Spoiler image attached to this post to tease you into clicking the album link above.
I added a PID, which controls boiler temp more exactly. I wired it to use the alarm function as a steam thermostat, so I can control steam temp more accurately too.
Then I used a PWM to control intrashot temp stability. I figured since the PID can't react fast enough, I'll go to an "open loop control" when I turn on the pump. The PWM causes the elements to pump out any amount of power I want, from ~5% to 95% of full power. On my very first attempt at pulling a shot with this, (more tuning necessary), my portafilter temp (measured with a thermocouple on top of the puck), only dropped 2*F, instead of the normal 10+ degrees. Two subsequent shots however dropped more like 4-5 degrees, so I've got some work to do still, (combo of me messing with PID setpoint and PWM duty cycle, as well as having new beans and therefore an inconsistent flowrate, is causing this variation so far, methinks). I can run the boiler boost, as I call it, in either auto mode, (kicks in when I hit the brew button), or manual mode (I control when it kicks in). Or I can shut it off entirely. I added the manual mode in case I need to "lead" the pump to get around boiler lag. If it turns out I need to kick over to PWM control say 4 seconds before I start a shot, I'll probably add in a timer in the end to trigger the PWM first, then the pump...but this is my first attempt.
Finally, I added a Hot Water switch, as I was tired of not having one, (dispenses hot water out of the steam wand).
Total cost was around $130-140. Auber PID, Auber RTD probe, Radio Shack control box, cheap SSR/heatsink from Amazon, switches/diodes/capacitors/relays from Digikey, 12v transformer from Amazon, PWM kit, (which I had to solder together), from electronics123.
Posted Sat Jan 19, 2013, 4:49pm Subject: Re: My Gaggia Classic Upgrade - DIYed PID, Steam control, and PWM Boiler Boost
Nice upgrades! Thanks for the mention of the Gaggia SBDU thread, I see that it get views, but not necessarily put to use. If I understand correctly, you used the same process of PID idle/brew temperature and the same Skene setup. Interesting use of pulse width modulation to control heater power. Your aim is to balance cold water calories with heat calories and keep the temperature stable. Stable intrashot and faster recovery for a second brew as the water is heated with brewing, not just after. I viewed your picture link, nice install.
I will add a bit to the discussion on the Gaggia SBDU thread about the methods, use of the steam switch, use of the second PID alarm and now PWM. Shot volume and time is a key component.
Posted Mon Jan 21, 2013, 6:35am Subject: Re: My Gaggia Classic Upgrade - DIYed PID, Steam control, and PWM Boiler Boost
Thanks D4F. Yep, that thread was a great, and dense, read.
You hit the nail on the head. PID controls idle temp, Skene setup controls steam temp, PWM controls temp during brew cycle.
I've got the auto-manual switch so I can figure out timing. Auto means PWM control kicks in when I hit the brew switch. Manual triggers the PWM regardless of the position of the other switches. I did this because of, I think, Andypanda's method of hitting the steam switch 4 seconds before starting brewing to overcome lag. I have yet to rig up a good thermofilter design to allow me to simulate shots to dial in the PWM, but if it turns out I need to activate the PWM BEFORE going into the brew, (to outpace the thermal lag of the system), I can manually do it with the Manual switch. If the manual method turns out to give me the best intrashot stability, I'll probably rig up a timer to trigger the PWM automatically, wait a predetermined # of seconds, then trigger the brew cycle.
One thing I've noticed so far is that my recovery *seems* faster with PWM, (to be confirmed once I make the aforementioned thermofilter). PID temp spikes when I start brewing, due to the heat input of the PWM, and after a 30 second shot I find the PID is reading out 250ish. However, in about 30-45 seconds it quickly settles right back to around 216*F with very little undershoot, which would seem to indicate that the extra heat "held" in the boiler walls matches pretty closely to the heat deficit inside the boiler due to the influx of cold water.
I thought of using rampsoak function on a PID as a timer, or setting the sequence of a timer. Think of preinfusion with the first run and rest of the preinfusion cycle also running the heater. That allows for heat before any volume. I decided to keep it simple. I flip on the steam for the first brew, and the retained heat takes care of that for a second brew in repid succession. In 5 minutes, is would be like brew one again.
Posted Mon Jan 21, 2013, 7:56pm Subject: Re: My Gaggia Classic Upgrade - DIYed PID, Steam control, and PWM Boiler Boost
I am curious to see what temperature you read after you brew. Peak temperature and then temperature about 30 seconds after you shut off the brew/pump heater and the water in the boiler has a chance to thermally mix.
I agree that open loop control is the right way to go during the brewing cycle. Preferably a power level taken from a time vs %power table from time -10 seconds to +30 seconds. It should result in almost any desired temperature profile (although the sudden change from no flow to flow might always cause some blip at the beginning). It might be worth having minor adjustments for ambient temperature and water tank temperature since these effect the btus needed. Come to think of it, heat loss to the air could be derived from the average steady state power used to maintain the boiler temp before brewing. Water tank temperature could usually be estimated from the boiler temperature probe when the system is first turned on (assuming that the system has been off for a long period).
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