The present invention relates to an apparatus for determining the oscillation parameters of a longitudinally oscillating band, for example a tape or ribbon, excited by a piezoelectric oscillator and whose oscillations are to be received and evaluated. The apparatus is used for measuring dust concentration in a fluid.
In the course of research work, discussed in a report entitled "Measurement of Emissions from Power Stations" by Theodor Gast and Karl Ulrich Kramm submitted to Bundesministerium fur Forschung und Technologie published October, 1984, tests have been made to determine to what extent the natural frequencies of a longitudinally oscillating band change when its inert mass changes. In an experimental arrangement employed for this purpose, a transmitter and a receiver are provided which are piezoelectric oscillators and which are arranged opposite one another at a defined distance. A band is held between the oscillators by means of first and second collet chucks which are each clamped in between the piezoelectric oscillators by way of a disc-shaped thickened portion.
The oscillations of the transmitter are transferred through the band and the second chuck to the oppositely disposed piezoelectric oscillator operating as the receiver, which thereby produces an output signal. In this experimental arrangement, the frequency of the transmitter is controlled in such a manner that a phase shift of 90.degree. exists between the phases of the excited and received waves, causing a standing wave to be formed having nodes at the clamping points (e.g., the first and second collet chucks). Experiments have shown that, for example, an increase in mass in the center region of the band results in a decrease in amplitude of the standing wave, and this change in amplitude could be used as a measuring value. In addition, it was found that the frequency is influenced, and this change in frequency could also represent a measuring variable. A chamber plunger structure and a pressure plate of this kind is described by Theodor Gast and Karl Ulrich Kramm in more detail in the European Patent Application No. 0 214 366 A2 and in the figure therein.
However, this "chamber-plunger structure" has considerable drawbacks. For example, the force generated by means of the piezoelectric element and acting on the plunger must be opposed by a counterforce produced by the enclosing chamber itself (i.e., by the corresponding front or rear pressure plate). Furthermore, in this device, synchronism of the piezoelectric element, which is necessary for conversion of input electrical energy to mechanical energy of the band, is not ensured. Therefore, it may be possible that not only the plunger--as is assumed in the ideal case--but the entire system of the chamber, the plunger, and the piezoelectric element oscillates, i.e. only part of the mechanically-produced energy is ultimately transferred to the band and detected at the receiver as a band signal. Consequently, bands having high material attenuation or a long length are not usable to produce a signal that can be evaluated.
Due to its complex mechanical structure, the foregoing system necessarily includes a plurality of "spring to mass" couplings, which all produce undesirable individual mechanical resonances. As a result of the external dimensions and material constants of the foregoing system, these undesirable individual mechanical resonances all lie in the intended useful frequency range of the measurements. However, an unequivocal evaluation of the resultant measurements can be made only if no natural resonance occurs in the transmitter or receiver below the uppermost measuring frequency. The prior art system exhibits these subordinate resonance phenomena beginning at 7 KHz, and therefore the measuring frequency has to be less than 7 KHz.
A practical embodiment of the apparatus with changed dimensions (larger plunger, chamber, and piezoelectric element) exhibits resonances over the entire frequency range (1 KHz to 40 KHz) measured, so that accurate and reliable results cannot be ensured. The resonant frequencies (e.g., subordinate resonances) of the undesirable "spring to mass" couplings are not frequency stable. They are subject to a great extent to extraneous influences such as, for example, tightening moments from the clamping screws, ambient temperature, mechanical shocks occurring during transport, and so on. The "tuning" of such systems is extremely difficult since there are interacting factors. That is, the effect of a change in one parameter can be evaluated only by observation of the total frequency spectrum of the system, e.g. a Fourier analysis, which, however, is possible in relatively few cases. For the above reasons, the aforementioned prior art device is not suitable for use in practice (especially for use in the rough surroundings of a power plant where this invention could be used).
Although piezoceramic high power transducers are known ("Technologie und Anwendungen von Ferroelektrika" [Technology and Uses of Ferroelectric Elements], published by VAG Leipzig, 1976, pages 360 et seq.), these serve to generate the acoustic output power of ultrasound transducers employed, for example, for cleaning purposes (in water as a prerequisite) or for underwater signal transmission (page 360, second paragraph, lines 4 and 5). Their use as elements for transferring forces to bands or the like that are mechanically coupled to them, for example under environmental conditions or in flue gas chimneys, is neither mentioned nor suggested.