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Multiple single-chip microcomputer approach tofire detection and monitoring systemA.J. AI-Khalili, MSc, PhDD. AI-Khalili, MSc, PhDM.S. Khassem, MScIndexing term : Hazards, Design, Plant condition monitoringAbstract: A complete system for fire detection and alarm monitoring has beenproposed for complex plants. The system uses multiple single chip architectureattached to a party line. The control algorithm is based on a two-level hierarchy ofdecision making, thus the complexity is distributed. A complete circuit diagram isgiven for the local and the central station with requirements for the software structure.The design is kept in general form such that it can be adapted to a multitude of plantconfigurations. It is particularly shown how new developments in technology,especially CMOS single chip devices, are incorporated in the system design to reducethe complexity of the overall hardware, e.g. by decomposing the system such thatlower levels of hierarchy are able to have some autonomy in decision making, andthus a more complex decision is solved in a simple distributed method.1 IntroductionRegulatory requirements for most high risk plants and buildings mandate theinstallation of fire detection and warning systems for all sensitive areas of the plant orthe building. Most fire codes state the requirement for monitoring and controlspecifically related to a type of a plant or building such as chemical plants, petroleum,nuclear plants, residential high-rises etc. A general conclusion of these codes can bespecified as the following requirements :(a) The source of all detector signals should be exactly identifiable by the centralstation(b) An extra path of communication between the central station and all localcontrollers(c) Direct means of control of alarm and central equipment by the central station(d) Means of communication between the central station and the fire department(e) Availability of emergency power supply. The codes usually also specify thetypes and frequency of tests for all equipment.A fire detection and alarm system is a combination of devices designed to signalan alarm in case of a fire. The system may also accomplish fan control, fire door holdor release, elevator recall, emergency lighting control and other emergency functions.These additional functions supplement the basic system which consists of detectionand alarm devices and central control unit.Technology has an influence on system architecture. When technology changes,the architecture has to be revised to take advantage of these changes. In recent years,VLSI technology has been advancing at an exponential rate. First NMOS and, in thelast year or two, CMOS chips have been produced with the same packing density withmore gates per chip yet at a lower power consumption than NMOS. Surely thischange in technology must affect our design of hardware at both the chip and thesystem level. At the chip level, single chips are now being produced which areequivalent to board levels of only the previous year or two. These chips havemicroprocessor, memory in RAM and ROM, IO Ports both serial and parallel, A/Dtimer, flags and other functions on chip. At the system level, the new chips make newarchitectures possible. The objective of this paper is to show how technology caninfluence system architecture in the field of fire control. The new high density singlechip microcontrollers are incorporated in the design of a large scale system and yet weobtain a smaller system with a better performance. In terms of fire detection andalarm monitoring, this is reflected directly in the local station hardware, because oftheir remoteness and power supply requirements. A complete local station can bedesigned around a single CMOS chip with power consumption of a few m Wdepending on system operation. This approach reduces the cost and complexity ofdesign, implementation and maintenance and provides easily expandable and portabledesign. This implementation was not possible with old technology. Most of firedetection/monitoring systems available are tailored towards a specific application andlack the use of recent advances in CMOS VLSI technology. In this study, we developa fire detection/monitoring system which is general in concept, readily implementablein a multitude of applications for early detection of a fire before it becomes critical,for equipment and evacuation of personnel. Here, we propose a central control anddistributed control/detection/monitoring with adequate communication, where use ismadeofsingle-chipmicrocontrollersinthelocalstations,thusimprovingcontrollability and observability of the monitoring process.2 Detection and alarm devicesA basic fire detection system consists of two parts, detection and annunciation.An automatic detection device, such as a heat, smoke or flame detector, ultraviolet orinfrared detectors or flame flicker, is based on detectingthe byproduct of a combustion. Smoke detectors, of both ionization and opticaltypes, are the most commonly useddetector devices. When a typical detector of this type enters the alarm state itscurrent consumption increasesfrom the pA to the mA range (say, from a mere 15pA in the dormant mode to 60mA) in the active mode. Inmany detectors the detector output voltage is well definedunder various operating conditions, such as thosegiven in Table 1. Themore sensitive the detector, themore susceptible it is to falsealarms. In order to control thedetector precisely, either of the following methods is used: a coincidence techniquecan be built into the detector, or a filtering technique such that a logic circuit becomesactive only if x alarms are detected within a time period T. The detection techniquedepends greatly on the location and plant being protected; smoke detectors are usedfor sleeping areas, infrared or ultraviolet radiation are used when flammable liquidsare being handled, heat detectors are used for fire suppression or extinguishingsystems. In general, life and property protection have different approaches.Alarm devices, apart from the usual audible or visible alarms, may incorporatesolid state sound reproduction and emergency voice communication or printers thatrecord time, date, location and other information required by the standard code ofpractice for fire protection for complex plants. Heaviside 4 has an excellentreview of all types of detectors and extinguisher systems.2.1 Control philosophy and division of labourOur control philosophy is implemented hierarchically. Three levels of systemhierarchy are implemented, with two levels of decision making. There is nocommunication between equipment on the same level. Interaction between levelsoccurs by upwards transfer of information regarding the status of the subsystems anddownwards transfer of commands. This is shown in Fig. 1 where at level 1 is thecentral station microcomputer and is the ultimate decision maker (when not in manualmode). At level 2 are the local controllers, which reside in the local stations. At level3 are the actual detectors and actuators. A manual mode of operation is provided at alllevels.Information regarding the status of all detectors is transmitted on a per area basisto the local controllers. Their information is condensed and transmitted upward to thecentral microcomputer. Transfer of status is always unidirectional and upwards.Transfer of commands is always unidirectional and downwards, with expansion at thelocal control level. This approach preserves the strict rules of the hierarchy for exactmonitoring detection and alarm systems associated with high risk plants.The classification of the two layers of controls is based upon layers of decisionmaking, with respect to the facts that(a) When the decision time comes, the making and implementation of a decisioncannot be postponed(b) The decisions have uncertainty(c) It will isolate local decisions (e.g. locally we might have an alarm althoughthere may be a fault with the system)3 General hardwareI :Fig. 2 depicts our design in the simplest of forms. The system uses an openparty line approach with four conductor cables going in a loop shared by all theremote devices and the control panel. This approach is simple in concept and iseconomically feasible. However, one major disadvantage is the dependency on asingle cable for power and signaling. In cases where reliability is of extremeimportance, two or even three cables taking differentroutes throughout the system may be connected in parallel. Fig. 3 gives thedriver circuitry required to derive an expandable bus. This design takes advantage ofrecent advances in the single chip microcomputer technology to reduce the interfacebetween the central station and the local stations.3. 1 Central control taskA central unit provides a centralized point to monitor and control the systemactivities. In the system to be described the central control unit serves a fivefoldpurpose.(i) It receives information from the local stations and operates the alarms andother output devices.(ii) It notifies the operator in case of system malfunction.(iii) It provides an overall system control manual and automatic.(iu) It provides a system test point of local stations and itself.(u) It provides a central point for observation, learning and adaptation.3.2 Local stationsThe local stations can take local decisions regarding recognition of a risksituation, and act independently on local affairs. In this technique we depend onload-type coordination, e.g. the lower level units recognize the existence of otherdecision units on the same level; the central or the top level provides the lower unitswith a model of the relationship between its action and the response of the system.It is evident that a powerful machine is required at this stage so that all therequired functions can be implemented. The availability of the new generation ofmicrochips makes this architecture a feasible solution.A single chip microcomputer was chosen over discrete digital and analoguedevices to interface to the field devices and to the central microcomputer. This is themain reason that previously this approach was not feasible.In selecting the microcomputer for the local stations, the criterion was therequirement for a chip which contains the most integration of the analogue and digitalports required for the interface and the utilization of CMOS technology owing toremoteness of the local stations. The choice was the Motorola 68HC11A4, for thefollowing reasons:(a) It is CMOS technology; this reduces power consumption.(b) It has a UART on board; this facilitates serial communication.(e) It has ana/d converter on board; this eliminates an external A/D.(d) It has 4K of ROM, 256 bytes of RAM, 512 bytes of EERROM with 40 1/0lines and a 16 bit timer; this satisfied all our memory and 1/0 requirements at the localstation side.4 System implementationThe local station: Fig. 3 is the block diagram of the circuit used to utilize theMC68HCllA4 as a remote fire detecting circuit while Fig. 4 illustrates the samecircuit in an expanded form. It can be seen that the single microcontroller can be usedto monitor more than one detector, thus reducing system cost.The loop power supply, which is usually between 28 and 26 V, is furtherregulated by a 5 V 100 mA monolithic low power voltage regulator to supply powerto the microcontroller. The onboard oscillator, coupled with an external crystal of2.4576 MHz, supplies the microcontroller with its timing signal which is dividedinternally by four to yield a processor frequency of 614.4 kHz, which is an evenmultiple of the RS 232 7 baud rate generator. In this Section the term supervisedinput or output will be used to mean that the function in question is monitored foropen- and short-circuit conditions in addition to its other normal functions. Moreinformation can be found in Reference 9.5Main loop6 ConclusionThis paper describes the development of a large scale fire detection and alarmsystem using multi-single chip microcomputers. The architecture used is a two-levelhierarchy of decision making. This architecture is made possible by the new CMOSmicrocontrollers which represent a high packing density at a low power consumptionyet are powerful in data processing and thus in decision making. Each local stationcould make an autonomous decision if the higher level of hierarchy allows it to do so.It has been tried to keep the system design in general format so it can be adapted tovarying situations. A prototype of the described system has been built and tested 10.The control part of the central station is implemented with a development card basedon MC 68000 microprocessor (MEX 68KECB, by Motorola), which has a built-inmonitor called Tutor. The application programs were developed using the featuresprovided by this monitor. The local stations controllers were designed using the MC68705R3, single-chip microcontroller.7 References
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