Clock Synchronization Diagnostics for Safety-Rated Network Nodes
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Solution Overview
Problem
Existing safety-rated industrial control systems face challenges in maintaining accurate time synchronization due to potential errors in clock signals, which can lead to undetected failures in network devices, compromising safety integrity levels (SIL) and time-synchronous operations.
Innovation Solution
A method for synchronizing clocks between nodes in a safety-rated application by transmitting diagnostic messages and calculating clock skew using timestamps, storing offset values in safety memory, and applying filters to detect clock drift, ensuring accurate time synchronization and fault detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If network devices are used to connect safety-rated nodes, then device complexity and cost are reduced, but time synchronization accuracy deteriorates due to potential clock signal errors
Solution Approach 1:
The patent implements a feedback mechanism where safety-rated nodes transmit timestamped diagnostic messages through network devices, and receiving nodes calculate round-trip times and clock skew to detect synchronization errors. This feedback loop enables continuous monitoring and detection of time synchronization accuracy degradation caused by network devices, allowing the system to identify when clock skew exceeds thresholds and take corrective action.
Solution Approach 2:
The patent introduces intermediary diagnostic messages with timestamps as mediators between safety-rated nodes connected through network devices. These timestamped messages serve as intermediaries to carry timing information through the potentially problematic network devices, enabling indirect measurement and detection of clock skew without requiring direct connection between nodes.
2Ease of operation
If standard network protocols are used for communication, then ease of operation is improved, but reliability deteriorates due to undetected clock drift compromising safety integrity
Solution Approach 1:
The patent applies preliminary action by establishing diagnostic communication channels and timestamp exchange mechanisms before safety-critical operations. Safety-rated nodes continuously exchange diagnostic messages and calculate clock skew in advance, detecting potential synchronization issues before they compromise safety integrity levels, allowing preventive action to be taken.
Solution Approach 2:
The system implements feedback by continuously monitoring clock skew calculations derived from timestamped diagnostic messages. When clock skew exceeds predetermined thresholds, the system generates alerts or takes corrective action, providing continuous feedback on synchronization health to maintain reliability while using standard network protocols.
3Device complexity
If clock synchronization is implemented without diagnostic monitoring, then device complexity is reduced, but measurement precision deteriorates as clock skew goes undetected
Solution Approach 1:
The patent implements self-service by enabling safety-rated nodes to autonomously perform diagnostic functions. Nodes independently transmit timestamped messages, receive responses, calculate round-trip times, determine clock skew, and compare against thresholds without requiring external monitoring equipment, achieving accurate clock skew detection while minimizing additional system complexity.
Data Source
AI summary
To improve integrity of time synchronization, a node in a safety rated system verifies that its clock remains synchronized to another clock. Two adjacent, time-synchronized nodes transmit diagnostic messages to each other at an agreed upon interval and generate timestamps when the diagnostic message is received from the other node. The nodes then transmit their respective timestamp back to the sending node. Clock drift is detected by comparing a difference between the two timestamps at which the messages were received against a threshold. To avoid accidental detection of clock drift, a difference in transmission delays between the two nodes is stored in a FIFO buffer. Each node monitors the average of the data in the FIFO buffer. If the average deviates from the target value by too great a value, then the node determines the values of the clocks have skewed beyond an acceptable range and generates a fault condition.