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Speedometer for a dolly?


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#1 Karel Bata

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Posted 25 July 2013 - 06:38 AM

It doesn't have to be accurate, but to act as an aid to getting a sequence of dolly moves at consistent speeds.

 

Any ideas? We're on a tight budget.

 

Cheers!  B)


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#2 Sanjay Sami

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Posted 25 July 2013 - 11:44 AM

Metronome ... clicks lining up to marks on the track


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#3 JD Hartman

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Posted 25 July 2013 - 01:10 PM

Practice, practice, practice.  Same as smooth starts and stops.  It's a learned skill.  Best bet is using an experienced dolly grip, but I see you say it's low budget.  Second best bet is a crew member who has pushed dolly before, not using just anyone who wants to or thinks it's "fun".  Some people just spaz out when put behind the pushbar.


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#4 Karel Bata

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Posted 25 July 2013 - 02:35 PM

You weren't to know JD, but we're going to be repeating the same move in 4 different setups in 3 locations over 3 days. I doubt there's a grip on Earth that can be that consistent. Especially after lunch.

 

I really do have enormous respect for experienced grips. But this is rather like expecting a focus-puller to do it by eye.

 

We could use a simple belt and motor, but I can't find one that doesn't come attached to a MoCo rig that costs the Earth. We've been quoted £5k.

 

Here's what we're doing

 

Any ideas gratefully received...


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#5 Igor Trajkovski

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Posted 25 July 2013 - 05:22 PM

Just some brainstorming...

 

Camera mounted on steady rig on car with smooth consistent cruise control?

 

Or have the dolly be pushed/towed by the cruise control car?

 

 

Best

 

Igor


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#6 Sanjay Sami

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Posted 26 July 2013 - 02:04 AM

what is the length of the move ?


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#7 Sanjay Sami

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Posted 26 July 2013 - 02:25 AM

I think a dolly grip can pull this off. You are tracking in, which already makes it easier to match than tracking across.
If you have marks every 6 inches, and the dolly grip has a clicking metronome app on his iPod that he matches click to mark, you can be extremely consistent. I have done this, even on IMAX, and it has not been a problem. It is more complicated if you are tracking across the scene, because the objects cross frame faster, and inconsistencies will be more apparent.
If I understood right, you will be stitching bits together, not layering them ?
I think its very possible to do. 
I have done layered moves this way with good results.
I also built a poor mans moco track, using a dolly with cable and a DC motor with rheostat attached to a cable drum. You set the rheostat to a particular speed on the dimmer, and go. The starts and stops are not the same, but the move after it ramps up is dead accurate.
For your job I would get a good dolly grip. Call John Flemming. PM me if you need his phone number. 

Edited by Sanjay Sami, 26 July 2013 - 02:26 AM.

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#8 Karel Bata

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Posted 26 July 2013 - 02:56 AM

We'll have 8 x 8ft of track, and anticipate that maybe 50ft of that is usable. In 3D you can't zoom like you can in 2D, so we have to do the move for real.

 

Originally we had a PeeWee booked, but we're not rising in shot, so It's a flatbed, bazooka, and paddle arm, all tied firmly down. For two days this stays locked together.

 

We're looking at a bicycle speedo. Put a magnet on the wheel, and use a magic arm (and gaffa tape!) to hold the sensor close by. Here's the manual http://sprockt.com.a...210W_ENG_v2.pdf The minimum programmable bicycle wheel size appears to be 102cm. A dolly track wheel would be a lot less and give a very high readout. Max displayable speed is 65.9 Mph. That's about 100ft/s. We'll be going at 3ft/s (shooting at half speed and doubling later), so the minimum wheel diameter is 3cm. So it could do it. But without drilling holes into the wheels, how would we attach the sensor magnet? The slightest movement over two days would render the system useless.

 

Scratching our heads here. It's fun thinking these things through, but time consuming. Still, we've got a week to figure something out. :)

 

 


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#9 Karel Bata

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Posted 26 July 2013 - 03:07 AM

Ah. Sanjay, we cross posted.

 

These are layered rather than stitched. You're right - looking down the axis of travel means the major motion cue is scaling which means there's some scope to do a little fix in post. But 3D is very unforgiving.

 

I'm really surprised that there's no gadget for this. There's a gadget for everything else!

 

We're doing this in London. I'll PM you.


Edited by Karel Bata, 26 July 2013 - 03:08 AM.

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#10 Darryl Richard Humber

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Posted 26 July 2013 - 05:22 AM

Sanjay's right, a good dolly grip can do this. He (She) just needs to be able to see the playback of past shots and a marking system like Sanjay mentions. Sanjay can probably recommend someone in London.


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#11 Phil Rhodes

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Posted 26 July 2013 - 06:12 AM

It wouldn't be impossible to create some sort of widget to give you an idea of speed, but nobody pushing on a dolly is ever going to be a repeatable motion control device.

 

You'd need some sort of reference. Perhaps some sort of optical pattern printed on a strip on the track, or something like that.


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#12 Phil Connolly

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Posted 26 July 2013 - 06:23 AM

I think Phil Rhodes has the right idea with and optical pattern.

 

Something along the lines of a turntable strobe trick:

http://petapixel.com...ng-a-turntable/

 

A cheap camera that allows you to adjust shutter speed to something fast, attached to the dolly pointing virtically down. Then a patten strip running along side the track - the patten on the strip designed to make the marks look static when then dolly is moving at the correct speed.

 

Theres probably some maths involved - but nothing too scary.


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#13 Karel Bata

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Posted 26 July 2013 - 06:27 AM

I was just typing much the same idea!

 

A strip with a printed series of bars laid on the floor next to or under the track. And a strobe light whose frequency you can adjust. Dial in the correct frequency, and as you push at the correct speed the pattern appears to be static. Go too fast or slow and the pattern appears to move telling you whether to slow down or speed up.

 

If anyone takes this idea forward into a marketable product you'll have to pay me royalties! :D

 

This would be a great idea if you expected to be doing this again, but for the one shoot the R & D is a bit much.


Edited by Karel Bata, 26 July 2013 - 06:28 AM.

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#14 Phil Rhodes

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Posted 26 July 2013 - 08:15 AM

You could easily do better than that.

 

Encoded strip on the floor (or maybe an ultrasonic thing on a C-stand, or whatever). Then you could push the dolly, establish a speed, and hit the "this speed" button, then have it indicate absolute speed in units of your choice and/or give a faster/slower indication.

 

P


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#15 Chris Millar

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Posted 26 July 2013 - 07:53 PM

A bicycle speedo is basically a 1 ppr encoder... (wont work well at low rpm, unless you glue/snot-tape on more magnets that is)

 

If you're going down the manual move path then it might be a simpler to use a 'digital tachometer' (google it) - never seen/used one - but it looks like a laser disto that operates in distance with respect to time rather then just distance, if it's update is zippy enough it would be more accurate and simpler to set up than a bike speedo.


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#16 Sanjay Sami

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Posted 26 July 2013 - 11:19 PM

.... but nobody pushing on a dolly is ever going to be a repeatable motion control device.

 

 

Maybe not ... but you can get EXTREMELY close with a good dolly grip. If you haven't worked with one you wouldn't know what they are capable of.

My recommendation is to speak to John Flemming, since you are in the UK, and you will probably get a good solution from him.


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#17 Karel Bata

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Posted 27 July 2013 - 04:04 AM

I've sent John a mail. Fingers crossed! ;)

 

I found this page reviewing hand-held laser tachometers interesting http://makezine.com/...ser-tachometer/  But hand held isn't too useful.

 

Do those made for motorbikes work on the same principle - a laser that makes no contact? This is not my area at all. :wacko:


Edited by Karel Bata, 27 July 2013 - 04:04 AM.

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#18 Phil Rhodes

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Posted 27 July 2013 - 08:02 AM

What you're really after is a giant optical mouse which could look down at any surface passing by beneath the dolly.


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#19 JD Hartman

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Posted 27 July 2013 - 11:36 AM

I think a dolly grip can pull this off. You are tracking in, which already makes it easier to match than tracking across.
If you have marks every 6 inches, and the dolly grip has a clicking metronome app on his iPod that he matches click to mark, you can be extremely consistent. I have done this, even on IMAX, and it has not been a problem. It is more complicated if you are tracking across the scene, because the objects cross frame faster, and inconsistencies will be more apparent.
If I understood right, you will be stitching bits together, not layering them ?
I think its very possible to do. 
I have done layered moves this way with good results.
I also built a poor mans moco track, using a dolly with cable and a DC motor with rheostat attached to a cable drum. You set the rheostat to a particular speed on the dimmer, and go. The starts and stops are not the same, but the move after it ramps up is dead accurate.
For your job I would get a good dolly grip. Call John Flemming. PM me if you need his phone number. 

 

DC stepper motor and controller would be a better low budget control situation (than a DC motor and rheostat).  It would be self correcting, but probably need more track to acount for startup and stop.


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#20 Jason Comparetto

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Posted 31 July 2013 - 03:02 PM

This has been an interesting problem solving challenge.  Best guess I can come up with so far-- silent metronome on an iPhone or similar.  let's say you get it to blink 1 per second.  each blink corresponds to a "tie" in the dolly track.  more than likely the ties in the track are at even distances.  if you need your dolly grip to speed up, tell him to change the metronome to 1.2 blinks per second, and the tip of his foot needs to hit the "tie" on every blink.  this will be pretty accurate as long as you give him 10 minutes to practice beforehand.


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