Magnetic Bearing Rotor Position Sensing with Asynchronous Sampling
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Solution Overview
Problem
Current position detection methods for magnetically levitated objects, such as those in turbomolecular pumps, face challenges due to varying phase shifts between excitation and sensor signals, leading to measurement errors that cannot be eliminated by synchronized sampling.
Innovation Solution
A device and method utilizing an asynchronous sampling frequency for the sensor output signal, where the sampling frequency is defined as (2M/2 ± 1) * excitation frequency, allowing for more robust and precise position detection based on the eddy current effect, and incorporating multiple sampling units with time offsets to enhance robustness against phase shifts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If synchronized sampling is used with excitation signal, then low-frequency noise is reduced, but position-dependent phase shift causes measurement errors
Solution Approach 1:
The patent changes the sampling frequency parameter from being synchronized with the excitation frequency to being asynchronous and higher than the excitation frequency. This parameter change allows the system to capture multiple samples per excitation cycle, enabling accurate position determination even when phase shift varies with position. The sampling frequency is specifically set to be at least twice the excitation frequency to satisfy the Nyquist criterion while maintaining asynchrony.
Solution Approach 2:
The patent employs periodic sampling at a frequency that is an integer multiple of the excitation frequency. This periodic action ensures that enough samples are taken during each excitation cycle to accurately determine the position signal despite the position-dependent phase shift. The periodic nature of both excitation and sampling creates a structured data pattern that can be processed to extract accurate position information.
2Device complexity
If phase shift is assumed constant for synchronized sampling, then peak value determination is simplified, but actual phase shift varies with object position
Solution Approach 1:
The patent uses excessive sampling by taking multiple samples per excitation cycle (sampling frequency at least twice the excitation frequency). This excessive action provides redundant data points that allow accurate position determination even when the phase shift assumption is incorrect. The extra samples compensate for the simplified control approach by providing enough information to accurately reconstruct the position signal.
3Measurement precision
If sensor coil inductance changes with object position, then position detection is enabled, but resistance simultaneously changes causing phase shift variation
Solution Approach 1:
The patent changes the sampling approach from synchronous to asynchronous with a higher frequency, which fundamentally alters how the position signal is captured. This parameter change in the sampling strategy makes the system robust against the position-dependent phase shift caused by inductance and resistance changes in the sensor coils. The asynchronous sampling captures the signal characteristics at multiple points, allowing accurate position determination despite varying phase conditions.
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 a more reliable and precise non-contact detection of magnetically levitated objects, particularly in turbomolecular pumps, by reducing the impact of phase shifts and improving measurement accuracy.
Implementation Method 1
The non-contact sensor system for detecting object movement along a respective measuring axis comprises a sensor coil unit with at least two sensor coils, which are subjected to a periodic electrical excitation signal
Implementation Method 2
as the object moves along the measuring track from one deflection to the next, the inductance of the first coil increases, while its resistance simultaneously decreases
Data Source
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AI summary
A device for detecting the position of an object that is at least partially magnetically levitated, in particular a rotor that is at least partially magnetically levitated, and especially a rotor of a vacuum pump, preferably a turbomolecular pump, comprises a non-contact sensor and an associated digital control and/or evaluation unit operating at a control frequency fc. The control and/or evaluation unit includes means for generating a periodic electrical excitation signal with an excitation frequency fe, which is defined by the following relationship: fe = M ⋅ fc, where M is a natural number. The non-contact sensor is configured to modulate the excitation signal depending on the object's position and provides a correspondingly modulated analog sensor signal as its output signal.The control and/or evaluation unit also includes means for A/D conversion of the modulated analog sensor signal into a digital sensor signal, by which the modulated analog sensor signal is sampled at a sampling frequency fs that is asynchronous with respect to the excitation frequency fe, for which the following relationship holds: fs ≠ fe/N, where N is another natural number independent of M. Furthermore, the control and/or evaluation unit includes computing means operating at the control frequency fc for calculating the object's position based on the digital sensor signal supplied by the A/D conversion means.