On Thu, 2007-12-06 at 12:09 -0800, Kirk Wallace wrote: > (Dooh, fixed title spelling) > > On Thu, 2007-12-06 at 11:43 -0800, Kirk Wallace wrote: > > I got the first pass on my Bridgeport X axis working and I am having > > trouble with tuning. I am using a brushed DC motor with a 3:1 belt ratio > > between the motor and ballscrew. The motor is driven by a Pico PWM amp > > and controlled by a Pico UPC and EMC 2.1.7. Because a glass slide was > > already mounted, I used it for my encoder. My primary concern was to do > > a sanity check on the motor because it is a cheap treadmill motor and > > not a true servo motor. The motor seems to have no trouble driving the > > axis at my theoretical rapid of 390"/minute or 6.5"/second. My next > > consideration is the low speed performance, which seems much more > > dependent on the system tuning than just raw system power. Since I have > > a similar system on my lathe, I started with its tuning parameters. I > > need to post pictures and Halscope plots but some general issues came to > > mind. > > > > Glass slides do not seem to be recommended for CNC applications. Could > > someone remind me why? I would think for positioning accuracy you would > > want as little as possible between the tool and the mechanism doing the > > position measuring. On the other hand for motor control, you would want > > as little as possible between the motor shaft position and the > > controller. Is there a way to cater to both? > > > > I seem to have a consistent (non-self energizing) oscillation across a > > wide P range (30 to 280). I am wondering, is this is due to the .002" > > screw backlash, or this backlash being on the motor side of the encoder, > > the relatively low encoder (slide) resolution or all of the above? Which > > has the greater influence? > > > > The end position always seems to come to within an encoder count > > (0.0005") but the rate seems to vary widely during the traverse. > > > > Anyway, I need to document and post better information, but if anyone > > has any thoughts, I'd appreciate hearing them. > >
I jury rigged an encoder to the motor and it tunes fairly well now, but now the travel end points are affected by the backlash. I am looking at getting the MicroKinetics ballscrew kit. http://www.microkinetics.com/convkit.htm (at the bottom of the page) which appears to have adjustable double nuts. With zero backlash, it seems that it shouldn't matter whether I use the linear slide encoder or the shaft encoder, though my shaft encoder has much higher resolution. -- Kirk Wallace (California, USA http://www.wallacecompany.com/machine_shop/ Hardinge HNC lathe, Bridgeport mill conversion, doing XY now, Zubal lathe conversion pending) ------------------------------------------------------------------------- SF.Net email is sponsored by: Check out the new SourceForge.net Marketplace. It's the best place to buy or sell services for just about anything Open Source. http://sourceforge.net/services/buy/index.php _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
