Measuring and testing – Vibration – Resonance – frequency – or amplitude study
Reexamination Certificate
1999-07-29
2001-05-29
Moller, Richard A. (Department: 2856)
Measuring and testing
Vibration
Resonance, frequency, or amplitude study
C073S514280, C073SDIG004, C310S31300R, C310S336000
Reexamination Certificate
active
06237417
ABSTRACT:
TECHNICAL FIELD
This invention relates to an apparatus and method for measuring the value of physical quantities which affect the impedance of an electrically resonant structure. One example of such a structure is a Surface Acoustic Wave (‘SAW’) resonator as used in oscillator circuits. More particularly the invention is directed towards applications where such a resonator is mounted remotely from the driving and sensing electronics.
BACKGROUND ART
The prior art most closely related to that of the present invention is the technique for matching the impedance of radio transmission aerials to the impedance of the transmitter I(as described in “Electronic and Radio Engineering” by Frederick Terman, Library of Congress card number 55-6174 and “Radio Handbook” by William Orr, Library of Congress card number 40-33904). These prior art references refer to the need to match the impedance of each element of the circuit (transmitter, transmission line and aerial) to maximise the radio frequency (RF) energy radiation of the circuit at the required operating frequency. This impedance matching is carried out during initial set up of the system to operate at the fixed carrier frequency of the transmitter. Once the impedances are initially matched, no more tuning is carried out as the circuit does not substantially change its impedance during operation. The measurement of the system impedance is usually carried out by a directional coupler, which measures the reflected current returned from an ill-matched circuit. From this current the ratio of maximum to minimum voltage along the line (termed the ‘voltage standing wave ratio’ or ‘VSWR’) can be calculated. A VSWR of unity indicates a perfectly matched system. The adjustment of the impedance of the system is usually carried out by varying the length of the aerial or the transmission line, or by adding lumped components such as inductors or capacitors to the circuit. The technique described is well understood by those skilled in the art of radio transmission of RF power for the transfer of information to a remote RF receiver. The present invention addresses the problem of measuring the impedance of a remotely mounted electrically resonant structure which is not designed for energy radiation. Such structures are usually used in oscillator circuits to control the frequency of oscillation. They are always mounted close to the controlling circuit because the impedance of connection wires between the structure and the electronics would cause the circuit to cease functioning. The impedance of these structures can vary with factors such as temperature, humidity and strain, and therefore they can be used as sensors. Previous applications have always mounted the controlling electronics close to the resonant structure, and provided separate connections (contact or non-contact) for power transfer to the control electronics and measurement of the signal. This technique leads to problems when the sensor is mounted in difficult situations, such as in a high temperature region, in an explosive environment (power to the sensor must be limited) or on a rotating shaft.
An object of this invention is to remove the need for the controlling electronics to be mounted near the structure, and to allow the impedance of the structure to be measured with very low signal power.
SUMMARY OF INVENTION
According to one aspect the present invention is an apparatus for measuring the value of a physical quantity which affects the impedance of an electrically resonant structure, said apparatus comprising an electrically resonant structure, an RF electrical energy source, a bidirectional RF transmission line connecting said source to said resonant structure, a directional coupler associated with said transmission line, said source providing an excitation signal to said structure, said directional coupler detecting the voltage or phase of a reflected signal returned from said resonant structure characterised in that the resonant
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structure is substantially non energy radiating and, at a given frequency of said source, has an impedance which varies continuously as a function of the value of said physical quantity.
Preferably the electrically resonant structure is at least partially composed of piezoelectric material. Suitable piezoelectric materials include quartz and directionally orientated zinc oxide.
Preferably the substantially non energy radiating resonant structure is a substantially non RF radiating structure. Preferably the electrically resonant structure is electrically excited by means of at least one interdigital array (IDA). Suitable resonant structures which incorporate IDAs are surface acoustic wave (SAW) resonators, shallow bulk acoustic wave (SBAW) resonators or the like. Preferably the variable impedance of the resonant structure results from the variation in the pitch of the IDA or mass loading of the resonant structure. Preferably the variation in the pitch of the IDA results from strain of the resonant structure.
Preferably the resonant structure is substantially rigidly mounted to a surface subject to strain, and this strain is therefore imparted to said resonant structure. Strain of said surface may be caused by physical quantities such as applied load, applied bending moment, applied pressure, or thermal expansion caused by temperature. Mass loading of the resonant structure may be caused by absorption of fluids into the surface of the resonant structure in the presence of specific fluids or by physical quantities such as humidity.
Preferably the directional coupler may be a transformer, Maxwell Bridge (wire line) or Lange coupler.
Preferably the RF transmission line incorporates a non-contacting in line coupler, which may be an untuned or tuned transformer, laser, optical, capacitive or RF coupler.
Alternatively the RF transmission line is a continuous electrical conductor between the source and the resonant structure.
Preferably the electrically resonant structure is mounted on the surface of a rotating member subject to strain, with the in line coupler allowing the transmission of said excitation signal and said reflected signal to and from said rotating member respectively, in a non-contacting manner.
Preferably the output impedance of the source should be substantially conjugately matched to any one of said bidirectional RF transmission line, resonant structure, directional coupler, and in line coupler.
REFERENCES:
patent: 3031615 (1962-04-01), Chase et al.
patent: 4524620 (1985-06-01), Wright et al.
patent: 4621530 (1986-11-01), Dwyer et al.
patent: 4668909 (1987-05-01), Hickernell et al.
patent: 4691714 (1987-09-01), Wong et al.
patent: 4932255 (1990-06-01), Brace et al.
McDonald, James B. “Mismatched Waveguide Calibrates VSWR Scope Display”. Electronics, vol. 51, No. 13, Jun. 1978, pp. 146 and 148.
Microwave Journal, vol. 33, No. 12, Dec. 1990 (Fig. 2), (To follow).
Bhimnathwala et al., “Measurement of the Sheet Resistance”, J. of Vacuum Science & Techn. B., vol. 12, No. 1, Jan. 1994, pp. 395-398, (To follow).
Terman, Frederick., “Electroni and Radio Engineering”, Library of Congress Card No. 55-6174, (To follow).
Orr, William, “Radio Handbook”, Library of Congress Card No. 40-33904, (To follow).
Lonsdale Anthony
Lonsdale Bryan
Davis & Bujold P.L.L.C.
Moller Richard A.
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