Changing SP Frequently PID

Greetings to all
Please see attached a file (RSS file, i couldn't attach the whole project, as i have a dial up internet, but the heart of the program is attached). I have not been able to test this program but i will be going to the site tommorrow to do so. Before i go to the site i would like to have expert's opinion on it. What i am trying to accomplish with the logic is to control the Speed and Bypass pids which in tern controlling the end devices. The question is......i am using two pids and i am changing the SPs quite frequently......Although i am trying to care of it. i am trying to Mov the new set point every time there is a change.....I dont like that idea but i coudn't think of anything else......just now i thought of another idea why not i RAMP the set point to its new value everytime its being changes.....either RAMP up or RAMP down......It seems to me like a good idea....but i dont know how to implement it....if you guys can help me out by putting some examples on it will be easier for me to catch them in this short time.......

Other than that, i initially thought that my pids should take care of this change in the SP.....So alll your thoughts and ideas will be appreciated......... Thanks in advance

greetings plcnovel,

sorry, but I don't have time to look at this latest posting ... but a quick glance shows that we still don't have your I/O Configuration list, etc. ...

also ... I'd suggest that you simply keep attaching your latest posts to your previously existing thread ... since this is like an ongoing conversation, it would be much easier for everyone to keep up with what's going on if you could stay with one single thread ...

I have done this exact same thing with flow and pressure setpoints in a PID loop. Sorry I can't add a logic example, I just received new laptop and only have logix5000 on it. You have two options:

What you do is have two registers. One register is your setpoint to the PID and the other is your applied register.

Scenario #1- you find the difference between the two and add a certain percentage of the difference every time interval. This all depends on how fast you want it to react.

ACTUAL_SP-PID_SP=SP_DIFF
SP_DIFF/10=SP_ADJUST
PID_SP+SP_ADJUST=PID_SP

150-50=50
50/10=5
100+5=105

This will gradually slow down as you approach equality. For smoother transistion increase the interval of executing the adjustment rung and decrease your divisor in rung two.

The second is to cascade another PID loop on it but wont go into detail unless you refuse the first idea lol.

hope this helps

Have a look at this thread for ramping

hI gUYS
thanks a lot for your help. I think i got it working today....but only one thing gave me a little problem.....Even when the machine is shut down and there is no pv and sps for the SPeed PID, it gives me an output of 9%(5.85 mA instead of 4mA= 0 %). WHen i put 0 in my integral field, it goes away and back to zero....After putting the reset value again , doesn't create the problem.....but i coldnt figure it out why its doing like this.....i have checked all my pv and sp, but there is nothing........so any help or ideas will be appreciated...thanks in advance

The integral term will remain if SP=PV. You need to manually reset it, or preferably, disable the PID loop when you aren't using it (Set the .SWM Bit to put it in manual).

The April issue of Control Design magazine had an interesting article on this very subject. The author proposed using a lead/lag filter on the changing setpoint rather than using a ramp to tame unruly PID behavior.

http://www.controldesign.com/articles/2006/051.html

I recommend you take a look at it.

Why lead lag? Why not a second order low pass filter or a first order low pass filter like Bradal suggested. What isn't mentioned is that a PID and plant can be tuned so the whole system has all of its poles on the negative real axis and therefore the response will not over shoot, even when making step changes to the SP. No ramp or filter is required. Adding the ramp is necessary if you don't want to thermally shock something or limit current on a motor or keeping the place from shaking apart. Motion control requires ramps most of the time although we do have a mode where the customer can just enter a set point and we go there. With no ramps or filters because the PID and the actuator is the filter.

In this case I think the ramp is being used to compensate for poor tuning. I get suspicious when both loops have the same gain and the gains are nice neat numbers.

Just what the author suggested...

Peter Nachtwey said:

Why lead lag? Why not a second order low pass filter or a first order low pass filter like Bradal suggested. ...

I get suspicious when both loops have the same gain and the gains are nice neat numbers.

Click to expand...

Lead/lag is what the author suggested and I just passed on the link.

I actually wondered about a second order low pass filter myslef
when I first skimmed the article a couple of weeks ago. I also noticed the integer gains (P=2 and I=7) but I didn't give it much thought. You are right, this system is probably poorly tuned.

Nevertheless, I did find the article to be of interest. I would tend to agree with the premise that a lead/lag or second order filter on the SP is a probably better method than a ramp if the ramp is being used solely to limit over/under-shoot of the PID.

Can u try to structure ladder it man, i really cant understand it .....i guess dont have too much time.....at the site right now......if you can try to implement it for me i can try it at the site tommorrow

plcnovel said:

Can u try to structure ladder it man, i really cant understand it .....i guess dont have too much time.....at the site right now......if you can try to implement it for me i can try it at the site tommorrow

Click to expand...

I'm not entirely sure I understand the question. Are you asking how to code a filter? Or are you asking someone to write your program for you?

Ya I'm not sure if your asking me or the other guys here, what do you want to be done, if your just looking for the ramp ability, you should totally be able to following what I described, its all just about add, subtract or divide.

Actually just wrote a little piece of logic I'll try to attach.

what the heck???? Try it zipped i guess.