Motor controlled free fall camera sled

Let's Make Robots Project Ideas
,
1

Dear community,

I’ve been prototyping a free falling camera sled for a while that I would just let go at the top and then it would fall onto a padded surface. This has worked already quite well but now I want to be able to control the speed with which the sled travels vertically.

Please see the attached schematic drawing and photo of the sled.

It’s basically a sled for a follow camera which travels vertically on one wire rope on either side of the sled to keep it in balance. (See photo)

There’s an extra rope that is suspended between two pulleys at the top and bottom.

I now want to control the speed with which it falls (it’s supposed to be a controlled free fall) and the moment it comes to a smooth stop before it hits the ground.

I did some research and chose the following parts for the setup:

  • Brushless DC motor
  • Controlled by Arduino with a Mosfet Shield

At the moment I’m still unsure how strong the motor has to be in order to slow down the sled and which technique to use for braking.

Accelerating the sled downwards doesn’t need much force since gravity is doing most of it. Only the air drag (and some friction on the wire ropes) would slow down the sled which I want to overcome with the motor.

The kinetic energy of the sled after a free fall of 8.5m is approximately 42J.

If I want to motor to do the braking, it has to be able to handle this energy. I tried to find the right formulas in order to calculate the resulting torque the motor has to be able to withstand but didn’t manage. If you have a hint here, great!

A question that follows from my assumption is, can the BLDC motor handle this strong braking at all? Or are BLDC motors in general not good for braking?

Any help would be greatly appreciated.

039_Camera_Rig_Schematic-01
039_Camera_Rig_Schematic-011754×1240 91.3 KB

2

Hi there,

Welcome to the forum.

First, this looks like a great project.

Building this all custom would be pretty nice but probably would require a lot of electric and hand tools.

This is why I would suggest you RatRig, they have some great kit with everything you need to do what you want to achieve.

100cm Slider: https://www.robotshop.com/en/ratrig-camera-v-slider-100cm-diy-kit.html
Fast speed motor: https://www.robotshop.com/en/ratrig-v-motion-fast-speed-motor.html
Speed controller: https://www.robotshop.com/en/ratrig-v-motion-lite-upgrade-kit-wo-motor.html

I hope this is a good start for you. You may inspire you from their design and I’m sure there are tons of video to create your own Rat Rig DIY on the web. This would allow you to create the 1.5m rig you need.

3

Thank you pjutras for your reply.

Unfortunately, the parts you suggested don’t meet the requirements. For instance, the speed of the motor is way too slow (1m/18s) whereas I need (14m/s).

4

Wow! I understand.

This is some serious speed to simulate a fall.

I would recommend you verify this section for a high speed motor: https://www.robotshop.com/en/bldc-motors.html
Note that you will also need a ESC (Electronic Speed Controller).

Make sure the structure is solid enough to support the speed of the mechanism.
This is probably all the recommendation I can give but I’m really curious to see your project in the robots section once it’s completed!

Good luck!