Accelerometers- What everyone from Nokia to Apple are using

An accelerometer is an electromechanical device that measures acceleration often caused by motion. But when they are standing still, the only acceleration the cheap articles accelerometer senses is due to gravity pulling down on it. To put it in simpler words an accelerometer measures acceleration forces, which may be dynamic or static in nature. They are completely modular and very tiny devices. Analogue accelerometer gives voltage proportional to acceleration.  A digital accelerometer gives PWM output or direct binary digital data.

Figure: Accelerometer ADXL-335

Accelerometers are used to measure the angle the device is tilted with respect to the axis of the earth by measuring the amount of static acceleration due to gravity. They also help to analyze the way device is moving by sensing the amount of dynamic acceleration. Measuring tilt and acceleration helps us to make many useful products. For example they can be used in mobile phones for screen rotation, tilt controlled scrolling, and ringer control based on orientation etc. In addition, mobile phone makers can use the accelerometer to improve accuracy and usability of navigation functions. IPod’s game, steps counter, remote console, airbag, seismic movement detector, camcorders all of them have accelerometers.

Accelerometers can be 2D or 3D depending on whether they measure acceleration in 2 dimensions or 3 dimensions. The simplest accelerometer is a seismic mass inside a box. A dashpot is used to absorb the movement between the mass and the box which is proportional to the acceleration. A variable resistor is used to convert the vibration into voltage. Because of inertia, the seismic mass is always moving, at its natural frequency. Since this frequency, it is considered that the sensor is really moving. The seismic mass is surrounded with a coil and is made with a magnet. Accelerometers can also work on other principles like the piezoelectric effect – they contain microscopic crystal structures that get stressed by accelerative forces, which causes a voltage to be generated. Capacitive accelerometers are also available. If you have two microstructures next to each other, they have a certain capacitance between them. If an accelerative force moves one of the structures, then the capacitance will change. We can then use a circuitry to convert from capacitance to voltage, and you will get an accelerometer. There are even more methods, including use of the piezoresistive effect, hot air bubbles, and light.

Some points to keep in mind while using accelerometers.

  1. Since it is very sensitive, don’t touch the accelerometer ic as static charge may alter the analogue reading.
  2. Check the voltage rating before connecting to the supply as most accelerometer works around 3.6v so if greater supply is attached to it, it may damage the IC.
  3. Read the datasheet and basic operating instructions before using.
  4. Select the accelerometer based on the sensitivity required.

Here we will give you a brief introduction how to use an accelerometer for finding angles(x-z plane tilt, y-z tilt)

  • For this we are using ADXL-335 module (you can use others too).
  • It is a 3 axis device and its sensitivity is ±3g (g=gravity) which makes it capable of angle finding. It has 5 degrees of freedom.
  • It gives three analogue output(X, Y, Z) of every position tilted.

 Figure: Value of X-out, Y-out and Z-out at different positions of the IC

  • While placing the IC at position 1 take the reading in our case Xout=336 (0g), Yout=336 (0g), Zout=412 (1g).The position 1 is the calibration position and the corresponding values are the calibration values.
  • X1=X-336, Y1=Y-336, Z1=Z-336
  • Now, X-Z plane tilt =atan²( X1/(Z1²+Y1²)½ ).
  • So now you have found the angle of x-z tilt in radians.
  • Similarly you can find Y-Z plane tilt.

NOTE:

  • The value of 0g in our case was 336 it can vary depending on the platform upon which the sensor is placed i.e. if the platform is tilted slightly the values will differ.
Education, Engineering, Projects , ,

Leave a Reply