Oscillators – Electromechanical resonator – Crystal
Patent
1989-02-27
1991-07-02
Pascal, Robert J.
Oscillators
Electromechanical resonator
Crystal
331 65, H03B 532
Patent
active
050292684
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a circuit arrangement for self-excitation of a mechanical oscillation system to natural resonant oscillations comprising an electromechanical transducer system which is arranged in the feedback circuit of an electronic amplifier circuit so that said system is stimulated by the output AC voltage of the amplifier circuit to mechanical oscillations and furnishes an AC voltage with the frequency of the mechanical oscillations to the input of the amplifier circuit.
In various fields of use of mechanical oscillations this known type of self-excitation of natural resonant oscillations presents problems. This is for example true for mechanical oscillation systems with oscillating rods which are used as sensors for detecting when a predetermined filling level in a container is reached, employing the fact that the oscillations on immersion of the sensor into the filling material stop because of the strong damping whilst the restarting of the oscillations indicates that the filling level has dropped below the installation level of the sensor. If in such a use the sensor is exposed in the process container to high temperatures then this can change the transmission factor of the sensor to such an extent that it can no longer start oscillating and as a result an erroneous indication of the filling level is given. Filling materials (e.g. lime, flour) with a pronounced tendency to form encrustations or deposits have the same effect: with pronounced encrustation the sensor can no longer start vibrating so that it is erroneously indicated that the sensor is covered although in reality it is not immersed in the filling material and only covered with the encrustation or the like.
If the gain of the amplifier circuit is increased to avoid the problems outlined above the external vibration sensitivity becomes too large. This means that when the sensor is covered vibrations at the container caused for example by agitators or filling material flowing past can produce output voltages of the amplifier circuit which simulate the sensor not being covered and lead to natural resonant oscillations, then erroneously indicating too low a filling level.
The problem underlying the invention is the provision of a circuit arrangement for self-excitation of a mechanical vibration or oscillation system which with small circuit expenditure ensures reliable buildup of oscillations even under unfavorable operating conditions and reduces the risk of erroneous indications of the oscillation state.
According to the invention this problem is solved in that the amplifier circuit has a non-linear gain characteristic which at small values of the input signal gives a greater gain than at larger values of the input signal
The circuit arrangement constructed according to the invention has a high response sensitivity at small values of the input signal of the amplifier circuit so that even by weak interfering effects, for example slight external vibrations, thermal noise or similar interference effects, oscillation is initiated which rapidly builds up. In contrast, at higher values of the input signal the input sensitivity is reduced so that a good insensitivity to external vibrations is achieved. For example, when the circuit arrangement is used in a filling level sensor of the type outlined above said sensor has a very good starting behavior in a large temperature range and a very good encrustation compatibility with simultaneous high insensitivity to foreign or external vibrations.
The necessary non-linear gain characteristic can be achieved with small circuit expenditure because all that is needed is a two-stage amplification which changes from a large value to a smaller value when the magnitude of the input signal exceeds a predetermined threshold value.
Advantageous embodiments and further developments of the invention are characterized in the subsidiary claims.
Further features and advantages of the invention will be apparent from the following description of an example of embodiment which is illustrated in the drawings, wher
REFERENCES:
patent: 3469211 (1969-09-01), Shoh et al.
patent: 4734659 (1988-03-01), Tanaka et al.
Crump, "Diode Function Generators", Wireless World, Band 73, No. 12, Dec. 1987, pp. 594-598.
Patent Abstracts of Japan, Band 8, No. 163.
U. Tietze et al., "Halbleiter-Schaltungstechnik", Springer-Verlag, 1983, pp. 137-139, 440-442, and 453-458.
Endress u. Hauser GmbH u. Co.
Pascal Robert J.
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