Rosemount Inc.

Emerson Process Management Limited

Rongtai Wang

John Paul Schulte

Terrance F Krouth

Kris Allen Wendorf

A single plate capacitance sensor includes a sensor capacitor and a reference capacitor that share common plate. A capacitance-to-digital sigma delta modulator provides separate sensor excitation and reference excitation signals to the sensor capacitor and the reference capacitor to provide high resolution detection. Programmable ratio-metric excitation voltages and adaptive excitation voltage sources can be used to enhance modulator performance.

High resolution sigma delta modulator for capacitance sensor terminal displacement measurement

A field device detects the presence of terminal leakage between a first terminal and a second terminal in the terminal block area of the field device using a voltage variation ratio that is based on variations in a terminal voltage located across output terminals of a field device and variations in a current regulation voltage located within a current regulation circuit. Preferable, the field device measures an initial voltage variation ratio &#x3ba;and a subsequent voltage variation ratio &#x3ba;. Based on the initial voltage variation ratio &#x3ba;and the subsequent voltage variation ratio &#x3ba;, the terminal leakage existing between the first terminal and the second terminal can be calculated.

Terminal leakage monitoring for field devices

A two phase, second order capacitance-to-digital (CD) modulator includes a first stage sigma-delta integrator that forms charge packets as a function of sensor capacitance during an auto-zero phase and integrates the packets during an integration phase to produce an output voltage. The first stage integrator holds its output voltage during the auto-zero phase, so that a second stage sigma-delta integrator can sample the first stage output voltage during the auto-zero phase and integrate the sampled voltage during the integration phase.

Capacitance-to-digital interface circuit for differential pressure sensor

A capacitance-to-digital (CD) modulator converts capacitance of a differential pressure sensor to a pulse code modulation output signal. The first stage of the CD modulator is a sigma-delta integrator having an auto-zero capacitor connected between an integrator input node and an amplifier input. During an auto-zero phase, a feedback capacitor is connected between the amplifier input and output, and the auto-zero capacitor stores a voltage that is a function of leakage resistance of the sensor capacitor connected to the integrator input node. During an integration phase, the feedback capacitor is connected to the integrator input node. If an overpressure/short circuit condition exists, the stored voltage on the auto-zero capacitor induces a current to flow to the feedback capacitor to drive the integrator to saturation and suppress foldback anomaly.

Foldback free capacitance-to-digital modulator

A pressure transmitter having a capacitance-to-digital modulator produces an output as a function of sensor capacitance and a reference capacitance. Transmitter also includes a sensor failure mode detector produces an output signal and identifies failure modes of the sensor capacitance and reference capacitance.

Capacitance-to-digital modulator with sensor failure-mode detection

A field transmitter for transmitting signals representative of process variables has both a normal operating mode and a diagnostic self-test mode. The field transmitter has a physical sensor for sensing a process variable and generating a physical sensor signal which is representative of the process variable. A signal processing circuit converts the sensor signal to a measurement which is transmitted to a control room. The field transmitter also has a surrogate sensor for producing a surrogate sensor signal which is independent of the process variable. During the diagnostic self-test mode, the surrogate sensor is connected to the signal processing circuit in place of the physical sensor. If the output of the signal processing circuit does not correspond to an expected value, a diagnostic code is produced.

Field transmitter with diagnostic self-test mode

Adaptive compensation of measurement error for industrial process control transmitters

A pair of sensing capacitors, each having a capacitance C and C respectively, based on a process variable, are coupled to a bridge node which is coupled to a summing node. A reference capacitor, coupled to the summing node, has a capacitance C greater than an expected maximum difference between the capacitances of the pair of sensing capacitors. Switches selectively couple the sensing capacitors and the reference capacitor to at least first and second voltages to derive charges representative of C -C and C . In one embodiment the sensing capacitors are operated to charge and discharge during respective first and second phases of first cycles and the reference capacitor is operated to charge and discharge during respective first and second phases of second cycles. In another embodiment, the reference capacitor is operated to charge and discharge during alternate phases of the first and second cycles and the sensing capacitors are operated to charge and discharge during respective first and second phases of all cycles.

Bridged capacitor sensor measurement circuit

A capacitor industrial process control transmitter includes a three-phase excitation circuit to charge a sensing capacitor and a compensation capacitor of the transmitter and transfer charges to an integrator. The sensing capacitor is charged during the first phase. During the second phase, the voltage to the sensing capacitor is reversed, and the charge on the sensing capacitor is pumped to the integrator. Also, the compensation capacitor is charged with the reversed voltage during the second phase. During the third phase, the voltage to the compensation capacitor is changed, and the charge on the compensation capacitor is pumped to the integrator.

Three-phase excitation circuit for compensated capacitor industrial process control transmitters

A pressure transmitter includes a pressure sensor comprising a pressure cell having an interior to form a main cell cavity and a deflectable diaphragm which deflects in response to an applied pressure. The pressure sensor includes electrodes to compensate for hysteresis. A charge inverter amplifier includes a switch capacitor such that it may be implemented in a single integrated circuit.

Sigma-delta analog to digital converter for capacitive pressure sensor and process transmitter

An industrial process control transmitter has a capacitive sensor having at least three outputs each responsive in differing amounts to a process condition and to sensor hysteresis. An excitation circuit charges the sensor and pumps the charges to a sigma-delta capacitance-to-digital converter. The excitation circuit includes a charge amplifier coupling one of the sensor outputs to a summing node for summing with a charge on at least one other output. An auto-zeroing switch re-sets the charge amplifier on each cycle of operation of the charge amplifier.

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Edina, MN, United States of America

Rongtai Wang