Abstrict A fluid mass flow meter that responds rapidly to changes in the
rate of flow of a fluid and that is insensitive to variations in
ambient temperature. The flow meter has a thermal clamp that clamps
the sensor tube adjacent the extremities of a sensor disposed along
the tube; a frequency compensating amplifier that, in cooperation
with the thermal clamp, increases the frequency response of the
meter; and an offset null bias circuit that, in combination with
a bridge balancing potentiometer, corrects for the effects of variations
in ambient temperature.
Claims Having thus described a preferred embodiment, what is claimed is:
1. A fluid mass flow meter comprising:
a tube having therethrough a flow path for the fluid;
a sensor disposed adjacent the tube, in thermal connection therewith,
and along said fluid flow path, the sensor having distal and proximal
extremities, the sensor having a first electrical terminal at its
distal extremity, a second electrical terminal at its proximal extremity,
and a third electrical terminal at an intermediate point between
the first and second terminals, the first and third terminals defining
a first sensor unit, the second and third terminals defining a second
sensor unit, each of said units operative, in response to a change
in the temperature thereof, to change the electrical resistance
therethrough;
thermally conductive means in thermal communication with the tube
at a first point adjacent said distal extremity and at a second
point adjacent said proximal extremity, operative to prevent said
first point from having a different temperature than said second
point;
resistance means having a first connection at a distal extremity
thereof, a second connection at a proximal extremity thereof, and
a third connection at an intermediate point between the first and
second connections, said first and second connections being in electrical
communication with the first and second terminals of the sensor,
respectively, the resistance means and the sensor in combination
defining a bridge;
a current source, operative to cause a current to flow through
the bridge;
differential amplifier means having a first input in electrical
communication with the third sensor terminal, a second input in
electrical communication with the third resistance connection, and
an output;
frequency compensating amplifier means in electrical communication
with the differential amplifier means and having a final output
and operative to produce at the final output a signal indicative
of the rate of flow of the fluid through the tube;
a feedback element in a negative feedback relationship with the
final output of the frequency compensating amplifier and a feedback
input thereof; and
frequency compensation means, in electrical communication with
the frequency compensating amplifier means and operative to alter
the gain thereof as a predetermined function of the rate of change
of the signal being amplified, said function being a positive function
over a part of its range.
2. A fluid mass flow meter according to claim 1 wherein the frequency
compensation means comprises a capacitor.
3. A fluid mass flow meter according to claim 2 wherein the frequency
compensation means comprises a resistor in electrical communication
with the capacitor.
4. A fluid mass flow meter according to claim 3 wherein the capacitor
is in electrical communication with the feedback input and with
a ground reference, and wherein the resistor is in a negative feedback
relationship with the frequency compensating amplifier.
5. A fluid mass flow meter according to claim 4 and further comprising
a second resistor in series relationship with the capacitor.
6. A fluid mass flow meter according to claim 4 wherein the feedback
element comprises a capacitor.
7. A fluid mass flow meter comprising:
a tube having therethrough a flow path for the fluid;
a sensor disposed adjacent the tube, in thermal connection therewith,
and along said fluid flow path, the sensor having distal and proximal
extremities, the sensor having a first electrical terminal at its
distal extremity, a second electrical terminal at its proximal extremity,
and a third electrical terminal at an intermediate point between
the first and second terminals, the first and third terminals defining
a first sensor unit, the second and third terminals defining a second
sensor unit, each of said units operative, in response to a change
in the temperature thereof, to change the electrical resistance
therethrough;
thermally conductive means in thermal communication with the tube
at a first point adjacent said distal extremity and at a second
point adjacent said proximal extremity, operative to prevent said
first point from having a different temperature than said second
point;
resistance means having a first connection at a distal extremity
thereof, a second connection at a proximal extremity thereof, and
a third connection at an intermediate point between the first and
second connections, said first and second connections being in electrical
communication with the first and second terminals of the sensor,
respectively, the resistance means and the sensor in combination
defining a bridge;
a current source, operative to cause a current to flow through
the bridge;
a differential amplifier having a first input in electrical communication
with the third sensor terminal, a second input in electrical communication
with the third resistance connection, and a differential output;
an operational amplifier having an inverting input, a non-inverting
input in electrical communication with the differential output,
and a final output, said operational amplifier being operative to
produce at said final output a signal indicative of the rate of
flow of the fluid through the tube;
a feedback element in electrical communication between the final
output and the inverting input; and
frequency compensation means including a resistor and a capacitor
in electrical communication with the operational amplifier and operative
to alter the gain thereof as a predetermined function of the rate
of change of the signal being amplified, said function being a positive
function over a part of its range.
8. A fluid mass flow meter according to claim 7 wherein the frequency
compensation means is characterized by a time constant of about
6.5 seconds.
9. A fluid mass flow meter according to claim 7 wherein the frequency
compensation means is characterized by a time constant that is about
equal to the time required for the resistance of one of the sensor
elements to achieve about 63 per cent of the change of resistance
that it will ultimately achieve in response to a change in the rate
of flow of the fluid.
10. A fluid mass flow meter according to claim 7 wherein the capacitor
is in electrical communication with the inverting input and with
a ground reference, and wherein the resistor is in electrical communication
with the inverting input and with the final output.
11. A fluid mass flow meter according to claim 10 and further comprising
a second resistor in series relationship with the capacitor.
12. A fluid mass flow meter according to claim 10 wherein the feedback
element comprises a capacitor.
13. A fluid mass flow meter comprising:
a tube having therethrough a flow path for the fluid;
a sensor disposed adjacent the tube, in thermal connection therewith,
and along said fluid flow path, the sensor having distal and proximal
extremities, the sensor having a first electrical terminal at its
distal extremity, a second electrical terminal at its proximal extremity,
and a third electrical terminal at an intermediate point between
the first and second terminals, the first and third terminals defining
a first sensor unit, the second and third terminals defining a second
sensor unit, each of said units operative, in response to a change
in the temperature thereof, to change the electrical resistance
therethrough;
thermally conductive means in thermal communication with the tube
at a first point adjacent said distal extremity and at a second
point adjacent said proximal extremity, operative to prevent said
first point from having a different temperature than said second
point;
a potentiometer having first and second connections at opposing
extremities of a fixed resistance element and a third connection
at a variable intermediate point along said element, said first
and second connections being in electrical communication with the
first and second terminals of the sensor, respectively, the potentiometer
and the sensor in combination defining a bridge;
a current source, operative to cause a current to flow through
the bridge;
differential amplifier means having a first input in electrical
communication with the third sensor terminal, a second input in
electrical communication with the third potentiometer connection,
and an output;
adjustable bias means for applying an offset null bias to one of
the inputs of the differential amplifier;
frequency compensating amplifier means in electrical communication
with the differential amplifier means and having a final output
and operative to produce at the final output a signal indicative
of the flow of the fluid through the tube;
a feedback element in a negative feedback relationship with the
final output of the frequency compensating amplifier and a feedback
input thereof; and
frequency compensation means, in electrical communication with
the frequency compensating amplifier means and operative to alter
the gain thereof as a predetermined function of the rate of change
of the signal being amplified, said function being a positive function
over a part of its range.
14. A fluid mass flow meter according to claim 13 wherein the frequency
compensation means comprises a capacitor.
15. A fluid mass flow meter according to claim 14 wherein the frequency
compensation means comprises a resistor in electrical communication
with the capacitor.
16. A fluid mass flow meter according to claim 15 wherein the capacitor
is in electrical communication with the feedback input and with
a ground reference, and wherein the resistor is in a negative feedback
relationship with the frequency compensating amplifier.
17. A fluid mass flow meter according to claim 16 and further comprising
a second resistor in series relationship with the capacitor.
18. A fluid mass flow meter according to claim 16 wherein the feedback
element comprises a capacitor.
19. In a fluid mass flow meter of the kind having:
a tube having therethrough a flow path for the fluid;
a sensor disposed adjacent the tube, in thermal connection therewith,
and along said fluid flow path, the sensor having distal and proximal
extremities, the sensor having a first electrical terminal at its
distal extremity, a second electrical terminal at its proximal extremity,
and a third electrical terminal at an intermediate point between
the first and second terminals, the first and third terminals defining
a first sensor unit, the second and third terminals defining a second
sensor unit, each of said units operative, in response to a change
in the temperature thereof, to change the electrical resistance
therethrough;
a bridge element having three electrical terminals, the first and
second terminals being connected to the first and second terminals,
respectively, of the sensor to define therewith a bridge;
a current source, operative to cause a current to flow through
the bridge; and
differential amplifier means having a first input in electrical
communication with the third terminal of the sensor, a second input
in electrical communication with the third terminal of the circuit
element, and an output, an improvement comprising:
a potentiometer comprised in the bridge element;
adjustable bias means for applying an offset null bias to one of
the inputs of the differential amplifier means;
thermally conductive means in thermal communication with the tube
at a first point adjacent said distal extremity and at a second
point adjacent said proximal extremity, operative to prevent said
first point from having a different temperature than said second
point;
frequency compensating amplifier means in electrical communication
with the differential amplifier means and having a final output
and operative to produce at the final output a signal indicative
of the flow of the fluid through the tube;
a feedback element in a negative feedback relationship with the
final output of the frequency compensating amplifier and a feedback
input thereof; and
frequency compensation means, in electrical communication with
the frequency compensating amplifier means and operative to alter
the gain thereof as a predetermined function of the rate of change
of the signal being amplified, said function being a positive function
over a part of its range.
20. A fluid mass flow meter according to claim 19 wherein the frequency
compensation means comprises a capacitor.
21. A fluid mass flow meter according to claim 20 wherein the frequency
compensation means comprises a resistor in electrical communication
with the capacitor.
22. A fluid mass flow meter according to claim 21 wherein the capacitor
is in electrical communication with the feedback input and with
a groud reference, and wherein the resistor is in a negative feedback
relationship with the frequency compensating amplifier.
23. A fluid mass flow meter according to claim 22 and further comprising
a second resistor in series relationship with the capacitor.
24. A fluid mass flow meter according to claim 22 wherein the feedback
element comprises a capacitor.
Description BACKGROUND
1. Field of the Invention
This invention is directed to mass flow measuring systems, in general,
and to a system wherein a vastly improved operation is achieved
with a novel isothermal structural configuration together with a
sensing circuit which is speeded up in a novel manner, in particular.
2. Prior Art
For years mass flow meters have been made using a thermal transfer
principle, wherein a length of tubing has a pair of heater/sensor
coils mounted thereon. Fluid enters the sensor tube at ambient temperature.
The fluid conducts heat away from the first coil--(heater/sensor).
The heated fluid then continues down the conduct to the second heater/sensor.
An additional quantity of heat is again transferred to the fluid.
The difference in temperatures of upstream and downstream sensors
is an indication of mass flow rate. However, this method of sensing
mass flow is relatively slow (a 10 second time constant is typical)
and is dependent on the thermal response of the sensor tubing, gas
conduction factor, temperature sensor thermal mass and thermal mass
of the insulation surrounding the sensor. Electrical compensation
has been tried in the sensor circuit but has only provided a small
improvement because of the multiple time constants associated with
the sensor.
In conventional mass flow measuring systems, a portion of the fluid
being detected is passed through a sensing tube. An electrical coil
(or dual coil) is wound on the tube. The coil forms a portion of
a bridge network. As the resistance of the coil changes, the bridge
network detects the flow characteristic. The resistance of the coil
changes as a function of its temperature. The coil temperature is
affected by the gas flow through the sensing tube. Typically, the
bridge network is balanced without fluid flow through the tube and
detection of fluid flow is a function of the bridge changes. Reference
is herewith made to the copending application entitled IMPROVED
MASS FLOW METER WITH REDUCED ATTITUDE SENSITIVITY by J. W. ABOUCHAR
and M. J. DOYLE; Serial No. 173282; filed on July 29 1980; now
U.S. Pat. No. 4440021 issued Apr. 3 1984 to Abouchar et al.
The cited copending patent application provides a suitable, detailed
description of conventional mass flow measuring systems.
In particular, the referenced patent application describes a dual
coil arrangement which is used in measuring the mass flow of a fluid
over a wide temperature range with reduced attitude sensitivity.
The referenced application specifies the use of an insulating material
made of a fibrous material which encases the sensor elements (including
the dual coil arrangement) of a measuring system. The apparatus
of the co-pending application is a distinct and important improvement
over the prior art systems such as those shown and described in
U.S. Pat. No. 3938384. The prior art system has several distinct
disadvantages, as described in the aforementioned application of
Abouchar, et al. The Abouchar, et al., application shows and describes
an improved system. However, because of the high degree of the sensitivity
and tolerance requirements of similar systems, better and improved
measuring systems are being sought.
For example, it is desirable to have a method of reducing the effect
of the multiple time constants so that the principal time constant
becomes dominant. Cancellation of the dominant principal time constant
in the electronic control circuitry can then provide a major improvement
in speed.
PRIOR ART STATEMENT
The most pertinent prior art known to Applicants is listed herewith.
1. U.S. Pat. No. 3938384; MASS FLOW METER WITH REDUCED ATTITUDE
SENSITIVITY; R. F. Blair. This patent teached a prior art system
with a less than adequate open-cell beaded foam insulator material.
2. U.S. Ser. No. 173282 (now U.S. Pat. No. 4440021 issued Apr.
3 1984); MASS FLOW METER WITH REDUCED ATTITUDE SENSITIVITY; J.
W. Abouchar and M. J. Doyle. This application is directed to a mass
flow measuring system with an improved operating capability due
to use of a superior insulating material.
3. U.S. Pat. No. 2813237; FLOW MEASURING SERVO SYSTEM; D. A.
Fluegel, et al. This patent shows another prior art system with
less than completely accurate operation.
4. U.S. Pat. No. 4056975; MASS FLOW SENSOR SYSTEM; D. B. Le May.
This patent shows a device including a sensing tube which includes
a loop or single turn helix formed therein.
5. German Patent 876484; E. Ebbinghaus. This patent shows a temperature
sensing bridge network.
SUMMARY OF THE INVENTION
This invention is directed to an improved mass flow measuring system
for measuring the flow rate of a fluid in a tube and doing so accurately
over wide ranges of attitude and temperature. The improvement comprises
using isothermal bands, planes or heat sink encasement to reduce
the effect of thermal conduction or convection along the tube of
the sensor. In addition, the sensor elements are encased in a thermally
insulating environment such as, a vacuum, layers of insulating material,
or the like, which is arranged to effectively reduce the thermal
flow at the outer surface of the sensor. The device provides improved
operation in terms of temperature and attitude sensitivity. In addition,
the invention includes a compatible circuit which uses a constant
current generator in conjunction with a bridge network to establish
a prescribed null signal for the sensing circuit. A precision reference
signal is also produced and used by the circuit. A very accurate
circuit portion controls both the current generator and the reference
signals wherein a highly accurate sensing circuit is produced. Also,
the circuit is matched to the operation of the sensor in order to
provide extended operational utilization of the system. The number
of components in this highly accurate circuit is not significantly
different from the ordinary sensing circuit known in the art.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an elevation view of a typical configuration of a sensor
system in accordance with the instant invention.
FIG. 2 is an elevation view of the improved sensor system of the
invention including one embodiment of an isothermal guard arrangement.
FIG. 3 shows a comparison of the temperature gradients of the sensor
systems, shown in FIGS. 1 and 2.
FIG. 4 is an elevation view of the sensor system partially broken
away with insulator materials applied thereto.
FIG. 5 is an elevation view of the improved sensor system, including
another embodiment of an isothermal guard arrangement.
FIG. 6 is a cut-away end view of the sensor system of the instant
invention showing the configuration of the various elements.
FIG. 7 is a partially broken away perspective view of the sensor
system of the instant invention with an outer casing.
FIG. 8 is a schematic diagram of a preferred embodiment of the
circuit of the instant invention.
FIGS. 9A, 9B and 9C are graphic representations of the operation
of the sensor system of the instant invention.
FIGS. 10A and 10B are graphic representations of the improved sensor
system compared with the prior art system operation.
FIG. 11 is a schematic representation of another embodiment of
an isothermal guard arrangement.
FIG. 12 is a partially broken away perspective view of the embodiment
shown in FIG. 11 in greater detail.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1 there is shown a schematic representation
of a sensor system of known configuration. This sensor system is
used for measuring the flow of fluid for use in a mass flow measuring
system. The system includes a base portion 10 which can be fabricated
of phenolic a thermo set or thermoplastic, or any material including
metal providing a cost effective mounting technique. In a preferred
configuration for use with the instant invention base 10 includes
a step portion 11 which is raised slightly above the upper surface
of base 10. Both base 10 and step 11 are substantially rectangular
in configuration (although this is not a critical requirement of
the system.)
The sensor system per se, consists of a thin walled tube 12 which,
in this configuration has an inverted U-shaped configuration with
a pair of vertical legs connected by a crossmember. The ends 12A
and 12B of tube 12 extend from the bottom of base 10 while the U-shaped
tube 12 projects from the upper surface thereof. A plurality of
electrical connector rods 13 are embedded in step 11 and base 10
to provide electrical connections to sensor coil 14. The coil 14
is wrapped around the cross-member of the U-shaped tube 12. The
coil 14 is connected to the connector rods 13 by means of wires
15. The wires 15 represent the end terminals and the center tap
of coil 14 (i.e. a dual coil). The wires 15 are wrapped around rods
13 at the junction points 16. Of course, any suitable soldering,
brazing, welding or the like can be used in effecting the junction.
As the fluid (gas or liquid) flows through the tube 12 heat is
transferred from the tube at heating/sensing element B-C, to the
fluid. (That is, it is assumed that the fluid entered the tube at
ambient temperature and the sensor portion is different from ambient.)
The transfer of heat to the fluid lowers the temperature of the
heating/sensing element B-C. A different transfer of heat takes
place on element C-D inasmuch as the fluid is no longer at ambient
temperature due to it passing through the element B-C. The different
heat transfer in B-C and C-D results in a change in electrical bridge
balance as described hereinafter. The output voltage produced across
the bridge is calibrated to be a measure of mass flow.
Referring now to FIG. 2 there is shown an improved embodiment
of the sensor system. In this instance, a band 18 of a thermally
conductive material is wrapped around the sensor tubing unit. The
thermally conductive material 18 which may be, for example, an
aluminum band or strip, is arranged to contact the vertical legs
of tube 12 whereby the entire strap 18 and the contacted portions
of tube 12 attain substantially the same temperature. Thus, strap
18 produces an isothermal band around the sensor unit. This isothermal
band around the sensor unit tends to provide substantial improvement
to the operation of the unit.
Referring concurrently to FIG. 3 there is shown, graphically,
the improved operation which is attainable with the modification
to the sensor which is shown in FIG. 2. For convenience, the sensor
tube has been shown in a straight configuration in FIG. 3. However,
the other aspects thereof remain the same. Thus, the tubing 12
coil 14 and the like remain the same. The reference locations A,
B, C, D and E are also considered to be the same. By showing the
tubing in a straight line configuration, the operation thereof can
be more readily described. Thus, in FIG. 3 the temperature gradient
for the prior art sensor unit is shown in line I. In particular,
the temperature gradient for the tubing unit without fluid flow
therethrough is, essentially, continuously variable along the length
of the unit as shown by line 300. The temperature gradient as shown
in line I extends from point A to point E in the tubing wherein
the temperature gradient increases with distance from A to C and
decreases from C to E. That is, heat is applied by the coil portion
BC which produces a large gradient in this area. A portion of this
heat is transferred along the tube 14 from B toward A so that the
gradient extends along the entire length of the tube. Likewise,
the temperature gradient varies from C through E because the sensor
tube 12 tends to be thermally conductive throughout the entire length
thereof.
As shown by the solid line 300 the temperature gradient reaches
a maximum value MAX IA as a function of the physical parameters
of the system. This gradient (solid line) is observed without gas
flow in the sensor tube and is substantially uniformly balanced.
Thus, the bridge network associated therewith is balanced or nulled.
When a fluid flows through the tubing, the temperature gradient
changes as a function of the temperature transfer from the tubing
to the fluid and vice versa. Thus, the temperature gradient for
the sensor, with fluid flow therethrough, is as shown by the dashed
line 301 in line I of FIG. 3.
That is, as the cooler (i.e. at or near ambient) fluid is passed
through the sensor tube, heat is transferred to the fluid from the
coil. Thus, the temperature gradient shifts (in this case, to the
viewer's right) relative to the sensor unit. The gradient reaches
the same maximum value but is shifted as shown by MAX IB.
It is seen that the slopes of the gradients, with and without fluid
flow, can be different. Also, it is clear that the gradients are
elongated with relatively low slopes. This tends to make the accurate
detection thereof more difficult. The known systems have tried many
different approaches, especially in terms of convection insulation,
to reduce the problem and to improve detection accuracy.
In contrast, the temperature gradient for the unique sensor unit
with the isothermal strap is shown in line II of FIG. 3. That is,
the isothermal strap 18 (shown schematically in dashed outline)
is connected across the sensor unit. In essence, the sensor leg
AB and the sensor leg DE (more precisely sensor tube points B and
D) are, effectively, joined together and maintained at the same
temperature by the application of the isothermal strap. Consequently,
there is no thermal gradient in leg AB or in leg DE. Thus, the thermal
gradient is limited to the sensor unit span BD. Ideally, this span
is limited to the length of tubing covered by coil BC and coil CD.
(If point C is a center tap on a single coil, then leg BD is a single
unit.)
Thus, with the isothermal strap in place, points B and D are held
at equal temperatures. Thus, tubing sections AB and DE do not change
temperature irrespective of fluid flow. Consequently, the thermal
mass of sections AB and DE of the sensor are removed from the system
response characteristics. As a result, the thermal gradient is confined
between sensor tube points B and D which is much shorter than the
distance between points A and E. The maximum gradient levels are
maintained the same, i.e. MAX I=MAX II. The gradient slope is much
steeper than in the prior art and is, therefore, more readily and
more accurately detected.
As in the prior art, the solid line 302 shows the "no-fluid"
balanced condition while the dashed line 303 shows the shifted gradients
caused by fluid flow in the tube. The maximum levels MAX IIA and
MAX IIB are basically the same as the values of MAX IA and MAX IB
in the prior art.
As noted, the principal thermal path of the sensor, with the isothermal
bar or strap in place, is the tubing wall of sections BC and CD.
With this arrangement, a greater heater power is required to raise
the temperature of point C to the same (i.e. MAX II) level above
the ambient temperature relative to the sensor as in the prior art
(i.e. MAX I). However, the output of the sensor increases because
there is no preheating of the gas stream through section AB. More
power is required to overcome the thermal conduction of the tubing
wall of sections BC and CD. That is, the temperature variations
in the novel invention occur only along section BD whereas in the
prior art sensors the changes occur along the entire length of the
sensor (i.e., from point A through point E).
However, as noted in the cited patents and application, there are
other problems which are found with the prior art systems. For example,
thermal convection and thermal conduction (secondary losses) along
and through the system create problems and diminish accuracy. Many
approaches have been tried to avoid these difficulties. In addition,
it is noted that the dissimilar metals at junctions 16 tend to cause
a thermocouple effect, i.e. generate an EMF between the two dissimilar
metals, especially when elevated temperatures are applied thereto.
What is more, if these temperatures are different, the different
thermocouple effects are such as to cause unpredictable readings
at the bridge network.
Referring now to FIG. 4 a sensor unit similar to the sensor unit
of FIG. 2 is shown. Similar components bear similar reference numerals.
However, in the device shown in FIG. 4 a layer 17 of fibrous insulating
material, for example of the type taught in the copending application
of Abouchar et al, is disposed over the sensor unit including tube
12 coil 14 and the connector rods 13. Insulating layer 17 can be
disposed within isothermal band 18. Typically, insulating layer
17 is fabricated of an insulating material with a high K factor.
This insulating layer 17 tends to reduce the thermal convection
currents along the sensor apparatus. Thus, the junctions 16 (see
FIG. 1) are maintained at a relatively constant temperature and
little or no convection current passes along the tube 12 irrespective
of the attitude of the sensor unit. This technique is an important
extension of the teachings in the copending application. That is,
reducing the impact of the thermocouple effects is significant.
As shown in FIG. 4 a further layer 22 of fibrous insulating material
can then be disposed over the entire sensing unit, including the
isothermal band 18. Insulating layer 22 is shown broken away in
FIG. 4 so that the internal arrangements of the components can be
better illustrated.
Referring now to FIG. 5 there is shown yet another embodiment
of the sensor system of the instant invention. In this case, the
fibrous insulator bat 17 is applied as shown in FIGS. 4 and 5. In
the embodiment of FIG. 5 relatively large masses 20 and 21 of thermally
conductive material are placed around the vertical legs of sensor
tube 14. These masses can be thermally tied together with straps
18 of high thermal conductivity material.
The purpose of the large masses 20 and 21 is to effectively reduce
any thermal gradient which may exist between the cross-member portion
of the U-shaped tube 12 and the ends 12A and 12B. This has a similar
effect to the strap 18 in FIG. 2. That is, the masses 20 and 21
produce an isothermal effect between points B and D of the sensor
tube.
Still referring to FIG. 5 there is shown outer casing 19 which
is used to encase the entire sensor unit, including the various
isothermal guard units and the insulating units, as well. The casing
19 also is adapted to snugly embrace step 11 to produce a friction
fit therewith. Of course, suitable fastening means between case
19 and step 11 can be provided in any fashion desired. In addition,
casing 19 can be fabricated of metal or wrapped in metal foil to
produce another isothermal shield.
Referring now to FIG. 6 there is shown a cut-away view of the sensor
system taken from one end thereof. In the illustration of FIG. 6
the relative dimensions of base 10 and step 11 are shown. The tube
12 and one end 12A thereof are also shown, together with a dashed
outline of the interconnection therebetween. The connector rods
13 per se, are not visible in the embodiment shown in FIG. 6. However,
an end thereof is shown in one typical implementation wherein the
end 13A is brought out of block 10 for connection with other portions
of the sensor unit such as, but not limited to, the bridge sensing
network described herein.
It is clear from the showing of FIG. 7 that the first insulating
layer 17 is draped over the sensor tube 12 (and coil 14) and positioned
adjacent thereto. In the embodiment shown in FIG. 7 the thermally
conductive strap 18 is disposed against the sides of insulator layer
17. Typically, guard band 18 is used to "pinch-in" the
sides of insulator 17 to closely engage connecting rods 13 thereby
to avoid any thermal convection thereat. The outer insulator layer
22 is then placed over the inner insulator 17 and the associated
isothermal guard apparatus as well. The casing 19 is then disposed
over the entire assembly to maintain the insulators in close proximity
to the sensor apparatus.
It should be understood that the cut-away view shown in FIG. 6
can also represent a cut-away view of the system shown in FIG. 5.
In this case, the element 18 would represent the opposite halves
of an element 20 or 21 when viewed from the end of the apparatus.
In either case, the construction would be substantially similar.
As noted above, a bridge circuit is typically used to permit a
highly accurate signal to be achieved though the balancing of representative
signals in the overall operation. However, many of the existing
circuits suffer from problems of, for example, offset voltages which
are generated in the various portions thereof. These offset voltages
produce inaccuracies and/or tolerance problems with the circuit
operation. In the past, attempts to reduce this offset voltage problem
have produced other problems such as lack of stability, unnecessary
rolloff or the like. As a consequence, the improved circuit of this
invention is highly desirable.
Referring now to FIG. 8 there is shown a schematic diagram of
the circuit of the instant invention. The major areas of the circuit
are a constant current generator 810 which includes an operational
amplifier 813 which is connected to a bridge circuit 811. A precision
signal circuit portion 816 is also connected to the current generator
810 and to the bridge 811. At node D there is produced a voltage
which indicates in a precision manner, the current through the bridge
811. Differential amplifier 812 is connected to the bridge circuit
811. An operational amplifier 814 is connected to the output of
amplifier 812 and an output or utilization device 815 is connected
to the output of amplifier 814.
In more detail, the bridge circuit 811 is typically, constructed
in any fashion known in the art. In this application of the circuit,
leg B, C, D of bridge 811 is formed of coil 14 shown in FIGS. 1-7.
A variable resistor R1 forms one side of the bridge so that the
wiper arm or tap of the variable resistor effectively forms one
node (A) of the bridge 811. This node is connected to the minus
(-) input terminal of differential amplifier A1 of typical design
(e.g. Fairchild UA 741 CP or Texas Instruments TL075CN).
The plus (+) input terminal of amplifier 812 is connected to the
opposite node (node C) of bridge network 811 so that the input signal
supplied to the amplifier 812 is, effectively, determined by the
operation of the bridge as is well known in the art. One of the
intermediate nodes (node B) of bridge 811 is connected to the emitter
electrode of NPN transistor Q1. The collector electrode of transistor
Q1 is connected to a suitable voltage source, in this instance +15
volts. The base electrode of transistor Q1 is connected to the output
terminal of operational amplifier 813 (which is designated as amplifier
A3) and is similar in structure to amplifier A1.
The voltage source generated at the cathode of Z1 (1.24V), is connected
to the plus (+) or non-inverting terminal of operational amplifier
813. The cathode of Zener diode Z1 is also connected to one end
of resistor 820 and to one terminal of variable resistor R3. The
other end of resistor 820 is connected to a suitable power source.
The anode of Zener diode Z1 is connected to ground. The other terminal
of variable resistor R3 is connected, via resistor 821 to another
voltage source, and to the anode of Zener diode Z2 which has the
cathode thereof connected to ground. This circuit arrangement produces
a precise voltage (-1.24 volts) at the anode of diode Z2. In addition,
the anode of Zener diode Z1 is connected to ground. Resistor R4
is connected between ground and the second intermediate node (node
D) of bridge circuit 811. Node D is also connected to the minus
(-) or non-inverting input terminal of amplifier 813 via resistor
816. Filter capacitor 840 is connected across amplifier 81. It is
seen that most of the components described above, with the exception
of bridge 81 form a constant current generator which supplies a
constant current to the bridge network.
The plus (+) input terminal of amplifier 812 is connected to ground
via resistors 822 and 824 as well as roll-off capacitor 823. The
common junction of resistors 822 and 824 is also connected via resistor
841 to the tap or wiper arm of resistor R3. It will be seen that
this network comprises a biasing resistor, as well as a roll-off
controlling network, which BIAS is capable of being precisely modified
via resistor R3.
The output terminal of amplifier 812 is connected to a resistor
network comprising fixed resistors 825 and 826 and a variable resistor
R2. This series resistor network is further connected to ground
with variable resistor R2 connected between resistors 825 and 826.
The tap of resistor R2 is connected to the gain control feedback
network which comprises resistor 827. Rolloff capacitor 828 is connected
in parallel with amplifier 812. In particular, this gain and time
constant control network is returned to the minus (-) input terminal
of amplifier 812.
The output terminal of amplifier 812 is connected to the plus (+)
input terminal of amplifier 814 via the RC network comprising resistor
829 and capacitor 830. The resistor and capacitor are connected
in parallel to provide a stability and noise eliminating input circuit
to amplifier 814. The output terminal of amplifier 814 is returned
to the minus (-) input terminal of amplifier 814 via the feedback
network comprising resistor 831 and capacitor 842. The minus (-)
terminal is also connected to ground via the series connected RC
network comprising resistor 832 and capacitor 833. This series RC
network R31 R32 and R33 around amplifier 813 produces a signal
phase cancellation as will be described hereinafter. In addition,
the output of amplifier 814 is connected to a suitable output or
utilization device 815 which can comprise a meter, a control circuit
or the like.
In operation, the circuit is arranged as shown. As noted, the bridge
811 represents the sensing element in the sensor apparatus. The
Zener diodes Z1 and Z2 are arranged to produce substantially constant
voltage drops thereacross. Consequently, the voltage level at the
cathode of Zener diode Z1 is +1.24 volts while the voltage at the
anode of Zener diode Z2 is -1.24 volts. These voltages are achieved
in accordance with the voltage source and the current limiting resistor
820 and 821 respectively.
Thus, the voltage developed across variable resistor R3 is between
-1.24 volts and +1.24 volts. The voltage developed across the resistor
R3 is then divided and a portion thereof is obtained at the variable
tap and supplied to the non-inverting (+) input of amplifier 812.
The signal from resistor R1 is supplied to the inverting (-) input
terminal of amplifier A1. By adjusting resistor R1 accurate control
can be obtained to adjust the offset voltage of the bridge as well
as the common mode characteristics of the system. By adjusting resistor
R3 a precise adjustment for the zeroing of the offset voltage to
amplifier 812 is achieved. While other known circuits use offset
nulling and adjusting circuits, the arrangement shown in FIG. 8
provides an extremely precise and accurate offset voltage nulling
operation while maintaining the common mode characteristics of the
system in check.
It should be understood that the connection of resistor R4 between
the bridge and signal ground is also highly advantageous. In known
circuits of the sensor type, this resistor is usually returned to
the plus (+) voltage supply. However, in the inventive circuit design,
the voltages for resistor R3 are readily available in a convenient
manner.
In addition, the constant current generator circuitry 810 is much
simpler to implement. The current generator 810 operates to control
the current flowing through bridge network 811. That is, when transistor
Q1 is turned on, the current therethrough flows through bridge 811
and resistor R4 to ground. The voltage at node D is returned to
the inverting input of amplifier 813. The non-inverting input of
amplifier 813 is tied to the fixed voltage at terminal Z (which
is also the cathode of Zener diode Z1). The amplifier 813 output
drives transistor Q1 to produce a null at the amplifier inputs.
This occurs when the voltage across R4 is equal to the voltage across
diode Z1. Since the voltage across R4 is held constant, the current
therethrough is also constant. The resistance of the bridge, however,
can vary with temperature. Therefore, the voltage at the emitter
electrode of transistor Q1 will vary as the bridge resistance varies,
thereby keeping the current constant through the sensor bridge 811.
Resistor R2 (connected at the output of amplifier 812) is used
to adjust the gain of the bridge amplifier 812. By using this arrangement,
a further control characteristic of the circuit is provided. That
is, the gain of amplifier 812 is adjustable and, if desirable, permanently
settable by fixing resistor R2 once the proper gain adjustment is
made.
As noted above, the time constant circuit comprising resistor 829
and capacitor 830 are used to provide equal impedance to the input
terminals of amplifier A2. The circuit comprising resistor 831 and
capacitor 833 has a time constant equal and opposite to the principal
time constant of the sensor device. In one embodiment resistor 831
is 6.5 megohms and capacitor 833 is 1 MFD. (This produces an RC
time constant of 6.5 seconds.) This RC time constant was selected
because the frequency response of the sensor establishes a principal
pole at 6.5 seconds (see FIG. 9A). The combination of resistor 831
capacitor 833 and amplifier 814 produces a frequency response with
a 6.5 second zero thereby balancing the sensor pole (see FIG. 9B).
Resistor 832 limits the effect of resistor 831 and capacitor 833
to a useful operating range and reduces high frequency noise. When
the frequency response characteristics of the sensor (FIG. 9A) are
combined with the frequency response characteristics of the circuit
(FIG. 9B), the combined system operating characteristic is shown
in FIG. 9C. That is, the response characteristic of the sensor is
substantially compensated by the response characteristic of the
circuit. Consequently, the compensated system characteristic is
substantially flat for at least 5 time constants. At this point
rolloff in operating characteristic is unimportant because the system
is in equilibrium. In this instance, the relatively simple compensation
electronic circuit shown in FIG. 8 is satisfactory. That is, the
RC circuit comprising resistor 831 and capacitor 833 is sufficient
to compensate for the time constant of the sensor. It must be noted
that in conjunction with many prior art sensor devices, an extremely
complex time constant relationship is produced. In those cases,
only a small correction of the time constant is possible without
an extraordinarily complex and expensive compensation network.
In the new circuit, a dramatic improvement in performance of the
sensor apparatus is achieved. That is, the speed of the sensor operation
is improved by approximately 2:1 but the ability to cancel the time
constant is improved by approximately 10:1.
The improvement in the sensor apparatus operation is suggested
in FIGS. 10A and 10B. In particular, in FIG. 10 there is shown
a graphic comparison of the system operation as related to the prior
art system. In particular, it is seen that the prior art system
represented by curve 10AP, has a first time constant (i.e. it reaches
about 63% of maximum) in about 10 seconds. Furthermore, this system
reaches about 1% from maximum in about one minute after operation
is begun. By adding the improvement of the isothermal bands to the
sensor device, the curve 10AN is produced. It is seen that with
this modification the system reaches its first time constant within
6.5 seconds. Moreover, the system is within 1% of maximum at ultimate
(or maximum) signal about 10 seconds. By one minute after start
up, the new system is within 0.1% of maximum operation. Thus, the
improvements over the prior art system are clear.
Referring now to FIG. 10B, there are shown waveforms for the operation
of circuits and systems of the prior art and the inventions described
herein. In this instance, the ideal situation is such that the signal
switches instantaneously from the zero level to the final value.
Thus, a perfect step wave is produced. However, in reality this
is rarely achieved. The waveforms depicted herein represent a typical
circuit operation by the prior art devices and the instant invention
within about 10% of maximum final value. In this case, it is seen
that the signal 10BP tends to have an undershoot or overshoot of
approximately .+-.10%. The circuit then responds and compensates
for this to error. In the waveform shown, the circuit does not settle
down to the ideal condition for over a minute (see FIG. 10A). It
is obvious that the circuit could continue to "hunt" or
"ring" as is known in many devices. However, some known
devices are able to settle down to an operating condition within
.+-.2% error within approximately 15 seconds after the signal change
that was initiated.
However, the operation of the circuit of the instant invention
is shown by waveform 10BN. In this instance, the overshoot is limited
to two percent (or less) and is generally limited to approximately
1%. In this inventive system, the overshoot and hunting to achieve
the optimum condition are virtually eliminated. That is, the inventive
system tends to reach the optimum level within approximately 1.5
seconds (or less). This rapid operation is achievable because the
sensor device is much more easily controlled and produces a relatively
simple operating characteristic. The circuit compensation is, therefore,
much simpler as well.
In comparisons of the existing devices and the device covered hereby,
it is observed that the prior art devices have errors in the order
of seven percent or more and take approximately 6-15 seconds (or
more) to settle to an operating condition of .+-.2% error. Conversely,
the circuit and system of the instant invention has a maximum error
of two percent or less and tends to settle to the optimum operating
condition within 1.5 seconds. Inasmuch as the circuit is frequently
used to control a mass flow meter of the type described in the copending
application, this rapid response time is highly desirable. That
is, the sensor device frequently controls critical measurements
or processes such as, but not limited to, semiconductor fabrication
techniques. Clearly, these controls are of great significance in
order to produce useful devices.
Referring now to FIG. 11 there is shown an improved embodiment
of the sensor. In this instance, a heat sink encasement 110 is bonded
to the sensor tubing 12 at points B and D in such a manner that
good thermal contact is made between the tubing and the walls of
the encasement. In this instance the walls of the encasement act
as a good thermal conductor to maintain the sensor tubing between
points B and D at an even temperature and to prevent heat from being
dissipated in the vertical legs of the sensor tubing thereby degrading
the accuracy of the sensor coil 14 readings. Additionally, an even
temperature gradient between points B and D is maintained regardless
of sensor attitude. Copper coating the sensor tubing 12 between
points B and D helps to insure good heat conductivity. In this way,
outside conditions normally affecting the accuracy of the sensor
readings are virtually eliminated. In the preferred embodiment,
the points B and D are immediately adjacent to the ends of the winding
14 in order to minimize the thermal gradient.
Normal radiation due to convection is eliminated by operating the
sensor coils in a vacuum, i.e. by evacuating the chamber defined
by encasement 110. Thus, the heat sink encasement 110 acts both
as an isothermal device to maintain an even temperature differential
across the critical area of measurement, i.e. coil 14 and a heat
sink to protect the sensor reading from being impacted by abnormal
radiation in the vertical legs of the tubing due to attitude changes.
Similar results can be attained by replacing the vacuum within the
heat sink encasement 110 with a high k-factor type of insulation.
Referring to FIG. 12 there is shown a partially broken away perspective
view of an embodiment of the sensor of the instant invention including
the encasement 110. In this embodiment, the encasement 110 is made
of a thermally conductive material which is connected to the tube
12 immediately adjacent to the ends of coil 14. Inasmuch as encasement
110 is evacuated, thermal convection does not occur relative to
coil 14. Therefore, the insulation layers shown in the other embodiments
can be eliminated. Moreover, the isothermal strap 18 is, effectively,
replaced by encasement 110 Of course, a high k insulation material
(not shown) can be used within the encasement 110 as well, if so
desired.
Thus, there is shown and described a new, unique and improved construction
for a mass flow measuring system. The system of the instant invention
provides a means for closely controlling the area of temperature
transfer and temperature gradients relative to the sensor. In particular,
an isothermal plane is created adjacent to the sensor coil. By controlling
the temperature transfers, the operation of the sensor system can
be greatly improved. Greater accuracy can be achieved through reduction
of temperature gradients and thermal shifts. In addition, a faster
operating circuit can be used with the sensing coil if so desired,
in order to produce substantially improved system operation. The
circuit can be more accurately matched to the operating characteristics
of the sensor, per se. This permits faster overall operation of
the sensor system. While the invention has been shown and described
in detail with great specificity, this teaching is intended to be
illustrative only, and is not intended to be limitative. The scope
of the invention is limited only by the claims appended hereto. |