In simple words, sensors measure a particular kind of physical quantity, such as force acting on device, light falling on a surface, or the temperature in a room. These are examples of a basic physical quantity that sensors can measure. Most Android phones come with advance sensors that can measure valuable information such as relative humidity, atmospheric pressure, magnetic field, steps taken, the rate of rotation of a device on the x, y, and z axes, proximity to an object, and many more. The majority of the sensors are Micro Electro Mechanical Sensors (MEMS), which are made on a tiny scale (in micrometers), usually on a silicon chip, with mechanical and electrical elements integrated together.
The basic working principle behind MEMS is to measure the change in electric signal originating due to mechanical motion. This change in electric signals is converted to digital values by electric circuits. The accelerometer and gyroscope are the main examples of MEMS. Most of the sensors in an Android phone consume minimal battery and processing power. We will discuss all the important sensors in detail in the coming chapters.
Sensor can be broadly divided into the following two categories:
- Physical Sensors: These are the actual pieces of hardware that are physically present on the device. They are also known as hardware sensors. Accelerometers, gyroscopes, and magnetometers are examples of physical sensors.
- Synthetic Sensors: These are not physically present on the device, and they are instead derived from one or more sensors. They are also called virtual, composite, or software sensors. Gravity, linear acceleration, and step detector are examples of synthetic sensors.
The Android platform doesn't make any distinction when dealing with physical sensors and synthetic sensors. The distinction is mostly theoretical to understand the origin of the sensor values.
Sensor values can be broadly divided into the following three categories:
- Raw: These values are directly given by the sensor. The operating system simply passes these values to the apps without adding any correction logic. Accelerometers, proximity sensors, light sensors, and barometers are sensors that give raw values.
- Calibrated: These values are computed by the operating system by adding extra correction algorithms, such as drift compensation and removing bias and noise over the raw values given by sensors. Step detector, step counter, and significant motion are sensors that give calibrated values by using an accelerometer as their base sensor. The magnetometer and gyroscope are special kinds of sensor that give both raw and calibrated values.
- Fused: These values are derived from a combination of two or more sensors. Generally, these values are calculated by leveraging the strength of one sensor to accommodate the weaknesses of other sensors. Gravity and linear acceleration give fused values by using the accelerometer and gyroscope.