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Sorry Olin:

   I didn't mean to show off any simple control principle, I just came to=
 this project, tried to write down what I thought and wait for anyone to =
correct me.
   Now that I agree that microcontroller would be a better choice. There =
is something else I may need help with. I cannot find any book in the mar=
ket that is more emphasized on "embedded control theory". I took quite a =
few control classes before in graduate school, and none of them taught me=
 how to build some adaptive or nonlinear control based on microprocessor =
or other circuits.I know something about both areas, but is there some go=
od reference to have integrated these two parts?


> From: olin_piclist@embedinc.com
> To: piclist@mit.edu
> Subject: Re: [EE]analog or digital? --PID temperature control
> Date: Sat, 21 Feb 2009 10:23:31 -0500
>=20
> gardenyu wrote:
> > I started looking at temperature control inside a thermal chamber.
> > That tank is actually a first-order system with some possible
> > disturbances from different input and ambient temperatures. Also a
> > temperature sensor can be considered to have a linear, very small
> > time constant (or: high bandwidth). Thus, the system should be stable
> > by its nature.
>=20
> I'm not sure what your point is here since you're not asking anything. =
 Do
> you think you've discovered something new and are trying to educate us,=
 or
> are you just trying to show off something you think you know?
>=20
> In any case, I think your conclusion is incorrect.
>=20
> > Since not much compensation might be needed, many
> > people just use PID to tune up the time response of a temperature
> > control system.
>=20
> So you are a expert on PID control systems?  Again, what is your point?=
  I
> agree that PID is a popular control system, but there are others out th=
ere.
> I have personally used a very different and simpler control system for
> temperature regulation of a small tank, and it worked superbly well.
>=20
> > Recently I read a book written by Robert Pease, called "analog
> > circuits, world class designs". The front chapters talked about how
> > to realize feedback control loops by using a series of opamps, caps
> > and resistors. It makes so much sense so that I found some link
> > between all the control courses I took to the real world controls.
>=20
> Look carefully at those circuits and you can probably spot integrators,
> differentiators, and high and low pass filters (zeros and poles).  They=
 may
> not be much different from PID schemes, or at least some subset of P, I=
, and
> D.
>=20
> > As to the PID control, I found some modern designs simply use A/D,
> > D/A, and a microprocessor, and let the software guys to do the math
> > (which takes time since microprocessor cannot do calculus that easy,
> > also discrete control involves different math).
>=20
> Even for a slow PIC, processor speed isn't going to be a issue when dea=
ling
> with temperature control of anything big enough to be called a "tank". =
 You
> are likely dealing with time constants on the order of seconds.  A slow=
 16F
> doing software floating point should easily be able to keep up.  I once=
 had
> a 16F876 at 20MHz do two nested full PID control loops using software
> floating point and a update rate of 100Hz.  That's probably a order of
> magnitude or two faster than you need.
>=20
> > I wonder which way might be more appropriate? In an analog way,
> > certainly a hardware guy can take control of most things; he can also
> > get an easier and more direct continuous system model.
>=20
> My first knee jerk reaction would be to feed the analog signals into a =
PIC
> as soon as possible and do all the control in firmware.  There you don'=
t
> have component tolerance and drift issues, fewer parts, and its easier =
to
> modify and tune.  I would only consider the discrete approach if this w=
ere a
> high volume cost-sensitive product and the thing it was supposed to con=
trol
> was well understood.  The discrete approach could possibly be a little
> cheaper, depending on how much you could pare it down and how tight the
> control needed to be.  I wouldn't start there though.
>=20
> > But a digital
> > way seems like easy-configurable and adjustable, but the credit won't
> > be mostly mine anymore :)
>=20
> Your first responsibility is to design the best solution for whoever is
> paying you to do si.  If you're worrying about credit, you should be in
> politics, not engineering.
>=20
> By the way, my temperature controlled water bath from 1980 hardly used =
any
> analog components and no programmable parts, but it worked very well.
>=20
>=20
> ********************************************************************
> Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
> (978) 742-9014.  Gold level PIC consultants since 2000.
> --=20
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