Open Wire Detection in 4-20 mA Current Loops
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Solution Overview
Problem
Existing 4-20 mA current loop systems for high-level analog field transmitters in process control systems face issues with voltage losses and noise sensitivity over long distances, and fail to rapidly detect open-wire conditions, leading to potential propagation of bad input values.
Innovation Solution
An open-wire detection system that uses a current transmitter, anti-aliasing filter, noise filter, and open-wire threshold detector to sense input signals with multiple frequency responses, allowing for rapid identification of open-wire conditions by sampling data at intervals correlated with digital signal frequencies and preventing bad data propagation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If voltage-based transmission is used over long distances, then signal transmission capability is improved, but noise sensitivity increases and voltage losses occur
Solution Approach 1:
The patent replaces voltage-based transmission with current-based transmission (4-20 mA current loop). Current signals are immune to voltage drops over long distances and have inherent noise rejection capabilities, eliminating the need for high-impedance devices and shielded wires.
Solution Approach 2:
The patent changes the transmission parameter from voltage to current. By using a current loop where the current magnitude (4-20 mA) represents the process variable, the system achieves long-distance transmission without voltage losses and with improved noise immunity.
2Object-affected harmful factors
If anti-aliasing filter with larger time constant is used, then noise filtering is improved, but open-wire detection speed deteriorates
Solution Approach 1:
The patent divides the filtering function into two separate filters: an anti-aliasing filter for noise reduction and a noise filter with smaller time constant for rapid open-wire detection. This segmentation allows each filter to be optimized for its specific function without compromise.
Solution Approach 2:
The patent introduces a noise filter as an intermediary component between the anti-aliasing filter and the open-wire detection logic. This intermediate filter with smaller time constant enables rapid detection of open-wire conditions while the anti-aliasing filter handles noise filtering for accurate process variable measurement.
3Object-affected harmful factors
If current loop system is used, then immunity against interference is improved, but open-wire condition detection speed deteriorates
Solution Approach 1:
The patent segments the signal processing into two parallel paths: one through the anti-aliasing filter for accurate process variable measurement and another through the noise filter for rapid open-wire detection. This allows the current loop to maintain interference immunity while enabling fast fault detection.
Solution Approach 2:
The patent performs preliminary open-wire detection by sampling the filter output multiple times before final processing. By checking for open-wire conditions in advance through multiple samples, the system can rapidly identify faults while maintaining the interference-rejection benefits of the current loop architecture.
Data Source
AI summary
An open-wire detection system and method includes a current transmitter that can be connected to one or more wires, wherein the current transmitter provides a minimum current and/or a current that is greater than the minimum current. An anti-aliasing filter is connected to an analog-to-digital converter, such that the anti-aliasing filter receives the minimum current provided by the current transmitter and provides an output signal to the analog-to-digital converter. A noise filter is generally connected to an open-wire threshold detector, wherein the noise filter and the open-wire threshold detector permit detection of input levels below the minimum current provided by the current transmitter. An output from the open-wire threshold detector can be sampled multiple times at intervals that correlate with a frequency of a plurality of digital signals to produce sampled data, such that if the sampled data is below the minimum current, one or more of the wires (e.g., a return field wire and/or a field wire connected to a power supply) is reported as constituting an open-wire.