On Saturday 03 September 2016 13:58:15 Chris Albertson wrote: > I want to understand stepper motor controllers. Yes I know I can > just read the specs and buy one but I want to understand what's > inside. Preferably someone has a link > > My use of stepper motors has been, I guess primitive. I can write > software to toggle bits on an output port and then I connect these > logic level bits to MOSFETs that switch a power supply to the coils in > the motor. I can do full and half steps this way. > > But now I see something like the Geko driver that takes 40 volts > input. I know that if it simply switched that 40V into the coils it > would burn out the motor in short order. As the motor is typical > rated for 7 volts DC or close to that. OK I can understand that if it > is switching an inductive load the voltage raise is slower and maybe > not get past 7V before the coil is switched off. But what if the > motor is running slow? So my final guess is that these kinds of > drivers are supplying a constant current and applying whatever volts > is required up to the supply voltage. > > Is it as simple as that? A constant current power supply and some > MOSFET H-bridge switches?
Commercial stepper drivers feed the coils full voltage, while watching the current, which in most drivers is adjustable to match the motors rated current. When that current has been exceeded, the power fets are reconfigured to allow the voltage to be taken away, but the reconfigure usually shorts the coil, in this manner allowing the current to continue to flow but decaying at the t=lr of the resulting short. When the current falls back below the desired amount by a few percent, the fets in the h-bridge are reset to feed full voltage to the coils, building up the current again. This switching it back and forth rate is generally ultrasonic, 16 to maybe 30 kilohertz. Your dog may be able to hear it, but humans may only feel it as a pressure sensation if its loud enough. But very rarely is it loud enough to "feel". Because this switching, if not timed to a fraction of a microsecond, can do odd things to hexfets, the home brewed design is a bit difficult to do, I much prefer to buy a known good driver even if I am a C.E.T. I use several of the now quite generic "2M542" driver to drive all my Nema 23's, as its about as bulletproof a driver as can be, with 4 in my toy mill, 2 more in the little monster lathe, 2 more running the table screws in my G0704 mill, one on a 4" rotary table, and my last "spare" soon to be used to move the X screw in an 11x36 Sheldon lathe, I've had zero losses in 7 years or so, and twice had one shut down because I hadn't wired the motor correctly. They are VERY good at protecting themselves. $30 to $40 a copy on ebay. Nema 34 motors generally will need 60 or more volts, and the DM860 driver handles that well up to nearly 8 amps into a motor. I hope this helps. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
