How hard is it to drive a 3-ph brushless motor?
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From: Rich Grise on 15 Dec 2009 12:55 On Tue, 15 Dec 2009 09:02:49 +0000, Peter wrote: > > 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. So, make up your mind. Do you want smooth rotation, or precision positioning? If you want both, it could be very pricey. What exactly are you trying to accomplish here? What's the goal? Thanks, Rich
From: DJ Delorie on 15 Dec 2009 18:10 Peter <nospam(a)nospam9876.com> writes: > The thing which concerned me with a brushless motor is that in the > standard driver chip implementation the rpm is totally open loop. If you have the inclination, replace the "standard driver chip" with a small MCU. The R8C family has many chips with built-in three-phase motor control, and can support either hall effect sensors or back EMF sensing. That gives you speed control and a digital channel to talk to it with (i2c, can, uart).
From: Joerg on 15 Dec 2009 18:29 Peter wrote: > Joerg <invalid(a)invalid.invalid> wrote > >> They are very responsive, it's just that I've had not so great >> experiences with EMI and internal noise pollution from the chips. We had >> sensors on the chassis that would pick up any mechanical shaft noise >> which the chip generated. > > Interesting... I will have a 4-layer PCB, with ground planes properly > done and decoupled. MOSFETs switch very fast so the potential for muck > is substantial. > 4-layer is a good thing here. > Why were you sensing mechanical shaft noise? > 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. >> True, mcro-stepping won't net you quite as many extra intermediate >> positions as there are micro-steps. The only way to do that would be to >> roll your own controller with a uC. Some day (when you have the time) it >> may be a good exercise for you to do that because you can use such >> acquired know-how over and over again. >> >> With a loop I wouldn't worry too much, essentially it gets tuned just >> like any other regulator such as a PID. But of course not need for that >> with a stepper. > > The thing which concerned me with a brushless motor is that in the > standard driver chip implementation the rpm is totally open loop. You > have the Hall sensors, and the coil currents are switched purely > according to the Hall sensor position feedback. So, if you have zero > load and zero friction, the motor will zoom up to an infinite speed :) > So, in terms of a control loop trying to hold the rpm, you need a > significant margin in there for stability under a wide variety of load > conditions. Whether this is done in software or with analog > components, makes no difference. > > Obviously this problem has been solved - brushless electronically > controlled motors have practically taken over the variable speed > drives market which is a multi billion sized field - and I would think > a more intelligent controller would not just blindly switch the coils > around from the Hall sensor feedback but impose a more intelligent > regime on the angular speed of the rotating field. And I know, from > peripheral involvement through my customers, that a large chunk of the > AC drive market uses motors with a shaft encoder because that is the > only way they can get any torque at low rpm. In fact I tried to learn > something from my project from these people but the stuff they are > doing is so different. They aren't using off the shelf brushless > controller chips; they have fast microcontrollers (often DSPs) running > some pretty complex algorithms. > > The brushless controller chips are great for model airplanes and I see > some amazing stuff there, but all they want is to stick 50 amps into a > tiny motor and get 20000 rpm from it driving a plastic prop :) > > But a stepper motor solves this neatly - if the parameters can be met > with a stepper which in my case they can be. Yes, your case almost screams for a stepper. 15-20 years ago that was different because the prices were outrageous but that has changed. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: VWWall on 15 Dec 2009 23:30 Peter wrote: > Hi All, > > I have been doing hardware/software (asm & C) development for 30 years > so not totally useless but have never done this, or stepper motor > control. > Have a look here: http://ecnmag.com/article-brushless-dc-motor-control-111609.aspx Getting from here to your application.... -- VWWall, P.E.
From: Meindert Sprang on 16 Dec 2009 02:54
"Peter" <nospam(a)nospam9876.com> wrote in message news:5ljei55mqe2p8voorhgr0hnn5ns6k42mpk(a)4ax.com... > That's a very impressive chip Joerg - thank you. Unfortunately it > needs a processor, which adds a whole dimension to the work involved. I can't remember the beginning of this thread: was this a one time solution or are we talking numbers here? For around EUR 100 you can buy 3 phase motor drivers. Why bother developing your own solution? Meindert |