Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CAP~CITANCE-TYPE MATERIAL LE~EL INDIC~TION
The present invention is directed to systems for
measuring physical characteristics of materials as a function
of electrical properties of the material, and more particularly
to a system and me-thod for indicating level of material in a
storage vessel as a function of material capacitance.
Use of capacitance-type detection techniques for
sensing level of material in a storage vessel has been widely
proposed and is reasonably well understood in the art. In
general, calibration in the field has been a time-consuming and
laborious process requiring the efforts of a skilled or semi-
skilled operator. There has been a need in the art for a system
embodying facility for automatic non-demand calibration which
requires little or no operator intervention.
U.S. Patent No. 4,499,766, dated February 19, 1985
entitled "Capacitance-Type Material Level Indicator" and
assigned to the assignee hereof, discloses a system and probe
for indicating the level of material in a vessel as a function
of material capacitance. The disclosed system includes a
resonant circuit having a capacitance probe adapted to be
disposed in a vessel so as to be responsive to variations in
capacitance as a function of material level. An rf oscilla-tor
has an output coupled to the resonant circuit and to a phase
detector for detecting variations in phase angle as a function
of probe capacitance. Level detection circuitry is responsive
to an output of the phase detector, and to a reference
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signal indicative of a predetermined level of material, for
indicating material level as a function of a difference between
capacitance at the probe and the reference signal. In the
preferred embodiments disclosed in such application, an
automatic calibration circuit adjusts the resonance
characteristics of the parallel resonant circuit or adjusts the
reference signal indicative of a predetermined reference
material level.
A material level indicating system has also been
proposed which includes a bridge circuit with a capacitance
material level probe in one bridge arm. An adjacent bridge arm
includes a plurality of fixed capacitors coupled to controlled
electronic switches for selective connection into the bridge
circuit. The bridge circui-t is powered by an rf oscillator, and
a differential amplifier is connected across the bridge circuit
for detecting balance conditions at the bridge~ An automatic
calibration circuit includes a digital counter having outputs
connected to the electronic switches. A operator is responsive
to the differential amplifier for enabling operation of the
counter during a calibration mode of operation for selectively
connecting the fixed capacitors into the bridge circuit until
a preselected balance condition, corresponding to a preselected
reference material level, is obtained. Thereafter, the
differential ampliEier is responsive to variation of probe
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capacitance from the reference level to indicate material
level.
A problem in the capacitance-type material level
indicating art arises in conjunction with materials which
stick to or coat the probe, and thus provide a capacitance
path to "short circuit" the bulk of material and give rise
to false indications of material level. Automatic calibration
technology hereinabove discussed has enjoyed substantial
commercial acceptance and success. Selective connection of
calibration capacitors into the level sensing circuitry is
effective for balancing the capacitive effects of material
coating on the probe within the capabilities or parameters of
the calibration circuitry. However, excessive coating on the
probe may exceed the capabilities of the calibration circuitry.
It is therefore desirable for an operator to be aware of build-
up of material on the probe, and more specifically to be
advised when such build-up of material exceeds the capabilities
of the calibration circuitry.
It is a general object of the present invention to
provide a method and system for measuring physical charac-
teristics of materials as a function of material electrical
characteristics sensed by a suitable probe, which include
facility for automatic calibration of the sensing circuitry
to establish a reference and facility for indicating material
conditions at the probe during the calibration operation. As
applied specifically to material level, it is an object of
the invention to provide a method and system for indicating
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conditions at the material probe during a calibration operation,
and more specifically for indicating the amount of material
coated on or adhered to the material probe.
The invention, together with additional objects,
features and advantages thereof, will be best understood from
the following description, the appended claims and the
accompanying drawings in which:
FIG. 1 is a functional block diagram of a presently
preferred embodiment of a material level indicating system in
accordance with the invention;
FIG. 2 is an electrical schematic diagram of a portion
of FIG. 1 which illustrates details of implementation; and
FIG. 3 is a fragmentary schematic diagram which
illustrates a modification to the embodiment of FIGS. 1 and 2
in accordance with the invention.
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FIG. 1 illustrates a presently preferred embodiment
o a material level indicating system in accordance with the
invention as comprising an rf oscillator 10 which provides a
periodic signal at a first output to a phase shift ~ninety
degrees) amplifier 12. The sinusoidal output of amplifier 12
is connected to an adjustable parallel LC resonant circuit 14.
Resonant circuit 14 is connected to the probe conductor 18 of
a probe assembly 20 (FIG. 1) mounted in the side wall o a
storage vessel 22. The output of amplifier 12 is also connected
through a unity-gain amplifier 24 having low outpu-t inpedance
to the guard shield 26 of probe assembly 20. The wall of vessel
22, which may be a storage bin Eor solid materials or a liquid
storage tank, is connected to ground. As is well-~nown in the
art, the capacitance between probe conductor 18 and the grounded
wall of vessel 22 varies with the level of the material 28
stored therein and with material dielectric constant. This
variation in capacitance is sensed by the remainder of the
system electronics to be described to provide the desired
indicationof material level. Guard shield 26, which isenergized
by amplifier 24 at substantially the ~ame voltage and phase as
probe conductor 18, functions to prevent leakage of probe energy
through material coated onto the probe surEace, and thus to
direct probe radiation outwardly into the vessel volume so as
to be more closely responsive to the level o material stored
therein. A presently preferred embodiment of probe assembly 20
is described in U.S. Patent No. 4,499,641 dated February 19,
1985 and assigned -to the assignee hereo.
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The sinusoidal output of amplifier 12 is fed
through a zero crossing detector 30 to one input of a phase
detector 3-2. Phase detector 32 receives a square-wave second
input from a second output of oscillator 10 one hundred eighty
degrees out of phase with the oscillator output directed to
amplifier 12. A first output of phase detector 32, which
is a d.c. signal at a level proportional to the phase relation-
ship between the respective inputs, and thus responsive to
variations in phase angle of the oscillator probe drive output
due to changes in probe capacitance, is fed to an automatic
calibration circuit 34. A second output of phase detector 32,
which is also a d.c. signal indicative of input phase relation-
ship, is directed to one input of a threshold detector 36.
The outputs of phase detector 32 are identical but effectively
isolated from each other. Automatic calibration circuit 34
provides a control input to adjustable LC resonant circuit 14,
which receives a second input for adjustment purposes from
oscillator 10. Calibration circuit 34 also provides a
reference input to threshold detector 36. The output of
threshold detector 36 is fed through material level indicating
circuitry 38 to external circuitry for controlling and/or
indicating vessel material level as desired.
In general, automatic calibration circuitry 34
functions to adjust the resonance characteristics of resonant
circuit 14 during a calibration mode of operation initiated
by an operator push-button 40 connected thereto so as to
establish, in effect, a reference capacitance level indicative
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of a preselected material condition in vessel 22 which exists
during the automatic calibration mode. Preferably, the level
of materia~ in vessel 22 is first raised (by means not shown)
to the level of probe assembly 20 and then lowered so as to
be spaced from the probe assembly. If material 28 is of a
type which coats the probe assembly, such coating will remain
on the probe and be taken into consideration during the ensuing
calibration operation. With the material level lowered, an
operator may push button 40 to initiate the automatic calibra-
tion mode of operation. The resonance characteristics of cir-
ciut 14 are then automatically varied or adjusted by calibra-
tion circuit 34 in a preselected or preprogrammed manner until
the output of phase detector 32 indicates that the return
signal from the parallel combination of resonant circuit 14 and
capacitance probe 18 bear a preselected phase relationship to
the oscillator reference input to phase detector 32, which
phase relationship thus corresponds to an effective reference
capacitance level at calibration circuit 34 indicative of a
low material level.
Thereafter, during the normal operating mode, the
output of phase detector 32 is compared in threshold detec-tor
36 to a reference input from calibration circuit 34 indicative
of the reference capacitance level, and threshold detector 34
provides an output to material level indicating circuitry 38
when the sensed material capacitance exceeds the reference
capacitance level by a predetermined amount which is selected
as a function of material dielectric constant. If probe
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assembly 20 is placed in the upper portion of vessel 22 as
shown in FIG. 1, such proximity would normally indicate a
full tank ~r high-level condition. If, on the other hand,
probe assembly 20 is disposed in the lower portion of tank 22,
material would normally be in proximity to the probe assembly,
and indeed would normally cover the probe assembly, so that
absence of such proximity would indicate an empty tank or
low-level condition.
FIG. 2 illustrates a presently preferred embodiment
of automatic calibration circuitry 34 and adjustable LC
resonant circuit 14. Resonant circuit 14 includes a fixed
capacitor 42 and an inductance 44 connected in parallel with
probe conductor 18 across the output of amplifier 12, i.e.
between the amplifier output and ground. Inductance 44 com-
prises a plurallty of inductor coils or windings having anumber of connection taps at electrically spaced positions
among the inductor coil turns. A plurality of fixed capacitors
46a-46f are each electrically connected in series with a
respective controlled electronic switch 48a-48f between a
corresponding connection tap on inductance coil 44 and
electrical ground. Switches 48a~48f may comprise any suitable
electronic switches and are normally open in the absence of
a control input. A digital counter 50 receives a count input
from oscillator 10 and provides a plurality of parallel digital
outputs each indicative of a corresponding bit of the count
accumulated and stored in counter 50. Each data bit output
of counter 50 is connected to control a corresponding electronic
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switch 48a-48f for selectively connec ing or disconnecting
the corresponding capacitor 46a-46f in resonant circuit 14
as a funct-ion of the state of the counter output bit.
Most preferably, the capacitance values of
capacitors 46a-46f and the number of coil turns separating
the connection taps of inductance 44 are selected such that
the effective capacitance added to the parallel LC resonant
circuit 14 by each capacitor 46a-46f corresponds to the
numerical significance of the corresponding counter output.
That is, assuming that counter 50 is a binary counter with
outputs connected to switches 48a-48f in reverse order of
significance, the values of capacitors 46e, 46f and the number
of turns at inductance 44 therebetween are selec-ted such that
the effective capacitance connected in parallel with fixed
capacitor 42 and probe 20 is twice as much when switch 48e
only is closed as when switch 48f only is closed. Likewise,
the effective capacitance added by switch 48a and capacitor 46a
is thirty--two times the effective value of capacitor 46f and
switch 48f. It will be appreciated that inductance 44 functions
as an autotransformer so as to establish the effective
capacitance of each capacitor 46 as a function of the cor-
responding connection point among the inductance coils. It
will also be appreciated that the number of inductance connec-
tion taps may be less than the number of capacitors 46a-46f,
with two or more capacitors connected to one tap. The values
of capacitors connected to a common tap should differ by
multiples of approximately two in correspondence with the
significance of the control bits from counter 50.
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Automatic calibration circuit 34 illustrated in FIG.
2 includes a one-shot 52 which receives an input from operator
pushbutto~ 40 and provides an output to the reset input of
counter 50 in resonant circuit 14 to initiate the automatic
calibration mode of operation. A differential comparator 54
has an inverting input connected to the output of phase
detector 32 and a non-invertin~ input connected to the wiper
of a variable resistor 56. Resistor 56 is connected across
a source d.c. potential. The output of comparator 54 is con-
nected to the enabling input of counter 50 in resonant circuit14. The output of comparator 54 is also connected through a
resistor 57 to the base of an NPN transistor 58 which functions
as an electronic switch having primary collector and emitter
electrodes connected in series with an LED 60, a resistor 61
and operator switch A0 across a source of d.c. potential. The
non-inverting input of comparator 54 is also connected through
an adjustable resistor 62 to threshold detector 36 (FIG. 1).
Depression of switch 40 by an operator initiates the
automatic calibration procedure by clearing or resetting
coun-ter 50. All capacitors 46 are disconnected from resonant
circuit 14. With material coated on the probe, circuit
operation is substantially removed from resonance on the
"inductive" side, and the output from phase detector 32 to
comparator 54 is high. Differential comparator 54 thus pro-
vides a low output to the enabling input of counter 50 andto the base of transistor 58, so that transistor 58 is biased
for non-conduction and de-energizes LED 60. With counter 50
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so reset and enabled, the pulsed counter input from oscillator
lO advances the count in counter 50, and thereby sequentially
and select-ively connects the various capacitors 46a-46f into
the parallel LC resonant circuit as controlled hy switches 48a-
5 48f. As previously indicated, the effective capacitance addedby connection of each capacitor is directly related and pro-
portional to the numerical significance of the corresponding
bit in counter 50.
As capacitors 46 are added in parallel connection
with inductance 44, capacitor 42 and probe 20, and as the paral-
lel combination approaches resonance at the frequency of
oscillator lt), the output of phase detector 32 decreases
toward the reference level determined by the setting of vari-
able resistor 56 at the non-inver-ting input of differential
15 comparator 54. Resistor 56 is preferably factory set to
correspond with a resonance condition at circuit 14 for a low-
level or "empty-vessel" nominal capacitance with no coating on
probe assembly 20 and all capacitors 46a-46f in circuit. The
empty-tank capacitance at probe assembly 20 may be fifteen
20 picofarads, for example. When the output of phase detector 32
reaches this reference capacitance level input to comparator 54,
which is preferably at substantially the resonance condition
of the LC resonant circui-t, the output of differential am-
plifier 54 switches to a high or one logic stage. Further
25 operation of counter 50 is inhibited and LED 60 is illuminated
through transistor 58 so as to indicate to an operator that
-the calibration operation has been completed. The operator
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may then release switch 40. Thus, the resonance circuit is
designed to be at resonance with all capacitors 46a-46f in
circuit and the probe uncoa-ted. The automatic calibration
operation functions to delete one or more capacitors 46a-46f
from the parallel resonance circuit to compensate for the coating
on the probe, cable capacitance, tank geometry, parasitic
capacitance, and variations in probe insertion length and circuit
operating characteris-tics.
All of the circuitry hereinabove (and hereinafter)
described receive input power from a suitable power supply (not
shown) energized by a utility power source. Preferably,
adjustable LC resonant circuit 14 further includes a battery
64 connected by the blocking diodes 66, 68 in parallel with the
power supply d.c. voltage to the power input terminal of counter
50 so as to maintain the calibration count therein in the event
of power failure. To the extent thus far described, the circuitry
of FIGS. 1 ansl 2 is similar to that disclosed in FIGS. 1 and 2
of above-noted U.S. Patent No. 4,499,766, with identical
reference numerals being employed to facilitate cross reference.
As previously indicated, the automatic calibration
operation is conducted in a preselected or preprogrammed manner.
In the specific embodiment disclosed, the calibration operation
is carried out by feeding counter 50 with a periodic signal at
controlled frequency - i.e., the output of oscillator 10. Thus,
in eEfect, the duration of the calibration operation reflects
conditions at the probe. Where condi-tions are
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nominal and the probe is uncoated, the calibration operation
will be relatively long ~twenty seconds) in the embodiment
disclosed. -When heavy coating on the probe taxes the cali- -
bration circuit capabilities, the duration of the calibration
operation will be relatively short (several seconds). Because
the calibration operation takes place in a preselected or pre-
programmed manner, the time required to complete the calibration
operation will accurately reflect conditions at the probe and
will remain substantially constant for given conditions. In
accordance with the present invention, this time duration is
measured and processed to indicate conditions at the probe,
including specifically the amount of material coating on the
probe.
More specifically, a timer 70 (FIG. 1) receives an
input from automatic calibration circuit 34 for timing or
measuring the duration of the automatic calibration operation.
The output of timer 70 is fed to an alarm circuit 72 and to a
meter or the like 74 which may be empirically calibrated, for
example, in units of thickness of material coated onto probe
assembly 20. Alarm circuit 72 receives a variable threshold
input from an adjustable circuit 76. In FIG. 2, timer 70 is
illustrated as comprising a clock 78, which may be an analog
or digital clock. FIG. 3 illustrates a modification to the
embodiment of FIG. 2 wherein the timer 70 comprises a digital-
to-analog converter 80 having inputs connected to the digital
outputs of counter 50. In the embodiment of FIG. 3, counter 50
thus performs the dual function of a duration clock and a
calibration controller. The outputs of clock 78 (FIG. 2) and
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converter 80 (FIG. 3) are fed to alarm 72 and meter 74 (FIG.
1). The outputs of clock 78 and converter 80 may also be fed,
where appropriate, to remote system circuitry instead of or in
addition to alarm 72 and meter 74.
In operation, timer 70 measures the duration of the
calibration mode of operation. Where no material is coated
onto probe assembly 20, circuit 14 is at resonance with no
capacitors 46a-46f removed as previously described, and the
calibration operation is consequently of relatively long
duration. ~s the material coating on probe assembly 20 increases
during use, the duration of the calibration operation decreases,
and a corresponding increase in thickness is indicated on meter
74. When such coating reaches the threshold set by variable
threshold circuit 76, alarm 72 is activated. Threshold 76 may
be empirically set to a coating beyond which the material level
sensing system will no longer operate properly. At this point,
it will then be necessary to clean probe assembly 20, and then
to recalibrate the circuit with a clean probe in order to
continue operation.
It will be appreciated that the principles of the
invention hereinabove discussed with reference to the
embodiments of the drawings may be applied equally as well in
o-therembodiments. For example, both of the specific embodiments
of FIGS. 2 and 3 may be implemented without modification in the
bridge-type circuits with automatic calibration features as
discussed above. Indeed, the principles of the invention find
ready utility and application outside of the material level
control arts where material char-
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acteristics are measured as a function of electr.ical char-
acteristics using a material-responsive probe, and where
varying conditions at the probe are removed or nullified in
an automated calibration procedure. Such calibration ~pera-
tion need not necessarily be implemented by a manual switch 40,of course, but may as readily be implemented by a remote control
system.
The invention claimed is:
