(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. >
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