Abstrict A redundant mass flow meter comprises a casing housing a main rotor
and an ntermediate rotor which is freely rotatable and is disposed
coaxially with a rotationally restricted turbine between the turbine
and the casing, a first measuring system for measuring and controlling
the position of the turbine relative to the intermediate rotor,
and a second measuring system, independent of the first measuring
system, for measuring and controlling the position of the intermediate
rotor relative to the casing.
Claims I claim:
1. A mass flow meter comprising a casing, a main rotor acted on
by a fluid flow mounted within said casing for rotation about an
axis, means for measuring the rotational speed of said main rotor,
a rotationally restricted turbine mounted in said casing spaced
axially from said main rotor, at least one rotationally free intermediate
rotor acted on by the same fluid flow as said main rotor mounted
coaxially with said turbine between said casing and said turbine,
first means for measuring the position of said turbine relative
to said intermediate rotor, second means for measuring the position
of said intermediate rotor relative to said casing, first means
for controlling the position of said turbine relative to said intermediate
rotor, and second means for controlling the position of said intermediate
rotor relative to said casing, wherein said first means for controlling
and said second means for controlling redundantly supply outputs
indicative of mass flow rates.
2. A mass flow meter according to claim 1 having a common shaft
on which said main rotor, said turbine and said intermediate rotor
are all mounted.
3. A mass flow meter according to claim 1 wherein said first and
second position measuring means are first and second position sensors
mounted concentrically on said intermediate rotor.
4. A mass flow meter according to claim 3 wherein said first and
second position controlling means are first and second torque motors
mounted concentrically on said intermediate rotor.
5. A mass flow meter according to claim 4 wherein said first torque
motor and said first position sensor each comprise a rotor element
fixed on said turbine and a stator element fixed on said intermediate
rotor.
6. A mass flow meter according to claim 4 wherein said second
torque motor and said second position sensor each comprise a rotor
element fixed on said intermediate rotor and a stator element fixed
on said casing.
7. A mass flow meter according to claim 1 including mechanical
stops disposed between said casing and said intermediate rotor and
between said intermediate rotor and said turbine.
8. A mass flow meter according to claim 1 wherein said first position
measuring means and said second position controlling means are mounted
concentrically on said intermediate rotor.
9. A mass flow meter according to claim 8 wherein said second
position measuring means and said first position controlling means
are mounted concentrically on said intermediate rotor.
Description BACKGROUND OF THE INVENTION
1. Field of the invention
The invention relates to a totally redundant mass flow meter, and
is particularly applicable to the measurement of the flow of a fluid
in an electronic control system in a turbomachine.
2. Summary of the Prior Art
In order to measure the mass flow rate of a fluid, it is known
to use a flow meter comprising a casing in which there is an axial
passage for the flow of a fluid propelled at a constant angular
velocity, a rotor which is caused to rotate by the fluid or an electric
motor, and a rotationally restricted turbine on which the fluid
exerts a torque proportional to the product of the mass flow rate
of the fluid and the angular velocity of the fluid. The value of
the torque is measured by controlling the position of the turbine
by means of a limited movement torque motor and an angular position
sensor. Measurement of the current which supplies the torque motor
makes it possible to calculate the torque acting on the turbine
and to deduce from it the mass flow rate of the fluid. Such a flow
meter is described in U.S. Pat. No. 4056976.
It is also known to provide a partially redundant flow meter by
disposing on the turbine two measurement systems each comprising
a torque motor and a position sensor. However, the two measuring
systems are not independent and cannot function simultaneously because
it is not possible to apply two restoring torques to the turbine
at the same time. These two systems can only be used alternately;
switching from one system to the other being effected in the event
of a breakdown in one of the measuring systems. However, in numerical
control where it is necessary to compare two different measurements
carried out by two measurement systems for fault probability or
detection tests, the two measurements must be carried out simultaneously.
SUMMARY OF THE INVENTION
The aim of the invention is to alleviate the drawbacks of known
mass flow meters and provide a totally redundant mass flow meter
comprising two measuring systems which are totally independent electrically
and which function simultaneously in such a way as to obtain two
simultaneous and independent measurements of the torque exerted
on the turbine, enabling two measurements to be deduced representing
the mass flow rate of the fluid.
To this end, according to the invention there is provided a mass
flow meter comprising a casing, a main rotor mounted within the
casing for rotation about an axis, means for measuring the rotational
speed of the main rotor, a rotationally restricted turbine mounted
in the casing spaced axially from the main rotor, at least one rotationally
free intermediate rotor mounted coaxially with the turbine between-the
casing and the turbine, first means for measuring the position of
the turbine relative to the intermediate rotor, second means for
measuring the position of the intermediate rotor relative to the
casing, first means for controlling the position of the turbine
relative to the intermediate rotor, and second means for controlling
the position of the intermediate rotor relative to the casing.
A first measuring system is constituted by the first position measuring
means and the first position controlling means, and a second measuring
system, independent of the first measuring system, is constituted
by the second position measuring means and the second position controlling
means.
In a first embodiment of the invention, the position controlling
means of the two measuring systems are two independent torque motors
mounted concentrically on the intermediate rotor, and the position
measuring means of the two measuring systems are two independent
position sensors mounted concentrically on the intermediate rotor.
In a second embodiment of the invention, the position controlling
means belonging to one measuring system and the position measuring
means belonging to the other are mounted concentrically on the intermediate
rotor.
The absolute values of the restoring torques exerted by the two
position controlling torque motors respectively on the turbine and
on the intermediate rotor are equal to the absolute value of the
torque exerted on the turbine by the fluid. Measurement of the two
currents which respectively supply the two torque motors makes it
possible to obtain two simultaneous and independent measurements
of the mass flow rate of the fluid.
Further preferred features and advantages of the invention will
become apparent from the following description of the preferred
embodiments, given by way of example only, with reference to the
appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a partial cross-sectional view through a first embodiment
of a totally redundant flow meter in accordance with the invention;
FIG. 2 shows diagrammatically one example of two loops for controlling
the positions of the turbine and the intermediate rotor in a flow
meter according to the invention; and,
FIG. 3 shows a partial cross-sectional view through a second embodiment
of a flow meter in accordance with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the first embodiment shown in FIG. 1 the flow meter comprises
a casing 10 within which are located a main rotor 12 arranged to
rotate freely on a shaft 14 a turbine 13 which is permitted restricted
rotation and is spaced axially from the main rotor 12 along the
shaft 14 the main rotor 12 and the turbine 13 both being provided
with fins 9 and an intermediate rotor 15 arranged to rotate freely
about the shaft 14 and disposed coaxially with the turbine 13 between
the casing 10 and the turbine 13. The flow meter further comprises
sensor means 11 for measuring the speed of rotation of the main
rotor 12 first means 16 for measuring the angular position of the
turbine 13 relative to the intermediate rotor 15 second means 17
for measuring the angular position of the intermediate rotor 15
relative to the casing 10 first means 18 for controlling the position
of the turbine 13 relative to the intermediate rotor 15 and second
means 19 for controlling the position of the intermediate rotor
15 relative to the casing 10. The angular movement range of the
turbine is less than 20.degree.. The main rotor 12 the turbine
13 and the intermediate rotor 15 are mounted on the shaft 14 by
means of bearings 20 and the turbine 13 is located inside the intermediate
rotor 15. The main rotor 12 may be located outside the intermediate
rotor 15 or inside it as shown in FIG. 1. The sensor means 11 for
detecting the rotational speed of the main rotor 12 may be fixed
on the casing 10 or on the intermediate rotor 15 when the main rotor
is located inside the intermediate rotor.
The first and second position measuring means 16 17 are position
sensors mounted concentrically on the intermediate rotor 15. The
first and second control means 18 19 are torque motors mounted
concentrically on the intermediate rotor 15. By way of example,
position sensors and torque motors of the type known by the name
"PANCAKE" having a large diameter, and a hollow disc-shaped
rotor may be used.
The torque motor 18 and the position sensor 16 belong to a first
measuring channel and are connected to a first connector 21 fixed
on the outside wall of the casing 10. They each comprise a rotor
element 181 161 fixed on the turbine 13 and a stator element 182
162 fixed on the intermediate rotor 15.
The torque motor 19 and the position sensor 17 belong to a second
measuring channel and are connected to a second connector 22 fixed
on the outside wall of the casing 10. They each comprise a rotor
element 191 171 fixed on the intermediate rotor 15 and a stator
element 192 172 fixed on the casing 10.
The positional control of the turbine and the intermediate rotor
is achieved by means of two conventional control loops described
below with reference to FIG. 2.
The flow meter operates as follows. A fluid whose flow rate is
to be determined passes through the main rotor 12 emerging from
it with a certain amount of angular motion, and then passes through
the rotationally restricted turbine 13 on which it exerts a torque
which is proportional to the product of the mass flow rate of the
fluid and the angular velocity of the fluid. The flow of the fluid
occurs on the one hand between the intermediate rotor and the main
rotor followed by the turbine, and on the other hand between the
casing and the intermediate rotor.
The main rotor 12 may be rotatably driven by an auxiliary electric
motor (not shown) or by the fluid itself. In the case where the
main rotor is driven by the fluid itself, the fluid is previously
set in motion, for example by means of a vortex generator (not shown),
in such a way as to be propelled at a constant angular velocity.
The angular velocity of the main rotor is equal to that of the fluid
and is measured by the speed sensor 11.
The torque motor 18 and the position sensor 16 ensure control of
the position of the turbine 13 relative to the intermediate rotor
15 by applying to the turbine a torque of a value equal to and in
a direction opposite to the torque exerted by the fluid. The two
actions exerted on the turbine by the fluid and by the torque motor
18 are intended to immobilise the turbine in a reference position,
the position of the turbine being measured by the position sensor
16. The intermediate rotor is then subjected by means of the torque
motor 18 to a torque which is equal and opposite to that exerted
on the turbine. The intermediate rotor, which is able to turn freely,
is positionally controlled relative to the casing 10 by means of
the torque motor 19 and the position sensor 17 applying to it a
torque which is equal and opposite to that exerted on the turbine
by the torque motor 18. The absolute values of the torques exerted
by the torque motors 18 19 are equal to the absolute value of the
torque exerted on the turbine by the fluid. The two torque motors
18 19 therefore draw currents of a strength proportional to the
mass flow rate of the fluid. Simultaneous measurement of the strength
of these currents and knowledge of the angular velocity of the fluid
as determined by the speed sensor 11 therefore make it possible
to obtain two simultaneous measurements of the mass flow rate of
the fluid.
Preferably, mechanical stops 23 24 are disposed between the casing
and the intermediate rotor and between the intermediate rotor and
the turbine in such a way as to limit the angular displacement of
the intermediate rotor 15 in relation to the turbine 13 and to the
casing 10 in the event of a breakdown of one of the torque motors.
Thus, in the event of a breakdown of the torque motor 18 so that
positional control of the turbine is no longer provided, the turbine
will be displaced rotationally as far as the stops and, by means
of these stops, will then drive the intermediate rotor with it.
Positional control of the intermediate rotor relative to the casing
continues to be carried out by the torque motor 19 and the position
sensor 17 so that the second measuring channel remains functional
and makes it possible to obtain a measurement of the mass flow rate
of the fluid. In the case of a breakdown in the torque motor 19
positional control of the intermediate rotor is no longer provided
but positional control of the turbine continues to be performed
by means of the torque motor 18 and the position sensor 16. The
first measuring channel therefore remains functional and makes it
possible to obtain a measurement of the mass flow rate of the fluid.
Only a double breakdown of a torque motor belonging to one of the
measuring channels and a position sensor belonging to the other
measuring channel, or a double and simultaneous breakdown of the
two torque motors gives rise to a loss of measurement of the fluid
flow rate. Such instances of a double breakdown are very rare.
FIG. 2 shows a block diagram of two loops for automatically controlling
the positions of the turbine and the intermediate rotor. The two
measuring channels of the flow meter are independent and are respectively
connected by the two connectors 21 22 to two devices 30 40 for
processing signals delivered by the position sensors 16 17 and
for supplying the torque motors 18 19. Each processing device 30
40 comprises a means 31 41 for comparing the signal received from
the respective position sensor 16 17 with a reference position,
and means 32 42 for supplying the respective torque motor 18 19.
The supply means 32 42 are respectively controlled by the comparison
means 31 41 and each delivers an electric current applied on the
one hand to the corresponding torque motor 18 19 and on the other
hand to calculating means 50 60. These two calculating means 50
60 also receive a signal from the sensor 11 which detects the speed
of the main rotor, and deliver at their outlet two simultaneous
and independent measurements of the mass flow rate of fluid.
In the second embodiment shown in FIG. 3 the disposition of the
torque motors and the position sensors is changed in relation to
the first embodiment. In this case the torque motor 18 belonging
to the first measuring channel for controlling the position of the
turbine 13 relative to the intermediate rotor 15 and the position
sensor 17 belonging to the second measuring channel are mounted
concentrically on the intermediate rotor 15. Likewise, the torque
motor 19 belonging to the second measuring channel for controlling
the position of the intermediate rotor 15 relative to the casing
10 and the position sensor 16 belonging to the first measuring channel
are mounted concentrically on the intermediate rotor 15.
This arrangement makes it possible to separate the motors of the
two measuring channels on a mechanical plane and thereby reduce
the problems of interference or successive breakdowns due, for instance,
to over-heating. As the motors are no longer concentric heating
is less localised, which improves their reliability.
It will of course be understood that the invention is not confined
to the embodiments which have been described herein. For example,
it is possible to obtain additional measuring channels by using
a plurality of concentric intermediate rotors associated with means
for measuring and controlling the position of each intermediate
rotor. |