Measuring transducer of vibration-type

Abstract

A measuring transducer includes a support body, a curved oscillatable measuring tube, an electrodynamic exciter, at least one sensor for registering oscillations of the measuring tube, and an operating circuit. The measuring tube has first and second bending oscillation modes, which are mirror symmetric to a measuring tube transverse plane and have first and second media density dependent eigenfrequencies f1, f3 with f3>f1. The measuring tube has a peak secant with an oscillation node in the second mirror symmetric bending oscillation mode. The operating circuit is adapted to drive the exciter conductor loop with a signal exciting the second mirror symmetric bending oscillation mode. The exciter conductor loop has an ohmic resistance R.sub.Ω and a mode dependent mutual induction reactance R.sub.g3 which depends on the position of the exciter. The exciter is so positioned that a dimensionless power factor ${\mathrm{pc}}_3=\frac{4.Math.R_{\Omega }.Math.R_{g3}}{{\left(R_{\Omega }+R_{g3}\right)}^2}$
has a value of not less than 0.2.

Claims

1. A vibration-type measuring transducer, comprising: a support body; at least one curved measuring tube for guiding a medium and having an inlet side end section and an outlet side end section, wherein the measuring tube is held by the support body at the inlet side end section and at the outlet side end section, wherein the measuring tube has a freely oscillatable section; an operating circuit; an electrodynamic exciter in an exciter conductor loop for exciting bending oscillations of the measuring tube; and at least one sensor for registering oscillations of the measuring tube; wherein a measuring tube longitudinal plane is defined as a plane in which an integral along a measuring tube centerline of the oscillatable section of a squared distance of separations between the measuring tube centerline in a resting position of the measuring tube and the plane has a minimum value; wherein a measuring tube transverse plane extends perpendicularly to the measuring tube longitudinal plane, and wherein the measuring tube is mirror symmetric to the measuring tube transverse plane; wherein the measuring tube has a first bending oscillation mode that is mirror symmetric to the measuring tube transverse plane and that has a first eigenfrequency that depends on a density of the medium guided through the measuring tube; wherein the measuring tube has a second bending oscillation mode that is mirror symmetric to the measuring tube transverse plane and that has a second eigenfrequency that depends on the density of the medium guided through the measuring tube; wherein the second eigenfrequency is greater than the first eigenfrequency; wherein the measuring tube has a peak secant that intersects points of an outer surface of a measuring tube wall that in the resting position of the measuring tube lie on a line of intersection between the measuring tube longitudinal plane and the measuring tube transverse plane, and wherein the peak secant has an oscillation node when the measuring tube oscillates in the second bending oscillation mode; wherein the operating circuit is adapted to drive the exciter conductor loop with a signal for exciting the second bending oscillation mode; wherein the exciter conductor loop has an ohmic resistance R.sub.Ω and a mutual induction reactance R.sub.g3 that depend on a position of the exciter; and wherein the exciter is positioned such that a dimensionless power factor ${\mathrm{pc}}_3=\frac{4.Math.R_{\Omega }.Math.R_{g3}}{{\left(R_{\Omega }+R_{g3}\right)}^2}$ has a value that is not less than 0.2 when the measuring tube is filled with water and is excited by the electrodynamic exciter with the eigenfrequency of the second bending oscillation mode to excite bending oscillations at a temperature of 300 K.

2. The vibration-type measuring transducer of claim 1, wherein the oscillation node of the peak secant in the second bending oscillation mode defines a node plane, wherein the node plane extends perpendicularly to the measuring tube transverse plane and perpendicularly to the measuring tube longitudinal plane, wherein the peak secant has no oscillation nodes in the node plane in the first bending oscillation mode.

3. The vibration-type measuring transducer of claim 1, wherein the measuring tube has an outer diameter in the measuring tube transverse plane, wherein a node plane is spaced from the intersection between the measuring tube centerline and the measuring tube transverse plane by no more than three outer diameters.

4. The vibration-type measuring transducer of claim 3, wherein a peak plane, which extends perpendicularly to the measuring tube transverse plane and perpendicularly to the measuring tube longitudinal plane and through the intersection between the measuring tube centerline and the measuring tube transverse plane, extends between the node plane and the electrodynamic exciter.

5. The vibration-type measuring transducer of claim 4, wherein the electrodynamic exciter is spaced from the peak plane by no more than two outer diameters of the measuring tube.

6. The vibration-type measuring transducer of claim 5, wherein the electrodynamic exciter is spaced from the peak plane by no more than one outer diameter.

7. The vibration-type measuring transducer of claim 3, wherein the node plane is spaced from the intersection between the measuring tube centerline and the measuring tube transverse plane by no more than two outer diameters.

8. The vibration-type measuring transducer of claim 1, wherein the operating circuit is adapted to drive the exciter conductor loop with a signal for exciting the first bending oscillation mode.

9. The vibration-type measuring transducer of claim 8, wherein the exciter conductor loop has a mutual induction reactance R.sub.g1 dependent on the first bending oscillation mode, which mutual induction reactance R.sub.g1 depends on the position of the exciter, wherein the exciter is positioned such that a dimensionless power factor ${\mathrm{pc}}_1=\frac{4.Math.R_{\Omega }.Math.R_{g1}}{{\left(R_{\Omega }+R_{g1}\right)}^2}$ has a value that is not less than 0.3 when the measuring tube is filled with water and excited by the electrodynamic exciter with the eigenfrequency of the first bending oscillation mode to execute bending oscillations at a temperature of 300 K.

10. The vibration-type measuring transducer of claim 9, wherein a total power factor $p{c}_{1.3}=p{c}_1.Math.p{c}_3$ is not less than 0.2.

11. The vibration-type measuring transducer of claim 10, wherein the total power factor pc.sub.1.3 is not less than 0.2.

12. The vibration-type measuring transducer of claim 10, wherein the total power factor pc.sub.1.3 is not less than 0.7.

13. The vibration-type measuring transducer of claim 9, wherein the exciter is positioned such that the value of the dimensionless power factor pc.sub.1 is not less than 0.8.

14. The vibration-type measuring transducer of claim 1, wherein the ohmic resistance R.sub.Ω is at least 90% caused by a coil or a plurality of coils of the exciter, a limiting resistance, or a plurality of limiting resistances.

15. The vibration-type measuring transducer of claim 1, wherein the at least one measuring tube comprises at least one pair of measuring tubes having a shared measuring tube transverse plane, wherein the electrodynamic exciter is adapted to excite oscillation of the measuring tubes relative to one another.

16. The vibration-type measuring transducer of claim 15, wherein the measuring tubes have parallel measuring tube longitudinal planes.

17. The vibration-type measuring transducer of claim 1, wherein the at least one sensor comprises a pair of sensors for registering oscillations of the measuring tube, wherein the pair of sensors are arranged symmetrically to the measuring tube transverse plane.

18. The vibration-type measuring transducer of claim 1, wherein the exciter is positioned such that the value of the dimensionless power factor pc.sub.3 is not less than 0.8.

19. The vibration-type measuring transducer of claim 1, wherein the ohmic resistance R.sub.Ω is at least 90% caused by a coil or a plurality of coils of the exciter.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The invention will now be explained in greater detail based on the example of an embodiment illustrated in the drawing, the figures of which show as follows:

(2) FIG. 1 shows an example of an embodiment of a measuring transducer of the invention;

(3) FIG. 2 shows a coordinate system for description of an example of an embodiment of a measuring transducer of the invention;

(4) FIG. 3a shows typical deflections of a measuring tube for the f1 mode and the f3 mode in plan view

(5) FIG. 3b shows typical deflections for the f1 mode and the f3 mode in orthogonal projection on the measuring tube transverse plane and the associated power factors as well as the product of the power factors;

(6) FIG. 4a shows a schematic model for description of an oscillator;

(7) FIG. 4b shows a schematic diagram of the effective resistances or reactances in an exciter circuit; and

(8) FIG. 5 shows a coordinate system to illustrate geometric elements referred to in the description of the invention.

DETAILED DESCRIPTION

(9) The example of an embodiment of a measuring transducer 100 of the invention shown in FIG. 1 includes a pair of curved measuring tubes 110. The measuring tubes 110 extend between an inlet end collector 120 and an outlet end collector 120 and are connected with these fixedly, for example, by roll expansion, welding, soldering or brazing. Extending between the collectors 120 is a sturdy support tube 124, which with both collectors are durably connected, whereby the collectors 120 are rigidly coupled together. The support tube 124 has on its upper side openings 126, through which the measuring tubes 110 exit and enter the support tube 124 in the vicinity of the collectors 120.

(10) The collectors 120 have terminal flanges 122, by means of which the Coriolis mass flow measuring device, and/or density measuring device, can be installed in a pipeline. Through central openings 123 in the flanges 122, a mass flow can traverse the measuring tubes 110, so that the mass flow, or its density, can be measured.

(11) Based on FIG. 2, some symmetry characteristics of measuring transducers of the invention will now be presented. Shown in this connection are measuring tube central axes 112a, 112b of the two measuring tubes 110, which form the oscillator. The measuring tube central axes 112a, 112b extend symmetrically to a first mirror plane Syz, which extends between the measuring tubes. The measuring tube central axes extend further symmetrically to a second mirror plane Sxy, the so-called measuring tube transverse plane, which extends perpendicularly to the first mirror plane Syz. Lying in the measuring tube transverse plane are peaks of the measuring tubes, and the measuring tube central axes. The measuring tube axes 112a, 112b extend preferably in planes, which extend in parallel with the first mirror plane. No symmetry of the measuring tubes is present relative to a third plane Szx, which extends perpendicularly to the first mirror plane and to the second mirror plane, and in which the measuring tube axes 112a, 112b extend into the collectors. The line of intersection between the first mirror plane Syz and the third plane defines a Z axis of a coordinate system of the measuring transducer. The line of intersection between the second mirror plane Sxy and the third plane Szx defines an X axis of the coordinate system, and the line of intersection between the first mirror plane Syz and the second mirror plane defines the Y axis of the coordinate system. With the coordinates defined in this way, we return to FIG. 1.

(12) The pair of measuring tubes 110 form an oscillator, which has especially a first bending oscillation mode mirror symmetric to the measuring tube transverse plane with a first eigenfrequency f1 and a second bending oscillation mode mirror symmetric to the measuring tube transverse plane with a second eigenfrequency f3, wherein the measuring tubes oscillate in the X direction with opposite phase relative to one another. For exciting the bending oscillation modes of the measuring tubes in the X direction, an electrodynamic exciter mechanism 140 is provided mirror symmetrically to the measuring tube transverse plane. The electrodynamic exciter mechanism 140 includes, for example, a coil on a first measuring tube and an element on the oppositely lying, second measuring tube for plunging into the coil. Details for the vertical positioning of the exciter mechanism in the y direction are explained below.

(13) For registering the oscillations of the measuring tubes, sensor arrangements 142 are provided symmetrically to the measuring tube transverse plane Sxy. The sensor arrangements 142 are embodied, in each case, as inductive arrangements with a coil on one tube and a plunge element on the other tube. Details of this are known to those skilled in the art and need not be explained in further detail here.

(14) For influencing the oscillation characteristics, the measuring tubes 110 are connected at their inlet and outlet ends via couplers 132, 134, wherein the positions of the two inner couplers 132, thus those, which are farthest removed from the nearest collector 120, establish a free oscillatory length of an oscillator formed by the two measuring tubes 110. This free oscillatory length influences the bending oscillation modes of the oscillator, especially their eigenfrequencies, with which the oscillator is preferably excited. Outer couplers 134, which are arranged between the inner node plates 132 and the collectors 120, serve especially to define other oscillation nodes.

(15) The variable h is the arc height of the freely oscillatable measuring tube curve between the two inner couplers 132, wherein the arc height is measured from the intersection of the coupler with the measuring tube centerline to the peak of the measuring tube centerline in the measuring tube transverse plane.

(16) The oscillatory behavior of a measuring tube 110 will now be explained based on FIGS. 3a and 3b.

(17) FIG. 3a shows examples of deflections X.sub.1(z) and X.sub.3(z) along the measuring tube centerline of the first and second mirror symmetric bending oscillation modes orthogonally projected onto the Szx-plane. The Z-coordinate of the sensor arrangements is shown with the lines S. The sensor arrangements are so positioned that they can register the deflections of both bending oscillation modes. The electrodynamic exciter is positioned in the measuring tube transverse plane Sxy, which coincides in this projection with the X axis. The deflection of the sensor arrangements is in a bending oscillation mode proportional to the deflection of the exciter. Thus, an efficient deflection of the exciter leads to an efficient deflection of the sensor arrangements. Starting from this deliberation, an especially optimized exciter position will now be sought.

(18) In FIG. 3b and FIG. 5, the lines X1(y) and X3(y) show the orthogonal projections of the height dependent amplitudes of the measuring tube centerline of a measuring tube in the first and second mirror symmetric bending oscillation modes on the measuring tube transverse plane Sxy, wherein the amplitudes in the peak of the measuring tube centerline are normalized to 1. The considered measuring tube has, measured from the inner couplers, an arc height h=0.4 m and inner couplers at y=0.1 m. Furthermore, the graph shows peak secants S1, S3, which extend through points on the surface of the measuring tube, which lie in the resting position of the measuring tube on the line of intersection of the measuring tube longitudinal plane Syz and the measuring tube transverse plane Sxy. These peak secants S1, S3 are of interest when an exciter is affixed mechanically on the measuring tube in the measuring tube transverse plane Sxy and so has a deflection, which depends not only on the deflection of the measuring tube centerline but also on the torsion of the measuring tube in the measuring tube transverse plane Sxy. Therefore, the expected deflection of an exciter as a function of its mounting separation from the measuring tube centerline is to be found on the peak secant S1, S3. In FIG. 3b and FIG. 5, K1 and K3 designate positions of oscillation nodes, in which an exciter in the first and second mirror symmetric bending oscillation modes, respectively, would experience practically no deflection. The exciter is thus to be arranged spaced from these positions. As shown in FIG. 5, these positions define node planes PK1 and PK3, respectively. FIG. 5 further shows a peak plane, which extends perpendicularly to the measuring tube transverse plane Sxy and perpendicularly to the measuring tube longitudinal plane Syz and through the intersection between the measuring tube centerline and the measuring tube transverse Sxy plane.

(19) Other considerations for arrangement of the electrodynamic exciter will now be explained based on FIGS. 4a and 4b.

(20) The measuring tube, or the measuring tubes, of an oscillator are excited to oscillate in bending oscillation modes by a force F, which is composed of a sum of modal forces Fi, which are given by the product of the modal contribution I.sub.i to the exciter current I and a constant e, thus
F.sub.i=I.sub.i.Math.e  (1)

(21) On the other hand, the oscillating oscillator induces in the exciter an induced voltage U.sub.gi, whose amplitude is given by the expression
U.sub.gi=X.sub.i−e  (2),

(22) wherein e in (1) and (2) is the same constant dependent on the inductance of the exciter.

(23) The amplitude Xi of the i-th bending oscillation mode at the considered site, for example, in the measuring tube transverse plane, depends on the oscillating mass m.sub.i, the resilience n.sub.i, and the quality Q.sub.i of the oscillator in a particular oscillatory mode.

(24) In the case of excitation with the resonance circuit frequency oi, the amplitude of the deflection is:
X.sub.i=n.sub.i.Math.Q.sub.i.Math.F.sub.i  (3).

(25) The velocity is:
X.sub.i=ω.sub.i.Math.n.sub.i.Math.Q.sub.i.Math.F.sub.i  (4).

(26) For the induced voltage U.sub.gi there follows then with (1) and (2)
U.sub.gi=e.sup.2.Math.ω.sub.i.Math.n.sub.i.Math.Q.sub.i.Math.I.sub.i  (5),
or
U.sub.gi=R.sub.gi.Math.I.sub.i  (6),
wherein R.sub.gi is the mutual induction reactance.
R.sub.i=e.sup.2.Math.ω.sub.i.Math.n.sub.i.Math.Q.sub.i  (7),

(27) The electrical induction power P.sub.i is given by the product of the induced voltage U.sub.gi and the electrical current I.sub.i, or by the product of the induction reactance R.sub.gi and the square of the electrical current I.sub.i. The electrical current is given by I=U/R, wherein R is the total resistance of the exciter circuit, also referred to as exciter loop 200, shown in FIG. 4b, which includes in series the ohmic resistance R.sub.e of the exciter, the inductive reactance ω.sub.i.Math.L.sub.e of the exciter and the mutual induction reactance of the exciter R.sub.gi. Additionally, a protective resistance element R.sub.ex can be provided for meeting an ignition protection type. When one takes into consideration that the inductive reactance ω.Math.L.sub.e of the exciter is significantly less than the ohmic resistances, the following holds for the electrical excitation power:

(28) $\begin{array}{cc}{P}_i=U_0^2.Math.\frac{R_{\mathrm{gi}}}{{\left(R_{\mathrm{ex}}+R_e+R_{\mathrm{gi}}\right)}^2}.& \left(8\right)\end{array}$

(29) This expression is maximum, when the mutual induction reactance R.sub.gi equals the ohmic resistance R.sub.Ω of the exciter conductor loop, thus equals the sum of the ohmic resistance R.sub.e of the exciter and the resistance of the, in given cases present, protective resistance element R.sub.ex, thus R.sub.gi=R.sub.Ω=R.sub.ex+R.sub.e. It is helpful to define a dimensionless power factor pc.sub.1 for the different bending oscillation modes for describing this situation:

(30) $\begin{array}{cc}{\mathrm{pc}}_i=\frac{4.Math.R_{\Omega }.Math.R_{\mathrm{gi}}}{{\left(R_{\Omega }+R_{\mathrm{gi}}\right)}^2}& \left(9\right)\end{array}$

(31) This power value assumes the maximum value pc.sub.i=1, when R.sub.gi=R.sub.Ω.

(32) For developing a measuring transducer, the above equations offer an approach for checking R.sub.gi via the resilience ni, which for the electrodynamic exciter depends on its position in the measuring tube transverse plane. In this way, the power factors of a measuring transducer can be established for the different modes. For a given measuring tube, the mode dependent amplitudes, velocities, forces and eigenfrequencies are ascertained, for example, by simulation, and experimentally checked. The quality Qi for an oscillatory mode is measurable via the breadth of a resonance, or via the decay behavior of an oscillation. Finally, the induced voltage in the case of a freely oscillating measuring tube can be determined experimentally for verifying calculated variables.

(33) FIG. 3b shows the power factors pc.sub.1 and pc.sub.3 for the first and second mirror symmetric bending oscillation modes as well as their product pc.sub.1.3 as a function of the position of the exciter. At the nodes K1 and K2 along the peak secants, the power factors assume the value 0, as expected. Furthermore, it is shown that an arbitrarily increased amplitude X.sub.i, to which the resilience n.sub.i is proportional, does not lead to an unlimited increasing of the power factor. In the present case, the maximum of pc.sub.3 as well as the maximum M1.3 of pc.sub.1.3 lie scarcely above the measuring tube curve. The electrodynamic exciter mechanism is mounted in FIG. 1 in this position.

(34) As a result, the present invention provides the bases for using optimized power factors to obtain a measuring transducer with efficient excitation.