ROV Teams - More Ideas

Pololu Robotics recently released a truly novel electronic speed controller (ESC) called the TReX. It will drive two bidirectional motors up to 13A continuous, 30A peak from 6V-16V and one unidirectional motor up to 15A continuous. It can be controlled through RC inputs, analog inputs, or through a serial port that has either RS-232 or TTL voltage levels available on it. The control mode can be changed on the fly, which makes it a truly unique device:

http://www.pololu.com/products/pololu/0777/

So here's the idea for teams building underwater ROVs for competition:

Several people have demonstrated that you can replace an RC transmitter antenna with a coax cable that can run underwater and terminate inside the dry box of an ROV. The dry box termination is basically to leave N inches of the coax core exposed as an antenna (N depends on your transmitter frequency, and needs to be wavelength-matched). The coax acts like a waveguide so the transmitter is essentially transmitting inside the dry box. Place the RC receiver nearby, but not in contact with the antenna coax. Voila, you now have RC feeding into your ROV.

Plug in two of the TReX speed controllers. Channels 1 and 2 drive the fwd/rev/turn thrusters. The TReX can be set up to do onboard channel mixing, so the primary joystick of the RC transmitter could be used as a fwd/rev/turn control, just like a video game. (For those who are slow to pick up on hints, by just like a video game I mean it gives your operator an interface they're already more than comfortable with, which means less seat time is required for them to become proficient in using the ROV!)

Channels 3 and 4 use the second TReX drives two vertical thrusters that can be used for ascent/descent/tilt. Considering how many ROVs wound up in a massive down-pitch after picking up a mission prop, having the ability to counter imbalances is a huge plus.

This leaves two aux outputs. Provided you are using at least a six channel radio, one could be used for a spring-loaded manipulator. The drive motor would have to fight the spring in order to close, but it would allow for pretty fine control over a manipulator.

An alternate idea would be to use the aux outputs to drive ballast pumps. Since there's always a chance an ROV challenge will involve lifting a payload, ballast tank designs may be more competitive than strictly thruster-based designs. This gives you a way to drive the pumps.

Now for Nifty Part #1: Your umbilical is now miniscule. Two 14-12ga wires carry DC power to the ROV (this is a requirement of some of the ROV competitions or I'd argue for on-board SLA batteries.) A single coax carries the RC signal to the ROV. A single coax can carry video back to the surface. You can get very narrow 75ohm coax that would do a dandy job of this, and not add much at all to the umbilical. The umbilical is much narrower and lighter weight than the typical umbilical, leading to less drag in the water and fewer issues in balancing it for neutral buoyancy.

Nifty Part #2: You now have a completely electronically controlled sub. If, at a later date, you decide to add an onboard microprocessor to the sub it's dead-easy. The TReX controllers can be daisy-chained (they're addressable, and treat the serial connection as a common bus), so a single UART micro would do the job. And since TTL connections are available you don't even need level shifters on board in order to drive the TReX controllers (though it supports that mode as well.)

Nifty Part #3: Already mentioned, but RC radios present a tried, true, and pretty common interface to the ROV. Any kid who's played video games or flown an RC helicopter or airplane will be more than familiar with the control layout. Since older AM proportional radios would do the job as well as a higher-end newer radio, a second-hand radio can be had for not much money off Ebay. I picked up a six-channel Futaba AM system for $45 at a garage sale years ago. They're even cheaper now.

Now let's talk cost, because none of this comes for free:

The TReX controllers retail for $99 each. Figure another $50-100 for a radio off of Ebay. Figure another $50-100 for an IP66-rated enclosure and a set of IP66-rated connectors such as the Bulgin Buccaneer connectors that are sold by Digikey. Voila. You now have two speed controllers mounted in a dry box with hermetic waterproof connectors to hook it up to your thrusters, and a complete user interface at the top.

I realize that tacking $250-400 onto the price of an underwater ROV may be more than most teams are in for. However at the last competition I saw there were teams that spent well into the thousands on their ROVs, so it's not out of the question. Considering how many teams were tripped up by their user interface, I'd consider $300 as money well-spent if it gives the team full proportional control over every thruster and high current outputs for driving additional devices like thrusters, pumps, and solenoid valves.

Tom