NL2012498A

NL2012498A - Coriolis flowsensor.

Info

Publication number: NL2012498A
Application number: NL2012498A
Authority: NL
Inventors: Conrad Lötters Joost; Ronald Katerberg Marcel; Bernardus Johannes Hakvoort Wouter; David Kruijswijk Willem; Van De Ridder Lubbert
Original assignee: Berkin Bv
Priority date: 2014-03-24
Filing date: 2014-03-24
Publication date: 2015-12-10
Also published as: NL2012498B1

Description

Title: Coriolis flowsensor

Description

The invention relates to a Coriolis flow sensor, comprising at least a Coriolis-tube with at least two ends being fixed in a tube fixation means, wherein the flow sensor comprises excitation means for causing the tube to oscillate, as well as detection means for detecting at least a measure of displacements of parts of the tube during operation. A Coriolis flow sensor having a loop-shaped Coriolis tube is known from EP 1 719 982 A1. Various types of loop-shaped Coriolis tubes are described therein, both of the single loop type and of the (continuous) double loop type. The present invention relates to any of these types, but is not restricted thereto. A Coriolis flow sensor (also indicated as flow sensor of the Coriolis type) comprises at least one vibrating tube, often denoted Coriolis tube, flow tube, or sensing tube. This tube or these tubes is or are fastened at both ends to the housing of the instrument. These tube ends serve at the same time as feed and discharge ducts for the liquid or gas flow to be measured.

Besides the flow tube (or tubes), a Coriolis flow sensor comprises two further subsystems, i.e. one for excitation and one for detection. The excitation system (exciter) is arranged for bringing the tube into vibration. For this purpose, one or several forces or torques are applied to portions of the tube. The detection system is arranged for detecting at least a measure of the displacements of one or several points of the tube as a function of time. Instead of this displacement, the force (or torque) exerted by the tube on its environment may alternatively be measured. The same holds true for the velocity, acceleration and strain of the tube; what will be described below with reference to displacement detection is equally valid for force detection, velocity detection, acceleration detection and/or strain detection.

As a fluid flows in the vibrating tube, it induces Coriolis forces, proportional to the mass-flow, which affect the tube motion and change the mode shape. Measuring the tube displacement using the detection system, the change of the mode shape may be measured, which allows for mass-flow measurements.

The vibration of the tube generated by the exciter takes place at a more or less fixed frequency which varies slightly as a function, amongst others, of the density of the medium flowing through the tube. The vibration frequency is almost always a natural frequency of the tube so that a maximum amplitude can be achieved with a minimum energy input.

Besides the effect of the mass-flow on the mode shape of the tube, external vibrations can introduce motions that cannot be distinguished from the Coriolis force induced motion. The external vibrations create additional components in the Coriolis flow sensor signals, and those additional components can introduce a measurement error. For low flows, the Coriolis force induced motion is relatively small compared to external vibrations induced motions. Coriolis flow sensors designed to be sensitive to low flows, are normally rather sensitive to external vibrations.

To minimize the influence of floor vibration on the measurement value, it is known in the prior art to use so called passive vibration isolation. According to the prior art, passive isolation consist of several stages of mass-spring-damper systems between the floor and the tube fixation means, the parameters are adjusted to achieve high-frequency attenuation, which is appropriate for many applications. The better the vibration isolation system the better the decoupling of the internal measurement system from any environmental disturbances.

It is an object of the present invention to provide an improved Coriolis flow sensor, in particular a Coriolis flow sensor designed to be sensitive to low flows, wherein the influence of external vibrations is reduced, in particular using passive vibration isolation.

To this end, the invention provides a Coriolis flow sensor, that is characterized in that it comprises a compliant connection between the casing and at least part of the detection means. The compliant connection according to the invention is constructed and arranged such that a relative displacement measurement of the tube, dependent on the tube actuation and mass-flow, though independent of casing excitations is obtained.

The flowsensor according to the invention is in an embodiment arranged for causing the detection means to oscillate in response to casing excitations, in substantially the same manner as the Coriolis-tube oscillates in response to casing excitations.

This may in an embodiment be obtained, tuning the resonance frequency and/or the damping of the compliant detection means. Tuning may be dependent on the actuation frequency of the Coriolis tube, and the first and second Coriolis mode.

The tuning may comprise that the resonance frequency and/or the damping ratio are substantially equal for the Coriolis-tube (i.e. internal mode of the tube-window) and the compliant mounted detection means.

The tuning may in practice comprise that the resonance frequency and/or the damping ratio are lower for the internal mode of the tube-window (Coriolis-tube), compared to the resonance frequency and the damping ratio of the compliant mounted detection means.

The tuning may in an embodiment comprise that the resonance frequency and/or the damping ratio for the internal mode of the tube-window (Coriolis-tube) are approximately 90% of the resonance frequency and the damping ratio of the compliant mounted detection means.

It is conceivable in an alternative embodiment that the resonance frequency and the damping ratio are higher for the internal mode of the tube-window (Coriolis-tube), compared to the resonance frequency and the damping ratio of the compliant mounted detection means.

In addition to the passive vibration isolation, active vibration isolation may be used, in particular as described in co-pending application NL 2011836, which is hereby fully incorporated by reference. According to an aspect of the invention, the active vibration isolation, in particular as described in NL 2011836, may be accomplished together with the passive vibration isolation according to the present invention.

In particular, the Coriolis flowsensor may comprise a reference mass element, further excitation means arranged for causing the reference mass to oscillate, as well as further detection means, as described in NL 2011836 in name of applicant. The further detection means are in an embodiment arranged for detecting at least a measure of displacements of the reference mass during operation. With this, it is meant that the further detection means are at least arranged for detecting relative displacements of the reference mass, for instance between the reference mass and the Coriolis tube, or between the reference mass and the housing. In this sense, it is thinkable that the further detection means are arranged for detecting at least a measure of displacements of the housing during operation. A detection of displacements of both the reference mass and the housing is also possible. An absolute displacement detection (of the reference mass and/or the housing) is conceivable as well. Additionally, control means are provided for controlling the excitation means and/or further excitation means based on vibrations measured by the detection means and/or further detection means.

The reference mass element together with the further excitation means and the further detection means provide in effect active vibration isolation means which are arranged for active vibration isolation, in order to minimize the effect external vibrations have on the relative measurement of the relative Coriolis-tube’s motion. This may in general be done by means of two principles, which are explained in more detail in NL 2011836.

The invention will be described below by means of the accompanying Figures, which show several embodiments of the invention. In the figures, it is shown:

Fig. 1a-c - a perspective view, a side view and a dynamical overview of a second embodiment of the flowmeter according to the invention;

Fig. 2 - an embodiment of a compliant sensor design for passive vibration isolation according to the invention.

Fig. 1a shows a perspective view of a flowmeter 301 of the Coriolis type (or Coriolis Mass Flow Meter, CMFM), and Fig. 1b shows a side view of said flowmeter 301, according to an embodiment of the invention. The flowmeter 301 of the Coriolis type has a looped tube 302 that is bent into a rectangular shape so as to follow a substantially circumferential path (substantially one full turn), and that comprises a flexible inlet tube 303 and a flexible outlet tube 304 for a fluid medium. Preferably, the loop 302 and the inlet and outlet tubes 303, 304 are portions of one and the same tube. The tube 302 in its entirety is bent into a rectangular shape, but the corners are rounded such that it can be bent into this shape. The inlet tube 303 is connected to a supply line 306 and the outlet tube 304 to a discharge line 307 via a supply/discharge block 305a, which forms part of a casing 305. The inlet and outlet tubes 303, 304 of this embodiment extend within the loop 302 and are fastened (i.e. clamped) to the casing 305 by fastening means 312. The fastening is provided in a location such that the free path length of the inlet and the outlet tube 303, 304 (i.e. the portion of the inlet/outlet tube 303, 304 between the connection of the second transverse tube portions 302a, 302b and the location of the clamping to the fastening means 312) is at least 50%, preferably 60% of the length of each of the lateral tube portions 302c, 302d, or even longer. The flexible inlet and outlet tubes 303, 304 do not form part of the loop 302 but provide a flexible fastening of the loop 302 to the casing 305. The loop 302 may thus be regarded as being flexibly suspended by means of the inlet and outlet tubes. The loop 302 and the inlet and outlet tubes 303, 304 may advantageously be manufactured from one integral piece of tubing. This may be, for example, a stainless steel tube with an outer diameter of approximately 0.7 mm and a wall thickness of approximately 0.1 mm. Depending on the outer dimensions of the loop 302 and the pressure the tube is to be able to withstand (for example 100 bar), the outer diameter of the tube will usually be smaller than 1 mm and the wall thickness 0.2 mm or less. It is noted that other dimensions may be used as well.

The tube 302 consists of a substantially rectangular framework comprising two parallel lateral tubes 302d and 302e, a first transverse tube 302c connected to first (lower) ends of the lateral tubes 302d and 302e, and two second transverse tubes 302a and 302b connected at one side to second (upper) ends of the lateral tubes and at the other side to the centrally returning inlet and outlet tubes 303 and 304, respectively. The rectangular loop 302 preferably has rounded corners. The tubes 303 and 304, which run closely together on either side of and symmetrically with respect to the main axis of symmetry S of the loop 302, are fastened to the fastening means 312, for example by clamping or soldering or welding, said means 312 in their turn being fastened to the casing 305. The inlet and outlet tubes 303, 304 are flexible and act as it were as a suspension spring for the loop 302. This suspension allows a motion of the loop 302 both about the main axis of symmetry S and about a second axis S' situated in the plane of the loop 302 and perpendicular to the main axis of symmetry S.

To close the loop 302 mechanically (i.e. to interconnect the beginning and end of the loop mechanically, directly or indirectly), the tubes 303, 304 are preferably connected to one another along the extent of their free path lengths, for example in that they are welded or soldered together.

In the construction of Figs. 1a and 1b the excitation means for causing the loop 302 to oscillate about the main axis of symmetry S (the primary or excitation axis of rotation) comprise a permanent magnet yoke 319 fastened to the casing 305, said yoke having two gaps 309 and 310 through which portions 302a and 302b (denoted the second transverse tubes above) of the looped tube 302 are passed, as well as means for introducing an electric current into the tube 302. These are means for inducing a current in the tube 302 in the present case.

The current is induced in the tube by means of two transformer cores 317a, 317b provided with respective coils (not shown) through which cores the respective tube portions 302d and 302e are passed. The combination of the magnetic fields generated in the gaps 309 and 310 of the permanently magnetic yoke 319, which fields are transverse to the direction of the current and are oppositely directed, and an (alternating) current induced in the tube 302 exerts a torque on the tube owing to which it starts to oscillate or rotate about the axis S (in the so-termed twist mode). When a medium flows through the tube, the tube will start to rotate about an axis S' transverse to the axis S (in the so-termed swing mode) under the influence of Coriolis forces. During operation the (sinusoidal) displacements of points of the tube portion 302c, which are representative of the flow, are detected by detection means 311 in the form of a first sensor 311a and a second sensor 311b, and optionally a third sensor 311c. The first and the second sensor are arranged on either side of the first axis of rotation S. A third sensor 311c may serve for correction purposes. The sensors may be, for example, of an electromagnetic, inductive, capacitive, or ultrasonic type. In the present case, however, optical sensors are chosen. The sensors 311a, 311b, and 311c each comprise, in the embodiment shown, a U-shaped housing that is fastened to the frame 315, with a light source (for example an LED) placed in the one leg and a photosensitive cell (for example a phototransistor) opposite the light source in the other leg. The lateral tube 302c is capable of moving between the legs of the U-shaped sensor housings 311a and 311b (and 311c, if present). It should be noted that other ways of detecting displacement (or at least a measure of displacement) of the Coriolis tube are conceivable as well.

The configuration described above is per se known to those skilled in the art, and is, for example, described in EP 1 719 982 A1, which document is incorporated by reference here.

In the embodiment shown, the tubes 303 and 304, are fastened to the fastening means 312, for example by clamping or soldering or welding, said means 312 in their turn being fastened to the main body 305b of the casing 305.

Note, in this sense, that the fastening means 312 are directly connected to the main body 305b of the casing 305.

Connected to the main body 305b of the casing 305 is further frame 325. The further frame 325 is resiliently connected, by means of suspension means 321, which comprise in the embodiment shown a total of three suspension elements 321a, 321b, 321c in the form of flexible beams, preferably spring-leaf like elements. On the further frame 325, the detection means 311 are provided.

According to the invention, the a compliant connection 321 between the casing 305 and the detection means 311 is present. The compliant connection 321 according to the invention is constructed and arranged such that a relative displacement measurement of the tube 302, dependent on the tube 302 actuation and mass-flow, though substantially independent of casing 305 excitations is obtained. In other words, the further frame 325 with the detection means 311 is arranged to oscillate, in response to casing 305 excitations, in substantially the same manner as the Coriolis-tube 302 oscillates in response to said same casing 305 excitations. Thus, the influence of external casing excitations does not result in a relative movement between the Coriolis-tube 302 and the detection means 311.

Fig. 1c shows a dynamical scheme of the embodiment of the flowmeter 301 as shown in Fig. 1a and 1b. Here it can be seen that the frame 325 with the detection means 311 is resiliently connected by means of the suspension means 321 to the “ground mass” formed by the casing 305. The Coriolis flow tube 302 defines a Coriolis mass Me, and is directly connected, in a resilient manner, by means of a resilient suspension 312’ (formed by internal elasticity of one or more of the Coriolis tube parts) to the “ground mass” formed by the casing 305. Thus, the detection means 325 and the Coriolis tube 302 are connected in parallel to each other.

The placement and the characteristics of the compliant connection 321, as well as design of the flow sensor (including design for masses Md and Me of the frame 325 with detection means, and the Coriolis tube, respectively, may be in order to obtain the desired effect. In particular, the compliant connection 321 is arranged and designed such that the resonance frequency and/or the damping of the compliant detection means 311 are tuned such that the dynamic response to external vibrations on the casing is substantially equal to said response of the Coriolis tube.

As followed from the above, the Coriolis flow-sensor thus comprises passive vibration isolation means. In the embodiment shown in Fig. 1a and 1b, the Coriolis flow-sensor also comprises active vibration isolation means, although these means are entirely optional. These active vibration isolation means are in detail described in NL 2011836. In summary, referring to Fig. 1a and 1b, a further excitation means 331 is provided, which may be used to actively excite the further frame 325 as a reference mass, and further detection means 316 are provided, to measure the oscillations of the reference mass.

The Coriolis-tube 302 (having mass Me) and the reference mass Md provided by the frame 325 are positioned in parallel to each other, each being connected to the housing 305. The further excitation means 331 are used to match the dynamic properties of the reference mass 325 with the dynamic properties of the Coriolis-tube 302. Disturbances due to external vibrations acting on the housing 305 act on both the reference mass 325 and the Coriolis-tube 302, in a similar manner. Thus, the relative displacements of the Coriolis tube are at least substantially dependent on the Coriolis force and independent on the external vibrations. It is noted in this regards that the sensors elements 311a-311c of the detection means 311 are in this embodiment provided on the reference mass 325, such that the sensor elements 311a-311c are able to measure the Coriolis force induced vibrations xc on the Coriolis-tube 302.

An embodiment of a device 401 having passive vibration isolation is shown in Fig. 2. The device 401 comprises a casing part 405a and a frame part 405b. The frame part 405b has a compliant connection 421 to the casing part 405a. In the embodiment shown, several wire springs 421a-421e are used, which are arranged in such a manner to provide an exactly constraint configuration with only one remaining degree of freedom, which is along the longitudinal length of cylinder 406b. The device 401 may be used in a flowsensor of the Coriolis type, wherein the Coriolis tube is connected to the casing 405a, and detections means are provided on the frame part 405b, such that the detection means are resiliently connected to the casing. The Coriolis tube is provided in such a way that the only remaining degree of freedom is out of plane of the tube-window. This results in an extra degree of freedom between the casing 405a and the frame 405, which preferably comprises a printed circuit board (PCB) with the detection means in the form of optical sensors.

The resilient connection, in the embodiment shown provided by leaf springs, provides damping and stiffness. The connection of the Coriolis tube to the casing also entails damping and stiffness. Perfect vibration isolation may be achieved when the following conditions are met: damping of the detection means dd is equal to damping of the Coriolis tube dc times the ratio between the mass of the detection means md and the mass of the Coriolis tube me (dd = (md/mc) dc; and stiffness of the detection means kd is equal to stiffness of the Coriolis tube kc times the ratio between the mass of the detection means md and the mass of the Coriolis tube me (kd = (md/mc) kc.

These conditions are in general achieved when the resonance frequency and the damping ratio are equal for the internal mode of the tube window and the compliant mounted detection means.

It is noted that for the design shown in Fig. 2, a perfect match of the damping ratio and the resonance frequency no longer results in perfect vibration isolation because of the higher order dynamics of the tube window. Fortunately, mistuning of those parameters can be used in our advantage to minimise the transmissibility in the region of interest by the introduction of an anti-resonance. Therefore the concept device 401 shown in Fig. 2 is mistuned on purpose.

In the case shown in Fig. 2, the damping and resonance frequencies for the internal mode of the tube-window (Coriolis-tube) are approximately 90% of those of the compliant mounted detection means (frame 405b). With the design, a 20 dB attenuation of the influence of external vibrations on the mass-flow measurement value of a CMFM may be obtained.

The device 401 shown in Fig. 2 -as well as concepts described above in relationship to Coriolis flow sensors- can in principle be used in every flowsensor of the Coriolis type, and even in other kinds of systems were specific internal deformations need to be measured independently of external vibrations.

The invention is described above by means of a number of embodiments. These embodiments and the description thereof are not to be construed limiting on the invention. The invention and its desired protection is defined by the appended claims.

Claims

A Coriolis flow sensor, comprising a housing and at least one Coriolis tube with at least two ends that are fixed in a tube fixing means, the flow sensor comprising excitation means for causing the tube to oscillate relative to the housing, and detection means for operating in operation. detect at least one measure of movements of parts of the tube, characterized in that the detection means are suspended resiliently relative to the housing.

2. Coriolis flow sensor according to claim 1, wherein the flow sensor is adapted to cause the detection means to oscillate substantially in the same way as the oscillation of the Coriolis tube as a result of movements of the housing, in particular by adjusting the resonance frequency and the damping ratio of the resiliently suspended detection means.

3. Coriolis flow sensor as claimed in claim 2, wherein the resonance frequency and / or the damping ratio is / are substantially the same for the Coriolis tube and the resiliently suspended detection means.

A Coriolis flow sensor according to claim 2, wherein the resonance frequency and / or the damping ratio is / are substantially smaller for the Coriolis tube than for the resiliently suspended detection means.

The Coriolis flow sensor according to claim 4, wherein the resonance frequency and / or the damping ratio for the Coriolis tube is approximately 90% / amounts of those for the resiliently suspended detection means.

The Coriolis flow sensor according to claim 2, wherein the resonance frequency and / or the damping ratio is / are substantially greater for the internal mode of the Coriolis tube than for the resiliently suspended detection means.

7. Coriolis flow sensor according to one of the preceding claims, wherein the Coriolis flow sensor is further designed as described in NL 2011836.