Sensorless Magnetic Bearing Control Using Inductive Voltage Sensing
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Solution Overview
Problem
Conventional magnetic bearing control systems for turbo-molecular pumps face challenges in achieving high accuracy due to high-frequency noise amplification and difficulty in detecting current ripples, especially when using displacement sensors, which complicates the estimation of positional displacement and increases costs and size.
Innovation Solution
A magnetic bearing control apparatus that eliminates the need for displacement sensors by using an inductive element connected in series with the electromagnet coil, extracting voltage at prescribed timings, and calculating the time derivative of the current to estimate displacement, allowing for accurate control of the magnetic bearing with improved signal-to-noise ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a displacement sensor is used to detect positional displacement of the rotating body, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The electromagnet coil serves dual functions: generating the magnetic field for bearing support and acting as a sensing element for displacement detection. By monitoring the current required to maintain the magnetic field, the system self-diagnoses displacement without requiring separate sensors.
Solution Approach 2:
The electromagnet coil performs multiple functions simultaneously: it generates the magnetic field necessary for magnetic bearing operation and serves as the sensing element for detecting positional displacement through current monitoring.
2Measurement precision
If a differentiating circuit is used to obtain rate of change of current, then displacement estimation is enabled, but high-frequency noise is amplified making detection difficult
Solution Approach 1:
The patent converts the harmful high-frequency noise into a useful signal by recognizing that the ripple current generated by PWM switching contains displacement information. Instead of filtering it out, the system uses band-pass filtering to extract the displacement-related ripple components.
Solution Approach 2:
The system utilizes the periodic ripple current generated by PWM switching at known frequencies. By synchronizing detection with this periodic action and using band-pass filtering at the PWM frequency, the system extracts displacement information from the periodic variations in current.
3Productivity
If PWM switching amplifier is used for magnetic bearing control, then control efficiency is improved, but current detection becomes difficult due to switching noise
Solution Approach 1:
The system exploits the periodic nature of PWM switching by using band-pass filtering tuned to the switching frequency. This allows extraction of the ripple current signal that contains displacement information while rejecting other frequency components.
Solution Approach 2:
The system implements feedback control by continuously monitoring the ripple current and using this information to adjust the PWM duty cycle, thereby maintaining stable magnetic bearing operation despite the noisy switching environment.
4Device complexity
If displacement sensors are eliminated to reduce size and cost, then device complexity is reduced, but measurement precision may deteriorate
Solution Approach 1:
The electromagnet coil serves dual functions: generating the magnetic field for bearing support and acting as a sensing element for displacement detection. By monitoring the current required to maintain the magnetic field, the system self-diagnoses displacement without requiring separate sensors.
Solution Approach 2:
The patent replaces mechanical displacement sensors with an electrical sensing method that monitors current variations in the electromagnet coil. This substitution eliminates mechanical contact and complex sensor assemblies while maintaining measurement capability.
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 solution enables high-accuracy magnetic bearing control without displacement sensors, reducing size and cost while maintaining or exceeding the accuracy of systems with sensors, and allows for stable detection of current ripples across a wide range.
Implementation Method 1
an electromagnet which supports a control object in a state floating in the air
Implementation Method 2
an inductive element connected in series to a coil of the electromagnet; voltage extracting means which extracts a voltage applied to the inductive element
Data Source
AI summary
A magnetic bearing control apparatus and a vacuum pump which do not require a displacement sensor, which enable control with high accuracy, and which are small and low cost. A rate of change (di/dt) that is a time derivative of a current value Im flowing through an electromagnet varies in accordance with a magnitude of a displacement of a gap between a target member and the electromagnet. The rate of change (di/dt) can be obtained by detecting a voltage value Vs that is generated at both ends of an inductive element. Therefore, by detecting the voltage value Vs, the magnitude of the displacement of the gap can be estimated by calculation. Inductive elements are connected in series to electromagnets and the voltage Vs between the inductive elements is detected by the differential input amplifier. A single period of switching of a PWM switching amplifier is constituted by a current control period of the electromagnet and a displacement detection period for detecting the rate of change (di/dt). In addition, the displacement detection period is further constituted by a current increase period and a current decrease period which are certain periods of time. The current increase period and the current decrease period are equal to each other.