Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1991-05-20
1993-03-02
Ng, Jin F.
Communications: electrical
Condition responsive indicating system
Specific condition
340620, 73290V, 310323, 310328, G08B 2100
Patent
active
051913160
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an apparatus for determining and/or monitoring a predetermined contents level in a container, comprising a mechanical oscillation structure having at least two oscillating rods projecting into the container and mounted spaced from each other on a diaphragm clamped at the edge, an excitation arrangement which sets the oscillating rods in opposite oscillations transversely of their longitudinal direction and includes an excitation transducer having at least one piezoelectric element excitable by an ac voltage, and a receiving transducer having at least one piezoelectric element which transforms the oscillations of the mechanical oscillation structure to an electrical output signal.
An apparatus of this type is known from DE-AS 1,773,815. Compared with other known apparatuses having a single oscillating or vibrating rod projecting into the container it has the advantage that the alternating forces exerted by each oscillating rod on the clamping of the diaphragm cancel each other out due to the opposite sense of the oscillations and consequently no oscillation energy is lost by transmission to the container wall and the clamping is also subjected to less mechanical stress. In this known apparatus, the ends of the oscillating rods connected to the diaphragm are connected to each other by two bridges mechanically decoupled from each other, each of which consists of a yoke bearing on the diaphragm and a stirrup member spanning the yoke. The one bridge is coupled to the excitation transducer and the other bridge to the receiving transducer so that the piezoelectric element of the transducer is clamped between the stirrup member and the yoke. The excitation transducer is connected to the output of an amplifier, the input of which is connected to the receiving transducer so that a self-excitation of oscillations takes place with the natural resonant frequency of the mechanical oscillation structure.
The evaluating circuit connected to the output of the amplifier is configured to respond to amplitude changes. The mode of operation of this apparatus is based on the fact that the mechanical vibrations or oscillations of the oscillating rods are dampened on immersion into the contents material so that a reduction of the oscillation amplitude results which is detected by the evaluating circuit and effects the initiation of an indicating and/or switching operation.
It has been found that due to the configuration of the mechanical oscillation structure and the arrangement of the piezoelectric transducers, this known apparatus requires a relatively strong damping of the oscillating rods by the contents material to ensure a reliable response to the amplitude changes. This apparatus can therefore be used to monitor contents materials which when they cover the oscillating rods result in pronounced damping, such as bulk goods or fluids of high viscosity.
Another apparatus of the same type having two oscillating rods projecting into the container is known from German patent 3,336,991. The subject of this patent has a bridge which is arranged on the side of the diaphragm remote from the oscillating rods and which is held spaced from the diaphragm by flexurally elastic rod-shaped supports connected at the respective ends rigidly to the oscillating rod. In this known apparatus the piezoelectric elements of the excitation transducer and of the receiving transducer are arranged in a stack which is clamped with convex prestressing of the diaphragm between the bridge and the portion of the diaphragm lying between the oscillating rods. The subject of German patent 3,336,991 already solves the problem of further developing an apparatus of this type having two oscillating rods projecting into the container so that it is suitable for contents material of any desired type and can be operated with a low excitation ac voltage and low excitation energy.
Proceeding from this apparatus, the present invention is based on the problem of providing an apparatus for determining and/or monitoring a predetermined contents level
REFERENCES:
patent: 3396285 (1968-08-01), Minchenko
patent: 3625058 (1971-12-01), Endress et al.
patent: 3944994 (1976-03-01), Fanshawe
patent: 4507583 (1985-03-01), Jensen et al.
patent: 4594584 (1986-06-01), Pfeiffer et al.
Endress u. Hauser GmbH u. Co.
Hofsass Jeffery A.
Ng Jin F.
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