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ROV power switch design


(Richard) #1

In my design, all power for the ROV is contained within the submersible ROV, with signals only going back and forth to the remote surface control unit. I need suggestions for switching ROV power on and off without opening the pressure vessel containing the battery power supply and contol electronics…

Thanks,

Richard


(Richard) #2

I just read a post where someone suggested a magnetically operated reed switch. Sounds like a good idea, I will look into it. Any other ideas?


(Rusty) #3

Hi Richard,

There are a few options here. The first (and my personal preference) is to turn on the power through the tether. For instance, if you are using a Cat5 cable with 4 pairs, you could use on of the pairs to send a 5V signal that turns on the rest of the ROV. To switch on the high-current power you can use relays or MOSFETs.

Here’s a MOSFET kit that can handle 30A. You can use multiple of these (or one per 1-2 thrusters, for instance).

The other option is to use an onboard switch. There aren’t too many pressure and water tolerant options available so I would use a reed switch and magnet. The reed switch can’t handle too much power so you’d still have to use the relay or MOSFET.

Just fyi, at BR we are working hard on a good solution for the tether and interface. We should have something available within about 3 months.

Best,

Rusty


(Richard) #4

Thanks for the suggestions. I am trying hard to keep all power interface out of the tether. I like the idea of a magnetic switch controlling a mechanical power relay.


(Rusty) #5

Hi Richard,

Okay. Any particular reason you don’t want to control power on/off through the tether? I’m not suggesting that you send usable power through the tether, just the signal to turn power on.

You can also consider hall-effect magnetic switches but they aren’t quite as simple as reed switches.

-Rusty


(TCIII) #6

Hi Rusty,

If you read the fine print on the Sparkfun MOSFET kit schematic, it says that the pwb traces are only sized for 3.5 amps.

Regards,

TCIII AVD


(Paul) #7

Although being able to shut off the power from the topside controller may be convenient, I prefer having a means of completely disconnecting the power source all together. On my build I plan on connecting the battery pod to the electronics pod via a waterproof connector. That not only allows me to completely disconnect the batteries (for transport, etc.) but also allows me to have several battery pods that I can swap while in the field.


Remote power shut off?
(Rusty) #8

@Tom - You’re right. Good catch. I’d solder directly to the legs of the FET instead as a hack workaround.

@Paul - I like the separate swapable battery pod as well. Until we can find a good high-current waterproof connector it’s a little tough to swap.

-Rusty


(TCIII) #9

Hi Rusty,

The only problem with a swap-able battery pod is if you are using a T100 with the built-in ESC. I have each one of my ESC power cables attached to the Power Junction Board that is inside the battery compartment WTC so there is really no way to detach the power cables from the WTC. Even if the Power Junction Board is externally mounted, you would still need at least two power cables coming from the battery compartment WTC: One for the Navigation Controller power and one for the Power Junction Board.

Regards,

TCIII AVD


(Paul) #10

@Tom - You’re right. Wouldn’t work with the way you have your ROV wired up.

@Rusty - Rather than looking for a 2 conductor connector capable of 12 amps I might try a 4 conductor connector capable of 6 amps and then split the load between the connectors. Twelve amps might not be heavy enough, but I’m hoping it will be.


(Rusty) #11

@Tom - Yeah you’re right. I actually really like the idea of having a second battery enclosure with a Deans/XT90 connector hanging loose inside to connect the battery. I think it’s just as fast to pop off the end-cap and change the battery as it is the change the whole tube, plus it costs a lot less. I assume this is basically what you are doing?

@Paul - Load sharing between pins is definitely an option. The problem is that 6 x T200 Thrusters could draw up to about 150 amps at full power. That’s way more than any connector can handle. Of course, there are few situations where you’d attain that kind of current, but you don’t want your connector melting if you do.

-Rusty


(Paul) #12
@Paul – Load sharing between pins is definitely an option. The problem is that 6 x T200 Thrusters could draw up to about 150 amps at full power. That’s way more than any connector can handle. Of course, there are few situations where you’d attain that kind of current, but you don’t want your connector melting if you do.
True. But if you try to draw 150 amps from a 5000 (or 10000) mAh battery I suspect a lot of things besides the connectors will start to melt. ;)

What I need to find out is just how much power I will need to draw for normal maneuvering. I suspect if will be much less than the maximum the ESC’s are capable of. Then design a means of limiting the power draw from the batteries.


(Harold Scadden) #13

One of my connectors was tested at 750 amps for 6.5 hours :slight_smile: Seriously … that is a lot of juice but it depends on your budget.

@Paul, are you willing to deal with a bulkhead mount receptacle and a mating connector? You can get “wet” mate connectors from Impulse, Seacon and Eaton Crouse-Hinds that could do the trick for you. It all boils down to budget and what you want to deal with.

 


(Rusty) #14

@Paul - A 10000 mAh 10C lipo would have a tough time with 150 amps but most lipos are rated to higher C ratings. 150 amps for 10 seconds is a pretty realistic scenario. The battery wires will be able to handle that but I’m not sure a 12 amp rated connector would!

@Harold - 750 amps. Wow.


(Harold Scadden) #15

@Rusty … another option in life is bus bars. You can run bus bars from one container to the other and instead of having your pressure proof fitting, have slits cut in the container and then use the epoxy to pot around it. The bus bars could be coated with epoxy or polyurethane on the outside of the containers. One thing is for sure, there is going to be one heck of an E-Field generated by the current and I can see cathodic corrosion starting to go hog wild on exposed metals.

 


(Paul) #16

@Harold - Depending on the amp draw I was toying with the idea of using some el cheapo waterproof connectors with pigtails. Otherwise I’d just use Seacon connectors.

Wouldn’t a heavy gauge wire be just as effective as a buss bar? And infinitely more flexible to boot? I use “Ancor” brand, tinned wire around the boat and have tons left that could use for this if needed.


(Harold Scadden) #17

@Paul Bus bars can handle a lot of current … the connector that I mentioned being tested to 750 amps is based upon a bus bar that is broken out into seven “O” gauge sockets etc. The temperature on this thing only raised 20 degrees F in that time period. That is what it settled out to with a voltage drop of 6 millivolts between it and a mating receptacle body that I built. You can do the math and figure out what the resistance was … VERY low.

When you go to heavy gauge wire, I would recommend getting silver or nickel plated copper. It will stand up better to oxidation and yes stranded wire is nice and flexible but remember for a DC current stranded wire is not ideal … a solid conductor will carry more.

Considering how much you are putting into your unit, don’t go cheap … use the Seacon connectors and do it right.

 


(TCIII) #18

Hi Rusty,

Like you have suggested, I plan to use either a XT90 or a EF5 connector in the battery compartment WTC to connect and disconnect the batteries from the Thrusters/Navigation Controller as removal and installation of the WTC End Cap is pretty easy.

Though I plan to have some control over the power to the Navigation Controller from Topside.

Regards,

TCIII AVD


(Harold Scadden) #19

For everyone who looks at Sparkfun for stuff … please note that their PCB’s use about the lowest weight copper I have ever seen! I have had many of their boards delaminate the traces while trying to solder components. Granted they have some neat stuff … but it is HOBBY grade at best. If you like one of their circuit designs, they have the files online … snag the file … modifiy the trace widths or get the boards made with more than whatever tiny oz weight they are using and go for it.

You can use PCBs for high current. I had to make a replacement board design for the Atari Hercules pinball machine which used Que balls for pinballs … want current? The flipper circuits on this thing was hideous. 3 Oz copper board and wide traces did the trick.

Control lines from top side to control power is simple enough. If you want to use something that is specifically made for controlling high current DC … check out Crydom’s site. http://www.crydom.com/en/

I use their products on high current heater applications at work and for my surface mount reflow oven at home. Sweet, simple and last forever.