Tether Breakdown

The requirements of the 2017 ROV's tether are quite similar to that of Blue Puttee's. However, BP's tether had some issues (system brownout) due to the thinness/long length of the wire in the tether. Issues are discussed below, with suggestions made.

Voltage Drop Characterisation

Reading the datasheet of the DC-DC converters used on BP and the 2017 ROV, it is found they shut off when their input voltage reaches 36V. This is assumed to be the cause of the brownout issue discussed in Issue 220. On November 26, 2016, an extremely rough test was undertaken to characterise the tether voltage drop. A single data point was recorded before the alligator clips that were used exploded.

Through the 76ft (23.15m) spare tether, ~23A was run, causing an 18V drop across the tether. Working the math out, that gives a resistance of 33.8mΩ/m, which makes sense, as two passes of 16AWG wire should have a resistance of 26.4mΩ/m (before heating losses) based on the table found here. The heating and uncertainty of measurement accounts for any discrepancy we see from the theoretical value.

Working out more math, our current tether limits our current to about 15.3A before browning out (assuming that the spare tether and actual tether have the same length). This is bad, because the T200 thrusters each claim that they can take up to 350W, meaning we will draw over 15A by having just two thrusters at full power.

Some alternatives to our current tether have to be investigated.

Chopping the tether

Let's assume that we can operate with a smaller tether. If we say that we're okay with a 10V drop across our tether (12V causes brownouts) different loads on our ROV are possible at different lengths. Doing some math, we see:

• At current length of 23.15m, we can get a load of 486W, running a current of 12.8A
• At a length of 19m, the length of Clarenville HS's tether in 2015 (nobody posts their tether lengths in their tech reports apparently) we get a load of 592W, running a current of 15.6A. I assume this length is good for everything we could want to do in the flume tank.
• At a 60% of our current length (13.9m), we get a load of 809W, running a current of 21.3A. This is the almost the maximum current rated for 16AWG wire, so nothing is gained by going shorter than this. People who actually know what robots are can tell you if this is too short.

Obviously, we need to characterise the load we expect before these numbers will make much sense. All I can say, from my professional opinion, is that if Leoni won't give us a new thether, we can run with a shorter version of our current tether, because the brownouts were an edge case in operation to begin with, and we can commission the writing of some fancy software by our software team to limit max draw by reading the current draw to the thrusters.

Getting a new order from Leoni

We're looking at having a tether donated to us by Leoni. It's currently on Clar's massive donation order for Newfoundland, which means it'll take forever. Michaela is to contact Leoni herself. Here's what Nick told Clar to ask for:

As you've suggested, we are looking at a minimum 120' length of tether containing:

2 x twin glass fibres - multi-mode will be used for its simplicity, and because the equipment we currently have works with it.

2 x 14 AWG stranded copper wire

Neutrally buoyant in fresh water

We do not want a pnuematic line in this tether, as the weight requirement means we will most likely not be using any pneumatics. If we change our mind, we can always use the 2015 tether.

We have no preference for the colour of the tether. The yellow we have used in the past is fine, but it doesn't really matter to us.

Best case, we get a free new tether, and can cut it into two 60' (18.3m) chunks. Based on the resistance of 14AWG wire, applying a 1.28x fudge factor to account for heating (seen on our current tether), we could get a 978W load running at a very safe current of 25.7A (32A is the max current for 14AWG).