Servo

This article explains how to connect Servos to flight controller, and how to control them from the transmitter or ground station (i.e. Mission Planner).

Overview

Copter, Plane and Rover can control servos for any purpose, including: triggering a camera shutter, releasing a parachute or dropping a tennis ball. These servo(s) can be controlled either directly by the pilot via a switch on the transmitter, through commands sent from the ground station or as part of a mission.

Either analog or digital servos may be used.

Connecting the servo to a Pixhawk

../_images/Servo_Pixhawk.jpg
  • If using Copter Connect the servo to AUX OUT 1, 2, 3 or 4. MAIN OUT 1 ~ 8 should be avoided because these update at 400hz.
  • If using Plane or Rover where all pins update at 50hz, any unused MAIN OUT or AUX OUT 1 ~ 4 may be used.
  • AUX OUT 5 and 6 cannot be used by default because they are setup as Relays. These pins can changed to Servo outputs by setting the BRD_PWM_COUNT parameter to 6 and setting RELAY_PIN and RELAY_PIN2 to -1.
  • The Pixhawk flight controller cannot provide power to the servos so an external BEC or ESC that can provide 5V should be used.

Connecting the servo to an APM2.x

../_images/OptionalHardware_Servo.jpg
  • On Copter connect the servo’s signal wire to either A10 or A11 on the left side of the APM
  • On Plane the servos may also be connected to any available channel RC1 ~ RC8 on the back of the APM.
  • Connect the power and ground wires to the rear power rail (if the JP1 jumper is removed) or to an external BEC (if the JP1 jumper is left in place). It is absolutely critical that the servos power and ground wires not be connected to the + and - pins on A10 or A11 because the servos will then be sharing their power source with the main CPU. Movements of the servo will cause ripples in the power supply to the CPU and could cause brown-outs. For the same reason the rear power rail should not be used if the JP1 jumper is left in place.

Controlling the servo as a camera shutter

Details on how to set-up and control the servo as if it were triggering the shutter of a camera can be found here on the Camera Gimbal wiki page. The advantage of this set-up is:

  • the servo can be triggered from the ch7 or ch8 auxiliary switch on Copter
  • if you wish the servo to move to one position, delay for a moment then return to it’s original position this can all be accomplished with a single mission command
  • the location and attitude of the copter will be recorded to the dataflash each time the servo is triggered

The disadvantage of using the camera shutter method is the servo can only be moved to two pre-configured positions.

Video example of a tennis ball dropped used at Sparkfun’s AVC 2012.

Controlling the servo as a servo

The traditional way to control a servo only works as part of a mission (i.e. AUTO mode). Follow these instructions:

  • Connect to your flight controller from the ground station

  • On the Config/Tuning > Full Parameter List page, ensure that the SERVOx_FUNCTION (or RCx_FUNCTION) is set to zero for the servo (i.e. SERVO9_FUNCTION = 0 if the servo is connected a Pixhawks AUX OUT2).

  • Press the Write Params button

    ../_images/MissionPlanner_ServoSetup.jpg
  • Create the mission you wish to fly and add a DO_SET_SERVO command and include the servo number (i.e. “10”) in the “Ser No” field and the PWM value (usually between 1000 ~ 2000) in the “PWM” field.

    ../_images/mp_move_servo.jpg

Note that the DO_SET_SERVO command is a “do command” meaning that it can only be run between waypoints so it must not be the first or last command in the mission. It will be executed immediately after the waypoint that precedes it.

Testing with the Mission Planner

The mission planner’s Flight Data screen includes a “Servo” tab on the bottom right that can be used to test that the servos are moving correctly.

../_images/Servo_TestingWithMP.jpg