PCB Eddy-Current Sensor Thermal Drift Compensation
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Solution Overview
Problem
Conventional eddy-current sensors using coils wound with thick magnet wires face issues with thermal drift due to temperature-dependent resistive components in the coil impedances, affecting the accuracy of position measurements in non-contact position sensing applications, particularly in Active Magnetic Bearing Systems.
Innovation Solution
The use of Printed Circuit Board (PCB) coils with high resistive components, where the voltage induced in a sensing coil is measured instead of impedance, and a feedback control maintains constant excitation current or its derivative, minimizing the impact of temperature changes and simplifying the drive electronics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional eddy-current sensors use coils wound with thick magnet wires, then the inductive impedance components dominate, but thermal drift occurs due to temperature-dependent resistive components affecting measurement accuracy
Solution Approach 1:
The patent changes the physical parameters of the coil by using PCB traces instead of traditional magnet wires, fundamentally altering the resistive and inductive properties. PCB coils have higher resistive components relative to inductive components compared to traditional thick magnet wire coils, which changes the impedance characteristics and reduces thermal drift effects on measurement accuracy
Solution Approach 2:
The patent substitutes the traditional mechanical coil winding process with PCB fabrication technology. Instead of manually or mechanically winding thick magnet wires, the coils are created through PCB trace deposition, which provides more stable electrical properties and reduces temperature-dependent variations in resistance
2Reliability
If PCB coils with high resistive components are used, then thermal drift is minimized and measurement stability improves, but the voltage induced by excitation magnetic field increases requiring more complex drive electronics
Solution Approach 1:
The patent extracts and compensates for the unwanted voltage component induced by the excitation magnetic field. By measuring and subtracting this known interference voltage from the total sensed voltage, the system isolates the voltage component caused by eddy currents, thereby simplifying the drive electronics requirements while maintaining measurement accuracy
Solution Approach 2:
The patent introduces an intermediary measurement approach where the voltage induced by the excitation field is measured separately and used as a reference. This intermediary measurement allows the system to differentiate between excitation-induced voltage and eddy-current-induced voltage, reducing the complexity of the drive electronics
3Measurement precision
If the voltage induced by excitation magnetic field is compensated, then measurement accuracy improves, but the sensing process becomes more complex
Solution Approach 1:
The patent merges the excitation coil and sensing coil into a single integrated PCB structure. This combination allows simultaneous excitation and sensing operations, and the compensation for excitation-induced voltage is performed through signal processing rather than separate physical components, thereby improving accuracy without proportionally increasing device complexity
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 provides accurate and stable non-contact position measurements by isolating the sensor output from temperature-induced changes in the sensing coil resistance, enabling simpler and cheaper electronics while maintaining sensitivity and accuracy.
Implementation Method 1
The excitation coil is energized with an alternating current generating excitation alternating magnetic field which induces electrical eddy-currents in the electrically conductive sensor target
Implementation Method 2
These eddy currents in the sensor target in turn induce their own alternating magnetic fields which couple to the sensing PCB coil and induce voltage across its terminals in accordance with the Faraday's law
Data Source
AI summary
Noncontact measurements of positions of electrically-conductive objects is achieved by placing two conductive coils formed by traces on printed circuit boards (PCBs) in the proximity of the object surface, energizing one of the coils (excitation coil) with alternating electrical current and measuring the amplitude of the voltage induced on the terminals of the second coil (sensing coil). The alternating magnetic field generated by the current in the excitation coil induces eddy currents in the object, which affect the amplitude of the voltage induced on the terminals of the sensing coil. The sensing coil voltage depends on the mutual position between the object and the sensing coil, allowing the object position measurement. The excitation coil is integrated into a series LCR circuit driven by an output of an adjustable gain amplifier at the resonance frequency. The adjustable amplifier gain is constantly adjusted to maintain the sensor sensitivity constant.