CA1256301A - Capacitance-type material level indication - Google Patents

Abstract

Abstract of the Disclosure A system for indicating the level of material in a vessel as a function of material capacitance comprising a resonant circuit including 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 oscillator 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 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 of the invention dis-closed, an automatic calibration circuit adjusts the resonance characteristics of the parallel resonant circuit in a pre-determined or preprogrammed manner during a calibration operation to a point indicative of the predetermined reference material level. A timer measures the duration of the automatic cali-bration operation for indicating the amount of material coated onto the probe.

Description

~2563~

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 ,,, ~k ~ 25~

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 , ., , .-~2~i~3~l 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 ~25~3~

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 ~2~63~

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 ~5~

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 ``` ~2~ilEi3~

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 --~.0--5~3~

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 ~.~s~

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 ~2S63Qi~L

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-tr~ -14-~ 25~i3~3~
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:

Claims

THE EMBODIMENTS OF THE INVENTION TO WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

A system for indicating level of material in a vessel as a function of material capacitance comprising probe means adapted to be coupled to a vessel so as to be responsive to variations in capacitance at said vessel as a function of material level; circuit means coupled to said probe means such that operating characteristics of said circuit means vary as a function of capacitance at said probe means; calibration means including means for initiating a calibration operation, means coupled to said circuit means for automatically varying operating characteristic of said circuit means in a predetermined manner during a said calibration operation, and means responsive to said circuit means during said calibration operation for detecting a predetermine operating characteristic at said circuit means, corresponding to a predetermined material level condition at said vessel, and for terminating said calibration operation when said predetermined operating characteristic is obtained; means responsive to variations in operating characteristics of said circuit means, including said probe means, from said predetermined operating characteristic for indicating level of material in said vessel; and means responsive to duration of said calibration operation for indicating operating conditions at said probe means during said calibration operation, including amount of material coated onto said probe means.

2.
The system set forth in claim 1 wherein said dura-tion-responsive means comprises a clock.

3.
The system set forth in claim 2 wherein said calibration means comprises a digital counter and means responsive to the output of said digital counter for varying said operating characteristics of said circuit means, and wherein said clock comprises means responsive to output of said counter.

4.
The system set forth in claim 1 wherein said dura-tion-responsive means includes means for indicating an alarm condition at said probe means.

5.
The system set forth in claim 4 wherein said alarm indicating means comprises means for establishing a duration threshold corresponding to maximum allowable coating of material onto said probe means, and means for indicating said alarm condition when said duration exceeds said threshold duration.

6.
The system set forth in claim 1 wherein said calibration means comprises a source of periodic signals at fixed frequency, a counter having a count input connected to said source and a count output, means for enabling operation of said counter during said calibration operation, and means coupled to said count output for progressively altering operating characteristics of said circuit means in a pre-determined manner as a function of the count accumulated in said counter.

7.
The system set forth in claim 6 wherein said means coupled to said count output comprises a plurality of con-trolled electronic switch means having control terminals con-nected to said count output and switch terminals, and a plurality of capacitors selectively connected by said switch terminals onto said circuit means.

8.
The system set forth in claim 1 wherein said duration-responsive means includes a meter empirically calibrated to indicate amount of material coated onto said probe means as a function of said duration.

9.
A system for indicating a physical characteristic of a material as a function of an electrical property of such material comprising probe means adapted to be disposed so as to be responsive to variations in said electrical property of the material, circuit means coupled to said probe means and responsive to said variations in said electrical property for varying operating characteristics of said circuit means, calibration means coupled to said circuit means and including means selectively operable during a calbiration operation for automatically varying operating characteristics of said circuit means in a pre-selected and predetermined manner as a function of time to obtain a preselected operating characteristic at said cir-cuit means corresponding to a predetermined reference physical characteristic of said material, means responsive to said calibration means for indicating duration of said calibration operation, and means responsive to variations in operating characteristics of said circuit means from said preselected operating characteristic to indicate said physical char-acteristics of the material.

10.
In a system for indicating level of material as a function of material capacitance and which includes a capacitance probe adapted to be disposed so as to be responsive to variations in capacitance as a function of material level, circiut means responsive to such variations in capacitance at said probe for indicating material level, and calibration means for automatically varying operation characteristics of said circuit means in a predetermined manner as a function of time during a calibration operation until said circuit means obtains a predetermined operating characteristic indicative of capacitance at said probe during said calibra-tion operation, a method of indicating operating conditions at said probe, including coating of material on said probe, comprising the steps of: (a) initiating a said calibration operation, (b) measuring duration of said calibration operation, and (c) indicating conditions at said probe as a function of measured duration of said calibration operation.