Thursday, June 7, 2012

Serving up the Robotic Claws

To carry the Sea Perch to its mission deployment site, you will equip the Sea Hawk deck with Robotic Claws. The claws hold the Sea Perch in place by its grappling poles until you are ready to send your Robosub on its dive.  You can order the claws from Sparkfun here:

Ideally you can mount the claws with three servos to get three axes of motion, but that adds to cost and complexity. To compromise, we eliminate the swivel servo at the base and fix the "arm" of the robot in place. This means that at the end of the mission the Sea Perch pilot must carefully drive the Robosub into the docking bay. Once it is all the way forward in the docking bay, the large "wrist" servo, which moves up and down, turning on the X axis, can be used to make the open claw descend oover the grappling pole.  Then the little servo which controls the opening and closing of the claw itself can be closed, gripping the pole.

Our design calls for two claws, one on each side of the Sea Hawk.  You can save time and money by running them simultaneously in parallel motion, or you can experiment with having independent control of each claw.

Each servo of each claw NEEDS ITS OWN BATTERY! They run off of 5 V, so you can use 4 battery holders that each have 4 AA batteries in them.  If you try to run two servos off of the same battery you will experience issues where sometimes it works, and sometimes it doesn't. To avoid that , give each servo its own battery.  You hook the positive and negative wires from the servo to the battery and the signal wire to a mini-breadboard, then from there to one of the pins on your Arduino.

What we do is to control the two claw Servos from the Adafruit Motor Shield in the Sea Perch control box. the Adafruit has two locations for Servos on it.  This way you send a command to the Adafruit shield to open the claws, dropping the Perch and allowing you to start driving it with the same Motor Shield, using the down propellor only. We hook up the to large wrist servos to pins on the Arduino in the Sea Hawk control box. This way, when the Sea Perch returns from its mission, you use the Adafruit to control its motors to get it in docking position and the Sea Hawk Arduino to control the wrists so they can move up and down over the grappling poles. When the position is right you turn off the Sea Perch motor and use the Adafruit to close the claws.  Simple? Is anything in robotics simple?

Once the Sea Perch is "docked and locked" in the Sea Hawk you use your bluetooth Android phone with Amarino to drive the pair back to shore.

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