I remember an article in Czech electronic magazine describing
electromagnetical scale using a big speaker, a tube glued with epoxy to the
cone holded a board on which you put the load. Also had some
optoelectronical sensor. Then you apply current to the speaker, until the
opto is some half way covered and measure the voltage on the speker
proportional to the current. This system had a phase shift due to the
mechanical part even at low frequencies and dependable on the load, so the
feedback part was a little bit tricky. Another drawback - it uses quite a
lot of current.

I am just not sure, if you can find that big speaker to load it directly
with 5 kg. Otherwise it is mechanicly very simple and cheap if you use
speaker with little damage, not suitable for HiFi applications.

Josef Hanzal

>From:    John Shreffler <johns@AVENUETECH.COM>

>I have seen electronic scales use current to electromagnetically counter
>gravity, with the equilbrium point being determined by light that was
>obstructed by a blade on the moving part.  The current servos until the
>light is dimmed to about half.  Over narrow ranges the
>response was quite linear.  But I would think that force transducers
>would be a more rugged approach.