Measuring and testing – Instrument proving or calibrating – Displacement – motion – distance – or position
Reexamination Certificate
1999-05-04
2001-09-11
Raevis, Robert (Department: 2856)
Measuring and testing
Instrument proving or calibrating
Displacement, motion, distance, or position
Reexamination Certificate
active
06286359
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Art
This invention relates to a method for testing frequency characteristics of laser displacement/vibration meters which are widely in use in industrial fields. More particularly, this invention concerns a method for testing frequency characteristics of a laser displacement/vibration meter for the purpose of establishing traceability in the measurement of high-speed micro-level displacements and micro-level vibrations, by comparing measurement data of a laser displacement/vibration meter and a reference laser interferometer with a reference laser beam or a laser interferometer of a similar class, employing a novel technology for generating high-speed micro-level fine displacements or vibrations.
2. Prior Art
Laser displacement/vibration meters have thus far been in wide use in. industrial fields for measurement of displacements and vibrations according to the principles of light-wave interference based on Doppler shifts and heterodyne light-wave interference. However, it has been infeasible to assess the reliability aBd accuracy of laser displacement/vibration meters of this nature due to absence of technology of generating high-speed micro-level fine displacements over a wide frequency range where the frequency is close to a DC component.
On the other hand, as evidenced by the developments in the so-called micromachine technology in advanced western countries, there have been increasing necessities for measuring movements of microstructures far smaller than conventional counterparts, over a frequency range broader than was necessary heretofore. It follows that severe requirements are imposed on a laser displacement/vibration meter especially on the reliability and accuracy of measurement. As a consequence, users and manufacturers of meters of this sort are in need of a new method for generating high-speed micro-level displacements or high-speed micro-level vibrations which are difficult to generate with conventional methods.
SUMMARY OF TEE INVENTION
In view of the foregoing circumstances, it is an object of the present invention to provide a method for testing frequency response characteristics of a laser displacement/vibration meter, which can cope with a broader frequency range and displacements of a finer level to ensure higher reliability in the measurement of micro-level fine displacements and vibrations.
It is a more specific object of the present invention to establish reliable traceability in the measurement of high-speed micro-level fine displacements and vibrations by determining a measurable frequency range of a laser displacement/vibration meter, not from a theoretical analogy based on the principles of Doppler shifts or the like but from comparison of actual measurement data of a reference laser interferometer using a reference laser beam or a laser interferometer of a similar class and of a laser displacement/vibration meter with unknown frequency response characteristics.
In accordance with the present invention, for achieving the above-stated objectives, there is provided a method for testing frequency response characteristics of a laser displacement/vibration meter, which essentially comprises the steps of: applying impact on one end face of a round metal rod to produce an elastic wave pulse propagating toward the other end of the rod to generate stepwise dynamic displacement of the other end face of the rod by reflection of the elastic wave pulse; measuring the dynamic displacement of the other end face of the rod simultaneously by a reference laser interferometer with a reference laser beam and a laser displacement/vibration meter with unknown frequency characteristics: and determining frequency response characteristics of the unknown laser displacement/vibration meter by comparison of measurement data with the counterpart data of the reference laser interferometer.
In the practice of the testing method just described, there may arise a situation where it is difficult to use a reference laser interferometer in an actual spot of measurement. In such a case, namely, in case a reference laser interferometer is not available in an actual spot of measurement, the performance of a laser displacement/vibration meter with unknown frequency response characteristics Is assessed preliminarily on the basis of measurement data obtained from a strain gage which is bonded on a lateral side of the round metal rod. and determining the frequency response characteristics of the laser displacement/vibration meter afterwards by application of a corrective function, which is determined by measurement with a reference laser interferometer As a result, the frequency characteristics of the laser displacement/vibration meter can be assessed as accurately as in the measurement using a reference laser interferometer simultaneously in a measuring spot.
In accordance with the present invention, there is also provided a method for testing frequency response characteristics of a laser displacement/vibration meter, suitable for use in the measurement of dynamic displacements in a frequency range higher than several hundreds kHz, the method comprising the steps of: applying an impulse voltage to a piezoelectric vibrator in the form of a piezoelectric element or a piezoelectric film to generate stepwise dynamic displacement on the surface of the piezoelectric vibrator; measuring the dynamic displacements of the piezoelectric vibrator surface simultaneously by a reference lase interferometer with a reference laser beam and a laser displacement/vibration meter with unknown frequency response characteristics; and determining frequency response characteristics of the unknown laser displacement/vibration meter by comparison of measurement data with counterpart data of the reference laser interferometer over a frequency range.
Further, according to the present invention, there is further provided a method for testing frequency response characteristics of a laser displacement/vibration meter, particularly suitable for use in a measurement of dynamic displacements in a frequency range higher than several hundreds kHz and for use in a case where an impulsive displacement or amplitude of vibration resulting from application of an impulse voltage to a piezoelectric vibrator is smaller than laser beam wavelength, the method comprising the steps of: providing a piezoelectric film on an end face of a stack of piezoelectric elements; applying an impulse voltage to the stack of piezoelectric elements to generate micro-level fine displacements on a surface of the piezoelectric film; measuring the fine displacements of the piezoelectric film surface by a reference laser interferometer with a reference laser beam to determine a phase correction value according to resulting measurement data; measuring high-speed micro-level fine displacements of the piezoelectric film surface simultaneously by the use of the reference laser interferometer and a laser displacement/vibration meter with unknown frequency response characteristics; and determining frequency response characteristics of the displacement/vibration meter by comparison of measurement data with the counterpart data of the reference laser interferometer as corrected with the correction value.
The above and other objects, features and advantages of the invention will become apparent from the following particular description of the inventions taken in conjunction with the accompanying drawings which show by way of example preferred embodiments of the invention.
REFERENCES:
patent: 3693400 (1972-09-01), Savit
patent: 4355900 (1982-10-01), Nussmeir
patent: 4659224 (1987-04-01), Monchalin
patent: 5039221 (1991-08-01), Layton et al.
patent: 5353642 (1994-10-01), Hasegawa et al.
Bouch et al., “Calibrators for Accepatnce and qualification Testing of Vibration Measuring Instruments”, pp. 1-13, 1963.*
Clark, “An Improved Method for Calibrating Reference Standard Accelerometers”, pp. 103-107, 1983.*
Bouche et al., Calibrators for Acceptance and Qualification Testing of Vibration Measuring Instrument pp. 1-13, Dec. 1963.
Director-General of the Agency of Industrial Science and Technol
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Raevis Robert
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