Advancements in Fault-Tolerant Control for Underwater Robotics

By Ashutosh RoyReviewed by Susha Cheriyedath, M.Sc.Aug 1 2023

The exploration of the vast oceans and their uncharted territories has been a long-standing human endeavor. To aid in this quest, underwater robotics has emerged as a vital tool for research, exploration, and various industrial applications. Autonomous Underwater Vehicles (AUVs) play a pivotal role in these endeavors, undertaking complex tasks in challenging marine environments. However, these AUVs are susceptible to faults and failures due to unpredictable sea conditions, which can jeopardize missions or lead to the loss of the vehicle.

To overcome these challenges, researchers have been focusing on developing Fault-Tolerant Control (FTC) systems. The primary objective of FTC is to enable AUVs to automatically compensate for component faults and maintain motion reliability, desired performance, and system safety. A review published in the journal Ocean Engineering explored the advancements and research progress in FTC technology for AUVs over the past two decades, enhancing the reliability and robustness of underwater robotics.

Different Approaches to Fault-Tolerant Control

Over the years, researchers have explored various approaches to FTC for AUVs. One notable strategy is Multi-Model Reconfiguring FTC (MMR-FTC), which involves selecting the most appropriate system or combination of models and corresponding control laws to achieve online controller reconfiguration when faults are detected. This approach has shown promising results in ensuring system stability and performance in the presence of faults.

Another approach is Actuator Thrust Reconfiguring FTC (ATR-FTC), where the desired thrust is redistributed among the remaining functional actuators to maintain the AUV's motion. This technique is particularly useful when the actuating system becomes faulty, and the AUV needs to compensate for the loss of effectiveness in the actuators.

Optimization-based FTC is another avenue of research where researchers use objective functions and constraints to optimize system performance under fault conditions. By formulating FTC problems as optimization issues, researchers can achieve desired goals, such as minimum energy input or fastest convergence speed, while compensating for actuator faults.

Enhancing Underwater Robotics: Recent Developments

In recent years, significant progress has been made in FTC for AUVs. Researchers have developed sliding-mode FTC schemes, robust control techniques, and model predictive control strategies, among others, to ensure the AUV's stability and mission success under different fault scenarios.

Sliding-mode control (SMC) is an intelligent control technique known for its robustness against disturbances and uncertainties. In FTC applications, SMC has demonstrated the ability to maintain stability and reliable performance even in the presence of actuator faults. Different variations of SMC have been proposed, such as integral sliding-mode control, linear sliding-mode control, non-singular terminal sliding-mode control, and adaptive neural sliding-mode control.

Robust control techniques have also been explored in FTC for AUVs. These methods maintain system stability and performance under various uncertainties and disturbances. Robust controllers are designed to handle both parametric and unstructured uncertainties, making them well-suited for the unpredictable and harsh marine environment.

Model Predictive Control (MPC) has been another popular choice for FTC in AUVs. MPC is an advanced control strategy that uses a model of the system to predict its future behavior and then optimize the control inputs to achieve desired performance. In FTC applications, MPC can be adapted to account for actuator faults and ensure the AUV's continued operation and stability.

Challenges in Fault-Tolerant Control for AUVs

While FTC for AUVs has made significant progress, there are still challenges. One of the primary challenges is the design and implementation of fault detection and isolation (FDI) algorithms. Accurate and timely FDI is crucial for identifying and diagnosing faults in AUV components. FDI algorithms should be able to differentiate between faults and normal system variations to avoid false positives and false negatives.

Another challenge lies in achieving fault tolerance without compromising the AUV's overall performance. Fault compensation strategies should be carefully designed to ensure that the AUV can achieve its mission objectives even with degraded components. Balancing fault tolerance and performance is a critical aspect of FTC for AUVs.

The integration of various FTC techniques and control algorithms presents another challenge. A multi-layered and integrated FTC system should be able to handle different types of faults and adapt to varying mission scenarios. Developing a comprehensive and flexible FTC architecture is essential for enhancing the reliability of AUVs.

Application of Fault-Tolerant Control in Underwater Missions

Integrating FTC in AUVs has opened up new possibilities for underwater missions. These fault-tolerant AUVs can be deployed in various applications, such as marine exploration, environmental monitoring, and offshore industries.

In marine exploration, FTC-enabled AUVs can venture into uncharted territories and explore underwater landscapes with enhanced reliability. The ability to compensate for component faults ensures that the AUVs can complete their missions even in harsh and unpredictable conditions.

Environmental monitoring is another critical application where fault-tolerant AUVs play a crucial role. These vehicles can be equipped with various sensors to monitor water quality, marine life, and environmental parameters. In case of sensor faults, the FTC system can adapt and continue data collection, ensuring the continuity of environmental monitoring efforts.

The offshore industry can benefit from FTC-enabled AUVs in various ways. These vehicles can be employed for pipeline inspection, underwater maintenance, and offshore platform monitoring. The fault tolerance provided by the FTC system ensures that AUVs can carry out these tasks efficiently and safely, even in challenging offshore environments.

Conclusion

As humanity continues to explore the mysteries of the deep ocean, AUVs will remain crucial in exploration and research efforts. Ensuring the reliability and success of these underwater robots requires fault-tolerant control systems. Thanks to the advancements in Multi-Model Reconfiguring FTC, Actuator Thrust Reconfiguring FTC, sliding-mode control, robust control, model predictive control, and autonomous underwater helicopters, researchers have made significant progress in enhancing the fault tolerance of AUVs.