JPH07160338A - Flow rate control method - Google Patents

Abstract

PURPOSE:To provide the flow rate control method which speedily performs the flow rate control without any overshoot by switching a flow rate control system halfway. CONSTITUTION:This flow rate controlling method controls the flow rate of fluid by controlling a flow control valve 10 interposed in a fluid flow passage 4 from a flow rate control part 22, and stationary manipulated variables of the flow control valve corresponding to plural flow rate set values are previously measured and stored as a table. If a flow rate set value abruptly varies, a stationary manipulated variable corresponding to the new flow rate set value is found on the basis of the table and then an initial manipulated variable which is smaller that said value is outputted to abruptly supply fluid by an amount up to a certain extent less than the flow rate set value; and then speed type PID control is started immediately and slow valve operation is performed on the basis of the initial manipulated variable. Consequently, the flow rate is speedily varied to the flow rate set value without causing any overshoot.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flow rate control method for controlling the flow rate of a fluid having a relatively small flow rate such as gas.

[0002]

2. Description of the Related Art Generally, in order to manufacture semiconductor products and the like, for example, CVD film formation and etching operations are repeatedly performed on semiconductor wafers and the like, but in this case, it is necessary to accurately control a small amount of processing gas. For example, a precision gas flow rate control device is used. This type of gas flow rate control device is mainly configured by a sensor unit that detects the mass flow rate of a trace amount of gas, a valve unit, and a control circuit unit that controls this. The sensor portion has a sensor in which an electrothermal coil is wound around a sensor tube through which a small amount of the total gas amount passes, and most of the gas flows through the bypass. Then, the control circuit unit controls the valve opening of the valve unit based on the detection value of the sensor unit to flow the gas flow rate of the set value. Further, in order to control the valve opening, since the entire gas flow rate itself is very small, it is necessary to control the valve opening with high accuracy within a stroke range of, for example, several tens of μm. Since a large thrust change can be produced within the stroke range, a laminated piezoelectric element body is generally used, which allows the valve opening of the valve body consisting of a diaphragm to be operated based on the set value. It has become.

By the way, as a method for controlling the gas flow rate by controlling the valve opening of the valve, that is, the operation amount,
Position PID control method and speed type PID control method are known, and when controlling the gas flow rate, particles that cause product defects do not wind up in the semiconductor processing chamber due to a sudden gas flow. Need to control. Therefore, it is necessary to suppress the occurrence of overshoot, which causes particles, as much as possible.

[0004]

By the way, the control equation of the above-described position PID control is generally expressed by the following equation 1 in a digital system.

[0005]

[Equation 1]

Where m _n is the operation amount of the n-th valve,
k is a proportional constant, Ts is the sampling time, Ti is the integration time (constant concept has entered time), e _n is the actual amount of operation of the deviation with respect to the set value, Td is determined by differentiation time constant, m ₀ is required Bias amounts added according to Accordingly, Ts / TiΣe _n term is accumulation of the deviation is determined represents the integral term, Td / Ts (e _n -e
The term _n-1 ) represents the differential term, and the gradient of the deviation is obtained.

According to this integral term, for example, even if there is a slight deviation in the temperature control of the processing furnace where strictness is required, this deviation is accumulated and corrected, but the response is slow as a whole. In addition, when the set value is changed, the value of the integral term may act in the direction of preventing the change.Therefore, in the actual control, the value of the integral term should be set to zero by software when the set value is changed. Has become.

Further, since the differential term takes the difference between the deviations, it does not work very well when the change in the manipulated variable is small, but it acts suppressively when the deviation changes abruptly. Then, in the actual flow rate control device, if the valve is set to the fully closed state at the initial stage, for example, the flow rate will be 50%.
Since a large deviation occurs when the set value of% is input, in order to prevent this, m ₀ is set to an operation amount of 50%, for example, and the valve is opened.

However, in general, the response speed of the sensor is not so fast. Therefore, for example, when the set value is sharply lowered, the sensor output which is feeding back shows the previous state for a short time and deviates. Becomes zero, then m
There was a problem that the valve opened to 50% due to _0, and as a result, gas rapidly flowed and overshoot occurred. On the other hand, the control formula of the speed type PID control is generally expressed by the following equation 2 in a digital system.

[0010]

[Equation 2]

Here, Δm _n represents the difference in valve opening,
This is an equation for recognizing the present valve opening degree m _n and what kind of change is given to it. According to this control, as is clear from the equation, only the change in the valve opening is calculated, and the sum of the output and the previous position is performed by the output processing unit. Therefore, the internally calculated output value exceeds the output signal range. It has the advantages that it does not change excessively, the calculated output value does not easily change excessively, and that there is no problem due to saturation in the internal calculation of the integrated value, but on the other hand, the step when the set value changes abruptly The response speed at the time of response is the above position PID
There is a problem that it is considerably inferior to the case of control. FIG. 6 shows this state. For example, when the set value is rapidly changed stepwise from 0 V to 5 V in a digital system, the valve opening (operation amount) gradually rises as shown by the dotted line, Therefore, the actual flow rate does not overshoot, but the flow rate slowly rises toward the set value, which causes a problem of poor response speed.

In particular, the actual flow rate characteristics of the valve section are such that the flow rate change is small when the valve opening degree is small, and the flow rate change becomes large when the valve opening degree exceeds a certain level. There was a problem that the responsiveness when the set value changed stepwise from zero in the fully closed state was considerably poor.

The present invention focuses on the above problems,
It was created to solve this effectively. An object of the present invention is to provide a flow rate control method that can perform flow rate control quickly and without overshoot by switching the flow rate control method on the way.

[0014]

In order to solve the above problems, the present invention controls the flow rate of the fluid by controlling a flow rate control valve provided in a fluid passage through which the fluid flows from a flow rate control section. In the flow rate control method, the steady operation amount of the flow rate control valve corresponding to a plurality of flow rate setting values is measured in advance and stored in a table, and when the flow rate is rapidly changed to a predetermined flow rate setting value, The flow rate control unit determines the steady state operation amount corresponding to the predetermined flow rate setting value based on a table, and outputs an initial operation amount smaller than the obtained steady state operation amount by a predetermined ratio to the flow rate control valve, afterwards,
Immediately, the initial operation amount is used as a reference to shift to speed type PID control.

[0015]

Since the present invention is configured as described above, when the flow rate of the fluid is greatly changed, for example, when the flow rate is rapidly changed from the zero flow rate state to the predetermined flow rate set value, first, the flow rate setting is performed. When a value setting signal is input, the flow rate control unit obtains a steady operation amount corresponding to this value from a table that is stored in advance, and a predetermined ratio, for example, 0.6 to 0.7 times, from this steady operation amount. The operation amount of is output as the initial operation amount. If this initial manipulated variable is calculated in advance and similarly tabulated, the value of this initial manipulated variable can be extracted quickly. If the flow rate setting value is not stored in the table, the interpolation method is applied to the flow rate setting values stored on both sides of the table to obtain the corresponding initial manipulated variable. The flow rate control unit immediately drives and controls the flow rate control valve based on the initial manipulated variable thus obtained, and opens the valve opening degree corresponding to the initial manipulated variable.

Immediately thereafter, for example, when a slight time elapses until the sensor section becomes stable, the speed type PID control is started. As a result, the fluid initially flows out suddenly, but since the initial manipulated variable at the flow rate setting value at that time is set smaller than the steady manipulated variable, it is not a rapid flow that causes overshoot. Then, by immediately shifting to the speed type PID control, the valve opening gradually increases toward the steady operation amount with reference to the valve opening at that time, and finally the flow rate corresponding to the flow rate set value flows. Become. In this way, even when the set flow rate value changes abruptly, it is possible to control the flow rate so as to quickly change to the desired set flow rate value without causing overshoot or the like.

In this case, the fluid shown in the flow rate set value in the table is actually flowing according to changes in the environment in which it is used, such as the gas differential pressure in the piping system to which the fluid passage is connected and the secular change in the fluid passage. There may be a discrepancy between the actual operation amount at this time and the steady operation amount at this time, but when the deviation becomes excessively large, it is preferable to rewrite the flow rate setting value of the table and keep it constantly updated. .

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a flow rate control method according to the present invention will be described in detail below with reference to the accompanying drawings. 1 is a schematic configuration diagram showing a gas mass flow rate control device for carrying out the flow rate control method according to the present invention, FIG. 2 is a block diagram showing the configuration of a flow rate control unit of the control device shown in FIG. 1, and FIG. 5 is a graph showing changes in the flow rate set value, flow rate and manipulated variable when the control method of the invention is carried out, FIG. 4 is a graph showing the relationship between the manipulated variable and flow rate of the flow rate control valve when the differential pressure is changed, FIG. 4 is a graph showing a part of characteristic curves extracted from FIG. 4.

First, a gas mass flow controller for carrying out the flow control method according to the present invention will be described. As shown in the figure, the gas mass flow rate control device 2 has a fluid passage 4 formed of, for example, stainless steel, and most of the flow rate is provided upstream of the fluid passage 4 in the flow direction of the gas fluid. A flow bypass 6 is provided, and a flow rate control valve 10 having a diaphragm 8 as a valve body is provided on the downstream side for controlling the flow rate of the gas fluid.

Sensor pipes 14 of the mass flow rate detecting sensor unit 12 are connected to both ends of the bypass 6 so that a small amount of gas fluid can be made to flow therethrough as compared with the bypass 4. A pair of electric heating coils 16 for control are wound around the sensor tube 14, and a sensor control circuit 18 connected to the electric coil 16 detects the mass flow rate of the gas fluid.

The detected value here is the A / D of 2 channels.
It is input to the flow rate control unit 22 including a microcomputer or the like via the converter 20. In addition, the setting signal indicating the flow rate setting value input from the outside is also the A / D
It is input to the flow rate control unit 22 via the converter 20. This setting signal is usually an analog signal of 0 to 5 V, and is input to the flow rate control unit 22 after being converted into a digital signal by the A / D converter 20.

Further, the flow control valve 10 has, for example, a laminated piezoelectric element 24 that produces a large thrust change within a small stroke range as an actuator for vertically driving the diaphragm 8, and the piezoelectric element drive section 26 Is controlled by the drive signal. The digital drive signal from the flow rate control unit 22 is input to the drive unit 26 after being converted into an analog signal by the D / A converter 28, and the flow rate control valve is controlled so that the flow rate corresponding to the flow rate set value flows. The operation amount of 10, that is, the valve opening is controlled.

In the present invention, the steady operation amount of the flow rate control valve corresponding to a plurality of flow rate setting values is measured in advance and stored in a table, and when the set flow rate value changes abruptly, it is newly added. A steady operation amount corresponding to the set flow rate set value is obtained based on the table, and an initial operation amount smaller than the obtained steady operation amount by a predetermined ratio is first output to the flow rate control valve 10, and thereafter, The flow rate control unit 22 is programmed so as to immediately shift to the velocity type PID control (represented by the above-mentioned equation 2) with the initial operation amount as a reference.

In other words, when the flow rate set value is changed, the valve opening degree (initial operation amount) slightly smaller than the valve opening degree (operation amount) corresponding to the newly set flow rate set value is first set. It is opened or closed all at once, and then immediately shifts to the speed type PID control which is characterized by stable control, and as a result, it quickly shifts to the set flow rate without causing overshoot.

The functional contents of the above-mentioned program are shown, for example, in FIG. 2, and the arithmetic unit 3 performs various arithmetic operations.
0 and a measurement result of the flow rate control valve measured in advance are rewritably stored, for example, in an EEPROM (electric)
al erasable programmable R
OM), a memory unit 31, a sensor stability confirmation unit 34 that indicates whether or not the sensor 12 is stable, and a rewrite command unit 33 that notifies the timing of rewriting the table.

The sensor stability confirmation unit 32 is a speed type PI.
It informs the timing to switch to D control,
When a sudden decrease or increase in the gas flow rate is detected, the sensor is recognized as stable and the arithmetic unit 30 is notified that the sensor is stable. The time until the sensor is stabilized (response time) is very short, for example, on the order of μmsec, whereas the response time of the actuator that operates the diaphragm 8 is relatively long, for example, on the order of msec.

Further, the rewriting command section 33 operates in the table of the actual operation amount output as a drive signal and the flow rate setting value at that time when the input flow rate value is stable (constant). A certain amount between the amount and, for example, 10%
If the above difference occurs, a rewriting signal for rewriting the flow rate setting value in the table is output, and the arithmetic unit 30 receiving this rewriting signal performs a predetermined calculation to perform rewriting. . All the above operations are processed by the program. Here, the contents shown in Table 1 stored in the storage unit 32 will be described.

[0028]

[Table 1]

In Table 1, the flow rate setting value Q represents a target flow rate input from the outside, and the maximum flow rate is divided into 10 steps of 10% from Q _{1 to} Q ₁₀ for convenience. It may be further subdivided. The manipulated variable indicates the valve opening degree of the control valve, and indicates the valve opening degree required when the flow rate represented by the flow rate setting value is allowed to flow stably. Here, in general, the characteristic of the operation amount (valve opening) of the flow control valve and the flow rate has a hysteresis characteristic as shown in FIG. 4, and the operation amount is increased or decreased. It has a complicated characteristic that the flow rate is different even when the valve opening is the same, and the flow rate is different even when the differential pressure is different.
For example, assuming that the valve openings are the same, the flow rate increases as the differential pressure increases, and the entire characteristic curve shows a similar shape. Therefore, as shown in Table 1, for the same flow rate setting value Q, there are an operation amount mA when the valve opening is increasing and an operation amount mB when the valve opening is decreasing, both of which are measured.

Incidentally, compensation for the point that the flow rate changes even if the valve opening is the same due to the difference in the differential pressure will be described later. These manipulated variables mA and mB are measured after the gas flow rate control device 2 is actually incorporated in, for example, a piping system of a semiconductor manufacturing apparatus, and the flow rate is increased or decreased by 10%, for example, and the flow rate is changed each time Reading the valve opening (equivalent to the drive voltage of the actuator) when the valve is stable, and manipulated variables mA and mB
Is stored in the storage unit 32.

Then, the respective manipulated variables mA and mB
Then, a predetermined ratio α (1 <0) is added to obtain the corresponding initial manipulated variables mAo and mBo, which are similarly stored. Therefore, the initial manipulated variables mAo and mBo are obtained by calculation as follows. mAo = α · mA mBo = α · mB Here, α is set to a value such that the flow rate does not overshoot even if the valve opening degree corresponding to the initial operation amount is established at once, and normally 0.6 to 0. It is set within the range of 7.

The differential pressure when forming Table 1 is set to the largest differential pressure expected in the environment in which this flow rate control device is installed. For example, the gas pressure of the processing gas piping system in a semiconductor manufacturing apparatus is usually 3 kgf / cm ^{2 at} maximum.
Therefore, when preparing Table 1, data should be taken under the pressure difference of 3 kgf / cm ² . As a result, even if the flow rate control device is installed in any piping system having a maximum differential pressure of 3 kgf / cm ² , no overshoot will occur.
Further, the differential pressure of the piping system in which the flow rate control device is arranged is smaller than, for example, 3 kgf / cm ² , for example, 2 kgf / c.
When that m ^2, and each of the operation amount corresponding to the flow rate set value Q represented in Table 1 and the initial operation amount are all increased (see FIG. 4).

Therefore, in order to ensure the quickness of the control speed, the learning function rewrites all the flow rate setting values Q in Table 1 so as to correspond. Specifically, the flow rate control valve 10 when the flow rate specified by the flow rate set value is flowing stably
Of the actual flow rate (corresponding to the voltage of the drive signal) is monitored, and based on the ratio between this flow rate set value and the flow rate set value in Table 1 corresponding to the above manipulated variable, the flow rate set value in Table 1 To be rewritten. The rewritten state is shown in Table 2.

[0034]

[Table 2]

In this case, since it is considered that the manipulated variable actually changes microscopically due to the hysteresis characteristic, the rewrite command section 34 rewrites when a difference in manipulated variable of 10% or more occurs. The replacement signal is output. Further, the flow rate set value Q not stored in Table 1, for example, the flow rate set value Q
₁ and it is between for example 15% when the flow rate set value is inputted Q _2, to seek a corresponding value by interpolation from the operation amount and initial operation amount corresponding to the flow rate set value Q _1, Q ₂ To In FIG. 1, 34 is a D / A converter that converts a flow rate output into an analog signal of 0 to 5 V, and 36 is a RS232C that communicates with the outside, for example.
This is a standard interface.

Next, an example of the method of the present invention which is carried out by using the gas mass flow rate controller constructed as described above will be specifically described. For example, the fluid passage 4 with a differential pressure of 3 kgf / cm ²
When the gas fluid flows into the sensor section 12, a part thereof flows through the sensor tube 14 of the sensor section 12, and most of the gas flows through the bypass 6, and the flow rate is controlled by the fluid control valve 10 toward the gas use system.

The flow rate of the gas fluid flowing through the sensor pipe 14 is detected by the constant temperature difference type sensor 12 to obtain the mass flow rate flowing through the entire fluid passage 4, and the mass flow rate is input to the flow rate control unit 22. Then, feedback control is applied so that the detected value becomes equal to the flow rate set value input from the outside, and the laminated piezoelectric element 24 of the flow rate control valve 10 is driven to control the valve opening degree. Become.

Now, in the present embodiment, a case where the flow rate set value changes drastically, for example, a case where the flow rate set value Q abruptly changes to Q _{3 in} Table 1 from the state of zero flow rate will be described. As shown in 3, time t
When the flow rate setting value Q ₃ is input as the setting signal in, the calculating unit 30 obtains the steady operation amount mA ₃ at the time of increasing the flow amount from Table 1 stored in the storage unit 31, and adds a predetermined ratio α to this value. The initial manipulated variable mAO ₃ at the time of increase is obtained by performing the calculation. If the initial manipulated variable is calculated in advance and tabulated in Table 1, the initial manipulated variable mAO ₃ (<mA ₃ ) may be directly read as a value corresponding to the flow rate setting value Q _3. Good. This initial manipulated variable mAO ₃ is immediately transmitted as a drive signal from the flow rate control unit 22 to the flow rate control valve 10 side, and the diaphragm 8 is opened all at once up to the initial manipulated variable mAO ₃ .

As a result, the gas suddenly flows out, and this gas flow is detected by the sensor 12 having a high response sensitivity. Then, the sensor stability confirmation unit 32 (see FIG. 2) informs the arithmetic unit 30 that the sensor is stable. . Then, the arithmetic unit 30
At the point A, the control method is switched to the speed type PID control as shown in the above-mentioned mathematical expression 2. As described above, this control method recognizes the current valve opening degree and adjusts the valve opening degree stably and slowly toward the set value based on this, so that the valve operation amount is set at point A. At the boundary, that is, the initial manipulated variable mAO ₃ rises steadily and slowly. During this period, the gas flow rate sharply increases toward the flow rate setting value Q ₃ , but the initial operation amount mAO ₃ is set smaller than the steady operation amount mA ₃ so that overshoot does not occur, so that the overshoot occurs. It is possible to rapidly increase the gas flow rate up to the flow rate set value Q ₃ without generating a chute. Therefore, since no overshoot occurs, in a semiconductor manufacturing apparatus, for example, it is possible to prevent the particles from being swirled up due to a sudden entrainment of gas.

Such an operation is the same not only when the flow rate setting value is increased stepwise from zero flow rate but also when the flow rate setting value is increased stepwise from the state where the gas fluid is flowing to some extent. Conversely, when the flow rate set value decreases stepwise from the state where the gas fluid flows to some extent, the same operation is performed. However, in this case, since the gas flow rate decreases, a value is selected or calculated from the fields of the steady operation amount mB at the time of decrease and the initial operation amount mBO at the time of decrease.

When a flow rate set value Q which is not stored in Table 1 is inputted (in practice, this type of case seems to be common), the flow rate set value Q is shown in Table 1. From the steady state operation amount corresponding to the flow rate setting values on both sides of the position located at the position and the initial operation amount at that time, the corresponding steady state operation amount and initial operation amount are obtained by an interpolation method such as proportional division. The time Δt from the time t to the point A is very small, and depends on the responsiveness of the sensor, for example, several μs
It is set to about c.

In the above operation, when preparing Table 1, actual measurement was performed under a differential pressure of 3 kgf / cm ² , and the differential pressure of the piping system equipped with the flow control valve was 3 kgf / cm 2.
Since it is the case of the piping system of ² , there will be almost no problem if the values shown in Table 1 are used as they are, but in reality, due to the nature of the gas used, it will be used in an environment with a differential pressure below 3 kgf / cm ^2. There are also cases where In this case, as described above with reference to FIG. 4, when the differential pressure is different, the flow rate characteristic is different even at the same valve opening, so that Table 1 needs to be corrected. As shown in FIG. 5, Table 1 measured with a differential pressure of 3 kgf / cm ² (corresponding to Table 1) shows, for example, a differential pressure of 0.5 kgf / cm 2.
^{When the} flow rate control device is used on the basis of No. ² , it is necessary to make the flow rate characteristic of the differential pressure 0.5 kgf / cm ² correspondingly, but in this embodiment, the learning function is provided. The table 1 is automatically rewritten to the table 2 corresponding to the differential pressure of 0.5 kgf / cm ² .

[0043]

[Table 2]

Specifically, for example, in the case of increasing the gas flow rate, initially, the initial operation amount is selected or calculated based on Table 1, and the valve opening is controlled by this initial operation amount. However, when the flow rate of the flow rate setting value Ql is stable, the actual manipulated variable mAk at that time is different from the steady-state manipulated variable mA1 in Table 1 at the time of increase. Therefore,
The flow rate set value Qk in Table 1 corresponding to the actual manipulated variable mAk at this time is obtained, and all the flow rate set values Q in Table 1 are rewritten by Ql and Qk to obtain the flow rate set value Q'shown in Table 2. create. This formula is given as follows. Q '= Q · Ql / Qk

In actual control, as shown in FIG. 2, the rewriting command section 33 constantly monitors the actual operation amount indicated by the flow rate set value and the drive signal, and in the state where the flow rate set value does not change, When the difference between the actual manipulated variable and the steady manipulated variable in Table 1 corresponding to the flow rate setting value at that time exceeds a predetermined range, for example, 10% or more, a rewrite signal is output and Rewriting operation was done,
A new table as shown in Table 2 is created and stored. After that, the above-mentioned control is performed based on Table 2.

By providing the learning function in this way and rewriting the table when necessary, not only can it be applied to a piping system having any pressure difference, but also a gas cylinder can be used. Even if the differential pressure changes due to a change in the source pressure, it can be dealt with. In the above embodiment, the control switching timing is set by the operation of the sensor stability confirmation unit 32, but this is only an example, and the speed type PID control is performed immediately after the output of the initial operation amount. Any switching function may be provided as long as it can be moved to.

[0047]

As described above, according to the flow rate control method of the present invention, the following excellent operational effects can be exhibited. When the flow rate set value is suddenly changed, the valve opening degree is immediately set to the initial manipulated variable corresponding to the new flow rate set value, and then the speed type PID control is immediately performed, so that no overshoot occurs. The desired flow rate can be set quickly. Therefore, when applied to the gas supply system of the semiconductor manufacturing apparatus, it is possible to prevent the particles from being wound up due to the overshoot and contribute to the improvement of the yield. Also, by rewriting the flow rate setting value on the table in correspondence with the actual manipulated variable, even if the differential pressure of the used system fluctuates from the beginning, or even if it is applied to the used system having a different differential pressure, for example. Can respond.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a gas mass flow rate control device for carrying out a flow rate control method according to the present invention.

FIG. 2 is a block diagram showing a configuration of a flow rate control unit of the control device shown in FIG.

FIG. 3 is a graph showing changes in flow rate set value, flow rate, and manipulated variable when the control method of the present invention is performed.

FIG. 4 is a graph showing the relationship between the operation amount of the flow control valve and the flow rate when the differential pressure is changed.

FIG. 5 is a graph showing a part of characteristic curves extracted from FIG.

FIG. 6 is a graph showing changes in a flow rate set value, a flow rate, and a manipulated variable when speed type PID control is performed from the beginning.

[Explanation of symbols]

 2 Gas mass flow rate control device 4 Fluid passage 6 Bypass 8 Diaphragm 10 Flow rate control valve 12 Mass flow rate detection sensor 14 Sensor tube 22 Flow rate control section 24 Multilayer piezoelectric element 26 Piezoelectric element drive section 30 Computing section 31 Storage section 32 Sensor stability confirmation section 33 Rewrite command section

Claims (2)

[Claims] 1. A flow rate control method for controlling a flow rate of the fluid by controlling a flow rate control valve provided in a fluid passage through which a fluid flows, from a flow rate control section, wherein the flow rate is preset according to a plurality of flow rate set values. The steady operation amount of the control valve is measured and stored in a table, and when the flow rate is rapidly changed to a predetermined flow rate set value, the flow rate control unit performs the steady operation corresponding to the predetermined flow rate set value. The amount is calculated based on the table, an initial operation amount smaller than the calculated steady operation amount by a predetermined ratio is output to the flow rate control valve, and immediately thereafter, the speed type PID control is performed with the initial operation amount as a reference. A flow control method characterized by the above. 2. The flow rate control unit monitors the actual manipulated variable of the flow rate control valve when the flow rate regulated by the flow rate set value is flowing stably, and monitors the flow rate set value and the actual flow rate. 2. The flow rate control method according to claim 1, wherein the flow rate set value in the table is rewritten based on a ratio with the flow rate set value on the table corresponding to the operation amount of.