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Abstract

A method for generating a valve drive signal using a variable gain PI controller in a mass flow controller. The method includes multiplying an error signal with a proportional gain factor to generate a proportional signal. The method also includes implementing an integral function on the error signal. The integral function includes an integral gain factor which functions as the variable gain in the PI controller. For set-point signals less than a normalized predetermined percentage of the maximum set-point, the gain factor is equal to ##EQU1## where A is a first gain constant and B is a second gain constant. Otherwise, the integral gain factor is ##EQU2## where A is the first gain constant, B is the second gain constant, and x is the normalized set-point signal. Implementing the integral function produces an integral signal. Lastly, the integral signal and the proportional signal are summed to generate the valve drive signal. The error signal is a function of the difference between a set-point signal and a feedback signal. The set-point signal is a function of a desired flow rate in the mass flow controller, while the feedback signal is a function of the actual flow rate through the mass flow controller.

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