Upgrading Manual Kiln With Digital Control 1

[paulcook]

Just bought a used kiln to help with some prototype work.

Unfortunately, it is not possible to program controlled heating schedules with this kiln. It has a Kiln Sitter, which basically uses a physical piece of material (a "cone"), which softens at a certain temperature allowing a physical switch to turn off power to the kiln.

The wiring of this kiln is a bit funky, and I want to keep my modifications to a minimum.

I'm not worried about super tight temperature control. I'd like the kiln to ramp up and hold heat within a +/- 15F range at specified points.

I'd like to keep the Kiln Sitter intact as a failsafe/for future resale.

The kiln is a Skutt 145.

Could I:

- Add SSRs, control board, and thermocouple as illustrated below:

- Program the controller to open/close the relays to maintain the schedule.

Would be programmed to work something like this:

1) Set target temperature and hold time

2) Once 15 degrees above target temp is reached, open relays (cut power)

3) When temperature drops to 15F below target, close relays

4) Continue for set amount of time, and then progress to next target temperature.

5) Hold at that temperature as described in steps 2/3

6) Continue process until schedule is complete.

Thoughts? Is it bad for the elements to cycle them on/off so frequently? (I have no idea how long it would take to fall 30F, raise it back up, fall, etc,)

 

[IRstuff]

Seems to me that you need to do a thermal survey to see if your kiln even has uniform temperature at a stable thermocouple reading; if the kiln interior is ±50F, isn't that a problem?

TTFN (ta ta for now)
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[paulcook]

@IRstuff

It's quite small, so I'm not worried about temperature consistency throughout the chamber.

I do wonder if my range is too wide for the holding spots (would be +/- 15F).

It's only for preparing molds for investment casting - I'm not sure how tightly the temperature needs to be controlled.

Castings/molds will be relatively large. 5-10lb A356 castings.

 

[3DDave]

They are heating elements like in clothes driers and electric ovens so cycling won't be particularly harmful. The temp is mainly limited by the heat absorbed by the chemical reaction in fusing the material. That is why the Kiln Sitter works. The temp doesn't rise to the point that the cone melts until the fusion takes place. Obviously the cone is selected to match the requirement.

There are solder flow controls that do the same sort of process control. They are Arduino based; you should be able to change the software. They are typically used with toaster ovens. You would change the power controls and temp sensors.

Full do-it-from-pieces example:

https://projecthub.arduino.cc/thedalles77/toaster-oven-conversion-to-smd-reflow-oven-0ccbd7

but there are dozens of examples to search.

 

[danw2]

I'm not sure that the typical "open-collector output" that provides 10's of milliamps to the coil circuit of the SSR are designed to drive two separate SSR's at the same time. I'd check on that.

 

[waross]

Why not use a PID controller and keep the temperature steady?

The PID control is supported by 64 ramp/soak actions

Ramp and Soak PID controller

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[paulcook]

@waross

That does makes sense, but I’m not sure how to wire the PID. The kiln seems to use the hots independently to run 120V elements. How can I use one PID to control both hots. What line voltage would I wire to PID?

 

[paulcook]

Does this seem like a reasonable way to wire this up? The two hots, each feeding their own set of elements is throwing me off a bit.

 

[waross]

That looks OK.
PID stands for Proportional, Integral, Derivative.
For a kiln, don't try to use Integral or Derivative.
Use Proportional control with about 10% proportional band.
There is a lag between the elements generating heat and the thermo-couple responding.
If the kiln is at 70 F and you put the setpoint at 300 F, the temperature will overshoot, and then undershoot several times befor settling.
The control will still be much closer than you have now, you just don't see the overshoot.
After a couple of minutes the control will settle down and remain quite close for small changes.
If you use the ramp feature you will have much less overshoot.
And with proportional control alone you will always have the temperature a little below the set point.
After a little use, you will be able to see how much low and put your setpoint a little higher to compensate.
Integral does this automatically, but it may be better to Keep It Simple, Sam. KISS


--------------------
Ohm's law
Not just a good idea;
It's the LAW!