Method and apparatus for self verification of pressured 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 method of controlling mass flow through a mass flow control system to a process and of self-verifying the accuracy of the mass flow control system when controlling a flow to a process, the method comprising:
controlling the flow of fluid through the system as a function of a control signal applied to a first control valve;
generating the control signal as a function of pressure sensed by a pressure sensor in the system and a set point; and
providing a reservoir of known volume of fluid downstream of the first control valve and self-verifying the mass flow control of the system by closing the first control valve and determining a rate of pressure decay within the reservoir based on the signal from the pressure sensor.
2. A method according to claim 1 , wherein controlling the flow of fluid includes controlling the flow of fluid through a flow restrictor so as to create choked flow conditions.
3. A method according to claim 2 , wherein controlling the flow through a flow restrictor includes controlling the flow through an orifice whose cross sectional area is adjustable.
4. A method according to claim 2 , wherein controlling the flow through a flow restrictor includes controlling the flow through a second control valve for providing an adjustable opening that defines the flow restrictor.
5. A method according to claim 1 , further including providing a pressure measurement signal representative of the measured pressure of fluid in the system; and providing a temperature measurement signal representative of the measured temperature of fluid in the system.
6. A method according to claim 5 , further including determining the measured flow of fluid Q p through the system as a function of the measured pressure and temperature of the fluid in the system as
Q p =C′·A ·ƒ( 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 gas, γ the specific heat capacity ratio of the gas, T the gas temperature, Pu the upstream pressure, and ƒ(m,γ,T) a mathematic function which is related to the gas molecular weight, the specific heat capacity of the gas, and the gas temperature.
7. A method according to claim 1 , further including providing a source of fluid from the reservoir of known volume positioned downstream from the first control valve such that the first control valve is closed when a zero flow set point is commanded, and allowing fluid to still be allowed to flow from the reservoir and measured by the system based on choked flow condition Q p , wherein another flow measurement Q v can be made by the rate of decay of the fluid from the reservoir as
Q
v
=
-
k
·
V
·
d
(
P
u
/
T
)
dt
,
wherein t denotes time, k denotes a conversion constant and V, Pu, and T respectively denote the volume of the reservoir, the pressure and temperature of the gas in the reservoir.
8. A method according to claim 7 , further including self-verifying system flow accuracy as a function of any differences between flow measurement made by the rate of decay of the fluid from the reservoir Qv, and the flow rate measured by the system based on choked flow condition Qp.
9. A method according to claim 7 , further including closing a second control valve to fulfill the zero flow set point command after the flow verification is completed.
10. A method 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 method according to claim 7 , wherein the reservoir is positioned between the first control valve and a flow restrictor.
12. A method according to claim 7 , further including sending an alarm to a host controller to warn of an out of accuracy condition if the deviation of Q p from Q v is above a predetermined accuracy tolerance limit.
13. A method according to claim 7 , further including adjusting the coefficients of a 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 as to recalibrate the system within the tolerance limits during the flow verification period.
14. A method according to claim 1 , further including generating a signal as a function of the pressure of the fluid upstream from a flow restrictor, and generating a signal as a function of the pressure of fluid downstream from the flow restrictor for measuring the flow of fluid during non-choked flow conditions, where the measured flow rate Qp is based on the following equation:
Q p =ƒ( P u ,P d ,T,m,γ,A ),
Wherein f is a mathematic function of the upstream pressure , the downstream pressure P d , the gas temperature T, the gas molecular weight m, the gas specific heat ratio γ and the effective office area A.