Rosemount Inc.

Earl A Grindheim

Roger L Frick

James R Ruesch

A circuit for measuring a parameter to be sensed by means of at least one variable impedance sensor responsive to such parameter. The circuit includes a frequency determining means coupled to the variable sensor for providing an oscillatory frequency signal responsive to the impedance of the sensor, a signal amplifier coupled to the frequency determining means for amplifying the frequency signal to provide an output signal representative of the frequency signal and to sustain circuit oscillation, a signal detector coupled to the output signal to provide a control signal representative of such output signal, and a control coupled to receive the control signal and coupled to the frequency determining means thus allowing the oscillatory frequency signal of the circuit to oscillate at a frequency representative of the measured parameter.

Parallel T impedance measurement circuit for use with variable impedance

A circuit for measuring the reactance of an AC reactance sensor which varies responsive to a condition to be sensed, such as pressure, is disclosed. The circuit comprises an oscillator coupled to the sensor for providing a time varying oscillator signal having a variable frequency and a substantially constant peak to peak voltage to the sensor, and a plurality of rectifiers coupled to the oscillator and to the sensor for providing the oscillator signal as a charging signal to the sensor and for providing discharging signals from the sensor, the discharging signals being a function of the reactance of the sensor, and an amplifier coupled to the rectifiers and to the oscillator for providing an input control signal to the oscillator to control the frequency of the oscillator signal as a function of the reactance of the sensor.

Reactance measurement circuit

A digital output capacitive transducer wherein two capacitors are used, one providing a reference current and the other providing a varying signal current. The signal current is obtained from an oscillator through a capacitor, which varies in capacitance as a function of the parameter to be measured. The reference current offsets the signal current to attain a known relationship between the sampled parameter and the signal current. The circuit has a small number of components and operates on a relatively low voltage with an accurate digital output.

Transducer for converting a varying analog current signal to a binary

Circuitry including a resistance sensor is excited by an external power source. A reference voltage is established, the voltage developed across the sensor resistance is compared at an amplifier input with a reference voltage signal and the voltage across a feedback resistor, and the total current drawn through the circuitry adjusts as a function of the sensor resistance to give a balanced amplifier input. Total current may be made a linear function of sensor resistance or a non-linear function by proper selection of certain resistors.

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Minneapolis, MN, United States of America

Earl A Grindheim