Control of a Flexible Rotor AMB Compressor Test Rig with Aerodynamic Cross Coupling

Many industrial companies are adopting magnetic bearings for high speed compressors at this time. A flexible rotor test rig of approximately 1.2 m in length was developed with four active magnetic radial bearings. It was employed to evaluate the control of destabilizing aerodynamic cross coupling stiffness Q in the center of the machine. The two end AMBs were used for support and one center AMB used to apply the cross coupled stiffness corresponding to a typical location of gas seal cross coupled stiffness sources. The fourth AMB was employed as a system exciter. Unfortunately for the compressor manufacturer and the end user, often the magnitude of the cross coupled stiffness changes with various compressor operating conditions. The test rig shaft operated up to about 18,000 rpm, through the first bending critical speed which was at approximately 14,000 rpm. m-synthesis control was used to achieve highly effective control and low vibration response with the selection of an averaged value of cross coupled stiffness Q and a set of useful system uncertainty weighting functions when the controller does not have exact advance notice of the cross coupling stiffness amplitude. Details of the application of the mu synthesis controller implementation are presented. A physical approach to modeling the Q uncertainty involved selecting an expected nominal value of the real part of the first rotor system eigenvalue for the rotor first bending mode with an expected range of uncertainly above and below that value. Then suitable weighting functions were developed for the mu synthesis controller design. The measured sensitivity function was below 3 at 7,000 rpm when the rotor was subjected to high cross coupled stiffness at the center.