JPH0410011B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a mass flow meter that utilizes the Coriolis force.

FIG. 1 is a configuration explanatory diagram for explaining the operating principle of a Coriolis flowmeter. Reference numeral 1 denotes a U-shaped tube through which a measuring fluid flows, a permanent magnet 2 is fixed to the center of the tip thereof, and both ends of the U-shaped tube 1 are fixed to a base 3. Reference numeral 4 denotes an electromagnetic drive/detection coil installed opposite to the U-shaped tube 1, 5 is a support beam that supports the electromagnetic drive/detection coil at its tip, and the other end is fixed to the base 3. The U-shaped tube 1 and the support beam 5 mutually form a tuning fork structure. That is, the U-shaped tube 1 and the beam 5 vibrate opposite each other, just like the teeth of a tuning fork vibrate, and like a tuning fork, the base 3 serves as a node of vibration, so that less vibrational energy is lost. It's summery. 6
1 and 62 are displacement detectors for detecting the displacement of both legs of the U-shaped tube 1.

The electromagnetic force acting between the driving coil 4 and the permanent magnet 2 fixed to the opposing U-shaped tube 1 causes the U-shaped tube 1 to move.
When oscillates at its natural frequency (longitudinal vibration (symmetrical flexural vibration): M ₁ , M ₂ , M ₃ in Fig. 2 A shows the pattern at each moment), Coriolis occurs in the fluid flowing inside the U-shaped tube 1. The force of is generated. The magnitude of this Coriolis force is proportional to the mass and velocity of the fluid flowing inside the U-shaped tube 1, and the direction of the force is proportional to the direction of fluid movement and the U-shaped tube 1.
coincides with the direction of the vector product of the angular velocities that excite. Further, since the direction of the fluid is reversed on the input side and the output side of the U-shaped tube 1, twisting (asymmetrical deflection) torque is generated in the U-shaped tube 1 due to the Coriolis force on both legs. This torque varies with the same frequency as the excitation frequency, and its amplitude value is proportional to the mass flow rate of the fluid. FIG. 2B shows the vibration mode (Coriolis vibration mode) caused by this torsional torque, and M ₄ , M ₅ , and M ₆ show the vibration patterns at each moment. Therefore, if the amplitude of this torsional vibration (asymmetrical flexural vibration) torque is detected by the displacement detectors 61 and 62 in the form of a pulse width, for example,
You can know the mass flow rate.

A mass flow meter using the above-mentioned principle has been known for some time (for example, Japanese Patent Application Laid-open No. 52570/1983), but this method has the following problems. That is, when determining the mass flow rate from the phase difference between the output signals from the displacement detectors 61 and 62, the phase and gain of the amplifier, the threshold of the comparator, the trigger level of the counter, etc. change depending on the temperature, etc. There is a tendency for output to drift.

The present invention was made to eliminate the above-mentioned drawbacks, and an object of the present invention is to realize a vibrating mass flowmeter that can provide a stable output without circuit drift.

In measuring the time difference between two detection signals of a Coriolis flowmeter, the present invention switches from measurement mode to calibration mode in a time-division manner, applies the same signal to two circuits in this calibration mode, and compares the outputs. However, by performing calibration, a stable time difference output can be obtained.

The present invention will be explained below based on the drawings.

FIG. 3 is a structural plan view showing the main parts of the device according to the present invention. The figure shows the case of a straight pipe Coriolis flowmeter, where 1 is a vibrating pipe, 3 is a base that fixes the vibrating pipe 1 at both ends, and 61 and 62 are detectors that detect the vibration of the vibrating pipe 1. For example, Piezoelectric elements and the like are used. 8 is an amplifier that amplifies the detection output e ₁ from the detector 61, and 10 is an amplifier that converts the output of the amplifier 8 into a binary signal e ₃
It is a comparator that converts to . 7 is the detector 61, 62
9 is an amplifier that amplifies the output from _the selector switch ₇ ; 11 is an amplifier 9;
This is a comparator that converts the output from _e4 into a binary signal e4. Reference numeral 12 denotes an arithmetic circuit which inputs the outputs from the comparators 10 and 11, calculates the time difference between them, and outputs a switching signal to the changeover switch 7. The arithmetic circuit can be composed of known circuits such as counters and controllers.

FIG. 4 shows a vibration pattern exhibited by the straight pipe Coriolis flowmeter. M ₇ and M ₈ are symmetrical deflection vibration patterns that the vibrating pipe 1 exhibits at each moment when excited by the driving means, and M ₉ and M ₁₀ are asymmetric deflection patterns that are caused by the Coriolis force acting on the fluid flowing in the straight pipe. It is a pattern of vibration. In reality, the vibration pipe 1 vibrates in a manner in which these two types of vibration patterns are superimposed.

FIG. 5 is a time chart of each signal shown in FIG. 3, where FIG. 5A shows the measurement mode and FIG. 5B shows the calibration mode. When the vibrating pipe 1 is performing asymmetrical flexural vibration as shown in FIG. 4, the detector outputs e ₁ and e ₂ vibrate with a time difference between them. In the measurement mode, the changeover switch 7 is in the state (solid line), so the output of the detector 62 is
e ₂ becomes the input of the amplifier 9, and the time difference Δt _n between both detection signals e ₁ and e ₂ is obtained by the arithmetic circuit 12. Figure 5A
As shown in , when the threshold value L ₂ of the comparator 11 deviates from the threshold value L ₁ of the comparator 10 to a value different from L ₁ due to drift, the error caused by this deviation becomes the time difference. Included in Δt _n . When the changeover switch 7 is switched to the state shown by the dotted line, the calibration mode is entered, and the output _e1 of the detector 61 becomes a common input to the amplifiers 8 and 9. At this time, the arithmetic circuit 12 obtains a time difference Δt _c due to the difference between the threshold values of the comparators 10 and 11, as shown in FIG. 5B. Next, in the arithmetic circuit 12, Δt
= Δt _n − Δt _c , obtain the time difference Δt that does not include the error due to drift, and output the corresponding output.
It is set as _e5 .

The measurement mode and the calibration mode can be switched for each cycle of vibration, but they can also be averaged for every multiple cycles. When the drift speed is slow, the calibration mode may be performed at a reduced rate.

The above explanation deals with the case where there is a drift in the threshold value of the comparator, but the error will be eliminated in the same way even if the phase and gain of the amplifier, the trigger level of the counter, etc. change due to temperature, etc. .

Further, if the detector output level is sufficient, the amplifiers 8 and 9 shown in FIG. 3 can be omitted.

In addition, in the embodiment shown in Fig. 3, the calibration
Although the output of the detector is used as the input signal in the mode, a standard signal may be prepared separately and used in place of the output signal of the detector.

Although the embodiment shown in FIG. 3 shows a straight pipe type Coriolis flowmeter, the present invention can be similarly applied to a U-tube type Coriolis flowmeter as shown in FIG.

As described above, according to the present invention, it is possible to realize a vibrating mass flowmeter that is not affected by circuit drift and can provide stable output.

[Brief explanation of drawings]

Figure 1 is a diagram of the operating principle of a Coriolis flowmeter, Figure 2
Figures A and B are explanatory diagrams of the operation in Figure 1, Figure 3 is a plan view of the main part configuration of an embodiment of the present invention, Figure 4 is an explanatory diagram of the operation in Figure 3, and Figure 5 is the device shown in Figure 3. This is a time chart to explain the operation. 1... Pipe line, 61, 62... Detector, 7... Changeover switch, 10, 11... Comparator, 12... Arithmetic circuit.

Claims (1)

[Claims] 1. In a mass flowmeter configured to flow fluid into a vibrating pipe and cause the pipe to deform and vibrate due to the Coriolis force generated by the flow and the angular vibration of the pipe,
A comparator 10 receives the output of one of a pair of detectors for detecting asymmetrical deflection vibration of the pipe and converts it into a binary signal, and a comparator 10 receives two output signals from the pair of detectors and converts these two A changeover switch that selects one of the detection signals, and a comparator 11 that receives the output signal from this changeover switch and converts it into a binary signal.
Then, the outputs of the comparators 10 and 11 are used as inputs, and a switching signal is applied to the changeover switch, and the time difference between the output signals of the comparators 10 and 11 is measured at each switching position of the changeover switch. A mass flowmeter characterized in that a calculation circuit is provided to calculate the difference between the two time differences, and a circuit drift component included in the flow rate output is removed.