CA 02287018 1999-11-OS
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Ray grnnnd and Summaxy of the Invention
The present invention relates to a processor apparatus and method for a
process measurement signal. More particularly, the present invention relates
to an
improved processor for time-of flight signals to provide an accurate
indication of the
location of an interface between a first medium and a second medium in a
vessel.
The process and storage industries have long used various types of
equipment to measure process parameters such as level, flow, temperature, etc.
A
number of different techniques (such as mechanical, capacitance, ultrasonic,
hydrostatic,
etc.) provide measurement solutions for many applications. However, many other
applications remain for which no available technology can provide a solution,
or which
cannot provide such a solution at a reasonable cost. For many applications
that could
benefit from a level measurement system, currently available level measurement
systems
are too expensive.
In certain applications, such as high volume petroleum storage, the value
of the measured materials is high enough to justify high cost level
measurement systems
which are required for the extreme accuracy needed. Such expensive measurement
systems can include a servo tank gauging system or a frequency modulated
continuous
wave radar system.
Further, there are many applications that exist where the need to measure
level of the product is high in order to maintain product quality, conserve
resources,
improve safety, etc. However, lower cost measurement systems are needed in
order to
allow a plant to instrument its measurements fully.
There are certain process measurement applications that demand other
than conventional measurement approaches. For example, applications demanding
high
temperature and high pressure capabilities during level measurements must
typically rely
on capacitance measurement. However, conventional capacitance measurement
systems
are vulnerable to errors induced by changing material characteristics.
Further, the
inherent nature of capacitance measurement techniques prevents the use of such
capacitance level measurement techniques in vessels containing more than one
fluid
layer.
CA 02287018 1999-11-OS
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Ultrasonic time-of-flight technology has reduced
concerns regarding level indications changing as material
characteristics change. However, ultrasonic level measurement
sensors cannot work under high temperatures, high pressures, or
in vacuums. In addition, such ultrasonic sensors have a low
tolerance for acoustic noise.
One technological approach to solving these problems
is the use of guided wave pulses. These pulses are transmitted
down a dual probe transmission line into the stored material,
and are reflected from probe impedance changes which correlate
with the fluid level. Process electronics then convert the
time-of-flight signals into a meaningful fluid level reading.
Conventional guided wave pulse techniques are very expensive due
to the nature of equipment needed to produce high-quality, short
pulses and to measure the time-of-flight for such short time
events. Further, such probes are not a simple construction and
are expensive to produce compared to simple capacitance level
probes.
Recent developments by the National Laboratory System
now make it possible to generate fast, low power pulses, and to
time their return with very inexpensive circuits. See, for
example, U.S. Patent Nos. 5,345,471 and 5,361,070. However,
this new technology alone will not permit proliferation of level
measurement technology into process and storage measurement
applications. The pulses generated by this technology are
broadband, and also are not square wave pulses. In addition,
the generated pulses have a very low power level. Such pulses
are at a frequency of 100 MHz or higher, and have an average
power level of about 1nW or lower. These factors present new
problems that must be overcome to transmit the pulses down a
probe and back to process and interpret the returned pulses.
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First, a sensor apparatus must be provided for
transmitting these low power, high frequency pulses down a probe
and effecting their return. Such appropriate sensor apparatus
is described in a U.S. Patent 5,661,251, entitled SENSOR
APPARATUS FOR PROCESS MEASUREMENT, and a U.S. Patent 5,827,985
entitled SENSOR APPARATUS FOR PROCESS MEASUREMENT.
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The sensor apparatus is particularly adapted for the measurement of
material levels in process vessels and storage vessels, but is not limited
thereto. It is
understood that the sensor apparatus may be used for measurement of other
process
variables such as flow, composition, dielectric constant, moisture content,
etc. In the
specification and claims, the term "vessel" refers to pipes, chutes, bins,
tanks, reservoirs
or any other storage vessels. Such storage vessels may also include fuel
tanks, and a host
of automotive or vehicular fluid storage systems or reservoirs for engine oil,
hydraulic
fluids, brake fluids, wiper fluids, coolant, power steering fluid,
transmission fluid, and
fuel.
The present invention propagates electromagnetic energy down an
inexpensive, signal conductor transmission line as an alternative to
conventional coax
cable or dual transmission lines. The Goubau line lends itself to applications
for a level
measurement sensor where an economical rod or cable probe (ise ,, a one
conductor
instead of a twin or dual conductor approach) is desired. The single conductor
approach
enables not only taking advantage of new pulse generation and detection
technologies,
but also constructing probes in a manner similar to economical capacitance
level probes.
The present invention specifically relates to a signal processor apparatus
for processing and interpreting the returned pulses from the conductor. Due to
the low
power, broadband pulses used in accordance with the present invention, such
signal
processing to provide a meaningful indication of the process variable is
difficult.
Conventional signal processing techniques use only simple peak detection to
monitor
reflections of the pulses.
The present invention provides signal processing circuitry co~gured for
measurement of the time-of flight of very fast, guided wave pulses. Techniques
used in
similar processes, such as ultrasonic level measurement are vastly different
from and are
insufficient for detection of guided electromagnetic wave pulses due to the
differences in
signal characteristics. For example, ultrasonic signals are much noisier and
have large
dynamic ranges of about 120 dB and higher. Guided electromagnetic waves in
this
context are low in noise and have low dynamic ranges (less than 10:1) compared
to the
ultrasonic signals, and are modified by environmental impedances. The signal
processor
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of the present invention is configured to determine an appropriate reflection
pulse of
these low power signals from surrounding environmental influences.
Standard electromagnetic reflection measurements are known as time
domain reflectometry (TDR). TDR devices for level measurement require the
measuring
of the time of flight of a transit pulse and a subsequently produced
reflective pulse
received at the launching site of the transit pulse. This measurement is
typically
accomplished by determining the time interval between the maximum amplitude of
the
received pulse. The determination of this time interval is done by counting
the interval
between the transmitted pulse and the received pulse.
The present invention provides an improved signal processor for
determining a valid reflective pulse signal caused by an interface of material
in contact
with a probe element of a sensor apparatus. 'The processor apparatus of the
present
invention is particularly useful for processing high speed, low power pulses
as discussed
above. In the preferred embodiment of the signal processor apparatus,
processing is
I 5 performed based on a digital sampling of an analog output of the
reflective pulses. It is
understood, however, that similar signal processing techniques can be used on
the analog
signal in real time.
It is well known that variations in operating conditions such as
environmental variations like temperature, humidity, and pressure; power
variations like
voltage, current, and power; electromagnetic influences like radio
frequency/microwave
radiated power which creates biases on integrated circuit outputs; and other
conditions
such as mechanical vibration can induce undesired drifts of electronics
parameters and
output signals. The present invention provides a processing means and method
for
compensating for signal drifts caused by these operating conditions.
According to one aspect of the present invention, a method is provided for
processing a time domain reflectometry (TDR) signal to generate an output
result
corresponding to a valid process variable. The method includes the steps of
processing
the TDR signal using at least two different techniques for detecting a valid
reflection
pulse generated by the process variable to calculate an, independent result
using each of
the at least two techniques, and applying a weighted factor to the independent
results
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from each of the at least two different techniques to provide weighted output
results. The
method also includes the steps of comparing the weighted output results, and
selecting
the valid output result from the weighted output results based on the
comparing step. In
the illustrated method, the comparing step includes the step of summing the
weighted
factors for each independent result.
According to another aspect of the present invention, a method is
provided for processing a time domain reflectometry (TDR) signal having a
plurality of
reflection pulses to generate a valid output result corresponding to a
reflection pulse
caused by a process variable. The method includes the steps of processing the
TDR
signal using a first method for detecting the reflection pulse generated by
the process
variable and for calculating a first output result, processing the TDR signal
using a
second method for detecting the reflection pulse generated by the process
variable and
for calculating a second output result, and pracessing the TDR signal using a
third
method for detecting the reflection pulse generated by the process variable
and for
calculating a third output result. The method also includes the steps of
comparing the
first, second, and third results, and selecting the valid output result based
on the
comparing step.
In the illustrated method, the comparing step includes the step of applying
a weighted factor to each of the first, second, and third results prior to the
selecting step.
The comparing step also includes the step of summing the weighted factors for
each of
the first, second, and third results.
Also in the illustrated embodiment, the first processing method includes
the steps of detecting a maximum value reflection pulse from the plurality of
reflection
pulses of the TDR signal and calculating the first result based on the maximum
value
reflection pulse of the TDR signal. The second processing method includes the
steps of
calculating a derivative of the TDR signal, determining a location of a zero
crossing
adjacent an absolute maximum value of the derivative TDR signal, and
calculating the
second result based on said zero crossing. The third processing method
includes the
steps of establishing an initial boundary reflection signal, determining a
baseline signal
by subtracting the initial boundary signal from the TDR signal, determining a
maximum
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value reflection pulse of the baseline signal, and calculating the third
result based on the
maximum value reflection pulse of the baseline signal.
The method further includes the step of processing the TDR signal using a
fourth method for detecting the reflection pulse generated by the Process
variable and for
calculating a fourth output result. The comparing step compares the first,
second, third,
and fourth results. Illustratively, the comparing step includes the step of
applying a
weighted factor to each of the first, second, third, and fourth results prior
to the selecting
step. The fourth processing method illustratively includes the steps of
calculating a
derivative of the baseline signal, determining a time position of a zero
crossing adjacent
an absolute maximum of the derivative of the baseline signal, and calculating
the fourth
result based on the zero crossing adjacent the absolute maximum of the
derivative of the
baseline signal.
According to yet another aspect of the present invention, a method is for
processing a time domain reflectometry (TDR) signal having a plurality of
reflection
pulses to generate a valid output result corresponding to a reflection pulse
caused by a
process variable in a vessel. The TDR signal is generated by a sensor
apparatus. The
method includes the steps of establishing an initial boundary signal for the
sensor
apparatus before the process variable is located in the vessel, and storing
the detected
initial boundary signal. The method also includes the steps of detecting the
TDR signal,
detecting a maximum value reflection pulse from the plurality of reflection
pulses of the
TDR signal, and calculating a first output result based on the maximum value
reflection
pulse of the TDR signal. The method further includes the steps of calculating
a
derivative of the TDR signal, determining a location of a zero crossing
adjacent an
absolute maximum value of the derivative TDR signal, calculating a second
output result
based on the zero crossing adjacent an absolute maximum value of the
derivative TDR
signal. The method still further includes the steps of determining a baseline
signal by
subtracting the initial boundary signal from the TDR signal, determining a
maximum
value of the baseline signal, and calculating a third output result based on
the maximum
value of the baseline signal. The method includes the steps of calculating a
derivative of
the baseline signal, determining a time position of a zero crossing adjacent
an absolute
CA 02287018 1999-11-OS
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maximum of the derivative of the baseline signal, and calculating a fourth
output result
based on the zero crossing adjacent the absolute maximum of the derivative of
the
baseline signal. The method also includes the steps of comparing the first,
second, third,
and fourth output results, and selecting the valid output result from the
weighted output
results based on the comparing step.
In the illustrated method, the comparing step includes the step of applying
weighted factors to the first, second, third, and fourth results to produce
weighted results.
The comparing step also includes the step of selecting the valid output result
from the
weighted results.
The illustrated method also includes the step of establishing a new initial
boundary reflection signal at a selected time, and storing the new initial
boundary signal
to update the baseline signal. The comparing step may include the steps of
comparing
the first, second, and third results to a previous output result, and
disregarding a
particular result which deviates from the previous output result by more than
a selected
amount.
According to a further aspect of the present invention, a method is
provided for processing a time domain reflectometry (TDR) signal having a
plurality of
reflection pulses to generate a valid output result corresponding to a
reflection pulse
caused by a process variable. The method includes the steps of processing the
TDR
signal using a primary detection method for detecting the reflection pulse
generated by
the process variable and for calculating a primary output result, and
processing the TDR
signal using at least one secondary method for detecting the reflection pulse
generated by
the process variable and for calculating at least one secondary output result.
The method
also includes the steps of comparing the primary result to the at least one
secondary
result to check the primary result, and selecting the valid output result
based on the
comparing step. In the illustrated method, the comparing step includes the
step of
applying a weighted factor to the primary result and each of the secondary
results prior to
the selecting step.
According to a still further aspect of the present invention, an apparatus is
provided for processing ~ time domain reflectometry (TDR) signal having a
plurality of
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reflection pulses to generate a valid output result corresponding to a
reflection pulse
caused by a process variable. The apparatus includes means for processing the
TDR
signal using a first method for detecting the reflection pulse generated by
the process
variable and for calculating a first output result, for processing the TDR
signal using a
second method for detecting the reflection pulse generated by the process
variable and
for calculating a second output result, and for processing the TDR signal
using a third
method for detecting the refleciton pulse generated by the process variable
and for
calculating a third output result, means for comparing the first, second, and
third results,
and means for selecting the valid output result.
In the illustrated embodiment, the apparatus further includes means for
processing the TDR signal using a fourth method for detecting the reflection
pulse
generated by the process variable and for calculating a fourth output result.
The
comparing means compares the first, second, third, and fourth results. The
comparing
means illustratively includes means for applying a weighted factor to each of
the first,
second, third, and fourth results.
According to an additional aspect of the present invention, an apparatus is
provided for processing a time domain reflectometry (TDR) signal having a
plurality of
reflection pulses to generate a valid output result corresponding to a
refleciton pulse
caused by a process variable. The apparatus includes means for processing the
TDR
signal using a primary detection method for detecting the reflection pulse
generated by
the process variable and for calculating a primary output result, for
processing the TDR
signal using at least one secondary method for detecting the reflection pulse
generated by
the process variable and for calculating at least one secondary output result,
for
comparing the primary result to the at least one secondary result to check the
primary
result, and for selecting the valid output result based on the comparison. In
the
illustrated embodiment, the comparing means includes means for applying a
weighted
factor to the primary result and each of the secondary results.
According to a further aspect of the present invention, a method is
provided for processing a time domain reflectometry (TDR) signal to generate a
valid
output result corresponding to a process variable in a vessel. The method
includes the
CA 02287018 1999-11-OS ' -
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steps of establishing an initial boundary signal before the process variable
is located in
the vessel; storing the initial boundary signal and detecting a TDR signal.
The method
also includes the steps of determining a baseline signal by subtracting the
initial
boundary signal from the TDR signal, establishing a signal pattern having a
time range
based on the width of reflection pulses in the baseline signal and comparing
the baseline
signal to the signal pattern until a reflection pulse in the baseline signal
matches the
signal pattern.
According to another aspect of the present invention, a method for
processing a time domain reflectometry (TDR) signal to generate a valid output
result
corresponding to a process variable in a vessel includes the steps of
establishing an initial
boundary signal before the process variable is located in the vessel, storing
the initial
boundary signal, and detecting the TDR signal. The method further includes the
steps of
determining a point on the initial boundary signal and a corresponding point
on the TDR
signal, calculating a correction factor by subtracting the point on the
initial boundary
1 S signal from the corresponding point on the TDR signal, and adding the
correction factor
to the TDR signal to establish a valid TDR signal. The method further includes
the step
of determining a baseline signal by subtracting the initial boundary signal
from the valid
TDR signal.
In the illustrated embodiment, the first processing method includes the
steps of establishing a threshold voltage prior to comparing the baseline
signal to the
signal pattern and converting negative-going components of the reflection
pulses to
positive-going components. Further, in the first processing method the step of
establishing a signal pattern includes the step of determining at least four
points within
the time range in proximity to the threshold voltage and the step of comparing
the
baseline signal to the signal pattern includes the step of searching for a
reflection pulse
where the four points are on the reflection pulse in proximity to the
threshold voltage.
In another illustrated embodiment, the second processing method includes
the step of converting the correction factor to a positive value prior to
adding the
correction factor to the TDR signal to establish the valid TDR signal.
In both illustrated methods, the processing methods include the steps of
CA 02287018 1999-11-OS
determining a maximum value of the baseline signal and
calculating an output result based on the maximum value.
5 According to a further aspect of the present
invention, an apparatus is provided for processing a time domain
reflectometry (TDR) signal having a plurality of reflection
pulses to generate a valid output result corresponding to a
process variable in a vessel. The apparatus includes means for
10 establishing an initial boundary signal before the process
variable is located in the vessel, means for storing the initial
boundary signal, means for detecting the TDR signal, and means
for determining a baseline signal by subtracting the initial
boundary signal from the TDR signal. The apparatus further
includes means for establishing a signal pattern having a time
range based on the width of reflection pulses in the baseline
signal and means for comparing the baseline signal to the signal
pattern until a reflection pulse in the baseline signal matches
the signal pattern.
According to yet another aspect of the present
invention an apparatus is provided for processing a time domain
reflectometry (TDR) signal having a plurality of reflection
pulses to generate a valid output results corresponding to a
process variable in a vessel. The apparatus includes means for
establishing an initial boundary signal before the process
variable is located in the vessel, means for storing the initial
boundary signal, and means for detecting the TDR signal. The
apparatus further includes means for determining a point on the
initial boundary signal and a corresponding point on the TDR
signal, means for calculating a correction factor by subtracting
the point on the initial boundary signal from the corresponding
point on the TDR signal, means for adding the correction factor
to the TDR signal to establish a valid TDR signal, and means for
CA 02287018 2003-05-08
75089-15D
IOa
point on the TDR signal, means for adding the correction
factor to the TDR ;.;igna:l to c ~~~h:uZ.is~o r_ v,~:l id 'f DR signal,
and means for determA.ni..r..g a k.~~,~el..ine :~ i~arn.l. by subtracting
the initial boundary signal from tl~e ~J<~l Lcl 'IDR signal.
The illustrated embodiments ~.~f t:he apparatus also
include means for c~eterrn~_ni:rzg ~a rr~<~«i.m~.zon 'vs:r:l~ae of 'the
ba,eline signal and means ror c:~a.l.cr.zla'= ~:og an output result.
based on 'the maximum value.
In accordaroce with the present a.nvention, there is
prc:wided as method fc~z proc;~s~~inc~ a t7.r°u.: dc~m<~:in
reflec::tomet:ry
(TDR) signal having ,~ plur.~~.it ~,% of r~~i--'.e,;;1-i.<,n pulses guided
by a transmission Brie to :~enera~:e a ~ml.:ic~ output result
corresponding to a p:e:ocE:s:~ vari.ak:S:L~a irr ,:~ v;-e:~sel, the method
comprising the steps of: establishing art initial boundary
signal before the process ~~ear ~~ab 1.e is Lacr~.~ted in the vessel;
storing the initial. l~ounlia,y ~~.~~rual., c~~ C:t,~cvt.Lr~g the ''L'L~R
signal; determining a base_Line signal by .ubtracting the
initial boundary signal f~~~om t~rn~ 'iDR .>>!.<~r~~.~l,~ establi:ahing a
signal pattern rnav.irrc~ a tune r<acnc:k~..= ba:.>c:~c:~ cn the widtkn of
reflection pulses in the baseline signal; comparing the
baseline ;~igrnal to t:he ~;:iclroal p~~t:te.r.rmrnt:a._1 a r~~f.L~ecl~ion
put se in t:he base~._ink_~ signal :mai:.c:has t k-,e ~.i.gnal pattern;
determining a maximurra value of thze reiriec:t ic>n pulse that
matches the signal pa:~ttez:n;. arne.~ c~a.lc:t,a -:a t:.i..r:,g an out:put::
z°esult
based on the maximum val..~eo
In accordance with the Lares~~r~t invention, t:.here is
further provided a metha~a j_or pr.o>>cess:v.og a ~;i.me domain
refl.ectometry (~I'DR) ~ign:~~:L koa~~ri.ruc~ t~ ~.0l~arali~:y of reflection
pulses to generate a valid output: res;lt corresponding to a
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process variable in a vessel, the method comprising the steps
of: establishing an initial boundary signal before the
process variable is located in the vessel; storing the initial
boundary signal; detecting the TDR signal; determining a point
on the initial boundary signal and a corresponding point on
the TDR signal; calculating a correction factor by subtracting
the point on the initial boundary signal from the correspond-
ing point on the TDR signal; adding the correction factor to
the TDR signal to establish a valid signal; determining a
baseline signal by subtracting the initial boundary signal
from the valid TDR signal; determining a maximum value of the
baseline signal; and calculating an output result based on the
maximum value.
In accordance with the present invention, there is
further provided a method for processing a time domain
reflectometry (TDR) signal having a plurality of ref lection
pulses to generate a valid output result corresponding to a
process variable in a vessel, the method comprising the steps
of: establishing an initial boundary signal before the
process variable is located in the vessel; storing the initial
boundary signal; detecting the TDR signal; determining a point
on the initial boundary signal and a corresponding point on
the TDR signal; calculating a correction factor by subtracting
the point on the initial boundary signal from the correspond-
ing point on the TDR signal; adding the correction factor to
the TDR signal to establish a valid TDR signal; determining a
baseline signal by subtracting the initial boundary signal
75089-15D
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from the valid TDR signal; establishing a signal pattern
having a time range based on the width of the reflection
pulses in the baseline signal; comparing the baseline signal
to the signal pattern until a reflection pulse in the baseline
signal matches the signal pattern; determining a maximum value
of the reflection pulse thus matches the signal pattern; and
calculating an output result based on the maximum value.
In accordance with the present invention, there is
further provided an apparatus for processing a time domain
reflectometry (TDR) signal having a plurality of reflection
pulses to generate a value output result corresponding to a
process variable in a vessel, the apparatus comprising:
means for establishing an initial boundary signal before the
process variable is located in the vessel; means for storing
the initial boundary signal; means for detecting the TDR
signal; means for determining a baseline signal by subtracting
the initial boundary signal from the TDR signal; means for
establishing a signal pattern having a time range based on the
width of reflection pulses in the baseline signal; means for
comparing the baseline signal to the signal pattern until a
reflection pulse in the baseline signal matches the signal
pattern; means for determining a minimum value of the reflection
pulse that matches the signal pattern; and means for calculating
an output result based on the maximum value.
In accordance with the present invention, there is
further provided an apparatus for processing a time domain
reflectometry (TDR) signal having a plurality of reflection
75089-15D
CA 02287018 1999-11-OS
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pulses to generate a valid output result corresponding to a
process variable in a vessel, the apparatus comprising: means
for establishing an initial boundary signal before the process
variable is located in the vessel; means for storing the
initial boundary signal; means for detecting the TDR signal;
means for determining a point on the initial boundary signal
and a corresponding point on the TDR signal; means for
calculating a correction factor by subtracting the point on
the initial boundary signal from the corresponding point on
the TDR signal; means for adding the correction factor to the
TDR signal to establish a valid signal; means for determining
a baseline signal by subtracting the initial boundary signal
from the valid TDR signal; means for determining a maximum
value of the baseline signal; and means for calculating an
output result based on the maximum value.
Additional objects and advantages of the invention
will become apparent to those skilled in the art upon
consideration of the following detailed description of the
preferred embodiment exemplifying the best mode of carrying
out the
75089-15D
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invention as presently perceived.
The detailed description particularly refers to the accompanying figures in
which:
Fig. 1 is a diagrammatical view illustrating a single conductor material
level sensor for measuring a level of a process variable such as a liquid in a
vessel, and
illustrating a block diagram of the pulse transmitter and receiver and the
processing
circuitry for determining the level of the process variable;
Fig. 2 is an analog signal output of the time domain reflectometry (TDR)
signal generated by the transmitter and a receiver;
Fig. 3 is an analog output signal indicating an initial boundary condition
of the inside of the vessel before the process variable is located in the
vessel;
Fig. 4 is a time aligned analog TDR output signal:
Fig. 5 is an analog derivative signal of the time aligned TDR signal of
Fig. 4;
Fig. 6 is an analog baseline signal generated when the initial boundary
signal of Fig. 3 is subtracted from the time aligned TDR output signal of Fig.
4;
Fig. 7 is an analog signal of a derivative of the baseline signal of Fig. 6;
Fig. 8 is a flow chart illustrating the steps performed by the processor
apparatus of the present invention to determine an actual, valid level
indication of the
process variable based on a reflective pulse caused by the process variable;
Fig. 9 is an analog baseline signal corresponding to the signal shown in
Fig. 6 illustrating the pattern recognition technique of determining the valid
baseline
signal;
Fig. 10 is an analog initial boundary or probe map time aligned signal
corresponding to Fig. 3;
Fig. 11 is an analog illustration of the drift of a real time initial boundary
signal relative to the initial boundary signal shown in Fig. 10 caused by
variations in
operating conditions;
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Fig. 12 is an analog illustration of a baseline signal after the application
of
a correction factor according to the present inventor to compensate for the
drift in the
signal shown in Fig. 11;
Fig. 13 is a segment of the flow chart illustrated in Fig. 8 incorporating
the steps performed by the processor apparatus of the present inventor to
determine and
apply the correction factor and to use the pattern recognition technique to
determine an
actual, valid level indication of the process variable based on a reflective
pulse caused by
the process variable; and
Fig. 14 is a flow chart expanding the steps performed in block 250 in Fig.
13 for calculating and adding the correction factor to the initial boundary
signal.
Referring now to the drawings, Fig. 1 provides a diagrammatical
illustration of operation of the surface wave transmission line sensor
apparatus for
process measurement. The apparatus 10 is adapted for use with level
measurement of a
process variable such as an interface between a first medium 11 and a second
medium 12
located within a storage vessel 14. Illustratively, the first medium 11 is air
and the
second medium 12 is a process variable such as a liquid or other material.
The present invention includes a mechanical mounting apparatus 16 for
securing a single conductor transmission line or probe element 18 to a surface
20 of the
vessel 14. The mechanical mounting apparatus 16 enables a transceiver 22 to
transmit
pulses onto the probe element 18 in the direction of arrow 24. Once the pulses
reach an
interface 26 between the first medium 11 and the second medium 12, such as a
top
surface of liquid, a reflective pulse is returned back up the probe element 18
in the
direction of arrow 28.
The transceiver 22 is coupled to processing circuitry which detects the
reflected pulses to interpret the return pulses and to generate an output
signal indicating
the level of second medium 12 in the vessel 14. Preferably, the transceiver 22
transmits
broadband pulses at very low average power levels such as about 1nW or less,
or l~cW or
less peak power. The frequency of the pulses is preferably about 100 MHz or
greater.
The transceiver 22 includes a transmit pulse generator 30 which generates
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13
a series of the high frequency pulses and transmits these pulses
via a cable 32 to mounting 16. Transceiver 22 also includes a
sequential delay generator 32 coupled to the transmit pulse
generator 30. A sample pulse generator 34 is coupled to the
sequential delay generator 32. A sample and hold buffer 36 is
coupled to sample pulse generator 34 and to the cable 37.
Illustratively, transceiver 22 is a micropower wide band impulse
radar transmitter developed by the Lawrence Livermore National
Laboratory located at the University of California located in
Livermore, California. It is understood, however, that other
transceivers 22 may also be used with the signal processor
apparatus of the present invention.
As discussed above, the mounting apparatus 16 must be
specially designed to transmit and receive the low power, high
frequency pulses. The above reference U.S. Patent No. 5,661,251
and U.S. Patent No. 5,827,985 provide a suitable mounting
apparatus 16 for transceiver 22. It is understood that the
electronics and processing circuitry may be located at a remote
mounting location spaced apart from the mounting apparatus 16.
An output from transceiver 22 on line 38 is coupled to
an amplifier 40. An output from amplifier 40 provides a TDR
analog signal on line 42. Although the preferred embodiment of
the present invention uses a digital sampling system and
processes digital signals related to the analog output signals,
it is understood that a processor apparatus in accordance with
the present invention may be built to process the analog signal
directly.
In the present invention, an analog-to-digital
converter 44 is coupled to amplifier 40. An output of the
analog-to-digital converter 44 is coupled to an input of
CA 02287018 1999-11-OS
13a
microprocessor 46. In the illustrated embodiment,
microprocessor 46 is a MC68HC711E9 microprocessor available from
Motorola. It is understood, however that any other suitable
microprocessor may be used in accordance with present invention.
Microprocessor 46 is used to implement both a fast clock and a
slow clock. A PRF clock implemented by microprocessor 46, which
is a square wave at about 2 MHz, is coupled to transmit pulse
generator 30. The microprocessor 46 also implements a sync
oscillator, which is illustratively a square wave having a
frequency of about 40 Hz. The sync
CA 02287018 1999-11-OS
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oscillator is coupled to sequential delay generator 32.
Microprocessor 46 is also coupled to RAM 48 and to EEPROM 50. An
output terminal of microprocessor 46 is coupled to an output 52.
Illustratively, output 52
provides a 4-20 mA output signal to provide an indication of the level of the
interface 26
between the first medium 11 and the second medium 12.
The TDR analog signal from amplifier 40 is an equivalent time signal
(ETS) of the real time signal traveling on the transmission line system. The
ETS is
expanded in time by way of digital sampling, thereby enabling the use of
conventional
hardware for signal conditioning and processing. The signal processor of the
present
invention provides means for determining a valid pulse reflection, whether in
real time or
from the ETS. These results allow flexibility to determine information
relating to the
position of mediums 11 and 12 relative to a top surface 20, a bottom surface
21, a sensor
launch plate, or an end 19 of the probe element 18. The process material
positional
information is derived from signal reflections caused by impedance
discontinuities on the
transmission line and subsequent signal processing.
The signal responses of a transmission line which includes cable 32,
mounting 16, and probe element 18 are dependent upon the inherent transmission
design
characteristics and impedance changes created by changing boundary conditions.
These
boundary conditions are used to determine changes in the sensor environment
and are
directly or indirectly related to the amount or position of the bulk process
materials being
measured. The impedance of the sensor at a given location can change with
variations of
the sensor's environment or boundary condition due to interaction of the
sensor, its
signal, and its surroundings.
An example of a time domain reflectometry (TDR) analog signal from
amplifier 40 is illustrated in Fig. 2. In Fig. 2, the first large voltage
fluctuation or pulse
54 is generated by the impedance change in the mounting 16. In the preferred
embodiment, the mounting 16 provides this impedance change as a reference
reflective
pulse. The second reflective pulse 56 in Fig. 2 is generated by an inherent
interference
within vessel 14. This interference reflection 56 may be caused by a ladder,
door, weld
seam, material buildup, o'r other internal factor from vessel 14. The third
reflective pulse
CA 02287018 1999-11-OS
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58 is provided by the interface 26 between the first medium 11 and the second
medium
12. The fourth reflective pulse 60 is generated by an end 19 of probe element
18.
The present invention initializes the signal processing function by
characterizing or recording sensor performance at a given time or under known
boundary
conditions so that this initial characterization can be used as an initial
boundary
condition. In other words, a reference or initial boundary signal is measured
and stored
before the first and second mediums 11 and 12 are placed in the vessel 14.
An example of an initial boundary signal (LB.) is illustrated in Fig. 3.
The initial boundary signal is used to help determine a valid impedance change
induced
reflective pulse caused by interface 26 between first medium 11 and second
medium 12.
In Fig. 3, the initial voltage peak or reflective pulse 62 is caused by the
interference in
the vessel 14. Pulse 62 of Fig. 3 corresponds to pulse 56 in Fig. 2. Pulse 64
in Fig. 3
corresponds to the end 19 of probe element 18:
The sensor characterization may include factory calibration,
environmental characterization or probe mapping, and sensor
recharacterization, or
recalibration. The characterization can be done in such a way to permit use of
only one
or a combination of initialization procedures to provide optimum performance.
The
characterization of the sensor and its signals inside or outside of its
installation
environment such as the mounting in the vessel 14 are referred to as its
initial boundary
conditions.
Factory calibration may include characterizing sensor performance in a
stable, known environment which provides a baseline for the system performance
while
neglecting the influences and effects that are encountered in field
installation. A field
installation, such as mounting the sensor in a tank or vessel 14, can present
an
environment for new boundary conditions to the sensor caused by the vessel or
permanent contents of the vessel which influence the sensor response due to
interaction
of the sensor with these vessel contents.
The present invention provides either an automatic recharacterization or a
manual recharacterization of the sensor which can be performed to re-establish
a new
baseline or probe map which enables these environmental changes to be
accounted for in
CA 02287018 1999-11-OS
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determining the valid signal indicating the desired process variable.
A second phase of the signal processor of the present invention involves
detecting the pulse reflection produced by a valid signal response of the
impedance
change along a conductor. In other words, the processor apparatus locates the
impedance
pulse reflection caused by the interface 26 between the first medium 11 and
the second
medium 12 in contact with the probe element 18. A number of mathematical
techniques
can be used to determine the positional information due to impedance changes
which
generate a signal reflection related in time to the position of the cause of
the impedance
change along the probe element 18.
Detection of impedance changes may include one or more of the
following techniques applied to the TDR analog output signal illustrated in
Fig. 2. One
detection method is a peak amplitude detection of a Time Aligned TDR signal
which is
illustrated in Fig. 4. In other words, the signal of Fig. 4 is shifted so that
time zero is set
as the time of the initial reflecting pulse 54 provided by the impedance
change at the
mounting 16. In Fig. 4, the first reflection pulse 66 is caused by the
interference within
vessel 14. Second reflection pulse 68 is caused by interface 26. The third
reflection
pulse 70 is caused by end 19 of the probe element 18.
Another detection technique is to determine the first zero crossing after
the positive peak of a first derivative signal of the Time Aligned TDR signal
of Fig. 4.
This derivative signal is illustrated in Fig. 5. Again, the first reflection
pulse 72 is caused
by the interference within vessel 14. The second reflection pulse 74 is caused
by
interface 26, and the third reflection pulse 76 is caused by end 19 of probe
element 18.
Using this technique, the processor apparatus determines the maximum absolute
value of
the peak reflective pulse, which is illustratively at location 78. If the
absolute maximum
was a negative value, the preceding zero crossing at location 80 is determined
to be the
location of interface 26. If the absolute maximum was a positive peak, the
next.
subsequent zero crossing is used as the indication of interface 26.
Yet another technique for determining the valid interface 26 is the use of a
baseline signal. The baseline signal is illustrated in Fig. 6. The baseline
signal is
determined by subtracting the initial boundary signal of Fig. 3 from the Time
Aligned
CA 02287018 1999-11-OS
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TDR signal of Fig. 4. Therefore, the pulse reflection 66 caused by the
interference
within vessel 14 is cancelled by the initial boundary pulse reflection 62. In
Fig. 6, the
initial pulse reflection 82 is therefore caused by the interface 26 between
the first
medium 11 and the second medium 12. Reflective pulse 84 is caused by the end
19 of
probe element 18. The processor determines the time of the greatest positive
peak 86 as
the pulse reflection caused by interface 26.
Still another technique for determining the actual position of interface 26
is to use the first derivative signal of the baseline signal of Fig. 6. The
derivative of the
baseline signal is illustrated in Fig. 7. Again, the first reflection pulse 88
is caused by the
interface 26 between first medium 11 and second medium 12. The second
reflection
pulse 90 is caused by end 19 of probe element 18. The processor determines the
peak
absolute value 92 of the pulse reflection 88. Since the peak absolute value is
associated
with a negative voltage, the processor proceeds to the first proceeding zero
crossing 94 as
the time for the interface 26. If the maximum absolute value was a positive
peak, the
next subsequent zero crossing is used as the interface level.
Some embodiments of the present invention use a combination of two or
more of the above-cited techniques to verify the data related to the valid
detection of
interface 26. The short term history of the signal can also be used to
substantiate the
validity of any change in position of the interface 26 and to verify that this
change is
possible within the process condition presently being used in the vicinity of
the sensor.
In a preferred embodiment of the present invention, the processor
determines the location of the valid impedance discontinuity caused by
interface 26
between first medium 11 and second medium 12 using each of the four techniques
or
methods discussed above. Each method is assigned a weighted factor. In the
illustrated
embodiment, the baseline signal calculation illustrated in Fig. 6 is assigned
a weighted
factor of 1.1, while the other three techniques are assigned a weighted factor
of 1Ø
These weighted factors provide means for showing the degree of agreement among
the
four methods. If the calculated boundary conditions as detected by the sensor
creates a
conflict among the four detection methods such that there is not a substantial
agreement
of all four methods, then a valid result is dependent upon whether there is
substantial
CA 02287018 1999-11-OS
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agreement between two or three of the detection methods. If there is
substantial
deviation in the detection of the valid impedance pulse by all four methods,
then the
method having the highest weighted factor is used as the valid detection.
In the present invention, the microprocessor 46 is programmed with
S software to calculate the position of the valid impedance change caused by
interface 26
using each of the four methods discussed above. Fig. 8 illustrates the steps
performed by
the microprocessor 46 of the present invention to determine the valid signal.
The
microprocessor 46 is first initialized as illustrated at block 100. Operation
mode of the
signal processor is illustrated at block 102.
The first operational mode is to set and store the initial boundary (LB.)
signal illustrated in Fig. 3. This initial boundary signal is generated before
the process
material is placed in vessel 14. Microprocessor 46 first receives an input
initial boundary
signal as illustrated at block 104. The data is then time aligned based on the
initial
impedance change caused by the mounting 16 as illustrated as block 106.
Microprocessor 46 then stores the time aligned data related to the initial
boundary
conditions in the EEPROM 50 as illustrated at block 108. Once the initial
boundary
signal is stored, microprocessor 46 returns to operation mode at block 102.
In one embodiment, the signal processor of the present invention may
establish the initial boundary conditions manually only during initial
installation of the
sensor apparatus 10 into the vessel 14. In another instance, the initial
boundary
conditions may be updated at predetermined times during operation of the
signal
processor.
During normal operation of the signal processor, microprocessor 46
receives an input TDR signal as illustrated at block 110. This input TDR
signal is a
digital representation from analog-to-digital converter 44 of the TDR analog
signal
illustrated in Fig. 2. Although reference will be made to the analog signals
in Figs. 2-7,
it is understood that the microprocessor 46 of the present invention uses the
digital
representation of these signals. It is also understood that an analog
processor may be
used to process the analog signals in accordance with the present invention.
Microprocessor 46 next provides a time alignment of the TDR signal as
CA 02287018 1999-11-OS
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illustrated at block 112. In other words, microprocessor 46 time shifts the
input TDR
signal so that the time zero begins at the location of the interface of
mounting 16 which
is indicated by the initial large reflection pulse 54 shown in Fig. 2.
In the illustrated embodiment, microprocessor 46 uses four different
detection methods to locate a valid pulse reflection indicative of the
interface 26 between
the first medium 11 and the second medium 12. In a first method,
microprocessor 46
detects a peak reflection pulse of the time aligned TDR signal (illustrated in
Fig. 4) as
illustrated in block 114 of Fig. 8. Peak 71 in Fig. 4 is the valid reflection
pulse
corresponding to interface 26. However, the peak detection step in this
example would
determine that peak 115 is the valid peak. Peak 115 actually corresponds to
interference
in vessel 14 to be the valid pulse. This explains why the peak detection
method of the
time aligned TDR signal, when used alone, may produce some inaccuracies.
Microprocessor 46 then determines a time corresponding to the position of the
maximum
pulse value as illustrated at block 116 in Fig. 8. The time value is then
converted to a
distance between the top surface 20 of vessel 14 and the interface 26. This
step is
illustrated at block 118. This distance result calculated using the first
detection method
is then stored.
It is understood that once a time position of an impedance change on a
sensor has been derived, there are a number of techniques that can be used to
convert the
detected time to a distance equivalent position of the interface 26 of the
process variable.
The time intervals between the impedance changes have a mathematical
relationship
such that the time relation between the impedance change is proportional to
the speed of
light and a continuous function of the relative dielectric constants of the
subject
materials. If the first medium 11 is air, the dielectric constant is
substantially equal to
1Ø The subject time of the interval can then be corrected by applying the
continuous
functional relation relative to the material dielectric and the environmental
surroundings.
Other techniques such as using a sensor or conductor of a known length
and then using the relationship changes of the pulse travel times form a
subject material
interface to an end 19 of the probe element 18 may be used. In other words,
once the
location of the valid impedance pulse is determined, a time or distance
between the
CA 02287018 1999-11-OS
-20-
impedance interface and the end 19 of probe element 18 can be used to
determine the
level of the interface 26. In the case of a sensor having a known length,
differential time
intervals from a material interface 26 to end 19 of the probe element 18
changes
proportionally with the thickness of the subject material 12 divided by
a.continuous
functional relationship of the material dielectric constant. Provided the
probe element 18
has a fixed location relative to the vessel 14, the material level or
thickness of the
material is an offset relative to sensor position. This posititional
relationship is
determined using a simple mathematical equations.
Similarly, the velocity of a pulse traveling on a sensor passing through
multiple material layers can be used to determine the level of each material,
provided the
relative dielectric constant of each material is known. When the sensor has a
fixed
location relative to vessel 14, the position of each material can be
determined as a
function of the time differential, with an offset to the sensor position. A
sensor can also
be designed having markers at known distances to create signal reflections
that can be
used for calibration and/or determining material dielectric values.
Microprocessor 46 also calculates a derivative of the time aligned TDR
signal as illustrated at block 120. An analog representation of this
derivative signal is
illustrated in Fig. 5. Microprocessor 46 then determines the location of a
first zero
crossing adjacent an absolute maximum value of the signal. If the maximum is
obtained
from a positive value, microprocessor 46 determines the next subsequent zero
crossing
after the positive peak. If the absolute maximum was obtained from a negative
value, the
microprocessor 46 determines the first zero crossing prior to the detected
absolute
maximum. This step is illustrated at block 122. Microprocessor 46 then
determines a
time value corresponding to the detected zero crossing as illustrated at block
124. This
time value is then converted to a distance corresponding to the level of the
interface 26
between first medium 11 and second medium 12 as illustrated at block 126. The
distance
calculated using the second detection Method is then stored.
In the third detection method, the microprocessor 46 calculates a baseline
(BL) signal by subtracting the initial boundary signal stored in EEPROM 50
(Fig. 3)
from the time aligned TDR signal which is illustrated in analog form in Fig. 4
as
CA 02287018 1999-11-OS
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illustrated at block 128. This baseline signal is illustrated in analog form
in Fig. 6.
Microprocessor 46 then determines a location of the positive maximum value of
the
baseline signal as illustrated at block 130. This positive maximum value is
illustrated at
location 86 in Fig. 6. Microprocessor 46 next determines the time value
corresponding
to the detected positive maximum value as illustrated at block 132.
Microprocessor 46
then converts the time value to a distance change indicating the location of
interface 26
between the first medium 11 and second medium 12 as illustrated at block 134.
The
distance calculated using the third detection methods is then stored.
In the fourth detection method, Microprocessor 46 generates a first
derivative of the baseline signal as illustrated at block 136. An analog
representation of
the first derivative of the baseline signal is illustrated in Fig. 7.
Microprocessor 46 then
determines a location of a zero crossing adjacent an absolute maximum value as
illustrated at block 138. If the absolute maximum comes from a positive value,
the next
subsequent zero crossing is used. If the absolute maximum is from a negative
value, the
first preceding zero crossing is used as a location of interface 26.
Microprocessor 46
then determines the time position of the zero crossing at block 140. In the
Fig. 7
example, the first preceding zero crossing 94 adjacent negative peak 92 is
used as the
time position. Microprocessor 46 then determines the time change as
illustrated at block
142. This time change is then converted to a distance change as illustrated at
block 144
to provide an indication of the level of the interface 26 between the first
medium 11 and
second medium 12. This distance change calculated using the fourth detection
method is
then stored.
Microprocessor 46 next checks the validity of the detected distances from
each of the four methods discussed above as illustrated at block 146. Each of
the
distance changes is rounded to a predetermined sensitivity level, for example,
one
millimeter. If all four stored results from each of the four methods are the
same,
microprocessor 46 determines that a valid output has been determined.
Therefore,
microprocessor formats the output into an appropriate form and sends the
result to the
output 52 as illustrated at block 150.
If the four stored results from the four detection methods are different,
CA 02287018 1999-11-OS
-22-
microprocessor 46 then takes into account weighted factors established for
each of the
detection methods as illustrated at block 152. At this point, microprocessor
46 may
compare the four stored method results to a previous result. If any of the
four stored
results deviates from the previous result by more than a predetermined amount,
the
microprocessor 46 may disregard such a stored result. Microprocessor 46
provides a
summation of the weighted results as illustrated at block 154. Examples of
this
summation by microprocessor 46 are provided below. Microprocessor 46 then
selects
the most appropriate distance as the valid impedance reflection from interface
26 using
the weighted results at block 156. Microprocessor 46 then outputs this
selected result at
block 150.
Three different examples are provided to illustrated the effect of the
weighted factors on the process measurement.
CA 02287018 1999-11-OS
-23-
et X ~cml W.~. Selected
Result
Peak TDR 29.0 1.0
Der. TDR 36.9 1.0
Max. BL 37.1 1.1 37.1
Der. BL I 37.3 I 1.0
Method XX (cml ~ Selected
Result
Peak TDR 36.9 1.0
Der. TDR 37.3 1.0 37.3
Max. BL 37.1 1.1
Der. BL 37.3 1.0
Method X (cml W,~ Selected
1_~esult
Peak TDR 37.1 1.0 '
Der. TDR 37.3 1.0
Max. BL 37.1 1.1 37.1
Der. BL 37.3 1.0
In Example 1, each of the detected results for the level or distance X of
the interface 26 is different. In this instance, the greatest weighted factor
indicates that
the maximum detected baseline value is used. Therefore, the selected result by
microprocessor 46 is 37.1 cm.
CA 02287018 1999-11-OS
-24-
In Example 2, the maximum baseline method still indicates a distance of
37.1 cm. However, both the derivative of the TDR signal method and the
derivative of
the baseline signal method provided a result of 37.3 cm. Therefore, the
distance of 37.3
cm has a weighted factor of 2.0 when the two identical results are added
together.
Distance 36.9 cm from the peak TDR signal method has a weighted factor of 1Ø
Distance 37.1 due to the maximum baseline method has a weighted factor of 1.1.
Therefore, microprocessor 46 selects the greatest weighted factor of 2.0 and
the
corresponding distance result of 37.3 cm during the selection step at block
156 in Fig. 8.
In Example 3, both the peak TDR method and the maximum baseline
method provided a distance result of 37.1 cm. The derivative TDR method and
the
derivative baseline method both produced a result of 37.3 cm. Therefore, the
distance
37.1 has a weighted factor of 2.1, while the distance 37.3 cm has a weighted
factor of
2Ø Therefore, microprocessor 46 selects the result of 37.1 cm during the
selection step
at block 156.
It is understood that other detection techniques may be used in accordance
with the present invention. In addition, one of the other detection techniques
may be
applied the highest weighted factor, if desired. In an alternate embodiment,
each of the
detection techniques may be assigned a different weighted factor. Such
weighted factors
are selected and applied on the basis of application knowledge and experience.
A further technique for determining the valid interface 26 is pattern
recognition using the baseline signal illustrated in Fig. 6. The pattern
recognition
technique uses the entire pattern of the reflected pulse 82 shown in Fig. 6
and a number
of sampled points taken after a reflected pulse 82 has reached a threshold
voltage. The
timing of the points must fall within specific boundaries for the pattern to
be considered
valid. This technique is an improvement over existing peak detection methods
in that it
protects against false readings due to signal-pulse spikes produced by noise
and other
phenomena.
Referring to Fig. 9 a reflected signal 200 includes a positive-going
component 202 and a negative-going component 204 (shown in broken lines) and
is
nearly sinusoidal in shape. The baseline reflected signal 200 is centered
about zero volts
CA 02287018 1999-11-OS
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as can be seen in Fig. 6.
In the baseline method for determining the valid interface 26, the center of
positive-going component 202 of the reflected signal 200 (i.e., the process
material
level) is determined by identifying two points 206 and 208 on the positive-
going
component 202 of the reflected signal 200 with respect to a threshold voltage
210. The
midpoint between these points 206 and 208 is the center of the positive going
component
202 of the reflected signal 200. Points on the negative going component 204
are
replaced with zeroes.
In the pattern recognition technique the points on the negative going
component 206 are not replaced with zeroes. Instead the negative points are
converted to
their absolute value using the 2's complement technique. The 2's complement
technique
is well known to those skilled in the art for determining absolute value of
negative signed
numbers and is described and explained in standard textbooks. See for example
the
textbook Di it 1 .on p~,s 8r ~nnlic'~ ations, published 1990 by Saunder's
College
Publishing (a division of Holt, Rinehart and Winston) p. 225. The result of
the use of the
2's complement technique is a second positive-going component 212 creating
dual
positive-going peaks 202 and 212.
According to the pattern recognition technique the valid interface 26 for
the process material is determined by using a four (4) point pattern and the
dual positive-
going peaks 202 and 212 of the entire reflected pulse 200. Once the first
point 206 is
detected relative to the threshold voltage 210 the second point 208, third
point 214 and
the fourth point 216 on the positive going peaks 202 and 212 must occur within
specific
time frames from the first point 206. The time frames are determined by the
overall 218
width of the valid reflected pulse 200. If the four (4) points 206, 208, 214
and 216 do not
occur within the specific time frames then the reflected pulse 200 is
considered invalid.
If the reflected pulse 200 is found to be valid, then the center of the first
positive-going peak 202 (i.e. the valid interface 26 for the process material)
is determined
by calculating the mid-point between the first point 206 and the second point
208. It will
be understood that the number of points in the pattern need not be limited to
four.
Additional points could be used without departing from the scope of the
present
CA 02287018 1999-11-OS
-26-
invention.
It is well known that variations in operating conditions such as;
environmental variations, (temperature, humidity, pressure,) power supply
variations
(voltage, current, power) electromagnetic influences (rf/uwave radiated power
creating
biases on IC outputs) and other conditions such as mechanical vibration can
induce
undesired drifts of electronics parameters and output signals.
In order to compensate for drifts in time and voltage in reflected signals
due to the above-described variations in operating conditions, a further
embodiment of
the present invention includes a corrective element or factor that is
calculated every time
the software executes a signal processing loop. The correction element or
factor is then
added to each signal sample prior to use of the baseline subtraction method
described
previously.
Refernng to Fig. 10, an initial boundary or probe map time aligned signal
220 that has been digitized and store in a microprocessor is shown. This
signal 220
corresponds to signal 62 shown in Fig. 3. The signal 220 is time aligned
relative a
starting voltage Vm;" which is located on the starting center line 222 of the
negative going
component 224 of the signal 220.
Figure 11 illustrates a situation where the real time TDR signal 226 has
drifted in both time and voltage relative to the initial boundary signal 220.
When the
baseline procedure is used in this situation, the results will not be valid.
This invalid
result can be overcome and corrected to compensate for these signal drifts
using the
correction element or factor according the present invention. The real time
TDR signal
226 has a new center line 228 which has shifted in time fit; and has shifted
in voltage
~V~n,p;.
The compensation can be accomplished by obtaining the time and voltage
variations Ot; and ~V~ompi ~d adjusting the digitized real time TDR signal 226
by the
drift Ot; and w~omP;. The correction factor V~o~ is calculated by subtracting
a specific
point 230 on the negative-going component 224 of the initial boundary .of the
probe map
signal 220 from its corresponding point 232 on the negative-going component
234 of the
real-time TDR signal 226, then inverting the result using the 2's complement
technique.
CA 02287018 1999-11-OS
-27-
This yields a number V~ort that is always added to the real time TDR signal
226,
regardless of offset polarity of the signals 220 and 226. The correction
factor V~~ is
represented algebraically by the formula:
V~on = -(V~i - VPm), where V~o~ = correction factor
V«, = point 232 on the real-time TDR signal 226
VPm = corresponding point 230 on the initial
boundary on the probe map signal 220
The compensated sample point V~mP (i.e. the center of the valid signal) is
determined by the formula:
Vcomp = V~aa + V~~, where V~omp = value of the compensated sample point
V~mP~e = value of the uncompensated point
V~n = correction factor
The baseline procedure can be performed upon completion of this compensation
in time
and voltage. The resulting baseline signal is shown in Fig. 12. This
compensated result
provides a valid reflection pulse that is easily analyzed providing the
desired valid and
accurate Otra,;d.
In order to implement the pattern recognition technique and the correction
factor shown illustrated in Figs. 9-12, the software programmed in the
microprocessor 46
is modified as shown in Figs. 13 and 14. Figs. 13 and 14 illustrate the
additional steps
performed by the microprocessor 46 as a result of the software modifications.
The
additional steps are shown inserted in the appropriate locations within the
steps
illustrated in Fig. 8. Thus reference numerals in Figs. 13 and 14
corresponding to
reference numerals in Fig. 8 are intended to denote the same steps. Further,
although not
shown in Figs. 13 and 14, it will be understood that the remainder of the
steps shown in
Fig. 8 occurring before and after steps 110 and 130 respectively would be
performed in
connection with the steps shown in Figs. 13 and 14. Steps 136-140, steps 120-
126 and
steps 114-118 would not be performed when using the pattern recognition
technique.
However, the correction factor could be used without the pattern recognition
technique in
which case all of the steps in Fig. 8 may be performed.
Referring to Figs. 13 and 14, the step for calculating and adding the
CA 02287018 1999-11-OS
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correction factor is shown in block 250 and is performed between blocks 112
and 128 in
the process illustrated in Fig. 8. A more detailed breakdown of the steps
performed in
block 250 is shown in Fig. 14.
Referring to Fig. 14, after the microprocessor 46 provides a time
alignment of the TDR signal in block 112, the microprocessor 46 then subtracts
the
specific point 230 on the initial boundary signal 220 from the corresponding
point 232 on
the real-time signal 226 in block 252 in accordance with the formula set forth
above. In
block 254, the microprocessor 46 then uses the 2's complement technique on the
negative
difference value between points 232 and 230.
After the 2's complement technique is applied then the correction factor
V~on determined in block 252 is added to the uncompensated sample point of the
real
time TDR signal to produce a value of the compensated sample point V~omp.
Thereafter,
the microprocessor 46 calculates a baseline (BL) signal by subtracting the
initial
boundary signal from the time aligned and corrected TDR signal to produce the
baseline
signal illustrated in analog form in Fig. 12. It will be understood that after
block 123 the
microprocessor 46 may proceed to block 136, block 120, block 114 or use the
pattern
recognition technique as shown in Fig. 13 at 260.
Using the pattern recognition technique the microprocessor 46 first uses
the 2's complement technique on the negative-going component 204 of the
baseline
signal 200 (See Fig. 9) in block 262. Thereafter the microprocessor 46
searches for the
predetermine four (4) point pattern (determined based upon the width 218 of
the signal)
in block 264 as shown in Fig. 9. If the predetermined pattern is not found
then the
microprocessor 46 continues to search baseline signal samples until a valid
pattern is
found. This step is performed in block 266. Once a valid patter is found, then
the
microprocessor 46 determines a location of the positive maximum value of the
valid
baseline signal in block 130 shown in Fig. 8.
Although the invention has been described in detail with reference to a
certain preferred embodiment, variations and modifications exist within the
scope and
spirit of the present invention as described and defined in the following
claims.
