Accelerometer: Advantages and Disadvantages

This page explores the advantages and disadvantages of accelerometer sensors, covering their basics and working principles. It details the benefits and drawbacks of accelerometers.

What is an Accelerometer?

• Acceleration refers to the rate at which velocity changes over time.
• An accelerometer is a sensor that measures acceleration and gravity. Its output is in units of gravity (represented by the letter ‘g’).
• Accelerometers are used in mobile phones to determine the orientation of the phone with respect to the ground. This enables the automatic rotation of the screen display for optimal viewing.
• Modern accelerometers are often packaged with gyroscopes and magnetometers. Gyroscopes measure the rate of rotation (angular velocity), while magnetometers measure the surrounding magnetic field to determine orientation relative to the Earth’s magnetic field.

• Figure 1 depicts a 3-axis accelerometer (ADXL335 from Analog Devices). It features five pins: VCC, GND, and three analog outputs. These analog outputs are interfaced with the microcontroller’s analog input pins for voltage measurement and decision-making. Often, these outputs are connected to a comparator IC (e.g., LM324) to generate digital outputs.

Accelerometer Working Principle | How Accelerometers Work

• Accelerometers operate based on Newton’s second law of motion: F = m*a (where ‘a’ is acceleration, ‘F’ is the applied force to the mass ‘m’ attached to a wall via a spring with coefficient ‘k’).

  • F = m*a = Fs = K*x, where x is the displacement of the body from its initial rest position.
  • m*a = k*x
  • a = f(x), meaning acceleration is a function of displacement.
  • Therefore, knowing ‘x’ allows easy determination of acceleration (‘a’). Various techniques exist to find displacement ‘x’, including resistive, capacitive, and inductive methods.

• Most accelerometers use a capacitive technique:

  • C = f (A/d), where ‘A’ is the area of the plate and ‘d’ is the distance between the plates. If the distance is known, capacitance (C) can be found. To mimic the spring-mass system, a setup with two plates is used: one fixed and one movable (Figure 2). Gravitational force causes variations in the distance ‘x’ between the plates, depending on the body’s acceleration ‘a’. This changes the capacitance between the plates. Capacitance measurement determines ‘x’ (from the equation C = f(a/x)), which allows calculation of ‘a’ (from the equation a = f(x)).

• As physically using a spring-mass system in an IC isn’t feasible, MEMS (Micro-Electro-Mechanical System) technology creates a similar setup within the IC (Figure 2). Movable assemblies have plates forming a capacitor with fixed assembly plates. These MEMS systems are used in three planes (x, y, z) in the accelerometer IC to determine acceleration in different directions.

Benefits or Advantages of Accelerometer Sensors

The following are benefits of accelerometer sensors:

• Simple to interface and rugged in design.
• High impedance.
• Offers higher sensitivity.
• High frequency response.
• Available at a lower cost due to advancements in MEMS technology.
• Uses a built-in signal conditioning circuit to measure capacitance.

Drawbacks or Disadvantages of Accelerometer Sensors

The following are drawbacks of accelerometer sensors:

• Measures changes in velocity only, not constant velocity.
• Cannot measure rotation around its own axis of movement; therefore, it is often used with a gyroscope to measure angular velocity.
• Sensitive to temperature and operates over a limited temperature range.
• Efficiency degrades over time.
• Requires external power for operation.
• Other disadvantages include less longevity and hysteresis error.