On Friday 08 October 2021 15:22:23 Peter Hodgson wrote: > Hello All, > > I'm hoping someone with more electrical control experience than me may > be able to offer advice on why I am seeing occasional rogue index > pulses which are messing up my G76 cycles on larger threads. > > The Halscope below shows an example of the occasional ghost index > pulse which can create a new 'thread start' and trash my parts as > photo above. > > A schematic of the encoder setup which is a quadrature 500ppr > incremental encoder can be downloaded here: > > > https://www.purbrookengineering.com/index.php?option=com_content&view= >article&id=11 > > > The signal goes to a Pico Systems USC board via a voltage level > shifter. We have a 0.75uF capacitor to filter the 5v supply to the > voltage shifter and 1kohm resistors in series on the input signals > from the encoder. The encoder cable is shielded, grounded only at the > encoder case end and is not close/parallel to any power cables. The > 12v supply to the encoder is from a voltage regulator and the 5v > supply to the level shifter is from the Pico control board. > > Jon @ Pico provided me with excellent support to get me this far with > the setup which now cuts smaller threads quite reliably but with > larger threads on my small lathe I need many passes and I haven't been > able to get rid of these occasional ghost pulses which eventually > cause a bad pass. > > I think these pulses are 'noise' but I cannot find what might be > causing them. > > Any suggestions please?
Most noise is caused by ground loops and less than optimum wireing practice. Good practice means all grounding is done at a single bolt, and nothing is allowed to be grounded /anyplace else/. Shielded wiring can help, but the shielding conductor is only grounded to this single bolt, with the far end of the shield wire cut off and insulated so it cannot touch anything on the far end. This limitation includes the power cord or supply hookups, there the buildings static neutral connection, done by the third rounded pin in the power plug here in the states (you didn't say where you are) must also be isolated such that once that conductor is in the equipment, it is isolated from ohmic contact to the machine frame anyplace but at this single bolt. Done correctly, a lightning strike on the buildings service may cause everything in the building to bounce 2 or 3 hundred thousand volts, but if wired properly according to those rules, it all bounces in unison and the voltage on the 5 volt logic line will still be 5 volts if measured to this bolt. In other words, no damage. I am a C.E.T., a retired broadcast chief engineer and I routinely bypass any encoder inputs that would be routed thru opto-isolators, connecting them directly to the inputs by cutting and bridging around any opto's in a BOB input circuit in order to get the high speed signals needed for good control back into LinusCNC. 3 machines in the garage, and one in a shop building in the back yard. Direct hits on the pole holding my supply can have probably hit more than once a year for the 31 years I've been here. I lost a computer modem about 29 years ago, so I took the house wiring to this room apart, which turned out to have been done by the flunky that wired this kit house at the National Homes factory in 1970, and was likely never on the same acre as a proper crimping tool, putting it back together properly, then 15 years ago, I added a 200 amp service, making the house a 60 amp branch. Lightning has not damaged anything here in that 29 years. I was typing, in this chair, when one strike occurred. I am not grounded as I have a full basement under me. I took a doorknob spark from the wired keyboard into a finger. Got my attention. So now I use wireless mice and keyboards. On everything. Follow the "star" ground rule I've outlined above, all radiating from that single bolt, and I expect your problem will be solved. If not, get back to me and we'll see if we can find it. A service oscilloscope with 30 megahertz or more bandwidth that can be set to trigger on the noise so you can send me pix of the noise would be most handy. > Pete > > > > _______________________________________________ > Emc-users mailing list > [email protected] > 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, 1940) 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 [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
