# Gas Foil Bearing Technology Enhanced with Smart Materials
## 5 Applications of Piezoelectric Transducers to Gas Foil Bearings  
PZTs are made of smart materials that perform the electromechanical coupling via piezoelectricity [41]. Specifically, the direct and converse piezoelectric effects govern the behavior of PZTs. The former effect describes the electrical response of a piezoelectric transducer subjected to mechanical deformation via the phenomenon of electric charge induction. The later effect refers to the converse energy transformation realized via electrically induced strains. Due to the electromechanical coupling in PZTs, these devices may be used as either sensors (accelerometers, force and torque sensors, etc.) or solid state actuators. Hence, various applications of piezoelectric transducers have been successfully developed to enhance the properties of GFBs. Below, an overview on the bearings equipped with PZTs is provided. The work [30] presents an application of the piezoelectric active radial injection system mounted in the GFBs' bushing. The authors report the results of experimental tests in which radial forces generated by the injected lubricants assure the proper operation of the bearing being disturbed. Specifically, independent injection subsystems allow us to adjust the position of the shaft's journal to remain within the acceptable trajectory bounds after external excitation is provided via hammer impact. The amount of injected gas is controlled via PZT valves mounted in the bearing's sleeves. As shown, the developed GFB enables safe operation across the critical rotational speeds and allows for the convenient control of the bearing's dynamic response. This capability considerably reduces the risk ofthe GFBs' instability. However, setting the proper values of the controller gains may be a challenging task to finally take advantage of the elaborated capability. In reference to the above-presented research, the authors of the work [35] develop a gray-box model for an active GFB. In the presented solution, PZT valves are used to control the lubricant injection, and, hence, excite the GFB system. The experimentally validated GFB model allows us to represent the dynamics of the bearing system for a given range of rotational speeds. One of the main goals of the study is to provide a reliable tool for the identification of the parameters of the controllers dedicated for active lubrication [30]. Eventually, the demanded significant growth of damping coefficients has been achieved based on the identified GFB model. An application of piezoelectric composites, i.e., Macro Fiber Composites (MFC), to control the operational properties of a GFB is presented in [34]. Making the reference to the concept of adaptive bearing, the authors focus on the modeling aspects. A single specialized active segment (shell) of a GFB equipped with MFC is analyzed, making use of the laminate theory, Hamilton principle and, finally, the Ritz method to yield an approximate solution. The authors consider flexure joints between the GFBs' shells. Additionally, the results of the conducted shape optimization are presented for the parameterized model of a specialized shell. The optimization aims at achieving shell deformation close to the given bore shape during MFC activation. A comprehensive description of the operation of the MFC-activated components (curved flexible shells), including both experimental and numerical outcomes, may be found in [4; 31]. The elaborated model explicitly introduces electromechanical coupling present in MFC, applying the electric field accordingly to the direction of piezoelectric fibers. The conducted experiments have allowed for model validation. High agreement obtained between the results of theoretical and experimental investigations confirms the usability of the elaborated model. The concept of circumferentially distributed and radially oriented PZTs stacks in a GFBs' bushing is presented in [32]. The proposed active control of the bearing operation allows for adjusting the GFBs' clearance via PZT activation.