Automatic position control devices

Claim 1

A POSITION CONTROL APPARATUS COMPRISING A FIRST PART THE POSITION OF WHICH IS TO BE CONTROLLED, A SECOND PART WITH RESPECT TO WHICH SAID FIRST PART IS TO BE POSITIONED, A SERVO MOTOR FOR DISPLACING SAID FIRST PART RELATIVE TO SAID SECOND PART COMPRISING A SOURCE OF ALTERNATING VOLTAGE, A POSITION SENSITIVE CIRCUIT ENERGISED BY SAID SOURCE AND COMPRISING A SCALE ON ONE OF SAID PARTS AND AN ELECTRICAL SLIDE ON THE OTHER WHICH CO-OPERATE TO PRODUCE AN ALTERNATING VOLTAGE OUTPUT THE AMPLITUDE OF WHICH VARIES CYCLICALLY WITH RELATIVE MOVEMENT OF SAID PARTS, EACH CYCLE OF VARIATION OF AMPLITUDE CORRESPONDING TO A UNIT OF RELATIVE DISPLACEMENT OF SAID PARTS, THE MAXIMUM RELATIVE DISPLACEMENT OF SAID PARTS BEING A PLURALITY OF SAID UNITS, MEANS FOR SETTING UP A COARSE DISPLACEMENT COMMAND SIGNAL REPRESENTING AN INTEGRAL NUMBER OF SAID UNITS AND MEANS FOR SETTING UP A FINE DISPLACEMENT COMMAND SIGNAL REPRESENTING A FRACTION OF SAID UNIT, A COUNTER FOR COUNTING CYCLES OF VARIATION OF THE AMPLITUDE OF SAID ALTERNATING VOLTAGE OUTPUT, MEANS FOR COMPARING SAID COARSE DISPLACEMENT COMMAND SIGNAL WITH THE COUNT IN SAID COUNTER TO PRODUCE A COARSE ERROR SIGNAL AND MEANS RESPONSIVE TO SAID FINE DISPLACEMENT COMMAND SIGNAL FOR ALTERNATING THE ENERGISATION OF SAID POSITION SENSITIVE CIRCUIT TO ALTER THE PHASE RELATIONSHIP BETWEEN THE CYCLES OF THE AMPLITUDE VARIATION OF SAID ALTERNATING VOLTAGE OUTPUT AND THE RELATIVE DISPLACEMENT OF SAID PARTS WHEREBY THE AMPLITUDE OF SAID ALTERNATING VOLTAGE OUTPUT FORMS A FINE ERROR SIGNAL, SAID SERVO MOTOR BEING RESPONSIVE TO SAID COARSE AND FINE ERROR SIGNALS TO TEND TO REDUCE SAID ERROR SIGNALS TO ZERO.