Introduction
Marine scientists would like to use underwater robot systems to improve their understanding of the underwater world. However, current underwater robot systems are limited to open waters where few obstacles exist and there is little need for real-time feedback and control. One reason for this limitation is that cluttered and dynamic environments prevent the use of a tether between the underwater robot and the user. Reefs, rocks, and other items will quickly tangle the tether, hindering and possibly damaging the vehicle or the environment. Our goal is to develop a system that allows real-time, high-bandwidth communication with an underwater robot system to enable operation in cluttered and dynamic waters such as in coral reef environments. On land, radio communication usually allows systems to operate wirelessly. Unfortunately, radio does not work underwater because water absorbs most electromagnetic radiation. Acoustic modems are the most commonly used underwater communication system with ranges of several km. Acoustic communications, however, is extremely slow (only hundreds of bits per second) with high latency due to reflections and the relatively slow speed of sound underwater. Thus, it is not possible to dynamically control underwater vehicles remotely using acoustic communication in real-time. Instead of using a tether or an acoustic modem, we developed a wireless underwater optical modem. In this paper, we present the design and experimental results of a system to control our underwater robot (Autonomous Modular Optical Underwater Robot or AMOUR) in real-time using our optical modem link. Our optical modem achieves high bandwidth (megabits per second) and low latency while maintaining good coverage of the area of operation of the robot. Figure 1 shows a picture of AMOUR and our optical communications system. Our optical modem allows a land-based user to remotely operate the robot using our human input device (HID) in real-time. The system achieves real-time control due to the high speed and low latency optical link. We analyze the performance of our system in a pool. In nearly all positions and orientations over a 100 square meter area, our robot successfully receives optical commands from a single stationary transmitter.
Fig 1 The optical transmitter mounted to a tripod (right) together with the optical receiver mounted to AMOUR.
We envision using this optical link to quickly update the mission goals and parameters of multiple autonomous robots. The optical link will also enable reception of high fidelity images and videos from the robots. Already, the optical modem has sufficient bandwidth available to allow the real-time operation of tens of robots in parallel with spare bandwidth for relay of images or video. The current system is uni-directional; however, we can easily add a transmitter
on the robot to enable a bi-directional link allowing, for instance, the transmission of live video from the robot.
System Design And Hardware
Our system allows control of an underwater robot via an optical link. The system consists of three high level components:
(1) The base station, which provides an interface for the user to control the robot,
(2) The optical modem, which forms the wireless communication link between the base station and the robot, and
(3) Our underwater robot, AMOUR, which is capable of motions in 6 degrees of freedom.
The Base Station is further divided into 2 parts:
(1) ?A laptop computer running a specially designed user interface (UI)
(2) And a human input device (HID) which allows the user to directly control the robot?s attitude.
Figure 2 presents an overview of the system.
Fig 2 Communication System between The User and Robot
System overview showing the data path through all modules. The optional HID forwards data to a notebook computer. The computer runs a user interface for desired robot attitude, depth, and speed control and forwards this data to the optical modem transmitter. The transmitter encodes the signal using DPIM and transmits it optically. The optical modem receiver decodes the received pulse train into the desired robot state and forwards this information to the Fit-PC located inside the robot. The Fit-PC forwards this information to the robot?s IMU. It also logs both the data from the optical modem as well as the robot?s current position which it receives from the IMU. Finally, the IMU uses the Modular Thrusters Control Algorithm to compute thruster?s updates.
Applications
1. The oil and gas industry uses AUVs to make detailed maps of the seafloor before they start building subsea infrastructure; pipelines and sub sea completions can be installed in the most cost effective manner with minimum disruption to the environment. The AUV allows survey companies to conduct precise surveys or areas where traditional bathymetric surveys would be less effective or too costly. Also, post-lay pipe surveys are now possible.
2. A typical military mission for an AUV is to map an area to determine if there are any mines, or to monitor a protected area (such as a harbor) for new unidentified objects. AUVs are also employed in anti-submarine warfare, to aid in the detection of manned submarines.
3. Scientists use AUVs to study lakes, the ocean, and the ocean floor. A variety of sensors can be affixed to AUVs to measure the concentration of various elements or compounds, the absorption or reflection of light, and the presence of microscopic life.
4. Many roboticists construct AUVs as a hobby. A simple AUV can be constructed for around US$99, consisting of a simple PVC pipe body and two or three waterproof motors with model airplane propellers. These AUVs can be fitted with a camera, lights and sonar like their commercial brethren, though usually their operational depth is around 50 to 100 feet, compared to the several-thousand-foot-depth capacity of some commercial models. They also tend to be less durable and are usually not oceangoing, being operated most of the time in pools or lakebeds. Several competitions exist which pit homemade AUVs against various objectives.
Future Scope
In the near future, we plan to add an optical transmitter to the robot to create a bi-directional link. This will enable the robot to transmit high-fidelity real-time data back to the user, such as video streams. Additionally we want to use a single optical link to control many robots at once. Using the current infrastructure, we have the bandwidth available to control tens of robots and the potential to expand the system to control over a hundred robots in the water. With the bi-directional system, the robots could communicate with high-speed links to enable groups of robots to autonomously and collaboratively perform tasks. We also plan to deploy the system in larger pools and continue our experiments in limited visibility water such as the Singapore Harbor. In these waters, we expect the link to be far more directional and lower quality. Since this could cause the robot not to receive commands for an extended period of time, we will need to take advantage of the autonomous capabilities of the robot. In these situations, the optical link may be best used to update mission parameters and obtain near real-time images and videos from the robot.
Conclusion
In this paper, we described a wireless underwater optical modem that allows real-time control of our underwater robot. The optical modem is a high-speed, low-latency link that can be used in environments with obstacles, where typical tethered operation would be impossible. We briefly described both the hardware and software systems. We verified the system in a number of experiments. First, we analyzed the range performance of the optical modem in air, pool, and Singapore?s Harbor. We then presented experimental data from a pool experiment. This experiment showed that a single stationary transmitter could transmit information to the robot throughout the pool regardless of receiver orientation with the exception of poor reception when pointed directly up. Finally, we presented experiments remotely controlling the robot with a human input device. The operation of the HID demonstrates robust optical link performance and sufficiently low latency to allow real-time control and operation of the robot.
We showed that optical communication for underwater robot control allows data rates on the order of megabits per second and latency on the order of a millisecond. In comparison acoustic communication achieves data rates on the order of kilobits per second and latencies of multiple hundred milliseconds. The disadvantage of underwater optical communication is the reduced communication range of tens of meters when compared to ranges of multiple kilometres achieved with acoustic communication. A further disadvantage of underwater optical communication is that ambient light can saturate the receiver. Ambient light is a predominant problem at low depths, as in the case of the system presented in this paper, which is designed to operate at depths of up to 100m. We showed that our system can deal with ambient light in cases where the receiver is not directly pointed at a light source, e.g. the sun or overhead lamps in the pool. A further challenge of shallow waters is the increased turbidity due to surface currents and waves.
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