Sending custom MAVLink commands in Drone Programming - Step-by-Step Execution

Concept Flow - Sending custom MAVLink commands

Start

↓

Create MAVLink command message

↓

Set command parameters

↓

Send command to drone

↓

Drone receives and processes command

↓

Receive acknowledgment

↓

Check if command succeeded?

No→Handle error

Yes↓

End

This flow shows how to build, send, and confirm a custom MAVLink command to a drone.

Execution Sample

Drone Programming

msg = create_mavlink_command(command_id=300, param1=1.0)
send_command(msg)
ack = receive_ack()
if ack.success:
    print('Command succeeded')
else:
    print('Command failed')

This code creates a custom MAVLink command, sends it, waits for acknowledgment, and prints success or failure.

Execution Table

Step	Action	Command Parameters	Drone Response	Output
1	Create command message	command_id=300, param1=1.0	None	None
2	Send command to drone	command_id=300, param1=1.0	None	None
3	Drone processes command	command_id=300, param1=1.0	Processing	None
4	Receive acknowledgment	None	ack.success=True	None
5	Check acknowledgment	None	ack.success=True	Print 'Command succeeded'

💡 Command acknowledged with success, so execution ends after printing success message.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
msg	None	{command_id:300, param1:1.0}	{command_id:300, param1:1.0}	{command_id:300, param1:1.0}	{command_id:300, param1:1.0}
ack	None	None	None	{success: True}	{success: True}

Key Moments - 3 Insights

Why do we wait for an acknowledgment after sending the command?

Because the drone needs to confirm it received and understood the command. See execution_table step 4 where ack.success is received.

What happens if ack.success is False?

The program would handle the error, possibly retry or alert the user. This is shown in the flow where 'Check if command succeeded?' leads to 'Handle error' if No.

Why do we set parameters like param1 when creating the command?

Parameters tell the drone what exactly to do with the command. Without them, the command might be incomplete or ignored. See execution_table step 1 for parameters.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'ack.success' at step 4?

A None

B False

C True

D Error

💡 Hint

Check the 'Drone Response' column at step 4 in execution_table.

At which step does the drone start processing the command?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Action' column in execution_table for when processing happens.

If the command parameters were missing at step 1, what would likely happen?

A Drone might ignore or error on command

B Drone processes command normally

C Acknowledgment would be success

D Program prints 'Command succeeded'

💡 Hint

Refer to key_moments about why parameters are important.

Concept Snapshot

Sending custom MAVLink commands:
1. Create command message with command ID and parameters.
2. Send command to drone.
3. Wait for acknowledgment.
4. Check if command succeeded.
5. Handle errors if any.
Always set parameters correctly for expected behavior.

Full Transcript

This visual trace shows how to send a custom MAVLink command to a drone. First, the program creates a command message with a specific command ID and parameters. Then it sends this message to the drone. The drone processes the command and sends back an acknowledgment. The program checks if the acknowledgment indicates success. If yes, it prints a success message; if not, it handles the error. Tracking variables like the command message and acknowledgment helps understand the flow. Key points include waiting for acknowledgment to confirm command receipt and setting parameters to define command behavior.

Practice

1. What is the main purpose of sending custom MAVLink commands to a drone?

easy

A. To update the drone's firmware automatically

B. To control the drone with commands beyond the standard set

C. To recharge the drone's battery remotely

D. To change the drone's GPS coordinates manually

Solution

1. Step 1: Understand MAVLink command basics

   MAVLink commands include standard and custom commands to control drones.

2. Step 2: Identify the role of custom commands

   Custom commands allow sending special instructions not covered by standard commands.

3. Final Answer:

   To control the drone with commands beyond the standard set -> Option B

4. Quick Check:

   Custom MAVLink commands extend control [OK]

Hint: Custom commands add new controls beyond defaults [OK]

Common Mistakes:

• Confusing custom commands with firmware updates
• Thinking commands recharge battery
• Assuming manual GPS change via commands

2. Which of the following is the correct way to send a custom MAVLink command with 3 parameters in Python using pymavlink?

easy

A. vehicle.mav.command_long_send(target_system, target_component, command_id, 0, param1, param2, param3, 0, 0, 0, 0)

B. vehicle.send_command(command_id, param1, param2, param3)

C. vehicle.mav.send_command_long(command_id, param1, param2, param3)

D. vehicle.command_long_send(command_id, param1, param2, param3)

Solution

1. Step 1: Recall pymavlink command_long_send syntax

   The correct method is vehicle.mav.command_long_send with parameters: target_system, target_component, command, confirmation, and up to 7 params.

2. Step 2: Match the correct parameter order

   vehicle.mav.command_long_send(target_system, target_component, command_id, 0, param1, param2, param3, 0, 0, 0, 0) correctly uses target_system, target_component, command_id, 0 (confirmation), then params, filling unused with zeros.

3. Final Answer:

   vehicle.mav.command_long_send(target_system, target_component, command_id, 0, param1, param2, param3, 0, 0, 0, 0) -> Option A

4. Quick Check:

   Use command_long_send with full parameters [OK]

Hint: Use command_long_send with all 11 parameters [OK]

Common Mistakes:

• Omitting target_system or target_component
• Using wrong method names
• Not filling unused params with zeros

3. Given this Python snippet using pymavlink:

vehicle.mav.command_long_send(
    1, 1, 300, 0, 10, 20, 30, 0, 0, 0, 0
)

What does the number 300 represent in this command?

medium

A. The command ID for the custom command

B. The first parameter value

C. The confirmation flag

D. The target system ID

Solution

1. Step 1: Identify parameter positions in command_long_send

   Parameters are: target_system, target_component, command, confirmation, param1...param7.

2. Step 2: Locate the third argument

   The third argument (300) is the command ID, which specifies which command to execute.

3. Final Answer:

   The command ID for the custom command -> Option A

4. Quick Check:

   Third argument = command ID [OK]

Hint: Third argument is always command ID [OK]

Common Mistakes:

• Confusing command ID with target system
• Mixing confirmation flag with command ID
• Thinking parameters come before command ID

4. You wrote this code to send a custom MAVLink command but the drone does not respond:

vehicle.mav.command_long_send(1, 1, 400, 1, 5, 10, 15)

What is the likely problem?

medium

A. The command ID 400 is invalid

B. Target system and component IDs must be zero

C. The confirmation flag should be 0, not 1

D. Missing required parameters; command_long_send needs 11 arguments

Solution

1. Step 1: Check command_long_send argument count

   command_long_send requires 11 arguments: target_system, target_component, command, confirmation, and 7 params.

2. Step 2: Identify missing arguments

   The code only provides 7 arguments; missing the last 4 parameters which should be zero if unused.

3. Final Answer:

   Missing required parameters; command_long_send needs 11 arguments -> Option D

4. Quick Check:

   command_long_send needs 11 args [OK]

Hint: Always provide 11 arguments to command_long_send [OK]

Common Mistakes:

• Passing fewer than 11 arguments
• Assuming confirmation must be zero
• Using invalid target IDs

5. You want to send a custom MAVLink command that sets a drone's LED color using command ID 2100 with parameters for red, green, and blue values. Which Python code snippet correctly sends this command assuming target system and component IDs are 1 and 1, and you want red=255, green=100, blue=50?

hard

A. vehicle.mav.send_command(2100, 255, 100, 50)

B. vehicle.mav.command_long_send(1, 1, 2100, 1, 255, 100, 50)

C. vehicle.mav.command_long_send(1, 1, 2100, 0, 255, 100, 50, 0, 0, 0, 0)

D. vehicle.command_long_send(2100, 1, 1, 0, 255, 100, 50, 0, 0, 0, 0)

Solution

1. Step 1: Use correct method and argument order

   command_long_send requires target_system, target_component, command_id, confirmation, then 7 params.

2. Step 2: Fill unused parameters with zeros

   Only 3 parameters used for RGB; remaining 4 must be zero to complete 7 params.

3. Step 3: Confirm target IDs and confirmation flag

   Target system and component are 1, confirmation is 0 (no confirmation needed).

4. Final Answer:

   vehicle.mav.command_long_send(1, 1, 2100, 0, 255, 100, 50, 0, 0, 0, 0) -> Option C

5. Quick Check:

   Correct method, full params, proper IDs [OK]

Hint: Fill unused params with zeros, use confirmation=0 [OK]

Common Mistakes:

• Omitting zeros for unused parameters
• Using wrong method name
• Swapping target system and component order