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Evan
11-08-2007, 06:42 PM
http://vts.bc.ca/pics3/shooter1.jpg

I'm still working on the camera/long lens project and have made a counterweight as well as added a stock to the rig. This will help tremendously to stabilize the system for hand held shots. Most important is the counterweight as it moves the center of gravity to the middle of the hand grip.

The counterweight must be very rigid with no flex occuring at low frequencies that correspond to the frequencies that your muscles respond at. To achieve that and to make the support as long as practical I used graphite-epoxy rods with an aluminum compression rod inside. The graphite rods are amazingly stiff and weigh almost nothing. The point is to add as little weight as you can to balance the system and to increase the polar moment of inertia. The position of the weight is adjustable via arm extension. Different size weights may be used to accomodate different configurations if simple adjustment won't do. This system has a natural resonant frequency of about 8-10 cycles per second, well above the required minimum resonance.

http://vts.bc.ca/pics3/shooter2.jpg

The addition of the counter balance and stock has moved the CG from the axis of the lens to the center of the hand grip, exactly where it should be. The system is perfectly balanced. I look forward to trying it out when the weather improves some. The stock is a commercial unit intended to be used as shown as well as a tripod. I bought it many years ago.


http://vts.bc.ca/pics3/shooter3.jpg

I am working on a further improvement for this and other optics I own. I'm experimenting with control moment gyros and am pretty sure I can build a very workable system. I have already had a 2" 250 gram flywheel spun up to 9000 rpm on 5 volts while drawing 300ma. The motor is a very small pancake motor that I have a bag full. I am right now designing a circuit to efficiently drive a 3 phase hard disk motor to spin the flywheels since structurally they are ideal for use as a flywheel motor. I'll be posting more on the gyro system as I make some progress. I plan to run the gyros in helium to reduce drag and improve electrical efficiency. I have a circuit worked out that will convert DC to 3 phase power up to 75 volts or so using just 3 chips. The question is how fast will a hard drive motor run on a battery powered VFD? I also have questions about flywheels and safety but the answers to that are hard to come by. I may have to just do spin testing and find out the old fashioned way.

BadDog
11-08-2007, 06:52 PM
I can see it now, wearing my sequined turban, headlines read:

"A Canadian man was killed today when his camera was mistaken for a sniper rifle."

Looks like a very neat little project, I hope my premonition wasn't accurate. ;)

Spin Doctor
11-08-2007, 06:54 PM
Just try taking that on an airplane in your carry on.

As usual, nice job

daryl bane
11-08-2007, 07:05 PM
Looking at this, I thought about I idea I had years ago. What about making a large NOGA type arm (like the indicator holders) with a pistol grip lock? You could swivel to any position and then lock it with a trigger? Just pay me some royalties
when you market it.:)

Todd Tolhurst
11-08-2007, 07:58 PM
Holy cow -- an assault camera!

Nice work.

Willy
11-08-2007, 10:51 PM
Evan as soon as I saw your thread title I immediately thought about my old Russian Photo Sniper outfit. Very much like yours, I bought it new in the mid 70's. It came without the Zenit camera shown in my link, and was labeled Photo Sniper in English.

Not sure where they got the production figures from in this link since I purchased it well before 1982. Perhaps they vary because my system did not come with the camera. I still have several older M42 screw mount cameras for it. Very well made case, and the optics always impressed me.

To take a picture one would open up the aperture ring manually, then focus the lens with the thumb wheel on the forearm of the stock. When you pulled the trigger a cable release would fire the camera and at the same time it would also trip the aperture ring and stop down to your preset f-stop.

Thanks for reminding me to take the old girl out of the closet and take a few rolls of film.

PhotoSniper (http://images.google.ca/imgres?imgurl=http://www.zenit.istra.ru/archive/fs/fs-5.jpg&imgrefurl=http://cameras.alfredklomp.com/fs12/&h=352&w=440&sz=9&hl=en&start=6&um=1&tbnid=RAXa3UxceMurTM:&tbnh=102&tbnw=127&prev=/images%3Fq%3Drussian%2Bphoto%2Bsniper%2B300mm%26um %3D1%26hl%3Den%26newwindow%3D1%26safe%3Doff%26sa%3 DN)

darryl
11-09-2007, 03:32 AM
That's an interesting assemblage.

As far as the 3 ph motor speed, I believe you'll be limited more by the voltage available to drive it than anything else. Even at 20,000 rpm, electronic commutation is still just loafing along, and windage should be minimal running a smooth flywheel in helium.

If the rotor is laminated, there will be some limits. I've had a cd player motor up to 80,000 rpm, and that's a laminated rotor, so the iron losses couldn't have been too much. Even a 9 pole laminated rotor should be good for 20,000 rpm without becoming too ineffecient.

If it's a true pancake motor with an ironless rotor, windage losses will probably be the larger factor. You'll still need higher voltage to reach higher rpms, of course, and you may find that heating will limit the ultimate rpm.

You may or may not have the choice of wiring it as a delta or a Y. I don't recall precisely, but I believe the delta winding gives you higher rpm for a given voltage. There is some interesting info on this in rc sites- there's one where they discuss hacking various cd rom motors for model use.

Your Old Dog
11-09-2007, 07:25 AM
Looks like a good idea Evan. Be interesting to see some pudding! Better eat your Cheerios before hauling that baby out of the closet :D

I don't think the video version of "Steady Cam" would work for still application. That system uses pulleys and counterweights to cancel one aonther out. Got to be beastly to carry.

john hobdeclipe
11-09-2007, 09:18 AM
I have a Novaflex pistol grip 300mm lens. It certainly raised a few eyebrows going through the airport security check a few years ago...and this was before 9-11.

A.K. Boomer
11-09-2007, 11:19 AM
It looks like it'll be about as steady as you can get for the aplication "at hand"

Be careful if other people are around (hunters) I could see a missunderstaning happening:eek:

Evan
11-09-2007, 11:31 AM
I'm not planning on building a conventional steady-cam. What I have in mind is already on the market and has been for 50 or more years. Control moment gyros are the original picture stabilisation system. Currently the main maker is Kenyon Labs.

http://www.ken-lab.com/stabilizers.html

Their units look like surplus WWII equipment which is probably when they were developed. The way they work is no secret and CMGs have been around since the early part of the 20th century when they were developed to stabilize ships. It also one of the attitude systems used on spacecraft. It works by allowing a gimbaled gyro to precess against a brake or damper which then transfers angular momentum to the entire system resisting the movement that caused the precession in the first place. The entire camera system is stuck in space like it is surrounded with syrup. This only applies to axial movements and usually on two axes, pitch and yaw. The camera is free to translate in X,Y and Z as a gyro cannot affect that.

The main issue I have is iguring out what is a safe rpm for a particular flywheel design. There is a lot of information out there but most of it is not oriented to the type of application I plan. For a hand held CMG system the first consideration is weight. That means that to produce sufficient angular momentum high rpms are necessary. The next consideration is size so that means that composite flywheels are not well suited as they generally need to be bigger to make up for the low density of the wheel.

There isn't a lot of information about solid metal flywheels except in the context of old steam and similar equipment. Those flywheels aren't designed with high rpms in mind and the available calculations don't apply to something weighing 4 ounces and doing 20K rpm. There is some info about basic designs and stress distributions but a lot of what is known is strictly empirically derived. Even then it can be hazardous. One person was killed and two injured in a flywheel test in Germany a couple of years ago at BMW. The calculation of stresses isn't straight foward except for a couple of simple designs. I have been contemplating making a semi composite design using an acetal core with a hardened bearing shell for the rim. They are already designed to withstand high centrifugal forces at high rpm and are made of the right kind of material.

What is clear is that a high rpm flywheel doesn't look anything like what your average model engine uses. The most efficient design for a solid material flywheel is what is called a "constant stress" shape.

http://vts.bc.ca/pics3/flywheel.jpg

It's also the most difficult to machine as the curve is a blended inverse hyperbola. Fortunately a reasonable compromise is available by simply making the sides straight instead of curved.


http://vts.bc.ca/pics3/flywheel2.jpg

What I plan to do is spin test my designs using a spin saw or router in a containment of some kind.

darryl
11-09-2007, 03:30 PM
Evan, I doubt that you'll explode a metal disc at 20,000 rpm with a diameter that might be suitable for what you're doing. But I would be just as leery as you because you can't really know how close to disaster it's getting. I like your idea of using a bearing outer race as the inertial element, and it would fairly easily be possible to spin test one to destruction, then keep the one you use well under that rpm. You'll probably find that you can't reasonably spin it that fast without having other problems such as balance and bearing heating issues, so in the final application, the ring is very unlikely to be a failure risk. That's my feeling on the matter, but I'm no flywheel engineer. I think one of the biggest issues you'll have is balancing it. The ring will expand with rpm, so there has to be some way to keep the hub in control of the ring.