Adaptive filtering for a vortex flowmeter

A method and apparatus for increasing the resolution and accuracy of flow measurements made by vortex flowmeters through the use of an adaptive filter. A vortex sensor signal is conditioned by a bandpass filter whose corner frequencies are dynamically altered as a function of the measured frequency of the vortex sensor signal. When the change in frequency is relatively small, the filters' corner frequencies are set to track the frequency signal in accordance with a specified bandwidth thereby improving the signal-to-noise ratio. For large frequency changes, a new frequency signal is searched for thereby avoiding tracking erroneous noise signals. Furthermore, compensation for additional or missed vortex pulses is made thereby generating a more accurate vortex frequency measurement.

BACKGROUND 
1. Technical Field 
This invention relates to industrial process control instrumentation, more 
particularly, adaptive filtering techniques. 
2. Background Art 
It has been known for many years that vortices are developed in a fluid 
flowing past a non-streamlined obstruction. It also has been known that 
with certain arrangements vortices are developed by alternate shedding at 
regular intervals from opposite edges of the obstruction to form 
corresponding rows of vortices. Such vortices establish a so-called von 
Karman "vortex street," which is a stable vortex formation consisting of 
two nearly-parallel rows of evenly-spaced vortices traveling with the flow 
stream. 
In a von Karman vortex street, the vortices of one row are staggered 
relative to those of the other row by approximately one-half the distance 
between consecutive vortices in the same row. The spacing between 
successive vortices in each row is very nearly constant over a range of 
flow rates, so that the frequency of vortex formation is correspondingly 
proportional to the velocity of the fluid. Thus, by sensing the frequency 
of vortex formation it is possible to measure the fluid flow rate. Devices 
for that purpose are often referred to as vortex meters or vortex 
flowmeters. 
Various types of vortex meters have been available commercially for a 
number of years. Typically, these vortex meters comprise a vortex-shedding 
body mounted in a flow tube together with a sensor for detecting the 
generation of vortex formation. Sensors used to detect the vortices often 
include diaphragms which fluctuate in response to alternating differential 
pressure variations generated by the vortices. The pressure applied to the 
diaphragms is transferred to a sensor or transducer which then produces 
electronic signals responsive to differential pressure variations applied 
to the diaphragms. This differential pressure measurement is used, in 
turn, to measure the frequency of vortex formation and ultimately the 
fluid flow rate or velocity. 
The sensor produces an analog sinusoidal voltage signal with frequencies 
ranging from 0 Hz to 3200 Hz. Various types of electronic components are 
used to condition and process the vortex sensor signal and thereby measure 
the flow rate. In many applications, the flowmeter circuitry is 
constrained by cost and, in addition, power consumption in order to adhere 
to industrial instrumentation standards. 
One such type of signal conditioning component is an electrical filtering 
circuit for sifting out noise signals associated with the acoustic, 
electrical, and mechanical vibration sources existing in the ambient 
flowmeter surroundings. Vortex sensor generated signals distorted by these 
noise signals result in errors in counting the vortex shedding frequency 
and, consequently, in measuring the flow rate. To alleviate this error, 
the signal passes through a bandpass filter which passes a specified band 
of frequencies while attenuating all signals outside the band. Due to the 
variable frequency range of the vortex signal arising from different size 
meters and process conditions, the filter needs to be tailored for a 
particular application. 
To avoid having to redesign the filter for each different frequency range, 
the prior art teaches of various adaptive filtering techniques for 
adapting or tuning the filter automatically to follow the vortex signal. 
Tracking filters and filter groups are commonly used techniques. 
Tracking filters have a frequency pass band that tracks or follows the 
changing frequency of a signal applied to its input. These filters consist 
of an active filter and a feedback means to control a preselected 
frequency pass band of the filter in accordance with the frequency of the 
output signal from the filter. However, tracking filters can lock on to 
noise signals rather than the vortex signal, thereby giving a false 
measure of the flowrate. 
Filters groups consist of a plurality of electronic filters having a 
control mechanism which switches on the appropriate filters in response to 
the measured vortex shedding frequency. This technique is costly, 
requiring complex circuitry and consuming a large amount of power. 
Both of these techniques are of limited value for industrial applications 
requiring low power consumption, low cost component construction, and 
reliable performance. Accordingly, there exists a need for an improved 
apparatus for adaptively filtering the vortex sensor signal used in 
industrial process instrumentation. 
It is an object of this invention to provide a reliable technique for 
filtering a variable frequency analog vortex sensor signal. 
It is a further object of the invention to provide a filtering technique as 
described above constructed with low cost components and utilizing low 
power consumption. 
It is a further object to provide a filter as described above which is 
dynamically tunable to the frequency of the input signal while avoiding 
locking onto noise signals. 
It is another object to provide a filter as described above which provides 
a clock-controlled tunable means for altering the corner frequencies of a 
band-pass filter in response to a variable-frequency analog sinusoidal 
input signal. 
Other general and specific objects of this invention will be apparent and 
evident from the accompanying drawings and the following description. 
SUMMARY OF THE INVENTION 
A method and apparatus for providing a micro-power adaptive filtering 
technique for a variable frequency analog vortex sensor signal is herein 
described. 
The apparatus of the invention comprises an analog signal conditioner and 
micro-controller for determining the flow rate of a process. An analog 
sinusoidal signal representative of the alternating differential pressure 
variations is generated by a vortex sensor. The vortex signal is used to 
calculate the fluid flow rate or velocity. It is processed by an analog 
signal conditioner which filters the signal to eliminate frequency signals 
attributable to acoustic, electrical, and mechanical vibration sources 
existing in the ambient surroundings in flow measuring environments. The 
filtered signal is then transmitted to a square wave generator producing 
an equivalent digital square wave pulse train for use in computing the 
measured vortex frequency and ultimately the flow rate of the process. 
The analog signal conditioner utilizes a bandpass filter comprising a low 
pass and high pass filter whose corner frequencies are under the control 
of the micro-controller. The settings of these corner frequencies are 
based on the measured vortex frequency of the vortex sensor signal and are 
altered to achieve a desired bandwidth about the vortex frequency. This 
bandwidth preserves a high signal-to-noise ratio which in effect produces 
a more accurate flow measurement over a wider flow range under adverse 
flow conditions. 
The method of the invention is designed to preserve the specified bandwidth 
about a changing vortex frequency. As the vortex frequency changes, the 
corner frequencies of the filters are dynamically adjusted to track the 
vortex frequency signal accordingly. However, for rapid changes in the 
vortex frequency, the bandwidth is opened wide and a renewed search for 
the vortex signal is initiated thereby avoiding locking onto an erroneous 
noise signal. Furthermore, the method compensates the vortex sensor signal 
for added or missed vortex pulses thereby producing a more accurate 
measurement of the vortex frequency. 
The electronic circuitry implementing the invention is designed to consist 
of low-cost components and to operate at micro-power levels being in the 
order of less than 10 mW. Both of these considerations are essential for 
industrial instrumentation.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
A method and apparatus for providing a micro-power adaptive filtering 
technique suitable for a variable frequency analog vortex sensor signal is 
herein disclosed. 
As is well known in the art, a vortex shedding flowmeter can be prepared by 
placing a non-streamlined obstruction (commonly referred to as a vortex 
shedder bar) in the fluid flow together with a sensor for detecting the 
generation of vortex formation. Sensors used to detect the vortices often 
include diaphragms which fluctuate in response to alternating differential 
pressure variations generated by the vortices. The sensor generates an 
analog sinusoidal signal representative of these altering differential 
pressure variations which is used to calculate the frequency of vortex 
formation and ultimately the fluid flow rate or velocity. 
Referring more particularly to the drawings and initially to FIG. 2, there 
is shown the basic components involved in generating an electrical output 
signal representative of the process flow rate. The vortex sensor, 10, 
generates an analog sinusoidal signal, 12, with frequencies ranging 
approximately from 0 Hz to 3200 Hz. However, this invention is not limited 
to this frequency range as others may be used. The vortex signal, 12, is 
then buffered and amplified by a standard preamplifier, 14, generating 
signal 16. Signal, 16, is transmitted further to an analog signal 
conditioner, 18, which filters out noise signals associated with the 
signal resulting from acoustic, electrical, and mechanical vibration 
sources existing in the ambient surroundings in flow measuring 
environments. Analog signal conditioner, 18, also transforms the signal 
16, into a square-wave pulse train, 20, of constant voltage height and 
having a frequency equal to the frequency of the vortex shedding. 
Micro-controller 22 counts the number of edges of square-wave pulse train 
20 occurring in a measured time interval. This information is used by 
micro-controller 22 to compute the frequency of the vortex signal. 
Micro-controller 22 then converts this frequency to a flow rate in 
engineering units, and instructs the signal output generator 24 to output 
the result as a 4 to 20 mA signal 26, digital signal 27, or, 
alternatively, as a scaled pulse signal 28. 
FIG. 1 depicts the basic components of analog signal conditioner 18. 
Referring to FIG. 1, analog signal conditioner 18 consists of a bandpass 
filter 35, composed of a cascaded high pass filter 30 and low pass filter 
34, for filtering out the noise distortions from input signal 16; 
square-wave generator 38 for converting the filtered input signal into a 
square-wave pulse train 40; and a dock pulse generator 46 which is used to 
adaptively set the corner frequencies of the low pass and high pass 
filters under the control of micro-controller 22. 
High pass filter 30 is a second-order active filter whose low corner 
frequency is dynamically tunable. The filter is controlled by clock pulse 
signal 48 and its inverted counterpart 50, which are generated by dock 
pulse generator 46. The frequency of dock pulse signals 48 and 50 
effectively determines the corner frequency of high pass filter 30. This 
frequency is determined by micro-controller 22 and its corresponding 
factor 42 is transmitted to dock pulse generator 46. Micro-controller 22 
determines this frequency as a function of the vortex signal frequency, 
F.sub.VOR. The measurement of the vortex signal frequency, F.sub.VOR. is 
compensated for missed and added vortices computed by micro-controller 22 
which will be discussed in detail below. The output of high pass filter 30 
is a partially filtered signal 32 which is transmitted to low pass filter 
34. 
Low pass filter 34 is a second-order active low pass filter controlled by 
clock pulse signal 52 and its inverted counterpart 54, which are also 
generated by dock pulse generator 46. The frequency of clock pulse signals 
52 and 54 determines the corner frequency of low pass filter 34. The clock 
pulse frequency is determined by micro-controller 22 and a corresponding 
signal 44 is transmitted to dock pulse generator 46. Micro-controller 22 
determines this frequency as a function of the vortex signal frequency in 
a similar fashion as discussed above. The output of low pass filter 34 is 
filtered signal 36 which is transmitted to square-wave generator 38. 
Square-wave generator 38 converts filtered signal 36 into a square-wave 
pulse train 40 having a constant voltage height and frequency equal to the 
frequency of the vortex shedding. The edges of the pulse train correspond 
to the zero crossings of the original vortex signal. Micro-controller 22 
counts a number of positive-going edges occurring over a time interval. 
Counters N.sub.P and N.sub.C are used for this computation and will be 
described in further detail below. Micro-controller 22 also determines 
whether the sampled pulse train needs to be corrected for added or dropped 
pulses. An added pulse is an erroneous pulse detected due to noise 
disturbances. A dropped pulse occurs when a vortex failed to form at the 
vortex shedder bar. Counter, N.sub.-, indicates the number of calculated 
dropped pulses and counter,N.sub.+ indicates the number of added pulses. 
Once the frequency of the vortex signal is determined micro-controller 22 
then, in turn, computes the appropriate frequency factors N.sub.L, 
N.sub.H, signals 42, 44 respectively which will adjust the corner 
frequencies for the low pass and high pass filters. Therefore, 
micro-controller 22 can be used to dynamically alter the filter settings 
of both the high pass and low pass filters based on the measured frequency 
of the vortex signal, herein denoted as F.sub.VOR. 
As detailed below, the function of the combination of the high pass filter 
and the low pass filter is to concentrate the electronic sensitivity on a 
limited range of frequencies in which the vortex signal is likely to be 
found. Turning now to FIG. 4, F.sub.LAS is the cut-off of the high pass 
filter, and F.sub.HAS is the cut-off of the low pass filter. To the left 
of F.sub.LAS and to the right of F.sub.HAS the signal is attenuated by the 
filters in order to reduce the likelihood of system noise from whatever 
source interfering with the vortex signal. The frequency of the passing 
vortices F.sub.VOR would be located approximately at a specified position 
between these two cut-offs for the filters when the filter tracking 
function is operating. Signals of large amplitude well outside of the two 
cut-offs (to the left or the right) would be substantially attenuated. The 
further a noise signal frequency is from the tracked frequency range, the 
more it is attenuated and the less likely to generate an erroneous pulse 
count. This depends on both signal frequency and amplitude to distinguish 
noise from passing vortices. 
Filter attenuation serves to optimize the signal to noise ratio. The 
filters in a tracking mode therefore fully optimize the signal to noise 
ratio by attenuating signals that are unlikely to have been caused by the 
passing vortices. If the frequency of the vortices F.sub.VOR begins to 
change due to an increase or decrease in flow, the tracking filter will be 
adjusted to move the cut-off filter limits to avoid the situation where 
the vortex signal is attenuated due to its being outside the high pass or 
low pass limits. When there are rapid changes in the vortex frequency, the 
filters are set in a search mode which has a lower signal to noise ratio 
but is more capable of picking up F.sub.VOR wherever it is occurring 
within the flowmeter's normal operating range. The search mode moves the 
filter cut-offs so that the signals within the bandwidth are unattenuated 
thereby increasing the signal to noise ratio as described below. Once 
tracking of the vortex signal begins, the bandwidth is preserved. 
FIG. 3 depicts the steps employed in the adaptive filtering scheme. Prior 
to discussing these steps, the filter settings used in this scheme will be 
presented first. There can be four classifications of settings: frequency 
range limits, base settings, adaptive filter settings, and virtual 
settings. The frequency range limits, F.sub.URL, and F.sub.LRL, define the 
frequencies for which the vortex flowmeter does not experience severe 
mechanical and/or electrical interferences while F.sub.URV is a user 
defined limitation. For example at certain flow velocities, the vortex 
flowmeter can be subjected to mechanical damage. The frequency threshold 
above which this velocity occurs can represent the upper frequency range 
limit F.sub.URL. The low frequency range limit F.sub.LRL represents the 
frequency threshold at which point the signal to noise ratio is high 
enough to distinguish the vortex signal. The upper frequency range value 
F.sub.URV is a user-defined limit which represents the highest frequency 
range that a particular application prefers. Once set these frequency 
range limits are not altered by the adaptive filtering scheme. 
The base settings, F.sub.LBS and F.sub.HBS, define the frequency range 
within which the adaptive filtering scheme operates. They are defined by 
the particular application and once set are not altered by the adaptive 
filtering scheme. The adaptive filter settings, F.sub.LAS and F.sub.HAS, 
can be used to preserve a specific bandwidth about the measured frequency 
of the vortex signal thereby preserving a high signal to noise ratio. The 
adaptive filter settings vary as the frequency of the vortex signal 
changes. Both the base and adaptive filter settings are "real" settings in 
that the filtering circuitry can be implemented to obtain one of these 
frequencies. By contrast the virtual settings are "virtual" as they can be 
derivations that are used to indicate whether the adaptive filtering 
scheme is to proceed in either a search or track mode as detailed below. 
Referring again to FIG. 4, the upper range frequency limit, F.sub.URL, is 
based on two flow velocity constraints that limit the upper range flow 
rate. Both are imposed to avoid mechanical damage of the flowtube and 
sensor. There is also a high frequency limit, F.sub.HEL, imposed by the 
electronics that can limit the upper range flow rate. The first velocity 
constraint, V.sub.PL, is that the differential pressure generated by the 
vortex shedding should not exceed a specified value set in accord with the 
following mathematical relation: V.sub.PL &lt;=200/.sqroot..rho. where .rho. 
is the process fluid density. The second is the sonic limit, V.sub.SL 
which is imposed to prevent supersonic conditions which can cause shock 
waves. Preferably, it can be set to the value of 600 feet/sec. 
The flow rate limits associated with these two velocity limits are Q.sub.PL 
=A*V.sub.PL, and Q.sub.SL =A*V.sub.SL, where A is the cross-sectional area 
of the flow tube. The corresponding upper range frequency limit F.sub.URL 
is given by K* Q.sub.PL, K*Q.sub.SL, or F.sub.HEL, whichever is smaller, 
where K is the meter factor at the relevant process conditions. In most 
applications the nominal upper range flow value Q.sub.URV is significantly 
less than the upper range flow limit Q.sub.URL. In this case the user sets 
the desired upper range flow value. The corresponding upper range 
frequency value F.sub.URV is given by K*Q.sub.URV, which is less than 
F.sub.URL. 
The lower range flow velocity limit, V.sub.LRL, which is the nominal flow 
velocity below which the output of the vortex sensor is set to zero is 
defined by the following mathematical relation: V.sub.L &lt;=k/.sqroot..rho., 
where k is a constant whose value depends on the sensor type process 
conditions. The lower range flow rate limit can be given by Q.sub.LRL 
=A*V.sub.LRL, where A is the cross-sectional area of the flow tube. The 
corresponding lower range frequency limit is F.sub.LRL =K*Q.sub.LRL, where 
K is the meter factor at the relevant process conditions. 
The lower and upper range frequency limits can be used to establish two 
base settings: a base setting for the corner frequency of the high pass 
filter, denoted as F.sub.LBS ; and another for the corner frequency of the 
low pass filter, denoted as F.sub.HBS. These base settings represent the 
operating range for the adaptive filtering scheme. The corner frequency of 
the high pass filter F.sub.LBS can be set equal to or the next closest 
setting below some fraction of the lower frequency range limit; preferably 
2/3 of F.sub.LRL. The corner frequency of the low pass filter Fuss can be 
initially set equal to or at the next closest setting above some fraction 
of the upper frequency range value, preferably 1/2 of the F.sub.URV. 
There can also be two adaptive filter settings based upon the frequency of 
the vortex signal, F.sub.VOR. For the purposes of this application, the 
adaptive setting for the corner frequency of the high pass filter is 
denoted as F.sub.LAS, and the adaptive setting of the corner frequency of 
the low pass filter is denoted as F.sub.HAS. The purpose of these settings 
is to preserve a specific bandwidth, .DELTA.F.sub.L +.DELTA.F.sub.H, about 
the frequency of the vortex signal. This frequency bandwidth is set to 
improve the signal to noise ratio thereby producing more accurate flow 
measurements for a wider flow range and under adverse flow conditions. 
Preferably, a high signal to noise ratio is desired so that the vortex 
signal is distinguishable from the noise signals. Additionally, the filter 
settings must not be too close to the vortex frequency signal otherwise 
the vortex signal will be attenuated. 
The low frequency part of the bandwidth is denoted as .DELTA..sub.FL and 
the high frequency part of the bandwidth is denoted as .DELTA.F.sub.H. The 
low frequency part of the bandwidth, .DELTA.F.sub.L, is narrower than the 
high frequency portion of the bandwidth, .DELTA.F.sub.H. This is due to 
the predominance of low frequency disturbances present in vortex 
flowmeters. An increase of these disturbances is seen with low flow rates 
where due to smaller signal levels filtering becomes more important. 
Therefore to accommodate these types of disturbances, the low frequency 
part of the bandwidth, .DELTA.F.sub.L, can be set to approximately one 
haft of the vortex frequency (0.5*F.sub.VOR), and the high frequency part 
of the bandwidth, .DELTA.F.sub.H, can be set to approximately three times 
the vortex frequency (3*F.sub.VOR). 
FIG. 5 is a schematic diagram illustrating the range of corner frequency 
settings for the high and low pass filters. However, it should be noted 
that this invention is not limited to these values as they are chosen for 
illustration purposes. For example, referring to FIG. 5, applications 
having a vortex signal with frequencies within the range of 0 Hz to 3200 
Hz, the range of the corner frequency of the high pass filter can be 
between 2 Hz to 145 Hz, and the range of the corner frequency of the low 
pass filter can be between 10 Hz and 2000 Hz. Within these ranges there 
can be a finite number of corner frequency filter settings uniformly 
distributed geometrically over the frequency range. The finite number of 
corner frequency filter settings is advantageous for obtaining a specific 
setting through a digital signal. For example, within the low filter 
frequency range, there can be m settings, a.sub.1, . . . a.sub.m, and 
within the high filter frequency range n settings b.sub.1, . . . b.sub.n. 
Preferably m and n can be set to a value of 32. In this instance, a 
specific setting can then be obtained by a digital signal having a value 
which can be represented by at least 5 bits in length (i.e. 2.sup.5 =32). 
Referring to FIG. 4, there are also six virtual settings which can be 
utilized to determine when the adaptive filtering scheme is to process in 
track mode or search mode. In search mode, the bandwidth about the 
measured vortex frequency is opened wide and a renewed search for the 
vortex signal is initiated. This usually occurs when rapid changes are 
present in the vortex frequency. This prevents the filtering scheme from 
locking onto an erroneous noise signal. By contrast in the tracking mode, 
the filtering scheme maintains a specified bandwidth about the measured 
vortex frequency. The virtual settings which relate to the low frequency 
part of the bandwidth are f.sub..gamma.L, f.sub..beta.L, 
f.sub..sub..alpha.L and those which relate to the high frequency part of 
the bandwidth are f.sub..gamma.H, f.sub..beta.H, f.sub..alpha.H. These 
settings are set in relation to the measured vortex frequency F.sub.VOR 
and are considered virtual as they may not be at one of the predefined 
corner frequency filter settings, a.sub.m or b.sub.n. However, these 
virtual settings are selected so that there is at least one predefined 
corner frequency filter setting between two consecutive virtual settings 
and between F.sub.VOR and the adjacent virtual setting. 
The range defined by the setting f.sub.65 L -f.sub..beta.L is the desired 
range for maintaining the low corner frequency. Signals below F.sub.LAS 
are attenuated whereas signals within the range f.sub..gamma.L 
-f.sub..beta.L preserve the low frequency part of the desired bandwidth, 
.DELTA.F.sub.L. This range is particularly useful for search mode 
processing. In search mode processing, the adaptive filter setting is 
adjusted at each processor cycle by one setting until it reaches the first 
setting within the range defined by f.sub..gamma.L -f.sub..beta.L. In the 
track mode depending on the position of the measured vortex frequency, the 
setting can be incremented or decreased. It will remain unchanged when 
F.sub.LAS stays within the range defined by f.sub..gamma.L -f.sub..beta.L. 
Similarly, the range defined by the settings f.sub..beta.H -f.sub..gamma.H 
is the desired range to maintain the high corner frequency. Signals above 
F.sub.HAS are attenuated whereas signals within the range preserve the 
high frequency part of the desired bandwidth, .DELTA.F.sub.H. This range 
is particularly useful for search mode processing. In search mode 
processing, the adaptive filter setting is adjusted at each processor 
cycle by one setting until it reaches the first setting within the range 
defined by f.sub..beta.H -f.sub..gamma.H. In the track mode depending on 
the position of the measured vortex frequency, the setting can be 
incremented or decreased. It will remain unchanged when F.sub.HAS stays 
within the range defined by f.sub..beta.H -f.sub..gamma.H. 
The frequency F.sub..alpha. defines the point at which the filtering scheme 
reverts to search mode processing. In search mode, the filtering scheme 
stops tracking the current signal, sets the filter back to its base 
settings, and then reinitiates its search for the vortex signal. This is 
done to prevent the filter from locking onto a noise signal. A measured 
vortex frequency greater than the frequency defined by f.sub..alpha.L or 
less than the frequency defined by f.sub..alpha.H triggers the adaptive 
filtering scheme to revert to the base setting and then to initiate the 
search mode. 
As the various settings used in the adaptive filtering scheme have been 
described above, a detailed description of the scheme is herein presented. 
Referring to FIG. 3, in step 72, the first step of the adaptive filtering 
scheme is to determine the frequency range limits and base settings of the 
bandwidth. These settings are determined and set as discussed above and 
remain constant for a given application. The adaptive filter settings, 
F.sub.LAS and F.sub.HAS, can vary throughout the adaptive filtering scheme 
and initially can be set to their respective base settings, F.sub.LAS 
=F.sub.LBS, F.sub.HAS =F.sub.HBS. 
Next, as illustrated in step 73, micro-controller 22 can be employed to 
detect the presence of flow from square-wave pulse train 40. If no flow is 
detected, satellite micro-controller 22 continues to poll. The presence of 
flow can be determined by measuring the periods of five successive pulses. 
If the periods for each of the five successive pulses are approximately 
identical, then it can be determined that flow is present. 
Once the presence of flow is detected, the vortex signal 40 is analyzed for 
dropped and added pulses in step 74. This is done in order to accurately 
determine the vortex shedding frequency. An added pulse may be considered 
when a sensor signal of sufficient amplitude is recorded to indicate a 
passing vortice. This signal must be compared with the frequency of vortex 
pulses most recently received from the sensor. A comparison of the period 
between the most recently received pulse and the median period for the 
present and four previous pulses should be closely matched. 
If the period between pulses is too short, shorter than a reasonable 
variation based on the recent median period as described below, then the 
pulse received is considered an added pulse. This analysis is based on the 
assumption that the flowrate and hence the vortex frequency will generally 
change in a gradual manner and will not be subject to extremely rapid 
changes or discontinuities. These added pulses can be caused by noise 
generated from any number of sources that then might be transmitted 
through the fluid or piping to the vortex sensor. Possible sources include 
nearby equipment tuning on and off, mechanical vibrations, and electronic 
noises. In such a situation, as described below, an internal counter in 
the controller is incremented to indicate an added pulse not generated by 
flow vortices passing the vortex sensor. 
In the converse of the above situation, the period between sequential 
pulses may be greater than can be reasonably expected based on the 
frequency of the most recent pulses and period between them. It can then 
be surmised that a vortex failed to form at the vortex shedder or was not 
detected for some reason. This can arise due to turbulence or debris in 
the flow stream flowing past the vortex shedder that result in the failure 
to form vortices of sufficient amplitude to be detected by the vortex 
sensor electronics. It should be noted that the vortex sensor signal is 
compared with a cut-off amplitude such that frequencies below a 
predetermined amplitude will not result in the generation of an electronic 
pulse. In these situations, the dropped pulse counter in the controller is 
incremented in order to artificially compensate for the missing pulse. 
This results in a more accurate average vortex frequency. A relatively low 
number of missing pulses, if not compensated for, could substantially 
affect the average vortex frequency calculation. The actual adding and 
dropping of pulses is described as follows. 
The added and dropped compensation scheme can utilize four counters: 
N.sub.P, the total number of sampled pulses; N.sub.C, the total number of 
clock cycles; N.sub.-, the number of dropped pulses over a sampling 
period; N.sub.+, the number of added pulses over the same sampling period. 
The scheme commences with controller 22 detecting the zero crossings of 
square-wave pulse train 40 for a number of clock cycles. For the purposes 
of this application, the term clock cycle denotes the micro-controller's 
internal clock pulse period. The sampling period constitutes a number of 
clock cycles. At a specific instant of time, the total number of pulses 
sampled or vortices is denoted as N.sub.P. and the total number of clock 
cycles is denoted as N.sub.C. A sampling period is denoted as 
.DELTA.N.sub.C which can be defined as the arithmetic difference between 
two clock cycle counts. Similarly, the number of vortices counted over a 
specified time interval can be denoted as .DELTA.N.sub.P and can be 
defined as the arithmetic difference between two vortex pulse counts 
computed over the same time interval. Referring now to FIG. 6, in step 
148, controller 22 can determine N.sub.P, N.sub.C, and hence 
.DELTA.N.sub.C, .DELTA.N.sub.P, in order to determine the average 
frequency F.sub.VOR. The average frequency can be determined as F.sub.C 
(.DELTA.N.sub.P /.DELTA.N.sub.C,), the ratio of the number of vortex 
pulses sampled over the sampling period, where F.sub.C is the frequency of 
the clock pulses. Counters .DELTA.N.sub.P and .DELTA.N.sub.C also be used 
in the determination of the vortex frequency. The average vortex period 
T.sub.N is given by the inverse of the average frequency. 
The median and average period is used in the determination of the number of 
added and dropped pulses. In step 150, the median period is calculated as 
the median of the present and four previous consecutive average periods, 
and is denoted herein as T.sub.m,n. A comparison of the current or last 
average period, T.sub.n, is made with the median period T.sub.m,n in step 
152 against a criteria which determines whether a missed pulse or an added 
pulse has occurred. It is expected that pulses should be present within a 
certain range of the median period. If pulses are not detected within this 
expected range, then it is assumed that a dropped pulse has occurred. 
Pulses that are detected earlier than the expected range are considered 
added pulses. Preferably, the expected range is +/-25% of the median 
period. However, this invention is not constrained to this particular 
range. Others may be used so long as they are large enough to capture the 
natural variations in the period and not loose any of the naturally formed 
vortices. Any expected range of less than 50% of the median period could 
meet this requirement. 
In step 154, if the current average period is less than 3/4ths of the 
median period, T.sub.n &lt;0.75*T.sub.m,n, then it is assumed that one added 
pulse has occurred. The computation proceeds, in step 156, to determine 
whether a second added pulse has occurred. This is assumed when the 
average period of the previous pulse is detected before the expected range 
and the sum of the current and previous pulses are detected before the 
expected range. This occurs if the following mathematical relation holds: 
T.sub.n-1 &lt;0.75* T.sub.m,n-1 and T.sub.n +T.sub.n-1 &lt;0.75* T.sub.m,n. The 
added pulse counter, N+, is incremented accordingly. 
The criteria set forth in step 158 determines whether a normal vortex pulse 
has occurred. This occurs if the following mathematical relation is met: 
0.75*T.sub.m,n &lt;=T.sub.n &lt;1.25*T.sub.m,n. Next, the criteria set forth in 
step 160 determines whether a dropped pulse has occurred. This happens 
when no pulse is detected after the expected range after one full median 
period has lapsed. This occurs if the following mathematical relation is 
met: 1.25* T.sub.m,n &lt;=T.sub.n &lt;2.25* T.sub.m,n. The detection of two 
dropped pulses is made in step 162 and occurs if no pulse is detected 
after the expected range after two full median periods have lapsed. This 
occurs if the following mathematical relation is met: 2.25*T.sub.m,n 
&lt;=T.sub.n. If either of these two previous conditions are met, the dropped 
pulse counter, denoted as N.sub.- is incremented accordingly. The average 
frequency will not be compensated for more then two consecutive missing 
pulses. Once the appropriate pulse counters have been updated, the set of 
four previous consecutive average periods, T.sub.n-1, T.sub.n-2, 
T.sub.n-3, T.sub.n-4, is updated to include the current average period and 
to exclude the oldest average period in step 164. 
The added/dropped pulse detection scheme is not constrained to a 
calculation based on average period measurements. Alternatively, the 
scheme can utilize frequency measurements in lieu of choosing the current 
and five previous average periods. Instead of basing the added and dropped 
pulse compensation on the average and median periods of the pulses 
contained in signal 40, the scheme can utilize the average and median 
frequencies of the pulses contained in signal 40. The scheme can proceed 
in the same manner as detailed above with the exception of the average and 
median frequencies are used in place of the average and median periods of 
pulse train 40. 
Referring back to FIG. 3, in step 75 the frequency of the vortex signal and 
the corresponding virtual filter settings are determined. This computation 
utilizes .DELTA.N.sub.P, .DELTA.N.sub.C, N.sub.-, N.sub.+ which were 
computed by micro-controller 22 previously. The vortex frequency can be 
determined in accord with the following mathematical relation: F.sub.VOR 
=F.sub.C [(.DELTA.N.sub.P +N.sub.- -N.sub.+)/.DELTA.N.sub.C ] where 
F.sub.C is the frequency of the clock pulses. Once the vortex frequency is 
determined, the virtual settings f.sub..gamma.L, f.sub..beta.L, 
f.sub..alpha.L, f.sub..gamma.H, f.sub..beta.H, f.sub..alpha.H are also 
determined in step 75. These setting can be set in accord with the 
following mathematical relations: f.sub..beta.L /f.sub..gamma.L =1.5; 
f.sub..alpha.L /f.sub..beta.L =1.3; F.sub.VOR /f.sub..beta.L =1.64, 
f.sub..beta.H/f.sub..gamma.H =1.5; f.sub..alpha.H /f.sub..beta.H =1.3; 
F.sub.VOR /f.sub..gamma.h =0.27. These settings were chosen to preserve 
the specified bandwidth .DELTA.F.sub.L, .DELTA.F.sub.H about the vortex 
frequency and may change accordingly. 
Next, in step 76, a determination is made as to whether the measured vortex 
frequency F.sub.VOR has experienced a rapid change. This check is made in 
order to prevent the filtering scheme from blindly locking onto a noise 
signal. This determination can be made relative to the low frequency range 
by checking the ratio of the latest vortex frequency measurement F.sub.VOR 
over the current adaptive filter setting for the low corner frequency, 
F.sub.LAS. If this ratio is less than a certain threshold then a rapid 
change in the vortex frequency can be assumed to have occurred in the low 
frequency range. Preferably, the threshold is the value of F.sub.VOR 
/f.sub..alpha.L from the most recent measurement of the vortex signal. The 
invention is not constrained to this value; others may be used so long as 
rapid changes in the vortex signal are detected. This check can be made in 
accord with the following mathematical relation: F.sub.LAS 
&gt;=f.sub..alpha.L. The filtering scheme then enters the search mode. 
Otherwise, the filtering scheme proceeds in the track mode as discussed 
below. 
In the search mode, step 77, the low portion of the bandwidth is opened 
wide and a renewed search of the vortex signal is initiated. This is 
performed by setting the low frequency adaptive filter setting to the low 
frequency base setting, F.sub.LAS =F.sub.LBS, and advancing the low 
frequency setting each processing cycle until the low corner frequency 
enters the f.sub..gamma.L --f.sub..beta.L range. This is accomplished 
through frequency factor signal N.sub.L, 42 which is determined by 
micro-controller 22 as described below and transmitted to clock pulse 
generator 46. 
Next, in step 78, the filtering scheme proceeds in track mode. In track 
mode the adaptive filtering setting, F.sub.LAS, is adjusted by one setting 
increments until F.sub.LAS comes within the range f.sub..gamma.L 
-f.sub..beta.L. For the case where F.sub.LAS is within the range 
f.sub..gamma.L -f.sub..beta.L, no adjustment is needed. If F.sub.LAS is 
within the range f.sub..beta.L -f.sub..alpha.L or F.sub.LBS 
-f.sub..gamma.L, an adjustment of one setting is made in the direction 
towards the range f.sub..gamma.L -f.sub..beta.L ; incremented by one for 
the case F.sub.LBS -f.sub..gamma.L, and decremented by one for the case 
f.sub..beta.L -f.sub..alpha.L. This adjustment is made through frequency 
factor signal N.sub.L, 42 and is described in further detail below. 
Similarly, a determination is made as to whether the measured vortex 
frequency F.sub.VOR has experienced a rapid change in the high frequency 
range as in step 79. This determination can be made by checking the ratio 
of the latest vortex frequency measurement F.sub.VOR over the current 
adaptive filter setting for the high corner frequency, F.sub.HAS. If the 
ratio exceeds a certain threshold, then a rapid change in the vortex 
frequency has occurred in the high frequency range. Preferably, the 
threshold is the value of F.sub.VOR /f.sub..alpha.H from the most recent 
measurement of the vortex signal. The invention is not constrained to this 
value; others may be used so long as rapid changes in the vortex signal is 
detected. This check can be made in accord with the following mathematical 
relation: F.sub.HAS &lt;=f.sub..alpha.H. The filtering scheme then enters the 
search mode. Otherwise, the filtering scheme proceeds in the track mode as 
discussed below. 
In the search mode, step 80, the high portion of the bandwidth is opened 
wide and a renewed search of the vortex signal is initiated. This is 
performed by setting the high frequency adaptive filter setting to the 
high frequency base setting, F.sub.HAS =F.sub.HBS, and tracking the vortex 
frequency signal until the high corner frequency enters the f.sub..beta.H 
-f.sub..gamma.H range. This is accomplished through frequency factor 
signal N.sub.H, 44 which is determined by micro-controller 22 as described 
below and transmitted to clock pulse generator 46. 
Next, in step 81, the filtering scheme proceeds in track mode. In track 
mode the adaptive filtering setting, F.sub.HAS, is adjusted by one setting 
increments until F.sub.LAS comes within the range f.sub..beta.H 
-f.sub..gamma.H. For the case where F.sub.HAS is within the range 
f.sub..beta.H -f.sub..gamma.H, no adjustment is needed. If F.sub.HAS is 
within the range f.sub..alpha.H, -f.sub..beta.H, or f.sub..gamma.H 
-F.sub.HBS, an adjustment of one setting is made in the direction towards 
the range f.sub..beta.H -f.sub..gamma.H ; incremented by one for the case 
f.sub..alpha.H, -f.sub..beta.H, and decremented by one for the case 
f.sub..gamma.H -F.sub.HBS. This adjustment is made through frequency 
factor signal N.sub.H, 44 and is described in further detail below. 
Once micro-controller 22 determines the recomputed value of the low corner 
adaptive filter setting, F.sub.LAS, it computes a frequency factor, 
N.sub.L, 42, which will obtain the desired frequency setting. Signal 42 is 
transmitted to clock pulse generator 46. Clock pulse generator 46 also 
receives signal 45 which can be a square-wave pulse train having a 
constant amplitude and frequency. From these input signals, it produces 
two signals 48, 50 which have identical amplitude as signal 45 (see FIG. 
1) but which are 180 degrees out of phase with respect to each other. 
Signals 48, 50 can then be used to establish the corner frequency of high 
pass filter 30 which is one of the m settings in the low frequency filter 
range. 
Signal 42, N.sub.L, and signal 44, N.sub.H, can be an eight-bit integer 
having values ranging from 1 to 256. Signal 45 can be a 50% duty cycle 
square-wave pulse train having a preferred frequency of approximately 500 
kHz. However, this invention is not limited to this frequency as long as 
the choice is divisible by frequency factor 42 resulting in one of the 
predetermined m low frequency range settings. 
Similarly, once micro-controller 22 determines the high corner frequency of 
the adaptive filter setting, F.sub.HAS, the corresponding frequency factor 
N.sub.H, 44, is computed and transmitted to clock pulse generator 46. 
Signal 44 is used by dock pulse generator 46 to generate the appropriate 
dock frequency which obtains the corner frequency of low pass filter 34. 
Clock pulse generator 46 proceeds to compute this corner frequency in the 
same fashion as described above. It results in generating signals 52 and 
54, which have identical amplitudes as signal 45 but which are 180 degrees 
out of phase with respect to each other. 
Turning now to the electrical schematics found in FIGS. 7, 8, and 9, the 
electrical circuitry implementing the adaptive filtering scheme of the 
cascaded high and low pass filter is discussed below in more detail. FIG. 
7 illustrates the circuit elements of high pass filter 30 which represents 
a novel combination of a known circuit topology, clocking signals, and 
circuit elements which results in micro-power consumption, particularly 
well-suited for industrial applications. Referring to FIG. 7, high pass 
filter 30 is an second order active filter encompassing switched 
capacitors in an equal-component-value Sallen and Key circuit topology. 
The benefits of this circuit is three-fold. 
First, the switched capacitors offer a significant improvement over 
conventional resistor-capacitor filters since the effective resistance 
values can be adaptively modified in accordance with the process flow 
frequency. By alternately dosing and opening the switched capacitors 66a 
and 66b at a dock frequency f.sub.c (signals 48, 50), the switched 
capacitors, effectively simulate a resistor having a value: R=1/C*f.sub.c, 
where C is the value of switched capacitors 66a and 66b. This use of the 
switched capacitors eliminates the use of resistors thereby diminishing 
power consumption which is crucial for industrial instrumentation. 
Second, the filter can be adapted to variable process fluid frequencies 
through the use of the dock frequency, f.sub.c, thereby eliminating the 
dependency on statically-sized circuit elements. For the high pass filter, 
this permits the corner frequency, F.sub.LAS of the high pass filter to be 
dynamically altered to accommodate a particular flow frequency in 
accordance with the following mathematical relation: F.sub.LAS 
=1/2.pi.*C/C.sub.1 *f.sub.c, where f.sub.c is the clock frequency, and 
C.sub.1 is the value of capacitors 64a and 64b. 
Lastly, the use of the equal-component-value second-order Sallen and Key 
circuit topology is advantageous for providing easy tuning and for 
independently adjusting damping of the filter response near the corner 
frequency. Each of these benefits are essential for achieving micro-power 
consumption, which is in the order of less than 10 mW of power. 
High pass filter 30 can consist of operational amplifier 60 having its 
inverting input terminal connected to the ratio of resistors 58a and 58b. 
Resistors 58a, 58b serve to set the gain and damping of the circuit. The 
noninverting input terminal is coupled to two equal-value-component 
capacitors 66a, 66b which are each coupled to charging switches 70a, 70b 
and discharging switches 68a, 68b respectively. Switches 68a and 70a are 
used to control the charge and discharge of capacitor 66a, and switches 
68b and 70b are used to control the charge and discharge of capacitor 66b. 
Switches 68a and 68b are controlled by dock signal 48 and switches 70a and 
70b are controlled by dock signal 50, the inverted counterpart of signal 
48. Charging switches 70a, 70b, and discharging switches 68a, 68b are 
alternately opened and dosed at the dock rate specified by the frequency 
of signals 48 and 50. 
Noninverting input terminal 60 is also coupled to resistor 62 and 
capacitors 64a and 64b. Resistor 50 provides a dc bias path to an 
electrical ground and is sized preferably at 20M ohms. Capacitors 64a, 64b 
are of equal value, preferably 0.039 uf and are sized in accordance with 
the frequency range of signals 48, 50. 
FIGS. 8 and 9 illustrate the components of low pass filter 34. Low pass 
filter 34 consists of two switched capacitor networks. The first network 
or stage one is depicted in FIG. 8 and the second network or stage is 
depicted in FIG. 9. Each stage produces a first order low pass rolloff 
with the combined stages resulting in an approximate second order filter 
response. 
Referring to FIG. 8, stage one consists of two low pass filters cascaded to 
a buffer amplifier 112. The first low pass filter consists of a switched 
capacitor network 106 being utilized as an equivalent resistor and coupled 
to capacitor 108. The purpose of this filter is to shift out low frequency 
signals as defined by the corner frequency. Preferably, the corner 
frequency range can be between 10 Hz through 3 kHz. The second low pass 
filter 110 is a standard RC low pass filter used to filter out noise 
components contained in the signal at frequencies greater than the flow 
range. Preferably, this can be set to 3k Hz. These noise components can be 
attributable to clock pulse signals 52, 54 which are used to control the 
switched capacitor network. 
Due to the large range of the corner frequencies, the range can be divided 
into two subranges: a first subrange spanning from 10 Hz to 400 Hz; and a 
second subrange spanning from 400 Hz to 3 kHz. Capacitor 98 is utilized to 
set a high corner frequency contained within the first range and capacitor 
96 is used to set a high corner frequency contained within the second 
range. Capacitor 98 can be preferably 150 pF and capacitor 96 can be 
preferably 1200 pF. The use of the specific capacitor and ultimately the 
high corner frequency range can be chosen from the switches controlling 
the respective capacitor. Switch 100a controls the selection of capacitor 
96 and switch 100b controls the selection of capacitor 98. Switches 100a, 
100b are controlled through signals 102 and 104 respectively which emanate 
from micro-controller 22. 
Switches 90a and 90b control the charging and discharging of capacitors 96 
or 98. These switches are under the control of signals 52 and 54 which are 
180 degrees out of phase with each other. The frequency of the switching 
as indicated by signals 52 and 54, and the size of the capacitor chosen 96 
or 98 determines an equivalent resistance. This equivalence resistance is 
coupled with capacitor 108. Preferably, capacitor 108 can be 0.15 uf. The 
combination of the switched capacitor network 106 with capacitor 108 
serves to define the high corner frequency of the low pass filter thereby 
providing a first order filter response. 
FIG. 9 illustrates the components of the second stage of the low pass 
filter. Referring to FIG. 9, the second stage consists of a switched 
capacitor network coupled to low pass filter 134 connected to an optional 
low pass filter 146 followed by buffer stage 138. An additional low pass 
filter 140 is cascaded to the output of buffer stage 138 and coupled to an 
optional low pass filter 142. The low pass filters and buffer stages are 
conventional circuit elements. 
Switched capacitor network 106 and capacitor 108 are identical to the same 
elements used in the first stage. Low pass filters 134 and 140 are 
identical filters which serve to filter out high frequency noise signals 
emanating from clock pulse signals 52, 54. Preferably, the high frequency 
corner setting is set at 3 k Hz. Low pass filters 136 and 142 are 
identical filters which serve to filter out low frequency noise components 
attributable to low flow rates. As this condition does not occur 
regularly, filters 136 and 140 are optional and are set when this event 
occurs. Signal 146 controls when these filters are activated and it is 
under the control of micro-controller 22. 
The output of the low pass filter is filtered signal 36 which is then 
transmitted to square wave generator 38 for additional signal processing. 
In summary, a method and apparatus for providing a micro-power adaptive 
filtering technique suitable for a variable frequency analog vortex sensor 
signal has been disclosed. 
The apparatus of the invention comprises an analog signal conditioner and 
micro-controller for determining the flow rate of a process from a vortex 
signal generated from a vortex sensor. The vortex sensor signal is 
filtered by a bandpass filter to eliminate frequency signals attributable 
to acoustic, electrical, and mechanical vibrations existing in the ambient 
surroundings in flow measuring environments. The bandpass filter is a 
cascaded low pass and high pass filter whose corner frequencies are under 
the control of the micro-controller. The settings of these corner 
frequencies is based on the measured vortex frequency of the vortex sensor 
signal and altered to preserve a specified bandwidth about the vortex 
frequency thereby preserving a high signal-to-noise ratio which in effect 
produces a more accurate flow measurement for a wider flow range under 
adverse flow conditions. The filtered signal is then transmitted to a 
square wave generator producing an equivalent digital square wave pulse 
train for use in computing the measured vortex frequency. 
The method of the invention is designed to preserve the specified bandwidth 
about the measured vortex frequency. As the vortex frequency changes, the 
corner frequencies of the filters are dynamically adjusted to track the 
vortex frequency signal accordingly. However, for large changes in the 
vortex frequency, the corner frequencies are reinitialized and a new 
search for the vortex signal is made, thereby avoiding locking onto an 
erroneous noise signal. Furthermore, the method compensates the vortex 
sensor signal for added or missed vortex pulses thereby producing a more 
accurate measurement of the vortex frequency. 
The electronic circuitry implementing the invention is designed to consist 
of low-cost components and to consume low power being in the order of less 
than 10 mW. Both of these considerations are essential for industrial 
instrumentation. 
Although the preferred embodiment of the invention has been described 
hereinabove in detail, it is desired to emphasize that this is for the 
purpose of illustrating the invention and thereby to enable those skilled 
in this art to adapt the invention to various different applications 
requiring modifications to the apparatus described hereinabove; thus, the 
specific details of the disclosures herein are not intended to be 
necessary limitations on the scope of the present invention other than as 
required by the prior art pertinent to this invention.