On Mon, 2010-12-06 at 08:08 -0700, Cathrine Hribar wrote: > Hi Kirk: > > Thanks for ur interest in my project. > > The reason that I am having to do something about my steppers is that my > machine started off life as a toy, mini desktop. > > I changed things and up graded the size of the whole machine so I could do > real work with it. > > The steppers I have are Bipolar, 1.8 amp. @ 30V. > > The controller is from stepmaster. It can only handle 2.8 amps @ 30 volts, > and > I can't find any tuning info at all, on this drive, except for the manual I > got with it when I bought it off e-bay.
It looks like other people have had problems with this class of controller: http://www.cnczone.com/forums/general_electronics_discussion/37937-confused_xylotex_alien_stepmaster.html (Short URL) http://alturl.com/krzs6 I suppose the good news is that there are things on this controller that can be improved, such as adding another power supply or replacing with a bigger one, adding heat sinks and/or a fan, changing a setting for a higher Vref or amperage limit(?). If your controller uses these Allegro chips: http://www.allegromicro.com/en/Products/Part_Numbers/3977/ then you may be able to tweak the board to get better performance. This chip has a thermal shutdown feature so keeping the chip cool may allow you to turn up the amp limit and make an improvement. Finding the weak points and correcting them may help. > Yes ball screws would be a great asset, but cost would be about $130 per > axis, > as far as I can tell. It's too bad that ball screws are so expensive, but backlash is probably the most important feature for a CNC,and ball screws are pretty much the only solution that allows one to preload the backlash out without driving the motor torque requirements too high. Lead screws are fine for manual machines, because the operator can compensate for the lash with coming up to a position from the same direction every time and using the axes locks while making a cut. A CNC with lead screws, or more accurately, backlash, is just an automated manual machine, which is okay if one accepts the limitations. > All axis and screws turn freely. Yes, the weight of the spindle head could > be > compensated for, but the small ampridge of the controller ends up being the > weak link when considering up grading the steppers to proper size. But a Z counter balance might be just enough to make the machine usable. You may need to tweak everything to get what you need. It might be fun to do and you will get to know your machine very well. > What would be the minimum that I would need from Mesa, as far as boards, to > start building toward a 4 axis system with the least expense? They have so > much that they offer, it becomes confusing. I don't know the Mesa products that well. Peter may chime in here, but it may depend on how much you want to change. I assume you want to go to servos, which would mean new motors, drivers, controller and maybe power supplies. The motors could be brushed DC or some sort of permanent magnet AC or DC. I tend to go with whatever happens along that seems to be a good deal (well used, odd ball, or junk). Currently, it seems buying new from a hobby CNC vendor or eBay is the only reliable way to find parts. With buying new, I would tend to go with a BLDC (brushless DC, permanent magnet AC or DC, and the like). Mesa most likely have a driver that would match your motor type and size, and could be used with their 7i43 controller. This will narrow your driver choices quite a bit. You will need to allow for encoders, so the motors will need encoders or a place to install them, or place the encoders on the lead screws (my preference). This is another place were backlash can be a problem. Backlash can make servo tuning a pain. It's common to use a pulley reduction between servo motors and the lead screws. The pulley ratio is a balance between motor speed, torque and axis speed. Ratios between 4 to 1, and 2 to 1, are common. Pulley reduction could also help with steppers that are too small. You can nearly double the torque by going from direct drive to 2:1, but it may reduce your maximum axis speed by half. A motor power supply will need to match the motor and driver specs. Antek type supplies are popular. Try http://www.antekinc.com/ and eBay. > One thing I don't understand; For a servo system, I need a servo amp, with a > PWM card, because EMC doesn't generate the PWM signal fast enough nor can > read > the encoders fast enough, to keep up with the movement of the axis. So then, > how does EMC control the process taking place? And if that's true, why would > the step and direction board be a less advantage than the Mesa amp? Doesn't > both boards take control of the operation using their own, separate, PID > control? > > Thanks, Bill No (mostly). With EMC2 using a simple driver, the PID and motion control intelligence is in the EMC2 software. The Mesa, Pico or Pluto card is a parallel port to signal generator/counter interface. The encoder signals go to a counter on the interface card, then the interface passes the count data to the parallel port and to EMC2. EMC2 then calculates the next motion command, sends it to the hardware signal generator on the interface card, which then makes a PWM signal for the driver. Course corrections based on the encoder input are made in EMC2. With a step/dir controller, EMC2 just handles a list of locations that the controller needs to get to, and can only monitor the encoders to set a following error if needed. Servo tuning issues (course correction characteristics) are handled with the controller separately from EMC2, using mostly Windows software or hardware settings (knobs or jumpers). Practically speaking, both systems can work well, but a machine integrator can be more creative if EMC2 handles more of the motion process. -- Kirk Wallace http://www.wallacecompany.com/machine_shop/ http://www.wallacecompany.com/E45/index.html California, USA ------------------------------------------------------------------------------ What happens now with your Lotus Notes apps - do you make another costly upgrade, or settle for being marooned without product support? Time to move off Lotus Notes and onto the cloud with Force.com, apps are easier to build, use, and manage than apps on traditional platforms. Sign up for the Lotus Notes Migration Kit to learn more. http://p.sf.net/sfu/salesforce-d2d _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
