Simpler Than A Thermostat?

Bret Cahill wrote:

What feedback? From where?


If sign is irrelevant, what does "slips past" mean?


I resent that characterization! The proper term is "jury rig". Look it up.



What can be prompting you to want to control a thing without knowing how
big it is or even whether it's more or less than you want? Off hand, it
sounds to me like the product of a fevered mind.

Jerry
--
Engineering is the art of making what you want from things you can get.
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At first glance this control system would seem even more primitive
than a conventional simple thermostat control.  It would work only if
the initial conditions were the right sign because it only cuts on
when the output is equal to or just above (or below) the reference
temp.

For example, in the morning you set the ac to 80 degrees when it was
75 degrees.  At noon the temperature reaches 80 degrees and the ac
automatically cuts in as it should.  The room cools down a degree or
two and then the ac shuts off as it should.  The room warms up again
and the ac cuts on again.

The controller seems to be working.

If you waited until the afternoon, however, when it was already 85
degrees in the room to set the ac at 80 degrees, the ac would not cut
on as it could not tell the difference between being above or below
the reference temp.

One less than satisfying solution might be for the controller to start
looking for the output.  It could only do this by _changing_ the
output until it reaches the reference.  In the thermostat example the
controller would wait a certain amount of time and if the output
doesn't reach the reference point, the controller turns on the ac for
x minutes.  If that didn't find the output it shuts off and turns on
the heat pump for 2x minutes.  If that doesn't work then back to the
ac for 4x minutes . . .

Obviously this isn't going to revolutionize the thermostatic controls
industry, but has anyone heard of any solutions that could end run
sign determination yet still be reliable?


Bret Cahill

Bret Cahill wrote:

Most of us know about thermostats.


Why not? What kind of mechanism has a set point but no way to determine
if it is exceeded or not reached? A built-in absolute-value function?


And that is better than an ordinary bi-metal thermostat?


Yes. Remove the sign suppressor.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

A thermostat-simple controller.


Absolute value is the best way to look at it.

The ref would be x = - b/m of the line y = |mx + b|.

The only difference is there isn't much of way to measure the
magnitude error.

Of course, conventional thermostats don't really care much about the
magnitude of the error either and they work just fine.


The thermostat was only proffered as an example.  I'm not interested
in temperature or ac, just controls solutions.


That would require a sign _recoverer_ which will complicate things.
(I'm one of the greatest apostles of the KISS religion the world has
ever known.)

There's some predictability to the time lags so it might be better to
have the output dither just above and just below the reference.  Once
the system is up and running the controller would allow a certain
amount of time where it is _guaranteed_ to pass the ref and then
reverse direction.  If the ref isn't found in that amount of time the
controller knows something is wrong.

This would be good enough for a prototype anyway.  Someone else can
get fancy if that's necessary.


Bret Cahill

Bret Cahill wrote:

Then KISS and KTS (Keep The Sign). Error signals come from subtractions
-- desired minus actual -- and subtraction yields a sign. Keep it.


We might be better able to help if you describe the actual problem
instead of stand-in systems for which existing trivial systems are the
clear answer. I'm unlikely to think of a solution to your problem
without understanding why it a problem.


Hmmm... Simpler than a thermostat, but fancy if necessary. Cryptic!

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

It might not be possible to get a useful magnitude from the sensor.
All you get is an indication that the output is where it should be.

That's why constantly dithering around the ref. pt. may be a good
prototype solution.


Bret Cahill

If you can't tell if it's not where it shouldn't be, how can you tell it
is where it should be?

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

On Mon, 13 Jul 2009 18:37:35 -0400, Jerry Avins wrote:


You may wish to check the Google Groups archives for
sci.electronics.design for this guy's behavior; it may save you some time
here.

--
www.wescottdesign.com

Tim Wescott wrote:

Thanks, but I'm done anyway.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

A delta function is out there somewhere.

Anyway here's a couple approaches, which may or may not be in the
literature.

1.  Several reference points [sensors] are closely spaced around the
desired ref. then simple diode logic determines status & +/- action.
This would be pretty cheap and easy.  You don't complicate your life
with a lot of transfer functions, stability analysis or software.

2.  Use two reference points sensors] just above and just below the
desired reference point.  The difference in the derivatives of both --
one should always be negative and one should always be positive -- to
determine action.  Maybe it's closer to conventional thermostat
controls than originally thought.

3.  Some combination of 1 & 2.

Are there any more?


Bret Cahill


"When compared to a genius, that is, he who begets or gives birth, the
scholar comes off a bit like an old maid.  He's not conversant with
the two most valuable functions of man."

-- Nietzsche