Earth Resistance Sensing in Battery Chargers Using Lock-In Detection
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Solution Overview
Problem
Existing methods for measuring earth resistance in battery charging systems are ineffective in noisy grid conditions, making it difficult to distinguish between the injected signal and noise components, particularly in non-insulated on-board charging systems where safety is a concern.
Innovation Solution
A method and device that inject a measuring signal in alternating current between a neutral and earth node, processing the voltage signal to extract a frequency-correlated component, allowing for accurate earth resistance calculation and using this value to protect the charging system by comparing it to a reference limit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a current signal is injected into the grid for earth resistance measurement, then the earth resistance can be calculated, but the measurement becomes unreliable when the grid frequency is occupied by significant disturbances
Solution Approach 1:
The patent applies periodic action by using a sinusoidal measuring signal with a specific frequency (e.g., 15 Hz or 25 Hz) that is periodically injected into the grid. This periodic signal allows for frequency-selective detection, where the measuring device can distinguish the periodic measuring signal from random noise components through synchronous detection techniques, thereby maintaining measurement reliability in noisy grid conditions
Solution Approach 2:
The patent introduces an intermediary approach by using a lock-in amplifier or synchronous detector as a mediator between the injected measuring signal and the voltage measurement. This intermediary device selectively amplifies only the voltage components that are synchronized with the injecting frequency, effectively filtering out noise at other frequencies and improving both measurement precision and reliability
2Object-affected harmful factors
If the current intensity for measurement is limited to just over 1 mA due to regulatory constraints, then safety requirements are met, but it becomes practically difficult to distinguish the injected signal from noise components
Solution Approach 1:
The patent applies parameter changes by modifying the frequency parameter of the measuring signal to a value distinct from the standard grid frequency (e.g., using 15 Hz, 25 Hz, or other non-standard frequencies). This frequency differentiation allows the measuring device to distinguish the measuring signal from grid noise through frequency-selective detection, thereby maintaining measurement precision while complying with the 1 mA current intensity safety constraint
Solution Approach 2:
The patent uses a lock-in amplifier or synchronous detector as an intermediary that can detect and amplify extremely weak periodic signals (at the level of microvolts or less) by synchronizing with the known measuring frequency. This intermediary device enables accurate signal detection even when the current intensity is limited to just over 1 mA, as it can extract the periodic signal component from the noise floor through coherent detection
3Adaptability or versatility
If a non-insulated on-board charging system is used with direct electrical connection to alternating current wiring, then charging functionality is achieved, but safety protection becomes more critical
Solution Approach 1:
The patent applies preliminary action by performing an earth resistance measurement before the charging operation commences. The measuring device injects a test signal and measures the earth resistance to verify that the earthing system meets safety requirements (typically requiring resistance below a threshold value). Only after successful verification does the system proceed to normal charging operation, thereby preventing unsafe charging conditions
Solution Approach 2:
The patent implements feedback by continuously monitoring or periodically re-measuring the earth resistance during charging operations. If the measured earth resistance exceeds the safety threshold at any point, the system automatically interrupts the charging process and alerts the user. This feedback mechanism dynamically adjusts the charging state based on the actual earthing conditions, maintaining safety while enabling versatile charging functionality
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 reliable and versatile earth resistance measurement, even in varying noise levels, enhancing safety by accurately determining earth resistance and isolating the converter assembly from the grid when necessary, thus improving user safety.
Implementation Method 1
injecting a measuring signal in alternating current between a first, neutral node of the grid and a second, earth node
Implementation Method 2
the voltage signal is processed so as to extract a first signal having a frequency equal to said pre-set frequency and a phase correlated to that of said measuring signal
Data Source
AI summary
A method for measuring an earth resistance in a battery charging system, including the steps of injecting a measuring signal in alternating current between a first, neutral node and a second, earth node, detecting a voltage signal representing an electrical potential difference between the first and the second node, processing the voltage signal so as to extract a first signal having a frequency equal to that of the measuring signal and a phase correlated thereto, and calculating an earth resistance value as a function of a ratio between the measuring signal and a reference value of said first signal.