Nomad Engineer
Member
I'm fairly new to the world of PLC's. So far I've managed to resolve the problems I've crossed but this one has me stumped.
I've got a Rotork actuator (IQT) thats fed a 4-20mA modulation signal that sets the position of the valve its attached to. This was working correctly for some months however I have now found that it continuously moves open and closed quite randomly. The first couple of times it failed it worked again for a time after before failing again but now it just moves randomly all the time the analogue drive signal is attached.
With a multimeter I measured the current entering the actuator and its totally solid however I did notice that the voltage driving the current is all over the place. Anywhere from 1-10v quite randomly. I suspected the output card of the PLC as that is driving the current so I disconnected the actuator signal wires and placed a dummy 250 omh resistor in its place. Both the current and voltage over the resistor where solid so I have no reason to think its the PLC card at fault. Interestingly I checked the resistance across the actuators input terminals and found that it was varying alot, quite randomly. Generally around 220 ohm but anywhere from 200-500ohm. My multimeter is cheap and I suspect that its missing alot of the variation. A scope would probably show the full extent of the problem.
Any ideas? It looks to me as if the analog input of the actuator is at fault but it seems unlikely that that would break in any way. I suspected that it might have been set up to take 1-5v not 4-20ma but that doesn't explain how it worked previously or why it has a resistance.
My next step is to get hold of the remote (supplier of the skid the actuator is part of removed it from the actuator) and check how the inputs have been setup.
Thoughts? Similar experiences?
I've got a Rotork actuator (IQT) thats fed a 4-20mA modulation signal that sets the position of the valve its attached to. This was working correctly for some months however I have now found that it continuously moves open and closed quite randomly. The first couple of times it failed it worked again for a time after before failing again but now it just moves randomly all the time the analogue drive signal is attached.
With a multimeter I measured the current entering the actuator and its totally solid however I did notice that the voltage driving the current is all over the place. Anywhere from 1-10v quite randomly. I suspected the output card of the PLC as that is driving the current so I disconnected the actuator signal wires and placed a dummy 250 omh resistor in its place. Both the current and voltage over the resistor where solid so I have no reason to think its the PLC card at fault. Interestingly I checked the resistance across the actuators input terminals and found that it was varying alot, quite randomly. Generally around 220 ohm but anywhere from 200-500ohm. My multimeter is cheap and I suspect that its missing alot of the variation. A scope would probably show the full extent of the problem.
Any ideas? It looks to me as if the analog input of the actuator is at fault but it seems unlikely that that would break in any way. I suspected that it might have been set up to take 1-5v not 4-20ma but that doesn't explain how it worked previously or why it has a resistance.
My next step is to get hold of the remote (supplier of the skid the actuator is part of removed it from the actuator) and check how the inputs have been setup.
Thoughts? Similar experiences?