[ODE] Servo to reach a target position

[David Whittaker]

> 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.'

That makes sense.  But since ODE will 'put the brakes on' (apply a
negative force/torque) if inertia is carrying your joint faster than the
set velocity, this is yet another proof that the D term is not needed.

> 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.

I agree.

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