1
SIGNAL PROCESSING TECHNIQUE WHICH
SEPARATES SIGNAL COMPONENTS IN A
SENSOR FOR SENSOR DIAGNOSTICS
This application is a continuation of application Ser. No. 5 08/744,980, filed Nov. 7, 1996, now U.S. Pat. No. 5,828, 567, issued Oct. 27, 1998.
BACKGROUND OF THE INVENTION
The present invention relates to process variable sensors of the type used in the process control industry. More 10 specifically, the invention relates to life expectancy estimation and diagnostics for such a process variable sensor.
Process control transmitters are used to monitor process variables in industrial processes. For example, a transmitter might monitor pressure, temperature or flow (e.g., process 15 variables) and transmit such process variables back to a control room, where a controller sends a control signal back to an actuator (e.g., valve, motor) to control the process. In order to monitor a process variable, the transmitter must include some type of a sensor. For example, transmitters 20 include sensors with resistances or capacitances which vary in response to temperature, deformations or strain which allow the transmitter to measure, for example, temperature, pressure, flow, level, pH or turbidity.
As sensors age or are subjected to harsh environmental 25 conditions, the accuracy of the sensor tends to degrade. Ultimately, the sensor will fail. Diagnostics can be performed on a sensor by monitoring the sensor output signal. For example, a simple diagnostic technique is to compare the sensor output to a maximum or minimum value and 30 provide an alarm indication if the threshold is exceeded. However, one difficulty in prior art diagnostic techniques is that the variations in the process variable being sensed should not be incorrectly interpreted as a sensor fault.
SUMMARY OF THE INVENTION 35
A device in a process control system includes a sensor input which receives a composite sensor signal from a process variable sensor. The composite sensor signal includes a process variable signal related to the process 40 variable being sensed and a residual sensor signal related to sensor operation. Wavelet preprocessing circuitry coupled to the sensor input separates components of the composite sensor signal and responsively provides the components of the sensor signal to diagnostic circuitry. Diagnostic circuitry 45 receives and responsively provides an output related to sensor health.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a process control system including a trans- 50 mitter in accordance with the present invention.
FIG. 2 is a simplified block diagram of diagnostic circuitry in accordance with the present invention.
FIG. 3 is a simplified block diagram of a process device in accordance with the present invention. 55
FIG. 4 is a graph showing a nominal base signature for a sensor signal.
FIG. 5 is a diagram illustrating an individual wavelet transformation.
FIG. 6 is a graph illustrating various components of a 60 process variable sensor output from a wavelet decomposition of a composite sensor signal.
DETAILED DESCRIPTION OF THE
PREFERRED EMBODIMENTS 65
FIG. 1 is a diagram of process control system 2 including field mounted transmitter 40 coupled to control room 4 over
2
a two wire process control loop 6. Transmitter 40 monitors a process variable (e.g., flow, pressure temperature) of process fluid in process piping 8. Transmitter 40 transmits information related to the sensed process variable to control room 4 over loop 6 by controlling the current flowing through loop 6. For example, the current flowing through loop 6 may be controlled between 4 and 20 mA and properly calibrated to indicate the process variable. Additionally or in the alternative, transmitter 40 may transmit digital information related to the sensed process variable over loop 6 to control room 4 such as in a HART® or Fieldbus protocol. Transmitter 40 includes circuitry described herein in more detail which provides advanced diagnostics including life expectancy information (e.g. health) related to sensor operation. The present invention may be implemented in, for example, a pressure transmitter, a magnetic flowmeter, a coriolis flowmeter, a level transmitter with a low power radar measuring means, a resistance based temperature transmitter, or any other type of transmitter.
FIG. 2 is a simplified block diagram of one example of circuitry for performing sensor diagnostics in accordance with the invention. As shown in FIG. 2, process variable sensor 20 and sensor compensation circuit 22 provide a composite sensor signal to measurement circuitry 11 and preprocessing function 14. Measurement circuitry 11 provides an output to output circuitry 13 representative of the process variable being measured. The composite sensor signal provided by sensor 20 includes a component related to the sensed process variable and a residual sensor signal due to mechanical or electrical characteristics (e.g., the transfer function, process noise, etc.) of sensor 20. Further, the signal related to the process variable may be separated into two components, one component due to repeatable sensor variations and another due to repeatable process variations.
Wavelet preprocessing function 14 receives the composite sensor signal and separates the individual signal components, including seasonal variations. These separate signals are provided to diagnostic circuitry 12 on data bus 15. As explained below, this allows diagnostic circuitry 12 to function on the separate, individual signals which make up the composite sensor signal, and, for example, provide a diagnostic output indicative of a failure of sensor 20.
Output circuitry 13 receives the process variable from measurement circuitry 11 and formats the output as desired. For example, the output may be coupled to loop 6 shown in FIG. 1. Further, output circuitry 13 receives the diagnostic signal from circuitry 12 which, for example, may be output on loop 6, used to inhibit the output of process variable or to alarm.
FIG. 3 is a simplified block diagram of transmitter 40 in accordance with the present invention including wavelet preprocessing circuitry 32. Transmitter 40 includes sensor 20 which provides a sensor signal to sensor circuitry 22. Sensor 20 can be a resistance based sensor for sensing pressure or temperature (e.g., an RTD or a strain gauge), a capacitive pressure sensor, etc. As discussed, the sensor signal is a composite signal which includes a process variable signal and a residual sensor signal. Sensor circuitry 22 performs initial compensation including optional scaling, on the analog sensor signal and the output of sensor circuitry 22 is converted into a digital format by analog to digital converter 24. A microprocessor 26 receives the digitized process signal and is also coupled to memory 28 and a system clock 30. Microprocessor 26 operates in accordance with instructions stored in a memory 28 to perform various functions. Two such functions in accordance with the
