August 20, 2017

Planar Magnetic Headphones, Part 1

A brief break from 3-phase electric motors, for a different kind of electromagnetic actuator.

Planar magnetic (a.k.a. "orthodynamic", "isodynamic", etc) drivers are a pretty neat topology of linear motor, which, as far as I can tell, has found no application besides speakers and headphones.  Here's a well-diagrammed explanation of how they work.

Instead of a voicecoil actuator which drives a stiff cone back and forth, like a conventional speaker driver, a thin membrane with conductive traces sandwiched within a funny magnet arrangement moves back and forth.

image credit: innerfidelity.com
I won't try to argue that this is "better" than a conventional driver in any way, because frankly I don't care.  It is cool, though, and also a perfect excuse to send out for some flex-PCB's, which my go-to PCB vendor, PCBWay, recently started offering for quite reasonable prices

I started out by playing with magnet arrangements in FEMM, to see how sensitive this arrangement was to changes in magnet dimensions and spacing, and I ended up here:

This uses 1/8" square N52 magnets, with a 2mm airgap, and 9mm pitch.  The array 2 inches long (into the page), and each bar is a pair of 1/8" x 1/8" x 1" magnets, which are cheaply available on ebay:


Here's the tangential flux density (which is the normal force producing component)  in the middle of the air gap:


Doubling the depth of the magnets got it up to .5T peak (the path around the magnet gets longer, so less flux leaks through it), but I decided it wasn't worth the more than double magnet cost (they are harder to find magnetized through the small face).

Here's what the actual flex pcb coil looks like.  The traces go up and down in the regions of ±.35T field in the plot above.  This was very unpleasant to draw in Eagle.  0.25mm trace width with .5mm pitch.  PCBWay claims to be able to do .1mm between traces, but I didn't want to push it.


For the actual PCB, I chose single layer, 75┬Ám flexible adhesiveless base, .1mm FPC thickness, no solder mask, and 1/3 oz copper, to keep the board as light and flexible as possible. Maybe a 2-sided trace would have been better, for a higher ratio of conductor to pcb material, but this whole thing is an experiment anyway.


Front:  A few of them came out a little wrinkly.  Not sure why, or if it will cause problems.  In retrospect, it might have been a good idea to put a thick copper trace around the outside of the whole board, to stiffen up the perimeter.  Coil resistance came out to 16 ohms.


Back:


I CNC milled half of a housing out of some Delrin:



Glued in the magnets:


There's a lip on the housing that will apply some tension to the flex pcb to hold it flat, once both halves of the housing are made, but it was secure enough for some testing with only one half.



It actually works even with only half of the magnet arrangement - half the magnet gets you half the flux density.  It should produce the same volume on roughly half the power, once the whole thing is assembled.



I'll reserve qualitative judgement until the whole thing is assembled, but from the brief testing I've done on this single partially assembled driver, it seems promising.

August 13, 2017

Reamer Regrinding

In the near future I'll need to make a bunch of things with tight-tolerance 22 mm bores, so instead of buying a 22 mm reamer, or just boring the holes like a normal person, I tried out re-grinding a 7/8" reamer (22.225 mm) reamer down to 20 mm by setting up my toolpost spindle as a toolpost grinder.

Here's the setup:


The reamer is set up between centers, and the toolpost spindle has a 4" grinding wheel on an arbor.  There's a mist-coolant dispenser to keep everything cool and lubricated.  The center on the chuck-side was turned in place out of some steel scrap, to ensure its concentricity with the spindle.

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All together it turned out pretty well.  I turned down the shank and ground it as well, so it will fit in the lathe's drill chuck.




I overshot my target 22 mm by 5 microns, but that's not too shabby, considering the ticks on the lathe's cross slide are 25.4 microns each.  It seems to cut a tiny bit oversized, so it works out in the end to give me a very tight slip fit onto a 20mm ground shaft.

This method leaves no relief behind the cutting edges, so as you might expect, they drag a little bit.  The surface finish inside the bore is okay, but definitely not as nice as I'm used to getting with reamers.  Over all, this was definitely way more time and effort than just boring the holes like I normally would, but it was a fun exercise, and good to get some experience using a toolpost grinder.