HARDWARE MANUAL Document reference MAMPS-HW/E
VM600 machinery protection system (MPS) Edition 18 - March 2022
4-10
Inputs and outputs
MPC4 / IOC4T CARD PAIR
2- DC component represents gap and is used for OK line check
In this case, the DC voltage contains information. This is applicable to proximity probes,
where the DC signal represents the gap (distance between the probe tip and the shaft),
and the AC signal represents the shaft vibration. The AC and DC components are both
available for processing.
As in case (1), the DC component is used for the OK line check.
4.4.1.3 Current-based input signal
The current-based inputs fall into two categories:
1- DC component is used for OK line check
In this case there is an AC current-based signal with a DC component, where the latter
represents the supply current of the sensor (standing current). The DC component is only
used for the OK system check (it must be compared with fixed OK levels). This is
applicable to systems using accelerometers, velocity transducers or dynamic pressure
probes. Either polarity can be handled by the MPS, depending on the polarity of the
sensor and/or signal conditioner's power supply. The AC current-based signal is
processed separately.
2- DC component represents gap and is used for OK line check
In this case, the DC current contains information. This is applicable to proximity probes
(for example, the current output of IQS45x signal conditioner), where the DC signal
represents the gap (distance between the probe tip and the shaft), and the AC signal
represents the shaft vibration. The AC and DC components are both available for
processing.
As in case (1), the DC component is used for the OK line check.
4.4.1.4 Unpowered sensors
An OK line check is performed on sensors that do not require a power supply. For this, a
resistor is internally connected between the signal high input (HI) and the unused sensor
power supply terminal (PS). This is configured automatically by the VM600 MPSx software
when the Sensor Power Supply field is set to No Supply. This technique enables open
circuits to be detected, but not short circuits.
NOTE: Refer to the relevant manual for further information: VM600 MPS1 software
manual or VM600 MPS2 software manual.
4.4.2 Speed signal inputs
These inputs (TACHO 1 and TACHO 2) handle several kinds of speed and phase reference
transducers:
• Proximity probes (TQ/IQS type) delivering a conditioned voltage or current signal.
A power supply of −27.2 V
DC
is available for these probes.
• Magnetic pulse pick-up sensors (SP type) delivering a voltage whose amplitude varies
widely with speed.
• Systems delivering a TTL output.
Only the input signal frequency is of interest for tacho processing, that is, the speed inputs
are used only to detect the edges of the signal. The edge of detection (rising or falling) is
software selectable.
The speed inputs can handle either a "one per revolution" (1/REV) phase signal coming from
a protrusion or notch on the shaft, or a speed signal generated by a toothed wheel (more than
one impulse per revolution).
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