Ultrasound sensor for detecting the level of liquids

Details Ultrasound sensor for detecting the level of liquids Ultrasound sensor for detecting the level of liquids

1999-05-12

2001-08-14

Chapman, John E. (Department: 2856)

Measuring and testing

Liquid level or depth gauge

Reexamination Certificate

active

06272921

ABSTRACT:

APPLICATION FIELD
The invention relates to liquid level indicators using measurement of sound waves parameters.
BACKGROUND OF THE INVENTION
Designs of ultrasonic liquid level detectors are known where resonators and tuning forks are driven with the help of sound waves. The acoustic parameters of resonators and tuning forks vary when contacting liquid, a medium with higher density than air, and these variations are detected.
The design according to DE patent N 4201360 can serve as an example. The device contains two or more vibratory rods put into a reservoir under control, which are connected with radiating and receiving transducers. The same principle is used with the “Device for detection and/or control of filled reservoir level,” according to DE patent N 4118793, or the “Device for measurement and/or keeping of given level in a reservoir,” according to the International application WO 92/21945.
The given designs of the liquid level detectors depend on operating conditions. Liquid and dirt remaining in the tuning fork spaces can influence measurement accuracy of the mentioned devices.
A device for liquid level detection is known from DE patent N 3011603 priority 26.03.80, Int. Cl. G 01 F 23/28. The device contains vibratory elements placed co-axially. The space between the vibratory elements is isolated from the medium and is located along the whole length of the rod. Piezoelements are fixed on membrane inserts connected to one of the vibrators. The case of the device is fixed to the membrane insert. The attachment joint of the device case to the external basement is made in the form of a threaded connection. The device differs from the offered invention in operating principle and design.
A further device for liquid level control is known from DE application N 2949162 priority 06.12.79 Int. Cl. G 01 F 23/28. The device has a hollow portion along the whole length of a waveguide, to which a radiator and receiver are fixed in different places. The case is not rigidly fixed to the waveguide and includes a threaded connection for fixing it to the external base.
Moreover an acoustic liquid level detector is known from FR application N 2596515 priority 28.03.86 Int. Cl. G 01 F 23/28. The detector contains a hollow rod along the entire length and is open to the surrounding medium. Another hollow pattern along the length of the waveguide is installed inside the rod on which transducers are installed. The waveguide attachment points to the external base are not considered in this design.
An ultrasonic liquid level indicator is known from SU Invenitor's Certificate N 231151, wherein the detector consists of two separated waveguides. The waveguides are fixed in the walls of a reservoir under control of the oscillation nodes.
The designs of all above mentioned ultrasonic detectors differ from the offered one. In particular, protection from the influence of condensate, any remaining liquid and dirt on measurement accuracy is not provided in the above designs.
The “Detector of Liquid Presence” according to EP patent N 409732 priority 19.07.90 Int. CL. G 01 F 23/28 is the closest in technical principle. The design of the detector includes a case, a measuring element connected with a pulse generator and a receiving device. The measuring element of this device includes an acoustical-electrical transducer and a compound acoustic waveguide connected to the transducer. The first part of the compound acoustic waveguide consists of a solid cylinder and the second one consists of a hollow cylinder. The first part of the waveguide is smaller in diameter than the second one.
The performance accuracy of the given detector design depends on conditions of the surrounding medium. Liquid and other contaminates can accumulate in the open space. Liquid and dirt can also gather on the transducer and on the waveguide case where the junction (transition) to a greater diameter occurs. All these can influence the accuracy of the detector performance.
SUMMARY OF THE INVENTION
It is an object of the present invention to reduce detector performance dependence on a surrounding medium and various operating conditions, thereby providing high accuracy and sensitivity.
The ultrasonic liquid level detector according to the present invention contains a detector case and a rod acoustic waveguide on one end of which there is an acoustical-electrical transducer and on the other—a hollow resonator. The acoustical-electrical transducer provides excitation of the rod acoustic waveguide, which will oscillate on operating frequency depending on the resonator. Oscillation duration of the excited resonator will depend on whether it is placed in a gaseous medium or has contact with liquid.
The novelty consists of the following.
The resonator cavity is isolated from the external medium. Such a design avoids accumulation of liquid and foreign matter, and thus diminishes measurement errors. The resonator space is placed on the end opposite to the acoustical-electrical transducer of the waveguide. The space is placed where it is necessary to control the liquid level. This allows to reduction in the influence of dirt and condensate, because foreign matter deposited on the waveguide part, which is not hollow, does not practically influence measurement accuracy.
The detector case is fixed rigidly and hermetically on the surface of the rod acoustic waveguide in the zone of minimum rod oscillations of the rod acoustic waveguide at detector operating frequency. Such case fixing on the rod acoustic waveguide avoids ingress of moisture and foreign matter to the acoustical-electrical transducer and onto the waveguide upper part. Therefore, operation accuracy and sensitivity of the detector are not reduced as a result of the rigid case being fixed to the rod acoustic waveguide, because it is fixed in the zone of minimum oscillations of the waveguide rod.
The detector case contains an attachment means for attaching said case to the external base. The location of the attachment means on the case also serves as the solution of the set task. Since the mass on the case, even though it is rigidly and hermetically fixed on the acoustic waveguide, influences the detector parameters only minimally, the case fixed on the external base by means of the attachment means does not practically influence detector parameters regardless of the base type.
In such a design there is no place on the rod acoustic waveguide where liquid, condensate and dirt can gather. The case isolates entirely the acoustical-electrical transducer and the waveguide part from external surroundings. Moreover, the case does not practically influence the detector parameters, regardless of where on whatever base the case is installed.
In a particular embodiment the resonator space is isolated from the external surrounding by a plate which is rigidly and hermetically fixed to the rod acoustic waveguide. The plate thickness is less than W/12, where W is the sound wavelength in the rod acoustic waveguide at operating frequency. Such a plate thickness does not practically influence the detector sensitivity.
Besides, the case fixing zone on the surface of the rod acoustic waveguide should be placed in the nodes of longitudinal waveguide oscillations.
Such zone position under concrete values of detector operating frequency provides minimum case influence on detector parameters. For this purpose the distance between the fixing place and the transducer should be equal to an odd number of wavelengths quarters that with regard to acceptable spreads of this distance (equal to W/12) leads to the condition
W/
4*[(2*
k+
1)+⅓]>
L>W/
4*[(2*
k+
1)−⅓],
where
W—sound wavelength in the rod acoustic waveguide at operating frequency;
k—integer value.
Additionally, the thickness of the detector case in the place of fixing on the rod acoustic waveguide is less than W/12, where W is the sound wave-length in the rod acoustic waveguide at operating frequency. It is necessary to provide minimum case influence on detector sensitivity.
In addition, the a

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