RTU Stress Testing (again)

We did some more stress tests on the RTU after re-tightening the bolts.

Another RTU Overhaul

Back at the lab, the RTU had to be removed so that we could service it.


After draining the oil and opening the housing, it was clear that some of the bolts holding the stator had worked themselves loose again. This is a known issue, caused by a shortcoming in the design of the RTU.

For now, the only thing we can do is re-tighten the bolts and re-assemble the unit.

The Search for Didemnum: Day 2 (Double Success, Double Failure)

On our second day out, we went a few miles further offshore to try some surveys in roughly 100 feet of water. From this far out, we could see the Boston skyline relatively clearly.


The day's first deployment went smoothly.

The goals for our second day out were to find out what altitude is best for taking pictures, and to see if Justin's changes to the survey behavior could help straighten out the tracklines when the vehicle is at depth.


In this test, we stepped through altitudes from 1.8 meters to 1.1 meters in increments of 10 centimeters. The results are exactly what we had hoped for: consistently good images, and a better idea of what range of altitudes will produce them.

Notice the difference in the amount of red channel as the vehicle approaches the bottom. To get good information from red light, we have to fly very close to the bottom. A difference of a few tens of centimeters makes a big difference in light quality!

We also had one flash misfire, the cause of which is unknown.

Overall, this set of images is a huge success; we can start doing image surveys in earnest now that we know the physical focus and aperture settings are in the right place.


The new survey behavior also worked well... mostly. The tracklines are much straighter, but what happened at the 4th corner?

Before we could wonder too much, Justin noticed that the vehicle was running in circles under remote control and had a problem with rolling over.

Uh oh.


Mike and Seth sprang into action to check it out.


Both thruster cables were still attached. So, we decided that recovering Odyssey would be the only way to fully determine the cause of the problem. Kyle got to try his hand at recovery.


Once the vehicle was on deck, the problem was fairly evident. We had lost one of the ninja pins that holds the thruster in proper orientation on the RTU shaft, and the thruster had worked its way to the opposite direction.

Only 3/4" of the thruster was still within the coupling. Had it worked its way fully loose, we would have lost it permanently!

Unfortunately, there was a second problem as well: the RTU had fallen out of alignment, and was unable to produce adequate torque on the shaft to properly position the thrusters. This is a game ender; we have to take the RTU back to the lab for disassembly.


Catching a dogfish lifted our spirits somewhat. We threw it back.


On the steam back to the harbor, I was able to put together a small mosaic of a few good images using the hugin panorama stitcher (not to be confused with the Hugin AUV).

When the RTU is fixed, we should be in very good shape to get more like this!

Bypassing Sliprings

Of all the users of Parker DriveBloks, we seem to be the only ones that experience frequent capacitor failures. Of the failures we have, only the thrusters that go through the RTU (and its sliprings) seem to suffer.

Solution: bypass the sliprings by drilling a hole in the fairing for the cable. As a safety precaution against continuous rotation of the RTU (disabled in software, but... you never know), we aren't using the plastic shells to hold the cables together.

RTU Shaft Stress Test

To make sure that the force produced by our thrusters would not bend (and bind) the RTU shaft, we hung 60lbs of weight from each arm (simulating 30lbs of thrust at the end of the arm).

The RTU survived without binding!