Mocap Offboard going to local position

shortcut21 · November 20, 2018, 8:18am

Hello
I connected my drone with a custom mocap device which determined the local position of the drone, so if the device is 0,0,0 the drone is -1,-2,-3 for example. I want to write a script that makes the drone why to the device, what is the best way. I tried the mavros offboard example, but it didn’t work well. It armed and took off gave me a message that it reached the goal but then instead of landing it crashed in my wall. What should I do?

JulianOes · November 20, 2018, 8:21am

I think you need to check the log and figure out why it crashed into the wall. Switching to e.g. the Dronecode SDK probably won’t help because it would do the same as MAVROS.

shortcut21 · November 20, 2018, 8:37am

Thanks.
https://review.px4.io/plot_app?log=33fd4851-3afd-410d-ac8e-bfbadada2c47

Can you help on the review?

JulianOes · November 20, 2018, 9:10am

So I can see that first the vehicle was in offboard mode for a short time. Then it lost offboard mode and went into descent, then at some point, offboard is back.

Also, during the second offboard face it doesn’t seem to track roll/pitch setpoints at all. Does the vehicle fly well in manual mode? And does it fly properly in position control? It’s the prerequisite that this works properly. Also, have you checked that all directions in the mocap system are correct and yaw/heading is correct?

shortcut21 · November 20, 2018, 9:24am

Thank you, @JulianOes! I will take a look and test again.

shortcut21 · November 22, 2018, 10:04am

Hello, @JulianOes! I tested my system again. It works well in manual mode. Last time I didn’t set the heading param, but now I did. I tried again the previous script, but the thing is that the initial position (smilar to -0.2, 0.3, 0.11) is close to the 0,0,0(the motion capture device) and I when want to go a meter above it - for example 0,0,1, some props refuse to spin/move. How to achieve that going over the center having the initial position, that is coordinates regarding the center? How to write appropriate script to first reach the z and then move to the x and y over the mocap device? Please help me out.

JulianOes · November 22, 2018, 6:19pm

I would first only set the new z and then compare the telemetry so that it reaches the altitude that you want. Once it reached the altitude, you give it the horizontal position that you want to go to. Also, make sure to test in SITL first to get a feel for what will happen.

shortcut21 · November 26, 2018, 2:58pm

Can you specify what do you mean by “compare the telemetry so that it reaches the altitude”? To what should I subscribe to get the altitude? Global position/rel_alt or get it from mavros_msgs/Altitude?

One more thing, how can I avoid movement after it has reached the desired position? How can I keep it in one like-“hold” position?

JulianOes · November 26, 2018, 3:10pm

Can you specify what do you mean by “compare the telemetry so that it reaches the altitude”? To what should I subscribe to get the altitude? Global position/rel_alt or get it from mavros_msgs/Altitude?

You can subscribe to this info:

github.com

mavlink/MAVSDK/blob/ef45e4ddb7a8c5dcd0ac20df9a330afdaa9a7be6/plugins/telemetry/include/plugins/telemetry/telemetry.h#L43


      
           */
          ~Telemetry();
          
          /**
           * @brief Position type in global coordinates.
           */
          struct Position {
              double latitude_deg; /**< @brief Latitude in degrees (range: -90 to +90) */
              double longitude_deg; /**< @brief Longitude in degrees (range: -180 to 180) */
              float absolute_altitude_m; /**< @brief Altitude AMSL (above mean sea level) in metres */
              float relative_altitude_m; /**< @brief Altitude relative to takeoff altitude in metres */
          };
          
          /**
           * @brief Position type in local coordinates.
           *
           * Position is represented in the NED (North East Down) frame in local coordinate system.
           */
          struct PositionNED {
              float north_m; /**< @brief Position along north-direction in meters. */
              float east_m; /**< @brief Position along east-direction  in meters. */

here:

github.com

mavlink/MAVSDK/blob/ef45e4ddb7a8c5dcd0ac20df9a330afdaa9a7be6/plugins/telemetry/include/plugins/telemetry/telemetry.h#L507-L512


      
          /**
           * @brief Subscribe to position updates (asynchronous).
           *
           * @param callback Function to call with updates.
           */
          void position_async(position_callback_t callback);

One more thing, how can I avoid movement after it has reached the desired position? How can I keep it in one like-“hold” position?

You just keep commanding that same position, so it should stay there. That’s if you’re commanding position. If you’re commanding velocity, you would command 0 velocity.

shortcut21 · November 27, 2018, 7:37am

Thanks a lot, Julian!
What about without DronecodeSDK? Can you give me the same information in MAVROS example? Since I am not using the Dronecode yet. Is it better to use it and use velocity commands or not?

JulianOes · November 27, 2018, 8:35am

I don’t know MAVROS but in the docs I found this:
http://wiki.ros.org/mavros#mavros.2BAC8-Plugins.setpoint_position
http://wiki.ros.org/mavros#mavros.2BAC8-Plugins.setpoint_velocity

Is it better to use it and use velocity commands or not?

It depends what you want to achieve. Do you want to move the drone to a specific point (position control) or do you want to move it at a specific velocity until you say stop (velocity control)?

shortcut21 · November 27, 2018, 9:32am

I want to achieve more like position control, so it’s okay. Thanks for the help and the fast responses. If I still struggle with something I will reach out to you.

shortcut21 · November 30, 2018, 3:20pm

I have some troubles. My drone refuses to take off to a certain distance and then move forward. It literally heads straight forward, what should I chance or do?

JulianOes · December 2, 2018, 10:17am

Give it a setpoint to go up first. Then after some time or when it has reached the point you can go forward. You’re in control with the setpoints and should be able to guide the drone how you want.