On Saturday 17 September 2016 17:02:51 andy pugh wrote:

> On 17 September 2016 at 20:24, Frederic RIBLE <fri...@teaser.fr> wrote:
> > This is why we are thinking about using mesa to implement this
> > function.
> It might be enough to "fool the drive" but there really is no
> information content in that velocity signal.

I just looked again Andy, and what I see is a faster error feedback to 
the drive than it would get by going around thru the PID in the probably  
much slower because its much older OEM controller.

Am I imagining that would make a difference?  I have seen similar 
treatments in tracking and error correction in high end broadcast 
videotape machines, some more than 30 years old now.  Not wearing the 
same names for the function block but it seemed to me the principle was 
the same.  A faster feedback path is one way around the Nyquist 
stability diagrams. I am recalling a robotic 2" videotape player that 
kept 30 sec to 2 minute commercials on small cassettes that the operator 
could program 4 or 5 station breaks at a time, from a library robot 
feeding two player decks that were self threading.  In addition to the 
robotic load and unload back to a storage rack, it had a several 
kilowatt rated headwheel motor drivers which supplied some of its magic. 
Both headwheels sitting dead, push the start button, the lights blinked 
and the wheel was turning 14,400 rpm in about half a second. Time base 
correctors locked to house synch could output a color phase locked 
picture in under a second, but you could also have the tapes vacuum 
shoes retracted 10 thou so the flying heads wouldn't saw the stopped 
tape in two, which allowed the wheels to be started a minute early, so 
that when the start signal came in, the shoes closed in 5 milliseconds, 
and one frame of video later (16 milliseconds) it was color locked and 
the commercial was playing.

It was a hungry machine, needing an additional 25 kw in AC for every such 
machine in use.  The technology would be considered ancient history 
today, and it cost north of a million in 1975 dollars.  But there was a 
need, and it worked well enough the stations that could afford it, 
bought it in pairs.

Cheers, Gene Heskett
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>

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