Accelerometers are used very often in digital health devices. Let us try to understand them better but before that lets understand acceleration since that is what they measure.
Acceleration:
Let’s consider a runner in a 100m dash like the fast runner, Usain Bolt. He can rapidly accelerate from zero to nearly 27 miles per hour which calculates to an incredible acceleration of 1g (acceleration due to earth’s gravity) as calculated in this paper in European Journal of Physics. However, this requires knowing the speed and the change of speed which are difficult to measure. Sir Isaac Newton had determined that acceleration was the amount of force that is required to move one unit of mass. So, if you can measure the force acting on a defined mass then you can measure the acceleration. For example, if you move you hand to grab a cup of coffee, your hand moves from steady and stopped state to moving by accelerating your hand towards the cup. Your muscles develop the force. However, if we had a defined mass and we could measure the force on that mass very accurately then we can measure acceleration.
What is it?
Accelerometers have been around for quite some time and have been used in different applications like rockets, airplanes and cars. However, the commercialization and commodity pricing enabled them to be commonly used in phones and everyday devices. Accelerometers are electromechanical devices that measure acceleration forces. This acceleration can be used to compute the angle of the device (gravity works in 1-axis in the direction we call “down”) or how fast is the device changing direction and velocity. These days many of them are tiny MEMS (Micro Electro Mechanical Systems) that are microfabricated and are just a few millimeters across.
How do they work?
Accelerometers have a defined mass (think of a ball inside a box) and the force experienced by that mass is measured very accurately (think of a spring that is exquisitely calibrated) to compute the acceleration (Force =mass x acceleration and thus acceleration = Force/mass) The force is measured by various means and the accelerometer report back on the acceleration experienced by the device whether static (gravity) or dynamic (like a moving car, hand or object). Currently there are 2 main technologies that enable measurement:
Piezoelectric effect: The piezoelectric effect is the generation of small amount of electricity by distortion of a crystal or the reverse – distortion of the crystal to produce a sound by the application of electricity. In the case of the accelerometer, the acceleration forces by the ball/mass are configured to mechanically distort the crystal and generate a small electric pulse. This pulse is then amplified and available to the outside of the integrated circuit.
Capacitance: Capacitance is the ability of any object to store an electric charge on their surface. This was dramatized by Nickola Tesla by understanding static electricity and creating lightning. The capacitance is a property of the material that is influenced by many factors and one of the major factors is distance between the two adjacent objects Thus, if the moving mass inside the accelerometer changes the distance, it changes the capacitance which can be measured.
Who makes them?
Almost all companies that manufacture electronic parts make a version of accelerometers. Some examples are Analog Devices, Bosch, Honeywell but there are several companies and each of them have a distinct characteristic that are application specific. Some comparison of companies and their accelerometer products are here.
Accelerometer axis:
The axis refers to the X-Y-Z axis in which you can measure acceleration. If you measure acceleration in only 1-axis then they are called single axis accelerometers and if multiple measurement units are aligned in X-Y-Z planes, then they are called 3-axis accelerometers.