Flow rate control apparatus, storage medium storing program for flow rate control apparatus and flow rate control method

Abstract

In order to keep a stable flow rate at a set flow rate value when a pressure fluctuation occurs in an upstream side of a valve, without providing an additional sensor for detecting a pressure fluctuation, a flow rate control apparatus is provided with: the valve; a flow rate sensor; a valve control part configured to control the valve so that a deviation between a set flow rate value and a measurement flow rate value is reduced, on the basis of the deviation and a set control coefficient; and a control coefficient setting part configured to set the control coefficient so that when a pressure rise occurs in the upstream side of the valve, a decreased amount in flow due to a decreased opening of the valve and an increased amount in flow due to an increased amount of a differential pressure before and after the valve, are balanced.

Claims

1. A flow rate control apparatus comprising: a valve provided on a flow path where fluid flows; a flow rate sensor provided on an upstream side of the valve in the flow path; a valve control part configured to control the valve so that a deviation is reduced, based on the deviation and a control coefficient which is set, and the valve control part is configured so as to control an opening position of the valve by a PID control; and a control coefficient setting part configured to set a control coefficient of the valve control part based on pressure in the upstream side of the valve or a set flow rate value, wherein the deviation is a difference between the set flow rate value and a measurement flow rate value measured by the flow rate sensor, and the control coefficient is a value which is set so that, in the case where a pressure rise due to disturbance occurs in the upstream side of the valve, a decreased amount in flow due to disturbance and an increased amount in flow due to disturbance are substantially balanced, the decreased amount in flow due to disturbance is a decreased amount of a flow rate corresponding to a decreased amount of the opening position of the valve in the case where the pressure rise due to disturbance occurs in the upstream side of the valve, and the increased amount in flow due to disturbance is an increased amount of a flow rate corresponding to an increased amount of a differential pressure before and after the valve in the case where the pressure rise due to disturbance occurs in the upstream side of the valve, and the control coefficient setting part is configured so as to set a proportional gain as the control coefficient and configured so as to set the proportional gain larger as the set flow rate value is larger so that the increased amount in flow due to disturbance and the decreased amount in flow due to disturbance are substantially balanced.

2. The flow rate control apparatus according to claim 1 further comprising: an inflow characteristic storage part configured to store inflow characteristic data indicating a relationship between the pressure in the upstream side of the valve and an inflow of the fluid flowing into an internal volume including at least a flow path between the flow rate sensor and the valve; and a valve flow rate characteristic storage part configured to store valve flow rate characteristic data indicating a relationship between the set flow rate value and the increased amount in flow due to disturbance per a unit pressure rise amount of the pressure in the upstream side of the valve, wherein the control coefficient setting part is configured so as to set the control coefficient so that the decreased amount in flow due to disturbance, and the control coefficient and the increased amount in flow due to disturbance are balanced, wherein the decreased amount in flow due to disturbance is calculated on the basis of the inflow characteristic data, and the increased amount in flow due to disturbance is calculated on the basis of the valve flow rate characteristic data and the set flow rate value.

3. The flow rate control apparatus according to claim 1, wherein the flow rate sensor is a thermal flow rate sensor.

4. The flow rate control apparatus according to claim 1, wherein the flow rate sensor is a pressure flow rate sensor.

5. The flow rate control apparatus according to claim 1, wherein the control coefficient setting part is configured so as to correct the control coefficient on the basis of molar heat of fluidic species.

6. A method of controlling a flow rate using a flow rate control apparatus which comprises: a valve provided in a flow path where fluid flows; a flow rate sensor provided on an upstream side of the valve in the flow path; and a valve control part configured to control the valve so that a deviation between a set flow rate value and a measurement flow rate value measured by the flow rate sensor is reduced, on the basis of the deviation and a control coefficient which is set, and the valve control part is configured so as to control an opening position of the valve by a PID control, wherein the method includes; setting the control coefficient so that, in the case where a pressure rise due to disturbance occurs in the upstream side of the valve, a decreased amount in flow due to disturbance and an increased amount in flow due to disturbance are substantially balanced; the decreased amount in flow due to disturbance is a decreased amount of a flow rate corresponding to a decreased amount of the opening position of the valve in the case where the pressure rise due to disturbance occurs in the upstream side of the valve, and the increased amount in flow due to disturbance is an increased amount of a flow rate corresponding to an increased amount of a differential pressure before and after the valve in the case where the pressure rise due to disturbance occurs in the upstream side of the valve, wherein a proportional gain as the control coefficient is set larger as the set flow rate value is larger so that the increased amount in flow due to disturbance and the decreased amount in flow due to disturbance are substantially balanced.

7. A flow rate control apparatus comprising: a valve provided on a flow path where fluid flows; a flow rate sensor provided on an upstream side of the valve in the flow path; a valve control part configured to control the valve so that a deviation is reduced, based on the deviation and a control coefficient which is set; and a control coefficient setting part configured to set a control coefficient of the valve control part based on pressure in the upstream side of the valve or a set flow rate value and, wherein the deviation is a difference between the set flow rate value and a measurement flow rate value measured by the flow rate sensor, and the control coefficient is a value which is set so that, in the case where a pressure rise due to disturbance occurs in the upstream side of the valve, a decreased amount in flow due to disturbance and an increased amount in flow due to disturbance are substantially balanced, the decreased amount in flow due to disturbance is a decreased amount of a flow rate corresponding to a decreased amount of an opening position of the valve in the case where the pressure rise due to disturbance occurs in the upstream side of the valve, and the increased amount in flow due to disturbance is an increased amount of a flow rate corresponding to an increased amount of a differential pressure before and after the valve in the case where the pressure rise due to disturbance occurs in the upstream side of the valve, and the control coefficient setting part is configured so as to set the control coefficient and configured so as to set the control coefficient larger as the set flow rate value is larger so that the increased amount in flow due to disturbance and the decreased amount in flow due to disturbance are substantially balanced.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic sectional view showing a configuration of a flow rate control apparatus according to one embodiment of the present invention;

(2) FIG. 2 is a schematic diagram showing a configuration of a program portion of the flow rate control apparatus in the same embodiment;

(3) FIG. 3(a) and FIG. 3(b) are schematic block diagrams showing a control structure of the flow rate control apparatus in the same embodiment;

(4) FIG. 4(a), FIG. 4(b) and FIG. 4(c) are schematic diagrams showing a control concept to a disturbance pressure of the flow rate control apparatus in the same embodiment;

(5) FIG. 5(a) and FIG. 5(b) are schematic block diagrams showing an influence of the disturbance pressure to the flow rate control apparatus in the same embodiment;

(6) FIG. 6(a), FIG. 6(b) and FIG. 6(c) are schematic graphs showing a feature of a differential pressure flow rate characteristic of a valve in the same embodiment;

(7) FIG. 7(a), FIG. 7(b) and FIG. 7(c) are schematic graphs showing a trend of parameters and control coefficients in the same embodiment; and

(8) FIG. 8 is a schematic sectional view showing a configuration of a flow rate control apparatus according to another embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

(9) The following describes one embodiment of the present invention with reference to each of the accompanying drawings.

(10) A flow rate control apparatus 100 of the present embodiment is a thermal mass flow controller used to continuously supply component gas and the like at a predetermined constant flow rate into a vacuum chamber of, for example, a semiconductor manufacturing apparatus.

(11) More specifically, as shown in FIGS. 1 and 2, the flow rate control apparatus 100 is equipped with: a body 1 of a generally parallelepiped shape having a flow path 11 formed inside thereof; a thermal flow rate sensor 2 and a valve 3 which are attached to the body 1; and a control mechanism 4 configured to control the valve 3 on the basis of an output of the flow rate sensor 2.

(12) The body 1 is provided with an inlet 12 and an outlet 13 formed in a bottom surface thereof respectively for introducing and deriving fluid, and the flow path 11 is formed so as to connect between the inlet 12 and the outlet 13. The valve 3 is provided on the most downstream side with respect to this flow path 11 and the flow rate sensor 2 is provided on the upstream of the valve 3. Here, although the flow rate to be controlled by the flow rate control apparatus 100 is a flow rate of the fluid after derived from the outlet 13, the flow rate per se to be controlled is not measured in the present embodiment, but the opening position of the valve 3 is to be controlled on the basis of the flow rate measured at another point in the upstream of the valve 3. That is, a target flow rate to be controlled has not been directly observed but it is to be indirectly measured at another point.

(13) The flow rate sensor 2 is configured of: a shunt element 21; a capillary 22; a detection mechanism 23; and a flow rate calculation part 24. The shunt element 21 is a fluid resistance provided in the flow path 11, and the capillary 22 is branched from the flow path 11 and it is provided so as to bypass the front and rear of the shunt element 21. The detection mechanism 23 is composed of two coils which are provided on the capillary 22 and detects a value related to the flow rate, and the flow rate calculation part 24 is configured using a calculation function of the control mechanism 4 and calculates a flow rate on the basis of the output of the detection mechanism 23. Each of the coils is an electrically heated wire and a temperature control circuit (not shown) is connected to each of the coils so that a temperature of each of the coils is kept at a predetermined temperature. A voltage value to be applied to each of the coils is outputted from the detection mechanism 23 to the flow rate calculation part 24 and the flow rate calculation part 24 calculates a flow rate on the basis of the respective voltage values.

(14) In the present embodiment, in the case where a flow rate is varied in an internal volume VL including at least the flow path 11 from the flow rate sensor 2 to the valve 3, the flow rate sensor 2 generates an output indicating that there is a variation in flow rate even if there is no variation in flow rate in the downstream side of the valve 3. That is, an actual flow rate Q.sub.out after the valve which is a flow rate in the downstream side of the valve 3 is not necessarily coincident with a measurement flow rate value y which is a flow rate measured by the flow rate sensor 2.

(15) The valve 3 is, for example, a solenoid valve or a piezo valve and an opening position (open degree) thereof is controlled in accordance with the measurement flow rate value y measured by the flow rate sensor 2. In the case where a differential pressure before and after the valve 3 is assumed to be constant, the opening position of the valve 3 substantially corresponds one-to-one to the flow rate of the fluid passing through the valve 3. Thus, the valve 3 has a valve flow rate characteristic indicating that, the larger the opening degree, the larger the flow rate passing through the valve 3 becomes.

(16) On the other hand, in the case where the opening position of the valve 3 is assumed to be constant, the valve 3 has a valve flow rate characteristic indicating that, the larger the differential pressure before and after the valve 3, i.e., the larger the pressure in the upstream side (primary side) of the valve 3, the larger the flow rate passing through the valve 3 becomes. In this case, the valve flow rate characteristic indicates a characteristic that, the larger the opening degree and the larger the flow rate passing through the valve 3, the larger the flow rate increased in the case where the pressure in the upstream side becomes larger.

(17) The control mechanism 4 is a so-called computer equipped with: a CPU; a memory; an A/D and D/A converter, input/output means and the like and a program for use in the flow rate control apparatus is stored in the memory. Upon execution of the program and cooperation of the equipment, the control mechanism 4 is configured so as to exhibit functions as at least the flow rate calculation part 24, a valve control part 41, a control coefficient setting part 44, an inflow characteristic storage part 45 and a valve flow rate characteristic storage part 46.

(18) Each part will be described below.

(19) The flow rate calculation part 24 calculates a flow rate of the fluid flowing in the flow path 11 formed inside the body 1 on the basis of the output of the detection mechanism 23, and the calculated flow rate as the measurement flow rate vale y is outputted to the valve control part 41.

(20) As shown in FIG. 2, the valve control part 41 controls the opening position of the valve 3 so that a deviation between a set flow rate value r and the measurement flow rate value y measured by the flow rate sensor 2 is reduced on the basis of the deviation and a set control coefficient. More specifically, the valve control part 41 is configured so as to exhibit a function as a controller configured to control a control target composed of the valve 3 and the flow rate sensor 2 in a feedback system having the set flow rate value r as an input thereof and the measurement flow rate value y measured by the flow rate sensor 2 as an output thereof, as shown in a control block diagrams of FIG. 3(a) and FIG. 3(b). As described above, in the flow rate control apparatus 100 of the present embodiment, the measurement flow rate value y which is the output of the feedback system is not coincident with the actual flow rate Q.sub.out after the valve 3 which is a flow rate actually desired to be kept at the set flow rate value r, and the flow rate in the downstream side of the valve 3 is represented as a value outputted after the block of the valve 3 in the control block diagrams of FIG. 3(a) and FIG. 3(b).

(21) As shown in the schematic diagram of FIG. 2 and the control block diagrams of FIG. 3(a) and FIG. 3(b), the valve control part 41 of the present embodiment is configured of a PID control part 42 executing a PID calculation for the flow rate control and a QV characteristic adjustment part 43 for keeping the control characteristic of the valve 3 to be substantially constant regardless of the flow rate and pressure of the fluid flowing in the valve 3.

(22) The PID control part 42 executes a PID calculation to the deviation between the set flow rate value r to be inputted and the measurement flow rate value y to thereby output a voltage to be applied to the valve 3. In the present embodiment, a proportional gain which is a control coefficient for use in the PID control part 42 is set by the control coefficient setting part 44. Note that, in order to simplify the explanation in each of the drawings used for explaining the present embodiment, there is shown a case where a differential coefficient of a differential term is zero. It is needless to say that the present invention can exhibit the same effect also in the case where the differential coefficient is not zero.

(23) As shown in FIG. 3(a), the QV characteristic adjustment part 43 stands in a relationship of an inverse function with respect to a transfer function of the valve 3. That is, when considering as a control block, only the PID control part 42 and a first-order lag element indicating a characteristic of the flow rate sensor 2 remain in the control loop as shown in FIG. 3(b). More specifically, a QV characteristic which indicates a relationship between the flow rate and the applied voltage (opening position) which is a characteristic when controlling the flow rate of the valve 3 is varied under the influence of the flow rate and pressure of the fluid. In the present embodiment, the QV characteristic adjustment part 43 constitutes a reverse characteristic map with respect to the QV characteristic of the valve 3 so as to obtain a similar QV characteristic also under other conditions using the QV characteristic of the valve 3 in the case of the predetermined set flow rate value r and the pressure as a reference.

(24) The control coefficient setting part 44 sets a control coefficient for use in the valve control part 41 so that the actual flow rate Q.sub.out after the valve which is the flow rate in the downstream side of the valve 3 does not deviate from the set flow rate value r even though there occurs a pressure fluctuation in the upstream of the valve 3. That is, the control coefficient setting part 44 sets the control coefficient of the valve control part 41 so that the decreased amount in flow due to disturbance ΔQ.sub.cl and the increased amount in flow due to disturbance ΔQ.sub.valve are balanced in the case where there occurs a pressure rise due to disturbance ΔP.sub.in in the upstream side of the valve 3.

(25) The following describes an influence on the flow rate control in the case where there occurs a pressure fluctuation in the upstream side of the valve 3 in a state that the actual flow rate Q.sub.out after the valve is stable at the set flow rate value r and explains what control coefficient is set by the control coefficient setting part 44. Note that, in the present embodiment, although the terms “pressure rise due to disturbance ΔP.sub.in”, “decreased amount in flow due to disturbance ΔQ.sub.cl” and “increased amount in flow due to disturbance ΔQ.sub.valve” are defined for easy understanding, these terms may be either positive or negative values. However, the codes representing the negative or positive amount are always common. Further, in the present embodiment, the pressure in the upstream of the valve 3 is usually kept substantially constant at a reference pressure P.sub.base, but in the case where there occurs a pressure rise due to disturbance by some disturbance, the pressure may be varied to P.sub.base+ΔP.sub.in.

(26) FIG. 4(a) shows a control block diagram in the case of modeling not only the flow rate control apparatus 100 of the present embodiment but also the disturbance such as a pressure fluctuation in the upstream side of the valve 3. As shown in FIG. 4(a), the pressure fluctuation in the upstream of the valve 3 affects at least two types of disturbance influences on the feedback control system.

(27) As shown in FIG. 4(b), one of the disturbance influences is that the flow rate of the fluid flowing into the internal volume VL (dead volume) including at least the flow path 11 between the flow rate sensor 2 and the valve 3 is increased by the pressure rise due to disturbance ΔP.sub.in and this results in causing deviation between the measurement flow rate value y measured by the flow rate sensor 2 and the actual flow rate Q.sub.out after the valve. In this case, the valve control part 41 controls the valve 3 so as to reduce the opening position of the valve 3 in accordance with the increment amount ΔQ.sub.in.sub._.sub.vol of the measurement flow rate value y. Therefore, this results in that, if there is no other influence, the decreased amount in flow due to disturbance ΔQ.sub.cl corresponding to the decreased amount of the opening position of the valve 3 appears in the actual flow rate Q.sub.out after the valve.

(28) The other disturbance influence is that the differential pressure before and after the valve 3 is increased due to the pressure rise due to disturbance ΔP.sub.in and the flow rate characteristic per se of the valve 3 is changed that the fluid tends to easily pass through the valve 3. In this case, since the flow rate passing through the valve 3 is increased due to the change of the flow rate characteristic even though the valve has the same the opening position, this results in that, if there is no other influence, the increased amount in flow due to disturbance ΔQ.sub.valve corresponding to the increment amount of the differential pressure appears in the actual flow rate Q.sub.out after the valve.

(29) In the present embodiment, by quantifying the decreased amount in flow due to disturbance ΔQ.sub.cl and the increased amount in flow due to disturbance ΔQ.sub.valve caused by the pressure rise due to disturbance ΔP.sub.in and setting the control coefficient so that these decreased amount in flow due to disturbance ΔQ.sub.cl and increased amount in flow due to disturbance ΔQ.sub.valve are substantially equal as shown in FIG. 4(c), this results in that the actual flow rate Q.sub.out after the valve is allowed to be kept at the set flow rate value r without being influenced by the pressure rise due to disturbance ΔP.sub.in. The following quantitatively describes each of the amounts.

(30) Since the increment amount ΔQ.sub.in.sub._.sub.vol of the flow rate of the fluid flowing into the internal volume VL caused by the pressure rise due to disturbance is proportional to the time derivative of the pressure rise due to disturbance ΔP.sub.in, the transfer function can be expressed as Equation 1.
[Equation 1]
Q.sub.in vol=α.Math.s.Math.ΔP.sub.in  (1)

(31) Here, α is a constant that does not depend on the set flow rate value r and this constant α is inflow characteristic data indicating a relationship between the pressure in the upstream side of the valve 3 and the inflow of the fluid flowing into the internal volume VL including at least the flow path 11 between the flow rate sensor 2 and the valve 3. This inflow characteristic data is previously stored in the inflow characteristic data storage part 45 and this inflow characteristic data is made available for reference by the control coefficient setting part 44.

(32) As to the block diagram of FIG. 4(a), by paying attention to only the influence of the internal volume VL while regarding the disturbance as the input and the actual flow rate Q.sub.out after the valve as the output, a block diagram as shown in FIG. 5(a) is obtained. Therefore, the transfer function of the decreased amount in flow due to disturbance ΔQ.sub.cl is expressed by Equation 2.

(33) $\begin{array}{cc}\left[\mathrm{Equation}2\right]& \\ \begin{array}{cc}\begin{array}{c}\Delta Q_{\mathrm{cl}}=\alpha .Math.s.Math.\Delta P_{in}*\frac{\left(\left(-\frac{1}{1+e_1.Math.s}\right)*\frac{\left(a_1.Math.s+1\right)}{b_1.Math.s}\right)}{\left(1+\left(-\frac{1}{1+e_1.Math.s}\right)*\frac{\left(a_1.Math.s+1\right)}{b_1.Math.s}\right)}\\ =-\frac{\alpha .Math.s}{b_1.Math.s+1}\Delta P_{in}\end{array}& \left(2\right)\end{array}& \end{array}$

(34) Here, a.sub.1 and e.sub.1 are set to be the same value.

(35) Meanwhile, regarding the valve flow rate characteristic that is a relationship between a flow rate and an opening position (applied voltage), as shown in FIG. 6(a), when the pressure is varied, the higher the pressure, the larger flow rate the fluid flows even at the same opening position. As shown in FIG. 6(b), the increased amount in flow due to disturbance ΔQ.sub.valve corresponds to a flow rate increased by the pressure rise due to disturbance ΔP.sub.in in the case of the same opening position. Based on this FIG. 6(b), when plotting on a graph the increased amount in flow due to disturbance ΔQ.sub.valve in the case where the pressure rise due to disturbances by 1 atm, i.e., a relationship between an increased amount in flow due to disturbance β per a unit pressure rise and the set flow rate value r which corresponds to the opening position, there can be obtained an approximately linear relationship as shown in FIG. 6(c), and it becomes possible to be modelled as a control block as shown in FIG. 4(a). Here, an inclination of β with respect to the set flow rate value r shown in FIG. 6(c) is created based on a change from a reference pressure P.sub.base. In the case where the upstream side of the valve 3 is maintained at a pressure higher than the reference pressure P.sub.base under a normal condition that there occurs no disturbance, the inclination of β with respect to r is decreased, and in the case where the upstream side of the valve 3 is maintained at a pressure lower than the reference pressure P.sub.base, the inclination of β with respect to r is increased. In the present embodiment, since the pressure of the valve 3 is changed on the basis of the reference pressure P.sub.base, only one graph of FIG. 6(c) is specified and used configured to set the control coefficient.

(36) The relationship between the set flow rate r and the increased amount in flow due to disturbance β per a unit pressure rise amount of the pressure in the upstream side of the valve 3 shown in FIG. 6(c) is previously stored in the valve flow rate characteristic storage part 46 as the valve flow rate characteristic data, and this data is available for reference by the control coefficient setting part 44.

(37) Then, since the increased amount in flow due to disturbance ΔQ.sub.valve at that time can be obtained by multiplying β determined every set flow rate value r by the pressure rise due to disturbance ΔP.sub.in, the increased amount in flow due to disturbance ΔQ.sub.valve can be expressed by Equation 3.
[Equation 3]
ΔQ.sub.valve=βΔP.sub.in  (3)

(38) Then, as to the block diagram of FIG. 4(a), by paying attention to only the influence of the change of the valve flow rate characteristic caused by the pressure rise due to disturbance ΔP.sub.in while regarding the disturbance as the input and the actual flow rate Q.sub.out after the valve as the output, a block diagram as shown in FIG. 5(b) is obtained. Therefore, the increased amount in flow due to disturbance ΔQ.sub.valve is expressed by Equation 4.

(39) $\begin{array}{cc}\left[\mathrm{Equation}4\right]& \\ \begin{array}{c}\Delta Q_{\mathrm{valve}}=\beta \Delta P_{in}*1/\left(1-\left(-\frac{1}{1+e_1.Math.s}\right)*\frac{\left(a_1.Math.s+1\right)}{b_1.Math.s}\right)\\ =\frac{\beta .Math.b_1.Math.s}{b_1.Math.s+1}\Delta P_{in}\end{array}& \left(4\right)\end{array}$

(40) Based on these Equations, the control coefficient setting part 44 sets the control coefficient so as to balance the increased amount in flow due to disturbance ΔQ.sub.valve expressed by Equation 4 and the decreased amount in flow due to disturbance ΔQ.sub.cl expressed by Equation 2.

(41) Specifically, Equation 5 is obtained.

(42) $\begin{array}{cc}\left[\mathrm{Equation}5\right]& \\ \Delta Q_{\mathrm{cl}}+\Delta Q_{\mathrm{valve}}=-\frac{\alpha .Math.s}{b_1.Math.s+1}\Delta P_{in}+\frac{\beta .Math.b_1.Math.s}{b_1.Math.s+1}\Delta P_{in}=0& \left(5\right)\end{array}$

(43) By solving this equation, Equation 6 is obtained and the control coefficient b.sub.1 can be determined so as to prevent the pressure fluctuation in the upstream side of the valve 3 from appearing in the actual flow rate Q.sub.out after the valve at all.

(44) $\begin{array}{cc}\left[\mathrm{Equation}6\right]& \\ b_1=\frac{\alpha }{\beta }& \left(6\right)\end{array}$

(45) Thus, the control coefficient setting part 44 sets b.sub.1 which is one of the control coefficients of the valve control part 41 using α and β. As shown in FIG. 7(a), α takes a constant value for the set flow rate value r, and β is a variable which becomes larger as the set flow rate value r becomes larger. Therefore, as shown in FIG. 7(b), the control coefficient b.sub.1 can be set so as to be substantially inversely proportional to the set flow rate value r.

(46) Further, the control coefficient b.sub.1 is in a relationship of being equal to a value obtained by multiplying a reciprocal of a proportional gain Kp in the PID control by a constant. Therefore, it is found that the proportional gain Kp may be merely set so as to be increased in proportional to the set flow rate value r as shown in FIG. 7(c).

(47) Thus, in the flow rate control apparatus 100 of the present embodiment, since it is configured that the control coefficient setting part 44 sets b.sub.1 and the proportional gain Kp so that the increased amount in flow due to disturbance ΔQ.sub.valve due to a pressure rise in the upstream side of the valve 3 and the decreased amount in flow due to disturbance ΔQ.sub.cl are balanced and offset, it is possible to prevent the influence of the pressure fluctuation from appearing in the actual flow rate Q.sub.out after the valve.

(48) That is, by setting the control coefficient as described above, the transfer function from the pressure fluctuation in the upstream side of the valve 3 to the flow rate in the downstream side of the valve 3 is made substantially zero in the feedback control system to thereby make it possible to cancel the influence of the pressure disturbance. Thus, the flow rate in the downstream side of the valve 3 can be made stable substantially constantly at the set flow rate value r.

(49) Moreover, since the pressure disturbance is prevented from appearing by using the characteristics of the feedback control system per se, there is no need to detect the pressure disturbance in the flow rate control apparatus 100 of the present embodiment. Therefore, in the case where the pressure in the upstream side of the valve 3 is substantially kept based on only the reference pressure P.sub.base under a condition of no disturbance occurrence, a robust flow rate control can be realized against the disturbance without providing an additional sensor for detecting a disturbance as in the conventional device.

(50) Other embodiments will be described.

(51) In order to set a control coefficient so as to be able to fully exhibit a flow rate control capability of the above embodiment even in the case where the kind of the fluid flowing through the flow path is changed, it may be configured that the control coefficient setting part corrects the control coefficient on the basis of the molar specific heat of the fluid species. More specifically, there has been already known a control coefficient b.sub.1 to be determined as to, for example, inert gas such as nitrogen or helium, and in the case where other kinds of fluid flows, the control coefficient can be corrected to b.sub.1 appropriate to the fluid species by multiplying b.sub.1 by a ratio of a molar specific heat of each of the fluid species. Note that, there is a tendency that the influence of the pressure fluctuation of the valve is less affected as the higher molar specific heat the fluid has.

(52) In the above embodiment, although it is configured that the decreased amount in flow due to disturbance and the increased amount in flow due to disturbance are offset by setting the gain b.sub.1 or proportional gain K.sub.p for the PID control as the control coefficient, the offset may be performed by setting other control coefficients appropriately.

(53) Regarding the control coefficient to be set, it may be appropriately calculated using α and β described above, or previously preparing, for example, in a table format, a relationship between the set flow rate values and the gains for offsetting between the decreased amount in flow due to disturbance and the increased amount in flow due to disturbance, and the control coefficient setting part may be configured so as to set the control coefficient of the valve control part by referring to this table.

(54) In the above embodiment, although a thermal flow rate sensor is used as the flow rate sensor, other types flow rate sensors using other measurement principles may be available. Specifically, a pressure type flow rate sensor may be used.

(55) In the above embodiment, although the control coefficient for use in the valve control part is appropriately set every set flow rate value by the control coefficient setting part, the control coefficient may be fixed by omitting the control coefficient setting part in a usage such that the flow rate control is performed only at, e.g., a predetermined constant set flow rate value. Further, the control coefficient setting part may be configured to set the control coefficient on the basis of not only the set flow rate value but also the pressure of the fluid flowing through the flow path so that the increased amount in flow due to disturbance and the decreased amount in flow due to disturbance are substantially balanced. More specifically, even in the case where the pressure in the upstream side of the valve 3 is changed to be kept from the reference pressure P.sub.base to other pressure even in the no disturbance condition and the inclination of β with respect to the set flow rate value r is changed, in order to balance the increased amount in flow due to disturbance and the decreased amount in flow due to disturbance, it may be configured that, as shown in FIG. 8, a pressure sensor 5 is previously provided on the upstream side of the valve 3 and the control coefficient setting part 44 changes β based on the measured pressure value and the set flow rate value and sets the control coefficient such as the proportional gain K.sub.p. Further, since β is a value which is varied in accordance with gas species, it may be configured that the control coefficient setting part accepts the gas species and sets the control coefficient corresponding to the gas species. By this arrangement, there can be realized a flow rate control further ensuring capability of dealing with the pressure disturbance.

(56) In the above embodiment, although the control coefficient, the increased amount in flow due to disturbance and the decreased amount in flow due to disturbance are defined on the basis that the pressure in the upstream of the valve rises, it is, of course, also possible to define them on the basis that the disturbance pressure is reduced. In this case, it can be explained that, in the case where there occurs a disturbance pressure reduction in the upstream side of the valve, the control coefficient is a value which is set so that the increased amount in flow due to disturbance which is an increment amount of the flow rate corresponding to the increment amount of the opening position of the valve that is increased by the valve control part in accordance with the decreased amount of the measurement flow rate value due to the disturbance pressure reduction and the decreased amount in flow due to disturbance which is a decreased amount of the flow rate corresponding to the differential pressure decreased amount before and after the valve due to the decreased amount in flow due to disturbance are substantially balanced.

(57) In the above embodiment, although the valve control part is configured to control the flow rate by the PID control, the flow rate may be controlled based on such as I-PD control and other control algorithms. Moreover, in the above embodiment, although the flow rate control is executed on the basis of the QV characteristic of the valve, the valve may be controlled on the basis of, for example, a relationship between the flow rate and the opening position per se of the valve or a relationship between the flow rate and a position of a valve body of the valve. More specifically, it may be configured that the valve is previously equipped with a displacement sensor capable of measuring an opening position of the valve or a position of the valve body and the valve can be controlled by feeding back the output of the displacement sensor. In this case, it is possible to realize a high-speed control by further improvement of the responsibility of the valve.

(58) In order to be able to realize the flow rate control as in the present invention by retrofitting even in an existing flow rate control apparatus, it is merely necessary that a program is installed in the existing flow rate control apparatus using a program storage medium storing, for example, a program for the flow rate control apparatus to thereby exhibit the functions of the valve control part and the control coefficient setting part of the present invention. Note that various kinds of medium such as CD, DVD, HDD, flash memory and the like can be used as the storage medium.

(59) It is needless to say that various combinations and modifications of the embodiments can be made in a range without departing from the spirit thereof.

REFERENCE SIGNS LIST

(60) 100: Flow rate control apparatus 1: Body 11: Flow path 2: Flow rate sensor 3: Valve 4: Control mechanism 41: Valve control part 42: PID control part 43: QV characteristic adjustment part 44: Control coefficient setting part 45: Inflow characteristic storage part 46: Valve flow rate characteristic storage part