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Vibration levels in the hull of the Collins class submarines as a result of
operation of the diesel engine are sufficiently high that a
further reduction is desirable to reduce the noise signature and subsequently enhance the performance. It is likely that at least part
of the vibratory energy that reaches the hull is being transmitted
through the engine mounts.
The purpose of this study is to investigate the effectiveness of using
active vibration control to reduce the vibration transmitted through
the engine mounts. The most appropriate cost function for in-situ use
of the control system is to minimize the motion of the intermediate
mass of the mount. The current work involves undertaking tests on an
experimental rig to verify that the control approach is feasible and
also optimize the actuator and controller configuration for the
particular application.
The experimental rig under investigation is shown in Figure 1, where the
excitation comes from a set of dynamic forces generated by inertial primary shakers attached to the upper panel. In practice, low order engine harmonics dominate vibration transmission into the hull. The engine signature was simulated here using several sinusoids with additional random noise.
Energy (both translation and rotation) is then transmitted to the intermediate mass
via the flexible support mounts. It is recognized that total six
(rigid body) modes of the intermediate mass (within the operating frequency range) are responsible for the energy transmitted from the
intermediate mass into the receiving base structure.
Figure
1 A photo of the experimental rig
The control forces are provided using inertial shakers which are mounted
on the intermediate mass, as shown in Figure 2. Since the intermediate mass has 6 degrees of freedom, at least 6 control shakers are required for effective control. Seven were used in the final configuration, with the additional shaker needed to retain symmetry. The shakers are
tuned to provide maximum force at the frequencies where the vibration
transmission from the intermediate mass is greatest.
Figure
2 A photo of the inertial shaker
Error signals
are detected using a set of accelerometers which are mounted on the
intermediate mass. These signals are then fed into an EZ-ANZ II ten channel
controller developed by Casual Systems as shown in
Figure 3.
Figure
3 A photo of the control system
Two control strategies are currently being investigated: kinetic energy control and modal
control. Kinetic energy control aims to minimize the sum of the
squared vibration levels from all the error sensors and modal control
on the other hand aims to sense and control each of the orthogonal
modes. This requires modal sensors and actuators which may be
achieved by summing either the outputs from the individual error
sensors or the inputs into the actuators.
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