Active Bearings

The use of actuators offers the possibility to automate former manual activities and electrify passive mechanisms. This can, for example, increase the efficiency of the application respectively reduce the emitted noise. The focus of our group is the efficient integration of actuators throughout incorporation of knowledge regarding the passive system. We therefore are conducting research on the overall understanding of passive systems in order to develop actuated systems with far exceeding performance compared to the one of a purely passive system.

The consideration of active measures already at the beginning of the product development phase does not only have the potential of replacing passive measures, but additionally results in the possibility of generating an improved overall system behavior. This can be reflected on costs, weight and efficiency: on the one hand passive measures are discarded and on the other hand tasks can be effectively fulfilled by active systems, thus allowing for reduced requirements for the design of the basic passive system.

The knowledge about the basic passive system is the most important factor for the development of an active system, when aiming for an optimal solution of a specific problem. That is why the research group Actuated Systems focuses on fundamental research regarding system theory knowledge besides their activities on active measures. This covers for example the description of vibrations in gearboxes and rotors, and identifying excitation sources and their influence on fatigue or failure. Throughout the consideration of the gained overall system knowledge within the design process and the control of an actuated system, a more efficient and performant system can be obtained compared to using standard methods.

Piezo bearings

Structure of an active piezo bearing

Piezo bearings are using piezoelectric actuators, which can shift the rotor highly dynamically. This leads to a change of dynamics of the rotor system. The general structure of an active rotor is depicted here. The actuators are kept in place during operation using prestressing springs. The bearing forces are measured with sensors at the actuators locations (collocated) and are used for different control approaches.

Different control approaches are investigated at IMS for piezo bearings. The most challenging part is the change of system properties during operation, which is caused by strong gyroscopic effects.

Until now many different control approaches such as

  • PDT1, Integral Force Feedback (PT1)
  • H2, H∞ and gain scheduled H∞
  • Adaptive feed forward (e.g. FxLMS)

have been investigated numerically and experimentally. Three test rigs are available at IMS for experimental investigation of control algorithms.

For the past years our main focus was on the implementation of control strategies. However, to make piezoelectric bearings interesting for real-world applications outside the laboratory, the required hardware has to be considered as well. Thus, we are currently exploring and developing an active bearing, which can be treated as a standalone machine element without the need of special knowledge on the technology. Furthermore, a significant reduction of costs is targeted by integrating former control aspects into the hardware functions.

Magnetic Bearings

In active magnetic bearings, electromagnets are used to support the rotor without contact. Compared to conventional roller and friction bearings, these have the advantage that they operate without friction, wear and lubricant. They also require little maintenance and are suitable for very high speeds. The structure of such bearings essentially consists of an electromagnet, the rotor, the sensors for position detection and the control and power electronics. To prevent damage to the system in the event of bearing failure, safety bearings are also required.

Due to their lower load capacity compared to conventional bearings, the necessary position detection as well as the safety gear and peripherals, magnetic bearings require a large amount of space and have high acquisition costs. In terms of rotor dynamics, magnetic bearings offer many interesting possibilities of influence, such as active unbalance compensation, adaptation of the system dynamics to the operating point, determination of the rotor's balancing status and system fault diagnosis.

Sensorless magnetic bearings are being researched at the IMS. The position of the rotor is estimated from the system parameters of the magnetic bearing. This makes it possible to dispense with position sensors, which results in a considerable cost saving potential and eliminates the required installation space for the sensors. In addition, the integration of the position detection into the magnetic bearing provides the control technology important property of collocation.

Previous research focused on the implementation of an estimation method that can be used for different magnetic bearing geometries. This estimation method is limited to small bearing sizes due to the non-linearity of the magnetic materials. The aim of further research is to improve the quality of the estimation method so that the use of the method is no longer limited by the bearing size.