The following is a conceptual representation of the <calibration /> element in the base-head joint. See code.
| switch
| deactivated
| ___________________ <- Falling edge (-1.5 radians)
| /
| /
| /
| /
| /_______________________ <- Joint movement direction
| \
| \
| \
| \ <- Rising edge (1.5 radians)
| -----------------------
| switch
| activated