The development of a radial active magnetic bearing / J.D. Nel
This dissertation presents the development of a radial active magnetic bearing (AMB). With AMBs the rotor of a machine can be suspended in the air without any direct contact between the stator and the rotor. This makes it a frictionless bearing and eliminates the need for lubrication. The AMB system implements a feedback control system to control the position of the rotor. The aim of this project is to develop a radial AMB with an air gap of 1 mm and a rotation speed of 3000 rpm. Through this project basic knowledge of magnetic suspension is gained and expertise is established at the Engineering Faculty. The model can be used for further studies and as a demonstration model to illustrate the concept of AMBs. The model constitutes one radial AMB and one conventional ball bearing supporting a rigid shaft. The AMB system constitutes 1) electromagnets, 2) power amplifiers, 3) position sensors and 4) a control system. Inductive sensors measure the air gap between the shaft and the stator in the vertical and horizontal axis. The sensor signal is fed back to a controller that provides a control signal to the power amplifiers. The power amplifiers control the current through the electromagnets that apply a force on the shaft. The shaft is then suspended in the air. An air pressure turbine is used to propel the shaft up to 3000 rpm. A homopolar AMB configuration is implemented using mild steel for the electromagnets. The four electromagnets used in the system are designed in terms of a required force. Linear power amplifiers are designed to activate the electromagnets and to eliminate possible noise problems on the sensors. Inductive position sensors are implemented producing a dc voltage proportional to the size of the air gap. dSpace® software is used to implement the controller. A position sensor value is read in through an analog-to-digital converter channel and subtracted from a reference signal for the position. The error signal is then the input of the controller. The controller sends a control signal via the digital-to- analog converter to the power amplifiers. A PID controller is created in sirnulink®. With the aid of dSpace® software the controller is downloaded onto the dSpace card. Different tests are performed to characterise the system. The step responses in both axes are measured and the percentage overshoots and settling times are determined. Impulse disturbance tests at different speeds are used to calculate the dynamic stiffness and damping of the system. Stable suspension was achieved with the final AMB system at rotation speeds of 3000 rpm. The maximum deviation was found to be less than 0.11 mm from the centre position. The settling time was less than 0.4 s and with no steady state error. The developed AMB system has a relatively low dynamic stiffness. Future studies can be done to find the effect that each PID parameter has on the dynamic stiffness. It is recommended that the controller be implemented on an embedded microcontroller to eliminate the computer and the dSpace® card.
- ETD@PUK