PID question

Stromberg Knives

Stromberg Knives

strombergknives.com
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Jan 3, 2015
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Hi guys!

I'd like to pick your brains on a matter regarding PIDs.

I have several of them, I use them for controlling temperatures in tempering, Kydex heating, oil heating etc.

My question relates to the PID used for the temper oven. It's an ordinary China PID, nothing fancy, but it works. It's got a TC mounted in a aluminum plate for a more even temperature reading (as per Stacys recommendation).

Though I need to use the oven at different temperatures for tempering, heating Kydex, curing wood stabilizer etc. I've found that the PID doesn't really like to switch between different setpoints. It takes really long time for it to acclimatize to the set temperature.

What would be the best way to remedy this? Are there PIDs with settings for multiple setpoints? If not, should I autotune at every temperature setting?

Thanks!
 
You have taught it to run at a certain temperature, and when you switch to a new one it is clueless. Run an auto-tune at the new temp to give it a chance to learn the new parameters.

If you use a programmable controller instead of a PID, you can set it to any temp and it will do the control pretty smoothly ... but they cost more. The plus on these is many have multiple memory settings, so you can run a program for kydex, one for 425°F HT, and one for warming a bagel.
 
PID as in PID controller I guess? If yes:
It depends on your system but in general a PID controller consists of P(roportional), I(ntegrating) and D(ifferentiating) part(if you want details -> wikipedia) that all react differently to a change in the actual value.
A good PID controller(what is probably what Stacy E. was talking about) can be parameterized/programmed to take every value and switch between them in the fastest way possible to the system. But programming it isn't an easy job at all; so you either have to get someone to do it for you, or get one that's already tuned correctly for your system.
Or you can get a PLC with analog in/outputs for a few hundrer dollars and tweek every last bit.
 
Okay!

So you guys don’t know any controllers that can be calibrated for multiple temperatures?

Otherwise I might build another cheap PID controller and connect to the same oven and run one at the time.
 
Hm, what about the controllers in the heat treat ovens? I've got a Paragon KM24T and when running a program with several ramp and soak steps the temperatures doesn't fiddle around as much as my PID controller. Sure it overshots a couple of degrees and then steps back.

How come the controller in my oven is that much better at this than my PID controller? And without auto tuning.

I the simple answer that my cheap China PID controller just doesn't cut it? The old, you get what you pay for, maybe?

It would be great to find a temperature controller that could handle different temperatures without lengthy auto tune and stabilizing processes.
 
currenthill said:
It would be great to find a temperature controller that could handle different temperatures without lengthy auto tune and stabilizing processes.
Click to expand...

I thought some, or most, had the ability to save each as a "program." Auto-tune for 400C and save, program #1, Auto-tune for 800C and save, program #2, etc. Then when you want a specific temperature, you call Program #X rather than setting a temp and allowing the PID to find it.
 
Have you tuned the PID controller(s) to the process(es)? This is usually done with the controllers autotune function, but can be done old-school. I remember using the Ziegler-Nichols method on pneumatic PID controllers back in the 1980s. It was a lot of faffing about. Personally, if I had a piece of equipment fitted with a PID controller that lacked autotune now, and needed to tune it, I'd replace the controller and use the autotune.

PID controllers come with a default set of PID parameters loaded. This is intended as a get-you-started setting: it will sort-of-work for almost any process you are likely to want to run, but will be almost always be way less than optimum. The assumption is always that you will run the autotune for the process.

The process itself has a greater bearing on the "correct" PID parameters than the temperature alone. A 2 kW heater heating 5 gallons of quench oil will have totally different characteristics to a 3 kW element heating half a cubic foot of air and ten ounces of blade in a well-insulated Heat-Treat oven and will need different PID values.

The output cycle time is often a significant factor and this is not normally something that the autotune has any control over. As a rule, it should be set as short as the process will reasonably stand. For a relay output this might be 30 seconds and is not usually less than ten seconds. For an SSR output, 5 seconds is usually a good place to start. SSRs can switch much faster than electromechanical relays or contactors.

It may be worth mentioning that there is usually a damping or filtering setting in the controller. It is not unusual for this to be set by the equipment manufacturer to filter/damp over a long period, making the process seem much more stable than it actually is.

Where a controller is supplied as part of an oven, it will have been tuned to that oven and really ought to perform well.

The AutomationDirect Solo-series and Omega 7200-,7500-,7800-series ramp/soak controllers (as far as I can tell, they are the same controllers with different badges) all have 4 tuning setpoints and can be set to automatically select the PID settings from whichever tuning temperature is nearest to the setpoint.
 
kuraki said:
I thought some, or most, had the ability to save each as a "program." Auto-tune for 400C and save, program #1, Auto-tune for 800C and save, program #2, etc. Then when you want a specific temperature, you call Program #X rather than setting a temp and allowing the PID to find it.
Click to expand...

That's interesting. I need to look into this. Unfortunately the supplied manual/instruction sheet is the kind that makes you giggle (a.k.a A brave attempt of auto translation).
 
timgunn1962 said:
Have you tuned the PID controller(s) to the process(es)? This is usually done with the controllers autotune function, but can be done old-school. I remember using the Ziegler-Nichols method on pneumatic PID controllers back in the 1980s. It was a lot of faffing about. Personally, if I had a piece of equipment fitted with a PID controller that lacked autotune now, and needed to tune it, I'd replace the controller and use the autotune.

PID controllers come with a default set of PID parameters loaded. This is intended as a get-you-started setting: it will sort-of-work for almost any process you are likely to want to run, but will be almost always be way less than optimum. The assumption is always that you will run the autotune for the process.

The process itself has a greater bearing on the "correct" PID parameters than the temperature alone. A 2 kW heater heating 5 gallons of quench oil will have totally different characteristics to a 3 kW element heating half a cubic foot of air and ten ounces of blade in a well-insulated Heat-Treat oven and will need different PID values.

The output cycle time is often a significant factor and this is not normally something that the autotune has any control over. As a rule, it should be set as short as the process will reasonably stand. For a relay output this might be 30 seconds and is not usually less than ten seconds. For an SSR output, 5 seconds is usually a good place to start. SSRs can switch much faster than electromechanical relays or contactors.

It may be worth mentioning that there is usually a damping or filtering setting in the controller. It is not unusual for this to be set by the equipment manufacturer to filter/damp over a long period, making the process seem much more stable than it actually is.

Where a controller is supplied as part of an oven, it will have been tuned to that oven and really ought to perform well.

The AutomationDirect Solo-series and Omega 7200-,7500-,7800-series ramp/soak controllers (as far as I can tell, they are the same controllers with different badges) all have 4 tuning setpoints and can be set to automatically select the PID settings from whichever tuning temperature is nearest to the setpoint.
Click to expand...

The PID controller is (auto-)tuned to the main process (tempering). I would say the controller has a pretty good understanding of the ovens heating capabilities after this tuning. It works rather well. But I haven't autotuned at other temperatures, because I thought it would overwrite the tuning made for the first autotune temperature.

Yeah, I get that a controller supplied as part of an oven is tuned to it, and that's exactly what I'm trying to do with the cheap PID controller. But it seem to want to tune to just one temperature whereas the Paragon doesn't mind which temperature I set, it gets there nice and smooth.

But thanks for giving me a couple of ideas and leads to try out!
 
I'll put it like this: A PID Controller is a block with about 15 variables and analog logics, so, very complex.
When you buy an oven with integrates temp controll all these variables are put exactly to the right value, by the guys designing the system.
If you autotune or buy an all-in-one solution, they may be in the ballpark of where they need to be, but they're far from perfect. There's a reason that the programming of PIDs is a very high-paying job.

And if I understood you correctly you've got an oven with integrated controller that you've also put an external one on? If yes: don't; they'll(or more it's likely that they will) fuck with each other big time
 
SV-97 said:
And if I understood you correctly you've got an oven with integrated controller that you've also put an external one on? If yes: don't; they'll(or more it's likely that they will) fuck with each other big time
Click to expand...

2 ovens. He's comparing the factory controlled one to his lower temperature DIY PID one.
 
SV-97 said:
I'll put it like this: A PID Controller is a block with about 15 variables and analog logics, so, very complex.
When you buy an oven with integrates temp controll all these variables are put exactly to the right value, by the guys designing the system.
If you autotune or buy an all-in-one solution, they may be in the ballpark of where they need to be, but they're far from perfect. There's a reason that the programming of PIDs is a very high-paying job.

And if I understood you correctly you've got an oven with integrated controller that you've also put an external one on? If yes: don't; they'll(or more it's likely that they will) fuck with each other big time
Click to expand...
I'm completely happy with the precision of my cheap PID controller for the low temp use in my oven (i run it between 90-250 degC).

I'm just trying to get it as precise at more than one temp, without having to wait for several hours for it to stabilize.
 
currenthill said:
I'm just trying to get it as precise at more than one temp, without having to wait for several hours for it to stabilize.
Click to expand...

Yep, that's it - most all those lower end (cheap) PID controllers do not have the ability to save different programs, which is what you're looking for. I don't think Auber has multi-program controllers in their lineup of controller. I did a quick search and didn't find any other than the controllers that are available with HT ovens. I suspect you'll be looking for a controller costing several hundred bucks. Less expensive might be to have separate controller for each function, and just switch controllers in the same oven.
 
You’re right. I found a couple of premium ones that could do Dual Setpoints.

But as you said, the more cost efficient solution would probably be to have several controllers.

Another solution would probably be to get a midrange controller with better auto tune and fuzzy logic than my super cheap china clone. It may not take me all the way, though.
 
One thing that can really affect how well the PID will hold temp (especially when shifting around between temps) is what soak you have in the body of your oven/kiln. So for instance, even if you autotune, things will shift a lot less if the kiln has a better soak in it. One thing I noticed on my kiln is that if I want to hold really stable temps - as measured on the blade - below 800 without getting a lot of extra pump from the IR from the coils it works better to soak the kiln at a much higher temp (1200 or so) then let it cool back down to my tempering temps. Then turn the PID back on and let it stabilize for about 30 minutes. Since I started doing an "overshoot soak" like that, I started getting a lot more stability at the lower tempering temperatures. Doesn't swing nearly as much and stabilizes much more quickly because of all the thermal mass in the kiln walls. A lot of the wandering around the controller does at lower temps are from the IR pulse from the coils (at least on my kiln).
 
After some thought, I'm going to build an enclosure with three PID controllers, connected to the same oven and that will just work one at a time. So I auto tune the temp per controller, for example; one for wood, one for Kydex and one for tempering steel.

The build is nearly at no cost, so it'll be a fun small project.

HkXlY8s.jpg
 
I built a similar unit to yours a good while back. I found that there wasn't sufficient variation in control, so the separate controllers weren't really necessary, but the triple reading was useful. It also allowed switching to a different controller if I had a suspicion that one wasn't running right.

I would suggest not use the rotary switch to select the PIDs. Instead, wire all with separate switches ... or just one power switch. Normally, turn all three on. That way you can put the desired PID in the RUN mode, but leave the others in the IDLE mode. You will get three separate readings of the oven temperature. This will clearly show any variance in a bad TC or PID.
 
That's an interesting suggestion, I thought about it early in the process, but decided against it. I wasn't sure the SSR would get confused (or even get damaged) from three controllers simultaneous controlling it.

But I'll give it some more thought, thanks!

Edit. Maybe I should put the rotary switch between the controllers and the SSR, as a safety measure. That would still make it possible to run all three controllers at once, but only one controls the oven at a time.
 
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