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 |