Instruction manual

57
Abbe Error
Abbe error is a condition which 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.