Eddy Current Scanner Circuit Stabilizes Display via FIR Filter
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Solution Overview
Problem
Eddy current inspection instruments with rotary scanners face challenges in maintaining a stable display of inspection results due to changes in rotary scanner speed, causing 'backwards 6' and 'figure 8' patterns to change in size and shape, making it difficult for inspectors to accurately identify abnormalities like cracks or corrosion.
Innovation Solution
The implementation of a signal processing method using Finite Impulse Response (FIR) filters and a Hilbert transform to provide a 90-degree phase shift, allowing for independent control of phase from low and high pass filter parameters, and linking these settings to the rotary scanner's RPM speed, resulting in a constant signal amplitude and image size, regardless of frequency changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an IIR filter is used to shift the phase of frequency response, then the phase can be adjusted for signals below certain frequency, but the display pattern changes in size and shape when the rotary scanner speed changes
Solution Approach 1:
The patent transitions from using an IIR filter to an FIR filter, changing the fundamental parameter of the filtering approach. The FIR filter with Hilbert transform provides a frequency-independent phase shift of exactly 90 degrees, eliminating the frequency-dependent phase variation that caused the display pattern instability. This parameter change resolves the contradiction by maintaining stable display patterns across different rotary scanner speeds while achieving the required phase shift.
Solution Approach 2:
The Hilbert transform acts as an intermediary mechanism that provides the desired 90-degree phase shift without the frequency-dependent behavior of IIR filters. By introducing this intermediate processing step, the system achieves phase shifting functionality while maintaining display pattern stability across varying operating conditions.
2Productivity
If the rotary scanner speed changes, then the inspection coverage and productivity improve, but the signal amplitude and image size vary making it difficult to identify abnormalities
Solution Approach 1:
The patent changes the filtering approach from IIR to FIR with Hilbert transform, which provides a frequency-independent phase shift. This parameter change ensures that signal amplitude and image size remain consistent regardless of rotary scanner speed variations, allowing inspectors to maintain high productivity without sacrificing measurement precision or abnormality detection capability.
3Measurement precision
If the frequency response is adjusted using IIR filter, then signals below certain frequency can be phase shifted, but the display pattern changes into different shapes at different frequency ranges
Solution Approach 1:
The patent fundamentally changes the filtering parameter from IIR to FIR with Hilbert transform. This provides a uniform 90-degree phase shift across all frequencies in the pass band, eliminating the frequency-dependent shape transformations that occurred with IIR filters. The display pattern maintains a consistent backwards 6 shape across the entire frequency range, resolving the contradiction between frequency response control and display pattern shape stability.
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 stabilizes the display of inspection results, reducing the need for frequent adjustments and improving the visibility of abnormalities, enabling more efficient and effective bolt hole inspections by maintaining consistent signal amplitude and image size across varying scanner speeds.
Implementation Method 1
The implementation of a signal processing method using Finite Impulse Response (FIR) filters and a Hilbert transform to provide a 90-degree phase shift
Implementation Method 2
An eddy current probe uses an alternating current that flows through a wire coil and generates an oscillating magnetic field. If the probe and its magnetic field are brought close to a conductive material like a metal test piece, a circular flow of electrons known as an eddy current moves through the metal and generates its own magnetic field, which interacts with the coil and its field through mutual inductance.
Implementation Method 3
a circular flow of electrons known as an eddy current moves through the metal and generates its own magnetic field, which interacts with the coil and its field through mutual inductance
Data Source
AI summary
A rotary bolt hole eddy current inspection scanner using a differential eddy current probe, the circuitry of the scanner is embodied with a filtering circuit with three filters: FIR (Finite Impulse Response), a low pass filter, and a phase control filter (by means of a Hilbert transform). The result from a scan of a bolt hole is an output signal on an impedance plane exhibiting a “backwards 6” shape of stable size when the scanner changes its rotating rate significantly.