Inertial Measurement Unit

Owned by Kiran Surendran
Last updated: 2021-08-20
4 min read

References

 

What does the sensor do? How do they work?

An Inertial Measurement Unit (IMUs) is used to detect movements and measure the intensity of movements. The sensor will measure acceleration (can derive force), magnetic field and angular rate.

IMUs are usually made up for a blend of 2-3 sensors (for a 6-axis IMU):

  • Accelerometer

    • Measures acceleration of the body

    • Work on the principle of inertia, where a body will resist acceleration or deceleration

    • As the system accelerates in one direction, the mass in the accelerometer will “accelerate” in the opposite direction relative to the system

    • By measuring a generated voltage or change in capacitance depending on the type of accelerometer, one can derive the acceleration experienced by the mass (thus the -ve of the systems acceleration)

    • Types of accelerometer:

      • Piezoelectric accelerometer

      • Capacitive Accelerometer

    • As technology has improved, this setup has been made smaller and smaller and now we are in the MEMS (Micro Electro Mechanical System) era:

      • Micro: all the dimensions are in the micrometer range

      • Electro: the electrodes form an electrical system

      • Mechanical: “mass and spring” (dielectric) form a mechanical system

      • System: the form a system!

 

  • Gyroscope

    • Measures angular velocity

    • Work on the principle of conservation of angular momentum and measuring the Coriolis effect

      • In a gyroscope sensor, a rotor or a spinning wheel is mounted on a pivot. The pivot allows the rotation of a the rotor on a particular axis which is called a gimbal.

      • Using two gimbals at a time (one gimbal will be mounted on another), will give the rotor three degrees of freedom, so whenever the rotor of the gyroscope is spun, the gyroscope will continue to point in the same direction. i.e., the rotor will maintain its spin axis regardless of the orientation of the outer frame

      • Coriolis Effect (really just a side effect of inertia)

        • When a mass is moving in a particular direction with a particular velocity and when an external angular rate will be applied, it will cause a perpendicular displacement of the mass

    • Types of gyroscope sensors (most common is vibration gyroscope)

      • Ring Laser

      • Fiber-optic

      • Quantum

      • Vibration

    • Vibration Gyroscope (MEMS)

      • Proof mass will vibrate/constantly move and oscillate

      • When an external angular rate is applied, the flexible part of the mass will move and make a perpendicular displacement

      • This displacement can be detected due to a change in capacitance, detection from a piezoelectric etc

 

  • Magnetometer

    • Measures a magnetic field, specifically its strength and orientation (usually Earth’s magnetic field and finds the vector pointing towards Earth’s magnetic North)

    • Types of magnetometers

      • Scalar magnetometers: performs an measurement of the magnitude of the magnetic field

        • Proton Precession

        • Overhauser

      • Vector magnetometers: performs an measurement of the magnitude and direction of the magnetic field

        • Hall Effect

        • Fluxgate

        • Magneto Resistive

        • SQUID

        • Search-coil

        • Rotation Coil

    • Hall Effect

      • Consider a conductive plate, through which we set a current to flow through. Bringing some magnetic field near the plate would disturb the straight flow (due to Lorentz force) and the electrons would be deflected to one side of the plate. This would create a voltage across side the plate, which can be measured to detect the strength and direction of the field

    • Magneto Resistive

      • Using materials that are sensitive to magnetic field (usually composed of Iron and Nickel)

      • Since these materials change their resistance when exposed to a magnetic field, this can be measured to figure out the strength and direction of the field

 

What specs/metrics should you be looking at when selecting an IMU?

Accelerometers

  • Range

    • Upper and lower limits of what the accelerometer can measure

    • Usually, smaller range means more sensitive output (i.e. more precise reading)

    • Listed in m/s^2 or G-force (g, 9.8m/s^2 on Earth)

  • Interface

    • How the device will interface with the rest of the system

    • Analog

      • produce a voltage that is directly proportional the sensed acceleration

      • generally easiest to work with, by using an ADC

    • PWM

      • Square wave with a fixed frequency, but duty cycle will vary depending on sensed acceleration

    • Digital

      • Use a serial interface such as SPI or I2C

      • Popular as they usually have more features and less susceptible to noise than the rest

  • Number of Axes Measured (self explanatory)

  • Power usage

    • Usually on the range of 100s of uA

  • Bonus Features

    • Selectable measurement ranges

    • Sleep control

    • 0-g detection

    • tap sensing

Gyros

  • Range

    • Angular velocity in rotations per minute (RPM) or degrees per second (deg/s)

    • 3 axes of rotations are referenced as either x ,y, z or roll, pitch, yaw

    • Make sure max angular velocity doesn't exceed max range of gyro. Also make sure gyro range isn't much greater than what you’re expecting

  • Interface

    • Analog

      • Most gyros have analog outputs

    • Digital

      • SPI

      • I2C

  • Number axes measured

  • Power Usage

  • Bonus Features

    • Temperature output, to compensate for drift

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