Equipment Latest change 2017-10-02
A description of the equipment made for the Chapel.
mount with Hall sensors to measure the behaviour of the
- Wire to suspend the bob.
- Bob, aluminium cylinder filled with ca. 6 kg
- Base assembly allowing precise
adjustment of the coil set consisting of:
- Zero cross
- Drive coil.
- Rim coil.
- Bob Control electronics to detect
center and rim passages and to control the drive coil.
- PC program which takes the data from
the Line Sensors and the Bob Control unit and presents it
graphically and logs the data for later study.
- Analysis program to study the
- Firmware for the BobControl and Power
- The story of the broken
With similar equipment at home I've seen a very regular behaviour
of the pendulum and a Foucault precession which very good matches
the calculated value for this lattitude.
The setup has no Charron ring or something equivalent, but obeys
the timing requirement from Schumacher, that is, to give the drive
impulse at a very particular moment in the swinging period. There
is a substantial amount of ellipse, changing periodically in
direction. But it has hardly any effect on the velocity of the
The Top mount is rigidly mounted to a purlin of the roof, next to
the open stairs. This gave me a height of about 4.5 meters for the
length of the pendulum.
In the Chapel the Top Mount is attached to a raft of the roof at
approx. 8.5 m height.
The wire at home is a piano wire of 0.75 mm diameter, in the
chapel it is 0.925 mm diameter (gauge 16).
The bob consists of an aluminium cylider filled with lead. A
central rod allows it to be attached to the wire. On the lower end
of this rod sits a magnet which induces electrical signals in the
Zero-crossing coil and in the Rim coil, and it experiences a
repelling force when the Drive Coil is engaged.
The base assembly contains the Coil assembly and the mechanism for
precise centering of the coil assembly
Zero cross detection coil
This small coil at the center of the base-assembly detects the
passing of the Bob. When the Bob is approaching the voltage will
rise. When the Bob is nearly overhead the voltage will fall and
cross zero at the moment that the bob is exactly in the center.
The Arduino software detects this center-crossing and starts the
timing for a.o. the DriveCoil pulse.
When the time for one half of the swing differs from the time for
the other half, our zero detection coil is not at the center of
the pendulum's swing. This can be corrected with the centering
The driveCoil, when engaged, provides a repelling force on the
magnet in the Bob's lower stem. This will replenish the energy
lost by air friction in each swing.
The rim coil sits at a somewhat large distance from the center.
Detecting the Bob's passages tells us quite precise what the
amplitude of the pendulum is. Based on this information the
duration of the drive pulse is modified to keep the amplitude as
constant as possible.
The electronics for the BobControl unit consists of some
operational amplifiers to condition the signals from the Center-
and Rim coils and the Hall sensors, and to engage the DriveCoil at
the proper moment during the proper time. At the hart of the
electronics sits an Arduino Nano.
Here you find the zipped code for the Nano.
Look here for the schematics and a detailed description of the
The whole system is supplied from a 24 Volts battery, which is
constantly charged by a mains power supply.
This program collects data from the Arduino processors in the
BobControl unit and the Power unit, present some data graphically
and logs all important data to a server in the network.It also
allows to change several parameters of the Pendulum Contol.
The Analisys Program, also written in FPC and Lazarus IDE shows us
the behaviour of the pendulum over somewhat longer time, as it
emerges from the logfiles.
The Arduino firmware for the units Bob Control and Power.