Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1994-06-01
1997-04-29
Hofsass, Jeffery
Communications: electrical
Condition responsive indicating system
Specific condition
340618, 340621, 324602, 73304R, 73290V, 73 173, G08B 2100
Patent
active
056253435
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
The invention pertains to a device for determining and/or monitoring a predetermined level with a sensor which is arranged in a container at the elevation of the level to be monitored in such a way that it comes in contact with the filling material once said material has reached the predetermined level, an electronic excitation circuit which excites the sensors to perform mechanical oscillations at its self-resonant frequency via a piezoelectric excitation and reception transducer, and with an evaluation circuit for triggering display and/or switching processes as a function of whether the frequency of the mechanical oscillations of the sensor exceeds or falls short of a operating frequency.
One example of such a device is disclosed in German Patent No. 3,931,453. In this known device, the sensor has two oscillating rods which are fastened onto a membrane at a distance from each other and may be excited such that they oscillate in opposite directions perpendicular to their longitudinal direction via a piezoelectric excitation and reception transducer which is excited by means of an electronic excitation circuit. However, there exists devices of this type in which the sensor only has one oscillating rod or is constructed in a different fashion without oscillating rods. The function of all such devices is based on the fact that the self-resonant frequency of the sensor is changed once the sensor comes in contact with the filling material. If the sensor is covered by the filling material, its self-resonant frequency is lower than if the sensor is not covered by the filling material. The operating frequency is selected such that it lies between the lower self-resonant frequency while the sensor is covered and the higher self-resonant frequency when the sensor is not covered. Once the evaluation circuit determines that the oscillation frequency of the sensor lies below the operating frequency, said fact indicates that the sensor is covered by the filling material, i.e., the level in the container has exceeded the elevation to be monitored. If the evaluation circuit determines that the oscillation frequency of the sensor lies above the operating frequency, said fact is an indication that the sensor is not covered by the filling material, i.e., the level lies below the elevation to be monitored.
The natural frequency of the sensors is different due to the manufacturing conditions. In addition, the operating frequency must be adjusted individually for each sensor since the self-resonant frequency of a covered and a non-covered sensor may lie relatively close together, in particular with liquids of low density. In such instances, the evaluation circuit is inevitably coupled with the sensor. If said evaluation circuit should be used in association with a different sensor, it must be readapted to this new sensor, which means that the self-resonant frequency of the new sensor must be known. This adaptation may usually not be carried out without certain problems at the place at which the device is used. This aspect makes it more difficult to carry out repair and maintenance procedures of the device.
The invention is based on the objective of creating a device of the type described initially in which each evaluation circuit may be used in association with any arbitrary sensor without requiring an individual adaptation of the evaluation circuit to the respective sensor.
According to the invention, this objective is attained by the fact that a plug connection is arranged between the electronic excitation circuit on one hand and the piezoelectric excitation and reception transducer on the other hand, whereby said plug connection produces the electrical connection between the electronic excitation circuit and the piezoelectric excitation and reception transducer and contains a receptacle mounting for a characteristic impedance which is connected electrically with the evaluation circuit when closing the plug connection, and whereby the value of resistance of the characteristic impedance is in definite relation t
REFERENCES:
patent: 3943440 (1976-03-01), Edwards
patent: 4594584 (1986-06-01), Pfeiffer et al.
patent: 4735240 (1988-04-01), Ziegler
patent: 4740726 (1988-04-01), Umezawa
patent: 5135002 (1992-08-01), Kirchner et al.
patent: 5191316 (1993-03-01), Dreyer
Endress & Hauser GmbH & Co.
Hofsass Jeffery
Lieu Julie B.
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