Magnetic Field Sensor AC Bias Immunity
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Solution Overview
Problem
Magnetic field sensors using DC biasing are susceptible to low-frequency magnetic interference, making it difficult to accurately detect ferromagnetic target profiles, especially in applications with significant magnetic interference sources like current-carrying wires and electric motors.
Innovation Solution
The use of an AC bias magnetic field, modulated by the movement of a ferromagnetic target, allows for demodulation to shift the desired signal portion to a higher frequency band, separating it from unwanted interference, which can then be filtered out, thereby improving signal clarity and accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a DC bias magnetic field is used to sense ferromagnetic target features, then the sensor can detect target profile information, but the sensor becomes susceptible to low-frequency magnetic interference from sources like current-carrying wires and electric motors
Solution Approach 1:
The patent applies periodic action by using an AC bias magnetic field instead of a DC bias field. The AC bias field oscillates at a specific frequency, causing the sensing element to produce an output signal that is modulated at this frequency. This periodic modulation allows the target profile information to be encoded in a frequency domain that is separated from low-frequency magnetic interference, enabling accurate detection even in electrically noisy environments.
Solution Approach 2:
The patent changes the frequency parameter of the bias magnetic field from DC (zero frequency) to AC (non-zero frequency). By modulating the bias field at a frequency higher than the interference sources, the target detection signal is shifted to a different frequency band where it can be easily distinguished from low-frequency magnetic interference through frequency-selective filtering or demodulation techniques.
2Object-affected harmful factors
If an AC bias magnetic field is used to shift the signal to a higher frequency band, then low-frequency magnetic interference is reduced, but the device complexity increases due to additional circuitry for AC bias generation and signal demodulation
Solution Approach 1:
The patent integrates multiple functions into a single sensor device: the AC bias magnetic field generator serves both to provide the modulating field for target detection and to act as a reference signal source for demodulation. The sensing element simultaneously detects target profile information and generates a signal that is inherently modulated at the AC bias frequency, eliminating the need for separate reference signal generators and reducing overall device complexity.
Solution Approach 2:
The sensor system is self-sufficient by using its own AC bias field as the reference signal for demodulation. The same AC bias coil that generates the modulating field also provides the reference frequency needed to extract the target profile information from the sensing element output. This self-service approach eliminates the need for external reference signal sources and simplifies the overall system architecture.
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 effectively reduces the impact of low-frequency magnetic interference, enabling more precise detection of target profiles and movement information, even in high-interference environments.
Implementation Method 1
an AC bias coil to produce an AC bias magnetic field capable of being modulated by movement of a ferromagnetic target with respect to the AC bias coil
Implementation Method 2
a magnetic field sensing element, such as a Hall element or magnetoresistive (MR) element
Implementation Method 3
circuitry including a demodulator to perform a demodulation of the magnetic field signal. When the magnetic field signal further includes an unwanted signal portion in a second frequency band based on a magnetic field interference, the demodulation results in the modulated signal portion being shifted from the first frequency band to a third frequency band and the unwanted signal portion being shifted to the first frequency band
Data Source
Figure 1
Figure 2A~2B
Figure 3
AI summary
A magnetic field sensor with enhanced immunity to external magnetic interference is presented. Included is a magnetic field signal generator and a demodulator. The magnetic field signal generator produces a magnetic field signal having a modulated signal portion in a first frequency band based on a sensed modulated AC bias magnetic field. The modulated AC bias magnetic field is produced by movement of ferromagnetic target relative to a bias coil when an AC signal is applied to the bias coil. When the magnetic field signal also includes an unwanted signal portion in a second frequency band based on external magnetic interference, demodulation performed by the demodulator results in the modulated signal portion being shifted from the first frequency band to a third frequency band and the unwanted signal portion being shifted to the first frequency band. The bias coil may be provided as part of the magnetic field sensor.