Adjusting Detector Technology to Real-Life Expectations

Adjusting Detector Technology to Real-Life Expectations

Real-life conditions could easily affect sensor sensitivity and optimum performance. In this feature, practical issues are addressed for you to build more resilient intrusion detection systems with minimized FARs.

Intrusion detection systems, or burglar alarms, are used to detect unauthorized entry into buildings or areas. While most of us know intrusion detection as basic security for residential apartments, commercial offices or retail stores, they are, in fact, used to safeguard high-value assets and high-security establishments such as banks, ATMs and even military bases. Today, intrusion detection forms an integral part of integrated security systems, which can also include video surveillance, access control and fire alarms.

Many different technologies and methods have been used for motion detection, and the biggest challenge for any motion detection device is and has always been falsealarm reduction. One such technology used for motion detection is heat sensing, or passive IR (PIR).

As the name suggests, PIR sensors do not transmit anything; they measure changes in IR radiation and register an electrical signal. All objects with a temperature above absolute zero (minus 273 degrees Celsius) emit IR energy. The PIR sensor consists of a Fresnel lens, a pyroelectric sensing element and a signal-processing unit. The Fresnel lens focuses IR energy onto the pyroelectric element, which then translates the measurements of change in IR energy into an electrical signal, which is then interpreted by the signal-processing unit.

The PIR sensor looks at a sensing area by dividing it into horizontal and vertical detection sectors or zones, each defined with specific boundaries. Detection occurs when an IR energy source (such as a human being) crosses two adjacent sector boundaries or crosses the same boundary twice within a specific time. The signalprocessing unit uses algorithms to decipher the wave form generated by the pyroelectric element and by studying its amplitude (or signal strength), frequency and timing between each signal pulse. This enables the sensor to differentiate human moving targets from other heat sources or false-alarm sources such as pets and animals.

Before installing a PIR sensor, it is important to understand that the intruder is “required” to walk across the PIR sensor's effective sensing area in order to maximize catch performance. Therefore it is important to take note of the following when deciding where to place the motion sensor.

*Place the intrusion sensor where there is expected human traffic, or place it as a trap for the intruder.
*Place the intrusion sensor at the correct height in accordance to the installation manual.
*Take note of the sensing area of the sensor and the sensing area that is required.
*Avoid having blind spots in your coverage area where it is not possible for the sensor to detect anyone.
*Walk test your sensor.

Since PIR motion sensors are sensitive to IR energy, it is important to note that heat sources that could be mistaken for human intruders should be avoided. They include windows (with or without curtains), air conditioning, air vents, fire places and direct sunlight.

Pets and rodents are notorious false-alarm sources. The problem can be circumvented by installing PIR sensors that are “pet-immune” or “pet-friendly.” Pet-friendly PIR sensors ignore IR radiation signals from rodents and pets since these signals are smaller than that of a human. This, of course, depends on the size of the animal, and usually the instruction manuals of the motion sensor would specify the performance limitations of its pet immunity features in terms of the size or weight of the animal.

Since PIR technology alone is not the ideal single technology for motion sensing, PIR sensors are often combined with other motion-sensing technologies in a single unit in order to provide optimum performance while canceling out each other's weaknesses. This type of motion sensing is commonly known as dual-technology motion sensors.

It is possible to configure the dual-technology motion sensor to work in “and” mode where both technologies must simultaneously sense an alarm condition before activating an alarm. In this mode, false-alarm immunity is increased while catch performance is lowered. Alternatively, it can be configured to operate in “or” mode where only one of the two technologies needs to send an alarm condition. Catch performance is increased this way, but false alarms are also increased. It is next to impossible to have both increased false-alarm immunity and catch performance at the same time. Therefore, expectations have to be clarifed before deciding on which is more important.

Microwave technology is based on the Doppler Shift Principle, which indicates that electromagnetic energy waves are sent out and then get reflected back to a built-in receiver. The sensor's built-in signal-processing unit compares the transmitted and reflected. If there is no movement in the area, the wave form remains constant. If the electromagnetic energy is reflected from a moving object, the wave form changes and initiates an alarm.

Microwave motion sensors require the expected intruder to move toward the sensor in order for it to see. The closer and faster the intruder moves, the stronger the signal sensed by the microwave sensor. However, microwave technology has its flaws. Microwave can see through walls, which means that if the range of the microwave sensor is not tuned to suit the required sensing area, the microwave sensor can potentially mistake a person walking behind the wall as an intruder.

Product Adopted:
Detectors / Sensors

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