How hard is it to drive a 3-ph brushless motor?
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From: Joerg on 16 Dec 2009 09:52 Peter wrote: > Joerg <invalid(a)invalid.invalid> wrote > >> It was an optics measurement setup where the slightest vibration >> modulated the optical path and showed up in the signal. The Allegro chip >> was sort of self-polluting. I didn't get very far with the support guys >> but assume that the internal oscillator became modulated by the power >> paths on the chip. I am not a fan of chips where the power path is >> integrated, I rather use external FETs. >> >> IOW the whole EMI dog fight seemed to have gone on inside the chip and >> there was nothing we could do on the outside, other than roll our own >> solution. > > I think I will design the PCB with some LC pi-type filtering in the > supply rails, to keep conducted muck from getting out. > >> Yes, your case almost screams for a stepper. 15-20 years ago that was >> different because the prices were outrageous but that has changed. > > Have you seen the price of that stepper? :) $500 plus... > Steppers can either come with cut-throat pricing when used in disk drives, cars or the like. Or they come with super-fat profit margins if you pick a boutique size. Shopping around pays big time if your quantities are high. > Of course it is much cheaper if purchased in Germany where they make > it... Order it there and have them send some Niederegger Marzipan along with it. Good stuff but not recommended by the surgeon general :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: JosephKK on 17 Dec 2009 21:59
On Tue, 15 Dec 2009 09:02:49 +0000, Peter <nospam(a)nospam9876.com> wrote: > >Joerg <invalid(a)invalid.invalid> wrote > >>Peter, just got an email from ST and in there was an announcement about >>this chip, in case you are still considering a stepper solution: >> >>http://www.st.com/stonline/products/literature/ds/16737.pdf >> >>Can supposedly do 128 microsteps, and maybe this could eliminate you >>gear box? Digikey doesn't carry it yet and if suitable you'd have to >>call ST about status and samples. >> >>Just thought it might be interesting for you. > >That's a very impressive chip Joerg - thank you. Unfortunately it >needs a processor, which adds a whole dimension to the work involved. > >Currently I am working with the Allegro 3987 >http://www.allegromicro.com/en/Products/Part_Numbers/3987/3987.pdf > >and to my suprise Allegro are really supporting it, with replies to >emails!! I have never seen that kind of support before. But then I am >used to Hitachi H8/300 etc ;) ;) ;) > >I don't think microstepping is quite what some people (incl myself) >expect[ed]. You do get smooth rotation IF the motor is actually >rotating continuously, so it cuts out the normal stepper motor noise. >But you don't get the angular precision which the microstep size might >imply - because the motor has no actual detent in between steps. You >get some kind of a fraction of the microstep precision, and there is >also less than the normal torque available between the full steps. > >At my speeds (200rpm max, maybe 10rpm min) x16 microstepping should >produce smooth rotation but I don't need the angular precision. In a >nutshell I am using a stepper rather than a brushless (which was the >original idea) because a brushless would need a tachometer and >feedback, to deliver any speed stability. A tacho is not hard to do >(most brushless controllers provide a pulse output which can be >lowpass filtered to give a voltage proportional to RPM) but the >control loop for the motor rpm obviously involves the usual control >loop parameters which will need to be developed with the appropriate >margins to ensure stability under all speed, load, temperature and >transient conditions. Whereas a stepper gives you implicitly accurate >rpm. There is one thing to remember about steppers, you need to ramp the stepping speed at the controller input. Even a little inertia can surprise you. |