Coriolis-type mass flowmeter having a straight measuring tube

A flowmeter of the Coriolis type in which the fluid to be metered is conducted through a straight measuring tube attached at its inlet and outlet ends to fixed supports, whereby the tube is free to vibrate in a circular path as well as to torsionally oscillate. Actuator means are provided to excite the tube at a point intermediate the inlet and outlet ends, causing the tube to vibrate in a circular path. When fluid flows therethrough, the tube is subjected to Coriolis forces, causing it to torsionally oscillate in accordance with mass flow. A pair of sensors are disposed at respective points between the inlet and outlet ends of the tube and the excitation point to yield in response to the torsional oscillations, sensor signals which are applied to a processing circuit from which a measurement signal is derived that depends on the relative phases of the sensor signals, after averaging several circular vibrations to provide a mass flow readout.

BACKGROUND OF INVENTION 
1. Field of Invention 
This invention relates generally to mass flowmeters, and more particularly 
to a Coriolis-type meter in which the fluid to be metered is conducted 
through a straight flow tube which is excited to vibrate in a circular 
path. 
2. Status of Prior Art 
A mass flowmeter is an instrument for measuring the mass of a fluid flowing 
through a conduit per unit time. Most meters for this purpose measure a 
quantity from which the mass can be inferred, rather than measuring mass 
directly. Thus, one can measure the mass flow with a volumetric flowmeter 
by also taking into account pressure, temperature and other parameters to 
compute the mass. 
A Coriolis-type mass flowmeter provides an output directly proportional to 
mass flow, thereby obviating the need to measure pressure, temperature, 
density and other parameters. In this type of meter, there are no 
obstacles in the path of the flowing fluid, and the accuracy of the 
instrument is unaffected by erosion, corrosion or scale build-up in the 
flow sensor. 
In the Roth U.S. Pat. No. 3,132,512, a Coriolis-type mass flowmeter is 
disclosed in which a flow loop vibrating at its resonance frequency is 
caused to oscillate about a torque axis which varies with fluid flow in 
the loop. This torsional oscillation is sensed by moving coil transducers. 
The Cox et al. U.S. Pat. No. 4,127,828 and U.S. Pat. No. 4,192,184 show a 
Coriolis-type meter having two U-shaped flow loops arranged to vibrate 
like the tines of a tuning fork, the torsional oscillation of these loops 
being sensed by light detectors to determine the mass flow. In the Smith 
U.S. Pat. No. 4,222,338, electromagnetic sensors provide a linear analog 
signal representing the oscillatory motion of a U-shaped pipe. 
Electromagnetic sensors are also used in the Smith et al. U.S. Pat. No. 
4,492,025, in which the fluid whose mass is to be measured flows serially 
through two parallel U-shaped pipes. 
The present invention provides a Coriolis-type mass flowmeter using a 
straight measuring tube. Of greatest prior art interest in this regard is 
the patent to Sipin, 3,329,019, which also discloses a straight measuring 
tube. In Sipin, this tube is electromagnetically excited to vibrate up and 
down, and because the tube is at the same time subjected to Coriolis 
forces, the vibrating tube oscillates torsionally. Strain gauge sensors 
are provided adjacent the inlet and outlet ends of the tube to sense the 
torsional oscillations. 
The advantage of the straight tube arrangement of Sipin over measuring 
tubes disclosed in prior art Coriolis-type mass flowmeters is that the 
latter flowmeters employ U-shaped or otherwise curved flow tubes which 
somewhat impede flow, whereas a straight flow tube offers minimal 
resistance to flow and therefore exhibits the lowest possible pressure 
drop. 
The drawback to the Sipin arrangement is that the measuring tube which is 
excited to vibrate up and down is sensitive to disturbances from external 
sources which act to impart vibratory motion to the flow tube, and these 
disturbances adversely affect the accuracy of the meter. 
SUMMARY OF INVENTION 
In view of the foregoing, the main object of the invention is to provide a 
Coriolis-type mass flowmeter which makes use of a straight measuring tube, 
the meter having a relatively low pressure drop and being substantially 
insensitive to external disturbances. 
A significant feature of the invention is that the flowmeter is capable of 
functioning in an environment which subjects the meter to vibratory forces 
of external origin, yet the meter continues to afford accurate mass flow 
readings. 
Also an object of the invention is to provide a Coriolis-type meter of 
exceptionally simple design and which can be manufactured at relatively 
low cost. 
Briefly stated, these objects are attained in a flowmeter of the Coriolis 
type in which the fluid to be metered is conducted through a straight 
measuring tube attached at its inlet and outlet ends to fixed supports, 
whereby the tube is free to vibrate in a circular path as well as to 
torsionally oscillate. Actuator means are provided to excite the tube at a 
point intermediate the inlet and outlet ends, causing the tube to vibrate 
in a circular path. When fluid flows therethrough, the tube is subjected 
to Coriolis forces, causing it to torsionally oscillate in accordance with 
mass flow. A pair of sensors are disposed at respective points between the 
inlet and outlet ends of the tube and the excitation point to yield in 
response to the torsional oscillations, sensor signals which are applied 
to a processing circuit from which a measurement signal is derived that 
depends on the relative phases of the sensor signals, after averaging 
several circular vibrations to provide a mass flow readout.

DETAILED DESCRIPTION OF INVENTION 
Referring now to the figure, a mass flowmeter in accordance with the 
invention includes a straight measuring tube 2 through which the fluid to 
be metered is conducted. Flow tube 2 is formed of high-strength flexible 
material such as stainless steel or other thin-walled tubing non-reactive 
with the fluid to be metered. Tube 2 is supported coaxially within a 
straight, rigid support pipe 12 of larger diameter by means of clamps 8 
and 12 at opposite ends of the pipe. Clamp 8 engages flow tube 2 at its 
inlet end 4, while clamp 10 engages the tube at its outlet end 6. The mass 
of the support pipe is large relative to that of the measuring tube. 
At the midpoint between inlet 4 and outlet 6 is an electromagnet actuator 
formed by electromagnet coils 14 attached to the inner wall of support 
pipe 12 and cooperating permanent magnets 16 attached to the outer surface 
of flow tube 2. The actuator serves to excite the tube to vibrate in a 
circular path, rather than up and down as in the Sipin patent, supra. For 
this purpose, three permanent magnets 16 are symmetrically disposed in a 
circumferential array on tube 2, to cooperate with a like array of coils 
14 on the inner surface of pipe 12. 
A drive generator 7 applies to the three coils 14 of the electromagnetic 
actuator, three-phase AC currents having a predetermined frequency. These 
three phase currents cause the excited flow tube 2 to vibrate in a 
circular path so that during this motion flow tube 2 is no longer coaxial 
with its support pipe. 
Preferably placed midway between inlet end 4 of tube 2 and the actuator 
excitation midpoint of flow tube 2 is a first electromagnetic sensor 
composed of a permanent magnet 20 attached to the exterior surface of flow 
tube 2 and a cooperating pick-up coil 18 attached to the inner surface of 
support pipe 18. And placed midway between outlet end 6 and the excitation 
point is a second electromagnet sensor composed of a permanent magnet 24 
and a cooperating pick-up coil 22. These sensors are responsive to the 
movement of the flow tube relative to the stationary support pipes. The 
invention is not limited to electromagnet sensors and other known types 
may be used. 
Measuring tube 2 which is excited to vibrate in a circular path, is at the 
same time twisted by Coriolis forces to oscillate torsionally as a 
function of mass flow. As a consequence, the signals yielded by the 
sensors which are on either side of the excitation point, are displaced in 
phase. 
In the absence of flow, when the flow tube is vibrating in a circular path, 
the signals yielded by the sensors are then in phase at the excitation 
frequency. But when fluid flows through measuring tube 2, the vibrating 
tube then twists as a result of the torsional oscillations, and the 
signals then yielded by the sensors are displaced in phase as a function 
of these oscillations. 
The sensors pick up the relative radial amplitude of measuring tube 2 at 
the points at which they are placed. These signals are applied to a 
processing circuit 26 to which is also applied a frequency signal from 
drive generator 7. Processing circuit 26 yields a measuring signal which 
depends on the differential between the relative phases of the sensor 
signals to produce a mass flow measurement signal that is applied to a 
suitable mass flow readout 9. 
Transitory or random disturbances from external sources which give rise to 
vibrations of the meter and produce movement of sensor elements 18-20 and 
22-24 relative to the center of the mass of the quiescent measuring tube 
2, act to modulate the processed signal that depends on mass flow. Also, 
disturbances which twist the flow tube or the support pipe act to modulate 
the processed signal. Such disturbances therefore degrade the accuracy of 
the mass flow measurement. However, the processing circuit 26, by 
averaging over several circular vibrations, yields a measurement signal of 
relative phase corresponding to the mass flow; hence the measurement 
signal is substantially independent of these disturbances. 
It is to be noted that external mechanical vibrations from external sources 
which affect the measuring tube do not normally have a circular movement, 
and therefore can be suppressed by the intelligent signal processor 26. 
In practice, the exciter elements 14-16 need not be located at the midpoint 
between the supported inlet and outlet ends 4 and 6 of flow tube 2. Also, 
it is not necessary that sensors 18-20 and 22-24 be midway between the 
inlet and outlet ends 4 and 6 and the excitation point. Nor is it 
essential that the sensors pick up the relative phases at a phase angle of 
90 degrees. 
The Coriolis effect producing the phase differential at sensors 18-20 and 
22-24 is proportional to the rotational speed V.sub.u. Since V.sub.u 
=W.multidot.r, where W is the angular velocity and r is the amplitude of 
the circular vibration of measuring tube 2, amplitude r remains constant, 
and value W, the angular velocity, is measured. And since angular velocity 
W changes with mass flow, the density of the fluid can also be determined. 
With a constant amplitude 4 of the circular vibrations, mass flow is then 
proportional to angular velocity W. Therefore, in the measurement system, 
the phase differential signal is divided by the measured angular velocity 
W to yield a value independent of it. 
While there has been shown and described a preferred embodiment of a 
Coriolis-type mass flowmeter having a straight measuring tube in 
accordance with the invention, it will be appreciated that many changes 
and modifications may be made therein without, however, departing from the 
essential spirit thereof.