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Tether Design for ROVs: Maximizing Flexibility and Minimizing Voltage Drop

Learn how to design a tether that balances flexibility and voltage drop for optimal performance of your ROV. Understand the factors affecting tether design and calculate the minimum wire resistance required.

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Tether Design for ROVs: Maximizing Flexibility and Minimizing Voltage Drop

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  1. 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

  2. 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!!!

  3. 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 OF THESE CAUSE PROBLEMS FOR THE OTHERS! THERE IS A BIG PROBLEM HERE!!!

  4. 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 in mind!!! OK, what are we supposed to do?

  5. 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?

  6. 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

  7. 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

  8. 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

  9. 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?

  10. 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

  11. 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.

  12. 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)

  13. 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)

  14. 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? • 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 distance

  15. 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?

  16. 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.

  17. 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? • ...

  18. 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 • ...

  19. 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?

  20. Module 7 – Power Systems: Tethers • Additional study material can be found on YouTube by searching for • Voltage Drop Tutorial • Have Fun!

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