Propulsion channels
We are basing our design on the approach taken by the UPenn Modlab team on their DARPA TEMP proposal: a motor pushes water through a narrow channel in either direction to control thrust - there are four propulsion channels, one in each corner mounted at a 45 degree angle, so that through combining directions on each of the channels any direction (and rotation direction) can be achieved.
The following picture shows the flow of water through the channel in each of the two possible paddle directions:
If the paddle turns clock-wise (yellow flow), the water moves right through the channel and the boat steers left. On the other hand, if the paddle turns counter clock-wise (red flow) the water comes out of the left channel exit, steering the boat to the right.
An example of how one of the propulsion channels would be mounted:
We have settled for a €5.95 Polypropylene toolbox as the hull for the dollbot, it will probably need some ballast to avoid overturning too easily but it is a lot more sturdy and water proof than the plastic food container (€1.99) that we had chosen originally.
This layout shows the internals of each of the four propulsion channels once mounted:
You can easily see how turning the motors in either direction the boat can be made to move on any direction or to rotate, the UPenn video includes towards the end a few seconds that show the propulsion system in operation.
We could not find any piece laying around our workshop that would lend itself to be used as the propulsion channel, so we have decided to create a model and 3d-print a few copies with translucid PLA, this is how it looks like:
We will put up some pictures once we have the completed modules in about a week's time.
UPDATE (28/May/2015): Just picked up the printed propulsion modules from our friends at Nativa 3D, who pointed out a flaw in our original design (it's difficult to fit a 30mm turbine through a 2mm hole, thanks Sergio!) so we had to create a new revision. The results are great:
We are already working on the turbines, will put up the final results once we put the thruster together.
Controlling the motors
The propulsion system requires the ship to control four DC motors, we have decided to run these off a separate battery (5500mAh, 7.4V 2S LiPo) to avoid frying the controller and keep the boat operational even in low-voltage conditions when the LiPo needs to disconnect. To control the engines we have chosen the Adafruit Arduino motor shield, which will allow us to address the motors through serial I2C communications and keep the number of components down. We will need to develop a library to control the motor vectoring, so that we can simplify the navigation code.
We still have to get the motors, we are limited to a maximum 3A stall current and to run at 7.4V - nothing fancy, nearly any motor will do given that we are just moving water around at relatively small speeds.
