EP1812775A1

EP1812775A1 - Measuring and operational circuit for a coriolis-mass flow meter comprising three measuring channels

Info

Publication number: EP1812775A1
Application number: EP05801324A
Authority: EP
Inventors: Philipp Alber; Christian Matt
Original assignee: Endress and Hauser Flowtec AG; Flowtec AG
Current assignee: Endress and Hauser Flowtec AG
Priority date: 2004-11-17
Filing date: 2005-11-02
Publication date: 2007-08-01
Also published as: US20080156108A1; DE102004055553A1; WO2006053829A1; US7469600B2

Abstract

The invention relates to a method for determining the mass flow rate with the aid of a Coriolis-mass flow meter, wherein the mass flow rate (I) is determined from the phase difference of the two sensor signals (X17, X18). Three measuring channels (K1, K2, K3) are provided for both of the sensor signals (X17, X18). Phase errors related to the different signal paths can be determined by switching the sensor signals (X17, X18) on the measuring channels (K1, K2, and/or K3) and can be taken into account when calculating the mass flow rate (I).

Description

description

MEASURING AND OPERATING CIRCUITS FOR A CORIOLIS MASS FLOWMETER WITH THREE MEASURING CHANNELS

The invention relates to a measuring and operating circuit for a Coriolis mass flow meter with three measuring channels.

Coriolis mass flow meters are widely used in process measurement for determining the mass flow of a fluid in a pipe section.

The Coriolis measuring principle is based on the measuring tube in which the fluid flows to vibrate and evaluate the resulting oscillatory motion.

Coriolis mass flowmeters therefore have one oscillation exciter and two oscillation sensors per measuring tube.

Measuring tube and fluid together form a vibratory system, which is usually excited at its resonant frequency. The resonant frequency depends, among other things, on the material and dimensions of the measuring tube. The density of the flowing fluid also has an influence on the resonance frequency.

In certain applications, however, the measuring tube is not excited at the resonant frequency but at an adjacent frequency.

The vibration sensors detect the oscillatory movement of the measuring tube at two spaced locations in the flow direction and convert them into sensor signals. The two sensor signals have the same frequency as the Meßrohr¬ vibration. If a fluid (liquid, gas) flows through the measuring tube, the two sensor signals are out of phase due to the Coriolis effect. The phase shift is a measure of the mass flow of the fluid through this section of the pipeline. The sensor signals are evaluated in a measuring / subcircuit to determine the exact value of the mass flow. The measured value can be displayed in a display unit on the Coriolis mass flow meter or for process control to a higher-level unit, eg. B. a controller (PLC, PLC) or forwarded to a control system.

In addition to the mass flow and other properties of the fluid, such as the density can be determined. For this purpose, a frequency evaluation of the measuring tube oscillation is additionally necessary.

Various types of Coriolis mass flowmeters are manufactured and sold by Endress + Hauser Flowtec AG.

[0010] US Pat. No. 4,801,897 describes an exciter / subcircuit for a Coriolis mass flow meter, which is designed in the manner of an analog phase logarithm. Loop control is established. The exciter frequency for the measuring tube automatically adjusts itself to the resonance frequency of the measuring tube even with variable fluid density.

The known measuring circuits operate either analog or digital. In the documents EP 698783, US 4,895,030, EP 702212 and US 4,529,002 such measuring circuits are described in more detail.

From EP 698783 a measuring circuit for a Corio lis-

Mass flow meter known, which has an analog control loop, which controls the two sensor signals to the same amplitude.

From EP 866319 a further measuring and operating circuit for a Coriolis mass flow meter is known. In this circuit, the two sensor signals are amplified prior to their further processing, wherein the amplification factor of an amplifier is variable. In a digital signal processor, the sum and the difference of the two sensor signals and one of the sensor signals are evaluated.

Recently, measuring circuits for Coriolis mass flow meters are known (see, for example, German published patent application DE10132603), which directly measure the phase shift between the two sensor signals and calculate the mass flow therefrom. Depending on the structure of the Coriolis mass flow meter, the occurring phase shifts can be relatively small. Maximum phase shifts of 1.5 ° are not unlikely. Additional phase shifts, which are caused by the two measuring channels for the two sensor signals, should be avoided as far as possible. These phase shifts, which are not caused by the Coriolis effect, are referred to as phase errors.

Due to the small measuring effect, especially at low flow speeds, even very small phase errors can have an extremely negative effect on the measurement result.

It is conceivable to determine in advance the phase error for the sensor signals in the two measuring channels and to consider them accordingly in the further evaluation.

Due to the time dependence of the influencing factors such as aging of the measuring transducer, ambient temperature, etc., a one-time determination would certainly not be sufficient. A regular repetition of the provision would be urgently needed.

Unfortunately, however, the actual measurement must be interrupted at least in the short term when measuring the phase error. This is undesirable and unacceptable in certain applications.

The object of the invention is to provide a method for determining the Masse¬ flow of a Coriolis mass flow meter, the above does not have said disadvantages, which in particular eliminates phase errors and at the same time allows a continuous measurement.

This object is achieved by the method specified in claim 1.

Advantageous further developments of the invention are specified in the subclaims.

The essential idea of the invention is not to provide two but three measuring channels for the two sensor signals. By corresponding switching of the sensor signals to the three measuring channels, the relative phase error of respectively two measuring channels can be determined and taken into account in the determination of the mass flow.

In a further development of the invention, the sensor signals are cyclically switched to the three measurement channels.

In an alternative embodiment of the invention, a measurement channel is permanently provided for a specific measurement signal.

According to a further embodiment of the invention, the relative phase error of two measuring channels is measured only every second Zy¬ klusschritt.

A significant advantage offered by the method according to the invention is that an interruption of the measurement is no longer necessary.

The invention also relates to a device for carrying out the method.

The invention is explained below with reference to an exemplary embodiment shown in the drawing.

In the drawings:

Fig. 1 sensor for a Coriolis mass flowmeter

FIG. 2 shows a block diagram of a measuring and operating circuit belonging to the measuring sensor according to FIG. 1

3 shows a block diagram of a measuring and operating circuit according to the invention

Fig. 4 Table Assignment Measuring Channel Sensor Signal

Fig. 5 flowchart with the individual process steps

1 shows a sensor 1 for a Coriolis mass flow meter in a schematic representation. The sensor 1 is arranged in a pipeline, not shown, in which a fluid F flows whose mass flow rate is one of the parameters of interest. The connection with the pipeline takes place via the two flanges 2,3.

The sensor 1 has a single straight measuring tube 4, which is fixed on the inlet side via an end plate 13 on the flange 2 and the outlet side via an end plate 14 on the flange 3.

The measuring and operating circuit according to the invention is not limited to this special sensor 1 with a single straight measuring tube. She can in Connection can be used with the various known sensors. To mention are, for example, sensors with a measuring tube with cantilever mass, as described for example in EP 97 81 0559, sensor with a curved measuring tube (EP 96 10 9242) and sensors with two parallel straight or curved measuring tubes (US 4793191 or US 41 27 028 ).

The flanges 2, 3 and the end plates are attached to or in a support tube 15.

To generate the measuring tube vibration, a vibration exciter 16 is arranged in the middle between the two end plates 13, 14 on the measuring tube 4. The vibration exciter 16 may be e.g. to act an electromagnetic drive consisting of a permanent magnet 161 and a coil 162.

The coil 162 is fixed to the support tube 15 and the permanent magnet 161 on the measuring tube 4.

[0041]

Over the current flowing in the coil 162, the amplitude and the

Frequency of the bending vibration of the measuring tube 4, which runs in the drawing plane control.

In the drawing plane, the Coriolis forces occur, which cause that no longer all points along the measuring tube 4 oscillate in phase.

The oscillation movement of the measuring tube 4 is recorded by means of two vibration sensors 17 and 18, which are likewise arranged symmetrically to the vibration exciter 16, likewise on the supporting tube 15. In the vibra tion sensors 17 and 18, it may be z. B. are electromagnetic transducers that are similar to the arrangement permanent magnet coil of the vibrator 16 are constructed.

The two permanent magnets 171, 181 are fixed to the measuring tube 4 and the two coils 172, 182 on the support tube 15. The movement of the measuring tube 4 causes via the magnets 171, 181, an induction voltage in the respective coil 172, 182, which is tapped as an analog sensor signal X17 and X18.

A Coriolis mass flowmeter generally consists of a measuring transducer and an associated measuring and operating circuit.

FIG. 2 shows a block diagram of such a measuring and operating circuit belonging to the measuring sensor 1, which, inter alia, carries out the evaluation of the two sensor signals and controls the oscillation excitation of the measuring tube 4.

The two sensor signals xl7 and xl8, which are received by the vibration sensors 17 and 18, are amplified in two preamplifiers VVl and VV2 and each digitized in an analog / digital converter AWl or AW2 in sensor signals X17, X18 and a digital Signal processor DSP supplied. The digital signal processor DSP supplies the measured value of the mass at a first output A1. flow m

, An output A2 supplies a signal which controls the excitation current I εrr for the oscillation excitation of the measuring tubes. As can be seen from the drawing, the two sensor signals X17 and X18 are supplied to the digital signal processor DSP via two measuring channels K1 and K2.

FIG. 3 shows a block diagram of a measuring and operating circuit according to the invention, with three measuring channels K1, K2, K3 in more detail. This measuring and operating circuit substantially corresponds to the circuit shown in FIG. The two sensor signals X17 and X18 are supplied from the two coils 172 and 182, respectively. These two sensor signals are amplified in two preamplifiers VVl and VV2, respectively, and fed to three switches SRI, SR2 and SR3 in each case. Each of these switches is assigned to a measuring channel K1, K2, K3. In each of the measuring channels Kl, K2 and K3 is in addition to the switches SRI, SR2, SR3 each have an amplifier Vl, V2, V3 with downstream analog-to-digital converter AWl, AW2, AW3. The outputs of the three analog-to-digital converters AW1, AW2, AW3 provide signals, each with one, two or three superscripts ', "or"', to a digital signal processor DSP. The digital signal processor DSP calculates the mass flow m

, which is output at the output Al, from the frequency of the Meßrohrschwingung f and the respective phase shift between two input signals z. The switches SR1, SR2, SR3 are controlled by three control signals S1, S2, S3 supplied by the digital signal processor.

An output A2 supplies a signal which controls the exciting current I for the oscillation excitation of the measuring tubes.

The table (FIG. 4) explains below how the two sensor signals X 17, X18 are switched to the three channels K1, K2 and K3. In a first step, the sensor signal X 17 is switched to the channel Kl and the sensor signal Xl 8 to the two channels K2 and K3. In the first cycle step, the phase error Δφ 23 = ς> 2 ς> 3 between the channels K2 and K3 can be determined. In cycle step 2 there is no new assignment of the signals. In the cycle step 3, the signal Xl 8 is switched to the channel and the channel 2. The signal Xl 8 remains on the channel 3. In this cycle step, the phase error Δφ 12 = φl-φ2 between the channels Kl and K2 can be determined. In cycle step 4 again no change of the channel assignment takes place. In the cycle step 5, the sensor signal X18 is switched to the channel K1 and the channel K3. The sensor signal Xl 8 remains on the channel K2. In this cycle step, the phase error Δ 13 = φ 1 -φ 3 between the two channels K 1 and K3 are determined. Other channel assignments are conceivable but not necessary, since all possible phase errors are already determined.

In the flowchart of FIG. 5, the method steps that are the basis of the first row of the table of FIG. 4, explicitly stated.

In the digital signal processor DSP, the respective phase error Δφ of the two channels is measured and stored with identical signals. When determining the phase difference Δφ of the two sensor signals X 17 and Xl 8, the phase error Δφ, which is caused by the different measuring channels, is taken into account accordingly (see table in FIG. 4). When calculating the mass flow rate rh, the phase error is also taken into account. As a result, the measurement accuracy of a Coriolis mass flow meter can be significantly increased. With the inventive method also temporally fluctuating influences, eg. B. the Tem¬ temperature dependence of the phase error detected.

For the frequency measurement and the amplitude determination, it has proved to be very advantageous if a signal, in this case the signal X 18, permanently associated with a particular channel, here the channel K3.

The change of channel switching takes place approximately every 10 seconds. Typical values for the measuring raw vibration are 800 Hz. The measuring tube diameter is 12mm.

The invention is particularly suitable even if the individual switches SRI, SR2, SR3 cause a phase error.

Claims

claims

Method for determining the mass flow with the aid of a Coriolis

Mass flow meter, in which from the phase difference between two sensor signals Xl 7, Xl 8 of the mass flow is obtained rix, wherein for the two sensor signals X17, X18 three measuring channels Kl, K2 and K3 are provided, via the signals a digital signal processor DSP are fed and a switch SR1, SR2, SR3 are provided, which enable a switching of the sensor signals X17, X18 to individual measuring channels K1, K2 or K3, with the following method steps: switching of the first sensor signal X17 to the measuring channel K1 switching of the second sensor signal X1 8 to the measuring channels K2, K3 Determining the phase error Δφ 23 of the measuring channels K2, K3 by measuring the phase shift of the output signals Xl 8 'and Xl 8 "of these two measuring channels K2, K3 Saving the phase error Δφ 23 Measuring the phase shift Δφ between the off ¬ gang signals of the measuring channel pair Kl and K2 determination of Masse¬ flow rix from the phase shift Δφ under consideration one to one earlier

Time gained phase error Δ 23 of the measuring channel pair Kl, K2. The method of claim 1, wherein the sensor signals Xl 7, Xl 8 cyclically to the

Measuring channels Kl, K2, K3 are switched. A method according to claim 2, characterized in that the measuring channel K3 is permanently provided for the measuring signal Xl 8. A method according to claim 3, characterized in that only every second

Cycle step takes place a measurement of the phase error Δφ two measurement channels. Apparatus for carrying out the method according to one of the preceding

Claims.