STI recommends performing double integration (Acceleration to Displacement)
when required by using a CMCP793V Velocity Output Accelerometer and a CMCP535(A)
Transmitter or Monitor. As each integration stage can require significant
gain to the raw dynamic signal this method keeps amplified noise to a minimum
as the first level of integration (acceleration to velocity) is performed
within the Accelerometer before the field wiring and external noise is introduced.
The second level of integration (Velocity to Displacement) is then performed
within the CMCP535 Transmitter or Monitor. Noise sources that create issues
with double integration include field wiring, hand held radios, ground loops,
VFD’s (Variable Frequency Drives) and AC power.
Portable data collection instruments and analyzers that are not connected
to the real world (battery powered) are not subject to the same noise and
interference sources as permanently installed systems and thus perform double
integration quite well.
As can be seen in the following calculation the machine speed must be
considered. Low speed machines in general are not good candidates for double
integration (<600 RPM). The following calculation can only be used for discrete
frequencies and cannot be used for broadband vibration.
D = 19.57 g/f2
D = Displacement (mils Peak to Peak)
g = Acceleration (g’s 0 to Peak) f = frequency Hz
As can be seen in the following table, machine speed and frequencies
of interest are critical to having a viable output for double integration
from a case mounted industrial accelerometer that will not be effected by
field wiring noise and interference.
| Displacement |
RPM |
g’s |
Output @ 100 mv/g |
| 2 mils |
3600 |
0.368 g’s |
36.8 mv |
| 2 mils |
1800 |
0.092 g’s |
9.2 mv |
| 2 mils |
900 |
0.023 g’s |
2.3 mv |
| 2 mils |
600 |
0.01 g’s |
1 mv |
| 2 mils |
300 |
0.003 g’s |
0.3 mv |
Displacement readings on rotating machinery have historically been requested
on Journal Bearing Machines where economic decisions do not justify the
installation of Proximity Probes that provide a true relative measurement
of displacement (shaft to bearing). Additionally as Operators have historically
used displacement on Journal Bearing Machines the same engineering term
was desired. Rolling Element Bearings (REB) should always be monitored using
Velocity (single integration) as provided by a standard 100 mv/g industrial
accelerometer and the CMCP530(A) Transmitter/Monitor.
Displacement readings provided by double integrating a acceleration signal
from a case mounted sensor are not the same nor will they agree with the
displacement signal provided by a Proximity Probe System. In general the
case reading may be 3 to 5 times lower than the actual shaft displacement
internal to the bearing.
- • Case Mounted Sensors are a Absolute Measurement (Case to Free Space)
- • Proximity Probes are a Relative Measurement (Shaft to Bearing)
- • Journal Bearings provide Damping (like a shock absorber)
Double Integration Checklist:
- • Machine Speed >600 RPM
- • Journal Bearing
- • CMCP793V (Accelerometer w/Velocity Output)
- • CMCP535(A) Transmitter or Monitor
- • Painted Steel or Stainless Steel Enclosure for Transmitter or Monitor
(if field mounted)
- • Extension Cable (793V to Transmitter) short as possible and in metal
conduit
- • Cross all power lines at 90 degree angle (Do not Parallel)
- • Extension Cable Shield landed at Transmitter/Monitor
- • No Ground Loops
- • Proper Grounds Established for Enclosure and Power Supply
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