1756-OF6CI at Power Up

[RZR]

Hello,

We have 1756-OF6CI module on Controllogix chassis controlling an actuator by applying 4-20 mA. The actuator is for opening and closing a valve. The 4mA is for Close command and 20mA is for Open.
The problem we face is at PLC power up.If the valve is in open position and a power off occur and PLC shutdown then when power come back the 1756-OF6CI at power up send close command to the actuator and the valve start to close during first 15 seconds and it goes toward close about 20% and after that it start to open again. All this happens during power up automatically without close command being issued from the HMI.

Is there any way to avoid that at power up and keep the valve at open position.

Thanks

 

[bernie_carlton]

The card may be transmitting the low current (possibly even 0 ma) as a default until the system is reset. You may have to provide for a timer to delay power to the actuator for sufficient time to allow the PLC to fully reset.

 

[ASF]

Presumably, your valve has four wires going to it: + supply, - supply, + signal, - signal (it could also have just three wires: the - supply and - signal could be common, but either way it will have little effect).

If you switch the + supply wire through a relay, that is controlled by your PLC output, then the valve will have no power to drive itself closed until the PLC has started up, and by this stage it should be receiving the full 20mA signal and will remain open. The output to control the relay could be "always on" as once your PLC is executing code, your 4-20mA should be all OK.

That said, there are a couple of things that need to be considered:
- My solution assumes that the valve is driven closed electrically, and that we can therefore stop it from driving anywhere by removing it's supply power. If this is not the case, give us some more information and we may be able to find another solution
- You should consider any safety aspects of the valve being open. If you have a PLC, output card, or relay failure, and you have wired the power supply to the valve through a relay off your PLC, it will be impossible to close the valve until you resolve the issue with your PLC. Could this cause any safety hazards to machinery or personnel? If so, you will again have to rethink your approach

 

[RZR]

@ bernie_carlton
@ ASF

Thanks for the idea.
Your solutions are good for our application and I will take safety issues inconsideration.