Methods and apparatus for pressure compensation in a mass flow controller

• US 8,751,180 B2
• Filed: 09/18/2008
• Issued: 06/10/2014
• Est. Priority Date: 07/19/2002
• Status: Active Grant

• Current Assignee: Brooks Instrument LLC (Illinois Tool Works Inc.)
• Original Assignee: Brooks Instrument LLC (Illinois Tool Works Inc.)
• Inventors: Wang, Chiun, Saggio, Joseph A. Jr., Lull, John Michael
• Primary Examiner(s): Kundu, Sujoy
• Assistant Examiner(s): Ngon, Ricky
• Application Number:
  US12/233,405
• Publication Number:
  US 20090078055A1
• Time in Patent Office: 2,091 Days
• Field of Search: 324/306, 702/100
• US Class Current: 702/100
• CPC Class Codes: G01F 1/696 Circuits therefor, e.g. constant-current flow meters
• CPC Class Codes: G01F 1/86 Indirect mass flowmeters, e.g. measuring volume flow and density, temperature or pressure
• CPC Class Codes: G01F 15/024 involving digital counting
• CPC Class Codes: G05B 13/042 in which a parameter or coefficient is automatically adjusted to optimise the performance
• CPC Class Codes: G05D 7/0635 by action on throttling means G05D7/0688, G05D7/0694 take precedence
• CPC Class Codes: Y10T 137/0379 By fluid pressure
• CPC Class Codes: Y10T 137/7759 Responsive to change in rate of fluid flow
• CPC Class Codes: Y10T 137/7761 Electrically actuated valve
• CPC Class Codes: Y10T 137/7762 Fluid pressure type

Abstract

Performance of mass flow controller may be vulnerable to pressure transients in a flow path to which the controller is coupled for the purpose of controlling the fluid flow. A system and method are provided for reducing or eliminate performance degradations, instabilities, and/or inaccuracies of a mass flow controller caused by changes in the pressure environment. In particular, a method and system are provided for compensating for pressure transients in the pressure environment of a flow path and mass flow controller.

Claims

1. A method performed by a mass flow controller for correcting a flow sensor output signal indicative of a sensed fluid flow through the flow path, the method comprising acts of:

measuring, using at least one thermal sensor, a mass flow rate of fluid flowing in a flow path of the flow controller;

generating the flow sensor output signal based on the measurement of the mass flow rate using the at least one thermal sensor;

measuring, using a pressure sensor located upstream of the at least one thermal sensor, at least one pressure of the flow path;

constructing a false flow signal indicating a pressure transient in the flow path based on the at least one pressure;

subtracting the false flow signal from the flow sensor output signal to generate a corrected flow signal indicative of the of the true flow flowing through the flow path of the flow controller; and

providing the corrected flow signal to a controller of the flow controller.

2. The method of claim 1, further comprising an act of providing a pressure signal indicative of the at least one pressure measurement.

3. The method of claim 2, wherein the act of constructing the false flow signal includes an act of delaying the pressure signal such that it is substantially aligned in time with the sensor output signal.

4. The method of claim 2, wherein the act of constructing the false flow signal includes an act of differentiating the pressure signal.

5. The method of claim 2, wherein the act of constructing the false flow signal includes an act of filtering the pressure signal with at least one filter, the at least one filter having a transfer function that emulates a response of the flow sensor to the pressure change in the flow path.

6. The method of claim 5, wherein the at least one filter includes a plurality of 2nd-order filters connected in series, and an output from each of the plurality of 2nd-order filters are scaled and summed to provide the false flow signal.

7. A flow meter comprising:

a flow sensor adapted to measure fluid flow in a flow path using at least one thermal sensor, the flow sensor providing a sensor output signal in response to sensed fluid flow in the flow path;

at least one pressure transducer located upstream of the a flow sensor to measure at least one pressure in the flow path, the at least one pressure transducer providing at least one pressure signal related to the respective at least one measured pressure;

a compensation filter to receive the at least one pressure signal, the compensation filter adapted to construct a false flow signal approximating a response of the flow sensor to pressure transients in the flow path, the false flow signal indicating an erroneous flow pressure not indicative of a true flow flowing through the flow path of the flow controller; and

a subtractor to receive the sensor output signal and the false flow signal and to provide a corrected flow signal by removing the false flow signal from the sensor output signal.

8. The flow meter of claim 7, wherein the compensation filter includes a delay block that delays the at least one pressure signal to be substantially aligned in time with the response of the flow sensor to pressure transients, and wherein the delay block provides at least one delayed pressure signal.

9. The flow meter of claim 8, wherein the compensation filter includes a differentiator to receive the delayed pressure signal, the differentiator being adapted to determine a derivative of the delayed pressure signal to provide a derivative signal.

10. The flow meter of claim 7 wherein the compensation filter comprises:

a differentiator receiving a pressure signal indicative of a pressure in a fluid path, the differentiator being adapted to determine a derivative of the pressure signal to provide a derivative signal; and

at least one filter having a transfer function adapted to transform the derivative signal into a false flow signal indicative of false flow information generated by the flow sensor in response to pressure transients.

11. A method performed by a mass flow controller coupled to a flow path, the mass flow controller having a control loop including a flow meter, a controller, a valve actuator and a valve, a method comprising acts of:

measuring, using at least one thermal sensor, a mass flow rate of fluid flowing in the flow path of the mass flow controller;

generating a flow sensor output signal based on the measurement of the mass flow rate using the at least one thermal sensor;

measuring at least one pressure in a fluid path environment upstream of the at least one thermal sensor;

providing at least one pressure signal indicating at least one pressure measurement;

determining at least one compensation signal based on at least one pressure measurement, wherein the act of determining at least one compensation signal includes constructing a false flow signal to recreate false flow information resulting from a response of the flow meter to pressure transients in the flow path environment, wherein the false flow information causes the flow sensor output signal to be not indicative of a true flow flowing through the fluid path of the mass flow controller;

generating a corrected sensor output signal indicative of the of the true flow flowing through the flow path of the flow controller by subtracting the at least one compensation signal from the sensor output signal; and

providing the corrected sensor output signal to the control loop of the mass flow controller.

12. The method of claim 11, further comprising generating a control signal to move the controlled portion of the valve an amount having an opposite sign of and substantially equal in magnitude to the calculated displacement.

13. The method of claim 11 in which determining at least one compensation signal based on at least one pressure measurement comprises predicting a valve displacement that will be caused by a pressure transient based on at least one pressure measurement;

and applying the at least one compensation signal to the control loop of the mass flow controller comprises sending a control signal to move the controlled portion of the valve to counter-act the predicted displacement.

14. A system for controlling a fluid flow along a flow path, the system comprising:

a mass flow controller including a flow meter adapted to sense fluid flow in a fluid flow path using at least one thermal sensor and provide a flow signal indicative of a mass flow rate in the flow path;

a valve actuator adapted to receive a drive signal from a controller;

a valve for controlling the flow rate in the fluid path, the valve coupled to the fluid flow path and adapted to be controlled by the valve actuator;

at least one pressure transducer located upstream of the flow meter to measure at least one pressure in a mass flow controller environment, and to provide at least one pressure signal indicative of measurement of the at least one pressure;

the controller coupled to the flow meter and adapted to provide the drive signal based at least in part on the flow signal; and

at least one compensation means to receive at least one pressure signal, and to provide at least one displacement compensation signal, wherein providing at least one compensation signal includes constructing a false flow signal to recreate false flow information resulting from a response of the flow meter to pressure transients in the flow path environment, wherein the false flow information causes an erroneous flow signal that is not indicative of a true flow flowing through the fluid flow path of the mass flow controller, the false flow signal subtracted from the flow signal of the flow meter to generate a corrected flow signal indicative of the of the true flow flowing through the fluid flow path of the flow controller.

15. A mass flow controller comprising:

a flow sensor adapted to measure fluid flow in a flow path using at least one thermal sensor, the flow sensor providing a sensor output signal in response to sensed fluid flow in the flow path;

at least one pressure transducer located upstream of the flow sensor to measure at least one pressure in the flow path, the at least one pressure transducer providing at least one pressure signal related to the respective at least one measured pressure;

at least one processor configured to receive the at least one pressure signal and construct a false flow signal approximating a response of the flow sensor to pressure transients in the flow path, the false flow signal indicating an erroneous flow pressure not indicative of a true flow flowing through the flow path of the flow controller and remove the false flow signal from the sensor output signal.