Hello!
I'm working on an idea for a filmmaking tool and hoping to lean on some electronics expertise! Thanks in advance.
The short version is that I need a way to create variable resistance for linear motion.
I frequently use a slider similar to the one below. I'm working on a way to create consistent and variable drag between the platform and the rails.


My first thought is to use a DC generator with a rubber wheel secured against the railing. The generator would then be wired to a variable load (ie pot+lamp) that would allow the operator to change the resistance.
Obviously, there are a few mechanical design challenges, but electrically speaking, is that possible? I apologize that this is such a vague explanation but any insight would be greatly appreciated.

Thanks!

 

Would it be easier to run a servo motor system that you can control the speed?
Just a variable drag would be hard to calibrate for consistency I think.

 

Believe it or not, moving a magnet along a conductor (like your aluminum track) causes a mechanical resistance.
You can simply use some big Neodymium magnets and a positioning system. To set close, medium and far from the rail. Close could be touching, medium, about 2-3mm and far, removed or 20 mm+.

They also are speed (force) sensitive, the faster you try to accelerate, the more resistive force.

 

Believe it or not, moving a magnet along a conductor (like your aluminum track) causes a mechanical resistance.
You can simply use some big Neodymium magnets and a positioning system. To set close, medium and far from the rail. Close could be touching, medium, about 2-3mm and far, removed or 20 mm+.

They also are speed (force) sensitive, the faster you try to accelerate, the more resistive force.

thank you! I am marveling at what an elegant solution that is. no bearings, no additional moving parts... genius.

 

Would it be easier to run a servo motor system that you can control the speed?
Just a variable drag would be hard to calibrate for consistency I think.

there are systems that use motors, the main issue is that the camera operator needs to be able to adjust quickly and organically, which is hindered by motorized systems.

 

thank you! I am marveling at what an elegant solution that is. no bearings, no additional moving parts... genius.

I hope it works for you. It is called Eddy Current Braking.
https://en.m.wikipedia.org/wiki/Eddy_current_brake

 

Sorry, I did some tests and the effect on a thin-wall aluminum track like yours will not produce a significant braking effect. You'll have to use a motor or other friction-based solution (light tightening down the axle bolts on one of those skateboard wheels).

 

As was mentioned above, eddy current braking is one of the simplest and smoothest techniques you could hope for in an application like this. If the operator needs to control the drag quickly you could substitute an adjustable permanent magnet for a fixed horseshoe-core electromagnet and drive it with a simple linear power supply. You could maybe even rig up an old power drill trigger to it instead of a standard rotary pot.

For best effect you'll want to create a complete magnetic circuit out of the electromagnet, through the rail and back into the electromagnet. I.E. a "horseshoe" magnet. Try running a small magnet along the outside of a piece of copper water pipe... then drop it through the middle. Same concept. The tighter the air-gap between the magnet and rail, the better the effect too.

Here's a real-world example of a practical linear eddy current brake. If I'm not mistaken, those pole shoes are probably oriented N-S-N-S to create a magnetic field which extends into the rail between each magnet.

If the track is not conductive enough, you may need to fit a length of copper or aluminum busbar or upgrade to a solid rail. Most steel houses carry pure copper square/flat bar and aluminum alloys which will probably be good enough for this application. It may even be easiest to use some thick flange aluminum angle, channel or I-beam for ease of implementation. Rigidity is going to be a consideration for consistency in braking force and avoiding contact between the magnet and rail which could introduce unwanted vibrations during the shot.

 

As was mentioned above, eddy current braking is one of the simplest and smoothest techniques you could hope for in an application like this. If the operator needs to control the drag quickly you could substitute an adjustable permanent magnet for a fixed horseshoe-core electromagnet and drive it with a simple linear power supply. You could maybe even rig up an old power drill trigger to it instead of a standard rotary pot.

For best effect you'll want to create a complete magnetic circuit out of the electromagnet, through the rail and back into the electromagnet. I.E. a "horseshoe" magnet. Try running a small magnet along the outside of a piece of copper water pipe... then drop it through the middle. Same concept. The tighter the air-gap between the magnet and rail, the better the effect too.

Here's a real-world example of a practical linear eddy current brake. If I'm not mistaken, those pole shoes are probably oriented N-S-N-S to create a magnetic field which extends into the rail between each magnet.

If the track is not conductive enough, you may need to fit a length of copper or aluminum busbar or upgrade to a solid rail. Most steel houses carry pure copper square/flat bar and aluminum alloys which will probably be good enough for this application. It may even be easiest to use some thick flange aluminum angle, channel or I-beam for ease of implementation. Rigidity is going to be a consideration for consistency in braking force and avoiding contact between the magnet and rail which could introduce unwanted vibrations during the shot.

I just tried it with some powerfulneodymium magnets and found the braking effect can slow a free falling Magnet but, attaching the magnet to a skateboard-sized sled, the braking force is so small, nobody will notice the resistance vs the force to move the skateboard.

 

I just tried it with some powerfulneodymium magnets and found the braking effect can slow a free falling Magnet but, attaching the magnet to a skateboard-sized sled, the braking force is so small, nobody will notice the resistance vs the force to move the skateboard.

If eddy brakes can be built to stop a 200 ton locomotive, they can most certainly be built to resist the movement of a skateboard. What did your testing arrangement consist of?

 

If you gear up a rotating copper disk so it spins faster, then the magnets close to it will work a lot better.

 

Good idea. Might be tricky getting involute gear teeth to mesh smoothly without introducing vibration into the camera shot though. Perhaps some sort of gearless friction drive driven directly off of the carriage wheels? Or a nice smooth round-belt drive?

 

A Google of "Mag Particle Brake" will bring up plenty of mag particle brakes. Since the dolly traverses a pair of rails a steel cable could likely be run end to end parallel to the rails and attached to the dolly. Mag particle brake on one end with the cable spooled on it. You may also get some results trying mag particle clutch. Tried a few when doing torque testing and liked mag particle simply because it was cleaner than using hydraulics.

Ron

Ron

 

It could also be done mechanically. Think of how the drag on a fishing reel works. A series of disks some connected to the reel body and some to the line spool. And a knob to set the drag value. The tighter the knob is the more the drag is. A quick Google will show you much more.

 

With the linear idea, would a thicker bar of metal increase the braking force? What about using copper vs. aluminum?

 

If eddy brakes can be built to stop a 200 ton locomotive, they can most certainly be built to resist the movement of a skateboard. What did your testing arrangement consist of?

You've seen a 200 ton locomotive with aluminum wheels? Who uses those?

Note that magnetic braking can only slow its own weight as the magnet falls though a copper pipe or aluminum tube. How much does that magnet weigh? Now, add 50x the weight to the magnet - will you notice the magnetic braking? I'm sure a camera, skateboard wheels, axels and mounting hardware will be more than 50x the weight of any magnets used.
I think @Reloadron was in the right track by suggesting some iron/steel be fitted inside the aluminum tracks and try again.

 

You've seen a 200 ton locomotive with aluminum wheels? Who uses those?

Note that magnetic braking can only slow its own weight as the magnet falls though a copper pipe or aluminum tube. How much does that magnet weigh? Now, add 50x the weight to the magnet - will you notice the magnetic braking? I'm sure a camera, skateboard wheels, axels and mounting hardware will be more than 50x the weight of any magnets used.
I think @Reloadron was in the right track by suggesting some iron/steel be fitted inside the aluminum tracks and try again.

Here's a real-world example of a practical linear eddy current brake. If I'm not mistaken, those pole shoes are probably oriented N-S-N-S to create a magnetic field which extends into the rail between each magnet.

As was also mentioned, introducing some sort of a belted, geared or friction drive to increase the speed of a spinning copper/aluminum disc passing through lines of flux could improve the net braking force over using linear rails as the force developed is velocity-dependent.

 

Look closely at the size of the magnet under the skateboard. If that's something that works (or even 1/4 that size works) with the camera, good for the OP.

 

Look closely at the size of the magnet under the skateboard. If that's something that works (or even 1/4 that size works) with the camera, good for the OP.

As was also mentioned, introducing some sort of a belted, geared or friction drive to increase the speed of a spinning copper/aluminum disc passing through lines of flux could improve the net braking force over using linear rails as the force developed is velocity-dependent.

E.G. utilize a higher velocity to dramatically reduce the field strength (magnet size) needed to achieve the desired effect. If my memory serves, EMF = lines of flux per second through a given conductor.

Though at this point it seems as though that thought process is drifting in the direction of a commercially available DC project motor coupled with some sort of belt drive.

 

I also thought of the eddy current brake immediately when reading the OP. I think it is the smoothest, most organic source of drag that anyone could hope for. And also the simplest to implement and probably the cheapest as well.

Don't take it personally but I think the concerns of the trolley weight vs magnet weight are unfounded, or reached too soon in the experimental process. There are plenty of counter examples.

Go to someplace that sells tools, look at the high end toolboxes. Most of them nowadays have a small magnet that provides eddy current braking in that last few inches of drawer movement so that the drawer doesn't slam shut. It is quite effective, even with a few hundred lbs of tools in the drawer. I believe even some higher end kitchen cabinet drawers have this feature now. Maybe you can demo it at Home Depot. Maybe you can even find the magnetically braked drawer slides for sale separately, and if you can, maybe you can use them for your project.