Edge following servo control

Claim 1

A SYSTEM FOR TRACING THE EDGE OF A PATTERN ON A CONTRASTING BACKGROUND COMPRISING A SCANNING DEVICE, SAID SCANNING DEVICE COMPRISING PHOTOSENTIVE MEANS HAVING A SENSITIVE AREA POSITIONED TO HAVE AN IMAGE OF THE SCANNING FIELD CONTAINING A PORTION OF THE PATTERN EDGE CAST THEREON, SAID PHOTOSENSITIVE MEANS ADAPTED TO CAUSE FLUCTUATIONS IN AN ELECTRIC SIGNAL PROPORTIONAL TO VARIATIONS IN LIGHT INTENSITY, MEANS FOR PERIODICALY INTERRUPTING THE IMAGE OF THE EDGE PORTION ON THE SENSITIVE AREA AND DEFINING A REFERENCE LINE, THE AMPLITUDE OF SIGNAL FLUCTUATIONS CAUSED BY THE PERIODIC INTERRUPTING OF THE IMAGE BEING INDICATIVE OF THE DISTANCE OF THE IMAGE OF SAID EDGE PORTION FROM SAID REFERENCE LINE, MEANS SYNCHRONIZED WITH SAID INTERRUPTING MEANS FOR PROVIDING A CONSTANT AMPLITUDE SIGNAL PHASED OPPOSITE TO SAID FLUCTUATING SIGNAL AND HAVING AN AMPLITUDE SUBSTANTIALLY LESS THAN THE MAXIMUM AMPLITUDE OF SAID FLUCTUATING SIGNAL CORRESPONDING TO AN ALIGNED POSITION OF SAID EDGE PORTION WITH THE REFERENCE LINE, MEANS FOR ADDING SAID SIGNALS AND MEANS RESPONSIVE TO THE AMPLITUDE AND PHASE OF SAID SUMMATION SIGNAL FOR MOVING SAID SCANNING DEVICE TO ORIENT SAID REFERENCE LINE TOWARD A REFERENCE POSITION WITH RESPECT TO SAID EDGE PORTION.