JP3662900B2 - Filter circuit for phase difference signal detection of Coriolis meter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention uses a switched capacitor filter that tracks the drive frequency to significantly attenuate pipe vibration, cavitation, mechanical noise of the sensor itself, etc. The present invention relates to a phase difference signal detection filter circuit.
[0002]
[Prior art]
The Coriolis meter supports the measurement tube at two points, applies vibration around the support point to the supported measurement tube, and detects the Coriolis force acting on the fluid flowing through the measurement tube. A mass flow meter. When the fluid is moved while the central portion of the measurement tube is driven alternately in a direction perpendicular to the axis, a phase difference is generated between the inflow side and the outflow side of the measurement tube, centering on the central portion of the measurement tube. This phase difference is based on the Coriolis force and is a value proportional to the mass flow rate. The mass flow rate can be measured by detecting this phase difference.
[0003]
The detection of the phase difference is performed by measuring a time difference when the measurement tube at the detection position passes through the reference plane with a plane passing through the tube axis of the measurement tube in a stationary state of the measurement tube as a reference plane. Specifically, the time difference is obtained by shaping the signals of the displacement detectors of the measuring pipes attached at the symmetrical positions on the inflow side and the outflow side of the measuring pipes into square waves, and starting up each of the shaped square waves. It can be determined from the number of constant frequency clocks counted between the time differences.
[0004]
However, the phase difference signal of the measurement tube generated at a predetermined symmetrical position on the inflow side and the outflow side of the measurement tube due to the Coriolis force is an extremely small amount compared to the drive amplitude of the measurement tube. Therefore, the time difference detected in proportion to the phase difference signal is very small. Further, since the Coriolis flowmeter is mounted between the upstream and downstream pipes, it is susceptible to external vibration effects and noise effects due to the pipes. Therefore, in order to perform stable and accurate time difference measurement, it is necessary to remove the influence of external vibration and noise.
[0005]
Conventionally, the filter used in the Coriolis meter is an analog filter, and a low-pass filter having a fixed cutoff frequency has been used. Further, the frequency to be removed by the analog filter is usually designed to be separated from the cut-off frequency by 100 times or more. This is because a sufficient attenuation rate is obtained and the maximum phase rotation is involved at the cutoff frequency. In the case of the Coriolis type mass flow meter, since a minute time difference is measured, the fact that the phase difference between the left and right sensor signals is caused by the filter is practically not suitable for measurement. Therefore, the cutoff frequency is set to a frequency sufficiently separated from the actual drive frequency. This can reduce the phase rotation and reduce the high frequency components.
[0006]
However, there are frequency components that cannot be removed by these filters. It is an Nth order harmonic or twist frequency very close to the drive frequency. The frequency of the sensor's natural frequency and its N-order harmonics (2nd, 3rd, 5th, 7th) and noise components such as twist are so close that it cannot be removed with a normal analog filter. It is. As described above, a filter with a cutoff frequency that is 100 times the drive frequency attenuates at -6 dB / Oct in the first order, so the second harmonic of the drive frequency can be attenuated only by -6 dB. It is also a story that the drive frequency that is the fundamental wave is also attenuated at the same time.
[0007]
These harmonics and twist frequencies are extremely disturbing frequency components in time difference measurement, and removing these components leads to improving the accuracy of the Coriolis mass flowmeter.
[0008]
[Problems to be solved by the invention]
The present invention can arbitrarily change the cut-off frequency, and automatically follows the frequency even if the drive frequency fluctuates due to changes in sensor diameter, fluid density, temperature, etc. An object of the present invention is to provide a phase difference detection circuit of a Coriolis meter that can change the frequency.
[0009]
Another object of the present invention is to cancel the phase difference by always making the phase rotation of the two filters always the same without causing a difference in the cut-off frequency between the left and right filters.
[0010]
In addition, the present invention makes it possible to measure by bringing a cutoff near the drive frequency, thereby eliminating vibrations caused by cavitation above the drive frequency and mechanical noise of the sensor itself. The purpose is to remove the frequency band that could not be made.
[0011]
[Means for Solving the Problems]
The filter circuit for detecting a phase difference signal of the Coriolis meter of the present invention is based on Coriolis force on the inflow side and the outflow side of the measurement tube with the center portion of the measurement tube being centered when the center portion of the measurement tube is driven. The phase difference that occurs is detected. The Coriolis meter has a pair of amplifier circuits provided corresponding to the left and right sensor signals to amplify the left and right sensor signals respectively detected by the left and right vibration detection sensors, and the waveform of one of the amplified sensor signals. A waveform shaping circuit for shaping the signal, a drive circuit for generating an output signal for driving one of the amplified sensor signals, and an output from the waveform shaping circuit as a clock signal, A phase-locked loop that multiplies and outputs the clock signal and a left-right sensor signal are provided correspondingly, and the output of the phase-locked loop is input as a clock, and each amplified left-right sensor signal is input. A pair of switched capacitor filters. The phase difference between the output signals from each of the pair of switched capacitor filters is measured as the mass flow rate flowing through the measuring tube.
[0012]
In the present invention, the cut-off frequency can be arbitrarily changed by using a switched capacitor filter (SCF) in which the clock frequency that is 1/100 (or 1/50) of the input frequency becomes the cut-off frequency. I can do it. The clock input to the SCF is multiplied by the frequency from the measurement tube drive signal using a PLL. As a result, even if the diameter of the sensor, the density of the measurement fluid, the temperature, and the like change, the cutoff frequency is automatically changed following the frequency.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described based on examples. FIG. 1 is a block diagram showing a first example of a phase difference detection circuit embodying the present invention. The drive device and the vibration detection sensor are each composed of a coil and a magnet. The left and right sensor signals respectively detected by the left and right vibration detection sensors are input to the amplification circuits. The output signal from the amplifier circuit is input to the corresponding switched capacitor filter (SCF), and the output signal of one amplifier circuit is driven to drive the driving device together with the waveform shaping circuit through the filter. Input to the circuit. Regarding waveform shaping, it is desirable to use a comparator with hysteresis or the like in order to prevent an erroneous frequency due to chattering or the like.
[0014]
The waveform-shaped signal is input to a phase-locked loop (PLL) as a clock. The multiplication factor of the PLL varies depending on the characteristics of the switched capacitor filter (SCF) used. For example, when using a 1: 100 filter, it requires at least 100 times (the cutoff frequency Fc is More than drive frequency.) In order to eliminate mechanical noise and harmonics, the cut-off frequency Fc should be sufficiently lower than twice the drive frequency, so the multiplication factor is 200 times or less of the drive frequency.
[0015]
If the SCF clock frequency becomes an obstacle due to the phase difference detection circuit method, it may be possible to remove the SCF clock using a lower clock by adding another SCF having a higher clock frequency. FIG. 2 is a block diagram showing a second example of a phase difference detection circuit embodying the present invention, and shows an example in which another SCF having a high clock frequency is added.
[0016]
The multiplication factor of SCF with a low clock frequency is, for example, 128 times. If the multiplication rate of the SCF having a high clock frequency is 50 times, for example, the low clock frequency is attenuated by about -30 dB. Therefore, with respect to the drive frequency f, the low clock frequency is 128f (cut-off frequency is 1.28f), and the high clock frequency is 50 × 128f = 6400f (cut-off frequency is 64f).
[0017]
If the drive frequency is 160 Hz and the cut-off frequency is 1.28 times the drive frequency, the cut-off frequency is 204.8 Hz. The second-order harmonic is 320 Hz, and the high-order filter based on this SCF is the fifth order. -59.18dB (about 1/1000) for the 3rd harmonic. It is possible to attenuate about -29 dB for the twist between the second order and the third order. The frequency due to cavitation exists between 1 and 5kHz, but 1kHz noise is -68.87dB, and 5kHz noise is -138.8dB, so it is almost unaffected.
[0018]
The drive frequency varies between 50Hz and 800Hz depending on the sensor type. FIG. 3 is a diagram showing the relationship between the drive frequency, the SCF clock frequency, and the cutoff frequency in the phase difference detection circuit having a two-stage SCF configuration as shown in FIG. In FIG. 3, the clock frequency and cut-off frequency of the first stage SCF are indicated by clk1 and fc1, respectively, and the second stage SCF are indicated by clk2 and fc2. Further, the second order, the third order, and the 3 kHz in FIG. 3 indicate the attenuation at the second harmonic, the third harmonic, and the 3 kHz frequency (cavitation frequency), respectively. Both have been shown to provide sufficient attenuation.
[0019]
In FIG. 1 or FIG. 2, the left and right sensor signals from which noise has been removed by the corresponding switched capacitor filters (SCF) are normally input to the phase difference measurement circuit. Then, the phase difference is measured and output to the output circuit, and is normally displayed as a mass flow rate on the display circuit.
[0020]
In the present invention, the cut-off frequency can be arbitrarily changed by using a switched capacitor filter (SCF) in which the clock frequency that is 1/100 (or 1/50) of the input frequency becomes the cut-off frequency. I can do it. The clock input to the SCF is multiplied by the frequency from the measurement tube drive signal using a PLL. As a result, even if the diameter of the sensor, the density of the measurement fluid, the temperature, or the like changes, the cutoff frequency is automatically changed following the frequency.
[0021]
Usually, the phase change becomes maximum near the cutoff frequency. Even if a constant is strictly matched with an analog filter, a difference in phase rotation occurs between the two filters due to temperature characteristics and frequency characteristics. A Coriolis type mass flow meter that measures mass flow by measuring a slight phase difference is taboo.
[0022]
However, even if a phase change occurs, if the filters used for the left and right sensor signals have the same phase rotation, the phase rotation difference is cancelled. Since the cutoff frequency of this filter is controlled by the same clock, there is no difference between the cutoff frequencies of the left and right filters, and the phase rotation of the two filters is always the same. This makes it possible to measure even if a cutoff is provided in the vicinity of the drive frequency, which has been impossible until now, for example, a frequency of 1.28 times (the clock frequency needs to be 128 times the drive frequency).
[0023]
As a result, vibrations (1 kHz to 5 kHz) due to cavitation above the drive frequency (for example, 160 Hz) and mechanical noise of the sensor itself (n times the drive frequency, twist frequency, etc.) could not be removed. It becomes possible to remove the frequency band, and no phase rotation difference is generated after filtering the left and right sensor signals.
[0024]
【The invention's effect】
According to the present invention, the cutoff frequency is fixed to the mechanical noise of the sensor itself and harmonics (nth harmonic, twist, etc.) that change according to the drive frequency by changing the cutoff frequency to the drive frequency. Therefore, it is possible to remove the 1 kHz to 5 kHz generated by cavitation as well as the harmonic frequency band in the vicinity of the drive frequency that could hardly be removed.
[0025]
The accuracy of the cut-off frequency depends on the accuracy of the clock frequency. By using the same clock frequency for the left and right filters, it is input to the subsequent time difference measurement block without causing a phase difference. It becomes. Thus, only the phase difference of the measurement tube vibration can be measured almost purely, and the mass flow rate can be measured with high accuracy.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a first example of a phase difference detection circuit embodying the present invention.
FIG. 2 is a block diagram showing a second example of a phase difference detection circuit embodying the present invention.
FIG. 3 is a diagram showing a relationship among a drive frequency, a clock frequency for SCF, and a cutoff frequency.

Claims (2)

For detecting the phase difference signal of a Coriolis meter that detects the phase difference that occurs based on the Coriolis force on the inflow side and the outflow side of the measurement tube around the center portion of the measurement tube when the center portion of the measurement tube is driven In the filter circuit,
A pair of amplifier circuits respectively provided corresponding to the left and right sensor signals in order to amplify the left and right sensor signals respectively detected by the left and right vibration detection sensors;
A waveform shaping circuit that shapes the waveform of one of the amplified sensor signals;
A driving circuit that receives one of the amplified sensor signals and generates an output signal that drives the driving device;
A phase-locked loop for inputting the output from the waveform shaping circuit as a clock signal, multiplying the clock signal and outputting it,
A pair of switched capacitor filters provided corresponding to the left and right sensor signals, respectively, wherein the output of the phase-locked loop is input as a clock, and each of the amplified left and right sensor signals is input;
A filter circuit for detecting a phase difference signal of a Coriolis meter, comprising: measuring a phase difference between output signals from each of the pair of switched capacitor filters. 2. The filter circuit for detecting a phase difference signal of a Coriolis meter according to claim 1, wherein another pair of switched capacitor filters is provided after the pair of switched capacitor filters.

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