Apparatus and method for calibration of mass flow controller

• US 7,136,767 B2
• Filed: 06/24/2002
• Issued: 11/14/2006
• Est. Priority Date: 06/24/2002
• Status: Expired due to Term

• Current Assignee: MKS Instruments Incorporated
• Original Assignee: MKS Instruments Incorporated
• Inventors: Shajii, Ali, Ambrosina, Jesse, Kottenstette, Nicholas
• Primary Examiner(s): Nghiem, Michael
• Assistant Examiner(s): Lau, Tung S.
• Application Number:
  US10/178,119
• Publication Number:
  US 20030236643A1
• Time in Patent Office: 1,604 Days
• Field of Search: 702/100, 702/12, 702/45, 702/50, 702/114, 732/04, 73/12.8, 731/17, 738/63, 73/10.6, 73/1, 137/10, 137/599, 137/486, 137/487, 137/468, 431/12, 431/15, 438/710, 298/90, 600/538, 128/207, 700/281
• US Class Current: 702/100
• CPC Class Codes: G01F 1/684 Structural arrangements; Mounting of elements, e.g. in relation to fluid flow
• CPC Class Codes: G01F 1/6965 comprising means to store calibration data for flow signal calculation or correction
• CPC Class Codes: G01F 1/86 Indirect mass flowmeters, e.g. measuring volume flow and density, temperature or pressure
• CPC Class Codes: G01F 25/15 specially adapted for gas meters G01F25/11 - G01F25/14, G01F25/17 take precedence
• CPC Class Codes: G01F 5/00 Measuring a proportion of the volume flow
• CPC Class Codes: G05D 7/0635 by action on throttling means G05D7/0688, G05D7/0694 take precedence

Abstract

A mass flow sensor calibrator employs a variable-flow fluid source, a receptacle of known volume, and a pressure differentiator. The variable-flow fluid source supplies gas at varying rates to the mass flow sensor being calibrated and at proportional rates to a receptacle of known volume. A pressure differentiator computes the time derivative of gas flow into the receptacle of known volume and, from that, the actual flow into the receptacle. Given the actual flow, the proportionate flow into the mass flow sensor may be determined and the flow signal from the mass flow sensor correlated to the actual flow.

Claims

1. A self-calibrating mass flow controller system comprising:

a mass flow controller including a flow channel with an inlet and an outlet, a laminar flow element disposed within the flow channel in a bypass section, a sensor tube coupled at one end to the flow channel at a location upstream of the bypass section and at a second end to the flow channel at a location downstream of the bypass section, an outlet control valve disposed within the outlet, and one or more resistive elements disposed about the sensor tube, wherein the one or more resistive elements are each operable to produce an electrical measurement signal corresponding to a mass flow within the sensor tube;

a variable gas flow source configured to supply gas to the mass flow controller;

a receptacle of predetermined volume configured to receive gas from the variable gas flow source;

a pressure differentiator configured to produce an electronic calibration signal representative of the time derivative of gas pressure within the receptacle; and

an electronic control unit configured to receive the electrical measurement signal, adjust the measurement signal using the calibration signal, and control the outlet control valve.

2. The system of claim 1, wherein the differentiator includes:

a pressure transducer configured to produce an electronic pressure signal representative of the pressure within the receptacle;

analog differentiator circuitry configured to produce an analog electronic calibration signal that is representative of the time derivative of said electronic signal representative of the pressure within the receptacle; and

an analog to digital converter (ADC) configured to convert one or more values of the analog calibration signals to one or more digital calibration signals corresponding to digital samples of the time derivative.

3. The system of claim 2, further comprising:

storage for storing the digital calibration signals.

4. The system of claim 1, wherein the differentiator includes:

a pressure transducer configured to produce an analog electronic pressure signal representative of the pressure within the receptacle;

an analog to digital converter (ADC) configured to convert one or more values of the analog pressure signal to corresponding digital pressure signals; and

a digital differentiator configured to receive the digital pressure signals and produce a plurality of digital values representative of the time derivative of the analog pressure signal.

5. The system of claim 4, further comprising:

storage for storing one or more samples of the calibration signal.

6. A self-calibrating mass flow controller system comprising:

a mass flow controller including a flow channel with an inlet and an outlet, a laminar flow element disposed within the flow channel in a bypass section, a sensor tube coupled at one end to the flow channel at a location upstream of the bypass section and at a second end to the flow channel at a location downstream of the bypass section, an outlet control valve disposed within the outlet, and one or more resistive elements disposed about the sensor tube, wherein the one or more resistive elements are each operable to produce an electrical measurement signal corresponding to a mass flow within the sensor tube;

a receptacle of predetermined volume configured to supply a variable gas flow to the mass flow controller;

a pressure differentiator configured to produce an electronic calibration signal representative of the time derivative of gas pressure within the receptacle; and

an electronic control unit configured to receive the electrical measurement signal, adjust the measurement signal using the calibration signal, and control the outlet control valve.

7. A gas flow calibration system comprising:

a variable gas flow source;

a receptacle of predetermined volume configured to receive gas from the variable gas flow source; and

a pressure differentiator configured to produce an electronic signal representative of the time derivative of gas pressure within the receptacle of predetermined volume so that the electronic signal can be used to calibrate mass flow measurement;

wherein the gas source is configured to provide an exponentially variable gas flow.

8. The system of claim 7, wherein the gas source is configured to provide an exponentially increasing gas flow.

9. The system of claim 7, wherein the gas source is configured to provide an exponentially decreasing gas flow.

10. A gas flow calibration system comprising:

a receptacle of predetermined volume configured to supply a variable gas flow; and

a pressure differentiator configured to produce an electronic signal representative of the time derivative of gas pressure within the receptacle of predetermined volume so that the electronic signal can be used to calibrate a mass flow measurement, wherein the gas source is configured to provide an exponentially variable gas flow.

11. The system of claim 10, wherein the gas source is configured to provide an exponentially increasing gas flow.

12. The system of claim 10, wherein the gas source is configured to provide an exponentially decreasing gas flow.

13. A method of calibrating a mass flow controller comprising:

(A) providing a variable gas flow to or from a source;

(B) flowing the variable gas flow into or out of a receptacle of predetermined volume at a variable rate;

(C) producing an electronic calibration signal that is representative of the derivative with respect to time of the gas pressure within the receptacle of predetermined volume as the variable gas flows into or out of the receptacle;

(D) using a thermal sensor to produce a mass flow measurement signal indicative of a flow rate within a mass flow controller; and

(E) using the calibration signal to adjust the mass flow measurement signal, so as to calibrate the mass flow controller.

14. The method of claim 13, wherein the step (C) comprises:

(C1) measuring the pressure within the receptacle of predetermined volume at least three times during the variable rate flow;

(C2) computing at least two pressure differences; and

(C3) dividing each pressure difference by the time between the two pressure measurements used to compute each respective pressure difference.

15. The method of claim 13, wherein the step (C) of producing an electronic calibration signal that is representative of the derivative of the gas pressure within the receptacle of predetermined volume includes:

(C4) producing an electronic signal representative of the pressure within the receptacle; and

(C5) producing a pressure time derivative signal that is representative of the time derivative of said electronic signal representative of the pressure within the receptacle.

16. The method of claim 15, wherein the step (C) of producing an electronic calibration signal that is representative of the derivative with respect to time of the gas pressure within the receptacle of predetermined volume includes:

(C6) converting one or more values of the pressure time derivative signal to digital samples of the pressure time derivative signal.

17. The method of claim 16, further comprising:

(F) storing one or more of the digital samples of the pressure time derivative signal.

18. The method of claim 13, wherein the step (C) of producing an electronic calibration signal that is representative of the derivative with respect to time of the gas pressure within the receptacle of predetermined volume as the variable gas flows into or out of the receptacle includes:

(C7) producing an electronic pressure signal representative of the pressure within the receptacle;

(C8) converting one or more values of the electronic pressure signal to a digital pressure signal; and

(C9) producing a plurality of digital values from the digital pressure signal representative of the time derivative of the pressure signal.

19. The method of claim 18, further comprising:

(G) storing one or more samples of the digital samples.

20. A method of producing a gas flow calibration signal comprising:

(A) providing a variable gas flow to or from a source;

(B) flowing the variable gas flow into or out of a receptacle of predetermined volume at a variable rate; and

(C) producing an electronic signal that is representative of the derivative of the gas pressure within the receptacle of predetermined volume as the variable gas flows into or out of the receptacle so that the electronic signal can be used to calibrate a mass flow measurement;

wherein providing a variable gas flow to or from a source includes providing an exponentially variable gas flow.

21. The method claim 20, wherein providing a variable gas flow to or from a source includes providing an exponentially increasing gas flow.

22. The method of claim 20, wherein providing a variable gas flow to or from a source includes providing an exponentially decreasing gas flow.

23. A mass flow calibrator for calibrating a mass flow controller that includes a mass flow sensor that produces a signal representative of mass flow, comprising:

a variable gas flow source, wherein the gas flow source is configured to supply the same gas at the same flow rate to both the mass flow controller and the receptacle of predetermined volume;

a receptacle of predetermined volume configured to receive gas from the variable gas flow source, the variable gas flow source configured to provide proportionate flow to a mass flow controller being calibrated and to the receptacle;

a pressure differentiator configured to produce an electronic signal representative of the time derivative of gas pressure within the receptacle of predetermined volume;

a comparator configured to compare the electronic signal from the differentiator to the signal from the mass flow sensor; and

a compensator configured to compute a correction factor from the difference between the signal from the differentiator and the flow sensor if such a difference exists.

24. The calibrator of claim 23, wherein the gas flow source is configured to supply gas to the mass flow controller and to the receptacle of predetermined volume in parallel.

25. The calibrator of claim 23, wherein the gas flow source is configured to supply gas to the mass flow controller and to the receptacle of predetermined volume in series.

26. A gas flow calibrator for calibrating a mass flow controller including a mass flow sensor that produces a signal representative of mass flow, comprising:

a receptacle of predetermined volume configured as a gas source to supply a variable gas flow, wherein the gas source is configured to provide an exponentially variable gas flow;

a pressure differentiator configured to produce an electronic signal representative of the time derivative of gas pressure within the receptacle of predetermined volume, the receptacle of predetermined volume configured to provide propartionate flow through a mass flow controller being calibrated and from the receptacle;

a pressure differentiator configured to produce an electronic signal representative of the time derivative of gas pressure within the receptacle of predetermined volume;

a comparator configured to compare the electronic signal from the differentiator to the signal from the mass flow sensor; and

a compensator configured to compute a correction factor from the difference between the signal from the differentiator and the flow sensor if such a difference exists.

27. The gas flow calibrator of claim 26, wherein the differentiator includes:

a pressure transducer configured to produce an electronic signal representative of the pressure within the receptacle;

analog differentiator circuitry configured to produce an electronic signal that is representative of the time derivative of said electronic signal representative of the pressure within the receptacle; and

an analog to digital converter (ADC) configured to convert one or more values of the analog time derivative signal to digital samples of the time derivative.

28. The gas flow calibrator of claim 26, further comprising:

storage for storing one or more samples of the signal representative of the time derivative of gas pressure.

29. The gas flow calibrator of claim 26, wherein the differentiator includes:

a pressure transducer configured to produce an electronic signal representative of the pressure within the receptacle;

an analog to digital converter configured to convert one or more values of the pressure transducer signal to digital form; and

a digital differentiator configured to produce a plurality of digital values representative of the time derivative of the pressure signal.

30. The gas flow calibrator of claim 26, further comprising:

storage for storing one or more samples of the signal representative of the time derivative of gas pressure.

31. The gas flow calibrator of claim 26, wherein the gas source is configured to provide an exponentially increasing gas flow.

32. The gas flow calibrator of claim 26, wherein the gas source is configured to provide an exponentially decreasing gas flow.

33. A method of calibrating a mass flow controller including a mass flow sensor, comprising:

(A) providing a variable gas flow to or from a source, wherein providing a variable gas flow to or from a source includes supplying gas to the mass flow controller and to the receptacle of predetermined volume in parallel;

(B) flowing the variable gas flow into or out of a receptacle of predetermined volume at a variable rate;

(C) producing an electronic signal that is representative of the derivative of the gas pressure within the receptacle of predetermined volume as the variable gas flows into or out of the receptacle;

(D) flowing gas into or out of the mass flow controller at a rate proportionate to that into or out of the receptacle;

(E) comparing the electronic signal from the differentiator to the signal from the mass flow sensor; and

(F) computing a correction factor from the difference between the signal from the differentiator and the flow sensor if such a difference exists.

34. The method of claim 33, wherein flowing gas into or out of the mass flow controller at a rate proportional to that into or out of the receptacle comprises flowing gas at the same rate to both the mass flow sensor and the receptacle of predetermined volume.

35. A method of calibrating a mass flow controller including a mass flow sensor comprising:

(A) providing a variable gas flow to or from a source, wherein providing a variable gas flow to or from a source includes supplying gas to the mass flow controller and to the receptacle of predetermined volume in series;

(B) flowing the variable gas flow into or out of a receptacle of predetermined volume at a variable rate;

(C) producing an electronic signal that is renresentative of the derivative of the gas pressure within the receptacle of predetermined volume as the variable gas flows into or out of the receptacle;

(D) flowing gas into or out of the mass flow controller at a rate proportionate to that into or out of the receptacle;

(E) comparing the electronic signal from the differentiator to the signal from the mass flow sensor; and

(F) computing a correction factor from the difference between the signal from the differentiator and the flow sensor if such a difference exists.

36. The method of claim 35, wherein the step of producing an electronic signal representative of the time derivative of gas pressure within the receptacle of predetermined volume includes:

producing an electronic signal representative of the pressure within the receptacle;

analog differentiator circuitry producing an electronic signal that is representative of the time derivative of said electronic signal representative of the pressure within the receptacle; and

an analog to digital converter converting one or more values of the analog time derivative signal to digital samples of the time derivative.

37. The method of claim 36, further comprising:

storing one or more samples of the signal representative of the time derivative of gas pressure.

38. The method of claim 35, wherein producing an electronic signal representative of the time derivative of gas pressure within the receptacle of predetermined volume includes:

producing an electronic signal representative of the pressure within the receptacle;

converting one or more values of the pressure transducer signal to digital form; and

producing a plurality of digital values representative of the time derivative of the pressure signal.

39. The method of claim 38, further comprising the step of:

storing one or more samples of the signal representative of the time derivative of gas pressure.

40. The method of claim 35, wherein supplying a variable gas flow from a receptacle of predetermined volume includes providing an exponentially variable gas flow.

41. The method of claim 40, wherein supplying a variable gas flow from a receptacle of predetermined volume includes providing an exponentially increasing gas flow.

42. The method of claim 40, wherein supplying a variable gas flow from a receptacle of predetermined volume includes providing an exponentially decreasing gas flow.

43. The method of claim 35, further including restricting the gas flow in order to differentiate the pressure over a greater period of time than would be available with unrestricted flow.

44. A method of calibrating a mass flow controller of the type including a mass flow sensor for producing a signal representative of the mass flow through the controller, comprising:

supplying a variable gas flow from a receptacle of predetermined volume, wherein supplying a variable gas flow from a receptacle of predetermined volume includes providing an exponentially variable gas flow;

producing an electronic signal representative of the time derivative of gas pressure within the receptacle of predetermined volume, the receptacle of predetermined volume providing proportionate flow through a mass flow controller being calibrated and from the receptacle;

comparing the electronic signal representative of the time derivative of gas pressure to the signal from the mass flow sensor; and

computing a correction factor from the difference between the signal representative of the time derivative of the gas pressure and the signal from the flow sensor, if such a difference exists.

45. The gas flow calibrator of claim 44, wherein supplying a variable gas flow from a receptacle of predetermined volume includes providing an exponentially increasing gas flow.

46. The gas flow calibrator of claim 44, wherein supplying a variable gas flow from a receptacle of predetermined volume includes providing an exponentially decreasing gas flow.

47. The method of claim 44, further including restricting the gas flow in order to differentiate the pressure over a greater period of time than would be available with unrestricted flow.