On Tue, 20 Oct 2009 23:26:27 +0200, you wrote: >several folks (including me) reported problems with threading - namely >oscillations of the spindle-synchronized feed (G33 and G76), mostly >with homebrew encoders. Most people looked at their encoders and >signal properties.
It's not only home made encoders, expensive commercial encoders have the same problem. >I've isolated the problem now with a *simulated encoder* - decoder >pair - it looks and sounds the same like a real encoder with same PPR. > >After trying with various PPR values in this setup, it turns out the >problem depends on PPR - if the PPR value is set high enough the >problem goes away. > >Here's my observation points: > >PPR behaviour >2000 fine optically + acoustically >1000 acoustically not as clean as 2000, but fine otherwise >200 ok, but Z axis definitely sounds "rough" >70 Z axis oscillation clearly audible >50 heavy Z axis oscillation >45 same (that's my encoder disk :-/) >4 same > >I had the sim-encoder RPM fixed at 240rpm. The problem shows on both >the threading.ngc and lathe-g76.ngc examples. The problem is, without using external hardware, anything above 150 ppr or so limits the spindle speed severely. 240 rpm is very slow for a CNC machine threading. Most of my threading is done at 700-1000 rpm, and that is well within the capabilities of my Z axis. Compared to commercial machine even that is slow. Here, with my hardware and PC, EMC can't read pulses reliably over about 200 rpm with a 1024 encoder, which is totally impractical. >Even if some folks have problems with encoder noise, I think the >problems more likely stem from the fact that homebrew encoders tend to >have a low PPR value - that might explain some of the observations. But we are stuck with low PPR, it would appear, due to latency. >I dont fully understand the trajectory planner, but my gut feeling >it's a control loop oscillation which gets excited with the encoder >quantization/noise spectrum gets too close to the loop frequency. Neither do I, but I tend to agree that there is some other design limitation that is not helping somewhere. Steve Blackmore -- ------------------------------------------------------------------------------ Come build with us! The BlackBerry(R) Developer Conference in SF, CA is the only developer event you need to attend this year. Jumpstart your developing skills, take BlackBerry mobile applications to market and stay ahead of the curve. Join us from November 9 - 12, 2009. Register now! http://p.sf.net/sfu/devconference _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
