Experiment on propulsion and maneuverability

1) Measurement system

The test system of the complete SPC-III is shown in Figure 4. IFLY40 Autopilot provides it with perfect telemetry function. This paper is focused on power of the propeller, as well as speed, maneuverability of the vehicle. All collected data are added into the protocol through Autopilot before they are sent to the ground station software. The transmission speed is adjustable within the range of 1~10 frames.

Fig. 11. SPC-III AUV electrical system

• Vehicle velocity measurement. IFLY40 Autopilot possesses various navigating modes such as UAV and RPV, which provides advantage for the vehicle to keep course or route for a long time to have tests. Therefore, this paper directly takes the longitude-latitude co-ordinate sent back by Autopilot as the original data to calculate distance and speed. Positioning of the GPS has accuracy as high as 2.5m(CEP). Comparatively accurate average speed can be achieved if distance long enough is used for calculation. In this paper, data of 200 seconds are used for the calculation of distance and speed.

• Power measurement. A 16-bit 8-channel A/D embedded computer is adopted for the measurement of power. Sampling frequency is 100Hz. Integrating Calculation over power is carried out every second. Through Autopilot, power obtained is sent to Ground Control System (GCS) software, which records the data frame. Under the modes of Vehicle velocity measurement and power measurement, transmission speed of Autopilot is one frame per second. Similar to Vehicle velocity measurement, average value is also got by calculating data of 200 seconds under power measurement. Nodes of current and voltage measurement are set on the circuit through which the batteries supply power to the amplifier; therefore the power measured is all the power that consumed by the propeller.

• Yaw rate measurement. Under the mode of maneuverability measurement, the speed of Autopilot increases to 10 frames per second. IFLY40 adopts data fusion algorithm to synthesize outputs of IMU and electronic compass into a course angle that is hard to be disturbed and can cause drift.

2) The comparison of propulsion performance

From the 9th to the 13th of October, 2006, Comparative experiment on SPC-III and its propeller was carried out on the coast of Qinhuangdao which is located in the west coast of the Bohai Sea. As is shown in Figure 5, the experiment was carried in calm offshore waters at ebb time every day. Firstly, static power and zero-load power of the propeller were measured. The results show that the power of caudal fin and screw propeller at static state are respectively 7 Watt and 3.5 Watt, correspondent to the static power of two amplifiers and one amplifier. Zero-load power refers to the motion power of the propulsion system except caudal fin and screw, including all transmission loss. At this stage, because of more complicated mechanical structures, the power of caudal fin thruster is higher than that of screw propeller. The former is 25 watt at 2.5Hz, while the latter is 18watt at the speed of 10 turnings per second.

Fig. 12. SPC-III biorobotics AUV (the right one) and the Comparison AUV (the left one) on the beach

Measurement in water. Firstly, the posture of the vehicle was adjusted to level. To ensure that GPS could receive stable signals and that the wireless data transfer device could work in good condition, draft was adjusted until GPS was 0.2m above water. To avoid influence from the control system, instead of starting the course control function of the Autopilot, the neutral position of caudal fin flapping or the deviation angle of rudder were adjusted manually to keep the vehicle in a linear trajectory. Note that the rudder is disabled when caudal fin thruster is used.

The measurement of power and Vehicle velocity was carried out continuously. After Autopilot was set with new frequency, rotation speed and flapping parameters through Ground Control Software GCS300, the vehicle was meticulously adjusted to keep it in a linear trajectory. This stable status was kept for a few minutes to allow the GPS time recorded. Relevant measurement data was searched according to the GPS time for later data processing. As is shown in Figure 6, 7,8, the final test results indicate that at 2.5Hz, H=1.5, caudal fin thruster achieves maximum speed 1.36m/s. At that moment, the total power of the propeller is 161watt; screw propeller achieves maximum speed 1.4m/s at the speed of 10 turnings/sec. At that moment, the total power of the screw propeller is 165watt. The pitch of the vehicle became unstable when the flapping frequency or rotation speed was increased to increase Vehicle velocity. Another reason may be that resistance at water surface is higher than that underwater, so that vehicle is prone to pitch and oscillation or it may submerge into the water, so that GPS can not work.

The Principle of motion law adjustment for caudal fin flapping is to generate comparatively greater thrust forces and speed for the comparison with screw propeller. Because of deformation of the carbon fiber caudal fin, there is some inconsistence between the actually applied motion law and the ideal value obtained through earlier work. Deformation of the caudal fin may be explained as the increasing of attack angle and the lag of phase. Finally, the values adopted in the experiment are: a=10°, ®=45°. At that moment greater propulsion speed is achieved. Measurement was carried out respectively when H=0.75, H=1, H=1.5, and it is discovered that when H=1.5, the output power is higher at similar frequency, and power consumption is lower at similar speed.

Fig. 13. Power of screw propeller in Linear Motion, „zero-load" means the screw was not installed.

180 160 140 120 gT100 ^ 80 60 40 20 0

Fig. 14. Power of caudal fin thruster in Linear Motion, „zero-load" means the screw was not installed.

Fig. 13. Power of screw propeller in Linear Motion, „zero-load" means the screw was not installed.



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