Re: [Emc-users] Slides, Endoders and Resulotion
On Thu, 2007-12-06 at 23:01 -0500, xtra209 wrote: I checked out some treadmill motors rescued from defunct treadmills and discovered they are really not reversible. Mine will run in both directions but one direction the armature arcs more than the other direction. A closer investigation of the motor showed that the brushes contact the commutator at an angle. Sooo... My treadmill motor isn't designed to run both directions. Maybe you are moving the brushes backwards causing problems? I think the motor I have is only suitable for the direction it is designed to run. Maybe this will help... Clint I was lucky. My motors have the brushes located axially. I have not noticed much arcing at all. I do have some other issues with the motors, such as, the rotor has straight windings where some motors have a twist to the laminations which I think is supposed to provide a more constant torque. I powered them with my lab supply and I was able to run them very slowly without any galloping or stopping at a particular location. The rear bearing is tiny compared to the front and is mounted in plastic, which shouldn't be a problem if I keep my belt tension at a reasonable level. The front bearing has a spring which biases the rotor towards the rear bearing. If I push on the rear shaft, I can move the rotor against the spring about .030. I haven't decided if this is going to be a problem. It may be, if I fit an encoder to the rear shaft, but there should be no side loads. This is the motor here: http://surpluscenter.com/item.asp?UID=2007120702032835item=10-1783-Acatname=electric At $20 each, I wanted to see if any money could be saved compared to a real servo motor with the same ratings. It may be a wash since I had to machine a new front bearing mount to provide for mounting, but it depends on the value of my time. With 3:1 (actually 40:13) ratio pulleys, it scoots pretty well. At certain loads, though, it makes a faint moose mating call. I find it hard to concentrate on my work when it does that. -- 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, Endoders and Resulotion
Kirk Wallace wrote: On Thu, 2007-12-06 at 14:08 -0800, Peter C. Wallace wrote: What is your deadzone? ... snip If you mean deadband, I have it set to 1e-05. I set it to 1e-04 and 1e-06 which had no effect. You need to set it to 3E-03 to get it above your stated backlash. (You can try 2.1E-03 and move up and down a little bit to find the minimum that supresses the hunting.) You said you had 0.002 of backlash. 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, Endoders and Resulotion
On Thu, 6 Dec 2007, Kirk Wallace wrote: Date: Thu, 06 Dec 2007 11:43:27 -0800 From: Kirk Wallace [EMAIL PROTECTED] Reply-To: Enhanced Machine Controller (EMC) emc-users@lists.sourceforge.net To: Enhanced Machine Controller (EMC) emc-users@lists.sourceforge.net Subject: [Emc-users] Slides, Endoders and Resulotion 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? One way is to use dual feedback, Dont know if EMC supports this or not. 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? I would say the backlash would be the major source of the oscillation. Even with dual feedback, It would be problematic to prevent 'hunting' . I guess it would be simpler to use a rotary encoder and have EMC do the backlash compensation. What is your deadzone? 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. -- 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 Peter Wallace Mesa Electronics (\__/) (='.'=) This is Bunny. Copy and paste bunny into your ()_() signature to help him gain world domination. - 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, Endoders and Resulotion
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? Low resolution and possible backlash between motor and encoder. 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? Yes, have the backlash reduced to the absolute maximum. Make sure the slider on the glass scale moves very freely (may need internal cleaning and work on the shield that keeps junk out of the encoder. Make sure the link between the encoder read head and the machine is straight. A kink in the linkage can allow it to bend under load, and so the encoder itself can develop backlash. This is especially true of the ones that use a piece of music wire to push/pull the head. 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? You're sunk! 0.002 backlash is HUGE! You need to have the ballscrews repaired, or the end bearings on the screws, wherever that backlash is, you can't expect proper operation with that much backlash. The only way to supress this right now is to set the deadband to .003 or so. The end position always seems to come to within an encoder count (0.0005) but the rate seems to vary widely during the traverse. Yes, the servo is trying to get to where it is commanded, but it is hard to do with the backlash. You probably need more gain, but the system must be stable, first, or the vibration will cause damage. Try some deadband, then maybe you can turn the P up. 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, Endoders and Resulotion
I checked out some treadmill motors rescued from defunct treadmills and discovered they are really not reversible. Mine will run in both directions but one direction the armature arcs more than the other direction. A closer investigation of the motor showed that the brushes contact the commutator at an angle. Sooo... My treadmill motor isn't designed to run both directions. Maybe you are moving the brushes backwards causing problems? I think the motor I have is only suitable for the direction it is designed to run. Maybe this will help... Clint - 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, Endoders and Resulotion
On Thu, Dec 06, 2007 at 08:22:15PM -0800, Dave Engvall wrote: Having said that it would be most interesting to have a system that used the information from both a linear scale and an encoder on the ball screw. I'll let the really bright guys dope out how to make that work. ;-) If nothing else you could use the scales to generate very nice screw comp tables. It would be interesting to do this periodically and see how it changes. - 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, Endoders and Resulotion
On Dec 6, 2007, at 6:56 PM, Jon Elson wrote: 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 think the resolution of glass scales adequate i.e. 10, 5 and 1 um. However, they require really tight systems to work well. That is, no backlash in the ball screws and almost perfect alignment with the table. Something like 0.005 over the length of the scale. If your system has backlash, going to extremes on resolution doesn't do much good; 5-10 times the accuracy you require. I'm always reminded of something Ray Henry once said and it goes like this, machine manufacturers are more concerned about really good performance than costs; and they use high resolution encoders on the end of the ball screw. If your system has 0.002 backlash then it will tune MUCH better with an encoder on the ball screw, and of course a tach on the servo motor. Been there, done that. Having said that it would be most interesting to have a system that used the information from both a linear scale and an encoder on the ball screw. I'll let the really bright guys dope out how to make that work. ;-) Low resolution and possible backlash between motor and encoder. 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? Yes, have the backlash reduced to the absolute maximum. Make sure the slider on the glass scale moves very freely (may need internal cleaning and work on the shield that keeps junk out of the encoder. Make sure the link between the encoder read head and the machine is straight. A kink in the linkage can allow it to bend under load, and so the encoder itself can develop backlash. This is especially true of the ones that use a piece of music wire to push/pull the head. 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? You're sunk! 0.002 backlash is HUGE! You need to have the ballscrews repaired, or the end bearings on the screws, wherever that backlash is, you can't expect proper operation with that much backlash. The only way to supress this right now is to set the deadband to .003 or so. The end position always seems to come to within an encoder count (0.0005) but the rate seems to vary widely during the traverse. Yes, the servo is trying to get to where it is commanded, but it is hard to do with the backlash. You probably need more gain, but the system must be stable, first, or the vibration will cause damage. Try some deadband, then maybe you can turn the P up. Dave 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 - 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, Endoders and Resulotion
Dave Engvall wrote: Having said that it would be most interesting to have a system that used the information from both a linear scale and an encoder on the ball screw. I'll let the really bright guys dope out how to make that work. ;-) Since EMC's PID loop is now part of HAL and easily replaceable, I can envision a special PID loop that has multiple feedback inputs. Position feedback for the I term (long term position accuracy) would come from the scale. Position feedback for the P term and velocity feedback for the D term would come from the motor encoder. Regards, John Kasunich - 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, Endoders and Resulotion
On Thu, 2007-12-06 at 14:08 -0800, Peter C. Wallace wrote: On Thu, 6 Dec 2007, Kirk Wallace wrote: Date: Thu, 06 Dec 2007 11:43:27 -0800 From: Kirk Wallace [EMAIL PROTECTED] Reply-To: Enhanced Machine Controller (EMC) emc-users@lists.sourceforge.net To: Enhanced Machine Controller (EMC) emc-users@lists.sourceforge.net Subject: [Emc-users] Slides, Endoders and Resulotion I got the first pass on my Bridgeport X axis working and I am having ... snip controller. Is there a way to cater to both? One way is to use dual feedback, Dont know if EMC supports this or not. 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? I would say the backlash would be the major source of the oscillation. Even with dual feedback, It would be problematic to prevent 'hunting' . I guess it would be simpler to use a rotary encoder and have EMC do the backlash compensation. What is your deadzone? ... snip If you mean deadband, I have it set to 1e-05. I set it to 1e-04 and 1e-06 which had no effect. -- 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
