User guide

Veloce 50 Operating Manual
10
6.2 PRINCIPLE OF THE VELOCE SIGNAL CONDITIONER
Optical
demodulator
A/D
converter
Signal
processing
D/A
converter
Analog output
Optical input
(light signal from the gauge)
Out
1
Out
2
Out
3
Out
4
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Figure 4 Schematic of the operation of the Veloce
Light from the gage is fed into the optical demodulator. The analog output signal of the demodulator Out
1
is then sampled by the analog-to-digital converter at the specified sampling rate (this is the sampling rate
that is specified with the conditioner, i.e. like 200 kHz). The digital output signal
Out
2
is then processed by
a digital signal processor (DSP). The digital output signal
Out
3
is then fed into the digital-to-analog
converter and then send to the analog output.
The signal coming out of the optical demodulator,
Out
1
, is a sinusoidal function of the time-dependent
cavity length of the gage:
Out
1
(t) = A×sin[2π × L
Cavity
(t) / VSF] (2)
where
A and VSF (the Veloce Scale Factor) are constants which are characteristics of the conditioner.
The
VSF typically ranges from 420 nm to 430 nm. The VSF is individually calibrated for each Veloce
module and is programmed into its flash memory. The DSP takes the
VSF into account in the calculations
so that 1 mV at the analog output always corresponds to 2 nm of gage cavity length.
The Veloce conditioner can read the gage cavity length
L
Cavity
in a range that goes from13 µm to 19 µm, or
13 000 nm to 19 000 nm. Outside these limits, the amplitude of the signal (the
A factor) becomes too
small to lead to precise calculations.
Eq. (2) shows that the maximum frequency of
Out
1
is proportional to the rate of change of the cavity
length. In other words, for a strain gage, it is proportional to the strain rate
. And because of the Veloce
proprietary signal processing algorithm, one has to ensure that the maximum change of the cavity length
between two samples of the A/D converter must never exceed one half of the
VSF, i.e. approximately
200 nm per sampling interval. In résumé one can say that the frequency bandwidth of the Veloce signal
conditioner is mostly limited by the strain rate, that is the variation of strain with time. However, for small
signals (maximum excursion lower than
VSF / 2), the signal bandwidth is limited to the Nyquist criterion of
half the sampling rate of the Veloce (i.e. 100 kHz).