I have been working on a way to make a Wi-Fi Topside with Tenda’s wireless module, the PW201A. There have already been a couple projects to allow a connection to the topside adapter over Wi-Fi. Erika and Eric built a long range wireless buoy which allows you to operate an ROV from miles away. Or, if you don’t want to bother with connecting cables to your computer, there is a community hack on Dozuki to build a Wi-Fi enabled tether management system. The Wi-Fi Topside is optimal for use as a short distance topside buoy or attached to a tether reel to simplify operation.
The PW201A is comprised of two boards just as the P200 that the OpenROV currently uses. The pins that connect the two boards are 3.3V, ground, and two signal lines. Conveniently, LEDs are built into the Wi-Fi module instead of the power board. Based on the current topside adapter, I designed a topside adapter for the wireless Tenda. The adapter PCB must provide 3.3V power, be powered by a battery pack, and send a voltage over the tether to turn the ROV on.
The first board I built to accomplish this used a linear regulator to power the Wi-Fi board. The AP1117 regulator that is used on the normal topside adapter cannot handle current spikes required by the Wifi Tenda, so I used the LM1085 linear regulator that is rated for 3A. However, the 3A regulator required a large heat sink, so I decided to use a switching supply instead. I replaced the linear regulator with a V7803-2000 switching power supply. Most of my testing so far has been with the V7803-2000 on the board pictured below.
I used a 3in plastic mail tube to build the Wi-Fi Buoy. The Wi-Fi Topside is powered with 9V from a normal battery tube, although this 26650 battery holder would be more compact. Scuba weights to keep the tube upright, and everything is mounted on a piece of 6mm acrylic. I added epoxy around the bottom endcap to waterproof the tube. A waterproof connector attaches to the neutrally buoyant tether that goes to the ROV. When the ROV is connected, I just flip the power switch to turn the buoy and ROV on.
I tested the buoy on a deployment to Tahoe. During the Tahoe testing, I had some latency issues, but it functioned well for most of the time.
I also tested the signal strength (RSSI) of the buoy at various distances with the following command. This command can be run on OSX by pasting the line directly into the application Terminal. The plot below shows the average (mean) RSSI reading from 4 different trials. I walked 2.5 meters with my computer and recorded the reading after it settled. Two trials were taken as I walked towards the buoy, the other two were recorded as I walked away. I used an area that was as free from obstructions as possible, but there were still some nearby metal buildings that could have had an impact on the signal. Without a better idea of an acceptable RSSI, this information is not terribly useful. However, this method could be useful in comparing different configurations or different Wi-Fi modules. For instance, repeating this experiment with the antenna at different distances out of the water will show how antenna height alters performance.
while x=1; do /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I | grep CtlRSSI; sleep 0.5; done
Apple computers can also show additional information about the Wi-Fi signal. The airport command below gives much more information about the signal. I did not measure the TX rate against distance, but it should be another useful measurement to determine if the Wi-Fi is good enough to use for ROV operation.
The second PCB replaced the V7803-2000 switching supply with discrete components to reduce cost. The switching IC is the RT8250, the same chip that supplies 5V to the OpenROV controller board.
Problems and Future Improvements
If the buoy is dropped or knocked, it will often reset. I’ve also had issues with the Tenda not connecting to the ROV even though the Wi-Fi still worked. The connection issue only happened after I handled the board while putting the buoy together. It is possible that I may have caused ESD damage to the electronics. If the electronics are still all right, this may be able to be fixed by resetting the Tenda with the Tenda software.
Future improvements include changing the battery holder and putting everything in a smaller enclosure.
Bill of Materials
The Bill of Materials below has links to places to purchase the materials I used for this project. The links do not necessarily show the exact product I used or the place where I purchased it, but I tried to be as accurate as possible. Note that many of these items can be found for cheaper or can easily be replaced with another item that is easier to find or less expensive.
- Wi-Fi Topside Interface Board
o Digikey Component List
o PCB on OSH Park
- Wi-Fi Tenda
- 3 in Mailing tube and endcaps
- 6mm acrylic mounting plate
- Detachable tether connector
- Battery Tube
- 3 x 26650 Batteries
- Power Switch
- 2 x 1 pound Scuba weights - This is a little more weight than necessary
- Glue – I initially used hot glue, but it hasn’t held well since it was exposed to the heat during the Lake Tahoe trip; I have not yet used super glue
- 2-Ton Epoxy
- Zip Ties
The following Github repository has the EAGLE files for the PCB and the ipt and dxf files for the acrylic mounting plate: