Yes, that works in your case where the cable is short. But what if this were a CAN bus in a very noisy environment. So noisy that only slow speed could work. Image an electric car and you want to keep the from and rear traction motors "balanced" so they don't work in opposition and you care about 0.1% gains in efficiency. These are 120 horsepower three-phase motors working at 400 volts DC. Your CAN bus is slow. Your bos is nuts and demands you solution have a 50 year design lifetime in an automotive environment. Tesla engineers solves this problem.
Now image you are given the same problem, Ok smaller moters, but this time the "cable" has to reach to Mars and has a 14 minute speed of light delay. And the environment is even worse than "automotive". JPL engineers seem to have solved this. (They could be the same engineers. Both places are within commute distance of my house and people here swap jobs every few years.) Here is one of by favorite "synchronized motors" videos. "Atlas is a 28 degree of freedom machine. All 28 motors are well synced https://youtu.be/knoOXBLFQ-s?t=42 Here is another. They built by mear mortals at a much lower cost. They use motors from quadcopter drones and some CNC'd metal parts. These are 12 "axis" machines, with three motors and three rotary encoders per leg. These are "Open Source" so you can download the design files. https://youtu.be/G6fMV1UPzkg?t=86 SO motor syncing is possible and is done. The machine tool use case is actually easy compared to other things On Tue, Aug 18, 2020 at 5:41 PM Gene Heskett <ghesk...@shentel.net> wrote: > On Tuesday 18 August 2020 18:58:01 Frank Tkalcevic wrote: > > > An off topic question, not directly LinuxCNC related... > > > > How are motors on a serial bus controlled and synchronised? > > > > My only real control experience is with LinuxCNC with Mesa and > > parallel port hardware where commands and feedback and precisely > > timed. Sending commands and receiving feedback over a serial link I > > would expect to cause synchronising problems, and the bandwidth would > > reduce the update rate. > > > > How are these issues handled? I tried googling this, but didn't find > > much - I wasn't sure what to search for. > > > By running the serial connection at many megabaud speeds. With spi, I am > issueing 32 bit command words with the pi's gpio's, writing to the 7i90 > at 40 megabaud, and reading the 7i90's replies at 25 megabaud. With a > transmission line only an inch long, its bulletproof. > > > Thanks, > > Frank > > > > > > > > > > > > > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > Cheers, Gene Heskett > -- > "There are four boxes to be used in defense of liberty: > soap, ballot, jury, and ammo. Please use in that order." > -Ed Howdershelt (Author) > If we desire respect for the law, we must first make the law respectable. > - Louis D. Brandeis > Genes Web page <http://geneslinuxbox.net:6309/gene> > > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > -- Chris Albertson Redondo Beach, California _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
