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Old 10th April 2008, 06:36 AM   #1
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Default Air Bearing Spindle DD Turntable

Hello everyone-

I'm in the first stages of designing my DIY turntable, and had the idea of using an air bearing spindle with a brushless dc motor (integral to the unit) to support/ drive the platter. These are often used in HDD verification, silicon wafer manufacturing, etc. The unit I have was manufactured by Air Bearing Technologies, and includes Hall Effect sensor feedback and a 1024 line optical encoder, presumably used to sense spindle position. I wonder if anyone has any experience with BLDC controllers/ drives, and if something like this would be suitable for running at low RPM. It seems to me that most available commercial controllers are way too complex (offering ramp up/ down speeds, position control, etc.) and don't like to be run at low RPM's. Any advice?

Thanks,

Christopher
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Old 10th April 2008, 07:12 AM   #2
Shaun is offline Shaun  South Africa
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Hi Christopher

Interesting thread! Will this be direct drive? I have as a background project a controller for the Technics SP10Mkii. At the moment I'm just gathering info on various controllers (haven't even read much of the data sheets). It might help to look at the SP10's service manual, or even that of the SL1200.

Do you have any specs on hand for the motor you are using?
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Old 10th April 2008, 08:36 AM   #3
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Here's what an applications engineer sent me:

Spindle Type: Air Bearing, 3-Phase Brushless DC
Back EMF Constant: 1.7 Volts / kRPM
Torque Constant: 0.144 in-lbs / Amp Peak
Recommended Maximum Speed: 20,000 RPM
Resistance: 0.33 Ohm
Inductance: 0.133 mH
Recommended Bus Voltage: 48 Volts DC
Recommended Continuous Max Radial Loading: 7 lbs
Approximate Dimensions: 8" Length, 6" Diameter
Req'd Air : 85-90 psi @ ~2 CFM
Commutation Control: 5 Volt Digital Hall Effect Sensors (120 Degree, Phases U/V/W)
Position Control: Line Driver, Incremental Encoder
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Old 10th April 2008, 09:10 AM   #4
Shaun is offline Shaun  South Africa
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Wow! That's a big motor!

I don't know enough about BLDC motors to answer most of your concerns. What does the app engineer say about minimum RPM? 33.3 rpm is exactly one universe away from 15000. I know that to make the rotation smooth (i.e., overcome cogging), one has to use sine wave drive. It also just ocurred to me that if the motor has unusually high torque the platter and plinth/base will have to be of very high mass to counter the cogging effect. And you'll need a compressor for the air (adds to complexity as well as introduces an unwanted noise source). Do they offer non-air bearing motors?
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Old 10th April 2008, 04:57 PM   #5
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I am a controls specialist for systems like those. There is likely no reason why you can't get waaaaay better performance out of that system than you can get out of a more conventional system. An air bearing system, when working properly will eliminate all the bearing nonsense that people chase in conventional systems.

You have a few things that you need to ensure:
- You need very low cogging in the motor. This is pretty easy to do. Turn on the air and see if the motor wants to pull to a position, like a ball detent, but much more subtle. If it is pronounced, you may end up like Sisyphus trying to deal with it. Many high power motors that will spin up quickly will have cogging.

- If the bearing needs 2 cfm, it is likely a high leakage bearing, which is ok for precision, but not ok for noise. Turn on the air , put a tonearm on it with the amps on and see If you can hear it. You'll find out if you have a project or a non-starter.

- You encoder is pretty coarse. A much higher resolution encoder would make the control system easier to deal with. But that can be dealt with later if everything works up to that point

- Commercial controllers may have too many features, but a good one will have a good control loop, which is going to be needed to give you a perfect 33 1/3

- You need sinusoidal commutation amplifiers. Trapezoidal amplifiers will be audible on the hall transitions.

I'm really interested in your progress. I made an air bearing turntable years ago and they sound great when they work right.
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Old 10th April 2008, 08:18 PM   #6
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Hi,

Why would you put an electric motor on it to drive such a system? Put some nozzles around the circumference of the platter to get it spinning. If it is heavy enough then it has enough mass to run stable. You will need then a control system to control air flow of course, to keep it on the right speed.

Cheers
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Old 11th April 2008, 05:26 AM   #7
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Thanks for the interest in my project...

Shaun- Unfortunately, the applications engineer doesn't have any experience with anyone running these at such low RPM's, but doesn't see why it can't be done. Speed seems to be limited by the controller/ drive more than the motor itself- but I'm not so sure the motor itself will be running at it's optimal. I do have some concerns about motor cogging, but won't be able to report on it until I can get to the shop and follow ebrewste's suggestion... I'm not convinced that a high-mass platter would be ideal just yet, as lower mass would allow for faster correction of speed variations, but we'll see how things go once I assemble a test rig of sorts.

Don't think they manufacture stand alone motors- nothing listed on their site... http://www.airbearingtechnology.com

erbrewste- I'll let you know about the cogging as soon as I get into the shop... As for noise, wouldn't it be isolated from the spindle (and platter) because of the physical decoupling provided by the air? That was one of my initial reasons for using an air bearing. But I'll try your suggestion, and see what happens.

What resolution do you suggest for the encoder?

I agree, commercial controllers offer way too many features- and most need to be controlled via PC. Any knowledge of an stand alone interface that would allow simple speed selection/ trim adjustment?

I'm aware of the need to use sinusoidal commutation, especially at low RPM's. Do you think I would achieve better results using an analog amplifier vs. digital?

Oh, any photos of the TT you made? Would be interesting to see how someone else implemented an air bearing spindle.

Christopher
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Old 11th April 2008, 06:18 AM   #8
fizzard is offline fizzard  Canada
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If you have problems with cogging you may need to blend in some harmonics with the sine wave.

If you have the resources, measure the spectrum of the current when you apply a sine wave to see what harmonics you're getting in the current. The less harmonics in the current the less cogging you should get.

They also use air bearings for mercury telescopes. A rotating fluid forms a perfect parabola that can be adjusted by adjusting the speed of rotation. This might be a source of slower speed air bearings and drives. They might be custom order and lots of $$$$.
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Old 11th April 2008, 07:18 AM   #9
Shaun is offline Shaun  South Africa
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Quote:
Originally posted by monsieurphot
I'm aware of the need to use sinusoidal commutation, especially at low RPM's. Do you think I would achieve better results using an analog amplifier vs. digital?
I don't believe there is much advantage to using switch-mode amplifiers in this application. The amount of power consumed in a turntable is not a lot (so less need for an efficient amp). The main concern I think will be bandwidth; depends on the frequency of the motor drive signal. So far I can see that there are purpose-made drivers available.

ebrewste

Can you give me a clue as to what BLDC controller chip and driver might be suitable?
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Old 11th April 2008, 08:02 AM   #10
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Air Noise
------
Regarding the noise, any significant noise source will bleed into the platter. It won't be louder than the signal coming from the groove (it should be pretty quiet by the time it got the cartridge), but you aren't making this just because it will play a record, you want the ultimate, right? If you wanted good enough, I doubt you would be looking at air bearings.

Current Encoder
------------------
Your encoder situation is tough. If your 1000 line encoder was perfect, you could come up with a scheme to do a phase locked loop with the pulse edges and get pretty good velocity info. The problem is that the average optical encoder is not very consistent on where the edges on the pulses are. They typically spec +/-45 degrees (electrically) or so. That's fine for reading position, but not for phase locked loops. The encoder is fine at 20 krpm, but the signal to noise, limited by quantization noise is pretty ugly in the 33 rpm range. Think 4000 counts (1000 lines x 4 for quadrature) / rev and 33 rpm (0.55 rev / sec) gives 2222 counts / sec. One count of error (smallest increment of error) and you get .05% speed error at 1 Hz bandwidth or 5% at 100 Hz. That's pretty bad by anyone's closed loop measure. Let's say that your encoder is good to +/- 18 degrees on edge phase accuracy (optimistic), You could see a 10x improvement by going to a phase locked interface, but that still isn't too great, and you can't really get a commercial product that would do that too easy.

Ideal Encoders
-----------------
I would shoot for 1,000,000 count/rev encoder, which would give you a 250x increase in velocity resolution, which should give you a fighting chance. Those were very exotic until the last decade or so and can be found on eBay from time to time. I've had good luck with Micro E encoders for a relatively low cost but super performance encoder. http://www.microesys.com/m1encoders/data.html In an ideal world, I would suggest a big ring encoder like this, but it's likely out of the budget http://www.renishaw.com/en/6453.aspx
Also, another option is something like a 1000 line sin/cos encoder. They have analog outputs and if you (your drive or controller) can interpolate the signal, 1000's X interpolation is not uncommon. The Micro E stuff really just does that and hides the details from you.

Controller
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A lot of smart drives have more than enough functionality to do the velocity ramp up / ramp down on a push button, like you need, but they will be industrial PWM drives. They are miracles of modern manufacturing and control, but from your cartridge, wiring, and preamp's perspective, they are like radio stations. No grounding scheme will protect you from the 16 kHz (varies, drive to drive) carrier frequency, and you will likely hear noise from the motor, which isn't good in a turntable, no matter now low the level. What you need is a linear amp, but that pretty much rules out smart drives. I would probably look at galil (www.galilmc.com). You might also look at the microcontroller based motion control market. I'm not super familiar with it, but they seem to be getting better all the time and you can get <<$100 controllers there. In any case, if you use a linear drive, you will likely need to deal with commutation in the controller because linear drives will need two phase inputs that control commutation.

Drives
-------
I don't think you will be able to get away with a PWM drive and get super low noise, audibly and electrically. I have used these linear drives with great success and the company has really great people. Remember that you need next to no power, so the smallest drive will do the job. http://trustautomation.com/cm/Standa...ts/Drives.html

Realism
---------
If the linear amps and controllers and fancy encoders put this over the top, take a look at this http://www.elmomc.com/products/solo-...ted-servo.htm. A buddy at work is using some of their stuff. As long as you don't get visions that it will be the most precise control loop in the world and realize that it is pwm, you get a very small, reasonably priced, stand alone product from a company that makes good stuff. A turntable project that gets done slightly less ambitiously than the ideal is much better than the ideal one that never gets done, and this might make the difference.

My Old Turntable
-------------------
I'll see if I can dig up any pictures of my old turntable. It didn't survive some re-evaluation of my priorities. It was two granite surface plates, mounted flat surface to flat surface. The top one was cut to be round. When some air was pumped inbetween the two surfaces, they made a great air bearing that would float << 1 micron height. After much experimenting, I found that a weed sprayer made a great compressor and would last more than a record on one pump. It solved the noisy pump problem. I used an old Gerrard turntable motor and a belt made of a bungee cord fiber. I was in the process of converting it to direct drive with a home brew brushless motor and an industrial controller system when I scrapped it. It was a lot of fun and shockingly good at sounding relaxed.

Let us know what path you choose, it sounds like you have a great project.

Erik
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