Resonant tube sensor

Details Resonant tube sensor Resonant tube sensor

2001-11-06

2004-12-07

Hofsass, Jeffery (Department: 2632)

Communications: electrical

Condition responsive indicating system

Specific condition

C340S616000, C340S618000, C340S621000

Reexamination Certificate

active

06828912

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to resonant tube level sensors. More particularly, this invention relates to the use of acoustic waves for point level sensing, continuous level sensing, and pressure sensing.
BACKGROUND OF THE INVENTION
It is often necessary to determine (sense) the level of a material. In general, there are two main types of level sensing: point level sensing and continuous level sensing. Point level sensing determines whether a material is above or below a particular level, while continuous level sensing determines the material level relative to a reference point. It is possible to approximate a continuous level sensor using many point level sensors. Level sensors can be used to determine other parameters. For example, a manometer is a pressure sensor that uses the level of a liquid to determine pressure.
Numerous types of level sensors are known. For example, some level sensors use floats, tuning forks, electrical conductivity, ultrasonic time-of-flight, microwaves, optical reflections, tapes, magnetostriction, capacitance, electromagnetic time domain reflectometry, thermal conductivity, and pressure. In U.S. patent application Ser. No. 09/854,500, entitled “Resonant Tube Level Sensor,” filed on May 14, 2001, I described a point level sensor based on acoustic resonance in a tube. The patent application is hereby incorporated by reference. Also, reference can be made to U.S. Pat. No. 5,128,656, entitled “Level Detecting Method and its Apparatus” issued on Jul. 7, 1992 to Watanabe for another systems that uses acoustic waves.
While all of the foregoing level sensors are useful, none is optimal in every application. For example, floats are often rather large and are subject to leaks and other failures, tuning forks thermal conductivity, and electrical conductivity sensors are sensitive to material build-up or are limited to certain types of materials, tapes are subject to breakage and require an operator, and magnetostriction, capacitance, ultrasonic, time domain reflectometry, and pressure are typically relatively expensive. While acoustic methods are very promising in that low cost, accurate systems that are relatively insensitive to material build-up and foams are achievable, some problems are evident. Problems with U.S. Pat. No. 5,128,656 and U.S. patent application Ser. No. 09/854,500 will now be discussed.
U.S. Pat. No. 5,128,656 and U.S. patent application Ser. No. 09/854,500 are both based on the physics of resonant tubes. A tube having an effective length L that is filled with a medium (usually a gas such as air) having a speed of sound of c can produce two different sets of resonant frequencies. If the tube is closed at both ends (or open at both ends) the possible resonant frequencies are:
f
nc
=nc
/2
L
, where
n
=1, 2, 3,
If the tube is open at one end and closed at the other end the possible resonant frequencies are:
f
no
=(2
n
−1)
c
/4
L
, wherein
n
=1, 2, 3,
It should be noted that the length L is the effective length. As is well known, the resonant frequency of a tube is subject to an “end effect” in that an open tube is acoustically longer than the actual length, with the additional length depending on the radius of the tube's opening.
Since acoustic resonance is a fundamental physical property, its use in level sensing is beneficial. However, U.S. Pat. No. 5,128,656 appears to have drawbacks in that its method of sensing resonance is not particularly easy to implement, it may have operational reliability problems, and it appears to be difficult to use with caustic vapors. Furthermore, the sensor described in U.S. patent application Ser. No. 09/854,500 did not perform well enough over time and temperature for most practical commercial applications.
One problem with the sensor described in U.S. patent application Ser. No. 09/854,500 was that it worked well only with certain tube lengths. Furthermore, those tube lengths depended on the tube type. For example, copper tubes, plastic tubes and tubes having different wall thicknesses and inner diameters worked best with different tube lengths. Furthermore, once acceptable operation was achieved with a given tube, temperature changes (say by 10° F.) made operation erratic. Operation also tended to change over time (say 24 hours). Another problem was that system operation did not always follow the simple physical theory described above. For example, when acceptable operation was achieved with a “closed end” tube, changing frequency to obtain closed resonance at n+1 did not always work. However, the sensor described in U.S. patent application Ser. No. 09/854,500 is highly advantageous in that it has no moving parts (except for the transducer movement), is easy to fabricate, is low cost, rugged, and is difficult to clog.
Therefore, an improved acoustic resonance level sensor would be beneficial. Particularly beneficial would be an improved level sensor that operates on the principles described in U.S. patent application Ser. No. 09/854,500. Furthermore, a new level sensor that extends the principles of acoustic resonance as described in U.S. patent application Ser. No. 09/854,500 to continuous level sensing would be beneficial. Additionally, a new level sensor that extends the principles of acoustic resonance to sensing other parameters, including pressure, would be highly beneficial.
SUMMARY OF THE INVENTION
The principles of the present invention provide for point level sensors and for continuous level sensors that can sense the level of a material. Advantageously, the principles of the present invention enable sensing of many materials, including very light solids, such as feather, cotton, and powders, and of almost all liquids, including highly viscous liquids that tend to cling. Additionally, the principles of the present invention enable both temperature and pressure sensing.
A point level sensor according to the principles of the present invention includes a tube having a sense position (such as the end of the tube) and an acoustic assembly that produces sound in the tube. The acoustic assembly is mounted to reduce or eliminate vibrations in the tube body. A beneficial way of reducing or eliminating such vibrations is to use a vibration dampening material, such as a rubber compound, between the tube and a source of acoustic waves. A driver circuit can then drive the acoustic assembly in an attempt to produce a standing wave in the tube at the sensing position. After a time sufficient to produce a standing wave the driver circuit stops driving the acoustic transducer. An electronic network then monitors the decay of the acoustic waves in the tube to determine if a standing wave was produced. Based on that determination, a signal is produced that indicates whether a material has reached the sensing position. Multiple acoustic frequencies can be used to attempt to produce resonance. Beneficially, the acoustic frequency (or frequencies) that are used in the attempt(s) to produce resonance depends on temperature.
According to one embodiment of the present invention, the driver circuit drives the acoustic assembly with a frequency that would produce a standing wave if an end of the tube is open (the end of the tube then being the sensing position). If a standing wave is produced, as determined by the acoustic decay, a level signal is produced that indicates that a material has not reached the end of the tube. Beneficially, the system compensates for temperature effect on the speed of sound (which impacts on acoustic resonance).
According to another embodiment of the present invention, the driver circuit drives the acoustic assembly with a frequency that would produce standing waves if an end of the tube is closed (the end of the tube again being the sensing position). If a standing wave is produced, as determined by the acoustic decay, a level signal is produced that indicates that a material has reached the sensor end. Beneficially, the system compensates for temperature effect on the speed of sound (which impacts on acoustic resonance).
Acco

Affiliated with

Also associated with

Rating

Resonant tube sensor does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Resonant tube sensor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Resonant tube sensor will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3332925