WO2002063249A2

WO2002063249A2 - Material level sensing system and method

Info

Publication number: WO2002063249A2
Application number: PCT/US2002/003463
Authority: WO
Inventors: Ernesto Barrantes
Original assignee: Venture Measurement Company
Priority date: 2001-02-08
Filing date: 2002-02-05
Publication date: 2002-08-15
Also published as: AU2002245391A1; WO2002063249A3

Abstract

A system for measuring the level of material (16) in a vessel (14) includes a transmission line probe (12) having first and second conductors (18, 20) adapted to be positioned for contact with material in the vessel. Electronics (22) are coupled to the transmission line probe for launching microwave radiation along the probe conductors simultaneously and 180° out of phase, such that portions of such radiation are reflected from the impedance discontinuity at the air/material interface within the vessel. A differential amplifier (34) subtracts the return signals at one probe conductor from the return signals at the other conductor so as to cancel noise received from the probe conductor while enhancing the strength of return signals from the material interface. Gain of the differential amplifier is controlled as a function of the apparent distance to the material surface, and level of the air/material interface is determined employing time domain reflectometry techniques.

Description

MATERIAL LEVEL SENSING SYSTEM AND METHOD

The present invention is directed to methods and systems for measuring the level

of material in a vessel employing time domain reflectometry techniques.

Background and Summary of the Invention

It has heretofore been proposed to employ so-called time domain reflectometry

techniques to detect or measure the level of material in a storage vessel. In general, this

technique involves placement of a conductive transmission line probe in the vessel at an

orientation to be contacted by the material in the vessel. Microwave pulses of short duration are

launched onto the transmission line probe, typically in a downward direction through air toward

the material surface. When the pulses encounter an electrical discontinuity, such as the change

in dielectric constant at the interface between air and the material surface, portions of the pulse

energy are reflected back along the transmission line probe to detection electronics. Time-

delayed gating techniques are employed in a process referred to as equivalent time sampling to

locate the position of the reflecting discontinuity along the transmission line probe, and thereby

to determine the level of the material surface with respect to the probe. See, for example, US

Patent 6,085,589 assigned to the assignee of the present application.

Although the material level sensing technique so described has overcome

problems and difficulties theretofore extant in the art, further improvements remain desirable.

For example, a problem is encountered, particularly in connection with single-conductor probes,

in that a significant amount of electrical noise is picked up by the probe and transmitted to the

system electronics along with the reflected pulse energy. This problem can be particularly acute

when the material level is at the far end of the transmission line probe, at which losses of the

transmitted and reflected signals can be significant as compared with the amplitude of the noise signals. It is a general object of the present invention to provide a system and method for

measuring the level of material in a vessel that increases measurement accuracy and resolution

by increasing the reflected signal-to-noise ratio at the measurement electronics.

A system for measuring the level of material in a vessel in accordance with the

present invention includes a transmission line probe adapted to be positioned for contact with

material in the vessel such that an impedance discontinuity is presented along the probe at the

air/material interface in the vessel. An electronic circuit launches microwave radiation along the

probe such that a portion of such radiation is reflected from the impedance discontinuity at the

air/material interface. The electronic circuitry is responsive to such reflected energy portion for

determining or measuring the level of the air/material interface with respect to the probe

employing time domain reflectometry techniques. In accordance with a first aspect of the present

invention, the transmission line probe has first and second conductors, and the electronic circuitry

launches microwave radiation along the conductors simultaneously and 180° out of phase. One

of the reflected signals is subtracted from the other, so that the true reflected signal energy is

enhanced while noise picked up by the two conductors is canceled by the subtraction. In

accordance with another aspect of the present invention, which may be implemented separately

form or more preferably in combination with the first aspect of the invention, the reflected

signals are fed through a variable gain amplifier having a gain controlled as an increasing

function of time after initiation of each measurement cycle. This variable gain amplification

helps compensate for loss in signal strength due to increasing distance between the electronics

and the material surface. Brief Description of the Drawings

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 material level sensing system in

accordance with a presently preferred embodiment of the invention;

FIG.2 illustrates the system electronics and transmission line probe mounted on

a material vessel;

FIG. 3 is a sectional view taken substantially along the line 3-3 in FIG. 2; and

FIG. 4 is a timing diagram that illustrates the signals transmitted along the probe

conductors in accordance with one aspect of the present invention.

Detailed Description of Preferred Embodiments

FIG. 1 is a functional block diagram of a material level sensing system 10 in

accordance with a presently preferred embodiment of the invention. A transmission line probe

12 is suspended within a material vessel 14 so that the material 16 within vessel 14 is in contact

with the probe. Probe 12 comprises a pair of spaced parallel conductors 18, 20 that are

surrounded by insulation 22 (FIG. 3) and held in spaced parallel relationship to each other by the

insulation. Conductors 18, 20 are electrically isolated form each other throughout the length of

the probe. A central processing unit or CPU 22 includes a pulse width modulator 24 that

transmits microwave pulses to the input of a differential buffer amplifier 26. Amplifier 26 has

two outputs, one of which transmits a signal V, to probe conductor 18 and the other of which

transmits a signal V₂ to probe conductor 20. As shown in FIG. 4, signals V,, V₂ are identical

and transmitted simultaneously, but are 180° out of phase with each other - i.e., of opposite polarity. Portions of these transmitted signals are reflected from air/material interface 28 within

vessel 14.

The pulse width modulated input to differential buffer amplifier 26 is also fed

through a programmable delay line 30 to the control input of a differential gate 32. The signal

inputs of gate 32 are connected to probe conductors 18, 20 so as to receive the signal portions

reflected from interface 28. The control input of programmable delay line 30 is connected to

CPU 22 by a serial bus 36. During each measurement cycle, programmable delay line 30

operates gate 32 to monitor for reflections from the air/material interface for brief time intervals

at increasing time delays from pulse transmission. In other words, at the beginning of a

measurement cycle CPU 22 operates gate 32 through delay line 30 to monitor for reflections near

the top of bin 14, while at the end of a measurement cycle gate 32 is operated to monitor for

reflections near the end of probe 12. (This measurement cycle could be reversed.) This

technique, commonly referred as equivalent time sampling, effectively divides the length of

transmission line probe 12 into a multiplicity of small discrete lengths that are monitored in

sequence for reflections from any electrical discontinuity. During operation of gate 32, the

signals or probe conductors 18, 20 are respectively fed to the non-inverting and inverting inputs

of a variable gain differential amplifier 34. Within amplifier 34, the reflected energy from probe

conductor 20 is subtracted from the energy at probe conductor 18. Any noise picked up at the

probe conductors and transmitted to the system electronics will be substantially equal at the two

inputs of differential amplifier 34, so that such noise will be effectively canceled by subtraction

of one input from the other. On the other hand, the reflected signals will be of opposite polarity,

so that subtraction of one from the other effectively doubles signal amplitude. Thus, the signal- to-noise ratio of the reflected signals is significantly enhanced employing this signal transmission

and differential amplification technique.

The gain control input of variable gain differential amplifier 34 is connected to

CPU 22 by bus 36 for controlling amplifier gain as an increasing function of distance along probe

12. That is, when the equivalent time sampling technique is monitoring sections of the

transmission line probe near the top of the vessel, the gain of amplifier 34 is relatively low.

When portions of the probe near the bottom of the vessel are being monitored, the gain of

differential amplifier 34 is relatively high. Amplifier gain may be controlled as any suitable

increasing function of apparent distance, such as a linear function or non-linear function. In this

way, signals returned from the bottom end of the probe receive a greater degree of amplification

at variable gain amplifier 34 to compensate for signal loss along the greater length of signal

travel to and from the material interface.

The output of amplifier 34 is connected to an a/d converter 38 within CPU 22 for

computation of material level. CPU 22 provides an RS 485 output at 40 to remote monitoring

electronics and/or display 41 for indicating material level and other monitored conditions at the

vessel. CPU 22 is also connected by bus 36 to a 4-20mA output circuit 42 for providing an

analog signal at a remote meter 43 indicative of material level. System 10 is powered at 45 by

current received through 4-20 mA output 42, with the current drawn being indicative of material

level. CPU 22 is also connected by a bus 44 on EEPROM 45, which stores programming and

control parameters for operation of the level measurement electronics.

FIG. 2 is a schematic diagram of a system installation, in which the system

electronics are mounted within a housing 50. Housing 50 is mounted to the top of material

vessel 14. Transmission line probe 12 is suspended from a connector block 52 within housing 50. A weight 54 is suspended from a terminal block 56 at the lower end of probe 12 for

maintaining vertical orientation of probe 12 within vessel 14 as material is added to or withdrawn

from the vessel. As previously noted, probe conductors 18, 20 are isolated from each other by

probe insulation material 22 throughout the entire length of the probe.

There have thus been disclosed a system and method for measuring the level of

material in a vessel that fully satisfy all of the objects and aims previously set forth.

Implementation of the invention in a currently preferred system for measuring material level in

a range up to 60 feet (18.3 m) achieves accuracy and repeatability within a range of ± 1 in. (25.4

mm), and a resolution of 0.5 in. (12J mm). Even at the maximum range of 60 ft., scan time for

a complete measurement cycle is less than 1 second, and a linearity of ± 1% is maintained. The

dead zone at the top of the vessel, within which material level cannot be measured, is 2 ft. (0.61

m). The system can be employed for measuring level of either dry or liquid materials having a

dielectric constant as low as 1.5.

A number of modifications and variations have been discussed, and other

modifications and variations will readily suggest themselves to persons of ordinary skill in the

art. The invention is intended to encompass all such modifications and variations as fall within

the spirit and broad scope of the appended claims.

Claims

1.

A system for measuring level of material in a vessel, which comprises:

a transmission line probe, having first and second conductors, adapted to be

positioned for contact with material in the vessel such that an impedance discontinuity is

presented along each conductor at the air/material interface in the vessel,

first means for launching microwave radiation along said conductors

simultaneously and 180° out of phase such that portions of such radiation are reflected from said

impedance continuity, and

second means for determining level of the air/material interface with respect to

said probe employing time domain reflectometry as a function of a difference between said

reflected signal portions.

2.

The system set forth in claim 1 wherein said second means includes a variable

gain differential amplifier and means for controlling gain of said amplifier as an increasing

function of distance along said probe.

3.

The system set forth in claim 1 wherein said first means includes a differential

buffer amplifier. 1 The system set forth in claim 1 wherein said first and second conductors of said

2 transmission line probe comprise parallel conductors held by insulation at fixed spacing from

3 each other.

5.

1 A method of measuring level of material in a vessel, which comprises the steps

2 of:

3 (a) suspending within the vessel a transmission line probe having first and second

4 conductors, such that an impedance discontinuity is presented along each said conductor at the

5 interface between air and material in the vessel,

6 (b) launching microwave radiation along each said conductor simultaneously

7 and at opposite polarity, such that portions of such radiation are reflected from said impedance

8 discontinuity, and

9 (c) determining level of material in the vessel as a function of a difference

,0 between said reflected signal portions, such that noise in said reflected signal portions is

[ 1 canceled.

1 The method set forth in claim 5 wherein said step (c) includes the step of

2 amplifying said reflected signal portions following said step (b) as an increasing function of

3 distance along said probe.

7.

A system for measuring level of material in a vessel which comprises:

a transmission line probe adapted to be positioned for contact with material in the

vessel, such that an impedance discontinuity is presented along the probe at the air/material

interface in the vessel,

first means for launching microwave radiation along said probe such that a portion

of such radiation is reflected for the impedance discontinuity, and

second means responsive to said reflected energy portion for determining level

of the air/material interface with respect to the probe employing time domain reflectometry,

including an amplifier that receives said reflected energy portion and means for controlling gain

of said amplifier as an increasing function of distance to said interface along said probe.