Pressure sensor diagnostics in a process transmitter

• US 5,665,899 A
• Filed: 02/23/1996
• Issued: 09/09/1997
• Est. Priority Date: 02/23/1996
• Status: Expired due to Term

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Willcox, Charles R.
• Primary Examiner(s): RAEVIS, ROBERT R
• Application Number:
  US08/605,941
• Time in Patent Office: 564 Days
• Field of Search: 73/4 R, 73/4 D, 73/4 V, 73/1 B, 73/1 D, 73/724
• US Class Current: 73/1.63
• CPC Class Codes: G01L 27/007 Malfunction diagnosis, i.e. diagnosing a sensor defect
• CPC Class Codes: G01L 9/0075 using a ceramic diaphragm, e.g. alumina, fused quartz, glass

Abstract

A transmitter in a process control system includes diagnostic circuitry for determining condition of a pressure sensor. The pressure sensor includes a pressure responsive structure and a capacitor coupled to the pressure responsive structure whereby capacitance of the capacitor varies in response to process pressure. Measurement circuitry determines capacitance which is transmitted over a process control loop using output circuitry. The diagnostic circuitry is coupled to the pressure sensor and provides a diagnostic input to the pressure sensor. The diagnostic input causes a change in the capacitance which is monitored to determine condition of the pressure sensor.

Claims

1. A transmitter adapted to couple to a two-wire loop in a process control system, comprising:

a pressure sensor comprising;

a pressure responsive structure for receiving a process pressure;

a capacitor having at least one capacitor plate coupled to the pressure responsive structure and having a capacitance related to the process pressure;

measurement circuitry coupled to the capacitor providing a pressure output related to the capacitance;

input circuitry for completely powering the transmitter from a loop current received from the two-wire loop and output circuitry for transmitting the pressure output on the two-wire loop; and

diagnostic circuitry coupled to the pressure sensor and powered solely from the loop current received by the input and output circuitry, the diagnostic circuitry providing a perturbation input to the pressure sensor and providing a sensor diagnostic output based upon a change in pressure output caused by a change in capacitance in response to the perturbation input.

2. The transmitter of claim 1 wherein the diagnostic circuitry includes a radiation emitting device and the perturbation input comprises electromagnetic radiation directed at the pressure responsive structure generated by the radiation emitting device.

3. The transmitter of claim 2 wherein the radiation emitting devices comprises a light emitting diode.

4. The transmitter of claim 1 wherein the perturbation input comprises an electrical input signal which is powered solely from the loop current received from the two-wire loop and is applied to the capacitor.

5. The transmitter of claim 4 wherein the electrical input signal comprises a voltage.

6. The transmitter of claim 1 wherein the diagnostic circuitry monitors a change in pressure output to provide the diagnostic output.

7. The transmitter of claim 1 wherein the perturbation input comprises an impulse and the diagnostic circuitry provides the diagnostic output based upon a ringing frequency in the pressure output.

8. The transmitter of claim 1 wherein the perturbation input comprises an impulse and the diagnostic circuitry provides the diagnostic output based upon a decay rate in the pressure output.

9. The apparatus of claim 1 wherein the perturbation input comprises an impulse and the diagnostic circuitry provides the diagnostic output based upon the amplitude of the pressure output.

10. The apparatus of claim 1 wherein the measurement circuitry compensates the pressure output based upon the sensor diagnostic output.

11. The apparatus of claim 1 wherein the sensor diagnostic output comprises an alarm output provided to the input and output circuitry for transmission over the process control loop.

12. The apparatus of claim 1 wherein the pressure sensor includes a cavity containing a vacuum reference.

13. The apparatus of claim 1 wherein the pressure sensor includes a cavity containing a non-vacuum reference.

14. The apparatus of claim 13 wherein the non-vacuum reference comprises an inert gas.

15. The apparatus of claim 1 wherein the pressure sensor is formed of sapphire.

16. The apparatus of claim 1 wherein the input and output circuitry provides an analog current level output on the process control loop.

17. The apparatus of claim 1 wherein the input and output circuitry provides a digital output on the process control loop.