Accelerometers

In everyday language, acceleration is the term given to a state of increasing speed. It is the rate of change of velocity over time. Many people only associate acceleration with the increasing speed of a vehicle – we understand that if a car company claims its vehicle will go from “zero to 50 in five seconds,” they mean it accelerates from a static start to 50 miles an hour in five seconds.

To calculate the rate of acceleration, we take the change in speed (velocity) and divide it by the time – 50/5, which works out at 10 miles per hour per second. In other words, every second that the car is driving it adds another 10 miles per hour to its speed.

In physics, however, acceleration is much more than increasing speed. It also includes decreasing speed and changing direction. It can be calculated using Newton’s second law of motion, incorporating mass, force and using the formula Acceleration = Force x Mass. In other words, acceleration is the amount of force required to move an object (mass).

How Do We Measure Acceleration?

Measuring acceleration is more complicated than measuring speed because it needs to take into account how the speed changes over time. This is done using an accelerometer. An accelerometer is one of the most common types of inertial sensors and can measure acceleration in up to three orthogonal axes.

They have three typical uses:

  • To record the inertial measurement of position and velocity
  • To sense tilt, orientation or inclination
  • To sense impact or shock

Not too long ago, accelerometers were only found on large commercial airliners or spacecraft. Nowadays, you’ll find them in almost every car, smartphone, laptop computer and other gadgets such as iPods and game controllers for Playstation, Nintendo Wii, etc.

Types of Accelerometer

There are two common types of accelerometer – capacitive and piezoelectric.

Capacitive – these give off a voltage dependent on the distance between two planar surfaces, one or both of which are charged with an electrical current. This is a highly sensitive and accurate method of sensing, and is less susceptible to noise and temperature variations.

Piezoelectric – The piezoelectric effect occurs when certain kinds of crystal are compressed, causing a charge to accumulate. This charge is then translated and amplified into either an output current or voltage. These accelerometers are best used to measure vibration and shock.