Method for operating a coriolis mass flowmeter and corresponding coriolis mass flowmeter

Abstract

A method for operating a Coriolis mass flowmeter having at least one controller, at least one electric actuating device, at least one electromagnetic driving mechanism with a drive coil as oscillation generator, at least one measuring tube and at least one oscillation sensor involves excited oscillation of the measuring tube being detected by the oscillation sensor and emitted as at least one output signal and the electric actuating device causing the electromagnetic driving mechanism to produce oscillation of the measuring tube largely in resonance by the output signal of the oscillation sensor, the drive voltage at the drive coil, and phasing of the drive current in relation to the phasing of the output signal of the oscillation sensor being determined and a new target phasing for the drive voltage derived from the determinations and supplied to the controller to generate a drive voltage with the new target phasing.

Claims

1. Method for operating a Coriolis mass flowmeter having at least one controller, at least one electric actuating device, at least one electromagnetic driving mechanism with a drive coil as an oscillation generator, at least one measuring tube and at least one oscillation sensor, the method comprising the following steps: generating a controller output signal u.sub.c for controlling the at least one electric actuating device with the at least one controller, providing an electric excitation signal u.sub.dr for exciting the at least one electromagnetic drive mechanism with the at least one electric actuating device, exciting the at least one measuring tube to oscillation in at least one eigenform with the at least one electromagnetic drive mechanism, detecting excited oscillation of the measuring tube with the at least one oscillation sensor and emitting the detected oscillation as at least one output signal u.sub.s, applying a drive voltage u.sub.dr and a drive current i.sub.dr to the drive coil of the at least one electromagnetic driving mechanism with the at least one electric actuating device so that the oscillation of the measuring tube occurs largely in resonance, determining a target specification u.sub.dr,soll for the drive voltage u.sub.dr, and thus also a target phasing .sub.dr,soll of the drive voltage u.sub.dr, by defining a zero phasing (.sub.s=0) of the output signal u.sub.s and determining the target specification u.sub.dr,soll for the drive voltage u.sub.dr using the equation
u.sub.dr,soll=e.sup.j.sup.idr(u.sub.drK.sub.B.Math.u.sub.s)+k.sub.B.Math.u.sub.s wherein k.sub.B is a real number mutual induction factor, determining the output signal u.sub.s of the at least one oscillation sensor, determining the drive voltage u.sub.dr at the drive coil, determining phasing .sub.idr of the drive current i.sub.dr in relation to phasing .sub.s of the output signal u.sub.s of the oscillation sensor, and deriving a new target phasing .sub.dr,soll for the drive voltage u.sub.dr based on the determinations and supplying the new target phasing to the at least one controller, using the at least one controller for generating a drive voltage u.sub.dr with the determined new target phasing (.sub.dr,soll) via the electric actuating device so that the at least one controller produces resonance operation of the flowmeter.

2. The method for operating a Coriolis mass flowmeter according to claim 1, wherein the impedance Z.sub.S of the drive coil is determined outside of resonance operation, wherein a quotient of the drive voltage and an adjusting drive current i.sub.dr is calculated, and the mutual induction factor k.sub.B is determined without taking the mutual induction voltage u.sub.B at the drive coil into account using measuring equation for a measuring network of the output of the electric actuating device and the electromagnetic driving mechanism with the drive coil.

3. The method for operating a Coriolis mass flowmeter according to claim 2, characterized in that, in order to determine the mutual induction factor k.sub.B, the following relation is used with the impedance Z.sub.S of the drive coil, the drive current i.sub.dr and the output signal u.sub.s of the oscillation sensor $k_B=\frac{{\underset{\_}{u}}_{\mathrm{dr}}-{\underset{\_}{Z}}_S.Math.{\underset{\_}{i}}_{\mathrm{dr}}}{{\underset{\_}{u}}_S}.$

4. The method for operating a Coriolis mass flowmeter according to claim 1, wherein a control mode in resonance is temporarily interrupted, wherein the controller is provided another phasing specification as a target value for the phasing of the drive current i.sub.dr corresponds to a phase difference of +45 as compared to the phasing of the output signal u.sub.s.

5. A Coriolis mass flowmeter, comprising: at least one controller, at least one electric actuating device, at least one electromagnetic driving mechanism with a drive coil as oscillation generator, at least one measuring tube, and at least one oscillation sensor, wherein the at least one controller is adapted for generating a controller output signal u.sub.c for controlling the at least one electric actuating device, wherein the at least one electric actuating device is adapted to provide an electric excitation signal u.sub.dr for exciting the at least one electromagnetic drive mechanism, wherein the at least one electromagnetic drive mechanism is adapted for exciting the measuring tube to oscillation in at least one eigenform, wherein the at least one oscillation sensor is adapted for detecting excited oscillation of the measuring tube and emitting as at least one output signal u.sub.s based thereon, wherein the at least one electric actuating device is adapted to apply a drive voltage u.sub.dr and a drive current i.sub.dr to the drive coil of the at least one electromagnetic driving mechanism so that the oscillation of the measuring tube occurs largely in resonance, wherein a target specification u.sub.dr,soll for the drive voltage u.sub.dr, and thus also a target phasing .sub.dr,soll of the drive voltage u.sub.dr, is determined by defining a zero phasing (.sub.s=0) of the output signal u.sub.s and wherein the target specification u.sub.dr,soll for the drive voltage u.sub.dr is determined using the equation
u.sub.dr,soll=e.sup.j.sup.idr(u.sub.drK.sub.B.Math.u.sub.s)+k.sub.B.Math.u.sub.s wherein a mutual induction factor k.sub.B is a real number, further comprising means for determining the output signal u.sub.s of the at least one oscillation sensor, the drive voltage u.sub.dr at the drive coil, phasing .sub.idr of the drive current i.sub.dr in relation to phasing .sub.s of the output signal u.sub.s of the oscillation sensor, and for setting a new target phasing .sub.dr,soll for the drive voltage u.sub.dr derived from the determinations and for supplying the new target phasing .sub.dr,soll to the controller, and wherein the controller is adapted for generating a drive voltage u.sub.dr with the new target phasing .sub.dr,soll via the electric actuating device by which operation in resonance is achieved.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically shows the structure of a Coriolis mass flowmeter as it is known from the prior art, but as it could be used for the method according to the invention and the Coriolis mass flowmeter according to the invention,

(2) FIG. 2 is an equivalent circuit diagram of a coil comprised in an electronic driving mechanism with an electric actuating device and

(3) FIG. 3 is a block diagram showing the method according to the invention for operating a resonance measuring system.

DETAILED DESCRIPTION OF THE INVENTION

(4) FIG. 1 shows a Coriolis mass flowmeter 1 with a controller 2 implemented in a digital signal processor, with an electric actuating device 3 and with an electromagnetic driving mechanism 4 having a drive coil as oscillation generator, not shown in detail in FIG. 1.

(5) The Coriolis mass flowmeter 1 has a measuring tube 5. The electromagnetic driving mechanism 4 has the task of exciting the measuring tube 5 with medium flowing through it to an oscillation in an eigenform. Depending on the type of the eigenform, only one, single electromagnetic driving mechanism is required for this, if higher modes are able to be excited, then two or more electromagnetic driving mechanisms 4 are required.

(6) The Coriolis mass flowmeter 1 is shown in two parts in FIG. 1. The one unit forming the Coriolis mass flowmeter 1 ends with one half at the right edge of the figure and begins, for a better overview, again with the other half at the left edge of the figure. It can be seen there that the Coriolis mass flowmeter 1 also has oscillation sensors 6 that each emit an output signal u.sub.s presently in the form of a speed signal that provides information about the speed v of the movement of the measuring tube. The electric state variables are shown underlined here, in order to make clear that they are normally harmonic signals with a phasing, i.e., can be described as indicator variables. Thus, it is possible for electric state variables, which are shown without being underlined, to assume that theyfor whatever reasonhave a zero phasing, i.e. are mathematically real.

(7) The controller 2 generates a controller output signal u.sub.c for controlling the electric actuating device 3, and the electric actuating device 3 subsequently generates an electric excitation signal u.sub.dr, for exciting the electromagnetic driving mechanism 4. A plurality of transfer elements 7 are connected to the oscillation sensor 6, which are essentially used for signal preparation, such as, for example, adaptation electronics 7a consisting of amplifiers, a hardware multiplexer 7b for implementing different switchable measuring channels, a further adaptation electronics 7c and an analog-digital converter 7d that supplies the measured analog signals to the controller 2 in the form of digital signals. The exact execution of these transfer elements is not of importance, they are described here for the sake of completeness.

(8) In the prior art, the control loop implemented in this manner forms a phase locked loop and is based on either the imprinting of a current i.sub.dr in a coil 8 of the electromagnetic driving mechanism 4 or the intrusion of an electric excitation signal in the form of an excitation voltage u.sub.dr at the clamps of a coil 8 of the electromagnetic driving mechanism 4. This concept is depicted in FIG. 2 for clarification. The electromagnetic driving mechanism 4 has a drive coil 8 here, which, in the equivalent circuit diagram according to FIG. 2, has a coil inductance L.sub.s an ohmic resistance R.sub.s and a voltage source u.sub.B induced proportional to speed. The controller, not shown in FIG. 2, supplies the controller output signal u.sub.c for controlling the further electric actuating device 3, comprised of a controllable energy source 9 and a digital-analog converter. The controllable energy source 9 is either a voltage-controlled current source or, however, a voltage-controlled voltage source, wherein both solutions have different advantages and disadvantages relating to the particular characteristics of the coil 8, for example, that stepped changes in current lead to significant changes in the clamp voltages.

(9) In the electromagnetic driving mechanism 4, which, as depicted in FIG. 2, has a coil 8, the coil current i.sub.dr is of particular importance, because the coil current i.sub.dr is the state variable of the electromagnetic driving mechanism 4 that is proportional to the force of the electromagnetic driving mechanism 4 on the measuring tube 5. In the case of a Coriolis mass flowmeter 1, the phase difference, in resonance, between the force F acting on the oscillation element 5 and thus between the coil current i.sub.dr and the detected speed v of the measuring tube movement is zero. The speed v of the measuring tube movement thereby corresponds or, respectively, is proportional to the detected output voltage u.sub.s of the oscillation sensor 6. The movement of the measuring tube 5, however, not only influences the oscillation sensor 6, in fact, it is also a retroactive effect on the oscillation generator in the form of the drive coil 8 since the movement of the measuring tube 5 leads to a corresponding movement of a normally-present permanent magnet in the drive coil 8, which itself generates the mutual induction voltage u.sub.B.

(10) The following holds true in the interstices formed from the output clamps of the actuating device 3 and the clamps of the coil 8 attached thereto
u.sub.dr=Z.sub.S.Math.i.sub.dr+u.sub.B(1)

(11) The challenge during operation of a Coriolis mass flowmeter 1 is to control the electric actuating device 3 using the controller 2 so that the drive coil 8 of the electromagnetic drive mechanism 4 is impinged with a drive voltage u.sub.dr and a drive current i.sub.d so that the oscillation of the measuring tube 5 occurs largely in resonance. Largely in resonance thereby takes into account that the resonance point is a strictly defined exact state of the system, which mathematically in practice can never be exactly achieved, but is always only as exact as is technically possible and permitted by the implemented regulation, i.e., what is meant is the resonance operation is as close to the resonance point as is permitted by the implemented technical solution.

(12) The method for operating the Coriolis mass flowmeter 1 is shown in FIG. 3, namely shown in the form of a block diagram. The controller 2 controls the electric actuating device 4 via the controller output signal u.sub.c, wherein the electric actuating device 3 controls the electromagnetic driving mechanism 4 by emitting the electric excitation signal u.sub.dr, the driving mechanism as oscillation generator deflecting the measuring tube 5. The electromagnetic driving mechanism 4 consists of a schematically depicted coil 8 with a permanent magnet as core, wherein the non-depicted permanent magnet carries out a movement when supplying the coil 8 with current and, in this manner, is able to excite the measuring tube 5 to oscillation. The oscillation of the measuring tube 5 is detected by the oscillation sensor, which, in the present case, is also a permanent magnet and has a coil 11, wherein the voltage u.sub.s induced in the coil 11 is used for evaluating the change of position of the measuring tube 5. The speed signal is thus present as an output signal u.sub.s of the oscillation sensor 6.

(13) According to the invention, it is now intended, in order to achieve resonance operation, to determine the output signal us of the oscillation sensor, to determine the drive voltage u.sub.dr at the drive coil 8, to determine the phasing .sub.idr of the drive current i.sub.dr compared to the phasing .sub.s of the output signal u.sub.s of the oscillation sensor 6 and to derive a new target phasing .sub.idr,soll for the drive voltage u.sub.dr from the derived variables and to supply them to the controller 2, so that the controller 2 generates a drive voltage u.sub.dr with the newly derived target phasing .sub.dr,soll via the electric actuating device 3.

(14) The method is based on the consideration that the phase difference between the force F on the measuring tube and the resulting measuring tube speed v is to be regulated to zero when possible, wherein this phase difference also corresponds to the phase difference between the coil current i.sub.dr and the measuring tube speed v or, respectively, the induced mutual induction voltage u.sub.B. This simultaneously corresponds to the phase difference between the coil current i.sub.dr and the phasing of the output signal u.sub.s of the oscillation sensor 6, i.e.:

(15) $\begin{array}{cc}\left(\underset{\_}{F},\underset{\_}{v}\right)=\left({\underset{\_}{i}}_{\mathrm{dr}},{\underset{\_}{u}}_B\right)=\left({\underset{\_}{i}}_{\mathrm{dr}},{\underset{\_}{u}}_s\right)=0.& \left(2\right)\end{array}$

(16) The electric excitation signal u.sub.dr, for exciting the electromagnetic driving mechanism, thus, has to be chosen so that the above-mentioned resonance requirements are met. Thereby, the mutual induction voltage u.sub.B is to be in phase with the output voltage u.sub.s of the oscillation sensor 6, accordingly:
u.sub.B=k.sub.B.Math.u.sub.s.(3)

(17) Under this stipulation, the mesh equation can also be written as:
u.sub.dr=Z.sub.S.Math.i.sub.dr+k.sub.B.Math.u.sub.s(4)

(18) The notation and calculation is particularly simple when the phasing of the output signal u.sub.s is defined as zero phasing, i.e., .sub.s=0. Under this stipulation, the simplified equation (4) can also be written as follows:
u.sub.dr=Z.sub.S|i.sub.dr|e.sup.j.sup.idr+k.sub.B.Math.u.sub.s(5)

(19) Since, under these requirements, i.e., in the case of resonance, it holds true that the phasing .sub.idr of the drive current i.sub.dr is equal to zero, a target specification u.sub.idr,soll for the drive voltage u.sub.dr, when the specification is correctly chosen, is:
u.sub.dr,soll=Z.sub.S.Math.|i.sub.dr|+k.sub.B.Math.u.sub.s(6)

(20) Thereby, after solving the mesh equation established above in the actual state and the target state, the drive current i.sub.dr is:

(21) $\begin{array}{cc}{\underset{\_}{i}}_{\mathrm{dr}}=.Math.{\underset{\_}{i}}_{\mathrm{dr}}.Math.e^{-j_{\mathrm{idr}}}=\frac{\left({\underset{\_}{u}}_{\mathrm{dr}}-k_B.Math.u_s\right)}{{\underset{\_}{Z}}_S}{\underset{\_}{i}}_{\mathrm{dr}}=.Math.{\underset{\_}{i}}_{\mathrm{dr}}.Math.=\frac{\left({\underset{\_}{u}}_{\mathrm{dr},\mathrm{soll}}-k_B.Math.u_s\right)}{{\underset{\_}{Z}}_S}& \left(7\right)\end{array}$

(22) If the actual state and the target state are compared to one another in terms of equations, then:

(23) $\begin{array}{cc}\frac{.Math.{\underset{\_}{i}}_{\mathrm{dr}}.Math.e^{-j_{\mathrm{idr}}}}{.Math.{\underset{\_}{i}}_{\mathrm{dr}}.Math.}=\frac{{\underset{\_}{u}}_{\mathrm{dr}}-k_B.Math.u_s}{{\underset{\_}{u}}_{\mathrm{dr},\mathrm{soll}}-k_B.Math.u_s}& \left(8\right)\end{array}$

(24) Thereby, for the drive voltage u.sub.dr, the target specification u.sub.dr,soll is:
u.sub.dr,soll=e.sup.j.sup.idr(u.sub.drk.sub.B.Math.u.sub.s)+k.sub.B.Math.u.sub.s(9)

(25) Thus, it makes sense to redetermine the target specification u.sub.dr,soll for the drive voltage u.sub.dr according to the above equation. When this is done continuouslywhich is common for a control technology sampling systemthe Coriolis mass flowmeter 1 can also be kept in resonance operation, when the resonance pointfor whatever reasondrifts during operation.

(26) The shown correlation requires that the mutual induction factor k.sub.B is known. The mutual induction factor can be comparably easily determined according to an advantageous further development of the method according to the invention. For this, it is intended that the impedance Z.sub.S of the drive coil 8 is determined outside of resonance operation of the Coriolis mass flowmeter 1, in that the quotient of the drive voltage u.sub.dr and the adjusting drive current i.sub.dr is calculated, and the mutual induction factor k.sub.B is determined without taking the mutual induction voltage u.sub.B at the drive coil 8 into accountwhich is permitted in this case. The determination takes place using the mesh equation for the network mesh, which consists of the output of the electric actuating device 3 and the electromagnetic driving mechanism 4 with the drive coil 8. When the measuring tube 5 is excited to oscillation outside of resonance, the mutual induction voltage u.sub.B can be neglected, so that the impedance can be easily calculated from the mesh equation:

(27) $\begin{array}{cc}{\underset{\_}{Z}}_S=\frac{{\underset{\_}{u}}_{\mathrm{dr}}}{{\underset{\_}{i}}_{\mathrm{dr}}}.& \left(10\right)\end{array}$

(28) However, when the impedance Z.sub.s is known, the mutual induction factor k.sub.B can be easily calculated with:

(29) $\begin{array}{cc}{k}_B=\frac{{\underset{\_}{u}}_{\mathrm{dr}}-{\underset{\_}{Z}}_S.Math.{\underset{\_}{i}}_{\mathrm{dr}}}{{\underset{\_}{u}}_S}.& \left(11\right)\end{array}$

(30) The method depicted is implemented by the Coriolis mass flowmeter 1 in the controller 2, so that, during operation, the Coriolis mass flowmeter 1 carries out the shown variations of the method for operating a Coriolis mass flowmeter 1.