Two-Wire Loop Diagnostic Signal Encoding
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Solution Overview
Problem
Existing process control systems face challenges in simultaneously transmitting process variables and diagnostic information using two-wire loops, as alarm signals outside the normal range disrupt the analog output, limiting predictive diagnostics and requiring additional infrastructure or HART hosts.
Innovation Solution
The method involves using an analog current signal on a two-wire process control loop to convey diagnostic information, allowing multiple current levels or a continuous range to indicate diagnostic conditions, enabling predictive diagnostics without disrupting the process variable transmission and utilizing existing control room equipment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If alarm signals are transmitted by fixing current outside predetermined threshold values (greater than 21 mA or less than 3.8 mA), then alarm indication is achieved, but the analog output is disrupted and process variable transmission is limited
Solution Approach 1:
The patent segments the current signal range into distinct zones: normal process variable transmission (4-20 mA), alarm conditions (outside 3.8-21 mA), and diagnostic information (specific levels within or near the normal range). This segmentation allows different types of information to be transmitted using different current levels without interfering with each other, resolving the contradiction between alarm transmission and process variable transmission.
Solution Approach 2:
The patent adds a new dimension to the single-wire communication by encoding multiple types of information (process variable, alarm status, diagnostic data) into different current level dimensions. Instead of using separate wires or communication protocols, the system uses the amplitude dimension of the current signal to carry multiple meaning layers simultaneously, allowing alarm and diagnostic information to coexist with process variable transmission.
2Reliability
If HART protocol or Fieldbus is used to transmit diagnostic information, then predictive diagnostics capability is improved, but additional infrastructure or complex updates are required
Solution Approach 1:
The patent makes the existing two-wire loop universally capable of carrying multiple functions: process variable transmission, alarm signaling, and diagnostic information transmission. By encoding diagnostic information in the same communication medium using distinct current levels, the system eliminates the need for separate communication infrastructure like HART hosts or Fieldbus devices, allowing a single loop to perform multiple functions simultaneously.
Solution Approach 2:
The system enables the two-wire loop to serve itself by transmitting diagnostic information about its own operation and the connected devices without requiring external diagnostic equipment. The loop uses its existing current signal to carry self-diagnostic data, eliminating the need for additional diagnostic infrastructure and making the system self-monitoring.
3Loss of information
If multiple current levels or continuous range are used to indicate diagnostic conditions, then diagnostic information transmission is improved, but signal interpretation complexity increases
Solution Approach 1:
The patent uses specific parameter changes in current levels to encode diagnostic information. By assigning predetermined current levels or ranges to specific diagnostic conditions (e.g., particular levels indicating specific fault types), the system creates a standardized language for diagnostic communication. This parameter-based encoding allows rich diagnostic information to be transmitted using simple, interpretable current level changes that build upon the existing 4-20 mA standard.
Data Source
AI summary
A process device for coupling to a two-wire process control loop—includes diagnostic circuitry configured to identify a failure or reduced level of performance or provide a prediction of a future failure or reduced level of performance. An output coupled to the two-wire process control loop is configured to provide an analog output on the loop related to an output from the diagnostic circuitry.