Andy Neil wrote:
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How about a loudspeaker voice-coil?
Something like that should be OK?

A VCM (Voice Coil Motor) is indeed a very fast actuator. I have worked at IBM's test and repair centre on the really big harddrives which use VCMs to move the head assemblies about. Mind you: you need a pretty big magnet (the ones in the IBM drives weighed around 20 kg.) and an amplifier to drive the coil that can deliver around 10A with good speed and accuracy. Furthermore you will have to ensure that the coil will always remain centred inside the magnet or it'll be toast.

Still want an electronic solution? :-D

Have a great weekend,
Rob.