>re: PWM
>>I'd suggest ditching PWM.  As I remember, heating/cooling elements (oven,
>>air conditioning, fan) are more efficient in on/off mode.  IE, for your
>>oven, turn on until temp probe reads somewhat above your set point.  Then,
>>wait for it to get somwhat cooler than that point, at which you'll turn it
>>on again.


Uh-oh!  Bang-Bang control scheme - run away fast!

This simple control scheme burned a lot of biscuits when I built it.  The
problem is, electric heating elements are like racehorses.  By the time your
thermostat or thermistor has responded to the electric heating elelment, it
is shining like the sun and your food is burnt outside.

In a PID duty cycle control, you essentially GUESS at a duty cycle, try it,
give the oven a few seconds to stabilize, measure, then GUESS at another
duty cycle, until your guesses get real good.  When the oven is cold, your
guess will be 100% (see rant below).

Don't try this bang-bang control scheme on electric ovens, believe me they
don't work.  Might do well in a house thermostat, where the heating system
doesn't have hte capacity to turn the house into a red-hot pile of molten
metal (ask me about abnormal tests required for UL if you don't believe
that.)



>
>   My thought was to get up to the selected temperature as quickly as
>possible which is the way it works now. Once I get within range, then I
>start to adjust the duty-cycle. I use to design industrial energy
management
>systems so I have some experience here. These are standard resistance
>heating elements with the top element used for Broil as you mentioned. The
>elements have a significant thermal mass so it's possible to save energy by
>adjusting the duty-cycle. This will have to be `tuned' in the PID loop.
>


I found the PID loop does this by itself.  When the oven is cold it guesses
100% duty cycle, and then begins to guess less as it approaches the
setpoint.

Tuning was real easy.  Maybe I was lucky.  I started out with a P constant
of 8 then changed it to 4.





>>hope it goes well!
>
>   Thanks, but this is all Lawrence's fault ;-). I've had this on the `back
>burner' but his recent discussion of PID control and thermistors brought me
>out of the `closet'.
>


Sorry to cause you so much trouble.  Back burner (ouch!) projects tend to
get hot (aargh!) every once in a while, because they are just simmering
(yeech!) along then suddenly boil (ooh!) over.  I don't know about you, but
many of my half baked (ack!) ideas turn out to be real well done (ouch!)
after a little stirring (aaag!)


'Scuse the puns, but it is all your fault, Tom :)

Lawrence