Auto/Manual Stations

Hello all,


I have a question around auto/manual stations and if anyone can recommend any products that can perform this function.


A better description on my application:

• We have an S7-300 PLC to is controlling a field device with a 4-20mA signal.
• 4-20mA feedback from the field device is connected to the S7-300 PLC.
• PID control loop is running in the PLC to control the field device.
• Under normal conditions (e.g. 99% of the time), the PLC will be controlling the field device, so everything is fine.
• However, we want to have the ability to still control the field device in open loop mode in case the PLC has failed.
• So effectively looking for a device that when selected to "Auto" will just repeat the 4-20mA signal from the PLC to the field device. When selected to "Manual" the operator can manually control the 4-20mA signal to the field device. If the 4-20mA signal from the PLC fails then the device will automatically switch to "Manual" mode.

I have found the Moore Industries 532 auto/manual station which does the job, but was wondering if anyone had any other suggestions for this sort of application. It also worries me that there aren't a lot of auto/manual stations out there, making me think I might be looking at this the wrong way.




Thanks,


Nick

Nick: How is the selection from manual to auto and viceversa? Is it directly on the device by means of some panel or magnets or something like this. Or are you thinking of a device that has an additional digital input so the PLC could set the auto mode by having programmed some behaviour when the PLC fails for the corresponding digital output in the PLC?

Perhaps a better solution is to upgrade your PLC if it fails so much that you need stuff like this? With a robust PLC in place, you can just create a manual switchover to the PID.

353 is obsolete. It'll be Ebay time if you go that route.

Honeywell app note on using a UDC 2500 as an auto-manual station is attached. It does what you describe: manual control from the UDC, and if you rig up a relay to close when the PLC 'fails' and connect it to a DI on the UDC, it will transfer to manual mode when the DI closes.

Dan

Here is another option(s)...

https://www.miinet.com/?page=search&cid=pid-controllers-and-backup-stat