Experimental measurements of actively controlled bearing damping with an electrorheological fluid

Selection criteria and design evaluations of several types of bearing dampe rs with active control for application to aircraft engines were described i n a companion paper. A disk type electrorheological (ER) damper was chosen for further study and testing. The results of the tests and the final concl usions of the study are described in this paper Experimental results includ ing stiffness and damping coefficients are presented for the ER bearing dam per with two types of ER fluid 350 CS and 10 CS (centistokes) viscosity. Th e vibration attenuation performance of the ER damper was measured on a roto rdynamic rest rig in the form of free vibration decay, rotor orbits, and ru nup unbalance responses. The results show that the ER fluid with lower visc osity has the better characteristics for rotordynamic applications. It was found that ER fluids produce both Coulomb and viscous damping, If only the damping is considered, the Coulomb type is less desirable, but with active control it can also achieve control of rotor stiffness. A feedback control system was developed and applied to the ER damper with the objective of imp roving the overall rotordynamic performance of the rotor bearing system, co nsidering both vibration amplitudes and dynamic bearing forces. A ''bang-ba ng'' (on and off) simple control logic was found to work better in practice than more sophisticated schemes. The measured runup response of the rotor- bearing system with this control approximated the desired vibration respons e curves fairly well. The tests highlighted some of the practical considera tions that would be important for aircraft engine applications, such as the ER fluid limitations, the electrical power supply requirements, the electr ical insulation requirements, the nonlinear relationship between the voltag e and the damping, and the relative benefits of active control. Ir is concl uded that active control of bearing damping is probably nor a practical imp rovement over the passive squeeze film dampers currently used in most aircr aft gas turbine engines.