A FRETTING WEAR TEST RIG WITH TIME-VARYING NORMAL FORCE BASED ON CLOSED-LOOP CONTROLLED PIEZOELECTRIC ACTUATOR

Dry friction damping is one of the principal vibration suppression methods for structures in turbomachinery. The friction and fretting wear tests are important ways to study the damping performance and contact parameters evolution of dry friction dampers. The normal force is one of the most important factors affecting friction and wear characteristics. Under service conditions, there are noticeable time-varying components in the normal force due to the coupling of vibration in the normal and tangential directions. It was verified in previous studies that the harmonic components of time-varying normal force had a significant influence on frictional hysteresis behaviors. In this paper, a fretting wear test rig with accurate time-varying normal force is developed. It can output normal force with stable constant (0-300N) and time-varying (0-50N) components with given amplitude and phase lag in the fretting wear test process. The limit of working frequency is up to 300Hz. The key is to control the output force of the piezo actuator in a closed-loop manner, where the force amplitudes and phase are adjusted by PID controllers in the frequency domain. The control strategy is realized by a DAQ-USB input/output board and the corresponding program of LabVIEW. The conventional "producer-consumer" data flow structure and the array output mode are employed, ensuring the effectiveness of control at high frequencies. The test rig can monitor the wear depth online in real-time through a strain gauge integrated into the piezo actuator. Results show that the time-varying normal force has a significant effect on the wear depth.