An optical encoder measures how far something has moved, optically. It does this by looking at a pattern of alternating black-and-white, and counting number of transitions.
A typical optical encoder will either use a break-beam sensor, or a singlepattern (dual for quad-phase) of alternating black and white. This is donebecause most designs are in a well-controlled environment, where there arereasonably well known, fixed levels for dark and light. In this case, we had afar less controlled environment:
- The optical encoder went directly on the wheel. As a result, there was an ambient light level that varied by several orders of magnitude.
- The optical encoder disk could be positioned at different distances from the sensor.
- The optical encoder disk could be printed on a variety of printers, where toner may either be more or less reflective than paper, by varying amounts.
For these reasons, traditional hobbyist optical encoders tend to be completedisasters. To mitigate some of these problems, we used a differential design. Inso much as the photosensors matched, so that the design was very insensitive toambient light level, or how much light was reflected from the surface, so longas different amounts of light were reflected.
Using the phototransistors back-to-back as a voltage divider (rather thanamplifying first) let the design to have a huge dynamic range.
The major disadvantage of the differential design was decreased resolution.We needed four rings for a quad-phase design, instead of two. As a result, theinner-most ring was on a much smaller radius, and so could not have very densestripes. In addition, the photosensors needed to be reasonably well matched.
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