MASSACHUSETTS INSTITUTE OF TECHNOLOGY Sea Grant College Program

An acoustic network is a relatively low-bandwidth, long time-delay communication link compared to links typically used to control robots. Layered control provides an attractive framework for remote control of an AUV under these conditions. Human intervention can be accomplished with no change at all to the fundamental structure of layered control.

There are a variety of methods by which operator input can be accepted into the structure, each providing a different level of control of the vehicle. This range of capabilities is critical since an AUV can expect to encounter degraded performance of the acoustic link on occasion during its operation inside a network. At one extreme, it is possible that round-trip communications may take fractions of a minute in a congested network, or even be indefinitely interrupted. On the other extreme, when immediately adjacent to a node, communications will have little delay and have high bandwidth. As the level of sophistication of control increases, the level of abstraction of the commands also increases, decreasing the bandwidth required for command. Three different levels of control are now described.

User Override Mode

The lowest level of control of the vehicle would be for the operator to generate commands for the dynamic controller (e.g. heading, depth, velocity, etc.). To accomplish this within the framework of layered control, one need only construct a behavior that takes operator input to generate vehicle commands. One advantage of using layered control to achieve this level of control is that one can preserve the functionality of avoidance behaviors that protect the user from dangerous consequences (e.g., a depth envelope behavior will prevent the user from driving the vehicle into the bottom). The vehicle acts like a docile robot with just enough intelligence to override commands that would cause it to endanger itself. This level of control clearly requires a high bandwidth link and little delay.

Behavior Modification Mode

The behavior modification mode allows the operator to influence the vehicle by changing the internal setting of vehicle behaviors. This is a higher level of control than is the user override mode described previously. It assumes the vehicle is basically operating as it would under autonomous operation. However, unlike autonomous operation, an operator can modify the performance of individual behaviors while the vehicle is active. For example, the user might change the threshold of an obstacle avoidance behavior or the parameters of a survey. This mode of control is tolerant to long delays and low bandwidth.

Mission Modification Mode

The mission modification mode provides the user with the ability to activate and deactivate behaviors, and set their priorities. In this mode, the vehicle operates as an autonomous agent that receives high-level instructions from the operator during run time. An example is the user commanding the vehicle to abort a survey to return to a docking station by deactivating the survey behavior and activating homing. Mission modification is the most powerful level of user control possible within the framework of the layered control. A very low rate of communication from the operator is necessary to achieve overall command of the vehicle.

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