Method and apparatus for self verification of pressure-based mass flow controllers
A mass flow control system can be self verified for its accuracy when controlling a flow to a process. The system comprises: a control valve for controlling the flow of fluid through the system as a function of a control signal; a controller for generating the control signal as a function of measured flow of fluid through the system and a targeted flow set point; a pressure sensor for measuring the controlling fluid pressure for use in measuring and verifying the flow rate; and a source of fluid for providing a known volume of fluid for use in verifying the system accuracy anytime between steps of the flow control process.
1. A self-verifying mass flow control system for real time accuracy verification when controlling a flow of a fluid to a process, the system comprising:
an inlet that receives a fluid under pressure;
an outlet that delivers the fluid under pressure;
a conduit through which the fluid flows under pressure;
a control valve that controls the flow of fluid within the conduit from the inlet to the outlet;
a reservoir connected to the conduit that stores a known volume of fluid;
a flow restrictor between the reservoir and the outlet that controllably restricts fluid flow between the reservoir and the outlet;
a single pressure sensor that is coupled to the conduit and senses the pressure of the fluid within the conduit providing as an output a signal indicative of the pressure; and
a controller that controls position of the control valve and uses the signal from the single pressure sensor for two different operations of the mass flow control system:
(i) flow control of the control valve for a process operation, based on the signal from the pressure sensor so as to cause the flow rate of fluid through the conduit to be equal to a flow set point; and
(ii) flow verification of the accuracy of the fluid flow control by determining a rate of pressure decay within the reservoir based on the signal from the pressure sensor.
2. A mass flow control system according to claim 1 wherein the flow restrictor is controlled to create choked flow conditions for the flow of fluid through the conduit.
3. A mass flow control system according to claim 2 , wherein the flow restrictor has an orifice whose cross sectional area is adjustable.
4. A mass flow control system according to claim 2 , further including a second control valve for providing an adjustable opening that defines the flow restrictor.
5. A mass flow control system according to claim 1 , further including a temperature sensor configured to provide a temperature measurement signal representative of the measured temperature of fluid in the conduit.
6. A mass flow control system according to claim 5 , wherein the controller is configured to determine the measured flow of fluid Q p through the conduit as a function of the measured pressure and temperature of the fluid in the system as
Q p =C′·A·f ( m,γ,T )· P u ,
where C′ is the orifice discharge coefficient of the flow restrictor, A the effective orifice area of the flow restrictor, m the molecular weight of the fluid, γ the specific heat capacity ratio of the fluid, T the fluid temperature, P u the pressure, and f(m, γ, T) a mathematic function which is related to the fluid molecular weight, the specific heat capacity of the fluid, and the fluid temperature.
7. A mass flow control system according to claim 1 , wherein the reservoir is positioned downstream from the control valve such that the control valve is closed when a zero flow set point is commanded, and fluid is still allowed to flow from the reservoir and measured by the mass flow control system based on choked flow condition Q p , wherein another flow measurement Q v is made by the rate of decay of the fluid from the reservoir as
Q
v
=
-
k
·
V
·
d
(
P
u
/
T
)
dt
,
where t denotes time, k denotes a conversion constant and V, P u , and T, respectively, denote the volume of the reservoir, a pressure and temperature of the fluid in the reservoir.
8. A mass flow control system according to claim 7 , wherein the system can self verify its flow accuracy as a function of any differences between flow measurement made by the rate of decay of the fluid from the reservoir Q v , and the flow rate measured by the system based on choked flow condition Q p .
9. A mass flow control system according to claim 7 , further including a second control valve wherein the second control valve is closed to fulfill a zero flow set point command after flow verification is completed.
10. A mass flow control system according to claim 7 , wherein verification occurs during a verification period anytime between steps of the flow control process, the verification period being between 100 and 300 milliseconds.
11. A mass flow control system according to claim 7 , wherein the reservoir is positioned between the control valve and the flow restrictor.
12. A mass flow control system according to claim 7 , wherein the system provides an alarm to a host controller to warn of an out of calibration condition if the deviation of Q p from Q v is above a predetermined accuracy tolerance limit.
13. A mass flow control system according to claim 7 , wherein the system can adjust the coefficients of the flow calculation equation for the measured flow rate Q p based on the verification results such that the flow error between Q p and Q v is minimized, at or below the predetermined accuracy tolerance limit so that the system is recalibrated within the tolerance limits during the flow verification period.
14. A mass flow control system according to claim 1 , wherein the pressure sensor is a first pressure sensor for generating a signal as a function of the pressure of the fluid upstream from the flow restrictor and further comprising a second pressure sensor for generating a signal as a function of the pressure of fluid downstream from the flow restrictor for measurement of the flow of fluid during non-choke flow conditions, where the measured flow rate Q p is based on the following equation:
Q p =f ( P u ,P d ,T,m,γ,A ),
wherein f is a mathematic function of upstream pressure P u , downstream pressure P d , fluid temperature T, gas molecular weight m, gas specific heat ratio γ and effective office area A.