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In lugnet.robotics.rcx, Kevin Karplus wrote:
> How much power do you want for the electromagnets, and how rapidly do
> you need to switch them on and off?
> ...
> If you need faster switching (like for pulse-width modulation), your
> best bet is to use power MOS transistors with Schottky protection
> diodes.
Or... avoid both stock relays and chip-based switching.
For a linear motor you want to trigger each electromagnet in a series, you
ideally you might like a "push-pull" system, where you aren't just turning a
magnet on and off, but reversing the polarity (this is true for either the track
magnets or the magnets on the "car" under active control, but perhaps easier to
visualize for dynamically switched track magnets). Why not make a "mechanical
multi-relay", where a motor-driven slider slides two electrical bridges over a
series of contacts, each powering a different magnet in a different polarity.
After all, the order the magnets will "fire" in is fixed. The timing is not
(ideally the later magnets in the series should fire faster, as the car is
moving at higher velocity), which can be varied by changing either the speed the
electrical bridges slide along the sequencer, or by spacing out the contacts on
the sequencer itself.
Not only can you see that easily, but it's easy to tune (mechanically), and
perhaps best of all, it can be made completely electrically isolated from the
RCX or NXT (an important point given the inductive loads and currents that might
end up running around).
Railgun-style linear accelerators are far easier... if you have very low
voltage, ludicrously high current power systems. I've always wanted to make a
simple one of those with a rolling armature (instead of one you have to dump
enough current through to vaporize it), just to see it work.
--
Brian Davis
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Message is in Reply To:
 | | Re: Controlling an Electromagnet (LSM)
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| Iain, How much power do you want for the electromagnets, and how rapidly do you need to switch them on and off? Old-fashioned relays will give you the least power loss, but can't be switched very quickly. They also have a limited life (between 10 (...) (13 years ago, 24-Dec-11, to lugnet.robotics.rcx)
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