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Subject: Re: Heated seat controller, open or closed loop temp.control
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Date: Sat, 2 Nov 2002 01:11:54 -0800
NNTP-Posting-Date: Sat, 02 Nov 2002 01:09:23 MET
Organization: @Home Network
"john jardine" wrote in message
> "Jeroen" wrote in message
> > >
> > > That's right but ... Essentially, what I'm suggesting, is that
> > > sticking a probe somewhere in the seat and allowing the setup to cycle
> > > within an undefined temperature window without PID control is
> > > actually still more complicated (even less comfortable!)than having a
> > > completely open loop setup.
> > Why is there an undefined temperature window? I will use a uC with a
> > tempsensor. When the temp is lower than the setpoint-1 cdeg the heater
> > turned on and when the temp reaches setpoint+1cdeg it is turned off
> > The ripple in temp. will be defined by the amount of hysteresis I define
> > (which can actually be quite small, nothing wears out by rapid cycling).
> > I will check the temp and act only once a second, any faster is not
> > necessary.
> > Jeroen
> With 'real world' closed loop control systems, the physical properties
> of the non electronic variables, gum up the electronic works and act
> as a grade #1 bitch!. It can't be half answered so this here is a bit
Thanx for the time taken for the explanation
I indeed forgot about the overshoot problem.
> This method is the (D)erivative or 'rate of change' part of a full
> PI(D) control setup.
I'm familiar with it, and yes, I know it can be damn tough to tune them
properly. I once made one that had to stabilize the light intensity in a box
(exactly 38 lux and 512 lux, switchable) for light sensor calibrations.
Well... that took *much* **much** longer to tune than I ever imagined... :(
The reason I choosed for the control loop was to compensate for lamp wear,
mains fluctuations etc.
Back to the seats:
From I what I can make up, the problem is that when you turn of the heater,
heating still continues for a while, so this need to be accounted for (as
someone once told me that this happens in house heating). And overshoot is
always a problem. The problem also with the heater is, the characterics
change depending on who sits on it (or nobody at all). I'll include a timer
that heats the seat some time before I'm going to use the car, so that I can
sit myself down on a nice warm seat :)
But do I need a fullblown PID?? Isn't the I-term just enough? E.g. constant
switching on/off, period 1 second, and increase/decrease PW by a fraction of
the error? The P term is not needed, because you can't say 'when continously
turned on the temp will be so and so much C deg', it also depends on the
Maybe I can make it an autotuning control loop? Digital home thermostats
seem to do this (not every house is the same). I've E2PROM in the AVR
anyway. Hmmm, very interesting stuff...
Are there any other control algorithms?
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