mike:
>I do get a slight overshoot with as step input, but adding even quite large
>amounts of D into my controller didn't seem to help overshoot very much at
>all.

the long time constant you are talking about suggests the system is pretty heavily damped anyway (i
think...), so the D term may not bring much to the party.
also in heating / cooling systems the drive is often asymetric - by which I mean that you are using
a different mechanism (or in the case of peltier devices one mechanism with differing efficiencies)
in each direction - and I guess that may well have an effect on the relative usefulness of the
different terms.

do remember that PID control is only a bodge really anyway - it saves you having to analyse the
system you are trying to control and model its transfer function 'classically' - but it's a bodge
(aka "engineering approximation") that is very often perfectly sufficient for the "real world" job
(as opposed to the first year control systems course where they will have you think that you need to
analyse every system individually and then design a custom controller... still at least it provides
a bit of background to the discovery later on that in fact 'almost everyone' just buys PID
controller units and has a the manufacturer tune it for them)

Regards,
Simon

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