22nd International Congress of Mechanical Engineering (COBEM 2013) November 3-7, 2013, Ribeirão Preto, SP, Brazil Copyright c ⃝ 2013 by ABCM OPEN LOOP RESPONSE IDENTIFICATION DURING NON-STATIONARY OPERATION OF ACTIVELY CONTROLLED ROTATING SYSTEMS Patrick Felscher Felix Dornbusch Richard Markert Technische Universität Darmstadt, Fachgebiet Strukturdynamik, Petersenstrasse 30, 64287 Darmstadt, Germany felscher@sdy-tu-darmstadt.de, dornbusch@sdy.tu-darmstadt.de, markert@sdy.tu-darmstdt.de Rodrigo Nicoletti University of São Paulo, São Carlos School of Engineering, Trabalhador São-Carlense 400, 13566-590 São Carlos, Brazil rnicolet@sc.usp.br Abstract. One of the challenges in designing actively controlled rotating systems is finding the appropriated controller gains of the feedback system. Some strategies of finding the controller gains require that the open loop response of the system be known or identified experimentally. This can be done by exciting the system in a given rotating speed (stationary condition). However, the most critical situations that justify controlling occur during run-up and run-down (non-stationary conditions). In these non-stationary conditions, unbalance strongly affects the results and must be con- sidered in the analysis. In this work, one presents a methodology for identifying the open loop response of an actively controlled rotating system during non-stationary conditions. One takes advantage of the fact that, in these conditions, frequency changeswith time and can be used for exciting the system. Unbalance can be disregarded in signal processing by measuring the unbalance response during run-up and forced + unbalance response during run-down. Experimental results show the feasibility of finding the open loop response of the system after a single run-up and run-down opera- tion. Phase identification is critical and some precautions during the response measurement procedure are suggested. At the end, the obtained open loop response functions can be further used to find the controller gains of the system for non-stationary operation. Keywords: rotor dynamics, identification, non-stationary condition, control system, frequency response 1. INTRODUCTION Rotating machines are vital elements in industry and, for this reason, they must present not only high performance, but also high availability to avoid interruptions in the production flow. One way of increasing the efficiency of rotating machines is the attenuation/control of the vibration levels, specially the vibration that occurs in the shaft. Unbalance, misalignment, or external forces due to operational conditions are the most common contributing factors for high lateral vibration in rotors, and they can reduce performance and cause energy losses, fatigue, or even failure (Adams, 2010). However hard to eliminate, controlling vibration levels within acceptable margins is essential for the safe and reliable operation of rotating machines. In this context, actuators and sensors have been incorporated into rotating machines, and control systems have been developed (Keogh et al., 1995; Sun and Krondkiewski, 2000; Santos et al., 2004; Pinte et al., 2010). In literature, one can find different control techniques applied to vibration control of rotating systems, most of them based on traditional control strategies: PID, optimum and robust control. A thorough review of control system design for rotating systems is presented in Schweitzer and Maslen (2009). In the experimental implementation of such control systems, it is usually necessary to have a mathematical model of the system to design the controller. In these cases, the successful control of vibration depends on the quality of the adopted model, including the model of sensors and actuators. Considering that imprecise models can jeopardize the performance of the controller, model free design of controllers began to be investigated. The most common strategy for designing the control system not depending on mathematical models is based on the experimental identification of open loop frequency responses. In this case, the actuators can be used as exciters and open loop frequency response is obtained for the global system composed of the actuator system + plant + sensor system (Keel and Bhattacharyya, 2008). Hence, all dynamic information is embedded in the global open loop response function, and the controller can be designed with no further data. Regarding stationary operation (constant rotating speed), successful results were achieved for a rotating system with electromagnetic actuators whose flexible shaft supported a single disk (Buttini et al., 2011), and two disks (Buttini and Nicoletti, 2012). The most critical situations that justify vibration control in rotating systems occur during non-stationary operation of the machine (run-up and run-down). In this non-stationary condition, unbalance has a strong impact in the dynamic behavior of the system, and must not be overlooked. In this work, one presents a methodology for identifying the open loop response of an actively controlled rotating system during non-stationary conditions. One takes advantage of the ISSN 2176-5480 423