[dafonso] purchased a nice 1500W smoker but was somewhat dismayed that it only had one cooking temperature. To compensate he designed his own PID control system which allows him to set the cooking temperature digitally. At the heart of the system is a PICAXE 18 micro which switches the smoker on and off using a solid state relay. Rather than testing the 110V system on the smoker itself (which would have been a pain indoors) he used a lamp instead. To see if he was getting the correct temperature he taped his thermocouple to the light bulb and let the PID switch the lamp on and off. Also be sure to checkout his video which does a good job of explaining how he was able to solder the surface mount components required for the control board.
“…he tapped his thermocouple to the light bulb…”
I am unfamiliar with this technique of adhering thermocouples. I’ve used the “tapping” trick for my monitors, keyboards, etc. but never thermocouples…
>:)
@Fritoeata
Yes, but do you have heat resistant tap like he does?
taping? or maybe its a special technique – “ducttapping”
whoops, all better now.
“Tape”: I did exactly that when building my coffee roaster. The goo on e-tape gets soft at relatively low temperatures, though.
I might have to grab a smoker like his… It wouldn’t take much to hook it up to my roast controller.
On/Off control is not PID… right?
Soon to become extinct.
Dan is switching the relay controlling the heading element off rapidly to simulate finer grained control.
I have the same cheepo electric smoker, got it from Homedepot $50 on sale. I just used a $7 Walmart smoker/bbq temperature gauge. And a $15 “router speed controller”.
This PID controlled smoker is way more sophisticated but mine cheepo method works perfectly fine for me. But it wouldn’t even be worth posting on HaD.
@ecor: It can be. PID just uses proportional, integral and differentials of the signal as feedback, so you not only know the state of your system, but the rate of change of the state. It allows the controller to adjust the controls before the system reaches the desired state, rather than reacting once it has reached it, as would happen with proportional alone.
How you choose to control the system with this predictive system is up to you. Obviously a gradual change would be better, but on/off will work just as fine.
@guff: and so the control keeps the smoker on longer or shorter depending on the value of the input, right?
@guffguff
PID is NOT a predictive control system, just as the proportional control system it is reactionary. The derivative and integral terms are to help minimize overshoot and steady state error respsctively. PID has no knowledge of the system and is thus incapable of predicting where it is going.
Also on a side note proportional does not react once the system has reached the desired state – that is a “Bang bang” control system and is much more crude than a proportional control system. A proportional control system increases the input to the system (in this case electrical power) proportional to how far you are away from the desired set point. The problem with proportional is it would only completely back off the power once you are past the set point often causing oscillation which is why the derivative term is used.
Half the fun of smoking food is staying up all night drinking cold adult beverages…and I am glad to see no one has called it a barbecue.
@Swighton
Yes, you are most probably right. I’m not primarily from a control background, so I was using the word ‘predictive’ in the sense that the PID knows rates of change of state, so it knows roughly what is happening in the future. As you said though, there is not a system model in the controller, so it can’t actually predict.
And again, when I meant a proportional controller ‘reacts’, I meant it only puts a negative input once it has reached the state, so you are likely to get an overshoot, whereas a PID is more ‘proactive’ in that it will reduce or negate inputs before the reached state. It’s still reacting, but has more information to do so with.