Excitation system for electromagnetic flowmeter

A technique for exciting the electromagnet of a magnetic flowmeter in which a fluid to be metered is conducted through a flow tube having detecting electrodes, the fluid intercepting a magnetic field established by the electromagnet to induce a signal in the electrodes indicative of flow in a high flow-rate range as well as in a low flow-rate range. In this technique, derived from the electrode signal is a control signal whose frequency is a function of the velocity of the fluid passing through the tube, the excitation current supplied to the electromagnet being governed by the control signal so that it has a frequency which is higher in the high flow-rate range and lower in the low flow-rate range.

BACKGROUND OF INVENTION 
This invention relates generally to an improved excitation technique for an 
electromagnetic flowmeter, and more particularly to a method and a system 
based thereon to effect automatic control of the frequency of the 
excitation current supplied to the electromagnet of the flowmeter so that 
it is higher in a high flow-rate range and lower in a low flow-rate range. 
In an electromagnetic flowmeter, a magnetic field is established by an 
electromagnet having an excitation coil, the field being intercepted by a 
fluid passing through a flow tube to induce a signal between a pair of 
diametrically-opposed electrodes, which electrode signal is indicative of 
flow rate. 
In flow rate measurement utilizing a magnetic flowmeter, it has heretofore 
been the practice to make use of an a-c excitation current in order to 
eliminate polarization effects which take place between the electrodes and 
the fluid being metered. However, when employing the a-c magnetic field, 
eddy currents are generated that are 90.degree. out of phase with the 
magnetic flux. 
Although, in an ideal state, these eddy currents in the cross-sectional 
plane of the flow tube which includes both electrodes, flow symmetrically 
with respect to the tube's axis, in actual practice these currents are 
asymmetrical with respect to this axis because of an unbalance in the 
geometry of the tube. This gives rise to unbalanced eddy currents which 
are changed by the capacitance between the electrodes and the fluid into 
components that are in-phase with the detected signal. These in-phase 
components result in zero drift, in that the eddy currents fluctuate from 
time to time. 
With a view to overcoming this drawback, it has heretofore been the 
practice to provide an excitation system in which the excitation frequency 
is decreased to a value below the frequency of the commercial power line 
in order to reduce the level of eddy currents generated in the flowmeter. 
However, when the excitation frequency is so decreased, it becomes 
impossible to measure the flow rate of fluids whose velocity lies in a 
high flow-rate range. 
SUMMARY OF INVENTION 
In view of the foregoing, the main object of this invention is to provide 
an improved excitation technique and a system based thereon whereby a 
higher response is obtained from a magnetic flowmeter in the high 
flow-rate range, the meter having good stability in the flow-rate range. 
Briefly stated, this object is attained in a technique for exciting the 
electromagnet of a magnetic flowmeter in which a fluid to be metered is 
conducted through a flow tube having detecting electrodes, the fluid 
intercepting a magnetic field established by the electromagnet to induce a 
signal in the electrodes indicative of flow in a high flow-rate range as 
well as in a low flow-rate range. In this technique, derived from the 
electrode signal is a control signal whose frequency is a function of the 
velocity of the fluid passing through the tube, the excitation current 
supplied to the electromagnet being governed by the control signal so that 
it has a frequency which is higher in the high flow-rate range and lower 
in the low flow-rate range.

DESCRIPTION OF INVENTION 
The System 
FIG. 1 schematically illustrates a magnetic flowmeter arrangement that 
includes an excitation system in accordance with the invention. The 
arrangement includes a magnetic flowmeter 1 provided with a flow tube 2 
through which the fluid to be metered is conducted, a pair of electrodes 3 
and 4 mounted at diametrically-opposed positions on tube 2, and an 
electromagnet having an excitation coil 5 acting to establish a magnetic 
field normal both to the longitudinal flow axis of the tube and to a 
transverse axis extending between electrodes 3 and 4. 
A resistor 8 connected in series with the excitation circuit that includes 
coil 5, a commercial power line source 6 and a switch 7, serves to detect 
fluctuations in the excitation current to produce a reference signal. A 
diode 9 connected across the excitation circuit functions to discharge 
energy generated by the counter electromotive force produced in excitation 
coil 5. A signal-receiving circuit 10 serves to amplify the reference 
signal developed across resistor 8 to produce a reference signal V.sub.r. 
When a fluid passing through tube 2 intersects the magnetic field, a 
voltage proportional to the volumetric flow rate is induced in the fluid 
which is transferred to electrodes 3 and 4 to yield a flow rate signal. 
The detected signal V.sub.i is amplified by a pre-amplifier 11 connected 
to the electrodes and then applied to the non-inverting input of a 
deviation amplifier 12 whose inverting input is connected through a 
semiconductor multiplier 13 to receiving circuit 10. 
The output of deviation amplifier 12 is applied through a sampling circuit 
constituted by semiconductor electronic switches 14 and 15 to each input 
terminal of a differential amplifier 16 which acts to determine the 
difference between the signals applied thereby and for smoothing. The 
output of amplifier 16 is fed to a voltage-to-frequency converter 17 whose 
output terminals are connected to a frequency-to-current converter 18 as 
well as to one of the inputs of an AND circuit 22 whose other input 
terminal is connected to a stable oscillator 21. The output of AND circuit 
22 is connected to a frequency divider 19 whose output terminals are 
connected to a timing-signal generating circuit 20. 
The arrangement is such that excitation circuit switch 7, semiconductor 
multiplier 13, and sampling semiconductor switches 14 and 15 are "on-off" 
controlled by the respective outputs of frequency divider 19, 
voltage-to-frequency converter 17 and timing signal generator 20. 
Operation 
The operation of the magnetic flowmeter in FIG. 1 will now be explained in 
connection with FIGS. 2(A) to (F). FIG. 2 (A) is a waveform showing the 
"on-off" operation of switch 7; FIG. 2 (B) is the waveform of the signal 
V.sub.r derived from receiver circuit 10; FIGS. 2 (C) and (D) are 
waveforms illustrating the "on-off" operations of sampling switches 14 and 
15; FIG. 2 (E) is a waveform of the flow rate signal V.sub.i yielded in 
the output of pre-amplifier 11; while FIG. 2 (E) is a waveform showing the 
output of differential amplifier 16. Although this output does not 
actually change in the step-like manner shown in FIG. 2 (F) because of the 
time-constant of amplifier 16, the operation is nevertheless illustrated 
in this manner in order to simplify the explanation. 
The basic operation of a magnetic flowmeter having an excitation circuit in 
accordance with the invention is as follows: When switch 7 is "on-off" 
controlled, as shown in FIG. 2 (A), an excitation current having the 
waveform shown by FIG. 2 (B), flows through excitation coil 5, and a 
reference signal V.sub.r proportional to the excitation current and 
reflecting fluctuations therein is detected by resistor 8 and receiver 
circuit 10. 
Now we shall explain in greater detail the function of the excitation 
circuit. Applied to deviation amplifier 12 are the flow rate signal 
V.sub.i from pre-amplifier 11 and the product obtained from multiplier 13 
of the reference signal V.sub.r and the output frequency F of 
voltage-to-frequency converter 17. Thus, the relationship of the signals 
at the input terminal of amplifier 12 can be expressed by the following 
equation: 
EQU V.sub.i =V.sub.r .multidot.F (1) 
Equation (1) can be rewritten as follows; 
EQU F=V.sub.i /V.sub.r (2) 
As a result, a frequency signal (F) that is free from fluctuations in the 
excitation current can be obtained. 
From deviation amplifier 12, the output yielded in the excitation period 
and the output yielded in the non-excitation period are applied 
selectively to differential amplifier 16 through sampling switches 14 and 
15 in a manner whereby the flow signal sampled in the non-excitation 
period is subtracted from the flow signal sampled in the excitation 
period, thereby eliminating unwanted d-c noise included in the output of 
deviation amplifier 12. To this end, the timing of the sampling effected 
by semiconductor switch 14 or 15 is arranged so that the flow signal is 
sampled in its substantially constant amplitude state. 
The frequency of the control signal CS for driving switch 7, which is shown 
by a solid line A in FIG. 3, is determined by dividing the sum of the 
constant frequency F.sub.o supplied by local oscillator 21 and the output 
frequency F of voltage-to-frequency converter 17. This is accomplished by 
supplying the beat frequency F+F.sub.o to divider 19. As shown by this 
figure, the excitation frequency which is determined by the frequency of 
control signal CS changes in accordance with the flow velocity. Frequency 
F.sub.o from the local oscillator functions to bias the excitation 
frequency so that excitation by a fixed low frequency continues even when 
the flow rate is zero. 
With an increase in the mean flow velocity, the interval for sampling the 
flow rate signal and the interval for sampling the reference signal are 
made narrow, whereas with a decrease in the main velocity, the sampling 
interval is expanded. 
Accordingly, a magnetic flowmeter having an excitation system in accordance 
with the invention has the following features: 
(1) When the mean flow rate of liquid flowing in tube 2 lies in a low 
flow-rate range and the accuracy necessary for flow rate measurement 
cannot be obtained unless zero drift is reduced considerably, the 
excitation frequency is then made low, whereby the "on" period of switch 7 
is widened and it becomes possible to sample the flow signal during its 
substantially constant amplitude state wherein the noise voltage resulting 
from fluctuations in the excitation current becomes minimal. In this way, 
highly precise flow rate measurement is made possible. 
(2) When the mean flow rate lies in a high flow-rate range and the accuracy 
necessary for flow rate measurement can be obtained without suppressing 
the zero drift to the extent required when operating in the low flow-rate 
range, the excitation frequency is made high, and, in turn, the sampling 
interval is made narrow. As a result, a flow rate measurement which has a 
higher response can be attained with an accuracy essentially identical to 
that in the low flow-rate range. 
Thus highly precise flow-rate measurement can be carried out with an error 
that is within a fixed limit for every measurement. This does not mean, 
however, that the error is reduced to a fixed value within the full scale 
of the flow signal. 
Since the response of the meter changes in proportion to the volumetric 
flow rate, desirable response characteristics can be obtained even though 
the flow rate signal undergoes large changes. 
Although this invention has been explained for a situation in which the 
excitation frequency changes linearly in proportion to the mean flow rate, 
as shown by the solid line A in FIG. 3, the invention may be used in a 
situation where the curve showing the relationship between excitation 
frequency and mean flow rate changes in a step-like manner, as shown by a 
dotted line B in FIG. 3, or in a situation in which the excitation 
frequency increases continuously with an increase of the mean flow rate, 
as shown by curve C in FIG. 3. 
While there has been shown and described a preferred embodiment of an 
excitation system for electromagnetic flowmeter in accordance with the 
invention, it will be appreciated that many changes and modifications may 
be made therein without, however, departing from the essential spirit 
thereof.