Polytec laser vibrometers use heterodyne interferometers with an acousto optic modulator in one arm of the interferometer. This generates a frequency modulated carrier signal in the RF region, whose center frequency is identical to that of the acousto optical modulator drive signal. The directionally sensitive Doppler information is thus contained in the RF carrier: The signed object velocity determines sign and amount of frequency deviation with respect to the center frequency.
The Doppler frequency is proportional to the surface velocity and the phase change Dj with respect to the phase j0 of the reference signal is proportional to the displacement of the object.
In laser Doppler vibrometry applications signals are continuous but disturbances, low scattering properties of the object and low signal amplitudes due to speckle effects will affect the signal-to-noise ratio. At the He-Ne laser wavelength of l = 0.6328 µm the Doppler frequency is 3.1606 MHz per m/s. High velocities therefore produce high Doppler frequencies which add to the complexity of the signal demodulation process.
Velocity Decoding
For velocity decoding, the RF detector signal is mixed with a variable local oscillator frequency before demodulation. The output signal of the velocity demodulator V(t) is related to the input frequency f(t) via a linear characteristic.
The frequency modulation Df of the RF carrier is proportional to the velocity v of the object:
Df = 2 v / l where l is the wavelength of the laser light.
The required modulation bandwidth can be estimated as 2 ( Df + fvib) where fvib is the vibration frequency of the object.
In order to cover the ranges of applications required, different demodulation techniques are used: the PLL (phase locked loop), the coincidence demodulator techniques and as a new development digital demodulation utilizing nearly real-time DSP techniques.
In a PLL , a voltage controlled oscillator (VCO) tracks the Doppler signal. The VCO is controlled via a feedback loop. The phase detector in this feedback loop detects a phase deviation which is amplified and filtered and used to control the oscillator. Sophisticated circuitry is used to vary bandwidth and amplification of the loop to provide an optimum response to Doppler signals of different amplitudes and signal-to-noise ratios. Model VD-01velocity decoder of Polytec's modular OFV-5000 vibrometer controller works according to this principle. This decoder offers the advantages of excellent linearity and well defined noise bandwidth.
The coincidence demodulator covers most of the laser vibrometer applications. It can demodulate Doppler signals with high resolution and frequency response (up to 1.5 MHz). At its full bandwidth velocities up to ± 10m/s and accelerations up to 107g can be measured.
A new technology, now available with Polytec laser Doppler vibrometers is digital demodulation. The heterodyne interferometer signal is converted into a homodyne signal ( I&Q) by quadrature demodulation. From the phase of the I&Q signal the phase respectively displacement is calculated with an incredible resolution which is only achievable by numerical methods. The digital demodulation is available either on a PC platform (VDD sytem) or as real-time plug-in boards for the OFV-5000 controller. The PC based version has been accepted by the German Institue of Standards (PTB) for traceable calibrations according to the ISO 16063-11 standard.
The digital VD-06 plug-in board for OFV-5000 controller utilizes quasi real-time phase demodulation. After numerical differentiation the decoder provides a digital and analog velocity output with the highest resolution of all Polytec velocity decoders. A combination of the digital VD-06 decoder with the analog VD-02 decoder gives the maximum performance in terms of resolution, linearity, accuracy, velocity and frequency range.
Displacement Decoding
The classical technique to measure displacement is the fringe counting technique. This technique is also available with Polytec vibrometers. Displacement is electronically derived by measuring the phase change at the detector as the surface motion of the object changes the total path length of the beam. As displacement by half the laser wavelength ( l/2 =316nm for He-Ne lasers) changes the phase by 360 degrees. By counting the 'zero-crossings' or 'fringes' the laser vibrometer measures how far the object has moved, in increments of l/2 per cycle. With a D/A converter the digital information is changed into a voltage and a time resolved output ( calibrated in µm/V) proportional to the surface displacement is produced. This technique is incorporated in model VD-100 displacement decoder.
With model VD-200 displacement decoder the resolution is increased as high as l/160 ( 2nm resolution) by using an additional phase multiplier circuit. This corresponds to an interpolation of the phase signal. It should be noted that, because of principle involved, the higher the phase multiplication becomes the lower the measurable velocity and vibration frequency of the object will be.
Whereas the VD-200 achieves high displacement resolution (2nm) only for low velocities (50mm/s) and low frequencies (25kHz) the new DD-500 overcomes this limitation: The DD-500 plug-in board for OFV-5000 controller must be combined with the digital VD-06 velocity decoder. Purpose of the DD-500 is to unwrap the phase information calculated by the VD-06 DSP. As a result of this unwrapping process the DD-500 provides quasi real-time displacement information with 15pm resolution for vibration frequencies as high as 350kHz and velocities up to 500mm/s. The displacement output is either analog (full bandwidth) or via a digital interface (limited bandwidth of 42 kHz).
For high frequency low amplitude displacement decoding, an analog phase displacement decoder (DD-300) that analyzes phase fluctuations within half a fringe period is required. Such a decoder can detect amplitudes below ± 75nm at frequencies up to 30 MHz. An internal reference signal that is derived from the original measurement signal removes relatively slow phase fluctuations due to environmental noise or mechanical vibrations and only the ultrasonic signal is measured.
Signal decoding for rotational and in-plane vibrometer
The rotational and in-plane vibrometers are based on the same physical principles as the standard out-of-plane vibrometers. The signal decoding schemes are therefore similar for all types of laser Doppler vibrometers. However, the continuous movement of a rotating object or a superimposed DC linear movement on an in-plane vibration produces a static or slowly varying Doppler frequency that is superimposed on the dynamic part.
Additional demodulating circuits are used to compensate for the static or low frequency part allowing the generation of an RF carrier that can be decoded as outlined before.
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