Adaptive Low-Pass Filter Circuit for Accurate Zero-Crossing Detection
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Solution Overview
Problem
Existing power quality measurement systems face challenges in accurately detecting zero crossing (ZC) events due to the introduction of phase shifts by low-pass filters and the impact of harmonics on signal precision.
Innovation Solution
A circuit and method that utilize a low-pass filter followed by a fixed all-pass filter and an adaptive all-pass filter to adjust phase and DC offset, ensuring a constant total phase shift and aligned zero crossings, thereby enhancing the accuracy of ZC detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a low-pass filter is used to remove noise and harmonics, then measurement precision is improved, but phase shift variability worsens
Solution Approach 1:
The system continuously monitors the phase shift between the original signal and filtered signal, and automatically adjusts the filter parameters to maintain a constant phase shift. This feedback mechanism allows the system to adapt to frequency variations while preserving measurement precision.
Solution Approach 2:
The patent dynamically changes the filter parameters (such as cutoff frequency or time constant) based on the detected frequency of the input signal. This parameter adaptation ensures that the phase shift remains constant across different operating conditions, resolving the contradiction between noise filtering and phase stability.
2Reliability
If a low-pass filter is used to reduce harmonics, then reliability is improved, but zero crossing accuracy worsens
Solution Approach 1:
The system introduces an intermediate phase compensation stage between the low-pass filter and the zero crossing detector. This intermediary component corrects the phase distortion caused by filtering, allowing the system to maintain both reliable noise rejection and accurate zero crossing detection.
Solution Approach 2:
The system performs preliminary phase alignment by adjusting the filtered signal's phase before it reaches the zero crossing detector. This preliminary action ensures that the zero crossing points of the filtered signal coincide with those of the original signal, maintaining detection accuracy while benefiting from noise reduction.
3Measurement precision
If hardware and software procedures are used to compensate phase shifts, then measurement accuracy is improved, but device complexity worsens
Solution Approach 1:
The patent replaces complex hardware-based phase compensation circuits with a digital signal processing approach. By implementing phase adjustment through digital algorithms in the filter circuitry, the system achieves accurate phase compensation with reduced hardware complexity and improved flexibility.
Data Source
AI summary
A circuit detects zero crosses in an input-signal and includes a low-pass-filter (LPF) receiving the input-signal and introducing a phase-shift dependent on the frequency thereof. Filter circuitry receives the output of the LPF, applies a fixed phase-shift thereto, and adjusts phase and DC-offset thereof based on control signals to produce a filtered output-signal. Control circuitry has a zero-crossing detector receiving the input-signal and the filtered output-signal, detecting zero-crossings of the input-signal and the filtered output-signal, asserting a digital zero cross signal at each zero crossing, and determining a phase-shift and DC-offset between the input-signal and filtered output-signal. The control circuitry has a controller generating the control signals, based upon the phase-shift and DC-offset, so a total phase-shift between the input-signal and the filtered output-signal is constant and there is a same duty-cycle between the input-signal and the filtered output-signal, providing for accurate zero-crossing detection.