# Advice for a first boat of bathymetric cartography

Hello,
We are in the process of making our first boat.

Frame for prototype: boogie board
2 TrusterT200
Pixhawk Cube black
Survey GNSS: Comnav T300+ in RTK mode, NMEA GGA, RMC and VTG input to GPS1 connector.
Sonar for prototype PingSonar, but we buy a ECT400.

We have some questions about the parameters to be entered regarding the offsets to be filled in for IMU, GNSS RTK and sonar.
The IMU is behind the center of gravity X=0.193,Y=0 and Z= 0.03. We must enter the offset values IMU to center of gravity or center of gravity to IMU?
The GNSS is just above the center of gravity X=0, Y=0 and Z=0.429 (IMU-GNSS X=0.193, Y=0 and Z=0.399). We must enter the offset values GNSS to center of gravity or GNSS to IMU or vice versa?
Is sonar the same protocol as GNSS?

About the data log, for the POS values (lat,long and alt), it is the position of the boat after the EKF2? At the center of gravity or the Cube?

Do you have advice for EKF2 settings with a GNSS RTK?

Thanks
Ludo41

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Hello,
I finish my boogie board!

Now, I like setup my GNSS, Sonar and IMU offset, but Iâ€™m alway the same question about this:

1. itâ€™s GNSS to CoG, IMU to CoG and Sonar to CoG
or
2. itâ€™s IMU to CoG, GNSS to UMI and Sonar to IMU?

Equip_position.pdf (13.6 KB)
According to my scheme it is necessary to setup:
1- GNSS offset Z : 0.395, IMU offset X: 0.18, Z: 0.04 and Sonar offset X: 0.387, Z: -0.098
2- IMU offset X: 0.18, Z: 0.04, GNSS offset Z: 0.355 and Sonar offset X: 0.207, Z: -0.138
or
1- GNSS offset Z : -0.395, IMU offset X: -0.18, Z: -0.04 and Sonar offset X: -0.387, Z: +0.098
2- IMU offset X: -0.18, Z: -0.04, GNSS offset Z: -0.355 and Sonar offset X: -0.207, Z: +0.138

Can you help me?

Best Regard

Ludovic

2 Likes

Does anyone have an idea? Please.

Thanks Matt.

Can you confirm your idea for setup my offset:

IMU offset X: 0.18, Z: 0.04, GNSS offset X: -0.18, Z: 0.355 and Sonar offset X: 0.207, Z: -0.138

For you itâ€™s good?

Normaly I test end of this week on a lac, but itâ€™s not easy in France to obtain autorisation with Cov19 restriction.

I would have a very similar setup, I will use ECT400 (I already have it), ardusimple for RTK. My boat have two T100, but I will need a little bit more power, so I will buy two T200.
@Matt_C could you point me out to the models you are trying to print? I could try them too!

BTW, good project!

Nice!
Do you test it? I found the motors on aliexpress and want to buy two of them, but Iâ€™m concern about the design.

Thank

Hello,
After some test on lac and river, we modify our boat, and we plan to go to 4 T200 (2 right and 2 left) in 18.5V.
Is it possible to connect the 2 Esc (left for example) to the same SERVO output of the Pixhawk?

Has anyone ever tested it?

Thanks

Hello,
I am giving news of our project. Maybe our experience will help other people.
We put 2 T200 powered to 18.5V.
The ESC Basic Bluerobotic will be changed because they heat too much.
A short video, going up a current of 2.4m/s on the Loire (French river). The boat was at 2-2.5m/s.
Best regards
Ludo41

Hi,
Almost 2 years later I always ask some questions, if anyone can help me:

And:

Now, I like setup my GNSS, Sonar and IMU offset, but Iâ€™m alway the same question about this:

1. itâ€™s GNSS to CoG, IMU to CoG and Sonar to CoG
or
2. itâ€™s IMU to CoG, GNSS to UMI and Sonar to IMU?

Equip_position.pdf (13.6 KB)
According to my scheme it is necessary to setup:
1- GNSS offset Z : 0.395, IMU offset X: 0.18, Z: 0.04 and Sonar offset X: 0.387, Z: -0.098
2- IMU offset X: 0.18, Z: 0.04, GNSS offset Z: 0.355 and Sonar offset X: 0.207, Z: -0.138
or
1- GNSS offset Z : -0.395, IMU offset X: -0.18, Z: -0.04 and Sonar offset X: -0.387, Z: +0.098
2- IMU offset X: -0.18, Z: -0.04, GNSS offset Z: -0.355 and Sonar offset X: -0.207, Z: +0.138

Can you help me?

Best Regard

Ludovic

Hi @Ludo41,

The boat is looking good! By the way if you find that leaves, etc are getting stuck in the thrusters it might be good to add 10cm standoffs so they are lower in the water.

Itâ€™s â€ś1â€ť. When setting up the sensor position offsets they should all be setup relative to the vehicleâ€™s center of gravityâ€¦ or actually in the case of a boat it should be the vehicleâ€™s center of rotation which, on a skid steering vehicle, will probably be the bottom of the boat between the thrusters.

Hi @rmackay9 ,

best regards

Hi @rmackay9,

About the data log, for the POS values (lat,long and alt), it is the position of the boat after the EKF2 or at the center of rotation or the center of the Cube?
And Depth value, on data log, is from the face of sonar to the bottom of the lake or from the center of rotation or the center of the Cube to the bottom of the lake?

Thanks

Best regards

Ludovic

POS will also be the center of rotation of the vehicle.

The depth value will be from the face of the sonar. We donâ€™t do any adjustment of that sensor value.

Thanks @rmackay9 for this information.

Hello
I juste like more information about DPTH Lat and Lng, what is the different with Lat and Lng POS or Lat and Lng GPS?

Best regards

Ludovic

The major difference is timing.

• The GPS is an unfiltered GPS lat,lon logged at the moment the GPS provides an update (5hz)
• The POS message is the output of the EKF (combination of GPS, IMU, compass, etc) and is separately logged at some rate independent of any particular sensor
• The DPTH message is the same as POS except it is logged at the same time as the reading from the depth sensor arrives.

Hope that helps.

@rmackay9
Thank you very much, now itâ€™s very clear for me.

Best regards

1 Like