Critical Technologies
- Precise, high-speed underwater manipulators.
- Task-level programming with an advanced finite-state machine paradigm [].
- Underwater video navigation, stationkeeping, tracking [].
- Coordinated underwater vehicle/manipulator control.
- Optimal control of combined vehicle/camera systems through bandwidth separation [].
Experimental Platforms
- At Stanford ARL, we have a test tank installation suitable for
testing underwater manipulators and thrusters. The test tank is 2.1
m in diameter and 1.5 m deep. A single link manipulator has been
outfitted with strain gauges and an accelerometer to both
characterize the dynamics of underwater motion and control the arm
for high performance slews. An elbow joint has been constructed and
will be added to form a two-link underwater manipulator system. For
thruster testing, we also have a thrust stand for direct force
measurement of thruster performance.
- The OTTER vehicle was designed for depths up to 1000 m. It has
6 oil-filled, 1/4 hp brushless direct-current motors for maneuvering
and 2 oil-filled, 1 hp brushless drive motors. All motors have an
integral electronics stack with an HC11 performing closed-loop
torque and velocity control. We have provisions for closed-loop
force control using duct-mounted strain gages for future research
efforts.
On-board batteries provide up to 1.5 kWh of power. While a tether is currently used to trickle charge the batteries, and provide a data link over a serial line, future plans call for tetherless operations using acoustic modems for communications. The sensor suite includes a flux-gate compass, roll-pitch sensor, pressure depth sensor, and an integrated angular-rate/accelerometer package from Systron Donner. The control electronics are centered around two on-board VME cages: one for the vehicle controller, the other for vision processing.
An advanced vision system provides real-time video processing at frame rates from a pair of stereo cameras. This vision system uses sign-correlation hardware and optical flow theory to provide velocity and range maps of the incoming video. Research is being performed to used the correlation of live video with stored video to create a video mosaic of a large region of the ocean floor and then to navigate the vehicle from the stored mosaic.
- In addition to our vehicle testbed, we have access to Ventana, a commercially-available tethered ROV, which is used daily by MBARI scientists in the Monterey bay. The eventual transfer of the technologies developed at ARL to MBARI research operations is greatly facilitated by incorporating the experience of MBARI submersible pilots, scientists and engineers early in our experiments.
Demonstrations to Date
- Although this program only began in 1991, we have made several
important contributions. At the beginning of the program, Tim McLain
designed, built and tested a tether tension measuring device that has
helped MBARI engineers better understand the tether forces which act
on their remotely-operated vehicle (ROV), the Ventana
[].
- We have used the first experimental submersible built by this program,
SPOTTER, to show that better overall system performance can be
obtained with intelligent compartmentalizing of control based on
bandwidth limitations. In this experiment, cameras were mounted on a
high-speed pan-and-tilt and used a simple vision processing scheme to
track a laser dot. The pan-and-tilt mechanism was itself mounted onto an
underwater vehicle. By combining the control of the pan-and-tilt
mechanism with that of the vehicle, we were able show that overall
tracking performance of the system was superior to either subsystem
alone.
- A second experimental submersible, OTTER, was built the following year. With this vehicle, we were able to demonstrate the tracking of arbitrary objects underwater []. It was during this effort, that we first used optical-flow based, sign-correlation technology to obtain range and velocity maps from a pair of stereo cameras.
Experimental Demonstrations in Progress
- A basic control architecture for the task-level control of underwater
vehicles has been formulated [] and research into the
particulars of the architecture is ongoing.
- In addition, fundamental research is ongoing in the area of hydrodynamics to better understand the interaction between underwater manipulators and the water itself. The culmination of this research will be much improved coordinated control of the vehicle/manipulator system.
For questions or comments contact: www@arl.stanford.edu
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