Step-wise tracking electronic filter with offset up and down transition

A step-wise tracking electronic filter comprises a parallel bank of a plurality of band pass filters respectively having a plurality of electronic switches and respectively having different band widths, and a command signal generating device generating a command signal from the frequency of a primary alternating electrical signal being transmitted through the step-wise tracking electronic filter and being conditioned thereby or from an auxiliary alternating electrical signal obtained for the sole purpose of generating the command signal therefrom; wherein the command signal closes one of the plurality of electronic switches belonging to one of the plurality of band pass filters having a band width substantially including the frequency of the primary alternating electrical signal and opens the electronic switch when the band width of the band pass filter substantially excludes the frequency of the primary alternating electrical signal in an operating mode wherein only one electronic switch is closed at a time, and the command signal closes the first of two electronic switches respectively belonging to an adjacent pair of the band pass filters and opens the second of the two electronic switches when the frequency of the primary alternating electrical signal is equal to a first value of a transition frequency, and closes the second of the two electronic switches and opens the first of the two electronic switches when the frequency of the primary alternating electronic signal is equal to a second value of the transition frequency offset from the first value of the transition frequency.

FIELD OF INVENTION 
This invention relates to an electronic filter for conditioning a primary 
flow signal generated in the form of an alternating electrical signal by a 
flowmeter such as a vortex flowmeter or turbine flowmeter, which primary 
flow signal is converted to a flow rate of fluid media moving through a 
flow passage included in the flowmeter by a data processor, wherein the 
electronic filter comprises a parallel combination of a plurality of band 
pass filters, one of which plurality of band pass filters having a 
bandwidth substantially including the frequency of the primary flow signal 
is switched on by a command signal generated from the primary flow signal 
or from an auxiliary flow signal provided by an accessory flow sensor or 
flow switch. The present invention is characterized by the controlling of 
transition from one band pass filter to another band pass filter, wherein 
the command signal voltage switching on one of the plurality of band pass 
filters and the command signal voltage switching off said one of the 
plurality of band pass filters are offset from one another, which control 
of switching on and off each of the plurality of band pass filters 
prevents an adjacent pair of the band pass filters from being switched on 
and off in an alternating and cyclic manner when the primary or the 
auxiliary flow signal generating the command signal controlling the 
switching on and off of the band pass filters is generated by a flow rate 
corresponding to the transition value between the adjacent pair of the 
band pass filter. 
BACKGROUND OF INVENTION 
With few exceptions, all of the existing versions of the electronic filters 
conditioning the flow signals such as those provided by the vortex 
flowmeters employ a parallel bank of band pass filters switched on and off 
one at a time by a command signal generated from the frequency of the flow 
signal provided by the vortex flowmeter in the form of an alternating 
electrical signal, wherein the frequency of the flow signal generating the 
command signal is detected at the output side of the parallel bank of band 
pass filters as taught by the conventional wisdom of the feedback control 
theory. As the frequency of vortex shedding measurable by an 
advanced-concept up-to-date vortex flowmeters varies in a range from a 
fraction of a Hz to a few thousand Hz, the conventional feedback method of 
controlling the parallel bank of band pass filters by using a command 
signal generated from the frequency of the alternating electrical signal 
detected at the output side of the parallel bank of the band pass filters 
does not work, because such a control method tends to make the parallel 
bank of band pass filters lock on to the noise signal instead of the 
vortex signal. 
In general, the error arising from the distortion of the flow signal of 
normally harmonic geometry occurring during the transition from a band 
pass filter to another band pass filter included in the parallel bank of 
the plurality of band pass filters is negligibly small as long as the 
transition from one band pass filter to another band pass filter occurs 
infrequently. However, when the flow of fluid media occurs at a rate that 
generates the command signal voltage having a value triggering the 
transition between two adjacent pair of the band pass filters, the 
transition between the two adjacent pair of the band pass filters can 
occur in an alternatively and cyclically repeating manner and, 
consequently, an unacceptably large error in the flow measurement can take 
place due to the distortion of the flow signal cause by a repeatedly 
occurring transition between the adjacent pair of the band pass filters. 
BRIEF SUMMARY OF INVENTION 
The primary object of the present invention is to provide an electronic 
filter assembly comprising a parallel combination of a plurality of band 
pass filters, wherein a command signal generated from the frequency of the 
primary flow signal provided by the flowmeter measuring the flow rate of 
fluid media or from an auxiliary flow signal provided by an accessory 
electromechanical or thermal flow sensor or switch, switches on each of 
the plurality of band pass filters one at a time when the command signal 
has a value equal to a first value assigned to the band pass filter and 
switches off the band pass filter when the command signal has another 
value equal to a second value assigned to the band pass filter, which 
first and second values of the command signal assigned to each of the 
plurality of band pass filters for switching on and off thereof are offset 
from one another to ensure that there will not occur transition between an 
adjacent pair of the band pass filters in an alternatively and repeating 
cyclical manner distorting the substantially harmonic geometry of the 
primary flow signal providing the flow rate of fluid media. When the 
command signal controlling the switching on and off of the plurality of 
the band pass filters is generated from the frequency of the primary flow 
signal providing the flow rate of fluid media, the frequency of the 
primary flow signal may be taken at either the input or output side of the 
electronic filter assembly depending on the design preference. The best 
result is obtained when a compound version of the electronic filter 
assembly is employed, wherein the electronic filter assembly comprises a 
first parallel bank of a small number of band pass filters controlled by a 
command signal generated from the frequency of the primary flow signal 
detected at theeoutput side of the first parallel bank of the band pass 
filters, and a second parallel bank of a significant number of band pass 
filters controlled by a command signal generated from the frequency of the 
primary flow signal detected at the output side of the first parallel bank 
of band pass filters and the input side of the second parallel bank of 
band pass filters, which most desirable version of the electronic filter 
assembly is illustrated in FIG. 2 of the parent patent application that is 
now U.S. Pat. No. 5,351,556. The performance of the electronic filter 
assembly can be further enhanced when the control of the band pass filters 
covering the lower range of flow rate is executed by a command signal 
generated from an auxiliary flow signal provided by an accessory flow 
sensor or switch as illustrated and described in the parent patent 
application Ser. No. 08/270,820, that is now U.S. Pat. No. 5,435,188, 
while the control of the band pass filters covering the middle and upper 
range of flow rate is executed by a command signal generated from the 
frequency of the primary flow signal provided by the flowmeter measuring 
the flow rate of fluid media. It should be also pointed out that the first 
and second values assigned to each of the plurality of band pass filters 
respectively for switching on and off the band pass filter when the value 
of the command signal is respectively equal thereto must be offset from 
one another by a finite and small value for the band pass filters covering 
the lower range of flow rate, while these first and second values of the 
command signal may be offset from one another by a very small value or 
even converge to the same value for the band pass filters covering the 
middle and upper range of flow rate as the transition between an adjacent 
pair of the band pass filters occurs less frequently even when the primary 
flow signal generates a command signal of a value coinciding with the 
first and second value of the command signal converging to an identical 
value triggering the transition between the adjacent pair of the band pass 
filters at the single value of the command signal as long as the adjacent 
pair of the bandpass filters cover the middle and upper range of flow 
rate. 
Another object is to provide the electronic filter assembly described in 
the afore-mentioned primary object of the invention that includes an 
output cut-off electric circuit controlled by a command signal generated 
from the auxiliary flow signal. 
A further object is to provide the electronic filter assembly described in 
the afore-mentioned primary object that includes an output cut-off 
electric circuit controlled by a command signal generated from the 
amplitude of the primary flow signal measured at the output side of the 
second parallel bank of filters. 
Yet another object is to provide the electronic filter assembly described 
in the afore-mentioned primary object, that includes an output cut-off 
electric circuit controlled by a command signal generated from the 
frequency of the primary flow signal measured at the output side of the 
first or second parallel bank of filters. 
Yet a further object is to provide an electronic filter assembly 
conditioning the primary flow signal such as an alternating electrical 
signal provided by a vortex flowmeter or turbine flowmeter, that comprises 
a parallel bank of filters controlled by a command signal generated from 
an auxiliary flow signal provided by a mechanical or thermal flow sensor 
or flow indicator, wherein the flow rate of media is determined from the 
frequency of the primary flow signal measured at the output side of the 
parallel bank of filters. 
Still another object is to provide the electronic filter assembly described 
in the afore-mentioned yet a further object of the invention, that 
includes an output cut-off electric circuit controlled by a command signal 
generated from the auxiliary flow signal, or the amplitude or the 
frequency of the primary flow signal detected at the output side of the 
parallel bank of filters. 
Still a further object is to provide the electronic filter assembly 
described in the afore-mentioned yet a further object of the invention, 
that comprises another parallel bank of filters disposed in a parallel 
relationship to said a parallel bank of filters, and controlled by a 
command signal generated from the frequency of the primary flow signal 
detected at the input or output side of the another parallel bank of 
filters. 
Yet still another object is to provide the electronic filter assembly 
described in the afore-mentioned a still further object of the invention, 
that includes an output cut-off electric circuit controlled by a command 
signal generated from the auxiliary flow signal, or the amplitude or the 
frequency of the primary flow signal detected at the output side of the 
parallel bank of filters. 
Yet still a further object is to provide the electronic filter assembly 
conditioning the primary flow signal such as an alternating electrical 
signal provided by a vortex flowmeter or turbine flowmeter, that comprises 
a parallel bank of filters controlled by a command signal generated from 
the frequency of the primary flow signal detected at the input side or 
output side of the parallel bank of filters, wherein at least the 
electronic filter or filters covering the lower range of flow rate of 
fluid media is assigned with a first value of the command signal switching 
on the electric filter and a second value of the command signal switching 
off the electronic filter, which first and second values of the command 
signal are offset from one another by a finite interval. 
An additional object is to provide the the electronic filter assembly 
described in the afore-mentioned yet still a further object of the 
invention, that includes an output cut-off electric circuit controlled by 
a command signal generated from an auxiliary flow signal, or the the 
amplitude or the frequency of the primary flow signal detected at the 
output side of the parallel bank of filters. 
Another additional object is to provide the electronic filter assembly 
described in the afore-mentioned an additional object of the invention, 
that comprises another parallel bank of filters disposed in a parallel 
relationship to said a parallel bank of filters, and controlled by a 
command signal generated from the frequency of the primary flow signal 
detected at the input or output side of the another parallel bank of 
filters, wherein the individual filters included in the another parallel 
bank of filters may be switched on and off respectively at two different 
and offset values of the command signal or at a common value of the 
command signal. 
A further additional object is to provide the electronic filter assembly 
described in the afore-mentioned another additional object of the 
invention, that includes an output cut-off electric circuit controlled by 
a command signal generated from the auxiliary flow signal, or the 
amplitude or the frequency of the primary flow signal detected at the 
output side of the parallel bank of filters.

DESCRIPTION OF ILLUSTRATED EMBODIMENTS 
In FIG. 1 there is illustrated a flow diagram of an embodiment of the 
compound electronic filter conditioning a primary flow signal occurring in 
the form of an alternating electrical signal generated by a vortex 
flowmeter or turbine flowmeter, that is called the primary flow signal 
because the flow rate is determined from that primary flow signal. The 
primary flow signal generated by a vortex flowmeter 1 or 2, or by a 
turbine flowmeter 3 in the form of an alternating electrical signal is 
preconditioned by a prefilter such as a band pass filter 4 or low pass 
filter 5 having a preset fixed band width with a frequency range covering 
only the range corresponding to the range of flow measurement, which 
primary flow signal is supplied to the input end 6 of the compound 
electronic filter. The compound electronic filter of the present invention 
selectively transmitting the primary flow signal by tracking or locking 
onto the frequency of the primary flow signal in a stepwise mode comprises 
a parallel bank of filters comprising a first set 7 of band pass filters 
L1 through L4, a second set 8 of band pass filters M1 through M4, and a 
third set 9 of band pass filters H1 through H4, wherein the three sets 7, 
8 and 9 of band pass filters respectively include three 
switch/multiplexers 10, 11 and 12. Of course, each individual band pass or 
low pass filter selectively transmits an alternating electrical signal 
with frequencies distributed within the band width of the band pass or low 
pass filter. It must be understood that the number of parallel filters 
included in each of the three sets of band pass filters may vary from a 
single band pass filter to any plurality of band pass filters depending on 
the operating requirements and design preference, while the particular 
illustrative embodiments shows an embodiment wherein each set of band pass 
filters includes four band pass filters. The individual band pass filters 
L1-L4, M1-M4 and H1-H4 included in the parallel bank of filters 
continuously cover the entire range of frequencies of the primary flow 
signal as the band widths of each adjacent pair of band pass filters 
over-lap one another extending over frequencies distributed over the 
boundary frequency between the adjacent pair of band pass filters in a 
sharply decaying manner. Of course, the band pass or low pass filter L1 
covers a band width of the lowest frequencies, L2 covers a band width of 
the next lowest frequencies and so on, and H4 covers a band width of the 
highest frequencies. It should be understood that, in an alternative 
design, the parallel bank of filters may include only one or a pair out of 
the three sets 7, 8 and 9 of band pass filters instead of all three sets 
included in the particular illustrative embodiment as the band widths of 
the individual band pass filters can be selected in such a way that a 
single or a pair of the sets of band piss filters continuously covers the 
entire range of frequency of the primary flow signal. Each of the 
switch/multiplexers 10, 11 and 12 respectively includes a plurality of 
switches switching on and off one at a time the individual band pass 
filters included in each of the sets 7, 8 and 9 of the band pass filters, 
wherein a switch assigned to a band pass filter becomes switched on when a 
command signal received by the priority encoder controlling the 
switch/multiplexer is equal to or greater than a specific threshold value 
assigned to the boundary frequency between that band pass filter and an 
adjacent band pass filter having a band width of next lower frequencies, 
and becomes switched off when the command signal is less than the 
afore-mentioned threshold value or greater than a threshold value assigned 
to the boundary frequency between that band pass filter and another 
adjacent band pass filter having a band width of next higher frequencies. 
The individual band pass filters included in the first set 7 are 
respectively switched on and off one at a time in the above-described 
manner by a command signal generated from an auxiliary flow signal 
provided by a flow sensor or flow indicator 13 that provides a less 
precise but highly reliable signal representing the levels of fluid flow, 
which may be a displaceable flap or target triggering on various proximity 
switches on its path of displacement as shown in the particular 
illustrative embodiment, or a nondisplaceable target experiencing a fluid 
dynamic drag or lift force that is converted to an electrical signal, or a 
thermal flow sensor that converts convective heat transfer to an 
electrical signal that is included in the embodiment shown in FIG. 2. The 
flow signal provided by the flow sensor or flow indicator 13 is called the 
auxiliary flow signal because this auxiliary flow signal is used only to 
generate a command signal controlling the switching of the individual band 
pass filters rather than to determine the flow rate. It should be 
understood that any inexpensive flow sensors, flow indicators, or 
flowmeters providing a crude but reliable flow signal can be used to 
generate the auxiliary flow signal. A transducer 14 converts the flow 
level detected by the flow sensor or flow indicator 13 into a command 
signal supplied to a priority encoder 15 that controls the 
switch/multiplexer 10. The function of the priority encoder 15 is to 
ensure that only one band pass filter is switched on "at a time" (the 
proper phrasing in English language is "only one filter is switched on at 
a time" rather than "only one filter is switched on one at a time") and 
that all other band pass filters are switched off in the previously 
described manner. In the particular illustrative embodiment, the band pass 
filter L1 becomes switched on only when the flap or target included in the 
flow sensor or flow indicator 13 is displaced from the resting position 
corresponding to zero flow rate and, consequently, the output from the 
parallel bank of filters remains zero as long as the flow rate is less 
than a preset minimum value that is set as required by adjusting the bias 
spring countering the fluid dynamic force on the flap or target. In an 
alternative design, the band pass filter L1 may stay switched on as long 
as no other band pass filters are switched on, which arrangement can put 
out erroneous flow data, particularly in the operation of a vortex 
flowmeter, as the mechanical vibration of the vortex flowmeter body can 
produce a non-zero flow rate read out when the actual flow rate is equal 
to zero. The switching on and off one at a time of the individual band 
pass filters included in the "set 2 of band pass filters and the set 3 of 
band pass filters" are controlled by command signals generated from the 
frequency of the primary flow signal. The frequency detector 16 detects 
the frequency of the primary flow signal at the output side of the 
parallel bank of filters and converts the frequency to a command signal, 
that is sent to a priority encoder 17 controlling the switch/multiplexer 
11. The priority encoder 17 ensures that only one band pass filter is 
switched on at one time in the previously described manner. The frequency 
detector 18 detects the frequency of the primary flow signal at the input 
side of the parallel bank of filters and converts the frequency to a 
command signal, that is sent to the priority encoder 19 controlling the 
switch/multiplexer 12, which priority encoder 19 ensures that only one 
band pass filter is switched on at one time. The high pass filter 20 with 
a preset fixed band width conditions the primary flow signal supplied to 
the frequency detector 18, whereby the frequency detector 18 does not 
miscount the frequency due to the low frequency noise produced by the 
mechanical vibrations of the flowmeter body and entrained in the primary 
flow signal. The multiplexer couplers 21 and 22 correlate the operation of 
all three switch/multiplexers 10, 11 and 12 to each other in such a way 
that only one of all of the individual band pass filters included in the 
three sets 7, 8 and 9 of band pass filters is switched on at one time in 
the previously described manner. A simple form of the multiplexer couplers 
21 and 22 can be the integration or interconnection of the three priority 
encoder 15, 17 and 19 into a single interrelated system, or the supplying 
of a command signal provided by one of the three command signal generators 
14, 16 and 18 to all of the three priority encoders 15, 17 and 19. While 
the probability of actual occurance is extremely low, there may occur a 
failure of the step-wise tracking or locking on to the frequency of the 
primary flow signal in the operation of the parallel bank of filters. In 
other words, a wrong band pass filter may become erroneously switched on 
and, consequently, no or very little primary flow signal becomes 
transmitted through the parallel bank of filters. When the over-ride 
scanner 23 detects a condition wherein the amplitude of the primary flow 
signal detected at the input side of the parallel bank of filters is 
greater than a preset minimum value and the amplitude of the primary flow 
signal detected at the output side of the parallel bank of filters is less 
than a preset threshold value, the over-ride scanner generates and 
supplies a series of ramp dc voltages starting with zero value and ending 
with a maximum value equal to or greater than the threshold dc voltage 
assigned to the band pass filter with band width of the highest 
frequencies to all three priority encoders 15, 17 and 19, whereby 
switching on and off one at a time of all of the band pass filters 
included in the parallel bank of filters in a sweep or scanning mode 
starting with the lowest frequency band pass filter L1 and ending with the 
highest frequency band pass filter H4, which switching in the sweep or 
scanning mode is continued until the amplitude of the primary flow signal 
detected at the output side of the parallel bank of filters becomes 
greater than the preset threshold value, at which instant the over-ride 
scanner 23 becomes automatically turned off allowing the regular control 
of the band pass filters by the command signals generated by the command 
signal generators 14, 16 and 18 to take over. The output signal 24 from 
the parallel bank of filters can be supplied to a data processor, that is 
not shown in FIG. 1, to determine the flow rate, or can be supplied to a 
second stage parallel bank of filters 25 for a further conditioning before 
being supplied to a data processor determining the flow rate. In a design 
wherein the lowest frequency band pass or low pass filter such as the 
filter L1 stays switched on even when the flow rate becomes zero, the 
output end of the parallel bank of filters comprising the band pass 
filters L1 through H4 or from the second stage filter 25 may include a 
minimum amplitude cut-off circuit 26 that cuts off the output signal when 
the amplitude thereof is less than a preset minimum value, or a minimum 
frequency cut-off circuit 27 that cuts off the output signal when the 
frequency of the output signal is less than a preset minimum value; 
whereby the noise signal of low amplitude or low frequency created by the 
mechanical vibrations of the flowmeter body does not create a false 
nonzero output signal when the actual flow rate is equal to zero. It 
should be pointed out that the most preferred version of the parallel bank 
of filters shown in FIG. 1, that provides a high level of performance and 
yet is economically priced, can be obtained by including two band pass 
filters in the set 9, two band pass filters in the set 8, and a reasonably 
small number of band pass filters in the set 7, which preferred version 
should be used as a first stage filter in conjunction with a second stage 
filter such as that shown in FIG. 6. In a further economized version of 
the above-described most preferred version of the parallel bank of 
filters, both or only one of the sets 7 and 8 of band pass filters may be 
included in the first stage filter without the set 9 of the band pass 
filters. 
In FIG. 2 there is illustrated a structural embodiment of the first set 7 
of band pass filters included in the parallel bank of filters shown in 
FIG. 1. The primary flow signal is supplied to the input end 6 of the 
parallel bank of filters. A low pass filter 28 and a plurality of band 
pass filters 29, 30, 31, etc. constitute the parallel combination or the 
set 7 of the band pass filters. The plurality of JFET switches 32, 33, 34, 
35, etc. respectively assigned to the individual low pass or band pass 
filters 28, 29, 30, 31, etc. constitute the switch-multiplexer 10. The 
plurality of combinations of Nand gates and inverters 36, 37, 38, 39, 
etc., and the plurality of comparators 40, 41, 42, 43, etc. constitute the 
priority encoder 15. The auxiliary flow signal generating the command 
signal, that switches on and off the JFET switches 32-35 one at a time in 
a mode of logic described in conjunction with FIG. 1, is provided by a 
thermal flow sensor 44 comprising a heater coil 45 disposed intermediate 
two temperature sensing coils 46 and 47. The temperature difference 
between the two sensing coils 46 and 47 is converted to a dc voltage that 
is supplied to the comparators 40-43 respectively defining the threshold 
voltages respectively switching on the JFET switches 32-35, wherein the 
threshold voltages respectively defined by an adjacent pair of comparators 
corresponds to a lower and an upper boundary frequency of the band width 
of the lower frequency band pass filter of the adjacent pair of band pass 
filters. It should be noticed that the magnitudes of the threshold dc 
voltages respectively switching on the band pass filters 29-32 
progressively increase from lower frequency band pass filters to the 
higher frequency band pass filters. The thermal flow sensor 44 is shown in 
place of the mechanical flow sensor 13 shown in FIG. 1 as the device 
providing the auxiliary flow signal in order to demonstrate the variety of 
flow sensors or flow indicators, which can be employed to provide the 
auxiliary flow signal. For example, in addition to the illustrative 
example of the flow sensors or flow indicators providing the auxiliary 
flow signal, a fluid dynamic target flow sensor or a turbine flowmeter can 
be used to provide the auxiliary flow signal in the operation of a vortex 
flowmeter in conjunction with the compound electronic filter of the 
present invention. 
In FIG. 3 there is illustrated a structural embodiment of the second set 8 
of band pass filters included in the parallel bank of filters shown in 
FIG. 1, which comprises components essentially identical or similar to 
those shown and described in conjunction with FIG. 2 with the following 
exceptions: The set 8 of band pass filters comprises all band pass filters 
48, 49, 50, 51, etc. without any low pass filters, and the command signal 
switching on and off one at a time the band pass filters 48-51 is now 
generated from the frequency of the primary flow signal detected at the 
output side 24 of the parallel bank of filters by a frequency to voltage 
converter 52. It should be understood that the threshold dc voltages 
respectively switching on the band pass filters 48-51 one at a time 
progressively increase from lower frequency band pass filters to the 
higher frequency band pass filters. In an alternative embodiment wherein 
the first set 7 of band pass filters is omitted from the parallel bank of 
filters shown in FIG. 1, the band pass filter 48 should be replaced by a 
low pass filter. 
In FIG. 4 there is illustrated a structural embodiment of the third set 9 
of band pass filters included in the parallel bank of filters shown in 
FIG. 1, which comprises components essentially identical or similar to 
those shown in FIG. 3. The command signal controlling the switching of the 
band pass filters 53, 54, 55, 56, etc. is generated from the frequency of 
the primary flow signal detected at the input side 6 of the parallel bank 
of filters by the frequency to voltage converter 57. The threshold 
voltages respectively switching on the band pass filters 53-56 one at a 
time progressively increase from lower frequency band pass filters to the 
higher frequency band pass filters. In this particular illustrative 
embodiment, the high pass filter 20 included in the version shown in FIG. 
1 is omitted as the parallel bank of filters work with or without the high 
pass filter 20. 
In FIG. 5 there is illustrated the relationship between the sources of the 
command signal and the threshold values of the command signal switching on 
various band pass filters constituting a parallel bank of electronic 
filters. The flow sensor 44 shown in FIG. 2 providing the auxiliary flow 
signal provides a command signal having a dc voltage equal to 
(V-)+(V+)!/2, which command signal is generated from the auxiliary flow 
signal by a command signal generator such as an amplitude-to-voltage 
converter or frequency-to-voltage converter. The plurality of comparator 
circuits 40 through 43 respectively controlling the closing and opening of 
the electronic switches 32 through 35 have hysteresis which add to or 
subtract from the dc voltage provided by the command signal a 
predetermined amount of hysteresis voltage depending on the transition of 
the switching of electronic switches takes place in an upward direction or 
a downward direction. In other words, when the transition of switching on 
of a band pass filters occurs in the upward direction due to the 
increasing value of the command signal voltage resulting from increasing 
fluid velocity, the actual switching on of various band pass filters 
occurs following the V+ curve or line shown in FIG. 5, while the actual 
switching on of various band pass filters occurs following the V- curve or 
line when the transition of switching on of the band pass filters occurs 
in the downward direction due to the decreasing value of the command 
signal voltage resulting from the decreasing fluid velocity. For example, 
when a lower band pass filter assigned to a velocity range bounded by 
U.sub.1 and U.sub.2 is switched on due to increasing velocity of fluid 
media, the switching on actually occurs at a fluid velocity U.sub.1 + 
slightly greater than the lower bound velocity U.sub.1, while the actual 
switching on of the band pass filter occurs at a fluid velocity U.sub.2 - 
slightly less than the upper bound velocity U.sub.2 when the switching on 
of the particular band pass filter bounded by the velocities U.sub.1 and 
U.sub.2 occurs due to the decreasing fluid velocity. The lowest velocity 
band pass filter 28 becomes switched on when the fluid velocity becomes 
slightly greater than the minimum velocity U.sub.min and remains switched 
on as long as the fluid velocity varies within a subregion bounded by a 
velocity slightly less than U.sub.min and a velocity slightly greater than 
U.sub.1. When the fluid velocity increases to a value in a subregion 
bounded by a value slightly greater than U.sub.1 and a value slightly 
greater than U.sub.2, the next lowest velocity band pass filter 29 becomes 
switched on and the lowest velocity band pass filter 28 becomes 
automatically switched off. It must be mentioned that the command signal 
generated from either the auxiliary flow signal or the primary flow signal 
effects the switching on of various band pass filters, while switching off 
of various band pass filters occurs in accordance with the logic built 
into the switching circuits shown in FIGS. 2, 3, 4 and 6, which logic 
makes only one band pass filter is switched on at one time and all of the 
remaining band pass filters become switched off as soon as the particular 
one band pass filter becomes switched on. When the mth band pass filters 
become switched on due to the increasing fluid velocity, the actual 
switching on occurs at a fluid velocity U.sub.m +, while the actual 
switching on of the mth band pass filter occurs at a fluid velocity 
U.sub.m - when the switching on of the mth band pass filter takes place 
due to the decreasing fluid velocity because of the hysteresis. It is 
obvious that, since the upward and downward transitions between an 
adjacent pair of band pass filters occur respectively at two different 
transition velocities offset from one another by two times the value of 
hysteresis built into each individual comparator circuit, there will not 
take place the alternative and repeated cyclic transition between the 
adjacent pair of band pass filters even when the fluid velocity coincides 
with one of the transition velocities between the adjacent pair of band 
pass filters. 
In FIG. 6 there is illustrated a structural embodiment of the second stage 
parallel bank of filters 25 included in the embodiment shown in FIG. 1, 
that receives the output signal of the parallel bank of filters shown in 
FIG. 1, which second stage filter comprises a parallel combination of a 
first set of low frequency band pass filters 58, 59, etc. controlled by a 
command signal generated by a frequency to voltage converter 60 from a 
frequency of the primary flow signal detected at the output side 61 of the 
second stage filter, and a second set of higher frequency band pass 
filters 62, 63, etc. controlled by a command signal generated by a 
frequency to voltage converter 64 from a frequency of the primary flow 
signal detected at the input side of the second stage filter, that is the 
output side 24 of the parallel bank of filter shown in FIG. 1 being used 
as the first stage filter. It is readily noticed that the first set of 
band pass filters 58, 59, etc. operates on the same principles as those of 
the set of band pass filters shown in FIG. 3, while the second set of band 
pass filters 62, 63, etc. operates on the same principles as those of the 
set of band pass filters shown in FIG. 4. The minimum amplitude cut-off 
circuit 65 cuts off the output signal from the second stage filter when 
the amplitude of the output signal detected by the amplitude detector 66 
is less than a preset minimum value. The minimum frequency cut-off circuit 
67 cuts off the output signal when the frequency of the output signal 
detected by the frequency detetector 68 is less than a preset minimum 
frequency. The minimum velocity cut-off circuit 69 cuts off the output 
signal when the fluid velocity is less than a preset minimum value, below 
which minimum value the flap 70 stays at the resting position shown in the 
illustrative embodiment due to the counter weight 71 exerting a greater 
torque on the flap than the fluid dynamic torque experienced thereby, 
whereat a command signal originating from the proximity sensor 72 keeps 
the minimum velocity cut-off circuit at the open position. The second 
stage filter shown in FIG. 6 may be used in conjunction with the parallel 
bank of filters shown in FIG. 1 in the following two different 
combinations: The first combination is to use them as a first and second 
stage filter in a series combination as exemplified by the switch 73 at 
the position shown in FIG. 6, wherein the output end 24 of the parallel 
bank of filters shown in FIG. 1 is the input end of the second stage 
filter shown in FIG. 6. The second combination is provided when the switch 
73 is flipped over to a position opposite to that shown in FIG. 6, wherein 
the output signal from the parallel bank of filters shown in FIG. 1 is 
supplied only to the frequency detector 64, while the primary flow signal 
provided by the vortex flowmeter or turbine flowmeter is supplied directly 
to the input end 74 of the second stage filter. It is readily recognized 
that the lowest frequency band pass filter 58 can be a low pass filter. 
The most preferred version of the second stage filter shown in FIG. 6 in 
terms of performance and economics is obtained when the first set of 
filters 58, 59, etc. is omitted, and the second stage filter comprising 
only the second set of filters 62, 63, etc. is used in the afore-mentioned 
series combination with the parallel bank of filters shown in FIG. 1 being 
used as the first stage filter. Of course, in such a series combination of 
the first and second stage filters, the plurality of band pass filters 62, 
63, etc. must cover the entire frequency range in a continuous manner, and 
the lowest frequency filter may be a low pass filter instead of a band 
pass filter. The flow rate is determined from the frequency of the primary 
flow signal measured at the output side of the second stage filter. 
In FIG. 7 there is illustrated a relationship between the frequency of the 
primary flow signal and the threshold dc voltages switching on various 
band pass filters included in the second stage filter shown in FIG. 6. The 
different frequencies of the primary flow signal corresponding to 
different fluid velocities and generating the threshold dc voltages of 
different magnitudes switch on various band pass filters one at a time in 
the same manner as that described in conjunction with FIG. 5. 
In FIG. 8 there is illustrated a structural embodiment of the minimum 
amplitude cut-off circuit. A signal level detector 75 detects the level of 
the primary flow signal, and a comparator 76 compares the detected level 
of the primary flow signal with a preset minimum value. When the detected 
level of the primary flow signal is less than the preset minimum value, 
the comparator 76 generates a command signal that opens the JFET switch 
77, thereby cutting off the output signal 78. 
In FIG. 9 there is illustrated a structural embodiment of the minimum 
frequency cut-off circuit. A frequency to voltage converter 79 generates a 
dc voltage, and a comparator 80 compares the magnitude of the dc voltage 
with a preset minimum value. When the magnitude of the dc voltage is less 
than the preset minimum value, the comparator 80 sends a command signal 
that opens the JFET switch 81, thereby cutting off the output signal 82. 
In FIG. 10 there is illustrated a structural embodiment of the over-ride 
scanner that re-establishes the locking on or tracking of the frequency of 
the primary flow signal in the stepwise mode in the operation of the 
compound electronic filter of the present invention. A ramp generator 83 
generates a series of ramp dc voltages increasing from zero or a value 
equal to the threshold value of the command dc voltage assigned to the 
lowest frequency low pass or band pass filter to a maximum value equal to 
or greater than the threshold value of the command dc voltage assigned to 
the highest frequency band pass filter, when an input primary flow signal 
level detector 84 detects a value greater than a preset minimum value and 
an output primary flow signal level detector 85 fails to detect a value 
greater than a preset threshold value. The ramp dc voltage switches on and 
off one at a time the plurality of band pass filters included in the 
parallel bank of filters, that may be the parallel bank of filters shown 
in FIG. 1 or 6, in a sweeping or scanning mode starting with the lowest 
frequency band pass filter and ending with the highest frequency band pass 
filter, which sequential switching on and off of the individual band pass 
filters one at a time is repeated as long as the output primary flow 
signal level detected by the output signal level detector 85 does not 
exceed the preset threshold value. As soon as a value of the output 
primary flow signal level greater than the preset threshold value is 
detected by the output signal level detector 85, the JFET switch 86 shuts 
off the over-ride scanner 83, whereupon the plurality of band pass filters 
are controlled by the regular command signal generated from the auxiliary 
flow signal and/or the frequency of the primary flow signal. In the 
particular illustrative embodiment, the over-ride scanner 83 is turned on 
when a value of the input primary flow signal greater than a preset 
minimum value is detected and a value of the output primary flow signal 
greater than a preset threshold value is not detected. In an alternative 
design wherein the input signal level detector 84 is omitted, the 
over-ride scanner 83 is turned on when the output primary flow signal 
level detector 85 fails to detect a value greater than the preset 
threshold value. 
In FIG. 11 there is illustrated an embodiment of the plurality of the 
comparator circuits respectively controlling the switching on and off of 
the plurality of electric switches respectively belonging to the plurality 
of the band pass filters included in a parallel bank of the band pass 
filters. Each comparator circuit adds to or subtracts from the reference 
voltage a hysteresis voltage whereby effecting the transition from a band 
pass filter to another filter included in the parallel bank of filters of 
the present invention. In this particular embodiment of the comparator 
circuit, the hysteresis voltage added to or subtracted from the reference 
voltage set forth in the mth comparator circuit is equal to 
##EQU1## 
where V.sub.CC is the supply voltage that is set equal to +5 volts in the 
particular illustrative embodiment. Therefore, when the fluid velocity or 
the corresponding frequency increases from a lower value to a higher value 
by crossing over the lower bound of the mth band pass filter, the 
transition from a lower band pass filter to the mth band pass filter 
occurs at a fluid velocity or the corresponding frequency generating the 
command signal voltage of the following value: 
##EQU2## 
When the fluid velocity or corresponding frequency decreases from a higher 
value to a lower value by crossing over the upper bound of the mth band 
pass filter the transition from a higher band pass filter to the mth band 
pass filter occurs at a fluid velocity or the corresponding frequency 
generating the command signal voltage of the following value: 
##EQU3## 
Therefore, the upward transition between an adjacent pair of band pass 
filters occurs at a fluid velocity slightly higher than the fluid velocity 
whereat the downward transition between the adjacent pair of band pass 
filters occurs and, consequently, the occurrence of the alternative and 
repeated cyclic transition between an adjacent pair of band pass filters 
due to the fluid flow taking place at a velocity coinciding with 
borderline velocity between the adjacent pair of band pass filters is 
prevented and the error in the flow measurement originating from the 
distortion of the wave shape of the primary flow signal resulting from the 
alternative and repeated cyclic transition between an adjacent pair of 
band pass filters is eliminated. In an alternative design of the 
comparator circuit, the mode of transition from one band pass filter to 
another band pass filter can be changed in such a way that the downward 
transition to the mth band pass filter occurs when the fluid velocity or 
the corresponding frequency generates the command signal voltage given by 
equation (2), while the upward transition to the mth band pass filter 
occurs when the fluid velocity or the corresponding frequency generates 
the command signal voltage given by equation (3). The particular method 
described in conjunction with FIG. 11, that makes the upward and downward 
transition to an individual band pass filter to occur at two different 
fluid velocities separated from one another by a small and finite 
interval, is only one of many available designs well known to those 
skilled in the art of switching electric circuits and relays and, 
consequently, other designs may be employed in the practice of the present 
invention without departing from the teachings of the present invention. 
The use of the compound electronic filter of the present inventions in 
conjunction with a well designed vortex flowmeter enables the vortex 
flowmeter to measure air flows under the standard condition as low as 1 
meter per second and water flows as low as 0.1 meters per second. While 
the principles of the inventions have now been made clear by the 
illustrative embodiments shown and described, there will be many 
modifications of the structures, arrangements, proportions, elements and 
materials, which are immediately obvious to those skilled in the art and 
particularly adapted to the specific working environments and operating 
conditions in the practice of the inventions without departing from those 
principles. It is not desired to limit the inventions to the particular 
illustrative embodiments shown and described and accordingly, all suitable 
modifications and equivalents may be regarded as falling within the scope 
of the inventions as defined by the claims which follow.