Mutual Inductance Current Sensor Harmonic Attenuation
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Solution Overview
Problem
Current current measurement systems using mutual inductance sensors face challenges in accurately measuring non-sinusoidal currents due to high harmonic content, leading to signal clipping and reduced dynamic range, especially when using digital integration methods.
Innovation Solution
A signal processing arrangement that combines analogue low-pass filtering with digital signal processing to attenuate harmonic components and compensate for frequency and phase responses, improving the dynamic range and accuracy of current measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If digital integration is used to recover current waveform from mutual inductance sensor output, then measurement accuracy is improved, but dynamic range is reduced due to signal clipping from high harmonic content
Solution Approach 1:
The signal processing is divided into two frequency domains: analogue domain for high-frequency harmonic attenuation and digital domain for low-frequency fundamental reconstruction. This segmentation allows each domain to handle specific frequency ranges optimally, resolving the contradiction between accuracy and dynamic range.
Solution Approach 2:
An analogue low-pass filter is introduced as an intermediary component between the mutual inductance sensor and the ADC. This filter mediates the signal by attenuating high-frequency harmonics before digitisation, preventing clipping while preserving the fundamental frequency for accurate digital reconstruction.
2Adaptability or versatility
If analogue integrator is used to recover current waveform, then dynamic range is maintained, but measurement accuracy deteriorates due to component tolerance and temperature instability
Solution Approach 1:
The analogue integrator is replaced with a digital integrator implemented in the digital signal processing stage. This substitution eliminates component tolerance and temperature stability issues inherent in analogue circuits, achieving high measurement accuracy while maintaining dynamic range through proper signal conditioning.
Solution Approach 2:
The integration function is transformed from the analogue domain to the digital domain. By changing the domain parameter from continuous analogue integration to discrete digital integration, the system achieves improved stability and accuracy while maintaining the ability to handle wide dynamic ranges.
3Reliability
If anti-alias low-pass filtering is applied before ADC, then aliasing is reduced, but harmonic signal attenuation increases
Solution Approach 1:
The low-pass filter is designed with a cutoff frequency that partially attenuates harmonics above the Nyquist frequency while preserving those below it. This partial action approach reduces aliasing for the fundamental frequency without excessively attenuating measurable harmonics, achieving a balance between reliability and precision.
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
The solution enhances the dynamic range of current measurements by reducing the amplitude of higher frequency signals through analogue low-pass filtering and digitally correcting for the effects of this filtering, thereby improving the accuracy of current measurements in systems with non-sinusoidal loads.
Implementation Method 1
Mutual inductance sensors, for example Rogowski coil sensors
Implementation Method 2
The signal from the current sensor is passed through a low pass filter having a second transfer function
Data Source
Figure 1
Figure 2
Figure 3~4
AI summary
Mutual inductance-type current sensing apparatus (1) is described which includes a mutual inductance current sensor (11) having a first transfer function. The apparatus (1) also includes a low-pass filter (12) which receives a signal from the current sensor (11). The low-pass filter (12) has a second transfer function configured to attenuate one or more harmonic components of the signal. The apparatus (1) also includes an analogue-to-digital converter (13) which receives and digitises a filtered signal output from the low-pass filter. The apparatus (1) also includes a controller (8) configured to process a digitised signal from the analogue-to-digital converter (13) using a digital processing chain configured to compensate for the frequency and phase responses of the first transfer function and the second transfer function.