User`s manual

ProScale M150, M250 and all products with General Purpose LCD Digital Displays 40 of 44
Abbe Error
Abbe error is a condition that may not be visible to the human eye, but will affect linear
measurements. Be sure to take precautions when installing ProScale in order to eliminate
the possibility for Abbe error.
Abbe error refers to a linear error caused by the combination of an angular error and a
dimensional offset between the sample and the measuring system. It is important to
understand that the information the encoder is providing is only the position of the
readhead on the scale. To illustrate this, see the figure, which shows a linear
measuring device. (The apparent distortion in the measuring device is intentional - for this
example - to show the measuring device with a curvature in its mounting.)
Suppose the curvature in the figure is sufficient to produce an angle of 40 arc-seconds. If
the measuring device moves 10 inches, the probe will be found to have moved 10.0039
inches, resulting in an error of +0.0039 inches. Abbe error could be lessened by moving
the measuring system closer to the sample. This effectively solves one half of the Abbe
error problem (offset) and leaves only the angular mounting problem to be solved.
Angular error can best be countered through proper design and placement of the linear
scale. Sources of angular error include:
1. Mounting the linear scale to an imperfectly flat surface.
2. Mounting the linear scale to an imperfectly straight surface.
3. Curvature of ways (or linear bearings) used to measure the sample.
4. Contaminants between the probe and item being measured.
5. Friction in any part(s) of the measuring device.