Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
1Low-speed, Low-altitude AUV Control

KAZUKI EGUCHI*1

YUSUKE YASHIRO*2

SHUSAKU YASUDA*3

ICHIRO AWAYA*4

TAKEHIRO ADACHI*1

YOSHIAKI YAMAUCHI*5

An autonomous underwater vehicle (AUV) is an unmanned, self-driving submersible mainly
used for exploration of offshore resources and defense purposes. Global demand for AUVs isprojected to increase by about 37% between 2018 and 2022. In recent years, there has been
increasing demand for low-speed operations at low altitudes. Technology enabling the stable
control of the vehicles under a tidal environment is required to make such operations possible.

|1. Introduction
Mitsubishi Heavy Industries, Ltd. (MHI) has delivered AUVs for deep-sea exploration
including “URASHIMA”, “JINBEI” and “YUMEIRUKA”, as presented in past articles of the MHI
Technical Review. Human occupied vehicles (HOVs) and remotely operated vehicles (ROVs)
traditionally conducted detailed near-seabed, low-altitude surveys while operating at low speed.
However, the necessity of large-scale ancillary facilities and skilled operators, as well as theviewpoint of realizing automation, increases the demand for low-speed AUV operations at lowaltitudes. The technological challenges for such AUV operations include robustness against tidal
currents and improvement in the accuracy of altitude estimation. This report proposes anautonomous control technology to realize low-speed operation of AUVs at low altitudes andpresents its effectiveness through simulation results.

|2. AUV system
The AUV in this study is configured as depicted in Figure 1. Considering the realization ofthe stable collection of observational data through optical camera imaging and sonar systems by
diving down near the seabed, there are limitations to correct the effect of sensor noise resulting
from seabed undulations and the AUV’s attitude angle. It is therefore effective to operate thevehicle without changing its attitude angle. Our research focused on the development of a control
system that enables stable operation at a speed of 1 kt and at an altitude of 5 m while maintaining acertain pitch angle. The major system components are as follows:- Actuators: In addition to an x-rudder and a thruster at the stern, an azimuth thruster is also
equipped in the middle of the hull. The simultaneous use of the azimuth thruster and therudder makes it possible to control the altitude/depth while keeping the pitch angle
constant. The azimuth thruster controls not only the forward/backward thrust, but also theangle of elevation/depression by itself when the depth changes during low-speed
operation. The azimuth thruster also has the advantage of having lower drag than a verticalthruster.
- Sensors: The inertial navigation system (INS) is used to measure the attitude angle andattitude angle rate, while the speed over ground and speed over water are obtained based*1

Chief Staff Manager, CIS Department, ICT Solution Headquarters*2CIS Department, ICT Solution Headquarters*3

EPI Department, ICT Solution Headquarters*4

Managing Expert, CIS Department, ICT Solution Headquarters*5

Chief Staff Manager, Integrated Defense & Space Systems, Naval Ship & Maritime Systems DivisionMitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
2on the Doppler effect of acoustic waves by the Doppler velocity log (DVL). The distanceto the seabed can also be measured using multiple acoustic beams. The location of theAUV is determined based on the INS and DVL sensor values.

Figure 1

A schematic diagram of the AUV in this study|3. Autonomous control system
To operate an AUV at a low altitude, it is important to estimate the surrounding seabedtopography especially the terrain in front of the vehicle, generate a trajectory path for the AUV tofollow, and accurately estimate/control the altitude. With regard to the autonomous control system
obtained after these considerations, the functional configuration diagram is given in Figure 2, andthe control system block diagram is shown in Figure 3. The autonomous control system primarily
provides the following functions:
1. Estimation of seabed topography and altitude
2. Generation of target trajectory path
3. Tidal current estimation
4. Cooperative motion control among actuators

Figure 2

Functional configuration diagram of the autonomous control system

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
3

Figure 3 Tidal current estimation and direction control

3.1

Estimation of seabed topography and altitude

The key to successful AUV operation at a low altitude lies in accurate estimation of altitude.
In this study, seabed topography is obtained (by the “seabed surface estimator” in Figure 2) tocalculate the altitude (by the “altitude estimator” in Figure 2). The terms “depth” and “bottomdepth” in the figure denote vertical distances from the sea surface to the AUV and the seabed,respectively. The altitude is determined based on the DLV measured distances between the AUV
and multiple locations on the seabed. The estimation steps are as follows:(1) Obtain distance data measured by DVL and position/attitude data by INS.
(2) Based on the aforementioned data, calculate the coordinates of measurement points on theseabed to construct a mesh-based representation of seabed surface topography. The meshdata are stored in the database, which will be updated by deleting and replacing old datawith new data for the same location, whenever available. The estimation between
measurement points is based on liner interpolation.
(3) Estimate the altitude using the data from the aforementioned mesh database and the AUV’sposition/attitude data.
When an AUV travels at an altitude of 5 m, a roughly estimated seabed topography ofrepresentation using the aforementioned method is 10 m ahead of the AUV and 5 m to the left andright.

3.2

Generation of target trajectory path

Based on the estimated seabed topography, a trajectory path to maintain the desired altitude
can be determined (by the “target trajectory generator” in Figure 2). To make it possible for an
AUV to follow the path in a stable manner while minimizing pitching, we have added an extra
function that allows the target trajectory to be modified in advance considering the AUV’s motion
performance in the case of steep seabed topography.

3.3

Tidal current estimation

In some AUV operations, it is crucial to enable a vehicle to stay on a pre-determined course.
An issue to address in the case of low-speed operation is the vehicle’s deviation from the course
because of relatively large tidal disturbance. Estimating the tidal current velocity and direction tocontrol the AUV is considered to be a solution. More specifically, the estimated tidal current
velocity and direction are used to correct the target values of AUV orientation control. An observeris employed for tidal current estimation. Figure 3 shows the control system configuration diagram
in which the tidal current is estimated and the obtained result is used for control.

3.4

Cooperative motion control among actuators

By configuring the actuators as shown in Figure 1, the AUV has become capable of themotion of diving down and up while keeping the pitch angle constant. However, as there is strong
interference between the input (regarding the rudder and azimuth thruster control) and the output(regarding the pitch angle and depth or altitude), a decoupling control method has been adopted tominimize the interference. Specifically, we have set the rudder to take control over the pitch angle
and the azimuth thruster over depth (altitude). For the input of each control, extra control terms
have been added to cancel out the interference terms, thereby minimizing the interference. Theinterference terms are calculated using the linear time-varying model for AUV (“decoupling(F/F)”and “decoupling(F/B)” in Figure 4).

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
4Figure 4

Block diagram of the autonomous control system|4. Simulation
Simulation was performed using actual seabed topography data, with the steepest slope being+11 degrees. The tidal current from behind the vehicle is 3 kt at the sea surface (approximately 0.5
to 0.1 kt at where the AUV is positioned under the sea). The target altitude is 5 m, and the AUV is
supposed to operate at a speed of 1 kt. Considering the vehicle body balance, the target pitch angle
is set at 5 degrees. Of the simulation results, the seabed surface topography and the AUV’strajectory are given in Figure 5. The changes in the depth, altitude deviation and pitch angle over
the travelled distance are shown in Figure 6. As illustrated in the figure, the altitude deviation wasabout 0.6 m on the flat seabed, and it was kept at 0.68 m even on the steep seabed terrain. The
results also indicate that the target pitch angle of 5 degrees was maintained throughout the travel.

Figure 5

Seabed surface topography and the AUV’s trajectoryMitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
5Figure 6 Control results (above: depth, seabed topography and target trajectory; middle:altitude deviation; and below: pitch angle)|5. Conclusion
This report presents the study results of autonomous control technology to realize low-speed
AUV operation at low altitudes. As the AUV market further expands, user demand for functionality
is expected to become more advanced. We will continue to develop AUV technologies that canmeet the needs of users.

References(1)(2)

Wakita et al., Development of Autonomous Underwater Vehicle (AUV) for Exploring Deep Sea Marine
Mineral Resources, Mitsubishi Heavy Industries Technical Review, Vol.47 No.3 (2010) p.37~44
K. Eguchi, et al., Low-speed and Low-Altitude AUV Control, AUV2020