On Fri, Jan 22, 2021 at 1:42 PM Ralph Stirling < ralph.stirl...@wallawalla.edu> wrote:
> I don't think you are going to use an ADS1256 for > servo feedback. 30khz sounds good, but when you > look at the datasheet fine print, there are caveats. > First, the inputs are multiplexed, so cycling the > multiplexer drops your 30khz data rate to an effective > 4.3khz rate. There is also a 0.2msec settling time > for the digital filter (which would be per channel). > I'm not sure how much it affects CNC control, but > I know in robotics applications non-simultaneous > sampling causes problems. The sequential multiplexing > causes each axis to be sampled at a different time. > With robots, the way this is fixed is that the ROS "TF" or geometry transform messages are timestamped. Then the TF package can interpolate the transform for any desired time. So if say you have an arm with 6 joints and each sensor is sampled at different rates it still works. THis works because the parts have mass and can't teleport instantly to random places. so you can compute the position of the "fingers" even if all the joints are sampled at random times. The TF package hides all this Basically if you know the transfom at any two points in time then you can estimate the transform at any point in time near those two measured points. And again, this is all hidden inside the infrastructure. I say "transform" because in general even though mostly we use rotation (angle) only it might be any kind of linear transform. Yes it is a lot of real-time math but 16-core Xeon computers are cheap now. > > A precision single-turn pot for a linear axis sounds > like a problem for other reasons. You would have a > giant discontinuity every revolution. > > Anyway, this ADS1256 on these Chinese cards does > look very interesting for other, non-realtime applications. > > -- Ralph > > ________________________________________ > From: Gene Heskett [ghesk...@shentel.net] > Sent: Friday, January 22, 2021 12:45 PM > To: emc-users@lists.sourceforge.net > Subject: Re: [Emc-users] Another Mesa vhdl question (STMBL summary with > added pico-pi?) > > On Friday 22 January 2021 14:37:56 andy pugh wrote: > > > On Fri, 22 Jan 2021 at 18:41, Gene Heskett <ghesk...@shentel.net> > wrote: > > > What comparison can be made between the very limited, fixed scale > > > A-D conversion of a mesa card's 1st 4 inputs on say a 7i76, and the > > > A/D speed and scaling that $4 pico-pi might be able to do when > > > running the STMBL code, and how fast could it do it? > > > > If you want lots of high resolution A to D, have a look for ADS1256 > > boards on eBay. > > That's a sweet price for that much precision, and 30ksps sounds like its > plenty fast enough to run a servo feedback from precision pots. Good > thing about the precision being 24 bits, as the max of 3 volts full > scale says it will need scaled at the inputs to live with 5 volt or > higher circuits. With an SPI interface, working that out to cohabit with > a 7i90 looks to be an interesting project. And I just ordered a 4 > channel digital scope so I can see this stuff better. A 2GS input gets > pricey though. > > Thanks Andy. > > 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 < > https://nam12.safelinks.protection.outlook.com/?url=http%3A%2F%2Fgeneslinuxbox.net%3A6309%2Fgene&data=04%7C01%7Cralph.stirling%40wallawalla.edu%7C7b16a513d0b74475828608d8bf16aa15%7Cd958f048e43142779c8debfb75e7aa64%7C0%7C0%7C637469451362247427%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=DjcpC5qnBuyCYsgQU71FG8kTK7DNym23XrYlK277ItQ%3D&reserved=0 > > > > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > > https://nam12.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.sourceforge.net%2Flists%2Flistinfo%2Femc-users&data=04%7C01%7Cralph.stirling%40wallawalla.edu%7C7b16a513d0b74475828608d8bf16aa15%7Cd958f048e43142779c8debfb75e7aa64%7C0%7C0%7C637469451362247427%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=a20RBPKaM0pedhfPRU4R6WsbazAyJpUMdYABIiImbs8%3D&reserved=0 > > > _______________________________________________ > 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
