14.10.2009
In the Working Group "Realization of Acceleration", a measuring set-up was developed within the scope of a diploma thesis at the University of Applied Sciences Braunschweig/Wolfenbüttel for the primary determination of the phase retardation of laser Doppler vibrometers (LDVs). To achieve measurement uncertainties as small as possible, the laser beam of the test object (LDV measuring beam) also supplies a Michelson interferometer which is operated in quadrature and serves as a reference.
For the primary calibration of acceleration transducers, laser Doppler vibrometers (LDVs) are used in the Working Group "Realization of Acceleration" to establish the traceability of the quantity "acceleration" back to the quantities "length" (wavelength) and "time". During the sine calibration of a mechanically excited acceleration transducer with regard to the signal amplitude and phase, the time curves of the reference acceleration, measured by means of an LDV, and of the acceleration transducer under test are recorded synchronously. To be able to determine the phase retardation of the acceleration transducer to be calibrated for sinusoidal excitations, the phase retardation (or the signal propagation time) of the optical and electronic components of the LDV must be known precisely.
The LDVs used in the Working Group, which are made by the Polytec company, consist of an optoelectronic measuring head and an electronic controller. To enable a primary traceability, the manufacturer adapted the controllers to the needs of PTB (frequency-modulated output, in short: FM output). So far, the phase retardation of the LDV controllers has been determined with regard to the FM output only, without considering an additional component due to the optoelectronic measuring head. In order to further improve the measurement uncertainties during the primary calibration of acceleration transducers, an "over-all" measurement of the phase retardation of the entire LDV, including the measuring head and the controller, was therefore necessary.
Figure 1: Experimental set-up (with sketched-in beam path) for the primary phase calibration of LDVs
To carry out the "over-all" measurement, a novel measuring set-up (Figure 1) was established which, by means of the test object (LDV), realizes the normal measurement mimic of the LDV as well as a so-called homodyne quadrature interferometer (which is a modified Michelson interferometer). The homodyne quadrature interferometer is supplied by the laser beam of the LDV and is the primary reference. The measuring beam, which is identical in both methods, detects the line-of-sight motion component of a mirror fixed at the armature of the acceleration exciter exciting sinusoidal oscillations. The homodyne quadrature interferometer, which is composed of various optical components, uses photo detectors with fast PIN photo diodes manufactured by FEMTO®. In the case of these detectors, the phase retardation can be neglected due to the short rise and fall times of about 1 ns. Therefore, the homodyne quadrature interferometer is regarded as a zero phase reference.
The synchronous data acquisition of the modulated measurement signals as well as the provision of the acceleration exciter signal are realized by means of a PXI system. The signal demodulation and the analysis are carried out in LabVIEW, whereby an automatic presentation of the results is carried out in Excel, if necessary. To validate the analysis procedure, a simulation software was developed in LabVIEW which describes the whole experiment.
Figure 2: Phase retardation of the FM output of an LDV test object
Figure 2 shows the determined phase retardation of the FM output of an LDV test object in the frequency range from 100 Hz to 20 kHz. At 10 kHz, the scattering of the measured data point (see enlarged section) is less than 0.008°. To extend the frequency range to higher frequencies, a modified evaluation algorithm has been developed which was used to carry out first investigations up to 80 kHz.
With the new measuring set-up, it is also possible to calibrate commercially available LDVs which have an analogous velocity output and are generally used in industry. Also in this case can a transducer under test be calibrated with regard to amplitude and phase.
Contact person:
Frank Blume, Dep. 1.3, WG 1.31, e-mail: frank.blume@ptb.de