Interesting so far... > From: Ralph Stirling [mailto:[email protected]] > > Linuxcnc with the bldc component can certainly drive an induction > motor or other 3-phase motor as a variable frequency drive, but I > have never heard the term "VFD" used for driving two-phase steppers > before. What Andy was suggesting, and what I looked up, was doing > the full FOC drive of steppers (or 3-phase motors for that matter) > with Linuxcnc directly. The bldc hal component does the relevant > calculations already. The only slightly tricky part is measuring > phase current. You could use an A/D converter with SPI interface > to the Mesa FPGA, but it is still harder than grabbing analog values > in a microcontroller. > > Personally, I am more interested in the approach you've been > advocating for years, of having a microcontroller per motor handling > commutation and drive, with the Linuxcnc/Mesa setup sending > step/direction pulses or communicating via sserial or Ethercat with > each motor. I've built a number of mosfet bridge boards for 2 or 3-phase > drive of small motors which have been good enough for in-house use, > and it isn't too difficult. Making them bullet-proof for other end-users > to connect to their own motors and power supplies would require some > design hardening. > > The bldc hal component is great for experimenting with driving brushless > motors, though, if you don't need phase current feedback in your control > scheme. > > -- Ralph
Back when it was still National Semiconducor and low cost stepper drivers from China didn't exist I used two LMD18245 as the full phase micro-stepping drivers embedded in my ELS product. If you look at the price of this product in singles it's $30Cdn from Mouser and with two required plus additional devices to create the 4 bits per device for micro-stepping make it more expensive than a far east driver. https://www.ti.com/lit/ds/symlink/lmd18245.pdf?HQS=dis-mous-null-mousermode-dsf-pf-null-wwe&ts=1642094728491&ref_url=https%253A%252F%252Fwww.mouser.com%252F What's interesting is I think the data sheet is one of the few places that explains how the micro-stepping actually works. For example, at any one point you only get 0.707*MaxA through both the windings. The other thing to keep in mind is that for every full step the current through the windings changes direction. That may appear trivial but if the motor is turning it's also a generator. To change the direction the applied voltage has to be opposite to and greater than this generated voltage. That's why the motor specifications may say 1V for the coil voltage (with low resistance) for 3A coil current but in fact, depending on the RPM you may need 50V. And since it's an inductor with a back EMF of say -49V from the motor RPM that extra 1V doesn't cause an instantaneous change in current in the other direction. And it's the motor inductance that determines how fast the current builds up to maximum which is why for slow or stopped the current is measured by the device and switches to chopped mode to hold it at 3A. This time to build up to maximum current is why torque falls off on stepper motors at higher step rates. A new step is required before maximum current has been reached. Closing the loop helps to know where the motor is but my experiments with commercial closed loop stepper motors shows that at higher RPM not a lot changes. They still have low torque and the difference is they will create a fault when the load exceeds the torque and the motor loses steps. Contrast that with a DC servo, designed quite differently from a magnet and winding perspective, and these motors can draw far more current without demagnetizing the motor like a stepper so they maintain full torque up to maximum RPM. The price for that architecture is quite a bit larger motor for the same low speed torque ratings of a stepper motor. Where you can direct drive a stepper on say the X axis, you need 3:1 reduction for a similar sized servo which can also run on much higher voltages to counter that back EMF. So whether LCNC does the closed loop control or a separate driver board messaged by step/dir or serial commands the limitations of the motor architecture still exist. And the price to get a motor that can create similar torque up to X RPM ends up now similar to a closed loop servo. John Dammeyer _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
