stage multiple sources and then a diesel engine will be fitted to the test rig. Four hydraulic actuators, which were used as
secondary sources and could each generate a force of 8.9 kN, are located symmetrically between the raft and the hull,
as shown in Fig. 1(b) and Table 2. The whole system weighs about 8 tons.
The active vibration isolation is depicted in Fig. 2. Fig. 2(a) is a photograph of the system in which the primary shaker,
which was suspended on a crane, and the control instrumentation, can be seen. A block diagram of the control system is
shown in Fig. 2(b). The vibration of the system was measured using 8 accelerometers. Four accelerometers (Nos. 1–4) called
error sensors were used to measure the vibration of the hull structure at the positions where the four hydraulic secondary
actuators were located, and four accelerometers (Nos. 5–8) were placed on the raft at the positions of the hydraulic actuators
to determine the effect of the control system on the raft.
A B&K 3560D system with Pulse software was used as a signal source for the primary shaker and for subsequent
analysis of the resulting vibration. Four hydraulic actuators with a four-channel valve driver acted as the secondary
sources. A TMS320VC33 based DSP board with 4-channel A to D and 4-channel D to A converters was used as the
controller. The system also included four-channel high-pass and low-pass filters. The transducers and instrumentation
are listed in Table 2.
The signal used to drive the primary shaker through a power amplifier was also used as a reference signal for the adaptive
controller. Four hydraulic actuators were operated simultaneously by feeding the acceleration from their locations on the
hull structure (measured by 4 error sensors) back to the adaptive controller. The frequency responses between the electrical
input to one of the hydraulic actuators and the four error sensors outputs, which are called error paths in the active control
system, are shown in Fig. 3. It can be seen that, except at some frequencies close to the natural frequencies of the whole
system, in which there is strong coupling between the raft and the hull as described in [17], the response of the structure
at the point where it is excited (point response) is almost 10 dB greater than at the other positions (transfer responses).
This means that a decentralized control system could be implemented, reducing the complexity drastically. Four
independent adaptive x-LMS based filters [18], which are described by the following equations, were used as controllers,