Same here. This is what the (ultra compact) TP-Link AV200 nano looks like inside.
I picked the MediaLink MHP-EA-200 because it was cheap, and it was easily available at Amazon.
Sometimes it's better to be lucky than it is to be good.
How about this adapter at $1.68 per unit and free shipping. It has a serial port but we can ignore that portion of the board and/or disconnect the D+/D- pins from the USB interface. The question is will it deliver enough current?
Ha! That's awesome!.. also (Walt, you may be thinking what I'm thinking since we talked about this a bit last week) the HomePlug adapters ignore signals under about 1MHz (ish). If the serial out from that devise is slower then that we should be able to put a secondary signal over the line. You could use that signal (which would talk to some other chip on board the ROV) to do basic functions with out the computer even being booted up! Think of it like a television remote that could turn parts of the ROV on and off, communicate basic serial data with the Arduino, etc. Ohh the possibilities!
if i have understood the datasheet correctly it seems to deliver up to 150mA, and this is not enough,(260mA according to walts picture above)1750-ds_pl2303HX_v1.6.pdf (777 KB)
Other ebay alternatives:
Thanks for all your work on this great job, I have ordered up all the components for this and now trying to decide on the wire to use. I was thinking 22awg is what should be used because of the power we want to push down to the ROV. What is your thoughts on this? can we get away with parallel cable or should we go twisted pair, should we have a jacket on the wire for some added strength or just go without. I would like to order the wire up this week and would like input as to what to order and were we source it from. Thanks in advance for your input.
Your question is not a simple one, and I believe it deserves a discussion all by itself. So I would suggest you start a new topic in the tethers category, with a title of "Wire Selection for the Tether" or something like that, and essentially repost your original letter. I think we'll see some interesting opinions on the subject.
This is also based on a single base board, and a logic board that sticks up verticaly(i'll take a picture of it)
But i think i will try to do some measurements on it. its probably usable with removing some components and injecting 3.3v or 5V on some point on the board.
I tried ordering the medialink adapters from amazon, but they wont ship this to Norway for some reason..
it would be interessting to se how this looks like on the inside: http://reviews.cnet.com/networking-and-wi-fi/actiontec-500-mbps-powerline/4505-3243_7-35567341.html
looks very compact!
Stefan: your TP-link adapter looks fairly easy buildt up. i would guess you can remove some components aswell to separate the AC to DC conversion part of the board to separate the tether and the low-voltage to drive the board.
What kind of power prongs are used in Norway? Maybe Amazon won't ship a product somewhere if the AC power connector is of the wrong configuration?
If that's the case, you'll have to scout around like Mike Cochrane did on page 2 of this thread, and find a re-badged version that has the correct power prongs for Norway.
I'm also in Norway, we are using the regular euro connector (~220V):
I've ordered the Medialink ones from Amazon using a US freight forwarder (Thomas S: Send me a message).
It turns out that you finding the re-branded version of the Medialink adapters (actually, Medialink is probably the rebranded version) has been really helpful! I think Tenda is probably the original manufacture Here's a page I found that is like 4000 times more helpful then what I could find elsewhere:
Thanks for the help!!
This is an excellent write up!
Is it possible to fully power the ROV over the tether using the Medialink adapters? (i.e. eliminate batteries and power both the Beaglebone and the motors) If so, where would you tap off the power on the ROV side adapter? Would there be a current draw issue with higher gauge (thinner) wires?
Thanks again for a great write up of a great idea.
The problem is going to be the power loss from driving the motor currents through the long thin tether wire. At the very least you are going to want a local capacitor bank for intermittent loads. Can someone who has the actual cape and software tell us how much current the ROV uses when all the motors are running in water?
Hi Wayne, Doug:
Powering the ROV over the tether is certainly on the roadmap for OpenROV. For the last couple of months the folks at the HQ have been preoccupied with getting the latest wave of kits out the door, and updating the documentation. Once the documentation process is complete, you'll start to see a wave of new developments, and dealing with power is one of those. The switch to Lithium cells has taken some of the heat off of this issue for now.
The MediaLink adapters are fully compatible with sending power over the tether, since they don't really care about what DC voltage is on the line. But it's not as simple as just hooking power wires up to the tether- you could do something close to this if you had a short tether with fat wires, but the current tether arrangement isn't going to be able to handle the surge current that the ROV draws at times. So what you really need is a charge controller working in concert with the ROV battery pack (sort of like the capacitor bank that Doug describes above), and pulls a fairly constant amount of power through the tether. When the ROV is relatively idle the batteries will charge, and when you're driving aggressively the batteries will discharge since the tether will not be able to provide the full amount of power needed by the vehicle.
As for Doug's question about current draw, there's an ROV at HQ now that has a current monitoring chip in the battery power lead, but I don't think anything's been done yet about wiring it back to an A/D on the Cape. Maybe this week, maybe not. I'm sure those guys wake up every day and have to choose between which of twenty things need to get worked on. If anyone at home has made current measurements on the entire vehicle (as opposed to the current monitor that measures just the Cape power), please share that with us.
I did some measurements on overall current a couple of weeks ago in a bathtub setup. I reduced the maximum thrust values for all three thrusters to a range from -30 to 30 (I'm using an older software version where the ESC values were handled as degrees between -90 and 90 rather than milliseconds as it is now) which I found to be optimal for controlling the ROV. I also use a different homeplug adapter (modified TP-Link AV200 nano), which draws a little less current than the MediaLink.
Here are the results (as far as I can remember):
Beaglebone including homplug adapter (base load): 680 mA
ROV with all three motors running at full speed (under water): approx. 3.5 A
With both LED arrays on and the tilt servo rotating, the whole ROV would draw a litte more than 4 Amps peak with my setup which is fine with my 4000mAh NiCd Cells.
There is now instructions+video on the wiki on how to build up a homeplug tether topside adapter!
Following your lead, I put together a powerline system and I've tested it over 500 feet of 22awg wire with no problems at all. I used it to connect my laptop to my router and I can stream video with no lag. I'm looking for a more definitive way to test latency, but right now it looks like we can stretch this well beyond the 100 meter rating. I'm considering getting another 500' of wire and testing it at 1000', just to see. I'll let you know what happens.
Thanks for all your help.
I managed to modifiy the TP-Link nano. Since Jon bought the same adapter, I post here some pictures of the modification of the (single) PCB:
The nano draws approx. 120mA whne idle and 180mA when transmitting, which is a little more than half of the Medialink's power consumption. At the top side I used the LM1117-33 to get 3.3V from the computer's USB port.
This is really interesting, Stefan. I've been doing some work on cutting down the existing MediaLink / Tenda homeplug adapter to make it smaller, but of course there's not much you can do about the power consumption. I might pick up a pair of these TP-Link nanos to do a side-by-side comparison. They only cost $38 for a pair on Amazon right now, which is pretty good. The lower power consumption would be nice in the ROV, but what will be really interesting is to see which can drive the longest length of tether.
One thing we really need to get around to doing at OpenROV HQ is some maximum tether length tests. Connect up a big spool of wire, drop it in the test tank, and then note the connection speed. Repeat for different lengths of tether. Repeat again for different adapters, MediaLink vs. TP-Link. Then repeat again at the park nearby, immersed in salt water as opposed to fresh water.