Mark, I understand the desire for as much precision as possible but do you regularly work to tolerances of less then .001? If not then dither of a count or two will not affect your part making precision in the slightest.
an old racing quip is "Speed costs money. How fast do you want to go?" apply that to precision "Precision costs money. How close do you need to cut?" My goal with the 5 axis Cinci at MPM was positioning the tool tip within a .010 volumetric accuracy. I felt I could cut +-.010 parts all day with the machine at that accuracy. I did not calculate the volumetric accuracy from the readings I saw on the 3 indicator device but I estimate about .005 volumetric accuracy. I figured if I put a part on the mill with a +-.001 tolerance it was my fault for putting it on the wrong machine. We cut very few parts with a positioning tolerance of less than .005 and almost none with less than .001? When we do we use other tools and machines. When I got the 5 axis Cinci an old shop owner (300 man shop) told me the check was to run a part and if it sold then it was good enough. I didn't run quite that loose but I soon reached the point of dimishing returns. The Cinci is not perfect but we ran a 5 axis drill fixture a few months ago. Imagine a large dinner plate (36 inches diameter) with 100 holes normal (perpendicular) to the contour. These holes have drill bushings and are to drill small holes in a pattern. This requires closer tolerance positioning as opposed to drilling the pattern directly. I do not know the tolerance on the tool print but usually it is less than true position of .007. We spot drilled, drilled, end milled (bored) and reamed the holes for the bushing. The CMM bought the tool on the first try. I could have spent a LOT more money trying to get more precision out of the machine. The Cinci "appears" to cut to +-.001 many times. Cutting a +-.010 part, slapping calipers on the wall thickness (or any other convenient measuring feature) will many times yield a measurement that looks as if it is repeating inside the +-.001 tolerance zone. That is a far cry (and a completely different cutting regime) from holding the part to within .001 of a particular dimension. That is a wall of text to say this. Get your machine to make the parts you regularly expect to make and start making parts. You will then learn your machine and how to use it in a precision fashion. Then, when you have a precision cut to make you can use the process necessary to allow your machine to make the cut you need to make. Sorry for the longwinded response. Especially if you need the +-.001 cuts on a regular basis. :) Stuart On Sat, May 30, 2015 at 2:20 AM, Mark Johnsen <[email protected]> wrote: > John, > > Thanks for the response. > > By dither, I am trying to explain how the motor is not stationary at an > idle or rest condition. When stopped at 0 Velocity with the drives and > linuxCNC enabled, the motor constantly seeks a position between linear > scale graduations (0.01mm in this case). I can see end of the ball screw > turning and the DRO readout in Axis will change between say 0.0004" and > .0008". The higher the Pgain the faster the movement. I've tried some > Igain, but doesn't straighten it out. > > I have not tried the deadband equal to one or two, but certainly will do > that in the morning. That was also a a forum suggestion from PCW, however > he also said w/ a coarse scale you might not want to loose the resolution. > This was partially why I thought the 1um scales might be a good idea > because I don't think I'd care about dithering between .00004 and .00008" > or setting a deadband (maybe of 2 or a tad more here?). > > The 7I87 is remote isolated eight channel +-10V analog input card that is a > RS422 smart serial interface. I've read on the forum w/ a resistor it can > be used to +-20V, which is good because my tachs are 9.5V/1000rpm w/ a max > speed of 2400rpm. Also, PCW thought this solution still might seek scale > graduations (dither), only at a slower frequency. If there was a consensus > this would work, I'd be thrilled to try it, but PCW didn't seem to think > this was the answer. At one point he suggested encoders on the ends of the > ball screws (hmmm, sounds familiar). > > To use rotary encoders on the motor, I'm assuming I need to leave the tachs > for the old West Amp drives? Also, the encoders should have finer > resolution than the linear scales, I think 4:1 for motor encoder to linear > is what I heard? Does that seem right? > > Lastly, to beat a dead horse, I thought the higher resolution linear scales > would be easier to mount because of the existing bracketry. To add the > rotary encoders, I need to add mounts to the end of the ball screws or > backs of the motors. Maybe I'm being dumb here... > > Thanks again, > Mark > > On Fri, May 29, 2015, at 09:23 PM, Mark Johnsen wrote: > > I have made huge strides on my Supermax conversion from an Anilam > Crusador > > M to a 5i25/7i77. I was very happy to have the encoder resolution and > > motor/encoder directions setup so that I can start tuning. I was able to > > follow John Thornton's Velocity tuning instructions, but I have the > > almighty dither... > > What exactly do you mean by dither? Axis moving back and forth when the > commanded position is constant? > > Have you tried deadband equal to one or two counts of the scale? > > > Specs for the system are West Amp Servo's w/ tach feedback and 0.01mm > > resolution glass scales connected to the 7i77. > > > > Has anyone been able to tune this system and remove the dither without an > > additional high-res encoder added to the system? > > > > I've read as much as I can, mostly on the forum and here are the options > > I've found. > > > > 1. Try to tune out the dither. PCW mentioned maybe add a touch of > > pid-bias or FF3 (need to add FF3 to PID comp). NOTE: I need to go and > > work this a little more, but not optimistic. > > > 2. 7i87 for tach feedback, but PCW says you're still dead reckoning and > w/ > > time you'll drift. (ie no velocity when idle). And, I didn't see any > more > > about this after about January 2013. I wish this would work because it > > would be rather inexpensive for hardware. > > Is the 7i87 an analog input module? > > With only velocity feedback from the motors, tuning is likely to be tricky. > It might work though. And if the tachs are already there it is probably > the > least expensive solution. > > > > 3. External encoders on ball screws or back of motors. I've heard > people > > mention it, but haven't actually read about someone being successful with > > it. > > I think the best approach if you have linear scales is to add rotary > encoders > on the motors and use both. Use dual PID loops, one loop getting its > feedback > from the scale, and the other getting its feedback from the motor encoder. > Sum the two PID outputs together. > > The scale PID should have I-gain only, for steady-state accuracy in spite > of things like screw lash and thermal expansion of the screw. The encoder > PID should have P-gain and D-gain for good dynamic response and stability. > > FF gains can be applied to either PID loop, since the same command goes > to both. > > This has been done successfully. I used to have a blog posting describing > how we did it on Stuart Stevenson's big horizontal boring mill. (Ten foot > long > X axis.) But my blog has been down since I upgraded my server. Sorry > about that. > > > 4. Higher resolution linear scales. > > I don't think that is the answer. Any lash at all between motor and scale > will make it very difficult to tune. > > > Ok - I'm going for the jugular here, I found on Ebay some 1um renishaw > > linear encoders and scales from the same guy. I'm thinking I can get a > > discount and get 3 axes of 1um linear scales for $400. It's more than > > adding rotary encoders, but I think the futzing around and simplicity of > > the final solution is compelling. Also, I can remove (and sell) the > glass > > scales, put the renishaw's in place and wire in. > > > > Suggestions? > > I would add motor encoders before buying higher grade linear scales. > > > Thank you, > > Mark > > > John Kasunich > jmkasunich@... > > ------------------------------------------------------------------------------ > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users > -- Addressee is the intended audience. 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