[time-nuts] Sensing pendulum position, speed, or height

[Dr Bruce Griffiths]

Hal Murray wrote:
> How do modern pendulum clock geeks measure what their pendulum is doing?  I'm 
> picturing a magnet on the bottom of the pendulum and a coil or hall effect 
> sensor.
>
>
>
>
> A variation on this would be optical as in CD reader technology.  Put a 
> pattern on the bottom of the pendulum and then shine a light (IR?) on it and 
> detect the reflections.  How far is the CD read mechanism above the surface 
> of the CD?  Is it "flying" with an air bearing like disk heads?
>   
Hal

A CD head is a non contact device. The head vertical position is servoed 
so that the recording layer is at the focus of a lens.
No air bearings are required. The clearance can be several millimeters.

Bruce