Inertial Measurement Unit (IMU)

Summary

The inertial measurement unit (IMU) is an umbrella term for three specific sensors that describe motion, namely the accelerometer, gyroscope, and magnetometer.

• The accelerometer measures linear acceleration.
• The gyroscope measures angular velocity
• The magnetometer measures absolute orientation.

While an accelerometer might tell you that the device is moving to the left really fast, it won’t tell you which way left actually is. The magnetometer will tell you that moving to the left means going East. Each of these sensors has 3 channels of measurements, each in a perpendicular direction in 3D space and would be labeled x, y, and z. This would be like measuring the acceleration up and down, left and right, and forward and backward. Device manufacturers can orient their accelerometers however they want, so we can’t assume that the z-direction is the vertical direction.

We won’t be dealing with gyroscopes and magnetometers in this course, but we will look at accelerometers. The animation below is a model of an accelerometer. There is a mass attached to springs and the mass moves a plate between two capacitors inside a circuit. This changes the capacitance depending on the location of the plate. Because the displacement of the mass on a spring is proportional to the force it sees, the changing voltage in the circuit is proportional to force and acceleration. If you combine three of these circuits together perpendicularly, you can measure acceleration in 3 dimensions.

This also means that accelerometers are affected by gravity. For example, if we rotate our accelerometer 90 degrees, we can see gravity pulls the mass down and the accelerometer will see a voltage associated with 1 g-force or the magnitude of the acceleration due to Earth’s gravity. The accelerometer only measures 0 if it’s in free-fall.

This also means that when the device is stationary, you can measure its orientation. If all the acceleration is in the downward z-direction, then it’s flat on a table. Or if it’s all in the x-direction, then it’s tilted on its side. We’ll take a more in-depth look at this another phenomena in the next lesson where we look at accelerometer traces in detail.

Not all accelerometers are implemented using this capacitor attached to a mass on a spring model, but they follow similar principles. For example, some accelerometers use a force sensitive resistor or a piezoelectric crystal to modulate a voltage in response to an acceleration.

New Vocabulary

• Inertial Measurement Unit (IMU): A collection of sensors that measure motion.
• Accelerometer: A sensor that measures linear acceleration.
• Gyroscope: A sensor that measures angular velocity.
• Magnetometer: A sensor that measures magnetic forces.
• g-force: The amount of acceleration on a body measured in units of acceleration due to gravity on earth (or roughly 9.8m/s^2).