Effect of Control Algorithms on Magnetic Journal Bearing Properties

Abstract

Magnetic journal bearings are coming into increasing use in industry today. They are primarily used to replace either rolling element or fluid film bearings in rotating machinery. The major advantages are elimination of oil systems and associated seals, expected very long life, very low power losses, and great potential for vibration reduction. Disadvantages include lack of field experience, unknown reliability over a long time, high cost (so far primarily due to the small quantity being made for a given application), and advanced automatic control design required. This paper discusses the design of a magnetic journal bearing with four electromagnets arranged radially around a shaft to fully support a rotor. Each electromagnetic is connected to a controlling electronics circuit which regulates the current to the magnet. For the measurements presented here, only the top magnet was tested and the shaft was not rotating. Thus a single control algorithm was isolated from other effects. This paper compares two control algorithms with differing circuit band widths of 1.2 kHz and 50 kHz. The wider bandwidth algorithm produced approximately a sixfold increase in magnetic bearing stiffness and a much greater stable operating region compared to the lower bandwidth algorithm. Overall, the calculated effective stiffness and damping coefficients were within 20 to 30 percent of the measured values.

Robust H ∞ control for uncertain nonlinear active magnetic bearing systems via Takagi-Sugeno fuzzy models

“…Unfortunately, the AMB is a highly complex nonlinear system and presents challenging difficulty from the standpoint of a closed-loop control design. To overcome this difficulty, many control methods have been applied to this system, such as linear control [1,2], PID control [3,4], sliding mode control [5], feedback linearization [6][7][8], and adaptive control [9]. However, many of the existing methods which deal with nonlinearity of the AMB usually require complicated algorithms or are effective only in the limited small neighborhood of the nominal equilibrium point, since they utilize linear control techniques by linearizing the dynamics of the AMB.…”

A rotor unbalance response based approach to the identification of the closed-loop stiffness and damping coefficients of active magnetic bearings

“…On the theoretical analysis of stiffness and damping, Humphris et al [2] derived the equivalent stiffness and damping coefficients from the AMB system transfer function, Yu [3] and Hu et al [4] analyzed the static and dynamic characteristics of the AMB, and Wang and Jiang [5] proposed a method to estimate the bearing stiffness by using the control current ratio. These studies were all based on a simplified model, and the support parameters of magnetic bearings are identified through theoretical analysis, which, in the absence of experimental verification, often leads to inaccurate results.…”

Analysis of Stiffness and Damping Properties of Active Magnetic Bearing Using Cross Feedback Control

“…Hereafter, most scholars analyse the stiffness characteristics of magnetic bearing with the found of Haberman. Humphris, et al [4] derived equivalent stiffness and equivalent damping coefficient basing on the transfer function of electromagnetic bearing system. With the analysis theory and method based on the stiffness and damping characteristic of system transfer function Yu Liang, et al [5] established the theory and method for analysing the rotor dynamic characteristic of electromagnetic bearing, and validated it by developing and testing a practical oxygen turbine expander prototype.…”