Accelerometer: Piezoelectric Accelerometer, Capacitive Accelerometer (For CBSE, ICSE, IAS, NET, NRA 2022)

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  • The compass application on your smart phone somehow knows in which direction the phone is pointing.
  • Same as the stargazing application in your phone knows which part of the sky you are looking at, to display constellations correctly. Ever think how they do it?
  • Smart phones and other mobile technologies identify their orientation with the help of a small device made up of axis-based motion sensing, and the device is known as an accelerometer.

What is Accelerometer?

  • An accelerometer is an electromechanical device that is used to measure acceleration forces. These forces may be static like the force of gravity or, they may be dynamic as in the case of mobile devices.
  • When two or more accelerometers are coordinated with one another, they can measure differences in proper acceleration, particularly gravity, over their separation in space — that is, the gradient of the gravitational field.
  • Gravity is useful the reason is that the absolute gravity is a weak effect and depends on the local density of the Earth that is quite variable.

How Does an Accelerometer Work?

  • An accelerometer is in the form of a simple circuit for a large electronic device. Despite the simple appearance, the accelerometer is made of different parts and works in many ways, which are the piezoelectric effect and the capacitance sensor.

Piezoelectric Accelerometer

  • The piezoelectric effect is very popular form and uses microscopic crystal structures that get stressed due to accelerative forces.
  • A voltage arises in the crystals from the stress and the accelerometer translates it to velocity and orientation.
  • Piezoelectric accelerometers measure motion against the inertial reference frame, as they do not need a nonmoving reference structure like displacement probes do.

Figure of Piezoelectric Accelerometer

Figure of Piezoelectric Accelerometer
  • These accelerometers use materials quartz crystal, that exhibit the piezoelectric effect. They produce an electric charge in response to mechanical deformation. The idealized structure of a piezoelectric accelerometer as given in Figure.
  • When the housing experiences acceleration, the crystal is squeezed between the mass and the housing with a force that is proportional to the acceleration.
  • The crystal produces an electric charge, which is also proportional to the acceleration. An electrical circuit called a “charge amplifier” which converts the charge into an easily measurable and transmittable voltage.
  • The accelerometers can be used to measure very low frequencies.
  • The low-frequency response is usually limited by the signal conditioning electronics in order to eliminate noise from sources such as thermal effects, strain on the accelerometer base, and tribo-electric noise generated in the connecting cable.
  • The low-frequency cut-off is typically set around 2 Hz or more if the lowest frequencies are not of interest to the user.

Capacitive Accelerometer

  • The capacitance accelerometer senses the changes in capacitance between micro structures.
  • If the accelerative force moves one of these microstructures, the capacitance changes and for interpretation, the accelerometer translates this into voltage.

Figure of Capacitive Accelerometer

Figure of Capacitive Acceleromoter
  • Capacitive accelerometers (vibration sensors) sense a change in electrical capacitance, with respect to acceleration, to change the output of an energized circuit.
  • The sensing element consists of two parallel plate capacitors acting in a differential mode.
  • The Accelerometers are made of multiple axes – two to determine two-dimensional motion with an option for a third for 3D positioning.
  • Smart phones make use of the three-axis model while cars make use of the two-axis to determine the moment of impact.

What is the Use of Accelerometer?

  • The applications of accelerometer extend to multiple disciplines as below given a few of its uses:
  • Any change in the location of the accelerometer chip will be registered as changes in the values it sends out.
  • One can use the fact that the numbers registered have changed to detect a movement, or poll the x, y, and z values and make calculations as to in what direction the motion has taken place and by how much it has moved.
  • Accelerometers in laptops protect the hard drive from damage. If the laptop were to fall while in use, the accelerometer would detect the fall and immediately turn off the hard drive to avoid hitting the reading heads into the hard drive platter.
  • Accelerometers in cars are used to detect car crashes and deploy airbags immediately.
  • Smart phones rotate their display between landscape and portrait mode depending on how one titles the phone.
  • Accelerometers are actively used in many electronics and it plays an active role in a wide range of functionalities.

How to Choose an Accelerometer?

  • The factors to keep in mind when to Choose an accelerometer are as below:
  • The vibration amplitude
  • The frequency range
  • The temperature range
  • The size and shape of the sample
  • Presence of electromagnetic fields
  • High level of electrical noise in the area
  • The surface where the accelerometer is to be mounted grounded or not
  • The environment corrosive or not
  • Is the area a wet or a wash down area?

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