An inertial measurement unit, or IMU, is a compact sensor package that helps a robot, drone, or phone sense how it is moving in 3D space. It usually contains a 3-axis accelerometer and a 3-axis gyroscope, and many IMUs also include a 3-axis magnetometer. These sensors provide measurements along the X, Y, and Z axes so a computer can estimate tilt, rotation, vibration, and heading.
IMUs matter because robots often need fast motion feedback even when cameras or GPS are unavailable.
Key Facts
- A 3-axis accelerometer measures specific force along X, Y, and Z, often in m/s^2 or g.
- A 3-axis gyroscope measures angular velocity about X, Y, and Z, often in rad/s or degrees/s.
- A magnetometer measures the local magnetic field and can help estimate compass heading.
- Acceleration from rest can be estimated by v = v0 + at, but small sensor errors grow over time.
- Angle change from a gyroscope can be estimated by theta = theta0 + omega t for constant angular velocity.
- Sensor fusion combines accelerometer, gyroscope, and sometimes magnetometer data to estimate orientation more reliably than any one sensor alone.
Vocabulary
- Inertial Measurement Unit
- An inertial measurement unit is a sensor module that measures acceleration and rotation, and sometimes magnetic field, to estimate motion and orientation.
- Accelerometer
- An accelerometer measures specific force along one or more axes, including the effect of gravity when the sensor is not in free fall.
- Gyroscope
- A gyroscope measures angular velocity, which tells how fast an object is rotating about an axis.
- Magnetometer
- A magnetometer measures magnetic field strength and direction, often used to help determine heading relative to Earth’s magnetic field.
- Sensor Fusion
- Sensor fusion is the process of combining measurements from multiple sensors to produce a better estimate of orientation or motion.
Common Mistakes to Avoid
- Treating accelerometer readings as pure motion acceleration is wrong because a stationary IMU still measures gravity as a specific force.
- Using only gyroscope integration for orientation is wrong because tiny bias errors accumulate into drift over time.
- Assuming the magnetometer always gives a clean compass heading is wrong because nearby motors, wires, and metal parts can distort magnetic field measurements.
- Mixing up body axes and world axes is wrong because the IMU measures in its own local coordinate frame, which rotates with the robot.
Practice Questions
- 1 A robot’s gyroscope reads 30 degrees/s about the Z axis for 4.0 s. If it starts at 10 degrees, what is its estimated yaw angle after 4.0 s?
- 2 An accelerometer at rest on a level table reads approximately ax = 0, ay = 0, az = 9.8 m/s^2. If the same sensor measures ax = 2.0 m/s^2 while the robot drives forward, what forward speed change occurs over 3.0 s if acceleration is constant?
- 3 A drone’s gyroscope shows a slow rotation even when the drone is sitting still, while the accelerometer indicates a stable gravity direction. Explain how sensor fusion can reduce the orientation error.