The Control Panel for Alarm Systems - Term Paper Example

Sensor Technology Name Institution Date Table of Contents Table of Contents 2 Introduction 3 Sensors 3 Passive Infrared Sensors 3 Microwave sensors 6 Point sensors - magnetic reed switch. 9 Smoke detectors 10 Ported coaxial cables 12 The alarm system and the control panel 13 House alarm system design 15 References 17 Introduction Intruder sensors are devices used to detect any unauthorized motions and converts the disturbance due to this intrusion into signals that trigger an alarm system. Several intrusion sensors exist, each one coming with specific performance characteristics and principles or operations. While volumetric sensors provide surveillance over large coverage areas and can monitor comparatively long distances, point sensors provide narrow line of sight surveillance. Different alarm systems may use different combinations of these sensors to ensure that all areas of interest have been cover effectively. This paper focuses on such sensors, and investigates their principles of operation and applications. The control panel for alarm systems, which is a central point of monitoring and communication in the system, has also been discussed. The paper also focuses on the communications between the alarm systems and the central monitoring station, which serves as a central point for monitoring and coordination of responses. Sensors Passive Infrared Sensors The passive infrared sensors (PIR) are pyroelectric devices that function by detecting changes in the infrared (radiant heat) from emitting bodies in its surrounding. Any movements in the surrounding are detected by monitoring any sudden changes in the IR pattern in the neighbourhood. When the device detects any movements, it sends out a high output signal. This signal may be sent and interpreted by a microcontroller or may be used to trigger other loads like an alarm system. The sensor is made up of crystalline material that produces an electrical charge when infrared energy falls on it. When the total infrared energy falling on the element changes, the voltages produced by the element also changes. These signals are then measured by an on-board amplifier which brings the signal to levels that can easily be used for triggering. The devices have an inbuilt Fresnel lens that increases the sensitivity by focusing the infrared energy on the element. Rapid changes in the ambient infrared signals cause the amplifier to trip the output so that motion is detected (Paralax, 2012). With the onboard jumper, users can choose between reduced sensitivity and normal operation. The device’s sensitivity is greatly influenced by the temperature (see figure below) and other environmental conditions. When the device is operated in reduced sensitivity, it will detect movements up to `half of the distance covered during normal operating mode. Figure: Effect of temperature on PIR Distance Detection The optics designed within the device for collecting signals is a major factor that will determine its quality and sensitivity. They will also determine the spectral composition and effects quantity of the radiation that is directed on the element. The crystalline element integrates the optical signal received on its surface and time, before converting it into an electrical signal. While amplification and signal filtration occurs, the frequency and noise characteristics of the device will determine the accuracy of the final analysis which influences object detection. For objects sizes smaller than the field of view of the optical sensor system, the device decreases the signal to noise ratio with the square of the distance between sensors and object (Madura, et al., 1998). Given its sensitivity and vulnerability to effect of environmental conditions, this device is designed for indoor use. It may be significantly affected extreme temperatures and other environmental conditions if assembled outside. The device may also operate undesirably when exposed to direct sunlight or to other sources of radiant heating. The PIR 555-28027 shown below operates with sources of up to 12 mA at 3V and 23 mA at 5V. It can detect a person from about 30 feet away in normal mode and up to 15 feet in reduced sensitivity. It has an inbuilt jamper that the user will use to select the mode of operation and onboard LEDs that illuminates the lens for fast visual feedback when motion is detected. It is also small size making it easy to conceal and to fit into critical positions without attracting attention (Paralax, 2012). These devices can operate in daylight and in darkness. They can also operate desirably in environments with reduced visibility, as opposed to sensors that operate within visible or near-infrared radiation. Since they detect radiant heat from bodies, which is an inseparable attribute of these bodies, they are very effective. Figure: 555-28027 PIR sensor Microwave sensors Microwave sensors are particularly useful when there is need to cover large areas. They can operate over large temperature ranges and can work effectively under the influence of strong environmental interferences like wind, moisture, fog, acoustic noise and so forth. Microwave sensors are categorised as active sensors since they provide their own excitation signal in the form of electromagnetic energy. The sensor operates by projecting the electromagnetic radiofrequency towards the protected area. The device sends a high frequency electromagnetic signal and waits to receive the signal back. The returned signal wave is then sampled and any changes in strength and frequency monitored to determine whether or not a disturbance has occurred (Mueller, 2005). When the waves are scattered back from intruding objects that are sizable enough to cause significant disturbance, the devices begins to operate. The reflected waves are received and amplified before they are analysed. The time period between the sending of the signal and its reception is used to determine the distance of intruding object. The device will interpret the frequency shift and estimate the speed of motion of the object (Fraden, 2010). Microwave sensors fall under the class of devices referred to as radars (RAdio Detection And Ranging). The radar frequencies are as below: Figure: radar frequencies The microwave signals refer to those with wavelengths shorter that 4 cm. The wavelengths are, however, long enough to pas to pass through several contaminants like dust and cloud, but get reflected by objects large enough. Several classes of microwave detectors have been developed and applied in various applications. The pulsed radars microwave systems can be arranged into two basic modes. In the monostatic mode, the device utilizes a single antenna to transmit and receive the signal pulse. The time delay for the received signal is found by the expression: T = 2 r/c Where r (in meters) represents the distance between antenna and the reflective object while c is the speed of light (3 x 108 m/s). With this system, however, the antenna needs to be switched very fast from transmitter to receiver making it difficult to detect intrusions that are too close to the antenna. The bistatic mode comes with two antennas for the transmitting and receiving ends (see figure below). The transmitted wave covers the region to be monitored while the receiver senses the existence, distance and exact position of intrusion (Polivka, 2001). Figure: bistatic (pulsed) radar The receiver combines the received signal in vector summation with signals reflected from objects in the neighbourhood and the ground. Any changes in this signal at the receiver due to movements in the coverage zone that exceeds the preset threshold values triggers the alarm. For improved pointing, the antennas could be arranged as a matrix and the mechanical antenna movements replaced by electrical scanning (Polivka, 1995). The detection zone for a bistatic mode detector is shown below: Figure: detection zone for a bistatic microwave Microwave sensors are affected by extreme weather conditions which may cause these sensors to generate false alarms. They are commonly affected during the first few drops of hard rain but they recognise the rain when it persists for some time and then adapts to the rain. When the detection area is extended to the ground, then false alarms may also arise from surface water. It is therefore important to ensure proper drainage within the detection area. These detectors are however, known for their abilities to overcome the challenges of falling stones and debris, cardboards and paper since these objects do not reflect back the electromagnetic waves. When these objects get wet or icy, however, there are chances that they will reflect the waves Point sensors - magnetic reed switch. A basic magnetic switch is made up of two low resistance, ferromagnetic, thin reeds that are hermetically sealed within a glass tube. This enclosure is controlled in a cantilever fashion making the ends of the two reeds to align and overlap with a small distance between them. Given the ferromagnetic properties of the two reeds, their extreme ends will develop opposite magnetic polarity when a magnetic field is brought close to them. As long as the field is close enough and strong enough, the force of attraction between the opposite poles exceeds the stiffness of the reeds causing them to be drawn towards one another, making the switch close. This switch can operate in this manner several times at extreme speeds. The magnetic reed switch is activated by the presence of a magnetic field. This may be achieved by reducing the distance between the switch and a permanent magnet or by passing current through an electromagnetic coil that is mounted near the switch. This switch is highly reliable since the contacts are hermetically sealed within a glass tube. It is therefore free from dust, oxidation, tamper and corrosion proof. The switch has a long service life, with the ability to operate over a million times at maximum rating as long as contact rating is observed and proper protection done. It can operate over a huge temperature range from -20oC to +250oC, although much higher temperatures may influence the switching function. Although he switch is not as fast as the electronic switches, it is comparatively faster than traditional switches. They will therefore give almost instantaneous triggering of alarm systems and are therefore ideal for monitoring doors that should remain closed. They can also be used to monitor windows and drawers or any openings that need to be kept closed (Gems sensors, 2008). Smoke detectors This device detects fire at an early stage by sensing the smoke development. The photoelectric types operate based on the scattered-light principle. In this type of detector, the light transmitters and receivers are arranged in the chamber in such a way that the light signals emitted from the transmitters do not directly impact the receiver. As the smoke builds up in the chamber, however, the floating smoke particles begin to scatter the light signals. As more smoke fills the chamber, more light is scattered until they impact the photocell which generates an electrical signal that triggers the alarm. The alarm panel must keep checking the condition of the smoke detector to ensure that it remains functional. Any faults detected on the smoke detector are indicated by a fault signal (sound and LED) at the panel. It is important to note that smoke detectors only monitor the situations around it, the designers must ensure that a sufficient number of detectors are installed to cover the whole room especially if the room is large (Gira, 2002). As shown in the figure below, enough detectors must be installed for efficient detection and alarm triggering. They should be mounted under the ceiling in the middle of the room to achieve optimal detection. It is advisable to maintain a minimum distance of 50 cm from the wall. A smoke detector is able to effectively monitor a room with no more than 60m2 in area, with a maximum of 6 m in height. Figure: smoke detector arrangement for effective fire detection Figure: smoke detector arrangement for storey building Ported coaxial cables These are active, covert terrain following sensors that are perfectly concealed from intruder’s view since they are buried into the ground. They are also referred to as radiating cable or leaky coax sensor. They respond to movements of bodies that have high dielectric constant or those with high conductivity (Garcia, 2007). They are made up of two ported coaxial cables and a control unit. The processor contained within these systems has a transmitter, a number of amplifiers and filter circuits, a receiver as well as a microprocessor. The sensor makes use of electromagnetic energy with two cables that are laid side by side in the ground. One of the cables is connected to the transmitter while the other is connected to the receiver (Mays, 2004). The outer conductor of both cables is designed with small holes that are specially spaced so that radiofrequency energy is radiated from the transmitter cable to the surrounding environment. The radiated energy is then coupled to the receiver cable through its own pores. This establishes a static field that couples the pair of cables. Any movements into the field disturb this coupling so that the receiver gets a signal that is distorted. The changes in the received signal are processed and assessed for appropriate action. If the changes detected are beyond the threshold values, a trigger signal is generated to activate the alarm (Mays, 2010). The detection area for these sensors is elliptical and can extend 3-4 metres wide, 1 metre high. The sensor can detect intrusions both above and below the ground to some extent but it has been revealed that sensitivity and detection area varies along the length of the cables. This may result from a number of reasons like cancellations of phase, variations in the distance between the cables, metallic bodies within the neighbourhood of the cables etc. The sensor is however, greatly effective since it remains hidden below the ground. It is safe from physical destruction by falling trees and is less likely to be tampered with by intruders. It can be effectively assembled around the building on the inner circumference of the fencing to detect any unauthorized intrusion (Barnard & Barnard, 1988). The alarm system and the control panel Alarm systems are usually made up of various components. The components can be categorised into: Initiating devices - these devices are responsible for placing the system in the alarm state. They are used to monitor the area that need to be protected. These devices can be heat detectors, smoke detectors, microwave sensors, and other sensors that can convert motion or other forms of danger into signals that trigger alarms. They form the starting point of the operations of alarm systems. Indicating appliances - these are devices used to show when they system has gone into alarm state to the occupants of the building or remote persons that are supposed to respond to the situation at hand. These devices include horns, bells, chimes, strobe lights or combination units. These devices are designed to withstand extreme weather conditions and can be used in harsh weather and other hazardous locations. Control panel – this is the heart of the alarm system. It contains programming and operating electronics, and a user interface where operators can interact with the system. The panel is fed by standard branch-circuit wiring and is designed with replaceable circuit cards for all the zones monitored by the system. The panel comes with an alphanumeric display that shows the state of the system and provides information used to troubleshoot as well as a touchpad used by the operating personnel to silence an alarm, reset the system and reprogram if required (see figure below). Figure: Fire alarm control panel with touchpad The control panel continuously assess the state of the monitoring devices and circuits for any shorts and open wiring using the applied DC voltage. The triggering devices are usually open and only close in the event of a fire or intrusion and become conductive close to zero ohms. The control panel is able to distinguish between a non-alarm state and when there is an open wire fault. This is achieved by using an end-of-line resistor. During the designing of the system, a 4.7 kilohm resister is usually installed across the line after the last device. When the control panel sees this resistance, it maintains its normal status. In the event that this resistance increases, the panel interprets this as an open fault and the panel shifts to trouble state. The panel causes a buzzer sound to bring this situation to the attention of the personnel and display something like “Open Circuit Zone Three” on its alphanumeric display. The panel also monitors the wiring and zone cards to find out whether or not they are functioning well. A low voltage signal is always applied to the indicating appliance circuits. This voltage is kept below that which can set off the horns, but is closely measured as part of the supervisory functions of the control panel. The moment this current stops flowing, the trouble alert buzzer sounds and the display shows the presence of an open circuit (Herres, 2006). The batteries and auxiliary devices make up the remaining components of the system. The sealed emergency light batteries used for alarm system are usually 6V that are connected in series to achieve 24V for a power limited system. The batteries are housed within the panel or can be in a separate enclosure. Auxiliary devices include remote annuciators with LEDs that indicate the state of the system, a switch for silencing the alarm, and visual LED that indicates the zone from which an alarm has been initiated (Herres, 2006) Communications to the central monitoring station The alarm system will always include a central monitoring station where the entire operation of the protection circuits and devices are controlled directly, recorded in, supervised and maintained. This station will usually house trained operators and qualified professionals who attend to the needs of the system all the time. When the alarm system is operational, the control panel which is the central computer receives signals from the sensors and automatically alerts the central monitoring station by means of built-in telephone communicator. The panel could also be linked to the station through a radio or cellular transmitter (Safe security, 2010). House alarm system design The alarm system below has used various motion sensors that have been carefully selected for specific positions. The system is monitored by an alarm control panel that has been installed as far away from the common areas of the house as possible. The specific corner in the laundry room was selected as a safe location to minimize accidental or intentional interference. The alarm keypad is positioned close to the door, but far away from the control panel. Along the corridor, PIR sensors have been positioned strategically to enhance the chances of accurate monitoring. The sensors are positioned so that intrusion cuts across the sensor field of view as opposed to having the intrusion move along the field lines. To achieve efficiency, the PIR sensors have been overlapped by the microwave sensor waves to detect any crawling intrusion and to cover other areas not entirely covered by the PIR sensors. All the windows are monitored using the magnetic reed switch. This will ensure that they remain secured. All the information will be communicated and processed at the control panel, which will be linked to the central monitoring station. The alarm system is as shown below: Figure: Home alarm system References Fraden J. (2010). Handbook of Modern Sensors: Physical, Design, and Applications. New York: Spring Science & Business Media Polivka J. (1995). Microwave Radiometry and Applications. International Journal of IR/MM Waves. Vol.16, No. 9, pp.1593-1672 Polovka J. (2007). An Overview of Microwave Sensor Technology, High Frequency Electronics. Santa Barbara, CA: Summit Technical Media, LLC. pp 34 – 42 Parallax. (2012). PIR Sensor (#555-28027). Retrieved on 17th October 2014 from < http://www.parallax.com/sites/default/files/downloads/555-28027-PIR-Sensor-Prodcut- Doc-v2.2.pdf > Madura, et al. (1998). Analysis and elaboration of IR detection systems applied in object protection. Report from work No. 103/WAT/98 Gems sensors. (2008). What is a reed switch? Retrieved on 17th October 2014 from < http://docs- europe.electrocomponents.com/webdocs/00b8/0900766b800b81b0.pdf> Gira. (2002). Installation and Operating Instructions: Smoke detector basics 114402. Retrieved on 17th October 2014 from < http://download.gira.com/data2/11441190.pdf> Herres D. (2006). Understanding Basic Fire Alarm Systems. retrieved on 1th October 2014 from < http://ecmweb.com/design/understanding-basic-fire-alarm-systems> Safe security. (2010). Alarm system operations manual. Retrieved on 17th October 2014 from < http://www.safesecurity.com/support/pdf/SAFE_Residential%20_Manual.pdf> Garcia M. L. (2007). Design and Evaluation of physical Protection Systems. Oxford: Butterworth-Heinemann Mays L. (2004). Water Supply Systems Security. New York : McGraw Hill Professional Mueller J.P. (2005). The Savvy Guide to Home Security. New York: Indy Tech Publishing Barnard R. L. & Barnard R. (1988). Intrusion Detection Systems. Texas: Gulf Professional Publishing Mays L.W. (2010). Transmission and Distribution. Denver, CO: American Water Works Association Garcia M. L. (2007). Design and Evaluation of Physical Protection Systems. Oxford: Butterworth-Heinemann Read More