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Old December 26th, 2017, 09:31 AM   #13
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Pete.S. is offline
Join Date: Mar 2016
Location: Fl
Posts: 464
Originally Posted by Aardwizz View Post
Are you aware of the existence of Single Loop Controllers ("SLC" -- not to be confused with the AB PLC with the same initials) ?

These cheap gizmos (~$100) do all the PID control for you, and have a display where the setpoints can be entered and temperature monitored. They also have communication capability (typically RS485 modbus) to talk to/from PLCs.

Your PLC would thus not need ANY AI/RTD or AO cards to talk to the thermocouples and glycol control valves, just a Prosoft module to talk to these guys. I think this is the kind of solution that your customer is expecting from you -- local control at the vat's; supervisory control at the HMI.

Technically / in theory you don't even need a PLC -- an open-source HMI like the AdvanceHMI that's popular on this site could do the Modbus interfacing with the SLCs. I've seen this sort of thing done; whether it's a good idea or not depends on one's ability to service/troubleshoot it down the road.

Anyway, just throwing it out there. Generally speaking, telling a client that he shouldn't want what he wants (local control) isn't a great way to get that client to come back to you. It's easier to keep clients than find new ones.

Good luck!
If all the client want to do locally is to change the setpoint then I think your solution is the best.

If modbus is fully implemented in the loop controller you could read and write all setpoints, process values, PID tuning/settings, operation mode etc from the PLC/HMI. That also means you can do full trends on the HMI if you read the SP/PV/OP trifecta on a regular basis.

Also good from a cost and installation point of view since you would wire up everything to the loop controller so there is a minimum of wires to run.
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