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Nimble underwater Sepios robot gets 6 axes of motion from four fins controlled by Zynq-based NI myRIO

Xilinx Employee
Xilinx Employee
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Four undulating fins serve as the sole motive force giving the student-built Sepios underwater robot six axes of freedom:

 

  • Up/down
  • Left/right
  • Forward/back
  • Roll
  • Pitch
  • Yaw

 

 

 

 

The biomimetic robot emulates cuttlefish, which sport long, side-mounted fins that generate thrust through undulating motion. This unique form of underwater propulsion allows the cuttlefish to rapidly switch direction, generate sideward thrust, and to sneak up on prey without giving alert. Here’s a video of two different cuttlefish species showing the motions that inspired this project:

 

 

 

 

A student design team at the Swiss Federal Institute of Technology Zurich decided to adapt the cuttlefish’s unique propulsion mechanism for an underwater robot. They also decided to go the cuttlefish one (or two) better by adding two fins for a total of four. These four fins are independently controlled with nine servomotors in each fin creating the undulation.

 

 

 

Sepios Underwater Robot.jpg

 

 

Each fin’s nine independent servomotors drive a ray through bevel gears. Each ray moves through 270 degrees around the fin’s front-to-back axis. Each fin has a separate servomotor controller and the servo controllers are in turn operated by a Zynq-based National Instruments (NI) myRIO data acquisition and control system, which was specifically designed for student design projects.

 

The Sepios underwater robot also contains a camera, an IMU (inertial measurement unit) to measure acceleration and gyroscopic forces, a laser for collision detection, a humidity sensor to detect internal leaks, and an underwater depth sensor. The NI myRIO controller collects and uses information from all of these sensors for navigation, communicating over a wired Ethernet link to an underwater, handheld 3D space mouse controller. Commands from the mouse controller and the sensor information help the myRIO controller decide how to operate the four fins.

 

The result is a highly maneuverable robot that creates very little wake compared to propeller-driven craft. This propulsion system is vaguely reminiscent of the fictional caterpillar drive that silently propelled the Red October submarine in Tom Clancy’s 1984 novel “The Hunt for Red October” and subsequent movie.

 

The first Sepios prototype was a single-finned boat driven by Lego NXT bricks. The hardware was built swiftly but the team struggled for more than a week to program even a simple sine wave motion using C. A team member suggested trying NI’s LabVIEW graphical system design environment instead. Within half a day, the robot was able to perform any function the team could devise. The hardware design plan immediately switched from the planned Raspberry PI system to the NI myRIO and the team gained NI as a sponsor.

 

Once the hardware was sorted out, everything else worked perfectly. The depth controller, which might have taken ages to implement, worked instantly under LabVIEW. The huge amount of time saved by using LabVIEW allowed the student team to focus on other important areas such as control allocation and the user interface.

 

Finally, here’s a video of the Sepios robot diving in the ocean:

 

 

 

 

 

Note: This student project was one of three highlighted at the NI Engineering Impact Awards dinner during the recent NI Week held in Austin, Texas. It was the student project award winner in the NI Global Student Design Showcase 2014.

 

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