On Sunday, June 03, 2012 08:11:48 PM andy pugh did opine: > On 3 June 2012 16:39, gene heskett <[email protected]> wrote: > > But its duty cycle was > > about 57 on to 43 off, > > I don't think duty cycle is all that important, unless it drifts far > enough to break the quadrature. > > If you use analogue-output sensors with comparators to square them up, > then you can change the threshold voltage to set the duty cycle.
Yes, but because the encoders velocity pin outputs a new sample/hold value for every edge that goes by, its velocity output is almost a random value, which at best jumps up and down by 10-40% of its average value on a sample by sample basis, so both the duty cycle of the signals, and the accuracy of the quadrature are in the real world, very important. A very minor tweak to the led current can make a huge difference in the peak to peak noise in the encoders velocity output. And that setting does not necessarily coincide perfectly with a 50% duty cycle as I found today after I tore up my circuit board and incorporated a 1k trimpot in series with each led so that I could set it for a 50% duty cycle. >From the docs on the encoder module: encoder.<chan>.velocity (float, Out) - Velocity in scaled units per second. encoder uses an algorithm that greatly reduces quantization noise as compared to simply differentiating the position output. When the magnitude of the true velocity is below min-velocity-estimate, the velocity output is 0 "greatly reduces quantization noise" everytime I read that, I'm like "in what universe does this happen?" It sure isn't happening in this one. Next I suppose is to get the disk to run a bit truer, it currently has about a 7 thou runout, probably because I fit the hole to the spindle a bit snuggly, so the side which happens to be sitting over the thread groove is possibly being pulled into the valley of the thread at that position by the thread peak 180 degrees away. I can put a dial on the edge, position the spindle to the high point and loosen the lock nut, but I can't tap it with a screwdriver handle and move it to reduce that runout. I should give it a good enough whack to mark the thread on the inside of the hole & sand the mark away, and might yet. There is also about 10 thou of wobble runout, but short of selective sanding on the bearing nuts to make the faces of the nuts run truer, or adding a layer or 4 of alu foil in the right place (I've done that a couple other places in this POJ), it simply isn't going to run any truer. In any event, it is now running noticeably better in mode 0 than the 39 cycle disk ever did in mode 1. Some pid tuning yet to do of course, but its fairly stable. It should cut some pretty high quality threads. Those I've already cut were better than I usually get with std dies if I keep the back end of a cutoff tool properly sharpened. 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> One way to stop a runaway horse is to bet on him. ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
