How Does an IMU Work?
Posted on November 22, 2022 by Zach Strout
IMUs (Inertial Measurement Units) are at the heart of the SageMotion system. They enable the system to measure how people move so that they can be trained to move better. For example, IMU sensor data fusion calculate segment orientation and finding the relative orientation between segments to find joint angles. Subjects could then be trained to movement with specific joint angles instructed via haptic biofeedback.
IMUs are an increasingly important and widespread means of movement analysis and training, so let’s take a deeper look at what’s inside and how they work.
Which sensors are inside an IMU and how do they work?
IMUs are typically composed of three different types of sensors. The first type of sensor is an accelerometer which measures acceleration or the rate at which things speed up or slow down. While there are many different sensor technologies for accelerometers, the most common by far for wearable applications is MEMS (Micro-electromechanical systems). MEMS are sensor systems composed of electrical and mechanical parts usually etched out of silicon at the micrometer scale.
Whenever a MEMS accelerometer experiences an acceleration, a proof mass experiences this acceleration too. Sets of etched springs resist this acceleration. Using Hooke's Law (spring force is proportional to spring compression distance) and Newton's 2nd law (force is proportional to acceleration), the distance that the proof mass moves is proportional to the acceleration that it experiences (see figure below). This movement is sensed using the electrical property of capacitance which is related to the distance between two conductors. A set of electronics is then able to measure the change in capacitance, calibrate the signals, and further process it to give the acceleration.
How a MEMS accelerometer senses acceleration
The second type of sensor in an IMU is a gyroscope which measures angular velocity or how fast and in what direction something is spinning or rotating. Gyroscopes typically also use MEMS technology, although it is more complicated than MEMS accelerometers. The main physics phenomenon that is used for gyroscopes is the Coriolis effect which describes the forces involved when an object moves in a rotating frame of reference.
MEMS gyroscopes have a mass that reciprocates at a constant frequency. During rotation of the gyroscope, the mass will induce a force perpendicular to the reciprocation direction due to the Coriolis effect. This force is countered by the etched springs and sensed by capacitive sensing arms like the accelerometer. The signal-processing electronics then processes this change in capacitance relative to the reciprocation of the resonating mass (see figure below).
How a MEMS gyroscope senses angular velocity
The last sensor that is commonly found in IMUs is the magnetometer which measures magnetic field strength and acts somewhat like a digital compass. Most magnetometers take advantage of the Hall Effect to measure magnetic field strength. The basic premise of magnetometers is that moving electrons in a conductor are deflected by magnetic fields that the conductor is exposed to. As charge travels through a conducting plate in a magnetic field, the magnetic fields deflect the electrons to one side of the conducting plate. With more negative charge building up on one side of the plate and more positive charge building up on the other side of the plate, there is a measurable voltage between the sides of the plates that is proportional to the magnetic field strength.
How a magnetometer senses magnetic field direction
What are the benefits of using IMUs?
Compared to other motion capture methods, IMUs have some distinct benefits.
Infinite capture volume
Since IMUs are stand-alone devices that can capture movement data without being tethered to lab equipment or needing to be in a certain capture volume, they can be used in many out-of-the-lab situations.
Flexibility of analysis
Since IMUs have a suite of sensors that accurately measure different things, there are many different ways that IMUs can be used for analysis. If the physics involved in an analysis task is simple, a physics-based model based on acceleration and angular velocity can be developed and the raw sensor values can be used. For models based on body segment orientation, sensor fusion for sensor orientation can be performed, and these orientations can be used in the model. For more complex tasks, machine learning and deep learning models can use the sensor data directly.
Find Out More
IMUs are useful for many different sports or medical-related research applications. To get a better idea about where IMUs are used, check out these review papers:
