Exterior Sensors

Exterior intrusion-detection sensors normally are used to detect an intruder crossing the boundary of a protected area. They also can be used in clear zones between fences or around buildings, and to protect materials and equipment stored outdoors within a protected boundary.

Exterior sensors are designed to operate outdoors. The detection function must be performed with a minimum of unwanted alarms such as those caused by wind, rain, ice, standing water, blowing debris, animals, and other sources. Criteria for selecting an exterior sensor include the probability of detection, the sensor's susceptibility to unwanted alarms, and the sensor's vulnerability to defeat.

An exterior sensor is much more vulnerable to the physical and environmental conditions of a site than an interior sensor. Many uncontrollable forces (such as wind, rain, ice, frozen soil, standing or running water, falling and accumulated snow, and blowing dust and debris) may affect an exterior sensor's performance. Although attention generally is directed to circumstances that cause a dramatic drop in the probability of detection, environmental factors also can cause short-term increases in the probability of detection.

Exterior sensors are more susceptible to nuisance and environmental alarms then interior sensors. Inclement weather conditions (heavy rain, hail, and high wind), vegetation, the natural variation of the temperature of objects in the detection zone, blowing debris, and animals are the major sources of unwanted alarms.

• Fence-mounted sensors. These sensors are part of a fence protection system that consists of a cable (often ported coaxial or fiber optic cable) or a series of wires whose physical properties are changed by attempts to climb, cut, or lift a chain link fence. The sensor’s processor receives updates of the physical properties of the cable or wires. If the change meets the processor’s criteria, the processor sends an alarm signal to the overall alarm system’s processor, an alarm is generated, and security forces (hopefully) respond.
  
  
• Electric-field or e-field sensors. Electric-field sensors consist of an alternating-current field generator, one or more field wires, one or more sense wires, and a signal processor. The generator excites the field wires, creating an electrostatic field. The electrostatic field induces electrical signals in the sense wires, which are monitored by the signal processor. Under normal operating conditions, the induced signals are constant. An intruder approaching the sensor alters the induced electrical signals, causing the signal processor to generate an alarm.
  
  
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  Capacitance sensors. Capacitance sensors detect an intruder approaching or touching a metal object (including a wire) by sensing a change in capacitance between the object (or wire) and the ground (another wire). A capacitor consists of two metal plates separated by a dielectric medium that does not conduct electricity. A change in the dielectric medium or electrical charge results in a change in capacitance. In practice, the metal object to be protected forms one plate of the capacitor and the ground plane surrounding the object forms the second plate. The sensor processor measures the capacitance between the metal object and the ground plane. An approaching intruder alters the dielectric value, changing the capacitance. If the capacitance change satisfies the alarm criteria, an alarm is generated.
  
  
• Buried-line or ported-cable sensors. A buried-line sensor system consists of detection probes or cable buried in the ground, ideally between two fences that form an isolation zone. The sensor’s cables are ported (leaky) coaxial cable. One cable is used as a transmitting antenna to generate a radiofrequency field in the secured area. The other cable is used as a receiving antenna. In a “secure” condition, the receiving cable detects a steady or unchanged cable signal. An approaching intruder alters the characteristics of the received signal. The alteration is interpreted by an electronic processing unit that generates an alarm if an intruder passes into or through the detection field.
  
  
• Line-of-sight sensors. Line-of-sight sensors, which are mounted above ground, can be active or passive.
  
  
  • Active sensors generate a beam of microwave or infrared light energy and
    
    detect changes in the received energy when an intruder penetrates the beam.
    
    
  • Passive infrared sensors do not generate a beam of energy; they simply look for changes in the thermal characteristics of their field of view. For effective detection, the terrain within the detection zone must be flat and free of obstacles and vegetation.
    
    
• Video motion sensors. A video-motion sensor generates an alarm whenever an intruder enters a selected portion of a video camera's field of view. The sensor processes and compares successive images from the camera, generating an alarm if differences between the images satisfy predefined criteria. Digital devices convert selected portions of the analog video signal into digital data that are compared with data converted previously; if differences exceed preset limits, an alarm is generated.
  
  
• Seismic or seismic/magnetic sensors. A seismic sensor detects ground vibrations. Seismic/magnetic sensors detect ground vibrations and ferromagnetic material commonly associated with ground vibrations (such as a truck driving in the sensor field).