WO1996000896A1

WO1996000896A1 - Gas or vapour detector

Info

Publication number: WO1996000896A1
Application number: PCT/GB1995/001533
Authority: WO
Inventors: Peter Alfred Payne; Krishna Chandra Persaud
Original assignee: Aromascan Plc
Priority date: 1994-06-30
Filing date: 1995-06-28
Publication date: 1996-01-11
Also published as: AU2800995A; EP0767908A1; GB9413105D0; JPH10502172A

Abstract

There is disclosed a gas or vapour detector comprising an array of sensor elements which experience changes in their electrical properties on exposure to gases or vapours and measuring means to measure such changes to indicate when the elements have been exposed to a gas or vapour, and further comprising control means operable to select from the array a subset of sensor elements for measurement.

Description

GAS ORVAPOUR DETECTOR

This invention relates to gas or vapour detection.

It is known that the electrical properties of some substances change in the presence of certain gases or vapours, and sensors for gases or vapours have been proposed - and indeed demonstrated - that use reactions from a plurality of different sensors as a means of identifying gases or vapours.

Sensing arrangements based on this principle are described in WO 86/01599 where the d.c. resistances of the sensors are measured, and in EP 0 286 307 where the a.c. impedance is measured. French Patent Application 92.11634 discloses the use of such devices, for example, in the location of truffles.

To deal with the volume of information from a plurality of sensors, say twenty or so, use is made of neural net technology, which involves a training phase in which the sensors are exposed to known gases or vapours and the hidden elements of the net adjusted so as to give the required output. Whilst such an arrangement may be trained against specific individual compounds and against specific mixtures, essentially it will recognise only individual compounds and mixtures to which it has been exposed and will be unable, generally speaking, to analyse a mixture in order to identify its individual components and will, again in general, be unable to recognize that two mixtures of the same components in different proportions do in fact contain the same components.

The present invention provides sensing arrangements having substantially improved analytical power as compared to previously proposed arrangements.

The invention comprises a gas or vapour detector comprising an array of sensor elements which experience changes in their electrical properties on exposure to gases or vapours and measuring means to measure such changes to indicate when the elements have been exposed to a gas or vapour, and further comprising control means operable to select from the array a subset of sensor elements for measurement.

The control means may be operable to select subsets of sensor elements for measurement which are known to have no or only minimal change in at least one electrical property on exposure to a particular gas or vapour, so that a sample gas or vapour can be investigated for other gases or vapours eliminating or substantially eliminating any signal due to said particular gas or vapour.

The control means may also be operable to select subsets of sensor elements for measurement which are known to have substantial changes in at least one electrical property on exposure to a particular gas or vapour, so that a sample gas or vapour can be investigated to confirm or otherwise the absence of such particular gas or vapour from the sample.

At least one of the sensor elements may experience changes in at least two of its electrical properties on exposure to at least one gas or vapour and the control means may then be operable to select said one of the sensor elements to be, or to be a member of, a subset of sensor elements for measurement of both or all of its said electrical properties. Said electrical properties may be selected from d.c. resistance and a.c. impedance at one or more frequencies.

The control means may be operative to select from the array a series of subsets of sensor elements for measurement, and may be programmable to select a predetermined series of subsets whereby a sample can be investigated for the presence or absence of individual components of a mixture of different gases or vapours. The control means may be operative to select a series of subsets whereby a sample can be investigated for the presence or absence of individual components of a mixture of different gases and to determine which series shall be selected on the basis of an initial measurement or measurements indicating the possible presence or absence of such individual components.

Embodiments of gas or vapour detectors according to the invention will now be described with reference to the accompanying drawings, in which :-

Figure 1 is a diagrammatic representation of one embodiment of detector;

Figure 2 is a typical response pattern for a first compound;

and Figure 3 is a typical response pattern for a second compound.

Figures 1 illustrates a gas or vapour detector 11 comprising an array 12 of sensor elements 13 which experience changes in their electrical properties on exposure to gases or vapours, and measuring means 14 to measure such changes to indicate when the elements 13 have been exposed to a gas or vapour, and further comprising control means 15 operable to select from the array 12 a subset of sensor elements for measurement.

The sensor elements 13, of which there may be twenty or more, though only five are shown for clarity's sake, are of various semiconducting polymer materials such as polypyrroles which exhibit different responses to different gases or vapours to which they may be exposed. In known arrangements, for example in British Patent Application No. 9503760.2 a neural net is employed to analyse the response of all of the sensor elements to gases or vapours, the array 12 being exposed to known gases or vapours. The net is trained to respond with an appropriate output - for example, the net can be trained to output "isoa yl acetate" by presentation of an input pattern corresponding to isoamyl acetate and can then be trained again against, for example, cineole to output the name "cineole" without upsetting the previous teaching of isoamyl acetate, and so the net can be trained to recognise a large number of compounds. Likewise, the net can be trained to recognise mixtures of compounds, but the response of the system to a mixture will naturally depend upon the relative proportions of compounds in the mixture, and the response will in general not be a simple summation of the responses to the individual compounds. To train the net to recognise mixtures of gases or vapours in all possible relative proportions and over all possible concentrations would be possible, but in general impractical.

Generally speaking, the more different sensor materials in the array 12, the better is the system able to discriminate between samples, because more information is available. And, since sensors will in general react differently at different a.c. frequencies, and differently to how they react to d.c. (which may be regarded as zero frequency a.c, however) more information still is available by operating in both d.c. and a.c. modes and, in the a.c. mode, at different frequencies. Other methods of sensor interrogation are also within the scope of the invention.

However, it is now found to be sometimes advantageous to utilise less than the total available information, and this can be achieved, according to the invention, by carrying out some measurements on a subset only of the sensor elements of a given array. In one manner of operation, the control means 15 are operable to select subsets of sensor elements 13 for measurement which are known to have no or only minimal change in at least one electrical property on exposure to a particular gas or vapour, so that a sample gas or vapour can be investigated for other gases or vapours eliminating or substantially eliminating any signal due to said particular gas or vapour.

Thus, of a twenty sensor array, sensors 5-12 and 17-20 might be known to have no or little response in their d.c. resistance to cineole - see Figure 2. When cineole is known to be present in a sample, therefore, the subset 5-12 and 17-20 can be selected for measurement in d.c. mode, and the system operated at, perhaps, enhanced sensitivity level to discriminate between other gases or vapours.

Conversely, the control means 15 may be operable to select subsets of sensor elements 13 for measurement which are known to have substantial change in at least one electrical property on exposure to a particular gas or vapour, so that a sample gas or vapour can be investigated to confirm or otherwise the absence of such particular gas or vapour from the sample. This can also improve the signal-to-noise ratio of the measurement. Thus if ten different compounds C.-C,₀ were known strongly to affect ten different subsets S_{l f}S₂ ... S₁₀ of sensors 13, each of those subsets can be checked in succession for elimination, or otherwise, of the possibility that a given sample contains one or other of the compounds.

The result of such a test may not be a proof of the presence or absence of any particular compound, but may rather be regarded as a probability of the presence or absence of such compound, which may be sufficient to trigger an alarm or may point to the need for further investigation, perhaps by other means.

Thus if subset S, , for example, comprised sensors 1-5, and all five responded to a sample, that might be regarded as reasonable evidence of the presence of compound C, , though the response might have been caused by other compounds. However, if none of the sensors 1-5 responds that might in many circumstances be taken as certain proof of the absence of compound C,, as it would be highly unlikely that all five sensors would be blocked by other compounds. In this way, therefore, a mixture that could comprise one or more of the compounds C,-C,_n could rapidly be shown not to contain compounds C,-C_q, say, leaving only compounds C_q and .^. A reading taken over all the sensors in the array could, however, give rise to a result not recognised by the neural net because the mixture C_g and C,_Q had not been taught to the neural net in all its possible relative proportions.

The sensor elements 13 will in general experience changes in at least two of their electrical properties, which may be d.c. resistance and a.c. impedance at different frequencies, on exposure to at least one gas or vapour. The control means 15 are operable to select one, say S,, of the sensor elements 13 to be a subset, or a member of a subset of sensor elements, for measurement of both or all of their electrical properties.

A source 16 of d.c. and a.c. voltage to be applied to the sensors is controlled from the control means 15 to apply different outputs to the selected sensor S. so that its responses can be measured at, say, 100 MHz and 250 MHz in addition to d.c. or zero frequency voltage. A particular sensor element or a subset of elements 13 known to have specific relation¬ ships between their d.c. and a.c. responses at different concentrations of one or more compounds can therefore be selected, after one of such compounds has been detected in a mixture, to measure its concentration and hence its proportion relative to other compounds in the mixture.

The control means 15 can be operated or programmed to select different subsets of sensor elements in turn for investigation of a sample. Where a sample's constituents are known to be limited to a number of compounds, for example, and it is desired to know which are present and in what proportions, the control means 15 can be programmed to select subsets in turn which will test for the absence, as described above, of each of the possible components, and then go on to select further subsets to measure the proportions, also as indicated above, of those compounds found to be present. The control means 15 may follow a preset routine, or its programming may be adaptive, missing out, for example, concentration measuring steps for compounds found not to be present.

It is convenient to select subsets of elements 13 by individually addressing the elements 13 through a multiplexer 17.

A display screen 18, neural net 19 and keyboard 20 or other input, such as a mouse, tracker ball or touch screen arrangement, together with a printer 21 are elements of a general-purpose gas or vapour detector according to the invention. It is possible to produce the equipment in a standard format, and particularly the sensor array 12, and adapt it for different specific purposes simply by training the neural net 19 and inputting instructions regarding the selection of subsets of sensor elements 13. The appropriate instructions may be generated having regard to knowledge of the behaviour of the individual sensor elements 13 to known substances, or may be developed in a training program for the neural net 19.

Figure 2 , as has been mentioned, is a typical response pattern of a set of twenty different sensors to cineole. Figure 3 is a typical response pattern of the same set of twenty sensors to isoamyl acetate. The response values assigned to each sensor are normalised, i.e. are actual resistance or impedance values multiplied by an arbitrary constant to give a histogram which optimises the relative values for recognition purposes. The arbitrary constant for isoamyl acetate may be different for that for cineole. Eliminating the high value sensor elements from the isoamyl acetate histogram can allow an increase in sensitivity for better discrimination between other compounds.

Claims

1. A gas or vapour detector comprising an array of sensor elements which experience changes in their electrical properties on exposure to gases or vapours and measuring means to measure such changes to indicate when the elements have been exposed to a gas or vapour, and further comprising control means operable to select from the array a subset of sensor elements for measurement.

2. A detector according to claim 1, in which the control means are operable to select subsets of sensor elements for measurement which are known to have no or only minimal change in at least one electrical property on exposure to a particular gas or vapour, so that a sample gas or vapour can be investigated for other gases or vapours eliminating or substantially eliminating any signal due to said particular gas or vapour.

3.^' A detector according to claim 1 or claim 2, in which the control means are operable to select subsets of sensor elements for measurement which are known to have substantial change in at least one electrical property on exposure to a particular gas or vapour, so that a sample gas or vapour can be investigated to confirm or otherwise the absence of such particular gas or vapour from the sample.

. A detector according to any one of claims 1 to 3, in which at least one of the sensor elements experiences changes in at least two of its electrical properties on exposure to at least one gas or vapour and the control means are operable to select said one of the sensor elements to be, or to be a member of, a subset of sensor elements for measurement of both or all of its said electrical properties.

5. A detector according to claim 4, in which the said electrical properties are selected from d.c. resistance and a.c. impedance at one or more frequencies.

6. A detector according to any one of claims 1 to 5, in which the control means are operative to select from the array a series of subsets of sensor elements for measurement.

7. A detector according to claim 6, in which the control means are programmable to select a predetermined series of subsets whereby a sample can be investigated for the presence or absence of individual components of a mixture of different gases or vapours.

8. A detector according to claim 6 or claim 7, in which the control means are operative to select a series of subsets whereby a sample can be investigated for the presence or absence of individual components of a mixture of different gases or vapours and to determine which series shall be selected on the basis of an initial measurement or measurements indicating the possible presence or absence of such individual components.