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Research and development

Working Group 1.71

Research and development for the design and investigation of acceleration exciters and angular acceleration exciters, and of precise methods for the measurement of motion quantities

Research and development work concerning exciters has been focused on both, the design and manufacture of high-quality linear and angular acceleration exciters as sub-systems of the standard measuring devices (national standards) and the identification of metrological characteristics (investigation of disturbing effects) of calibration exciters used in calibration laboratories.

As its influence on the quality and the mode of operation of the standard measuring devices (national standards) is of decisive importance, the associated linear or angular acceleration exciter must be almost free from any disturbing motion (e.g. harmonics, transverse motion). In view of these high requirements, new methods and techniques have been developed by Working group 1.71 to generate stationary or transient accelerations and angular accelerations. Various acceleration and angular acceleration exciters have been designed, manufactured and optimized, such as:

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Highly precise measuring methods

Research and development work concerning the precise measurement of motion quantities has been focused, on both the establishment of highly accurate laser interferometer systems as sub-systems of the standard measuring devices (national standards), and the identification of the metrological characteristics (investigation of disturbing effects) of those measuring methods and devices which are considered candidates for use as reference standards in any calibration laboratory (e.g. reference standard laser vibrometers).

Laser interferometry applied worldwide for primary calibrations at the level of the national metrology institutes has been further developed in recent years by PTB Working group 1.71 into a uniform methodology and technique for primary measurements of translational motion quantities (e.g. acceleration) and rotational motion quantities (e.g. angular acceleration) at sinusoidal, shock-shaped and other, user-defined time dependencies. A traceability chain has been defined and established which allows the respective physical quantities to be realized and disseminated at those time dependencies that reflect the conditions of use (e.g. measurement of the peak value of 50000 m/s² of a shock-shaped acceleration). A heterodyne interferometer(of the Mach-Zehnder type) combined a frequency conversion technique and digital data processing has proved its worth as a universal interferometric method of measurement. Special signal processing procedures and algorithms have been developed to process interferometer signals responding to sinusoidal or shock-shaped motion. They are also applicable to translational and rotational motion quantities provided the angular acceleration exciter is equipped with a special diffraction grating(sine-phase grating) developed for this purpose. The special version of the universal for the measurement of sinusoidal motion quantities method developed at PTB on the basis of a heterodyne interferometerwas tested over the frequency range from 0,1 Hz to 50 kHz and the amplitude range from 1 nm to 0,5 m and, within the framework of international comparison measurements, proved to be very accurate (uncertainty of measurement of 0,1 %). The version developed for sinusoidal vibration and referred to as the "sine-approximation method" has been incorporated into the international standard ISO 16063-11:1999 "Primary vibration calibration by laser interferometry". Incorporation of the version which, for the measurement of rotational quantities, makes use of the diffraction grating, into the first international standard for the calibration of rotational transducers is in preparation (new standardization project ISO 16063-15 "Primary vibration calibration of angular transducers using laser interferometry").

In addition to the new "sine-approximation method", the conventional "fringe counting method" with a Michelson interferometer (also specified in ISO 16063-11:1999) is used at PTB to measure sinusoidal vibration within the framework of primary calibrations or of comparison measurements between national metrology institutes. The application range of this method, originally restricted to frequencies from

20 Hz to 800 Hz (cf. ISO 5347-1:1993), has been extended to the frequency range from 0,01 Hz to 20 kHz (displacement amplitude range: 5 nm to 0,5 m). Various international comparison measurements have proved not only the worth of the extended frequency range (0,5 Hz to 20 kHz was applied) but also of the improved accuracy (frequency-dependent; measurement uncertainty of 0,1 % at a reference frequency of 160 Hz) achieved by the "modified fringe counting method" developed by PTB Working group 1.71.

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