Temperature problem

Roasters and roasting

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Temperature problem

Postby bruceb » Mon Jun 27, 2011 9:48 pm

Firstly, as I have written before I am quite happy with the results I have been getting from my Coffee-Tech Maggiolino. The roaster does have a characteristic that I have to compensate for and the thought keeps gnawing in me that perhaps there is a more elegant solution to the problem.

As you can see in the upper diagram the temperature at any given setting undergoes a sine-wave variation as the electric heater is cycled from full on to off and on again. The recording was made using GreenBean's OCR monitoring software, the drum was empty and the thermostat was set to 150°C. The picture is so small you probably can't read the scales, but the swing is from 139°C to 159°C. The peak-to-to peak time is 8 minutes. This very large hysteresis is probably the result of a number of factors; the slow thermocouple, the slow reacting temperature controller, the slow reacting heating element, placement of the thermocouple above the bean mass and so on. I understand that with this type of setup some hysteresis is unavoidable, but this does seem extreme.

I compensate for this problem by increasing the temperature to 190°C while the temperature is on the upswing, dump in the beans and then the increase is smooth (heater remains on). At higher temperature the bean mass keeps the temperature more stable, but of course the reaction time is still very slow. The curve at the bottom shows a typical roast and as one can see it is quite smooth and predictable. The heater gets shut off at 189°C and I let it coast into second crack before ejecting the beans.

My question is obvious, I guess: How can I improve this situation? Using the same temperature probe in the same position, would a PID be able to reduce the amplitude of the swings? Or would controlling the voltage to the heater be a better solution? Or do I really need to go for a different type of thermocouple or probe, perhaps positioned differently (not a simple job!)? Or should I just leave it as it is and stop thinking of it as a problem?

If anyone has any thoughts on this or suggestions I would be most indebted.
Attachments
Heatup_06_11_cut_a.jpg
Temperature set at 150°C - Amplitude 139-159°C.
Heatup_06_11_cut_a.jpg (7.85 KiB) Viewed 6400 times
Roast_191a.jpg
Normal roast - Brazil Daterra Bourbon Collection
Roast_191a.jpg (9.97 KiB) Viewed 6400 times
Three Francesconi (CMA) espresso machines - Rossi, San Marco, LaCimbali, Faema and 2 Mazzer Major grinders- CoffeeTech Maggionlino, Hottop, Alpenröst and HW Precision roasters.
I decided I needed a bit of a change so I roasted some Monsooned Malabar. That was a change!
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RE: Temperature problem

Postby lukas » Tue Jun 28, 2011 9:37 am

Very interesting Bruce. Given the first graph, I'd think a properly setup PID would definitly be able to reduce the hysteresis to a minimum. The reason the temperature probe is not in the bean mass but in the air is probably that that's about the only temperature you can actually control with a thermostat or PID in such a roaster. Every change in heat input (especially with electric elements) would take way too long to manifest in the bean temperature to be of any use to control directly (if that sentence makes any sense). See for example this roast (image too big to be included, only look at the foreground red and blue lines:
http://lukas.einfachkaffee.de/wusel/equador-valle-de-intag-12-10.png

At 10min, I reduced the power about 30%, and it took nearly two minutes to manifest itself in the bean mass temperature probe. By that time, it would have been way too late to add power to rescue the roast from stalling.

The bean temperature follows the environmental (air inside or directly outside the drum) temperature, which is the only thing you can more or less control directly.


The advantage of a PID is that you can set and forget your temperature - it will maintain a near constant environmental temperature for you once it's properly setup. Though you might need different setpoints throughout the roast. The advantage of a voltage regulation is that you can set your elements to a constant heat output and will be able to lower that as much as you need, when you need it. But I believe a PID is better at regulating such a system than humans are - I alwasy ask myself 'how much do I need to reduce power to reach an ET of 240degC at rolling first crack and when the heck am I supposed to up the voltage again so that it doesn't stall?'
With a PID, you'll have an easier time learning stuff like 'it takes 1.5min for the PID to reach the setpoint', which might be very helpful.

Coffee-Tech doesn't sell the Maggiolino anymore, I suppose the newer models are evolutions of it. You could ask them for what modifications would work well with the Maggiolino, I found them (especially the boss) to be very kind people :)

All the best,
Lukas
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Postby GreenBean » Tue Jun 28, 2011 10:36 am

A PID could be set up to give a much steadier temperature before drop in but it seems massive overkill to me and if you wanted to use it to control the roast after drop in you would need to reset the PID parameters to suit the different characteristic with beans in the chamber.

What you do now seems an eminently sensible approach and will produce results (as far as drop in temperature is concerned) as repeatable as a well set up PID.

You do not state where the thermocouple is placed in relation to the heating element. The worst case for the hysteresis would be with the thermocouple above the heating element and in direct line of site. In this case the thermocouple would be influenced by radiation and convection from the heating element and would not be very representative of the average bulk temperature in the chamber. I know you say it is difficult to reposition the thermocouple but that is what I would be looking at as without a reasonable representation of the bulk temperature in the chamber no control system based on offsets from a set temperature (PID) will be very satisfactory.

If you do want to make serious modifications I would suggest heater power control. With this you can easily achieve a steady temperature despite the problems with thermocouple placement as it is not based on feedback from the thermocouple reading. With my roasters, for example I know that 50% power towards the end of the roast will hold the current temperature. In your case you would set 100% power to warm the roaster to the drop in temperature then drop the power to the point that holds the temperature for your roaster. The same control can be used to drop the heater power before first crack starts to achieve whatever timing you want between first and second crack. It is easy to create a nice smooth curve such as the following:

Image
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Izzo Alex Duetto | Gaggia XD 2 Group | Mazzer Super Jolly | La Cimbali Max | Solis 166 | Dalian 1 kg roaster | Hottop P | Hottop B | French Press (several) | Kettle modded, no really, added digital thermometer |
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Postby bruceb » Tue Jun 28, 2011 11:48 am

Thanks for the very useful input. Controlling the voltage to the heating element would not be difficult and a good deal cheaper than a PID. I agree, GB, that it seems like a bit of a palaver to have to change the PID parameters each time you roast. I may eventually try that, too, but I think I will first go for the voltage control.

The heating element is beneath the heavy, non-perforated drum and the thermocouple is inside the drum above the bean mass, ie. in the upper third of the drum. If anyone is interested there are pictures on my website here.
Three Francesconi (CMA) espresso machines - Rossi, San Marco, LaCimbali, Faema and 2 Mazzer Major grinders- CoffeeTech Maggionlino, Hottop, Alpenröst and HW Precision roasters.
I decided I needed a bit of a change so I roasted some Monsooned Malabar. That was a change!
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Postby GreenBean » Tue Jun 28, 2011 4:52 pm

Controlling the voltage to the element would do the trick but heater power control by switching the heater on and off in time slices to provide any chosen power level would be easier and cheaper. As your roaster already has an SSR it is only necessary to switch the control signal (12 volts and a few mA) between the thermostat and the SSR and not mains voltage at high current.

We discussed this in a previous thread somewhere. It can be done with many circuits, I originally used 555 circuits and now use arduinos to do just this on my Hottops. The easiest way is to use a 555 circuit that can be powered by the control signal so that when the thermostat is calling for heat the 555 is powered and switches the SSR on and off to provide any power level desired controlled by a simple potentiometer.

I have just had a rummage around and have found the original circuit board I used to test the idea in the Hottop. It is not in good condition but is working fine. If you want to try this approach sometime let me know and I will send you the circuit board. It is already wired with a suitable potentiometer and an LED so all that is required is to connect it between the control signals 8). No permanent changes to the roaster are required to test it.
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Izzo Alex Duetto | Gaggia XD 2 Group | Mazzer Super Jolly | La Cimbali Max | Solis 166 | Dalian 1 kg roaster | Hottop P | Hottop B | French Press (several) | Kettle modded, no really, added digital thermometer |
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