[JohnP]

in reply to: Mods to control a fan with the second relay? #3970

I noticed this as well – the fan actually slows the heating cycle, probably based on the air cooling the radiant heat gained from the lamps.

At this point, I am more concerned with cool-down times in a well insulated oven – see my post over under tuning for details. My conceptual hurdle is that the PID stuff seems to be aimed at a single control mechanism – a valve, a heater, etc. Can the notion of “up” and “down” be managed within a single instance (rather than globally setting the “Direct” setting)? That is, turn on heating coils to increase heat, turn off to stop raising and start slowly lowering it, open door (with a servo…) or turn on fan to lower it more/faster.

I could imagine this being simple if the output of the PID algorithm went from +100 to -100, but it only goes from 100 to 0.

BTW, many many thanks for everything you have done with this topic/product! I’m learning a lot, albeit slowly.

-John

[JohnP]

in reply to: Autotuning #3848

I’m trying to tune a toaster-based reflow oven I’m building (http://www.spcoast.com/wiki/index.php/ReflowOven)

Running on manual, I can get the oven to heat at the desired ramp rates (or faster, the 4 elements have enough oomph to easily keep up with the leaded and unleaded curves), and by modulating the door opening by hand, I can track the cool down ramps as well.

I’ve tweeked the settings to KP=0.78, KI to 0.09 and KD=0 so that when I run osPID automatically on a reflow curve, the temperature ramp up curve rises as fast as the reflow curve, but it overshoots the top of the curve a bit, and the cool down is way too slow (probably due to the insulation and heat-mass of the oven).

I can live with the 5 degree overshoot, but the thermal mass of the oven is making it hard to track the soak and cool curves. This seems to call out for a “cooling element” to balance the heating one. I suspect I could add a fan or automatic door opener or … under control of the PID algorithm to help cool the oven on the ramp down side of the reflow curve. In other words, use the heating coils to raise the temp when {input < setpoint} and to cool it down when {input > setpoint}.

Unfortunately (at least for my brainstorming 🙂 with these tuning settings, the PID output is zero for the whole cool side of the curve. Would it be “proper” to apply an offset to the output such that (say) anything above 20 is heating, anything below 20 is cooling, and 20 is taken to be steady state – or is there another way to address the problem?

Thank in advance for any clue dispensed 🙂
-John

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