Difference Between an Accelerometer and a Gyroscope

An inertial measurement unit (IMU) is an electronic system which comprises accelerometers, gyroscopes, and sometimes even magnetometers, to measure a craft's acceleration, velocity, orientation, and gravitational forces. It is commonly used for complete motion control in aircraft, spacecrafts, satellites, etc.

Newton's laws of motion govern the movements of all the objects that we see around us. They allow us to study, control, and manipulate these motions to our advantage.

For the mathematics behind these laws to work, appropriate measurements of the different physical quantities associated with the motion need to be made. This requires the use of a number of different instruments and sensors. Among these instruments are the accelerometer and gyroscope.

They are useful for the measurement of certain physical quantities which can allow us to control and manipulate the motion that they are associated with. In the following lines, we shall find out what these instruments are, what they are used to measure, and run a comparison between them.

As its name suggests, an accelerometer is a device that is capable of measuring non-gravitational acceleration. When the object that it is mounted on jumps from stand-still or a fixed velocity to a higher velocity, the accelerometer responds and measures the amount of acceleration that the object undergoes.

The most popularly used type of accelerometers make use of special piezoelectric micro-crystals which go under stress in response to the forces resulting from acceleration. Correspondingly, they generate a voltage which is proportional to the amount of stress on them. This voltage signal can then be conditioned and used to measure the amount of acceleration.

A gyroscope is a device that utilizes the Earth's gravitational pull for determining the respective orientation of an object. In the basic form, its design comprises a rotating disk, known as a rotor, which is mounted on a 'spin axis'. The spin axis in turn is fixed at the center of a larger and more stable wheel.

The gyroscope works on the principle of conservation of angular momentum. When the rotor is set spinning around the axis, each and every little bit of it tries to continue moving in the same direction, thereby resisting any changes in its angle of orientation.

As such, even when the axis of the gyroscope turns or tilts, the spin axis continues to remains stationary, indicating the downwards gravitational pull.

This enables the gyroscope to be used in order to measure the degree of tilt, and to maintain orientation.

The main point of difference between these two devices is that, a gyroscope is capable of sensing rotation, while an accelerometer cannot.

A 3-axis accelerometer can sense the orientation of a stationary object in relation to the Earth's surface. However, if the object is in a free-fall, the acceleration measured is zero.

Also, if the object accelerates in a particular direction, that acceleration gets added to the one induced by gravity, making it impossible to distinguish between the two. This significantly reduces the effectiveness of an accelerometer for certain applications such as aircraft orientation.

A gyroscope, on the other hand, is able to measure any tilt or turn about its axis. Thus, when it is used in an aircraft, it is able to accurately gauge the degree of roll around the aircraft's axis. While the aircraft continues to tilt, the gyroscope helps indicate a proportional value to it, until the aircraft stabilizes (levels) once more.

Accelerometers typically measure linear acceleration. Multi-axial accelerometers can also be used to measure angular acceleration.

A gyroscope is only capable of sensing the rotation about its axis, and cannot measure the acceleration in any direction.

A dual axis accelerometer can be used to obtain the direction of gravitational pull. This makes it useful in balancing instruments.

Based on the principle of rigidity of space, a gyroscope helps measure the angular position of an object.

An accelerometer is a sensor circuit comprising several parts, including the actual acceleration sensor, amplifiers, signal conditioners, etc. The signal needs to pass through it completely for the reading to be obtained at the output. As such, a small delay is introduced in the measurement process. This device also has a low signal-to-noise ratio (SNR).

A gyroscope immediately responds to the rotation about its axis, and thus has minimum measurement delay. It has a high SNR.

To obtain displacement using an accelerometer, a complicated double integration has to be performed on the acceleration data collected through it.

A single integration is sufficient to obtain displacement from the measurement data obtained from a gyroscope.

Accelerometers find widespread use in several applications, including monitoring and controlling industrial and construction machines, navigation, airplanes, consumer electronics, smartphones, fitness devices, etc.

Gyroscopes are used in navigation, radio controlled aircraft, unmanned aerial vehicles (UAV), spacecraft controls, etc.

Thus, in conclusion, the accelerometer and the gyroscope are two separate measuring instruments which are devised to gauge distinct motion-related physical quantities. In many modern systems, both these instruments are used in conjunction with each other, in order to be able to completely measure and control motion.