Driver for very small brushless DC motors?
in [Design]
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From: Phil Hobbs on 23 Nov 2009 00:07 dagmargoodboat(a)yahoo.com wrote: > On Nov 22, 9:06 pm, Phil Hobbs wrote: >> dagmargoodb...(a)yahoo.com wrote: >>> On Nov 22, 5:43 pm, Phil Hobbs wrote: > >>>> I'm mostly interested in very smooth motion at small scales, which is >>>> why I want an ironless BLDC. The gizmo's operation will require a lot >>>> of curve fitting to pull out the amplitude and phase of a >>>> small-amplitude tone burst of about 10k cycles over about 5 degrees of >>>> shaft rotation, once per rev. Any cogging or other bad behaviour of the >>>> motor will cause nasty spurious peaks in the spectrum, among other problems. >> >>>> Steppers are never sufficiently well made to avoid periodic errors--I'm >>>> at the level where I have to worry about whether the ball bearings are >>>> smooth enough, or whether I need to use jewels, which would be fragile >>>> and expensive enough to dim my enthusiasm quite a bit. (A galvo is >>>> another possibility, but those cost the Earth.) My hope is that because >>>> the balls' motion doesn't have the same period as the shaft rotation, I >>>> can sort out the bearing junk from the desired signal. >> >>>> In the real system, I'm expecting to have optical clues as to what the >>>> actual motor phase is, but I'm not too worried about that at this point. >> >>>> I'm currently gearing up to do a sanity test with a nice Maxon brush >>>> motor from my junk box, a He-Ne, and an HP 35665A dynamic signal >>>> analyzer to do the data acq and so on. (I just got a Prologix >>>> GPIB-Ethernet gizmo, so I don't have to use the floppy drive to get data >>>> in and out.) >> >>>> Cheers >> >>>> Phil Hobbs >> >>> Even microstepped, steppers shake, rattle,& roll. And they sing >>> (resonate). I never imagined how much until I tried a few. >> >>> As far as COTS, CD, DVD& hard disk spindle motor drivers? They use 3- >>> phase BLDC motors& integrated controllers. >> >>> Here's an old BLDC datasheet off ye old hard drive: >>> http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MC34929 >> >>> But won't you be wanting ultra-fine control over commutation, PWM, >>> position-interpolation and such? You'll probably have to do that >>> yourself. >> >>> Atmel, Microchip, and Freescale all have good application notes on >>> BLDC-driving with uCs. >> >>> e.g. Atmel AVR444: Sensorless control of 3-phase brushless DC motors. >> > > >> I'm actually just going to spin it up and do the measurement as it spins >> down unpowered. That way I should have zero cogging and no jitter due >> to commutation. > > The unpowered motor will still cog of course, just not nearly as much. > > For just testing VCR spindles might be interesting. They're an > endangered species now, but they're 3-phase BLDC motors, with > integrated drivers, flywheels, and impressively low run-out bearings. > That level of precision& longevity has got to imply a certain > smoothness of rotation& lack of vibration too. Couldn't hurt, > anyhow. > > Probably kid stuff by your standards. > > -- > Cheers, > James Arthur There are no iron teeth (or any other iron) in the rotor, so when it's unpowered, the only things left to cause angular acceleration are the bearings, the slip rings, air friction, and probably some slight eddy current loss due to remanent magnetization. They call it 'zero cogging'. Whether it's close enough to zero, I'm not sure. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: dagmargoodboat on 23 Nov 2009 00:30 On Nov 22, 9:23 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Sun, 22 Nov 2009 17:23:32 -0800 (PST), dagmargoodb...(a)yahoo.com > wrote: > > > > >On Nov 22, 5:43 pm, Phil Hobbs > ><pcdhSpamMeSensel...(a)electrooptical.net> wrote: > >> John Larkin wrote: > > >> > What a coincidence... I've been thinking about the same problem. > >> > > >> > How about a small, cheap stepper. One could run it in microstep mode > >> > and tweak its drive waveform to get very smooth rotation; I know that > >> > works. Then couple it to the load platform through something > >> > torsionally compliant, like a spring or a rubber tube or a piece of > >> > piano wire or something. Maximize the mass of the load platform to > >> > make a mechanical lowpass filter. > >> > > >> > Over the top, but I suppose one could make a multipole rotational > >> > lowpass filter by adding mass to the motor and/or insert an > >> > intermediate mass and use two compliant couplings. I've seen > >> > Collins-type mechanical filters like this, and it resembles a > >> > microstrip lowpass filter in concept. > >> > > >> > The stepper gives exact, controllable rotational speed open-loop, > >> > which is nice. And small steppers are cheap and easy to drive. > >> > > >> > We could program one of our multichannel arbs to test some motors and > >> > find a nice pre-distorted waveform that gives smooth rotation. I think > >> > adding some third harmonic is classic here, but whatever works. How > >> > would one instrument the resulting angular rotation? Optically, I > >> > guess, or maybe drive a variable capacitor? > > >> I'm mostly interested in very smooth motion at small scales, which is > >> why I want an ironless BLDC. The gizmo's operation will require a lot > >> of curve fitting to pull out the amplitude and phase of a > >> small-amplitude tone burst of about 10k cycles over about 5 degrees of > >> shaft rotation, once per rev. Any cogging or other bad behaviour of the > >> motor will cause nasty spurious peaks in the spectrum, among other problems. > > >> Steppers are never sufficiently well made to avoid periodic errors--I'm > >> at the level where I have to worry about whether the ball bearings are > >> smooth enough, or whether I need to use jewels, which would be fragile > >> and expensive enough to dim my enthusiasm quite a bit. (A galvo is > >> another possibility, but those cost the Earth.) My hope is that because > >> the balls' motion doesn't have the same period as the shaft rotation, I > >> can sort out the bearing junk from the desired signal. > > >> In the real system, I'm expecting to have optical clues as to what the > >> actual motor phase is, but I'm not too worried about that at this point. > > >> I'm currently gearing up to do a sanity test with a nice Maxon brush > >> motor from my junk box, a He-Ne, and an HP 35665A dynamic signal > >> analyzer to do the data acq and so on. (I just got a Prologix > >> GPIB-Ethernet gizmo, so I don't have to use the floppy drive to get data > >> in and out.) > > >> Cheers > > >> Phil Hobbs > > >Even microstepped, steppers shake, rattle, & roll. And they sing > >(resonate). I never imagined how much until I tried a few. > > But they can be silky-smooth if you drive them right, in the speed > range they like. > > > > >As far as COTS, CD, DVD & hard disk spindle motor drivers? They use 3- > >phase BLDC motors & integrated controllers. > > >Here's an old BLDC datasheet off ye old hard drive: > > http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MC34929 > > >But won't you be wanting ultra-fine control over commutation, PWM, > >position-interpolation and such? You'll probably have to do that > >yourself. > > >Atmel, Microchip, and Freescale all have good application notes on > >BLDC-driving with uCs. > > >e.g. Atmel AVR444: Sensorless control of 3-phase brushless DC motors. > > I think of a BLDC as a 3-pole stepper that hard commutates based on > crappy Hall sensors. And I think of a stepper as a 100-pole BLDC that > soft commutates using precisely the waveform that produces the > smoothest rotation. > > So there. > > John Depends on how you drive 'em, of course, and how fast. I think of BLDCs and kin as linear motors--almost like a voice-coil motor--wrapped around a spindle: drive them with sinusoids at low speeds, and interpolate smoothly between positions. Or you can drive them all--steppers too--at high speeds with rectangular or crapezoidal waveforms for higher torque, & the mechanical low-pass of the rotor's inertia still yields smooth rotation. Stepper resonances aren't a problem at all if you crawl, or if you fly, but they sure are a pain at mid-band. But for super-fine angular resolution stepper poles just aren't mechanically or magnetically accurate enough. I'd think ironless rotors would still have several once-per-rev periodic errors, but at least they don't have a magnetized cog with 50 hungry poles, lusting for iron fingertips across a small gap. So, that's my boneheaded appreciation of it. Phil's app sounds like it needs a 1,000,000 line optical encoder (or a 100,000 line analog encoder and a 14-bit a/d)! -- Cheers, James Arthur
From: Phil Hobbs on 23 Nov 2009 00:42 dagmargoodboat(a)yahoo.com wrote: > On Nov 22, 9:23 pm, John Larkin > <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Sun, 22 Nov 2009 17:23:32 -0800 (PST), dagmargoodb...(a)yahoo.com >> wrote: >> >> >> >>> On Nov 22, 5:43 pm, Phil Hobbs >>> <pcdhSpamMeSensel...(a)electrooptical.net> wrote: >>>> John Larkin wrote: >> >>>> > What a coincidence... I've been thinking about the same problem. >>>> > >>>> > How about a small, cheap stepper. One could run it in microstep mode >>>> > and tweak its drive waveform to get very smooth rotation; I know that >>>> > works. Then couple it to the load platform through something >>>> > torsionally compliant, like a spring or a rubber tube or a piece of >>>> > piano wire or something. Maximize the mass of the load platform to >>>> > make a mechanical lowpass filter. >>>> > >>>> > Over the top, but I suppose one could make a multipole rotational >>>> > lowpass filter by adding mass to the motor and/or insert an >>>> > intermediate mass and use two compliant couplings. I've seen >>>> > Collins-type mechanical filters like this, and it resembles a >>>> > microstrip lowpass filter in concept. >>>> > >>>> > The stepper gives exact, controllable rotational speed open-loop, >>>> > which is nice. And small steppers are cheap and easy to drive. >>>> > >>>> > We could program one of our multichannel arbs to test some motors and >>>> > find a nice pre-distorted waveform that gives smooth rotation. I think >>>> > adding some third harmonic is classic here, but whatever works. How >>>> > would one instrument the resulting angular rotation? Optically, I >>>> > guess, or maybe drive a variable capacitor? >> >>>> I'm mostly interested in very smooth motion at small scales, which is >>>> why I want an ironless BLDC. The gizmo's operation will require a lot >>>> of curve fitting to pull out the amplitude and phase of a >>>> small-amplitude tone burst of about 10k cycles over about 5 degrees of >>>> shaft rotation, once per rev. Any cogging or other bad behaviour of the >>>> motor will cause nasty spurious peaks in the spectrum, among other problems. >> >>>> Steppers are never sufficiently well made to avoid periodic errors--I'm >>>> at the level where I have to worry about whether the ball bearings are >>>> smooth enough, or whether I need to use jewels, which would be fragile >>>> and expensive enough to dim my enthusiasm quite a bit. (A galvo is >>>> another possibility, but those cost the Earth.) My hope is that because >>>> the balls' motion doesn't have the same period as the shaft rotation, I >>>> can sort out the bearing junk from the desired signal. >> >>>> In the real system, I'm expecting to have optical clues as to what the >>>> actual motor phase is, but I'm not too worried about that at this point. >> >>>> I'm currently gearing up to do a sanity test with a nice Maxon brush >>>> motor from my junk box, a He-Ne, and an HP 35665A dynamic signal >>>> analyzer to do the data acq and so on. (I just got a Prologix >>>> GPIB-Ethernet gizmo, so I don't have to use the floppy drive to get data >>>> in and out.) >> >>>> Cheers >> >>>> Phil Hobbs >> >>> Even microstepped, steppers shake, rattle,& roll. And they sing >>> (resonate). I never imagined how much until I tried a few. >> >> But they can be silky-smooth if you drive them right, in the speed >> range they like. >> >> >> >>> As far as COTS, CD, DVD& hard disk spindle motor drivers? They use 3- >>> phase BLDC motors& integrated controllers. >> >>> Here's an old BLDC datasheet off ye old hard drive: >>> http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MC34929 >> >>> But won't you be wanting ultra-fine control over commutation, PWM, >>> position-interpolation and such? You'll probably have to do that >>> yourself. >> >>> Atmel, Microchip, and Freescale all have good application notes on >>> BLDC-driving with uCs. >> >>> e.g. Atmel AVR444: Sensorless control of 3-phase brushless DC motors. >> >> I think of a BLDC as a 3-pole stepper that hard commutates based on >> crappy Hall sensors. And I think of a stepper as a 100-pole BLDC that >> soft commutates using precisely the waveform that produces the >> smoothest rotation. >> >> So there. >> >> John > > Depends on how you drive 'em, of course, and how fast. > > I think of BLDCs and kin as linear motors--almost like a voice-coil > motor--wrapped around a spindle: drive them with sinusoids at low > speeds, and interpolate smoothly between positions. > > Or you can drive them all--steppers too--at high speeds with > rectangular or crapezoidal waveforms for higher torque,& the > mechanical low-pass of the rotor's inertia still yields smooth > rotation. > > Stepper resonances aren't a problem at all if you crawl, or if you > fly, but they sure are a pain at mid-band. > > But for super-fine angular resolution stepper poles just aren't > mechanically or magnetically accurate enough. > > I'd think ironless rotors would still have several once-per-rev > periodic errors, but at least they don't have a magnetized cog with 50 > hungry poles, lusting for iron fingertips across a small gap. > > So, that's my boneheaded appreciation of it. > > Phil's app sounds like it needs a 1,000,000 line optical encoder (or a > 100,000 line analog encoder and a 14-bit a/d)! > > -- > Cheers, > James Arthur Nah, just Newton's laws and good timing accuracy, hopefully. I used to pal around with a guy named Ed Yarmchuk, who invented self-servowriting for hard disks--he replaced insane laser interferometer spin-stands for writing the servo tracks, with a bit of drive firmware, good timing, and Mr. Newton. You couldn't make terabyte hard disks without it. He retired a year or so ago (very young). Smart guy. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: dagmargoodboat on 23 Nov 2009 00:46 On Nov 23, 12:07 am, Phil Hobbs wrote: > dagmargoodb...(a)yahoo.com wrote: > > On Nov 22, 9:06 pm, Phil Hobbs wrote: > >> I'm actually just going to spin it up and do the measurement as it spins > >> down unpowered. That way I should have zero cogging and no jitter due > >> to commutation. > > > The unpowered motor will still cog of course, just not nearly as much. > > > For just testing VCR spindles might be interesting. They're an > > endangered species now, but they're 3-phase BLDC motors, with > > integrated drivers, flywheels, and impressively low run-out bearings. > > That level of precision& longevity has got to imply a certain > > smoothness of rotation& lack of vibration too. Couldn't hurt, > > anyhow. > > > Probably kid stuff by your standards. > > > -- > > Cheers, > > James Arthur > > There are no iron teeth (or any other iron) in the rotor, so when it's > unpowered, the only things left to cause angular acceleration are the > bearings, the slip rings, air friction, and probably some slight eddy > current loss due to remanent magnetization. They call it 'zero > cogging'. Whether it's close enough to zero, I'm not sure. I was thinking both eddies and modulation of aerodynamic drag by / at the poles. Also, even completely electrically open, the rotor poles form L-C tanks with their winding capacitances. I've no idea how much those will matter, but they'll suck a little energy at each pole crossing, and kick or drag, depending. -- Cheers, James Arthur
From: Phil Hobbs on 23 Nov 2009 00:49
dagmargoodboat(a)yahoo.com wrote: > On Nov 23, 12:07 am, Phil Hobbs wrote: >> dagmargoodb...(a)yahoo.com wrote: >>> On Nov 22, 9:06 pm, Phil Hobbs wrote: > >>>> I'm actually just going to spin it up and do the measurement as it spins >>>> down unpowered. That way I should have zero cogging and no jitter due >>>> to commutation. >> >>> The unpowered motor will still cog of course, just not nearly as much. >> >>> For just testing VCR spindles might be interesting. They're an >>> endangered species now, but they're 3-phase BLDC motors, with >>> integrated drivers, flywheels, and impressively low run-out bearings. >>> That level of precision& longevity has got to imply a certain >>> smoothness of rotation& lack of vibration too. Couldn't hurt, >>> anyhow. >> >>> Probably kid stuff by your standards. >> >>> -- >>> Cheers, >>> James Arthur >> >> There are no iron teeth (or any other iron) in the rotor, so when it's >> unpowered, the only things left to cause angular acceleration are the >> bearings, the slip rings, air friction, and probably some slight eddy >> current loss due to remanent magnetization. They call it 'zero >> cogging'. Whether it's close enough to zero, I'm not sure. > > I was thinking both eddies and modulation of aerodynamic drag by / at > the poles. > > Also, even completely electrically open, the rotor poles form L-C > tanks with their winding capacitances. I've no idea how much those > will matter, but they'll suck a little energy at each pole crossing, > and kick or drag, depending. > > > -- > Cheers, > James Arthur True, but hopefully a small effect at a few hundred RPM--they'll be very far from resonance. After all that build-up, I'd better go take some data, or people will start thinking I'm like that guy who used to brag all over Usenet about making diamonds by the pound... Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net |