Method for determining parameters, for example level,...

Details Method for determining parameters, for example level,... Method for determining parameters, for example level,...

1999-05-07

2002-01-15

Williams, Hezron (Department: 2856)

Measuring and testing

Liquid level or depth gauge

C073S579000

Reexamination Certificate

active

06338272

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention concerns a method for the determination of parameters, e.g. filling level, pressure, gas composition in the head space and material condition, in closed containers, wherein a container wall is excited to produce mechanical oscillations and the resultant oscillations are picked up and analysed in respect of their chronological sequence or their frequency spectrum.
The filling level of liquids in containers is normally determined optically by means of light barriers, by checking of the weight of the container or by measurement of a high-frequency electromagnetic radiation absorbed by a container.
Methods are known for testing containers for leakages and for ascertaining the internal pressure, in which the cap of a container is displaced electromagnetically (U.S. Pat. No. 1,956,301). Furthermore, it is known that inferences on container states can be drawn not only from the displacement but also from the vibration frequency of the container walls (U.S. Pat. No. 2,320,390). Initially, the displacement and the vibrations of the container walls were measured mechanically. Since these vibrations result in acoustic signals, electronic circuits such as microphone arrangements (U.S. Pat. No. 3,290,922) or electrostatic sensors (U.S. Pat. No. 3,441,132) can also be used for this purpose.
It is further known that only sub-ranges of the signal spectrum are relevant to the checking of tightness or pressure, so that the electronic measures can be reduced to the evaluation of specific frequency ranges (U.S. Pat. No. 3,802,252). It thus became possible to digitise the filtered oscillation signal and, for the purpose of checking pressure, to count the number of periods during a measurement interval (U.S. Pat. No. 4,187,718). In order to enable the correct frequency of the cap oscillation to be measured constantly over a longer period, measures were taken by which the vibration signal was fed back to the exciter arrangement to achieve an undamped cap oscillation through repeated excitation with the cap frequency (U.S. Pat. No. 4,406,157). Furthermore, instead of concentrating on one frequency range in the evaluation of the vibration signal, it is possible to analyse the entire signal with the use of signal processors. Initially, the spectrum alone was examined for the presence of particular frequencies for the purpose of thereby detecting the presence of absence of defects (U.S. Pat. No. 5,144,838). It subsequently became possible to compare the measured frequency spectrum with stored reference spectra of containers with a known pressure in order to permit determination of a pressure value for the current container (U.S. Pat. No. 5,353,631).
The object of the invention is to create a method for the comprehensive quality testing of containers which can be performed with a particularly small amount of apparatus and provides very accurate results.
BRIEF SUMMARY OF THE INVENTION
This object is achieved according to the invention with a method of the type initially referred to, wherein the secondary oscillations which are excited by the primary mechanical oscillations of the cap and which are produced in the container within the excited container wall are picked up and analysed.
If a face of a container, e.g. the cap of a beverage container, is briefly raised with an electromagnetic pulse and then released, it is excited to produce a free, primary oscillation. The frequency of this oscillation is a measure of the cap tension which is determined by, amongst other factors, the pressure prevailing in the container. As the tension of a drum-head determines the sound of the drum, the lid oscillation varies in dependence on the internal pressure of the container. This primary oscillation of the cap is highly damped, so that the oscillation determined by the internal pressure decays rapidly. The short oscillation period is however sufficient to emit an acoustic signal into both the air space outside the container and the container volume itself. If this signal is picked up by means of a vibration pick-up, e.g. a microphone arrangement, this primary oscillation frequency can be ascertained in a first time measurement window by means of a relatively simple circuit through measurement of the duration of an oscillation period. The position and the length of the measurement window can be individually adapted to each container type. In the case of crown cap closures, for example, it can be said that this first time measurement window commences approximately 0.3 ms after the excitation of the cap oscillation and lasts for approximately 0.4 ms.
In order to obtain a sufficient number of primary oscillation amplitudes for the excitation of secondary oscillations, the method according to the invention is used primarily for containers which have at least one rigid container wall, e.g. a metal crown cap closure. Containers which are made from plastic or which have plastic closures are less suitable. The natural frequency of the container wall, e.g. the closure, preferably lies within the range from {fraction (1/10)} to 10 times the expected signal of the secondary oscillations, for example between 2 and 12 kHz in the case of beverage bottles.
Within the container, the cap signal is propagated at the sound velocity of the gas in the head space or at the sound velocity of the liquid. The junctions between liquid and gas and between liquid and container material each reflect the sound waves, so that standing waves can form as secondary oscillations within the container. Since the wavelength of the standing wave in the head space and that of the standing wave in the liquid depend upon the intervals of the phase transitions, if the composition of the gas in the head space is known the frequency of the secondary oscillation is a measure of the filling level within the container. Since the secondary oscillations within the container are excited and supplied with energy only by the original, primary cap oscillations, it is only after the decay of the highly damped cap signal that the standing waves become pronounced and detectable as a vibration signal, e.g. an acoustic signal, through the vibration of the side walls of the container, particularly the cap. It is thus possible for this signal also to be picked up by means of a vibration pick-up and for the filling level in the container to be determined through appropriate selection of a second measurement window.
In a first embodiment of the invention, therefore, the primary oscillation of the cap is differentiated from the standing waves or secondary oscillations within the head space of the container and within the volume of the liquid, which are excited by this oscillation, in that the second measurement window is located so that the primary oscillations of the cap have themselves essentially decayed and the picked-up oscillations signals consequently originate essentially from the standing waves or secondary oscillations.
If the two measurement windows cannot be sufficiently separated, the effective period of the primary cap oscillation continues to be influenced by the forming standing wave in the head space. However, if following expiry of the second measurement window, the frequency of the standing wave, as filling level information, and the frequency of the primary cap oscillation are known, the filling level information can also be used to compensate fully the impairment of the pressure information within the scope of the measuring accuracy. It is precisely in this connection that a further advantage of the time-differentiated analysis of a single signal becomes apparent, since there is no need for any adjustment between different sensors and thus the high accuracy is achieved with simple means.
Further information on parameters of a closed container can be obtained from the signals of the primary and secondary oscillations. The frequency of the secondary oscillation depends principally on the filling level, the temperature and the gas composition, e.g. the mixture ratio of two gases. If the filling level is measured in a conv

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