[ODE] Servo to reach a target position

[Nate W]

On Fri, 11 Apr 2003, David Whittaker wrote:

> > You have to think about what D is for. D back-compensates fluctuations.
> > Say you're running along nicely and suddenly hit an ice patch that
> > causes your engine to rev up due to lack of friction. D-term would help
> > greatly minimize that by responding to the sudden change in
> > error. 
> 
> Ok, so D is basically a reflex parameter.  Then it stands to reason that
> if you step a particular system more often, it's going to have better
> reflexes than another one...  So trying to make the same system behave the
> same with respect to D at different timesteps may be impossible.  

A friend of mine tunes PID loops as a part of his job, and he described
the D term of P+I+D as 'the only one that will actually change sign when
the present value is rapidly approaching the target value.  The other
terms will shrink, but the D term will actually put the brakes on to
prevent it from overshooting.'

Also, I doubt it's possible to change the update rate of a feedback look
without changing all three gain coefficients anyway.  A higher rate means
a lower latency, and allows higher coefficients and better performance all
around.  A lower rate won't be able to keep up with the same feedback
produced by existing coefficients.

-- 

Nate Waddoups
Redmond WA USA
http://www.natew.com