IMU

See also: Terms and Technical Terms

An Inertial Measurement Unit (IMU) is an electronic sensor device that measures and reports a body's specific force, angular rate, and sometimes the orientation of the body, using a combination of accelerometers, gyroscopes, and often magnetometers. IMUs are fundamental components in Virtual Reality (VR) and Augmented Reality (AR) systems for tracking the orientation of HMDs and Input Devices like controllers.

Components and Function

A typical IMU integrates multiple sensor types onto a microchip:

• Accelerometers: Measure proper acceleration (g-force), which includes both acceleration due to movement and the constant pull of gravity. When the IMU is relatively static, accelerometers can determine tilt (pitch and roll angles) relative to the direction of gravity. When moving, they measure linear acceleration.
• Gyroscopes: Measure angular velocity (rate of rotation) around one or more axes. In VR/AR, they detect rotational movements corresponding to pitch (nodding 'yes'), yaw (shaking 'no'), and roll (tilting head side-to-side). Gyroscopes provide fast and responsive rotational data but are prone to sensor drift over time.
• Magnetometers (Optional but common): Measure the strength and direction of the local magnetic field, typically the Earth's magnetic field. They act like a compass to provide an absolute reference for the yaw orientation, helping to correct gyroscope drift around the vertical axis. However, they are susceptible to interference from nearby magnetic materials or electronic devices.

When an IMU includes all three sensors (accelerometer, gyroscope, and magnetometer), it is sometimes referred to as a 9-axis IMU or a MARG (Magnetic, Angular Rate, and Gravity) sensor.

Sensor Fusion

Raw data from individual sensors can be noisy (for example accelerometers during fast movement) and inaccurate (for example gyroscopes drift). Sensor fusion algorithms, such as Kalman filters or complementary filters, are essential. These algorithms intelligently combine the data from the accelerometers, gyroscopes (and magnetometers, if present) to produce a single, more accurate, stable, and low-latency estimate of the device's orientation in real-time.

Role in VR/AR

IMUs are crucial for providing low-latency rotational tracking, which is essential for creating a sense of immersion and preventing motion sickness.

Importance in Head Tracking

IMUs provide the rapid orientation tracking needed to update the virtual view in sync with the user's head movements. This low latency is critical for user comfort. The typical update rate of modern IMUs used in VR headsets is between 500Hz to 1000Hz, much faster than most visual tracking systems can achieve alone.

3 Degrees of Freedom (DoF)

An IMU inherently provides 3 DoF tracking, measuring orientation changes (pitch, yaw, roll). This is sufficient for basic VR experiences like 360-degree video viewing on mobile VR headsets where the user's physical position in the room is not tracked.

6DoF Tracking Systems

For full 6DoF tracking (which includes positional tracking translation along X, Y, and Z axes), IMU data is combined via sensor fusion with data from other tracking systems. These can include:

• Inside-out tracking: Cameras on the HMD observe the external environment.
• Outside-in tracking: External sensors (like cameras or base stations) observe markers on the HMD and controllers.
• Camera-based tracking: General term encompassing various visual tracking methods.

In these systems, the IMU provides the high-frequency orientation updates, while the positional tracking system provides absolute position data and periodically corrects for any accumulated IMU drift.

Limitations and Correction

While essential, IMUs have inherent limitations:

• Sensor Drift: Gyroscopes accumulate small errors over time, leading to a gradual mismatch between the tracked orientation and the real-world orientation. This is particularly noticeable in yaw if uncorrected.
• Magnetic Interference: Magnetometers can be disturbed by ferrous materials or strong magnetic fields in the environment, leading to inaccurate yaw readings. Advanced sensor fusion algorithms may attempt to detect and compensate for such interference.
• No Positional Data: By themselves, IMUs cannot determine a device's position in space; they only measure rotation and linear acceleration, not absolute location or translational velocity relative to the world.

VR/AR systems address these limitations, particularly drift, through:

• Visual correction using cameras or external reference points (in 6DoF systems)
• Complementary filtering combining accelerometer (gravity vector) and gyroscope data for tilt correction
• Kalman filtering algorithms integrating multiple sensor inputs and predictive models
• Magnetometer data (if available and reliable) for absolute yaw correction
• Zero velocity updates (ZUPTs) during periods of detected stillness to reset velocity error accumulation

IMU Specifications for VR/AR

For optimal performance in VR/AR applications, IMUs typically require:

• Low latency (< 2ms sensor processing time desirable)
• High update rate (500-1000Hz)
• High precision gyroscopes (< 0.01 degrees/second drift)
• Low noise accelerometers
• Efficient power consumption
• Small form factor
• Integrated processing capabilities (sometimes including basic sensor fusion)

Future Developments

Next-generation IMUs for VR/AR are focusing on:

• Reduced power consumption for longer device battery life
• Smaller form factors for integration into lighter HMDs and glasses
• Integrated ML capabilities for improved motion prediction and pattern recognition
• Enhanced sensor fusion algorithms, potentially running on the sensor itself
• Further reduction in sensor noise and drift characteristics

Key IMU Manufacturers

Several companies manufacture IMUs used in consumer electronics, including VR/AR devices:

• TDK Invensense - Major provider for consumer electronics.
• Bosch Sensortec - Produces high-performance MEMS sensors.
• STMicroelectronics - Manufacturer of various MEMS sensors.
• Analog Devices - Often provides higher-grade IMUs.
• Xsens - Specializes in high-precision motion tracking modules often incorporating IMUs.

Notable IMU Models in VR/AR

• MPU-6050: A popular low-cost 6-axis IMU (accelerometer + gyroscope) from InvenSense, used in hobbyist projects and early devices like the Oculus Rift DK1.
• MPU-9250: An InvenSense 9-axis IMU (adds a magnetometer to the MPU-6xxx series capabilities). Used in some dev kits and controllers.
• ICM-42688-P: A high-performance 6-axis IMU from TDK InvenSense, known for its low noise and stability, used in the Meta Quest 2 headset.
• BMI085/BMI270: Bosch IMUs optimized for VR/AR applications, found in devices like the Valve Index controllers.
• LSM6DSO/LSM6DSOX: STMicroelectronics 6-axis IMUs used in various HMDs and AR glasses, including the HoloLens 2.