obtained from the correspondence between the opening of the control valve 2 and the flow rate obtained by the above-described method, and the normalized proportional constant Kz and the specific resistance R0 at the maximum opening degree of the control valve 2 are respectively obtained and input to the arithmetic control device 7 in advance as process constants.

The specific resistance R0 of the control valve 2 is a fixed resistance defined at the maximum opening dp of the control valve 2. Further, since the first pressure loss ΔP generated between the inlet 4 and the outlet 5 of the control valve 2 is proportional to the flow rate Fp at the maximum opening dp of the control valve 2, the formula (6) is established.
ΔP = R0 × Fp (6)

  In order to obtain the specific resistance R0, the control valve 2 is opened to the maximum opening dp, the pressure at the inlet 4 of the control valve 2 is changed, and a plurality of first pressure losses ΔP and a plurality of flow rates Fp at the maximum opening dp are obtained. And substituting it into equation (6) to solve the simultaneous equations.

  For the second pressure loss ΔE and the total resistance r of the maximum opening dp of the control valve 2, for example, provisional values may be stored in the arithmetic control device 7 as initial values.

  On-line, the first pressure loss ΔP of the fluid generated between the inlet 4 and the outlet 5 of the control valve 2 in step S2 and the flow rate by the flow meter 3 are obtained every time, and a plurality of sets of data are taken in and calculated. Store in the control device 7.

Here, the first pressure loss ΔP is expressed by equation (7) in which the relationship of total resistance r = R0 + r ′ when the control valve 2 is at the maximum opening dp is substituted into equation (4).
ΔP = ΔE− (r−R0) × F (7)

  In step S3, the flow rate F obtained by the flow meter 3, the plurality of sets of data of the first pressure loss ΔP, and the resistance R0 input in advance are combined by a plurality of equations respectively substituted into the equation (7). When the equation is solved, the second pressure loss ΔE during operation and the total resistance r of the maximum opening dp of the control valve 2 can be obtained. The obtained second pressure loss ΔE and the total resistance r of the maximum opening dp of the control valve 2 are replaced with the stored values so far and stored in the arithmetic and control unit 7.

Further, in step S4, when the second pressure loss ΔE obtained in step S3 and the total resistance r are substituted into the equation (5), the maximum flow rate Fp at the maximum opening dp of the control valve 2 is newly calculated. The proportional gain (1 / Kf) is obtained according to the following equation (8) from the maximum flow rate Fp and the normalized proportionality constant Kz that is initially set in the off-line step S1. The obtained proportional gain (1 / Kf) is updated as a new optimum value.
1 / Kf = 1 / (Kz × Fp) (8)

  In step S5, the control valve 2 is driven using this proportional gain (1 / Kf).

  In this way, according to the procedure of steps S2 to S5, the optimum proportional gain (1 / Kf) is obtained every time and is automatically updated by the arithmetic and control unit 7 to perform proportional operation control by feedback using this proportional gain. As a result, the flow rate control with good responsiveness can be continued.

  Up to now, the maximum opening dp of the control valve 2 and the flow rate corresponding to the maximum opening dp have been defined as the maximum flow rate Fp. However, the maximum opening dp does not necessarily have to be the opening when the control valve 2 is fully opened. At this time, an opening range in which the control valve 2 normally operates may be defined, and a state where the control valve 2 is opened most within the opening range may be set as the maximum opening dp.

  FIG. 7 is an operation flowchart of the second embodiment. Steps S11 and S12 are added between steps S2 and S3 in FIG. 6 of the first embodiment.

  In fact, when the fluid flow rate is controlled on-line, the change in the first pressure loss ΔP is more frequent than the proportional gain is frequently changed in response to a slight change in the pressure at the inlet 4 of the control valve 2. In some cases, the control is smoothed by updating the proportional gain only when a predetermined threshold value is provided for the quantity and a change width of a certain level or more is obtained.

Therefore, in step S11 of the second embodiment, the resistance r ′ (r ′) of the control valve 2 is determined from the stored second pressure loss ΔE based on the first pressure loss ΔP and the flow rate F generated by the control valve 2. = R-R0), and the difference from the value obtained by subtracting the pressure loss is defined as parameter M. This parameter M is expressed as in equation (9).
M = | ΔE− (r−R0) × F−ΔP | (9)

  During operation, it is always monitored whether the currently set proportional gain is optimal based on the change in parameter M. If parameter M exceeds the threshold in step S12 and the deviation from the optimal state of proportional gain is confirmed, it is updated. The operation to be performed is performed in the next step S3. That is, when the parameter M exceeds the threshold value, the second pressure loss ΔE and the total resistance r are obtained in step S3, a new proportional gain (1 / Kf) is obtained in step S4, and the process proceeds to step S5.

  On the other hand, if the parameter M is smaller than the threshold value, it is determined that the currently set proportional gain is sufficient, and steps S3 and S4 are omitted, and the process proceeds to step S5.

  In step S5, control similar to that in the first embodiment is performed using the obtained proportional gain (1 / Kf).

It is a block diagram of a control system. It is a graph of the relationship between the opening degree and the flow rate at a constant pressure. It is a graph of the relationship between the opening degree of the control valve and the flow rate based on a plurality of input pressures. It is a graph of the relationship between the flow volume after normalization and the opening degree of a control valve. It is an equivalent circuit diagram. FIG. 3 is an operation flowchart of the first embodiment. FIG. 6 is an operation flowchart of the second embodiment.

Explanation of symbols

1 Piping 2 Control valve 3 Flow meter 4 Control valve inlet 5 Control valve outlet 6 Differential pressure sensor 7 Arithmetic controller 8 Flow meter outlet

Claims (4)

When the flow rate of fluid is measured by a flow meter and the control valve is driven by proportional control operation using proportional gain to perform flow control, the control process conditions are obtained offline in advance, and these conditions are set online. In the flow rate control method in which the proportional gain is updated using and the flow rate control is performed using the updated proportional gain,
In the offline mode, the pressure on the inlet side of the control valve is made constant, the control valve is driven between the fully closed state and the maximum opening, and the opening of the control valve and the flow rate obtained from the flow meter Reciprocal of the gradient obtained from the corresponding relationship, normalized proportionality constant obtained by dividing the gradient by the flow rate at the maximum opening of the control valve, and the specific resistance of the control valve at the maximum opening of the control valve, Is obtained in advance and set in the arithmetic and control unit, and the second pressure loss that occurs between the inlet of the control valve, the flow meter and the outlet of the flow meter, and the control valve when the control valve is at the maximum opening degree. , Having a preliminary step of inputting the initial value of the total resistance generated in the pipe and the flow meter to the arithmetic control device in advance,
In the online, a first step of obtaining a first pressure loss that is a differential pressure before and after the control valve generated by the operating control valve and a flow rate at that time by the flow meter,
A second pressure loss caused by a pipe including the control valve and the flow meter based on a plurality of data including the first pressure loss and the flow rate obtained in the first step; A second step of calculating the total resistance by piping including the control valve and the flow meter at the maximum opening of
A third step of calculating the flow rate at the maximum opening of the control valve based on the second pressure loss and the total resistance determined in the second step;
The gradient is calculated based on the flow rate at the maximum opening of the control valve obtained in the third step and the normalized proportionality constant set off-line, and a new proportional gain is obtained from the reciprocal thereof. A fourth step of setting in the arithmetic and control unit;
A fifth step of feedback controlling the flow rate by driving the control valve by a proportional control operation using the proportional gain while measuring the flow rate by the flow meter,
A flow rate control method characterized by repeating the first to fifth steps.   The pressure loss caused by the resistance obtained by removing the specific resistance of the control valve from the total resistance generated in the control valve, the pipe, and the flow meter when the control valve is at the maximum opening degree is calculated from the second pressure loss. 2. The fifth step is executed by omitting the fourth step when a difference between the subtracted value and the first pressure loss is smaller than a predetermined threshold value. The flow control method described in 1.   The flow rate control method according to claim 1, wherein the flow rate at the maximum opening is a maximum flow rate at the maximum opening within an opening range using the control valve.   The flow rate control method according to claim 1, wherein the first pressure loss is obtained by a differential pressure sensor.

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