{HEAD ~ A displacement} of {TAIL ~ the laser beam spot} outputted from the actuator is differentiated by a differentiator and converted to a signal corresponding to the velocity of the movement of the laser beam spot 
of
{ HEAD ~ A displacement } of { TAIL ~ the laser beam spot } outputted from the actuator is differentiated by a differentiator and converted to a signal corresponding to the velocity of the movement of the laser beam spot
HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH