Amphenol Wilcoxon TN14 Rozwiązywanie problemów z instalacją - Strona 4
Przeglądaj online lub pobierz pdf Rozwiązywanie problemów z instalacją dla Przyrządy pomiarowe Amphenol Wilcoxon TN14. Amphenol Wilcoxon TN14 7 stron. Accelerometer
Drukuj stronęis very rare to have a short inside the sensor. The most common fault location is in junction box terminations.
Check to make sure that a frayed shield is not shorting across the signal leads. Many times a crushed cable
can produce a short. Use an ohmmeter to check electrical isolation between the leads. Disconnect the cable
from all other devices and measure between all signal leads and shields. When measuring the resistance be-
tween the cable conductors, the ohmmeter should measure infinite or at least above 50 megaohms.
Damaged sensor: Low bias, high bias
Out of specification bias readings other than those listed above usually indicate sensor damage. Common
sources of sensor damage are exposure to excessive temperature, shock impacts, mispowering, and electro-
static discharge. Excessive temperature is the most common cause of sensor failure. Sensors caught in a fire
are usually destroyed and can show various bias readings depending on the failure mode within the sensor.
Long term temperature failures are marked by a slowly rising or declining bias voltage. In many cases bias
returns to normal as the temperature decreases. However, the damage to the amplifier is permanent and the
sensor amplifier may continue to deteriorate.
Figure 4 shows the bias trend of a sensor failing
from long term temperature degradation in a
paper machine dryer section.
Excessive shock, mispowering and electrostatic
discharge can permanently damage the amplifier
of unprotected sensors. Industrial sensors
typically contain protection devices to prevent
these types of failures.
Erratic bias and time waveform
The bias voltage should remain stable and unchanging for properly
operating sensors. Shifting bias indicates a very low frequency
signal that is not filtered out by a DC meter. In rare cases this
indicates an actual low frequency signal, however in most cases
this indicates a fault. Primary causes of erratic bias are thermal
transients, poor connections, ground loops, and signal overload.
Each of these faults will also be visible in the time waveform as
erratic jumping or spiking of the signal. Thermal transients cause
uneven thermal expansion of the sensor housing materials. This
can be detected by the sensor as a low frequency signal. The
problem is most evident when using low frequency sensors.
Poor or contaminated connections can also cause low frequency
bias and contact noise. Look for corroded, dirty, or loose
connections. Repair or replace the connection as necessary. Non-
conducting silicone grease should always be applied to connectors
to reduce contamination.
Ground loops are developed when the cable shield is grounded at
two points of differing potential. Always ground the shield at one
end only! An easy test for ground loops is to disconnect the shield
at one end of the cable. If the problem disappears it was probably a
ground loop fault. Figures 5a and 5b show a connection susceptible
to ground loops and a correct installation where the shield is tied at
one end only.
Figure 4: BOV trend of a sensor exposed to long-term
excessive temperatures
Figure 5a: A poorly grounded installation
Figure 5b: A properly grounded installation
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