Monday, December 15, 2008

Linear motors: a little thought

I was doing a little looking at linear motors. Got an idea that it would be a great replacement for the geartrain of a homemade airsoft or Nerf gun, plus I've always been looking at how to build them. I'm focusing on the tubular variety instead of the linear rail as I believe these will be easier to build with minimal components, but I suspect they're either harder to design or build with, as there are two major manufacturers making them, Copley Controls and California Linear Devices.

Construction notes:

First, I've been researching the shaft. From what I can tell, it looks like the magnets are placed end to end, NS-SN-NS and held together mechanically. I'm looking at using a stainless steel sleeve with a series of high energy rare earth cylinder magnets inside with spacers. Down the center will be a possible sleeve and a threaded rod. I'm not certain of the material yet. I'll need a jig to hold the magnets on the rod, then nut each end. I plan to custom cut some aluminum end blocks to suspend the magnets in the middle of the sleeve, and provide me with some semi-disposable ends to mount hardware to.

So far, if I build my first arrangement, I think I'll use this:

They're not the largest or the strongest grade, but this should keep the cost down, too. I'd like a higher strength and temperature grade, but I'd settle for a higher temp grade. Well, one thing at a time.

I've found both online and at my local American Science and Surplus good supplies of small gage magnet wire. I need bobbins to spin this on. Modern stators are often made of steel, but I'm considering building a modular "coreless" coil system to drive this. I might be able to build these as plastic bobbins to wind the coils around and possibly use to hold everything together. I'd probably still dip the coils in epoxy (probably thin fiberglass resin). I have a lot of research to do on this. The hope is to fit all this into an aluminum tube to protect, align, and provide a heat sink.

I also have to find and do some calculations to determine what power/drive I can do. I want to look at 12 or 24V power systems, not 180V DC buses (120VAC). Probably too dangerous. But this means high current, large conductor systems. I need a motor design guide for brushless motors.

I'll also have to have a module to hold the sinusoidal hall effect sensors. Still lots to look into, let alone building a drive module.

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