Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
1999-07-09
2003-06-10
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
C073S204150
Reexamination Certificate
active
06575027
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to systems and methods for operating a mass flow controller (MFC) and, more specifically, to systems and methods for measuring mass flow within a mass flow controller by sensing the resistance change of a sense resistor or resistors in response to gas flow.
BACKGROUND OF THE INVENTION
Many manufacturing processes require the flow of process gases to be strictly controlled. To do so, the gas mass flow rate must be sensed and determined. Gas mass flow controllers sense the mass flow rate of a gas substantially independent of gas temperature or pressure, provide a measurement, and meter gas flow to adjust the mass flow rate as desired based on such sensing and metering. Mass flow controllers that operate on heat transfer principles have been widely adapted in the industry.
A common form of mass flow sensor for a gas incorporates a small diameter tube (a capillary tube) having two coils of wire wound on the outside in close proximity to each other, with one coil being positioned upstream of the other. The coils are formed from metallic material having a resistance that is temperature sensitive. The coils are heated by an electrical circuit in a bridge-type electrical circuit incorporated into a sensor to provide equal resistance in the absence of gas flow and hence a balance condition for the bridge-type circuit, i.e., a null output signal.
When gas flows within the capillary tube, the cool incoming gas is warmed as it flows past the upstream element, and this warmer gas then flows past the downstream element, resulting in differential cooling of the two elements. The difference in temperature is proportional to the number of molecules per unit time flowing through the capillary tube. Based on the known variation of the resistance of the coils with temperature, the output signal of the bridge circuit can provide a measure of the gas mass flow.
However, prior art mass flow sensor interface circuits have certain undesirable characteristics. First, prior art circuits compromise the ideal situation where a circuit is simply driving the sensing elements because they include resistances in parallel with the sensing elements for trimming the circuit output to zero volts with zero flow. By doing so, they compromise the apparent gain of the sensor. Sensor gain is traded off for controllability of zero-volt/zero-flow conditions. It is normally undesirable to attenuate the sensor output. If too many impedances of commensurable value are placed in parallel with the sense resistors, they degrade the maximum signal voltage that can be derived from the sense resistors in response to flow.
Secondly, in prior art circuits the relationship between the output voltage (which is proportional to flow) and the difference between the upstream and downstream sense elements is nonlinear, which is an undesirable feature of the prior art. Because the typical resistance values placed in parallel with the upstream and downstream sense elements are not significantly larger than the resistance values of the sense elements, the non-linearity in the relationship between the sense elements and the output voltage is not negligible.
Furthermore, prior art circuits require a large amount of amplification, typically on the order of a 35-70 gain factor, to produce a zero to five-volt output indicative of the gas flow. This requires additional circuitry and complexity. These prior art circuits require that the output from the sensing circuit be connected to a high input impedance amplifier stage because any load placed on the common junction point of the upstream and downstream sense elements increases the loss of sensor output and the circuit non-linearity.
Further, prior art circuits are designed to be calibrated manually using a gain control potentiometers (pots). In addition, the zero-volt/zero-flow condition also is adjusted manually by an operator using a multimeter. This results in increased circuitry and complexity as well as a greater opportunity for inaccurate mass flow values due to drift in the gain and zero control devices (potentiometers).
Prior art circuits are also susceptible to ambient temperature deviations common to both sense elements. Because prior art mass flow sensing circuits are essentially voltage dividers that can be arbitrarily trimmed to zero output by means of a virtual ground and variable resistors, any ambient temperature change will be reflected in the circuit output voltage. This occurs because prior art systems compare the absolute change in the resistance of the sense resistors. In such a case, even if both elements are cooled or heated equally, the absolute resistance change in each will likely be different.
FIG. 1
shows the basic topology of a prior art flow sensing bridge circuit
100
. Flow sensing bridge circuit
100
is a modified Wheatstone bridge driven by an ideal current source
20
. An ideal current source is characterized by a very high internal impedance, which means its output current will not change with a change in the voltage drop across a load. Ideal current source
20
can thus supply the same current regardless of the voltage drop across the load.
One branch of prior art flow sensing bridge circuit
100
of
FIG. 1
consists of two sense elements, R
U
and R
D
. These sense elements are used to sense the gas flow and are representative of the respective dynamic resistance values of the upstream and downstream sensor coils wound on the outside of the capillary tube; R
U
represents the upstream sense element and R
D
represents the downstream sense element. The upstream sensor coil is cooled more by the gas stream flow than the downstream sensor coil, therefore the resistance value of R
U
is less than that of R
D
. With no gas flow, R
U
is equal to R
D
and the bridge is balanced by means of variable resistor RV
1
. Under nonzero flow conditions, prior art flow sensing bridge circuit
100
output voltage e
out
30
is given by Equation 1 below. As shown by Equation 1, the relationship between output voltage e
out
30
and (R
U
−R
D
) is nonlinear.
e
out
=
(
R
D
R
1
-
R
U
R
2
)
i
(
R
U
+
R
D
)
(
1
+
R
1
+
R
2
R
p
)
+
(
R
1
+
R
2
)
[EQN. 1]
The other branch of flow sensing bridge circuit
100
includes resistors R
8
, R
9
and variable resistor RV
1
. The impedance value of variable resistor RV
1
is only a small fraction of the values of resistors R
8
and R
9
. Variable resistor RV
1
is used to adjust the offset of flow sensing bridge circuit
100
so that output voltage e
out
30
is zero. Resistance value R
1
represents the combined value of resistor R
8
and the portion of variable resistor RV
1
on the resistor R
8
side of variable resistor RV
1
's wiper arm, and resistance value R
2
represents the combined value of resistor R
9
and the portion of variable resistor RV
1
on the resistor R
9
side of variable resistor RV
1
's wiper arm. Typically, the values of R
1
and R
2
are about eight times as large as those of sense resistors R
U
and R
D
. Additionally, resistor R
p
is connected in parallel with sense resistors R
U
and R
D
and resistors R
8
and R
9
. The value of R
p
is about four times as large as that of R
1
and R
2
. The non-linearity of the circuit is therefore non-negligible.
As can be seen in Equation 1 above, the relationship between output voltage e
out
30
and sense element resistances R
U
and R
D
is inherently non-linear. Additionally, Resistors RV
1
, R
8
, R
9
and R
p
connected in parallel with sense elements R
U
and R
D
reduce the effect of sense elements R
U
and R
D
. Ideally, the entire circuit current should run through sense elements R
U
and R
D
to obtain the maximum output signal.
FIG. 1
therefore demonstrates both the non-linear characteristics of the prior art as well as the reduction in output voltage e
out
30
resulting from the use of resistances in parallel with sense elements R
U
and R
D
.
FIG. 2
is a more detailed representation of the prior art flow sensing bridge circuit
100
of FIG.
1
. Indivi
Larsen Dwight S.
Pattantyus Tamas I.
Gray Cary Ware & Freidenrich LLP
Mykrolis Corporation
Patel Harshad
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