AU635039B2

AU635039B2 - Particle detectors

Info

Publication number: AU635039B2
Application number: AU58306/90A
Authority: AU
Inventors: John Charles Edwards
Original assignee: First Technology Fire & Safety Ltd
Current assignee: First Technology Fire & Safety Ltd
Priority date: 1989-06-15
Filing date: 1990-06-14
Publication date: 1993-03-11
Anticipated expiration: 2010-06-14
Also published as: GB9103206D0; GB8913773D0; WO1990016053A1; GB2242521B; AU5830690A; EP0434790A1; GB2242521A

Description

PARTICLE DETECTORS

SPECIFICATION

This invention relates to particle detectors, and is particularly concerned with smoke detectors. The invention is more particularly concerned with aspirated detectors in which air passes through the detector and any particles in the air are detected.

It is an object of the present invention to provide a particle detector which can be used for small rooms, cabinets, etc., as well as for large volume premises. It is a further object of the present invention to provide a particle detector in which one has a relatively fast air flow and in which there is still a high percentage chance of detecting particles within the air flow.

In accordance with the present invention there is provided a particle detector comprising an emitter arranged to direct radiation towards a detection zone through which air is arranged to flow, sensor means arranged to sense radiation reflected from particles in the air flow, and an electrical control circuit including said sensor means and arranged to generate an output signal which in the absence of particles in the air flow is a compensated signal but which in the presence of particles in the air flow is a signal proportional to the number of particles present. In a preferred embodiment of the invention the emitter generates a cone of radiation and a proportion of this emitted radiation is continuously reflected towards the sensor means to produce a base signal.

Preferably, the emitter source has an output in the infra-red portion of the electromagnetic spectrum. Preferably, it is a pulsed source. In a preferred embodiment the sensor means comprises a sensor having a large surface area, as opposed to the use of a lens. This provides a large detection surface and also ensures a good response rise time.

The particle detector of the present invention also preferably incorporates temperature compensation. This can be achieved in various alternative ways, examples of which will be described hereinafter. In order that the invention may be more fully understood, an embodiment of particle detector in accordance with the invention will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 shows details of the particle detector;

Fig. 2 is a circuit diagram illustrating one form of electronic control circuit for use with the detector; and,

Fig. 3 is a circuit diagram showing an alternative electronic control circuit for use with the detector.

Referring first to Fig. 1 , this shows a smokehead device 10 which is an aspirated unit, with air being directed to the unit by way of an inlet pipe 12 and taken from the unit by way of an outlet pipe 14. The air can be blown through the smokehead device 10 or drawn through it. The smokehead device 10, which is preferably an aluminium block with suitable holes and passages therein, is provided with a main flow passage 16 therethrough, connecting the inlet pipe 12 with the outlet pipe 14. This flow passage 16 comprises a first portion in alignment with the inlet pipe 12 and a second portion which is in alignment with the outlet pipe 14 and which is therefore at right-angles to the first portion. The detector incorporates a further passage, indicated generally at 18, which connects with the main flow passage 16 and the longitudinal axis of which lies at an angle of about 30° to the longitudinal axis of the first portion of the main air flow passage 16. At the outer end of the further passage 18 is fitted a radiation source, indicated generally at 20. The radiation source 20 is fitted into a bore in the block 10 and is secured in place by appropriate screws or bolts. The source 20 has a high output, preferabl with an emission wavelength of the order of 950 πm, i.e. in the infra-red portion of the electromagnetic spectrum. The source 20 is a pulsed emitter source. The pulsed radiation from the source 20 is emitted into the passage 18 as a cone of radiation which impinges on the air flowing through the passage 16. Radiation striking any particle, such as a smoke particle, within the air flowing through the passage 16 will therefore be reflected.

A sensor, indicated generally at 22, is fitted into a bore in the block 10 in alignment with the outlet pipe 14 but on the opposite side of the block. The sensor 22 is thus "looking" at the cone of radiation emitted by the source 20, and radiation reflected from particles in the air flow through the passage 16 will be directed back towards the sensor 22. As will be appreciated from the configuration of the unit as shown in Fig. 1 , a certain amount of radiation emitted by the source 20 will strike the internal walls of the passage 16, notably the wall surfaces 25a, 25b, and be reflected directly or indirectly to the sensor 22. In other words, even in the absence of any particles within the air flow, the sensor 22 will produce a standing base signal as an output. As will be described hereinafter, this standing signal is monitored and it is changes in the signal level which are measured and used as an indication of particles within the air flow. With the detector of the present invention one is measuring changes in the amount of radiation picked up by the sensor. The sensor 22, as shown, has a flat outer surface 24 with no lens. The use of such a large surface area detector means that one can keep the detection surface as large as possible and, as a consequence, achieve a good rise time in the response of the sensor.

Fig. 2 schematically shows one form of control circuit for use with the particle detector of the present invention. With this control circuit one provides automatic compensation for temperature effects occurring on the emitter, sensor or chamber and also for changes due to dust deposited on the chamber walls. As shown, the emitter source 20 is powered from a pulse generator 26. The pulse output from the sensor 22 is fed via an amplifier A1 to one input of a differential amplifier A2. The output of the differential amplifier

A2 is fed to a peak detector 28 whose output voltage is used as one input of a differential amplifier A3. The other input to differential amplifier A3 is connected to a potentiometer 30 which can be varied to set the

"smoke" output from the circuit on line 32 to a desired standing output current level I₀. The peak detector provides at the output an analogue display of the particle level, for example the smoke level.

The second input to differential amplifier A2 is connected to a track and hold circuit 32 which comprises an analogue switch 34, a voltage comparator VC, a voltage follower A4, a charge resistor R and a hold capacitor C. A power up reset circuit 36 is connected to the voltage comparator VC to ensure that the hold capacitor C charges to the quiescent condition when the circuit is switched on. In this circuit arrangement a separate device 38 is 5 provided to monitor air flow, with this device being linked to control means (not shown) which can adjust the air flow in order to ensure a constant flow of air through the detector unit 10. In order to achieve this, the flow of air is monitored by a solid state device 38, for example a thermistor, mounted within the passage 16. The output from the airflow monitor 38 is fed via amplifiers A5 and A6 to an airflow signal output 40. The compensation circuit functions as follows. With the analogue switch 34 closed the hold capacitor C charges to the voltage appearing at the output of the peak detector 28. This voltage, via the voltage follower A4, controls the second input of the differential amplifier A2. The circuit stabilises when the output of voltage follower A4 is almost equal to the peak of the pulse coming from amplifier A1. Any slow change of pulse amplitude due to changes of temperature of the emitter or sensor or of the chamber, or due to dust on the chamber wall, is followed on capacitor C, and thus automatic compensation is made to provide a constant base output current I₀.

The voltage comparator VC compares a voltage slightly less than the output of the peak detector 28 with the output of A4 and normally maintains the analogue switch 34 closed. When smoke enters the chamber the pulse out of amplifier A1 increases in amplitude, resulting in a rise in the output of the peak detector. This rise is faster than the time constant RC and the output of A4 remains substantially* constant while the comparator is forced to change state, resulting in the analogue switch 34 being switched to its "off" state. The output of A4 now holds the input to A2 constant, and thus the output current I₀ changes in proportion to the quantity of 6 particles passing through the passage 16. When the passage 16 is clear of smoke particles the output of the peak detector 28 falls back to its quiescent value, the comparator VC switches the switch 34 back to its 5 "on" state, and the circuit continues to track as before.

In the alternative embodiment of control circuit shown in Fig. 3, the particle detector has two particle sensors 22a and 22b. The emitter is again indicated at

10 20 and the radiation emitted therefrom is reflected from a reflector plate 42 towards one of the sensors 22a. The plate 42 is a fixed reflector plate and the radiation beam is arranged to pass through the particle-carrying air flow in its path from the emitter

15 20 via the plate 42 to the sensor 22a. The second sensor 22b receives the radiation emitted from the emitter 20 via an attenuator 44. This attenuator 44 is necessary in order to prevent saturation of the sensor 22b. With this attenuator the sensor 22b is then able

20 to operate on the linear part of its characteristic. The sensors 22a and 22b have the same thermal characteristics. In this embodiment a feed-back loop to the emitter 20 is provided for temperature compensation. The feed-back loop includes a peak

25 detector 46 connected to the output of sensor 22b and a current control circuit 48 connected between the peak detector 46 and the emitter 20.

Alternatively, the second sensor 22b may be used to balance out any unwanted change of output from sensor

30 22a, for example by means of a differential amplifier circuit, such that only the change of output from sensor 22a due to smoke particles is seen.

Although in the embodiments described above the particle detector unit uses a detector block having an

35 air flow passage therethrough, one could alternatively mount the emitter and sensor or sensors in free space on a printed circuit board, for example with a fixed reflector positioned opposite the emitter and with the emitter and sensor or sensors positioned close to a nozzle through which the air enters the detection zone.

Claims

CLAIMS:

1. A particle detector comprising an emitter arranged to direct radiation towards a detection zone through which air is arranged to flow, sensor means arranged to sense radiation reflected from particles in the air flow, and an electrical control circuit including said sensor means and arranged to generate an output signal which in the absence of particles in the air flow is a compensated signal but which in the presence of particles in the air flow is a signal proportional to the number of particles present.

2. A particle detector according to claim 1, in which the emitter generates a cone of radiation and a portion of this is continuously reflected towards the sensor means to produce a base signal.

3. A particle detector according to claim 2, in which the detection zone is a passage within a housing and radiation is arranged to be reflected to said sensor means from a wall or walls of the housing on the side of the passage remote from the emitter.

4. A particle detector according to claim 3, in which said passage comprises an inlet portion- and an outlet portion at right-angles to the inlet portion, and the emitter is positioned so that the longitudinal axis of the emitted beam is at an angle of the order of 30° to the longitudinal axis of the inlet portion of the passage.

5. A particle detector according to any preceding claim, in which the emitter has an output in the infra- red portion of the electromagnetic spectrum.

6. A particle detector according to any preceding claim, in which the emitter produces output pulses of radiation.

7. A particle detector according to any preceding claim, in which the compensated signal is compensated for relatively slow changes in the amplitude of the output from the sensor means due to temperature changes and dust on walls defining the detection zone.

8. A particle detector according to any preceding claim, in which said control circuit includes a track and hold circuit which includes switch means which are controlled so that in the absence of particles in the air flow the track and hold circuit tracks the output from the sensor means and automatically compensates for slow changes in the output from the sensor means , whereas with the presence of particles in the air flow the track and hold circuit permits the output signal to be representative of the particles in the detection zone.

9. A particle detector according to any preceding claim, in which an airflow monitor is provided within the air flow, associated with control means to enable a constant flow of air through the detection zone to be maintained.

10. A particle detector according to claim 1 or 2, in which the sensor means comprises two sensors, a first of which receives radiation from the emitter reflected by a reflector plate and the second of which receives radiation from the emitter by way of an attenuator, one of said sensors feeding the control circuit and the other sensor forming part of a temperature compensation circuit.