Document reference MAMPS-HW/E HARDWARE MANUAL
Edition 18 - March 2022 VM600 machinery protection system (MPS)
4-9
Inputs and outputs
MPC4 / IOC4T CARD PAIR
The signal is then low-pass filtered again by the DSP to reduce its bandwidth to 1 Hz. The
GAP value (or any other DC function) can now be measured and the OK levels monitored by
comparators.
(3) Processing of AC component
The first stage in the AC path is a low-pass pre-filter (Figure 4-4, Ref. 4) whose cutoff
frequency is controlled by the analog-to-digital converter (A/D). The anti-aliasing filter
(Ref. 10) is also controlled in this manner.
The first programmable gain amplifier (PGA, see Ref 6) is followed by either i) an integrator
stage with high-pass filters having a fixed cutoff frequency of 2.5 Hz (Refs 5 and 8) or
ii) directly connected through the second PGA (Ref. 9). The amplifier gains are software
controlled (namely by the FSD or Signal Dynamic parameters, whichever leads to the higher
signal-to-noise ratio).
The analog integrator (Ref. 7) is switched on when the BBAB function is selected and if the
raw vibration (in a natural unit such as "g") is not required. Otherwise, if the measurement is
required in the natural unit, then the analog integrator is by-passed and the integration is done
digitally by the firmware on the DSP (Ref. 11) if required. This last solution, however, results
in a higher noise level.
The selection of analog or digital integration is transparent to the user. A warning message
may appear if the noisier solution is selected.
Finally, the digital signal is broad-band filtered (Ref. 12). Several stages of multi-rate finite
impulse response (FIR) digital filters are used.
Rectification (RMS, RMS scaled to Peak, True Peak, True Peak-to-Peak) is performed on the
digitised and filtered signal.
The AC signal component is used in the following processing functions:
• Broad-band absolute bearing vibration (BBAB)
• Broad-band pressure (BBP)
• Narrow-band (tracking) vibration (NB)
• Relative shaft vibration (RS) – output 1
• Eccentricity (EC) with peak-peak rectifier.
The DC signal component is used in the following processing functions:
• Eccentricity (EC) with “peak-to-peak per revolution” rectifier
• Relative shaft vibration (RS) – output 2
• Position (PS)
• Absolute housing expansion (HE)
• Relative shaft expansion with pendulum (SEP)
• Quasi-static pressure (QSP).
4.4.1.2 Voltage-based input signal
Two types of voltage-based signals (AC+DC) can be considered, differing only in the
meaning of the DC component:
1- DC component is used for OK line check
In this case there is an AC signal with a DC component, where the latter represents a
level only used for the OK system check (it must be compared against fixed OK levels).
This is applicable to systems using accelerometers, velocimeters or dynamic pressure
probes. The DC voltage can be positive or negative, depending on the polarity of the
sensor and/or signal conditioner's power supply. The AC signal is extracted and
amplified separately.
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