Employ MCU to maintain wooden wall clock's accuracy

I recently contacted Max to chat about his cunning chronograph and to introduce him to my own wooden wall clock. Called Synchronicity, this is a unique exposed wood gear pendulum timepiece that uses a microcontroller to maintain accuracy. Based on our chat, Max invited me to contribute this column describing the electronics I developed to control Synchronicity.

Many wood gear clocks are driven by weights. These clocks need to be wound periodically, as often as every day. I wanted to build a wood gear clock that did not require this frequent winding.

Some designers use an electromagnetic pendulum to drive their clocks. The pendulum contains a rare earth magnet that passes by one or more coils. As the magnet passes by the coil, it induces a current in the coil. This triggers the electronics to inject a pulse of current into the coil (or into a second coil), thereby repelling the magnet, giving the pendulum a push, and powering the clock. This is the technique I chose to power Synchronicity.

Oftentimes, wood clocks of this type are not very accurate because wood expands and contracts with changes in temperature and humidity. I wanted my clock to be accurate, however, so I decided to use a microcontroller as part of my electromagnetic pendulum drive to ensure that Synchronicity keeps perfect time.

The magnet, coil, electronics, and battery are all hidden from view, giving the impression that Synchronicity is a perpetual motion machine. In fact, I like to challenge guests to figure out how the clock keeps itself going!

Some time ago, I wrote an Instructables column on Synchronicity. In that column, I focused on the construction and assembly of the clock itself. Now, in this column, I will elaborate on the electronics sub-system and associated software development.

The mechanism

In most pendulum clocks, the swing angle of the pendulum is very small, and so the actual swing angle has no bearing on the period of the pendulum. However, the period of a pendulum does increase with increased swing angle. Here is an equation for the period of a simple pendulum:

From this equation, we can see that the pendulum's length is the largest determinant of its period. Having said this, if the swing angle is relatively large and is varied between about 15 to 25 degrees, the period of the pendulum can be altered by over 0.5%, which equates to several seconds per day.

The Synchronicity software and electronics vary the swing angle of the clock's pendulum to slightly speed it up or slow it down as necessary to keep perfect time. (The clock's mechanicals were designed to tolerate this variance.)

The electronics
As the clock was to be battery-powered, I chose a low-power Texas Instruments (TI) MSP430 microcontroller to implement the heart of the electronics. In fact, most of the time, the microcontroller is in "sleep" mode, thereby drawing virtually no power at all.