What exactly Is a MEMS based IMU?

H4-lab’s entire MEMS based IMU catalog

In order to understand what MEMS based IMUs are, let’s first define what each accronyms are.

MEMS: The technology of tiny devices

MEMS stands for microelectromechanical systems, which is the technology of micro-fabricated devices. The main idea behind this technology is to combine basic mechanical parts and tools – like cantilevers and membranes – with silicon technology of the type found in computer processors.

The result is a microfabricated electronic chip that unites multiple sensors in a small space, and which can be used to build various systems, including accelerometers, gyroscopes, and magnetic sensors. Accordingly, MEMS’ small size has made them ideal for smartphones, GPS, game controllers, and other modern inventions.

IMUs: Tiny devices for sensing

An inertial measurement unit, or IMU, is an electronic chip composed of several micro-fabricated sensors: an accelerometer, which senses force or gravity; a gyroscope, which reports the angular rate; and sometimes a magnetic sensor, which acts as a compass.

These sensors collect data that are processed by an external or onboard microprocessor to provide information about the attitude, orientation, and rotational speed of an object.

Depending on the model and the manufacturer, each of these internal sensors is available in 1-, 2-, or 3-axis versions. However, the 3-axis devices are the most commonly used today, since they provide extra information which may be required for certain applications such as robotics. 

How MEMS-based IMUs work

MEMS-based IMUs usually package an accelerometer and a gyroscope in a single chip with the magnetometer added as a separate chip on the sensor. Each of those components use different physical principle to measure its value in all 3 axes.

The MEMS accelerometer chip consists of a mass attached with springs inside a cavity that make use of Newton’s second law to measure the acceleration. When the sensor is accelerated, a force is applied on the mass which create a small displacement of the mass inside the cavity. This displacement creates a change in capacitance which induces a small variation in tension that can be measured. This analog value is then amplified and converted to a digital acceleration value that can be used directly by the user.

The MEMS gyroscope chip again consists of a mass attached with springs inside a cavity. However, this time it uses the Coriolis Effect, which is using a vibrating mass deflection resulting from rotation to measure the angular acceleration. Just like for the accelerometer, this displacement creates a change in capacitance which can be measured as a variation in tension.

The MEMS magnetometer chip again measures the displacement of a mass inside a cavity. This time, the mass moves through the application of the Lorentz force. This force is applied on all charged particle that is moving perpendicular to a magnetic field. The displacement caused by this force once again results in a change of capacitance which can be measured as a variation in tension.

Inside of a 3D MEMS gyroscope sensor

Other types of IMUs

For tactical-grade applications which require high accuracy, Ring Laser Gyros (RLGs ) are generally considered to be the most accurate option. The RLG-based IMUs have excellent stability and repeatability characteristics. However, they are also very expensive. 

Fiber-optic (FOG) technology is an alternative to the more expensive RLGs. FOGs are more accurate than their MEMS counterparts. They provide more precise rotational rate information, since they are less dependent on using mechanical movement to conduct the transduction. However, while FOGs are not quite as costly as RLGs, they are still significantly more expensive than MEMs. 

Conclusion
As MEMS inertial measurement sensors improve in performance, they’re more accepted for use in the latest industrial and research applications, like unmanned vehicle control and biomechanics research, to name just a few. With their small size, low cost, and and multi-axis sensing, MEMs-based IMUs are rightfully poised to become the most popular type of sensors.