Electricity: motive power systems – Positional servo systems – With particular 'error-detecting' means
Patent
1993-03-25
1995-04-18
Ro, Bentsu
Electricity: motive power systems
Positional servo systems
With particular 'error-detecting' means
340621, 340620, 310323, 310328, 73290V, G01F 2328, B06B 106
Patent
active
054081688
DESCRIPTION:
BRIEF SUMMARY
The invention pertains to a device to determine and/or monitor a predetermined filling level in a container with a mechanically oscillating structure that comprises at least two vibrating rods that protrude into the container and are arranged at a distance from each other on a membrane that is fixed at its edges, an excitation arrangement that effects opposite oscillations of the oscillation rods transverse to their longitudinal extent, an excitation transformer that contains at least one piezoelectric element that can be excited by an alternating voltage, and a reception transformer with at least one piezoelectric element that transforms the oscillations of the mechanically oscillating structure into an electrical output signal.
A device of this type is known from the Patent No. DE-AS 17 73 815. As compared to other known devices with a single oscillation rod that protrudes into the container, this device provides the advantage that the alternating forces that are applied by each oscillation rod, and act upon the fixing of the membrane, mutually eliminate each other due to the opposite direction of the oscillations, which means that no vibrational energy is lost by a transfer onto the container walls, and that the fixing is exposed to less mechanical stress. In this known device the ends of the oscillation rods which are connected with the membrane are also connected with each other by two bridges that are mechanically decoupled from each other, whereby each of these bridges consists of a yoke adjoining the membrane and a bar spanning the yoke. One bridge is coupled with the excitation transformer and the other bridge is coupled with the reception transformer by the fact that the piezoelectric element of the transformer is clamped between the bar and the yoke. The excitation transformer is connected to the output of an amplifier, the input of which is connected with the reception transformer, so that a self excitation of oscillations with the natural resonance frequency of the mechanically oscillating structure is executed.
The evaluation circuit that is connected to the output of the amplifier is constructed such that it responds to amplitude changes. The function of this device is based on the fact that the mechanic oscillations of the oscillation rods are damped during submersion into the material, so that a reduction of the oscillation amplitude results that is detected by the evaluating circuit and effects the triggering of a display and/or switching process.
It was established that this know device requires relatively strong damping of the oscillation rods by the material due to the construction of the mechanical oscillation structure and the arrangement of the piezoelectric transformers in order to ensure a reliable response to the amplitude changes. This device can thus be utilized to monitor materials that cause a strong damping if the oscillation rods are covered, for example, bulk materials or fluids with high viscosity.
A different device from this type with two oscillation rods that protrude into the container is known from Patent No. DE-PS 33 36 991. The device of this patent has a bridge arranged on the side of the membrane that opposes the oscillation rods and is retained at the ends within a distance from the membrane by rod-shaped supports that are connected with the oscillation rods. The piezoelectric elements of the excitation transformer and the reception transformer are in this particular device arranged in a stack which is clamped between the bridge and the portion of the membrane lying between the oscillation rods. The device of Patent No. DE-PS 33 36 991 already attained the objective to construct a device of this type with two oscillation rods that protrude into the container such that it is suitable for materials of any type, and additionally make it possible to operate the device with a low alternating, excitation voltage and low excitation energy.
A further device of the same type which also comprises two oscillation rods that protrude into the container is known from Patent N
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Endress & Hauser GmbH & Co.
Ro Bentsu
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