1) Description of SPC-III System
SPC-III is designed to be a mission-scale vehicle; therefore its hull should have the ability to bear the pressure of water. The forward and middle sections of the hull are constructed with carbon fiber and aluminum alloy framework. The caudal section is an installed caudal fin thruster. Due to its complex shape, it is molded in the die using engineering plastics. SPC series biorobotics vehicle has two important characteristics: (1) the caudal fin thruster does not use flexible structure but expose Drive link assembly which has small volume and low weight in water. Thus conventional pressure hull and seal style can be adopted without considering the additional power consumed by flexible material for swinging deformation. (2)caudal fin is driven by two actuating DC motors, which is a comparatively mature robotics technology realizing accurate motion of caudal fin. Compared with other modes, DC actuating motors has higher electro-mechanical conversion efficiency. As is shown in Figure 1, the forward half of SPC-III is comprised of equipment and the payload capsule, with the IFLY40 Autopilot used to control Small UAVs installed at the front. IFLY40 was successfully applied to the 24th Chinese National Antarctic Research Expedition in December 2007, finishing the task of sea ice investigation . It can provide the data of posture, course angle, position and altitude and has five PID control devices. The only difference is that the barometric sensor is replaced with the water pressure sensor, which provides depth control of 0 ~ 50m. Below the Autopilot is an instrument for AD transfer and recording. At the back is the movement controller of caudal fin thruster, which will be introduced in details in the following part of this paper. The middle section is the dynamic cabin, which is composed of dynamic units formed by 28 pieces of 84-watt Li-poly batteries and providing permanent endurance for SPC-III. On the top of the dorsal fin is GPS antenna of the Autopilot.
2) Caudal fin thruster and its control law
As is shown in Figure 2, caudal fin which generates thrust forces is installed at the end of the Drive link assembly. The caudal fin is made of lmm-thick carbon fiber and imitates the shape of caudal fin of tuna, but with a lower aspect ratio. Dimension data is shown in Table 1. Link 1 and Link 2 are respectively driven by a Maxon RE40 24V motors trough reducers with reduction ratio of 30. Let flapping amplitude of the caudal fin be A, attack angle of relative stream be a, and 01, 02 be the respective output angle of the two motors through the reducer. And then the relationship can be expressed as:
In itinerant state, the performance effectiveness of caudal fin is affected by the following parameters . They are (1) Dimensionless flapping amplitude, which is defined as H=A0/c0, where A0 is the peak of flapping. (2) attack angle amplitude of caudal fin:a0. (3) phase difference of the former two: O. (4) Strouhal number, which is defined as St.=fA0/V, where. V is the speed of inflow. Motion law of caudal fin of caudal fin propulsion marine animals like dolphin and tuna can be expressed as:
Thus it is clear that accurate adjustment of the former three factors can be realized only by adjusting motion law of output0102 of motors. As the control system of caudal fin thruster, a 2-Axis motion coordinator is used generate the above motion law. As for the optimum range of the four parameters, previous work has derived preliminary conclusions [7-9]. Part of the results obtained by the self-propelled SPC-III in open sea will be shown in the experiments introduced in the following part of this paper.
As it is difficult to install two driving units in the narrow cone space of the caudal section, a very thin spur gear reducer is customized. At the same time, motors —reducer-sealing assembly are put in staggered arrangement to complete the assembly of caudal fin thruster. Thus it is impossible to add more torque and speed sensors.
Fig. 9. The mechanical sketch of caudal fin thruster e(t)=e2
Fig. 9. The mechanical sketch of caudal fin thruster
3) Propeller comparing AUV
The building of a AUV experimental platform used for comparison is actually to directly replace caudal fin thruster with screw propeller on SPC-III, while dynamic units, motion coordinator and amplifier remain the same. Thus difference factors are reduced. The selection and production of the propeller have got the help of China Ship Scientific Research Center (CSSRC). Open water efficiencyr|o of the propeller in five sections is predicted to be 0.67 . The structure of screw propeller is shown in Figure 3. Parameter comparison between it and caudal fin thruster is shown in Table 1.
Fig. 10. Structure diagram of the screw propeller
Caudal fin propeller
Area of caudal fin: S (mm2) Maximum chord length: Co (mm) Average chord length:
C (mm) Lead edge sweepback (deg.) Airfoil
Length of Link1,3 (mm)
Length of Link2,4 (mm)
Reducer Weight (kg.)
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