Inductive Displacement Sensor Ripple Filtering via Synchronous Detection
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Solution Overview
Problem
Inductive displacement sensors used in magnetic bearing systems suffer from reduced accuracy and precision due to ripple generation in displacement signals, which lowers the control accuracy of magnetic bearings.
Innovation Solution
A displacement sensor system comprising a first and second coil unit, a sensor drive means, and a detection circuit that includes a synchronizing unit, sample-and-hold circuit, band pass filter, and low pass filter, where the coils are differentially connected and power source voltages have a 180-degree phase difference, allowing for improved frequency characteristics of the output signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If a low pass filter with low cutoff frequency is used to remove ripples in displacement signal, then ripple removal is improved, but the output displacement signal frequency becomes lower than drive frequency, reducing accuracy and precision
Solution Approach 1:
The patent applies periodic sampling action synchronized with the drive frequency. The detection circuit samples the differential signal at specific phases of the drive frequency cycle, effectively capturing displacement information while avoiding ripple frequencies. This periodic sampling approach allows the system to maintain high measurement precision without requiring aggressive low-pass filtering that would attenuate the signal frequency.
Solution Approach 2:
The patent converts the harmful ripple effect into beneficial information by using synchronous detection. The ripple frequency components, which are harmonics of the drive frequency, are actually useful for determining the displacement through phase-sensitive detection. By correlating the detected signal with the known drive frequency, the system extracts accurate displacement information while rejecting unrelated ripple noise.
2Measurement precision
If separate differential circuit is used to obtain difference between electrical signals from coils, then displacement detection is achieved, but device complexity increases due to minimum one differential circuit needed per displacement sensor
Solution Approach 1:
The patent merges the differential detection function directly into the coil structure itself. The first and second coils are connected in a differential configuration where their outputs are naturally combined, eliminating the need for separate external differential circuits. This integration reduces device complexity while maintaining the displacement detection capability through the inherent differential nature of the coil arrangement.
3Measurement precision
If half-wave rectification or full-wave rectification is used in displacement detection circuit, then displacement signal detection is enabled, but ripples with harmonic component of drive frequency are generated
Solution Approach 1:
The patent replaces the mechanical rectification process with an electrical synchronous detection method. Instead of using diode-based half-wave or full-wave rectification that inherently generates ripple harmonics, the system uses a detection circuit that multiplies the coil output signals and filters them through a low-pass filter with cutoff frequency higher than the drive frequency. This electrical approach eliminates the ripple generation problem while preserving displacement detection accuracy.
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 proposed solution enhances the accuracy and precision of displacement sensors for high-speed rotary bodies, thereby improving the control accuracy of magnetic bearings by filtering out unwanted frequency components.
Implementation Method 1
a displacement sensor which uses a differential signal due to a change in a mutual inductance according to a change in a position of an object is referred to as an inductive displacement sensor
Implementation Method 2
As a low pass filter having a low interruption frequency is used to remove the ripples, an output displacement signal has a frequency lower than a drive frequency
Data Source
AI summary
In a displacement sensor and a magnetic bearing system using the same, the displacement sensor includes: a first coil unit including at least one first coil; a second coil unit including at least one second coil differentially connected to one side of the at least one first coil of the first coil unit; a sensor drive means for supplying a first power source voltage and a second power source voltage having the same drive frequency to the first coil unit and the second coil unit, respectively; and a detection circuit for extracting displacement information from a differential signal produced by the first coil unit and the second coil unit due to a change in inductances of the at least one first coil and the at least one second coil according to a position change of a displacement measurement object.