Parasitic flow correction method and apparatus
Mass flow controllers and methods for correcting flow inconsistencies associated with parasitic flow of a fluid in mass flow controllers are disclosed. A method includes obtaining a pressure measurement signal of the fluid generated by a pressure sensor and receiving a flow sensor signal of the fluid generated by a flow sensor. An estimated parasitic flow signal is generated using the pressure measurement signal, and the flow sensor signal is accelerated to produce an accelerated flow sensor signal with a bandwidth that is comparable to that of the estimated parasitic flow signal. A corrected flow signal is generated using the accelerated flow sensor signal and the estimated parasitic flow signal to control the mass flow controller.
1. A method for correcting for parasitic flow of a fluid in a mass flow controller, the method comprising:
obtaining a pressure measurement signal generated by a pressure sensor;
receiving a flow sensor signal generated by a flow sensor;
generating an estimated parasitic flow signal using the pressure measurement signal;
accelerating the flow sensor signal to produce an accelerated flow sensor signal with a bandwidth that is comparable to that of the estimated parasitic flow signal without slowing down the estimated parasitic flow signal; and
generating a corrected flow signal using the accelerated flow sensor signal and the estimated parasitic flow signal to control the mass flow controller.
2. The method of claim 1 , including:
determining with the estimated parasitic flow signal when to disengage a closed loop control of the mass flow controller;
controlling the mass flow controller based on the pressure measurement signal when the closed loop control has been disengaged; and
re-engaging the closed loop control when the corrected flow signal is reliable.
3. The method of claim 2 , wherein the closed loop control is disengaged when the estimated parasitic flow signal exceeds a threshold.
4. The method of claim 3 , wherein the closed loop control is reengaged when the estimated parasitic flow signal falls below the threshold.
5. A mass flow controller comprising:
a valve configured to adjustably control a flowrate of a fluid responsive to a control signal;
a pressure sensor configured to provide a pressure measurement signal that indicates a pressure of the fluid;
a thermal mass flow sensor configured to provide a flow sensor signal; and
a controller configured to:
generate an estimated parasitic flow signal using the pressure measurement signal;
accelerate the flow sensor signal to produce an accelerated flow sensor signal that has a bandwidth that is comparable to that of the estimated parasitic flow signal without slowing down the estimated parasitic flow signal;
generate a corrected flow signal using the accelerated flow sensor signal and the estimated parasitic flow signal to control the mass flow controller.
6. The mass flow controller of claim 5 , wherein the controller includes a multimode control component configured to:
determine, with the pressure measurement signal, when to disengage a closed loop control;
control the valve based on the pressure measurement signal when the closed loop control has been disengaged; and
re-engage the closed loop control when the corrected flow signal is reliable.
7. The mass flow controller of claim 6 , wherein the multimode control component is configured to determine when to disengage the closed loop control based upon the estimated parasitic flow signal exceeding a threshold.
8. The mass flow controller of claim 7 , wherein the multimode control component is configured to re-engage the closed loop control based upon the estimated parasitic flow signal falling below the threshold.
9. The mass flow controller of claim 8 , wherein the multimode control component is configured to re-engage the closed loop control based upon the estimated parasitic flow signal falling below another threshold.
10. The mass flow controller of claim 7 , wherein the multimode control component is configured to enable the threshold to be manually set.
11. The mass flow controller of claim 7 , wherein the multimode control component is configured to automatically set the threshold based upon a level of measured parasitic flow noise.
12. The mass flow controller of claim 5 , wherein the controller includes an acceleration filter with a time constant of between 1 millisecond and 5 milliseconds to accelerate the flow sensor signal to produce the accelerated flow sensor signal.