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Old June 15th, 2017, 07:53 AM   #1
MrRobot2you
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SMC vs PID control

Hi all,

I read an old thread on here about sliding mode control (SMC) and how it should be considered more in temperature applications as apposed to PI control. I was wondering if anyone has put any effort into implementing this and how it compares to a PID controller. I assume the time to get the controller coded and working would be a lot longer than just placing a PID block.

Basically just looking for discussion on the comparison between PID and SMC control. If it is worth the effort, pros and cons and the likes.

New here and loving the community and threads.

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Old June 15th, 2017, 01:49 PM   #2
RonJohn
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Welcome to the forum! Kudos for searching through old forum posts; there is plenty of good information to be found using the "search forums" tool.

I implemented SMC on a steam/water heat exchanger controlling a steam shutoff valve a couple of years ago (digital output to an air solenoid). This was on an A-B PLC5 that had no analog outputs available.

If I have analog control available, I would probably go with the PID instruction. However, in my circumstance, the SMC has worked quite well and I would recommend checking it out. It never hurts to acquire another "tool" for your experience/knowledge base. I have found a few other applications in our plant that I'd like to try SMC on, if I ever get the time!

A couple words of advice from my experience:
- My calculated gains were way too aggressive (probably due to my miscalculation). I had to play around with that a bit to get the desired response.

- I do get occasional "chatter" in my solenoid output. Make sure your control output and the downstream hardware can handle this. We had to change our heat exchanger gaskets to a more heavy duty type after blowing through the regular gaskets more often than desired.
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Old June 15th, 2017, 03:23 PM   #3
rupej
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I've also been interested in SMC but haven't been able to find any information on it anywhere, even basic info. Would love to get a link or have someone explain it!
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Old June 15th, 2017, 03:41 PM   #4
Bob O
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Rupej...See if this helps.
http://www.plctalk.net/qanda/showthr...&highlight=SMC
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Old June 15th, 2017, 04:05 PM   #5
RonJohn
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rupej, not sure how familiar you are with the "search forums" tool here. In case you're not, look for the menu bar on this website - it has the following items:

Profile FAQ Calendar ... Search QuickLinks LogOut

Click the down arrow next to Search and enter in "Sliding Mode Control". You'll see about 3 dozen topics to choose from. There are several helpful topics in that list. This one in particular got me started in using SMC. It has an example in either SLC or PLC5 format buried down inside the thread. Lots of posts to dig through but worth the time.
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Old June 15th, 2017, 11:14 PM   #6
Peter Nachtwey
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Most of you know that I/we make motion controllers. We have not implemented SMC in our product yet but we have written user programs in our motion controller that do SMC and it works well.

SMC is much different that PID. PID is all about where the poles and zeros are. SMC is about "sliding" along a path or what some call a manifold to where the error is 0.

SMC does not like dead time but if you add a Smith Predictor there is no dead time and SMC works well
This video about using SMC with the Smith Predictor is the most viewed video on my channel. I think it is because it is original content.
https://www.youtube.com/edit?o=U&video_id=uhLMyOlwCoM

SMC will also work for motion control and non-linear systems.

There is also a recent thread on eng-tips.com where I am giving a person that claims to be an associate professor grief for his lack of experience and misleading people. Actually he is just plain wrong. He challenges me to control a system that is unstable. I show him how to do it with SMC. No problem.
http://www.eng-tips.com/viewthread.cfm?qid=425421
Students aren't allowed to post on this forum but you guys may find it entertaining. I pitty his students. This is truly sad.

Again, I don't expect anybody on this forum to be a control expert but when an associate professor wants to make a fool of himself I will oblige.


I know that most of you are taught the integrator gain does this, the proportional does that and the derivative gain should only be used when necessary. This is OK if you are just trying to make a system work but it NEVER should be good enough for engineers.
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"Living is easy with eyes closed, misunderstanding all you see...." Strawberry Fields Forever, John Lennon
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Old June 19th, 2017, 04:39 AM   #7
MrRobot2you
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Thanks for all the replies. In my current job I'm writing a 4-20 mA signal to a control valve for the temperature control. Definitely want to try get stuck into SMC for the next ideal application.

Thanks for the links. Making my way through them when I have time.
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