[time-nuts] D term (was no subject)
jimlux at earthlink.net
Sun Jan 25 21:12:00 EST 2015
On 1/25/15 1:30 PM, WarrenS via time-nuts wrote:
> I second Poul-Henning Kamp's comments concerning D-terms,
> (mostly) as done in the TBolt and likely other GPSDOs.
Bear in mind that a PID loop is basically a fairly simple control loop
that is easily susceptible to linear analysis.
They're simple to implement with analog controls, they're simple to
analyze, and for a whole lot of applications, they'll work just fine.
And there's decades, if not centuries, of experience with P, PI and PID
controllers in a practical sense. A lot of people know how to *tune*
the parameters based on observed system dynamics.
But for a lot of systems:ones where there are significant nonlinearities
and/or time delays and/or saturation/limiting effects a PID loop might
not be a good choice. (for instance, if you're doing a closed loop
position control with a stepper motor as the actuator, with a small
motor and a big heavy load, with low friction...)
For myself, I am seduced by the idea of a control system that builds and
adjusts a model of the system being controlled, and then derives the
needed control inputs from inverting that model (whether arithmetically,
or by some clever algorithmic means).
For instance, a PID controller doing temperature control doesn't have a
*good* way of incorporating side information like the outside
temperature. There's all kinds of schemes for doing this (double loops,
extra terms, etc.)
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