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Katy1163

RSLogix PID

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I am new to using the PID control in RSLogix and I am trying to switch over from using a PID controller to using the RSLogix PID control with a PLC. Can anyone give me some basics on what I should know before trying to do this? I have some specs we used on the PID controller, but there seems to be alot of information that doesn't quite match up between the two types of controllers. What information is actually necessary for the PID function to execute? Also, if there are more (any) details I can supply that would be useful, please let me know.

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Which version of RSLogix are you using and which PLC are you using? Do you have an instruction set reference manual for your PLC? The PID instructions of RSLogix5 and RSLogix500 are different. In RSLogix500 it will matter whather you are using a SLC500 family processor or a Micrologix family processor because the data structure is different. And in RSLogix5000 there are standard PID and enhanced PIDE instructions, both of which are different.

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I am using RSLogix 5000 with a CompactLogix controller, but I am using the Mini edition of Logix and this version does not support Function Block programming. Therefore, I have to use the standard PID, since the PIDE is only available as a function block.

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The Logix 5000 PID instruction is quite simple to use but I have still seen many people come unstuck with it. The most common problems I have seen are:- i) The instruction must be triggered periodically. You can do this by generating a clock pulse using a timer, using a periodic task or even using the timestamp from a 1756 analogue card but something has to tell the instruction to execute. The trigger interval must match the Loop Update Time on the configuration tab for the PID instruction. ii) Always place the loop in manual when the process is not running and fix the output where you want it to be (typically at 0%). Unless you have a hard wired manual system with a tieback signal for the PLC you should use the software manual (SWM) bit to do this and write the desired CV to SO. If you do not do this the loop will still respond to errors and may restart with the the output in a potentially dangerous state (ie a fully open steam or fuel valve). iii) If you are familiar with "normal" single loop controllers which use a PID algorithm similar to the ISA ideal model you should set the PID Equation on the configuration tab to Dependent. This will allow you to use the tuning techniques you are used to. iv) Always use the Scaling tab to apply any scaling to the controller output. This might seem obvious but I have lost count of the number of times I have seen people use the CVmin and CVmax settings to do this and end up totally confused about what is going on. Andybr

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As it stands right now your quesiton is still vague. A little more detail and some information about what you don't understand will be helpful.

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