Commutation

Anything not specifically related to NBTV, but at least of some technical nature that might be of interest to NBTV members. Items for sale and links to retailers do not belong here.

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Commutation

Postby Harry Dalek » Tue Dec 04, 2012 6:01 pm

Just thinking about what Steve was asking about in my lens disk topic the idea of commutation for lights leds and how it might be done today .

On the Slip rings idea i think Steve is not keen on ? i didn't know what they were , these pictures are an idea like that i think?

On a electronic idea i can see 2 problems the power and nbtv communication to the electronics that would have to rotate with the leds lights what ever .

The power part might be a way.... 2 motors one used as a generator but the nipkow would have to be mounted with the generator part as well see very messy picture ...better to explain with a picture .

The communication between the nbtv signal to the leds electronics a light communication systems perhaps using a mask and a ldr and a little trany amp to each 32 led ?

Well just an idea perhaps a workable system least theres no mechanical brushes apart from the motor used as a generator .
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Just an idea to power electronics on a nipkow
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Postby gary » Tue Dec 04, 2012 6:58 pm

Harry I think that is just a fancy brush system.

Slip rings are quite simple, they are just a continuous band of conductive material, usually copper, that are mounted on, but insulated from, the rotor. There are usually 2 of these bands (or rings) and a conductor is connected to each of the slip rings and they go to whatever is to be powered on the rotor (usually a coil as in a motor or alternator). A brush rides along the band/ring as the rotor spins to conduct the current.

Both slip rings and commutators do the same basic thing but the difference is a commutator ring is made up of insulated segments of copper which switch the polarity of the current every time the brush pass over one segment on to another - thus on an electric motor a DC current is turned into an AC current to feed the rotor winding. In the case of Baird's matrix screen instead of alternating the polarity the same principle was used to switch the video signal to another lamp.

Slip rings are a much more reliable method of conducting current to a rotating device because the brush is always in contact with with the ring and therefore arcing is eliminated (in an ideal implementation).

The problem from an NBTV constructors point of view is that they, generally, have to be constructed from scratch and therefore suffer from the usual problems of "kitchen table engineering".

I think, however, that a slip ring system, for transferring power to a "fire wheel" is quite within the capabilities of most constructors - commutation is another kettle of fish altogether.

You must also learn to temper Steve's comments somewhat by appreciating that he has a horror of all things mechanical - in the near future when we are all being waited on hand and foot by robot servants he will refuse to have one insisting he will wait until the all electronic versions come out ;-)

On a final note I would say that power can be transferred to a disk by inductance thus eliminating the mechanical parts. In fact your generator idea is really just a form of that. Have a look at all those POV projects on YouTube - they all solve the same problem one way or another.
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Postby Steve Anderson » Tue Dec 04, 2012 7:24 pm

gary wrote:You must also learn to temper Steve's comments somewhat by appreciating that he has a horror of all things mechanical -


Absolutely correct! If it moves I don't want to know about it - includes humans! Though I do have a soft-spot for motorcycles and piston-powered aircraft. Oh, and throw in the odd Saturn V too...

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Postby Steve Anderson » Tue Dec 04, 2012 7:37 pm

To continue...in the mechanical lash-ups I have tried....though not necessarily NBTV, I have used rechargeable batteries for the power source, mounted near the axis if you want to reduce the moment if inertia or away from it if you want the flywheel effect.

Data/signal transfer via on-axis IR LEDs and photo-transistors. Other methods include rotating transformers as used in VCR head-drum assemblies and radio, including Wi-Fi.

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Postby Harry Dalek » Tue Dec 04, 2012 8:24 pm

Hi gary thanks for the detailed explanation its some thing not really been looked into much here,i find it amazing it can work ...
looking at a post from a few years back on i think bairds matrix tv theres a good photo of the workings ...looks me it would wear out pretty quick .

Yes the induction sounds good the nipkow could be a transformer thats a good idea.

Oh robot servants yakes they might take over my mowing run !

Wonder if Steve ever made a mechanical television ? I really liked the look of that Aperture Drum he made a while back .
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Postby gary » Tue Dec 04, 2012 8:44 pm

harry dalek wrote:looks me it would wear out pretty quick


Well, like many of his products they were really just designed for demonstration purposes, but if manufactured to, say, electric motor tolerances etc. I am sure it could last decades with just the occasional brush change. I wonder if Baird's commutator (or bell labs') was the most complex (in terms of segments) ever constructed?

Oh robot servants yakes they might take over my mowing run !


...and you would have more time for NBTV!

(for general gardening robots you can't beat the drones in the classic movie "silent running" - highly recommended if you haven't seen it).
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Postby Steve Anderson » Tue Dec 04, 2012 9:06 pm

harry dalek wrote:Wonder if Steve ever made a mechanical television ? I really liked the look of that Aperture Drum he made a while back .


Yes, (he says scratching his head), I had high hopes for that but the VCR head-drum motor simply wasn't up to the task, it didn't have enough torque to get the thing moving or overcome the air-drag. I was looking for a direct-drive system as adding in the loop-filter the effect of a knicker-elastic drive just makes it even more variable.

There are stepper motors that will run at 750 RPM, but often the torque at those speeds is limited...so that little beast awaits a solution.

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Postby gary » Tue Dec 04, 2012 9:14 pm

Steve what you want is one of these beasties:

http://secure.oatleyelectronics.com//pr ... e6718260be

Ain't nuffin' they can't turn and easily controlled.

Relatively cheap too.

and when your done with it you can electrify your motor bike... ;-)
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Postby Steve Anderson » Tue Dec 04, 2012 9:40 pm

Yep, sure up to the task but when I mentioned direct drive I meant it in the sense of direct on the end of the shaft (as per) AND fully synchronous too...a brute of a stepper motor really. That would eliminate the requirement for feedback from the disk/drum thereby eliminating Albert's beloved 4046. I don't have any problems at all with using the 4046 or its derivatives. It works as advertised. But this is not really its realm...at least as applied to cassette motors and knicker-elastic, but many have had success, and well done to them all.

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P.S. Motor suggested can't be adapted for nitrous...
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Postby gary » Tue Dec 04, 2012 10:00 pm

Steve Anderson wrote:Yep, sure up to the task but when I mentioned direct drive I meant it in the sense of direct on the end of the shaft (as per) AND fully synchronous too...a brute of a stepper motor really. That would eliminate the requirement for feedback from the disk/drum thereby eliminating Albert's beloved 4046. I don't have any problems at all with using the 4046 or its derivatives. It works as advertised. But this is not really its realm...at least as applied to cassette motors and knicker-elastic, but many have had success, and well done to them all.

Steve A.


Oh Steve - no, I use this motor direct drive and a simple bistable circuit for sync - no problems and it would scale to much higher torque. 4046 is convenient but 5 GP transistors PLUS a driver is all you need for sync (ok a bit of signal conditioning from a 555 or similar is helpful). This is just a great big DC motor and as long as your driver transistor can handle the current you are sweet.

The only real problem with the 4046 is coming up with the correct filter characteristics - every different motor needs a new variation and hence it's a real trial an error nightmare - using a discrete form of phase lock loop makes it much easier to do that tweaking. Actually, I say that from personal experience and someone may have a better approach to that tweaking for the 4046.

PS: I used this motor because I made a 20" 1/4" thick MDF bead disk.

PPS: of course direct or indirect drive is not an issue here
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Postby Steve Anderson » Tue Dec 04, 2012 10:31 pm

gary wrote:The only real problem with the 4046 is coming up with the correct filter characteristics - every different motor needs a new variation and hence it's a real trial an error nightmare - using a discrete form of phase lock loop makes it much easier to do that tweaking. Actually, I say that from personal experience and someone may have a better approach to that tweaking for the 4046.


And there's a case of QED, Again I'll quote Gary, "every different motor needs a new variation and hence it's a real trial an error nightmare".

That's what I have been banging on about within this forum...OK at low-level...for years. Each mechanical arrangement will ideally require different loop-filter characteristics, the fact to so many appear to work with the default published values is probably down to the fact that most are using the similar intertia/+moment/-moment parameters...similar motors, similar masses, similar knicker-elastic reduction ratios. But it's not really scientific...

When it don't work where do you go? When they hunt and eventually settle down, that is clue for marginality in stability. Working through the maths for a known electronic PLL is not for the mathematically faint-hearted, throw in the mechanical variables which are unknown...I'll see you down the pub.

There simply is a better way than cassette motors and knicker-elastic, I know it. I have yet to prove it. First I need to be convinced. Either by others or myself.

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Postby gary » Tue Dec 04, 2012 11:21 pm

To quote Shakespeare: "The lady doth protest too much, methinks"

There are many circumstances where a certain amount of "adjust on test" is required, this is nothing unusual.

I haven't yet been unable to control a motor with the simple bistable circuit, and I must have used dozens of them.

Although I do understand that what you are after is a kind of "plug and play" arrangement.

The 4046 circuit is generally recommended for use with the club cassette motor, however, I think it is implied that it *should* be able to be made to work with other motors. The main problem is, I think, is that there has never been any definitive description of how the circuit works and how the components for the filter are calculated. I don't want to belabour that subject though as, although I have successfully implemented that circuit, in general I use the bistable circuit because I feel it is easier to adapt to different arrangements.

In addition I recommend the bistable circuit because it is easier for the constructor to understand how it works.

With the bistable circuit it eventually comes down to determining the value of the filter capacitor which is reasonably easily done by trial and error (binary method).

The ideal motor for our purpose is obviously a synchronous motor - alas these are hard to source and expensive when they are.

Stepper motors are true synchronous motors but have characteristics, such as resonance speeds which must be understood and designed for to make good use of.

In addition stepper motors of the torque we are currently considering are difficult, but not too difficult to source - but are quite expensive - ~ 50 US dollars or more + shipping - and they are heavy.

None-the-less I have successfully used one (as previously reported) and thoroughly recommend their use when appropriate.

Personally, I think it is possible to build a synchronous motor specifically designed for NBTV use. It is something I have been experimenting with and will explore in much greater detail when I have the time. Such a motor will have relatively low torque and will probably need to be used in conjunction with another motor to reach synchronous speed. That is not a problem and is obviously not without precedent.

It should also be noted that the literature provides examples of self-built synchronous motors specifically designed for this purpose (not specifically Steve's drum of course).
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