Table 1. Dimensions of ISiMI 3.2 Mechanical system
The hull size of the ISiMI AUV is constrained by the space for the onboard instruments, and the hull shape is constrained by the AUV's hydrodynamic characteristics. The standard hull length and diameter are 1.2 and 0.17 m, respectively, considering the installation of the onboard equipment, which will be described in the following sections. The hull shape of the ISiMI AUV was designed based on the Myring hull profile equations (Myring, 1976), which are known as the best contours for minimizing the drag coefficient for a given ratio of body length and diameter. The dimensions of the ISiMI AUV are shown in Table 2. Its hull contour is shown in Figs. 3. Its mid-section is a pressure housing made of aluminum, and its nose section is a flooding hull made of polyurethane. Its tail-section tube, which includes a mounting jig for a BLDC motor and three linear actuators, is covered by a buoyant material. The NACA 0012 cross-section was adopted for the control plane. The design parameters are listed in Table 3. Linear stepper motors were chosen for the fin actuators. The pair of rudders is controlled by a linear stepper motor, and the two stern elevators are independently driven by two linear stepper motors. Assuming the maximum torque required by the control fin is 10 kg cm, including the torque margin for mechanical loss, and assuming that the distance between the fin shaft and the driving axis is 5 cm, we chose a 25N stepper motor for each elevator and a 50N motor for the rudders. The capacity of the thruster motor was estimated based on the drag equation, described as wherein Fd is the form drag acting on the hull, Cd is the drag coefficient, Af is the maximum cross-sectional area of the hull, u is the advanced speed, and p is the water density. The required power capacity was obtained using:
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