Electromagnetic flowmeter utilizing magnetic fields of a plurality of frequencies

An electromagnetic flowmeter has first and second magnetic field generating units. The first magnetic field generating unit applies a first magnetic field having a square wave of 4 to 8 Hz to a fluid to be measured. The second magnetic field generating unit applies a second magnetic field having a square wave of 25 to 35 Hz independently of the first magnetic field. A first electromotive force signal induced by the first magnetic field and a second electromotive force signal induced by the second magnetic field are detected by electrodes. A determination circuit determines whether the first electromotive force signal is normal and outputs a determination result. A first signal processing circuit generates a first flow rate signal on the basis of the first electromotive force signal. A second signal processing circuit generates a second flow rate signal on the basis of the second electromotive force signal. A calibration circuit outputs one of the first flow rate signal and the second flow rate signal calibrated by the first flow rate signal as a signal representing a flow rate of the fluid when the determination result represents a normal state, and outputs one of the second flow rate signal and the first flow rate signal calibrated by the second flow rate signal as a signal representing the flow rate when the determination result represents an abnormal state.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electromagnetic flowmeter and, more 
particularly, to an electromagnetic flowmeter for relatively accurately 
measuring a flow speed or rate of a fluid to be measured regardless of a 
type of fluid and a diameter of a measuring pipe. 
2. Description of the Related Art 
An electromagnetic flowmeter generally measures a flow rate of a fluid by 
utilizing the Faraday's law. 
In other words, when a magnetic field is applied to a conductive fluid 
flowing through a measurement tube, an electromotive force which is 
proportional to a flow rate of the fluid to be measured is generated. 
By detecting the electromotive force, the flow speed and the flow rate of 
the fluid can be obtained. 
In an industrial electromagnetic flowmeter, a fluid to be measured is 
usually a fluid containing an electrolyte. When a fluid contains an 
electrolyte, a kind of a battery is formed by electrochemical phenomenon 
on electrodes. An output voltage of the battery is larger than a voltage 
of the signal proportional to the flow rate. Therefore, an accurate 
electromotive force cannot be measured. For this reason, the 
electromagnetic flowmeter using the DC magnetic field cannot use for 
measuring a flow rate of a fluid containing an electrolyte. 
In order to measure the flow rate of a fluid containing an electrolyte, an 
electromagnetic flowmeter for exciting a magnetic field generation coil by 
using a commercial AC power source is developed. In this electromagnetic 
flowmeter, however, variations in output signal levels caused by 
variations in zero point of the output signal may occur due to an AC 
phenomenon. 
In order to eliminate the drawbacks of these conventional electromagnetic 
flowmeters, a flowmeter for exciting a magnetic field generation coil by 
using a square (rectangular) wave signal was developed. In this 
electromagnetic flowmeter, an electromotive force obtained in a stable 
area of a magnetic flux density B of a square wave magnetic flux is 
measured to obtain a flow rate of a fluid to be measured. In this 
electromagnetic flowmeter, an excitation frequency is low. Therefore, this 
electromagnetic flowmeter, however, is susceptible to noise having 
frequencies close to that of the magnetic field, e.g., noise called (1/f) 
noise and aliasing noise. It is possible to prevent noise by increasing an 
excitation frequency. When the frequency is increased, a rise time of a 
magnetic flux is undesirably prolonged due to an iron loss and the like. 
For this reason, an area where the magnetic field is kept stable is 
shortened. Therefore, the square wave magnetic flux behaves like an AC 
magnetic flux. The AC phenomenons which affect measurement precision are 
impaired. This drawback becomes more conspicuous in accordance with the 
increase of the diameter of the measurement pipe. 
In order to eliminate the above drawback, a dual frequency excitation type 
electromagnetic flowmeter is proposed in "NEW INTELLIGENT MAGNETIC 
FLOWMETER WITH DUAL FREQUENCY EXCITATION", ISA, 1988-Paper #88-1566. In 
this electromagnetic flowmeter, a magnetic field excitation coil is driven 
by a signal obtained by superposing square wave excitation signals of a 
low frequency (about 6 Hz) and a high frequency (about 100 Hz) on each 
other. The electromotive force signal detected by the electrodes is 
separated into a signal induced by low-frequency excitation using a filter 
and a signal induced by high-frequency excitation. The separated signals 
are processed to obtain a flow rate. 
In the magnetic flowmeter with dual frequency excitation in the above 
literature, a rise time of a magnetic flux of the high-frequency 
excitation signal is prolonged due to an iron loss. For this reason, an 
area where the magnetic flux is kept stable is shortened, the magnetic 
flux obtains AC characteristics and loses square wave characteristics. 
Therefore, an AC phenomenon for varying the zero point of the output 
signal also occurs. The drawback of the high-frequency excitation signal 
becomes more conspicuous in accordance with the increase of the diameter 
of the measuring pipe. In addition, unless a difference between the 
frequencies of the two excitation signals is large, the resultant flow 
rate signal cannot be accurately separated into two signals by using a 
filter. 
It is, therefore, difficult for these conventional electromagnetic 
flowmeters to accurately measure a flow rate of a fluid to be measured. 
SUMMARY OF THE INVENTION 
The present invention has been made in consideration of the above 
situation, and has as its object to provide an electromagnetic flowmeter 
capable of accurately measuring a flow rate of a fluid. 
It is another object of the present invention to provide an electromagnetic 
flowmeter capable of maintaining the nature of square wave excitation and 
accurately measuring a flow rate of a fluid regardless of a type of fluid 
and a diameter of a measuring pipe. 
In order to achieve the above objects according to an aspect of the present 
invention, there is provided an electromagnetic flowmeter, comprising: 
measuring pipe means through which a fluid to be measured flows; 
first magnetic field generating means for forming a first magnetic field in 
the measuring pipe means by using a first square wave excitation signal; 
second magnetic field generating means for forming a second magnetic field 
in the measuring pipe means by using a second square wave excitation 
signal having a frequency different from that of the first square wave 
excitation signal; 
first detecting means for detecting a first electromotive force signal 
induced by the first magnetic field; 
second detecting means for detecting a second electromotive force signal 
induced by the second magnetic field; 
determining means for determining whether the first electromotive force 
signal is normal and outputting a determination result; and 
signal processing means for outputting a signal representing a flow rate of 
the fluid in accordance with the determination result in response to the 
first and second electromotive force signals. 
In order to achieve the above objects according to a second aspect of the 
present invention, there is provided an electromagnetic flowmeter 
comprising: 
path means through which a fluid to be measured flows; 
first magnetic field generating means, having a first wiring and a first 
magnetic path, for generating a first magnetic field in the path means to 
generate a first electromotive force signal upon supply of a first 
excitation signal of a substantially square wave to the first winding; 
second magnetic field generating means, having a second winding and a 
second magnetic path, for generating a second magnetic field in the path 
means to generate a second electromotive force signal upon supply of a 
second excitation signal of a substantially rectangular wave to the second 
winding, 
the first excitation signal having a frequency higher than the second 
excitation signal, 
the first and second magnetic fields being independently applied to the 
fluid to be measured, 
the second magnetic path having better magnetic characteristics than those 
of the first magnetic path, and 
the first and second electromotive force signals being substantially free 
from mutual interference; 
means for independently detecting the first electromotive force signal 
induced by the first magnetic field and the second electromotive force 
signal induced by the second magnetic field; 
means for determining whether the first electromotive force signal is 
normal and outputting a determination result; 
first flow rate measuring means for generating a first flow rate 
designation signal representing a flow rate of the fluid on the basis of 
the first electromotive force signal; 
second flow rate measuring means for generating a second flow rate 
designation signal representing a flow rate of the fluid on the basis of 
the second electromotive force signal; and 
calibrating means, responsive to the determination result from the 
determining means, for outputting a signal representing the flow rate of 
the fluid on the basis of the first and second flow rate designation 
signals. 
According to the first aspect of the present invention, the first and 
second flow rate designation signals are obtained by utilizing the 
frequency characteristics of the square wave excitation signals having 
different frequencies. In addition, since the determining means determines 
whether the first electromotive force signal is normal, and the signal 
processing means outputs the signal representing the flow rate in 
accordance with the determination result. Therefore, the flow rate or 
speed of the fluid to be measured can be accurately measured. 
According to the second aspect of the present invention, since the 
characteristics of the second magnetic path are better than those of the 
first magnetic path although the second frequency is higher than the first 
frequency, the second magnetic field has a waveform relatively closer to 
the square wave. Since the first and second magnetic fields are 
independently applied to the fluid to be measured, it is very easy to 
separate the electromotive force signals induced by both the magnetic 
fields. In addition, the determining means determines whether the first 
electromotive force signal is normal. The calibrating means outputs the 
signal representing the flow rate in accordance with the determination 
result. Therefore, the flow rate or speed of the fluid to be measured can 
be accurately measured.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An electromagnetic flowmeter according to an embodiment of the present 
invention will be described with reference to the accompanying drawings. 
A structure of a measuring pipe portion of a flowmeter according to this 
embodiment will be described with reference to FIGS. 1A to 1C. 
A measuring pipe 11 through which a fluid to be measured flows consists of 
a magnetically permeable material. The measuring pipe 11 is fixed in the 
pipe and has a diameter (inner diameter) of 500 mm. A main magnetic field 
generating coil 12 is arranged to oppose the outer wall of the measuring 
pipe 11. In this embodiment, the main magnetic field generating coil 12 
comprises a saddle coil. The main magnetic field generating coil 12 
generates a main magnetic field to obtain a flow rate signal almost free 
from an eccentric flow included in the fluid to be measured and its flow 
state. 
The direction of the magnetic field generated by the main magnetic field 
generating coil 12 is perpendicular to the direction of movement of the 
fluid to be measured in this embodiment. A pair of signal electrodes 13 
are located on a line which crosses the axis of the measuring pipe 11 and 
is perpendicular to the direction of the magnetic field generated by the 
main magnetic field generating coil 12 and the direction of movement of 
the fluid to be measured. Since the signal electrodes 13 detect 
electromotive force signals induced by the fluid to be measured, the 
electrodes 13 are mounted on the inner wall of the measuring pipe 11. 
An auxiliary magnetic field generating coil 14 is arranged at a position 
which does not overlap the position of the main magnetic field generating 
coil 12 mounted on the outer wall of the measuring pipe 11 in this 
embodiment. In the layout shown in FIGS. 1A to 1C, the auxiliary magnetic 
field generating coil 14 is located at a position so as to interpose the 
outer electrode 13. The auxiliary magnetic field generating coil 14 
comprises a U-shaped iron core 14a and a coil 14b wound around the iron 
core 14a. The U-shaped iron core 14a consists of an iron core material 
having a small loss such as a small iron loss (e.g., a laminated core or 
an amorphous iron core), or steel. The auxiliary magnetic field generating 
coil 14 generates an auxiliary magnetic field to obtain a flow rate signal 
almost free from (1/f) noise and aliasing noise. 
The measuring pipe 11, the coils 12 and 14, and the like are protected by 
an outer casing 15. The outer casing 15 consists of steel and also serves 
as a magnetic path for the magnetic flux generated by the main magnetic 
field generating coil 12. 
An example of circuits for outputting an excitation signal, detecting a 
flow rate signal, and processing the flow rate signal will be described 
with reference to FIG. 2. 
An excitation power source 16 includes a DC power source and supplies a 
power to first and second excitation circuits 17 and 18 to excite the 
excitation coils 12 and 14. 
Each of the first and second excitation circuits 17 and 18 comprises a 
switching circuit for switching a DC power supplied from the excitation 
power source 16 and outputting a square wave excitation signal. 
The switching circuit in the first excitation circuit 17 is controlled in 
response to a signal from an operation instruction circuit 21 (to be 
described in detail later) and generates a first low-frequency square wave 
excitation signal ES1. In the following description, the frequency of the 
excitation signal ES1 is set to be about 6 Hz. The excitation signal ES1 
is supplied to the main magnetic field generating coil 12. 
The switching circuit in the second excitation circuit 18 is controlled in 
response to a signal from the operation instruction circuit (to be 
described in detail later) 21 and generates a second low-frequency square 
wave excitation signal ES2 having a frequency of 25 Hz to 35 Hz higher 
than the first low-frequency. In the following description, the excitation 
signal ES2 has a frequency of about 30 Hz. The excitation signal ES2 is 
supplied to the auxiliary magnetic field generating coil 14. 
The main magnetic field generating coil 12, the excitation power source 16, 
the first excitation (signal generating) circuit 17 constitute a main 
magnetic field generating unit. The main magnetic field generating unit 
applies a square wave magnetic field having a frequency of about 6 Hz to 
the entire sectional portion including the electrodes of the measuring 
pipe as shown in FIG. 4. 
On the other hand, the auxiliary magnetic generating coil 14, the 
excitation power source 16, and the second low-frequency excitation signal 
generating u it 18 constitute an auxiliary magnetic field generating unit. 
The auxiliary magnetic field generating unit generates a rectangular wave 
magnetic field having a frequency of about 30 Hz at part of the sectional 
portion including the electrodes 13 of the measuring pipe 13. 
The operation instruction circuit 21 includes an internal clock and outputs 
an operation control signal SC whose level is changed every predetermined 
period of time. When the operation control signal SC is set at high level, 
the first excitation circuit 17 is operated. However, when the operation 
control signal SC is set at low level, the second excitation signal 
generating circuit 18 is operated. 
An electromotive force signal E induced in the fluid by the magnetic field 
generated by the main or auxiliary magnetic field generating unit is 
detected by the pair of electrodes 13. 
The electromotive force signal E detected by the electrodes 13 is supplied 
as a flow rate signal SA to a signal distribution circuit 23 through an 
amplifier 22. The signal distribution circuit 23 comprises a switching 
circuit having a plurality of terminals and switching a signal output 
terminal in response to the control signal. This switching circuit 
responds to the operation control signal SC from the operation instruction 
circuit 21. When the operation instruction signal SC is set at high level, 
the switching circuit supplies the 6-Hz flow rate signal SA1 to a first 
signal processing circuit 24 and a signal evaluation circuit 26. When the 
operation control signal SC is set at low level, the switching circuit 
supplies the 30-Hz flow rate signal SA2 to the second signal processing 
circuit 25. 
The first and second signal processing circuits 24 and 25 comprise sync 
circuits, sample/hold circuits, and subtracters and are alternately 
operated in response to the operation control signal SC and output signals 
representing the flow rates. 
More specifically, when the operation control signal SC from the operation 
instruction circuit 21 is set at high level, the first signal processing 
circuit 24 is operated to receive the 6-Hz flow rate signal SA1 from the 
signal distribution circuit 23 and synchronizes it with the flow rate 
signal SA. The first signal processing circuit 24 then samples the flow 
rate signal SA1 during an area when the signal level of the flow rate 
signal SA1 is kept stabilized, i.e., time intervals SP1 and SP2 in FIG. 5. 
The first signal processing circuit 24 subtracts the sampled negative 
signal level from the positive signal level obtained by sampling after a 
half the period has elapsed from the positive signal level. Even if DC 
noise and the like are superposed on the flow rate signal SA1, the DC 
noise and the like components are canceled to each other. In the case 
shown in FIG. 5, a signal level is obtained as an E1+E2 signal. The signal 
levels of the signal from the first signal processing circuit 24 are 
averaged, and the average value is output as a signal S1 to a signal 
calibration circuit 27. The signal S1 represents a flow rate measured on 
the basis of the electromotive force signal E induced by the main magnetic 
field. When the operation control signal SC goes to low level and supply 
of the flow rate signal SA1 from the signal distribution circuit 23 to the 
first signal processing circuit 24 is kept interrupted, the first signal 
processing circuit 24 keeps outputting the signal S1 output during the 
high-level period of the operation control signal SC. 
The second signal processing circuit 25 is operated when the operation 
control signal SC from the operation instruction circuit 21 is set at high 
level. The second signal processing circuit 25 receives a 30-Hz flow rate 
signal from the signal distribution circuit 23 and synchronizes it with 
the flow rate signal SA2. The second signal processing circuit 25 samples 
the flow rate signal during a period when the signal level of the flow 
rate signal SA2 is kept stabilized, e.g., the time intervals SP1 and SP2 
in FIG. 5. The second processing circuit 25 then subtracts the negative 
signal level from the positive signal level obtained by sampling when a 
half the period of the sampled signal has elapsed from the positive signal 
level. The second signal processing circuit 25 averages the signal levels 
of the signals and outputs the average value as a signal S2. The signal S2 
represents a flow rate measured based on the electromotive force signal E 
induced by the auxiliary magnetic field. The signal S2 is supplied to a 
signal calibration circuit 27. When the operation control signal SC goes 
to high level and supply of the flow rate signal from the signal 
distribution circuit 23 to the second signal processing circuit 25 is kept 
interrupted, the second signal processing circuit 25 keeps outputting the 
signal SC output during the low-level period of the operation control 
signal SC. 
The signal evaluation circuit 26 comprises, e.g., a sync circuit, a 
differentiator, a comparator, and a reference voltage generator. The 
signal evaluation circuit 26 is operated when the operation control signal 
SC is set at high level and determines whether the 6-Hz flow rate signal 
SA1 supplied from the signal distribution circuit 23 is normal. The signal 
evaluation circuit 26 supplies a signal representing a determination 
result to the signal calibration circuit 27. Note that the signal 
evaluation circuit 26 keeps outputting an immediately preceding 
determination result signal during a low-level period of the operation 
control signal SC. More specifically, the signal evaluation circuit 26 
differentiates a signal level during a period when the signal level of the 
flow rate signal SA1 is kept stabilized, i.e., in the range between the 
time intervals SP1 and SP2 in FIG. 5. The signal evaluation circuit 26 
compares the differentiated value with a reference value. When the 
differentiated value is almost zero (i.e., dA1/dt.apprxeq.0), the signal 
evaluation circuit 26 evaluates that the signal SA1 is normal. However, 
when the differentiated value falls outside the predetermined range 
(dA1/dt.noteq.0), the circuit 26 evaluates that the signal SA1 is abnormal 
and outputs a signal representing the evaluation result. 
The signal calibration circuit 27 comprises an equalizer. When the signal 
calibration circuit 27 receives a signal representing the normal state of 
the signal SA1 from the signal evaluation circuit 26, the signal 
calibration circuit 27 amplifies the signal S2 so that the signal level of 
the signal S1 coincides with that of the signal S2. The signal calibration 
circuit 27 outputs a calibrated signal from the second processing circuit 
25 in the normal state. When the signal calibration circuit 27 receives a 
signal representing the abnormality of the signal SA1 from the signal 
evaluation circuit 26, the signal calibration circuit 27 outputs the 
signal S2 from the second signal processing circuit 25, for example, 
without any processing (without any calibration operation). 
A signal converter circuit 28 converts the signal from the signal 
calibration circuit 27 into a signal representing a flow rate or speed, 
e.g., a pulse signal or a digital signal, and outputs the converted 
signal. 
An operation of the electromagnetic flowmeter having the above arrangement 
will be described below. 
The operation instruction circuit 21 outputs the operation control signal 
SC shown in FIG. 3A. The period and waveform of the operation instruction 
signal can be arbitrarily set in accordance with the type of fluid to be 
measured and the diameter of the measuring pipe. FIG. 3A shows a case 
wherein a high-level period of the signal SC is set to be about 2 sec, and 
its low-level period is set to be about 0.1 sec. The first and second 
excitation circuits 17 and 18 are alternately operated in response to the 
signal SC in this embodiment and the excitation signals are alternately 
supplied to the main and auxiliary magnetic field coils 12 and 14, thereby 
applying the magnetic field having waveforms shown in FIG. 3B to the fluid 
to be measured. 
In order to facilitate understanding, assume a timing T1 in FIG. 3A. The 
operation control signal SC is set at high level, and the first excitation 
circuit 17 is operated. Since the excitation signal output from the first 
excitation circuit 17 is a low-frequency signal having a frequency of 
about 6 Hz, the rise time of the magnetic flux is relatively short, and a 
magnetic field having a waveform close to the square wave can be 
generated. For this reason, noise having the nature of magnetic flux 
differential value as a function of time (dB/dt) is not much generated. 
The zero point of the output signal becomes stable regardless of the size 
of the diameter of the measuring pipe 11. As shown in FIG. 4A, since a 
magnetic field is formed in the almost entire sectional area including the 
electrodes 13 of the measuring pipe 11, the flow rate can be measured 
without being adversely affected by the eccentric flow and the flow state. 
However, the induced signal is susceptible to an influence of so-called 
1/f noise. 
Upon application of the magnetic field of the 6-Hz square wave to the fluid 
to be measured, the 6-Hz electromotive force signal E is induced in the 
fluid to be measured. This electromotive force signal E is detected by the 
electrodes 13 and is amplified by the amplifier 22. The amplified signal 
is supplied to the signal distribution circuit 23. 
The signal distribution circuit 23 supplies the flow rate signal SA to the 
first processing circuit 24 and the signal evaluation circuit 26 in 
response to the control signal SC. 
The first signal processing circuit samples the signal supplied from the 
signal distribution circuit 23 at timings represented by SP1 and SP2 in 
FIG. 5. The first signal processing circuit 24 subtracts the negative 
signal level from the positive signal level obtained by sampling when a 
half the period of the sampled signal has elapsed from the positive signal 
level. For example, in the waveform shown in FIG. 5, an output is given as 
E1-(-E2)=E1+E2. By this subtraction processing, when DC noise is 
superposed on the flow rate signal, the DC noise components are canceled 
to each other. The levels of the resultant signal are averaged by the 
first signal processing circuit 24. The average value is output to the 
signal calibration circuit 27. 
The signal evaluation circuit 26 differentiates the signal level of the 
signal SA1 during an area when the level of the signal SA1 is relatively 
kept stabilized. When the differentiated value is close to zero, the 
signal evaluation circuit 26 outputs a signal representing that the signal 
SA1 is normal. Otherwise, the signal evaluation circuit 26 outputs a 
signal representing abnormality of the signal SA1. 
During the time interval T1, since the operation control signal SC is set 
at high level, the second signal processing circuit 25 does not perform 
signal processing. However, the second signal processing circuit 25 keeps 
outputting the signal S2 output during the time interval T0. 
In response to a high-level signal from the signal evaluation circuit 26, 
the signal calibration circuit 27 amplifies the signal S2 so that the 
levels of the signals S1 and S2 coincide with each other when the 
evaluation signal represents the normal state. The amplified signal S2 is 
supplied to the signal converter circuit 28. However, when the evaluation 
signal represents abnormality, the signal S2 is supplied to the signal 
converter circuit 28, for example, without any processing. 
The signal converter circuit 28 converts the input signal into, e.g., a 
digital signal and outputs it. 
Assume that a fluid to be measured is a slurry, and that a large amount of 
1/f noise is contained in an electromotive force signal induced by the 
main magnetic field. In this case, the signal evaluation circuit 26 
determines that dS1/dt is not equal to zero. For this reason, the signal 
evaluation circuit 26 outputs a signal representing abnormality of the 
signal SA1. In response to this signal, the signal calibration circuit 27 
outputs the output signal S2 from the second signal processing circuit 25 
to the signal converter circuit 28. The frequency of the auxiliary 
magnetic field is as relatively high as 30 Hz, and the signal S2 contains 
a small amount of 1/f noise. In addition, since the auxiliary magnetic 
field generating unit uses a high-quality core material and generates a 
magnetic field in part of the measuring pipe 11, a magnetic field has a 
waveform relatively close to the square wave even if the diameter of the 
measuring pipe is large. For this reason, even if the signal S2 is used as 
an output from the signal calibration circuit 27, the flow rate can be 
relatively accurately measured. 
Assume that a fluid to be measured includes an eccentric flow. The main 
magnetic field passes the entire sectional area of the fluid to be 
measured. For this reason, the electromotive force signal induced by the 
main magnetic field is almost free from an influence of the eccentric flow 
or the like. In this case, when the evaluation circuit 26 evaluates that 
the signal SA is normal, the signal S2 which has received the influence of 
the eccentric flow can be calibrated by the signal S1 free from the 
influence of the eccentric flow. For this reason, even if the signal S2 is 
influenced by the eccentric flow, a measured value is accurate. 
Assume that time has elapsed and the level of the operation control signal 
SC goes to low level. The operation of the first excitation circuit 17 is 
stopped, and the second excitation circuit 18 starts supplying the 
excitation signal ES2 to the auxiliary excitation coil 14. The auxiliary 
excitation coil 14 is formed by using a compact, relatively high-quality 
iron core 14b and generates a magnetic field within a relatively narrow 
area. For this reason, although the excitation frequency is as relatively 
high as about 30 Hz, the rise time of the magnetic flux can be shortened 
and a magnetic field having a waveform close to the square wave can be 
obtained regardless of the diameter of the measuring pipe 11. For this 
reason, noise having the nature of the magnetic flux differentiated as a 
function of time (dB/dt) which tends to be generated using a 
high-frequency magnetic field is not much generated. The zero point of the 
electromotive force signal E is kept stabilized. This point will be 
further described with reference to mathematical expressions. The 
electromotive force signal E appearing across the pair of electrodes 13 is 
expressed by equation (1) below: 
EQU E=-(d.PHI./dt) (1) 
where .PHI. is the magnetic flux generated by the magnetic field generating 
unit, S is the effective area through which the magnetic flux .PHI. 
passes, and B is the magnetic flux density. 
The magnetic flux .PHI. is represented by equation (2) below: 
EQU .PHI.=S.multidot.B (2) 
The electromotive force signal E is decomposed into noise and a signal as 
equation (3). In equation (3), the first component is the noise component 
and the second component is the signal component. 
EQU E=-{S.multidot.(dB/dt)+B.multidot.(ds/dt)} (3) 
When a magnetic field has a short rise time, the magnetic flux has a long 
constant period, and dB/dt=dk/dt=0. Only the signal component having no 
noise can be extracted. If noise is mixed by other factors, condition 
dB/dt=0 is established. Further, the zero point noise (d.sup.2 B/dt.sup.2) 
caused by the eddy current in the metal parts also becomes about zero, 
thereby stabilizing the zero point. 
The detected electromotive force signal is supplied to the second signal 
processing circuit 25 through the signal distribution circuit 23. The 
signal supplied to the second signal processing circuit 25 is processed, 
and the signal S2 is output. At this time, the first signal processing 
circuit 24 and the signal evaluation circuit 26 do not perform signal 
processing. The first signal processing circuit 24 and the signal 
evaluation circuit 26 keep outputting signals obtained during the 
high-level period of the signal SC. For this reason, in response to the 
signal output from the signal evaluation circuit 26, the signal 
calibration circuit 27 amplifies the signal S2 so that the signal levels 
of the signals S1 and S2 coincide with each other when the evaluation 
signal represents the normal state. The amplified signal S2 is supplied to 
the converter 28. When the evaluation signal represents abnormality, the 
signal S2 is supplied to the converter 28. 
The above operation is repeated in accordance with levels of the operation 
control signal SC. 
According to this embodiment, the characteristics of the two low-frequency 
magnetic fields are enhanced to compensate for the drawbacks of the 
respective magnetic fields, thereby measuring a flow rate of the fluid to 
be measured. The magnetic field applied to the fluid to be measured is not 
limited to the one shown in FIG. 3B but can be arbitrarily determined in 
accordance with the type of fluid to be measured and the diameter of the 
measuring pipe. For example, the magnetic field may be applied to the 
fluid to be measured, as shown in FIG. 8. 
According to this embodiment, two magnetic flux having two frequencies are 
applied to the fluid. However, magnetic flux having three or more 
frequencies may be applied to the fluid to be measured. 
Since the signal distribution circuit 23 is operated in response to the 
control signal from the operation instruction circuit 21, the induced 
signals can be clearly separated from each other even if the frequencies 
of the two excitation signals are relatively close to each other. 
The present invention is not limited to the particular embodiment described 
above. Various changes and modifications may be made within the spirit and 
scope of the invention. 
In the above embodiment, the main and auxiliary excitation units are fixed 
in one measuring pipe. The pair of electrodes 13 detect the electromotive 
force signals induced by the main magnetic field and the electromotive 
force signal induced by the auxiliary magnetic field. However, the main 
and auxiliary excitation units may be fixed in different pipes, and an 
electrode for detecting an electromotive force signal induced by the main 
magnetic field may be arranged separately from an electrode for detecting 
an electromotive force signal induced by the auxiliary magnetic field. 
FIG. 6 shows an arrangement of such an electromagnetic flowmeter. Referring 
to FIG. 6, a main magnetic field generating coil and an electrode 13 are 
arranged in a measuring pipe 11, and an auxiliary magnetic field 
generating coil 14 and an electrode 131 are arranged in a pipe 111 
connected to the measuring pipe 11. For this reason, even if a 6-Hz 
magnetic field and a 30-Hz magnetic field are simultaneously generated, 
they are not interfered with each other. For this reason, in this 
embodiment, first and second excitation circuits 17 and 18 and the coils 
12 and 14 are always operated. The electromotive force signals detected by 
the electrode 14 and 131 are supplied to the first and second processing 
circuits 24 and 25 and the signal evaluation circuit 26, which circuits 
are also kept operated. 
When the arrangement shown in FIG. 6 is employed and the pipe 131 having 
the auxiliary magnetic field generating coil 14 and the electrode 131 is 
connected to a conventional electromagnetic flowmeter, the same effect as 
in the above embodiment can be obtained. 
An arrangement of the auxiliary excitation coil 14 and the electrode, which 
is suitably applied in the system shown in FIG. 6, will be described with 
reference to FIG. 7. 
FIG. 7 shows an auxiliary excitation coil which can be inserted into a 
measuring pipe 11. Referring to FIG. 7, a coil 142 is wound around an iron 
core 141 constituted by ferrite, iron, etc. A 30-Hz rectangular excitation 
signal is supplied from a second excitation circuit 18 to a lead wire 142' 
of the coil 142. A resin 144 is molded on the inner surface and the coil 
periphery of the iron core 141 so as to cover the auxiliary magnetic field 
coil 14. A flange 143 is fixed to the resin 144 to fix the auxiliary 
magnetic field generating coil 14 to the side wall of the measuring pipe 
11. The outer surface of the coil is in contact with the outer surface of 
the iron core 141 and the flange 143 and is protected by a feedback 
magnetic path 145 which covers the iron core 141 and the flange 143. 
Reference numeral 146 denotes an O-ring packing. A pair of auxiliary 
electrodes 147 are arranged at a predetermined interval on a surface of 
the auxiliary magnetic field generating coil 14 which is inserted into the 
measuring pipe 11. The electromotive signal E induced between the 
auxiliary electrodes 147 is supplied to a second signal processing circuit 
25 through an amplifier 22 and a signal classification circuit 23. In the 
arrangement of FIG. 7, the auxiliary excitation unit and the auxiliary 
electrodes can be attached to the pipe by forming only a hole matching 
with the flange in the side surface of the pipe. 
The arrangements of the coils 12 and 14 are not limited to the above 
embodiments. For example, the coil 12 may comprise a conventional coil 
having a yoke. In addition, the coil 14 may generate a magnetic field not 
to part of the sectional area of the measuring pipe 11 but to the entire 
area thereof. 
The excitation power source 16 may output an AC signal. In this case, the 
excitation circuits 17 and 18 process the AC signal to generate an 
excitation signal having a rectangular wave. 
In each embodiment described above, the two excitation circuits 17 and 18 
are used. However, one or a plurality of excitation circuits which change 
a switching speed in response to a control signal from the operation 
instruction circuit 21 may be used. 
The signal distribution circuit 23 need not be used. For example, the 
output signal from the amplifier 22 may be directly supplied to the signal 
processing circuits 24 and 25 and the signal evaluation circuit 26, and 
the operations of the circuits 24, 25, and 26 may be switched in response 
to a control signal from the operation instruction circuit 21. 
The signal processing circuits 24 and 25 may comprise any conventional 
circuits if they can process flow rate signals having a square wave. 
Various techniques for determining whether a signal is normal are known. 
The signal evaluation circuit 26 may employ any technique except for the 
one described in the above embodiments. For example, the signal evaluation 
circuit 26 (1) analyzes a frequency of an input signal and determines 
whether the analyzed frequency falls within a predetermined allowable 
range, (2) determines whether an amplitude or peak of an input signal 
falls within a predetermined range, or (3) determines a fluctuation in a 
signal of "0" level falls within an allowable range. Any one of the 
references may be used to check whether the signal is normal. 
In the above embodiment, when the signal SA1 is determined to be normal, 
the calibration circuit 27 outputs the signal S2 calibrated by the signal 
S1. When the signal SA1 is determined to be abnormal, the signal S2 is 
directly output. The present invention is not limited to the particular 
embodiments described above. For example, the signal calibration circuit 
27 may be operated as follows: (1) when the signal SA1 is determined to be 
normal, (A) the signal S1 is directly output, or (B) the signals S1 and S2 
are alternately output on the basis of the operation control signal SC; or 
(2) when the signal SA1 is determined to be abnormal, (A) the signal S1 
calibrated by the signal S2 is output, or (B) amplification factors (their 
average value or a final value) upon amplification of the signal S1 for 
calibration are stored in a memory or the like, the signal S2 is 
calibrated using a stored amplification factor, and the calibrated signal 
S2 is output. 
In the above embodiment, the signal S2 is amplified (an amplification 
factor may be 1 or more or less than 1) to calibrate the signal S2. 
However, the signals S1 and S2 are averaged to perform a calibration 
operation. In addition, signal calibration can be performed by using any 
conventional calibration method. 
When the signal SA1 is normal and one of the signals S1 and S2 is output to 
the signal converter 28, one of the signals S1 and S2 is arbitrarily 
selected in accordance with the diameter of the measuring pipe 11 and the 
type of fluid. For example, when an eccentric flow tends to be formed in 
the fluid to be measured, the signal S1 is supplied to the signal 
converter 28. 
In each embodiment described above, the signal S2 is amplified to calibrate 
the signal S2. However, the present invention is not limited to this. The 
levels of the signals S1 and S2 may be averaged to perform a calibration 
operation. 
In each embodiment described above, the pair of electrodes 13 are arranged 
in correspondence with the pair of main magnetic field generating coils 
12. However, plural pairs or plurality of electrodes may be arranged. The 
frequency of the first excitation signal is set within the range of 4 to 8 
Hz, and the frequency of the second excitation signal is set to fall 
within the range of 25 to 35 Hz. However, the frequencies are not limited 
to these values. 
According to the present invention, since a magnetic field can be formed to 
cover almost the entire sectional area of the measuring pipe, a flowmeter 
almost free from an influence of an eccentric flow of the fluid can be 
obtained. In addition, the main magnetic flux has a low frequency, so that 
the rise time of the main magnetic field can be shortened regardless of 
the structure of the core and the outer casing and the diameter of the 
measuring pipe. Square wave excitation can be effectively utilized. 
In the above embodiment, the 6-Hz flow rate signal is determined whether to 
be normal. However, the present invention is not limited to the embodiment 
described above. The 30-Hz flow rate signal may be checked whether to be 
normal. In this case, the first signal processing circuit processes the 
30-Hz flow rate signal, and the second signal processing circuit 25 
processes the 6-Hz flow rate signal. In this case, the signal calibration 
circuit 27 performs calibration by using a conventional calibration 
technique. Evaluation of the signal as a normal signal based on one of the 
6-and 30-Hz signals is arbitrarily determined in accordance with the type 
of fluid to be measured and the diameter of the measuring pipe. 
On the other hand, the auxiliary magnetic field generating unit uses a core 
having good characteristics and generates a magnetic field which covers 
part of the sectional area of the measuring pipe. The rise time of the 
auxiliary magnetic field is short regardless of the diameter of the 
measuring pipe, and the advantage of the square wave excitation can be 
effectively utilized. In addition, since the auxiliary magnetic field has 
a higher frequency than that of the main magnetic field, it is almost free 
from an influence of so-called 1/f noise. 
In the above embodiment, the main and auxiliary magnetic fields are applied 
to the fluid independently with respect to time and positions. Eve if the 
frequency of the main magnetic field is close to that of the auxiliary 
magnetic field, the flow rate signals can be perfectly separated. 
Each embodiment described above exemplifies a case wherein magnetic fields 
having two different frequencies are applied to the fluid to be measured. 
However, the present invention is not limited to this. Magnetic fields 
having three or more frequencies may be applied to the fluid to be 
measured. For example, magnetic fields having frequencies of 8 Hz, 25 to 
35 Hz, and 55 to 65 Hz may be used. 
According to this embodiment described above, by using the magnetic fields 
having two different frequencies, the advantages of the respective 
magnetic fields are enhanced, and their disadvantages are compensated, 
thereby measuring a flow rate. 
Additional advantages and modifications will readily occur to those skilled 
in the art. Therefore, the invention in its broader aspects is not limited 
to the specific details, representative devices, and illustrated examples 
shown and described herein. Accordingly, various modifications may be 
without departing from the spirit or scope of the general inventive 
concept as defined by the appended claims and their equivalents.