Motion sensors are the sensor devices that are used to detect moving objects. Normally, the motion sensors are incorporated with an automated system that performs a task or alert a user in case of detection of motion in a particular area. Motion sensors are the building blocks components in advance security systems, home control, automated lightning systems and other automated system used in industries.
Motion sensors have two basic types: active motion sensors and passive motion sensors. An active sensor needs to emit a signal first and collects the reflected signal after the signal hits an object. When an object is moving, there are also changes on the reflected signals, which activate the system to function. As the name suggests, a passive sensor does not send out any signal but only detects the infrared radiation from surrounding objects. Passive sensor detects object based on temperature difference, so it is more suitable for detecting people by getting their body temperature.
Motion sensing could be achieved through different technologies, such as infrared, microwave, ultrasonic and optics etc.
Infrared: Infrared method includes active infrared (AIR) and passive infrared (PIR). In AIR sensors, the emitter gives off a beam of infrared that can be detected by an infrared receiver. When the signal is interrupted, a motion is detected. In PIR, there is no infrared emitted but the sensor passively detects any infrared irradiation from surrounding environments. PIR sensors are sensitive to human body’s temperature and can detect body heat. So PIR sensors have become the widely used motion sensors in home security system.
Microwave: The operating principle in microwave motion sensors is the same as what is used in Doppler radar and speed gun. In microwave motion sensor, a continuous microwave is emitted and a phase shift is measured in reflected microwave due to the motion of an object. Microwave motion sensor usually covers a larger detection area compared to infrared sensors, but they are also more vulnerable to electrical interference.
Ultrasonic: In ultrasonic motion sensor, an ultrasonic wave is emitted and reflection from nearby objects is received. When a movement is detected, the ultrasonic wave pattern is disrupted and alarm can be triggered. If no object is detected, the pattern of ultrasonic wave emitted and the time to receive reflection remain the same.
Optics: Digital camera technology has made it possible to detect the motion of a moving object. It is very convenient when one wants to record video triggered by motion detection, as the hardware requirements for this system is just a camera and a computer. Recent advancement combines video cameras with advanced signal processing. The sensitive motion sensors begin to record the video if there is a motion being detected. The advantage of triggered recording is that cameras only record when it is necessary to do so because motion sensors only triggers the signal after detecting a motion. This can largely save the memory storage and does not require recording hundreds of hours of useless footage. Only important information will be recorded in this case.
The motion sensors are widely employed in commercial applications and can be found in almost every field of latest electronics, such as smartphone, wearable electronics, playstation and virtual reality etc. Besides, they are used in intruder alarm in home, offices, banks and shopping malls for property protection. They are now also found in counting machines and automatic light control. With further combination of other sensor technologies, motion sensors can be used in energy efficient systems, home intelligent systems, gesture recognition, robots and smart city etc.