Vehicle Chassis Isolation Resistance Measurement
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Solution Overview
Problem
Existing methods for measuring electrical isolation resistance between a vehicle chassis and high voltage battery terminals in electric vehicles are prone to errors due to sensitivity to battery voltage fluctuations and require lengthy stabilization times, leading to delayed and inaccurate results, especially when the vehicle is in operation.
Innovation Solution
A method involving the measurement of open and closed voltages across test resistors connected between the battery terminals and the chassis, using ratios and differences to calculate isolation resistance, allowing for accurate and rapid assessment without excessive filtering or averaging, even during dynamic battery conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the SAE J1766 method waits for voltage stabilization after connecting test resistor, then measurement accuracy improves, but measurement time increases significantly
Solution Approach 1:
The patent measures the open-circuit voltages VP(open) and VN(open) before connecting the test resistor to the chassis. These preliminary measurements are used to calculate a compensation factor that accounts for the differential voltage between battery terminals. By performing this preliminary measurement and calculation, the system eliminates the need to wait for voltage stabilization after test resistor connection, as the compensation factor pre-adjusts for any subsequent voltage variations.
2Device complexity
If the SAE J1766 method uses single measurement after stabilization, then device complexity is reduced, but measurement precision deteriorates due to sensitivity to differential voltage variations
Solution Approach 1:
The patent implements a feedback mechanism where the open-circuit voltage measurements VP(open) and VN(open)) are used to calculate a compensation factor (1 + VP(open)/VN(open))). This compensation factor is then applied to the measured voltages after test resistor connection to correct for differential voltage variations. The feedback loop continuously monitors and compensates for voltage fluctuations, maintaining high measurement precision without requiring complex filtering or averaging hardware.
3Measurement precision
If multiple measurements are averaged or low frequency filtering is applied, then measurement precision improves, but response time and productivity decrease
Solution Approach 1:
The patent changes the measurement parameters by using voltage ratios instead of absolute voltage values. By measuring the ratio VP/VN both before and after test resistor connection, and using the difference in these ratios to calculate isolation resistance, the system becomes insensitive to differential voltage variations. This parameter transformation allows single-measurement accuracy equivalent to multiple averaged measurements, achieving high precision without sacrificing response time or productivity.
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 approach provides highly accurate and quick isolation resistance measurements, reducing errors and response time, enabling real-time monitoring of insulation health without the need for prolonged stabilization periods or averaging multiple samples.
Implementation Method 1
the RC time constant caused by test resistor and the suppression and parasitic capacitors in the system
Implementation Method 2
the decay of the voltage to steady state has a very long time constant (typically in the range of 15 seconds) due to the combination of: the electromagnetic compatibility (EMC) suppression filter capacitors; the parasitic capacitance between the chassis and the high voltage positive and negative terminals
Data Source
AI summary
A method for measuring the isolation resistance between a chassis and a battery terminal in an electric vehicle. The method can include measuring an open voltage (VP(open)) of the positive terminal; measuring an open voltage (VN(open)) of the negative terminal; measuring a voltage (VN(S1 closed)) between the negative terminal and the chassis with switch S1 closed; measuring a voltage (VP(S1 closed)) between the positive terminal and the chassis with switch S1 closed; calculating the ratio (VP(open)/VN(closed)) of the voltages VP(open) and VN(closed); calculating the open ratio of the open voltages (VP(open)/VN(open)); calculating the difference between the ratio (VP(S1 closed)/VN(S1 closed)) and the open ratio (VP(open)/VN(open)); and multiplying the value of the test resistor (R0) by the difference between the ratio (VP(S1 closed)/(VN(S1 closed)) and the open ratio (VP(S2 open)/VN(S1 open)) to obtain the isolation resistance.