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Start by
Mansoor Ikram
03-24-2014 12:37 PM

Inductive Sensor Loop Powered Biasing Voltage & Signal Current Fluctuation Problem

We have 2-wire loop powered (24VDC/4~20mA) inductive sensor installed on Steam Turbine Turning Gear Motor for engaging/ disengaging purpose. This sensor basically senses the movement of plate gap (mm) and transforms this gap (mm) information into mA.
The problem is when its target moves, sensor mA & biasing voltage also starts fluctuating simultaneously (Volts fluctuates from 24VDC to 6VDC & signal current fluctuates from 5mA to 19mA).
The sensor is loop powered from INFI90 Bailey DCS (Analog Input Slave Card having Termination Unit#NTAI05). Though the Slave Card Termination unit supply remains constant.
We did this checks on several new sensors but observing the same results.
Wire loop from DCS till sensor is OK. DCS card also checked, verified and found ok.
Please suggest what could be the possible reason for such fluctuation?
Sensor Model# PEPPERL+FUCHS NJ2-12GM-N
03-24-2014 02:47 PM
Top #2
Tiberiu Nicolau-Stoika
03-24-2014 02:47 PM
The sensor NJ2-12GM-N is an On/Off device. It detects the presence of a target, does not measure the distance to a target.
03-24-2014 05:11 PM
Top #3
03-24-2014 05:11 PM
put a dial indicator on the surface of your drive and check for discrepancies on the machined surface.
03-24-2014 07:42 PM
Top #4
sumesh p k
03-24-2014 07:42 PM
Dear friend,
PEPPERL+FUCHS NJ2-12GM-N is an on off device , which will get on in the presence of metal and off in the absence of the metal......Otherwise please send the detailed part number.....
03-24-2014 09:43 PM
Top #5
Nate Bergeron
03-24-2014 09:43 PM
As mentioned, the prox sensor you have is a switch, not a transducer. So it's not "biasing" voltage that you see fluctuating, but rather the DCS is limiting the loop power to prevent an overcurrent situation.

Are you sure you're measuring 5 mA and 19 mA? I would expect you would see something more like 0 mA and then it goes straight to 21-22 mA (overcurrent limiting).
03-24-2014 11:56 PM
Top #6
Ahmad KolahiZonouzi
03-24-2014 11:56 PM
In the technical manual we have consumption "Measuring plate not detected >= 3mA" and "Measuring plate detected <= 1mA", and as you told that you get fluctuation in range between 5 and 19mA, so I think the distance of the sensor to the plate is not correctly set. First correct the distance such that you get correct consumption, and then program the DCS to interpret <= Min mA as detected, and >= Max mA as not detected. The Min and Max could be empirically defined, something like 1 mA and 3 mA stated in the manual.
best regards,
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