HomeRoast Digest


Topic: "PID" stuff (2 msgs / 93 lines)
1) From: Deward Hastings
Jeff:
<Snip>
"normally"
used to "control" the temperature. 
Actually, commercial machines almost all control temperature with a
pressurestat on the boiler . . . more reliable, and easier to set the
pressure and differential.  If I put an electronic controller on one to
"fine tune" temperature I'd leave the mechanical system in place (just =
set
the cutout a bit higher) as a safety backup.
But your general point is correct . . . "PID" (especially "autotune")
controllers are best suited for *stable* systems (like an espresso =
machine)
and of not much use at all in highly *variable* systems (like a roaster,
where it's hard enough to get a useful temperature reading at all).  =
It's
kind of fun (in a geeky sort of way) to watch an autotune controller =
start
up and "hunt" around the setpoint while it "sizes up" the rest of the =
system
.
In a roaster you're controlling for the constantly changing (and =
difficult
to measure) temperature of the beans, by controlling for heat transfer =
into
the bean, itself determined by the temperature and heat availability in =
the
rest of the system (air, drum, burner, heater etc.).  What, again, do we =
use
for a setpoint ? ? ? 
SSR v. mechanical relay: I've watched SSRs fail "on" (always a =
disconcerting
experience around a pressure vessel ), but I've seen mechanical =
relays
fail "on" also (welded contacts), and industry wide experience (I don't =
have
enough to make an independent judgment) seems to favor SSRs.  "Off" =
state
leakage is hardly an issue when running a heater, even if measured in =
more
than microamps.  The fast, quiet and reliable switching counts for much
more.  The probable issue in espresso machines is environmental (finding =
a
cool enough mounting location) . . . people don't want the bother, or =
the
noise, of a fan on the heat sink, and it gets toasty warm inside the =
box.
Deward

2) From: Jeff Braman
Deward,
Totally agree with your observations, most of my experience is not in
the "coffee" world!
I believe that the SSR offers the speed needed when trying to control
the radical temperature "upset"after a "pull" on the "Silvia". I'm
guessing that the mechanical relay would be "slower", so recovery and
stability wouldn't be as fast or accurate. (mechanical relays, life
expectancy would also be relatively short, due to continuous and fast
cycling).
The SSR in my Isomac "Tea" is heat sinked and seems to "live" just
fine in that atmosphere. Due to the Mechanical design of HX machines,
a "controller", wouldn't add much................this topic has been
discussed on alt.coffee and other places!
Temperature Controls are also produce with "ramp and soak" capability;
the process is programmed in "segments" with time and temperature,
however for "roasters",  sensor placement for accurate temps along
with stability (where do you put setpoint?!) for any "modded"
appliance would be the issue. These units are generally much ,much
higher priced vs. single point digital units.
-- 
Jeff
jeff.braman


HomeRoast Digest