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Subject: Re: Heated seat controller, open or closed loop temp.control
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Fri, 1 Nov 2002 00:03:57 -0800
NNTP-Posting-Date: Fri, 01 Nov 2002 00:01:20 MET
Organization: @Home Network
"john jardine" wrote in message
> Sylvan Butler wrote in
> > On 25 Oct 2002 14:41:14 -0700, john jardine
> > > "Jeroen" wrote in message
> > >> I'm making a controller for my heated seats in my car, and still
> > >> a decision whether it should be a closed or open loop temp control.
> > >> have 4 settings. In openloop that would simply be 25,50,75 and 100%
> > > For simplicity and sanity, keep it open loop.
> > > If you go for closed loop then be prepared to implement (and tune) a 3
> > > term PID control sytem.
> > Why??? A simple thermostat is closed loop.
> Agreed. A T/S is an excellent closed loop conrol system. I wish people
> would use them more often in the simple applications that many times
> I've seen implemented in horrendously complicated uP/DAC/ADC PID
> Jeroen though is after 4 fixed settings. This would imply some kind of
> 4 position, presettable 'click stop' T/S (electrical or mechanical)
> with a probe placed somewhere in/on the car seat. Hopefully, not where
> Fred suggests. Dependant on the local thermal time constant the unit
> would then temperature cycle between its own (unpredictable)
> upper/lower limits.
> > > The heater has a significant time lag to setpoint and this creates all
> > True.
> > > the control problems.
> > False.
> > > Do you really need +/- .5degC control ;-).
> > Trying for that level of control is what creates control problems.
> > A simple closed loop which maintains temperature at a setpoint +/-
> > several degrees is not complex.
> 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 is
turned on and when the temp reaches setpoint+1cdeg it is turned off again.
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). And
I will check the temp and act only once a second, any faster is not
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