On Friday 14 December 2012 23:02:12 andy pugh did opine: > On 15 December 2012 02:57, Gene Heskett <ghesk...@wdtv.com> wrote: > > I never got far enough in math to know what a 'derivative' is. Does > > anyone have a URL where I could learn a bit about this function? > > It is largely the same as a differential, a measure if how one > quantity changes as a function of an input. > In general, in the context of your current project velocity is the > derivative of position, acceleration is the derivative of velocity, > and jerk is the derivative of acceleration. > > I feel sure that you actually do know what a derivative is, if not > necessarily by that name. The voltage drop through an inductor is > proportional to the derivative of current (by time) > > http://en.wikipedia.org/wiki/Derivative
After a bit of googling, that seems to be what it is. I was hoping for something that would linearize the response of the switching controller that actually controls the motor speed, something that would serve as a reverse of its function. At low drive levels, say up to 4 or 5 rps, it can be adjusted to perform well, but the controller itself seems to have a quite positive gain vs level function. It only takes about a 10% rise in input control once the 5rps line has been crossed, to drive it to what is effectively 100% and 21 rps. So I need a math function that can linearize the control, located between the pid.output and the pwmgen.value input. One thing I haven't checked yet is to time the pwm output and see if it changes by a fixed amount per requested rps, but with the feedback, I'd expect that to show some compression because of the outlandishly high rps obtained per rps requested at the high end of the range. This is difficult to characterize in a dynamic, feedback controlled system, but can be visualized easy enough by looking at the encoders velocity out and listening to the acceleration as a I limit the delta v going into pid.command, even when I set maxv so its a couple seconds to go from zip to 15 rps, it gets the first 3 rps in the first second, and the next 12 rps in less than the next second. The speed curve out of the velocity starts out at about a 20 degree angle for the first, then turns up at an ever steeper rate until it hits the target speed and come under feedback control till it shuts off 10 second after it started up. The decel rate OTOH, is very close to a straight, friction slowed line. There, the limit2 effect can be seen if I slow it down as that curve also turns concave if the slowing is gradual enough to be under feedback Igain control. The accel phase reminds me a wee bit of watching one of the Green Monster dragsters in about 1957. Rolls-Royce Allison 1710 cid engines out of the old P-51 fighter plane direct driving (with clutches of course) big truck rear ends with dual tires on both sides. Governors defeated, when the Christmas tree turned green, they were sitting at about 1500 revs and brought those truck tires up to about 80 mph in the first 3 feet, and they just sat there, rolling huge clouds of tire smoke until they had covered perhaps 330 of the 440 yards to the clocks. But at about the 330 yard mark, those tires got warm enough to hook up and they hit the clocks in the middle 140's, all in the last 110 yards or so of the run. The tachs would show peaks in the 3000-3100 range, a full 1000 revs above the red line when it was in an airplane. The biggest problem was the quill driving the blower scroll, designed for a max boost at red line of about 29 inches, but it was making 75 inches by the far end of the traps & that twisted off the blowers drive quill once in a while. To hear the tires finally hook up, and the throttle slam wide open in the last second of the run was to me, defying the laws of physics. Art Arfons, the driver/owner said it was like being hit from behind by a runaway freight train. They hadn't yet discovered that a jug of clorox dumped on each tire improved the grip quite a bit. That is about how this thing accelerates, the last 25% of the time is 50% of the acceleration. I miss those days, when you could take the family grocery getter to the drags on Sunday & if it was somewhere near right, bring home the trophy in your rides class. Cheers, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) My web page: <http://coyoteden.dyndns-free.com:85/gene> is up! May not be combined with other discounts. I was taught to respect my elders, but its getting harder and harder to find any... ------------------------------------------------------------------------------ LogMeIn Rescue: Anywhere, Anytime Remote support for IT. Free Trial Remotely access PCs and mobile devices and provide instant support Improve your efficiency, and focus on delivering more value-add services Discover what IT Professionals Know. Rescue delivers http://p.sf.net/sfu/logmein_12329d2d _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
