Want to control servo from laptop with tether.

[webshaark]

Hi guys, I'm wondering if anyone out there can help. I am currently constructing an ROV for underwater surveying and need some help controlling the unit underwater. What I would like to do in run a tether from the surface and use the transmitter I currently use for FMS to control servos that are :cool: :cool: connected through maybe a serial port? Any suggestions?

[AndyKunz]

Depending on the type of water (fresh/salt/clean/dirty), you can use RF links to 70' (20m) or deeper.

If you really need a tether, use the trainer jack like you have now. You can use the decode portion of a receiver (there's another recent thread here about re-programming a Hitec receiver) or you can get custom hardware to work with it. It's really not a difficult problem.

How many channels?

Andy

[Endurance]

You could use a USB or Serial servo controller attached to a laptop. Then read in the stick positions using a software application and send them to the controller.

You would need to find an interface cable that can be seen as a windows direct input device (joystick) or if you are OK with using a PC joystick or game pad this software could do it: www.endurance-rc.com/servocommander.html

[BushmanLA]

Depending on the type of water (fresh/salt/clean/dirty), you can use RF links to 70' (20m) or deeper.



Any idea what freqs work best? I know 2.4Ghz hates water. Is lower the better?

[Malc C]

As Endurance stated there are lots of serial servo controllers on the market that would work. These connect to the laptop (via USB or the COM port) and you then plug the servos directly into the controller. You can then control the ROV directly from the PC. However, the tether (even with common supply lines) could become quite thick if you had a lot of servos (some of these controllers can drive 32 or more servos), so it might affect the handling of the ROV.

As Andy mentioned traditional RF transmitters can operate in water (thats why RC subs work up to safe depths of 20 - 30 feet) - but you would need to fit a fail safe so that if the signal was lost the ROV will surface automatically

[Brandano]

An USB cable has only 4 conductors, two carrying power as well. I'd run a custom made USB cable to an USB hub on board the ROV and to another powered hub attached to the laptop, to protect the laptop's ports. The servos could be driven from an USB interface attached to the ROV's hub or from an USB to serial adapter... whadever is more convenient for developement. And you would still have some USB ports on the ROV to read sensors and maybe a sealed webcam or two

[jeffs555]

Need to know what depth you need to go. Usb needs a repeater every 15 ft, so would not be a good choice. RS-232 will work over much longer distance.

Another option would be to run a thin coax cable to the antenna connection on a receiver in the ROV and loosely couple it to the transmitter. You most likely have some type of video camera in the ROV, and if it is just a baseband video camera you could use the same coax to do double duty. Just put high and low pass filters on the ends of the cable.

[Malc C]

Jeff, you got there before me (couldn't google fast enough :) )

The alternative that i've seen used is to have a small floating bouy that has the antenna fitted to it and this is connected to the receiver in the ROV by the thin coax. The ROV is then controlled via a normal transmitter

[hargard]

Hi
Yes Jeff and Malc you have got the best solution there.
I am a RoV builder and after using the Micro processor - RS422 - Micro processor connecting a coax to the receiver aerial and establishing it on the surface is my favorite method.
It gives me loads of channels speed controllers and is easy to do.

Method :-
Remove receiver aerial and solder coax core to same spot.
At other end of coax solder small whip aerial same length as original aerial.
Use transmitter in the usual way.

I have a 200 meter length umbilical with this method and it works a treat.
you will need to put floats on the coax to make it neutral buoyancy.
Make sure the coax is waterproof as some are not.
Maplin in the UK make a flexible Scart connecting cable that OK and it contains 2 coaxes plus other small cables.
Very simple but it works.
Regards Antony(UK)

[webshaark]

So what you are telling me is that basically if I extend the receiver antenna to surface, it will continue to receive signal?

[jeffs555]

I wouldn't call it extending the antenna. You would actually be moving the antenna to the surface and connecting it to the receiver with a coax cable.

[rcBluebird]

Submarine uses 30KHz - 35KHz carrier freq for communication under deep blue sea.

[AndyKunz]

Any idea what freqs work best? I know 2.4Ghz hates water. Is lower the better?

Lower is better, yes. A 27MHz system would be much better than a 75.

Andy