Extruder PLC control


Thread Starter

Pat Levinson

Hello list members, I recently replaced the 11 single-loop temperature controllers on a plastic pipe extrusion machine with a PLC based system. It has worked flawlessly, however I'm a bit worried because the t/c (thermocouple) inputs (double ended) in the PLC are non-isolated, and the module's specs call for a maximum common mode voltage of 5 volts between inputs.

I've measured common mode voltage between all inputs and it never exceeds a few millivolts, which is to be expected since the t/c's are in direct contact with their respective barrel and die zones, and all the zones are mechanically (and therefore electrically) linked.

However, this is what worries me, since I guess that the heating resistor in particular certain zone could short out and/or make contact with the barrel or die, possibly affecting my t/c input modules either through a voltage spike at a t/c input, or a common mode voltage mismatch between zones.

Has anybody got any experience with this, or possible problems/precautions that I should take into account? is this normal practice in extruders? By the way, I'm using an Automation direct d4-450 cpu with two 8-input t/c modules. The machine is supposedly protected against resistors shorting out, shutting down inmediately.

Also, the manual recommends connecting the t/c's negative terminal to the module's common in case of grounded thermocouples.. should I do that in this particular case?

Regards, Pat

Burak Cedetas

I would strongly suggest to use opto isolated
analog inputs. A short circuit or a spcan burn your PLC. Also RFI travelling through the analog inputs can effect the CPU.
I would use isolator modules from companies like
weidmuller or similar.
In you case you can use transducer with Thermocouple input and 4/20mA output. This transmitter will be between the PLC board and the thermocouple.

Burak Cedetas
[email protected]

Peter Anzalone

Hello list members, and Pat, Red Lion recently developed a DIN rail mounted dual loop temperature controller w/Autotune that has full input and output isolation. We developed this product specifically for use with non-grounded T/Cs and it takes care of any ground loop issues that may occer. We've supplied many plastic processors with this product. It is Model DLC00001. You can contact Red Lion for further details at www.redlion-controls.com