I wanted to test out a LIM (Linear Induction Motor) or and LSM (Linear Synchronized Motor) to launch a small car along the track. It is pretty much just using electromagnets that pull and push a car. The car would trip sensors which would turn on the next magnet. One guy said that using sensors and relays you could turn on and off the two different poles of the magnet so that it pulls in he car and then pushes it to the next magnet. Does anyone know how to do this? A diagram would be helpful.

 

You could simply use reed switch's to activate individual coils. Placment of switches and coils, including the attraction between coil and car, weight of car etc.. would have to be critical if smooth operation was required. Not sure about the pull push approach though, I think I would try the pull approach first and see how that goes. Also I think it would be really hard to get an accurate scaled speed using either method (although I might be wrong!).
Using a micro (PIC or AVR) would enable more dynamic control of the electro magnets and would result in a more reliable maybe smoother result, but would require quite a bit of work.
Have you ever made model roller coasters before? I was just thinking if so you may want to start by building the logic / mock PLC system that controls timing and safety of multiple cars etc.. this would be a good start and may give you the sort of grounding required to start messing with electro magnets!

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OLLY_K