Externally pressurized air journal bearing has been widely used in high-speed spindle system and precision machinery because of its characteristics such as substantially low frictional loss, low heat generation and averaging effect leading better running accuracy. But, like other fluid film bearings, air journal bearing has self-excited instability which is commonly called "whirl instability". Journal speed at the onset of this instability is the limit speed of journal operation. So, to increase the operation speed of spindle, which is supported by air journal bearing, suppression of the whirl instability is a very important subject.
In this thesis, active control of bearing is proposed as a stabilization method of air journal bearing and results of theoretical and experimental investigations of the characteristics of actively controlled externally pressurized air journal bearing are presented. Linearized perturbation method, step jump method and nonlinear orbit method are selectively used to obtain the stability characteristics of active air journal bearings operating at zero steady-state eccentricity.
Proportional-derivative control and synchronous control are used for the control algorithm of active air bearing. In proportional-derivative control, the speed at onset of instability is raised for both proportional and derivative control of bearing. Proportional control increased the stability threshold without affecting the whirl ratio. But in the case of derivative control, stability threshold increase is accompanied by a parallel reduction of the whirl ratio. For synchronous control of bearing, the stability of air bearing is greatly influenced by phase difference of bearing with respect to the rotor, there exists optimum phase difference which gives maximum stability threshold. The whirl ratio also influenced by the synchronous control of bearing.
With respect to the running accuracy, actively controlled air bearing is investigated to overcome the defects of air bearing such as low stiffness and damping coefficients. Active thrust bearing and active journal bearing are designed and manufactured. The active air bearings are composed with air bearings, gap sensors, a controller, and piezo actuators. Proportional-Integral-Derivative controller is employed. Active air bearing is investigated numerically and experimentally. There is good agreement between the simulation and the experimental results.
These results show that active control of bearing can be adopted for the stability improvement method of air journal bearing, also the stiffness and damping characteristics and positioning accuracy of air bearing can be improved by means of active control of bearing.
