Intrinsic safety process control system

Claim 1

IN A PROCESS CONTROL SYSTEM, AN ELECTRONIC RECORDER COMPRISING AN AMPLIFIER, MEANS FOR APPLYING AN INPUT SQUARE WAVE SIGNAL HAVING AN AMPLIFIER MEANS, A MEASURES VARIABLE TO THE INPUT OF SAID AMPLIFIER MEANS, SYNCHRONOUS DEMODULATOR MEANS CONNECTED TO THE OUTPUT OF SAID AMPLIFIER FOR DEVELOPING A PAIR OF DIRECT CURRENT OUTPUTT SIGNALS OF OPPOSITE POLARITIES PROPORTIONAL IN AMPLITUDE TO THE SQUARE WAVE OUTPUT OF SAID AMPLIFIER, A SHAFT POSITIONED WITHIN A MAGNETIC FIELD AND HAVING A RECORDING ELEMENT CONNECTED THERETO, A PAIR OF ROTOR WINDINGS POSITIONED WITHIN SAID FIELD AND CARRIED BY SAID SHAFT, MEANS FOR SUPPLYING SAID DIRECT CURRENT OUTPUT SIGNALS TO SAID ROTOR WINDINGS SO THAT SAID SHAFT IS POSITIONED IN ACCORDANCE WITH THE AMPLITUDE OF SAID DIRECT CURRENT OUTPUT SIGNALS, MEANS CONNECTED TO SAID SHAFT FOR DEVELOPING A SQUARE WAVE FEEDBACK SIGNAL HAVING AN AMPLITUDE PROPORTIONAL TO THE POSITION OF SAID SHAFT, AND MEANS FOR APPLYING SAID SQUARE WAVE FEEDBACK SIGNAL TO THE INPUT OF SAID AMPLIFIER IN OPPOSITION TO SAID SQUARE WAVE INPUT SIGNAL, THEREBY TO LIMIT THE CURRENT DRAWN BY SAID ROTOR WINDINGS TO A RELATIVELY SMALL VALUE DURING PERIODS WHEN SAID RECORDING ELEMENT IS AT REST.