Abstrict The output from the sensor detecting the flow rate of the fluid
passing through the conduit is compared with the ceiling value and
the appointed compensating signal is added to the output from the
sensor to display the signal obtained on the basis of this addition
result as the value of flow rate or reduce the openness of the control
valve on the basis of the above described signal when the output
from the sensor is larger than the threshold value, so that the
mass flow meter can accurately display the flow rate even when the
fluid passes in an excessive flow rate. In addition, even though
the mass flow meter and the mass flow controller are fallen into
the abnormal operating condition due to an excessive flow rate of
fluid, they can be quickly returned to the normal operating condition.
Claims What is claimed is:
1. A mass flow meter control system comprising:
a fluid flow control valve configured to be in a fluid path and
controlled by a control signal;
a fluid flow sensor for detecting the flow rate of a fluid in the
fluid path, and providing a corresponding flow rate signal;
means for comparing the detected flow rate signal of a fluid with
a predetermined value to determine an abnormal flow rate;
means for providing a compensation signal when an abnormal flow
rate is detected by the comparing means;
means for adding the compensation signal to the flow rate signal
to provide an output signal, and
means for comparing the output signal with a predetermined desired
flow rate signal and providing a control signal to be applied to
the fluid flow control valve in response to the comparison to adjust
the flow rate to the desired flow rate if necessary.
2. The mass flow meter control system of claim 1 wherein the fluid
flow sensor includes a pair of thermal flow rate sensors.
3. The mass flow meter control system of claim 1 wherein the fluid
flow sensor includes a constant-temperature bridge circuit.
4. The mass flow meter control system of claim 1 wherein the means
for comparing the detected flow rate signal includes a predetermined
flow rate value that is above a predetermined normal flow rate signal.
5. The mass flow meter control system of claim 1 wherein the means
for comparing the detected flow rate includes means for limiting
the provision of a compensation signal to a range of flow rates
between a minimum flow rate Q.sub.1 and a maximum flow rate Q.sub.2
above a predetermined normal flow rate.
6. A mass flow meter control system comprising:
a fluid flow control valve configured to be in a fluid path and
controlled by a control signal;
a fluid flow sensor means for detecting the flow rate of a fluid
in the fluid path and providing a corresponding flow rate signal;
means for compensating for an excessive flow rate above a predetermined
operative flow rate, including:
means for comparing the detected flow rate signal of a fluid with
a predetermined value to determine an abnormal flow rate and for
providing a compensation signal when an abnormal flow rate is detected;
and
means for adding the compensation signal to the flow rate signal
to provide an output signal, and
means for comparing the output signal with a predetermined desired
flow rate signal and providing a corresponding control signal to
be applied to the fluid flow control valve to adjust the flow rate
to the desired flow rate if necessary.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a mass flow meter and a mass flow
controller.
2. Description of the Prior Art
For example, in the case where an apparatus for producting semiconductors
is supplied with various kinds of gas used for producing semiconductors,
their supply passages are provided with a mass flow controller,
respectively, to control flow rates of the respective gases.
FIG. 5 schematically shows a construction of a control system of
the general mass flow controller 50. Referring to FIG. 5 reference
numeral 51 designates a flow rate-measuring portion comprising a
thermal flow rate sensor 54 provided in a measuring passage 53
a bridge circuit 55 and an amplification circuit 56 for measuring
a flow rate of a gas G passing through a passage 52 to put out a
flow rate signal (x) corresponding to said flow rate of said gas
G. And, reference numeral 57 designates a by-pass passage provided
so as to bypass said measuring passage 53. Said by-pass passage
57 is provided with a by-pass portion 58 having constant-flow rate-ratio
characteristics. In addition, reference numeral 59 designates a
control valve provided on the downstream side of the confluence
of the measuring passage 53 and said by-pass passage 57. Reference
numeral 60 designates a comparison circuit for comparing said flow
rate signal (x) put out from said flow rate-measuring portion 51
with a flow rate-setting signal (y) from a flow rate-setting portion
(not shown), reference numeral 61 designating a control circuit,
and reference numeral 62 designating a control valve-driving circuit.
And, with the mass flow controller 50 having the above described
construction, when the flow rate signal (x) is larger than said
flow rate-setting signal (y), an instruction for reducing an openness
of said control valve 59 is given from said control circuit 61
while, when the flow rate signal (x) is smaller than the flow rate-setting
signal (y), an instruction for increasing said openness of the control
valve 59 is given from the control circuit 61 so as to supply an
apparatus (not shown) provided on the downstream side of the mass
flow controller 50 with an appointed flow rate of gas G.
However, in the conventional mass flow controller 50 in order
to prevent an excessive flow rate of gas G from passing through
the measuring passage 53 the control valve 59 of small flow rate
has been incorporated. The openness of the control valve 59 in the
mass flow controller, of which full-scale flow rate is for example
5 ml/min, has been set so that the gas G may pass at a flow rate
of merely about 7 to 8 ml/min when a differential pressure is 0.5
kg/cm.sup.2 G. And, even though said differential pressure was 3.0
kg/cm.sup.2 G, the gas G could be passed at a flow rate of merely
about 50 ml/min.
Accordingly, in the mass flow controller 50 with the control valve
59 of small flow rate incorporated therein, as above described,
it has taken a long time to conduct a formation of a vacuum in a
gas line, a displacement of an inside of said gas line with an inert
gas and the like.
On the contrary, a by-pass line 63 has been provided in parallel
to a mass flow controller 50 as shown by for example an imaginary
line in FIG. 5 but, in this case, a disadvantage has occurred in
that the whole construction is complicated and large-scaled.
So, the above described problems can be solved by incorporating
a control valve, which can pass a gas at a sufficiently large flow
rate, in a mass flow controller of small flow rate.
However, in the case where said control valve, which can pass a
gas at a sufficinetly large flow rate, is incorporated in said mass
flow controller of small flow rate under the conventional technical
condition, the following problems occur anew.
That is to say, flow-rate output characteristics of a general flow
rate-measuring portion 51 as shown in FIG. 5 produce an inversion
of phenomenon when a flow rate exceeds an appointed limit, as shown
in FIG. 6. In FIG. 6 an axis of abscissa shows said flow rate of
gas (fluid) passing through said mass flow controller 50 while an
axis of ordinate shows an output from said flow rate-measuring portion
51.
And, the mass flow controller 50 having such the characteristics
is usually used within a normal operation range N encircled by an
imaginary line in FIG. 6 but, when an excessive flow rate of gas
G exceeding said normal operation range N is suddenly passed through
the mass flow controller 50 said output is reduced in spite of
said excessive flow rate of gas G in the case where for example
an internal volume of a gas passage 52 from a thermal flow rate
sensor 54 to a control valve 59 is large or a response speed of
said control valve 59 is small. Accordingly, although a signal for
closing the control valve 59 is ought to be originally put out from
a control circuit 61 contrarily a signal for further opening said
control valve 59 is put out from said control circuit 61 and thus
the mass flow controller 50 is fallen into an abnormal operating
condition when for example the flow rate exceeds Q.sub.x in FIG.
6.
Such the problems have occurred also in a mass flow meter having
a construction almost same as that of the above described mass flow
controller 50 from which the control valve 59 and a member for opening
and closing the control valve 59 are removed. That is to say, in
this mass flow meter, a value of flow rate indicated has been reduced
in spite of an excessive flow rate of gas G and thus an indication
corresponding to the real flow rate has not been given.
SUMMARY OF THE INVENTION
The present invention has been achieved paying attention to the
above described matters and it is an object of the present invention
to provide a mass flow meter and a mass flow controller having no
above described disadvantage.
In order to achieve the above described object, a mass flow meter
according to the present invention is adapted to compare an output
from a sensor detecting a flow rate of a fluid passing through a
conduit with the threshold value and add an appointed compensating
signal to said output from said sensor, whereby temporarily displaying
a signal obtained on the basis of the result of the addition as
a value of flow rate at that time, when the output from the sensor
is larger than the threshold value.
And, a mass flow controller according to the present invention
is adapted to compare an output from a sensor detecting a flow rate
of a fluid passing through a conduit with the threshold value and
add an appointed compensating signal to said output from said sensor,
whereby reducing an openness of a control valve on the basis of
a signal obtained on the basis of the result of the addition, when
the output from the sensor is larger than the threshold value.
In the case where said flow rate of said fluid passing through
said mass flow meter exceeds a normal operation range, said value
of flow rate displayed exceeds the maximum value within said normal
operation range, in short a full-scale value, and thus a display
corresponding to the flow rate of the fluid passing through the
mass flow meter is made by adding the compensating signal to the
output from the sensor.
And, in the case where said flow rate of said fluid passing through
said mass flow controller exceeds a normal operation range, the
compensating signal is added to the output from the sensor to reduce
said openness of said control valve on the basis of said signal
obtained on the basis of the result of the addition.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing a construction of a control system
in a mass flow controller according to one preferred embodiment
of the present invention.
FIG. 2 is a diagram showing flow rate-output characteristics for
describing an operation of said control system shown in FIG. 1.
FIG. 3 is a diagram showing a control system in a mass flow controller
according to another preferred embodiment of the present invention.
FIG. 4 is a diagram for describing an operation of said control
system shown in FIG. 3.
FIG. 5 is a diagram showing a construction of a control system
in the general mass flow controller.
FIG. 6 is a diagram showing flow rate-output characteristics of
said mass flow controller shown in FIG. 5.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The preferred embodiments of the present invention will be below
described with reference to the drawings.
FIG. 1 shows an example of a control system in a mass flow controller
according to the present invention. Referring to FIG. 1 reference
numeral 1 designates a conduit as a passage to be measured through
which a fluid F passes in the direction shown by an arrow. Reference
numerals 2u, 2d designate thermal flow rate sensors (hereinafter
referred to as sensors) wound around said conduit 1 independently
to each other. Said sensors 2u, 2d form a bridge circuit 6 together
with resistances 3 4 a variable resistance 5 and the like. In
addition, reference numeral 7 designates a resistance for detecting
an electric current passing through said bridge circuit 6.
Reference numeral 8 designates an error amplifier operating so
that a voltage converted by said current-detecting resistance 7
may always equal to a reference voltage from a reference voltage
source 9. Reference numeral 10 designates a transistor as a switching
element and reference numeral 11 designates a power source for supplying
the bridge circuit 6 with an appointed electric power.
Reference numerals 12 13 designate a differential amplifier provided
on an output side of the bridge circuit 6 and the above described
construction is unchanged from that of the conventional mass flow
controller. And, an output (a) of said differential amplifier 13
expresses a flow rate of a fluid passing through the conduit 1 {hereinafter
referred to as a sensor output (a)}.
Reference numeral 14 designates an abnormal output-detecting and
compensating signal-putting out circuit (hereinafter referred to
as a compensating signal-putting out circuit) detecting whether
said sensor output (a) exceeds a normal operation range of said
mass flow controller or not and putting out an appointed compensating
signal (h) merely in the case where the sensor output (a) exceeds
said normal operation range so that the sensor output (a) may be
put in one input terminal, that is an input terminal on the - side,
of a comparator 15 and for example a ceiling value V.sub.err, which
is slightly larger than the maximum value (full-scale value) of
flow rate within the normal operation range, may be put in the other
input terminal, that is an input terminal on the + side, of said
comparator 15. In addition, reference numeral 16 designates a diode
for preventing a back current and reference numeral 17 designates
a resistance.
Reference numeral 18 designates an addition circuit for adding
said compensating signal (h) to the sensor output (a) and an output
from said addition circuit 18 is put in a linearizer 20 through
an inversion amplifier 19. Reference numeral 21 designates an amplifier,
reference numeral 22 designating an output terminal of a display
portion displaying a value of flow rate, reference numeral 23 designating
a control circuit for controlledly driving a control valve 24 provided
in said fluid passage of the mass flow controller, and reference
numeral 25 designating a buffer for putting a set value from a set
input terminal 26 in said control circuit 24.
Next, an operation of the mass flow controller having the above
described construction will be below described with reference to
also FIG. 2.
At first, when said fluid F passes through the conduit 1 at a small
flow rate and the sensor output (a) exists within the normal operation
range N shown in FIG. 2 the sensor output (a) from the differential
amplifier 13 is smaller than the threshold value V.sub.err, so that
the compensating signal (h) is not put out from said compensating
signal-putting out circuit 14. Accordingly, said output put out
from the addition circuit 18 is equal to the sensor output (a) and
transmitted to said control circuit 23 and said output terminal
22 of said display portion, respectively. And, in the control circuit
23 the output put out from the addition circuit 18 is compared
with said set value from said set input terminal 26 to conduct an
appointed control for said control valve 24 on the basis of the
obtained comparison result. In addition, in the display portion,
said value of flow rate is displayed on the basis of the signal
put out from the addition circuit 18.
And, when the fluid F passes through the conduit 1 at an excessive
flow rate exceeding the threshold value V.sub.err, the sensor output
(a) becomes larger than the threshold value V.sub.err, so that the
compensating signal (h) is put out from the compensating signal-putting
out circuit 14 to be put in the addition circuit 18 and thus a signal
(shown by a mark H in FIG. 2) larger than the threshold value V.sub.err
is put out from the addition circuit 18. Accordingly, said signal
H is put in the control circuit 23 through said inversion amplifier
19 and said linearizer 20 whereby the control valve 24 is controlled
so as to reduce an openness thereof. As a result, the mass flow
controller can be returned to the normal operating condition. In
addition, in this time, although the value of flow rate exceeding
the full-scale value is temporarily displayed in the display portion
on the basis of the signal H, the normal display is made with the
return of the mass flow controller to the normal operating condition.
In addition, referring to FIG. 2 the compensating signal (h) is
adapted to be put out merely at the flow rate of Q.sub.1 to Q.sub.2
and not to be put out at the flow rate exceeding Q.sub.2 the control
valve 24 is usually operated until the flow rate arrives at said
value exceeding Q.sub.2 no problem occurs.
Although the bridge circuit 6 has a so-called constant-current
type construction in the above described preferred embodiment, the
present invention is not limited by it. The present invention may
be applied to a mass flow controller using a so-called constant-temperature
type bridge shown in FIG. 3.
Referring to FIG. 3 T.sub.1 and T.sub.2 designates a constant-temperature
control circuit comprising a sensor 2u, 2d as a constituent element
of a bridge circuit 35 40 which will be mentioned later, respectively.
Said constant-temperature control circuits T.sub.1 T.sub.2 are
composed of parts identical with each other and conduct the control
so that the respective temperatures of said sensors 2u, 2d may be
always constant. That is to say, one constant-temperature control
circuit T.sub.1 is composed of a bridge circuit 35 comprising the
sensor 2u, a temperature-setting resistance 31 for this sensor 2u,
bridge resistances 32 33 and a variable resistance 34 and a control
circuit 36. And, the other constant-temperature control circuit
T.sub.2 is composed of a bridge circuit 40 comprising the sensor
2d, a temperature-setting resistance 37 for this sensor 2d and bridge
resistances 38 39 and a control circuit 41.
Reference numeral 42 43 designates a subtraction circuit and an
addition circuit, respectively, for subtracting and adding the respective
outputs V.sub.1 V.sub.2 from the respective constant-temperature
control circuits T.sub.1 T.sub.2 to put out a subtraction output
(V.sub.1 -V.sub.2) and an addition output (V.sub.1 +V.sub.2), respectively.
In addition, reference numeral 44 designates a division circuit
for dividing said subtraction output (V.sub.1 -V.sub.2) by said
addition output (V.sub.1 +V.sub.2) to put out a division output
(V.sub.1 -V.sub.2)/(V.sub.1 +V.sub.2). Reference numeral 45 designates
an amplifier.
And, in this preferred embodiment, a signal based on an output
V.sub.u from the sensor 2u on the upstream side in a conduit 1 is
put in a comparator 15 of a compensating signal-putting out circuit
14 to be compared with the threshold value V.sub.err and an output
(h) from said comparator 15 is added to said substraction circuit
42.
An operation of said mass flow controller shown in FIG. 3 is nearly
same as that of the mass flow controller shown in the above described
FIG. 1 so that its description is omitted.
In addition, although said signal based on said output V.sub.u
from the sensor 2u on the upstream side in said conduit 1 is compared
with the threshold value V.sub.err in a preferred embodiment shown
in FIG. 3 a signal V.sub.d from the sensor 2d on the downstream
side may be compared with the threshold value V.sub.err. However,
as shown in FIG. 4(A), (B), an advantage occurs for the case where
the signal based on the output V.sub.u from the sensor 2u on the
upstream side is compared with the threshold value V.sub.err in
that a response speed is increased. In addition, FIG. 4(C) shows
flow rate-output characteristics in the case where the signal based
on the output V.sub.u from the sensor 2u on the upstream side is
compared with the threshold value V.sub.err. V' shows an output
after said compensating signal (h) from said compensating signal-putting
circuit 14 was added.
As above described, according to the present invention, the output
from the sensor detecting the flow rate of the fluid passing through
the conduit is compared with the ceiling value and the appointed
compensating signal is added to the output from the sensor to display
the signal obtained on the basis of this addition result as the
value of flow rate or reduce the openness of the control valve on
the basis of the above described signal when the output from the
sensor is larger than the threshold value, so that the mass flow
meter can accurately display the flow rate even when the fluid passes
in an excessive flow rate. In addition, even though the mass flow
controller is fallen into the abnormal operating condition due to
an excessive flow rate of fluid, it can be quickly returned to the
normal operating condition.
And, according to the present invention, the control valve capable
of passing a sufficiently large flow rate of gas therethrough can
be incorporated in the mass flow controller of a small flow rate,
so that the evacuation of the gas line, the displacement of the
gas line with the inert gas and the like can be conducted within
a short time. |