During reversal {HEAD ~ the servo system} which is driven into {TAIL ~ saturation} senses the occurrence of zero motion of the drive mechanism and reverses the direction of energization of the electromagnetic drive 
During reversal { HEAD ~ the servo system } which is driven into { TAIL ~ saturation } senses the occurrence of zero motion of the drive mechanism and reverses the direction of energization of the electromagnetic drive
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL REL REL 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