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Claims  |
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What is claimed is:
1. A circuit for sensing a difference between the temperature at a first
location and the temperature at a second location, said circuit
comprising:
A) a bridge including
(i) first, second, third and fourth nodes;
(ii) means for defining a first resistance between the first and second
nodes;
(iii) means for defining a second resistance between the second and third
nodes;
(iv) first temperature sensing means, disposed at said first location, for
defining a third resistance between the first and fourth nodes, having a
resistance value as a function of the temperature at said first location;
and
(v) second temperature sensing means, disposed at said second location, for
defining a fourth resistance between the fourth and third nodes, having a
resistance value as a function of the temperature at said second location;
B) first control means for controlling a voltage across said first and
second resistances so that a voltage at said second node is maintained at
a predetermined reference level; and
C) second control means for maintaining a voltage at said fourth node at
said predetermined reference level and for generating an output signal as
a function of the difference between the resistance values of said third
resistance and of said forth resistance and representative of the
difference in temperatures between said first and second locations.
2. A circuit according to claim 1, wherein said predetermined reference
level is virtual ground.
3. A circuit according to claim 2, wherein said first and second
resistances are substantially identical, and said first control means
includes means for applying a current through said first and second
resistances so as to provide substantially equal voltages across said
first and second resistances.
4. A circuit according to claim 3, wherein said first and second
temperature sensing means each includes resistive means having
substantially identical temperature coefficients.
5. A circuit according to claim 1, wherein said first control means
includes an amplifier for generating a compensation signal as a function
of any sensed difference between the voltage at said second node and a
system reference level, and means, responsive to said compensation signal,
for adjusting the current flow through said second resistance so as to
maintain said voltage at said second node at said predetermined reference
level.
6. A circuit according to claim 5, wherein said means for adjusting the
current flow through said second resistance includes transistor means
connected to said third node and having its base connected to receive said
compensation signal.
7. A circuit according to claim 5, wherein said system reference level is
ground.
8. A circuit according to claim 1, wherein said second control means
includes an operational amplifier for generating said output signal.
9. A circuit according to claim 8, wherein said operational amplifier
includes an input connected to said fourth node, a second input connected
to a system reference level, and feedback means for providing a
compensation signal to said fourth node as a function of said output
signal so as to maintain said fourth node at said predetermined reference
level.
10. A circuit according to claim 9, wherein said predetermined reference
level is virtual ground.
11. A circuit according to claim 9, wherein said system reference level is
ground.
12. A circuit according to claim 1, wherein said first and second control
means includes a constant current source for providing current through
said first, second, third and fourth resistances.
13. A circuit according to claim 1, wherein said temperature sensing means
each include a thermal element.
14. A mass flow control system including (a) a control valve for
controlling the rate of fluid flow through a conduit, (b) a transducer for
sensing the rate of fluid flow through said conduit as a function of the
difference in temperature between first and second regions of said conduit
and for generating a control signal as a function or said rate of fluid
flow, and (c) a mass flow controller for controlling the operation of said
valve as a function of said control signal; said transducer comprising:
A) a bridge including
(i) first, second, third and fourth nodes;
(ii) means for defining a fast resistance between the first and second
nodes;
(iii) means for defining a second resistance between the second and third
nodes;
(iv) first thermal element means, disposed in said first region, for
defining a third resistance between the first and fourth nodes, having a
resistance value as a function of the temperature in said first region;
and
(v) second thermal element means, disposed in said second region, for
defining a fourth resistance between the fourth and third nodes, having a
resistance value as a function of the temperature in said second region;
B) first control means for controlling a voltage across said first and
second resistances so that a voltage at said second node is maintained at
a predetermined reference level; and
C) second control means for maintaining a voltage at said fourth node at
said predetermined reference level and for generating an output signal as
a function of the difference between the resistance values of said third
resistance and of said forth resistance and representative of the rate of
flow of fluid through said conduit.
15. A control system according to claim 14, wherein said predetermined
reference level is virtual ground.
16. A control system according to claim 15, wherein said first and second
resistances are substantially identical, and said first control means
includes means for applying a current through said first and second
resistances so as to provide substantially equal voltages across said
first and second resistances.
17. A control system according to claim 16, wherein said first and second
thermal element means have substantially identical temperature
coefficients.
18. A control system according to claim 14, wherein said first control
means includes an amplifier for generating a compensation signal as a
function of any sensed difference between said voltage at said second node
and a system reference level, and means, responsive to said compensation
signal, for adjusting the current flow through said second resistance so
as to maintain said voltage at said second node at said predetermined
reference level.
19. A control system according to claim 18, wherein said means for
adjusting the current flow through said second resistance includes
transistor means connected to said third node and having its base
connected to receive said compensation signal.
20. A control system according to claim 18, wherein said system reference
level is ground.
21. A control system according to claim 14, wherein said second control
means includes an operational amplifier for generating said output signal.
22. A control system according to claim 21, wherein said operational
amplifier includes an input connected to said fourth node, a second input
connected to a system reference level, and feedback means for providing a
compensation signal to said fourth node as a function of said output
signal so as to maintain said fourth node at said predetermined reference
level.
23. A control system according to claim 22, wherein said predetermined
reference level is virtual ground.
24. A control system according to claim 22, wherein said system reference
level is ground.
25. A control system according to claim 14 wherein said first and second
control means includes a constant current source for providing current
through said first, second, third and fourth resistances.
26. An improved mass flow transducer of the type including a balanced
bridge comprising a first node and a second node and two, substantially
identical, thermal elements forming two sides of the bridge between the
first and second nodes of the bridge, the two elements being adapted to be
positioned in two regions of a laminar flow tube in a symmetrical tandem
arrangement so as to sense flow through the tube, one element being
upstream from the other; wherein the improvement comprises:
first control means for monitoring a voltage level as a function of the
voltages at the first and second nodes of the bridge and for maintaining
said voltage level at a predetermined reference level; and
second control means for monitoring the current required to maintain a
voltage at a third node between the thermal elements at said predetermined
reference level.
27. A transducer according to claim 26, wherein said first control means
includes means for maintaining the voltages at the first and second nodes
at respective levels which are equal in magnitude and opposite in polarity
to each other.
28. A transducer according to claim 27, wherein said second control means
includes an operational amplifier having two inputs, one of said inputs
coupled to said third node and the other of said inputs coupled to a
system reference level, and a feedback resistor connected between the
output of the amplifier and said one input.
29. A transducer according to claim 28, wherein said system reference level
is ground.
30. A transducer according to claim 26, wherein said balanced bridge
includes two resistive elements connected between the first and second
nodes and to each other at a fourth node, said transducer further
comprising a constant current source for providing constant current
through said resistive elements and said thermal elements.
31. A transducer according to claim 30, wherein said first control means
includes switching means for controlling current between said forth node
and said second node and an amplifier having an input coupled to said
fourth node and a second input connected to a system reference level and
its output for controlling said switching means.
32. A transducer according to claim 31, wherein said system reference level
is ground.
33. A transducer according to claim 26, wherein said predetermined
reference level is ground.
34. A mass flow control system including (a) a control valve for
controlling the rate of fluid flow through a conduit, (b) a transducer for
sensing the rate of fluid flow through said conduit as a function of the
difference in temperature between first and second regions of said conduit
and for generating a control signal as a function of said rate of fluid
flow, said transducer being of the type including a balanced bridge
comprising two, substantially identical, thermal elements forming two
sides of the bridge between a a first node and a second node of the
bridge, the two elements being adapted to be positioned in two regions of
a laminar flow tube in a symmetrical tandem arrangement so as to sense
flow through the tube, one element being upstream from the other; and (c)
a mass flow controller for controlling the operation of said valve as a
function of said control signal; the transducer further comprising:
first control means for monitoring a voltage level as a function of the
voltages at the first and second nodes of the bridge and for maintaining
said voltage level at a predetermined reference level; and
second control means for monitoring the current required to maintain a
voltage at a third node between the thermal elements at said predetermined
reference level.
35. A control system according to claim 34, wherein said first control
means includes means for maintaining the voltages at the first and second
nodes of the bridge at respective levels which are equal in magnitude and
opposite in polarity to each other.
36. A control system according to claim 35, wherein said second control
means includes an operational amplifier having two inputs, one of said
inputs coupled to said third node and the other of said inputs coupled to
a system reference level, and a feedback resistor connected between the
output of the amplifier and said one input.
37. A control system according to claim 36, wherein said system reference
level is ground.
38. A control system according to claim 34, wherein said balanced bridge
includes two resistive elements connected between the first and second
nodes of said bridge and to each other at a fourth node, said transducer
further comprising a constant current source for providing constant
current through said resistive elements and said thermal elements.
39. A control system according to claim 38, wherein said first control
means includes switching means for controlling current between said forth
node and said second node and an amplifier having an input coupled to said
fourth node and a second input connected to a system reference level and
its output for controlling said switching means.
40. A control system according to claim 39, wherein said system reference
level is ground.
41. A control system according to claim 34, wherein said predetermined
reference level is ground. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The present invention relates generally to mass flow transducers for
precisely measuring the mass flow rate of a fluid flowing through a
conduit. More particularly, the invention relates to an improved mass flow
transducer comprising a bridge circuit including two thermal resistive
elements disposed in different regions of the conduit.
BACKGROUND OF THE INVENTION
Flow transducers are typically used to measure the mass flow rate of fluid
through a conduit. Various mechanical mass flow transducers, which monitor
fluid induced movement of mechanical components, are known in the art and
provide rough measurements of the mass flow rate of a fluid. Electrical
transducers are known to provide more accurate measurements of fluid flow.
Electrical transducers of the thermal type typically rely on one or more
temperature-sensitive, resistive elements disposed typically around the
conduit. These latter types of transducers are based upon a well known
relationship that the rate of heat transfer to a fluid in a laminar flow
channel from the walls of the channel is a rather simple function of the
temperature difference between the fluid and the walls of the channel, the
specific heat of the fluid and the mass flow rate of the fluid within the
channel. Since the specific heat of a gas does not vary greatly with
pressure or temperature, a thermal mass transducer calibrated for a
particular gas will give true mass flow readings over a wide range of
operating conditions.
Thermal mass flow transducers therefore include one or more heating
elements to transfer heat energy to a fluid stream flowing in a small
laminar flow tube, sometimes known as a sensor tube. The heating elements
are usually made of a metal alloy having a high resistance and high
temperature coefficient of resistance. The sensor tube is usually a thin
stainless steel tube, and the elements are wound tightly around the
outside of the tube to provide effective heat transfer to the fluid
without disturbing the fluid flow within the tube. The high temperature
coefficient makes these heating elements also very good devices for
sensing the temperature of the tube, and they are often employed in that
double capacity. For clarity, such double duty heating/sensing elements
will be referred to herein as "thermal elements". These thermal elements
are well known and are described, for example, in U.S. Pat. Nos.
4,464,932, entitled THERMAL MASS FLOWMETERING and issued to Ewing; and
4,984,460, entitled MASS FLOWMETER and issued to Isoda.
While a one element fluid flow transducer has been described in U.S. Pat.
No. 5,142,907 (Hinkle), thermal fluid flow transducers have tended to
develop into two basic varieties, which may be designated the differential
voltage variety and the absolute voltage variety. In the differential
voltage variety of flow rate transducer, such as disclosed in U.S. Pat.
Nos. 3,851,526 (Drexel) and 4,548,075 (Mariano), two identical thermal
elements surround a laminar flow tube in a symmetrical tandem arrangement,
one element being upstream from the other. The temperature differential
between the elements is used as the measure of mass flow. In one
traditional arrangement, shown in FIG. 1 and referred to as the
two-element, constant current, differential voltage type, a constant
current electrical source feeds both elements in a series circuit
arrangement.
In FIG. 1 the prior art thermal fluid flow transducer 10 measures the mass
flow of a gas flowing through a sensor tube 22 from, for example, a
reservoir 24 to a process chamber 26. For small flow rates, the sensor
tube 22 is of capillary dimensions and the transducer measures the flow
rate directly through the conduit 22. For larger flow rates, both the
sensor tube 22 and a by-pass tube 28 couple reservoir 24 to chamber 26, as
shown in FIG. 1. Sensor tube 22, and by-pass tube 28 each draw a fixed
percentage of the total gas flow. In such a system, the total gas flow
between reservoir 24 and process chamber 26 is determined by multiplying
the flow measured through sensor tube 22 by a scale factor. Different
ranges of mass flows can be sensed by such a device by switching between
different sized by-pass tubes 28.
Transducer 10 is shown as the two element, constant current differential
voltage type. Specifically, transducer 10 includes a bridge of four
resistors, 12, 14, 16 and 18. Resistors 12 and 14 are standard electrical
resistors, such as ceramic resistors, and are chosen such that R.sub.12
(the resistance provided by resistor 12) equals R.sub.14 (the resistance
provided by resistor 14). Resistors 16 and 18 are thermal elements in the
form of coils that have an electrical resistance as a function of their
temperature, preferably the resistance of each coil increasing as a
function of temperature. Resistors 16 and 18 are chosen such that their
temperature coefficients are equal, i.e., at any given temperature
R.sub.16 (the resistance of coil 16) equals R.sub.18 (the resistance of
coil 18). Further, the resistors R.sub.12 and R.sub.14 need to match the
resistors R.sub.16 and R.sub.18, both in resistance (at zero flow) and in
their temperature coefficients, in order to provide a reliable circuit. An
example of this type of transducer circuit is shown and described in the
Isoda patent.
The thermal elements, resistors 16 and 18, are typically wrapped around the
sensor tube 22 and heated to the same initial temperature above the
ambient temperature forcing the same current through each resistor. For
this purpose a constant current source 20 provides current to the bridge,
and specifically to the resistors 16 and 18. When gas from reservoir 24,
which is usually at ambient temperature, flows through sensor tube 22 (as
shown in FIG. 1), the flowing gas has a cooling effect on coils 16, 18 and
lowers their temperature as a function of mass flow. The flowing gas cools
coil 16 more than coil 18 because coil 16 is disposed upstream from coil
18. Transducer 10 measures the mass flow rate of gas flowing through tube
22 by measuring the difference in temperatures between coils 16 and 18,
i.e., by measuring the difference in resistances between the two. Thus,
when no gas is flowing through tube 22, coils 16 and 18 are at the same
temperature and therefore, R.sub.16 equals R.sub.18. Since R.sub.12 equals
R.sub.14, the voltage at node 32 equals the voltage at node 34. When gas
flows through tube 22, R.sub.16 drops below R.sub.18 due to the
differential cooling effect. Therefore, the voltage at node 32 drops below
the voltage at node 34. Operational amplifier 36 generates a signal
indicative of the difference between the voltages at nodes 32 and 34. This
signal is fed to mass flow controller 30 which determines the mass flow
rate through conduit 28 and compares it to a set point (the desired flow
rate). Controller 30 in turn controls valve 32 to selectively adjust the
gas flow rate if the rate sensed by the transducer 10 is not equal to the
set point.
The transducer shown in FIG. 1 has several disadvantages. First, the
difference between the voltages at nodes 32 and 34 is typically very
small, even when gas is flowing at a maximum rate. Measuring this small
voltage difference is difficult and the measurement is very susceptible to
noise. Further, since the voltage difference is very small, the difference
can not be measured remotely as is often desirable. Rather, the voltage
difference must be measured by equipment that is in close proximity to the
bridge. Secondly, the output of this device is non-linear. Typically,
linearization circuitry is required to calibrate such a device.
Another type of differential (voltage) sensing variety of flow rate
transducer is described in U.S. Pat. No. 4,624,138 (Ono, et al.), which
can be referred to as the two-element, constant temperature, differential
type. This transducer uses a heat producing resistor, which is heated to a
constant temperature, and two thermal elements in the form of
temperature-sensitive resistors. The heat producing resistor is disposed
in a region of the conduit, and the two temperature sensitive resistors
are disposed so that one is upstream and the other is downstream from the
heat producing resistor. When gas flows through the conduit, the gas
conducts heat from the heat producing resistor to the down stream
temperature sensitive resistor. By measuring the differential voltages
across the temperature-sensitive resistors, this device calculates the
mass of gas flowing through the conduit.
This transducer has several disadvantages. Since this device relies on a
constant temperature process (because the heat producing resistor is
heated to a constant temperature) the device is only useful in a limited
range of environmental temperatures. This device has further disadvantages
when it is used in connection with a by-pass conduit such as shown at 28
in FIG. 1 because such by-pass instruments can produce undersirable
effects when used with constant temperature sensors. When mass flow
transducers are used in combination with a sensor tube and a by-pass
conduit, it is generally assumed that the mass of gas flowing in the
sensor tube is a fixed percentage of the total gas flow. This assumption
is only correct if the temperature of the gas in the by-pass conduit is
fixed relative to the temperature of gas in the sensor tube. This is true
because the viscosity of a fluid depends upon its temperature. So if the
temperature of the by-pass conduit varies with respect to the temperature
of the sensor tube, the mass of gas flowing in the sensor tube will not be
a fixed percentage of the total gas flow. Since the Ono et al. transducer
heats the gas in the sensor tube to a constant temperature and the gas in
the by-pass conduit can fluctuate with the ambient temperature, this
transducer requires temperature compensation equipment if it is to be used
in conjunction with a by-pass conduit.
A third type of differential (voltage) sensing variety of transducer can be
described as the two-element, floating temperature, differential voltage
transducer. Such a transducer is described in U.S. Pat. No. 4,984,460
(Isoda). This device requires four temperature-sensitive resistive
elements. Two are disposed around the conduit and two are disposed in the
ambient air. The device requires the temperature-sensitive resistors that
are disposed in the air to have the same values of resistance and the same
temperature characteristics (i.e., the same temperature coefficient of
resistance) as the temperature-sensitive resistors that are disposed
around the conduit. Requiring four temperature-sensitive resistors rather
than two, and requiring that their resistances and temperature
coefficients of resistances be matched, makes the implementation of the
circuit much more difficult, adding significantly to the cost of the
device.
U.S. Pat. No. 4,464,932, entitled THERMAL MASS FLOWMETERING, issued to
Ewing et al. describes an example of the absolute voltage type of
transducer, in which three thermal elements are used. This transducer can
be described as the three element, constant temperature, absolute voltage
transducer. It suffers from the same disadvantages as described in
connection with the two element, constant temperature, differential
voltage type transducer, and in addition the zero point is less stable
since the measurement is absolute rather than differential.
OBJECTS OF THE INVENTION
It is an object of the present invention to substantially reduce or
overcome the above-identified problems of the prior art.
Another object of the present invention is to provide an improved mass flow
transducer.
And another object of the present invention is to provide an improved mass
flow transducer that is believed to be more accurate than provided by the
prior an systems.
Yet another object of the invention is to provide an improved mass flow
transducer that operates in a more linear fashion than prior art
traditional two element, constant current, differential voltage type of
mass flow transducer.
Still another object of the present invention is to provide an improved
mass flow transducer in which the output signal can be reliably measured
remotely from the transducer.
And another object of the present invention is to provide an improved mass
flow transducer that can be used reliably in conjunction with a sensor
tube and a by-pass conduit for measuring the mass flow through the by-pass
conduit.
And yet another object of the invention is to provide an improved mass flow
transducer that operates reliably over a wide dynamic range.
And still another object of the present invention is to provide an improved
mass flow transducer including temperature sensing elements capable of
operating at a temperature that floats at a specified value above the
environment so as to increase the environmental temperature range and
reduce the effects of differences between the sensor channel and by-pass
channel.
And yet another object of the present invention is to provide an improved
mass flow transducer that generates a differential measurement so as to
provide a stable zero indication.
And still another object of the present invention is to provide an improved
mass flow transducer that generates a current at ground representative of
mass flow making the current less susceptible to noise.
And yet another object of the present invention is to provide an improved
mass flow control system comprising the improved mass flow transducer of
the present invention.
SUMMARY OF THE INVENTION
These and other objects are provided by an improved mass flow transducer of
the type including a balanced bridge comprising two, substantially
identical, thermal elements forming two sides of the bridge between the
top and the bottom of the bridge, the two elements being adapted to be
positioned in two regions of a laminar flow tube in a symmetrical tandem
arrangement so as to sense flow through the tube, one element being
upstream from the other. The preferred transducer includes control means
for monitoring the voltage at the top and bottom of the bridge and
monitoring the current required to maintain the node between the thermal
elements at virtual ground.
In one aspect the control means includes a voltage divider of two
substantially identical resistors forming the other two sides of the
bridge, an operational amplifier coupled between the resistors and a
transistor for adjusting the current flowing through the divider.
Still other objects and advantages of the present invention will become
readily apparent to those skilled in the art from the following detailed
description wherein several embodiments are shown and described, simply by
way of illustration of the best mode of the invention. As will be
realized, the invention is capable of other and different embodiments, and
its several details are capable of modifications in various respects, all
without departing from the invention. Accordingly, the drawings and
description are to be regarded as illustrative in nature, and not a
restrictive or limiting sense, with the scope of the application being
indicated in the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
For a fuller understanding of the nature and objects of the present
invention, reference should be had to the following detailed description
taken in connection with the accompanying drawings in which the same
reference numerals are used to indicate the same or similar parts wherein:
FIG. 1 shows a partial schematic and partial block diagram of a prior art
mass flow transducer shown in use with a mass flow controller system for
sensing flow through a by-pass tube by sensing the flow through a sensor
tube;
FIG. 2 is a partial schematic and partial block diagram of a preferred
embodiment of the mass flow transducer of the present invention;
FIG. 3 is a partial schematic and partial block diagram of another
preferred embodiment of the invention;
FIG. 4 is a graph comparing the errors of a transducer constructed
according to the principles of the present invention and a prior art
transducer of the type shown in FIG. 1; and
FIG. 5 is a partial schematic and partial block diagram of a preferred
embodiment of the mass flow transducer of the present invention shown in
use with a mass flow controller system for sensing flow through a by-pass
tube by sensing flow through a sensor tube.
DETAILED DESCRIPTION OF THE DRAWINGS
In the drawings the same numerals are used to refer to the same or like
parts.
In FIG. 2 the preferred transducer 60 measures the mass flow rate of gas
flowing in sensor tube 22. Transducer 60 includes a balanced bridge 58 of
four resistive elements, 64, 66, 68 and 70. Resistors 64 and 66 are
standard resistors and are chosen so that they are substantially matched,
i.e., such that R.sub.64 (the resistance of resistor 64) and R.sub.66 (the
resistance of resistor 66) are equal and each has substantially the same
thermal coefficient. Resistors 68 and 70 are preferably thermal elements
and are chosen so that they are matched, i.e., have the same coefficient
of temperature so that for any given temperature, R.sub.68 (the resistance
of thermal element 68) equals R.sub.70 (the resistance of thermal element
70). The thermal elements 68 and 70, however, do not necessarily have to
match the resistive elements 64 and 66. The node 82 of the bridge between
resistive elements 64 and 66 is connected to the inverting input of an
operational amplifier 74, the latter having its non-inverting input
connected to system ground. The output of amplifier 74 is in turn
connected to the base of pnp transistor 76. The emitter of transistor 76
is connected to node 80 of the bridge (connecting the resistive element 66
to the thermal element 68), while the collector of the transistor 76 is
connected to a negative voltage source, e.g., -15 volts DC. The node 78
connecting the resistive element 64 to the thermal element 70 is connected
to a constant current source 72, the latter being powered by a positive
voltage source, e.g., +15 volts DC. Finally, the node 86 is connected to
the inverting input of operational amplifier 88, the latter having its
non-inverting input connected to system ground and its output connected
through feedback resistor 90 to node 86. The output of amplifier 88 forms
an output terminal 92 of the transducer 60.
The constant current source 72 provides a current through resistive
elements 68 and 70, and through resistors 64 and 66. Resistive elements 64
and 66, amplifier 74, and transistor 76 form a control system 84 that
operates to maintain the voltages at nodes 78 and 80 equal and opposite to
one another so that the voltage at node 82 remains at virtual ground.
The transducer 60 operates in the following manner. With respect to the
control system 84, if the negative voltage at node 80, V.sub.80, decreases
in magnitude such that the voltage at node 78, V.sub.78, becomes greater
in magnitude than the negative voltage V.sub.80, or V.sub.78 increases in
magnitude relative to V.sub.80 (i.e., .vertline.V.sub.78
.vertline.<.vertline.V.sub.80 .vertline.), then the voltage at node 82,
V.sub.82, will tend to drift above ground. When the voltage V.sub.82
drifts above ground, the amplifier 74 provides a signal to the base of
transistor 76 so that the latter becomes more conductive and thus draws
more current so as to pull V.sub.80 down forcing the voltage at node 82,
V.sub.82, to virtual ground. If V.sub.80 increases in magnitude such that
the voltage V.sub.78 becomes smaller in magnitude than the negative
voltage V.sub.80, or V.sub.78 decreases in magnitude relative to V.sub.78
(i.e., .vertline.V.sub.78 .vertline.<.vertline.V.sub.80 .vertline.), then
V.sub.82 will tend to drift below ground. When V.sub.82 tends to drift
below ground, difference amplifier 74 adjusts transistor 76 to draw less
current thereby raising V.sub.80. When V.sub.82 is at exactly ground,
difference amplifier 74 controls transistor 76 to continue drawing the
same amount of current thereby maintaining V.sub.82 at the current level.
When no fluid is flowing through sensor tube 22, T.sub.68 (the temperature
of thermal element 68) equals T.sub.70 (the temperature of thermal element
70), and therefore, R.sub.68 equals R.sub.70. Since the magnitude of
V.sub.78 is maintained to equal to the magnitude of V.sub.80, the voltage
across elements 68 and 70 will remain equal and opposite to one another,
with the node 86 (V.sub.86) remaining at virtual ground.
Fluid flowing through tube 22 cools thermal element 68 more than thermal
element 70 because element 68 is upstream of element 70. Therefore, fluid
flowing through tube 22 lowers R.sub.68 below R.sub.70 so as to cause the
voltage at node 86, V.sub.86, to tend to drift below ground. As V.sub.86
tends to drift below ground, differential amplifier 88 supplies current to
node 86 through feedback resistor 90 in order to maintain node 86 at
virtual ground. The amount of current required to maintain V.sub.86 at
ground 8 is representative of the mass flow of fluid flowing through tube
22. This current can be measured by measuring the voltage across R.sub.90,
or by directly measuring the current at the non-inverting input of
amplifier 88. Since this signal is a current at ground level, it can be
reliably measured remotely from the transducer 60 components. Further,
current through the bridge is supplied by constant current source 72 so
that the initial steady state temperatures (the temperatures at zero flow
after warm up), T.sub.68 and T.sub.70 essentially remain fixed above
ambient temperature, but are allowed to float with ambient temperature.
Transducer 60, therefore, overcomes the limitations of the two- and
three-element constant temperature devices.
FIG. 3 shows one modification to the embodiment of FIG. 2. In this
embodiment, control system 84 includes a potentiometer 94 disposed between
resistors 64 and 66 for compensating for any mismatches between the two
resistors. Potentiometer 94 thus facilitates balancing the voltages at
nodes 78 and 80.
FIG. 4 shows a comparison of the errors in the outputs of a transducer
constructed according to the invention (CURVE A) and the prior art
transducer (CURVE B) illustrated in FIG. 1. FIG. 4 shows the relationship
of error (as a percentage of full scale) and the flow rate (as a
percentage of full scale). As FIG. 4 shows, the transducer 60 constructed
in accordance with the present invention provides reduced error and an
output response that is more linear than the prior art transducer of the
type shown in FIG. 1.
The invention has been discussed in connection with balancing voltages
V.sub.78 and V.sub.80 such that V.sub.78 equals -V.sub.80. As those
skilled in the art will appreciate, rather than tying the bridge circuit
between +15 Volt and -15 Volt rails, the circuit could be tied between,
for example, +10 Volt and ground rails. In this case, the sensor would
maintain V.sub.82 and V.sub.76 at a system reference level of +5 Volts
rather than at virtual ground. The sensor would still operate in the same
fashion, and V.sub.78 and V.sub.80 would still be balanced at an identical
amount above and below, respectively, the system reference level.
The transducer 60 is particularly useful when used in combination with
other components so as to control the mass flow rate through the tube 22.
For example, as shown in FIG. 5, the output of the transducer 60 is shown
connected to the input of a mass flow controller 100. The output of the
controller 100 is used for controlling a proportional control valve 102,
the latter for controlling the rate of flow of gas from reservoir 24 to
chamber 26. Controller 100 can be coupled to the output of amplifier 88,
as shown in FIG. 5, in which case controller 100 measures the voltage
across resistor 90 as the output signal; or controller 100 can be coupled
to the input of amplifier 88 in which case controller 100 measures the
current at ground as the output signal. As is well known, the proportional
control valve can be connected upstream of transducer 60, or as shown,
downstream of the transducer.
The transducer 60 thus provides an improved mass flow transducer for
sensing the mass flow rate of a fluid through a channel. Mass flow
transducer 60 is believed to be more accurate because it operates in a
more linear fashion than prior art traditional two element, constant
current, differential voltage type of mass flow transducer, such as the
type shown in FIG. 1. Because the output signal at the input of amplifier
88 is a current relative to ground level, the output can be reliably
measured remotely from the transducer, provide a stable zero indication,
and be less susceptible to noise. The mass flow transducer 60 can be used
reliably in conjunction with a sensor tube and a by-pass tube for
measuring the total mass flow through both tubes as shown in FIG. 5. The
device operates reliably over a wide dynamic range with thermal elements
operating at a temperature that floats at a specified value above the
environment so as to increase the environmental temperature range and
reduce the effects of irregularities between the sensor channel and
by-pass channel. Finally, an improved mass flow control system comprising
the mass flow transducer 60 can provide more accurate control.
Since certain changes may be made in the above apparatus without departing
from the scope of the invention herein involved, it is intended that all
matter contained in the above description or shown in the accompanying
drawing shall be interpreted in an illustrative and not a limiting sense.
* * * * *
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