# Module 7 – Power Systems: Tethers What is a Tether?

```Module 7 – Power Systems: Tethers
What is a Tether?
It is the lifeline for your ROV
The Tether provides your ROV with
Physical Connection to the Surface
Electrical Power to run the ROV
Communications link between the ROV and Surface
Video Link to see what the ROV is doing
Module 7 – Power Systems: Tethers
What is one of the biggest problems
with tethers in small ROVs?
Length?
Weight in Water?
Flexibility?
Drag?
Voltage Drop?
Strength?
ALL
OF
THESE!!!
Module 7 – Power Systems: Tethers
Good Tether design takes all these
factors into consideration
Length – Just as long as needed
Weight in Water – Neutral if possible
Flexibility – nice and flexible
Drag – as small diameter possible
Voltage Drop – the least possible
Strength – nice and strong to pull the ROV out if
it dies.
EACH
CAUSE HERE!!!
THEREOF
ISTHESE
A BIG PROBLEM
PROBLEMS FOR THE OTHERS!
Module 7 – Power Systems: Tethers
WHAT Do you mean?
Too much length causes more voltage drop
If you fix that by making it bigger the tether is less
flexible. It also becomes heavier and now is a
big drag on the ROV.
If you try and make it really small and just the right
length, you may also have too much voltage
drop.
Design a tether with all these items
OK,
what are we supposed to do?
in mind!!!
Module 7 – Power Systems: Tethers
First we need some information.
1. What is the worst case current draw that your
ROV will demand?
2. What is the maximum distance that your ROV
will need to go for this mission?
3. What is the Maximum Voltage your ROV will be
supplied with?
4. What is the Minimum Voltage that your ROV can
receive and still operate in an acceptable
manner?
Module 7 – Power Systems: Tethers
First we need some information.
1. What is the worst case current draw that your
ROV will demand?
Three thruster ROV: Each thruster at full power
draws 3 amps. Total current = 3 Amps * 3 = 9
amps
Module 7 – Power Systems: Tethers
First we need some information.
2. What is the maximum distance that your ROV
will need to go for this mission?
DEPTH/TETHER LENGTH
EXPLORER class ROVs must be capable of operating in a maximum pool depth of 5.2 meters
(17 feet). RANGER class ROVs must be capable of operating in a maximum pool depth of 3.7
meters (12 feet). All underwater missions will take place within 10 meters from the side of the
pool. The mission station will be no more than 2 meters from the side of the pool. Tether length
should be calculated accordingly.
Using the above criteria, we determine that a 18 meter tether will work for
your mission. 2 meters to pool + 10 meters from pool edge + 3.7
meters deep + 2 meters extra = 17.7m
Round to 18 meters
Module 7 – Power Systems: Tethers
First we need some information.
3. What is the Maximum Voltage your ROV will be
supplied with?
Maximum will be our power supply of 12V
4. What is the Minimum Voltage that your ROV can
receive and still operate in an acceptable
manner?
We don't have any electronics on board but
would like at least 85% of the voltage delivered to
the ROV, 12*0.85 = 10.2V
Module 7 – Power Systems: Tethers
Now we have all our information.
We want to design our tether for maximum flexibility
and with a minimum voltage drop of 1.8 volts
(12v – 10.2v = 1.8v)
But what is this
voltage drop
stuff?
Module 7 – Power Systems: Tethers
To understand voltage drop, we have
to go back to Ohms Law .
E=I*R
Also, EVERY wire has resistance associated with it
and the bigger the wire the smaller the
resistance.
Already know two of the values in the equation
above.
E = 1.8 volts and I = 9 amps
Module 7 – Power Systems: Tethers
That means we have to select our
wire that has a value of R that
satifies the equation.
R=E/I
R = 1.8 / 9
R = 0.2 ohms
OK, But where do these ohms come from in
the wire? I thought wire didn't have any
resistance.
Module 7 – Power Systems: Tethers
Every wire has resistance. How much depends upon
material, temperature, length and diameter of that
wire.
There are many conductor characteristic charts that will
provide you with this information.
One such chart is at:
Wire Chart
This chart is for Copper at 75C and gives ohms per
foot and sizes using American Wire Gauge
(AWG)
Module 7 – Power Systems: Tethers
Lets look at our tether
While our electrons are going from the power supply to
the ROV and back, they have to make a total round
trip of 36 meters or 118 feet.
Total Wire resistance is based on total length of
the electrons trip OR 2 * the distance between
power and load. (supply and ROV)
Module 7 – Power Systems: Tethers
From the wire characteristic chart:
24 AWG copper has a resistance of 25.67 ohms/kft
12 AWG copper has a resistance of 1.588 ohms/kft
24 AWG is standard stranded networking wire
12 AWG is standard stranded speaker wire
OK, lets see what we have for wire resistance in our
tether.
24 AWG: (25.67 ohms /1000 feet) * 118 feet = 3.029 ohms
12 AWG: (1.588 ohms /1000 feet) * 118 feet = 0.187 ohms
What resistance did we need?
Module 7 – Power Systems: Tethers
OK, with these two wires, we have
24 AWG: (25.67 ohms /1000 feet) * 118 feet = 3.029 ohms
12 AWG: (1.588 ohms /1000 feet) * 118 feet = 0.187 ohms
Earlier we calculated that we
needed a resistance of
R = 0.2 ohms (or less)
It looks like the 12 AWG will just work!
But...
Can we make the tether more flexible?
Module 7 – Power Systems: Tethers
Now we need wire with less resistance.
We can.......
1. Get bigger wire – BAD, bigger = less flexible
2. Parallel smaller wires – maybe better, smaller wires
are more flexible.
Module 7 – Power Systems: Tethers
Paralleling wires:
The resistance of the wires will decrease by the number
of pairs of wire in the tether.
If there are 2 pairs, it decrease by 2, 4 pairs by 4, etc.
24 awg with 4 pairs = 3.029/4 = 0.757 ohms
Not quite there yet, but is there something inbetween 24
AWG and 12 AWG?
...
Module 7 – Power Systems: Tethers
Is that the best you can do?
By trying different combinations of wire sizes and
number of pairs, you can come up with a combination
that will meet the original criteria of
0.2 ohms.
Exercise:
Go to the Voltage Drop Calculator and see if you can
come up with a 4 pair combination of wire sizes that
equal to 0.2 ohms and has a voltage at the ROV of
10.2 volts.
NOTE: The calculator uses distance from source to
load, not total length, so use 18m or 59 feet
Module 7 – Power Systems: Tethers
WELL???
What did you come up with?
You should have settled on four pairs of 18AWG in
parallel for your tether and a voltage at the ROV of
10.3 Volts.
How did you do?
Module 7 – Power Systems: Tethers
for
Voltage Drop Tutorial
Have Fun!
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