Frequency feedback on a current loop of a current-to-pressure converter

An electrical control apparatus for providing an output fluid pressure at an output which is a function of a magnitude of a variable input DC current, the apparatus comprising:

• first and second terminals adapted to be connected in a circuit through which the variable input DC current flows, and wherein the control apparatus is energized solely by the variable input DC current flowing between the first and second terminals;
• first and second conductors connected to the first and second terminals, respectively;
• first circuit means for receiving the input DC current and providing an electrical signal representative of the magnitude of the input DC current;
• pressure sensor means for receiving the output pressure and providing an electrical signal representative of the output pressure;
• amplifier means for receiving the electrical signal representative of the magnitude of the input DC current and for receiving the electrical signal representative of the output pressure and providing a control signal which is a function thereof;
• output transducing means for providing the output pressure controlled by the control signal;
• circuit means for receiving the electrical signal representative of the output pressure and providing a signal having a frequency representative of the output pressure;
• and
• interface means for receiving the signal having a frequency representative of the output pressure and non-interferingly superimposing a time varying signal having the frequency representative of the output pressure on the input DC current.