When analyzing movement sensors, they can be broken down into two main types of movement sensors: active motion detectors and passive motion detectors. Active movement sensors utilize sensors that emit a type of signal that is then reflected back and detected. Passive movement sensors do not actually emit a signal like active sensors, but instead detect the signals being emitted by the objects in their field of view and have a predetermined baseline reading for the surrounding area.
Active movement sensors are mainly used in places where a rapid response from a change in detection is needed (ultrasonic or microwave). For example, a motion sensor for a car garage uses an active ultrasonic radar-like sensor that emits sound waves. When the sound waves are emitted, the sensor detects the pattern or the time it takes for those waves to bounce off of its surroundings and return back to the sensor. When an object is approaching the garage door or entering the space under the garage door while it is closing, the sound waves are emitted and bounce off of the object crossing the path of the sensor causing the return time to be faster. This is then computed by the sensing system and tells the garage door mechanism to open so as to allow a car to enter or to prevent the door from closing on something or someone.
Passive movement sensors detect a change in their surroundings by reading the energy of their surroundings. These types of sensors come in various forms, some of the most common being infrared or photo sensors. These sensors detect and measure the energy that is being produced by the objects in their line of sight and are commonly used in home security systems or businesses to warn them of someone entering the facilities. Bodies that generate heat, be they humans or animals, also generate infrared energy (for humans usually ranging between 9 and 10 micrometers). With this range in mind, infrared sensors are programmed to detect emissions within the range of 8 and 12 micrometers with the use of a photo detector. This sensor measures the light being transmitted to it, converts it into an electrical current that is sent to the sensor’s processing core. “The alarm is triggered when the photo detector detects large or fast variations in the distribution of the emitted infrared energy.” (1). This form of detection allows for the movement sensor to ignore slight variations in heat that occur over the length of the day, such as the slow change in temperature of objects in the sensor’s field of view as their temperatures cool down over night.
These sensors are very inexpensive and can offer a very secured environment when combined to cover all entry points of the desired area.
I found it interesting in my classmate Matthew Tedesco's post (http://ae-510-ay12-13.blogspot.com/2013/02/flow-measurement-of-fluids-including.html) how he explained the various applications a specific type of sensor may have. Movement sensors aren't limited to only sensing movement, but as Matthew described, optic sensors (which are a type of movement sensor) can also be used to measure mechanical flow, fluid velocity and flow.
Sources:
(1) http://www.ehow.com/how-does_4596955_motion-sensor-work.html
(2) http://home.howstuffworks.com/home-improvement/household-safety/security/burglar-alarm2.htm
(3) http://home.howstuffworks.com/home-improvement/household-safety/security/question238.htm
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