GB2281995A

GB2281995A - Alarm systems

Info

Publication number: GB2281995A
Application number: GB9319478A
Authority: GB
Inventors: Witold Adam Szpunar; Roy Holmes Bance
Original assignee: PROTECTOR ALARMS Ltd
Current assignee: PROTECTOR ALARMS Ltd
Priority date: 1993-09-21
Filing date: 1993-09-21
Publication date: 1995-03-22
Anticipated expiration: 2013-09-21
Also published as: GB9319478D0; GB2281995B

Abstract

An alarm system has a control device 1 coupled in parallel with a detection device 5 and a signalling devise 6. The system is operable in a first normal operating mode in which the state of the detection device is continuously monitored and, in response to an activation signal, in a second, alarm mode in which the control device cyclically switches between a state in which the signalling device is activated and a state in which the detection device is monitored. In the normal operating mode, a voltage of a predetermined polarity is applied across lines 3, 4. In the alarm mode, the voltage alternates so that the signalling device is operated whilst allowing the detection device to continue to be monitored. <IMAGE>

Description

ALARM SYSTEM This invention relates to alarm systems and in particular, though not necessarily, to fire alarm systems.

There is described in United Kingdom Patent No.

1491222 a signalling apparatus comprising detection devices and signalling devices connected in parallel with two output terminals of a central control unit by way of a two wire cabling system. The signalling devices may each be coupled to the output terminals by way of a diode arranged with a first polarity whilst the detection devices may each be coupled to the terminals by way of a diode arranged with the opposite polarity. In the normal operating condition a voltage is applied across the terminals by the control unit with a polarity such that current can flow through the detection devices but not through the signalling devices. In the event that one of the detection devices is activated, causing a short circuit to occur therethrough, current is drawn from the control unit.

This current flow is detected by monitoring means of the control unit, triggering a reversal in the polarity of the terminal voltage. Thus, in this signalling mode current can flow through the signalling devices, setting off alarms, but cannot flow through the detection devices. Monitoring of the detection devices can only be recommenced by manually resetting the terminal voltage polarity.

According to a first aspect of the invention there is provided an alarm system comprising a control device coupled in parallel with a detection device and a signalling device, the control device having a detection state in which it can monitor the detection device and a signalling state in which it can actuate the signalling device, there being oscillator means arranged cyclically to switch between the states.

In cases where it is desired to use existing detection and signalling devices, it is desirable to minimise the duration of the detection state in order that a conventional, unidirectionally operating, signalling device can be maintained in operation without a significant level of audibly detectable low frequency component. Thus, and depending upon the overall capacitance of the cabling system and attached devices, this duration is preferably greater than 1 ms and less than 10 ms, e.g. less than 4 ms and perhaps about 2 ms. The signalling state can be substantially longer than the detection state e.g. of the order of 30 ms.

According to a second aspect of the invention there is provided a control device for an alarm system comprising means for applying a unidirectional voltage of given polarity across an output of the device, means for sensing a change in current flow at the output when the voltage across the output has the given polarity and means responsive to the sensing of said change to change the mode of operation of the device to one in which the voltage across the output is alternating.

It will be apparent that embodiments of the present invention may provide an alarm system which can continue monitoring detection devices at the same time as actuating signalling devices.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, wherein: Fig. 1 shows a circuit diagram of a system according to a first embodiment of the invention; Fig. 2 shows a single pole switching configuration for use with the system of Fig. 1; Fig. 3 shows an implementation of the switching configuration of Fig. 2 using field effect transistors; and Fig. 4 is a timing diagram for the operation of the implementation of Fig. 3.

Fig. 1 shows a simplified circuit diagram of a single zone fire alarm system according to a first embodiment of the present invention. The system comprises a central control unit 1 which acts as the monitoring device and contains a pair of DC voltage supplies B and C which can be alternately connected across two output terminals T1 and T2 of the control unit by way of a pair of two-way switches S1 and When the switches S1 and S2 are connected to their terminal b positions, the voltage supply B is connected across the output terminals and the voltage supply C is disconnected from the output terminals. Similarly, when the switches S1 and S2 are connected to the terminal c positions, the voltage supply C is connected across the output terminals and the voltage supply B is disconnected therefrom.The voltage supplies B and C are arranged so that they apply voltages of opposite polarity to the output terminals.

A sense resistor R5 is connected in series with the voltage supply B so that a current drawn from the voltage supply B develops a voltage across the resistor R5. Connected in parallel with the resistor R5 is a voltage measuring device A which is in turn connected to a microprocessor 2. The microprocessor is arranged to operate the switches S1 and S2 by means of connections not shown in Figure 1. Coupled to the output terminals of the control unit are a pair of conductors 3 and 4 which are terminated by a load resistance RL. Connected between the conductors and in parallel with the voltage supply terminals are a detection device 5 and a signalling device 6.The detection device may be for example a smoke, flame or heat detector whilst the signalling device may be a bell, sounder or lamp. The detection device and the signalling device are connected directly to the conductor 3 and indirectly to the conductor 4 by way of respective diodes Dd and D5. The diodes are arranged with opposite polarities such that the diode Dd is forward biased and diode D5 is reverse biased when the voltage supply B is connected to the conductors; diode D5 is forward biased and diode Dd is reverse biased when voltage supply C is connected to the terminals.

During the normal operating mode of the system, switches S1 and S2 are configured such that voltage supply B is connected across the output terminals T1 and T2. In this mode current may flow through the detection device 5 but not through the signalling device 6. However, unless activated the detection device effectively acts as an open circuit. In this state only that current which flows through the load resistance RL will be drawn from the supply B. Whilst this current is arranged to be relatively small it is noted that its presence can be used to verify the integrity of the conductors 3 and 4. However, if the detection device 5 is activated, thereby substantially shortcircuiting the conductors 3 and 4, a significant current will be drawn from the voltage supply B so that the voltage measuring device A will detect an increased level of voltage across the sense resistor Rs. The resulting change of voltage is passed to an analogueto-digital converter of the microprocessor 2 which, upon detecting that this increased voltage has exceeded a preset level, causes the system to enter an alarm mode. Whilst in the alarm mode the system alternates between a signalling phase and a detection phase. The first phase entered is the signalling phase in which both the switches S1 and S2 are moved to the terminal c positions, connecting the voltage supply C to the conductors.In this state, current will cease to flow through the detection device 5 and will instead flow through the signalling device 6 causing the signalling device to activate. After a short time, e.g. one second, the microprocessor switches to the detection phase, connecting the switches back to the terminal b positions and thus reconnecting the detection device and effectively disconnecting the signalling device.

The system will remain in this phase only long enough to determine the state of the detection device, e.g.

for a few milliseconds.

The microprocessor continues to switch alternately between the signalling and monitoring phases until the control unit is manually reset, whereupon it switches back to the normal operating mode.

Whilst operating in the alarm mode, the microprocessor can continue to monitor the state of the detection device whilst operating the signalling device. The interruption of the operation of the signalling device can be made so brief as to be imperceptible to anyone listening to and/or watching the signalling device. With some makes of existing sounder, it has been found that they require an electrolytic capacitor to be placed across them to reduce the effect of the low frequency component (e.g.

of about 30 Hz) introduced.

Whilst Figure 1 shows a system having only a single detection device and a single signalling device it will be apparent that any number of such devices can be connected between the conductors 3 and 4.

A typical alarm system will comprise a number of single zone alarm systems, of the type shown in Figure 1, connected to a common microprocessor of type 68HCll having several analogue-to-digital converters. In order to reduce the complexity of the overall system the voltage supply means B and C may also be common to each of the zone systems. Each zone system may comprise one or more detection device and one or more signalling device. The detection and signalling devices of the single zone systems will each be located in separate regions of a building to be protected. The system may be arranged such that when the activation of a detection device is detected by the microprocessor the alarm mode is entered for all of the single zone systems in order to alert the entire building to the danger of fire.It will be apparent, however, that the microprocessor is able to continue monitoring the state of all the detection devices in order to determine the extent of the fire.

Considering the timing involved in the alarm mode, whilst the duration of the detection phase is extremely short in comparison to that of the signalling phase, it must be sufficiently long to allow any current drawn by the detection devices to stabilize after the voltage supply B has been connected. The minimum duration of the detection phase is largely determined by the capacitance of each zone. This capacitance is dominated by the capacitance of the conductors 3 and 4 and for a single zone fire resistant cable of length lkm the settling time will be of the order of 1 millisecond. Therefore a detection phase duration of 2 milliseconds would normally be appropriate. The signalling phase might then be of the order of 30 ms, giving a cycle time of about 32 ms.

For a multi zone system, the detection and signalling phases can be synchronized. After the initial 1 millisecond settling time of the detection phase, the microprocessor can poll each of the separate zone systems in turn. With a plurality of zones the microprocessor might be programmed to interrogate only eight zones. A further eight zones will be interrogated in the subsequent detection phase, and so on cyclically around the zones.

Figure 2 shows a possible implementation of the system of Figure 1 in which switch S1 is replaced by two single pole switches S3 and S4 and switch S2 is replaced by two single pole switches Sg and Switches S3 and S4 are operated by a common control line as are switches Sg and S6. The switches are operated on a "brake-before-make" principle in order to prevent the two voltage supplies B and C from being simultaneously connected to the conductors 3 and 4. A suitable interval between the breaking and the making of the connections, the "dead" time, is of the order of 100 ps.

The following table lists the technical details appertaining to the switching elements of Fig. 2.

These details are derived from the specification of a typical fire alarm system which allows for a maximum short circuit current of 80 mA in the normal operating mode and a maximum current of 1A in the alarm mode.

Table 1. Technical details of the switches shown in Figure 2.

S3:- open Vac max = - 30V Ia Ia max = +/-lOuA closed Ia max = lOOmA Ron max = 5 ohm (Vac max = -0.5V) S4:- open Vbc max = 30V Ib max = +/-10uA closed Ib max = -1A Ib peak max = -5A Ron max = 0.3 ohm (Vcb max = 0.3V) S5:- open Vdf max = 30V Ic max = +/-lOuA closed Ic max - -lOOmA Ron max = 5 ohm (Vfd max = -0.5V) S6::- open Vef max = -30v Id max = +/-10uA closed Id max = 1A Id peak max = Ron max = 0.3 ohm (Vef max = 0.3V) Whilst the single pole switches of the embodiment of Figure 2 may be provided by electromechanical relays, in order to allow the detection phase duration to be reduced to the desired 2 milliseconds, the preferred embodiment of the present invention makes use of solid state, semiconductor devices e.g. bipolar or field effect transistors, which are fast acting, reliable and free of contact bounce. Complimentary pairs of devices may be used. For example, in the case of bi-polar transistors, S3 and S6 could be PNP devices and S4 and S5 could be NPN devices. Alternatively, if enhancement type FETs are used, S3 and S6 could be P channel devices and S4 and Sg could be N channel devices.

Figure 3 shows an alarm system which uses the latter arrangement of enhancement type FETs, where +R and its associated circuitry provides the voltage supply B and +V provides the voltage supply C.

Transistors TR1 to TR4 perform the functions of switches S3 to S6 respectively. Also shown in Figure 3 is a timing circuit 7 which provides the appropriately timed switching signals to the gate electrodes of transistors TR1 to TR4.

When the alarm mode is entered, the microprocessor transmits a sequence of positive rectangular voltage pulses of amplitude V to the input H of the timing circuit 7. On the rising edge of the voltage pulse the diode D3 is forward biased and the capacitor CB is charged almost instantaneously to the voltage V. The pulse is therefore conducted to the comparators 8 and 9 without any significant delay. This causes the output of comparator 8 to go high and the output of comparator 9 to go low. As a result the complimentary pair of transistors TR3 and TR4 are turned off disconnecting the conductors 3 and 4 from the voltage supply C and hence disconnecting the signalling device.

The rising edge of the voltage pulse also causes diode D4 to be reverse biased and the capacitor Cc is charged through the resistor R3. Hence, the voltage applied to comparators 10 and 11 increases exponentially towards V. The outputs of the comparators 10 and 11 only switch from their normal operating points when the voltage has risen above some threshold level determined by the resistors R1 and R2.

When the outputs are switched, however, transistors TR1 and TR2 are turned on connecting the conductors 3 and 4 to the voltage supply B. By varying the values of R3 and Cc the time between the switching off of TR3 and TR4 and the switching on of TR3 and TR4, i.e. the dead time, can be varied.

In a similar manner, on the falling edge of the pulse diode D4 is forward biased and diode D3 is reverse biased. Hence the outputs of the comparators 10 and 11 switch almost instantaneously whilst the outputs of comparators 8 and 9 are switched only after a short delay causing the voltage supply B to be disconnected from the conductors 3 and 4 before the voltage supply C is re-connected. In this case the dead time can be changed by varying the values of R4 and Cb.

Figure 4 shows the voltages at various points in the timing circuit of Figure 3 which result from the application of the rectangular voltage pulse to the input H of the timing circuit.

Whilst the voltage measuring device A has not been described in detail it will be appreciated that it could be provided by any one of a number of conventional means. For example, a comparator could be connected across the sense resistor R5, the comparator producing an output signal when the voltage across Rs exceeds a given threshold voltage. The output signal could be passed to a storage cell such as a flip-flop which is gated to store any signal produced during the detection phase. In a multizone system, the storage cells for the respective zone systems could then be polled in turn by the microprocessor. Alternatively, the comparator and storage cell could be replaced by a sample and hold circuit connected across the sensor resistor, the sample and hold circuit being triggered by the microprocessor.The voltage held by the sample and hold circuit or, in the case of a multizone system, by each of the sample and hold circuits may then be interrogated by a low frequency A/D converter coupled to the microprocessor. As a further alternative a high speed AID converter may be used. One such A/D converter may be coupled directly across each of the sense resistors in a multizone system or a single A/D converter may be coupled to the resistors by way of a multiplexing system controlled by the microprocessor.

Finally, it is noted that not all types and makes of smoke, flame or heat detectors in their standard form will operate satisfactorily with the abovedescribed system.

Most detector modules comprise two parts, namely a common detector base which is fixed to the ceiling and wired to the circuit and a detector head. Into the base can be secured the appropriate type of head by a push and turn action which also makes the necessary electrical connections. In some cases the base is totally passive and in others signalling electronics are housed in the base. In one case where the signalling electronics are housed in the base, although the head is bi-polar in operation, the head is disabled by the electronics in the base during the signalling phase. It was found that with a 2 mSec long monitoring phase there was insufficient time for the detector head to respond to smoke/heat and operate the signalling electronics in the base. The solution in that case is to modify the electronics in the base so as to allow the head to operate in the signalling phase.

Because of the varied partitioning of the electronics and its functionality between head and base by different manufacturers, some devices may need to be modified in different ways.

Whilst the above description has been confined to the use of the present invention in a fire alarm system, it will be apparent that the invention is equally applicable to a variety of other applications including intruder alarms and other building and security management systems.

Claims

1. An alarm system comprising a control device coupled in parallel with a detection device and a signalling device, the control device having a detection state in which it can monitor the detection device and a signalling state in which it can actuate the signalling device, there being oscillator means arranged cyclically to switch between the states.

2. An alarm system according to claim 1, wherein the control device is operable in each of two modes, a first of the modes being a normal operating mode in which the detection state is continuously maintained and the second of the modes being an alarm mode in which the cyclic switching of states occurs.

3. An alarm system according to claim 2, wherein the control device is arranged to switch from the normal operating mode to the alarm mode when an activating signal is detected.

4. An alarm system according to claim 1, 2 or 3, wherein oscillator means is arranged to maintain the signalling state for a greater period of time than the detection state.

5. An alarm system according to any one of the preceding claims, wherein the oscillator means is arranged to maintain the detection state for a period of less than lOms.

6. An alarm system according to claim 5, wherein the cyclic operation is arranged to maintain the detection state for approximately 2ms.

7. An alarm system according to any one of the preceding claims, wherein the oscillator means is operable to apply first and second voltages alternately across the detection and signalling devices.

8. An alarm system according to claim 7 and comprising a first unidirectional device, arranged with a first polarity, coupling the detection device to the control device, and a second unidirectional device, arranged with a polarity opposite to that of the first unidirectional device, coupling the signalling device to the control device, the first voltage being arranged to forward bias the first unidirectional device and reverse bias the second unidirectional device and the second voltage being arranged to reverse bias the first unidirectional device and forward bias the second unidirectional device.

9. An alarm system according to claim 8, wherein the applying means comprises first and second voltage supply means for supplying the first and second voltages respectively and switching means for selecting one of the voltage supply means for application across the signalling and detection devices.

10. An alarm system according to claim 9, wherein the switching means comprises an arrangement of four single pole switches, a first pair of the switches being arranged to select the first voltage supply means and the second pair being arranged to select the second voltage supply means.

11. An alarm system according to claim 9 or 10, wherein the switching means are solid state, semiconductor switches.

12. An alarm system according to claim 10, wherein the switching means are transistors.

13. An alarm system according to claim 12, wherein the switching means are field effect transistors.

14. An alarm system according to claim 10 or to any one of claims 11 to 13 when appended to claim 10, wherein the control device comprises timing control means arranged to simultaneously operate the switches of the first pair and to simultaneously operate the switches of the second pair.

15. An alarm system according to claim 14, wherein the timing control means is arranged to separate in time the operation of the first and second pairs of switches with a dead band interval.

16. An alarm system according to claim 15 wherein the dead band interval is approximately 100 ps.

17. An alarm system according to claim 3 or to any one of claims 4 to 16 when appended to claim 3, wherein the control device comprises monitoring means for detecting the activation of the detection devices and for generating said activation signal in response to said detection.

18. An alarm system according to claim 17 when appended to any one of claims 9 to 16 wherein the monitoring means comprises a sense resistance connected in series with the first voltage supply means and means for measuring the voltage across the resistance.

19. An alarm system according to claim 18, wherein the measuring means generates said activation signal when the voltage across the resistance exceeds a threshold level.

20. An alarm system substantially as hereinbefore described with reference to Figure 1, Figures 1 and 2, or Figures 1 to 4 of the accompanying drawings.

21. A multizone alarm system comprising a plurality of alarm systems according to any one of the preceding claims.

22. A multizone alarm system according to claim 21, wherein the plurality of alarm systems are systems according to claim 3 or to any one of claims 4 to 19 when appended to claim 3, wherein said activation signal is generated when at least one of the detection devices is activated.

23. A control device for an alarm system comprising means for applying a unidirectional voltage of given polarity across an output of the device, means for sensing a change in current flow at the output when the voltage across the output has the given polarity and means responsive to the sensing of said change to change the mode of operation of the device to one in which the voltage across the output is alternating.

24. A control device for an alarm system, substantially as hereinbefore described with reference to Figure 1, Figures 1 and 2, or Figures 1 to 4 of the accompanying drawings.

Amendments to the claims have been filed as follows 1. An alarm system comprising a control device coupled in parallel with a detection device and a signalling device, the control device having a detection state in which it can monitor the detection device and a signalling state in which it can actuate the signalling device, there being oscillator means arranged cyclically to switch between the states.

23. A control device for use in an alarm system according to any one of the preceding claims, the control device comprising means for applying a unidirectional voltage of given polarity across an output of the device to define the detection state, means for sensing a change in current flow at the output when the voltage across the output has the given polarity in order to detect an alarm condition, and means responsive to the sensing of said change to change the mode of operation of the device to one in which the voltage across the output is alternating such that, when in use with a parallel connected signalling device and detection device, the control device is in its detection state when the alternating voltage has the given polarity and, when the alternating voltage has the opposite polarity, that voltage provides a polarity to activate the signalling device.

24. A control device according to claim 23 and substantially as hereinbefore described with reference to Figure 1, Figures 1 and 2, or Figures 1 to 4 of the accompanying drawings.

25. A control device for an alarm system comprising means for applying a unidirectional voltage of given polarity across an output of the device, means for sensing a change in current flow at the output when the voltage across the output has the given polarity and means responsive to the sensing of said change to change the mode of operation of the device to one in which the voltage across the output is alternating.