How does an IMU sensor work?
H4-lab’s QMU102-OEM with an IMU chip on board.
An IMU sensor usually consist of 2 or 3 different sensors packaged in one unit. The sensors inside an IMU are an accelerometer, a gyroscope and sometimes, but not always, a magnetometer. Each one work in a different manner.
Accelerometer:
The principle behind the measurements of acceleration is based on Newton’s second law of motion which states that the sum of all forces acting on a body is equivalent to its mass times the acceleration 𝐹=𝑚𝑎. It is thus possible to conclude that when a mass is accelerated, a resulting force (an inertial force) is applied on said mass which will lead to its displacement. Now, if we suppose that the mass is attached to a spring with its constant known 𝐹=−𝑘𝑥, it is possible to know exactly the force applied based on the displacement. In reality, the displacement is not measured with a ruler since it would not be easily feasible. It is instead using an electrical principle called capacitance where the variation in the distance between two charged plates (in a particular circuit) will lead to a variation in voltage that can be measured easily. This voltage measurement, which is an analog signal is then amplified and converted into a digital acceleration value that is easy to use.
Gyroscope:
The principle behind the measurements of angular rotation is based on the Coriolis Effect. This effect creates an inertial force on an object in rotation which can again lead to a mass displacement. Simplified, the Coriolis force is acting sideway on a body that is moving towards (or away from) its center of rotation. In order to measure this force, some sensor are using an oscillating mass that will move sideways only when there is a rotation applied to the sensor. The faster the rotation, the higher the force and thus the displacement. Just like for the accelerometer, this displacement is usually measured as a change in capacitance due to the variation in distance between to charged plates. With the right circuitry, it is then possible to easily measure the variation in voltage and convert this analog value to a digital acceleration measure.
Magnetometer:
The principle behind the measurements of magnetic field is the Lorentz force. The Lorentz force is generated inside a current carrying conductor when it is inside a magnetic field. The force is perpendicular to both the current direction and the magnetic field direction. It is thus possible to again measure the body displacement related to the Lorentz force and convert this measure into a magnetic field digital value.
Note that there is more than one way to measure the same phenomenom and that those presented here are some of the most commonly used.
IMU Sensors
Most IMU sensors can be bought as a chip that you can solder on a PCB to program yourself. However, there is multiple packaged sensor available that has already done this work for you and even added sensor fusion to the package so that you can not only read acceleration, rotational rate and magnetic field measurements, but also the roll, pitch and yaw as well. Such device, like Quark’s QMU101 even add a USB connector to easily read and log the data on your computer. Quark’s complete IMU line covers different need for easy to use devices and EtherCAT ready solutions as well.
