In one of his classes, Farin told us that when they were developing the early electronic controls for elevators that if they did not limit jerk (or maybe it was snap) that people riding the elector would loose their breakfasts. Apparently the fluid in the inner ear is quite sensitive to subtle changes in motion, and motion derivatives.
Now if you have ever ridden a moderns elevator (one made and installed in the last 20 years) in really tall buildings, they have very smooth motion profiles. It would be interesting to see just how fine they tune them. EBo -- On Dec 23 2015 7:50 AM, Pete_Gruendeman wrote: > Hi: >> I like to think of what happens when I apply the breaks on a car. >> It >> slows down, and if I do not let up a little near the end it grabs... >> I >> think that is technically jerk.< > Good example! Absolutely that is jerk as the curve for > acceleration has a step function in it. The higher derivative curves > would have a step in them also. They are frequently named Snap, > Crackle & Pop. While I don't know the exact significance of them in > regards to machine tools, when it comes to rotating cams, if all the > derivatives through Pop are smooth or at least have no step in them, > you can pretty much turn as fast as you want without developing > unintended motions and deflections. Sine waves and their derivatives > pass this test very well but there are other motions that can as > well. > > Just easing off the brakes as the car comes to a stop would be a > big improvement for fast moving machinery. That potentially solves > the overshoot problem. > > Pete Gruendeman > -------------------------------------------- > On Tue, 12/22/15, EBo <[email protected]> wrote: > > Subject: Re: [Emc-developers] jerk limited trajectory > To: [email protected] > Date: Tuesday, December 22, 2015, 10:44 PM > > On Dec 22 2015 8:33 PM, > andy pugh wrote: > > On 23 December 2015 at > 02:02, Jon Elson <[email protected]> > > > wrote: > >> So, > the spindle's > >> rotational > inertia makes jerk MECHANICALLY impossible. > > > > I don't think > that there is a mechanical limit on jerk. > > > > This is why you > stumble when a tube train stops hard. > > I like to think of what happens when I apply > the breaks on a car. It > slows down, and > if I do not let up a little near the end it grabs... I > think that is technically jerk. As for > applying gas, if you pop the > clutch... > > Seriously though. The VLA > example, they installed two motors (each > feeding the opposite way, and would feed them > propositionally to control > jerk, and > possibly higher order derivatives). They told me that when > > they originally had one motor on the > system, the initial impulse would > send a > shock through the entire antenna. I could have the story > wrong, > but that is what I remember. > > Back to LCNC land... I can > see this being useful for most cases, but > not critically necessary unless I was spinning > something less than a 24" > chuck. I > only had a chance to run a machine that large once, and that > > was decades ago. > > EBo -- > > > > ------------------------------------------------------------------------------ > _______________________________________________ > Emc-developers mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-developers > > > ------------------------------------------------------------------------------ > _______________________________________________ > Emc-developers mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-developers ------------------------------------------------------------------------------ _______________________________________________ Emc-developers mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-developers
