A Complete Guide to Inertial Measurement Unit (IMU)

What Are the Components of IMU and How Do They Work?

An Inertial Measurement Unit operates by detecting linear acceleration through accelerometers, measuring rotational rate with gyroscopes, and, in some cases, employing a magnetometer for heading reference.

Each principal axis—pitch, roll, and yaw—typically includes one accelerometer, gyroscope, and magnetometer, providing comprehensive data on the object's motion and orientation in three-dimensional space.

Accelerometers

Accelerometers, the nifty devices that gauge and relay specific forces, come in a variety of flavors, including mechanical, quartz, and MEMS accelerometers.

Mechanical accelerometers can achieve in-run bias stabilities less than 1 µg but are mainly used in navigation-grade applications due to their size and cost.

Quartz and MEMS accelerometers offer in-run bias stability ranging from 1000 µg to 1 µg, covering various performance categories. With high precision and stability, they are often employed in industries where accurate measurement of acceleration is essential, such as aerospace, defense, automotive testing, and geophysical exploration.

In IMUs, the accelerometers often use MEMS technology, which stands for microelectromechanical systems.

These accelerometers have a tiny mass connected to a reference system by a spring.

This setup allows them to measure how fast something is speeding up or slowing down.

They keep track of the mass's movement using capacitors, and special electronic components.

When the accelerometer is not moving, the mass creates a specific capacitance, like a baseline, showing no acceleration.

But when there's acceleration, the mass moves, and this changes the capacitance.

The accelerometer then measures this change electronically, adjusts it for accuracy, and processes the data to figure out how much acceleration is happening.

Source from siliconsensing.com

This whole process follows Hooke's law and Newton's second law, where the mass moves in proportion to the applied acceleration.

In simple terms, it's like a tiny sensor that senses how fast something is speeding up or slowing down.

Gyroscopes

Gyroscopes in an IMU measure angular velocity, indicating how fast and in what direction something is rotating.

There are various types of modern gyroscopes, including mechanical gyroscopes, fiber-optic gyroscopes (FOGs), ring laser gyroscopes (RLGs), and quartz/MEMS gyroscopes.

Quartz and MEMS gyroscopes find applications in consumer, industrial, and tactical markets, while fiber-optic gyroscopes cover all performance categories.

Ring laser gyroscopes have in-run bias stabilities ranging from 1 °/hour to less than 0.001 °/hour, suitable for tactical and navigation grades.

Mechanical gyroscopes, the highest-performing, can achieve in-run bias stabilities of less than 0.0001 °/hour.

MEMS gyroscopes, commonly used, rely on the Coriolis effect - a phenomenon describing forces when an object moves in a rotating frame of reference.

The Coriolis effect in MEMS gyroscopes is illustrated by a circular platform rotating clockwise.

If a vehicle moves northward from the center, the Coriolis effect causes it to follow a curving arc to the west.

To maintain a northward course, the vehicle applies Coriolis acceleration.

In Earth's frame of reference, the Coriolis effect causes apparent deflection due to the Earth's rotation and latitude.

For instance, launching a rocket northward results in a left-curved path due to Earth's rotation.

In a typical Coriolis MEMS gyroscope, a vibrating proof mass is attached to a reference frame, inducing a secondary vibration perpendicular to the drive axis when rotated.

This secondary vibration sensed through changes in capacitance, provides a signal proportional to the Coriolis force and the sensed rotation.

Magnetometers

A magnetometer is a device that measures the magnetic field, including its strength and orientation.

Common examples include compasses, which determine the direction of the Earth's magnetic field.

Some magnetometers measure magnetic dipole moments, which involve closed loops of electric current or pairs of poles.

Source from advancednavigation.com

Various magnetometers work by aligning with magnetic fields, canceling forces through the Hall effect, magneto induction, or magnetoresistance.

Hall Effect Magnetometers

The Hall effect involves the generation of a voltage difference (Hall voltage) across a conductor when exposed to an applied magnetic field perpendicular to the current flow.

This principle allows magnetometers to use semiconducting materials, passing current through them and detecting changes in current due to nearby magnetic fields.

The resulting Hall voltage is proportional to the strength of the magnetic field.

Magneto Induction Magnetometers

Magneto Induction assesses how magnetized a material becomes when exposed to an external magnetic field.

This involves creating demagnetization curves, also known as B-H or hysteresis curves.

These curves measure the magnetic flux and force experienced by a material when subjected to varying magnetic fields.

Scientists and engineers use these curves to classify materials based on their magnetic properties and responses to external magnetic fields.

Magneto-Resistance Magnetometers

Magnetometers using this method assess an object's ability to change electrical resistance when exposed to an external magnetic field.

Specifically, they leverage the anisotropic magneto-resistance (AMR) of ferromagnets.

When subjected to a magnetic field, the spins of the electron orbitals in the material redistribute themselves, affecting electrical resistance.

The resistance is highest when the current aligns with the external magnetic field.

IMU Without Magnetometer vs. With Magnetometer

The inclusion or exclusion of a magnetometer in an IMU can have a significant impact on its capabilities and the type of information it can provide.

IMU Without Magnetometer:

When we talk about an IMU devoid of a magnetometer, we're essentially discussing a sensor system that relies solely on accelerometers and gyroscopes to glean information about an object's motion.

In this configuration, the IMU excels at providing fundamental data on linear acceleration and angular velocity.

Its focus lies in deciphering basic changes in orientation, with the accelerometers capturing linear acceleration and the gyroscopes tracking angular rate.

However, the absence of a magnetometer presents certain limitations, particularly in the realm of heading accuracy.

Without the capability to directly sense the Earth's magnetic field, determining precise heading becomes a challenge.

The system may encounter drift over time due to inherent gyroscopic errors, making it less suited for applications requiring highly accurate directional information.

In terms of applications, an IMU without a magnetometer finds its niche in scenarios where rudimentary orientation data suffices.

Moreover, it is often preferred in environments where concerns about magnetic field interference are pronounced.

IMU With Magnetometer

Conversely, an IMU equipped with a magnetometer introduces a new dimension to its capabilities, significantly enhancing the system's ability to provide accurate heading information.

The magnetometer measures the Earth's magnetic field, allowing the IMU to correct for gyroscopic drift and offer improved stability in determining orientation changes.

While this integration of a magnetometer enhances heading accuracy, it also introduces considerations related to calibration.

IMUs with magnetometers typically require calibration on the vehicle or in situ to compensate for static magnetic interference that may introduce heading errors.

Magnetometer calibration becomes a critical step post-installation if precise magnetometer heading is a requirement.

Despite potential challenges, the advantages in accuracy, especially in scenarios demanding precise heading information, make an IMU with a magnetometer well-suited for applications like navigation systems, robotics, and augmented reality.