Intrusion detection devices must maintain a delicate balance. Highly sensitive devices can cause too many false alarms, while devices with insufficient sensitivity are easily bypassed by sophisticated intruders. Innovations in intrusion detection technology have enabled significant advances in performance, resulting in excellent capture performance while virtually eliminating false alarms. In this article, Tom Mechler, Product Marketing Manager for Anti-Intrusion Products for Bosch Security Systems in North America, explains that well-designed intrusion detection devices have the ability to ignore non-intruder events such as animals. domestic and other, changes in ambient temperature or lighting conditions, moving shadows, moving electrical appliances and other harmless events. These detectors integrate and analyze data from multiple sensors, including a passive long-range infrared (PIR) sensor, a short-range PIR sensor, a microwave sensor, a white light sensor, and a temperature sensor.
The concept of collecting data from multiple sensors is not new. Many detectors combine data from various sensors, but simply building multiple sensors into one detector does not increase its efficiency. Using multiple sensors as checks and balances can increase a detector’s false alarm immunity, but does so at the expense of detection performance. The critical difference lies in the way this data is handled.
Sensor data fusion collects information from all sensors and feeds it into a microprocessor that analyzes the data using complex algorithms. This allows the detector to balance the sensors and adjust the sensitivities in order to make truly intelligent decisions to determine valid alarm conditions.
Sensor data fusion technology in intrusion detectors at work
|Since white light sensors prevent false alarms, burglars try to trick detectors with a flashlight|
Here are some examples of how data fusion from sensors works in practice under a wide range of conditions that otherwise have a high potential for triggering false alarms.
Car Headlight Scan Detector
When a car enters the parking lot at night, the headlights scan the detector mounting location. Because PIR sensors are sensitive to slight changes in temperature, they detect both short and long range headlights. Of course, the white light sensor also detects the headlights.
The key to this scenario is the synchronization of the data from the PIR sensors and the white light sensor. Heat and light change in tandem each time the headlights sweep the detector. The microprocessor algorithm is aware of this timing and determines that this is not a valid alarm condition. Therefore, the security system does not trigger a false alarm.
Burglar tries to trick intruder detector with flashlight
Aware of the white light sensor and its role in preventing false alarms, a burglar attempts to trick the detector with a flashlight.
Due to the burglar’s location, it only appears on the long range PIR sensor, not the short range. And since the disturbance is detected at a long distance, the sensitivity of the microwave sensor is increased and its alarm threshold lowered. More importantly, the microprocessor’s algorithm knows that the data from the PIR sensor, white light sensor, and microwave sensor is not in sync and determines that this is a valid alarm condition.
Mouse cross room 4.5 m from the detector
Conventional intrusion detection devices that degrade capture performance for false alarm immunity or vice versa are replaced by technology that eliminates the need to compromise between the two critical functions
When a mouse is only 4.5m away from the detector, only the short-range PIR sensor responds. A human intruder near the detector would reflect a large area towards the microwave sensor. So when only the short range PIR detects a signal, the microprocessor increases the threshold of the microwave sensor. The distance data provided by the two PIR sensors allow the microprocessor to vary the threshold of the microwave sensor. And because the mouse reflects a small area back to the microwave sensor, the newly high threshold is not reached and the security system does not trigger the alarm.
These are just a few examples showing how sensor data fusion technology can work. There are countless scenarios where it comes into play to catch real intruders, while minimizing false alarms.
In addition to the sensor technology described above, there are other sensors that play an important role in intrusion detection. A description of these follows.
Pet immune detectors
Pet immune detectors use precise optics and specialized signal processing to distinguish between signals caused by humans and those caused by pets. This allows them to ignore signals caused by one or two animals up to about 45 kg or signals caused by many rodents.
Some detectors with temperature sensors simply increase the gain of the sensor as the ambient temperature increases. This gain increase continues linearly when the ambient temperature exceeds the expected surface temperature of an intruder (33.3 Â° C). Thus, the risk of false alarms increases with the ambient temperature.
With advanced detectors, the sensor gain increases as expected, but only until the ambient temperature reaches 33.3 Â° C. By using two PIR sensors with multiple high quality lenses, the signal to noise ratio can only be increased in this narrow temperature band. Once the ambient temperature rises above the surface temperature of an intruder, the sensor gain decreases again to minimize false alarms.
Anti-mask technology alarms when an intruder attempts to defeat the detector by masking it with an infrared blocking material, such as paper, tape, film or spray. Typically, an intruder will use these strategies during normal business hours when the security system is disarmed, and then return later knowing the motion detectors are compromised.
Multi-point anti-mask with integrated spray detection technology recognizes attempts to mask black paper, aluminum and acrylic sheets, white styrofoam, clear self-adhesive vinyl, sprayed plastic skin , clear lacquer (applied with a brush), spray paint, etc. . This is accomplished with the following technologies:
Bounce Technology determines if the detector is covered or blocked by an object, such as a shoebox or a piece of paper. Using rebound technology, the detector creates an infrared energy bubble extending approximately 30 cm in front of the detector. This bubble radiates from the IR emitter. If the detector is blocked, a higher than normal level of IR energy will reflect through the detector lenses. The detector uses multiple IR photodiodes to detect additional IR energy levels, then sends a trouble signal to the security system indicating that someone has obscured the detector.
Retro reflector Technology determines if someone is attempting to compromise the detector with sprayed material, such as paint. The detector’s LED light pipe contains the advanced multi-prismatic structure of the Retro Reflector. Under normal conditions, the Retro Reflector structure reflects IR energy back into the detector where it is detected by a dedicated IR sensor. When the prism structure is covered with a spray material, the reflective properties of the prisms are negated, allowing infrared energy to pass through the prisms and out of the detector. A dedicated IR photodiode detects the reduced reflected IR energy level, then sends a fault signal to the security system.
Through the lens The technology determines whether a material is placed directly over the detector lens, such as tape or other IR-opaque material. The detector emits IR energy onto itself from two specialized prisms located at the bottom of the detector. If the lens is masked, a photodiode receives reduced IR energy levels. The detector then sends a trouble signal to the security system indicating that someone has obscured the detector.
As you can see, there have been many innovations in intrusion detection technology over the past decade. Conventional intrusion detection devices that degrade capture performance for false alarm immunity or vice versa are replaced by technology that eliminates the need for tradeoffs between the two critical functions.
Take a close look at the sensor technology you are using for your next project to make sure you are providing your customers with the best performance for their security systems. Not all detectors are the same.