View Full Version : Re: EMD

06-17-2005, 02:19 AM

I think it is important to note the difference between 'delay' and
'phase.' Delay is fixed time regardless of frequency. Phase is frequency
dependent. Phase can look like delay and vice versa but they are
different phenomena and require different control and analysis techniques.
In the case of a force acting on a mass, (f=ma), there is 180 degrees of
phase between the input force(f) and the output(a). This phase can be
thought of as a delay if you pick a particular frequency. But this
shouldn't be confused with a pure time delay.
Signals traveling along neural pathways(or the internet,etc) have true
time delay, not phase. It takes a certain amount of time to get from
point a to point b.

From a classical control systems point of view, a system is stable in
feedback if the open loop phase is less than 180deg at the cutover
frequency(where open loop gain=1).
A pure time delay will add a nonlinear phase delay in the frequency
domain. Pure time delays make the system nonlinear and more difficult to
analyze with classical control techniques. Phase is something well
understood and capable of being handled by classical control techniques.


Richard Baker wrote:
> Dear all,
> Just to put a different perspective I've never heard any consideration of
> what might be called the "inertial delay" associated with forces moving
> Any comments?
> Richard

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