Fluid handling – Line condition change responsive valves – Pilot or servo controlled
Reexamination Certificate
2000-03-09
2001-06-19
Rivell, John (Department: 3753)
Fluid handling
Line condition change responsive valves
Pilot or servo controlled
C137S012000, C251S129050
Reexamination Certificate
active
06247493
ABSTRACT:
TECHNICAL FIELD
The invention relates to mass flow controllers and, in particular, to highly accurate flow rate controllers for gases.
BACKGROUND ART
The control of flow for liquids and gases is a major problem in various manufacturing processes. Certain industries, notably the manufacture of silicon wafers, require very accurate gas flow rates, for many different gas species and with precise start flow and end flow points.
There have been a number of responses to the industry's need for accurate flow controllers. The prevalent type senses gas flow by measuring the thermal properties of gas flow over sensor elements. See for example, U.S. Pat. No. 4,658,855 to Doyle. The sensor flow measurement is combined with the control of a valve to proportionally enlarge or decrease the cross sectional area of the passageway through the valve and thereby control the flow of gas. If the sensor detects that the measured flow differs compared to the desired or set point flow, then a controller will partially close or open the valve until the measured flow equals the set point flow.
Although such control devices have resulted in acceptable products there is a need for a more universally applicable approach. The difficulty with the present methods arises because the thermal sensors used in such devices are quite dependent upon the heat transport properties of the gas species involved. The second is the difficulty associated with controlling the cross-sectional area of a valve opening for flow control. Minute changes in area result in substantial changes in flow imposing a need for a highly accurate method of valve opening. In practice, this means restricting the operations of the controllers to near their maximum displacement of the valve opening, which constitutes the smallest possible fraction of the valve area. Furthermore, for any given valve there is a relatively restricted range of flow over which changing the cross-sectional area is effective. The result of these restrictions is the widespread use of many different flow controllers each set up for a particular gas and maximum flow. However, they are largely non-interchangeable.
There are further difficulties inherent in the flow sensing concept of flow control. There has to be some flow to measure and this creates the “overshoot” problem wherein during the first few moments of flow after flow start, the actual flow will differ from the set point flow until the controller has had time to adjust the system. There is also the potential problem of oscillation wherein the inherent minimal adjustment of the controller might lag the measured flow. In such a situation the flow may oscillate between high and low states compared to the set point flow. Ideally this dampens out quickly to the set point flow but in any case magnifies the time period of the overshoot problem. In the worst case, oscillation drives the valve opening/closing sequence to a widely uncontrolled situation.
An objective of the invention was to provide an improved highly accurate flow rate controller, especially for small volumes of gas used in scientific, biomedical and engineering applications.
SUMMARY OF THE INVENTION
The above objective has been achieved in a mass flow controller using valves operated in a pulse mode. A housing is provided with one inlet valve, an inlet orifice, and at least one outlet valve and outlet orifice. The valves have poppets operating to close orifices etched through a silicon substrate, a material allowing the size of the orifices to be precisely known. Each poppet is enclosed in a bellows within the housing so that the poppets are protected from the gas in the housing. Each poppet is actuated by a shape retentive membrane, deformable in response to heat, either opening or closing the respective orifice. The temperature and pressure in the housing are measured so that real time control of the poppets by a controller will result in a known mass flow through the outlet apertures.
The controller modulates gas pressure at the gas inlet orifice about a set point so that pulses of gas enter the housing. At the same time, the controller monitors gas pressure at the outlet orifice, also about the set point, as gas exits the housing. The controller coordinates the duration of the inlet pulse to produce the desired gas flow. The inlet pulses may be formed by varying the time the orifice is open, or by fixing the times to establish an incremental flow, then counting cycles to obtain a desired flow rate.
REFERENCES:
patent: 4658855 (1987-04-01), Doyle
patent: 4706703 (1987-11-01), Takeuschi et al.
patent: 4842017 (1989-06-01), Reynolds
patent: 5038821 (1991-08-01), Maget
patent: 5061914 (1991-10-01), Busch et al.
patent: 5082242 (1992-01-01), Bonne et al.
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patent: 5323999 (1994-06-01), Bonne et al.
patent: 5325880 (1994-07-01), Johnson et al.
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patent: 5388984 (1995-02-01), Meslif
patent: 5417235 (1995-05-01), Wise et al.
patent: 5487378 (1996-01-01), Robertson et al.
patent: 6032689 (2000-03-01), Tsai et al.
patent: 6062256 (2000-05-01), Miller et al.
Krishnamurthy Ramesh
Rivell John
Schneck Thomas
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