Reactance measurement circuit

A two wire current transmitter for measuring the capacitance of a capacitive sensor means having at least two plates which capacitance varies as a function of a parameter to be sensed and for providing a DC signal representative of the parameter to be sensed, the transmitter comprising:

• transformerless oscillator means for providing a time varying oscillator signal for exciting the sensor means to produce a sensor signal representative of the parameter to be sensed;
• rectification means coupled to the sensor means and to the oscillator means to supply the oscillator signal to the sensor means and to rectify the sensor signal;
• control means coupled to the rectification means having an input terminal means for receiving the rectified sensor signal and having an output terminal means coupled to the oscillator means for controlling the frequency of the oscillator signal as a function of the rectified sensor signal;
• and
• a pair of terminals for connection to an external power supply and series connected load by means of two wires to power the oscillator means and the control means and to carry the DC signal representative of the parameter.

In a two wire transmitter having capacitive sensor means for sensing a parameter, the improvement comprising:

• transformerless voltage controlled oscillator means for providing a time varying peak to peak voltage oscillator signal having a frequency controlled by a control signal;
• rectification means coupled to the sensor means and to the oscillator means for applying the oscillator signal to excite the sensor means and for providing a DC current path from the sensor means for a sensor current responsive to a change in capacitance as a function of the sensed parameter;
• control means having an input coupled to the rectification means to receive the sensor current and an output for providing the control signal to the voltage controlled oscillator means as a function of the sensor current to control frequency of the oscillator signal;
• and
• current control means responsive to the sensor current and including means for providing a feedback signal for controlling total current in the two wire transmitter as a function of the sensed parameter.

A two wire transmitter having a pair of terminals for connection by means of two wires to an external supply and series connected load so that a total two wire current from the transmitter flows through the two terminals and the two wires, the two wire transmitter comprising:

• voltage regulator means powered through the two terminals to provide the transmitter with a regulated voltage;
• feedback means coupled to the two terminals to provide a feedback signal as a function of the total two wire current;
• variable AC reactance means having a reactance which changes as a function of changes in a parameter to be measured;
• transformerless oscillator means powered by the voltage regulator means for providing a variable frequency oscillator signal;
• rectification means coupled to the oscillator means and the variable AC reactance means to apply the oscillator signal to the variable AC reactance means and to rectify an output signal therefrom, such that the output signal is representative of the parameter to be measured;
• control means coupled to the rectification means for receiving the output signal and for providing an oscillator control signal to the oscillator means to control frequency of the oscillator signal as a function of the parameter to be sensed;
• means for providing a current control signal as a function of the output signal and the feedback signal;
• and
• current control means coupled to the two terminals and the feedback means for controlling total two wire current as a function of the current control signal.

A circuit for measuring reactance of AC reactance means, the circuit comprising:

• oscillator means for providing a constant amplitude time varying oscillator signal having a frequency which is a function of a control signal;
• coupling means coupled to the oscillator means and to the reactance means to apply the oscillator signal to the AC reactance means;
• rectification means coupled to the variable AC reactance means to provide a DC current path for a signal from the variable AC reactance means;
• control means coupled to the rectification means for providing the control signal as a function of the signal from the variable AC reactance means;
• output means coupled to the rectification means for providing an output signal which is a function of the reactance of the variable AC reactance means;
• means to couple said circuit to a pair of wires in turn connectable to a power source and load;
• and
• current control means coupled to the output means for controlling the total current in the pair of wires as a function of the output signal.