WO2009024774A1

WO2009024774A1 - Gas sensor operation with feedback control

Info

Publication number: WO2009024774A1
Application number: PCT/GB2008/002804
Authority: WO
Inventors: John E. A. Shaw
Original assignee: Thorn Security Limited
Priority date: 2007-08-22
Filing date: 2008-08-18
Publication date: 2009-02-26
Also published as: GB0716399D0

Abstract

A gas sensor and method of operating a gas sensor with reduced energy requirements employing a sensor material providing on exposure to a target gas a primary response which is used in a control circuit to control energy delivered as heat or photon radiation to the sensor material, a measure of which delivered energy provides a secondary sensor output. The feedback control of energy delivery to the gas sensor material allows operation at low power at target gas concentrations below a selectable threshold and elevation of energy inputs above said threshold allowing fast sensor response to target gas exposure and recovery after cessation of exposure. The sensor and mode of operation provides low energy requirements for sensor applications with rare or intermittent exposure to target gases such as in fire detection and security applications.

Description

Gas sensor operation with feedback control

There is a wide range of chemical sensors based on sensor materials having one or more properties dependant on their exposure to species to be sensed. Sensor materials usually have at least one property dependant on the presence of a target chemical species, and in particular target gases which by a transduction process can generate a signal. It is commonly necessary to provide an energy input to the sensor materials to achieve desired response characteristics, including sensitivity to target chemical species, response speed, selectivity, and recovery times after exposure. The energy input is most generally achieved by heating the materials or structures incorporating the materials, but exposure to photon radiation has also been described (US 20050224360, US 20060000259). These energy requirements can limit applications, and it is desirable that materials and methods be developed reducing such requirements.

Sensor materials are in many cases provided with catalytic material, or have catalytic sites either incorporated or located at some surface. Such catalytic materials or sites may act to promote reaction, enhancing interaction between the target chemical species and the sensor material or structure. Such catalytic materials or sites may alternatively or additionally act to promote reaction, enhancing the removal of target chemical species or derivatives. Without sufficient energy input, rapid response and rapid recovery at low or moderate temperatures may require substantial quantities of active catalyst. As these are commonly based on expensive platinum group materials, it is advantageous to reduce the requirement by providing additional energy inputs to promote reaction.

Excessive or prolonged energy inputs to sensor materials may promote degradation of the materials, including unwanted reactions with normal environmental species, especially oxygen and water vapour, or annealing out of active sites. It is desirable, therefore, to operate devices in a mode whereby such energy inputs as are required to maintain desired response characteristics are limited to periods where exposure to target chemical species occurs. Sensor and sensor material lifetime may thus be increased. Materials used or proposed for chemical sensing (including gas sensing) include those having electronic properties dependant on environmental effects including some metal oxides such as SnO₂ and TiO₂ and organic compounds including conducting polymers, polymer/conductor composites, phthlocyanins, and other poly-aromatic molecules and their derivatives including metal complexes and nanostructured carbon materials including nanotubes and graphene.

Operation of sensing materials at lower or ambient temperatures enables reduction or elimination of energy inputs; but, under these conditions, sensors can suffer from low or slow response and especially slow recovery after termination of exposure. Sensors which show non-reversible effects in response to target chemical species, which is sometimes referred to as a dosimeter type response, while acceptable for some applications is not acceptable for applications where recovery is required. In some cases, use of sensing materials in nanoparticulate or nanostructured form enhances response, but the issue of slow recovery after termination of exposure to a target chemical species commonly remains.

Moreover, while application of heat or illumination can raise sensitivity and signal levels for selected sensor materials, continuous application of heat or illumination to sensor materials can result in excessive diminution of the signal elicited by exposure to a given concentration of the species to be sensed, whilst increasing sensor power requirements. Intermittent or pulsed application of heat or illumination to the sensor materials can allow adequate build up of sensor response, while promoting sensor recovery from transient exposures, while power requirements remain lower than for continuous application of heat or illumination. Variations in system output arising from variation in sensor characteristics resulting from the intermittent application of heat or illumination may be removed by time- gating the system output, by using differential output for sensing and reference structures which are both exposed to the applications of heat or illumination, or by combination of such methods. Whilst it is known to reduce energy requirements by use of pulsed heating of devices, which may be combined with time-gated monitoring of sensor material properties (US 20040084308) this has limited utility unless the power application is a small proportion of the pulse time. It has also been described (US 20040099047) that heater control be linked to gas sensor output, but merely to initiate a high temperature sensor clean cycle once a sensor device has passed a threshold level. US 20040099047 does not teach operation with a feedback arrangement continuing through gas sensing, or the use of feedback control generating a secondary response value, or the use in such system of photoactivation of the gas sensor which is the subject of the present invention. Operation of sensor devices with feedback control is known as in operation of electrochemical cells in potentiostatic mode, and in operation of calorimetric combustible gas sensors in a constant temperature mode, as pellistors. Potentiostatic operation of electrochemical sensors operates by feedback control of the current through a sensing electrode, not by feedback control of a thermal or photonic energy flux to a sensor material as described in the present invention. Pellistors operate by feedback control of electrical heating based on the output of a temperature sensor used to monitor the temperature of a catalyst body. The temperature sensor and the heater used are commonly the same item, typically a platinum resistor wire or film. Heat output from reaction of a combustible gas at the catalyst varies with the gas concentration and the heater current variation produced by feedback maintains a fixed temperature which correlates with the gas concentration. For a pellistor, feedback is based on a measurement of a temperature, not on a measure of a material property of a sensor material which varies with target species concentration. Further, a pellistor requires heat output from the reaction of a combustible gas, and operates under relatively high temperature conditions where such combustion reactions proceed rapidly at a catalyst. The power to maintain such temperatures is required even when the combustible gases are absent, and only falls significantly in the presence of relatively-high concentrations of combustible gases. Power requirements for pellistors are not, therefore, very low, and this limits the application of pellistors in safety, fire, or security systems where target species are,' for the majority of the time, very low or negligible.

The present invention provides a chemical sensor apparatus comprising: a sensor material having an intrinsic material property sensitive to a target species; access means allowing transfer of the target species to the sensor material; monitor means for monitoring the intrinsic material property to provide a primary sensor response; energy input means for inputting energy to the sensor material; feedback control means for controlling the energy input means in dependence upon the primary sensor response; and means for providing a measure of the energy input to the sensor material, the measure of the energy input constituting a secondary sensor response.

In a preferred embodiment, the apparatus further comprises indicator means for providing an indication that the target species has been detected, the indicator means being actuable by the primary sensor response and/or the secondary sensor response.

The intrinsic material property of the sensor material may be one selected from a group of material, electronic and optical properties, including or deriving from electronic conductivity, ionic conductivity, work function, chemical or electrochemical potential, optical absorbants, fluorescents, phosphorescents and photo-bleaching. Preferably, the feedback control means is such that a deviation of the primary response from a predetermined value controls the energy input to the sensor material so as to return the primary response towards the predetermined value, and the predetermined value is a value corresponding to zero or a low value of target species concentration.

Thus, the primary sensor response deriving from the sensor material properties is used to control the application of energy input to the¹ sensor material where such energy input increases the speed of response, and where a measure of the energy input provides a secondary sensor response. In the absence, of changes in the primary sensor response away from values corresponding to zero or low exposure to target chemical species, the control acts to maintain the energy input at a zero or low level. When the primary sensor response deviates from values corresponding to zero or low exposure to target chemical species, the control acts to increase the energy input to the sensor material so that the sensor material recovery processes are enhanced and the primary response tends to return towards values corresponding to zero or low exposure to target chemical species. A measure of the energy input or power required to drive the processes returning the sensor material and its properties to the condition corresponding to zero or low exposure to target chemical species is taken as a secondary sensor response. This secondary response is related to the concentration of the target chemical species. The relationship is not necessarily linear.

Apparatus according to the present invention may provide primary and secondary responses to the presence and concentration variation of a target species, and these responses individually or together may by known means, such as conventional transduction and signal processing means, be manifested as changes in electrical output from a measuring device, and may be provided to a display device or as an input to circuits or computation systems which provide display of information relating to that output, or to target species presence or concentration, or notification of events or alarm conditions.

Preferably, the energy input means is constituted by a heat source or a photon source, and the energy input means is operated in either a continuous or pulsed mode.

The apparatus may further comprise a filter or grating to restrict the wavelength of radiation reaching the sensor material from the energy input means.

Advantageously, the apparatus further comprises a porous or permeable filter for restricting access of materials passing from the environment to the sensor material. In this case, the sensor material may be provided on an at least partially transparent medium, and the energy input means is provided on the opposite side of said medium to the porous or permeable filter.

In a preferred embodiment, the feedback control means controls the energy input means in dependence upon the primary sensor response and a reference value. Conveniently, the reference value is the predetermined value.

Preferably, the feedback control means includes a comparator whose inputs are the primary sensor response and the reference value, the output of the comparator controlling the energy input means. > ,

Advantageously, the reference value is chosen in dependence upon the primary sensor response during a period when the target species concentration is low. Alternatively, the reference value is constituted by the output of a long-term averaging of the primary sensor response.

In another preferred embodiment, the apparatus further comprises a second sensor material, and a barrier positioned between the second sensor material isolating it from the target species. Alternatively, the apparatus further comprises a second sensor material, the second sensor material being a material less sensitive to the target species than the first- monitored sensor material, the monitor means being arranged to monitor the intrinsic material property of the second sensor material.

In either case, the output of the monitoring of the intrinsic material property of the second sensor material constitutes the reference value. The invention also provides a fire detector comprising a chemical sensor apparatus as defined above.

Preferably, the indicator means is such as to generate an alarm signal when a predetermined amount of the target species has been detected. In a preferred embodiment, the fire detector further comprises at least one other type of fire detecting sensor.

Advantageously, said at least one type of fire-detecting sensor is one or more of a temperature sensor and a smoke sensor.

Conveniently, the fire detector further comprises a local signal assessment and control unit.

The invention also provides a fire detector system comprising a plurality of fire detectors, each defined above, and a system assessment and control unit. Preferably, the system assessment and control unit is communicatively coupled to each of the local assessment and control units. The invention further provides a method of sensing a chemical, the method comprising the steps of monitoring a sensor material having an intrinsic material property sensitive to a target species to provide a primary sensor response; inputting energy to the sensor material; using feedback control means for controlling the energy supplied to the sensor material in dependence upon the primary sensor response; providing a measure of the energy input to the sensor material, said measure constituting a second sensory response; and providing an indication that the target species has been detected, in dependence upon the primary sensor response and/or the secondary sensor response.

The invention further provides a chemical sensor apparatus comprising: a sensor material having an intrinsic material property sensitive to a target species; which property is from the group of material, electronic, and optical properties, including or deriving from electronic conductivity, ionic conductivity, work function, chemical or electrochemical potential, optical absorbance, fluorescence, phosphorescence, and photo-bleaching, access means allowing transfer of target species to the sensor material monitor means for monitoring the intrinsic material property, alone or in combination with time and property of a reference material or structure, to provide a primary sensor response; energy input means comprising source and transmission means for inputting energy to the sensor material; feedback control means for controlling the energy input means dependant on feedback input values including at least the primary sensor response; means for providing a measure of the energy input to the sensor material, the measure of the energy input constituting a secondary sensor response.

Preferably, the feedback control means, in dependence on the primary sensor response or a differential or variation of the primary means, sensor response with respect to one or more of time and reference values, acts to return the primary sensor response to a value or value range corresponding to the presence of the target species within a preselected low concentration range.

Advantageously, the feedback control means includes a comparator whose inputs are the primary sensor response and a reference value, the output of the comparator controlling the energy input means.

In a preferred embodiment, the feedback control means acts to increase the energy input when the primary sensor response, or differential of the primary sensor response output with respect to one or more of time and reference values, deviates from a value range corresponding to the presence of the target species within a preselected low concentration range, which low concentration range may be between zero and a preselected low level

The feedback control means may act to reduce the energy input to a preselected minimum level when the primary sensor response, or differential of the primary sensor response with respect to time and/or one or more reference values, is within a value range corresponding to presence of the target species within a preselected low concentration range, which low concentration range may be between zero and a preselected low level

The feedback control means may act to control the energy input in dependence on the differential of the primary sensor response with respect to reference values, and wherein said reference values correspond to, or are a derivative of, the output of the monitor means for a period included within which is a period when the target species is within a preselected concentration range, which range may be between zero and a preselected low level, and which derivative may be a long term average value.

The feedback control means may act to control the energy input in dependence on the differential of the primary sensor response with respect to reference values, and wherein said reference values are derived from a reference material or structure.

Preferably, the reference material or structure has a lower sensitivity to the target species than the sensor material, said lower sensitivity being provided by the difference in composition between the reference material and the sensor material.

The reference material or structure may have a lower sensitivity to the target species than the sensor material, the lower sensitivity being provided by provision of a barrier restricting target species access to the reference material or structure.

The energy input may be continuous or pulsed.

Advantageously, means are provided to vary the output of the energy input means, which variation may include intensity, pulse frequency and pulse duration.

Control means may be provided for actuating the monitor means at set times, which set times may include times set with respect to changes in the energy input levels.

Preferably, the secondary sensor response comprises, or is derived from, a measure of energy which may include a measure of energy supplied to a source from which at least part of said energy is transferred to the sensor material.

The secondary sensor response may comprise, or be derived from, a measure of energy which may include a measure of energy flux from said source, which measure of energy flux may include the output of a sensor provided to measure said flux.

The energy input means may comprise a heat source or a photon source. Energy transmission means may provide additional energy transmission pathways, which additional pathways may be to reference materials or structures, or to energy flux measurement means or combinations thereof.

Part of said energy transmission means may comprise an at least partially energy transmissive medium forming a substrate or housing for the sensor material, and the energy source may be provided on the opposite side of said medium to the access means.

Preferably, the energy input comprises a photonic radiation input, wherein the photonic radiation may consist of radiation with a wavelength range extending from microwave radiation to deep ultraviolet radiation, including single or multiple bands or discrete wavelengths within said range, and wherein the provided wavelength range or component thereof may be fixed or varied.

The energy input may comprise a photonic radiation source, which may include components or filaments subject to electrical resistive heating including heated incandescent light sources, radiation-emitting semiconductor devices including light emitting diodes, lasers, including solid state lasers, and discharge lamps, which one or more sources may be integrated or combined on common substrates or holders with structures comprising sensor materials and reference structures or materials.

The sensor materials may include insulating, semiconducting, ionically-conducting materials, and materials comprising or consisting of combinations or mixtures of such types, wherein the sensor materials may be in nanomaterial form including nanoparticulates or other nanostructured forms including nanotubes, fullerenes and aggregates or assemblies of such nanomaterials, wherein the sensor materials may include ceramics and metal containing compounds, elemental and compound semiconductors, organic or carbon based materials, and derivatives, doped compositions, mixtures, mixed compounds and composites of such materials, and wherein the sensor materials may be provided with, incorporate, or be doped with catalytically-active material.

The sensor materials may be photochemically active, which materials include but are not limited to, TiO₂, SnO₂, In₂O₃, ZnO, V₂O₃ and related compounds including oxyhydroxides. The target species or precursors thereof may be transported, contained, dissolved, dispersed, including as gases, vapours, droplets, suspensions, aerosols or emulsions in one or more fluid phases, and wherein the target species or precursors thereof may include combustible gases or products of combustion or fire.

The invention still further provides a contamination sensor comprising apparatus as defined above, wherein the primary and secondary sensor responses indicate the presence of contamination, and wherein the time integrated value of the secondary sensor response provides a measure of contamination loading.

The access means may be provided with one or more permeable filter structures having selective permeability to the target species or precursors, or relative impermeability to one or more interfering substances.

Indicator means may be provided for providing an indication that the target species has been detected, the indicator means being actuable by the primary sensor response and/or the secondary sensor response, and which indicator means provides an indication of a measure of the target species concentration, and optionally provides or actuates a signal or alarm if said indication reaches a preselected threshold.

The target species may be an event marker, which event may include fire, combustion, generation or release of products affecting environmental quality, chemical or biological presence or hazard, or security hazards, and wherein indicator means are provided for providing an indication of a measure of said marker, the indicator means being actuable by the primary sensor response and/or the secondary sensor response, which indicator means provides or actuates a signal or alarm signifying event detection if said indication reaches a preselected threshold.

A system incorporating one or more apparatus each as defined above, wherein the system may include use of different sensor materials, and wherein the system may be operated to provide redundancy or sensitivity to multiple target gas species.

A system incorporating one or more apparatus each as defined above, may optionally additionally incorporate other sensors, to monitor the condition of an environment and provide a signal or notification of said condition or alarm in case of deviations from a normal or standard environmental condition, wherein said deviations may include but are not limited to, deviations characteristic of a fire condition, deviations characteristic of a security or safety breach condition which conditions may include unauthorised access, or release or generation of toxic or nuisance gases, vapours, or biological agents.

The proposed mode involves a feedback control system whereby deviations of the primary sensor response away from an initial or long term average value is used to control one or more energy 'inputs which tend to return the primary response to that value. The energy input means may be, for example, a heat source or a photon source, and may be pulsed or continuous. The energy input means may also be varied by changes in intensity or pulse rate or duration or a combination of those factors.

The primary sensor response used may be selected to be that from a period without energy input or with energy input at a predetermined level, and may be measured at some selected time after changes in energy input. The secondary sensor response is a measure of the energy input, and may be derived directly from power supplied to the heat source or the photon source, or from measurements of the energy fluxes and/or duration or frequency of application of such power or fluxes. Secondary sensor response values may be taken as a difference between a measure of the energy input under exposure to the target chemical species and those corresponding to the zero or low exposure condition. A threshold level may be set for the primary sensor response to operate a feedback circuit to increase the energy input to the sensor material, and thus the secondary sensor response value.

Low level continuous or intermittently elevated levels of energy inputs may be required or desirable to maintain the sensor material in a responsive condition. Also according to the invention, measurements of the primary sensor response used to provide input to the feedback operation may be restricted to periods of zero or lower energy input to the sensor material, or to periods of elevated energy input to the sensor material, or to a combination of these. Where the primary sensor response is set to include measurements during elevated energy inputs to the sensor material, establishing a baseline for the primary sensor response requires that such intermittent elevated energy inputs are applied under conditions of zero or low exposure to target chemical species. This power requirement for operation at zero or low exposure conditions may be set at a level consistent with application requirements and may typically represent 1% or less of the energy requirements for operation in continuously heated or illuminated mode. Generally, however, operation according to the invention will include operation where base temperature and illumination levels at the sensor material may be maintained above ambient levels to induce a sufficient primary sensor response, and devices operated in the proposed mode so that temperature or illumination is increased when the target chemical species is present to improve sensitivity and recovery times.

Application of the invention is not limited to sensor devices or materials which show only very slow recovery after exposure or dosimeter type response, but may advantageously be applied where, under low energy input conditions, the sensor material response and recovery is sufficiently slow to cause issues with applications of the device. Operation of a device with feedback controlled energy supply to the sensor material is advantageous where a rate of sensor response time improvement is required, and where a secondary sensor response provides a separate measure of the level of target chemical species. Where the sensor system employs a sensor material structure for exposure to the test environment, and another reference structure is isolated from that environment, a differential primary response for the system may be derived from difference in sensor material properties for the exposed and reference structures. The secondary sensor response is a measure of energy input applied by a feedback control system tending to drive the differential primary sensor response towards a zero or low target gas exposure value. The proposed method includes providing energy inputs to both exposed and reference sensor material structures, or to the exposed sensor material structure only.

Operation of sensor devices according to the proposed mode ensures that energy requirements are maintained at a low level when the concentrations of target chemical species are low. A measure of the energy inputs applied by the feedback system comprises at least part of the response of sensor device or system referred to here as the secondary response which is related to the quantity of target chemical species to which the sensor system is exposed. This is of particular value for applications in the security field and especially for fire detection where target gases are only rarely or intermittently present at significant levels. One of the proposed methods of operation includes using controlled input of energy to the sensor material as heat. Heat may be transferred to the sensor material wholly or in part by thermal conduction. The heat may be generated by a resistive structure coupled closely to the sensor structure or substrate bearing the sensor structure. Heat may also be introduced by other means including inductive heating, heating by radiation including by microwave radiation or contact to material, solid or fluid, held or existing at an elevated temperature. The secondary response may be derived from power delivered from such sources to the sensor material or where appropriate to the heat source such resistive heating structure. Preferably the heater structure, sensor structure and any linking substrate should have low thermal capacity and relatively high thermal conductivity so as to allow rapid heating and cooling of the sensor structure whilst minimizing power requirements.

Another proposed method of operation particularly includes using input of energy to the sensor material by means of photon radiation, especially with sensor materials showing photochemical activity. Sensor materials having photochemical activity are those where reaction of adsorbed or absorbed species is promoted by photon radiation. Depending on the system and materials this radiation may be from a wide selection of wavelengths but most generally these will be in the visible to UV range. Radiation at the shorter wavelengths is able to promote more energetically demanding transitions and photochemical processes. Use of selected wavelengths has been proposed as a means to enhance chemical selectivity (US 20060000259). The present invention may be applied with a range of wavelengths and may employ multiple devices and radiation sources with different wavelength characteristics. Radiation sources may include UV discharge lamps or solid state LED or laser devices, especially those emitting in the visible blue to UV range. The present invention is however not limited to the use of radiation at wavelength short enough to promote electronic transitions in the sensing material. Photonic radiation at longer wavelengths operated at fluxes sufficient to promote thermal transitions or cause local heating of sensor materials may also be applied according to the invention.

What is further proposed in this invention is the selection of sensor materials for use with the proposed method where those materials show sensor properties at low or moderate temperatures. Additionally sensor materials may be selected which exhibit thermal enhancement of sensor characteristics. Additionally sensor materials may be selected which exhibit photochemical activity at low or moderate temperatures. Such materials may include materials exhibiting characteristics which are varied in response to exposure to target chemical species which characteristics may include electronic or ionic conductivities, work function, electrochemical potential, and changes in optical properties which may include absorbance, fluorescence, and phosphorescence. Such materials may be ceramics including metal oxides and chalconides, organic species including polymers, conductive polymers, organometallic compounds, phthalocyanins, porphorins, polyaromatics, carbon based conductors, and composites of such materials. Such materials may include but are not limited to nanoform materials. Such photochemical active sensor materials include but are not limited to oxides of titanium, vanadium, and tin and to carbon nanostructures including carbon nanotubes. The sensor materials may incorporate or be decorated with dopants or catalyst materials which may include Platinum group metals.

Operation of chemical sensors generally involves absorption of the target chemical species onto the sensor structure. Interaction between absorbed species and electronic configuration of the sensor structure produces a change which may be monitored or converted to provide a signal, hi some cases the target chemical species is desorbed or converted or consumed and when target chemical species concentration falls the sensor structure returns to an original condition until more of the target chemical species is presented to the sensor structure. In other cases the sensor structure may remain in a condition produced by interaction with the target chemical species even when the concentration of the target chemical species in the adjacent environment falls. Without wishing to be bound by theory, it is believed that this is either because the target chemical species or some derivative of it remains bound to the sensor structure or the change produced in the sensor structure by interaction with the target chemical species is not readily reversible. Sensor response which is cumulative and is not removed when the supply of target chemical species is removed may be referred to as dosimeter type response. While such cumulative or dosimeter response is useful for some applications, for many it is desired that the sensor response rises or falls with concentration of the target chemical species either immediately or with relatively little delay. This is certainly the case for fire detection devices where transient or low level exposure to products from non fire sources may otherwise elicit a false alarm response.

Sensor recovery after exposure to a sensed species may be promoted by energy transfer to the sensor material. Without wishing to be bound by theory, it is believed that such recovery may involve one or more of a variety of processes. These processes may include desorption of that species or derivative thereof, reaction to convert adsorbed molecules of the sensed gas to a form which does not cause a response or is desorbed which process may include consumption or decomposition of adsorbed molecules, and relaxation or reversal of changes in the sensor material which may include structural rearrangements and changes in oxidation state or atomic or molecular binding. Such relaxation or reversal of changes in the sensor material binding may involve species deriving from background gases such as oxygen, water, or hydroxyl groups. It is believed that for some sensor materials the processes of consumption or decomposition of adsorbed molecules may involve their oxidation ultimately to small molecular species which may include CO₂ and H₂O. Energy inputs, generally as heat or photon radiation may be required to enable these processes to occur at acceptable rates.

These photochemical processes operating on materials showing chemical sensor activity may result in changes in electronic properties such as conductivity, work function, or electrochemical potential by which the chemical sensor activity can be recognized, measured or transducer. The sensor materials operated according to this invention may also include those where chemical sensitivity results in changes in optical properties such as absorbance, fluorescence, phosphorescence or photobleaching.

The application of energy inputs to sensor materials by heating or by illumination with photon radiation to enhance rates of such desorptive or reaction processes of sensor materials may be varied both in terms of exposure times and levels or intensity or combinations thereof. For photon radiation the wavelength or wavelength bands used may also be varied. Such variations may be regulated by a control means and according to the present invention such regulation can be made to depend on an input signal or primary sensor response derived or transduced from properties of the sensor materials affected by environment which may contain the target chemical species. A feedback arrangement is employed such that a change in said primary sensor response input signal from a value corresponding to a base level of target chemical species is followed by an output from said control means causing variation in energy inputs to the sensor material tending to decrease said change in signal input or primary sensor response. The feedback arrangement delivering energy inputs to sensor structures has, as input, a primary sensor response arising from interaction of sensor material and target chemical species. This primary sensor response may be a differential response with respect to a reference device or structure, and may include a time dependant element such as where the primary response depends on a rate of change of measured output from the sensor material and structure. The feedback control driving energy inputs to the sensor material may depend on threshold values for the characteristic properties of the sensor material or primary output and can be made dependant on or independent of direction of change of primary sensor response.

The values, direction, and time dependence of the primary sensor response may be taken together with secondary response relating to energy input values to provide further information on the sensed environment. This may provide information not only on the concentration of gases sensed but also on the identity or at least the oxidizing or reducing (redox) properties of the gas. For example direction of primary response change for a reducing gas such as CO may be opposite to that for an oxidizing gas such as NO₂, while energy input such as by exposure to suitable photon radiation may drive recovery for each and a measure of the required energy input provide a measure of concentration. It is generally necessary to carry out calibration with test gases to determine these relationships. There may be cross talk and combinations of oxidizing and reducing gases may tend to generate signals lower than for the individual components. This and other interferences or additive effects may be minimized by providing the sensor with filter materials to remove one or more component e.g. using an active carbon filter will remove NO₂ but pass CO resulting in a response characteristic of the CO level only. By use of multiple devices with differently selective filters, which may include no filter, the multiple primary and secondary responses may be processed to determine the mix of sensed gases present in the environment being monitored.

According to the present invention a measure of the output from the control means regulating energy inputs to the sensor material or of the resultant energy transfer to the sensor material is employed as a secondary sensor response or indicator of the concentration of the target chemical species. Such a mode of operation may enhance sensor material recovery rates and restrict the elevation of energy consumption to periods when target chemical species are present. In response to a rise in the concentration of target chemical species, this feedback arrangement may take the form of an increase in application of heat or illumination and a corresponding increase the value of the corresponding concentration indicator or secondary sensor response. For applications like fire detection where such rises in relevant species are relatively rare or abnormal conditions the needs for increased power are limited so that overall sensor power requirements remain low. Where the amount of reaction can be related to energy input, for example by the quantum efficiency of a photochemical process, the rate of energy input may be directly related to concentration of the species being sensed. This relationship may be dependant on controlling transport of species to the sensing material e.g. by means of a structure or material limiting diffusive transport. It will not be necessary to have full knowledge of the processes, quantum efficiencies, or transport as concentration versus signal dependence may be determined experimentally.

Although the method and devices have been described predominantly in terms of gas sensors, the invention is not limited to sensing gaseous species or to operation in a gaseous environment. The method may be applied where material to be sensed is transported as droplets or aerosols or in fluids other than air. It is known that photochemical sensor devices, especially titania-based gas sensors, show response to chemical contaminant such as oils and that such contaminants are eliminated by sufficient application of UV illumination. According to the invention a measure of the delivered UV power to return the sensor device to an initial precontaminated state as evidenced by a primary output signal level provides a secondary response which is a measure of the contaminant level. Such operation may be performed in a gaseous environment such as air, but may also be performed where sensors are immersed in a liquid environment and particularly in an aqueous environment. For operation in a liquid environment it is preferable that energy transfer to the sensor material is by photonic radiation at wavelengths capable of exciting electronic transitions in the sensor material. Thermal excitation in a liquid environment will tend to be inefficient due to the relatively higher thermal capacity and conductivity of liquids relative to gases. The operation mode and sensors operated according to the mode may be applied to a range of applications according to the invention. Fire detection, toxic gas detection, environmental monitoring and security applications are favored as generally these may require operation for long periods without presence of target chemical species and then to detect exposure events and their duration. Further deployment of sensors for such applications can be limited by available power so that low power operation when target chemical species are absent or below acceptable threshold levels is advantageous. While target chemical species selectivity may be an advantage, and accessible by use of selected sensor materials, catalysts, optical wavelength selection, or selectively permeable filters, it is not always necessary to identity specific gases so that targets may include classes of gases, polar or non polar, acidic or basic, and also non-gaseous chemical species such as aerosols or mists of organic contaminants which may include biological material such as microbes, viruses, spores and fragments thereof as long as those or their derivatives can affect sensor material characteristics and can be removed, degraded, or desorbed by the action of energy inputs, especially photonic radiation, to the sensor materials.

Input signals controlling the variation of application of heat or illumination to structures in a sensor system may be based on the characteristics of sensing material structures or on differential response of sensing and reference structures. The control of the variation of application of heat or illumination to structures in a sensor system may be applied at some threshold of the sensor system response or according to an algorithm which may depend on the level and duration of sensor system response. Fire detection may be based directly on the primary sensor response for the system even when limitation of sensor response or sensor recovery are affected by application of energy inputs, by thermal processes or by photon radiation. Alternatively fire detection may be based on secondary sensor response signals corresponding to the level of, or level and duration of, the application of heat or illumination to the structures in the sensor system, which signals may include measures of or be indicative of the energy inputs, duration, intensity, or the power or energy requirements for such application.

The application of heat or illumination to enhance rates of desorptive or reactive processes at chemically sensing materials may be varied both in terms of exposure times and levels. This variation may be controlled such that exposure times and levels depend on the sensor output. A feedback arrangement may be employed such that an increase in sensor response to the species to be sensed is followed by variation in application of heat or illumination tending to decrease the sensor response and enhance sensor recovery rates. For sensing structures based on chemically sensing materials this feedback arrangement will normally take the form of an increase in application of heat or illumination in response to a rise in the concentration of species to be sensed. Sensor output controlling the variation of application of heat or illumination to structures in a sensor system may be based on the characteristics of sensing structures or on differential response of sensing and reference structures. The control of the variation of application heat or illumination to structures in a sensor system may be applied at some threshold of sensor system response or according to an algorithm which may depend on the level and duration of sensor system response. For applications like fire detection where such rises in relevant species are relatively rare or abnormal conditions the requirements for increased power are limited so that overall sensor power requirements remain low. Application specific system responses, such as fire detection system responses, may be to either primary or secondary sensor response values or to those values in combination, and to the time evolution of those primary and secondary sensor response values.

Sensor materials to which the invention may be utilized in sensors operated according to the invention can include semiconductor sensor materials known to the art, for example, organic semiconductors, inorganic semiconductors, semiconductors including inorganic and organic components, and the like. Sensor materials which can be utilized by the present invention include, but are not limited to, sensing materials having photocatalytic activity which sensing materials include compositions containing TiO₂, or SnO₂, or V₂O₃ and related oxyhydroxides.

For example, organic semiconductor sensing materials to which the invention may be applied can include organic materials known to have conducting or semiconducting properties under appropriate doping conditions, e.g., carbon nanotubes; fullerenes, e.g. C60, C70 and the like; conjugated oligomers and polymers, e.g. polyacetylene, polythiophene, polyphenylene, poly(para-phenylene)vinylene, poly(para-pyridyl)vinylene, polyaniline, polypyrrole and the like.

Typically, organic semiconductors will be doped according to methods well known in the art. See for example "Organic Semiconductors" Gutmann, L.; Lyons, L. R. E.

Krieger Pub. Co., Malabar, FIa. (1981), "Organic Molecular Semiconductors: Structural,

Optical and Electronic Properties of Thin Films" Zahn, D. R. T.; Kampen, T. U.; Scholz,

R. (2004), pub.NY: John Wiley and Sons and "Handbook of Conducting Polymers"

Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R.; Eds. pub. Marcel Dekker, New York, 2.sup.nd Ed (1997). The entire teachings of these documents are incorporated herein by reference.

19 Sensor materials to which the invention may be applied can include any inorganic semiconductor known to the art, typically selected from family II- VI, III-V or column IV semiconductors/insulators, metal oxides, sulfides, selenides, and nitrides. For example, semiconducting substrates typically include, or more preferably consists of inorganic 5 materials which may include but are not limited to: CdTe, CdSe, ZnS, AlGaN, InGaN, GaP, InP, InAsP, Ge, CrO_2-x, Ti_xO₃, SiC, MoO₃, CaTiO₃, (La₅Sr)FeO₃, (La₅Sr)CoO₃, SnO₂, TiO₂, ZnO, WO₃, Fe₂O₃, In₂O₃, Ga₂O₃, SrTiO₃, BaTiO₃, CdS, GaN, GaAs, and Si. In some embodiments, semiconducting substrate is SnO₂, TiO₂, ZnO, WO₃, Fe₂O₃, In₂O₃, Ga₂O₃, SrTiO₃, BaTiO₃, CdS, GaN, GaAs, or Si. 0

APPLICATIONS IN GAS SENSING

Sensors operated according to the mode of the invention can be employed to sense any gas of interest in a vacuum, in a closed system having a background of other gases, in atmosphere, in space, and the like. For example, gases that can be sensed include H₂, O₂,5 O₃, H₂O, halogens (e.g. F₂, Cl₂, ClF₃, and the like), acids (e.g. HF, HCl, and the like), nitrogenous gases (e.g., NH₃, NOX, NF₃, and the like), hydrocarbon or carbonaceous gases (e.g. CO, CO₂, C1-C4 aliphatic gases such as CH₄, cyclopropane, cyclobutane, ethylene oxide, CH₂CH₂, CH₃CHCH₂, HCCH and the like), organic solvents (e.g., benzene, toluene, xylenes, tetrahydrofuran, acetone, diethyl ether, ethanol, methanol, and the like) O halocarbons (e.g. C₂F₆, C₂HF₅, CF₄, C₃F₈, CHF₃, C₄F₈, CH₂F₂, C₃F₈, C₄F₈O, CH₃F, and the like), boronic gases (BF₃, BCl₃, B(CH₃)₃, and the like), silicon, germanium, and arsenic gases (e.g. SiF₄, SiCl₄, Si₂H₆, SiH₂Cl₂, SiH₃CH₃, SiHCl₃, GeF₄, AsH₃, AsF₅, and the like), sulfurous gases (H₂S, SO₂, SF₆, and the like), and metal halides (e.g. WF₆, and the like). hi some embodiments, a toxic gas is detected. As used herein, toxic gases include 5 any gas known to the art to be injurious to health, e.g. corrosives such as HCl, HF and the like, oxidizers, e.g. F₂ and the like; chemical poisons such as CO, HCN, H₂S and the like; gases injurious as a result of concentration and the like.

In some embodiments, a combustible gas is detected. As used herein, a combustible gas is any gas that can burn or explode, typically in reaction with an oxidizing gas, e.g. H₂ O burning in O₂ and the like. Combustible gases can include, for example, H₂, hydrocarbon or carbonaceous gases, gases associated with flammable solvents or fuels (e.g. gases or vapors emitted from petroleum and petroleum derived fuels), vapors of organic solvents (e.g. benzene, toluene, xylenes, tetrahydrofuran, acetone, diethyl ether, ethanol, methanol, and the like), hydrogen containing gases (ammonia, silanes, boranes, and the like) and the like. In various embodiments, a gas is detected from a combustion process, e.g. in an exhaust stream from an internal combustion engine, an exhaust stream from a furnace, an open fire, and the like. Gases emitted by such processes are well known to the art and can include unburned fuels (derived from petroleum, coal, biomass, natural gas and the like), products of combusted fuels (H₂O, CO, CO₂ and the like) products of nitrogen, e.g. nitrogen oxides; combusted products of contaminants in the fuel (e.g. sulfur oxides from coal or diesel containing sulfur) and the like.

In various embodiments, a gas is detected from a chemical process, for example, any gas or vapor associated with a chemical process, e.g. reagents, solvents and products in chemical synthesis, reagents, solvents and products in semiconductor manufacturing; products of refining petroleum, coal, biomass and the like and solvents in coating processes and the like.

In various embodiments, a gas is detected from a bacterial process, for example, any gas or vapor emitted by the action of bacteria, e.g. bacteria employed in bioreactors to produce chemicals or biochemicals, bacteria used in fermentation to produce ethanol; bacteria used to prepare consumable/food products (wine, beer, liquor, cheese, cured meats, and the like); bacteria in waste treatment and the like. Gases or vapors emitted by such processes can include, for example, metabolic products such as ethanol, carbon dioxide, hydrogen sulfide, methane, ammonia, acetone and the like.

In various embodiments a gas is detected from a food source, for example, a gas emitted during food processing, storage, cooking, and the like.

In various embodiments a gas is detected from a subject (e.g. humans, mice, rats, dogs, cats, monkeys, chimpanzees, chickens, pigs, cattle, sheep and the like) that is indicative of a characteristic of the subject such as presence, health and the like. Such a gas can be any metabolic product, e.g. carbon dioxide, nitrogen dioxide, water, ethanol, acetone, methane, ammonia, hydrogen sulfide, acetaldehyde and the like or can be a gas which is used by the subject, e.g. oxygen or can be a gas administered to a subject during

21 medical treatment, e.g. oxygen or an anesthetic such as nitrous oxide or can be a gas or metabolic product thereof associated with exposure of the subject to a toxic gas, e.g. carbon monoxide, hydrogen cyanide and the like.

In various embodiments, a gas is detected to monitor indoor air quality, e.g. in environments such as vehicle cabins, e.g. automobiles, planes, trains and the like, buildings, e.g. homes, industrial buildings, hospitals, laboratories, clean rooms and the like.

In various embodiments, a chemical warfare agent, or a chemical precursor or decomposition product thereof is detected. Chemical warfare agents include, for example, nerve agents including tabun, sarin, soman, cyclohexyl methylphosphonofluoridate, methylphosphonothioic acid S-(2-(bis(l-methylethyi)amino)ethyl) O-ethyl ester), phosphonofluoridic acid, ethyl-, isopropyl ester), phosphonothioic acid, ethyl-, S-(2- (diethylamino)ethyl) O-ethyl ester), Amiton, phosphonothioic acid, methyl-, S-(2- (diethylamino)ethyl) O-ethyl ester) blister/vesicant agents, e.g. lewisite, mustard-Lewisite, nitrogen mustards (HN-I, HN-2, HN-3), phosgene oxime, sulfur mustards (H, HD, HT), cyanogen chloride, hydrogen cyanide, chlorine, chloropicrin, diphosgene, phosgene, and the like. A chemical precursor includes gases or vapors known to the art to be used in the preparation of chemical warfare agents. A decomposition product includes gases or vapors known to the art to result from reaction or decomposition of a chemical warfare agent with oxygen, water, sunlight, biological tissue, and the like. In various embodiments, a chemical indicative of a high explosive is detected. Such compounds include explosives themselves (e.g. trinitrotoluene, hexogen, octogen, pentaerythritol tetranitrate, triamino trinitrobenzene and the like) and compounds emitted by the explosives, e.g. nitrogen oxides.

APPLICATION IN A FIRE DETECTION_. SYSTEM

Combustion of fuel in a fire generates heat and material products. The material products may include products of combustion and of pyrolysis. Fire detection generally involves sensing of temperature, radiation, or material transferring from the seat of the fire. The combustion and pyrolysis product materials transferred include soot as particulates, solid and liquid aerosols and gases and vapors. Soot particulates or aerosols may form

22 from gases or vapors, for example, by condensation processes, and gases and vapors may absorb or desorb from particulate and aerosol materials. Convective, advective, and diffusive processes may be involved in transfer and dispersion of fire products in the surrounding air and carry those products to detector devices.

5 Gases formed during the burning of the combustible material are generally designated as combustion gases. Most generally the fuels are organic materials resulting in CO, CO₂, and H₂O as the predominantly formed oxides. The starting phase of fires often yield CO, saturated and unsaturated hydrocarbons, alcohols, and acids due to incomplete combustion though these may continue through to well developed fires, especially if0 oxygen supply is limited. Other products depend on the composition of fuel and other materials, including suppressants, at or adjacent to the fire and on the oxygen supply. Chlorinated polymers such as PVC can give rise to HCl or Cl₂ fumes. Sulfur containing materials can give rise to oxides of sulfur (SOX) including SO₂ and/or SO₃ and under poorly oxygenated conditions to H₂S. Depending on oxygen supply at the fire seat,5 nitrogen-containing fuels such as polyurethanes can produce oxides of nitrogen (NOX) and hydrogen cyanide (HCN) while nitrogen oxides can arise by combination of oxygen and nitrogen in the air at temperatures above 2θO degree C. In the presence of water, including water vapor or droplets, acid fumes may be generated including sulfuric acid (H₂SO₄) and nitric acid (HNO₃). O Detection targets produced by fires which may provide useful indication include O₂ depletion, and a rise in levels of H₂O, CO₂, CO, oxides of nitrogen (NOX), and oxides of sulfur (SOX), HCl and a range of gaseous and volatile organic molecules including hydrocarbons, including acetylene, ethylene, ethane, and benzene, and organic molecules incorporating oxygen including products with alcohol and carbonyl groups including for5 example methanol, formaldehyde, formic acid, acetaldehyde, acetic acid, and acrolein. Changes in concentrations of fire product gases for relatively early stage fires may be of the order of 100 ppm up to a few percent for O₂, CO₂, and H₂O, and 10 to lOOppm or more for CO. Other gas and vapor concentrations will generally rise to only a few ppm during the early stages of a fire. Variation due to non-fire causes and relatively high background O levels has mitigated against widespread use of O₂, H₂O, and CO₂ sensing as nuisance fire indicators although their variation in concert with other indicators such as heat, and smoke may provide useful confirmatory indications. False alarms in fire detection systems can arise by a variety of routes and in some cases sensing levels of gaseous products may improve discrimination between real nuisance fires and false alarm stimuli. The pattern of absence or presence of particular gaseous products with or without detection of aerosols activating smoke detectors, ion, or optical scatter types can be indicative of whether the stimuli arise from fire or non-fire sources. For example, a response from a gas sensor sensitive to a simple hydrocarbon known to be used as aerosol propellant or as a fuel (e.g. butane) without response from another sensitive to more oxygenated products such as CO, methanol, formaldehyde may indicate simple vapor emissions rather than a nuisance fire scenario. Response by a smoke detector coupled with detection of hydrocarbons but not CO may indicate that the signals arise from propellant and aerosols produced by spraying cleaning products, insecticide, air fresheners or hair spray rather than from fire. An absence of a rise in gases other than H₂O vapor may indicate that the aerosol is condensed water droplets associated with bathroom showers, washing equipment, or cooking rather than fire.

Providing sensors that yield a recognizable gaseous output signature of other known false alarm initiating events such as smoking and cooking, including by use of suitable combinations of signal or algorithms for processing output signals, may be used to enhance discrimination between fire and false alarm events. Some known gas sensor systems require catalyst or conductive structures which need to be operated at elevated temperatures to provide adequate response and response times. While such devices provide a range of sensitivities useful for fire gas detection, power requirements have limited the use of such devices to niche applications. Some other gas sensors based on conduction or optical changes in polymeric materials at ambient temperatures have generally shown inadequate response or selectivity to species of interest and in some cases excessive recovery times.

Gas sensors operating according to the invention may be employed in fire detection systems alone or in combination with other sensors used for fire detection. Operation of pulsed energy inputs to the sensor material operated with feedback control according to the present invention may be related to the system polling times used for such fire detection systems.

24 An embodiment of the present invention concerns a fire detector or fire detector system incorporating at least one sensor responsive to a gas for which response is based on operation of a gas sensor according the proposed feedback mode providing primary and secondary sensor responses. The fire detector or detector system may incorporate a group of sensors which in addition to the at least one sensor responsive to a gas for which response is based on operation of a gas sensor according the proposed feedback mode providing primary and secondary sensor responses may include one or more fire detection sensors from a group including heat or temperature sensors including thermistors, smoke sensors based on optical obscuration, smoke sensors based on optical scattering, smoke sensors based on mobility changes in ionized air, optical flame detectors responding to radiant emissions from flames, electrochemical carbon monoxide sensors, and other sensors. An embodiment of the present invention includes a fire detection system incorporating a sensor group where at least one sensor within the sensor group is a gas responsive sensor responsive to a gas for which response is based on operation of a gas sensor according the proposed feedback mode providing primary and secondary sensor responses, and where the fire detection system incorporates a control and evaluation device or system which is connected to the sensor group, set up to evaluate the one or more signals supplied by the sensor group, and if necessary, set up to output at least one control signal. The at least one control signal may be used to activate an alarm or notification process. The at least one control signal may be used to modify the operation or signals of devices within the sensor group.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig.l depicts an embodiment of a gas sensor system comprising a feedback arrangement according to the invention. Fig.2 depicts a further embodiment of a gas sensor system comprising a feedback arrangement according to the invention.

Fig.3 depicts a further embodiment of a gas sensor system comprising a feedback arrangement according to the invention.

Fig.4 depicts a further embodiment of a gas sensor system comprising a feedback arrangement according to the invention. Fig.5 depicts a further embodiment of a gas sensor system comprising a feedback arrangement according to the invention.

Fig.6 depicts a further embodiment of a gas sensor system comprising a feedback arrangement according to the invention. Fig.7 depicts a further embodiment of a gas sensor system comprising a feedback arrangement according to the invention.

Fig.8 depicts a further embodiment of a gas sensor system comprising a feedback arrangement according to the invention.

Fig.9 depicts a further embodiment of a gas sensor system comprising a feedback arrangement according to the invention.

Fig.10 schematically depicts electrical response of a sensor structure and energy input characteristics, especially optical stimuli, as these change on exposure to target gas species, with both conventional operations and with operation according to the invention.

Figure 11 is a schematic diagram of an exemplary fire detector and signaling system incorporating one or more sensors operating in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Fig.l depicts an embodiment of a gas sensor system comprising a feedback arrangement according to the invention, incorporating a sensor structure 1 with a sensitive material 2 linked to a measurement device 3 providing a primary output 4 dependant on exposure to target gas species in a gas 5 which can access the sensitive material 2. A comparator 6, for which inputs are the primary response 4 and a reference value 7, provides output to a control unit or driver 8 which actuates an energy source 9, the output of which is directed onto the sensitive material 2. In the embodiment represented, the energy source 9 is a source of photonic radiation. Output from the driver unit 8 providing a measure of the energy delivered by the source 9 constitutes secondary output 10. In the embodiment represented, the sensor structure 1 incorporating sensitive material 2 is a chemically-sensitive resistor structure provided with two conductive links. Alternatively, in this and other embodiments, the sensor structure may incorporate more conductive links to measurement or operating circuitry for example as in four-point probe arrangements, in transistors such as field effect transistors where the sensitive material is linked to the transistor gate, in Kelvin probe arrangements, or in electrochemical cells.

The reference value 7 may be a predetermined reference value chose in dependence on the properties of the sensor material and the sensitivity required, or may be a value derived from the sensor output as described below with reference to the embodiment of Figure 2, or may be a reference signal supplied by another sensor material component to be described below with reference to the embodiments of Figures 7 and 8, or any combination of these.

The circuit elements performing measurements, comparator, and energy source drive may be readily realized using combinations of circuit elements which may include operational amplifiers and discrete components and possibly computational systems in ways that will be familiar to those moderately skilled in electronic design.

Fig.2 depicts a further embodiment of a gas sensor system comprising a feedback arrangement according to the invention, incorporating a sensor structure 1 with a sensitive material 2 linked to a measurement device 3. Output from the measurement device 3 is combined with a time input or a reference device input 11 in a differential operation 12 to provide a differential primary output 4 dependent on exposure to target gas species in a gas 5 which can access the sensitive material 2. A long term averaging operation 13 on the primary output 4 may provide input to a reference value setting unit 7. A comparator 6, for which inputs are the primary response 4 and reference value unit 7, provides output to control a supply unit 8 which drives an energy source 9, the output of which is directed onto the sensitive material 2. In the embodiment represented, the energy source 9 is a source of photonic radiation. A secondary output 10, providing a measure of the energy delivered by the source 9, may be derived from the output from the driver unit 8 or from a sensor 14 receiving part of the energy output from the source 9.

Fig.3 depicts a further embodiment of a gas sensor system comprising a feedback arrangement constructed according to the invention, wherein the measurement, reference, comparator, and driver units described with reference to Figures 1 and 2 are represented by a single circuit block 40. The circuit block 40 operates a feedback function whereby control of drive output through the energy source 9 to the sensor material 2 derives from values measured from that sensor material, and provides a primary response output 4 and a secondary response output 10, representing respectively the sensor material response to presented target gas species, and a measure of the energy input to the sensor material. In the embodiment represented, the energy source 9 is a source of photonic radiation which is provided with a filter or grating 15 to provide a restriction on wavelength range or ranges impinging on the sensor material. Such selection of radiation wavelength may provide selectivity to the response of the sensor material 2. A porous or permeable filter 16 is provided to restrict material or species able to pass from the environment to the sensor material 2, thereby providing selectivity in species presenting to and therefore capable of generating the sensing response of the sensor material. Fig.4 depicts a further embodiment of a gas sensor system comprising a feedback arrangement constructed according to the invention, wherein the measurement, reference, comparator, and driver units are represented by a single circuit block 40. The circuit block 40 operates a feedback function whereby control of drive output through the energy source 9 to the sensor material 2 derives from values measured from the sensor material, and provides primary and secondary outputs representing respectively the sensor material response to presented target gas species, and a measure of energy input to the sensor material. In the embodiment represented, the energy source 9 comprises multiple sources of photonic radiation which may have different wavelength and intensity characteristics. A porous or permeable filter 16 is provided to restrict material or species able to pass from the environment to the sensor material 2, thereby providing selectivity in species presented to, and therefore capable of generating the sensing response of, sensor material.

Fig.5 depicts a further embodiment of a gas sensor system comprising a feedback arrangement constructed according to the invention, wherein the measurement, reference, comparator, and driver units are represented by a single circuit block 40. The circuit block 40 operates a feedback function whereby control of drive output through the energy source 9 to the sensor material 2 derives from values measured from the sensor material, and provides primary and secondary outputs representing respectively the sensor material response to presented target gas species, and a measure of energy input to the sensor material. Ih the embodiment represented, the energy source 9 comprises a resistive heater linked by a thermally-conductive pathway to the sensor material 2. A porous or permeable filter 16 is provided to restrict material or species able to pass from the environment to the sensor

28 material 2, thereby providing selectivity in species presenting to, and therefore capable of generating the sensing response of, the sensor material.

Fig.6 depicts a further embodiment of a gas sensor system comprising a feedback arrangement constructed according to the invention, wherein the measurement, reference, comparator, and driver units described for figures 1 and 2 are represented by a single circuit block 40. The circuit block 40 operates a feedback function whereby control of drive output through the energy source 9 to the sensor material 2 derives from values measured from the sensor material, and provides primary 4 and secondary 10 outputs representing respectively the sensor material response to presented target gas species, and a measure of energy input to the sensor material. In the embodiment represented, the energy source 9 is a source of photonic radiation, where radiation from the source passes to the sensor material 2 through an at least partially transparent medium 99, which may form a substrate on which the sensor material is formed or deposited. This allows a more convenient placement of porous or permeable filter material 16. Housing and filter material components may be provided with radiation reflective or scattering components or coatings to reduce optical losses and enhance transfer of radiation to the sensor material 2.

Fig.7 depicts a further embodiment of a gas sensor system comprising a feedback arrangement constructed according to the invention, wherein the measurement, reference, comparator, and driver units described for figures 1 and 2 are represented by a single circuit block 40. The circuit block 40 operates a feedback function whereby control of drive output through the energy source 9 to the sensor material 2 derives from values measured from the sensor material, and provides primary and secondary outputs representing respectively the sensor material response to presented target gas species, and a measure of energy input to the sensor material. In the embodiment represented, the sensor structure 1 comprises two sensor material components, one sensor material component 2 being accessible to a gaseous environment, and the other sensor material component 17 being provided with a barrier 18 isolating it from the gaseous environment, so that contact with a target gas species is prevented or much reduced or delayed. Alternatively, the reference sensor component 17 may have a composition rendering it less sensitive or insensitive to a target gas species, for example by absence of catalytic material conferring sensitivity, and in such arrangements the barrier 18 is not required. The two sensor material components 2 and 17 provide sensing and

29 reference structures from which a differential primary output may be derived by the components within the circuit block 40.

Fig.8 depicts a further embodiment of a gas sensor system comprising a feedback arrangement constructed according to the invention, wherein the measurement, reference, comparator, and driver units described for figures 1 and 2 are represented by a single circuit block 40. The circuit block 40 operates a feedback function whereby control of drive output through the energy source 9 to the sensor material 2 derives from values measured from the sensor material, and provides primary and secondary outputs representing respectively the sensor material response to presented target gas species, and a measure of energy input to the sensor material. In the embodiment represented, the sensor structure 1 comprises two sensor material components, one sensor material component 2 being accessible to a gaseous environment, and the other sensor material component 17 being provided with a barrier 18 isolating it from the gaseous environment, so that contact with a target gas species is prevented or much reduced or delayed. Alternatively, the reference sensor component 17 may have a composition rendering it less sensitive or insensitive to target gas species, for example by absence of catalytic material conferring sensitivity, and in such arrangements the barrier 18 is not required. The two sensor material components 2 and 17 are linked by a link structure 19 which allows an output directly related to the difference in response to presented species, which link may be a conductive structure which may be an ionic conductor or may be an electronic structure such as linked field effect transistors. Such structures provide a measurable output relating to differences in chemical or electrochemical potential or to work function differences.

Fig.9 depicts a further embodiment of a gas sensor system comprising a feedback arrangement constructed according to the invention, wherein the measurement, reference, comparator, and driver units described for figures 1 and 2 are represented by a single circuit block 40. The circuit block 40 operates a feedback function whereby control of drive output through the energy source 9 to the sensor material 2 derives from values measured from the sensor material, and provides primary and secondary outputs representing respectively the sensor material response to a presented target gas species, and a measure of energy input to the sensor material. Li the embodiment represented, the energy source 9 is a source of photonic radiation, and the sensor structure 1 comprises a sensor material 2 which changes optical properties on contact with a target gas species and optical components consisting of an

30 emission source 20 and an optical detector 21 acting with components of the circuit block 40 to provide a measure of changes of the optical properties of the sensor material 2. These measurements may be by transmission, absorbance, fluorescence, phosphorescence, or scattering of radiation depending on the material characteristics and placement of the components. The sensor device structure may further incorporate optical filters or gratings (not shown). The emission source 20 may be different from the energy source 9, or these components may be combined or identical.

In each of the embodiments described above, the primary sensor response and/or the secondary sensor response may be used to provide an indication that a predetermined amount of a target gas species has been detected. For example the primary and secondary sensor responses, either individually or together, may use known means such as conventional transduction and signal processing means to provide a change in the electrical output of a measuring device. This electrical output change may then be provided to a display device or as an input to circuits or computation systems which provide display of information relating to those outputs, or to provide a target species presence or concentration measurement, or a notification of events or an alarm condition.

Systems employing multiple sensors according to the invention and the embodiments may be operated together to provide redundancy or sensitivity to multiple target gas species. This may involve different sensor materials, sensor structures, light sources and optical filter or gratings or selectively-permeable filters.

The representations in Figures 1 to 9 show sensor devices and energy sources such as light sources which may be light emitting diodes, lasers, incandescent sources or discharge lamps as discrete components. It may readily be seen that, within the scope of the invention, these may be integrated or combined on common substrates or holders. One skilled in the art would recognize that various combinations of components may be combined differently to provide systems according to the present invention.

Operation of the invention may be understood with reference to the device characteristics illustrated in Figure 10.

Figure 10 schematically depicts the electrical response of a sensor structure and energy input characteristics, especially optical stimuli with, operations according to prior art and with operation according to the invention.

31 This representation includes energy input characteristics for operation according to the invention, where energy inputs are pulsed and feedback control affects the periods between pulses so that the pulse rate or interval provides a measure of the energy input to the sensor material 2 and thus the secondary sensor output. This representation includes energy input characteristics for operation according to the invention where energy inputs are continuous and feedback control affects intensity, so that the energy input intensity provides a measure of the energy input to the sensor material 2 and thus the secondary sensor output

In Figure 10, device characteristics and applied energy inputs are shown on the vertical axis over a period of time represented as the horizontal axis. Exposure to a target gas is initially zero, and exposure is to a fixed target gas concentration for a limited interval 22 before returning to zero. The sensor characteristics shown in Figure 10 are similar to those obtained with resistive carbon nanotube sensors exposed to low concentrations (e.g. around

200 ppm) of ammonia in air, but will be representative of responses of a wide range of sensor materials to which the invention may be applied. The curves and the lines 23 to 27 represent operation according to the prior art. The curves and the lines 28 to 31 represent operations according to the invention.

The solid curve 23 represents the primary response to gas exposure in the absence of an energy input sufficient to stimulate desorption or consumption of an absorbed target gas species. Response rises on exposure, but falls only slowly when exposure ceases. This is a dosimeter type response.

The solid curve 24 represents the primary response to gas exposure where there is a continuous period of energy input represented by the dashed line 25, where such energy input, as by UV illumination, is sufficient significantly to accelerate desorption or consumption of the target gas. This provides a relatively rapid rise of primary signal on exposure to gas, and relatively rapid fall of primary signal when the gas supply is removed. The continuous energy input is not acceptable for some applications.

The solid curve 26 represents the primary response to gas exposure where there is energy input as a series of pulses at fixed intervals represented by the dashed line 27, where pulse duration is small compared with the interval between pulses. This pulse energy input operation can reduce energy load, as the primary sensor response is dependent on pulse duration and interval, the response and recovery times can remain excessive for some applications.

The solid curve 28 represents the primary sensor response to gas exposure where energy input is controlled by a feedback arrangement according to the present invention. The energy input pulse rate, represented as the dashed line 29, is increased as the primary sensor response rises on first exposure to the gas, and falls off only after the primary sensor response falls. Response and recovery times are relatively rapid. Energy requirements are elevated only during and shortly after the gas exposure, allowing an overall low energy consumption where exposure is relatively rare. This allows the interval between pulses during non-exposure periods to be kept relatively long. The pulse interval variation, shown as the dashed line 29, provides the secondary sensor response representative of the quantity of gas sensed, which is supplementary to primary sensor response which may be made small by operation of the feedback arrangement.

The solid curve 30 represents the primary sensor response to gas exposure where energy input is controlled by a feedback arrangement according to the present invention. Energy input intensity, represented as the dashed line 31, is increased as the primary sensor response rises on first exposure to the gas, and falls off as the primary sensor response falls following cessation of exposure. Response and recovery times are relatively rapid. Energy requirements are elevated only during and shortly after the gas exposure, allowing an overall low energy consumption where exposure is relatively rare. This allows intensity of energy inputs during non-exposure periods to be kept low. The intensity variation, shown as the dashed line 31, provides the secondary sensor response representative of the quantity of gas sensed, which is supplementary to the primary sensor response which may be made small by operation of the feedback arrangement. It will be obvious that the operation according to the invention may be achieved by combination of energy input pulse width and amplitude, and that other means of feedback controlled variation, including use of multiple sources or variation in radiation wavelength, fall within the scope of the invention.

Figure 11 is a schematic diagram of an exemplary fire detector and signaling system 100 constructed in accordance with an embodiment of the present invention. The fire detector and signaling system 100 may include one or more fire alarm units 101 spaced about an area to be monitored for fire. Each fire alarm unit 101 may include one or more sensor groups 102 with provision for gas transfer between the sensor groups 102, and an ambient space external to the fire alarm units 101 that is to be monitored for fire. The sensor groups 102 may include, for example, one or an array of gas sensors 104 operated in feedback mode according to the invention, a temperature sensor 106, and a smoke sensor 108. Each fire alarm unit 101 may include one or more apertures 109 (or other access means) through an outer cover 110 that provides for gas transfer between the surrounding environment and its sensor groups 102. In the exemplary embodiment, protection is provided within one or more apertures 109 to one or more sensors of the sensor group 102 from ingress by contaminants including particulate materials, which may damage sensor function. Such protection from ingress of contaminants may be provided by gas-permeable membranes and/or filters positioned within one or more of the apertures 109.

In various embodiments, the apertures 109 provide selective access to gases in the environment to protect against damaging contaminants, and to facilitate improving selectivity of the sensors within the sensor groups 102. The apertures 109 also provide protection from radiation, for example!, optical radiation, which may affect the sensor output or induce degradation of the components of the fire alarm unit 101. The outer cover 110 is configured to facilitate reducing excessive air movement impinging on the sensors of the sensor groups 102, which may induce stresses affecting the sensor output or induce degradation of the sensors.

At least one of the sensors 104 is responsive to gases within the ambient space based on the chemically-responsive electronic properties of the incorporated sensor material. Sensor materials incorporated in the gas sensors 104 are, or have been, rendered chemically sensitive, such that the one or more structures respond by a change of electronic properties to the presence of one or more predetermined gases, for example, fire detection indicative gases or vapors. Such gases and vapors may include those that are generated or consumed by combustion or by fuel pyrolysis, or are associated with false fire alarm conditions. The sensor groups 102 may additionally include one or more other types of fire detection sensors such as temperature sensors,¹ heat sensors including thermistors, ionization-type smoke sensors, smoke sensors based on mobility changes in ionized air, smoke sensors based on optical obscuration, smoke sensors based on optical scattering,

34 electrochemical gas sensors including electrochemical carbon monoxide sensors, and flame detectors responding to radiant emissions from flames.

The sensor groups 102 may be selected to detect emissions of at least one of the products associated with a fire including combustion gas, smoke, flame, and heat. One or an array of gas sensors 104 is selected to provide one or more output signals related to the presence of gases associated with a fire using a change in the electronic properties of nano- particulate materials, and especially using a change in the electronic properties of carbon- based nano-structures. Signals relative to a concentration and/or presence of the products associated with a fire may be transmitted to a local signal assessment and control unit 112 that includes a microprocessor and an analog-digital converter for converting the signals supplied by the sensor groups 102 into corresponding digital signals. The signals received from each of the sensor groups 102 may be evaluated, and a result of the evaluation may be transmitted through a communications bus system 114 to a system assessment and control unit 116. Such evaluation may include a combination or integration of the various sensors in the sensor groups 102 with sensor signal conditioning and evaluation systems with output to alarm or notification devices.

In the exemplary embodiment, an overall signal assessment and control function may be performed using the system assessment and control unit 116 at a single location, hi an alternative embodiment, the overall signal assessment and control function may be performed using the system assessment and control unit 116 and/or one or more local signal assessment and control units 112 communicatively coupled together in a distributed network. In the exemplary embodiment, a plurality of gas sensors 104 may be configured to respond to two or more gases, wherein those gases may include gases generated in fires or associated with false alarm stimuli. The gases to which the gas sensors 104 respond may be selected based on the materials present in the monitored space, and the gases those materials generate when combusting or being subject to pyrolysis. A range of gaseous emissions may be associated with various fire types depending on fuel type, ignition conditions, fire progression, and ventilation.

A plurality of sensors may provide sufficient information to permit a range of conditions to be recognized to indicate fire or non-fire situations. Signals received from the sensor groups 102 may be processed to condition, modify, or combine the signals, and the

35 resultant is transmitted to the system assessment and control unit 116 and/or to one or more alarm, notification, or display units. The sensors of the sensor groups 102 may incorporate a low power requirement to permit operation in battery-operated equipment, and/or in systems where a plurality of sensors is powered by one electrical circuit.

While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention.

36

Claims

1. A chemical sensor apparatus comprising: a sensor material having an intrinsic material property sensitive to a target species; access means allowing transfer of the target species to the sensor material; monitor means for monitoring the intrinsic material property to provide a primary sensor response; energy input means for inputting energy to the sensor material; feedback control means for controlling the energy input means in dependence upon the primary sensor response; and means for providing a measure of the energy input to the sensor material, the measure of the energy input constituting a secondary sensor response.

2. Apparatus as claimed in claim 1, further comprising indicator means for providing an indication that the target species has been detected, the indicator means being actuable by the primary sensor response and/or the secondary sensor response.

3. Apparatus as claimed in claim 1 or claim 2, wherein the feedback control means is such that a deviation of the primary response from a predetermined value controls the energy input to the sensor material so as to return the primary response towards the predetermined value.

4. Apparatus as claimed in claim 3, wherein the predetermined value is a value corresponding to zero or a low value of target species concentration.

5. Apparatus as claimed in any one of claims 1 to 4, wherein the energy input means is constituted by a heat source or a photon source.

6. Apparatus as claimed in any one of claims 1 to 5, wherein the energy input means is operated in either a continuous or pulsed mode.

7. Apparatus as claimed in any one of claims 1 to 6, wherein the feedback control means controls the energy input means in dependence upon the primary sensor response and a reference value.

8. Apparatus as claimed in claim 7 when appendant to claim 2, wherein the reference value is the predetermined value.

9. Apparatus as claimed in claim 7 or claim 8, wherein the feedback control means includes a comparator whose inputs are the primary sensor response and the reference value, the output of the comparator controlling the energy input means.

10. Apparatus as claimed in any one of claims 7 to 9, wherein the reference value is chosen in dependence upon the primary sensor response during a period when the target species concentration is low.

11. Apparatus as claimed in any one of claims 7 to 9, wherein the reference value is constituted by the output of a long term averaging of the primary sensor response.

12. Apparatus as claimed in any one of claims 1 to 8, further comprising a second sensor material, and a barrier positioned between the second sensor material isolating it from the target species.

13. Apparatus as claimed in any one of claims 1 to 8, further comprising a second sensor material, the second sensor material being a material less sensitive to the target species than the first-monitored sensor material, the monitor means being arranged to monitor the intrinsic material property of the second sensor material.

14. Apparatus as claimed in either of claims 12 and 13 when appendant to any one of claims 7 to 9, wherein the output of the monitoring of the intrinsic material property of the second sensor material constitutes the reference value.

15. Apparatus as claimed in any one of claims 1 to 14, wherein the access means comprise a filter structure having selective permeability to the target species or relative impermeability to an interfering substance.

16. A fire detector comprising a chemical sensor apparatus as claimed in any one of claims 1 to 15.

17. A fire detector as claimed in claim 16 when appendant to claim 2, wherein the indicator means is such as to generate an alarm signal when a predetermined amount of the target species has been detected.

18. A fire detector as claimed in claim 16 or claim 17, further comprising at least one other type of fire-detecting sensor.

19. A detector as claimed in claim 18, wherein said at least one other type of fire- detector sensor is one or more of a temperature sensor and a smoke sensor.

20. A detector as claimed in any one of claims 16 to 19, further comprising a local signal assessment and control unit.

21. A fire detector system comprising a plurality of fire detectors, each as claimed in any one of claims 16 to 20, the system further comprising a system assessment and control unit.

22. A fire detector system as claimed in claim 21 when appendant to claim 20, wherein the system assessment and control unit is communicatively coupled to each of the local assessment and control units.

23. A method of sensing a chemical, the method comprising the steps of monitoring a sensor material having an intrinsic material property sensitive to a target species to provide a primary sensor response; inputting energy to the sensor material; using feedback control means for controlling the energy supplied to the sensor material in dependence upon the primary sensor response; providing a measure of the energy input to the sensor material, said measure constituting a second sensory response; and providing an indication that the target species has been detected, in dependence upon the primary sensor response and/or the secondary sensor response.