Variable drag module using Generator

strantor

Joined Oct 3, 2010
6,782
I tried to look up an example to post of the magnetically damped drawer slides and I can't find a single one. Everything i found with soft close drawers is using fluid dampening. I swear though I saw a sticker on on a toolbox sometime back that touted magnetic dampening and then more and more tool boxes had dampening, I just assumed it was magnetic. Now I'm wondering if I made the whole thing up. I apologize if that's the case.

Anyway, fluid dampening is also a good method. You could find a long pneumatic cylinder and attach a flow control valve to the exhaust. Then you would have adjustable dampening. It should also be very smooth, if possibly a little more prone to stick-slip.
 

Reloadron

Joined Jan 15, 2015
7,501
I tried to look up an example to post of the magnetically damped drawer slides and I can't find a single one. Everything i found with soft close drawers is using fluid dampening. I swear though I saw a sticker on on a toolbox sometime back that touted magnetic dampening and then more and more tool boxes had dampening, I just assumed it was magnetic. Now I'm wondering if I made the whole thing up. I apologize if that's the case.

Anyway, fluid dampening is also a good method. You could find a long pneumatic cylinder and attach a flow control valve to the exhaust. Then you would have adjustable dampening. It should also be very smooth, if possibly a little more prone to stick-slip.
I can give you an example of magnetic dampening on my powder scales.


The magnets are embedded in the scale where the arrows point. The end of the pivot arm is turned in copper:


Ron
 

MrAl

Joined Jun 17, 2014
11,389
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.
View attachment 231221

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!
Hello,

Heavy felt pads are sometimes used to create this kind of damper combined with a pressure screw type adjustment and possibly an added spring. They may wear with time though so there may be some maintenance required.

But what i would like to know is what exactly is driving this platform. Usually there is some sort of motor and a control for that motor that of course controls the entire movement of the platform.
As a quick example, a motor and lead screw. As the screw turns, the platform moves, and this gives us tremendous control over the movement of the platform. Using a good motor control method we can get the platform to dance the Nutcracker suite ballet.
There are other methods too such as precision chain drive which is very fast.

But what exactly drives the platform currently, that is what causes the platform to move in its current state? The answer to this may lead to a very elegant solution rather than trying to use some sort of damper.

As to the damper, if it really is required, another form of damper is an air pressure damper where a rod drives a piston in a cylinder with a small hole in the end. As the piston moves the rate of air escape is limited so it causes some back force. As the piston withdraws, the air hole restricts the rate of air intake so again it causes a back force that adds resistance to the movement. Adjusting the diameter of the hole varies the amount of resistance.
There are also fluid based versions where the fluid flow is restricted as the fluid circulates.
All these kinds of devices that are based on mechanical resistance are called "viscous dampers". They often involve some sort of friction.

My opinion is to try to accurately control the movement rather than try to dampen it, but i dont yet know what is driving the platform. If you can fill us in that will help a lot i think.

Depending on your project budget you can get some pretty darn sophisticated linear drive mechanisms these days that do just about everything and very precisely too. Some will be more expensive of course.
One i built myself some time ago though was very cheap because i used an old line printer converted to a linear drive. A second line printer acts as a second axis forming an x y drive. Using stepper motor microstepping you can get precise movement down to hundredths of an inch or better over the entire range of the movement.
 
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strantor

Joined Oct 3, 2010
6,782
I can give you an example of magnetic dampening on my powder scales.


The magnets are embedded in the scale where the arrows point. The end of the pivot arm is turned in copper:


Ron
Yeah that is a good example. I have a powder scale as well and never realized that's what the copper tab is for. Unfortunately I think the example bolsters the argument against eddy current damping more than the argument for it. There is very little force on the end of the balance beam, particularly when it's properly balanced, so it works well.

Perhaps I am wrong. Probably so. I concede.
 

Ya’akov

Joined Jan 27, 2019
9,069
thank you! I am marveling at what an elegant solution that is. no bearings, no additional moving parts... genius.
This is an interesting solution but one feature of it will be that the drag varies with the speed, increasing the faster you move and decreasing suddenly if you stop. The effect may not be too great for the application, or possibly even desirable in some cases, but for a slider it *seems* to lack the sort of control you’d want.
 

Ya’akov

Joined Jan 27, 2019
9,069
Though it might be mechanically complicated a possible solution which comes from the same domain as the slider might be to use a fluid head which, of course, is designed to provide the sort of drag an operator needs. Gearing it so it travels its full range from the start to end of the slider might work.
 

MrAl

Joined Jun 17, 2014
11,389
This is an interesting solution but one feature of it will be that the drag varies with the speed, increasing the faster you move and decreasing suddenly if you stop. The effect may not be too great for the application, or possibly even desirable in some cases, but for a slider it *seems* to lack the sort of control you’d want.
Yes i was going to mention that myself. Unlike Coulomb damping the magnetic current damping effect will vary greatly with speed of travel. It will be maximum during times of maximum speed, and zero near the lowest speed, and of course when the speed is zero which.

Now the question is what happens when instead of using a purely electrically active conductor we use a high permeability magnetically active material coupled with a very strong rare earth magnet.
If the speed does not vary to much we might see the speed/current/backemf/reversefiled effect get lost in the noise floor under the more significant effect of the direct magnet/iron static attractive force.
There is a chance this will work if the required damping is not too large.
There is also a chance that some friction will still have to be introduced into the movement because most of the magnetic force will be perpendicular to the direction of travel.
The other question then is how to vary the force. Possibly an electromagnet with controlled current.

As with most other control systems, some sort of feedback greatly helps to manage the performance which includes the accuracy. The feedback could come in the form of a linear motion sensor coupled with a control circuit that varies the current to the electromagnet. If the system works at all to begin with, this would make sure that the motion remains completely predictable and absolutely constant.
 
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MrSalts

Joined Apr 2, 2020
2,767
Yes i was going to mention that myself. Unlike Coulomb damping the magnetic current damping effect will vary greatly with speed of travel. It will be maximum during times of maximum speed, and zero near the lowest speed, and of course when the speed is zero which.
those properties are exactly why it would be a perfect damper for a person pushing the sled down the track. The effects you mentioned will greatly reduce the variations in speed as the camers operators hand slightly changes force. As his force increases, more resistance will slow the sled snd less force will allow the sled to continue moving. The whole concept, though, is like an amplifier of the pressure the camera operator's hand feels for a given change in sled speed. Changing the distance of the magnet to the rail would change the amplification.
 

MrAl

Joined Jun 17, 2014
11,389
those properties are exactly why it would be a perfect damper for a person pushing the sled down the track. The effects you mentioned will greatly reduce the variations in speed as the camers operators hand slightly changes force. As his force increases, more resistance will slow the sled snd less force will allow the sled to continue moving. The whole concept, though, is like an amplifier of the pressure the camera operator's hand feels for a given change in sled speed. Changing the distance of the magnet to the rail would change the amplification.
Yeah i never got the full story on how the thing was being driven although i did ask.
 
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