GB2078414A - Improvements in particle detecting systems - Google Patents

Abstract

A particle (smoke) detecting system uses both a photoelectric detector 1 and an ionization detector 2. The respective detectors 1, 2 sense particles of different ranges of sizes and each detector 1, 2 produces respective electrical analog signal I1, I2 proportional to the density of the particles sensed thereby. The two electrical analog signals I1, I2 are combined in a summation circuit N which arithmetically adds the two signals I1, I2, and a transresistance amplifier 4, R1 converts the summation to an analog voltage V. The voltage V representing the sum of the two signals I1, I2 is applied to a threshold device 5 which responds when the sum exceeds a predetermined value to energize an alarm H. <IMAGE>

Description

SPECIFICATION Improvements in particle detectors The present invention relates in general to particle, e.g. smoke, detectors and systems.

There are currently two major types of widely used smoke detectors: the photoelec tric type and the ionization types. Both types of detectors are so well known as to need no detailed description except as to their respective sensitivities. A photoelectric smoke detector is quite sensitive to the combustion products of early fires such as smoldering mattresses, plastics or fabrics, whose large, grey or black smoke particles roughly range from 0.1 to 1 microns in size. An ionization smoke detector is relatively, if not dangerously, insensitive to such products, but is relatively more sensitive to combustion products ranging below 0. 1 micron in size such as are present in less frequently encountered fires close to combustion or involving clean wood.

Ionization detectors are also prone to false alarm in households when they encounter cooking and aerosol vapours usually ignored by a photoelectric detecrtor. It is nevertheless desirable to extend the range of particle size sensitivity of the photo-electric detector to include the range of the ionization detector if its false alarm sensitivity could be reduced.

Speculatively, a particle detecting system using both photoelectric and ionization detectors might combine them in a logic OR, or a logic AND system. In the logic OR system, an alarm signal would be generated if either detector sensed particles above a predetermined threshold of density. In the logic AND system both detectors must exceed the threshold. In both OR and AND logic systems the susceptibility to false alarms of either detector remains undiminished.

It is therefore the object of the present invention to provide an improved particle detector system.

Acording to the present invention there is provided a particle detecting system comprising; a first detector for sensing particles in a first range of sizes and serving to produce a first analog signal corresponding to the density of particles sensed, a second detector for sensing particles in a second range of sizes and serving to produce analog signal corresponding to the density of particles sensed, summation or combining means which includes means for arithmetically adding the first and second analog signals, and threshold means coupled to the summation or combining means and serving to produce an alarmindicative signal when the arithmetic sum of the first and second signals exceeds a predetermined value.

Preferably the first detector is a photoelectric detector and the second detector is an ionization detector.

A detector system made in accordance with the invention can be thought of as a dual system (e.g. photoelectric and ionization) and is able to detect particles in an extend range of sizes with the summed sensitivity of both detectors with reduced susceptibility to false alarm conditions.

Usually, the first and second analog signals are analog currents and this may necessitate the use of conversion amplifiers. The combining or summation means may at least include a pair of analog current sigals. Preferably the combining or summation means also comprises a transresistance amplifier which produces an output voltage proportional to the produce of a resistance and the arithmetic sum of the first and second analog signals.

The invention may be understood more readily and various other features of the invention may become apparent, from consideration of the following description.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing, wherein: Figure 1 is a schematic diagram depicting a particle detector system made in accordance with the invention; and Figures 2 and 3 are graphs, not to scale, of signal strength versus time comparing the operation of the detector system of Fig. 1 with other systems.

As shown in Fig. 1, a particle or smoke detector system in accordance with the invention comprises a conventional photoelectric particle detector 1 such as is described in United States patent specification No.

3,863,076, and an ionization particle detector 2 such as is described in United States patent specification No. 4,104,619. Each other detector 1, 2 produces an analog signal (current or voltage) corresponding to the density of smoke particles which it senses. The photoelectric detector 1 produces a first analog signal corresponding to the density of the relatively large particles which it senses, and the ionization detector 2 produces a second signal representing the response to its different range of smaller particles. Each analog signal is boosted through amplifiers 3 (type CA 1458, or equivalent) which preferably produce electrical analog current signals proportional to the respective particle densities sensed.

The first photoelectric analog current signal 11 and the second, ionization analog current signal 12 are applied to a summation current network of device N including diodes D1 and D2 (type 1 N4454) and a summing junction J.

The diodes D1 and D2 constitute two inputs to the summation network N and arithmetically add the first and second signals to produce a summation signal at the junction J.

The arithmetic sum of currents 11, 12 for example, at junction J is applied to a transre sistance amplifier 4 (CA 3078, or equivalent) which multiplies the summation of the currents 11 and 12 by the resistance R1 in parallel with the amplifier 4 to produce a voltage V at the transresistance amplifier output which is expressed as: V=R1(l1 +12) The transresistance amplifier output voltage V is applied to a voltage threshold detector 5 whose sensitivity is set for a predetermined value T of input voltage. The threshold detector 5 responds to a voltage higher than a preset threshold by changing its output to a state signifying an alarm signal which is, for example, applied to a power amplifier 6 driving a horn H or other alarm indicator.

The advantages of the detector system of Fig. 1 over logic OR and logic AND systems are graphically illustrated in Figs. 2 and 3.

Fig. 3 illustrates the reaction to large particle, grey or black smoke, of the present system (solid line curve J), and of systems using OR logic or AND logic combinations of photoelectric responses 11 (dot-dash curve) and ionization response 12 (dashed line) to the same smoke densities increasing with time.

The summation signal J rises faster than either signal 11 or 12 to a time t1 when it crosses the threshold T of detector 5. In the example of Fig. 2, the photoelectic detector response 11 alone is slower than the summation response and would not cross the threshold level T until time t2 with a time delay OR TD in the case of an OR logic system. In the case of an AND logic system the response 12 of the ionization detector 2 is, for grey-black smoke, much slower than the photoelectric response 11, further delaying until time t3 (AND-TD) the generation of an alarm signal, since both the photoelectric 11 and ionization 12 signals must cross the threshold in an AND logic system.

Fig. 3 illustrates the reaction of the same three systems, summation, OR logic and AND logic, to a fine-sized smoke signal to which an ionization detector is relatively sensitive. Again the summation response is faster than either the photoelectric or ionization, reaching the alarm threshold at time t1. After a time delay (OR-TD*) the ionization detector signal 12* crosses the threshold T; and later (AND-TD*) the photoelectric signal 11 crosses the threshold.

It is apparent from the foregoing description of Figs. 1 to 3 that (1) The summation response J is faster than either the photoelectric 11 or ionization 12 response; (2) The ionization false alarm response 12 is reduced by reason of being only a part of the summation response; (3) At the summation junction J the signal includes responses to the particle size ranges of both the photoelectric detector 1 and the ionization detector 2 but without requiring that particles in both ranges be present as with a logic AND system; and (4) The present summation detecting system is far more reliable than a logic AND system since the summation system will detect smoke if either the photoelectric or ionization detector fails, whereas the logic AND system fails if either detector fails.

Claims (6)

1. A particle detecting system comprising; a first detector for sensing particles in a first range of sizes and serving to produce a first analog signal corresponding to the density of particles sensed, a second detector for sensing particles in a second range of sizes and serving to produce a second analog signal corresponding to the density of particles sensed, combining means which includes means for arithmetically adding the first and second analog signals, and threshold means coupled to the combining means and serving to produce an alarm-indicative signal when the arithmetic sum of the first and second signals exceeds a predetermined value.

2. A system according to claim 1, wherein the first detector is a photoelectric detector and the second detector is an ionization detector.

3. A system according to claim 1 or 2, wherein the combining means at least includes a pair of diodes connected to the first and second detectors respectively and connected to a common summing junction.

4. A system according to claim 3 wherein the first and second analog signals are analog current signals.

5. A system according to claim 4, wherein the combining means futher comprises a transresistance amplifier which produces an output voltage proportional to the product of a resistance and the arithmetic sum of the first and second analog current signals.

6. A particle detecting system substantially as described with reference to, and as illustrated in, Fig. 1 of the accompanying drawing.