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'PIC-based Oven Controller'
1999\05\05@154656 by Tom Handley

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  Lawrence Lile earlier started a topic relating to PID control of an oven.
I'm starting this new topic as I've been working on a very similar project.

  In my case, I have an old oven that my elderly mother still uses to bake
the same great things she baked when I was a child and I'm 49... The nice
thing about this old oven is the insulation. Once heated, it stays hot for
hours after you turn it off. The bad thing about this oven is the old,
mechanical, thermostat which is becoming `extinct'... I can still get
heating elements. Right now the thermostat keeps the average temperature
around +50 degrees F above the setting on the dial. Ok, let's get down to
basics. First. I could modify the dial settings with a pen or I could ask my
mother to subtract 50 degrees from the dial setting. This still does not
address the issue of an old, failing, thermostat that I may not be able to
replace and it's hysteresis which varies widely above and below the setting.
Then there is the fact that one needs to occasionally open the door to baste
a turkey or whatever. This leads to a confusing `thermal-lag' that can
`trick' the thermostat into providing a hotter than desired temperature
which, due to the thermal mass, tends to overheat the oven. The thermostat
use to provide fairly good control but it is failing...

  My first question is to Lawrence. Where `in the heck' did you find a
thermistor probe that will deal with 500F? I've looked at thermocouples
and cold-junction/interface ICs from Analog Devices and Linear Tech.
Right now I'm looking at the more expensive option of a Platinum RTD.

  As far as the user-interface, one side of me wants a digital display with
a keypad and an embedded internet interface with a web page. That way, I
could fly over to the former Soviet Union and visit with our friend Dmitry
and call home to start the oven, using a touch-tone phone, for the BBQ ribs.
Then Dmitry and I could fly back to Oregon and enjoy the ribs over a couple
of bottles of beer and vodka ;-)

  The practical side of the user-interface has to deal with my mother...
I've selected a rotary encoder with an absolute binary output. This would
provide a range from 150F to 500F in 25F increments with 0x00 = Off...

  I've looked at both PID and Fuzzy methods. The Fuzzy method would allow
me to tailor the oven to my mother's experience with the oven but it would
require a great deal of testing. I'd rather use PID which would require less
testing...  The oven was fine about a year ago from my mother's standpoint.
So I think using PID would bring the oven environment back to what she was
use to.

  To control the heating element, I was considering simple On/Off as
opposed to PWM. I want to use standard logic/AC solid-state relays. I have a
few 20A `blocks' in stock but this will require 40A with a good heat sink.
These are widely available and they all provide zero-crossing but I'm still
interested in using PWM.

  Rather than ask specifc questions, let's open this topic up for ideas
given the constraints of a typical oven that you use for cooking with
electrical heating elements.

  - Tom

1999\05\05@160347 by John Mitchell

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On Wed, 5 May 1999, Tom Handley wrote:


>    The practical side of the user-interface has to deal with my mother...
> I've selected a rotary encoder with an absolute binary output. This would
> provide a range from 150F to 500F in 25F increments with 0x00 = Off...

good idea (simpler is better.)  Make a big lighted switch which functions
as a on/off button -- you dont want Mom to try turn off the oven by
setting the dial *almost* off!

>    To control the heating element, I was considering simple On/Off as
> opposed to PWM. I want to use standard logic/AC solid-state relays.
> [...] I'm still interested in using 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.

My oven has two elements, one at the top and bottom of the stove.  The top
one only comes on in Preheating and Broiling mode, just keeping things hot
is the duty of the lower one.  Dunno about your stove, ask your mom.


hope it goes well!

- j


John Mitchell         "Beer is living proof that God loves us
Magnet Interactive    and wants to see us happy"
spam_OUTjohnmTakeThisOuTspammagnet.com      - Benjamin Franklin

1999\05\05@161818 by jamesp

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Tom,

Regarding the thermocouples, check with a vendor that deals in
semiconductor fabrication.  When I worked at TI, I worked in
the FAB, and there we used thermocouples that would deal with
up to 1000 to 1500 degrees C.  Believe me, this is hot.  If
you want, I can try to find the name of a vendor like this.
Just let me know.

                                         Regards,

                                           Jim


{Quote hidden}

1999\05\05@162014 by Geier, David

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Hello

Didn't Bob Pease have an Electronic Design article about using thermistors
or RTD to replace the thermostat in old ovens?  It was an analog PID, but it
should work just fine.  I took a quick look, but couldn't find the article.

David

> {Original Message removed}

1999\05\06@125907 by Tom Handley

picon face
  John, thanks for the feedback.

re: Rotary encoder with absolute binary output.
>good idea (simpler is better.)  Make a big lighted switch which functions
>as a on/off button -- you dont want Mom to try turn off the oven by
>setting the dial *almost* off!

  Saftey is a big concern but I want to simulate, even use, the same dial
and markings on the stove that she is use to. I plan on adding a mechanical
thermal switch if the temperature gets too high. The rotary switch will have
detents with a range from Off, 150 to 500 degrees F, and Broil.

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.

  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.

>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'.

  - Tom

At 04:02 PM 5/5/99 -0400, John Mitchell wrote:
{Quote hidden}

1999\05\06@125910 by Tom Handley

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  David, thanks for the pointer! I'll `fumble' through my stack of
Electronic Design mags. "What's all this Oven stuff Anyhow?" ;-)

  - Tom

At 04:10 PM 5/5/99 -0400, David Geier wrote:
>Hello
>
>Didn't Bob Pease have an Electronic Design article about using thermistors
>or RTD to replace the thermostat in old ovens?  It was an analog PID, but it
>should work just fine.  I took a quick look, but couldn't find the article.
>
>David
[snip ]

1999\05\06@172638 by Harold Hallikainen

picon face
On Wed, 5 May 1999 12:44:46 -0700 Tom Handley <thandleyspamKILLspamTELEPORT.COM>
writes:

       While I like the proposed web interface, the single knob for
temperature control is probably easier to work with!

>   The practical side of the user-interface has to deal with my
mother...
>I've selected a rotary encoder with an absolute binary output. This
would
>provide a range from 150F to 500F in 25F increments with 0x00 = Off...
>

       I kinda like the use of quadrature encoders with a display.  The
use of the rotary control has a nice feel to it while the display gives
precision in setting.  A quadrature encoder should be less expensive than
one with a binary output and takes less pins on the PIC.  Also, you can
do some dynamic scaling on it so that when you turn it quickly it makes
big changes in the setting, while turning it slowly makes small changes.
       Also, probably cheaper than an encoder is just a pot!  Run it
into an analog input on the PIC and do the A/D.  Only uses one pin!
       I think on/off control is fine for this application.  You may
want to experiment with how much hysteresis to put in the loop.  If you
do go for something fancier, a simple proportional control driving phase
control of the solid state relays seems pretty straightforward.  Just
don't try listening to the radio while baking!  Of course, you CAN move
all the way to PID control.  I'm not sure what this will get you.  You
state the heat losses through the insulation are low, so the 'droop" due
to the heating load should be minimal.  Even with proprtional control,
you have to "tune" the loop (or at least keep the loop gain down) to keep
the oven from oscillating.


Harold




Harold Hallikainen
.....haroldKILLspamspam.....hallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm

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1999\05\07@055619 by Russell McMahon

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A remarkably effective rotary encoder is a stepper motor. I haven't
used this in anything serious, but in bench tests it worked very
well.
A typical unipolar stepper with 2 coils produces two quadrature
voltage outputs when rotated. While amplitude is proportional to
speed of rotation (up to a limit) the voltages output are very
adequate to swing a comparator or even a transistor base. If you
don't mind committing two amplifiers or 2 comparators to the task you
can turn this slower than you would ever need to.

Small steppers are available at very reasonable prices surplus (often
new but in vast quantity if desired).




               Russell McMahon







>        I kinda like the use of quadrature encoders with a display.
The
>use of the rotary control has a nice feel to it while the display
gives
>precision in setting.  A quadrature encoder should be less expensive
than
>one with a binary output and takes less pins on the PIC.  Also, you
can

1999\05\07@094104 by Lawrence Lile

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face
>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

1999\05\07@094647 by Lawrence Lile

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-----Original Message-----
From: Harold Hallikainen <EraseMEharoldhallikainenspam_OUTspamTakeThisOuTJUNO.COM>
To: PICLISTspamspam_OUTMITVMA.MIT.EDU <@spam@PICLISTKILLspamspamMITVMA.MIT.EDU>
Date: Thursday, May 06, 1999 4:26 PM
Subject: Re: PIC-based Oven Controller

>        Also, probably cheaper than an encoder is just a pot!  Run it
>into an analog input on the PIC and do the A/D.  Only uses one pin!


Bingo, Harold!



>        I think on/off control is fine for this application.  You may
>want to experiment with how much hysteresis to put in the loop.  If you
>do go for something fancier, a simple proportional control driving phase
>control of the solid state relays seems pretty straightforward.  Just
>don't try listening to the radio while baking!  Of course, you CAN move
>all the way to PID control.  I'm not sure what this will get you.


P or PI control works pretty well.  I didn't see any need for the full PID
treatment.

I'd stay away from phase control.  I hate listening to my oven on my
walkman.

1999\05\07@131810 by Lawrence Lile

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-----Original Message-----
From: Barry King <KILLspambarryKILLspamspamnrgsystems.com>
To: RemoveMElilelTakeThisOuTspamtoastmaster.com <spamBeGonelilelspamBeGonespamtoastmaster.com>
Date: Friday, May 07, 1999 11:33 AM
Subject: Re: PIC-based Oven Controller


{Quote hidden}

Let's see.  My comments will apply most to small ovens, and your mileage may
vary using large ovens.  The thermostat controlled ovens I work on cycle at
10-20 second intervals at high temperatures (425F) and 30 sec to 1 minute
intervals at low temperatures (350F).   During these periods the unit will
cycle "ON" for 2 seconds (hot end) to 10 seconds (cool end) or so.

Hysteresis band is about 5 degrees C (9 degrees F)  meaning the unit will
vary in temperature by this much while trying to maintain a setpoint.  A
more massive oven might operate much differently.

I'm thinking that the radiation effects will be smaller in a large oven.
radiation is a square law phenomenon, and being 6 or 8 inches from a heating
element is a lot different from being 2 inches from it.

Besides, gas ovens have no radiation at all, just heat.  I'm beginning to
think the radiation effects are much more pronounced in a tabletop oven.

THermostats work in a wierd fashion.  They turn ON in response to oven
temperature.  But they contain a small internal heating element, caled an
anticipator.  They turn OFF when the anticipator runs the temperature of the
stat up too high.  This is how they prevent the stat from doing bang-bang
control.  Selection of the anticipator is a black art.



>

1999\05\08@063128 by Tom Handley

picon face
  Harold, thanks. As far as a quadrature encoder, I wanted to know the
absolute position. For example, what if there was a power failure when the
oven was set to 300F. How would I know where the dial was set? I could use
battery backup but I want to keep this simple and fairly `bullet proof'. I
like Lawrence's approach using a Pot.

  I also like his PID method which should be easy to implement with some
tuning. The proof is in the pudding and it sounds like his results are
pretty tasty ;-)

  - Tom

At 05:25 PM 5/6/99 EDT, Harold Hallikainen wrote:
{Quote hidden}

1999\05\08@070704 by Peter van Hoof

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this is actually used in the NSK Megatorq motors , it has a large
micrstepping motor part and a smaller encoder, the encoder in it is special
in that next to its normal phase outputs (or should i say inputs) it has a Z
winding to detect a reference position.

works excelent

Peter

{Original Message removed}

1999\05\08@073447 by Byron A Jeff

face picon face
>
>    Harold, thanks. As far as a quadrature encoder, I wanted to know the
> absolute position. For example, what if there was a power failure when the
> oven was set to 300F. How would I know where the dial was set? I could use
> battery backup but I want to keep this simple and fairly `bullet proof'. I
> like Lawrence's approach using a Pot.

Two or three possibilities:

1) It seems to me that any controller needs feedback. One could get the
current temp from a temp sensor.

2) Write the current temp into a EEPROM. a 24L00 are like a nickel each
and only requires two lines to talk to.

3) The obvious answer: start with the oven off. While debateable for short
glitches, clearly if the power is off for a long time, refiring up the oven
may not be a good idea.

BAJ

1999\05\08@083755 by Russell McMahon

picon face
Why would you want to know where the dial was set after a power
failure ! :-)
You wanna start a fire?
For very short power outages it is perhaps OK to carry such settings
through but after any protracted period (seconds?, minutes?) the
failsafe thing to do is to shut it down. Users may complain but ... .
Of course, this is what a mechanical dial does.

I have a product which has a similar speed setting device (here a
faster / slower self centering toggle switch) and I save the speed to
eeprom after each change. Not a problem over the design life in my
application. If the oven was to be adjusted with a frequency that
would overtax the EPROM's storage life if the setting was stored
after every change you could wait a few seconds after the last change
and then save it. This gives you 27 temperature adjustments per day
for 10 years with 100,000 writes to 1 eerom location. Using 10
locations would handle most tasks.




Russell McMahon

From: Tom Handley <RemoveMEthandleyspamTakeThisOuTTELEPORT.COM>

>   Harold, thanks. As far as a quadrature encoder, I wanted to know
the
>absolute position. For example, what if there was a power failure
when the
>oven was set to 300F. How would I know where the dial was set? I
could use
>battery backup but I want to keep this simple and fairly `bullet
proof'. I
>like Lawrence's approach using a Pot.
>
>   I also like his PID method which should be easy to implement with
some
>tuning. The proof is in the pudding and it sounds like his results
are
>pretty tasty ;-)
>
>   - Tom

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