I agree... but the question is whether it is possible. Proportional control will vary the duty cycle by percentage, but the cycle period (sum of on and off time) will have to be set to act in correlation to the 0.3 to 1.0 second on-time requirement.
It would help to know more about process, the purpose of the timing requirements and how they were established. The heating element must be rather fast acting and heat retention of the process quite high to maintain a set temperature cycling on only once every 30 seconds for a second or less.
let me clarify what this is going to be used for... as usual, i'm looking to fix something that isn't broken.
and it isn't even for profit, or honest work. i was trying to shield you folks from the extent of my obsession
with BBQ and smoking food, but you are all in now....
i've got a pellet fed smoker. there is an existing controller that maintains a set
point quite well, but is not able to be changed remotely. it controls two things,
the electric ignitor, and the pellet feed screw. combustion airflow is constant when the unit
is running, from a muffin fan. it also won't compensate for ambient temperature,
so you have to keep an eye on it.
the unit has a TC mounted at cooking level for control.
it maintains the electric ignitor until temp hits 140 degrees,
then maintains the fire and controls the temp with two presents,
low heat, and high heat.
low heat keeps the fire from going out with a small amount of pellets (1.5 seconds, adjustable)
if TC is showing below setpoint, the auger turns a longer period of time (up to 12.0 seconds, adjustable)
sample rate is every 30 seconds.
and, it works pretty well, but some tweaking is necessary to get it to hold temperature.
if your high heat setting is too low, obviously you won't hit your set point, and if you
are overfueling the smoker, the temp will fluctuate widely.
usual high heat setpoint is from 3.0 to 10.0 seconds, depending on pellets used, and ambient
temperature.
so, i bought a wifi controller for monitoring temperature of the cook. the unit has addressable inputs,
and addressable outputs so you can drive a blower on a uncontrolled smoker, and control your
cook... and i thought... hmmm... it'd be cool to be able to control the setpoint of the unit with
this wifi deal as well.....
so the first idea was simply to set the factory control to 500 degrees, so it was always calling
for high heat, then interrupt the feed to the auger motor with a ice cube relay, so when the
new controller was sending an output calling for high heat, it closed the relay, and the pellets
got fed, etc.
the problem is, when the new controller was satisfied, it'll open the power to the auger,
and there won't be any trickle of pellets to keep the smoker afire until you next need heat.
so that won't work well.
so then, that leaves a micro PLC to replace the factory controller entirely. so i can put
one TC in smoker to control the ignitor, use the call for heat from the new wifi temp
controller to toggle between high heat, and low heat. same basic functionality.
then the real problem with that, is that i continued thinking...... and why not put a second
thermocouple in ambient air, so it can correct automatically for air temperature, and
save having to fiddle with the high heat duration manually. i mean, as long as i've had
to go and buy a PLC to do this...... why not?
that's when i decided to post here, and see if anyone has a simple solution to this,
or can recommend a medication to keep me from having these thoughts.
either one will do, 'cause while i was thinking about this, the thought popped into my
head... "you are so batchit crazy, why don't you also put a wind speed indicator
on top of the smoker, so you can compensate for wind chill as well..."
it's a slippery slope for me, as you can see.....