Temperature control in PLC or Controller

[brucechase]

I have an older machine here that we want to upgrade. It is currently running a SLC 5/04 with Wonderware as the HMI. The process has a couple of servo drives, a couple of DVT cameras, and 10 heaters.

The heaters are on their own controllers (Parlow 8 zone MLC9002 and Partlow 2 zone MLC9002) talking to the PLC with Devicenet.

I would like to upgrade this to a controllogix platform (for servo control mostly) and I am wondering how everyone would handle the temperature inputs.

Would it be better to wire the type J thermocouple directly to the PLC using 2 of the 1756-IT6I2 cards or should I use a temperature controller and input using ethernet/IP? I understand that calibrations would be different on the 2 setups.

I haven't had much experience with temperature inputs and controls so any insight would be helpful.

Thanks

 

[TConnolly]

I would wire the T/Cs to the PLC and let the PLC do the temperature control. My recommendation is also that if you don't have the FB language extension you get it and use the enhanced PIDE instruction.

 

[Old No. 7]

We have done a lot of systems using the PLC (SLC, Micrologix, CompactLogix, and ControlLogix) for temperature control and also a lot of systems using loop controllers (Honeywell, Eurotherm, and others). My recommendation would be to keep the loop controllers and send setpoints from the PLC for a couple of reasons. If your system is already setup that way, you wont have any setup or wiring changes regarding the heating system. If you do it in the PLC, I think you will have a lot of time invested in getting the PID setup and then tuning the system once you are done. I also think that loop controllers are generally much easier to tune than the AB PID loops. In my experience the PIDE was horrible and we ended up switching back to the PID in CompactLogix systems. We have also had a lot more noise issues with thermocouple inputs on PLCs. I've seen a duplex thermocouple hooked to a PLC and Honeywell UDC and the PLC will jump all over the place and the Honeywell will be rock solid. VFD's and electric heating systems (especially VRT's with high current) cause lots of noise problems with PLCs. The only problem I've had with the PLC-Loop Controller setup is losing communications between the PLC and loop controllers.

Obviously all of this can be overcome, and there may be good reasons to have everything in the PLC. If you want to get this done as fast as possible, I'd go the loop controller route. If you have time and want to learn a lot about PIDs go the PLC route.

 

[Rube]

I agree with Alaric UNLESS you need really tight control of your temps. Then you should stay with the Parlows, Watlows, etc. We use the CL to control our wax heating in wax injection presses--but it's WAX and there's no need for very tight temp control. A differential of 2 degrees is easily obtainable with proper programming in the CL, depending on the size of your heaters and the substance being heated.