Calibration of a Drone

Contributed by Abhimanyu Vijayraghavan

Calibration in drones is the process of setting or correcting errors which could cause inaccurate sensor measurements leading to faulty altitude estimation and positioning. It checks and determines parameters by comparing against a standard reference model to make corrections. Things like differing magnetic pulls, a hard landing, and other factors skew the drone’s sensors; in particular the accelerometer and magnetometer (compass) and in the case of a self-build, the frequent loading and unloading of parts and the subsequent change in their positions no matter how miniscule can add up to significant errors on flights.

Calibration is required to meet the accuracy and reliability levels demanded of the drone. If not performed or done incorrectly, the drone will not be able to achieve its intended mission targets, be it capturing and transmitting data from a specific place(s), or delivering an object remotely, ruining the mission and potentially risking a crash. When performed successfully, the target will be reached accurately enough to carry out photo/ video capture as planned.

Prerequisites:

The drone’s battery needs to be fully charged

Calibration has to be done relatively close to the site of flying only. Yes, having a set-up drone will ensure a quicker calibration but nonetheless will need to be re-calibrated as close as possible to where the flying will take place.

Open, relatively obstruction free area during calibration

A cold climate is preferred to avoid heating the drone. This is essential for IMU calibration

Magnetometer:

The easiest way to know if the magnetometer is not aligned with the Earth’s true north is to point the front of the drone towards a landmark and then look at Mission Planner to see if the red heading line from the drone’s icon is also pointing at the same landmark. If it is more than a few degrees off, then this is a telltale sign that the compass needs to be re-calibrated. Another giveaway is if the drone flies like it’s in a “toilet bowl” (flying in circular, swirling movements) while in position hold/ altitude hold mode.

To recalibrate the magnetometer, it must collect a series of data points so it can correctly orient to the earth’s local magnetic field. Start by opening Mission Planner and select Initial Setup, then Mandatory Hardware, followed by Compass. Make sure the Manual Rotation is set to NO and then select Live Calibration.

When in Live Calibration, it will ask you to rotate the drone in circular motions. This allows for the GPS module to read the gravitational pull at your location. As you revolve the drone in your hand, data points surrounding the drone are collected and start to build a ball surrounding your drone’s center. You must fill in the points that are shown on the mission planner.

A similar pattern is followed for store bought drones too, wherein the application (iFly for example) or the transmitter will prompt you to spin the drone horizontally and vertically until calibration is complete.

Note: The compass always has to be kept away from magnetic objects at all times. Any magnetic interference or electronic disturbance during calibration or flight could result in flyaways.

IMU or Inertial Measurement Unit (Accelerometer and gyroscope):

The IMU is responsible for keeping the drone stable and balanced as it flies and for speed and directional control. If not properly calibrated, the flight path may become erratic and potentially unsafe.

To calibrate, open Mission Planner, select Initial Setup, then Mandatory Hardware, followed by Accel Calibration. It will guide you through the process asking you to hold the drone level on it’s supports, then lean it on it’s left side, then it’s right side, then angle it nose down then nose up, and finally rotate it on its back so the belly of the drone is facing the sky.

​The Accel Calibration will guide you through the entire process and signal at each step. Once completed, your accelerometer will read a correct artificial horizon when level.

Pitot/ Airspeed sensor:

During a basic Pitot probe calibration, the total and static pressure ports are pneumatically connected to a test instrument. The instrument applies a sequence of pressures and records the corresponding airspeed readings. More elaborate test equipment also simulates the variation of static pressure with altitude and temperature. Once assured of the pitot tube's accuracy, it needs to be calibrated as part of the preflight calibration after powerup to set ground barometric pressure and airspeed sensor using Mission Planner.

This airspeed reading is essential for autonomous flight modes particularly for fixed wing and VTOLs.

Gimbal:

The gimbal being a sensitive part needs calibration after handling or changing the camera filter, rough handling/ landing as well as after every IMU calibration. It can be performed through the transmitter in most store bought drones

Vision sensors:

The front camera and sensors placed around the drone help it interpret its surroundings and avoid collisions. DJI provides a camera calibration feature wherein the user can connect their drone to a laptop/ PC and follow a step by step guide to correct their camera.

How often do they need to be done?

IMU and Magnetometer calibrations have to be performed when you are flying after a long time or are flying at a new location. Additionally, if there are compass interference messages or if the drone is flying awkwardly and not in a straight line

The gimbal has to be calibrated after each IMU calibration. It only takes a few moments. Additionally, it can be manually tilted by a few degrees along its axes through the transmitter to a new ‘mean’ position if there is a horizontal tilt

Conclusion:

In all, a well calibrated drone can help complete your intended mission objectives successfully as well as quickly without expending too many resources or time. It will be a performance booster, adding to trouble free flying that will make the entire experience gratifying.

Link to Mission Planner documentation : https://ardupilot.org/planner/docs/mission-planneroverview.html#:~:text=Mission%20Planner%20is%20a%20ground,supplement%20for%20your%20autonomous%20vehicle.