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