System architecture
The AUH system and main sub-components are shown in Fig. 1. The electronics module includes a waterproof housing for the camera, microcomputer, main control chip, motor driver, battery, inertial navigation and gyroscopic stabilization system. The housing is composed of two acrylic domes surrounding an aluminium ring and waterproofed with an O ring(passed 100m pressure test). Inlet wire channels are inserted on the side face of the acrylic ring. Outside the electronics module is the attitude adjustment system assembly, consisting of two turn plates with buoyancy material (white) and leads (black) fixed to them. Four equally-spaced vectoring propellers are mounted to the outer turn plate. The vectoring propeller assembly is composed of a steering motor and a propeller. The system is covered by a 3D-printed shell, the shape of which is hydrodynamically optimized. Two fluid channels (horizontal and vertical) are preset on the shell for each vectoring propeller.
Attitude adjustment
The attitude adjustment system assembly includes two turn plates that can rotate freely on ball bearings. Two buoyancy material cubes and leads are fixed to each turn plate. By rotating the turn plates, the centre of gravity and centre of buoyancy can be changed, and the attitude of the AUH can be adjusted (Fig. 5): (i) At the initial position, the buoyancy cubes and leads are aligned in a staggered manner, and the AUH is in a horizontal attitude. (ii) The inner turn plate rotates 180 degrees relative to the initial position;, the centre of gravity is on one side of the AUH, while the centre of buoyancy is on the other side, and the posture of the AUH becomes vertical. (iii) The inner turn plate rotates 90 degrees relative to the initial position, and the posture of AUH becomes vertical. The propulsion direction of the vectoring propeller is 45 degrees.
Vectoring propulsion
Four vectoring propellers are equidistantly arranged on the outer ring of the AUH. Integrating posture adjustment with vectoring propulsion, the AUH is capable of accomplishing three-dimensional movement with a zero turning radius, making it the most agile underwater vehicle. 1) Horizontal movement. The AUH adjusts to a horizontal posture, and two opposite propellers propel forward and backward at the same speed. 2) Vertical movement. For large-scale vertical movement, the AUH adjusts to a vertical posture, and two opposite propellers propel forward and backward at the same speed. For fine-tuned movement, the AUH adjusts to a horizontal posture, and the four propellers propel forward and backward at the same speed. 3) Oblique movement. The AUH adjusts to an oblique posture, and two opposite propellers propel forward and backward at the same speed. 4) Steering. Two opposite propellers propel the AUH forward and backward at different speeds. 5) Spin turn. Two opposite propellers propel in reverse directions.
Acoustic positioning
The positioning system of the AUH consists of an inertial measurement unit (IMU), an inverse ultra-short baseline (I-USBL) system and depth a sensor (Fig. 6). I-USBL is a new system for underwater acoustic positioning. Normally, the transmitter of the USBL is placed on the underwater vehicle, and multiple receiving unit arrays are placed on the ship/shore base station. After receiving signals sent from the underwater vehicle, the ship/shore base station determines the location and attitude information. Additional acoustic communication is compulsory for the underwater vehicle to acquire its own position. This paper presents an inverse ultra-short baseline system in which a six-element receiving array on the AUH and one transmitter on the base station (the helipad) are used. The AUH vehicle can obtain positioning information directly from the receiver module rather than waiting for conventional USBL results transmitted from the acoustic modem on the surface vessel. Provided accurate clock synchronization on both the transmitter and receiver, the AUH could range itself once the transmitted signal arrives while being acoustically passive. In this case, the acoustic communication equipment can be omitted, and the passive receiver array on the AUH would consume much less energy.
The acoustic transmitter is housed in the underwater helipad, which incorporates an embedded processor, a power amplifier and a synchronized clock into an aluminium cylindrical waterproof cabin (ϕ202 mm*352 mm). The pre-designed signal is generated by a digital signal processor (DSP, TMS320C6748 from Texas Instruments) and transmitted through a field programmable gate array (FPGA, Spartan 6 from Xilinx) triggered by the rising edge of a synchronous PPS clock. We chose a 200-Watt class-D power amplifier placed ahead of the spherical omnidirectional underwater speaker to ensure a reasonable effective propagation zone. The pulse-per-second (PPS) signal of the synchronous clock is pre-calibrated using a GPS antenna above the water and kept synchronized underwater utilizing a chip-scale atomic clock (CSAC)(35).
The acoustic receiver consists of a six-element circular hydrophone array(57,58), a front-end amplifier and an embedded processor that incorporates a DSP and FPGA. In contrast to the transmitter scenario, the hydrophones of the receiver convert the arrived acoustic signal to a measurable electrical analogue signal; subsequently, the front-end amplifier delivers a voltage-level signal to a high-speed analogue-to-digital converter (AD7606) for sampling. The FPGA continuously keeps sampling triggered by the receiver PPS clock synchronized to that on the transmitter.
The transmitted signal is a pre-recorded 10 ms, 8–14 kHz linear up-chirp in consideration of the propagation distance and auto-correlation coefficient. The transmitter and receiver transmit and receiver signals at the same moment, which is the rising edge of the PPS. There is a meaningful time delay in the received signal standing for time of flight (TOF) that can be determined by a correlation process using known signal characteristics in both the time and frequency domains. Provided accurate conductivity, temperature and depth, the sound velocity can be determined and used to range the distance between the transmitter and receiver.
Azimuth is an essential parameter in underwater localization in addition to the distance mentioned above. To precisely estimate the azimuth, we set a horizontal circular 6-element array with hydrophones uniformly spaced in a diameter of 130 millimetre, whose geometry suits our disc-shaped AUH well. Beamforming is widely used for azimuth estimation. We use the deconvolved conventional beamforming (dCv) algorithm, which can achieve a narrow beam width with low sidelobes, providing a high directivity factor (DF).
Optical guided docking
In the far end, the AUH determines its position with an acoustic transducer and navigates towards the helipad. On approaching the helipad, the AUH gradually adjusts its posture to align with the target orientation. In the near end, the AUH tracks the navigation lights with a machine vision algorithm to adjust its position and posture precisely and dock with the helipad successfully.
When the distance between the AUH and helipad is within a given value, the AUH switches to visual navigation mode. The camera will first detect whether the light source is within the camera view. If the light source is not found, the AUH will keep moving to obtain a large field of view for a second detection. If the second detection fails, the AUH determines that the helipad is still at a distance away will switch back to acoustic navigation mode until the light source is finally detected.
Once the light source is detected, a program written in Python will call the camera to capture an image frame. Image pre-processing is conducted afterwards, including de-interference, noise reduction, sharpening, de-distortion and binarization.
According to the image processing algorithm, the features of the image are extracted and analysed to obtain the relative deviation angle and pixel distance between the AUH and the helipad. These processing results are sent to the STM32 through the serial port. The propellers are modulated according to the calculated deviation angle and distance through closed-loop PID control. Once the docking conditions are met, the propellers adjust the AUH to vertical attitude and push the vehicle downwards into the helipad(See Fig. 7).
Underwater Helipad
The helipad is the home port of the AUH. The supporting frame of the helipad is made of 980 steel pipes, 2.5 m in length and width, and 1.2 m in height. The main external frame of the support is the main bearing structure; it is welded with steel pipe with an outer diameter of 50 mm and a wall thickness of 5 mm. The internal supporting steel pipe is a square pipe with a wall thickness of 5 mm and a diameter of 50 mm. The helipad is a truss-like structure that fully utilizes its material strength with a minimal main frame weight. The helipad is small on the top and large at the bottom, which increases its standing stability. Its upper end is equipped with a guiding light and an acoustic transducer, and the lower end is equipped with mounting brackets, an electronic control cabin, a standby power supply cabin, and an underwater camera. The cabins are made of 6061 aluminium alloy. When the AUH docks with the helipad, the lander lands on a porous mounting plate.
After the AUH docks with the helipad, a locking device is activated, and wireless charging starts. Meanwhile, cruising data is uploaded, and a new task is downloaded to the AUH. A compact wireless power transfer system was built to enable contact-less power transmission from the helipad to the AUH. The WPT system, consisting of a DC voltage source, H-bridge inverter, magnetic coupler, compensation network, rectifier, capacitive filter, and controllers, is separated into two isolated parts, i.e., the primary side and the secondary side, as shown in Fig. 8. On the primary side, the H-bridge inverter outputs high frequency AC current to the primary side coil, establishing a high-frequency magnetic field. The high-frequency magnetic field is transferred to the secondary side coil through magnetic coupling, and then the secondary side rectifier converts the AC voltage to DC voltage, which is finally used to charge the battery with constant current output. As the equivalent resistance of the battery varies throughout the charging process, the WPT system should be able to maintain a constant output current and constant input current phase to retain stable and efficient operation. An LCCL compensation topology enables the system to output a constant current throughout the charging process.
A mechanical guidance frame ensures alignment of the transmitter and receiver coils. The electromagnetic coupling structure is shown in Fig. 8. As the acoustic receiver array is placed at the bottom of the AUH, the receiver coil is placed a distance away to reduce electromagnetic interference and increase efficiency.
Open-sea experiments
We tested the ultra-mobility of AUH in the open ocean, in which AUH takes typical actions like spin, vertical rotary, vectoring propulsion and hovering at any attitude in a complex underwater environment. Three dives were conducted over the course of 2 days, exploring in the Luhuitou Bay in Sanya, China. This environment offers complicated seabed topography with varying tidal conditions, allowing AUH to be evaluated in real-world conditions.
The AUH conducted about 30 min of continuous motion during each dive, at an average depth of 8 m and a maximum depth of 12 m. All of these tests evaluated the effectiveness of the AUH in ultra mobile motion control. The AUH’s trajectories were recorded by two divers using GoPro from distance of several metres.