Zero detector for a positioning system

Claim 1

IN COMBINATION WITH A TOOTHED POSITIONING RACK, A POSITIONING SYSTEM COMPRISING A CARRIAGE, A MAGNETIC HEAD CARRIED BY SAID CARRIAGE, A PAWL ASSOCIATED WITH SAID CARRIAGE AND ADAPTED TO BE IN AN EXTENDED OR RETRACTED POSITION, A MAGNETIC DRUM INCLUDING A PLURALITY OF INFORMATION TRACKS, A SERVO MECHANISM FOR DRIVING SAID CARRIAGE TO A SELECTED POSITION TO/ LOCATE SAID MAGNETIC HEAD OVER A SELECTED INFORMATION TRACK OF SAID MAGNETIC DRUM, MEANS FOR PRODUCING A DIRECT CURRENT ERROR VOLTAGE REPRESENTATIVE OF THE POSITION OF SAID MAGNETIC HEAD AND THE LOCATION OF THE INFORMATION TRACK BEING SOUGHT BY SAID MAGNETIC HEAD DURING A POSITIONING OPERATION, MEANS FOR PRODUCING A DIRECT CURRENT VELOCITY VOLTAGE REPRESENTATIVE OF THE VELOCITY OF SAID CARRIAGE DURING SAID POSITIONING OPERATION, SAID ERROR AND VELOCITY VOLTAGES BEING OF THE SAME POLARITY, A SOURCE OF REFERENCE VOLTAGE COMPRISING A SERIES OF PULSES OF OPPOSITE POLARITY TO THE POLARITY OF SAID ERROR AND VELOCITY VOLTAGES TO PRODUCE A COMBINED VOLTAGE, MEANS FOR APPLYING SAID COMBINED VOLTCOMBINED VOLTAGE, MEANS FOR APPLYING SAID COMBINED VOLTAGE TO DRIVE SAID SERVO MECHANISM, A CONTROL CIRCUIT NORMALLY OPERATIVE IN A FIRST STATE TO PERMIT OPERATION OF SAID SERVO MECHANISM AND TO MAINTAIN SAID PAWL IN A RETRACTED POSITION, SAID CONTROL CIRCUIT BEING OPERATIVE IN A SECOND STATE TO RENDER SAID SERVO MECHANISM INOPERATIVE AND TO CAUSE SAID PAWL TO BECOME EXTENDED TO ENGAGE ONE OF THE TEETH OF SAID POSITIONING RACK, MEANS FOR APPLYING SAID COMBINED VOLTAGE AND SAID SERIES OF PULSES TO SAID CONTROL CIRCUIT, SAID CONTROL CIRCUIT BEIG RESPON SIVE TO OPERATE IN ITS FIRST STATE WHEN THE AMPLITUDE OF SAID COMBINED VOLTAGE IS GREATER THAN THE AMPLITUDE OF SAID SERIES OF PULSES, AND SAID CONTROL CIRCUIT SWITCHING TO ITS SECOND STATE WHEN THE AMPLITUDE OF SAID SERIES OF PULSES IS GREATER THAN THE AMPLITUDE OF SAID COMBINED VOLTAGE.