Roast-Pi

[Dimal]

Looks like you've got the control pretty well nailed Gonzo...

What are the results in the cup like and have you tried changing profiles slightly, to see if you if you can detect cup outcomes? The idea being, that you could then control roasts to give you exactly what you're looking for in the end results.

Looking really good mate...

Mal.

[gonzob]

UPDATE...


Just to let you know that the Roast-Pi is continuing to perform. The latest print-out is attached. 300g Tanzanian Machare Estate. I now use a 2-minute 50C "starting platform" at the beginning to make sure all is up to temp before starting the serious heating. Note the ~45sec delay between the heat starting and the bean temp beginning to rise. I've made a thicker and tighter-fitting chip-board cover for the breadmaker to keep chaff from going everywhere, and to keep better control of the temp. A side benefit is that the stirrer noise is reduced, and the cracks are easier to hear.

Gonzo.

[Nickgb]

I don't have anything worthwhile to post, but I just wanted to say this was an entertaining read. I was thinking whilst modding my popper recently if anyone had created Pi/etc.. as it'd make a great little controller. Looks like you really have a neat little system going there Gonzob, nice work!

[gonzob]

Hi all.

Ok, thanks to Andy I was able to do a roast today and won't run out of roasted beans!!

I trimmed all the settings since my first feed-forward-power run and I got a really good run today. See graph. The actual-bean-temp curve is barely distinguisable from the target line. It was all hands-off - the system did the adjustments.

For those geeks amongst us, here's the maths:

Remington "Big Loaf" breadmaker
Mass of beans 530g
Ryobi heat gun max power 2000W
Thermocouple in the bean mass


The system splits the power% required using the following equation:

P = K1*(TT-TB)/LA + K2*(TB-TA)

The first term is the power needed to raise the bean temperature (TB) to the look-ahead target temperature (TT) over the lookahead time (LA) in seconds. The term can be positive or negative, depending on the temperatures

The second term is the power needed to keep the system at the current temperature. It assumes an ambient temperature of (TA) of 25. The term is always positive.

K1 and K2 are constants
K1 is 250
K2 is 0.28
LA is 45 secs

So,
to raise the temp from 175 to 179 deg in 45 secs, we need 250 x (179-175)/45 = 22 (22% of 2000W)
to maintain a temp of 175C, we need 0.28 x (175 - 25) = 42 (42% of 2000W)


The system looks at the required temp profile:

Minutes, Temp
0,50
6,130
12,160
16,190
24,210

and it interpolates to find the target temp in 45 secs from now. Why 45 secs? Well I did some analysis of my results and found it fitted.

K1 is "trimmed" on the fly so that if it is consistently under the target, it increases/decreases to compensate. A consistent error could be caused by a lower ambient temp or more/less beans than usual.

So, if anyone else wants to do something similar, that's what I'm doing.

Gonzo

[gonzob]

Ahh... good point. Yes, that could have made a difference. Although I can tell you that the outside end of the copper tube was getting mighty hot, so the heat was going somewhere.

Unfortunately it's a bit late as I've pulled the copper one apart to make the stainless one, and I've calibrated the new one against my dial thermometer, and they're pretty close.

The REALLY good thing is that Andy sent my beans yesterday when I ordered them, and they arrived today, so I can do a roast tomorrow. CS to the rescue!!

Gonzo

[MrJack]

Perhaps the problem wasn't so much conduction, but just very poor heat transfer between the probe and the thermocouple itself? Perhaps try filling the end of the tube (where the thermocouple bead is) with a thermal paste and see what happens?

[gonzob]

Learning...


Since may last PC roast (Pi-controlled) I have fiddled with a few things. And I've been anxiously waiting for the level in the roasted bean container to drop to my trigger point for the next roast. I've tried inventing excuses to do one early, but I was firm, in control, ... yeah right.

So one of the things I did was make a new thermocouple for my Yocto-Thermocoule. The thermocouple I had is a 100mm one, and I'd really like to poke it into the beans a bit more, so I need a longer one. Yocto supply two really tiny TCs (just the wires) with each board, but with no housing. "Well, I can make a housing", I said. "How hard can it be?"

So, fired with enthusiasm, I get some 3/16" copper tube, silver solder the end, round it off, poke in the TC wires and wrap them nicely. It all looks good. I check that it's working... yes seems to be. OK, it's time for the next roast....

The roast starts, and the PWM calls for MAX POWER. That's OK, I say, 'cos it's just starting. Five minutes into the roast, it's still at MAX POWER, and the bean temp is under the target by 5 deg. I start to think something's wrong. 10 mins into the roast I stick in a Tel-tru type thermometer, and it's reading 40 deg above the TC. That roast gets scrapped.

The leaning is: DONT MAKE A THERMOCOUPLE HOUSING OUT OF STANDARD THICK-WALLED COPPER TUBE. It conducts the heat away too much and the junction sees a temp much lower than the actual. It's a pity, really, 'cos it looked very profesh.

And now I don't have enough green beans to cover my new (thin-wall-stainless-steel-housed) thermocouple to do another roast, and I may run out of roasted beans this weekend. If only I'd done a roast earlier....

G

[Dimal]

yes it does change but would be a terrible way to control heat input

Yep, I agree...

Not how I'd go about it.

Mal.

[brettreaby]

Still lost...never heard of a temperature coefficient of a heating coil...please explain.

G

PS Just Googled it. Is this the increase in resistance as a result of increased temperature?

i assume he is talking about the change in the resistance of nichrome wire with heat: yes it does change but would be a terrible way to control heat input.

the resitance does change but not much- about 6% for 600 degrees C.

[gonzob]

An annoying contraction of "Temperature Coefficient" I believe Gonzo....

Mal.

Still lost...never heard of a temperature coefficient of a heating coil...please explain.

G

PS Just Googled it. Is this the increase in resistance as a result of increased temperature?

[Dimal]

Tempco?? you've lost me...

An annoying contraction of "Temperature Coefficient" I believe Gonzo....

Mal.

[gonzob]

Tempco?? you've lost me...

Re power monitoring, just get one of those plug in power meters. I use mine for doing nerdy things like checking how much power my appliances consume so I can reduce my carbon footprint. They're cheap and measure real power, VA, power factor, average power over 24 hours (good for fridges), voltage, and so forth.

Gonzo

[Hildy]

Actually, that's a very interesting idea re: power monitoring the mains. Somewhere I have small shunt amplifiers that are designed for power feedback measurement.

Do you know what the tempco of the heating element is? Could try using that as super dodgy feedback!

[gonzob]

Yeah, taking it apart is a step down the darkside. If you have two systems for running the heat gun, you instantly place your system at risk. If you turn on the heat, you have to make sure you turn on the fan too. If you forget the fan, it'll overheat, and the overtemp cutouts are single-use in my experience.

I like the idea of driving the temp knob with a servo. However, if you go down that path, see if you can calibrate the knob rotation to the power output. Put a power meter on the mains to see what it draws relative to the angle you turn the knob. You may find that it is no-where near linear.

As far as the PID is concerned, I wrote a PID program, but very quickly determined that the lag time meant that it would be difficult to control. As I mentioned in another post, we're not trying to build a temperature control system which reacts to sudden changes in temperature, we're trying to roast beans, and we KNOW what the temperature profile is going to be. Therefore, we know the input power profile pretty repeatably. The only variables are the outside temp, the bean moisure and the quantity of beans.

Sorry. I'll stop preaching now.

I'm looking forward to the servo.

Gonzo

[Hildy]

Looks very nice! Yay for not doing PID!

Having bought one of the Ozito variable temp heat guns (little knob on the side) I'm now tempted to mount a hobby servo on it as a super dodgy temp controller rather than taking it apart.