December 14, 2018 at 13:59 #3063steven.lebarsParticipant
I did a rov survey mission on a seawall this week with my BR2 and your waterlinked system pole mounted from a boat.
The objective for the client was to see if we could get the acurrate position of the ROV on a dwg map supplied by client with the drawing of its seawall and position of each block on it.
The survey area is located between 0 and 4m depth, the rov navigates then between 0.5 and 3m depth. The poles were immerged of 1.5 and 2m WD roughly 40m away from the survey area. I can send you pictures by email of the pole mounted system and the area we have to survey.
We took the global position of the ROV and input it in a GIS software for the positionning on the dxf map. Furthermore, we used an external RTK GPS to get a more accurate global position than with the supplied antenna.
I encountered several doubts and problems with the waterlinked for which I would appreciate clarifiations in order to evaluate feasability of the project in case I try it again:
1. There was no way to calibrate the master D1 IMU, I turned it in 8 for a while then a yellow arrow on top became green then after a couple of minutes it turned yellow again and the same IMU error appeared each time. Can you please tell me what did I do wrong? Because of that, the position was never accurate of course…
2. Is it a problem if the locator is above the receivers (when the ROV is at 0.5/1m WD)?
3. When using an external GPS antenna. Can the position of the receivers be configured relative to the antenna instead of the master D1? Or does the antenna absolutely need to be exactly above the master D1?
4. The location of the ROV would often be unstable even when I did not move but I could not confirm if this was because of the reflectivity of the seawall or of the IMU. Do you think a submetric precision is achievable in such a reflective and shallow water environment?
Thank you in advance
Steven LE BARS
ID OCEANDecember 14, 2018 at 14:10 #3064
I found the same behavior when working close to walls, so we thought that it is due to acoustic reflections from the wall. There where also water pumps working close to the ROV, so we are not sure about the real reason… I have also seen the IMU calibration issue since the last Software update.
Looking forward to see some light into these issues!
EnriqueDecember 17, 2018 at 14:33 #3075Oliver SkislandKeymaster
1. To achieve optimal results, we recommend using an external IMU just like you did with the RTK GPS system. The onboard IMU can be challenging to keep calibrated. We are aware of this issue and are looking into ways to make it perform better.
2. This is normally not a problem. Both the Locator and Receivers are omnidirectional and will transmit/receive signals in all directions. There is one potential issue though; On the BlueROV2, the normal Locator placement is on one of the sides and all the way to the top of the ROV. With this setup and the receivers located deeper than the ROV, the ROV will be in the way of direct acoustic communication when the ROV is located such that the Locator is furthest away from the Receivers. To mitigate this, you can try to lift the receivers all the way up to just below the surface. If you have limited waves this can work just fine.
3. This is actually the correct way to do it. If you are using an external GPS antenna, the Receivers should be measured relative to the antenna. That way the GPS antenna will be the center and the location of the Master-D1 will not matter.
4. In highly reflective environments, we suggest that you reduce the search area as much as possible. Especially in the direction where the reflective area is. If you had a reflective area 40 meters away from the receivers, you should reduce the search area to ~40 meters in that direction. You can also try to expand the distance between the receivers and to keep the receivers at different depths.
If you are able to send us some pictures of your setup as well as the config screens and diagnostic screen in the GUI, we will look into those and see if we can see any potential issues.
OliverDecember 17, 2018 at 18:52 #3076
Thanks for these notes! Do you have any recommendation for an external IMU?
EnriqueDecember 18, 2018 at 08:10 #3082steven.lebarsParticipant
Ok thank you for these clarifications.
Too bad your IMU has so much trouble on a boat, this added to the difficulty of mobilization limits the capacities of the system in open seas.
I would like to do another test in january in order to confirm all that, try to reduce search area and put the sensors closer to surface. Can you advise on the IMU model you would recommend for easy integration on the GUI? From what I understood, there is no roll/pitch compensation in the master D1, it is only a heading sensor right? If yes, can’t we just use 2 external aligned GPS antenna to get the heading like in bathymetric survey? If roll/pitch is considered, can a mems based IMU do the job like the ones from xsens?
I would love to send pictures but I cannot find how do it on the forum. I did not record anything from the diagnostic page though.
StevenDecember 18, 2018 at 08:22 #3083wlkhKeymaster
If you are operating from a boat that has a compass module available over NMEA this is a good option.
We’ve been feeding the compass (and GPS) from NMEA on the boat to the Underwater GPS using this script:
https://github.com/waterlinked/examples#about-nmeainputpyDecember 18, 2018 at 09:33 #3084wlkhKeymaster
You are right, only heading is needed. This means that using 2 aligned GPS antennas will be a good way of getting a high precision heading.
Keep in mind that the Underwater GPS expects the heading to be aligned with the Y axis. The easiest is to keep Y pointing towards the front of the boat when measuring the distances to the receivers.
-KristianDecember 29, 2018 at 11:19 #3254
Thanks for your answer, I will try getting the boat’s heading!
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