A transmitter (10) in a process control system includes input/output circuitry (74) for coupling to a process control loop (12). A first sensor (40L) having a first impedance is responsive to a first sensed parameter. A second sensor (40H) having a second impedance is responsive to a sensed parameter. First and second excitation signals (S1, S2) are applied to the first and second sensors (40L, 40H). A summing node (44) sums the outputs (OL, OH) of the first and second sensors (40L, 40H). An analog to digital converter (54) provides a digital output representative of the summed signals. Digital signal processing circuitry (70) coupled to the analog to digital converter (54) provides an output related to the outputs (OL, OH) of the first and second sensors (40L, 40H) to the input/output circuitry (72) for transmission over the process control loop (12).

Process control transmitter

A constant-capacitance capacitor is series-connected with a variable-capacitance capacitor whose capacitance varies with the pressure. The other end of the variable-capacitance capacitor is connected to the ground potential terminal of a D.C. power source. The terminal voltage of the variable-capacitance capacitor is compared with a threshold voltage provided by the D.C. power source by means of a comparator. An output voltage of the comparator is stored into a memory in synchronism with a clock pulse signal. An output voltage of the memory is integrated by an integrator. An output voltage of the integrator is supplied to a buffer amplifier for lowering the signal source impedance with respect to the output voltage. An output voltage obtained from the buffer amplifier is fed back to the other end of the constant-capacitance capacitor and at the same time supplied to an output terminal.

Variable-capacitance type sensor and variable-capacitance type sensor system using the same

The present invention comprises a circuit for measuring the capacitive differences of small capacitors. The circuit comprises a reference capacitor and a sensor capacitor. Connected to one of the plates of each capacitor is a switch which connects the capacitors to one of two reference voltages. The other plate of the capacitors are connected to an input terminal of a voltage comparator. The comparator compares the input voltage with a third reference voltage. Differences in voltages detected by the comparator are applied to a feedback loop for generating an offset voltage at the input terminal. The offset voltage applied at the input terminal is proportional to the capacitive difference between the reference capacitor and the sensor capacitor. The feedback loop comprises a successive approximation register for digitizing the offset voltages and a digital to analog converter for converting the digitized voltages into analog voltages which are applied at the input terminal. Digitized offset voltages can be measured at the output of the successive approximation register.

Digital technique for precise measurement of variable capacitance

A pressure transducer having a redundant fluid pressure sensor can be used to determine if the pressure transducer needs to be recalibrated. The pressure transducer comprises a fluid pressure sensor for measuring differential pressure. The fluid pressure sensor includes a pair of substantially identical pressure sensors for providing a variable electrical output as a function of a differential fluid pressure and a reference sensor for providing an electrical output independent of the differential fluid pressure. A conditioning circuit is provided for generating a frequency-based signal whose frequency is a function of the electrical output for each fluid pressure sensor. The electrical output of the first and second pressure sensors can be compared to determine whether the pressure transducer needs to be calibrated. In one embodiment, the pressure sensors comprise variable capacitors. The first and second capacitors each have one fixed plate and one movable plate responsive to differential fluid pressure across the movable plate. The reference capacitor has two fixed plates. The first variable, second variable, and reference capacitors are all located in proximity to one another and in communication with the differential fluid pressure. The pressure transducer also includes a switching network connected to the capacitors for selectable connecting at least one of the capacitors to an electronic circuit.

Transducer having redundant pressure sensors

A pressure sensor measures pressure by measuring capacitance between two capacitive plates. Pressure change appears as a change in capacitance. Stray capacitance interferes with this measurement. The stray capacitance arises between the capacitor plates and surrounding material. Circuitry reduces stray capacitance by maintaining substantially no potential difference between a capacitor plate and surrounding material responsible for stray capacitance.

Capacitive pressure sensor having circuitry for eliminating stray capacitance

A capacitive type converter device, such as used for measuring pressure, comprising first and second rectifying circuits, each including an operational amplifier with a non-inverting input terminal connected to a reference point, to which a variable condenser is connected, the variable condenser having a capacitance which varies in response to a force being measured, such as pressure, and to which a reference condenser is connected, the reference condenser having a capacitance which is fixed independent of the force being measured. The device further comprises an oscillator, whose oscillation output is applied to the variable and reference condensers,and which is controlled by a control means in response to output of the first rectifying circuit, or in response to the output of the second rectifying circuit together with the output of the first rectifying circuit, whereby an output responsive to the capacitance of the variable condenser is obtained from the output of the second rectifying circuit, or an output relating to the difference between the capacitances of the condensers is obtained from the outputs of the first and second rectifying circuits. The device eliminates adverse influences of stray capacitances, non-linearity, offset signals, and parallel capacitances and forward voltages.

Capacitive type converter device

PURPOSE:To remove the influence of stray capacitance by connecting a variable capacitor corresponding to a capacitor to be measured and a reference capacitor to the 1st rectifying circuit, controlling the oscillators of both the capacitors by the output of the 1st rectifier circuit, and obtaining an output from the 2nd rectifier circuit for the reference capacitor. CONSTITUTION:Respective fixed electrodes 16, 17 of variable and reference capacitors 11, 12 in a detecting part 10 are connected to the inverted input terminals of operational amplifiers A1, A2 of detecting circuits 51, 52 constituting the 1st and 2nd rectifier circuits respectively and the positive input terminals are connected to a common electrode 15 in the detecting circuit 10 through a reference point oscillator 20. On the other hand, the outputs of the amplifiers A1, A2 are fed back to respective inverted input terminals through feedback circuits consisting of resistors R3, R4, etc. respectively. A DC voltage from the amplifier A1 and a negative reference voltage are inputted to the inverted input terminal of an operational amplifier A3, the oscillator 20 is controlled by the output of the amplifier A3 and an output EOUT related to a capacitance value of the capacitor 11 corresponding to the capacitance to be measured is extracted from the circuit 52.

Capacitance type converter

PURPOSE:To transmit no abnormal current signal to a load side when the power source of the two-wire type transmitter which is supplied with electric power from the load side through two transmission lines to convert a physical quantity to be measured into an electric signal, processes the electric signal by a microprocessor, and sends the current signal to the load side through the transmission lines is turned on. CONSTITUTION:The two-wire type transmitter is equipped with a 1st power circuit PC1 which rises in voltage first when supplied with the electric power and supplies a constant circuit voltage, a 2nd power circuit PC-2 which has the voltage settled after the circuit voltage and supplies the processor source voltage, and a control circuit CNT which is supplied with the circuit voltage and outputs a reset signal for resetting the microprocessor at specific time after the processor source voltage settles and a switching signal after the reset signal, a fixed voltage source which sets the current signal supplied to the load to a specific fixed current, and a switching means which performs switching from the fixed current source to the output of the microprocessor with the switching signal.

Hisashi Tamura

Papers

Capacitive type converter device

Capacitance type converter

Two-wire type transmitter