Set Yourself Straight on Inertial Measurement Units (IMUs)

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Source : Mouser Electronics

My childhood was a long time ago. No handheld electronics, no video games, no portable devices. Instead, it was a time of Play-Doh, Silly Putty, Magic 8-Ball, Frisbee, and the Slinky. One toy I remember fondly is the Mattel Wiz-z-zer. It is essentially a gyrostat, a rotating wheel inside a case, a toy that introduced the spinning top to modern children (Figure 1). The twist was using a super-spinning, high-tech bearing that allowed the top to spin at a very high speed so it could remain standing for an extended period of time. I can still hear the sound it would make as I revved it up for high-speed spinning before releasing it to spin freely across the floor.


Figure 1: A spinning top. (Source: RODRIGO SAINZ/

This gyrostat top is a modified gyroscope. A gyroscope remains upright while spinning because of the conservation of its angular momentum (Newton (1687), Laplace (1799), Foucault (1852), Rankine (1858), others). Because of this phenomenon, gyroscopes can be employed for measuring or maintaining orientation and angular velocity.


Today’s modern electronics designs have adopted this mechanical knowledge and made it into a sensor. Such a sensor can be manufactured by employing microelectromechanical systems (MEMS). MEMS sensor technology enables sensor fusion whereby multiple sensors and software solutions are packaged as an integrated whole. Thus, it helps provide solutions in various large industries, including information and communications technology (ICT), the Internet of Things (IoT), and automotive. The semiconductor manufacturer calibrates these integrated solutions and utilizes embedded compensation and sensor processing, plus a simple programmable interface.

At the forefront of humanizing MEMS sensor technology is TDK InvenSense. InvenSense is part of the Sensor System Business Company within TDK. It is the industry leader in MEMS Motion, Audio, and Pressure Solutions for the consumer, industrial, automotive, and IoT market segments. With a robust portfolio of MEMS 3/6/7/9 axis motion sensors, accompanied by the highest performing MEMS audio microphones and pressure sensors, TDK continues to push the boundaries of performance and quality, setting new standards of innovation across multiple industries. Let’s examine how MEMS inertial measurement units (IMUs) from TDK InvenSense help design engineers keep straight on where things are positioned.


MEMS technology allows for multi-axis combinations of precision gyroscopes, accelerometers, magnetometers, and pressure sensors to be assembled into one device. Integrated devices employing these types of sensors are often called inertial measurement units (IMUs). IMUs are electronic devices that measure and report a body’s specific force, angular rate, and often the body’s orientation. Isaac Newton (1642–1726/27) described inertia as the first of the Laws of Motion (Mathematical Principles of Natural Philosophy, 1687) by defining it this way: “Every body perseveres in its state of rest, or of uniform motion in a straight line unless it is compelled to change that state by forces impressed thereon.” MEMS technology reliably senses and processes multiple degrees of freedom (DoF), even in highly complex applications and under dynamic conditions.

Multiple Degrees of Freedom

One vital criterion in IMU selection is the degrees of freedom. IMUs are commonly available with specifications from two to 10 degrees of freedom.

The word freedom means different things in different contexts. In this case, we are not talking about freedom of personal choice or political freedom, but rather in the context of physics, specifically mechanics. In mechanics, degrees of freedom correspond to translation components and rotation that define its configuration or state. For example, for a rigid body in space, both translation and rotation have three components, yielding a total of six degrees of freedom (Figure 2).

  1. Translation: Forward/backward
  2. Translation: Left/right
  3. Translation: Up/down
  4. Rotation: Side-to-side (Roll)
  5. Rotation: Forward/backward (Pitch)
  6. Rotation: Left/right (Yaw)

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Figure 2: Six degrees of freedom. Possibilities of movement of a rigid body in 3D space. Forward, backward, left, right, up and down, plus rotations about the three axes. (Source: Peter Hermes Furian/

Accelerometers (measure change of velocity → obtain position) and gyroscopes (measure angular velocity → acquire orientation) can be combined to collect information allowing the device to compute up to six degrees of freedom. So, where does the idea of more than six DoF come from? Providers of IMUs have noticed that they can obtain improved performance if they employ further sensor fusion. They add another sensor to improve their readings, reduce errors, and get additional compelling data to support internal adjustments and compensation. Adding a magnetometer provides new sensor information. The magnetometer senses the earth’s magnetic field providing data that allows directional heading to be obtained. When this information is sensor-fused with the accelerometer and gyroscope, sensor manufactures say they have three more degrees of freedom. Thus, a nine-degree of freedom IMU is born.

To be clear, this is a bit of mixing of terms. Technically, physics defines six degrees of freedom. However, the IMU device employs three different sensors in three separate axes in a sensor fusion scheme, bringing nine sensor inputs to the solution.

Earlier I stated that some IMUs come with 10 degrees of freedom. I just explained nine. How does one get to 10?

Simple, add another sensor. In this case, adding a barometric pressure sensor will provide more information. Incorporating a barometric sensor now leads to what is termed by IMU manufacturers as 10 degrees of freedom:

3 DoF accelerometer

3 DoF gyroscope

3 DoF magnetometer

1 DoF barometer (pressure)

10 DoF


Unlike the many toys from my youth, modern electronics require more and more sensing. Sensor fusion combines multiple sensors and software solutions that enable ICT, IoT, and automotive. You have learned how IMUs combine numerous sensors into one integrated monolithic device. Like a spinning top, I hope I have set you straight that TDK InvenSense is the best place to look so that you realize the benefit of IMUs in your next design.

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