Determining the actual angle around the polar axis relies not only
upon the panning motor position, but on the angle of the 45
mirror
as well. We devised a simple experiment to determine both at the same
time.
In a room, we aim the laser horizontally at a wall so it is roughly
perpendicular, marking its position. We then move the scanning mirror
over the pole 180,
aiming at the opposite wall, and mark
that position. We then pan the device 180
,
where it should
coincide with the first mark. We mark that position, and again move
the scanning mirror over the pole to point at the second mark. We
mark its position. If all the hardware were perfect, the two marks on
each wall would be coincident, but due to errors, they are not. From
the separation of the points and knowing the distance from the
rangefinder to the points, the panning error and mirror error are both
determined. The values we found are 14,039 steps in 180
,
and 44.89
(the mirror error also affects the latitude). At
a distance of 4m, this moves the data sample more than 3cm.