Re: [Emc-users] Slides, Encoders, and Resolution
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
Re: [Emc-users] Slides, Encoders, and Resolution
I don't know if you're set up to do this but you could use your shaft encoder for feedback to a drive and your linear encoder for feedback to EMC. That's what my Anilam controls do (one has been replaced with EMC, works great) but they use tach feedback to the drives. The drives are tuned to the motor and I use mostly P gain and a little I from the EMC PC. I can have backlash and the system is still stable though it delays getting in position. Roger N - Original Message - From: Kirk Wallace [EMAIL PROTECTED] To: Enhanced Machine Controller (EMC) emc-users@lists.sourceforge.net Sent: Friday, December 07, 2007 2:30 AM Subject: Re: [Emc-users] Slides, Encoders, and Resolution 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 - 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
Re: [Emc-users] Slides, Encoders, and Resolution
Kirk Wallace wrote: 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. You were using a linear encoder with an Acme screw? No wonder it didn't work. Jon - 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
Re: [Emc-users] Slides, Encoders, and Resolution
On Fri, 2007-12-07 at 12:46 -0600, Jon Elson wrote: Kirk Wallace wrote: 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. You were using a linear encoder with an Acme screw? No wonder it didn't work. Jon No, I have brand new ballscrews, but the dummy that ordered them (me) didn't insure that they had zero backlash. Anyone want to buy some barely used ballscrews? -- 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
Re: [Emc-users] Slides, Encoders, and Resolution
(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. - 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
