Large Balance Wheel

The balance wheel of this clock has a 5 second period and is electromagnetically impulsed. One of the balance weights has a small cylindrical magnet attached to it. When the magnet passes over the small rectangular box under the wheel it is passing over a coil with a soft iron core. This generates a small voltage in the coil and a sensing circuit then applies 5v to the coil for a few milliseconds, repelling the magnet and thus impulsing the wheel.

I had hoped that the wheel and balance spring combination would be isochronous but it is clearly not so. Larger amplitudes of swing are markedly slower in period than small amplitude swings.

The way to get over this is to use a microcomputer to detect the small voltage and to apply a larger timed voltage to the coil. I like to use Arduino Nanos because they are small, cheap and easily programmed from a PC.

The amplitude of the swing and hence its period can be controlled by the length of the pulse applied to the coil. The microcontroller can measure the period of the wheel and by using a control program can adjust the pulse to get an exact 5 second period. By repeatedly assessing the average period over a 30 second  time scale the period can be adjusted very accurately to account for the changing environment.

This clock is probably as accurate as any digital clock. to date I have run it for one month and can discern no difference in time from my digital watch.

More details to follow.