Shadow Function Validation for Safety Monitoring Diagnostics
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Solution Overview
Problem
Current diagnostic coverage methods for safety functions in safety instrumented systems are either impractical due to high costs or inefficiencies, such as full redundancy, which reduces processing power and increases costs, or require extensive analysis of every potential point of failure, making them impractical for complex systems.
Innovation Solution
Implementing a shadow function that replicates the output of safety functions during limited time windows, rather than continuously, to reduce computing resources and costs, while ensuring diagnostic coverage through time-division multiplexed redundancy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If full redundancy is implemented for diagnostic coverage, then reliability is improved, but device complexity and cost increase
Solution Approach 1:
The shadow function executes periodically at predetermined intervals rather than continuously, providing diagnostic coverage only when needed. This periodic execution reduces computational resource consumption and system complexity while maintaining adequate diagnostic capability to detect faults in the safety function.
Solution Approach 2:
A shadow function is created as a copy of the safety function to perform diagnostic comparison. The shadow function replicates the safety function's logic and receives the same sensor inputs, allowing output comparison to detect faults without requiring full redundant hardware systems.
2Reliability
If full redundancy is implemented for diagnostic coverage, then reliability is improved, but cost increases
Solution Approach 1:
By executing the shadow function periodically rather than continuously, computational resources are utilized more efficiently. This reduces the overall processing power requirements and associated hardware costs while maintaining sufficient diagnostic coverage to ensure safety function reliability.
Solution Approach 2:
The shadow function serves as a software-based copy that provides diagnostic capability without requiring duplicate hardware systems. This software copying approach significantly reduces manufacturing costs compared to full hardware redundancy while maintaining adequate fault detection capability.
3Reliability
If continuous monitoring is performed, then reliability is improved, but processing power is reduced
Solution Approach 1:
The shadow function executes periodically at predetermined intervals rather than continuously monitoring at all times. This periodic execution maintains fault detection capability by regularly comparing outputs while significantly reducing the continuous processing power requirements compared to continuous monitoring approaches.
Data Source
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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.