What's Going On Down There (a signal processing adventure)

A safety feature unique to the Hyball ROV and its descendants is vacuum leak detection.  This is a system that allows the operator to draw some air out of the hull before launch, creating a slight vacuum.  A barometric pressure sensor inside the hull lets the operator know if any air or water is leaking back into the vehicle through one or more of its thirty-two O-ring seals.  Ideally, leaks are identified topside, but if a seal fails during a dive, this system acts as the first line of defense for the vehicle and its operators.

Since the original vacuum sensor was missing, I replaced it this, a TDR-120 pressure transducer.

The tricky thing about this sensor is that it produces a differential voltage signal.  Instead of offering one signal terminal whose voltage relative to a 0-volt ground is proportional to the vacuum pressure, it has two terminals at very close positive voltages.  The difference (hence "differential" signal) between these voltages is proportional to vacuum pressure.

I used this little circuit from the LM124 application examples to ground-reference the signal and amplify it so that 0 to -700 torr gauge pressure maps to 0-5V, which can be fed to one of the Arduino's input pins.

It looks slightly different in real life.  The two 714 op-amps are inside the LM324 chip (at bottom):

Rather than narrating the two signal processing operations the circuit performs (ground referencing and amplification), I'll let the simulated oscilloscope and captions do most of the talking:

1) The raw signal coming out of the sensor
(each of the two traces represents a signal pin.)

2) The ground-referenced version of the signal in (1)

3) The signal in (2) amplified for a 5V maximum peak.

The traces above simulate what you'd see while drawing a 700-torr vacuum with the hand pump (below.)

 

I tested the output scale of the signal processing circuit by connecting it to a voltmeter, hooking the pump hose directly to the sensor, and comparing readings between the meter and the pump's mechanical gauge.

Geek on and happy holidays!

-Jacob

Motors

They save the best for last, eh? Anyway the motors...

They're 12VDC, size 550 Duratrax Starter Motors (they're meant for starting internal-combustion engines in radio-controlled vehicles). 55o is the standard size used in electronic radio-controlled vehicles, so why not use them in a remotely-operated vehicle?

Anyway the motors draw up to 148 amps when stalled, and produce about .55 horsepower (this is roughly equivalent to the power output of the motors that the ROV originally came with-- ~380 watts each. Also, the Duratrax motors were the only ones small enough and with enough power to meet the requirements of our project [hence the outrageous stall current]).

Now, all motors have different face-patterns (the arrangement of the tapped holes in the front of the motor for mounting). To adapt the new face pattern to that machined into the motor housing--with tight tolerance to reduce vibration--I commissioned a quick-turn prototyping machine shop (emachineshop.com) to fabricate adapter plates from ~1/8 inch, 6061-T6 multipurpose aluminum alloy (for strength, weight, and--to some extent--cost).

That's about all there is to the motors (the physical bit at least).

Cheerio~

On Yellow Submarines

The propulsion trilogy will finish up soon with a word from Aidan on the motors, which he did most of the work on.

Thanks to Tamrynn Clegg for the Antipodes shots.

In the meantime, there have been some cool happenings at the Sea Center tangentially related to this project.  On November 11 and 12, the team from OceanGate Inc. made a whistle stop on the wharf with their manned research sub Antipodes.




In terms of technical history, Antipodes has a surprising amount in common with our ROV (notwithstanding that it carries people.)  They are both products of the North Sea oil boom, and both feature approximately the same depth rating, at around 300 meters, along with an unbelievable amount of acrylic.  They're also both yellow.


Yellow is likely the most popular body color for research subs, manned and unmanned alike, due to its high visibility.
Nonetheless, one can't help but wonder:
Which came first, the Beatles album or a yellow vehicle?
Finding the answer to this question took some searching, but as it turns out, none other than Jacques Cousteau painted his organization's new submersible yellow in 1966.  The vehicle, believed to have inspired the title of the Fab Four's 1968 album and  animated film, is shown on display at the Oceanographic Museum in Monaco. 
Yet another way marine science affects us all!

image credit to user 4dmin of imagesearch.blogspot.com

Launch is nigh for our yellow sub. Stay tuned!

- Jacob