RSLogix 5000 PIDE Cascade

d.dinh

Member
Join Date
Mar 2011
Location
Brisbane, QLD, Australia
Posts
15
Hi all,

Having some issues setting up a PIDE Cascade control loop.
Have been trying to go through the Wp008 doc but there's probably something easy/small that I'm missing.

I have a tank with an oil jacket which is directly heated up using resistive heaters (SSR). The oil jacket in turn heat transfers to the tank product (wax).

The application is heat only, as the client generally turns on the oil jacket heaters and leaves overnight for the wax to melt.

Am I correct in assuming that this calls for a PIDE Cascade Loop?
Primary Loop - Wax Temperature - 4-20mA - 0-200C with SP cutoff at 150.
Secondary Loop - Oil Jacket Temperature - as above.
Secondary Loop output goes to SRTP for the SSR output.

Thanks ahead for any help.
d.dinh

PIDE_Error.jpg
 

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right click on all your "wires" that go from the PID on the right to the PID on the left, and click "Assume data available"

Thanks mate. Did work it out in the end. But was a quite frustrating process trying to figure what I had done wrong.

Does anyone know whether "assuming data available" is in the white papers/manuals anywhere?
 
I'm not seeing where you need cascaded loops here...Honestly, I'm not seeing where you even need two, perhaps a diagram?

Here's a quick sketch.

I am not exactly sure whether the client/operators control the wax temperature as the setpoint, or the oil jacket temperature as the setpoint.

But essentially, they heat up the oil using a pair resistive heaters that are turned on using solid state relays(on/off only).
As the oil heats up, the wax product heats up.

They generally leave this on overnight at the set temp so the wax melts overnight without supervision (apart from PLC).
 
I am not exactly sure whether the client/operators control the wax temperature as the setpoint, or the oil jacket temperature as the setpoint.

I'd try to find out more from them about their process/product.

I'd like to think the melting wax behaves as any other melting-solid process. Specifically, the solid temperature rises until it reaches a solid/liquid transition temperature (or temperature range). The temperature of the melting product remains relatively constant until all is melted and then the all-liquid product temperature begins to rise. As an analogy, think of heating a block of ice; the ice block temperature will rise until it reaches 0C/32F and stay at that temperature until it is completely melted. The water temperature will then rise towards the temperature of its surroundings.

For a process as described above, I would simply control oil temperature at some high level, wait for the wax temperature to indicate all has melted, and then set the oil temperature to some melt-maintenance temperature. (And note different temperature schedules/recipes might be necessary for different wax products).
 
I really don't see any need for cascaded loops there.
Run one PID for the oil temp, and one for the wax temp. Done.

Sorry. You have me a little confused (and probably my limited understanding).
If I were to use two separate PIDs, would the output of the Wax temp PID control? I only have the one SSR to control the heaters that heats up the oil only.
 
I'd try to find out more from them about their process/product.

I will have to double check at the client's old site and with the previous OEM they used for the PID loop controllers.

But how it was explained to from an operator's PoV:

When they start the heating process:
- the heaters will turn on and get the oil jacket temp up to 145C
- the wax product temp SP will be something like 120/130
- as the wax temp PV gets towards the SP, the heaters back off so the oil temp PV also backs off to keep the Wax temp at its SP.
- main reason for the over temp of the oil vs the wax is so the wax melts faster to reduce waiting time
- heaters turn on/off as required to maintain wax temp PV at the SP.

Hopefully that makes sense. Anyone have further thoughts on this?
Cascade PIDE control or not? Similar applications done in the past?

Many thanks,
d.dinh
 

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