# Experimental Validation of a Gyroscope Wave Energy Converter for Autonomous Underwater Vehicles
## 4 Test Environment  
The prototype is designed based on the formulas described previously in this paper. The economic cost of actual at-sea or wave tank experiments is high. Therefore, the hardware-in-the-loop simulation can not only save costs but also obtain the general performance of the prototype before the actual marine test. Figure 3 shows the prototype system and experimental configuration, where the CAD and physical models of the GWEC are shown in Figure 3a,b, respectively. Figure 3: Prototype system and experimental setups: (**a**) CAD of assembly gyroscope; (**b**) GWEC physical prototype; (**c**) CAD of assembly experimental setups; (**d**) experimental environment. Figure 2: RAO for the float under waves. GWEC is mainly composed of gyroscope, speed increase and PTO. The flywheel rotates in vacuum flywheel housing at a high rotational speed in a single direction, thus minimizing air resistance. The function of the flywheel housing is to support the overall gyro structure and transmit its motion to the speed increase. The lower precession angular velocity must be increased to a high speed through the speed increasing device to meet the rated velocity requirement of PTO. Therefore, a four-stage closed gearbox with a transmission ratio of 1:108 was chosen in this paper, which has the advantages of small backlash and smooth transmission. In addition, the sides of the GWEC frame are covered with circular matrix holes, which can effectively reduce stress concentration and unnecessary weight, thus improving conversion efficiency. The main parameters of GWEC are shown in Table 1.  
In order to reduce the testing time and cost of GWEC at-sea experiments, a six-degree-of-freedom (6-DOF) test rig capable of simulating regular wave motion is developed in this paper to evaluate the prototype capability and validate the GWEC numerical model. All the 6-DOF of the AUV will be affected by waves in actual sea conditions. However, from the gyroscope perspective and combined with Equation (8), only pitch and roll will directly affect the power absorption of the GWEC. Pitching motion stimulates the gyro system to produce precession, which is the main reason for the power output. The roll, on the other hand, is the result of the precession torque acting on the output shaft, which will cause the floating body to rotate and generate unnecessary water interaction forces. Therefore, the experimental platform was designed to provide only the pitching motion for the GWEC. The CAD and actual experimental scenes of the prototype experimental platform are shown in Figure 3c,d. The technical parameters and rated operating conditions of the experimental platform are shown in Table 2.  
The workflow of the dry test is shown in Figure 4. The drive board is powered by the power supply to rotate the flywheel. At the same time, the hydraulic station controls the 6-DOF platform to simulate regular wave motion under the instructions of the motion controller. The angular velocity of precession generated by GWEC is transmitted to PTO through the speed increase to generate electric energy. Finally, the monitor displays the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Symbol** & **Values** & **Units** \\ \hline GWEC volume & \(V\) & \(\phi 400\times 450\) & mm\({}^{3}\) \\ Total weight of GWEC & \(m_{G}\) & 50 & kg \\ Weight of flywheel & \(m\) & 30 & kg \\ Moment of inertia of rotor in \(x\) axis & \(J_{x}\) & 0.211 & kgm\({}^{2}\) \\ Moment of inertia of rotor in \(y\) axis & \(J_{y}\) & 0.211 & kgm\({}^{2}\) \\ Moment of inertia of rotor in \(z\) axis & \(J_{z}\) & 0.372 & kgm\({}^{2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: GWEC parameters.