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Different Types of Motion Sensors and How They Work

Motion sensors detect the movement of a person or object, including gravity, linear acceleration, rotation vector, vibration frequency, and more. They are often used to monitor device movements such as tilt, shake, rotation, or swing, and some frequently used sensor types are the rotation vector sensor and the gravity sensor. For example, rotation vector sensors are widely used for various motion-related tasks like monitoring movements, angular changes, and shifts in orientation. Those developing games, augmented reality applications, 2D or 3D compasses, or camera stabilization programs will find that the rotation vector sensor is particularly helpful. It is worth noting that different motion sensors use varying technologies such as infrared radiation, sound pulses, and vibration-based fluctuations to function. To learn more about various types of motion sensors and how they work, read on.

Passive Infrared Sensors

The first type of motion detector we will be discussing in this article is the passive infrared sensor (PIR), also called a body sensor. All natural objects radiate infrared light, and this detector measures the infrared distance between the target and the background to obtain different infrared images. It will detect the infrared light emitted by a moving person or animal and generates a switch signal. Essentially, this pyroelectric device detects motion by monitoring the change of radiant heat in surrounding objects. It is also worth noting that passive infrared detectors are built of optical systems, thermal sensors, and alarm controllers.

A pyroelectric sensor is the core component of a passive infrared detector and is made of a ferroelectric material that is polarized under the action of an external electric field. When this field is removed, the polarization state remains the same, and this is called spontaneous polarization. When the polarized ferroelectric material is exposed to infrared radiation, the polarization intensity decreases while the surface charge decreases, and this part of the charge is converted to an output voltage by the amplifier. If radiation continues, the temperature of the ferroelectric material will stabilize and produce no voltage output.


Another type of motion sensor is the accelerometer, which measures the linear acceleration of a carrier. Often used in smartphones and tablets, accelerometers are inexpensive and easy to use. They are also used to measure gravity, and thus find use in gravimetry. Often applied to the airbag system of modern cars to detect the sudden deceleration of a vehicle when a collision happens, many accelerometers rely on Micro Electro Mechanical Systems (MEMS) technologies and use deflection and circuits to measure acceleration. Moreover, most accelerometers are built on the principle that the reference mass is connected to the housing by a spring, and the relative displacement between it and the housing reflects the magnitude of the acceleration component. The mass is securely connected using a thin elastic rod and shell, and the load caused by acceleration deforms the rod. Lastly, the output comes in the form of an electric signal proportional to the size of the acceleration disc.

Tilt Sensors

Tilt sensors are like accelerometers, but have a much simpler design. Also referred to as motion detectors or tilt switches, these types of sensors detect tilt, and are usually composed of a rolling ball sensor switch. For instance, when the rolling ball tilts to a critical position, the either makes contact with or detaches from two or more conductive pins. Furthermore, tilt sensors are used in a variety of applications, from being used to warn the overturn of construction equipment to being used for automobile lighting control and anti-lock brake systems.

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