On 5/23/2013 11:29 PM, Jon Elson wrote: > Dave wrote: > >> I did a gantry system with LinuxCNC a few years back. The gantry was >> very stiff and self squaring when powered off. The gantry was 10-12 >> feet across and very heavy. It used used 2 - 1 KW servo drives driving >> two ball screws on each end of the gantry for the Y axis. In order to >> keep things simple I used step and direction on the servo drives and fed >> the same signals to both of the Y axis servo drives. >> > Thanks for the info. This machine has servo motors on each end of the > gantry, > so it really needs LinuxCNC in the loop for BOTH motor/encoder units. > What you did works great for step/dir drives, but won't work for servos. > > When LinuxCNC starts, the encoder counters are zeroed. When it is moving > toward the home switch, it is going to keep them synched the same way > as at startup. But, when it arrives at home, the first axis' counter is > going to get zeroed. At this point, something special has to happen to > prevent > the two motors from diverging. Assuming it has moved 10000 counts > from startup to the home switch, the first motor to find the index mark > on its encoder will suddenly have the encoder count jump from 10000 > to zero. I'm not sure what the other motor will be following at that > instant. > > I can think of a couple ways to do this. One is to simply make the 2nd > motor coast by disabling the servo amp until the master motor has completed > it's homing sequence. Then, enable the 2nd servo amp and home > that motor, which will be VERY close to home. Except that it would have > accumulated a bunch of movement while the master motor homed. > I can easily see a few HAL components could fiddle the commanded position > to make this work. > > What seems important is to keep the machine out of joint mode, so maybe > that eliminates gantrykins. The two motors must NEVER be moved > separately on this machine. The customer assures me this will cause > structural damage to the guideways. But, maybe gentrivkins will > solve the problems. > > Jon > >
Right, what I did with the step/dir servos won't work with analog servos. What I am saying is that the homing routine needs to be remade in hal probably using custom hal components so it can work with two drives while using Trivkins. The one that is there will not work with Trivkins and two parallel axes. On the large machine I did before, there was no way that one motor could drag the other motor to the home position, even if the drive was disabled. The gantry was stiff, but over a 10+ ft span that would simply not work. The gantry would bind up and jam. Dave Cole ------------------------------------------------------------------------------ Try New Relic Now & We'll Send You this Cool Shirt New Relic is the only SaaS-based application performance monitoring service that delivers powerful full stack analytics. Optimize and monitor your browser, app, & servers with just a few lines of code. Try New Relic and get this awesome Nerd Life shirt! http://p.sf.net/sfu/newrelic_d2d_may _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
