MAVLINKHUD

The Output Map: SERVOn_FUNCTION

Executive Summary

In ArduPilot, "Motor 1" is not physically tied to "Pin 1". Instead, there is a layer of abstraction called SRV_Channels. The Motor Mixer outputs to a logical "Function" (k_motor1), and you map that Function to any physical pin (SERVO5_FUNCTION) using parameters.

Theory & Concepts

1. Hardware Abstraction Layers (HAL)

A Hardware Abstraction Layer is a software bridge between the generic "Autopilot Brain" and the specific "Physical Pins" of a chip.

  • The Logic: ArduCopter doesn't know what an STM32 or an H7 chip is. It just knows "Motor 1".
  • The Bridge: SRV_Channels acts as the dispatcher. This allows the same ArduCopter firmware to run on hundreds of different boards with different pinouts.

2. Functional Mapping vs. Pin Mapping

  • Legacy Systems: You had to plug Motor 1 into Pin 1. If Pin 1 broke, the board was useless.
  • ArduPilot: Every pin is universal. If Pin 1 is dead, you can move the wire to Pin 8 and set SERVO8_FUNCTION = 33. The "Brain" never needs to know the hardware changed.

Architecture (The Engineer's View)

1. The Function Enum

ArduPilot defines a list of ~150 possible output functions in Aux_servo_function_t.

  • 33: Motor 1
  • 34: Motor 2
  • 35: Motor 3
  • 36: Motor 4
  • ...
  • 1: Manual (Passthrough)

2. The Conversion Pipeline (calc_pwm)

When the Mixer says "Motor 1 at 50%", here is what happens:

  1. Normalization: The mixer outputs a float 0.5.
  2. Scaling: SRV_Channel::calc_pwm() takes this float.
  3. Endpoint Lookup: It looks up the SERVOx_MIN and SERVOx_MAX parameters for the assigned pin.
  4. Math: 
    PWM = MIN + (Scaled_Value * (MAX - MIN))
    • Example: 1000 + (0.5 * (2000 - 1000)) = 1500us.
  5. Trim: If the function uses trim (like a servo surface), it centers around SERVOx_TRIM.

3. Safety Interlocks

The SRV_Channels library also handles the Safety Switch.

  • If the Safety Switch is active, SRV_Channels forces the output to 0 (or disarmed_pwm), regardless of what the Mixer requests.

Debugging Tips

  • "My Motor 1 is on Pin 5": This is valid. Set SERVO5_FUNCTION = 33 (Motor 1).
  • "My Servo moves backwards": Set SERVOx_REVERSED = 1. This flips the math logic (1.0 input becomes MIN pwm).

Source Code Reference

Practical Guide: Configuring a Gripper (Servo)

Let's say you have a 4-motor quadcopter (Channels 1-4) and you plug a Servo Gripper into Pin 5. You want to control it with Channel 9 on your transmitter.

Step 1: Physical Connection

  • Plug the Servo Signal wire into Pin 5 (MAIN OUT 5).
  • Warning: Most Autopilots do NOT provide 5V power on the servo rail. You must use a separate BEC to power the servo's Red/Black wires.

Step 2: Function Mapping

Tell ArduPilot what Pin 5 is.

  • Parameter: SERVO5_FUNCTION
  • Value: 59 (RCIN9).
  • Meaning: "Whatever PWM value comes in from Radio Channel 9, send it directly to Output Pin 5."

Step 3: Endpoints

Servos burn out if you drive them too far.

  • Open the Gripper: Toggle your switch. If it buzzes, reduce SERVO5_MAX (e.g., from 1900 to 1800).
  • Close the Gripper: Toggle switch back. If it buzzes, increase SERVO5_MIN (e.g., from 1100 to 1200).

Step 4: Reversing

If the switch is backwards (Up = Closed, Down = Open):

  • Parameter: SERVO5_REVERSED
  • Value: 1.