An inertial measurement unit (IMU) is a device equipped with many sensors that provides time-series data, used in human activity recognition (HAR) problems, tracking and navigation problems and many more.
What Is an Inertial Measurement Unit (IMU)?
An inertial measurement unit (IMU) is a device that uses various sensors to track the acceleration and angular velocity of an object over a period of time. On occasion, an IMU will also track the Earth’s magnetic field and air pressure.
In the artificial intelligence era, this cheap and reliable device can provide large volumes of data. By pairing it with machine learning and deep learning, we can generate useful insights across a variety of fields.
What Is an Inertial Measurement Unit?
An inertial measurement unit is one of the most common devices in the navigation field. It contains an accelerometer and gyroscope, and on occasion a magnetometer and a barometer. The accelerometer is responsible for acceleration measurements, while the gyroscope is responsible for angular velocity measurements. Each of these measurements are represented along a three-axis coordinate system. So, generally speaking, they yield a six-dimensional measurement time series stream.
Where Are IMU Sensors Used?
Actually, almost everywhere. Let’s start with our smartphones. Most smartphones contain an IMU based on micro-electromechanical systems (MEMS) technology, which supports small-scale, low-power motion sensors. It’s also placed in many tablet devices. IMU sensors are also very common in the automotive and aerospace industries, enabling a better determination of the vehicle position and orientation.
How Does an Inertial Measurement Unit Work?
Let’s divide the discussion into four parts based on the four different sensors.
Four Inertial Measurement Unit Components
- Accelerometer
- Gyroscope
- Magnetometer
- Barometer
1. Accelerometer
This is a sensor that measures the specific force (the body mass normalizes the force). It provides the acceleration across the x, y and z axes in its local frame.
2. Gyroscope
This is a sensor that measures angular velocity around the x, y and z axes, in its local frame. Measurements produced by a gyroscope can be computationally integrated to estimate orientation changes.
3. Magnetometer
This is a sensor that measures the Earth’s magnetic field and provides the heading (the compass is one such device). If it is included in the IMU, we commonly describe it as a “nine-axis IMU” since an IMU that includes an accelerometer, gyroscope and magnetometer now has three sensors that each provide data along three axes.
4. Barometer
This is a sensor that measures air pressure and can provide altitude.
Inertial Measurement Unit Limitations
One of the major disadvantages of IMUs is their accumulated error when you integrate them into navigation equations on their own. Accumulated error is often the result of drift, which refers to an IMU gradually losing its ability to accurately measure motion and location data over time. IMUs are also susceptible to noise that results from variables like shifts in temperature and vibrations, leading to accumulated error.
For example, double integration of the noisy acceleration leads to an inaccurate position solution that eventually results in a huge drift. Hence, many tracking and navigation applications use additional sensors, such as global navigation satellite system (GNSS) receivers and cameras. These sensors provide position information in lower frequency and reset the accumulated errors.
Advantages of An Inertial Measurement Unit
One of the famous time-series datasets is the human activity recognition (HAR) that contains recorded IMU signals, which was released in 2014. A collection of activities were tagged and stored at 50 Hz frequency. It is widely used to evaluate state-of-the-art time-series classification algorithms.
Many algorithms were established to recognize human activity based on IMU signals only, as it can be formulated as a classification problem. The major advantage of using IMU signals for HAR over images or video is the small size of data. This allows a compact representation of the HAR classifier.
Ultimately, the IMU is one of the common devices that uses sensors worldwide. It’s used almost everywhere for a wide variety of tasks.
Frequently Asked Questions
What is an inertial measurement unit (IMU)?
An inertial measurement unit (IMU) is a device that measures the acceleration and angular velocity of an object using different types of sensors. It’s often used in applications like smartphones, cars and spacecraft.
How does an IMU work?
An IMU uses various sensors, including accelerometers, gyroscopes, magnetometers and barometers. Together, these sensors capture data about an object’s motion, location and orientation.
What are the limitations of an IMU?
IMUs are susceptible to accumulated error. This can be caused by drift, where an IMU slowly loses the ability to measure an object’s location and motion data. IMUs are also sensitive to noise, which can result from temperature shifts, vibrations and other variables and contribute to accumulated error.
