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Advantages of Accelerometer | disadvantages of Accelerometer

This page covers advantages and disadvantages of Accelerometer sensor including its basics and accelerometer working principle. It mentions Accelerometer advantages or benefits and Accelerometer disadvantages or drawbacks.

What is Accelerometer?

• Acceleration refers to rate at which velocity changes over time. Accelerometer is a sensor which measures acceleration as well as gravity. Its output is in units of gravity (i.e. letter 'g').
• It is used in the mobile phones to determine orientation of the phone with respect to ground. This helps to rotate the picture or screen display appropriately for user to view.
• Now-a-days accelerometer is packed along with gyroscope and magnetometer. Here gyroscope measures rate of rotation of the device (i.e. angular velocity) in which it is mounted. Magnetometer measures magnetic field surrounding to itself. This helps in determining its orientation relative to the magnetic field of the earth.

3-Axis Accelerometer, ADXL335

• The figure-1 depicts 3-axis accelerometer with part number ADXL335 from Analog devices. It has five pins viz. VCC, GND and 3 analog outputs. The analog outputs are interfaced with microcontroller analog IN pins to measure voltage and take decision accordingly. Often these outputs are interfaced with comparator IC (e.g. LM324) to derive digital outputs.

Accelerometer Working principle | How Accelerometer works

• Accelerometer works based on Newton's second law of motion i.e. F = m*a, where 'a' is acceleration, 'F' is applied force to the mass 'm' attached to the wall through spring (having coefficient 'k').
➨F = m*a = Fs = K*x, here x is displacement of the body from initial rest position.
➨m*a = k*x
➨a = f(x), acceleration is the function of displacement
➨Hence if x is known, acceleration ('a') can be found out easily. There are various techniques of finding displacement 'x' viz. resistive techniques, capacitive techniques and inductive techniques.

MEMS Accelerometer

• Most of the accelerator use capacitive technique as explained below.
➨C = f (A/d), where 'A' is the area of plate and 'd' is the distance between the plates. If distance is known, capacitance (C) can be found out. To have the similar functionality to initial spring mass system, a modified version is employed using two plates, one fixed and the other movable as shown in figure-2. Due to gravitational force, distance between plates 'x' varies which depends on acceleration 'a' of the body. This results into change in the capacitance between plates. Capacitance measurement helps in determining value of 'x' ( from equation C= f(a/x) ) which helps us calculate value of 'a' (from equation, a = f(x) ).

As physically it is not possible to use spring mass system in the IC, concept of MEMS (Micro-Electro Mechanical System) is used to develop similar approach in the IC with diagram as shown in the figure-2. As shown, movable assembly has plates which form a capacitor with plates of outer fixed assembly. Such MEMS systems are used in three different planes (x, y, z) of accelerometer IC to determine acceleration in different directions of the body.

Benefits or advantages of Accelerometer sensor

Following are the benefits or advantages of Accelerometer sensor:
➨It is simple to interface and rugged in design.
➨It has high impedance.
➨It offers higher sensitivity.
➨It has high frequency response.
➨It is available at lower cost due to advancement in MEMS technology
➨It uses built-in signal conditioning circuit to measure capacitance.

Drawbacks or disadvantages of Accelerometer sensor

Following are the drawbacks or disadvantages of Accelerometer sensor:
➨An Accelerometer measures chance in velocity only. It does not measure a constant velocity.
➨An Accelerometer can not measure rotation around its own axis of movement. Due to this, it is used in conjunction with gyroscope to measure angular velocity.
➨It is sensitive to temperature and operates over limited temperature range.
➨Its efficiency degrades over time.
➨It requires external power for its operation.
➨The other disadvantages are less longevity and hysteresis error.



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