A low cost eddy current displacement sensor for active magnetic bearings

Abstract

The McTronX research group of the North-West University is involved in active magnetic bearing (AMB) research. An AMB is a mechatronic system that levitates a rotating axis with electromagnetic forces. The group has successfully implemented radial and axial AMBs as well as a complete flywheel energy storage system operating at 22 000 rpm. Research is also done on using the actuator voltage and current to derive rotor position, also known as self-sensing, but these methods have not been perfected. Position measurement is very important in AMBs, since it is the main control variable. The literature indicates that the eddy current phenomenon is well suited for displacement measurement, since it is relatively noise immune and insensitive to process medium when a nonmagnetic and non-conductive substance is used. Printed circuit board (PCB) sensors must be considered when low cost is a requirement. The goal of this project is to design, simulate, manufacture and test a PCB based, low cost eddy current displacement sensor for AMBs. This project will focus on the sensor (probe) that converts the physical rotor movement to an electric signal. An evaluation platform, used to test the PCB sensor, is also designed and manufactured as part of this project. The first step in the sensor design is to establish a suitable software model. A finite element method (FEM) software package, Comsol® , is used to realise a FEM model of the sensor. This FEM model is used to simulate sensor behaviour in various configurations. The trends found in the FEM model results are used to compile a design flow diagram. This diagram is illustrated by applying it to single, double- and five-layer designs. The single- and double-layer sensors are manufactured and tested to validate the accuracy of the FEM model and design flow diagram. Close correlation between the practical and predicted results is found for the single- and double layer sensors. The sensitivity and working point rms voltage correlated exceptionally well for both sensors. Linearity does not correlate as closely due to the evaluation platform and circuitry but is still within acceptable limits when compared to other displacement sensors used in AMBs. This project laid the foundation for PCB sensor design in the McTronX group. A comparison between the different sensors showed that the double-layer sensor is the best choice in terms of cost and performance. It is concluded that the PCB displacement sensor presented in this dissertation is a viable low cost option for displacement measurement in AMBs.

The McTronX research group of the North-West University is involved in active magnetic bearing (AMB) research. An AMB is a mechatronic system that levitates a rotating axis with electromagnetic forces. The group has successfully implemented radial and axial AMBs as well as a complete flywheel energy storage system operating at 22 000 rpm. Research is also done on using the actuator voltage and current to derive rotor position, also known as self-sensing, but these methods have not been perfected. Position measurement is very important in AMBs, since it is the main control variable. The literature indicates that the eddy current phenomenon is well suited for displacement measurement, since it is relatively noise immune and insensitive to process medium when a nonmagnetic and non-conductive substance is used. Printed circuit board (PCB) sensors must be considered when low cost is a requirement. The goal of this project is to design, simulate, manufacture and test a PCB based, low cost eddy current is placement sensor for AMBs. This project will focus on the sensor (probe) that converts the physical rotor movement to an electric signal. An evaluation platform, used to test the PCB sensor, is also designed and manufactured as part of this project. The first step in the sensor design is to establish a suitable software model. A finite element method (FEM) software package, Comsol, is used to realise a FEM model of the sensor. This FEM model is used to simulate sensor behaviour in various configurations. The trends found in the FEM model results are used to compile a design flow diagram. This diagram is illustrated by applying it to single, double- and five-layer designs. The single- and double-layer sensors are manufactured and tested to validate the accuracy of the FEM model and design flow diagram. Close correlation between the practical and predicted results is found for the single- and double layer sensors. The sensitivity and working point rms voltage correlated exceptionally well for both sensors. Linearity does not correlate as closely due to the evaluation platform and circuitry but is still within acceptable limits when compared to other displacement sensors used in AMBs. This project laid the foundation for PCB sensor design in the McTronX group. A comparison between the different sensors showed that the double-layer sensor is the best choice in terms of cost and performance. It is concluded that the PCB displacement sensor presented in this dissertation is a viable low cost option for displacement measurement in AMBs.