====== A Linear Positioning Sensor ======

==== Overview ====
This page describes how to use a linear image sensor to position a robot along a tape measure to within one-thousandth of an inch of the desired location.

==== Background ====
Gantry robots are the basis of many dedicated commercial robots.  Examples include 3D printers and pick-and-place machines.  Positioning on these robots is usually via stepper motors and belts or stepper motors and lead screws.  Precise positioning is difficult or expensive if the gantry travel length is too long for either a belt or a lead screw.

==== Our Approach ====
We use the 3648-by-1 Toshiba TCD1304 linear image sensor to view the tick marks on a tape measure.  Each pixel in the TCD1304 is 0.008 centimeters wide.  The array of 3648 pixels can image a line a little less than three centimeters long. By knowing the size and spacing of the tick marks and by knowing our target position, we can compute what the sensor should see at the target.  We move the robot until the target image is as close as possible to the sensed image.  That is, we try to minimize the number of pixels that differ between the target and real images.

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