Help? How many HP, torque, etc to spin this

[Nick Walters]

I myself am the artistic type, and was unsure where to look for help, so I'm posting here.

I'm trying to build a camera rig to spin around 10 pounds max with a 10ft. diameter at 300rpm.

I'm wondering what exact forces are required to do such a thing.

If anyone needs more specifics please let me know what they would be, and I'll provide them.

I'll now explain exactly what I'm trying to do...I want to ceiling mount, or just basically mount above talent, a camera rig to re-create the matrix style movement. I'm planning on using a Casio EX-F1 camera at 300 frames per second, which at 300rpm should take about 30 pictures in a 1/10 second period on a 180, and when played it should effectively re-create the "matrix" style of slow-mo kind of effect.

I want to use an electric motor which has a long straight piece attached to the shaft that is a total of 10 feet in length. This piece would have the camera mounted on one side, while the opposite side would have an amount of weight equaling the other side(camera and mounting equipment), to balance the rig.

Any ideas?

Thanks a BUNCH in advance.

 

[rcgldr]

10 lb_mass = 0.31081 slugs. (Note 1 slug = 32.174 lb_mass.) 10 foot diameter, 300 rpm => 5 foot radius, 5 revolutions / second = 10 π radians / second. Linear velocity = 50 π ft / s. Acceleration = v² / r = (50 π ft / s)² / 5 ft = 500 π² ft / s² = 153.38 g's (1 g = 32.174 ft / ²).

Tension due to moving camera = mass times acceleration = 0.31081 slugs * 500 π² ft / s² ~= 1533.8 lbs. The camera will have to operate under a 153.38g load.

Can someone here check my math? Don't forget to include the weight of the rig.

The amount of power is a function of friction and drag in the system. You're going to need a strong rig though.

 

[Nick Walters]

I realized at the beginning that this wouldn't be an easy project, and I've tackled a few hard projects, but mostly in the realm of stabilization, but I think my real world experience can help at least a little bit in this...although I greatly realize the need to pay attention to the experts, so would you have any suggestions for this rig? At first I was thinking wood, but after not a lot of thought I believe this is a pipe dream(cheap), and I've come to the realization that this might require a carbon composite?

I have no good grasp of physics, beyond the art of physically maneuvering her properties to my will...as in actually handling a rig with human power.

I also lack an understanding and ability to know what formulas to use to convert to the factors to supply to a manufacturer...I apologize for my ignorance, but when you say roughly 1500 pounds do you mean per square foot?...I'm sure I could find a friend to help me with all the details, but I enjoy trying to learn new things, so please forgive me for my curiosity into the physics of this.

I suppose what I'm trying to say, is what should I exactly be looking for in an electric motor.

Am I fooling myself that can cheaply build this system on electric, or should I be trying to use a gasoline engine to power such a system?

 

[Nick Walters]

Oh, also I was allready thinking about drag when this was just a sparkle in my head, and thought that a couple high speed fans with ducts pointing directly onto the outside of the rig might help to reduce drag...as well as just applying general intelligent drag design into the rig...as in having a basically ice cream cone shape in the entire shape...sharp on the one end and blunt on one end.

I was thinking the carbon composite(which is usually hollow), could have a duct system set up so that I could blow air directly into the tubes, and have them exhaust on the rear side of their movement to reduce air friction further.This all may seem a bit extreme, but the alternative is hundreds of cameras...which I think would cost way more, lol

 

[Bob S]

How fast (or how many degrees or revolutions) are required to accelerate from zero RPM to 300, and how many from 300 RPM to zero? Are there cables atatched to the camera system? The major power requirement might be the torque required for acceleration and deceleration.

 

[Nick Walters]

for the actual camera system there would be zero cable attachments. The camera would be powered and store images to itself...it by itself and mounting system would weigh under two pounds...of course the motor would require wiring but this would be independent of the shaft...

If a system for counteracting the drag was included, this of course would require a more elaborate system, but if possible it would be included inside of the rotating components, and the air force would be pushed through the shaft in some sort of alternating pattern to avoid any cabling problems.

 

[Nick Walters]

Oh and actual buildup to 300 RPM wouldn't be much of a problem...I estimate that if it could come up to speed in 5-10 minutes that I could effectively get the shot.

 

[Bob S]

I wll calculate the horsepower required to spin your camera system, along with a counterweight, to 300 RPM. The moment of inertia is for two m= 10 pound weights (= 9.1 Kg) times R² = (3.05 m)² = 84.5 Kg m². 300 RPM => w = 31.4 radians per second. So the total rotational energy is

W = (1/2) I w² = 41,700 joules.

Because 1 HP = 746 joules per sec, it would take 56 HP to spin this rig up to speed in 1 second, 5.6 HP in 10 seconds, or about 0.56 HP in 100 seconds. So I would opt for a 1/2 HP, 250 RPM Bodine gearmotor, with either a direct or belt drive. Because the Bodine motor is a DC gear motor, using the motor as a generator and dumping the energy into a resistive load will slow the rig down. I would also choose a 1" dia vertical support shaft with suitable thrust bearings.

 

[DavidSnider]

This sounds so cool. If you get this working would you mind posting some test footage?

Won't you end up getting pretty bad motion blur with this technique, even with a high speed camera?

 

[Nick Walters]

I started trying to calculate all this, and got no-where...I need to take a refresher in my physics, lol.

Am I right in thinking that you just calculated for two 10 pound loads(20lb=9.1kilos)? cause I actually was guessing that the entire load would be about 10 pounds. the camera is literally a small still camera, so the most weight will come from the all the stuff to mount to the shaft...

camera:1.5 pounds
mount:1 pound maybe?
counter balance:2.5 pounds(depending on end balance of mount)

What I actually attach to the shaft I'm still unsure about, but I wouldn't mind experimenting with wood first(from a safe distance in the country)...so I'm thinking a length of 1x2x8(best size I could find for experimentation) will be around 4 pounds, so If I bet on the safe side I think 10 pounds would be good.

So am I right in thinking that the HP is more like 1/4hp?

As for blurring...I'm unsure, lol! The camera will be recording 300 frames per second, and it has a maximum shutter speed of 1/40,000 of a second, so my best guess is that the subject in the middle might be ok, but even with that shutter speed the background will prolly go out.

I'll definitely post footage! It's the least I can do(as well as crediting everyone here helping me with the calculations).

Thanks again for everyones help, and I'll keep ya'll updated on the progress...feel free to friend me on facebook(http://www.facebook.com/nickbrianwalters" )

One last thing I can say...is I can see the possibility of this rig ripping itself apart, lol...so I can definitely see the HP needs going up as the equipment is re-inforced...I think I might start at lower speeds like 10rpm, and work my way up...that way I can ease into a final prototype.

 

[Nick Walters]

I guess I'm right in thinking that if I get a lower HP motor...like a 1/4 instead of 1/2 that it'll basically double the amount of time to get up to speed?...the reason I wonder about this is that I see that a Bodine 1/2hp 250rpm costs around $700(couldn't find an Ebay one), while a 1/8 on E-bay costs around $100...$100 ain't bad, and spinnin up in 5 minutes(depending on weight), doesn't sound too bad.

Perhaps I'll just keep an eye out on E-bay and a 1/2 will show up...

Thanks for the help...YOU GUYS ROCK!

 

[Bob S]

I guess I'm right in thinking that if I get a lower HP motor...like a 1/4 instead of 1/2 that it'll basically double the amount of time to get up to speed?...the reason I wonder about this is that I see that a Bodine 1/2hp 250rpm costs around $700(couldn't find an Ebay one), while a 1/8 on E-bay costs around $100...$100 ain't bad, and spinnin up in 5 minutes(depending on weight), doesn't sound too bad!

I agree that 1/4 HP is adequate, and even an 1/8 HP if you are careful revving it up. The Bodine dc brush gearmotor can easily work at lower RPM (use a Variac to control RPM), as long as the DC current is limited to the gearmotor's rated value.

 

[Nick Walters]

Is accurate control of amperage and ramping up voltage neccesary? The cheaper motor use 130vdc, and I was planning on using 11 cheaper lead acid batteries in series to provide 132v(close enough), and directly connect to the motor through a throw switch.

Would there be a lot of problems at start up? Would a hand crank for start up help this thing get going?

 

[Bob S]

Hooking up 10 or 11 12-volt lead-acid batteries in series sounds like a lot of unnecessary hassle. If you put a 100-watt or 150-watt incandescent bulb in series with the motor and connect this to the batteries, this will eliminate the current surge at startup, when there is no back-emf in the motor. Incandescent bulbs are very nice current limiters.
Another suggestion is to go to Radio Shack and buy this nice 8-amp full-wave bridge rectifier for $2.59:
http://www.radioshack.com/product/index.jsp?productId=2062583
You will also need a soldering iron to make the connections. Just connect the bridge rectifier output to the motor and incandescent lamp in series, and plug the rectifier input into any 120-volt wall socket. Unless you put this circuit in a box, be extra careful about electrical safety.

 

[Pupil]

The 360 degree slo-mo effect the Wachowski brothers used in The Matrix was an assembly of (if my memory is correct) 150 cameras taking pictures almost simultaneously. I would wager that if you rotate the camera around an object in the axis of rotation (center), then the object would blur very little, but the background is going to be hell. Not to mention, a camera of the mass you're talking about is going to experience some pretty hardcore g's. I'm curious how this will turn out, though. Interesting.

 

[Nick Walters]

The camera is completely solid state...no moving parts besides shutter, which I'm not even sure if it's a real physical shutter, but I have a good feeling that since there is a minimum of moving parts(the focus , zoom, and iris will be locked) that it'll work out ok...I'm a big fan of solid state, and actually own and operate a Panasonic HVX-200, and I don't think I EVER want to deal with tape or film again, lol...theres just so many amazing things you can do with a solid state camera that can't be achieved with the other stuff.

As for the light bulb, I didn't even think of using it for the start up process...I was actually thinking about hooking in a light for after I was done getting the shot and pulled the power. As for the soldering iron, wiring, and such things, I actually have all that...I just need to figure out housing, framing, mounting, and generally just purchase the bits and bobs that are the most important(wood, hardware, motor, camera, etc.).

 

[rcgldr]

Just remember 300 rpm with a 5 foot radius, tranlates into 153g's of acceleration, at a speed of 107 mph.

 

[Nick Walters]

Just remember 300 rpm with a 5 foot radius, tranlates into 153g's of acceleration, at a speed of 107 mph.

You know, I was actually thinking a little on the forces at work here, and I realize it will be insanely high...B4 I run her at top speed I may build a circular track on which the end of the rig travels, in an attempt to stabilize the whole thing so it doesn't fly apart...10 pounds of 150g's as you said is 1500lbs I have to support on the ends(half on each), I'll prolly end up with more weight then I've initially thought of, cause I'll need multiple wheels mounted on the end to support such a weight...we may have to say more like 3000lbs...the surrounding system which the rails go on will prolly have to be insanely beefy as well...the speed shouldn't be a problem, but the amount of force? that'll be the kicker. This also means that the center of the system is sustaining a high amount of stress...in fact I think this means that if the system were to break, the break would happen near the center, sending quite a wallop of energy out and about...this takes wood out of the equation, and means I'll have to upgrade to steel or a carbon composite...which in turn increases weight.

I will state this for the record...I, and no one else will be anywhere close to this rig on it's initial tests. I'll drag it out to my friends farm and power it far away from anything that can be damaged, and my initial tests will only be with counterweights on both sides and at a low speed

I in no way condone doing something like this...it's insanely dangerous! I have experience rigging up stuff, but no experience doing something this hardcore, which is why I will be studying, re-inforcing, and re-re-inforcing this rig in every way possible along the way.

In fact the more I think on it, the more I'm completely sure that my first rig will NEVER be tested at full speed. I'll most likely hook in a rpm gauge so that I know exactly where I am and then I'll pull the plug...at the end of testing I'll have a little bit of footage, a proof of concept, and I'll seek a little funding and professionals to assist in building a final prototype.

I'm guessing a more suitable final prototype would be made of all steel, use a 1-2 hp motor, and prolly look like something you'd usually see astronauts strapped into, lol

Again ...don't try this at home :)

 

[rcgldr]

I was thinking more along the lines of a medium scale centrifuge, with a stationary platform above the rotating parts. You'd need a open center, horizontal cylindrical bearing, attached to 2 horiztonal I-beams, one of which would have a small vertical I-beam that supports the camera, and the other I-beam end with a counter balance. Perhaps a square box structrure would be used to mount the I-Beams to the cylindrical bearing. The hollow bearing would allow a healthy diameter supporting pipe to extend through the bearing to support a non-moving platform that the actor/victm would stand on. A conventional chain or belt drive system could be used to drive the rotating parts with a motor hidden under the platform. The stationary pipe that goes through the bearing could be part of the main bearing.

The main danger would be getting hit by the beam ends moving at 107 mph, or the camera mount breaking and the camera being ejected at 107mph (perhaps more depending on any oscillation). Another danger would be bearing seizure.

 

[Nick Walters]

I've actually seen this exact design of rig in use with 35mm cameras...I'm not the first to think of this by any means, but I do believe my rig will be the fastest.

From what I've been able to find your ideas on design seem to mesh exactly(although I have no access to plans or anything) with Ewe Boll's rig in "House Of The Dead"...The only info I've been able to find on this rig is video in some behind the scenes on the DVD. You can see some of the shots here : http://www.gamecrap.com/moviestv/hotd/hotd.avi"

I think the major advantage of a rig that is suspended above is that you have more area in which to move around, and no need to make a platform...I'm basically taking advantage of the fact that the Earth exists, and utilizing it to reduce building more stuff. Another big kicker is that to make a platform you need a pretty large turntable bearing, and I'm not sure if you've ever priced one...but they certainly don't go cheap.

You know...just having this conversation is giving me ideas... to reduce stresses on some parts of the system, if I just removed the camera mount/wheel assembly and placed it directly on the circular path, then the "blade" that sticks out only has to push/pull the thing around the track...I'll still have the push/pull stress to worry about, but not quite as much pulling force outward on the blade...not just in the hardware, but there would be less stress on the motor and it's bearings as well.

 

[rcgldr]

suspended above

I thought of that also, but then there's the issue of hiding whatever frame work is being used to suspend the "centrifuge".

 

[mbunds]

You know, I was actually thinking a little on the forces at work here, and I realize it will be insanely high...

Why rotate the mass of the camera at the end of the moment arm at all? Place it on a rotating mount at the center of the apparatus, and aim it at a pair of adjustable mirrors. Mylar mirrors may be stretched over a light frame of any shape and have very little mass.

 

[Nick Walters]

Why rotate the mass of the camera at the end of the moment arm at all? Place it on a rotating mount at the center of the apparatus, and aim it at a pair of adjustable mirrors. Mylar mirrors may be stretched over a light frame of any shape and have very little mass.

Very good point. I havn't had the chance to purchase the camera yet, so the whole rig is still just on paper...this will definitely be used in the design! Thanks :)