When the codestrip moves in one direction, Channel A leads Channel B by 90 electrical degrees. When the codestrip moves in the other direction, Channel B will lead Channel A by the same amount. This concept is illustrated in Figure 7. Resolution higher than that of the codestrip is achievable via quadrature decoding of the encoder outputs, where different levels of decoding exist. Counting every rising edge of one channel (e.g., Channel A) is called 1X decoding. The codestrip resolution can be doubled by counting every rising and falling edge of one channel to further increase the resolution. This is called 2X decoding. When every transition of both Channel A and Channel B is utilized (or every logic state), 4X decoding is achievable.

The Codestrip

The codestrip surface must be reflective and specular (mirror-like) so that the image of the pattern is reflected back onto the photo-diodes of the AEDR-8400 en coder. Potential materials include metal and reflective film. One method to determine whether the code strip will work with the reflective optical encoder is by using a Scatterometer.

Reflective surfaces with a specular reflectance of 60 percent or higher, as measured by the device, are compatible with the reflective encoder. The non-reflective areas should have a reflectance of less than 10 percent.

When testing for specular reflectance, reflective surfaces should be tested separately from non-reflective surfaces. It is recommended to test the reflective surface by itself and to then test the non-reflective surface, and to not perform tests on the patterned surface since this will only provide an average reflectance across the pattern.

Future Encoder Technology

New encoder technology is currently being developed that integrates an index channel to the two existing channels of digital output. This index channel will help eliminate the need for photo-interrupters to indicate the limit or end of travel range for the lenses. In addition, the next-generation encoder will feature a built-in interpolator that allows users to set the interpolation factors to one, two or four times the base resolution of 304LPI.

This article was written by Foo-Hong Thong, Worldwide Marketing Manager, Motion Control Products Division, Avago Technologies (San Jose, CA). For more information, contact Mr. Thong at This email address is being protected from spambots. You need JavaScript enabled to view it., or visit http://info.hotims.com/34450-201.

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