Inertial Measurement Units IMUs

The locations of vehicles such as drones, robots and self-driving cars, remote control ships etc. are determined by the sensors that is embedded into these vehicles. This operation occurs by calculating the acceleration, displacement and orientation of vehicles. There are many sensors do this operation like as shown below.


1. Accelerometer 

it is an electromechanical device that sense static acceleration (gravity) and dynamic acceleration (vibrations & movement), its function is measuring the acceleration in one or more directions. Also, the position can be deduced by integrating the acceleration equation and orientation sensing by tilt sensor and vibration sensing. There are many types of accelerometer such as :


1.1. Mass spring accelerometer

 It is an instrument that measures the rate at which the acceleration of an object is changing. The accelerometer  measures  the  force  exerted  by restraints  that  are placed on a reference mass to hold its position fixed in an accelerating body. The acceleration  is  computed  using  the  relationship between restraint force and acceleration given by Newton’s second law    Force = mass x acceleration 


1.2. MEMS accelerometer 

MEMS  is the shortcut of MicroElectroMechanical Systems. These  techniques  create  mechanical  sensing structures of microscopic size, typically based on silicon when coupled with microelectronic circuits. There are two types of MEMS accelerometers: variable capacitive and piezoresistive. Variable capacitive VC MEMS accelerometers have lower range and high sensitivity and used for structural monitoring in Buildings and constant acceleration measurements such as gravity. Piezoresistive (PR) MEMS accelerometers have higher range and low sensitivity and used in shock and blast applications.


Mass spring Accelerometer
VC MEMS Accelerometer

Piezoresistive MEMS Accelerometer

Variable capacitive  MEMS Accelerometers

2. Gyroscope

It is measures the orientation or angular velocity or the rate of rotation along 3-axes. Their are Pitch(X angle), Roll (Y angle) and Yaw (Z angle). It is used in automatic pilots on ships and aircraft, in the steering mechanisms of torpedoes, and in the inertial guidance systems installed in space launch vehicles, ballistic missiles, and orbiting satellites.

Gyroscope rotation in all 3-axes

3. Compass


It is an instrument used for navigation and orientation that shows direction relative to the geographic cardinal directions . Digital or electronic Compass Sensor is actually a magnetometer that can measure the Earth’s magnetic field by use Hall Effect sensor and by calculating the very low frequency signals coming from the North or South direction, this sensor can calculate the orientation and direction. It is used in navigation systems, Localization and marine applications etc.

Hall Effect Sensor of Compass

4. Gyrocompass, it is similar to a gyroscope. But it is a non- magnetic compass that finds true north by using an (electrically powered) fast-spinning wheel and friction forces in order to exploit the rotation of the Earth. They have an advantage over magnetic compasses.They are not affected by ferromagnetic metal (iron, steel, cobalt, nickel,etc). However, magnetic compasses are still widely in use due to they can be small, use simple reliable technology,  cheap, easier  to  use  than  GPS  and  they require no energy supply unlike GPS.Also they are not affected by objects such as the walls or big trees which block the reception  of electronic signals.

5. Inertial Measurement Unit IMU


It is an electronic device which consists of a set of sensors like accelerometers, gyroscopes and magnetometers. It is measures a force, angular rate and orientation of the object. The IMU detect acceleration using accelerometers, heading reference by using magnetometer and the orientation by using gyroscopes.  Each axis of the three object axes (pitch, roll, yaw) contain one accelerometer, gyroscope and magnetometer.

6. Attitude and Heading Reference System AHRS, it is a type of 3-axes inertial measurements units. Also, it is an electronic device which has a set of sensors like MEMS gyroscopes, accelerometers, magnetometers and on-board processor. It can measure a heading (yaw), pitch and roll angles of any object moving in space. 

The main difference between of IMU and AHRS that is, the AHRS has a data processing unit (on-board processor) in the same device. So, AHRS capable for fusioning the data from various sensors and determine the attitude and heading information of the moving object in the same device. So, we can say that AHRS is real time orientation tracking. Also an  on-board  Extended  Kalman  Filter  (EKF)  handles  the  tricky  data fusion problem and estimate the future measurements for both accelerometers and gyroscopes. But IMU not capable for processing the data in the same device but it transmits the sensor’s data to another device to determine the attitude and heading information.

AHRS Model

Application on Positioning