S800IO - AO810 signal out intermittent
Currently, my control panel is using AO810 with TB810 for controlling our field instrument which supports 4-20mA signal for opening & closing temperature control valve(TCV).
However, recently we found out that when we are opening/closing the TCV from faceplate, the signal out from A0810 is intermittent to the device thus causing the opening/closing will not be smooth, for example, it will open awhile then pause then again continue to open then again pause repeatedly until it reaches the setpoint. This applies the same for closing.
We have tried to test on different AO channel on same AO unit and it is still the same. We have changed the new valve and also new AO810 unit, the result is the same.
Can anyone advise/share what will be the root cause and solution for this matter?
Voted best answer
I hope this is the problem with only one perticular AO810 module connected with some TCVs (control valves)? Does this problem is a recent finding? I mean it was working before and not it is giving problem?
As you have already changed the valve (positioner??) and AO810 module, still the problem persists! Hence i think there wouldn't be any problem with AO810 and Valve individually!!
I suggest you to check the AO810 output with mutlimeter or another recorder by disconnecting valve. And check the behavior of the 4-20mA output. If this seems to be ok, then test the valve by feeding 4-20mA from mA source from the marshalling termianls. This is helpful to check any problem (eletromagnetic interference) with cable from marshalling to valve.
If the AO810 module output when recorded by disconneting valve is not ok, then another suspected component would be TU810 base. This might be faulty??
If the valve doesn't operates smoothly when operetd by feeding 4-20mA from a source
(feeded from marshalling terminals), then feed the 4-20mA signal to the valve by connecting directly on valve positioner.
If the valve operates smoothly by feeding 4-20mA signale directly on positioner then cable would be problem (EMI) else the valve itself.
If both AO810 and valve are working fine in isolation then obiviously the combination would be problematic. But this case is very rare.
In such case you may install some resistor (resistance) in parallel to the signal wire. Please share the positioner model number too in case of above.
Hope this helps you.
It is unlikly that a hardware fault in the AO810 will cause this type of behavior. The unit either gives a current output (ie the vale is open) or it doesnt (ie the valve slams shut)
You could make the valve behave like this if you programmed it specifically ( or just really badly), but ...
It is **MUCH** more likely that the valve actuator is packed too tightly and its sticking. This is a very common behavior - called "stiction"