Digital Input Wire Break Detection via Optocoupler Ground Isolation
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Solution Overview
Problem
Industrial controllers using input-powered digital input circuitry cannot detect broken wires, as they lack field-side power, making it difficult to differentiate between an open switch/sensor and a broken wire, leading to a design quandary between using more expensive field-side power circuitry or losing wire break detection capability.
Innovation Solution
Incorporating an optocoupler in series between the field ground pins and the industrial controller's ground, along with a capacitor to field ground and a resistor from the input pin to the digital input circuitry, allows for wire break detection by isolating the digital input circuitry with a test pulse and monitoring the charge rate of the capacitor to determine if the wire is broken.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If field-side power digital input circuitry is used, then wire break detection capability is achieved, but device complexity and cost increase
Solution Approach 1:
An optocoupler is introduced as an intermediary component between the field ground and the digital input circuitry. This optocoupler enables wire break detection by isolating the detection circuit from the field ground, allowing the system to detect wire breaks without requiring complex field-side power circuitry. The optocoupler acts as a mediator that enables detection functionality while maintaining system simplicity.
Solution Approach 2:
The patent creates a simplified copy of the field-side power detection mechanism by using an optocoupler to simulate the ground connection behavior. Instead of implementing full field-side power circuitry, the system uses the optocoupler to create an equivalent detection capability through optical coupling, thereby achieving wire break detection with reduced complexity.
2Device complexity
If input-powered digital input circuitry is used, then device complexity is reduced, but wire break detection capability is lost
Solution Approach 1:
The system performs a preliminary test by temporarily activating the optocoupler to establish a test ground connection before normal operation. This preliminary action allows the system to detect wire breaks by comparing the test state with the normal operating state, enabling detection capability in otherwise simple input-powered circuitry without requiring continuous complex circuitry.
Solution Approach 2:
The wire break detection is implemented through periodic test pulses that temporarily activate the optocoupler. Instead of maintaining continuous complex detection circuitry, the system uses periodic test actions to detect wire breaks, thereby achieving detection capability with simple input-powered circuitry that operates in normal mode most of the time.
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
Enables the use of simpler input-powered digital input circuitry while still allowing for wire break detection, ensuring reliable operation without the need for complex and costly field-side power circuitry.
Implementation Method 1
an optocoupler is placed in series between the field ground pins of the digital input circuitry and the field ground of the industrial controller
Implementation Method 2
the digital input circuitry being disconnected from ground allows the capacitor connected between the input and ground to charge
Data Source
AI summary
An optocoupler is placed in series between the field ground pin of digital input circuitry and the field ground of an industrial controller. A capacitor to field ground is provided for each digital input. A resistor is provided to the input pin of the digital input circuitry. To detect a broken wire a test pulse is provided to the optocoupler connected in the ground path. This test pulse isolates the digital input circuitry from field ground. As current is always being provided from the field when the wire is not broken, the capacitor connected between the input and ground charges. After the test pulse has completed, the output signal of the digital input circuitry is examined. If the level indicates the input is high, the wire is not broken. If, however, the output remains low indicating that the input is low, the wire has broken.