Measuring and testing – Gas analysis – By vibration
Reexamination Certificate
2000-02-23
2001-11-13
Williams, Hezron (Department: 2856)
Measuring and testing
Gas analysis
By vibration
C073S031060, C073S03200R, C073S054410, C073S061790, C310S31300R
Reexamination Certificate
active
06314791
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a surface acoustic wave sensor comprising at least two oscillating passive components having oscillator circuits as measuring oscillators disposed in a housing through which a medium to be tested is conducted.
Such a sensor is known for example from DE 44 17 170.
Modified acoustic surface wave—or Surface Acoustic Wave (SAW) building components can be employed for the chemical sensing of gases or liquids by applying a respective chemically reactive coating to the components. With the ab- and adsorption of the analyte, the mass of the coating as well as the elastic parameters thereof change, whereby the sound propagation speed of the surface wave changes. In order to measure the change of the sound propagation speed of a surface wave in a simple manner, it is common practice to include in an oscillator circuit a coated SAW component, which is used as a frequency determination element.
The sound propagation speed change results in a proportional change of the oscillator frequency, which can be determined with a good approximation and can be measured with a high resolution of typically 10
−6
. With an appropriate selection of sorption layers, almost any number of gaseous analytes can be examined with this technique. Of greatest interest are those materials for which qualitative and quantitative determinations are difficult to perform with other chemical microsensors. Those are organic solvents such as hydrocarbons (hexane octane, decane, various fuels), alcohols (methanol, ethanol, halogenized hydrocarbons (CKW's, FCKW's), and aromatics (benzene, toluene).
EP 0 509 328 A2 discloses an arrangement of 3 SAWs, which are arranged, however, in series. This results in different flow conditions in the individual sensors.
EP 0 477 684 A1 discloses an arrangement of more than 2 SAWs with different coating. The sensors are not arranged however in any particular way.
If several oscillators are operated in a small space at almost the same frequency, there is a cross-influence as a result of the electromagnetic transgressions. In an extreme case, this may result in a lock-in situation (oscillators vibrate at the same frequency). This behavior limits the possibilities of miniaturization of HF oscillators with similar frequencies. The problem can be circumvented if the oscillators are switched on and off. When switched on again, the oscillators drift because of thermal changes whereby the measuring sensitivity of for example a sensor is reduced. In order to avoid this, it would be necessary to wait after each switching on until a thermal equilibrium has been established. This however results in a long down time.
It is the object of the present invention to provide a sensor of the type described above, wherein, however, electromagnetic transgression of the oscillators is avoided without sensitivity losses and extended down times.
SUMMARY OF THE INVENTION
In a sensor comprising a housing including at least two passive building components capable of oscillating and oscillator circuits and with means for passing a medium to be examined through the housing, the oscillator circuits each include an amplifier and a variable phase shifting component providing for a phase shift range sufficiently large to switch the oscillating components off while the amplifier remains operating.
The particular advantage of the invention resides in the combination of damping-tolerant oscillators (high amplification of the active component) with adjustable phase (phase shifter by means of coil and capacity diodes) with the capability to switch the oscillators off. This eliminates the need for a separation of the sensor chambers since the oscillators can be multiplexed and operated at the same time at their optimal phase points.
This kind of switching potentially permits the miniaturization of sensor oscillator arrays (BAW (QMB), SAW, capacitive oscillator principles) and, by a selection of the optimal operating points of the oscillators (phase), provides at the same time for an increase in the sensitivity.
In front of the amplifier input and output of an oscillator, a phase shifter is arranged, which is electrically adjustable by way of capacity diodes. By applying a predetermined voltage the oscillator vibrates at a predetermined phase condition. This means that the arrangement can be adjusted to the phase point of the passive regenerative component, which is optimal for the particular purpose or to fully exit the transmission range of the passive component by totally de-tuning the phase shifter with the voltage applied thereto. The phase condition for the oscillation is then no longer fulfilled and it breaks off. The same “switching” behavior could be achieved by the use of PIN diodes.
A similar principle is utilized in connection with quartz-stabilized VCOs (Voltage-Controlled Oscillators). In that case, however, the phase is only shifted; the oscillator is not switched.
Below, an embodiment of the invention will be described on the basis of the accompanying drawings.
REFERENCES:
patent: 3921093 (1975-11-01), Lewis
patent: 5076094 (1991-12-01), Frye et al.
patent: 5117146 (1992-05-01), Martin et al.
patent: 5661226 (1997-08-01), Bowers et al.
patent: 5918258 (1999-06-01), Bowers
patent: 0 542 469 (1993-05-01), None
patent: 0 618 446 (1994-10-01), None
patent: 0 750 192 (1996-12-01), None
patent: WO 93/07463 (1993-04-01), None
patent: WO-99/21001-a1 (1999-04-01), None
Rapp Michael
Voigt Achim
Bach Klaus J.
Forschungszentrum Karlsruhe GmbH
Miller Rose M.
Williams Hezron
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