User`s manual

Biaxial Displacement Follower User’s Manual Version 1.4, April 19, 2004 IM1013
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Theory of Operation
The Model 5600 Biaxial Displacement Follower is a non-contacting electro-optical displacement follower designed to track
the motion of a target along any axis. The moving target under study must show a sharp discontinuity in the intensity of its
reflected or emitted light. The tracker is, in fact, locking onto that light/dark interface.
The lens system focuses the image of the discontinuity onto the photo cathode of an image dissector tube. A simplified
drawing of this tube is shown in Figure 1. The backside of the photo cathode emits electrons at a level proportional to the
intensity of the projected light. These electrons are then accelerated to refocus on an aperture plate containing a small hole
or aperture. This converts the optical image to an electron image, with an electron density proportional to the original light
intensity of the target. As electrons enter the small aperture, they are amplified to produce a current output proportional to
the number of electrons entering the aperture. Moving the electron image across the aperture plate varies the number of
electrons entering the aperture.
Figure 1
The system detects change in the photo tube’s output current, and through a servo loop, causes the electron image to refocus
on the aperture. The servo loop circuit re-centers the electron image by passing current through coils creating a magnetic
field that deflects the electron beam back to its original position. Since the current needed for deflection is directly
proportional to the distance that the electron image has moved from center, it acts as a measure of displacement. The actual
interface (target) can be moved over the diameter of the aperture by adjusting the reference potential, or lock-on, in the
servo loop.
If the electron image moves so that fewer electrons are admitted through the aperture, the photo-multiplier output decreases
and changes the servo control voltage so that the electron image returns to its original locked-on position. If the image
moves in the opposite direction, letting more electrons through the aperture, the servo control is polarized, again returning
the image to its locked-on position.