Na 16.9.2010 17:22, Jon Elson je pisal: > Slavko Kocjancic wrote: >> Hello.... >> >> I'm use EMC with 25uS base period. This came little coarse now as I get >> stronger motors for my machine. So I look under the hood of emc how >> works. Now I get confused how all thing works. >> >> If I understand correctly the stepgen (in base period) just make pulse >> if needed but all the accelerations/deceleration are calculated in servo >> period thread. Is that true?!? >> >> If it's true then acceleration isn't smoth as we think. Assume Servo >> thread at 1ms and base thread at 25us. >> Let's assume we accelerate from 0 to 10000 step's /second >> >> When step rate is under 1000 the every step can have different length >> and over 1000 steps the more steps in series have same length not >> accelerating lineary. Near the end the motor got 10 steps at same length >> and next 10 steps at little shorter time. >> >> San someone verify that this is true? And how that affect performance? >> (acceleration curve not straight line but staircase one) >> > Sure, if you have the BASE_PERIOD set for 25 us, and your maximum step rate > is 10,000 steps/second, then that is 4 base periods. The next step down > in speed > is 5 base periods. There is no way, given how stepgen works, to produce > steps > at any rate between every 125 us and 100 us. So, to produce a rate of, > say, 9000 > steps/second, it has to hop back and forth between 100 and 125 us > periods between > steps. It is not very easy for larger motors to follow these large > fluctuations in > step timing. >> Is mesa and pluto board done in same way? >> > I can't answer for those boards, but my Universal Stepper Controller > (USC)works in a fairly > similar fashion. But, instead of the BASE_THREAD being 25 us, the > "clock" is 100 ns, or > 250 times finer timing granularity. So, instead of a 25 us jump in step > timing out > of 100 us, or 25% variation, it would have a 100 ns granularity out of > 100 us, or > 1 part per thousand. > > You can't do this with a microcontroller, as they are sequential > processors, and might > have to be shared between axes. Typical microcontrollers just won't be > much faster > than a Pentium-class CPU, even if dedicated. The USC board I make has > completely separate hardware > for each axis, and all the counters are running at 10 MHz (100 ns per > count). > And, yes, with the USC, you do not use a base thread at all, just a > servo thread. > Vastly lighter load on the CPU. A 600 MHz Pentium III is perfectly > adequate. > > Jon > >
I'm confused now. With microcontroller I can get 0ns jitter (without jitter calculation can got under 1us) and can set base period to aprox 5uS. And seem's to have just enougth time to calculate for 3 to 4 axis. ...running MCU at 20 MIPS. And with acceleration recalculation I probably confuse here (sorry my english isn't so good) so I try to make example. Let's get that stepper has already 4000 steps/second (250us/step) and accelerating so for motor the next step should be 250, 249, 248, 247, 246, 245, 244, 243, 242, 241, 240 microsec later of last one to got linear acceleration. but with EMC that wil be like that 250,250,250,250, (new servo thread calc), 245, 245, 245, 245,.... I thinking to put stepgen in microcontroller as finding good PC with lattency under 15us is not easy. So base period under 20uS is not likely on PC but for $2 microcontroller the 5us is easy reach. ------------------------------------------------------------------------------ Start uncovering the many advantages of virtual appliances and start using them to simplify application deployment and accelerate your shift to cloud computing. http://p.sf.net/sfu/novell-sfdev2dev _______________________________________________ Emc-developers mailing list Emc-developers@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-developers
