PID Help

It has been a long time since I've posted, and you guys were a lot of help then... I am back for more.



I am still working on a graduate project involving an AB1200 and temperature control. I have done a lot of research and received help to get me where I am now. Thanks!!! I finally have grasp on PID.



I have also revised my project. I still am trying to create a PID Temp control system for educational purposes but now even more basic.



I am using an AB Micrologix 1200, an analog module, a data logger to observe my results, a TC as my input, a resistor as my output, and a small plastic canister as my oven.



I have information on how to use the analog module and the TC to scale my input. My next step is to implement this into a PID function block. I have read the manual… and a lot of help that was… can anyone give me further assistance on building this function block and code to references to create the code I will need


Thanks,



Matt

First, keep your posts on the same topic in the same thread. Otherwise, people will lose track of what you're talking about.

Second, how do you intend to control the output relay for temperature? (Digital output with PWM, analog output)?

I'm not sure about the size or requirements of the load, but I would venture to guess that you will need additional circuitry to control power to the load. Has this been designed yet?


As for the PID, you mentioned that you read the manual. You can probably skip over some of the extra features of the PID block (manual mode, ...) for this case and just go with a simple controller.

You are on the right track that this control can probably be done with a PI loop. That just involves leaving the kd term = 0 and entering other values for Proportional and Integral. There are several tutorials on here for how do do that.

Ron Beaufort has posted 3 excellant dissertations on PID
They are entitled What is X in PID
X can be substituted for P or I or D.

The Allen Bradley manual can be hard to read and get an understanding from let us have no doubts about that.

MAKE SURE you have no interferance on your thermocouple that will SURELY throw off any control algoritym you use.
(I learned all about that the HARD way - never overcame the problem due to lack of equipment).
Run all power conductors as far as possible from the TC and shield them (in my case it was the heaters themselves "radiating" to the TC).

I have not used the 1200 so I cannot comment further.

Dan Bentler

Greetings Matt,



you’ve mentioned using a plastic container as an oven for your process - and a resistor as the heater ... first question: is there any chance that this thing will get hot enough for a “meltdown” situation? ... how about using a thin metal container? ...



next: what will you be using to simulate a “load” on the system? ... if the only load is “heat escaping to the room air” then you’re not going to be able to learn as much about PID as you might expect ... basic idea: once the system has stabilized at the setpoint, the next logical step is to increase (or decrease) the load on the system and see how well the PID controller can recover ... secret handshake: if the load never changes, then you don’t really need PID ... just experiment to find the “magic number” that gets the system “on target” and then manually dial it in ... that doesn’t sound like a lot of fun does it? ... so how about a small computer cooling fan? ... try mounting it to blow across the oven and varying the load by blocking the air intake with various sized cardboard dampers? ...


once you’ve got the process nailed down, then hold off on the PID control for just a little while ... instead, manually crank in some values for the controller output and try to graph a “system response” curve like the one shown in this post ...



post a picture and let us take a look at it ... we’ll probably have some more suggestions before you really dig into the PID subject ... basic idea: some “experimental-just-learning” processes are just “too easy” and some are just “too hard” ... others are downright impossible to control ... there’s no sense in spending time and energy barking up the wrong tree ... we’ll be glad to help guide you in the right direction ...



next suggestion: read this thread ...



I haven’t heard from this guy lately ... but most (all?) of the advice I offered him seems to apply to your situation too ...



final thought: sounds like fun ... if I’m invited, let’s get started ...

Thanks for your help guys. Please excuse my slow reply, but I am working on this project part-time while I am working a full-time job.

Ron, thanks for the mass amount of information. I would appreciate any assistance you can provide me with... welcome to the team.

I ran an experiment this morning to get my feet wet and attack this project a piece at a time. I hooked up the analog module, the AB Micrologix 1200, and a TC (Omega Type K )to get the analog input portion of the project simulated. I used RSLogix to created working code using the SCP and GRT functions to turn on an output when the TC sensed it at 25%. I could not get a valid reading from the input though... just of noise.

i suspect that it is because this TC is producing millivolts and the analog module is looking for volts. Do you have any suggestions on how to rectify this or of another input device?

Thanks for your suggestions,

Matt

Why are you using an analog module instead of a thermocouple module? (catalog # 1762-IT4)

This is more than likely what you really need. And for your dining and dancing pleasure, here's the 144 page manual...

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1762-um002_-en-p.pdf#xml=http://127.0.0.1/texis/search/pdfhi.txt?query=1762-it4&pr=literature.rockwellautomation.com&prox=page&rorder=500&rprox=750&rdfreq=0&rwfreq=0&rlead=250&sufs=1&order=r&cq=&id=42d8a79ea

Be sure to do the ENTIRE CONFIGURATION (including the "enable") for your particular thermocouple type.

Matt,

I'll look into the cost of the theromocouple module for the lab. I too want to thank all of you for your help in his project.

Thanks Jimmie for the suggestion,

Matt's Professor -- John

Greetings Matt,



I’ll assume (gosh I hate that word) that your input module accepts a 4-to-20mA signal ... if so, then another solution (other than buying a special thermocouple-type input module) is to buy a “thermocouple transmitter” which will change the thermocouple's millivolt output into a 4-to-20mA signal ... just as an example to help get you started, take a look at the TX-91 series from Omega shown here ... if you decide to go this route, be sure to match the model number you order with the thermocouple type and the temperature range that you’ll be working with ...



note: there’s really nothing wrong with using the module that Jimmie Ohio mentioned ... in fact, it might end up being even more educational than using a separate transmitter ... but since you asked, I’m just offering one other possible solution for you to consider ...



also ... I raised a couple of specific issues (questions) in my earlier post ... I know that you’re busy trying to juggle both work and school (been there and done that) ... but ... we’ll be able to cover a LOT more ground throughout this project if you’ll be careful to answer my (and everyone else’s) questions as we move along ... we can’t be very specific with our suggestions if we’re not sure exactly what we’re working with ... a few extra minutes of typing can save hours (if not days) of needless work ...

Gentlemen thanks for your information. I have gotten your replys and are looking into them. John, I know you are looking into the TC module but, I will also explore the options Ron has discussed also.

Thanks for the manual jimmie. I will make sure that I follow all the steps to set it up.

Ron and everyone else, please excuse my slow replies and non-reply to some of your questions. I will look at those again tonight and get back to you guys tommorrow morning.

Thanks

Matt

Please excuse the delay but here is the information I promised you guys.

My output is based on the limitations of the PLC. At this point, I am planning on using a simple resistor as my output with analog control. I think the current draw of this is minimal enough.. I will have to test it and see. But if is too much I will use additional cicuitry. So, theroetically the more volatge I send through th resistor the hotter it should get and vice-versa. If anyone foresees a problem with this please let me know.

I plan to show the output with a data logger so I get pretty graphs. When I am at this point, I will figure out how to show you guys them.

The plastic container idea was to try to keep the project easily repeatable and simple. But, once I get the parts assembled and at least operating, I will look into a nicer box. But until then I will be careful not to have a melt-down as interesting as it may be.

For a load on the system, I am planning on using a computer fan to blow across the system. The cardboard baffles to limit the load are even better. Thanks.

Thanks for all your help and suggestions. Please keep up the feedback it is extremely helpful.

Thanks

matt

I don't understand the problem

leitmotif said:

PLCRugger said:

t if you have a data logger handy use that - it will give you a
permanent record of all selected parameters and it will not be lost - I found it easy to lose the data with trend.

Dan Bentler

Click to expand...

The RSLogix trend files are just .dbf files. They are save like any other file. I have never had any problems with them. Ron Beuafort had no problem sending me these trend (.dbf) files and I had no problem using them to build a model of his hotrod system.

Click to expand...

Ot

Dan on each post in the bottom right is a "quote" button, if you click on it a reply window will open. You can edit any part of the quote but remember that quote and /quote must be at the beginning and end.

I am not complaining, its just that sometimes your replies blend into what someone has posted so it makes it harder to read.

Here is an update on the project.

The department has ordered the TC module and it should be here in a week or two(out of stock). I am going to try to have everything ready so when I get it I can quickly get some more information to you guys.

I will look into the possible use of the trend in RSLogix, but more than likely I will use the data logger because we know how to use it already and probably once the system is up and running.

I know you guys are used to this whole thing going a little faster but with working full time and getting married... tomorrow... I am working on this part-time. So, thanks again for the help you are really adding in my understanding and ablity to move farther along in my project.

Later

Matt