Hey guys thanks for the replies the homing is a interesting issue
I had a bit of a play and made a copy of my config that i hacked. What i found was that if i combined my original 2 PID loops into 1 loop and used the scale for position and used the motor velocity on encoder for velocity all in one PID then homing to index enable worked perfectly. but doing it that way the motor tuning was not as good. backlash is a constant of 0.1mm and the motor just jumped across that backlash whenever it moved. in practice that meant that the absolute scale position hovered by +-0.01 or so. this was not bad and totally usable. If i had done this option first i would have called it good and just used it. but the movement was much louder and less smooth than the two PID loops which get run through a sum component. As the original two PID loops was much much smoother, it was perfect and once i come to a stop the drive rapids to within 0.01 or 0.02 and then creeps the last 0.02mm (using the integral value which takes about a second) and ends up in the right place within 0.005mm or less. basically perfect every time with no dithering. It is now my most accurate machine. and really nice also the homing when homing I home all axis at the same time. X and Y get homed to index enable like normal. and the Z value i trick the home switch by checking the joint state. How this works is the Joint2 switch state goes form 7 when searching for switch the first time to 12 when latching on to switch slowly the second time so in classic ladder i set up two compare functions that basically choose whether to listen to the main home switch or the z axis motor encoder Z index pulse. (this is different to the index enable pulse and this is possible to miss when only checking in software not hardware. but at a slow speed i haven't found any issues yet, and when testing with a dial indicator i can't see any changes between homing. I can rehome linuxcnc up the top of the travel beside the home switch and then rapid down to the table which is about 1m away and the dial indicator is definitely within 0.01 each time. i can't really see any movement on it. So pretty happy all in all would like to fix the index enable issue in the future but for now it's working fine. this is a 40 year old machine and weighs 12 ton. and when i got it the backlash was 0.40mm. I replaced all the thrust bearings and changed from reduction gearbox to a nice mazak flexible coupling and direct drive. The timing belt was T8 timing belt so not even HTD profile which has more backlash which didn't help. The FIL factory actually got the location of the Z axis 2nd ballscrew mount out by 2.4mm which i was shocked by. i double checked measuring against the slideways at the top and bottom of the screw and yeah it was out 2.4mm from factory! so that is why all the thrust bearings were stuffed. anyway just about to get it running into production. this machine will be running like 16 hrs a day shortly probably. cheers Andrew On Thu, 23 Apr 2026, 5:11 am gene heskett, <[email protected]> wrote: > On 4/22/26 10:44, Dave Engvall wrote: > > Hi Andrew et al > > > > I have a cinci contourmaster set up with US made glass scales and a > rotary encoder on the ballscrew. I’m in transition to stepper-servos but > progressing slowly. > > When I started this process I did a manual check with the hand crank vs > the X axis scale. Proceeding in the positive direction I manually moved in > 1” increments, ie 4 turns of the hand wheel. Even in this early 1960’s > machine the glass scale was dead on. Since I’m protective of the US scale I > removed it and installed a Chinese scale. > > Both scales have a reference channel spaced with markers 50 mm apart I > suspect the US made scale has guard ticks either side of the reference > mark. I know the Chinese one's do. > > Homing procedure was simple. I installed a laser diode and a tab with a > hole in it at the end of the axis and manually homed to that before kicking > off a home of the machine. Z was done by manually moving Z to the upper > stop. If I positioned the tab and or Z stop correctly it never had to move > more than ~ 3 mm to index. All of this used the normal homing procedure. > Scan past the index and capture the position of index then move as > necessary to home. It is my impression that homing should be done in that > manner and not dual channel but after homing position seeking uses both > signals but I have no data to back this up! However, given the nature of > linear scales one might do just as well to use linear scale for position > and shaft encoder on the motor for velocity. The system on my Mazak used a > resolver on the servo motor geared (?) 7:1 and 1.6:1 to the ball screw. > Those numbers are from memory and a long time ago i.e a couple of years > before we did the Mazak at Galesburg. > > When I get the machine running with stepper-servos I will probably try > using the R channel for homing. Thanks for listening. Rant OFF. > > > > Dave > I have a much simpler setup, the only fully stepper/servo is my sheldon > lathe which is all stepper/servo, or about 1/2 of my 4 axis 6040 mill. > I have motors and drivers to do the same for my ball screw converted > go704. configurable as 3 or 4 axis. Home is the closing of the $2 > switch on each axis. The 6040 is the odd man out, There, z & b are > stepper/servo's where xy are just steppers. I can have a 1 second power > failure in the middle of a threading operation, remove the tool, rehome > the machine, I have notches in my r8's to replace the lack of tooing > keys go thru the rehome, replace the tool and hit r, all in the middle > of a 50 tpi threading opertion. That is plenty accurate enough for the > girls I might go with. The only none hard switch is the inductive home > switches on the 4th axis for z & a or b. My biggest problem is the.3mm > back lash in g7O4 z if the gibs are too tight. & I forget to oil it. > >> On Apr 18, 2026, at 10:49 PM, andrew beck <[email protected]> > wrote: > >> > >> hey guys > >> > >> just reporting back here with my success with the dual feedback > >> > >> i have 0.1mm backlash still in the ballscrew or somewhere > >> > >> and i have a magnetic scale on the machine also > >> > >> with a encoder on the end of the ballscrew > >> > >> i copied the examples on online of how they did that massive mill and > its > >> working awesome > >> > >> the mill rapids down or feeds down to 0.02 to 0.05 and then the I value > >> from the second PID takes over and moves the last tiny bit > >> > >> which takes a second or so > >> > >> and it stops right where it should everytime within 0.01mm the scale > says > >> it's perfect to 0.003 on the screen. and no dithering > >> > >> next issue thought is homing with index > >> > >> either to the motor encoder or scale z pulse > >> > >> > >> > >> i have my x and y axis homing perfectly with encoder index > >> > >> but i can't get linuxcnc to home this z axis to index. homing without > >> index works fine though. > >> > >> i think it is because linuxcnc internals throws a hissy fit because i > have > >> a hacked up PID system with two forms of feedback getting summed > together > >> > >> any ideas would be great > >> > >> cheers > >> > >> Andrew > >> > >> On Mon, Jan 5, 2026 at 11:04 AM gene heskett <[email protected]> > wrote: > >> > >>> On 1/4/26 01:36, gene heskett wrote: > >>>> On 1/4/26 00:23, andrew beck wrote: > >>>>> hey stuart > >>>>> > >>>>> yeah the ballscrew is connected both ends. i think one end has the > big > >>>>> angular contact bearings etc > >>>>> > >>>>> and the other only has deep groove ends probably (its much smaller > >>>>> than the > >>>>> driving end) > >>>>> > >>>>> on my other two cnc mills they have angular contact bearings both > >>>>> ends and > >>>>> the screws are stretched between them slightly with a tiny bit of > >>>>> preload > >>>>> > >>> Probably should clarify here. The tension isn't meant to stretch the > screw, > >>> but to pull it tight enough there's no lengthwise backlash under normal > >>> working loads despite a few microns of wear. On my sheldon, the z > screw > >>> has only a sliding ball bearing on the right end, so the double > bearing on > >>> the left controls the end play. After a decade of light use, I'm still > >>> under 2 > >>> thou of play acc a .0001 dial. > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > 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> >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>>>> hey guys > >>>>> > >>>>> i was reading the link that sam sent two and it looks like the two > Pid > >>>>> loops are running side by side and then the outputs are added > together > >>>>> before going to the servo drive > >>>>> > >>>>> would i be correct in thinking that they are using the "sum" > >>>>> component in > >>>>> linuxcnc to do that? > >>>>> > >>>>> if so this is getting pretty simple > >>>>> > >>>>> cheers > >>>>> > >>>>> andrew > >>>>> > >>>>> On Fri, Jan 2, 2026 at 3:07 PM Stuart Stevenson <[email protected]> > >>>>> wrote: > >>>>> > >>>>>> HI, > >>>>>> I don't think the LinuxCNC control scheme is cascaded or nested. I > >>>>>> would > >>>>>> call it parallel. > >>>>>> > >>>>>> The only backlash problem I can imagine is if the backlash causes > the > >>>>>> motor to dither trying to maintain position because cutting forces > >>>>>> move the > >>>>>> axis enough to cause the control to try to correct the axis > position. > >>>>>> > >>>>>> Is the ball screw supported on each end? If it is, then, is it > >>>>>> possible > >>>>>> the screw has some stretch preload. > >>>>>> > >>>>>> regards > >>>>>> Stuart > >>>>>> > >>>>>> On Thu, Jan 1, 2026 at 4:10 PM andrew beck < > [email protected]> > >>>>>> wrote: > >>>>>> > >>>>>>> hey guys > >>>>>>> sam i just looked at that link you sent > >>>>>>> > >>>>>>> that looks pretty cool > >>>>>>> > >>>>>>> only thing is i don't know is if it will work to reduce the > backlash > >>>>>> issue > >>>>>>> i am getting. I have about 0.10mm of backlash on each axis. half > of > >>>>>> that > >>>>>>> is from the thrust bearings which is fixable with new bearings > >>>>>>> the rest is from the ball nut i think and i am not willing to fix > that > >>>> Those tiny ball screws stuart sold me 25 years ago were just a hair > >>>> sloppy. > >>>> I found the balls .001 bigger on ebay and restuffed the nuts. One of > them > >>>> is in the Y of my sheldon, and has about a thou of backlash today I > had > >>>> to make a cage for the nut with teeny grub screws to stop the nuts > end- > >>>> play in the cage. So oversized balls are one possibility. Ticklish > >>>> work tho. > >>>>>>> but i can only try it. > >>>>>>> > >>>>>>> does anyone want to have a go with making a config like they did? > one > >>>>>> axis > >>>>>>> would be best I am struggling to get my head around the different > >>>>>> layers. > >>>>>>> i probably need to write it all out or use a diagram or something. > >>>>>>> > >>>>>>> regardless i am sure i will work it out. just might take some > brain > >>>>>> cells > >>>>>>> > >>>>>>> cheers > >>>>>>> > >>>>>>> Andrew > >>>>>>> > >>>>>>> On Fri, Jan 2, 2026 at 4:54 AM Sam Sokolik <[email protected]> > >>> wrote: > >>>>>>>> This was also John k.. I would trust his solution. He is the one > >>>>>>>> that > >>>>>>>> came up with hal in linuxcnc... (Among other things). > >>>>>>>> > >>>>>>>> On Thu, Jan 1, 2026, 9:49 AM andy pugh <[email protected]> > wrote: > >>>>>>>> > >>>>>>>>> On Thu, 1 Jan 2026 at 15:41, andy pugh <[email protected]> > wrote: > >>>>>>>>>> On Wed, 31 Dec 2025 at 22:14, andrew beck < > >>>>>> [email protected]> > >>>>>>>>> wrote: > >>>>>>>>>>> just looking for a config example if possible for a mesa card > >>>>>> from > >>>>>>>>> someone > >>>>>>>>>>> that has this nested PID loop setup. > >>>>>>>>>> This may not be as effective as you would hope. Cascaded PID > >>>>>> loops... > >>>>>>>>> Actually, what was done with the MPM jigmill wasn't a cascaded > PID. > >>>>>>>>> What was done there was to run a P + FF control based on the > motor > >>>>>>>>> encoder and an I control based on the linear scales. > >>>>>>>>> > >>>>>>>>> Then the two controllers were summed together to create a PID > >>>>>>>>> controller, but with I using a different feedback source than P > and > >>>>>> D. > >>>>>>>>> -- > >>>>>>>>> atp > >>>>>>>>> "A motorcycle is a bicycle with a pandemonium attachment and is > >>>>>>>>> designed for the especial use of mechanical geniuses, daredevils > and > >>>>>>>>> lunatics." > >>>>>>>>> — George Fitch, Atlanta Constitution Newspaper, 1912 > >>>>>>>>> > >>>>>>>>> > >>>>>>>>> _______________________________________________ > >>>>>>>>> Emc-users mailing list > >>>>>>>>> [email protected] > >>>>>>>>> https://lists.sourceforge.net/lists/listinfo/emc-users > >>>>>>>>> > >>> Cheers, Gene Heskett, CET. > >>> -- > >>> "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 > >>> Don't poison our oceans, interdict drugs at the src. > >>> > >>> > >>> > >>> > >>> _______________________________________________ > >>> Emc-users mailing list > >>> [email protected] > >>> https://lists.sourceforge.net/lists/listinfo/emc-users > >>> > >> _______________________________________________ > >> Emc-users mailing list > >> [email protected] > >> https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > > _______________________________________________ > > Emc-users mailing list > > [email protected] > > https://lists.sourceforge.net/lists/listinfo/emc-users > > Cheers, Gene Heskett, CET. > -- > "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 > Don't poison our oceans, interdict drugs at the src. > > > > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users > _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
