Fluid control device, fluid control method, and fluid control program

Abstract

To improve flow rate measurement accuracy in a state in which a valve is fully closed.SOLUTION: A fluid control device comprises: a fluid resistive element 33 which is provided on a flow path 2R; an upstream pressure sensor 31a which detects an upstream pressure P1 of the fluid resistive element 33; a downstream pressure sensor 31b which detects a downstream pressure P2 of the fluid resistive element 33; a flow rate calculation unit 4a which calculates a flow rate Q for a flow through the flow path 2R based on the upstream pressure P1 and the downstream pressure P2; a valve 32 which is provided upstream of the upstream pressure sensor 31a or downstream of the downstream pressure sensor 31b; and a valve control unit 4b which controls the valve 32 based on the flow rate Q calculated by the flow rate calculation unit 4a. When the valve 32 is fully closed, the flow rate calculation unit 4a calculates the flow rate Q by switching from a first flow rate calculation method Eq1 used when the valve 32 is open to a second flow rate calculation method Eq2 different from the first flow rate calculation method Eq1.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a fluid control device, a fluid control method, and a fluid control program. [Background technology]

[0002] Conventionally, as shown in Patent Document 1, a fluid control device has been considered that includes a control valve provided upstream of a throttle section, a first pressure sensor provided upstream of the throttle section and downstream of the control valve, a second pressure sensor provided downstream of the throttle section, an inflow pressure sensor provided upstream of the control valve, and a control circuit that controls the control valve. In this fluid control device, with the control valve fully closed, the outflow (seat leak) of the control valve is detected based on the time change in the inflow pressure, upstream pressure, or downstream pressure measured by the inflow pressure sensor after the upstream pressure measured by the first pressure sensor and the downstream pressure measured by the second pressure sensor reach an equilibrium pressure. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-87164 Summary of the Invention [Problem to be solved by the invention]

[0004] When a seat leak occurs in a conventional fluid control device, gas accumulated on the primary side (upstream side of the control valve) flows into the secondary side (downstream side of the control valve), resulting in an increase in pressure on the secondary side. Specifically, both the upstream and downstream pressures on the secondary side increase.

[0005] Conventional fluid control devices use a flow rate calculation formula that uses the difference between the power of the upstream pressure and the power of the downstream pressure (specifically, "the square of the upstream pressure" minus "the square of the downstream pressure"). Therefore, even if the differential pressure between the upstream and downstream pressures remains the same, an increase in the upstream and downstream pressures due to a seat leak increases the difference between the square of the upstream pressure and the square of the downstream pressure. As a result, as shown in Figure 4, the flow rate calculated using the above flow rate calculation formula increases, causing a shift in the zero point.

[0006] Therefore, the present invention has been made in consideration of the above-mentioned problems, and its main object is to improve the accuracy of measuring the flow rate when the valve is in a fully closed state. [Means for solving the problem]

[0007] That is, the fluid control device of the present invention comprises a fluid resistance element provided in a flow path, an upstream pressure sensor that detects the upstream pressure of the fluid resistance element, a downstream pressure sensor that detects the downstream pressure of the fluid resistance element, a flow rate calculation unit that calculates the flow rate flowing through the flow path based on the upstream pressure and the downstream pressure, a valve provided upstream of the upstream pressure sensor or downstream of the downstream pressure sensor, and a valve control unit that controls the valve based on the flow rate calculated by the flow rate calculation unit, and is characterized in that when the valve is in a fully closed state, the flow rate calculation unit calculates the flow rate by switching from a first flow rate calculation formula used when the valve is in an open state to a second flow rate calculation formula different from the first flow rate calculation formula.

[0008] With such a fluid control device, when the valve is in a fully closed state, the first flow rate calculation formula used when the valve is in an open state is switched to a second flow rate calculation formula that is different from the first flow rate calculation formula, so that the flow rate can be calculated accurately even if pressure fluctuations occur due to the outflow (seat leak) from the valve.

[0009] As a specific embodiment of the flow rate calculation unit, when the valve is in a fully closed state, it is desirable that the flow rate calculation unit calculates the flow rate by switching from the first flow rate calculation formula using the difference between the power of the upstream pressure (where the power number is greater than 1) and the power of the downstream pressure (where the power number is greater than 1) to a second flow rate calculation formula using the difference between the upstream pressure and the downstream pressure.

[0010] As a specific embodiment of switching the flow rate calculation formula, it is desirable that the flow rate calculation unit switches from the first flow rate calculation formula to the second flow rate calculation formula to calculate the flow rate when the difference between the upstream pressure and the downstream pressure is smaller than a predetermined switching threshold.

[0011] As a further specific embodiment of switching the flow rate calculation formula, it is desirable that the flow rate calculation unit switches from the first flow rate calculation formula to the second flow rate calculation formula when the flow rate calculated by the first flow rate calculation formula is in a stable state. With this configuration, it is possible to prevent frequent switching between the first flow rate calculation formula and the second flow rate calculation formula.

[0012] Furthermore, if the first flow rate calculation formula and the second flow rate calculation formula are simply switched based on a comparison with a predetermined switching threshold, the switching of the flow rate calculation formula may occur frequently, resulting in a distorted change in the flow rate. To suitably solve this problem, it is desirable to provide hysteresis for the predetermined switching threshold. Specifically, it is desirable that the flow rate calculation unit switches from the first flow rate calculation formula to the second flow rate calculation formula to calculate the flow rate when the difference between the upstream pressure and the downstream pressure is smaller than the value obtained by subtracting hysteresis from the predetermined switching threshold.

[0013] As a specific embodiment of switching from the second flow rate calculation formula to the first flow rate calculation formula, it is desirable that the flow rate calculation unit switches from the second flow rate calculation formula to the first flow rate calculation formula to calculate the flow rate when the difference between the upstream pressure and the downstream pressure is equal to or greater than a predetermined switching threshold.

[0014] Furthermore, when a conventional fluid control device is used in semiconductor manufacturing equipment, air pressure valves are provided upstream and downstream of the fluid control device to improve maintenance, such as by removing the fluid control device, or to prevent damage to wafers due to valve leakage (seat leaks). This air valve is often designed to close simultaneously when the flow rate setting of the fluid control device is 0% (fully closed), so the gas that was flowing before the air valve was closed remains inside the fluid control device, and the pressure inside the fluid control device is maintained at a relatively high level. In this state, when performing zero point adjustment of the flow rate in the fluid control device, a special sequence for evacuating the internal flow path of the fluid control device must be performed in addition to the sequence required for the semiconductor manufacturing process.

[0015] Therefore, the fluid control device of the present invention comprises a fluid resistance element provided in a flow path, an upstream pressure sensor that detects the upstream pressure of the fluid resistance element, a downstream pressure sensor that detects the downstream pressure of the fluid resistance element, a flow rate calculation unit that calculates the flow rate through the flow path based on the upstream pressure and the downstream pressure, a valve provided upstream of the upstream pressure sensor or downstream of the downstream pressure sensor, and a valve control unit that controls the valve based on the flow rate calculated by the flow rate calculation unit, and is characterized in that when the valve is in a fully closed state, the flow rate calculation unit detects that the difference between the upstream pressure and the downstream pressure has remained within a certain range for a predetermined period of time, and corrects either the upstream pressure or the downstream pressure to match the other. With this configuration, the difference (offset caused by drift) between the upstream pressure detected by the upstream pressure sensor and the downstream pressure detected by the downstream pressure sensor is automatically detected, and the upstream pressure and downstream pressure are set to the same value, so that changes in flow rate accuracy over time can be reduced without performing a special sequence for adjusting the flow rate zero point.

[0016] As a specific embodiment of the correction by the flow rate calculation unit, it is desirable that when the valve is in a fully closed state, the flow rate calculation unit detects that the standard deviation of the difference between the upstream pressure and the downstream pressure remains within a certain range for a predetermined period of time, and corrects either the upstream pressure or the downstream pressure to match the other.

[0017] Since the upstream pressure is high and the downstream pressure is low (vacuum), if the upstream pressure is corrected to match the downstream pressure in a flow rate calculation formula using the difference between the power of the upstream pressure (where the power number is greater than 1) and the power of the downstream pressure (where the power number is greater than 1), an error in the flow rate calculation may become large. Therefore, it is desirable for the flow rate calculation unit to correct the downstream pressure to match the upstream pressure.

[0018] Furthermore, the flow rate control method of the present invention is a flow rate control method that uses a fluid resistance element provided in a flow path, an upstream pressure sensor that detects the upstream pressure of the fluid resistance element, a downstream pressure sensor that detects the downstream pressure of the fluid resistance element, a flow rate calculation unit that calculates the flow rate through the flow path based on the upstream pressure and the downstream pressure, a valve provided upstream of the upstream pressure sensor or downstream of the downstream pressure sensor, and a valve control unit that controls the valve based on the flow rate calculated by the flow rate calculation unit, and is characterized in that when the valve is in a fully closed state, the flow rate calculation unit switches from a first flow rate calculation formula used when the valve is in an open state to a second flow rate calculation formula different from the first flow rate calculation formula to calculate the flow rate.

[0019] Furthermore, the flow control program of the present invention is a flow control program used in a fluid control device comprising a fluid resistance element provided in a flow path, an upstream pressure sensor that detects the upstream pressure of the fluid resistance element, a downstream pressure sensor that detects the downstream pressure of the fluid resistance element, a flow calculation unit that calculates the flow rate through the flow path based on the upstream pressure and the downstream pressure, a valve provided upstream of the upstream pressure sensor or downstream of the downstream pressure sensor, and a valve control unit that controls the valve based on the flow rate calculated by the flow calculation unit, and is characterized in that when the valve is in a fully closed state, the flow calculation unit is caused to perform the function of calculating the flow rate by switching from a first flow rate calculation formula used when the valve is in an open state to a second flow rate calculation formula different from the first flow rate calculation formula.

[0020] Furthermore, the flow control method of the present invention is a flow control method that uses a fluid resistance element provided in a flow path, an upstream pressure sensor that detects the upstream pressure of the fluid resistance element, a downstream pressure sensor that detects the downstream pressure of the fluid resistance element, a flow calculation unit that calculates the flow rate flowing through the flow path based on the upstream pressure and the downstream pressure, a valve provided upstream of the upstream pressure sensor or downstream of the downstream pressure sensor, and a valve control unit that controls the valve based on the flow rate calculated by the flow calculation unit, and is characterized in that when the valve is in a fully closed state, the flow calculation unit detects that the difference between the upstream pressure and the downstream pressure has remained within a certain range for a predetermined period of time, and corrects either the upstream pressure or the downstream pressure to match the other.

[0021] In addition, the flow control program of the present invention is a flow control program used in a fluid control device that includes a fluid resistance element provided in a flow path, an upstream pressure sensor that detects the upstream pressure of the fluid resistance element, a downstream pressure sensor that detects the downstream pressure of the fluid resistance element, a flow calculation unit that calculates the flow rate through the flow path based on the upstream pressure and the downstream pressure, a valve provided upstream of the upstream pressure sensor or downstream of the downstream pressure sensor, and a valve control unit that controls the valve based on the flow rate calculated by the flow calculation unit, and is characterized in that when the valve is in a fully closed state, the flow control program causes the flow calculation unit to perform the function of detecting that the difference between the upstream pressure and the downstream pressure remains within a certain range for a predetermined period of time, and correcting one of the upstream pressure or the downstream pressure to match the other. [Effects of the Invention]

[0022] According to the present invention as described above, it is possible to improve the accuracy of measuring the flow rate when the valve is fully closed. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a diagram schematically illustrating the configuration of a fluid control device according to an embodiment of the present invention. [Figure 2] 5 is a schematic diagram showing switching of a flow rate calculation formula in the same embodiment. FIG. [Figure 3] FIG. 10 is a diagram showing offset correction in a fluid control device according to a modified embodiment. [Figure 4] FIG. 1 is a schematic diagram showing a change in the measured flow rate due to an outflow (seat leak). DETAILED DESCRIPTION OF THE INVENTION

[0024] A fluid control device according to an embodiment of the present invention will be described below with reference to the drawings. Note that, for ease of understanding, all of the drawings shown below are drawn in a schematic manner with appropriate omissions or exaggerations. Identical components are given the same reference numerals, and their descriptions will be omitted where appropriate.

[0025] <Device configuration> The fluid control device 100 of this embodiment is used in, for example, a semiconductor manufacturing process, and is provided on one or more gas supply lines to control the flow rate of the process gas flowing through each gas supply line.

[0026] Specifically, the fluid control device 100 is a so-called differential pressure mass flow controller (differential pressure MFC), and as shown in Figure 1, it comprises a flow path block 2 in which multiple internal flow paths 2R are formed, and a fluid control device 3 mounted on the flow path block 2.

[0027] The flow path block 2 is provided with an inlet port 21 that introduces a fluid into the internal flow path 2R and an outlet port 22 that discharges the fluid from the internal flow path 2R. An upstream pipe H1 is connected to the inlet port 21, and an upstream air pressure valve V1 is provided in the upstream pipe H1. A downstream pipe H2 is connected to the outlet port 22, and a downstream air pressure valve V2 is provided in the downstream pipe H2.

[0028] The fluid control device 3 controls the fluid in the internal flow path 2R, and has a flow rate sensor 31 that measures the flow rate of the fluid flowing through the internal flow path 2R, and a fluid control valve 32 that is provided upstream of the flow rate sensor 31. The valve opening of the fluid control valve 32 is feedback-controlled by the control unit 4, which will be described later.

[0029] The flow rate sensor 31 is a differential pressure type flow rate sensor, and has an upstream pressure sensor 31a provided upstream of a fluid resistance element 33, such as a restrictor or orifice, provided in the internal flow path 2R, and a downstream pressure sensor 31b provided downstream of the fluid resistance element 33. Then, a flow rate calculation unit 4a of the control unit 4, which will be described later, calculates the flow rate Q flowing through the internal flow path 2R using the upstream pressure P1 of the fluid resistance element 33 detected by the upstream pressure sensor 31a and the downstream pressure P2 of the fluid resistance element 33 detected by the downstream pressure sensor 31b.

[0030] The fluid control valve 32 is provided upstream of the differential pressure flow sensor 31. Specifically, the fluid control valve 32 controls the flow rate by moving a valve element back and forth relative to a valve seat using a piezoelectric actuator. The fluid control valve 32 is controlled by the valve control unit 4b of the control unit 4.

[0031] The control unit 4 has a flow rate calculation unit 4a that calculates the flow rate Q through the internal flow path 2R based on the upstream pressure P1 and the downstream pressure P2, and a valve control unit 4b that controls the fluid control valve 32 based on the flow rate Q calculated by the flow rate calculation unit 4a. The control unit 4 is a so-called computer that includes, for example, a CPU, memory, A / D and D / A converters, and input / output means, and performs the functions of the flow rate calculation unit 4a, the valve control unit 4b, etc. by executing a flow rate control program stored in the memory and causing various devices to work together.

[0032] Therefore, when the fluid control valve 32 is in a fully closed state, the flow rate calculation unit 4a of this embodiment switches from the first flow rate calculation formula Eq1 used when the fluid control valve 32 is in an open state to a second flow rate calculation formula Eq2 different from the first flow rate calculation formula Eq1 to calculate the flow rate Q.

[0033] Here, the first flow rate calculation formula Eq1 used when the fluid control valve 32 is in an open state can be simply expressed as the following formula using the power of the upstream pressure P1 (here, the power number is 2) and the power of the downstream pressure P2 (here, the power number is 2). Eq1=k1×(P1 2 -P2 2 ) Here, k1 is a value determined by the temperature, viscosity, and compressibility of the fluid, and the shape of the fluid resistance element 33. The exponent is not limited to 2 and can be changed to a value greater than 1.

[0034] The second flow rate calculation formula Eq2 can be simply expressed as follows using the difference (|P1-P2|) between the upstream pressure P1 and the downstream pressure P2. Eq2=k2×|P1-P2| However, k2 is a value determined by the temperature, viscosity, and compressibility of the fluid, the shape of the fluid resistance element 33, and the like.

[0035] More specifically, the second flow rate calculation formula Eq2 is the flow rate Q obtained using the first flow rate calculation formula Eq1 in a steady state. stable can be expressed by the following formula using Eq2=Q stable ×(P1-P2 / ΔP stable ) However, ΔP stable is the difference between the upstream pressure P1 and the downstream pressure P2 in a steady state.

[0036] When the fluid control valve 32 is in a fully closed state, the flow rate calculation unit 4a switches from the first flow rate calculation formula Eq1 to the second flow rate calculation formula Eq2 to calculate the flow rate Q. When the fluid control valve 32 is in a fully closed state, the upstream air pressure valve V1 provided in the upstream pipe H1 and the downstream air pressure valve V2 provided in the downstream pipe H2 are both in a closed state.

[0037] When the difference between the upstream pressure P1 and the downstream pressure P2 is smaller than a predetermined switching threshold, the flow rate calculation unit 4a switches from the first flow rate calculation formula Eq1 to the second flow rate calculation formula Eq2 to calculate the flow rate Q. In this embodiment, when the difference between the upstream pressure P1 and the downstream pressure P2 is smaller than a value obtained by subtracting hysteresis from the predetermined switching threshold, the flow rate calculation unit 4a switches from the first flow rate calculation formula Eq1 to the second flow rate calculation formula Eq2 to calculate the flow rate Q when the flow rate Q calculated by the first flow rate calculation formula Eq1 is in a stable state.

[0038] On the other hand, when the difference between the upstream pressure P1 and the downstream pressure P2 is equal to or greater than a predetermined switching threshold, the flow rate calculation unit 4a switches from the second flow rate calculation formula Eq2 to the first flow rate calculation formula Eq1 to calculate the flow rate Q.

[0039] As described above, by changing the threshold for switching from the first flow rate calculation formula Eq1 to the second flow rate calculation formula Eq2 ("switching threshold" - "hysteresis") and the threshold for switching from the second flow rate calculation formula Eq2 to the first flow rate calculation formula Eq1 ("switching threshold"), it is made difficult to return to the first flow rate calculation formula Eq1 after switching to the second flow rate calculation formula Eq2, thereby preventing frequent switching of the flow rate calculation formula.

[0040] The following describes the conditional expressions for switching between the first flow rate calculation formula Eq1 and the second flow rate calculation formula Eq2 in the flow rate calculation unit 4a. Switching between the flow rate calculation formulas Eq1 and Eq2 using the following conditional expressions is shown in Figure 2. The function for switching between the first flow rate calculation formula Eq1 and the second flow rate calculation formula Eq2 can be switched ON / OFF.

[0041] (1) Switching from the first flow rate calculation formula Eq1 to the second flow rate calculation formula Eq2 (when the previous flow rate was calculated using the first flow rate calculation formula Eq1) (Switching threshold - hysteresis) > |P1-P2| and in a stable state

[0042] Here, the switching threshold is a predetermined value. The hysteresis is, for example, 1 Torr, and is provided to prevent frequent switching of the flow rate calculation formula. Furthermore, the stable state is a state in which the flow rate Q calculated by the first flow rate calculation formula Eq1 is continuously within a predetermined range (here, ±0.1%) of the target flow rate (for example, a flow rate setting of 0%) for a predetermined period of time.

[0043] (2) Switching from the second flow calculation formula Eq2 to the first flow calculation formula Eq1 (when the previous flow rate was calculated using the second flow calculation formula Eq2) (Switching threshold)<|P1-P2| Here, the switching threshold is the same as the switching threshold in the above conditional expression (1).

[0044] <Effects of this embodiment> In the fluid control device 100 of this embodiment configured as described above, when the fluid control valve 32 is in a fully closed state, the first flow rate calculation formula Eq1 used when the fluid control valve 32 is in an open state is switched to a second flow rate calculation formula Eq2 that is different from the first flow rate calculation formula Eq1. Therefore, even if pressure fluctuations occur due to the outflow (seat leak) of the fluid control valve, the flow rate can be calculated with high accuracy.

[0045] <Other embodiments> For example, in addition to the configuration in which the flow rate calculation formula is switched as in the above embodiment, the following configuration may also be used.

[0046] Specifically, as shown in FIG. 3, when the fluid control valve 32 is in a fully closed state, the flow rate calculation unit 4a detects that the difference between the upstream pressure P1 and the downstream pressure P2 remains within a certain range for a predetermined period of time, and corrects either the upstream pressure P1 or the downstream pressure P2 to match the other.

[0047] Specifically, when the fluid control valve 32 is in a fully closed state, the flow rate calculation unit 4a detects that the standard deviation of the difference between the upstream pressure P1 and the downstream pressure P2 remains within a certain range (for example, less than ±0.5) for a predetermined period of time (for example, 30 minutes), and corrects either the upstream pressure P1 or the downstream pressure P2 to match the other.

[0048] Here, since the upstream pressure P1 is high and the downstream pressure P2 is low (vacuum), in a flow rate calculation formula using the difference between the power of the upstream pressure P1 (where the power number is greater than 1) and the power of the downstream pressure P2 (where the power number is greater than 1), correcting the upstream pressure P1 to match the downstream pressure P2 may result in a large error in the flow rate calculation. Therefore, the flow rate calculation unit 4a corrects the downstream pressure P2 to match the upstream pressure P1. Since the downstream pressure P2 is matched to the upstream pressure P1 in this way, it is possible to reduce the error in the flow rate calculation using the difference between the power of the upstream pressure P1 (where the power number is greater than 1) and the power of the downstream pressure P2 (where the power number is greater than 1).

[0049] Alternatively, the upstream pressure P1 may be set to the downstream pressure P2, or the upstream pressure P1 and the downstream pressure P2 may be set to a pressure between them (for example, a median value).

[0050] In the above embodiment, the fluid control valve 32 is configured to be provided upstream of the flow rate sensor 31, but it may also be configured to be provided downstream of the flow rate sensor 31.

[0051] Furthermore, in the above embodiment, hysteresis is provided in the conditional expression for switching from the first flow rate calculation formula to the second flow rate calculation formula, but hysteresis may not be provided and only a switching threshold may be used.Furthermore, the conditional expression for switching from the first flow rate calculation formula to the second flow rate calculation formula does not need to include the stable state of the flow rate obtained using the first flow rate calculation formula.

[0052] In addition, various modifications and combinations of the embodiments may be made as long as they do not go against the spirit of the present invention. [Explanation of symbols]

[0053] 100 Fluid control device 2R...flow path 31a...Upstream pressure sensor 31b Downstream pressure sensor 32 Fluid control valve 33 Fluid resistance element 4a...Flow rate calculation section 4b Valve control section

Claims

1. a fluid resistance element provided in the flow path; an upstream pressure sensor for detecting an upstream pressure of the fluid resistance element; a downstream pressure sensor for detecting a downstream pressure of the fluid resistance element; a flow rate calculation unit that calculates a flow rate through the flow path based on the upstream pressure and the downstream pressure; a valve provided upstream of the upstream pressure sensor or downstream of the downstream pressure sensor; a valve control unit that controls the valve based on the flow rate calculated by the flow rate calculation unit, The flow rate calculation unit, when the valve is in a fully closed state, calculates the flow rate by switching from a first flow rate calculation formula using the difference between the power of the upstream pressure (where the power number is greater than 1) and the power of the downstream pressure (where the power number is greater than 1), which is used when the valve is in an open state, to a second flow rate calculation formula using the difference between the upstream pressure and the downstream pressure, which is different from the first flow rate calculation formula.

2. 2. The fluid control device according to claim 1, wherein the flow rate calculation unit calculates the flow rate by switching from the first flow rate calculation formula to the second flow rate calculation formula when a difference between the upstream pressure and the downstream pressure is smaller than a predetermined switching threshold.

3. A fluid resistance element provided in a flow path; an upstream pressure sensor for detecting an upstream pressure of the fluid resistance element; a downstream pressure sensor for detecting a downstream pressure of the fluid resistance element; a flow rate calculation unit that calculates a flow rate through the flow path based on the upstream pressure and the downstream pressure; a valve provided upstream of the upstream pressure sensor or downstream of the downstream pressure sensor; a valve control unit that controls the valve based on the flow rate calculated by the flow rate calculation unit, the flow rate calculation unit, when the valve is in a fully closed state, the difference between the upstream pressure and the downstream pressure is smaller than a predetermined switching threshold, and the flow rate calculated using a first flow rate calculation formula used when the valve is in an open state is within a predetermined range of a target flow rate for a predetermined continuous period of time, switches from the first flow rate calculation formula to a second flow rate calculation formula different from the first flow rate calculation formula to calculate the flow rate.

4. A fluid resistance element provided in a flow path; an upstream pressure sensor for detecting an upstream pressure of the fluid resistance element; a downstream pressure sensor for detecting a downstream pressure of the fluid resistance element; a flow rate calculation unit that calculates a flow rate through the flow path based on the upstream pressure and the downstream pressure; a valve provided upstream of the upstream pressure sensor or downstream of the downstream pressure sensor; a valve control unit that controls the valve based on the flow rate calculated by the flow rate calculation unit, When the valve is in a fully closed state and the difference between the upstream pressure and the downstream pressure is smaller than a predetermined switching threshold minus hysteresis, the flow rate calculation unit switches from a first flow rate calculation formula used when the valve is in an open state to a second flow rate calculation formula different from the first flow rate calculation formula to calculate the flow rate.

5. 5. The fluid control device according to claim 2, wherein the flow rate calculation unit calculates the flow rate by switching from the second flow rate calculation formula to the first flow rate calculation formula when a difference between the upstream pressure and the downstream pressure is equal to or greater than a predetermined switching threshold.

6. a flow rate control method using a fluid resistance element provided in a flow path, an upstream pressure sensor that detects an upstream pressure of the fluid resistance element, a downstream pressure sensor that detects a downstream pressure of the fluid resistance element, a flow rate calculation unit that calculates a flow rate through the flow path based on the upstream pressure and the downstream pressure, a valve provided upstream of the upstream pressure sensor or downstream of the downstream pressure sensor, and a valve control unit that controls the valve based on the flow rate calculated by the flow rate calculation unit, a flow rate control method in which, when the valve is in a fully closed state, the flow rate calculation unit calculates the flow rate by switching from a first flow rate calculation formula using the difference between the power of the upstream pressure (where the power number is greater than 1) and the power of the downstream pressure (where the power number is greater than 1), which is used when the valve is in an open state, to a second flow rate calculation formula using the difference between the upstream pressure and the downstream pressure, which is different from the first flow rate calculation formula.

7. a flow rate control program for use in a fluid control device comprising: a fluid resistance element provided in a flow path; an upstream pressure sensor for detecting an upstream pressure of the fluid resistance element; a downstream pressure sensor for detecting a downstream pressure of the fluid resistance element; a flow rate calculation unit for calculating a flow rate through the flow path based on the upstream pressure and the downstream pressure; a valve provided upstream of the upstream pressure sensor or downstream of the downstream pressure sensor; and a valve control unit for controlling the valve based on the flow rate calculated by the flow rate calculation unit, a flow rate control program that causes the flow rate calculation unit to perform a function of calculating the flow rate by switching, when the valve is in a fully closed state, from a first flow rate calculation formula that uses the difference between the power of the upstream pressure (where the power number is greater than 1) and the power of the downstream pressure (where the power number is greater than 1), which is used when the valve is in an open state, to a second flow rate calculation formula that uses the difference between the upstream pressure and the downstream pressure, which is different from the first flow rate calculation formula.

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