blueroomelectronics
Well-Known Member
I remember when fuzzy logic was all the rage for temperature control.
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I remember when fuzzy logic was all the rage for temperature control.
Any heating element starts cooling immediately after power is interrupted, that is a given. To what degree depends to a large extent of the mass of the heating element and the air flow and temperature of its environment.{snip}
Rolf: Its not overkill and quite necessary as one of the requirements is that the heating system is that it needs to turn on, be able to deliver the desired heat(with decent precision) with in 5 seconds of operation signal, maintain the target temp range while the flow rate of the air changes slightly, and immediately begin cooling when the operating signal stopped. I highly doubt a house thermostat can do that..
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Is there no way to add the loop mentioned earlier?
Ugh... okay... let me try to explain one more time how this thing works. What the air is heating isn't really important....
Maybe, but it's going to take a little putzing around. Your thermistor is near the value of the pot: 250k at room temp. The resistance goes DOWN when it heats up, so if you disconnect the leg of the pot that's on the side you turn it to go LOWEST on the pwm, connect the thermistor between the 1k resistor and the wiper on the pot, so it works like that side of the pot... See what I'm getting at? Instead of lowering the resistance with the pot, you now lower it with the thermistor. PWM goes down, heater cools off, thermistor cools off, PWM goes back up, etc.
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If so... its less than 3 seconds because its very thin nichrome that has the ability to heat up quickly.
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Now you just have to figure out how long it takes for the ceramic tube to reach thew same temp!
Because the air is being heated by the nichrome wires and at the same time being cooled by the ceramic tube.
Unless the ceramic tube's temp is close to the set point, then I can visualize stability problems. You didn't forget to insulate the tube did you?
In any oven or heater, whether the air is flowing or not, you can not treat the heating element as an entirely different element from the the chambers walls. They both has to be the same temperature before the air therein will be at that same temp.
But if the air is flowing then only the last section of the tube (chamber) has to meet that criteria. But that is even more difficult to achieve.
SteveB: it must be some sort of proportional control. ON/OFF is unacceptable....
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So... I would imagine that tube wouldn't ever really get hot... just warm...
Plus, if I have a semi decent control system... the controller should be able to compensate for the heating element enclosure being cool by bumping up the juice... and when it is already warm not requiring as much power.
I mean.. that is supposed to be the whole idea behind proportional control right?
Hi
We are trying to create an intellgent heater that will control to ±10C?
1. With such a wide tolerance why then the need for such a responsive sensor with such little thermal mass?
2. The desired temperature of ±10C is easlity attainable through simpler means but for some reason you have need to narrow the duty cycle through PWM. Would it be correct to assume therefore that the temperature is not as important as the method of heating, PWM in this case?
3. Why is the substance being heated not important? Surely the thermal mass has a massive role to play in the design your element and circuit.
Actually....
Here's a bit more info about operation.
A typical example of the typical work routine for this component would be:
30 Seconds on, while ideally providing the target temp.
Followed by maybe..... 5 minutes of non-operation time.
Followed by another 30 seconds, et cetera, et cetera, and so on and so forth following that pattern.
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Why not keep the tube at the target temperature with both external and internal heating elements? Then you could turn the air flow on and off at the inter-wall needed.
Hey blueroomdude thanks for the utterly useless criticism. It motivates me even more. **broken link removed**
And lastly, in response to your post Andrew, the substance being heated is solely, good old fashion, air.
Unless you're just blowing hot air.
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I remember Rolf mentioning that the temperature of the ceramic tube be added to this equation so ill just say a few things about that. Since the heating element contained in the tube is like the coil elements inside a hair drier except, symmetrically covering the entire inner "area" of the tube so no air flows by without being subject to heating... I have not really worried about the tubes temperature. Being that is is very thin AND only about 4" long. I don't think that tube temperature is going to be a very heavily influencing factor. Recall.. The temperature feedback device(either Thermistor or thermocouple) is at the end of that 4 inch tube so the air doesn't really have much time to be affected by the tube temp...
Don't you think the surface area of the tube is at least a thousand times greater than the heating element? And the air flows by both at the same speed.
I don't know about your hair dryer, but the one my wifes uses has be ON for over 30 seconds before the temperature stabilizes at its max.
What could cause that?
BTW what is the air flow velocity in your apparatus?