Active vibration control for a free piston stirling engine with linear alternator FPSE/LA
New regulations introduced by the European Network of Transmission System Operators for Electricity (ENTSO) have brought further requirements for grid connected generators into action in 2013. The β-type Stirling engine (FPSE/LA) used for micro combined heat and power systems (MCHP) is a synchronous machine that is designed and tuned to operate at 50Hz ± 0.5Hz. This type of technology has to comply with the new regulations that imposes a wider operating envelop (47Hz-53Hz). This engine suffers from continuous self-induced vibrations caused by the reciprocating motion of a permanent magnet attached to its piston inside a linear alternator. Currently, the damping of the vibrations in the FPSA/LA is achieved by employing a passive tuned mass damper (TMD) tuned to damp vibrations at 50Hz. While passive devices provide a simple and a reliable way to tackle many vibration problems, there exists distinct performance limitations associated with the use of only passive devices. As for this particular application, the main limitation of the TMD in its passive form is its narrow bandwidth of operation that cannot cope with the new bandwidth. Consequently, this could expose the engine to physical damage and failure. Therefore modifications to the existing TMD have to be introduced. This research involved the design and development of an active tuned mass damper (ATMD) and the suitable control strategies using an electromagnetic actuator, namely a linear voice coil motor (VCM). Starting with a validated vibration model of the engine based on two degrees of freedom system (2-DOF), two control techniques, Feedforward/Zero-Placement control with relative/absolute position and Linear Quadratic (LQ) optimal control, were investigated with numerical simulation in the frequency and time domains. For the purpose of testing and implementation, a test rig featuring an electromagnetic shaker, a VCM, and a TMD besides an embedded system was assembled. An electromechanical model of the test rig was also developed and simulated with the integration of the control strategies. A set of experimental tests were carried out and the concept of active vibration control was successfully illustrated. In addition to that, an in depth investigation of the effect of time delays on the control methodology was conducted. The study resulted in the identification of a time delay margin where below, stability is guaranteed. Furthermore, a set of comprehensive equations of the power and actuator force requirements to perform active damping with a VCM based on any general 2-DOF system are obtained for both the feedforward and the LQ control strategies.
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