System and method for a mass flow controller
A system and method for controlling a mass flow controller to have a constant control loop gain under a variety of different types of fluids and operating conditions, and for configuring the mass flow controller for operation with a fluid and/or operating conditions different from that used during a production of the mass flow controller. Further, the system and method includes providing control by reducing the effects of hysteresis in solenoid actuated devices by providing a non-operational signal to the solenoid actuated device.
1. A mass flow controller having a control loop, the mass flow controller comprising:
a flow meter adapted to sense fluid flow in a fluid flow path and provide a flow signal indicative of the mass flow rate in the flow path;
a controller coupled to the flow meter and adapted to provide a drive signal based at least in part on the flow signal;
a valve actuator adapted to receive the drive signal from the controller; and
a valve adapted to be controlled by the valve actuator and coupled to the fluid flow path;
wherein the control loop of the mass flow controller includes the flow meter, the controller, the valve actuator, and the valve each having a respective gain term;
wherein the control loop has an overall gain term that includes each of the respective gain terms, and
wherein the mass flow controller is adapted to maintain the overall gain term at a substantially constant value independent of changes in one or more of the respective gain terms.
2. The mass flow controller of claim 1 , wherein the overall gain term includes a reciprocal gain term formed by taking a reciprocal of the product of at least one of the respective gain terms.
3. The mass flow controller according to claim 2 , wherein the reciprocal gain term is a function of at least a set point of the mass flow controller.
4. The mass flow controller according to claim 2 , wherein the reciprocal gain term is a function of at least an inlet pressure of the valve.
5. The mass flow controller according to claim 1 , wherein the overall gain term includes a composite gain term associated with at least the flow meter, the valve actuator, and the valve.
6. The mass flow controller according to claim 5 , wherein the overall gain term includes a reciprocal gain term formed by a reciprocal of a product of the composite gain term, the reciprocal gain term being a function of the at least one of the respective gain terms.
7. The mass flow controller according to claim 6 , wherein a product of the reciprocal gain term and the composite gain term provides the overall gain term.
8. The mass flow controller according to claim 1 , wherein the flow meter includes at least one of a normalization circuit, a response compensation circuit and a linearization circuit.
9. The mass flow controller according to claim 8 , wherein the flow meter includes the linearization circuit.
10. The mass flow controller according to claim 9 , wherein the linearization circuit uses a cubic spline fit to a set of control points to linearize an output of the flow meter.
11. The mass flow controller according to claim 10 , wherein the set of control points are part of configuration data obtained during characterization of the mass flow controller on a test fluid under test operating conditions.
12. The mass flow controller according to claim 11 , wherein the control points are point pairs indicating a sensor output and an actual fluid flow, respectively, for a selected set of set points.
13. The mass flow controller according to claim 6 , wherein the controller is a gain/lead/lag (GLL) controller.
14. The mass flow controller according to claim 13 , wherein the reciprocal gain term is provided to a first input of the GLL controller.
15. The mass flow controller according to claim 14 , wherein a flow signal from the flow meter and a set point signal are provided to a second input and a third input of the GLL controller, respectively.
16. The mass flow controller according to claim 15 , wherein the GLL controller provides an error signal based on a difference between the flow signal and the set point signal.
17. The mass flow controller according to claim 16 , wherein the GLL controller multiplies the reciprocal gain term by the error signal.
18. A method of controlling a gain of a control loop of a mass flow controller, the mass flow controller including a flow meter adapted to sense fluid flow in a fluid flow path and provide a flow signal indicative of the mass flow rate in the flow path, a controller coupled to the flow meter and adapted to provide a drive signal based at least in part on the flow signal, a valve actuator adapted to receive the drive signal from the controller, and a valve adapted to be controlled by the valve actuator and coupled to the fluid flow path, the method comprising an act of:
maintaining an overall gain term of the control loop at a substantially constant value independent of changes in one or more of a plurality of gain terms,
wherein the plurality of gain terms includes a flow meter gain term, a controller gain term, a valve actuator gain term, and a valve gain term.
19. The method of claim 18 , further comprising an act of forming a reciprocal gain term by taking the reciprocal of a product of at least one of the flow meter gain term, the controller gain term, the valve actuator gain term and the valve gain term.
20. The method of claim 19 , wherein the reciprocal gain term is a function of at least a set point of the mass flow controller.
21. The method of claim 19 , wherein the reciprocal gain term is a function of at least an inlet pressure of the valve.
22. The method of claim 18 , further comprising an act of forming the overall gain term using a composite gain term associated with at least the flow meter, the valve actuator and the valve.
23. The method of claim 22 , further comprising acts of:
forming a reciprocal gain term as a product of the composite gain term; and
including the reciprocal gain term in the overall gain term,
wherein the reciprocal gain term is a function of at least one of the flow meter gain term, the controller gain term, the valve actuator gain term and the valve gain term.
24. The method of claim 23 , further comprising an act of forming the overall gain term as a product of the reciprocal gain term and the composite gain term.
25. The method of claim 18 , further comprising an act of including at least one of a normalization circuit, a response compensation circuit and a linearization circuit in the flow meter.
26. The method of claim 25 , further comprising an act of including the linearization circuit in the flow meter.
27. The method of claim 25 , further comprising an act of employing the linearization circuit to apply a cubic spline fit to a set of control points to linearize an output of the flow meter.
28. The method of claim 27 , further comprising an act generating the set of control points as a part of configuration data obtained during characterization of the mass flow controller on a test fluid under test operating conditions.
29. The method of claim 28 , wherein the control points are point pairs indicating a sensor output and an actual fluid flow, respectively, for a selected set of set points.
30. The method of claim 23 , wherein the controller is a gain/lead/lag (GLL) controller.
31. The method of claim 30 , further comprising an act of providing the reciprocal gain term to a first input of the GLL controller.
32. The method of claim 31 , further comprising acts of providing a flow signal from the flow meter to a second input of the GLL controller; and providing a set point signal to a third input of the GLL controller.
33. The method of claim 32 , further comprising an act of generating an error signal, with the GLL controller, based on a difference between the flow signal and the set point signal.
34. The method of claim 33 , further comprising an act of multiplying, with the GLL controller, the reciprocal gain term by the error signal.
35. A computer readable medium encoded with a plurality of instructions that when executed on at least one processor implement the method of claim 18 .
36. The method of claim 18 , further comprising acts of:
predicting how the flow meter gain term, the valve actuator gain term and the valve gain term will change with a change in operating conditions in the fluid flow path; and
forming the controller gain term as a constant times a reciprocal of a product of the flow meter gain term, the valve actuator gain term and the valve gain term to maintain the overall gain term at the substantially constant value independent of changes to at least one variable operating condition.