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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,

andy (ganglion)


I think you will find all of these applicable.

In general if you want to measure the distance ultrasonics or a laser should work BUT most of the systems use a measurement of the magnetic field rather than an actual distance measurement.

On the other hand you could use invisible thread!

Some years ago I made a life sized marionette controlled by servos this was suspended on invisible thread you couldn't see it fron a foot or so away. Very spooky.

One problem your going to have to deal with is the tenancy to spin.

Good point about the spin. I think it might work if I have another magnet in the back of the puppet attracted by a weak electromagnet in the back of the frame.

I could use hall effect sensors, but I'm not sure how well these would cope with sideways movements of the puppet. The idea I'm working towards is to have 3 coils at the top, arranged as a 3-way division of a circle, and change the balance of current between the coils to make the puppet dance from side to side. I was thinking that the method I was talking about would work better for this.

Maybe the thing is to just try both and see which works best.

The thread idea is nice, but I'd like to use magnetic suspension really.

This is a simple dancing puppet - using string again I am afraid :-)


5 years ago

First the microprocessor is a magnitude or two faster then the
electromagnet coil and that means PWM will be all you need.

The field will be greatly influenced by the diameter ( square too ) area
of the pole piece and the winding length somewhat less.

A bigger pole piece will not guarantee the puppet to be in the center
rather more on the edge ( flux lines repel each other ).

Sensing puppet headistance from the pole piece is paramount.
Photo sensors are easy and  most common.
Hall sensor is probably the next more common.
A coil reaction to the NIB flux if you are an EE wizard.
An embedded hf inductive or capacitive coupling should work.
Ultrasonics is a possibility too. 

ButIf you are looking for  esoteric,  try the radioactive position
technique using the source from a fire detector to give a
distance estimate by alpha presence detection.


-figuring out the wire exactly, not sure
Force in newtons = mass * acceleration, 9.81 m/s^2, so it differs with the mass.
There are lots of electromagnet calculations, but its been a LONG time since I remember them

-shape of magnet -- matters more for the amount of wraps you can get around the core

--height sensing -- it's been done before, I've seen it done with a laser rangefinder, and with a feedback coil -- If I recall they used the 'off' phase of the lifting magnet coil to sense the object's distance based on how quickly the magnetic field collapsed and from the induced kickback current. Like how a metal detector uses a coil to detect nearby disturbances in the magnetic field, by sensing the different feedback between pulses, you can probably figure out how to adjust the pulses.


Schematic using a hall effect sensor is the 2nd one on the page.