A while ago I posted a video showing a pendulum driven by a pulse motor:
It worked by using a single coil to detect the field from a magnet embedded in the pendulum base. An op-amp (wired as a comparator) detects when the pendulum reached the equilibrium point (vertical) a voltage (5-12v) is applied to the coil for around 20ms. This pushes the pendulum away. The accelerating force on the pendulum is therefore two-fold – the iron core of the coil attracting the pendulum on its approach, and then a small “kick” pushing it away.
Additionally I’ve found that the pendulum can keep still moving, at a reduced amplitude, if the kick is applied every second or third crossing of the equilibrium point. A uController like the Arduino [BlogCred + 10!!] makes it very easy to control this – along with other parameters like pulse width, and offset.
The interesting thing about pendulum mathematics, is that it’s only the force of gravity and the length of the pendulum that affect the period. Not the mass of the pendulum. So to get a 1 second, full cycle pendulum, the length needs to be around 24 cm long. – Just under the length of a standard ruler.
(Additionally, it appears the energy required to keep a pendulum swinging is not directly connected to the mass either – so in theory you can keep a 6-ton pendulum swinging with the same energy as a 6-ounce one. Of course, friction and air resistance will also play a part, and could be radically different for either pendulum).
Of course, this is for a light, inextensible, frictionless pendulum, operating in a vacuüm (gotta love O-level physics!)
In this new video, I added a simple ratchet mechanism, so that the oscillation of the pendulum could be converted to rotary motion, suitable for driving a clock:
In an attempt to control the amplitude of the pendulum I added a sliding weight, and damping springs. These allowed some degree of control, but both the amplitude and the frequency are affected.
There are some problems with this ratchet arrangement. Since the gear is able to spin freely, there is nothing keeping it engaged against the lock after each advancing “tick” of the pendulum. This may resolve itself once the rest of the clockworks are added.
It would also be nice if a mechanical arrangement could be devised to allow the use of a 60 tooth ratchet – eliminating the need for 15:60 conversion gears.
More to come…