June 2010

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Photoelectric scanning in accordance with the interferential scanning principle and single-field scanning

Dear Abbe

Question: Why should encoders with interferential scanning be used when accuracy requirements increase?

Answer:
Encoders using traditional scanning methods will typically operate down to a 10 micron grating while those utilizing an interferential system work down to 4 micron, 2 micron, and points below.  These graduations are smaller than is possible with any other scanning principle. This ability to obtain smaller graduations results in a smaller signal period during the measurement process.  Smaller signal periods result in smaller short wave errors (assuming constant signal quality).  Because short wave error is difficult to compensate in real time, it becomes a critical determinant of machine accuracy. By decreasing the incident of short wave error as done with interferential scanning systems, machine accuracy is naturally increased.

The science behind interferential scanning is based on a light source.  Here, light reflects but also refracts when it reflects off (or passes through) a surface. The refraction causes bending of the light; this bending effectively blurs the light/dark patterns that encoders use to generate signals. Rather than be limited by refraction, interferential scanning uses refraction to generate the signals.