Posted Sun Aug 25, 2013, 1:53pm Subject: Re: Gaggia Classic SBDU Preheat PID Temperature Recovery and Stability, Steam
Bronc, welcome to CG. The simple answer is no Sestos experience. The following link is how I understand the terms P, I, and D. P to me is Proportional Band also PB. P may also be proportional constant, Kp.
For espresso use, overshoot is not all bad. If you had something where 1 degree overshoot ruined a reaction, then it could not be tolerated, but you can get more rapid rise in initial temperature if a little overshoot is allowed. That may also mean that you get a little initial oscillation of temperature, but the idle temperature should still stabilize and not drift. See Fig 1 in the pdf. For espresso, I use something between the middle and R side graphs.
Some articles have noted that PB, or P, is a % of full scale/range of the sensor or thermocouple used. What is your thermocouple? How is it attached, and where? The Auber brand RTD sensor fits the threaded hole of the boiler tstat. That place was chosen by Gaggia for the tstat because it is maximally and quickly reactive to the heater surrounding it and the cold water inlet. A very reactive place is a good place for the PID to control.
I do know that Auber recommends a D of about 25% of I, so those may be ok. I am wondering if you are being shown setting over range in those P,I, and paramaters. If so, the P is about 11% (1050/9999) and I am used to 3% of a what I believe is about 1500F total range of PT100 RTD. That would make it active at a 45F range around a set of 214F.
I am assuming that you hade to pick a thermocouple or sensor type. You should be able to find a temperature rang for that type sensor on the internet.
You should have a code to enter the P,I, and D set values. Try to halve the P value and see what happens, assuming that P means PB as above. If worse, then double the original until you get an idea of how it reacts. I looked at a Sestos manual and "P" is differential and M50 is I. I could not find P as PB.
jonr Senior Member Joined: 25 Jun 2013 Posts: 298 Location: Americas Expertise: I like coffee
Posted Sun Aug 25, 2013, 2:08pm Subject: Re: Gaggia Classic SBDU Preheat PID Temperature Recovery and Stability, Steam
I bought a PID but solved the understanding and brew issues by switching to a computer and writing my own code. Anyway, I would try turning off the derivative and integral functions and use a very high proportional gain. This should give you easily verifiable on/off control (if boiler is too hot, turn off the heater, otherwise turn on the heater). Similar to the stock thermostat but with much less hysteresis (ie, water temp will actually be pretty stable). Once that is working you can turn down the proportional value for less oscillation (ie, if it just slightly too cold, only apply partial heat).
Another thought - thermocouples are very noise sensitive signals and that could cause it to occasionally/briefly do the wrong thing. Try the digital filter.
Posted Mon Aug 26, 2013, 2:39pm Subject: Re: Gaggia Classic SBDU Preheat PID Temperature Recovery and Stability, Steam
Looks interesting though not simple :) How much of the system will be off the shelf available? Advantages and disadvantages versus PID controller in terms of easy of install and day to day use? Perhaps a new thread with an appropriate title that will make search more useful, something about Analog Thermostat and Gaggia, or Analog Thermostat Temperature Control...
Posted Mon Aug 26, 2013, 11:30pm Subject: Re: Gaggia Classic SBDU Preheat PID Temperature Recovery and Stability, Steam
I don't know any programming language so I have to stick to analog. The circuit is quite simple. Top section consists of 2 resistive bridges with trimpots, relay to switch between brewing and steam temperatures and comparator driving main SSR. Middle section feeds 5.1V DC (derived from AC line) to PVA3055 Mosfet switch. Transformer voltage is half rectified by D3 and goes via PVA3055 switch to main SSR to get 50% heater power. 5.1V DC also feeds adjustable RC delay. Pump and solenoid will be energised via T2 and second SSR (low power). Bottom section is simple 9V DC supply. Non-standard component will be only PCB. As suggested, I will start another thread after testing the circuit.
I don't know any program language so I used PID. It will be interesting to see where you go with analog. While it may be simple as circuits go, using a PID is probably easier. It can just be a black box thermostat. You do not need programing language, just follow the instructions to set your desired temperature and then either autotune or set the P,I,and D parameters. You would obviously easily understand and connect the wiring, and the concept. The box is your computer with language, preprogrammed algorithms. I had not even heard of PID until I got the Gaggia and started reading here, a little over a year ago. It is however something that many are familiar with, as the cruise control in a car is quite similar.
If you want ease, consider the PID, nothing to program and nothing to build, just install. Most of us do mods because we enjoy the project process and better espresso. It looks like you enjoy electronics. I will look forward to your analog thread, and have seen the switch repair that you posted.
A little update on settings and use. I now have the "brew alarm" setting at 240F off and 241F off and use about 3 - 3.5 seconds of steam switch on and about the same time off before hitting brew. that will have the PID readout at about 219 - 220F. That seems to work for 30 - 35 gms of brew out.
If the brew volume/weight is about 50 ml/gms, then about 250F for the "brew alarm" as noted before. Volume or weight of brew between 30 and 50 gms should use temperature between and adjust to taste. The "on and off" of the steam switch should be about the same.
I finally adjusted the thermofilter to about 25ml/25sec and set the alarm for brew between 234F and 235F. 3.5 - 4 seconds of preheat steam switch and then about same time of "rest" and then the brew. I had been using this setting by taste and got a chance to thermofilter it. 199F - 200F. I use Redbird Espresso and Blue Jag, so just right. I could have done a video, but it looked like below with a little temperature variation on the upper number on the double PID and SV of 214F instead of 212F that I used a year ago.
Setting the alarm temperature manages the heater "on" time to adjust out intrashot drop. Preheat is still necessary to allow time for heat from element to boiler wall to the water while cool water is direct. I also tried 3 doubles of 25 ml with a minute between for "prep" and each ran 199F. I did not need the steam preheat other than the first shot as there is residual heat in the boiler. I waited 4 minutes and then tired a double and used the same preheat as first, and again, 199F. 3 different alarm settings are noted based on brew volumes. The system works fine if the pull rate and volume is kept constant. That means grinder adjustment as needed to compensate.
An interesting side note is that the "hole" necessary to run 25ml/25sec was 0.009" dia. I could not find the 0.0095" that should have made closer to 30 ml. It is an infrequently found guitar string. If I understand correctly, the puck for a double at 9 bar on my Gaggia, probably similar on others, is equivalent to a single 0.009 - 11" hole in the bottom of the basket. Hole size, grind equivalent with tamp and fresh coffee, is very critical.
I am sure that it can also be done with computer boards, and even with the steam switch if wired on a separate switch from the 3 way, but the PID is fairly simple, easily understood and at reasonable cost.
My data shows that a steadily rising boiler wall temperature is needed to maintain a flat (or slightly dropping which is what I prefer) coffee temperature profile during brew. I'm measuring boiler wall temp at the top of the boiler, but I don't know that that differs much from anywhere else.
It would be very interesting if someone sold a PID controller that could be programmed to ramp temperature at a specified rate. Or just switch to a defined power level (a constant 40% during brew works pretty well).
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