S7 - Changing PID Terms

Hi,

I am currently looking into a very odd process that we have previously controlled using custom embedded controllers. At certain stages of the process, we need either switch off PID or change terms and reset (whilst holding output steady).

Does anybody know if the terms can be changed 'on-the-fly'?

Thanks

eskay

Yes , you can do all of that if you wish . To make life a bit easier , I have encapsulated the standard FB41 inside another block - this way I am presented with inputs and outsputs that I want , rather than the ones Siemens wants to give me .Looking at your application , I would tend to set up enough instances of the PID controller to suit your application , one "cascaded" of the back of the next , each one running the PID terms you require , as each phase of the process is reached , the next loop is enabled and takes over control - you can of course just use one and a sequencer to control the way it operates .

All the inputs, outputs, in_outs and stats for each PID block will be held in the corresponding instance data block. So if you have FB41 called 3 times with 3 separate IDBs, you will already have complete access to all the terms via the standard DBx.DBDy type of addressing for all three instances.

There's nothing to stop you writing code to move different values in to these DBs whenever your process requires - you don't even need to specify actual parameters for the block's formal parameters.

Regards

Ken

Agree with you Ken , I tend to do this fairly infrequently as whilst it is a nice smart way of doing the job , a less experienced programmer / someone with less S7 experience may struggle to fault find later .
Up to the individual though !

You could copy the current output of the PID into a memory, then transfer that to the manual setpoint & set pid control into manual.
I had a problem with a mixing system that we needed to start at the normal running point found when the PID control had stabilised so we new that 62% output was approximately the settled down output, so loaded the man setpoint with 62%, set it in manual, then after a short time switched back to auto on PID this gives bumpless changeover, well no overshoots & gave us a very stable system.