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Re: linear motion encoder

John Denker wrote:

The problem with a picket fence is that it is
insensitive to direction. A lot of kinematics
experiments involve a change in direction....

A commercial probe called "Smar Pulley" is a
picket fence; it has a built-in photogate
measuring instant spoke-couning rates.

A more sophisticated probe, able to distinguish
directions, is called "Rotational Motion Detector".
It contains two photogates separated by angle which
is very small in comparison with the angle between
spokes. We used it yesterday in the "wheel barrow
mode" (pushing back and forth with the forcemeter
rod, randomly). Some technical problems prevented
us from collecting clean data.
Ludwik Kowalski