How hard is it to drive a 3-ph brushless motor?
in [Embedded]
|
Prev: And for some Mechanical binary addition
Next: .NET micro framework ... end of embedded world???
From: Rich Grise on 10 Dec 2009 16:13 On Wed, 09 Dec 2009 21:59:19 +0000, Peter wrote: > > Rich Grise <richgrise(a)example.net> wrote > >>> This seems to do the lot - except that the control voltage seems to >>> control the % pulse width 0-100%. There is no feedback control of motor >>> speed. >> >>Sure there is. Drive it with a variable constant current source or sink, >>and find what current runs it at 13 RPM. Then take the amount of current, >>make a fixed current source/sink, and the speed will be exactly >>proportional to the duty cycle. I've also done this before. :-) > > How does this work? The current drawn will be proportional to the torque, > so at a constant current you should get a constant torque. Yes. That's why for the final product you PWM it to control the speed. > I must be missing something big :) > > Luckily in this application the load variation on the motor should be > very small; nearly negligible. The gearbox I am looking at is 600:1 and > the load will be pretty light even at the end of the gearbox. Exactly. What I just described are two separate processes. First, determine at what DC current the motor drives your motor at at max. speed. Then, use that current value to design a constant current source/sink that can be PWM'd, and the speed, at that current/torque, will be proportional to the duty cycle. Hope This Helps! Rich
From: Rich Grise on 10 Dec 2009 16:21 On Wed, 09 Dec 2009 22:02:50 +0000, Peter wrote: > Rich Grise <richgrise(a)example.net> wrote > >> A different option is to use a stepper directly, 0-13rpm, no gearbox. >> This >>> might do but a failure of the electronics could make it go round very >>> fast, >> >>In what failure mode can this possibly happen? Shorted drivers will lock >>up the motor, open drivers will let the motor free-whee, kinda - steppers >>don't coast very well. If the micro locks up, no steps, no motion. I've >>worked with stepper motors before, and they have an inherent top speed - >>if you drive it too fast, it will just skip steps. >> >>I could spec an appropriate motor and possibly gearbox, and write a uP >>program to drive it at a speed proportional to your input control >>voltage. > > The sort of thing I was thinking of was where you use e.g. a 10000rpm > motor with a 100:1 gearbox, so your max rpm is 100. If one achieved the > 13rpm max by limiting the control voltage, then a fault at that point > would cause the motor to go to max rpm. Aren't we talking about stepper motors here? I reiterate, what _possible_ failure mode could drive a stepper to max. speed? This would be outside the realm of what I know about stepper motors and controllers. Please enlighten me. Thanks, Rich
From: Rich Grise on 10 Dec 2009 17:06 On Thu, 10 Dec 2009 21:42:54 +0000, Peter wrote: > > Rich Grise <richgrise(a)example.net> wrote > >>Aren't we talking about stepper motors here? I reiterate, what _possible_ >>failure mode could drive a stepper to max. speed? This would be outside >>the realm of what I know about stepper motors and controllers. >> >>Please enlighten me. > > I think it would only be a fault a long way up the chain, where one is > dealing with a straight control voltage. > > If one can scale the control voltages such that they are nearly up to the > supply rail then not a lot can happen. > > If you see my other post, I am moving to a stepper solution. A stepper is > much weaker than a brushless (10x weaker for a similar size) but even a > weak stepper can produce enough torque to take the gearbox output shaft to > its design limit, so I think a brush or brushless motor would be an > overkill, and a motor of that size is needed only because a smaller motor > cannot be mated with a gearbox whose output shaft is sufficiently thick > for this application (6mm). > > The only failure I can imagine with a stepper solution would be one which > causes the output of the V to F converter to go to a very high frequency. How did a V to V get into this? > OTOH if I configure the Allegro controller to do microstepping then that > scales down the motor RPM v. input frequency relationship, and a x16 > microstepping would limit the max motor RPM to a few hundred, no matter > what frequency one applied to the controller 'step' input. EXACLTY! Whew! ;-) Thanks, Rich
From: Joerg on 14 Dec 2009 20:20 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. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 15 Dec 2009 10:35
Peter 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. > It is quite self-contained but yes, you need to feed it the speed and all that via SPI. > 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 ;) ;) ;) > 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. > 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. 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. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |