1. A FLUID-OPERATED ERROE SENSING CIRCUIT COMPRISING SENSOR MEANS FOR GENERATING FIRST FLUID SIGNALS HAVING A FREQUENCY REPRESENTING THE MONITORED VALUE OF A CONTROL SYSTEM VARIABLE, RESONANT MEANS IN COMMUNICATION WITH SAID SENSOR MEANS FOR GENERATING SECOND FLUID SIGNALS REPRESENTING THE MONITORED VALUE OF THE CONTROL SYSTEM VARIABLE AS A FUNCTION OF TWO RESONT FREQUENCIES OF SAID RESONANT MEAN WHEREIN THE TWO RESONANT FREQUENCIES DEFINE A PREDETERMINED RANGE OF VALUES OF THE CONTROL SYSTEM VARIABLE, AND A FLUID AMPLIFIER DEVICE HAVING A POWER FLUID INLET, TWO CONTROL FLUID INLETS AND TWO POWER FLUID OUTLETS, A FIRST AND SECOND OF SAID CONTROL FLUID INLETS IN COMMUNICATION WITH SAID RESONANT MEANS, SAID FLUID AMPLIFIER DEVICE PROVIDING FLUID OUTPUT SIGNALS AT SAID POWER FLUID OUTLETS INDICATIVE OF THE DEVIATION OF THE CONTROL SYSTEM VARIABLE FROM A DESIRED VALUE THEREOF WHEREIN THE DESIRED VALUE IS REPRESENTED BY A FREQUENCY INTERMEDIATE THE TWO RESONANT FREQUENCIES, SID FLUID AMPLIFIER DEVICE HAVING NO MECHANICAL MOVING PARTS THEREBY AVOIDING FRICTIONAL WEAR AND PROVIDING HIGHLY RELIABLE CIRCUIT OPERATION.