Aircraft-mounted videocam mounting 1

[Botanist]

Newbie, non-engineer, here.

I wish to mount 2 Sony HVR-A1U HD videocams in the rear compartment of a Cessna 185 amphib. Weight and stability are the primary important factors, so the cameras will be mounted on short pedestals permanently bolted to a plywood base. I believe that vibration will be the single greatest impact on image resolution so I am looking for a (cheap!) way to minimize vibration.

The airplane cruises at between 2250 and 2450 RPM so I believe that the dampening method should be most effective in that range and harmonics of that range.

It looks to me like a fairly dense vibration-absorbing mat placed between the plywood and the airframe might be the most cost-efficient solution.

Is this the right approach? If not, where should I look for a solution?

If this is a reasonable approach, what qualities should I look for in vibration dampening material? Do manufacturers know about the frequency attenuation of their materials? How is it reported?

Thanks in advance for your patience and assistance,

Botanist

 

[40818]

Ok, your Cessna 185 has a 3 blade prop, and operating RPM of 2250 to 2450.

Your prop frequency will be banded between:

Low cruise (2250) = 2250 x (1/60sec) = 37.5Hz
High cruise (2450) = 2450 x (1/60sec) = 48.8Hz

Your Blade Passage frequency will be banded between:

Low cruise (2250) = 2250 x (3 blades/60sec) = 112.5Hz
High cruise (2450) = 2450 x (3 blades/60sec) = 122.5Hz

The harmonics from BPF are therefore:

2BPF = 225Hz & 245Hz
3BPF = 337.5Hz & 337.5Hz

Now, it is intelligent to ensure your natural frequency of the support structure is separated from the excitation frequencies of the aircraft, becasue if they both coincide the support will resonate and sustain repeatedly high loading.
General sinusoidal vibration in the airframe will occur up to around say 15Hz. This value and the above calculated values need to be avoided, but you also need to accoutn for prop drift and other uncertainties. Therefore you should adopt a decoupling factor of root2.

You should avoid the 1st blade passage frequency (including decoupling factor)like the plague, so you will have a minimum frequency to avoid of:

15hz x root2 = 21.2Hz

BPF: (low cruise) = 112.5Hz/root2 = 79.5Hz
BPF: (high cruise) = 122.5 x root2 = 173Hz

So you end up with having to design your support structure to avoid bands of up to 21Hz and between 79Hz to 173Hz. Probably also for the general prop frequencies (37.5 & 48.8 i havnt decoupled them).

You should model your support structure in FE and determine its normal modes and tinker with the design until you avoid those ranges.

Now to practicalities, your damper mat i dont think will work very well. I recommend you should employ some anti-vibration mounts in part of your support structure.
This all depends upon your expertise,available tools, money.
If all your after is for a home-built mod then you could just try "trial and error" and see what you can do to make it as good as you can get.

Hope this helps.

 

[GBor]

40818,

Nice job! A star for you!

Botanist

My only comment would be that, in my past experience, we used 20% above and below the various frequencies instead of root 2 (which is about 40%). Certainly 40% is better, but if you can only achieve 25-30%, it may be sufficient.

Instead of going with my passion for FEA, check out some standard springs and hand calc them. Probably use 4 in parallel, one on each corner of the mount. This might even help smooth the motions when you are doing your aerial stunts .

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group
Magnitude The Finite Element Analysis Magazine for the Engineering Community

 

[Botanist]

My mistake guys, for providing insufficient information. The 185 in this case is 2 bladed, single prop. As my screen name suggests, my background is botany, not engineering (though I did deceive several physicists and mathematicians on my rite-of-passage as an undergrad).

Currently, I work for an agency that has full-fledged mapping systems with all manner of damping and what have you in our high-flying, fully dedicated observation platforms.

In contrast, my colleagues and I have this little amphib airplane that we use to we fly low-altitude (often 500 ft and below) observational flights. We usually take a hand-held videocam or still camera to capture images on the fly. We're automating the process by tying the onboard GPS time stamp to two Sony HDR-FX1 camcorders along with voice comments from the in-cabin mikes, each to a separate camera, each time the mikes are keyed. So the frame gets a GPS time stamp along with the comment. The result is that observer's comments automagically correspond to a geographic location (and altitude) and that position relates to the oblique observation captured in the video. The prototype works, at least when truck mounted and driving around.

Now my butt-mounted accelerometer informs me that the prop vibration overwhelms the engine ignition and exhaust (6 cyl Lycoming) pattern but not the essentially low altitude aerodynamic buffeting. However, I think we'll just have to eat aerodynamics.

But I do think that knowing the fundamental freq (as you have demonstrated above) can allow us to tune-out or damp that source.

Thanks for the hand-holding, folks. It is more help than you know.

Botanist

 

[Botanist]

My mistake, I provided teh wrong camera model in the above post. The model is HVR-A1U. Sorry.

B

 

[CESSNA1]

BOTONIST: Stabilized camera mounts are availabe. I suggest that you check with a TV station that has a helicopter or a an aerial video service that provides them.

Regards
Dave Hall

 

[IRstuff]

Your problem isn't quite that straight-forward. Vibration comes in two flavors, translational and rotational, and they require different approaches. The translational can be handled by springs, but they need to be matched and directed through the center of mass of the payload, otherwise, they add to the rotational vibration. The rotational vibrations require active gyro stabilization.

While the links show rather complicated gizmos, they're what's needed for a fully stabilized system:

http://www.camerasystems.com/gyronfs.htm

http://www.axsys.com/p-scs.php

http://www.tylermount.com/equipment.html

http://www.cloudcaptech.com/gimbal.shtm

TTFN

FAQ731-376

 

[thruthefence]

Don't forget a form 337, if this thing is mounted to structure.

 

[Botanist]

thruthefence, that is exactly what I want to avoid.

We have a vertical CIR digicam mounted through the fuselage and had to go thru that FAA process. It's a good process but I don't propose any permanent airframe mods. As I understand it, if we do not permanently mount then we are okay and a permanent mount is that which requires a tool to install and remove. So we are looking at hard-mounting the cameras to a plywood sheet ("table") and then C-clamping (hand-tight) the "table" to the luggage compartment. We will isolate the "table" using vibration-damping material between the "table" and the deck as well as between the C-clamps and the "table". (I hope that's clear). In that way the "table" is temporary and is not an airframe modification.

Concerning gyro stabilization, we haven't the money. Plain and simple is our budget. Minimize, not eliminate is our goal.

I like the suggestion of springs and that brings me to an earlier idea that we had discarded -- suspending the cameras within independent PVC frames using bungies. Our thinking was we could tune the system by loading the bungies -- loosen the bungies would act like a low band-pass filter and tightening would act like a high band-pass filter -- like tuning a guitar. What I didn't like about the suspension method was this: I could imagine the camera bouncing chaotically after each bump of the airplane. Then we thought about adding mass to the camera, but nobody likes to add mass to an airplane.

Anyway, I could see using a combo of both springs and a foam material to gain the benefit analogous to an automobile spring and shock-absorber system. Changing the C-clamp pressure would be analogous to loading the bungies.

I think we are ready to prototype a system in the Suburban.

Thanks for your insight and guidance. When we get this in the aircraft I'll let you know how it works.

 

[IRstuff]

What resolution is your camera?

Unless the EVERYTHING, payload, and bungie cords, are PERFECTLY balanced, the slight mismatches in spring constants and off-center forces will result in wobbling the camera and probable make the problem worse.

Additionally, springs will not cure your angular vibrations, nor will they behave well when the aircraft banks or manuevers.

Rubber bands and chewing gum might be cheap, but the results will be commensurate.



TTFN

FAQ731-376

 

[Botanist]

You flippantly mention chewing gum, but if you think about it, chewing gum would probably be an effective shock absorber if contained within a membrane. I know a fellow who uses breast implant gel packs for his stereo system, for example.

I agree with the bungie thing, which is why we abandoned it early-on without even drawing it up. But if somebody had a working system I'd sure be interested in seeing it. To me, it would be too complex to model effectively and the solution would necessarily be wholly empirical. I don't have the time nor inclination for that. It'd be a great science fair project for a kid.

 

[IRstuff]

But, you not trying remove shocks, are you? And stereos don't care about milliradian-level vibrations.

And no, it wouldn't be a good science project for a kid, because he doesn't have the budget for it.

We've spent on average about $5M to develop each gimbal we produce. Admittedly, they're designed to provide better that 20 microradians stability. But, if you expect good performance, then you'll need to spend the money.

You've still have not really given up the requirements for your system. Until you do so, I don't see how you expect to get useful answers.

TTFN

FAQ731-376

 

[Botanist]

Good point at the end there. Your reference to microradian gimbals is an excellent starting point. I think I can figure out how to answer the question but to get to that answer I'll have to understand some issues that I frankly, haven't thought about.

We acquired low altitude vertical CIR video immediately post Hurricane Rita in order to conduct a rapid assessment of environmental impacts. When I saw frames captured from the video displayed as stereo pairs, I noticed what appeared to be a focusing issue on the occasional frame -- in which hard edges of actual objects, when zoomed, fuzzed-out on the edges. The fuzziness was consistent within a frame but that fuzziness was not necessarily consistent between frames. This indicated that something different was operating between frames, which eliminates focusing issue, IMO.

At this point I'm guessing that fuzziness is probably the route to defining minimal stability requirements.

Since the frames are a constant pixel density and the nadir pixels are approximately equal in area of ground coverage, then I should be able to count fuzzy pixels to compute the displacement from nadir experienced by a given frame and the effect on ground resolution of that displacement. Right? There are minimal sizes of objects that I'd like to recognize so if the displacement masks that size object, then the displacement is too great.

I'm learning as I go here, so jump in anytime.

Once I have the minimum pixel shift then I'm off to the geometrical races. arctan(Acceptable maximum lateral pixel-shift divided by number of pixels in the frame (in either direction)) = Max acceptable displacement in radians (in that direction), at a given pixel resolution.

Right?

Botanist

 

[IRstuff]

You'll still need to cough up the angular resolution of the camera pixel to get much farther. Something that gives the ground sampling distance divided by the aircraft altitude will give you the pixel instantaneous field of view (IFOV).

The fact that your images look "fuzzy" says that your vibration effect is angular, and not translational, so most of the previous discussion is not applicable.

Then, you'll need to compare "good" vs. "bad" vs. some baseline with no motion whatsoever to determine the degree of focus contributon vs. vibration. Find high contrast objects and look at the intensity profile across the edges. The width of the transition between white and black is a measure of the optical resolution, focus, and vibration. Assuming that you can separate out the vibration component, the rms vibration would be roughly 1/6 of the transition width caused by the vibration measured in fractions of a pixel. So, your IFOV times that rms vibration is the angular vibration.

As an example, if you determine that your pixel ground sampling distance is 10 ft and you're flying at 10,000 ft, your IFOV is 1000 microradians. If the transition width from vibration is 3 pixels, then the rms vibration is about 0.5 pixels and the vibration is therefore 500 microradians. You need to keep the vibration below about 1/4 of the IFOV to keep the images sharp.

That level of vibration requires a gyrostabilized gimbal, there's just no way around that.


TTFN

FAQ731-376

 

[sreid]

I would try cable mounts. Simple, rugged and reliable. For example;

http://www.vibrationmounts.com/Products5.htm

What you are trying to do with an anti vibration mount is to have a spring mass system with a low frequency. The mass is your camera, etc. The springs are the isolators. In general, the bigger the mass and the lower the spring rate, the better the isolation.

 

[IRstuff]

General vibration mounts work great if all you're concerned about is mechanical vibration. However, rope and cable isolators are notoriously unmatched and are generally ineffectual against rotational vibrations.

The fact that the current system shows blurring against ground targets says that angular vibrations are already present, and a rope or cable isolation will pass that vibration anyway, and the mismatching of the isolators will introduce additional rotational vibration.

TTFN

FAQ731-376

 

[floattuber]

Back when I was a freshman in college I built a vibration isolating platform for my portable CD player for use in my car. I ended up using a spongy foam that I found at a hobby store to act as both a spring and a damper. I cut it to shape, stood them on end and glued them onto some wood. Then I added lead weights to increase the mass. Having never taken a vibration class at that time, I was rather proud of myself.

While I was building it, I thought the ideal solution would be to use oil filled, coilover shocks from RC cars. Of course, for that project it would be cost prohibitative since they were coming out with anti-skip CD players for about $100.

But for a camera mount, I think the shocks would be perfect. They have changeable spring rates, adjustable damping coefficients and they're probably not that expensive now-a-days.

 

[40818]

As this thread has kinda went into aerial pictures.
Have a look at his guys work. I saw a picture in yesterdays Daily Mail (Uk paper), where he took an amazing long exposure photo over london.

http://www.jasonhawkes.com

 

[IRstuff]

Not bad. The resolution seems to be around 200 microradians or so. Given the long exposure required, the camera had to have been in a gimbaled platform.

TTFN

FAQ731-376

 

[IRstuff]

Oops, a bit premature on the resolution, since I didn't bother buying the higher pixel count image.

If your exposure times per frame are relatively short, i.e., video during daylight, then you could potentially post process, e.g., electronic image stabilization.

http://www.ovation.co.uk/Video-Stabilization.html

A good example of aerial video is here:

http://www.axsys.com/p-scs.php

The wires in the video are probably about 1 inch in diameter, and you can see that the image is angularly stable, but either linear motion comp was not commanded or implemented.

TTFN

FAQ731-376