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View Full Version : Cascading PIDs in RSLogix5000


robertmee
October 28th, 2008, 09:53 AM
I've been doing cascaded loops for awhile, so hopefully I don't need a primer on the control theory :) What I'm specifically after is the mechanics of setting up the PID in RSLogix5000 (not to be confused with PIDE, a customer request). I've done them in PLC5 and SLC5, no problems. Here's the two points I'm interested in:

According to the instruction help and one example given, you configure the PID blocks themselves and tell the Master its the Master and tell the Slave it is the slave and point it to the Master PID control word. Easy enough. However, the example shows a tag for the Master Control Variable. I'm assuming this is just a throw away tag as by linking the two PIDs with the Master/Slave attributes, the Master.CV automatically gets sent to the Slave.SP. At least that's how I read the manual and the example shown. So, the Master CV tag really is just a dummy placeholder?

Next question, is how do I disable the Master and throw the Slave into single PID operation. There are times when there is no Master PV to measure and we need to control the slave loop by the slave PV only. In the past, I just turn off the Master loop, and move a different SP into the Slave. But since the Master/Slave link seems to be behind the scenes, how do I ensure that the slave SP is not coming from the Master PID control word?

I think I could accomplish both of the above by just treating each PID block as a Master (as far as RSLogix5000 is concerned) and copy the Master CV tag to the Slave SP tag when in cascade, and use an external SP copied to the Slave SP when not in cascade.

Anyone done this specifically with PID in ladder in RSLogix5000?

averytc
October 28th, 2008, 12:41 PM
I had a similar issue and could not find a way to disable the Master. I chose to have two master loops with one feeding the SP to the other. When I don't need the first loop it is set to SWM and the other is fed it's SP from the last state - which is controlable from the HMI.

robertmee
October 28th, 2008, 12:58 PM
I had a similar issue and could not find a way to disable the Master. I chose to have two master loops with one feeding the SP to the other. When I don't need the first loop it is set to SWM and the other is fed it's SP from the last state - which is controlable from the HMI.

Thanks! That's the same line of thinking I had and is what I routinely do in PLC5 and SLC.

averytc
October 28th, 2008, 01:03 PM
Yep, doing it so much in PLC5 and SLC it seemed like the obvious option. There might be a better way, but it works and is consistent with the other systems in the plant.

RSL
October 28th, 2008, 03:07 PM
I have used the built in cascade functionality you are talking about. I found it to be way to rigid too. I now always use code to cascade them. This allows me to be a lot more flexible.

I had similar issues.

RSL