Making a magnetically suspended puppet. Answered
I'm thinking of making a magnetically suspended puppet. The idea is to make a puppet with a neodymium magnet in its head, and suspend it under an electromagnet, using a microcontroller to control the (DC) suspension current. To regulate the current, I'll probably use a D/A converter chip controlling an op-amp current sink, though I might use low-pass filtered PWM D/A conversion.
I was thinking of a way of sensing the vertical height where I put a small chunk of metal in the head as well as the magnet, modulate the suspension field slightly at a high frequency (20 kHz?), and have an extra sense coil around the suspension coil. The idea is there would be an induced current in the sense coil from the modulation. This would change as the metal chunk came nearer to the centre of the coil. The signal from the sense coil would go through a high-pass-filter (to remove any low frequency components like mains), an op-amp rectifier, and then a low-pass filter, to produce a sense voltage. I might try both para- and dia- magnetic metals (iron and aluminium) to see which works better.
I'm not entirely sure whether this will work, but I'd like to try it as it seems more elegant than using a hall effect sensor.
The questions I have are:
- How would I go about calculating roughly what length of wire and current I'm likely to need to suspend a given weight? Just a pointer to the relevant maths should be OK - don't feel like you to have to spell it out.
- Does it make any difference whether the electromagnet is long or short along its central axis? I had an idea that the field from a longer coil might extend further below the magnet than one from a shorter one, but I don't know if this is so.
- Does the method I've described for sensing the height sound practical?
Thanks for any help,