[time-nuts] D term (was no subject)

Jim Lux 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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