Research On Sensorless Position Magnetic Bearing

The magnetic bearing, which is much better than the traditional bearings, attracts much attention because it has a series of merits, such as non-contact, frictionless, low energy consumption and non-lubrication. At present mostly magnetic bearings use the position sensors to realize rotor position control of magnetic bearing system. In order to reduce the cost, magnetic force bearing axial size and enhance the system reliability, research and application of the sensorless position magnetic bearing is irresistible.The mathematical model of magnetic bearing is established, and the stability of magnetic bearing is analyzed at the base of analyzing the single degree of freedom magnetic bearing.Two methods of obtaining rotor displacement of sensorless magnetic bearing are mainly studied. The methods are small test-signal injection method and differential transformer method. The basal principle of small test-signal injection method is injecting a small signal to the input of the power amplifier. The alternating current component of high frequency signal is extracted from terminal voltage of coil. Through band-pass filter circuit, amplifier circuit, differencing subtraction circuit, demodulation circuit and low pass filter circuit, the direct current voltage proportional to the rotor displacement is obtained. The basal principle of differential transformer method is to distill mutual induct voltage signal from bias coil. Through rectification circuit and low pass circuit, the direct current voltage signal proportional to the rotor displacement is obtained. After carrying out the comparative analysis on two kinds of position self-sensing methods by simulation, the simulation results indicated that the small test-signal injection method obtains the displacement information of the rotor more accurately, and the design of position self-sensing circuit is easier than differential transformer method. Therefore the small test-signal injection method is adopted.After designing position self-sensing circuit of sensorless magnetic bearing, the controller and power amplifier of sensorless magnetic bearing are also designed. To carry out the simulation on the designed controller, the simulation results indicated that the designed controller has very good disturbance rejection ability. In order to enhance the performance of sensorless position magnetic bearing system, the linear power amplifier which is suitable to the small test-signal injection method is designed. Simulation results indicated that the designed power amplifier can make the output voltage of rotor position detection proportional to the position. It also validates the feasibility of detecting rotor's displacement with small test-signal injection method.