JP2003130704A - Coriolis mass flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a Coriolis mass flowmeter that an filter even when the sensor frequency suddenly varies by selecting a filter coincided with a sensor frequency. SOLUTION: The Coriolis flowmeter using a pair of Coriolis signals from measurement tubes on the upstream and downstream, is composed of a pair of filters having the same phase that filter each of Coriolis signals by the same phase to individually output a first and a second filter signals, a pair of filters having different phases that convert each of the signals to different phases to output them as a third and fourth filter signals, a pair of means for computing phases that compute the ratio of the first to the third filter signal and the ratio of the second to the fourth filter signal to compute the first phase and the second phase from each ratio, a means for computing the difference between phases that computes the difference between the first phase and the second phase to output the signal of the phase difference corresponding to the mass flow rate, and a means for changing a filter that changes the frequency bands of the same phase filter and the different phase filter corresponding to the frequency of the Coriolis signal.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a pair of Coriolis signals obtained upstream and downstream of a measurement tube by causing a fluid to be measured to flow in a measurement tube whose both ends are fixed and vibrating the measurement tube in a predetermined mode by an exciter. The present invention relates to a Coriolis mass flow meter for measuring mass flow rate.

[0002]

2. Description of the Related Art FIG. 3 is a configuration diagram showing a configuration of a sensor portion of a known Coriolis mass flowmeter. FIG. 4 is a waveform diagram for explaining the operation of the sensor unit of the Coriolis mass flowmeter shown in FIG. FIG. 5 is a configuration diagram showing a configuration of a conversion unit that calculates a mass flow rate in combination with the sensor unit shown in FIG. 3, and is disclosed by the applicant in Japanese Patent Application Laid-Open No. 7-181069.

A conventional Coriolis mass flowmeter will be described below with reference to FIGS. The measuring tube in this case may be another method such as a U-tube method, but for simplicity, a straight tube method will be described below.

Reference numeral 1 is a measuring tube through which a fluid to be measured flows, and both ends of the measuring tube 1 are fixed to supporting members 2 and 3. A vibrating device 4 for mechanically vibrating the measuring tube 1 up and down is installed near the center of the measuring tube 1.

Then, the supporting members 2, 3 of the measuring tube 1
The upstream sensor 5A and the downstream sensor 5B that detect the vibration of the measuring tube 1 are fixed near the fixed position. A temperature sensor 6 used for temperature compensation is provided near the support member 3. The sensor unit SNS is configured as described above.

From the shaker 4 to the measuring tube 1 shown in FIG.
When the fluid to be measured flows through the measurement tube 1 in a state where vibration is applied in the shape of the primary mode as shown by M1 and M2, the measurement tube 1 becomes a shape of the secondary mode as shown by M3 and M4. Vibrate.

Actually, the measuring tube 1 vibrates in a form in which these two types of vibration patterns are superimposed. This deformation of the measuring tube 1 is detected by the sensors 5A and 5B and the displacement signal S
A and SB are sent to the conversion unit TR1 shown in FIG.

Next, the conversion unit TR for processing the signal output from the sensor unit SNS will be described. The clock signal oscillator 17 has a timing signal TC having a predetermined sampling period regardless of the vibration of the measuring tube 1.
To generate.

On the other hand, the displacement signal SA is, for example, A · sin.
The displacement signal SA is sequentially sampled / held by the timing signal TC that determines the sampling time, and is output to the truck-and-hold (T & H) circuit 18 in the form of (ωt0). Where A is the amplitude, ω is the angular frequency,
t0 indicates an arbitrary time point.

The held displacement signal SA is analog /
It is output to the digital converter 19, where it is sequentially converted into the digital signal DA2 and output to the low-pass filter (LPF) 20 which is processed in a digital format.

A low-pass filter (LPF) 20 removes frequency components higher than the vicinity of the vibration frequency of the measuring tube, and a FIR (Finite) which is a kind of digital filter.
Impulse Response Filter 21
It is output to A as a digital signal DA3.

The FIR filter 21A is an in-phase digital filter for converting an input signal into an output signal having the same phase as the input signal. The FIR filter 21A basically outputs a digital signal DA4 of the form A · sin (ωt0).

The digital signal DA is also output to the FIR filter 21B which is a kind of digital filter. This FIR filter 21B has an input signal of 90 °.
It is a different-phase digital filter for converting output signals of different phases, and basically outputs a digital signal DA5 of the form A · cos (ωt0). The FIR filter 21A and the FIR filter 21B constitute the Hilbert transformer 21.

The phase calculator 23 uses the digital signal DA4.
Of digital signal DA5 and [A · sin (ωt0)
/ A · cos (ωt0) = tan (ωt0)], and tan ⁻¹ is calculated to calculate the phase signal θA2 (= ωt0).

The displacement signal SB is, for example, B · sin.
In the form of (ωt0 + ΔΦ)
H) The displacement signal SB is output to the circuit 24, and the displacement signal SB is sequentially sampled / held by the timing signal TC that determines the sampling time. Here, B indicates the amplitude, and ΔΦ indicates the phase difference with respect to the displacement signal SA at time t0.

The held displacement signal SB is analog /
The signal is output to the digital converter 25, where it is sequentially converted into the digital signal DB2 and output to the low pass filter (LPF) 26 which is processed in a digital format. This low-pass filter 26 has the same configuration as the low-pass filter 20, and gain characteristics and group delay characteristics are also selected in common.

Like the low-pass filter 20, the low-pass filter 26 removes a frequency component higher than the vicinity of the vibration frequency of the measuring tube and outputs it as a digital signal DB3 to an FIR filter 27 which is one type of digital filter.

Similar to the FIR filter 21A, the FIR filter 27A is an in-phase digital filter for converting an input signal into an output signal of the same phase, and its output end is basically a digital signal of the form B · sin (ωt0 + ΔΦ). The signal DB4 is output.

Further, the digital signal DB3 is output to the FIR filter 27B which is one type of digital filter. The FIR filter 27B is an out-of-phase digital filter that converts an input signal into an output signal having a phase different by 90 °, and basically outputs a digital signal DB5 in the form of B · cos (ωt0 + ΔΦ). And these FIs
The Hilbert transformer 27 is configured by the R filter 27A and the FIR filter 27B.

The phase calculator 29 uses the digital signal DB4
To the digital signal DB5 [B · sin (ωt0
+ ΔΦ) / B · cos (ωt0 + ΔΦ) = tan (ωt0 +
ΔΦ)] is calculated, and its tan ⁻¹ is calculated to obtain the phase signal θ.
Calculate A2 = (ωt0 + ΔΦ).

The phase difference calculation circuit 30 calculates the difference (= ΔΦ) between the phase signal θA2 sequentially output from the phase calculator 23 and the phase signal θB2 sequentially output from the phase calculator 29 to calculate the phase difference signal θ2. Are sequentially output as. This phase difference signal θ2 is proportional to the mass flow rate of the fluid to be measured.

The time delay element 32 outputs the phase signal θA2 (= ωt0) output from the phase calculator 23 to the sampling period T.
It is delayed by C and then output. Therefore, at time t0, the phase signal θA2 ′ (= ωt
₋₁ and t ₋₁ are immediately previous sampling points) and are output to the frequency calculator 33.

The frequency calculator 33 receives these phase signals θA.
The operation of dividing the difference between 2 and θA2 ′ by 2πT [(ωt0
−ωt ₋₁ ) / 2πT = fC] is performed to obtain the sensor frequency fC at the time point t0. The averaging circuit 34 outputs this as an average excitation frequency fC ′ of the sensor frequency fC obtained at a large number of sampling points.

Further, the displacement signal SA is input to the excitation circuit 35, an excitation voltage corresponding to the displacement signal SA is output to the exciter 4, and the exciter 4 is driven in, for example, a sine wave shape. On the other hand, the temperature signal ST1 is output from the temperature sensor 6 to the trunk-and-hold circuit 37, and the many temperature signals held by the timing signal TC that determines the sampling time point are converted into digital signals by the analog / digital converter 38. It is converted and output to the averaging circuit 39, where it is averaged and output as the temperature signal ST2.

The density calculator 40 receives the vibration frequency fC 'and the temperature signal ST2, and calculates the density of the fluid to be measured based on the following equation. At the reference temperature, the resonance frequency when the measured fluid is filled in the measurement tube 1 is fV, and the resonance frequency when the measurement tube 1 is empty is fV.
When 0, K1 and K2 are constants, the density signal D is obtained as fV = fC ′ + K1 · ST2 (1) D = K2 (f0 ² −fV ² ) / fV ² (2)

The mass flow rate calculator 41 receives the density signal D, the sensor frequency fC ′, the phase difference signal θ2 (= ΔΦ), and the temperature signal ST2, and calculates the mass flow rate QM based on the following equation. QM = f (ST2) · f (D) · tan θ2 / fC ′ (3) where f (ST2) is a temperature correction term and f (D) is a density correction term.

[0027]

[Problems to be Solved by the Invention] Hilbert transformer 2
Regarding the configuration of the FIR filter forming 1, 27,
Since it is described in detail in JP-A-7-181069, the description thereof will be omitted. FIR filters 21A and 21B
Is a bandpass filter characteristic having a frequency band that allows only the vicinity of the vibration frequency of the measurement tube 1 to pass, but the group delay characteristic in the passing frequency band is 90 degrees in phase in the FIR filter 21B than in the FIR filter 21A. The coefficient of each FIR filter is set so that This relationship also applies to the relationship between the FIR filter 27A and the FIR filter 27B.

When the frequency band of the FIR filter is fixed to a bandpass characteristic that allows passage only near the vibration frequency of the measuring tube 1, a sharp change in the frequency of the sensor signal due to a sudden change in the measuring fluid, When a sensor signal deviating from a preset frequency band is input due to a change in frequency due to replacement, the frequency band of the FIR filter becomes unsuitable, and accurate measurement becomes difficult. That is, there is a problem that the versatility of the conversion unit TR is low.

A first object of the present invention is to perform digital filtering by selecting in-phase and phase-shifting digital filters in a frequency band that matches the frequency of the sensor signal even when the frequency of the sensor signal suddenly changes. It is to realize a Coriolis mass flowmeter capable of

A second object of the present invention is to perform digital filtering by selecting in-phase and phase-shifting digital filters in a frequency band matching the frequency of the sensor signal, even when noise is mixed in the sensor signal, etc. It is an object to realize a Coriolis mass flowmeter equipped with means for selecting in-phase and phase-shifting digital filters in a frequency band that matches the frequencies of the upstream sensor signal and the downstream sensor signal without error.

[0031]

The present invention which achieves the above objects is as follows. (1) In a Coriolis mass flow meter that measures a mass flow rate using a pair of Coriolis signals obtained by an upstream sensor and a downstream sensor installed on the upstream and downstream sides of a measurement tube, an analog that converts each Coriolis signal into each digital signal / Digital converter, a pair of in-phase digital filter means for filtering the respective digital signals in the same phase and outputting as first and second filter signals respectively, and a third phase for converting the respective digital signals into different phases. Fourth
A pair of out-of-phase digital filter means for outputting as a filter signal, and a ratio of these first / third filter signals and a ratio of the second / fourth filter signals are calculated to obtain the first / second phase from each ratio. A pair of phase calculating means for calculating; a phase difference calculating means for calculating a difference between the first phase and the second phase and outputting a phase difference signal corresponding to the mass flow rate;
A Coriolis mass flowmeter, comprising: a filter changing unit that changes the frequency bands of the in-phase digital filter unit and the different-phase digital filter unit according to the frequency of the Coriolis signal. (2) The in-phase digital filter means and the out-of-phase digital filter means are composed of a plurality of sets each having a different frequency band, and the filter changing means selects an optimum set according to the frequency of the Coriolis signal. The Coriolis mass flowmeter according to (1), which is characterized. (3) The Coriolis mass flowmeter according to (1) or (2), wherein the filter changing means is controlled by the frequency of the Coriolis signal derived from the output of the phase calculating means. (4) The Coriolis mass flowmeter according to (1) or (2), wherein the filter changing means is controlled by the frequency of the Coriolis signal obtained by the upstream sensor or the downstream sensor. (5) Amplitude measuring means for monitoring the output amplitude of the in-phase digital filter means or the different-phase digital filter means is provided, and based on the output of the amplitude measuring means, the Coriolis signal of the Coriolis signal derived from the output of the phase calculating means is provided. The Coriolis mass flowmeter according to (1) or (2), characterized in that the filter changing means is controlled by selecting any of a frequency, a frequency of the Coriolis signal obtained by the upstream sensor, and a frequency of the Coriolis signal obtained by the downstream sensor. (6) The Coriolis mass flowmeter according to any one of (1) to (5), characterized in that a finite impulse response filter is used as the in-phase digital filter means or the different-phase digital filter means. (7) The Coriolis mass flowmeter according to any one of (1) to (5), wherein the finite impulse response filter has a bandpass characteristic. (8) A pair of low-pass filters is provided in the subsequent stage of the analog / digital converter, and each output is used as each digital signal (1).
The Coriolis mass flowmeter according to any one of (5). (9) In relation to the vibration frequency of the measuring tube obtained by dividing the phase difference measured at a predetermined time difference in the first or second phase by this time difference, and the temperature of the fluid to be measured. The Coriolis mass flowmeter according to any one of (1) to (5), wherein the frequency correction and the temperature correction are performed on the phase difference signal using the temperature signal thus generated.

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a functional block diagram showing a main part of a converter TR of a Coriolis mass flowmeter to which the present invention is applied. The same elements as those described with reference to FIG. 5 are designated by the same reference numerals, and the description thereof will be omitted. The block diagram of FIG. 1 shows the main part of the Coriolis mass flowmeter according to the present invention, and the blocks other than this main part are exactly the same as the conventional Coriolis mass flowmeter described in FIG. The description will be omitted because it is redundant.

In the Hilbert transformers 21 and 27,
The FIR filters 1 and 2 (21A) and the FIR filters 1 and 2 (27A) are a plurality of in-phase digital filter groups having different frequency bands (two FIR filters 1 and 2 in the minimum configuration).

Similarly, in the Hilbert transformers 21 and 27, the FIR filter (21B) and the FIR filter (27B) are composed of a plurality of 90 ° phase shift digital filter groups (in the minimum configuration, FIR filter 1 and FIR filter 2). Of two).

Reference numerals 42 and 42 'are input-side and output-side change-over switches for selecting one of the plurality of in-phase digital filter groups 21A (FIR filter 1 and FIR filter 2 in the minimum configuration), similarly 43 and 43'. Is a plurality of in-phase digital filter groups 27A (FI in the minimum configuration
These are input side and output side changeover switches for selecting one of the R filter 1 and the FIR filter 2).

44 and 44 'are input-side and output-side change-over switches for selecting one of the plurality of 90 ° phase digital filter groups 21B (FIR filter 1 and FIR filter 2 in the minimum configuration), similarly 45 and 45 '
Are input-side and output-side selector switches that select one of the plurality of 90 ° phase shift digital filter groups 27B (the FIR filter 1 and the FIR filter 2 in the minimum configuration).

These changeover switches are operated by the operation signal S1.
And operates in tandem with the in-phase digital filter group 21.
A, 27A and 90 ° phase shift digital filter group 21
A set of digital filters that matches the frequency of the upstream sensor signal is selected from B and 27B.

Reference numeral 46 is a filter changing means for comparing the frequency fC 'obtained by the averaging circuit 34 with the frequency value of the frequency band of the digital filter by software to select the digital filter whose frequency fc' is within the band. The operation signal S1 of is output.

In the case of the hardware configuration, the frequency fC 'is converted into a voltage proportional to the frequency fC', and the voltage is similarly compared with a voltage proportional to the frequency of the frequency band of the digital filter by a comparator to obtain a digital filter. The operation signal S1 for selecting may be output.

On the other hand, when the present invention shown in FIG. 1 is applied, if the noise component is superimposed on the signals of the upstream sensor and the downstream sensor and the frequency fc 'outputs the frequency of the noise component, the filter The changing unit 46 may select a digital filter corresponding to the frequency of the noise component. Even if the noise component disappears, the signal from the upstream sensor or the like is attenuated by the digital filter selected at this time and normal measurement cannot be performed.

When the frequency of the signal of the upstream sensor and the signal of the downstream sensor suddenly changes due to the change of the fluid type (including the empty state), the selected digital filter is for the frequency before the change, so The signal is attenuated and normal measurement cannot be performed.

The embodiment of FIG. 2 is characterized by the addition of a function capable of selecting a digital filter that matches the frequency of an upstream signal or the like so that normal measurement can be performed even when noise is mixed in or the signal frequency suddenly changes. And

Reference numeral 47 is a frequency measuring means for directly inputting the signal SA of the upstream sensor, which includes means for removing noise (for example, noise removal by an analog filter or a comparator having a hysteresis characteristic). The frequency of the signal is obtained and output to the filter changing means 48.

The filter changing means 48 has the same function as the filter changing means 46 described in FIG. 1, and compares the frequency fa of the signal SA of the upstream sensor with the frequency value of the frequency band of the digital filter to obtain the frequency fa. Outputs an operation signal S2 for selecting a digital filter whose band is within the band.

Reference numeral 49 is an operation signal changeover switch for switching between the operation signal S1 and the operation signal S2, and the switching operation is executed by the operation output S3 of the amplitude measuring means 50.

The amplitude measuring means 50 has a function of monitoring the amplitude of the digital filter (for example, 21A, 21B) output, and outputs the operation signal S3 when the attenuation of the amplitude becomes less than a predetermined level. The switching operation signal of the digital filter is switched from S1 to S2.

If the digital filter selects a band in which the frequency of the noise component matches, the frequency component of the signal of the upstream sensor is attenuated and its amplitude becomes small after the noise component disappears. Measuring means 5
It is possible to select a digital filter that matches the frequency fa of the signal of the upstream sensor by determining whether the amplitude of the output of the digital filter is less than or equal to a certain threshold value by 0.

As the amplitude measuring means 50, the output of the digital filter 21A is Asinωt and the output of 21B is A
Since it is cos ωt, it is also possible to use the square (= corresponding to the amplitude) value of the amplitude A obtained by the sum of the respective squares.

Even when the frequencies of the signals of the upstream sensor and the downstream sensor change rapidly due to changes in the fluid type (including the empty state), the amplitude of the signal of the upstream sensor after the frequency change remains constant as in the above case. Since it becomes less than the threshold value, the digital filter is selected by the operation signal S2 of the filter changing means 48, and the signal f of the upstream sensor after the frequency change is selected.
A digital filter matching a can be selected.

In the embodiment described above, the number of digital filters to be selected is shown to be two for simplification. However, in the concrete design, a plurality of digital filters having different frequency bands are prepared and selected. Is possible. Further, the filter can be changed by constructing a table of coefficient parameters that determine the characteristics of the FIR filter and selecting a predetermined parameter from the table by software.

In FIG. 1, the frequency fC 'can be output from the downstream sensor side to realize the filter selection.
Also, in FIG. 2, the frequency fC ′ is output from the downstream sensor side, and the filter selection can be similarly realized even if the frequency measuring means 47, the filter changing means 46, 48, and the amplitude measuring means 50 are provided on the downstream sensor side. You can

Although the embodiments shown in FIGS. 1 and 2 have been described as being realized by using discrete circuit elements, the signal processing of the analog / digital converters 19, 25 and 38 and thereafter is not limited to the micro. The functions in each block can be sequentially executed by software arithmetic processing using a processor, a memory, etc., and a mass flow rate signal can be obtained.

[0053]

As is apparent from the above description,
According to the present invention, for a sensor group having a wide frequency band depending on the diameter of the sensor and the fluid density, the in-phase and phase-shifting digital filters in the (= optimal) frequency band matching the frequency of the sensor signal are selected and digitalized. It becomes possible to perform filtering, and it is possible to realize the Coriolis mass flow rate with general versatility in the conversion unit.

Further, even when noise is mixed in the sensor signal or the like, it is possible to realize a Coriolis mass flow rate capable of obtaining a normal output by quickly selecting a digital filter that matches the sensor signal frequency after the noise disappears. You can

Further, even when the frequency of the sensor signal changes abruptly due to the inclusion of bubbles or the empty state from full water, it is necessary to promptly select a digital filter that matches the sensor signal frequency and obtain a normal output. It is possible to realize a Coriolis mass flow rate capable of

[Brief description of drawings]

FIG. 1 is a functional block diagram showing a main part of a conversion unit TR of a Coriolis mass flowmeter to which the present invention is applied.

FIG. 2 is a functional block diagram of a main part of a conversion unit TR showing another embodiment of the present invention.

FIG. 3 is a configuration diagram showing a configuration of a sensor unit of a known Coriolis mass flowmeter.

FIG. 4 is a waveform diagram illustrating an operation of a sensor unit of the Coriolis mass flowmeter shown in FIG.

5 is a configuration diagram showing a configuration of a conversion unit that calculates a mass flow rate in combination with the sensor unit shown in FIG.

[Explanation of symbols]

20, 26 Low pass filter 21, 27 Hilbert transformer 21A, 27A in-phase digital filter 21B, 27B 90 ° out-of-phase digital filter 23, 29 Phase calculator 34 Averaging circuit 42-45 'changeover switch 46 Filter changing means

Claims (9)

[Claims] 1. A Coriolis mass flowmeter for measuring a mass flow rate using a pair of Coriolis signals obtained by an upstream sensor and a downstream sensor installed upstream and downstream of a measuring tube, wherein each Coriolis signal is converted into each digital signal. An analog-to-digital converter, a pair of in-phase digital filter means for filtering the respective digital signals with the same phase and outputting them as the first and second filter signals, respectively, and converting the respective digital signals into different phases. A pair of out-of-phase digital filter means for outputting as the third and fourth filter signals, and a ratio of the first and third filter signals and a ratio of the second and fourth filter signals are calculated to calculate the first and second ratios from the respective ratios. A pair of phase calculation means for calculating a second phase, and a difference between the first phase and the second phase for calculating the mass flow rate. Phase difference calculating means for outputting a phase difference signal, and filter changing means for changing the frequency band of the in-phase digital filter means and the different-phase digital filter means according to the frequency of the Coriolis signal. Coriolis mass flow meter. 2. The in-phase digital filter means and the out-of-phase digital filter means are composed of a plurality of sets each having a different frequency band, and the filter changing means selects an optimum set according to the frequency of the Coriolis signal. The Coriolis mass flowmeter according to claim 1, characterized in that 3. The Coriolis mass flowmeter according to claim 1, wherein the filter changing means is controlled by the frequency of the Coriolis signal derived from the output of the phase calculating means. 4. The Coriolis mass flowmeter according to claim 1, wherein the filter changing means is controlled by the frequency of the Coriolis signal obtained by the upstream sensor or the downstream sensor. 5. Coriolis derived from the output of the phase calculating means based on the output of the amplitude measuring means, comprising amplitude measuring means for monitoring the output amplitude of the in-phase digital filter means or the out-of-phase digital filter means. 3. The Coriolis mass flowmeter according to claim 1, wherein the filter changing means is controlled by selecting either the frequency of the signal or the frequency of the Coriolis signal obtained by the upstream sensor or the downstream sensor. 6. A Coriolis mass flowmeter according to claim 1, wherein a finite impulse response filter is used as said in-phase digital filter means or said different-phase digital filter means. 7. The Coriolis mass flowmeter according to claim 1, wherein the finite impulse response filter has a bandpass characteristic. 8. The Coriolis unit according to claim 1, wherein a pair of low-pass filters are provided at the subsequent stage of the analog / digital converter, and the respective outputs are used as the respective digital signals. Mass flow meter. 9. A vibration frequency of the measuring tube obtained by dividing a phase difference measured at a predetermined time difference in the first or second phase by the time difference and a temperature related to the fluid to be measured. 6. The Coriolis mass flowmeter according to claim 1, wherein the frequency correction and the temperature correction are performed on the phase difference signal using the temperature signal measured by the above.