Wiring a PID for Your Tempering Toaster - Full Walk Through

J. Keeton

J. Keeton

Joined
Nov 15, 2005
Messages
1,201
This guide is a direct result of the help I received in this thread: https://www.bladeforums.com/threads/pid-toaster-preliminary-questions.1647527/

As with most of my DIY knife projects, I like to post a summery / tutorial when I'm finished so someone else looking to do the same thing can find their answers in one place.

Purpose:

The goal of wiring up a PID controller for your tempering toaster is to be able to maintain a tighter temperature band in the toaster. The stock controllers tend to be very inaccurate and also tend to "walk" on you... Meaning that you can put a knife, set your toaster for 400 ... but end up with a 425 temper.

Here is an example of the stock toaster VS the PID (NOTE that I'm not done tuning this toaster oven PID yet... Still working on the settings):



Required Items:


Cost:

My cost for this build was around $54.40. Like I said, if I had to do things differently I would have spent an additional $20 on a better controller (may still do this down the road) and I would have bought a plastic enclosure off of ebay.


Wiring:

I go into extreme detail on the wiring of this PID controller in the video below, but here is the diagram:



The Temper Cage:

Based off of advise from Stromberg Knives Stromberg Knives and Stacy E. Apelt - Bladesmith Stacy E. Apelt - Bladesmith I built a tempering cage for the knife. The termocouple is threaded into the plate on the bottom of the cage. It goes 1/2 of the way though the plate. The cage also has a roof to shield the knife from the elements.

Another feature of the cage is that it sits in the middle of the rack with gaps on all sides. This is done so that the hot air can circulate around the cage.





Tuning:

With a legitimate controller you should be able to utilize the auto-tuning feature. Since mine was garbage I'm having to use trial and error to tune it. I'm getting closer every day... I'm painstakingly making a change on the PID then recording the ramp manually.

Currently my best results are at a P of 70 an I of 400 and a D of 30.

I found this document very helpfull: https://web-material3.yokogawa.com/Adjusting PID Manually (short).pdf

Edit (7/2019) : Tuning issues resolved towards the end of page 2 and the start of page 3 of this thread. @Cushing H. is a life saver!

Video Walk-though:


If anyone has any suggestions on how to make this system better please let me know. Or if I missed anything in this description.

Also, I can't overstate how helpful the participants in the original thread were to the sucess of my PID! Major shout out to Stacy E. Apelt - Bladesmith Stacy E. Apelt - Bladesmith Stromberg Knives Stromberg Knives @Ken H> R Randy3000 @timgunn1962 L Lieblad and anyone else I may have missed!

Cheers,
JKeeton

Edit: Before rigging up the PID I took the toaster apart and lined the shell with some spare Kaowool I had from my forge build.

 
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So here is a tuning update... been messing with the settings and recording the data all day:


Findings:


Getting aggressive with P makes big changes. I went from 70 to 75 and it over shot the temp by 12 degrees as seen in Figure #1 (Orange)

*Figure #1


Not sure what my conclusion with changing I is... but here is my data thus far zoomed in. The Green, Yellow, and Light Blue all have a P of 70 but I varied the "I" (350 , 400, 650 respectively)

*Figure #2


I ran a specific test on the I component with a P of 60 to see the difference. Looks like a smaller I will dampen the aggression of the peak and then slowly rise before leveling off.

*Figure #3


Lastly here is a comparison of P values when holding I and D flat. I was held at 400, D was held at 30. The Orange curve is at a P of 75, the Blue curve is at a P of 72, the yellow curve is at a P of 70, and the Brown curve is at a P of 60.

*Figure #4


I'll keep working it, but in the short term my yellow curve (P70-I400-D30) is the one that seems best to use. It will get up to 391 F within 45 min and hold flat. I can adjust the set-point to bring that up some when targeting 400F.

If anyone has any tuning suggestions, I'm all ears!

Cheers,
JKeeton
 
If anyone has any suggestions on how to make this system better please let me know. Or if I missed anything in this description.
Click to expand...
For its main on-off, Its better practice to use a double pole switch connected to both conductors, active and neutral.
Its good practice on any plug connected appliance, and sometimes a requirement.
47344638542_9352048845_c.jpg
 
So I just had a thought... and want yall's opinion on this. I just re-ran the Auto tune on the controller. It's plotted as the PURPLE line below.

Musing the thought that just popped in my head:

This whole time I've been reading the temperature off the probes I have sitting in the rack resting on the plate (As seen in the picture below). Do yall think that since these probes are "open air" they're showing a larger swing then what the PID's thermocouple screwed into the plate is actually reading? Maybe the plate is actually being held at the 397 F set-point and this Chinese controller isn't trash after all!?! :eek:

Would it be better to drill completely though the plate and have the TC tip right at the level of the plate itself and exposed to the air?

*Figure 5


*Testing Set-up


Any thoughts appreciated!

Cheers,
JK
 
why two tc's I only run one. if they are both on the same controller then it could create a problem. both being the same calibration wouldn't seem likely. I use a rex c-100 and the probe is screwed into the bottom plate. it rises around 10c above set point and then it settles to stable at set point, after that it stays stable within 1 deg c. and I checked it with analog thermometer as well.
 
No expert here, but I would think that you would want the control thermocouple tip in air, since air is the transfer medium for the heat. Having it slaved to any kind of heat sink is going to slow the response time of the controller by giving it a reading that is almost always lagging behind the actual air temperature in the oven.
The control TC should be the most heat sensitive thing in the oven, you want it reading air temp in the vicinity of the blades/parts being tempered. Your test TCs can be used to verify the reading of the control TC in air, and can also be used in contact with the cage/blades to measure actual temps of objects in the oven.
 
kn4wd said:
why two tc's I only run one. if they are both on the same controller then it could create a problem. both being the same calibration wouldn't seem likely. I use a rex c-100 and the probe is screwed into the bottom plate. it rises around 10c above set point and then it settles to stable at set point, after that it stays stable within 1 deg c. and I checked it with analog thermometer as well.
Click to expand...

Those 2 probes on the rack are not associated with the PID. Just there to monitor the temperature.
 
Justin points out a problem I may have caused.
I used to say to place the TC in an aluminum plate under the knife. At the time I first did that, the TC/PID was a read-out device. I later made it a PID controlled device, and it seemed to work fine. I soon went to another type oven for tempering and got rid of the toaster oven conversion. I have realized that the issues with temperature swing as the PID tries to "learn" the control parameters is caused by the plate being too much heat sink for the control program to figure out. Placing the bare TC tip in the air right above the blade would be far better .. as Justin pointed out.
 
Justin King said:
No expert here, but I would think that you would want the control thermocouple tip in air, since air is the transfer medium for the heat. Having it slaved to any kind of heat sink is going to slow the response time of the controller by giving it a reading that is almost always lagging behind the actual air temperature in the oven.
The control TC should be the most heat sensitive thing in the oven, you want it reading air temp in the vicinity of the blades/parts being tempered. Your test TCs can be used to verify the reading of the control TC in air, and can also be used in contact with the cage/blades to measure actual temps of objects in the oven.
Click to expand...

Stacy E. Apelt - Bladesmith said:
Justin points out a problem I may have caused.
I used to say to place the TC in an aluminum plate under the knife. At the time I first did that, the TC/PID was a read-out device. I later made it a PID controlled device, and it seemed to work fine. I soon went to another type oven for tempering and got rid of the toaster oven conversion. I have realized that the issues with temperature swing as the PID tries to "learn" the control parameters is caused by the plate being too much heat sink for the control program to figure out. Placing the bare TC tip in the air right above the blade would be far better .. as Justin pointed out.
Click to expand...


Ahh yes, this makes sense. I think yall are right! I can drill the hole all the way though the plate and thread the TC so that it's 1/8'' - 1/4'' proud of the plate. The tip would be in air.

Before I drill I can test the theory by rigging up the TC on top of the plate "in air."

Thanks for the thoughts guys.. I think we're getting somewhere!
 
So I had some time to get an initial test run.

This is the setup:



And these are the results of the first auto tuning cycle run by the controller with the TC in the air.

 
I mentioned binder clipping the probe in the original thread. Good to see scientific data backing that up. I just did it because it was convenient and made sense.
 
Stacy E. Apelt - Bladesmith said:
Justin points out a problem I may have caused.
I used to say to place the TC in an aluminum plate under the knife. At the time I first did that, the TC/PID was a read-out device. I later made it a PID controlled device, and it seemed to work fine. I soon went to another type oven for tempering and got rid of the toaster oven conversion. I have realized that the issues with temperature swing as the PID tries to "learn" the control parameters is caused by the plate being too much heat sink for the control program to figure out. Placing the bare TC tip in the air right above the blade would be far better .. as Justin pointed out.
Click to expand...

Haha, I received this recommendation from you when I built my PID controllers a couple of years ago. Thought it was a good idea, and have been doing it like this since, and it's been doing its job without any problems at all (at least from my layman knowledge and experience of PID controllers in ovens).

But, despite the old saying "don't fix what ain't broken" I think I'm going to do some testing with alternate TC placements. Just for the fun of it.
 
I think it largely depends on the oven. It may cause a 10° swing +/- in some ovens and the normal +/- 5° in others. In air it would be the most sensitive to the radiation from the elements … and thus the fastest response.

One thing left out of these discussions is that the TC itself is only so accurate. 0.3% accuracy may sound pretty tight … but that is a +/- 5° rang at 1500°.
 
I don’t think the probe in the air fixed the auto-tuning errors on the PID. Still a pretty big swing around the set point.

I ordered a legit made in Japan RKC Rex-C100 to try out. We’ll see if it does better...

With my manual settings I can get the Chinese controller to hold flat around 7-10 degrees under the set point... Which in all reality is fine for my application as long as I know the delta, but for the sake of science I want to see if the RKC can actually nail the target...
 
J. Keeton said:
I don’t think the probe in the air fixed the auto-tuning errors on the PID. Still a pretty big swing around the set point.

I ordered a legit made in Japan RKC Rex-C100 to try out. We’ll see if it does better...

With my manual settings I can get the Chinese controller to hold flat around 7-10 degrees under the set point... Which in all reality is fine for my application as long as I know the delta, but for the sake of science I want to see if the RKC can actually nail the target...
Click to expand...

Probably a good move based on the amount of time it’s taking you to get this working.

My builds were more or less a couple of hours work to assemble, press auto tune and be done with it. They hold the temps extremely steady.
 
Stromberg Knives said:
Probably a good move based on the amount of time it’s taking you to get this working.

My builds were more or less a couple of hours work to assemble, press auto tune and be done with it. They hold the temps extremely steady.
Click to expand...

Hahaha, I hope mine works that easy! I’m stubborn... and at times cheap; which can be a deadly combination. That being said I’m enjoying the process. I end up learning more when I’m in the weeds. Lessons I won’t easily forget.
 
Ok... I think we're getting somewhere here. Got the new Rex-C100 RKC made in Japan PID controller wired up. And I swapped out the TC since my new controller came with a new TC and I trust it better than the one that came with the Chinese knockoff.

Testing Setup:



Head to Head Auto Tune performance (Knockoff Chinese version in PURPLE and New RKC PID in RED):




Notes / Vague Questions:

  • The AutoTune on the new controller yielded a P of 13 and I of 198 and a D of 49.
    • This may be a good starting place to tweak via manual tuning, I think.
  • Once the new RKC PID got up to the SV (setpoint) of 203C, the PV (measured via TC) never faltered from 203C. o_O
    • So... Maybe the cooking probe I'm using to record temp is slightly more sensitive than the TC? Or would that make the TC more sensitive than the TC... ?
    • Maybe the slight distance between the probe and the TC is what's causing the oscillation in the data (Red line on the graph)...?
      • They ended up being about 0.75'' apart in the test rig.
  • After getting up to temp, the total variance from the set-point of 397.4F was +6.1F above the set-point and -8.4F below the set-point (403.3F and 389F respectively).
    • According to the cooking probe


What do you guys think? :cool:

Cheers,
JKeeton

Edit:

To attempt some manual tuning... it seams like I'll need to decrease my reset (I value). I think this means to make the I value bigger on the controller... So maybe a good next test is bringing the I from 198 to 300 and see how it changes.



Edit #2:

This is the new RKC RexC100 controller I purchased: https://www.ebay.com/itm/REX-C100-Temperature-Controller-1-K-Type-1M-M6-Thermocouple-AC-100-240V/282567746730?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

 
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So... These are the results I've been able to achieve with a P of 17 an I of 450 and a D of 56 (GREEN). It has an 8 degree total delta from peak to valley.



In yalls opinion, should I base my tempering targets off the peaks? For instance, if I want to temper at 400 should I set my set-point at a spot that gives me peaks of 400?

Or since the steel is thicker then the probes are and likely less sensitive; should I set the PID so that my average will be 400?

Cheers,
JK29
 
I would set it at the set point desired. The peaks and valleys will average out in tempering. A swing of +/- 10 degrees makes almost no change.
 
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