PID Implementation and Ladder Logic

<i need the temperature to be stable at 140 c with accuracy of 0.5%.>

This is achieved by tuning the proportional, integral and derivative terms within the control algorithm and is not a function of the ladder logic or whatever embraces the control algorithm. They are user-selectable terms.

Some loops will not achieve this level of stability on account of the plumbing that they contain (been there, done that, secondhand T-shirt now on eBay...). While the software and the tuning parameters are naturally important, they cannot be looked at in isolation in some cases.

What happens when the steam conditions change (maybe another user goes on-line suddenly)?

What happens when the heater battery steam trap goes "stable 2"?

What happens when the condensate return pump trips out?

Dear PWSlack,

thanks for your reply. here are the answers to your questions.

1. What happens when the steam conditions change (maybe another user goes on-line suddenly)?

Ans: If steam conditions are not in the required range, the whole system is shut off.

2. What happens when the heater battery steam trap goes "stable 2"?

Ans: i am using the steam from utility,so there is no heater. Hence this problem is not in existance.

3. What happens when the condensate return pump trips out?

Ans: there is a vacuum pump in addition to it that helps in drawing out the condensate in such case of failure.

can you explain how to configure the PID using PL7 pro for the before mentioned objective( in previous post). I am completely new to it.

1) Is it not possible to implement just a stand alone temperatuur PID controler, to control only the steam temperature?

2) Is it not possible to add an inplug module which contains the specific PID software ?

3) I used a few times a LOGO! from Siemens. This small plc is not expensive (+/- 150 euro) is easy to program with logic symbols and incorporates a PID function.

Good luck

hey rudy,

thanks for taking interest... here are the answers to your questions:

1) Is it not possible to implement just a stand alone temperatuur PID controler, to control only the steam temperature?

Ans: YES! Infact i already have it. But i want to eliminate it by controlling with the plc that also exists there for some other purposes(MODICON TSX Micro 3722) so as to save the extra expenses.

2) Is it not possible to add an inplug module which contains the specific PID software ?

Ans: Modicon has PID software, but i don't knw how to configure it. That's what i am looking for.... :)

3) I used a few times a LOGO! from Siemens. This small plc is not expensive (+/- 150 euro) is easy to program with logic symbols and incorporates a PID function.

Ans: you may be right about that, but i already have this plc and hav to go with the available resources and minimum expenses :( ...... so this is the whole scenario.

I hope i have cleared your doubts. If in case you want to ask anything else, plz feel free... and plz plz don forget to put your valuable suggestions also.

thanks.

I once tried creating a PI controller through ladder logic using a spare Allen-Bradley PLC5 just to see if I could do it. This was about 14 years ago and I now can't remember how I did it. I only remember that it was very difficult and took a couple of weeks of working during lunchbreaks. Of course, there was no point in doing it really since the PLC5 had a built-in PID instruction. As I said, it was just for fun.

Further, I tested my program using a process simulator also created through ladder logic. I have no idea if it would have worked as well in the real world.

Basically, I used a timer to run the program at 0.1 second intervals. It calculated the output due to P, then due to I and, finally to D and just added them together. Lots of trial and error. In the end I got it working to my satisfaction.

P, was easy. Just calculate the error and multiply by the proportional gain.

I, was really difficult. I used repeats per minute to calculate the output by multiplying it by the proportional output and dividing it by 600 (the number of 0.1 second intervals per minute). This one needed an accumulator to make it work.

D, I never tried.

Sorry if I can't give a more detailed explanation but it was a long time ago. I also can't guarantee that it will maintain your temperature within 0.5%.

regards,

Vulcan

hey vulcan,

thanks for sharing your experience....

and i must say you are a real genious as hardly anybody wud think of doing wat u did 14 years ago... hats off to u man...

i m aware that plc program provides the PID configuration facility, but the problem is that i m new to plcs and don knw how to configure PID on it. I wud appreciate if you can give any suggestion regarding this matter.

thanks.

I am not familiar with Modicon, unfortunately. However, I'm guessing that you don't have a manual, otherwise you'd have already figured it out.

I googled for Modicon and found their manuals download page:

modicon manuals

Personally, I wouldn't advise you to do the PID through ladder logic. It will take more memory to implement it than using the PID instruction and, I tell you, memory can be a precious thing with PLCs. I can probably try duplicating what I did before but it will take time and learning how to do it with a Modicon PLC is going to take a long time (besides, you're going to have to pay me to do it ).

So my advise is, learn how to implement the PID instruction and use it. The experience will also give you more knowledge than just how to use the PID but other things as well.

Good luck and let us know how it goes.

regards,

Vulcan

Dear vulcan,

thanks for your suggestion...

Although I never thought of paying you or anybody for this , yet thanks for increasing one more option to me, if in case every thing goes wrong...(which i feel, wouldn't coz of your valuable suggestions and kind help ).

Yeah, i've figured out some ways to configure PID on the modicon PLC n m working on it... surely, i'll inform u all guys when i get it right.... OFCOURSE, WITHOUT ANY CHARGES... as the real value for that would be your time and interest that u all have shown in helping me...

thanks everybody for the help...

and especially thanks to u for the modicon manuals...

take care...

regards.

hie ..

me ritesh pursuing ma btech in ece currently m doing ma summer interns in plc...

can u plz help for the ladder logic programming implementaTION of pid in plc ..

plzzz reply soon waiting for ur reply ,,ma id is sood986@gmail.com

SIR PLZZ HELP ME , I M NEW IN THIS FIELD , ACTUALLY M PURSUING MA INTERN IN INSTRUMENTATION DEPT OF AN MNC ,, N THEY HV GIVEN ME WITH AN ASSIGNMENT TO IMPLEMENT LADDER LOGIC FOR PID ,, I DNT KNW MUCH ABOUT THIS TECHNIQUE ,, SO KINDLY ASSIST ME HOW WOULD I START WID..REPLY ASP ,, MA MAIL ID IS SOOD986@GMAIL.COM

If you had read my post carefully, you'd find that I mentioned that doing this will take a long time and I do not have the time to do it. I'm sorry but there it is. This is not a simple thing that you're asking.

You can search for an explanation about PID on the internet and, from there, develop your ideas (that's what I did before).

regards,

Vulcan

There should be a built-in PID block and then the TI should come into the analog card or perhaps an RTD card and will be the PV for the PID block. Set point goes into the PID block, compares with the PV, generates an error and based on your tuning parameters, changes the output to the valve. The 0-10000 I would imagine is the counts that the PLC uses (similar to AB 819-4095?).

Like PW says, the tuning of your valve (and the sizing of it and the process around it) will determine the accuracy. Sometimes your tuning can't solve your sizing problems.

Vinay,

I run the risk of being a spoilsport but..why do you need to go the PLC way...?

There are plenty of temperature controllers (For eg.Eurotherm or Omron,...) who are PID based controllers and will happily give you 4-20 ma output.

It would be cheaper and more reliable...unless you have PLC in your inventory which you want to put to use.

cheers.

dear chaterpilar,

thanks for showing interest.

As i mentioned in the earlier post that my device already has a plc (Which is a MUST for the application and the plc is capable enough for this extra burdon) and i am trying to eliminate some expenses ( about $3000) for the PID controller...

i hope that answers your question.

regards

Do you really need a PID controler ? Perhaps is a PI controler also enough....

Hope that you have solved the problem. I am curious about your statement that 4 ~ 20ma output to control that solenoid valve ( 3~ 15PSI). If i am not wrong you are talking about Valve Positioner or an I/P transducer. Also, i would like to say that standalone controllers for Temperature control applications are not expensive as you mentioned. You can buy them for few hundred dollars. However, i agree with you to implement the temperature control in your existing PLC system. RANGANATHAN

Hello.

We have many machines doing exactly that, same plc, same molules and same type of valve and ip transducer for steam heating. The ladder logic is fairly simple especially is u dont intend to use a controlled gradient in temp. raise.

Anyway i will be happy to help u if you still needed. Im not registered yet in this forum so for now u can write me to oficina.tecnica@incatops.com is a textile company in Peru where we use this pids algorithms to control dyieng recipes. Im trying to improve the temp control so maybe we can help eachother.