More PID?

Last summer I started a thread about sliding mode control. This summer I may start one on PID depending on the response. What I propose is using the same system as last year's. ( Ron Beaufort's hot rod system ). I still have the model I can implement a PID instead of a SMC to control it. I have noticed that there are many that are not familiar with the Micrologix's PID. In addition, RSLogix has a trend capability that can be most informative and can be used to show how to model a system. The RSLogix trends can be loaded into Excel and Excel's LINEST instruction can be used to model the system. I have mentioned that LINEST can be used to model a plant about 3 or 4 times over the last year, but no one seems interested enough to try.

I would like to help.

Peter,

I have a Micrologix 1500 with analog I/O cards and would be interested in helping if like.

Thanks,
Bob

Need actual systems to verify results.

Bob, where I could use some help is with finding actual systems to verify the results on. I don't have an analog in or output card. I will simulate the different plants in software.
Maybe you have one or can setup an clothes iron with temperature feedback. You may need to take the analog control signal can convert this to a PWM to switch on and off AC. It is best to have a system with relatively fast time constants so we can verify the test quickly. Last summer I used the data from Ron Beauforts 'hot rod' system which is actually just a soldering iron or wood burning iron. That worked well.

I won't need this for a while.

Test System

Peter and All Members,

I will have to create a system. I don’t have a TC card for the MicroLogix so I will try and locate some temperature transmitters and a good heat source. Once I get something put together I will post pictures of the test system.

Thanks,
Bob O

The hot rod system I made for testing the SMC control last year was a $1.89 hot melt glue gun from Hobby Lobby and a thermocouple stuck underneath the silicone sleeve that holds the glue stick. I also placed a second thermocouple on the outside of the sleeve so that I would have a delayed temperature point. I works very well and I use it frequently to test temperature control methods that I come across.

This summer I may start one on PID depending on the response.

Click to expand...

Mr N.

I'm interested!

Anything thing you have to say, or teach,
gets my full attention.

I am working on it.

I need to get more orgranized before I start. I am planning an outline and what must be done at each step. I plan to go slow and drag this out through the summer. Once I get started I plan to make one major posting each weekend. I have decided to do much on an excel spread sheet because Excel is more available than RSLogix. I am choosing RSLogix because of the trend feature that allows real data to be included in excel or copied into bitmaps for all to view. The RSLogix programs will be necessary to verify the techniques on real systems.

Peter,

Right now I am still learning about PID and in my laptop already installed RSLogix500 and I have Micrologix1500 demo box, of course I will following your instruction...go ahead Sir..!

Deer

Please keep it simple

I have allot of temperature PID loops in my plant. And PID is probably one of my greatest weaknesses.
I have been trying to get a complete handle on the topic for some time.
All I ask is that the explanations are keep simple for us slower people with a non-math brain.
Thanks

the Hotrod is available ...

Greetings to all,

Peter, if you want to run any of this stuff through the Hotrod, just let me know ... I haven't done any PID classes lately so it might take me a couple of days to get it set up and ready but it won't be any trouble ...

for those of you who have been talking about using your own devices, I'd sure like to see whatever Peter comes up with applied to more than just my one system ... the more applications, the better ...

may I make a suggestion ... go ahead and fire up your systems and make some graphs of their responses ... this can be done completely in manual mode (as in: without any tuning at all) ... if you'd like some examples, I did this for the Hotrod in two or three posts at the beginning of this:

old thread which includes Hotrod trends

this will give everyone a much better idea of all of the systems' characteristics ... it will also get everyone ready to graph the final results of whatever Peter finally comes up with ...

one more thing ... PLEASE change the colors on your graphs from the "dark-blue-on-black" default settings ... it's really hard to see what's going on once the graphs have been converted to JPG formats ... that's why I finally started doing mine as dark colors on a white background ... it's easier to look at ... and easier to print out paper copies too ...

as always, I'm looking forward to learning something new ...

All,

I was thinking about building a box out of Lexan and installing a heating element inside along with temperature feedback of some sort and then having a small fan mounted to the wall of the box to change the load on the heating element. All feedback is welcome.

Thanks,
Bob

Bob,

before I designed the "Hotrod", I built a "Hotbox" similar to what I think you have in mind ... I used a handy junction box instead of Lexan though ... my heating element was nothing more than a 150 watt lamp bulb ... I eventually had to stick a little muffin fan inside the box to distribute the heat around - I was always getting very erratic results without that minor detail ... I also screwed the thermocouple right to the side of the box as I think you have in mind ...

this whole rig worked fine ... I could tune it and get good control ... EXCEPT ...

the oscillation was something like twenty minutes per cycle ... since I was developing this for student hands-on exercises, this was unusable ... specifically, once a student made an attempt to tune the heat loop, he had to wait at least three cycles (or a total of one hour) before he knew whether his tuning was heading in the right direction or not ... there was absolutely no way to fit this application into a two-hour lab session ... the poor guys would only have been able to manage (at most) two tries per one week lab period ... I've been accused of being sadistic before but this would have been extremely cruel and unusual punishment ...

anyway ... I'd go ahead and try the rig you mentioned ... just be aware that you might have to spend some extra time to baby-sit with it ...

finally ... you might want to take a quick look at the Hotrod's cycling in this:

old post with Hotrod cycling graph

this might give you an idea of what I'm talking about ... as I said there:

My calculations indicate that this system has a period of 3.54 minutes per cycle.

Click to expand...

that works out fine for student experiments by giving them a response which they can gauge in about ten or fifteen minutes ...

Bob O,

For 2 months I've had a 1 ft cube of aluminum and lexan for just that purpose but just haven't had time to get it put together. I did do a desk top test and as Ron says, the fan is a must. Think about it, the bulb can heat up relative fast but turn it off and cooling isn't near as fast. The fan will provide some ompf (highly scientific term there) in the opposite direction.

The basic system I'm setting up has a 90 Volt DC drive to control the bulb and a thermister for temp feedback. The thermister is non-linear but real inexpensive. I think I'm actually going to set up a bridge with 2 thermisters to add the signal.

I'm using a Moeller PS4-341-MM1 for control. It actually has quite a bit of computing power to it. It will be fun to compare the canned Autotuning in the Moeller with what Peter comes up with.

Now that Dr. Nachtwey has started this thread, I have to finish my setup. ... in my spare time...

nOrM

I can contribute a larger scale process model for temperature control, if needed. It would involve group participation and give me the opportunity to add PLC control to my house. It's my sauna. The problem is that like Ron's 'hotbox' it will take considerable time per cycle and this is where the group participation would come in...you guys could provide the beer. beerchug

Greetings, Norm,

just to be specific ... the fan I mentioned in my post above was installed on the INSIDE of the box ... it served to stir the air around ... without it, the very top of the box got hot really quick ... but the sides and bottom stayed pretty cool ...

you are totally correct though about needing ANOTHER fan on the outside of the box to provide a reasonable load on the system ... it takes quite a bit of experimenting to get a system that will both heat up and cool down at reasonable rates ... you might have to work with how the "load" fan is positioned ... and how it's angled ... and you might even need to rig some type of damper on it to limit the amount of air it moves ...