Timelapse Rail vs. Video Dolly

 

I have been a firm believer a proper time lapse system and video system are 2 different tools. Many have tried to merge these two and unless you have a very large budget and a very in depth understanding of mechanics, engineering, and electronics it seems that at best you get a system that is excellent in one and workable in the other.

Lets look at a very basic dolly move with a travel of 6 feet over a 3 second period using 30fps video capture. It travels 2 feet per second and gathers 90 frames of footage.

Lets say you want to do that same basic move with timelapse photography. And you want to capture one frame every 5 seconds, that is a total of 90 frames at 450 seconds.

The speed is reduced at a magnitude of 150. if you wanted to do night time lapse with 30 second intervals using 25 second exposures it would be reduced a magnitude of 2,700.

If you want to achieve good timelapse and video speeds you should probably be targeting a speed range of 1 – 3000 in order to have headroom for proper dolly speed, and the slow speed needed for timelapse photography, or else you will end up limiting yourself on one end or the other. If you start limiting yourself on either end then you have made a compromise which ends up impairing the tool for one or both of the uses.

So one way to look at what you need is a motor that is capable of 1 to 3,000 rpm. Not only does it have to do this, it has to do it accurately, and have the needed torque at all speeds to perform the way you want.

DC motors will not be able to provide a range like this. in fact they must use heavy reduction just to get down to a few RPM. With that reduction they will not be able to hit the high speeds.

Stepper motors might work better for this, I have not tested my stepper motors for their low speed capability, but the slowest i have ever driven them is about .5rpm, and on the high speed depending on the drivers (generally i use setups that require low power due to portability concerns) i can get about 400-500rpm, and the faster the motor goes the weaker it becomes, generally the fastest speed i can get for actual movement is about 400rpm, anything faster and it starts missing steps.

Servos as far as i know are nowhere near capable of the speed ranges demanded.

One way to try to beat the problem is to cut one movement sequence into multiple moves using encoded dc motors or stepper motors. So on the high speed it can get from A to B pretty quickly, and when running timelapse it can make a small movement, stop, shoot, wait, make another small movement, shoot, then move.

This eliminates the need for such slow movements but puts a new demand on positioning resolution and movement speed.

Lets look at 3 different systems, all using the same 200 step per rotation stepper motor max speed 480rpm, 36 inch track. Lets look at the high speed, and resolution for a 360 frame timelapse.

 

===Setup number 1

Using a belt drive setup with a 1 inch pully (3.14 circumference)

3.14 inches travel per rotation

11.46 rotations from end to end

2292 steps from end to end

max end to end travel speed = 2.8 seconds

slowest end to end travel speed = 11.46 minutes.

Full step = 6.3 steps per movement for 360 frames (.3 is dropped so it is actually 6 steps)

16x microsteps = 101.86 steps per movement.

 

With this setup you can hit nice video speeds, but the larger pulley will weaken the output of the motor, it is not likely to be able to do any sort of vertical lift with standard 12v DC power unless moving very slow. The motion resolution for timelapse is poor. when using microstepping it gets better, and you might be able to pull off some ramped movement if you keep the shot count low and move the full end to end, the accuracy will likely suffer, and you will be required to shoot in microstepping mode which will draw power constantly, so this is a major consideration especially if doing star timelapse as those can run for 5-6 hours.

 

Good video speed

Low payload handling, vertical travel with very small lightweight cameras.

Poor timelapse resolution

Unable to do ramped movement in full step mode.

Unable to do continuous timelapse motion, requires SMS.

Inefficient, must remain powered between motions.

Incapable of short timelapse movements.


===Setup number 2

Belt drive setup using a .31 inch pulley (1 inch circumference)

1 inch per rotation

36 rotations end to end

7,200 steps from end to end

max end to end travel = 9 seconds

slowest end to end travel in continuous = 36 minutes.

Full steps mode = 20 steps per movement

Microstepping mode = 360 steps per movement

 

With this setup you have slow and limited video applications. The smaller pulley allows heavier payloads but nothing heavy.. For timelapse it offers far better resolution than setup 1, however it will still not be capable of ramping unless microstepping. When using microstepping the resolution is acceptable but will suffer for shorter runs, and it will be required to be powered the entire time resulting on faster battery drain.

 

Slow video speeds

Acceptable timelapse resolution when microstepping

requires microstepping on timelapse

capable of some short timelapse runs with no ramping.

capable of continuous movement for timelapse for routines under 36 minutes.

This can do timelapse and video speeds, but it does neither well.

 

On the belt drive setups, you can also try using a geared motor, this will increase the resolution to acceptable levels and greatly help the payload capacity, but video speeds will even further suffer.

 

==Setup number 3

20 Thread per inch Lead screw (no belt)

.05 inches per rotation

720 rotations from end to end

144,000 steps from end to end

Max end to end speed approx 3 minutes

Slowest end to end speed approx 12 hours.

400 steps per interval on full step mode

6,400 steps per interval on microstep mode.

 

With one foot per minute as the high speed this is useless for video unless doing macro work. The end to end resolution is fantastic for timelapse, the lead-screw design means it can handle very heavy payloads 35lbs+, it is also capable of running ramped movements and shutting off the motor between shots to increase battery life. Microstepping is only needed when doing macro timelapse, and it is more than capable of running timelapse routines with 400-500 frames even with ramping at distances as short as one inch. The lead screw also means that on vertical lifts it can shut the motor off as well. This is the most efficient design and works well with continuous movement too.

No good for video work.

Excellent time lapse capability.


So what can be done to achieve high level timelapse resolution and good video speeds? Well my answer is to consider them two separate systems. But some people want to have their cake and eat it too. One possibility is to swap out motors, use a high speed motor and a low speed motor, I’m not a fan of this due to extra wear on the system.  Another method which interests me is to have a transmission of sorts but there is nothing i have found on the market that would fit this purpose, nothing affordable at least.

The Chronos Project | Time Lapse Motion Control
©2013