On Sunday 15 July 2012 16:45:27 Jon Elson did opine: > Gene Heskett wrote: > > But the internal calculations for velocity etc are done at the servo > > threads granularity of nominally 1 millisecond. So there is no chance > > of the figures being accurate when the count may be up to a > > millisecond old. Perhaps with more resolution than my 39 cycle wheel, > > which is 156 counts per turn or a new position every > > 2.3076923076923076923 degrees of rotation, it might improve. > > Right, the count may come in just before or just after the sample is > taken. > > > But the addition of the noise caused by the 1 millisecond granularity > > of the floating point error process is an apparently huge error > > sample to sample. > > It has nothing to do with floating point, it is just the nature of a > sampled system. > The coarser the encoder resolution is, then the greater the difference > in whether > you got that count or didn't, on this servo period. The worst case is > when the > encoder is providing less than one count every servo period. Say 500 > counts/second > with a 1 ms period, you alternately get zero, one, zero, then one count > per period. It APPEARS the axis is stopping and starting 500 times a > second. > > > Its very difficult to determine without running that signal thru a > > lowpass block with a gain setting of .001 before feeding it to a > > halmeter, because even at that rps, the halmeter flickers between 17 > > and 23 from this (to me utterly false) digital noise! > > It isn't digital noise, it is the downside of quantization. > Its the same destroying the validity of the data effect Jon.
> > When cutting threads with the G76 canned cycle, with the spindle > > between 2 and 6 rps, it cuts great threads, but the racket as the z > > motor tries to track this noise is similar to slowly dropping a > > shovel full of pea gravel into a washtub. I should post a video with > > audio, its very rough sounding. > > > > I am not privy to how the encoder module works internally, but it > > seems to me that if a timestamp could be put on the base thread > > sample > > The software encoder component, Mesa encoder counters and the Pico > Systems UPC controller all can do this. For the UPC you have to enable > this feature with a parameter on the command line. I'm not sure how > much it can help with > very low-res encoders, there is a certain speed at which is switches > over from > timestamp to position delta, but as long as it is less than two counts > per servo > period it should be using the timestamp. If it is using time stamps at low rps, below 0.012820512820512820513 rps in my case, I don't see it. The error output is so close to rail to rail. It will accel for about 30 degrees, the coast for 30 degrees at my pwm's minimum setting, which IIRC is about a .078% duty cycle at creep speed, perhaps 1/3 rps. With 156 edges/rev in my setup, and your 2 edges per servo thread the rev is then divided into 78 pieces. So to get 2 edges in 1ms, it has to turn about 6.45 rps. If my button pushing on the calculator is correct... Somehow, it seems to me there really ought to be a better way to develop a smoother error signal. One totally based on the elapsed time between edges, and perhaps smoothed over the last 4 edges when in the quadrature mode. > Jon > > ------------------------------------------------------------------------ > ------ 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 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! It was a JOKE!! Get it?? I was receiving messages from DAVID LETTERMAN!! YOW!! ------------------------------------------------------------------------------ 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
