Shadow Function Validation for Safety Function Error Detection
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Solution Overview
Problem
Current diagnostic coverage methods for safety functions in safety instrumented systems, such as full redundancy, are costly and impractical due to the high computational resources required, and still face challenges in detecting real errors, leading to potential false negatives and increased risks.
Innovation Solution
Implementing a shadow function diagnostic system that replicates safety function outputs during limited time windows, reducing computational demands and using a time-division multiplexed approach to validate safety function outputs, thereby reducing costs and improving error detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If full redundancy is implemented to achieve diagnostic coverage of safety functions, then reliability is improved, but device complexity and cost increase significantly
Solution Approach 1:
The patent implements a shadow function that copies the safety function's processing logic and compares its output to detect errors. Instead of duplicating the entire safety function continuously, the shadow function replicates the processing logic and periodically compares results, achieving diagnostic coverage with reduced complexity compared to full redundancy.
Solution Approach 2:
The shadow function operates periodically rather than continuously, comparing its output with the safety function at designated intervals. This periodic comparison approach provides diagnostic coverage while reducing the computational burden and system complexity associated with continuous full redundancy.
2Reliability
If full redundancy is implemented to achieve diagnostic coverage of safety functions, then reliability is improved, but cost increases due to doubled processing power requirements
Solution Approach 1:
The shadow function creates a simplified copy of the safety function's processing logic rather than maintaining full redundant processing capabilities. This copying approach enables error detection through comparison while consuming significantly less processing power than full redundancy would require.
Solution Approach 2:
The shadow function performs partial redundancy by implementing only the essential processing logic needed for comparison, rather than full redundancy. This partial action provides sufficient diagnostic coverage to detect errors in the safety function while avoiding the excessive processing power consumption of complete duplication.
3Measurement precision
If analysis is performed on every electrical and electronic element down to code and memory bytes, then measurement precision is improved, but device complexity and cost become impractical
Solution Approach 1:
Instead of analyzing every individual element, the patent copies the functional output of the safety function and compares it with the shadow function's output. This functional copying approach provides comprehensive error detection at the system level without the impractical complexity of analyzing every code byte and memory element individually.
Data Source
AI summary
A diagnostic system is provided and includes a plurality of processors configured to receive data acquired by a sensor. A first processor can execute a safety function that determines a plurality of sensor measurement from the received data, compares selected sensor measurements to predetermined alarm set points, and determines a safety function output for the selected sensor measurements based upon the sensor measurement comparison. A second processor can determine a shadow function output corresponding to the safety function outputs during different respective portions a diagnostic interval. The shadow function output can be configured to replicate the safety function output under conditions where the safety function output is free from error. A third processor can be configured to validate the safety function output by comparing the safety function output to its corresponding shadow function output and outputting a condition based upon the validation comparison.