Magnetic actuator
Posted: 25 Feb 2019, 13:22
This thread is to detail the magnetic actuators I'm making for my ATtiny85 'galloping ghost' project. I know that magnetic actuators aren't really galloping ghost, but the same driver circuit and decoder electronics/code works for both.
You see these sort of actuators used on lightweight indoor models - they're basically just a moving magnet inside a coil. When the coil is energized it causes the magnet to rotate, reversing the connection rotates the magnet the opposite way. The moving magnet is directly connected to the control surface, so there's only one moving part. For operation on a rudder, the magnet poles point left and right, and the coil axis is aligned with the direction of flight. I wanted a bigger more powerful version than the ones sold for indoor models.
3-D printing a bobbin on which to wind the coil is tricky because whichever way you orient the print there will be steep gradients or big overhangs - these don't print well without also printing support material and the support material is a pain to remove. So I opted to print the bobbins in two halves and glue the halves together. The bobbin then prints as two 'top hat' shapes with the 'lids' missing.
Click for larger view.
The magnets I had lying around are really too small for this job: 2.03mm long by 3,02mm diameter, so I designed and printed a 'holder' that holds a central row of four such magnets, with a row either side consisting of three magnets - so ten tiny magnets working together as a large one - all the north poles point one way and all the south poles the other. The magnets are fixed to the holder with a drop of c.a. (superglue).
To wind the coil I used some old enamelled copper wire from a spool I've had for about forty years! There are no markings on the spool, but measuring the diameter with a micrometer, it's about 0.22 mm which I think makes it 34 SWG or 31 AWG. If and when I need to buy a new spool, I would get something slightly thinner to get more turns on the bobbin and produce a higher resistance.
I 3D-printed a mandrel to fit a small DC motor and hold/spin the bobbin for winding the coil - much easier and neater than winding by hand.
I designed the parts in OpenSCAD. I'll attach the files to the next post, but I expect if anyone else decides to make similar actuators they may decide to use different magnets and/or alter the diameter/length of the coil to suit.
You see these sort of actuators used on lightweight indoor models - they're basically just a moving magnet inside a coil. When the coil is energized it causes the magnet to rotate, reversing the connection rotates the magnet the opposite way. The moving magnet is directly connected to the control surface, so there's only one moving part. For operation on a rudder, the magnet poles point left and right, and the coil axis is aligned with the direction of flight. I wanted a bigger more powerful version than the ones sold for indoor models.
3-D printing a bobbin on which to wind the coil is tricky because whichever way you orient the print there will be steep gradients or big overhangs - these don't print well without also printing support material and the support material is a pain to remove. So I opted to print the bobbins in two halves and glue the halves together. The bobbin then prints as two 'top hat' shapes with the 'lids' missing.
Click for larger view.
The magnets I had lying around are really too small for this job: 2.03mm long by 3,02mm diameter, so I designed and printed a 'holder' that holds a central row of four such magnets, with a row either side consisting of three magnets - so ten tiny magnets working together as a large one - all the north poles point one way and all the south poles the other. The magnets are fixed to the holder with a drop of c.a. (superglue).
To wind the coil I used some old enamelled copper wire from a spool I've had for about forty years! There are no markings on the spool, but measuring the diameter with a micrometer, it's about 0.22 mm which I think makes it 34 SWG or 31 AWG. If and when I need to buy a new spool, I would get something slightly thinner to get more turns on the bobbin and produce a higher resistance.
I 3D-printed a mandrel to fit a small DC motor and hold/spin the bobbin for winding the coil - much easier and neater than winding by hand.
I designed the parts in OpenSCAD. I'll attach the files to the next post, but I expect if anyone else decides to make similar actuators they may decide to use different magnets and/or alter the diameter/length of the coil to suit.