Two supervisory control and data acquisition systems (SCADAS II) of Leuven Measurement Systems (LMS) with each 24 channels are used to store the time-domain as well as frequency domain data [249]. The name plate data of both SCADAS are listed in Fig. F.2.

Figure F.2: Name plate data of LMS-DIFA SCADAS II data acquisition system.

The programmable dual filter amplifier (PDFA) module in the LMS-DIFA SCA-DAS II is a universal signal conditioning module providing two identical, but each independently programmable differential amplifiers and analogue anti-aliasing filters. The PDFA includes a current source to provide power to integrated circuit piezoelectric transducers.

Five commonly used types of analogue filters are Butterworth, Elliptic, Cheby-shev (Type I and II), and Bessel. These so-called infinite impulse response (IIR) filters all approximate the ideal filter in different ways. Note that an ideal low-pass filter would have no attenuation in the pass band (i.e. 0 < f < fco), infinite attenuation in the stopband (i.e. f > fco), and a zero phase characteristic. In practice, a linear phase characteristic is often regarded as "ideal". After all, a linear phase characteristic introduces a time shift but preserves the wave shapes. Of the five mentioned filter types, elliptic filters usually meet a given set of filter performance specifications with the lowest filter order.

The PDFA anti-aliasing filters are eighth-order elliptic filters. They are built up by cascading four identical second order filter stages to approximate the ideal filter. Three filter characteristics are available, viz. PDFA-ETD (Equal Time Delay), PDFA-ELL (Elliptical), and PDFA-US (Ultra Sharp). We have used the PDFA-ETD anti-aliasing filter. The cutoff frequency fco of this filter can be varied from 10 Hz to 10.23 kHz in 10 Hz steps to meet the sample frequency and frequency range of interest requirements. The ETD filter uses 8 poles and 8 zeros to approximate the ideal filter. The result is a linear phase response within 2.2 ° while the stopband attenuation of 70 dB is reached at 3.7 • fco. The Bode diagram of this filter is depicted in Fig. F.3.

The sampling and quantisation is carried out by a 16-bit A/D converter. The accuracy of the sampling and quantisation process depends on the number of quantisation levels available in the A/D converter (an n-bit converter has 2n quantisation levels). In practice, the measured signal should occupy as much of the range of the

LMS-DIFA SCADAS II Data Acquisition Systems

LMS-DIFA Measuring Systems

• Serial number: 81904002

LMS-DIFA Measuring Systems

. Block size: 32768

• Serial number: 81904003

Normalised cutoff frequency f/fco [-]

Figure F.3: Amplitude and phase characteristic of the Equal Time Delay antialiasing filter as a function of the normalised cutoff frequency f/fco. Dashed line: ideal low-pass filter with phase proportional to f/fco.

A/D converter as possible. Provided that is done, quantisation errors are normally insignificant [170, 265]. The data were collected with a sample rate of 1000 Hz (measurement M1) and 500 Hz (measurement M2-M10), and with the cutoff frequency of the ETD low-pass filter set to 330 and 160 Hz respectively.

Normalised cutoff frequency f/fco [-]

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