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kirtley24
06-20-2005, 07:28 PM
Dear all,

Well, I think Richard has opened a nice can of worms here!

While I take Daniel's point that inertial and pure time delays are not
the same, it seems to me that they have very similar practical
implications for movement control. Richard pointed out that the body
CoM displacement is 180 degrees out of phase with its acceleration.
This makes good sense biomechanically, since the acceleration is at a
maximum during the push-off phase of the cycle, which is a likely
source of the acceleration. It makes me wonder if the people doing
induced acceleration analysis take this into account? I assume they
must be aware of it, but it seems to me that they use integration of a
very short time interlude so maybe not?

Chris

On 6/17/05, daniel paluska wrote:
> Richard,
>
> 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.
>
> -Dan
>
>
> 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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--
Dr. Chris Kirtley MD PhD
Associate Professor
Dept. of Biomedical Engineering
Catholic University of America
Washington DC 20064

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