U.S. patent application number 10/256917 was filed with the patent office on 2004-04-01 for flow pickup circuit.
Invention is credited to Luchner, Stefan.
Application Number | 20040064270 10/256917 |
Document ID | / |
Family ID | 32029391 |
Filed Date | 2004-04-01 |
United States Patent
Application |
20040064270 |
Kind Code |
A1 |
Luchner, Stefan |
April 1, 2004 |
FLOW PICKUP CIRCUIT
Abstract
A flow pickup circuit for receiving a flow signal from a flow
sensor and providing a flow indicator signal corresponding to flow
characteristics through the flow sensor includes an inner bridge
circuit, an outer bridge circuit, and a processor. The inner bridge
circuit operates in a constant current mode, and produces a first
sensing signal. The inner bridge circuit includes four components
connected in a series loop, two of which include the flow sensor.
The outer bridge circuit operates in a constant temperature mode,
and produces a second sensing signal. The outer bridge circuit
includes four components connected in a series loop, one of which
includes the inner bridge circuit. The processor receives the first
sensing signal and the second sensing signal, and produces the flow
indicator signal therefrom. The processor subtracts an ambient
temperature component from the second sensing signal to produce a
constant temperature flow indicator.
Inventors: |
Luchner, Stefan; (Munchen,
DE) |
Correspondence
Address: |
MCDERMOTT, WILL & EMERY
ATTN: INTELLECTUAL PROPERTY DEPT.
28 STATE STREET
BOSTON
MA
02109
US
|
Family ID: |
32029391 |
Appl. No.: |
10/256917 |
Filed: |
September 27, 2002 |
Current U.S.
Class: |
702/45 |
Current CPC
Class: |
G01F 1/698 20130101;
G01F 1/696 20130101 |
Class at
Publication: |
702/045 |
International
Class: |
G06F 019/00 |
Claims
What is claimed is:
1. A flow pickup circuit for receiving a flow signal from a flow
sensor and providing a flow indicator signal corresponding to flow
characteristics through the flow sensor, comprising: an inner
bridge circuit constructed and arranged so as to operate in a
constant current mode, and to produce a first sensing signal,
wherein one or more of the components of the inner bridge is the
flow sensor; an outer bridge circuit, constructed and arranged so
as to operate in a constant temperature mode, and to produce a
second sensing signal, wherein the inner bridge circuit is one of
the components of the outer bridge circuit; and, a processor for
receiving the first sensing signal and the second sensing signal
and producing the flow indicator signal therefrom.
2. A flow pickup circuit according to claim 1, wherein the inner
bridge circuit includes four components connected in a series loop
characterized by a first pair of diagonally-situated nodes and a
second pair of diagonally-situated nodes, and the first sensing
signal includes a voltage potential between the first pair of
diagonally-situated nodes.
3. A flow pickup circuit according to claim 2, wherein two of the
four components include the flow sensor, disposed between the
second pair of diagonally-situated nodes.
4. A flow pickup circuit according to claim 1, wherein the outer
bridge circuit includes four components connected in a series loop
characterized by a first pair of diagonally-situated nodes and a
second pair of diagonally-situated nodes, and the second sensing
signal includes a voltage potential between one node of the first
pair of diagonally-situated nodes and a reference voltage.
5. A flow pickup circuit according to claim 4, wherein one of the
four components includes the inner bridge circuit, disposed between
the second pair of diagonally-situated nodes.
6. A flow pickup circuit according to claim 1, wherein the
processor multiplies the first sensing signal by the second sensing
signal so as to produce a product, and divides the product by a
workpoint current value so as to produce a constant current flow
indicator.
7. A flow pickup circuit according to claim 1, wherein the
processor subtracts an ambient temperature component from the
second sensing signal, so as to produce a constant temperature flow
indicator.
8. A flow pickup circuit according to claim 7, wherein the ambient
temperature component is generated by subtracting the second
sensing signal from the first sensing signal so as to produce a
difference signal, and passing the difference signal through a
low-pass filter.
9. A flow pickup circuit according to claim 8, wherein the low-pass
filter includes an analog filter.
10. A flow pickup circuit according to claim 8, wherein the
low-pass filter includes a digital filter.
11. A flow pickup circuit according to claim 8, further including
at least one scaling module for scaling at least one of the first
sensing signal and the second sensing, so that the first sensing
signal and the second sensing signal have compatible
magnitudes.
12. A method of receiving a flow signal from a flow sensor and
providing a flow indicator signal corresponding to flow
characteristics through the flow sensor, comprising: producing a
first sensing signal via an inner bridge circuit, constructed and
arranged so as to operate in a constant current mode, wherein one
or more of the components of the inner bridge is the flow sensor;
producing a second sensing circuit via an outer bridge circuit,
constructed and arranged so as to operate in a constant temperature
mode, wherein the inner bridge circuit is one of the components of
the outer bridge circuit; and, receiving the first sensing signal
and the second sensing signal and producing the flow indicator
signal therefrom.
13. A method according to claim 12, further including producing the
first sensing signal via the inner bridge circuit that includes
four components connected in a series loop characterized by a first
pair of diagonally-situated nodes and a second pair of
diagonally-situated nodes, wherein the first sensing signal
includes a voltage potential between the first pair of
diagonally-situated nodes.
14. A flow pickup circuit according to claim 12, further including
producing the second sensing signal via the outer bridge circuit
that includes four components connected in a series loop
characterized by a first pair of diagonally-situated nodes and a
second pair of diagonally-situated nodes, wherein the second
sensing signal includes a voltage potential between one node of the
first pair of diagonally-situated nodes and a reference
voltage.
15. A method according to claim 12, further including multiplying
the first sensing signal by the second sensing signal so as to
produce a product, and dividing the product by a workpoint current
value so as to produce a constant current flow indicator.
16. A method according to claim 12, further including subtracting
an ambient temperature component from the second sensing signal, so
as to produce a constant temperature flow indicator.
17. A flow pickup circuit according to claim 16, further including
generating the ambient temperature component is generated by
subtracting the second sensing signal from the first sensing signal
so as to produce a difference signal, and passing the difference
signal through a low-pass filter.
18. A method according to claim 17, further including scaling at
least one of the first sensing signal and the second sensing via at
least one scaling module, so that the first sensing signal and the
second sensing signal have compatible magnitudes.
19. A flow pickup circuit for receiving a flow signal from a flow
sensor and providing a flow indicator signal corresponding to flow
characteristics through the flow sensor, comprising: an inner
bridge circuit constructed and arranged so as to operate in a
constant current mode, and to produce a first sensing signal, the
inner bridge circuit including four components connected in a
series loop characterized by a first pair of diagonally-situated
nodes and a second pair of diagonally-situated nodes, and the first
sensing signal includes a voltage potential between the first pair
of diagonally-situated nodes, wherein two of the four components
include the flow sensor, disposed between the second pair of
diagonally-situated nodes; an outer bridge circuit, constructed and
arranged so as to operate in a constant temperature mode, and to
produce a second sensing signal, the outer bridge circuit including
four components connected in a series loop characterized by a first
pair of diagonally-situated nodes and a second pair of
diagonally-situated nodes, the second sensing signal including a
voltage potential between the first pair of diagonally-situated
nodes, wherein one of the four components includes the inner bridge
circuit; a processor for receiving the first sensing signal and the
second sensing signal and producing the flow indicator signal
therefrom, wherein the processor subtracts an ambient temperature
component from the second sensing signal, so as to produce a
constant temperature flow indicator.
20. A flow pickup circuit for receiving a flow signal from a flow
sensor and providing a flow indicator signal corresponding to flow
characteristics through the flow sensor, comprising: means for
producing a first sensing signal, wherein the means for producing a
first sensing signal includes the flow sensor and operates in a
constant current mode; means for producing a second sensing signal,
the outer bridge circuit including four components connected in a
series loop characterized by a first pair of diagonally-situated
nodes and a second pair of diagonally-situated nodes, the second
sensing signal including a voltage potential between the first pair
of diagonally-situated nodes, wherein one of the four components
includes the inner bridge circuit. means for receiving the first
sensing signal and the second sensing signal, and for subtracting
an ambient temperature component from the second sensing signal, so
as to produce a constant temperature flow indicator, wherein the
ambient temperature component is generated by subtracting the
second sensing signal from the first sensing signal so as to
produce a difference signal, and passing the difference signal
through a low-pass filter.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] Not Applicable
REFERENCE TO MICROFICHE APPENDIX
[0003] Not Applicable
BACKGROUND OF THE INVENTION
[0004] The present invention relates to sensing circuitry, and more
particularly to circuitry, associated with flow sensor, that
provides significantly faster response times as compared to
circuitry in similar prior art sensors.
[0005] There exists a demand for a mass flow controller (MFC)
having a step response of less than 300 mS. The flow indicator
signal of a flow sensor operating in a constant current supplied
bridge circuit has been shown to exhibit a time constant of about 3
seconds, so that it takes more than 8 seconds for the signal to be
appreciably close to its final value. The flow indicator signal of
a flow sensor operating in a constant temperature (of sensor)
configuration has been shown to be much faster (about 50 mS), but
characteristics of the indicator signal are affected by the ambient
temperature. In brief, a MFC operating in a constant current mode
is stable, but slower than desired. A MFC operating in a constant
temperature mode can provide the desired step response, but
provides a signal that may be biased by the ambient
temperature.
SUMMARY OF THE INVENTION
[0006] In one aspect of the present invention, a flow pickup
circuit for receiving a flow signal from a flow sensor and
providing a flow indicator signal corresponding to flow
characteristics through the flow sensor comprises an inner bridge
circuit, an outer bridge circuit, and a processor. The inner bridge
circuit is constructed and arranged so as to operate in a constant
current mode, and to produce a first sensing signal. One or more of
the components of the inner bridge is the flow sensor. The outer
bridge circuit is constructed and arranged so as to operate in a
constant temperature mode, and to produce a second sensing signal.
The inner bridge circuit is one of the components of the outer
bridge circuit. The processor receives the first sensing signal and
the second sensing signal, and produces the flow indicator signal
therefrom.
[0007] In another embodiment, the inner bridge circuit includes
four components connected in a series loop characterized by a first
pair of diagonally-situated nodes and a second pair of
diagonally-situated nodes. The first sensing signal includes a
voltage potential between the first pair of diagonally-situated
nodes.
[0008] In another embodiment, two of the four components include
the flow sensor, disposed between the second pair of
diagonally-situated nodes.
[0009] In another embodiment, the outer bridge circuit includes
four components connected in a series loop characterized by a first
pair of diagonally-situated nodes and a second pair of
diagonally-situated nodes. The second sensing signal includes a
voltage potential between the first pair of diagonally-situated
nodes.
[0010] In another embodiment, one of the four components includes
the inner bridge circuit, disposed between the second pair of
diagonally-situated nodes.
[0011] In another embodiment, the processor multiplies the first
sensing signal by the second sensing signal so as to produce a
product, and divides the product by a workpoint current value so as
to produce a constant current flow indicator.
[0012] In another embodiment, the processor subtracts an ambient
temperature component from the second sensing signal, so as to
produce a constant temperature flow indicator.
[0013] In another embodiment, the ambient temperature component is
generated by subtracting the second sensing signal from the first
sensing signal so as to produce a difference signal, then passing
the difference signal through a low-pass filter. The low-pass
filter may include an analog filter (i.e., for filtering analog
signals) known in the art, or a digital filter (e.g., FIR, IIR,
etc.) known in the art.
[0014] Another embodiment further includes at least one scaling
module for scaling at least one of the first sensing signal and the
second sensing, so that the first sensing signal and the second
sensing signal have compatible magnitudes.
[0015] Another aspect of the invention comprises a method of
receiving a flow signal from a flow sensor and providing a flow
indicator signal corresponding to flow characteristics through the
flow sensor. The method comprises producing a first sensing signal
via an inner bridge circuit, constructed and arranged so as to
operate in a constant current mode, wherein one or more of the
components of the inner bridge is the flow sensor. The method
further includes producing a second sensing circuit via an outer
bridge circuit, constructed and arranged so as to operate in a
constant temperature mode, wherein the inner bridge circuit is one
of the components of the outer bridge circuit. The method further
includes receiving the first sensing signal and the second sensing
signal and producing the flow indicator signal therefrom.
[0016] Another embodiment further includes producing the first
sensing signal via the inner bridge circuit that includes four
components connected in a series loop characterized by a first pair
of diagonally-situated nodes and a second pair of
diagonally-situated nodes. The first sensing signal includes a
voltage potential between the first pair of diagonally-situated
nodes.
[0017] Another embodiment further includes producing the second
sensing signal via the outer bridge circuit that includes four
components connected in a series loop characterized by a first pair
of diagonally-situated nodes and a second pair of
diagonally-situated nodes. The second sensing signal includes a
voltage potential between the first pair of diagonally-situated
nodes.
[0018] Another embodiment further includes multiplying the first
sensing signal by the second sensing signal so as to produce a
product, and dividing the product by a workpoint current value so
as to produce a constant current flow indicator.
[0019] Another embodiment further includes subtracting an ambient
temperature component from the second sensing signal, so as to
produce a constant temperature flow indicator.
[0020] Another embodiment further includes generating the ambient
temperature component is generated by subtracting the second
sensing signal from the first sensing signal so as to produce a
difference signal, and passing the difference signal through a
low-pass filter.
[0021] Another embodiment further includes scaling at least one of
the first sensing signal and the second sensing via at least one
scaling module, so that the first sensing signal and the second
sensing signal have compatible magnitudes.
[0022] In another aspect, the invention comprises a flow pickup
circuit for receiving a flow signal from a flow sensor and
providing a flow indicator signal corresponding to flow
characteristics through the flow sensor. The flow pickup circuit
includes an inner bridge circuit constructed and arranged so as to
operate in a constant current mode, and to produce a first sensing
signal. The inner bridge circuit includes four components connected
in a series loop, characterized by a first pair of
diagonally-situated nodes and a second pair of diagonally-situated
nodes. The first sensing signal is given by a voltage potential
between the first pair of diagonally-situated nodes. Two of the
four components include the flow sensor, disposed between the
second pair of diagonally-situated nodes. The flow pickup circuit
further includes an outer bridge circuit, constructed and arranged
so as to operate in a constant temperature mode, and to produce a
second sensing signal. The outer bridge circuit includes four
components connected in a series loop, characterized by a first
pair of diagonally-situated nodes and a second pair of
diagonally-situated nodes. The second sensing signal is given by a
voltage potential between the first pair of diagonally-situated
nodes, wherein one of the four components includes the inner bridge
circuit. The flow pickup circuit also includes a processor for
receiving the first sensing signal and the second sensing signal,
and producing the flow indicator signal therefrom. The processor
subtracts an ambient temperature component from the second sensing
signal, so as to produce a constant temperature flow indicator.
[0023] In another aspect, the invention comprises a flow pickup
circuit for receiving a flow signal from a flow sensor and
providing a flow indicator signal corresponding to flow
characteristics through the flow sensor. The flow pickup circuit
includes means for producing a first sensing signal. The means for
producing a first sensing signal includes the flow sensor and
operates in a constant current mode. The flow pickup circuit also
includes means for producing a second sensing signal that includes
four components connected in a series loop characterized by a first
pair of diagonally-situated nodes and a second pair of
diagonally-situated nodes. The second sensing signal includes a
voltage potential between the first pair of diagonally-situated
nodes, and one of the four components includes the inner bridge
circuit. The flow pickup circuit also includes means for receiving
the first sensing signal and the second sensing signal, and for
subtracting an ambient temperature component from the second
sensing signal, so as to produce a constant temperature flow
indicator. The ambient temperature component is generated by
subtracting the second sensing signal from the first sensing signal
to produce a difference signal, and passing the difference signal
through a low-pass filter.
BRIEF DESCRIPTION OF DRAWINGS
[0024] The foregoing and other objects of this invention, the
various features thereof, as well as the invention itself, may be
more fully understood from the following description, when read
together with the accompanying drawings in which:
[0025] FIG. 1 shows a block diagram view of one embodiment of a
flow pickup circuit according to the present invention;
[0026] FIG. 2 shows the second element and the first amplifier of
FIG. 1 in greater detail;
[0027] FIG. 3 shows, in schematic form, another embodiment of the
flow pickup circuit of FIG. 1; and,
[0028] FIG. 4 shows a signal flow diagram that may be used to
implement an embodiment of the flow pickup circuit of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0029] One embodiment of the invention combines aspects of the
constant current sensor mode and the constant temperature sensor
mode in a single circuit, in order to benefit from the advantages
of each individual mode (i.e., fast yet stable mass flow
information). FIG. 1 shows a block diagram view of one embodiment
of a flow pickup circuit 100 according to the present invention.
The circuit 100 includes a bridge 102 that consists of a first
bridge component 104, a second bridge component 106, a third bridge
component 108, and a fourth bridge component 110, electrically
coupled in a series loop as shown in FIG. 1. This diamond-shaped
bridge architecture is well known in the art, and may be referred
to as a "Wheatstone" bridge. The bridge 102 is thus characterized
by a first pair of diagonally-situated nodes (node A 114 and node B
116), and a second pair of diagonally-situated nodes (top node 120
and bottom node 122). An operational amplifier 112 (or other
similar comparative element known in the art) monitors the voltage
potential V.sub.AB across node A 114 and node B 116, and controls
the current source 118 so as to drive the voltage V.sub.AB to zero.
The current source 118 provides current to the top node 120 of the
bridge 102 as a function of the output signal from the amplifier
112. Current that passes through the bridge 102 sinks, via the
bottom node 122, to a common ground 124. A first amplifier 126
receives two signals from within the second bridge component 106
and generates a first sensor signal BRIDGE 128 as a function of
those two signals. A second amplifier 130 generates a second sensor
signal CURRENT 132 as a function of the voltage at node B 116 and a
reference voltage V.sub.REF. In one preferred embodiment, the first
amplifier 126 and the second amplifier 130 include instrumentation
amplifiers, such as the INA2126E from BurrBrown (TI), but other
similar difference amplifiers known in the art may also be
used.
[0030] FIG. 2 shows the second element 106 in more detail, along
with the first amplifier 126. The element 106 includes an inner
bridge structure 138, consisting of a first inner element E21 140,
a second inner element 142, a third inner element 144, and a fourth
inner element 146, electrically coupled in the diamond-shaped
bridge architecture shown in FIG. 2. The inner bridge 138 is
characterized by a first pair of diagonally-situated nodes (node C
150 and node D 152), and a second pair of diagonally-situated nodes
(top node 146 and bottom node 148). The top node 146 of the inner
bridge 138 is electrically coupled to the top node 120, and the
bottom node 148 is electrically coupled to node B 116. Node C 150
of the inner bridge 138 is electrically coupled to the inverting
input of the first amplifier 126, and node D 152 is electrically
coupled to the non-inverting input of the first amplifier 126.
[0031] FIG. 3 shows, in schematic form, another embodiment of a
flow pickup circuit 200 according to the present invention. The
flow sensor (FS) 202 and resistors R21 204 and R22 206 correspond
to the inner bridge 138 that is shown within the second element 106
in FIG. 2. The resistor R21 204 corresponds to the first inner
element 140 of the inner bridge 138, the resistor R22 corresponds
to the second inner element 142 of the inner bridge 138, and the
flow sensor 202 corresponds to a series of the third inner element
144 and the fourth inner element 146 of the inner bridge 138. The
inner bridge 138 operates in a mode similar to what is typically
known in the art as a "constant current mode." The inner bridge 138
is used as a sensor in a configuration that is typically known in
the art as a "constant temperature mode." The inner bridge is a
half bridge with inverse sensitive sensor, i.e., the sensor
response is directly proportional to the bridge supply current. The
sensor thus imparts a flow signal to the flow pickup circuit 200
via the inner bridge 138. The operational amplifier 112 controls
the outer bridge so as to maintain a diagonal voltage (i.e., the
voltage potential V.sub.AB across node A 114 and node B 116) of at
or near zero volts, by varying the supply current for the inner
bridge. Since the amount of supply current through the inner bridge
138 can be measured via the first amplifier 126, subsequent signal
processing resources can use the measured inner bridge supply
current to interpret the BRIDGE signal as a constant current mode
flow signal. A flow indicator FCC may be formed as a function of
the BRIDGE signal, the CURRENT signal, and the current at
workpoint, as follows:
F.sub.CC=(k)(BRIDGE)(CURRENT)/I.sub.WP (1)
[0032] Where
[0033] F.sub.CC=flow at constant current,
[0034] I.sub.WP=current at workpoint
[0035] k=proportionality constant
[0036] F.sub.CC will be a flow indicator as good as the signal of a
constant current bridge.
[0037] Similarly, the outer bridge may be used to interpret the
CURRENT signal as a constant temperature mode flow signal F.sub.CT,
as follows:
F.sub.CT=(k)(CURRENT-T.sub.AT) (2)
[0038] The signal F.sub.CT includes the ambient temperature (AT),
but assuming that the rate of change of the ambient temperature is
much less than the rate of change of the measured flow, the ambient
temperature component is approximately equal to the result of
low-pass filtering the difference of the BRIDGE signal and the
CURRENT signal, i.e.,
I.sub.AT.about.LP[CURRENT-BRIDGE] (3)
[0039] The low pass filter (LPF) represented by the symbol "LP[ ]"
in equation (3) may include any signal filtering architecture known
in the art, including digital and analog structures. The filter
cutoff frequency and the rolloff characteristics may vary in
different embodiments, depending upon the nature of the flow being
measured. Note that when the flow is relatively constant, the low
pass filter function LP[ ] passes the result of CURRENT-BRIDGE
without substantial change (i.e., with little or no filtering), so
that
F.sub.CT=(k)(CURRENT-[CURRENT-BRIDGE])=(k)(BRIDGE) (4)
[0040] Equation (4) shows that when the flow is relatively
constant, the flow signal F.sub.CT is proportional to the BRIDGE
signal.
[0041] FIG. 4 shows a signal flow diagram that may be used to
implement one embodiment of the present invention. The CORRECT
CURRENT block 250 receives the BRIDGE signal and the CURRENT
signal, and produces a corrected BRIDGE signal that is corrected
according to the amount of supply current flowing though the inner
bridge. The CALB block 252 and the CALC block 254 modify (i.e.,
scale) the BRIDGE and CURRENT signals, respectively, so that these
two signals have similar magnitudes in terms of flow. Similar
magnitudes make the signals compatible for subsequent processing
operations. In other embodiments, only one of the signals are
scaled In operation, a flow pickup circuit constructed according to
the signal flow of FIG. 4 scans the sensor over several flow
samples (e.g., 10 samples in one embodiment) and equalize the
corresponding values of BRIDGE and CURRENT according to the
calibration tables within the CALB block 252 and the CALC block
254.
[0042] In one embodiment of the invention, non-linear
implementations of the LPF 258 may be used to provide an output
signal that is specifically tailored for a particular application.
For example, one embodiment of the invention may include a
non-linear filter that allows the output to follow the actual flow
characteristics as long as the rate of change of the flow does not
exceed a predetermined limit. When the flow rate of change exceeds
this predetermined limit, the output is "frozen," i.e., held at the
value of the output at the time the rate of change exceeded the
limit. When the rate of change of the flow falls back below the
predetermined limit, the non-linear filter once again allows the
output to follow the actual flow characteristics. A step in the
output may occur when the output transitions from the "frozen" mode
to the "flow following" mode, which may be smoothed by passing the
output through a linear low pass filter following the non-linear
filter.
[0043] The invention may be embodied in other specific forms
without departing from the spirit or essential characteristics
thereof. The present embodiments are therefore to be considered in
respects as illustrative and not restrictive, the scope of the
invention being indicated by the appended claims rather than by the
foregoing description, and all changes which come within the
meaning and range of the equivalency of the claims are therefore
intended to be embraced therein.
* * * * *