PID question

You only control the SSR with one PID. The other two are IDLE and only read temperature. Any of the three can be the one selected to RUN.

You are correct that running two or three PID at the same time and having them all control the same SSR would cause the electronic version of schizophrenia.
 
Yeah, I see what you mean about RUN and IDLE. But I haven't seen that kind of actions on these cheap controllers.

Sure, on my kilns controller I can choose to run it or idle. But these small controllers seem to be in auto run mode when you turn them on.

The only way I know of to have them in idle mode is to set the SV to a lower number than the PV.

If I don't find a easy fix for that, I'll go for the rotary switch, but put it between the controllers and the SSR as I mentioned earlier.

SgxH5cy.jpg
 
The PID has a button (usually called a key) that starts and stops the program. Normally it is something like an up and down arrow. Push the ON/RUN key and the program starts. When pushed again ( or push the OFF key), it stops or idles the program. These are the RUN/IDLE settings. When the program is not running, the SV will still show (the set value), and the PV will show (the current reading). The PV is what should be showing the same on all three PID readouts. You only turn the program ON on the one that has been autotuned to the desired parameters (kydex, oil, HT).

Now, lets consider your last drawing. With the SSR control voltage output being selected for one PID only .... If all were in the RUN mode, the two not attached would be going crazy, and all auto-tuning would be messed up. If #1 was tuned for 1500F, #2 was tuned for 250F, ands #3 was tuned for 130F you can see how the two not being in control would be trying like crazy to either lower or raise the temp, but to no avail. They would try and "learn" the new control, but would default as getting no response. Obviously, this would cause the same schizophrenia as all three trying to run the show at the same time.

If you want the rotary switch between the SSR and the PIDs, that is OK, but the two not in control must not be in the RUN mode either way. Just connecting all three to the SSR ( use two 4 screw terminal boards) would be the same thing, and simpler.

BTW, your drawings are excellent.
 
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I took a look around the interweb and it seem the Run/Idle/Start/Stop functionality is only present in more premium PID controllers with a ramp/soak feature. So no luck there, since this is a low cost build.

Edit: It seem I need to correct myself. Some low cost controllers without ramp/soak feature do have this functionality. It's called Automatic and Manual modes, if I understand correctly.

Thank you for the kind words about the drawings, I'm certainly no expert at this, but I like clean and comprehensible overviews.
 
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Currenthill, I'm having a little difficulty visualizing your process. Can you post any photos?

I'm particularly struggling with the alloy plate. What is it's intended function? It seems from your description that it is intending to provide a physical mechanism for damping out signal noise? If so, it seems likely to be a contributing factor to the slow response. Which controller are you using and what are the P, I & D settings?

Edit: What is the damping (input filter) time?

Costs aside, I'd expect to spend between 2 and 4 hours mounting the 2 extra controllers, wiring them up and generally faffing about, possibly much more. I think you'd probably be better served spending that amount of time reading about tuning PID controllers manually online and then manually tuning the controller you have. I don't envy you. Take notes. Drink strong coffee. Expect to become frustrated at some point.

FWIW, my guess is that you have a small Proportional band and/or a large Integral. Possibly also a long damping (input filter) time.

Stacy, most of the low-end controllers "just" have an autotune. Once the autotune is run and the calculated P, I & D terms are accepted, they don't change. It's only usually the higher-end controllers with "fuzzy logic" or "adaptive" tuning that continuously monitor the control response and make adjustments to the terms whilst running.

Having 3 controllers and selecting the output from only one to drive the SSR does not seem likely to cause an issue, but it may not help much with the control issue Currenthill is having.

When the middle temperature controller is selected, the low temperature controller will see the actual temperature above its setpoint and will try to reduce it by reducing its cycle "on" time. After a period of time, it will have reduced the "on" time to zero and will stay there until it sees the process temperature move towards (or through) the setpoint fast enough or far enough to respond.

The high temperature controller meanwhile, will see the actual temperature below the setpoint and will try to increase it by increasing the cycle "on" time. After a period of time, it will be 100% "on" and will stay there until it sees the process temperature move towards (or through) the setpoint fast enough or far enough to respond.

Switching from a stable middle controller output to one of the others will give an immediate large response, but is likely to result in severe overshoot unless the tuning parameters are correct for the process. This brings us back to the original problem, which seems to be that the PID terms are probably not correct for the process.

Once another controller output is selected, the middle controller will see the temperature either above or below its setpoint and its output will go to either zero or 100%.
 
My first thought was with your rotary switch controlling the PID's is make sure your rotary switch CONTACTS are rated for low voltage and low amperage. If not you will eventually or possibly soon see a failure there. My simple PID doesn't have a run manual or idle setting either as well as I can remember.
 
The auberins PID with ramp/ soak has the ability to be programmed with many different programs. It's not real user friendly, but basically you can program steps that are the different ramp/ soak times. On mine I have steps 1-3 for 1080 then steps 4-9 for AEB-L. To run the AEB-L "program" I just skip to step 4 and run it. It will start at step 4 and continue through my pre heat and target heat steps then stops.
 
timgunn1962 said:
Currenthill, I'm having a little difficulty visualizing your process. Can you post any photos?

I'm particularly struggling with the alloy plate. What is it's intended function? It seems from your description that it is intending to provide a physical mechanism for damping out signal noise? If so, it seems likely to be a contributing factor to the slow response. Which controller are you using and what are the P, I & D settings?

Edit: What is the damping (input filter) time?

Costs aside, I'd expect to spend between 2 and 4 hours mounting the 2 extra controllers, wiring them up and generally faffing about, possibly much more. I think you'd probably be better served spending that amount of time reading about tuning PID controllers manually online and then manually tuning the controller you have. I don't envy you. Take notes. Drink strong coffee. Expect to become frustrated at some point.

FWIW, my guess is that you have a small Proportional band and/or a large Integral. Possibly also a long damping (input filter) time.

Stacy, most of the low-end controllers "just" have an autotune. Once the autotune is run and the calculated P, I & D terms are accepted, they don't change. It's only usually the higher-end controllers with "fuzzy logic" or "adaptive" tuning that continuously monitor the control response and make adjustments to the terms whilst running.

Having 3 controllers and selecting the output from only one to drive the SSR does not seem likely to cause an issue, but it may not help much with the control issue Currenthill is having.

When the middle temperature controller is selected, the low temperature controller will see the actual temperature above its setpoint and will try to reduce it by reducing its cycle "on" time. After a period of time, it will have reduced the "on" time to zero and will stay there until it sees the process temperature move towards (or through) the setpoint fast enough or far enough to respond.

The high temperature controller meanwhile, will see the actual temperature below the setpoint and will try to increase it by increasing the cycle "on" time. After a period of time, it will be 100% "on" and will stay there until it sees the process temperature move towards (or through) the setpoint fast enough or far enough to respond.

Switching from a stable middle controller output to one of the others will give an immediate large response, but is likely to result in severe overshoot unless the tuning parameters are correct for the process. This brings us back to the original problem, which seems to be that the PID terms are probably not correct for the process.

Once another controller output is selected, the middle controller will see the temperature either above or below its setpoint and its output will go to either zero or 100%.
Click to expand...

You seem pretty knowledgeable about this, I'm not sure I follow everything you say and/or ask. My "problem" isn't very complex. I'm just trying to get my temper oven to, as fast as possible, attain my temperature of choice - and keep it there with minimal flapping.

So my current PID controller works just fine, it will get the oven to 175 degC pretty fast and precise - since it's been auto tuned to 175 degC.

My problem is introduced when I need to, for example, curing wood stabilizer and I need it to be 100 degC. Since it's been auto tuned to 175 degC it will take forever to stabilize to 100 degC.

That's why my approach of choice is to install three controllers (but I only need one SSR since I'll only use one controller a time). I choose to use three thermocouplers since I've read they can be pretty sensitive when introducing connectors. So I'll just auto tune each of the three controllers at the temperature of choice per application (for example 100 degC for wood, 175 degC for steel and 190 degC for Kydex). They will then be fast and accurate at their respective temperature.

The aluminium plate is put in the oven and the thermocouplers are mounted in it to ensure an even reading of the temperature, and thus avoid hotspots.
 
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