Optical Mouse Navigation

[phrontist]

Originally Posted by gnormhurst

Devil's Advocate here again...

Is there a findamental problem with using an optical mouse? The mouse reports a delta distance since the previous frame, as best as it can determine by performing a correlation between the current and the previous frame. But any fractional part of that measurement is not carried forward to the next measurement, and is lost.

For example, let's say you were trying to get the robot to move perfectly straight, but it was moving very slightly to the right, so slightly that the X axis change was less than one pixel of the mouse's imager per frame, say 1/3 pixel. The mouse will output zero change (because the best correlation would be "zero" offset).

On the next frame the mouse again compares the frame with the previous frame, and again the motion is so slight (1/3 pixel, not 2/3 because we only compare adjacent frame pairs) that the mouse again reports zero change. But the robot is moving steadily to the right.

Our program accumulates the deltas to measure position, but zero plus zero plus zero will be zero -- the slight drift to the right is never observed.

Compare that with a pair of wheel encoders. The difference between the two wheel encoders might be only 1/3 tick, but the error is "remembered" mechanically, so that the next time the difference is 2/3 tick, and finally the third time one side reports an extra tick compared to the other side, and the drift is detected.

The lack of memory for fraction pixel offsets seems fundamental to the optical mouse. Am I missing something? Is the resolution so high that my example would result in a trivial guidance error over the length of the field? If I knew the effective "size" of one pixel (on the carpet) I could calculate the potential error accumulation.

-Norm

While I can't give you an exact figure (after optical modification), I'm using a chip with 800 dpi resolution. Even if that is reduced 10 fold, it's still many, many times more precise AND accurate than encoders. Your arguement is not a logically flawed one, but this is happening on such a small scale, I think it will actually be more straight than gyros or encoders. We'll see!