5.6 Recommendations for improving signal quality
The data collected by the AMS 6500 system can only be as good as the signals presented at
the AMS 6500 inputs. The system is capable of resolving microvolt-level dynamic signal
components. Typically, signals from accelerometers mounted on operating machinery are
millivolt level signals. Signals of a magnitude this low can easily be overwhelmed by
interference from many sources in an industrial environment.
5.6.1 Choosing a sensor cable
Emerson recommends low capacitance shielded-twisted-pair cable for all AMS 6500
system sensor inputs. This cable protects from low frequency interference such as 50
Hz–60 Hz sources due to conductor twisting, and from RF and static discharge sources due
to overall shielding. Conductor size may vary from 22–16 AWG.
Excessive cable capacitance will affect the high frequency response of accelerometer
signals. Emerson recommends low capacitance cable (<15 pF/ft) for longer cable runs,
especially for channels used for PeakVue measurements. There is evidence that braided
shield cables are more effective than foil shield cables because they reduce impedance of
the shield conductor. Consider using braided shield cable for long cable runs, electrically
noisy installations, or critical sensor channels.
Emerson does not recommend coaxial cable or other non-twisted cable types because
they have lower immunity to 50 Hz–60 Hz interference than twisted pair cable. When
using multiple-conductor cable, consider individual isolated shields for each twisted signal
pair and an overall shield isolated from all cable pair shields.
5.6.2 Routing sensor cables
Route sensor cables in grounded conduit or in cable trays reserved for low voltage control
type signals. Do not route sensor cables in conduit or in cable trays containing AC power
lines, including the unit enclosure cable entries. If low voltage sensor cables are routed in
cable trays containing AC power cables, line frequency components will likely be induced
into the sensor signals.
When electrical equipment is switched on or off, the changes in current can induce large
spikes in nearby sensor signals. Maintain a minimum of 3 ft between sensor lines and AC
power lines. Allow larger distances for higher voltage AC power lines.
Limit the distance of accelerometer, velometer, and passive magnetic tachometer cable to
500 ft. Limit the distance between the displacement sensor cable and the amplifier to
1,000 ft.
Note
When high amplitude, high frequency signals are measured, particularly for PeakVue measurements,
the maximum cable length may be much shorter unless low capacitance cable is used.
Cabling requirements
30 MHM-97125-PBF-EN, R0
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