Micro mass flow control apparatus and method

A method of controlling mass flow rate of a fluid, comprising:

• sequentially starting and stopping sonic choked flow of the fluid through a flow restricter in a substantially digital, time-modulated, oscillatory manner to impose a duty cycle on the flowing fluid defined as a ratio of time the fluid flows in a cycle to total cycle time, where total cycle time is a sum of time the fluid flows through the restricter in a sonic choked manner plus time the fluid does not flow in the cycle, and wherein the mass flow rate is a product of what maximum flow rate through the flow restricter would be, if flow was not interrupted, multiplied by the duty cycle.

Mass flow control apparatus for setting a gas flow rate from a plenum, through an aperture, to an outlet at a desired mass flow rate M* in situations wherein inlet plenum pressure P.sub.1 and gas temperature T.sub.1 are measureable and type of gas is known, comprising:

• a combination of a digitally openable and closeable valve with said aperture, said valve having (i) a closure member positioned such that, when closed, said valve prevents flow of said gas through the aperture and, when fully opened, the flow of said gas is sonically choked at one cross-sectional area A in said aperture, and (ii) an actuator connected to said closure member, said actuator being capable of moving said closure member between closed and fully opened virtually instantaneously in response to control signals such that gas flow is effectively either stopped or sonically choked at said one cross-sectional area A;
• and
• a controller that generates said control signals to oscillate said valve between closed and opened positions in a time modulated duty cycle that produces said desired flow rate M*, said duty cycle being ratio of desired mass flow rate M* to maximum instantaneous mass flow rate M, i.e., M/M, where ##EQU3## and where C* is constant for said type of gas.