TMR Battery Current Sensing With Offset Cancellation
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Solution Overview
Problem
Current sensing systems for electric vehicles face challenges in non-invasively measuring both DC and AC currents over a wide range with high resolution, as existing methods such as shunt sensors are invasive and power-loss prone, Hall Effect sensors are sensitive to magnetic fields, and non-invasive technologies like current transformers and Rogowski coils are limited in their applications.
Innovation Solution
A non-invasive current sensor utilizing a Tunnel Magnetoresistance (TMR) magnetic-field sensor, which generates a linear differential-output voltage proportional to the magnetic field, allowing for the measurement of currents from 10 mA to 150 A with 10 mA resolution, and employs a differential arrangement and noise-cancellation techniques to reject common-mode noise and stray fields.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If shunt sensors are used for current measurement, then current can be measured, but significant power loss occurs during high current operation
Solution Approach 1:
The patent introduces a magnetic field as an intermediary between the current-carrying conductor and the sensor. The TMR sensor does not directly contact the current path but instead measures the magnetic field generated by the current, eliminating direct electrical contact and associated power losses while maintaining measurement capability
Solution Approach 2:
The patent replaces the direct electrical contact-based shunt measurement method with a magnetic field-based measurement approach. This substitution eliminates the need for current to pass through a resistive element, thereby eliminating the I²R power losses inherent in shunt-based systems
2Reliability
If Hall Effect sensors are used for current measurement, then galvanic isolation is provided, but sensitivity to magnetic fields causes significant errors in small current measurements
Solution Approach 1:
The patent exploits the dramatic change in resistance parameter of the TMR effect (up to 1000x or more) in response to magnetic field changes. This extreme parameter sensitivity enables detection of very small current-induced magnetic fields that would be undetectable by conventional Hall Effect sensors, while the differential configuration maintains galvanic isolation
Solution Approach 2:
The patent uses a differential arrangement with two TMR sensors positioned symmetrically on opposite sides of the conductor. One sensor detects the magnetic field from the current, while the other detects the opposite field, creating a differential output that copies and subtracts common-mode noise while preserving the signal of interest
3Loss of energy
If non-invasive current sensing is implemented, then power loss is reduced, but measurement resolution and accuracy deteriorate
Solution Approach 1:
The patent achieves high-resolution non-invasive measurement by utilizing the extreme sensitivity parameter change of the TMR effect. The resistance change of up to 1000x or more in response to magnetic field changes enables detection of very small current values (down to 10 mA) without direct electrical contact, maintaining both non-invasive operation and high measurement resolution
Solution Approach 2:
The patent replaces direct electrical measurement methods with magnetic field-based detection using TMR sensors. This substitution enables non-invasive measurement while achieving superior resolution compared to traditional non-invasive methods like current transformers, because the TMR effect provides much higher sensitivity to magnetic field changes
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 accurate, high-resolution measurement of currents in electric vehicles, reducing power loss and sensitivity to magnetic fields, while maintaining non-invasive operation and wide-range capability.
Implementation Method 1
A non-invasive current sensor utilizing a Tunnel Magnetoresistance (TMR) magnetic-field sensor, which utilizes a Tunnel Magnetoresistance (TMR) effect in a Magnetic Tunnel Junction (MTJ) to generate a linear differential-output voltage proportional to the magnetic field perpendicular to its package
Implementation Method 2
When installed on a current-carrying conductor of an electric vehicle, the presently disclosed current sensor enables measurement of currents ranging 10 mA-150 A with a resolution of 10 mA
Data Source
AI summary
A method of sensing a current in a conductor includes controlling a digital to analog converter output to cancel residual offset voltage in a magnetic tunnel junction device prior to sensing the current with the magnetic tunnel junction device. The method includes switching input to the magnetic tunnel junction device between a fixed voltage and an output of a digital to analog converter while switching input to a band pass filter between a lower and an upper voltage output of the magnetic tunnel junction device. The output of the digital to analog converter is modified to provide a low-amplitude unsaturated sine-wave at an output of the band pass filter, at which point changes in the output of the band pass filter are associated with the amount of current in a sensed conductor.