Improving Accuracy of Pendulum Clock

Maker of Merit prize won at Maker Faire Prague 2018!




Spring-powered pendulum clocks have accuracy of 10 seconds to a few minutes per day, and the inaccuracy accumulates every day.  The clock in the picture for this project is probably 50-year old and I could not get the accuracy better than 30 seconds per day.   We will see how it gets improved.


The period of pendulum swing cycle is
T = 2 * pi * sqrt( L / g )
where L is the length of the pendulum and g is the acceleration of gravity.
The idea of pendulum tuning is to add/subtract magnetic force to/from the gravity force and change the T.
 T' = 2 * pi * sqrt( L / (g + A/M) )
where A is the magnetic attraction force to the pendulum, and M is the mass of the pendulum.


Top view of the device.

Below is the side view.

Schematic diagram

The clock above should make 130 pendulum cycles per 147 seconds according to the gear train reduction ratio.  The time of the clock in seconds is
tc = C * 147 / 130
where C is the cycle count of the pendulum.
Every 60 seconds, tc is compared to the CPU time, and depending on the difference between the two, appropriate power is applied to the coil.
CPU time is calibrated every 5 minutes with NTP time server.


The device has web server function and any browser can access through WIFI connection.  The device status as well as the accuracy and coil power history for the last 15 hours can be displayed.  Below is a tablet screen showing a part of the accuracy history (in seconds).  In addition, device parameters can be modified with web browser.

Below is a graph showing time error and relative current on the coil for 26 hours.  A big delay on the left is because the pendulum was stopped for about 25 seconds during spring winding.  Other than that, the accuracy was within 2 seconds.

The latest hardware and software are here.




Click here to see another project for a wood foliot clock.  (in Japanese)