Advant OCS - PIDCONA sampling time
I am trying to figure out how to play with the parameter TS - Sampling Time of the PIDCONA controller. There is nothing about it in the documentatnion of PIDCONA.
For example let say TS = 2s and Ti = 1,4s - how could I understand it?
TS (Sampling Time) is related to the execution interval of the PID algorithm. You can include the PIDCONA in a 1 second control module and then set TS to a larger multiple of that time to slow down execution of the block and help to reduce CPU load.
There is a description of "sampling time" in the PIDCONA Functional Unit manual (3BSE 002 433R0101), and a search will find quite a few references to it.
In practical terms this is not really a usefull feature. Unless you are trying to add a LOT of PID controllers where you have an open loop process response time of more than 30 seconds or so, then you will have no opportunity to extend TS. For most purposes, configure TS to be the same as the CONTRM execution interval and execute PIDCONA's at either 500ms or 1 sec. This compromise works for most control loops.
For "good" control, most textbooks will tell you that your PID execution interval should be 5 to 10 times faster than your open loop time contant. This means that if your flow loop has a typical 2 to 3 second time contant, then your execution rate for the PID controller would be in the order of 200 to 500 msec. But realistically, your AC450 controller cannot do this and a 1 sec execution interval will work adequately. You absolutely do not want to increase TS in this case. And the same rule of thumb says you do not want to increase TS unless your loop has a long time constant.
As ever, if you have deadtime in your loop ( and yes, you always have some deadtime ) then the loop becomes harder to control. Increasing TS also effctively increases the deadtime of the control loop.
In your example, if Ti = 1.4 secs, then a TS of 2secs is far too long. Even a 1 seconds execution interval is a bit slow.
Finally, PIDCONA really is not needed in most cases. A regular PIDCON has the same PID algorithm and takes a lot less CPU time. The biggest "advantage" of a PIDCONA is Autotuning. This is great in theory, but in my experience this actually works pretty badly. I have seen it shut down the plant on more than one occasion (because the control bumps are too agressive) and the resulting tuning really cannot handle loops with any noticeable deadtime.