Diagnosis of a two-conductor field instrument
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Solution Overview
Problem
Existing two-conductor field instruments face challenges in ensuring high functional safety, particularly in reliably transmitting measurement data and failure information in safety-critical applications, where current modulation methods may not adequately verify the accuracy of electrical currents within the 4-20 mA range.
Innovation Solution
A method and apparatus for a two-conductor field instrument that includes a diagnostic operating mode, where input voltages and output currents are modulated during short time intervals to check the functionality of the instrument, allowing for the verification of correct failure current transmission and component functionality, including the ability to output predeterminable alarm values and detect errors in electronics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If current modulation methods are used for data transmission in two-conductor field instruments, then data transmission is achieved, but functional safety and reliability of measurement data transmission are insufficient
Solution Approach 1:
The patent applies preliminary action by performing diagnostic tests before normal operation to verify instrument functionality. The system executes self-diagnostics during startup or maintenance modes, checking critical components such as the measurement cell, electronics, and communication interfaces before the instrument is put into service. This ensures that potential failures are detected and addressed before they can compromise measurement accuracy or safety.
Solution Approach 2:
The patent implements feedback mechanisms by continuously monitoring the electrical current loop and comparing actual measurements against expected values. The system uses the 4-20 mA current loop not only for data transmission but also for diagnostic purposes, analyzing current characteristics to detect anomalies. Feedback loops verify measurement plausibility and trigger alarm conditions when deviations exceed predetermined thresholds, thereby maintaining functional safety.
2Reliability
If diagnostic operating mode with modulated input voltages and output currents is implemented, then functionality verification is improved, but device complexity increases
Solution Approach 1:
The patent applies universality by designing the diagnostic operating mode to utilize the existing two-conductor fieldbus infrastructure for multiple purposes. The same electrical current loop that transmits process data also carries diagnostic information. The system can operate in both normal measurement mode and diagnostic mode using the same hardware components, eliminating the need for separate diagnostic circuits and reducing overall device complexity despite the enhanced functionality.
Solution Approach 2:
The patent implements periodic action by executing diagnostic routines at scheduled intervals or under specific conditions rather than continuously. The diagnostic operating mode can be activated periodically during normal operation, or triggered by specific events such as maintenance schedules or detected anomalies. This approach verifies functionality without requiring constant diagnostic monitoring, thereby managing complexity while maintaining reliability.
3Reliability
If failure information transmission is enhanced for safety-critical applications, then functional safety is improved, but loss of time in detecting and transmitting errors increases
Solution Approach 1:
The patent applies continuity of useful action by maintaining continuous monitoring of the electrical current loop characteristics even during normal operation. The system continuously analyzes current modulation patterns, voltage levels, and transmission quality without interrupting the primary measurement function. This continuous surveillance enables immediate detection of failures and instantaneous transmission of alarm information, eliminating delays between error occurrence and notification.
Solution Approach 2:
The patent uses the electrical current loop itself as an intermediary for dual purposes: transmitting both process measurement data and diagnostic/failure information through the same communication channel. By encoding diagnostic information within the existing 4-20 mA signal framework and utilizing the fieldbus protocol's inherent diagnostic capabilities, the system transmits failure information without requiring separate communication channels, thereby minimizing time loss while enhancing safety monitoring.
Data Source
AI summary
Disclosed is a method for diagnosis of a two-conductor field instrument and a corresponding two-conductor field instrument. In a normal operating mode, an input voltage is provided and an output current is output. In a diagnostic operating mode, the method includes: providing a first diagnosis-input voltage and outputting a first diagnosis-output current during a first time interval, providing a second diagnosis-input voltage and outputting a second diagnosis-output current during a second time interval, determining the second time interval from the first time interval, registering a first and second diagnosis-output voltage as a function of the first and second diagnosis-output current, and checking the functionality of the two-conductor field instrument by the first and second diagnosis-input voltage, the first and second time interval, the first and second diagnosis-output electrical current, the first and second diagnosis-output voltage based on the input voltage and/or based on the output electrical current.