Send position and velocity commands

What you will do

You will write and run the offboard node from lesson M6-T11, extend it to velocity control, and verify the stream rate and the failsafe when the stream stops.

Before you start

• Lab “Connect ROS 2 with PX4” completed (topics visible, px4_msgs built).
• Lesson M6-T11 (offboard control) and lesson M1-T4 (frames).

Part 1: position setpoints

1. Create a Python ROS 2 package and add the offboard node from lesson M6-T11 (check the message field names in your px4_msgs).
2. Run PX4 SITL, the agent, and the node. The vehicle should switch to offboard mode, arm and climb to 5 m.
3. Change the setpoint to a square of 5 m sides, advancing to the next corner when within 0.5 m (read /fmu/out/vehicle_local_position).
4. Measure the setpoint publication rate with ros2 topic hz.

Part 2: velocity setpoints

1. In OffboardControlMode set velocity = True and position = False. In TrajectorySetpoint set the position fields to NaN and fill velocity in NED m/s.
2. Command 1 m/s north for 5 s and then stop. Check that the vehicle’s measured velocity follows and plot setpoint against measurement from the ULog.

import math
# In the setpoint publisher, for velocity control (NED, m/s). NaN means "not controlled":
sp.position = [math.nan, math.nan, math.nan]
sp.velocity = [1.0, 0.0, 0.0]        # 1 m/s north
sp.yaw = 0.0

Part 3: failure experiments

• Pause the publisher for 2 s (for example time.sleep in the timer callback) and record what the vehicle does.
• Send a position setpoint with a positive $z$ (underground) and observe, then correct.
• Run the planner computation on the same thread as the publisher with an artificial 300 ms delay and measure the stream gaps.

How to know it worked

• The vehicle enters offboard mode and flies the square with an acceptance radius.
• The velocity setpoint is tracked (plot from the log).
• You observed the dropout failsafe and can state how the setpoint stream timing caused it.

If it does not work

References

• PX4: Offboard mode
• PX4: ROS 2 offboard control example

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