JPH0477849B2 - - Google Patents

Description

[Detailed description of the invention] B Industrial application field This invention relates to an electromagnetic flowmeter.

B. Prior art An electromagnetic flowmeter using a rectangular wave excitation method is designed to maintain a stable magnetic flux so that it is not affected by the delay in the excitation current when the flow signal switches the excitation direction and the electromagnetic induction noise that occurs when the magnetic flux changes. The flow rate signal is sampled for a certain period of time. The excitation frequency was constant, and sampling was performed in synchronization with the excitation current for each excitation.

That is, as shown in FIG. 1A, the flow rate signal was sampled only during the sampling time Ts at the sampling period T in synchronization with the excitation current having the excitation period Tex as shown in FIG. 1B (T=Tex). In Figure B, the excitation current is simplified and shown as a rectangular wave, but in reality, as shown in Figure 2, there is a delay before it stabilizes after switching, and as shown in Figure 2 D, the excitation current is shown as a rectangular wave. Electromagnetic induction noise is superimposed on the signal. Therefore, sampling is performed for a time Ts after a dead time Te has elapsed from when the direction of the excitation current is switched until the magnetic flux becomes stable and the influence of noise can be ignored. The sampling period T should be set as small as possible considering the response of the electromagnetic flowmeter to flow rate fluctuations. However, there is a dead time Te due to the excitation current delay and electromagnetic induction noise mentioned above, so there is a limit to shortening the sampling period T.

By the way, when the fluid to be measured is, for example, a fluid pumped by a piston pump, periodic flow rate fluctuations (pulsations) occur in synchronization with the rotation of the pump. Furthermore, in a system in which the flow rate is controlled by controlling the pump rotation speed, the pump rotation speed changes over a wide range, so the period of the pulsation may coincide with the sampling period of the electromagnetic flowmeter. The flow rate in such a case is shown in FIG. 1C, where the flow rate Q is pulsating, and its average value is shown by a broken line.

Therefore, the magnetic flux due to the excitation current shown in Figure B and the flow rate signal obtained from the flow rate Q become as shown in Figure D. When this flow rate signal is synchronously sampled for a short period of time Ts using the sampling signal shown in FIG. 2A, a sampled flow rate signal as shown in FIG. In Figure 1, the sampling time ts happens to coincide with the period when the flow rate Q is small, so even if the sampled flow rate signal is smoothed, it will be a lower value than the average value of the flow rate, resulting in a negative sampling error. .

If the sampling timing is shifted by half the period T compared to the case shown in FIG. 1, the maximum value of the flow rate Q will be sampled, resulting in a positive sampling error.

If the pump rotation speed deviates slightly from the case shown in Figure 1, and the flow rate Q fluctuates at a slightly different pulsation frequency n from the sampling frequency s = 1/T (Hz), the sampled flow rate signal fluctuates slowly at |s−n| (Hz). In other words, the sampling error slowly fluctuates at |s−n| (Hz). Therefore, the signal obtained by smoothing the sampled flow rate signal also fluctuates slowly, which is inconvenient.

In the prior art, since the pulsating flow rate is sampled for a short period of time at a fixed period, there is a drawback that the sampling error becomes particularly large when the pulsating period and the sampling period are close to each other.

C. Purpose of the Invention The purpose of the present invention is to propose an electromagnetic flowmeter that can reduce the sampling error when measuring a pulsating flow rate.

D. Structure of the Invention The electromagnetic flowmeter of the present invention is a rectangular wave excitation type electromagnetic flowmeter that samples a flow signal during a stable period of the magnetic field, and the sampling period and the excitation period are changed once or every multiple times. It is characterized by causing

E. Example In Fig. 3, 1 and 1' are electrodes provided in the conduit 2, 3 is an excitation coil, 4 is an excitation switching circuit that alternately supplies excitation current to the excitation coil in opposite directions, and s.
1 is a preamplifier that amplifies the flow signal induced in the electrodes 1 and 1', 6 is a sampling switch, 7 is a synchronous rectification calculation circuit, 8 is a smoothing circuit, and 9 is a control circuit that connects the excitation switching circuit 4, sampling switch 6, and synchronous rectification calculation. An excitation switching signal 10, a sample signal 11, and a calculation signal 12 are output to each circuit to control the operation of the entire electromagnetic flowmeter.

The sampling and excitation periods can be switched by a control circuit 9.

Figure 4 shows the sampling period T and excitation period Tex.
FIG. 2 is a diagram illustrating an embodiment in which two values T ₁ and T ₂ are used alternately.

In FIG. 4, A is a sampling signal, which is sampled alternately at sampling periods _T1 and _T2 . The sampling time for each sampling is ts, and in this embodiment, the relationship is defined as T ₁ =4ts and _T2 =5ts. As shown in Figure B, the excitation current is determined by the excitation period.
T ₁ and T ₂ are used alternately, with the relationship T ₂ =T ₁ +tw, and the waiting time tw is set equal to the sampling time ts in this case. In this example, when the flow rate is pulsating with a period T ₁ as shown in Figure C, positions with different phases of the pulsation are sampled in order, and the same phase is sampled at the 8th sampling. I will do it.
Therefore, by smoothing the sampled flow rate signal with the smoothing circuit 8, an average flow rate without sampling error can be obtained.

C' in the same figure shows the flow rate when two periods of flow rate pulsation are equal to T ₁ + T ₂ , but in this case, sampling is performed at different phases alternately, so the maximum value of sampling error is reduced. Ru.

Although the flow rate pulsation period shown at C' in FIG. 4 has a specific relationship with the sum of the sampling periods T ₁ and T ₂ , it will be understood that even if the relationship is different, the sampling error will be reduced.

FIG. 4A' shows another embodiment of the sampling signal, but it is the same as in FIG. 4A in that the sampling periods T ₁ and T ₂ are used alternately. In this case, the sampling time is alternately ts and 2ts, and the sampling time is increased, so that the sampling error due to the influence of pulsation is further reduced.

By the way, in the embodiment shown in FIG. 4, an excitation period T ₂ in which the excitation current is switched after a waiting time tw is used, and an excitation period T ₁ in which the waiting time is not used are used to alternately excite and perform sampling. Regarding the time, T ₁ and T ₂ are also used alternately, but it is preferable to set the waiting time tw to be shorter than the excitation period T ₁ in which no waiting time is used.
This is because sampling at T ₂ when the waiting time tw is set to T ₁ or more is compared with sampling at T' ₂ = T ₂ - T ₁ , because there is a pulsation period that is not sampled, making it impossible to measure the flow rate with a quick response. be.

By the way, in the above embodiment, the sampling period or excitation period has two values T ₁ and T ₂ , and the periods T ₁ and T ₂ are used alternately as time passes.
Using three periods T ₁ , T ₂ , T ₃ , T ₁ ~ T ₂ ~ _{T 3}
The sampling period and the excitation period may be controlled to be changed in the order of , or in the order of T ₃ to T ₂ to T ₁ .

Although various patterns can be used as such combinations of cycles, typical ones will be described below.

(a) T ₂ ~ _{T 1} ~T ₂ ~T ₁ ~T ₁ ~T ₂ ~T ₁ ~T ₁ ~ _{T 1} ~
T ₂ 〜…… (b) T ₁ 〜T ₁ 〜T ₁ 〜T ₂ 〜T ₁ 〜T ₁ 〜T ₁ 〜T ₂ 〜…… (c) T ₁ 〜T ₁ 〜T ₂ 〜T ₂ 〜… … (d) T ₁ ~ _{T 1} ~T ₂ ~T _{1 ~T 1} ~ _{T 1 ~} T ₁ ~T ₂ ~…… (e) T ₁ ~T ₁ ~T ₁ ~T ₂ ~ _{T 1} ~T ₃ ~... (f) T ₁ ~ T ₁ ~ T ₂ ~ T ₂ ~ T ₃ ~ T ₃ ~... This invention aims to reduce sampling errors that tend to occur when measuring a flow rate that has periodic pulsations. Another feature is that the sampling timing is not fixed at a fixed phase of the pulsation.

Therefore, in order to maximize the effects of this invention, the sampling period and the excitation period must be controlled by the control circuit 9.
It is best to change it randomly.

Further, when using the sampling signal shown in FIG. 4A, the excitation current may be cut off during the waiting time tw, and the excitation power may be saved accordingly.

F. Effects of the Invention According to this invention, when measuring a pulsating flow rate, the sampling timing is not fixed to the same phase of the pulsation, so errors in sampling are reduced and the accuracy of flow rate measurement can be improved. effective.

[Brief explanation of drawings]

Figure 1 is a diagram explaining the operation of the prior art;
3 is a diagram showing an excitation current waveform, FIG. 3 is a block diagram showing an example of the configuration of the electromagnetic flowmeter of the present invention, and FIG. 4 is a diagram illustrating the operation of the electromagnetic flowmeter of the present invention. 3... Excitation coil, 4... Excitation switching circuit, 5... Preamplifier, 6... Sampling switch, 9... Control circuit, T, T ₁ , T ₂ ... Sampling period, Tex,
T ₁ , T ₂ ... excitation period, ts ... sampling time.

Claims (1)

[Claims] 1. An electromagnetic flowmeter that uses a rectangular wave excitation method and samples a flow rate signal during a stable period of the magnetic field, and is characterized in that the sampling period and the excitation period are changed once or every multiple times.