Homebuilt ROVs Forum

Posted: **Jun 24th, 2011, 8:43 am**

After calming down a bit (+ half bottle whiskey) I consider the following options.

1.Go with 12v topside but change to a shorter 60m, 2.5mm (10awg) power cable (If required even shorter). Should give me a 35% loss at 10A?
2.Get the 24V to 12V 20A 240W Step-down DC to DC Converter as recommended by Magic (thanks for that by the way)

First option is urgent and temporary since I am just about to leave for the “real world” test and could use that time to check the other 100 details (seals, motors in a salt water environment, etc.) Second would be after I return to make a more permanent build with the experiences of this testing phase.

Although there is not really room left in the ROV for the converter I think I could thermally glue it to the frame of the aluminum tube inside and that way also take care of some heat issues. Or in worst case change the camera which really is way too huge for this project (have several laying around doing nothing)

Vancouver – I will do the voltage measurement at the pins of the regulators for the sake of it but don’t think I will get alternative caps, solder and mess-up the whole thing just because I am stubborn. However I would appreciate your opinion on the board design and layout if you get the chance/time please look through the pdf manual I posted above. Think you can easily identify other potential limitations which could be useful.

Magic – Is a converter like that reliable? I checked on alternative designs/brands but they cost like 10 times more. What is the worst thing that can happen if it breaks/burns up? (Board goes to hell I guess) Will a converter like that always give a steady output?

Big Tx to both of you

Posted: **Jun 27th, 2011, 1:39 pm**

one simple way to get your testing started would be to mount a resistor (20-50ohms? / a few watts) and a 15V / 5 Watts Zener diode at the input of the receiver board supply, this would insure that the board input is locked to a maximum of 15Vdc. Now, this might be only a temporary solution because of heating of the resistor and/or the zener, but it would most likely be OK long enough for you to do live test at full upside voltage (24Vdc).

You put the resistor in series with the + line, and the zener across the + and - (cathode to +, anode to -, zener do their work with reversed polarities. The cathode is the end with a band).

Posted: **Jun 27th, 2011, 4:16 pm**

AC-power

I still mean the best solution is to run AC down to the ROV.

manhattan wrote:Magic-I have the equipment for AC - DC but I dont wont to run hight voltage in the ocean.

I wont' call 120/ 230V AC high voltage. According to the IEC (International Electrotechnical Commission) high voltage is voltages over 1000V AC or 1500V DC. I would rather call 120 or 230V mains electricity.

The mains cable can be dangerous if the wires are exposed and someone touches them. Like SoakedinVancouver says, if you have a faulty grounding and the capsule/ frame has mains voltage over itself you can also get electrocuted when you for instance are finished ROV-ing, stands at the beach with your big toe in salt water and lifts the ROV up. (Saltwater is ground.)

I think the electrocution problem can be solved using a residual-current device, which will cut the power and let you know if there is a voltage leak to the ground during operation. I would also turn the power off when I was about to lift the ROV out of the sea, so that I was sure not to get electrocuted. This way I would have made some precations.

DC-power

Using a Cat 5 cable for DC power sounds.. well, not good. The thin cable doesn't give you a reliable voltage when you start to draw some current.

If you want to run DC down you must have another cable for that, and we first have to find out what cable thickness you need. Like you say, the voltage at the end of the Cat cable drops significantly when you turn one of the engines on. This is because the voltage drop in a cable are depending on these factors: Cable thickness, cable length, cable material, the voltage and current through it. To reduce voltage loss you can either get a fatter cable, shorter cable, (choose a better material?), heighten the voltage or reduce the current through it.

If you want to run DC down you must provide us with cable thickness, cable length, cable material, the voltage and current through it. The formula we will be using is:

Code: Select all

Conductor area in mm^2 = (p x I x l x 2) / Ul

Where:
p = the resitivity of the condutor ( (ohms x mm2)/m, 0,0175 for copper)
I = the current
l = the length of cable in one direction
x 2 = because you need two wires in the cable (+ and -)
Ul = the acceptable voltage loss

In house installations the voltage loss is maximum 4% according to NEK (the Norwegian Electrotechnical Commiteee). The ideal in your ROV would be something like that.

I think the wires in a Cat 5 cable has the area of 0.8 mm^2

So how are you going to find out how many Amperes your ROV can draw at a maximum? The ROV Asia manual says the "down unit" needs 12-16V and 20A depending on user requirements. Can this mean depending on what engines, lights etc. you are using?

I would suggest measuring the current draw on your ROV with everything running (given you have an amperemeter that can handle 20A). Connect the amperemeter is series with one of the power wires between the ROV and the battery. You could do the test using a 12V car battery and some short, fat cables. Make sure to load your engines by putting them into water and have the lighting and everything else running.

When we have found out what cables you need we can get back to how to deal with the DC alternative. You can not use the tether cable for power.

These are just my ideas

Posted: **Jun 28th, 2011, 4:52 pm**

Vancuver - will do exactly that. Have a friend electrician that will help me build temporary solution based on your input and I have ordered the previously recommended 24-12 converter for the real deal. We also found out that the 12v-16v in the manual means exactly ergh…… that. When the voltage drops to less than 10.5 the board shuts off one or two engines. So only option is to stabilize between the set limits?

Magic – thanks for your input (do you even know you are helping a swede?..lol) Where did you get the idea that I was planning to run power over cat5? As stated in the above post “ill change to a shorter 60m, 2.5mm (10awg) power cable (If required even shorter). Should give me a 35% loss at 10A” Original idea was to use an awg18 but never a cat5. It is however obvious that you know this subject.

I have the tether ready with 1 Cat5 cable for video feed minus two wires for tx-rx and two AWG 10 (2.6mm conductor) power cables (minus and plus). According to my calculations I should be running periodically on 10A with 15A peak. I will not run any lights until I get this power sorted out. Tested dry with all 4 motors running and it topped out to 6A. This is without props and like Vancuver said not working the way it’s supposed to, under water…lol.

Did a wet test some time ago where one motor topped out above 10A, but I don’t think I was fair to the current by holding and pushing the motor against where it wanted to move? See for yourself:

http://www.youtube.com/watch?v=MWH1YChxzQ0

Finally, I really don’t want to go any-ware-near 120 – 240V. Any voltage that would get my testicals to fry is high voltage to me. I blow up things above and underwater for a living (I am not al-Qaida) and as principle I don’t mess with things I am not fully confident in. We only work with low voltage.

Anyway, I will put all together when I get the converter (fiddling with Bulgin connectors now) and will post a video of my first saltwater test in the appropriate forum. I will also post my build log (if successful) if anybody is interested. Again, Thanks for your help.

Posted: **Jun 28th, 2011, 5:08 pm**

Allright, so you are going with the big umbilical, eh? You will probably need to make it neutrally, or slightly positively, buyoant, so as to lighten the load on the thrusters.

Keep us posted!

Posted: **Jun 30th, 2011, 1:24 am**

MagicHat wrote:AC-power

I still mean the best solution is to run AC down to the ROV.

manhattan wrote:Magic-I have the equipment for AC - DC but I dont wont to run hight voltage in the ocean.
I wont' call 120/ 230V AC high voltage. According to the IEC (International Electrotechnical Commission) high voltage is voltages over 1000V AC or 1500V DC. I would rather call 120 or 230V mains electricity.

The mains cable can be dangerous if the wires are exposed and someone touches them. Like SoakedinVancouver says, if you have a faulty grounding and the capsule/ frame has mains voltage over itself you can also get electrocuted when you for instance are finished ROV-ing, stands at the beach with your big toe in salt water and lifts the ROV up. (Saltwater is ground.)

I think the electrocution problem can be solved using a residual-current device, which will cut the power and let you know if there is a voltage leak to the ground during operation. I would also turn the power off when I was about to lift the ROV out of the sea, so that I was sure not to get electrocuted. This way I would have made some precations.

DC-power

Using a Cat 5 cable for DC power sounds.. well, not good. The thin cable doesn't give you a reliable voltage when you start to draw some current.

If you want to run DC down you must have another cable for that, and we first have to find out what cable thickness you need. Like you say, the voltage at the end of the Cat cable drops significantly when you turn one of the engines on. This is because the voltage drop in a cable are depending on these factors: Cable thickness, cable length, cable material, the voltage and current through it. To reduce voltage loss you can either get a fatter cable, shorter cable, (choose a better material?), heighten the voltage or reduce the current through it.

If you want to run DC down you must provide us with cable thickness, cable length, cable material, the voltage and current through it. The formula we will be using is:
Code: Select all
Conductor area in mm^2 = (p x I x l x 2) / Ul

Where:
p = the resitivity of the condutor ( (ohms x mm2)/m, 0,0175 for copper)
I = the current
l = the length of cable in one direction
x 2 = because you need two wires in the cable (+ and -)
Ul = the acceptable voltage loss
In house installations the voltage loss is maximum 4% according to NEK (the Norwegian Electrotechnical Commiteee). The ideal in your ROV would be something like that.

I think the wires in a Cat 5 cable has the area of 0.8 mm^2

So how are you going to find out how many Amperes your ROV can draw at a maximum? The ROV Asia manual says the "down unit" needs 12-16V and 20A depending on user requirements. Can this mean depending on what engines, lights etc. you are using?

I would suggest measuring the current draw on your ROV with everything running (given you have an amperemeter that can handle 20A). Connect the amperemeter is series with one of the power wires between the ROV and the battery. You could do the test using a 12V car battery and some short, fat cables. Make sure to load your engines by putting them into water and have the lighting and everything else running.

When we have found out what cables you need we can get back to how to deal with the DC alternative. You can not use the tether cable for power.

These are just my ideas

A/c voltage and ocean and small nick in wire equals dead fish possible shock from any system connected to the water Toilet,sink, engine block/shaft plus possible blown inverter. I can't see how that would be safe for an ROV when there are so many options that are safer.

D/C losses can be mitigated. The the problems with a/c umbilical can get really bad in case of control box leakage or umbilical failure ie. being cut on sharp rock.

Posted: **Jul 19th, 2011, 10:32 pm**

I have the ROC-Asia control board and really like it.

A couple things you can do.

1) You can use a switching power supply on the top side running off of a invertor connected to a 12 trolling motor battery, adjusted to maintain 16 volts DC. This was a suggested solution by ROV-Asia.

2) I have tested and had great success in using an old computer power supply, and running the 3.3 volt output in series with a 12 volt battery, for a total output of 15.5 volts. The computer power supply is powered off of a 115v invertor connected to a 12 volt battery.

I am using a 100 foot tether and run both thrusters at full load in the water without the ESC dropping out from low voltage. I found and am using a small shielded 18 gauge wire.... using both the wire and the shielding. (one for the pos and one for the neg power leads)

I trick the ESC, by powering them up with 7.2 volts (2C equivilent) and then ramp the tether voltage up to 15.5 volts. The ESC doesn't kick out that way until it senses a low voltage as if it were being powered by a 2C lipo battery. (approx 6.5 volts)

Posted: **Jul 21st, 2011, 3:56 pm**

Rover – Thx for additional suggestions. I went with the previously proposed 24v-12v-20A DC/DC Converter. I haven’t tried it yet but will in the coming days when I do the first Mediterranean dive.

As for the ESCs, I disabled low voltage protection for all ESCs. Now they dont shut down when they sense lover voltage (or drop)

Stay tuned

Posted: **Jul 25th, 2011, 5:04 pm**

You can make your own DC-DC converter from 24V to 20V or lower. It's not so hard. You can use LM2577. With good cooller it can manage up to 5A. It has 80-90% usefull efficiency.

Regards
Fluffy

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