The self validating sensor rationale definitions and examples

A multi-level process monitoring system comprising a process monitoring unit, at a higher level of the system, and a plurality of sensors at a lower level of the system, at least one of the sensors having self-validating capability the sensors being adapted to provide respective measurement values of respective process variables to said monitoring unit, said monitoring unit being so arranged as to monitor the outputs of the sensors and to identily any significant apparent change in the process conditions based on a comparison of apparent process status information, which is obtained by a combination of said sensor outputs, with reference information, and on detection of an apparent significant change, to request additional status information from at least one of the SEVA sensor/s to determine whether the apparent change is in reality due to a change in the characteristics of a particular SEVA sensor rather than an actual significant change in the process conditions.

The transmitter (24) generates a first output signal related to the value of the variable. Typically, the measurement data is a signal that approximates the value of a process variable (a measurand).

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The transmitter (24) also generates a second output signal based on a dynamic uncertainty analysis of the first output signal. A method of compensating for variations in a transfer function of a sensor (12), comprising: receiving a measurement produced by the sensor; and characterised by comprising: identifying a desired transfer function for the sensor; determining an actual transfer function for the sensor; modifying the measurement to produce a modified measurement by first modifying the measurement with a transfer function derived from the actual transfer function, so as to produce a compensated measurement, and then by modifying the compensated measurement with the desired transfer function; and providing the modified measurement as a measurement value for the sensor. The method of claim 1, wherein the desired transfer function is identified from design criteria for the sensor. The method of claim 1 or 2, wherein the actual transfer function varies with time, and wherein the step of determining the actual transfer function comprises determining the actual transfer function for a particular time or period of time for which a measurement value is to be produced. The method of claim 1, 2 or 3, wherein the sensor (12) comprises a thermocouple, and wherein the step of determining the actual transfer function comprises using current injection tests. The method of any preceding claim, wherein the step of modifying the measurement comprises modifying the measurement based on an inverse of the actual transfer function to produce the compensated measurement. The method of claim 5, wherein the step of modifying the measurement comprises modifying the compensated measurement based on the desired transfer function to produce the modified measurement. The method of any preceding claim, further comprising generating an uncertainty measure for the measurement value. The method of claim 5 or 6, and 7, wherein the step of generating an uncertainty measure comprises generating a first uncertainty measure for the compensated measurement, generating a second uncertainty measure for the desired transfer function, and combining the first and second uncertainty measures to produce the uncertainty measure for the measurement value. The method of claim 8, wherein the step of generating the first uncertainty measure for the compensated measurement comprises: generating a third uncertainty measure for the measurement produced by the sensor; generating a fourth uncertainty measure for the inverse of the actual transfer function; and combining the third and fourth uncertainty measures to produce the first uncertainty measure for the compensated measurement. A sensor (12) for providing a measurement value, the sensor comprising: a transducer (22) configured to generate a measurement; and characterised by comprising: a transmitter (24) configured to: identify a desired transfer function for the transducer (22); determine an actual transfer function for the transducer; receive the measurement generated by the transducer; modify the measurement to produce a modified measurement (see claim 1); and provide the modified measurement as a measurement value for the sensor (12). The sensor of claim 10, wherein the transmitter (24) identifies the desired transfer function from design criteria for the sensor. The sensor of claim 10 or 11, wherein the actual transfer function varies with time, and wherein the transmitter (24) determines the actual transfer function for a particular time or period of time for which the measurement value is to be produced. The sensor of claim 10, 11 or 12, wherein the transducer (22) comprises a thermocouple, and wherein the transmitter determines the actual transfer function using current injection tests. The sensor of claim 10, 11, 12 or 13, wherein the transmitter (24) modifies the measurement based on an inverse of the actual transfer function to produce the compensated measurement. The sensor of any of claims 10 to 14, wherein the transmitter modifies the measurement based on the desired transfer function to produce the modified measurement. The sensor of any of claims 10 to 15, wherein the transmitter is further operable to generate an uncertainty measure for the measurement value. The sensor of claim 14, wherein the transmitter (24) generates a first uncertainty measure for the compensated measurement, generates a second uncertainty measure for the desired transfer function, and combines the first and second uncertainty measures to produce the measurement value. The sensor (12) of claim 14 and 17, wherein the transmitter (24) generates the first uncertainty measure by generating a third uncertainty measure for the measurement, generating a fourth uncertainty measure for the inverse of the actual transfer function, and combining the third and fourth uncertainty measures to produce the first uncertainty measure for the compensated measurement. In practice, the signal does not perfectly represent the value of the process variable.

Instead, the signal also includes effects resulting from the sensor (such as sensor faults or distortion) and other process influences (including those attributable to "faulty" process behavior).

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