Design of an autonomous underwater vehicle : vehicle tracking and position control.

The deepest studied point in the ocean is at a depth of about 11,000 m at the southern end of the Mariana Trench in the Pacific Ocean. Research and exploration in the oceans can be accomplished through the use of autonomous underwater vehicles.

Background Information and Research Objectives

Highly accurate sensors can be obtained to track the vehicle's position or movement, but they are very expensive. Another goal of this project is to build the capacity for AUV design and development in South Africa, as it is almost non-existent.

Problem Statement and its Significance

The goal of this research is to try to find low-cost solutions to these navigation problems and develop a small, low-cost AUV. It also discusses the obstacles and design steps to develop and successfully implement such a vehicle, and shows the components used to build the vehicle.

System Specifications

Vehicle x and y control: The vehicle control system must be able to control the x and y position of the vehicle with an error of less than 0.5 m. This control system is only required to navigate to waypoints and does not need to maintain its position.

Research Publications

Once a waypoint is reached, the AUV must proceed to the next waypoint.

Definitions, Assumptions, Limitations and Delineation

Dissertation Outline and Chapter Overviews

• Literature Review
• Mechanical Design
• Electronic Design
• Sensors
• Data Fusion
• Software Design
• Control System Design and Implementation
• Validation and Conclusion

Figure 5.5 shows individual measurements of the sensor's magnetic field in µT on the Y-axis, and the number of samples taken at the same sampling rate as before is shown on the X-axis. If the error is greater than 0.5 m, the set point is moved within 0.5 m of the current position.

Figure 2.1.: Floats and drifting/moored buoy locations in existing ocean observation network

Literature Review 11

Review of Existing Systems

It also has a movable mass inside to change the position of the vehicle's center of mass. The vehicle has a maximum operating depth of 200m and the newer version of the SLOCUM glider will have an operating depth of 1000m.

Figure 2.3.: The Puma AUV hangs from a deployment crane off the side of a ship [22]

Mechanical and Electrical Design

According to [9], hydrodynamic forces are forces that act against the movement of the vehicle in the water. This section described some of the mechanical and electronic considerations involved in the design process of an AUV.

Typical Sensor Systems and Guidance

According to [19], the accuracy of the magnetic heading sensor can be the main source of error in the operation of the entire navigation system. A study by [19] proposes the use of deterministic state estimators, where knowledge of vehicle dynamics is used to derive diffusion-based path estimators.

Vehicle Control

Other systems such as the one implemented in [21] show how a discrete time-delay controller can be used to control the yaw position and depth of the AUV. Another system discussed in [4] seeks to calculate the optimal kinematic controls and the corresponding curvature and rotation of the AUV path.

Vehicle Software and Underwater Communication

The study by [26] discusses some of the most common acoustic communication systems used for underwater data transmission. The most popular system used in AUVs is the Woods Hole Oceanographic Institute acoustic modem, or WHOI micromodem for short.

Conclusion

In Figure 5.6, the azimuth or angle from north is calculated on the Y-axis, and the sample number is again on the X-axis. The placement of the thrusters can be seen in the AUV mechanical design section.

Figure 3.1.: SolidWorks design of the AUV hull, protective frame and thrusters

Mechanical Design 29

Vehicle Manoeuvrability and Controllability

The AUV designed for this project has the ability to turn and maneuver in small confined spaces. The thrusters can provide up to 28 N of thrust in water, which is more than enough to accelerate and steer the AUV at comfortable speeds.

Vehicle Layout and Stability

Since the AUV is naturally stable in roll and pitch, there was no actuation or control in these degrees of freedom. Although the front and rear propellers can allow pitch to be activated, this was not implemented in this project.

Figure 3.5.: Component layout of the top level of the AUV

Thruster Torque Output

Conclusion

The rotation matrix is ​​used to represent the attitude of the vehicle, as described in [30]. The distances to known positions can then be used to calculate the vehicle's position.

Electronic Design 39

IO Board

The board has six PWM interfaces which can be used to control servos or DC motor speed controllers. It also contains eight analog channels that can be used for analog to digital conversion.

Actuators

• Thrusters
• Ballast System

Four signals are needed as inputs so that each piston tank can be controlled independently of the other. Therefore, each piston tank uses two digital input signals, and both signals determine the direction of the piston.

Power Distribution

A main power switch was added to the board to disconnect all power to the hardware if needed. A power indicator LED was also added, along with a fairly large current-limiting resistor, to indicate whether power is being applied to the system.

Communication System

System Integration

The sensors are not discussed in this chapter, because an entire chapter has been reserved for them.

Figure 4.4.: The complete hardware diagram of the AUV

Conclusion

The horizontal and vertical axes in the graph are the X and Y positions of the tool. This section will discuss the control system implemented to control the horizontal position of the vehicle.

Figure 5.1.: Six degrees of freedom of an AUV

Sensors 47

Vector Transformation between Reference Frames

A vector AP is defined as vector P in reference frame {A} where AP ∈ R3 and ABR is defined as a 3×3 rotation matrix in reference frame {B}relative to reference frame {A} where ABR∈R3×3. The rotation matrix between the global reference frame and the local reference frame is defined as shown in equation 5.3 to 5.6 according to [30].

Inertial Measurement Unit

The six-bit address is always a register address in the IMU's memory bank to be written to or read from. Also in a read operation, the second byte or command part of the data frame will be ignored by the device.

Magnetometer

In the figure, the Y-axis shows individual measurements of the sensor's magnetic field in µT, and the X-axis shows the sample number. The result is shown in Figure 5.5, and Figure 5.6 then shows the calculated direction of the vehicle against time.

Figure 5.3.: Example of the earth’s magnetic field vector

Water Pressure Sensor

The water pressure sensor used in this project is a LM series low pressure media isolated pressure sensor. The vehicle is held at that depth, using ropes, while measurements are taken by the pressure sensor.

Figure 5.7.: Depth measurement accuracy from water pressure sensor

Acoustic Transducers for Trilateration System

The accuracy of the trilateration system was verified using a laser range finder. The data can therefore be used to track the position of the vehicle very accurately and compare it with the results of the trilateration system.

Figure 5.8.: Trilateration simulation results

Wheel Encoders for Ballast System

Conclusion

This section will discuss the control system implemented to control the depth of the AUV. The design and implementation of the trilateration system was also a contribution to the electronic systems implemented in the AUV.

Data Fusion 65

Vehicle Yaw Estimation

The sensor package was then rotated on top of the potentiometer while data was collected. The values ​​given in Equations 6.7 through 6.13 were then used in Equations 6.3, 6.4, and 6.5 to calculate the estimate of the vehicle yaw iteratively as new data was collected from the sensors.

Figure 6.1.: Results from yaw estimation experiment

Vehicle Pitch and Roll Estimation

In the case of this implementation, u is taken as the gyro measurement and y is taken as the pitch or pitch calculation from the accelerometer data. In both figures, the black line is the slope calculation using the accelerometer data as explained above.

Conclusion

The purpose of the IO board is to provide low-level interfaces for sensors and actuators. Read Input Pins - This command is used to read the value of input pins.

Figure 6.2.: Results from pitch estimation experiment

Software Design 75

Player Robotic Control Software

Before moving on to the on-board computer software, this section will first discuss the robot control software used for this project. Control software is then implemented in the form of clients, which connect to the sockets provided by the server to access sensor data or send actuator commands.

Onboard Computer Software Implementation

It provides yaw position and the measurement is stored in the yaw position field of the position3d interface. The data stored in the standard interfaces is also constantly fed to the client.

Figure 7.3.: Structure of the Player implementation

Base Station Control Software

When any of these values ​​change, the new set point is sent to the AUV for control. Boxes are provided to show the current depth and the current X and Y position of the AUV.

Conclusion

This section shows the control system implemented for controlling the piston rod position of the ballast system. Because the AUV's yaw control is very sensitive, the gains used for the controller had to be quite small.

Figure 8.1.: Basic structure of a PID controller

Control System Design and Implementation 87

Ballast System Position Control

Since the motor speed of the piston tanks could not be changed, the controllability of the ballast system was limited. In this case, the band around the goal point was chosen as 0.25 % of the entire range of the piston rod to either side.

Depth Control

This control system essentially regulates the amount of water absorbed by the piston tanks, as the position of the piston rod determines the volume to be filled with water. In other words, if the piston rod position is set to 50%, it means that the piston will be filled with water to 50% of its capacity.

Heading Control

The controller will then add 360 degrees to the lower value of the two to make sure the controller is going in the right direction. The figure also shows the steady state of the controller when it holds its position for 80 seconds.

Vehicle Position Control

The position error has also been limited to reduce the controller output and ensure that the vehicle does not accelerate too quickly. The vehicle then drove to the final waypoint where it ended the experiment.

Figure 8.3.: Heading controller step response

Conclusion

Sensor technology has improved significantly and the accuracy of the cheap sensors will continue to improve. The low interfaces to sensors can also be shown as a contribution to the electronic design of the AUV.

Figure 8.4.: AUV way point navigation results

Validation and Conclusion 97

Conclusions

The study showed that it is possible to obtain relatively accurate AUV navigation using low-cost sensors. If the IMU is used for AUV navigation, it must be a high-quality IMU, as the error of a cheap IMU is still too large.

Summary of Contributions

Although it made a small contribution to the accuracy of the navigation system, it was not as effective as those discussed in the literature. Various navigation systems, control systems, and vehicle safety measures that have been implemented in the software can also be cited as contributions to the success of this AUV's software systems.

Suggestions for Further Research

The prototype to control the rotational movement was applied to an inertial navigation system equipped with an autonomous underwater vehicle. Proceedings of the International Foreign and Polish Engineering Conference, 2008. In Proceedings of the 7th Conference on Maneuvering and Control of Marine Vessels (MCMC2006) .