What is a Gimbal?

A gimbal is a pivoted support that lets an object rotate about an axis.

Definition

The Collins dictionary describes a gimbal as a device of two or three pivoted rings at right angles to each other. The rings give the object free suspension in all planes. Gyroscopes and compasses are the usual examples. The word was first recorded in 1570 to 1580.

With three gimbals, the object on the innermost ring stays independent of the rotation of its support. A ship’s compass stays upright as the ship pitches and rolls.

Gimbals in aircraft instruments

The FAA uses the gimbal in its description of gyroscopic flight instruments. A gyroscope on a set of gimbal rings can rotate freely in any direction. When the rings tilt, twist or move, the gyro stays in the plane in which it first spun. The FAA calls this rigidity in space.

The FAA glossary defines a gimbal ring as a support that lets an object, such as a gyroscope, stay upright when its base tilts. A double gimbal lets the gyro move freely in two planes. The turn-and-slip indicator uses a single gimbal, which limits the planes in which the gyro can tilt.

These are mechanical gimbals inside an instrument. A camera gimbal on a drone is a related mount that holds a camera and uses motors and sensors.

Drone gimbal

In UAV work, a gimbal is a device that maintains the orientation of the camera and dampens vibration. It connects the drone to the camera. A drone gimbal has electric motors, usually brushless servo motors, on the yaw, pitch and roll axes.

A multirotor drone shakes its camera with motor vibration and sudden altitude changes. A study in the journal Sensors recommends a gimbal on such drones to get sharp images and the planned endlap and sidelap. Fixed-wing flight is more stable, and fixed-wing drones rarely use gimbals.

Other sensors can ride on a gimbal too. The MAVLink gimbal protocol controls the attitude of cameras or other sensors that sit on a drone.

How the stabilization works

The control loop of a gimbal has five steps. This description follows the Sensors study of a low-cost 3-axis gimbal.

1. An IMU on the camera holder measures motion. It has a 3-axis accelerometer and a 3-axis gyroscope.
2. Filters process the raw readings. A Kalman filter is the usual choice.
3. An attitude and heading reference system (AHRS) calculates the orientation angles.
4. A PID controller calculates the motor commands.
5. A motor driver moves the camera to cancel the motion.

The gimbal in that study ran its control loop at 700 Hz. Other gimbals use other rates. The gimbal also had a second IMU on the controller board, mounted independently of the motors. The authors say a second IMU adds accuracy and keeps the yaw axis more stable.

Some gimbals have their own stabilization controller. ArduPilot calls these brushless direct-drive gimbals. The flight controller then sends only the target angle, by PWM, SBUS, MAVLink or a serial interface. With a servo gimbal, ArduPilot does the stabilization.

2-axis and 3-axis gimbals

A 3-axis gimbal controls pitch, roll and yaw. A 2-axis gimbal controls pitch and roll only. ArduPilot supports gimbals with up to 3 axes. Some controllers, such as SimpleBGC and SToRM32, exist in 2-axis and 3-axis versions.

Yaw differs from the other two axes. Gravity defines pitch and roll. Yaw is relative to the drone or to north.

Angles, frames and modes

The pitch angle sets the tilt of the camera. In ArduPilot, a negative pitch points the camera down. A pitch of -30 degrees in the Earth frame is 30 degrees below the horizon. In QGroundControl, Tilt 90 points the mount directly downwards, which is the nadir position.

Two frames exist. The body frame measures the angle from the mount itself. The Earth frame measures it from the horizon and the compass heading.

Systems name the yaw behavior in different ways.

• ArduPilot has Yaw Lock and Yaw Follow. Lock keeps an Earth-frame yaw target. Follow moves the camera with the drone.
• MAVLink has flags for yaw relative to the vehicle and yaw relative to north.
• PX4 can hold a constant attitude. It keeps roll level, yaw at north and a set pitch.

A region of interest (ROI) locks the camera on a ground point. The gimbal then tracks the point as the drone moves. A command to cancel the ROI returns the mount to its default mode. In the MAVLink protocol, a gimbal manager resolves conflicting commands and passes them to the gimbal device. One manager serves one device.

What a gimbal does for survey images

A gimbal holds the camera in a set position. This is mostly horizontal or vertical, and other positions are possible within the limits of the gimbal.

The Sensors study tested a low-cost 3-axis gimbal. In a flight simulation, the camera roll and pitch stayed within a range of 2.56 and 1.97 degrees. The standard deviations were 0.46 and 0.36 degrees. The platform itself moved over a range of 69.90 and 93.18 degrees. In real flights, the stability of the camera orientation improved 3 times with a 15 degree tilt limit indoors. It improved 6 times with a 25 degree tilt limit outdoors. The authors conclude that the gimbal gives better image geometry, endlap and sidelap. These numbers come from one low-cost gimbal. Another gimbal gives other results.

Nadir and oblique images

The gimbal tilts the camera, so one camera can take nadir and oblique images. A 2021 ISPRS paper describes a MAVLink command that links camera triggering to gimbal movement. A gimbaled camera then takes oblique images in the same pass as nadir images, without a multi-camera rig.

Oblique images matter for maps and 3D models. James and Robson found that UAV image networks with near-parallel viewing directions can show a broad vertical doming error. Adding oblique images reduces it. The nadir imagery entry explains the geometry.

Gimbal calibration

Gimbal calibration mostly means the calibration of the IMU. Low-cost IMU sensors have imprecise scale factors and axes that are not exactly perpendicular. Calibration reduces these errors.

The Sensors study used a 1-point calibration. The IMU stayed still on a level surface while the controller measured the accelerometer data. The authors say to avoid vibration during calibration and to calibrate at an air temperature close to the temperature of use.

Other gimbals calibrate in other ways. One archived ArduPilot controller runs an IMU calibration at power-up, and the pilot must not touch the gimbal for the first 10 seconds. A solid red LED shows that the calibration is complete. The steps differ for each controller. Follow the manual of the gimbal that you use.

Drift is a related effect. The low-cost gimbal in the Sensors study drifted about 1 degree per minute on the roll axis. The authors reduced it almost completely when they set the AHRS to use only accelerometer data. They think the software could not calibrate the gyroscopes.

Camera calibration is a different process. It finds the interior orientation of the camera, such as its focal length and lens distortion.

Specifications

Gimbal specifications are product data, not a standard. For example, the DJI Zenmuse H20 data sheet lists an angular vibration range of ±0.01 degrees. It lists a mechanical range of -132.5 to +42.5 degrees in pitch, -90 to +60 degrees in roll and ±330 degrees in yaw. The controllable range is -120 to +30 degrees in pitch and ±320 degrees in yaw. Compare the data sheets of the products that you use.

Gimbals in airfield inspection

TarmacView surface scans hold the gimbal at -90 degrees, the nadir position. The flight plan of TarmacView stores a gimbal pitch for each waypoint. After a flight, TarmacView compares the flown gimbal pitch with the pitch that the plan commanded. It flags a flown pitch that stays outside the commanded range for longer than a planned gimbal action takes, which points to a manual input. The check is advisory and does not change the result.