Fire alarm system

Abstract: A fire alarm system includes a receiver (1) adapted to detect the presence of a fire on the basis of analog detection data contained in a received signal which is connected by means of a transmission line (2) to a plurality of analog type sensors (3), each of which is arranged in a respective monitoring area monitored by the receiver and is adapted to produce analog detection data representative of the value of a parameter, or a change thereof, caused by a fire and to transmit the data to the receiver. Associated with each analog type sensor (3) is a plurality of on-off type sensors (7) which are arranged in the same monitoring area as the associated analog type sensor, are adapted to be switched to produce a fire signal when the change in the said parameter exceeds a predetermined threshold value and are connected to a common signal line (6) connected to the associated analog type sensor (3).

Description: The present invention relates to fire alarm systems and is concerned with that type of system which comprises a plurality of groups of sensors located, in use, in respective monitoring areas, each of which groups includes an analog type sensor having analog detection means for producing analog detection data corresponding to changes in a surrounding physical phenomenon caused by a fire and one or more on-off type sensors connected to the said analog type sensor and adapted to produce a fire signal when the changes in the surrounding physical phenomenon exceed a preset threshold value, a receiver adapted to detect a fire based on the said analog detection data contained in an output signal from the said analog type sensor, and a transmission line interconnecting the said receiver and the said analog type sensor in each of the said groups of sensors for transmitting the said output signal. A popular type of fire alarm system is that which includes a plurality of so-called on-off type sensors each having a switching element which is turned on when a fire is detected and is connected via a transmission line to a receiver. Such on-off type sensors sense changes in one or more physical phenomena or changes in a parameter caused by the fire, such as temperature or smoke concentration. When the magnitude of the change exceeds a predetermined threshold value, the switching element is actuated to transmit a fire alarm signal, that is to say a current signal, to the receiver via the transmission line. Thus with an on-off type sensor, it is the sensor itself that decides whether or not a fire has broken out. On the other hand, a fire alarm system of the generic type referred to above is known in which it is not the sensor itself but the receiver that has the function of determining whether or not a fire has broken out. In this case, an analog type sensor, which senses the magnitude of the change in one or more physical phenomena caused by the fire as an analog value, is used. The analog detection signal produced by the sensor is transmitted via the transmission line to the receiver where it is determined from the received analog detection data whether or not a fire has broken out. With such an analog fire alarm system it is the receiver which makes the judgement as to the possible presence of a fire, and it is thus possible to perform various complex signal processing routines, such as reaching a predictive judgement on the occurrence of a fire. Since the analog data itself is evaluated, no erroneous fire indication, as is often made by on-off type sensors, can be made, so that early fire detection can be achieved with the minimum risk of errors. With such analog fire alarm systems, it is necessary to distinguish the output analog data from a plurality of analog sensors connected to the same transmission network. For this reason, an address is set for each of the analog sensors and each sensor is adapted to sequentially transmit its own analog detection data to the receiver via the transmission line by time divisional multiplexed transmission according to a polling system. In this case, a reset pulse and a plurality of clock pulses are transmitted at a predetermined time interval from the receiver to the analog sensors. The clock pulses start to be counted at the sensor from the time when the reset pulse is received. When the count value reaches the clock count allocated to a specific sensor, it is determined that the sensor has been interrogated, and the analog detection data contained in the sensor at this time is sent to the receiver after conversion into a corresponding electrical current value. However, in such a conventional analog fire alarm system, since the sensors are sequentially interrogated by relying upon the addresses allocated thereto, the time period involved in interrogating the sensors, that is the polling period, is necessarily increased with an increase in the number of the sensors. The result is that there is a limitation on the number of the sensors that can be interrogated within a predetermined time period consistent with the allowable limit of the delay in fire detection and hence on the number of the sensors that can be provided in each network. Thus, in a monitored area of larger size, such as a large hall, it frequently occurs that the number of sensors to be installed exceeds the allowable limit per network, with the result that a number of analog sensor networks need be installed in the same monitored area with associated receivers or relays thereby complicating the construction and increasing the cost. On the other hand, provision of an address sensor in one monitored area results in increased costs and a complicated control operation at the receiver on the occurrence of a fire with a corresponding broad application to the control section. It is thus a principal object of the present invention to provide a fire alarm system in which the number of the installed analog type sensors can be minimised and yet a monitoring function achieved which is comparable with the case in which analog sensors are installed in all of the monitored areas and in which the system is simplified at reduced cost. Another known system comprises an analog sensor and a plurality of on-off type sensors all connected in parallel with each other on the same signal line, and arranged in the same monitoring area. The system is described in GB-A-2173622. According to the present invention a fire alarm system of the type referred to above is characterised in that the said analog type sensor comprises fire detection means for detecting the said fire signal from the said on-off type sensor connected thereto and transmission control means for transmitting to the said receiver the said output signal containing the said analog detection data from the said analog detection means and the data concerning the presence or absence of the said fire signal from the said fire detection means. The or each analog type sensor preferably includes a mounting base section adapted to be connected to a surface, e.g. a ceiling or wall, of a building and a detection head section detachably connected to the mounting base section, the fire detection means and the transmission control means being provided in the mounting base section and the analog detection means being provided in the detection head section. In the fire alarm system of the present invention the sensors are thus grouped in a number of sets each of which comprises an analog sensor and one or more, generally a plurality, of on-off type sensors connected to a common signal line which in turn is connected to the associated analog type sensor. Thus the number of analog type sensors which require an address is very considerably reduced so that only one analog sensor circuit and associated address is sufficient for all the sensors in any one monitoring area. If a plurality of on-off type sensors are installed around the associated analog type sensor, the condition of a fire in the vicinity of the surrounding on-off type sensors can be determined to a certain extent by the single analog type sensor for the monitoring area covered by that group of sensors. The site of the fire can be determined from the address allocated to the analog type sensor in question. Thus with the use of the present invention the number of address settings on the basis of which the sensors are interrogated by the receiver may be considerably reduced even over a relatively large monitoring area, such as a large hall, in which it is necessary to instal a relatively large number of sensors since each address setting applies not only to a single analog type sensor but also to all the associated on-off type sensors. Since each group of sensors is identified by a single address the site of a fire can be located promptly so that preventative measures may be taken at the earliest possible time. Since one sensor of each group of sensors is necessarily of analog type, the detailed condition of the monitoring area covered by each group of sensors, such as smoke density or temperature, can be determined so that a more satisfactory fire monitoring operation is achieved as compared with a system which employs only on-off type sensors. In addition, the total number of analog type sensors which is necessary to give satisfactory coverage of a large area can be significantly reduced as compared to a system employing only analog type sensors so that the construction of the system including the receiver can be simplified and the capital cost reduced. Further features and details of the present invention will be apparent from the following description of one preferred embodiment of the invention which is given by way of example with reference to the accompanying drawings, in which:- Figure 1 is a block diagram showing an embodiment of the present invention; Figure 2 is a schematic view showing one mounting configuration of the sensors; Figure 3 is a timing chart showing the state of interrogation of and response by the sensors in the embodiment of Figure 1; and Figure 4 is a flow chart showing the operational sequence of the sensors in the embodiment shown in Figure 1. Figure 1 is a block diagram showing an embodiment of the present invention. Referring to Figure 1, an analog type sensor 3 is connected to a transmission line 2 which in turn is connected to a receiver 1 and which is adapted to supply power to the system and to transmit signals. The analog sensor 3 comprises a base section 4 and a head section 5. The head section 5 is provided with a detection circuit 9 for analog detection of the value of, or changes in, one or more physical phenomena, such as smoke concentration, caused by a fire. The analog data detected by the circuit 9 is supplied via an analog output circuit 10 to the base section 4. The base section 4 is provided with transmission control circuit 11 for converting the analog detection data from the analog output circuit 10 into electrical current signals and transmitting them via the transmission line 2 to the receiver 1. The transmission control circuit 11 is responsive to polling control signals from the receiver 1 to transmit analog detection data from the output circuit 10 in the current mode when interrogated. More specifically, the receiver 1 repeatedly performs an interrogating operation by sending out a plurality of clock pulses on the transmission line 2 after sending out a reset pulse. The control circuit 11 clears a counter, not shown, with the reset pulse transmitted from the receiver 1 and counts the clock pulses following the reset pulse. When the count of the clock pulses coincides with a present address set in the analog type sensor 3, i.e. a predetermined count, the circuit 11 decides that it is being interrogated and converts the analog detection data from the head section 5 into a corresponding electrical current which is sent to the receiver 1. In order to make this decision, an address setting unit 14 is provided in the control circuit 11. The base section of the analog sensor 3 is provided with a fire detection circuit 12 from which a signal line 6 serving simultaneously as a power supply line, leads to the exterior. A plurality of on-off type sensors 7 are connected to the signal line 6, to the end of which a terminal resistor 8 for disconnection detection is connected. Each on-off sensor 7 is switched when the change in a physical parameter caused by a fire exceeds a predetermined threshold to emit a fire signal in a known manner. Thus the sensors 7 have the function of determining the occurrence of a fire. More specifically, the on-off type sensors 7 emit an alarm current via the signal line 6 by a switching operation when a fire is detected. This alarm current is detected by the fire detection circuit 12 provided in the base section 4 of the analog type sensor 3 and transmitted to the control circuit 11. A disconnection detection circuit 13 is provided in the base section 4 connected to the signal line 6. The disconnection detection circuit 13 senses if the disconnection monitoring current flowing in the terminal resistor 8 is interrupted due to disconnection of the signal line 6 and then emits a disconnection detection signal to the transmission control circuit 11. Thus the control circuit 11 not only has the function of transmitting the analog detection data from the output circuit 10 of the head section 5 to the receiver 1 but also has the function of transmitting a fire detection signal from the circuit 12 or a disconnection detection signal to the receiver 1 after conversion by the circuit 13 into electrical currents, similarly to the analog detection signal. Figure 2 is an explanatory view showing a mounting example of the sensors in a monitoring area covered by the fire alarm system of the present invention. In this embodiment, as shown in Figure 2, the receiver 1 covers three monitoring areas 14,15,16 in each of which 7 to 9 sensors, for example, are required, dependent on the surface area of the areas 14 to 16. For each of these monitoring areas 14 to 16, there are provided in the fire alarm system of the present invention an analog sensor 3 at, for example, the centre of each of the areas 14 to 16, and a plurality of on-off type sensors 7 around the analog type sensor 3. The on-off type sensors 7 of each of the areas 14 to 16 are connected to the signal line 6 connected to the fire detection circuit 12 of the associated analog sensor 3. With this sensor disposition, it is necessary to provide three addresses for the receiver 1 to make interrogations. If the system were of the same type as the known system with all of the sensors being of analog type, an address would be required for each sensor and hence 23 addresses would be necessary for the three monitoring areas 14 to 16. However, in the construction of the present invention the number of addresses which is required is reduced drastically to three, in the embodiment of Figure 2. Due to the fact that the analog sensor 3 which is capable of collecting detailed fire data, is provided at the centre of each of the monitoring areas 14 to 16 and the on-off sensors 7 are placed around it, detailed fire data such as smoke or temperature caused by a fire in the detection regions of the on-off sensors 7 may be roughly measured by the centrally disposed analog sensor 3. For example, if a fire is detected by one of the on-off sensors 7 in the monitoring area 14, analog detection data such as smoke concentration is obtained by the centrally disposed analog sensor 3 and displayed in the receiver to allow the situation after the start of the fire in the monitoring area 14 to be monitored by the receiver. The operation of the embodiment shown in Figure 1 will be explained with reference to the timing chart of Figure 3 and a flow chart for the analog sensor of Figure 4. In the steady state the receiver 1 repeatedly sends out via the transmission line 2 a reset pulse followed by clock pulses A1, A2, A3, A4,... for determining the sensor interrogating addresses. As shown in Figure 4, the transmission control circuit 11 of the analog sensor 3 is responsive to the clock pulses from the receiver 1 to count the clock pulses from the receiver at step S1 to decide if the count coincides with its own preset address. If the count is decided at step S1 to be the preset address, the program proceeds to step S2 where it is first checked whether an alarm is issued from the on-off sensors 7, that is, whether a fire detection output is produced from any of the on-off sensors 7 connected to the fire detection circuit 12 by the signal line 6. The program then proceeds to step S3 where the analog output value of the detection circuit 9 from the analog output circuit 10 is checked. The program then proceeds to step S4 where it is checked whether a disconnection detection output is produced by the detection circuit 13. The program then proceeds ultimately to step S5 where the results from steps S2, S3 and S4 are converted into electrical currents for transmission to the receiver. Data transmission from the analog type sensor 3 to the receiver 1 with the current mode at step S5 is performed as indicated by the terminal response current shown in Figure 3b. With regard to the response current from the analog sensor, if the clock pulse A1 shown in Figure 3a is decided to coincide with the preset address, nine states 0 to 8 are set in the present embodiment in the time interval between reception of the clock pulses A1 and A2. The fire detection data issued by the on-off sensors is sent out at the time of state 3, the analog detection data is sent out at the time of state 4 and the check data such as disconnection data are sent at the time of the state 5, in each case after conversion into corresponding electrical currents. Thus the terminal response current following the reception of the clock pulse A1 indicates that the on-off sensors are turned off at state 3, thus indicating no alarm has been produced, that the value of the analog detection data is as shown at state 4 and that there is no functional check output, such as a disconnection output, at state 5 so that the response current is zero. Looking at states 3 to 5 of the response current from another analog sensor which has decided that the count of the next clock pulse A2 coincides with its own preset address, the on-off sensors are turned on at state 3, i.e. an electrical current indicating an alarm is produced, the value of the analog detection data is as indicated at step 4 and a current is produced at step 5 indicating that check data reveals that a fault, such as disconnection, is present. Although one embodiment of the present invention has been described and illustrated in detail, it is to be understood that the same is by way of illustration and example only and that many modifications may be effected.

Claims: A fire alarm system for detecting a fire condition in a plurality of monitoring areas, comprising a plurality of groups of sensors located, in use, in respective monitoring areas, each of which groups includes an analog type sensor (3) having analog detection means (9) for producing analog detection data corresponding to changes in a surrounding physical phenomenon caused by a fire and one or more on-off type sensors (7) connected to the said analog type sensor (3) and adapted to produce a fire signal when the changes in the surrounding physical phenomenon exceed a preset threshold value, a receiver (1) adapted to detect a fire based on the said analog detection data contained in an output signal from the said analog type sensor (3), and a transmission line (2) interconnecting the said receiver (1) and the said analog type sensor (3) in each of the said groups of sensors for transmitting the said output signal, characterised in that the said analog type sensor (3) comprises fire detection means (12) for detecting the said fire signal from the said on-off type sensor (7) connected thereto and transmission control means (11) for transmitting to the said receiver (1) the said output signal containing the said analog detection data from the said analog detection means (9) and the data concerning the presence or absence of the said fire signal from the said fire detection means (12). A system as claimed in claim 1 characterised in that the analog type sensor (3) includes a mounting base section (4) adapted to be connected to a ceiling or wall of a building and a detection head section (5) detachably connected to the mounting base section (4), the fire detection means (12) and the transmission control means (11) being provided in the mounting base section (4) and the analog detection means (9) being provided in the detection head section (5). A fire alarm system as claimed in claim 1 or claim 2 characterised in that the said analog type sensor (3) further includes disconnection detection means (13) for detecting disconnection of a signal line (6) interconnecting the said analog type sensor (3) and the said on-off type sensors (7), the said transmission control means (11) being adapted to transmit the said output signal containing data relating to the presence or absence of a disconnection detection signal from the said disconnection detection means (13) over the said transmission line (2) to the said receiver (1).

Inventors: YUCHI SADATAKA, YUCHI, SADATAKA
Applicants: HOCHIKI CO, HOCHIKI CORPORATION
Application year: 1989
Main classification: G08B17
IPC codes: G08B17, G08B26, G08B25