Loop Current Readback Isolation in Automation Field Devices
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing field devices in process automation technology face challenges in accurately reading back the analog output signal transmitted via a two-wire line, particularly due to residual currents from defective explosion protection and electromagnetic compatibility measures, which affect the determination of tolerable risk in safety-critical applications.
Innovation Solution
A galvanically isolated current transformer is used instead of a shunt resistor to read back the loop current, positioned between the connection terminals and safety devices, with an inductive or capacitive current converter and insulating means to prevent electrical flashover, ensuring compliance with IEC DIN EN 60079 standards for explosive areas.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a shunt resistor is used to read back the loop current, then the current measurement is simple and direct, but residual currents from defective Ex and EMC measures cannot be detected and are included as errors
Solution Approach 1:
A galvanically isolated current transformer is introduced as an intermediary device between the two-wire line and the measurement circuit. This transformer provides galvanic isolation that separates the measurement system from potential interference sources (defective Ex and EMC measures), allowing accurate current measurement while enabling detection of residual currents that would otherwise contaminate the measurement.
Solution Approach 2:
The current measurement function is segmented into two independent parts: the galvanically isolated current transformer for measurement and the evaluation circuit for error detection. This segmentation allows the measurement system to operate independently from potential interference sources while maintaining the ability to detect residual currents through separate evaluation pathways.
2Device complexity
If the current transformer is positioned behind all Ex and EMC measures, then the existing architecture is maintained, but residual currents from defective measures cannot be detected
Solution Approach 1:
Instead of placing the current transformer behind the Ex and EMC measures (as in conventional architecture), the invention inverts the sequence by positioning the current transformer directly at the connection terminals, in front of all safety devices. This inversion enables the measurement of residual currents before they are processed by potentially defective Ex and EMC measures.
3Measurement precision
If the current transformer is positioned directly at the connection terminals, then residual currents can be detected accurately, but the device requires additional safety measures for explosive areas
Solution Approach 1:
The galvanically isolated current transformer acts as an intermediary that inherently provides electrical isolation suitable for explosive areas. This isolation capability reduces the need for additional complex safety devices while maintaining accurate residual current detection at the connection terminals.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution enhances the accuracy of reading back the analog output signal, reducing undetected errors and improving the safety integrity level of field devices by isolating the current measurement from potential interference sources, thus minimizing tolerable risks in safety-critical applications.
Implementation Method 1
a galvanically isolated current transformer (7) is used instead of a shunt resistor, which reads back the loop current (4) in one of the two lines (6) between the safety devices (5) and the connection terminals (2)
Implementation Method 2
an insulating means (8) is provided between the line (6) and the current transformer (7), which prevents an electrical flashover up to a predefined voltage
Data Source
Figure 1~3
AI summary
The invention relates to an automation field device (1) with at least two connection terminals (2) to which a two-wire line (3) or a four-wire line (3) can be connected such that a loop current (4) can be supplied to the field device (1) via the connection terminals (2). A safety device (5) is provided which is used to ensure the electromagnetic compatibility and/or the explosion proofing of the field device (1). The safety device (5) is connected to each connection terminal (2) via a respective line (6), and at least one current converter (7) is arranged around the line, said current converter back-reading the loop current (4) at least in one of the two lines (6a, 6b) between the safety device (5) or parts of the safety device (5) and the connection terminals (2) in a galvanically isolated manner.