On Wed, 2007-11-07 at 12:55 +0000, Ian Wright wrote: > Hi, > > This is probably a silly question but is it possible to mix stepper and > servo drives on one machine? I'm a bit tight on funds just now but I > would really like to get a bit more speed out of my rotary axis so that > I can use it for screwcutting. I can screwcut on it now using its > present stepper drive but it is SO pedestrian. I would be thinking of > using a Gecko 320 which seems to act like a stepper but I'm not sure if > the axes would still maintain their coordination one to another - am I > talking rubbish??
I have had a strong bias against steppers, so I have not studied how they work until now. I have been working on a Rutex driver step/direction setup for some brushless DC motors I have (actually permanent magnet AC motors ?). I am finally getting over some of my bias. Here is my understanding so far. If you are currently using a step/direction stepper drive like one of the G200 series drives, you can feed the G320 the same step direction signals, except instead of matching the number of stepper motor poles or micro-steps you match the number of encoder pulses per revolution. With my Rutex setup, I quickly found that a parallel port pin can not drive the signals fast enough to utilize the full speed of my motors. The options here are to use the step multiplier function in the drive, if it is available, or use a high speed data pin on an interface like the Pluto, 5i20, Pico USC and etc. One of my biggest gripes with the step/direction setup, is that there seemed to be no position feedback to EMC which is true for stepper motor systems, but is not entirely true for step/direction DC drives. With analog and PWM DC amp systems, encoder signals are connected to a PID loop in EMC. EMC constantly tries to keep the position error as close as it can to zero (or actually a set number of encoder counts called deadband?). Lose a step on a stepper and you're out of luck, and you may not notice until later, due to the user interface displayed position can only be the commanded position. With the step/direction DC drives, the PID loop moves out to the drive, so you still have a PID covering your back. Plus I believe you can route the encoder signals to an axis.encoder input to get your actual position readout in addition to the commanded position. I just need to verify that EMC will only use this information for position display and soft limits. With the PID in the drive, you now have to talk to the drive to tune the loop. It appears that the G320 can be tuned with an oscilloscope and a test point on the drive. The Rutex requires a Windows tuning utility connected through its SPI connection. So, tuning will be an issue you will need to address. I liked using Halscope for tuning my PWM system, but I think that since EMC already has the commanded and actual position information Halscope may still be used to monitor the tuning procedure. It also looks like on the G320 you only have P and D, no I and FF's. As far as axis coordination is concerned, i believe EMC's trajectory planner works that out, As long as each axis follows its commands, you should be okay. As my experience with this has been mostly theoretical, take the above with a grain of salt, or maybe a Jack Daniel's -- Kirk Wallace (California, USA http://www.wallacecompany.com/machine_shop/ Hardinge HNC lathe Bridgeport mill conversion pending Zubal lathe conversion pending) ------------------------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Still grepping through log files to find problems? Stop. Now Search log events and configuration files using AJAX and a browser. Download your FREE copy of Splunk now >> http://get.splunk.com/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
