Saturday, November 16, 2013

A different perspective - filming with the CableCam

This Blog post is one of an entire series
Motivation and Design
Download Link for CAD drawings (updated regular)
Bill of Materials
Motor and ESC considerations
CableCam rope
Building the DSLR CableCam
CableCam controller board

Free Download

All files needed to build this cablecam are found here.

Motivation

How do you film a moving person? From far away using a tripod and a fluid head following the person constantly? This is nice for overview shots but for tracking a single moving person not the right thing. You want to stay with the person.
The other option is to use a steadycam/glidecam running behind the person or driving in front of them. I actually like this method a lot and a good example is Devin Graham's work.
http://www.youtube.com/watch?v=gKwuvyNNwhA
The main advantage is a steady image, you are very flexible as you can walk and run wherever you want, turn, circle around the person and stay at the same distance always.
But for what I am doing most of the time, this is not an option either. I would need to run next to a snowboarder, across water or through woods. And the other thing is when filming from higher up, that gives the entire scene a dramatic sense. Using a camera equipped copter is certainly the most flexible method but then you can fly for a few minutes only etc. The other problem is, following somebody at same speed from the side probably through thick underbrush, again a very intense perspective, is hard if not impossible.

When I found these two videos, I knew what I wanted to get
http://vimeo.com/45441024

http://vimeo.com/54435407

Design

The one thing I decided right from the beginning was to use a Brushless Gimbal, this is a motorized camera head that can be moved but has sophisticated stabilizing sensors and software to offset any unwanted movement, usually used in copters. This approach is a great cost saver as you can spare the large, expensive and power consuming Gyros and use a 200USD GoPro gimbal for starter and upgrade to larger DSLR or Red Epic Gimbals later.
The best place for brushless gimbals is the fpv-community.de (German but English is welcome as well) as they created a freeware stabilizing software that made its way into various products. Very quickly a new thread (in German) was started and a couple of people owning a CableCam already for professional filming did help out with their experience as well.
http://fpv-community.de/showthread.php?34224-CableCam-mit-BL-Gimbal
http://www.rcgroups.com/forums/showthread.php?t=2103895


To cut a long story short, these are the main building blocks

  • The frame CNC milled out of 3mm AlMg1 alloy, 1 meter in span, 2kg weight and a view 8mm parts
  • Motor is a 1800 watt brushless motor with sensor and Electronic Speed Control spec'd for 1/8th scale buggies and rock crawlers
  • Any brushless gimbal from GoPro to large cameras, including RED

After a few iterations we had the final design nailed down.

And the first prototype got CNC milled.

To my surprise the end result was working almost perfectly. Everything we had not been sure about initially we designed to be flexible. For example, how much should the wheels be offset vertically? If it is little the CableCam might skid on the cable, if it is much then the load is so high that we do not reach a high speed. Turned out that there is not a single answer to that, for high speed the offset is just 20mm, for steep lines down a hill 50mm might be perfect. You can adjust that in four different positions.

The interesting part is the brushless gimbal and its operation. If you haven't seen such yet, here I recorded a demonstration. As you see, no matter what you do, the camera remains in the same position.
This gives you steady videos even if the CableCam itself is moving.

Some more pictures



The first thing I learned was that using a 22.2V setup with my motor is too much adjusted towards speed. Our initial calculations have indicated that already, but a speed of over 100 km/h is just ridiculous. Especially because this was not even top speed.

So instead we turned to this motor here: Turnigy 1900kV sensored Trackstar
It runs with 14.8V and gives you 1800 watt power, but I am running it with 11.1V battery only. The ESC used is a SkyRC TS120 for 3s applications or the TS150 for larger Gimbals. Both are built for cars and rock crawlers and you should get a programming box or the bluetooth adapter in addition. The rock crawler functionality is important because then your ESC allows to drive forward and reverse without any difference and allows for an initial motor brake, meaning the CableCam will not move even if the rope is rather steep.

With that setup the top speed is about 60 km/h and you can move the CableCam even at very low speed nice and smoothly.

For larger cameras the LRP Dynamic 8 1600kV Motor at 4-6s turned out to work nicely, like here in a Video with the RED camera and a HD downlink.
https://vimeo.com/116372172






Speaking about rope, we all seem to use a Dyneema rope in various strengths and diameters. The one I opted for is a 6mm diameter rope capable of handling up to 1ton of force. From my experience it is important that the rope has a sheeting so the cablecam runs with less vibrations.
The people using the RED gimbal opted for a 12mm diameter.
https://www.bootskram.com/liros-dynamic-plus-spulen.html

Now the CableCam looks like this (without electronics)
CableCam in the stand for better transportation

Moved the battery below the rope

The rope is inside a cage, the bottom screw can
be removed

Other side; As you can see we left the option for a
two stage gear in case we find use cases for that

But the belt works so well, at the moment I do not
see a reason for switching to gears

A few noteworthy details

  • The CableCam is asymmetrically built, meaning the wheels are not between the frame as with most others, but on one side of it. The advantage is, when you want to hook the CableCam into the rope, no wheel has to be disassembled. Center of gravity is still below the rope because of motor, battery and gimbal.
  • If needed however, it can be assembled symmetrically as well, the frame is prepared for that.
  • The rope runs inside a cage near the front and back wheel, closed at the bottom by a long screw. This does ensure that even if the cablecam accidentally travels over the end of the rope and gets off the wheel, it can never fall off.
  • Any gimbal can be mounted at the bottom using an adapter plate.
  • The rope winch is a great time saver. Using it, the prep time is cut down to 10 minutes currently.
  • Currently we are in the process to develop a little electronic device, which is hooked between the RC and the Motor Controller, acting as an acceleration limiter. This does ensure the cablecam stays at the same position and does brake soon enough to stop before the end of the rope is reached.




Use

The biggest downside of the CableCam is the setup and preparation time. Currently it takes about 10 minutes to install the 2kg rope. Once that is done getting the CableCam ready is a matter of minutes. You remove the two screws in the cage, set the CableCam on the rope, lift the rope into the middle wheel, the one that is powered by the motor and then you power up the RC and the ESC. The brushless gimbal needs 20 seconds where there is no moving and no shaking in order to align itself with gravity, then I turn on the GoPro camera via Wifi and you are good to go.

While I obviously haven't used it much yet, a first video should give you an idea of how interesting such a scene from higer above can look like.








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