Thomas Bruns

Investigation of primary vibration calibration at high frequencies using the homodyne quadrature sine-approximation method: problems and solutions

When homodyne quadrature techniques are used for primary vibration calibration at high frequencies the problem of low displacement amplitudes arises, which inhibits the accurate measurement of acceleration amplitude and initial phase. By exploiting the effect of disturbing motion caused e.g. by hum of the power amplifier or environmental vibrations, it is possible to still recover the displacement time history by phase unwrapping. However, it is then necessary to deal with the superimposed drift-like motion components. This contribution proposes two techniques to either include these components in the sine approximation or to apply a wavelet-based detrending technique. The performance and consistency of both methods are demonstrated by simulation tests, taking into account all significant influences known from real acceleration measurements, and by comparison of accordingly performed real calibrations with the respective calibration results with validated heterodyne techniques.

Modifications of the sine-approximation method for primary vibration calibration by heterodyne interferometry

The paper presents modifications of the sine-approximation method (SAM) specified in ISO 16063-11:1998, focusing on two of the various well-established versions: (i) SAM version using heterodyne quadrature signals (HQS) and (ii) SAM version using time interval analysis (TIA), and their implementation in the national medium-frequency vibration standard of NIM, China. For the HQS version, modified techniques and procedures have been introduced applicable to suppress disturbing effects from frequency instability of the carrier signal and from displacement drift. For the TIA version, a new algorithm has been developed for which the frequency demodulation is based on the estimation of time intervals between neighbouring peaks and valleys of the laser Doppler interferometer instead of successive positive or negative zero crossings. A spline approximation has been implemented allowing relatively slow sampling rates (e.g. 5?MS?s?1) to be applied. The low requirements on hardware techniques allow primary vibration calibrations with high accuracy to be performed efficiently and cost effectively. Some results of investigations of the modified versions of SAM are demonstrated. The results achieved by these modified SAM versions are compared with each other and with PTB results measured within the complementary key comparison CCAUV.V-K1.1 piloted by PTB from 2006 to 2007.

Recent progress in accurate vibration measurements by laser techniques

In order to create the scientific foundation of measurement, the national metrology institutes (NMIs) realize the units in compliance with the definition agreed on the international level and disseminate the units to the users through comparison. The PTB - as the National Metrology Institute of Germany providing scientific and technical services for Germany and other countries - has focused major activities to respond to the needs of manufacturers and users of laser vibrometers and of calibration systems applicable to laser vibrometer calibration and/or based on laser interferometry. Theoretical and experimental investigations and comparisons of different standard measuring devices (national standards of Germany) resulted in extended measurement ranges and increased accuracy. PTB has achieved, among other upgraded capabilities, primary calibration of laser vibrometer standards in the frequency range from 0.4°Hz to 20°kHz with a measurement uncertainty of 0.1°% to 0.2°% (frequency dependent, coverage factor k°=°2). This enabled one of the calibration laboratories in Germany, within the framework of the Deutscher Kalibrierdienst (DKD), to be accredited for the service of primary calibration of laser vibrometers in the frequency range of 0.4 Hz to 20 kHz with a best measurement uncertainty 0.25°% to 0.5°%. Improved methods and techniques of laser interferometry, and results of their experimental investigation will be presented. A survey will be given on proposed specifications of laser vibrometers and a first international standardization project for the calibration of laser vibrometers.

Traceability of dynamic force and torque calibrations by means of laser Doppler interferometry

In contrast to the fact that a huge class of industrial force and torque measurement applications are of dynamic type, the standard calibration methods for the facilitated transducers are still of static type. The presented paper gives an introduction to recent developments at the Physikalisch-Technische Bundesanstalt (PTB), the German national metrology institute, towards new dynamic calibration methods for force and torque. In addition to the already established sinusoidal force calibration, two new facilities have been developed over the last three years which are concerned with the generation of impact forces and periodic torque respectively. For the primary dynamic force and torque calibration facilities the preferred way to provide traceability is the measurement of the motion of an accelerated mass or mass distribution. The means of measurement for primary calibration therefore are laser-Doppler-interferometers (LDI), which are utilized in very special ways.

Calibration of bridge-, charge- and voltage amplifiers for dynamic measurement applications

Measuring amplifiers are used for transducer output signal conditioning in many dynamic measurement applications. For a traceable measurement, a calibration of all components of the measuring chain—and therefore of the conditioning amplifiers, too—is mandatory. In this paper methods for a dynamic calibration of different types of conditioning amplifiers are presented. Measurement uncertainties and calibration results for typical amplifiers are discussed.

Linear Mixed Models: Gum and Beyond

Abstract In Annex H.5, the Guide to the Evaluation of Uncertainty in Measurement (GUM) [1] recognizes the necessity to analyze certain types of experiments by applying random effects ANOVA models. These belong to the more general family of linear mixed models that we focus on in the current paper. Extending the short introduction provided by the GUM, our aim is to show that the more general, linear mixed models cover a wider range of situations occurring in practice and can be beneficial when employed in data analysis of long-term repeated experiments. Namely, we point out their potential as an aid in establishing an uncertainty budget and as means for gaining more insight into the measurement process. We also comment on computational issues and to make the explanations less abstract, we illustrate all the concepts with the help of a measurement campaign conducted in order to challenge the uncertainty budget in calibration of accelerometers.

Bilateral comparison CCAUV.V-S1: Primary angular vibration calibration

A comparison CCAUV.V-S1 was organized to compare measurements of sinusoidal angular accelerations in the frequency range from 0.4 Hz to 1 kHz. This was a bilateral comparison between the Korea Research Institute of Standards and Science (KRISS), Republic of Korea, and the Physikalisch-Technische Bundesanstalt (PTB), Germany. Both NMIs applied laser interferometry in compliance with ISO 16063-15:2006 Methods for the calibration of vibration and shock transducers—Part 15: Primary angular vibration calibration by laser interferometry. The complex voltage sensitivity (magnitude and phase) of one angular transfer standard in the frequency range of 0.4 Hz to 100 Hz and the complex charge sensitivity (magnitude and phase) of one angular reference standard in the frequency range of 1 Hz to 1000 Hz were measured. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCAUV, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Experimentelle Bestimmung von Massenträgheitsmomenten als Rückführungsgröße für die dynamische Drehmomentkalibrierung (Experimental Determination of Moments of Inertia for Traceability in Dynamic Torque Calibration)

Die Physikalisch-Technische Bundesanstalt (PTB) entwickelt neue Verfahren für die Kalibrierung von Drehmomentaufnehmern mit zeitlich variierendem Drehmoment. Die Rückführung erfolgt dabei auf die Größen Massenträgheitsmoment und Winkelbeschleunigung. Die vorliegende Arbeit beschreibt die experimentelle Bestimmung des beteiligten Massenträgheitsmomentes der Normal-Messeinrichtung (NME). Zur Messung wurde ein Torsionspendel aufgebaut und dessen Schwingung mit dem interferometrischen Messsystem der NME erfasst. The Physikalisch-Technische Bundesanstalt (PTB), the National Metrology Institute of Germany, is developing new methods for the calibration of torque transducers with dynamic torque. In this case the traceability is based on the measurands of moment of inertia and angular acceleration. This contribution describes the experimental determination of the moment of inertia intrinsic to the standard device. For the measurement a torsional pendulum was realized and its oscillation was traced by the laser interferometric measurement system of the device.

An enhanced primary shock calibration procedure to reduce the zero shift effect of piezoelectric transducers by using a virtual amplifier

The low-frequency response of a charge amplifier induces an accelerometer (a combination of a piezoelectric transducer and a charge amplifier) output voltage with zero shift. Hence, a virtual amplifier with same input–output characteristics as the charge amplifier was designed to reduce the zero shift effect. The charge shock sensitivities of a piezoelectric transducer were evaluated by applying the input acceleration to the virtual amplifier. The results of the study indicated that the charge shock sensitivities were comparable to the vibration calibration results.

A Model-Based Approach for the Dynamic Calibration of Torque Transducers

The demand for a traceable dynamic torque measurement increased over the last years. Procedures for a dynamic calibration of torque transducers have been developed in an European metrology research project. In the scope of this project, a dynamic measuring device was developed, and the corresponding mechanical modelling, which also includes the transducer under test, was investigated. The dynamic behaviour of the torque transducer is described by the parameters of its model. These model parameters will be identified from measurement data. The model-based approach for the description of the dynamic behaviour of torque transducers and model parameter identification procedures are described in this paper.