--- Roman Black <fastvid@EZY.NET.AU> wrote: > >

> Yes, you do need the negative feedback to get the
> ball nice and stable. You can remove the bottom
> light
> sensor and just use one sensor, and for that matter
> you only need one electromagnet. the lifting one.
>
> For the moment the biggest issue is the mechanics,
> not the PID method. :o)
>
> If you try a constant magnetic field you find the
> field strength needed to hold the ball weight
> suspended
> is VERY close to the field that will slam the ball
> full upwards. To make it work in that way you need
> VERY fine control of field strength which is a real
> pain especially since you are using relatively slow
> LDR sensors.
>
> A better technique is to use a strong field in an
> on-off fashion, switched very fast based on a simple
> comparator using the LDR. This will rely on the
> relatively high inertia of the ball, so it never
> has time to lift or fall too far.
>
> I think you can use something as simple as a LDR
> + comparator that switches for a set (monostable)
> off period whenever the ball gets too high, so when
> the ball gets to set point it switches off, and
> the off period and gravity are known so the ball
> falls a known amount, then it switches back on.
>
> I have seen commercial desk toys using just a few
> transistors, and noticed micro oscillation of the
> load. Basically it is just a mechanical oscillator
> regulated by ball position. :o)
> -Roman


  Thanks for the ideas, I4ll give it a try and will
tell you how did it go...

* Carlos *

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