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Eddy Current Magnetic Door Closer

Control door closing speed without failure-prone hydraulics or pneumatics
  [vote for,

One item of everyday technology that passes entirely unnoticed to all but the pathologically curious, is the hydraulic door closer <link>. Unnoticed that is, until they fail, dribbling hydraulic fluid down the door and causing a ~100lb fire door to go crashing into the frame every time someone passes through it.

These items are simple, essentially a fairly powerful spring that forces the door closed, a feature essential in a fire door and a damper. With just a spring, the system would have a tendency to accelerate the door toward the frame, and amplify any force applied by a person, resulting in a loud, room-shaking crash*.

To prevent this, the door's motion is controlled by a hydraulic damper. Similar to a car shock absorber, the door's motion is linked to a piston that moves through a hydraulic fluid. This system provides resistive force proportional to velocity, it doesn't stop the spring holding the door closed at all, it specifically resists when the door is moving quickly. This is great, you don't want a door slammed in your face, you especially don't want a spring- assisted 100lb steel door hitting you.

There is an alternative system used on car hatches, sometimes hoods/bonnets <link>, and on lighter doors such as screen doors <link>. These provide damping by moving air through a metered orifice and by making the shaft proportionally large, they can make the unit operate as a pneumatic spring also.

Ultimately they all fail because they rely on seals**. At some point the oil/air escapes and you're flinching every time you hear the door or propping your car open with your head. So, a solution.

Eddy-current dampers, also called magnetic dampers <link> or eddy-current brakes <link> are used in various applications including high-speed trains and roller coasters to provide braking. The effect is named "Lenz's law" and the simplest demonstration of this is the dropping of a magnet through a copper tube <link>. The tube doesn't need to be made of a magnetic material, it just needs to be electrically conductive. The moving magnet induces an electrical field in the conductor, this creates an opposing electromagnetic field proportional to the velocity of the magnet.

So, how do we put this on a door? Simple, no need to re design everything, instead of a piston moving through oil, we remove the oil and have the piston be a powerful permanent magnet. The magnet will be close to, but not touching the walls of the cylinder, so no wear. The seals are no longer needed, in their place super long life linear bearings or bushings. To modulate the damping effect, different sized magnets can be specced at the factory, changes in the leverage can be used for tuning (this is already used) and modulating the diameter of the cylinder through the stroke.

*Have a guess what I'm hearing every ~20s?

**Sea lions are unwieldy, but otters are used in lightweight racing applications.

bs0u0155, Mar 29 2021

Hydraulic door closer/damper https://www.homedep...er-608810/308265607
[bs0u0155, Mar 29 2021]

Pneumatic screen door closer/damper https://www.amazon....-315239372073&psc=1
[bs0u0155, Mar 29 2021]

Gas spring damper https://www.explain...com/gassprings.html
[bs0u0155, Mar 29 2021]

Eddy-current brake https://en.wikipedi.../Eddy_current_brake
[bs0u0155, Mar 29 2021]

Lenz's Law magnet/tube demonstration https://www.youtube...watch?v=N7tIi71-AjA
[bs0u0155, Mar 29 2021]

Eddies in the space-time continuum. https://hitchhikers...m/image/69807383581
Eddie's sofa. [whatrock, Mar 29 2021, last modified Mar 30 2021]

Lifting hinge https://www.hingeou...lygAXxoC0ecQAvD_BwE
[bs0u0155, Apr 05 2021]


       //Have a guess what I'm hearing every ~20s?//   

       A small girl playing Christmas carols on a tuba?
pocmloc, Mar 29 2021

       Eddie's indoors now, is he? Is he also still in the space- time continuum?
sninctown, Mar 29 2021

       (+) If I may make one suggestion? Design a mechanism which slightly retracts the cylinder in half along its length when the interior bar is pushed or the motion sensor sets it to open which then retracts when reaching the end of the doors swing.
This would allow the door to open easily while not letting it smack you in the face when closing.

       Hmm, I suppose I do need much more significant damping in the closing direction rather than the opening. I think the way to do it, is to have the magnet slide with a relatively modest spring on the opening side.
bs0u0155, Mar 30 2021

       Also, after rummaging around in my big box of nostalgia, I've found an RC car, a Tamiya Grasshopper. Now, the rear "shock absorbers" are just springs with a plastic tube imitating what would be a hydraulic damper in the full-sized car. Looking at it, it's going to be trivial to replace that plastic tube with metal, and glue in a neodymium magnet on top of the shaft. I think I might have the world's first magnetically damped RC car on my hands here.
bs0u0155, Mar 30 2021

       To achieve dampening in only one direction, the tube could have a gap extending the length of the tube. When the magnet moves in one direction the tube is compressed from the sides so the gap is closed (thus eddy currents can form); when the magnet moves in the other direction the tube is no longer compressed from the sides and the gap opens (thus eddy currents cannot form). A switch at each end of the tube mechanically activates/deactivates the tube compression.   

       Instead of compressing the tube, the gap could be filled with a member that can switch between conducting and non- conducting. For example, each side of the gap could have an alternating series of conductors and non-conductors. The gap-filling device would also have a series of alternating conductors and non-conductors. When the gap-filling device is pulled, it aligns its conductors with the gap conductors, thus allowing the current to flow around the tube.
xaviergisz, Mar 30 2021

       Perhaps a slightly conical shaped exterior casing instead of cylindrical? It would progressively slow down when closing but progressively get easier as you open.
AusCan531, Mar 30 2021

       Could have a kind of ratchet system. When you open the door, the ratchet would slip making the door as easy to open as you like. As soon as the door started closing, the ratchet would bite and the impeder would start to resist the closing force.
pocmloc, Mar 30 2021

       //the ratchet would slip making the door as easy to open as you like.//   

       The problem is, with slip, you don't move the magnet out for it then to do the damping on the return, you need a way to get the magnet out to full extension at the start of the closing. BUT you don't need THE magnet, you could do it with recirculating ball magnets and then the ratchet system would be fine. It would look great, but it's not as elegant as the two spring sliding magnet solution.
bs0u0155, Mar 30 2021

       C'mon - seals are good. It's just that they carry on so when they get a flipper caught in the door.   

       The ratchet could drive a brake disk from a rack and pinion. Geared, the speed would be higher, allowing braking force with slower door movement. In rotary motion, your magnets don't get lost somewhere.
lurch, Mar 30 2021

       You could use a directional planetary gearbox from the rack- n-pinion (or at the hinge); one gear in one direction, a different gear in the other (ie. the input is 2 different directions; the output is 2 different SPEEDS in the SAME direction). Magnets spin in their "shell" with eddy currents doing their magic.
neutrinos_shadow, Mar 30 2021

       Does the eddy field need a closed loop around the magnet? In other words would a magnetic falling through a copper C rail still work? If the field eddies are local wall patches around the magnet, a C track would be very easy to implement.
wjt, Apr 02 2021

       //need a closed loop around the magnet?//   

       No, roller coaster brakes are often just a plate on the car that runs into a slot with the magnets on either side. The effect depends on the strength of the magnets and how close they are.
bs0u0155, Apr 02 2021

       So the door could have a slightly curved rod magnet on an arm and the wall could have the flanged reciprocal curved copper c channel . Easy.
wjt, Apr 03 2021

       Angling the frame slightly and using the door weight would take way any springs or mechanisms but may need slightly upgraded hinges.   

       Then again, nothing is simple. The complexity would be in in the angles and setup alignment.
wjt, Apr 03 2021

       //Angling the frame slightly and using the door weight would take way any springs or mechanisms but may need slightly upgraded hinges. //   

       There's already a type of hinge that lifts the door by rotating on a helix-type arrangement <link>
bs0u0155, Apr 05 2021

       As the piston/magnet moves through the coil, this device should put the current generated through an LED strip running around the door frame. So, as the door closes, there would be a brief, satisfying pulse of light around the door. If you slam the door really hard, the pulse will be shorter but brighter.
hippo, Apr 06 2021

       True, that pitch cut on the hinge will make the door slightly harder to open but hopefully the door will slip back down the pitch and close by itself.   

       No matter what happens, to have the door close, the energy has to come from the traverser*.   

       * door not plugged in.
wjt, Apr 09 2021


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