Abstract:
A photoelectric smoke detecting apparatus capable of generating analog data accurately indicating a smoke density regardless of a presence or absence of contamination. A control unit for outputting the analog data corresponding to the smoke density based on a detection value derived from the output of a smoke sensor includes a smoke density arithmetic module having a characteristic function for converting the detection value to a smoke density value, a zero-density detection value storage device for storing a detection value at a time point when the smoke density is zero as a zero-density detection value, a change ratio arithmetic module designed for arithmetically determining a ratio of change representing a change of the zero-density detection value, and a compensation arithmetic module designed for compensating conversion characteristic for converting the detection value to the smoke density value based on the ratio of change. The compensation arithmetic module is designed to cause the smoke density arithmetic module to generate the smoke density value in such a manner to counter a change of an output characteristic of the detection value for of the smoke density, which change depends on the ratio of change. A self- or auto-compensation function against aged deterioration of the detection value due to contamination is realized.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a photoelectric smoke detecting apparatus (also known as the smoke detector) for generating analog data concerning smoke density indicating occurrence of fire or the like event with the aid of a microcomputer or microprocessor. More particularly, the present invention is concerned with a photoelectric smoke detecting apparatus which is imparted with a self- or auto-compensation capability of automatically or spontaneously compensating for a time-dependent change or aged deterioration of a detection characteristic (light reception sensitivity) of a light receiving element incorporated in a smoke sensor of the smoke detecting apparatus due to contamination thereof. 
     2. Description of Related Art 
     Heretofore, a well known type of photoelectric smoke detecting apparatus is arranged such that a light emitting element is disposed within a well-ventilated chamber of a smoke sensor and is electrically driven periodically at a predetermined time interval for enabling a microcomputer or microprocessor to fetch the detection signal from the output of the smoke sensor thereby processing the same in order to decide whether a fire event is taking place in a place where the smoke sensor is installed or to detect the density of smoke prevailing in that place. 
     More specifically, the detection signal outputted from the light receiving element of the smoke sensor disposed for receiving light rays scattered by smoke particles is amplified by an amplifier circuit provided in association with the smoke sensor. The amplified signal is supplied to a microcomputer or microprocessor after analog-to-digital conversion (A/D conversion), whereon the digital data as fetched by the microcomputer is converted to corresponding smoke density data, which is then sent out in the form of an analog data signal to receiver equipment installed at a center station. 
     In the photoelectric smoke detecting apparatus of this type, contamination of an inner wall of a casing, a light emitting element and/or the light receiving element which constitute the smoke sensor will bring about a variation or change in the sensitivity characteristic of the smoke sensor and hence a change in the detection signal level which of course depends on the color of a contaminant. 
     Thus, when the contamination of the smoke sensor is detected, there arises the necessity of cleaning the sensor in order to restore the original state thereof to thereby prevent erroneous or false detection of the fire state. When such cleaning is difficult or practically impossible for some reason, it will then be required to take other appropriate measures such as exchange of the smoke sensor itself. 
     For having better understanding of the concept underlying the present invention, description will first be directed to the conventional photoelectric smoke detecting apparatus known heretofore by reference to FIGS. 6 and 7 of the accompanying drawings. FIG. 6 is a functional block diagram showing schematically a structure of a conventional photoelectric smoke detecting apparatus, whereas FIG. 7 is a circuit diagram of the same. 
     Referring to FIG. 6, the conventional photoelectric smoke detecting apparatus includes a smoke sensor  10  which is composed of a light emitting element  11  and a light receiving element  12 . A shielding plate  13  is interposed between the light emitting element  11  and the light receiving element  12 . It is noted that the light emitting element  11 , the light receiving element  12  and the shielding plate  13  are disposed within a chamber enclosed by a labyrinth inner wall  14  which is employed for implementing the smoke sensor in an antireflection structure. By virtue of this structure, the light receiving element  12  can receive only the scattered light rays L 2  of the light rays L 1  emitted by the light emitting element  11 , whereby the detection value D indicating the smoke density within the chamber enclosed by the labyrinth inner wall  14  can be acquired in the form of a detection signal outputted from the smoke sensor  10 . 
     A control unit  20  which may be constituted by a microcomputer or microprocessor is designed or programmed to process the detection signal D outputted from the smoke sensor  10  to thereby output an analog data signal E indicative of the smoke density prevailing within the smoke sensor  10 . At this juncture, it should be mentioned that a plurality of photoelectric smoke detecting apparatuses each composed of the smoke sensor  10  and the control unit  20  may be disposed at various locations within a building or the like where the smoke detection is required. 
     The output data signals (analog data signals E) of the individual photoelectric smoke detecting apparatuses installed at various places are supplied to receiver equipment  30  installed at a center station through signal transmission via signal lines (not shown). 
     As can be seen in FIG. 6, the control unit  20  includes a driving circuit  21  for generating a driving pulse signal P for driving the light emitting element  11 , and A/D (analog to digital) converter  22  for converting the detection value D into digital data Dd and a smoke density arithmetic module  23  for determining arithmetically a smoke density value VKe on the basis of the digital data Dd by referencing a characteristic function table  23 T incorporated in the smoke density arithmetic module  23 . The control unit  20  is provided with a sender or transmission circuit  24  for sending or transmitting the smoke density value VKe in the form of an analog data signal E to the receiver equipment  30  of the center station. 
     In the characteristic function table  23 T, there are stored characteristic functions each approximated by a positive linear function (represented by a straight line), as described later on. 
     Next, description will be made by reference to FIG. 7 in which reference characters  10  to  13 ,  20 , L 1  and L 2  denote same items as those described above by reference to FIG.  6 . 
     Referring To FIG. 7, the microcomputer  40  constituting a major part of the control unit  20  includes a CPU (Central Processing Unit) which serves for the functions of the A/D converter  22  and the smoke density arithmetic module  23  shown in FIG.  6  and other peripheral components. A light emitting circuit  41  corresponds to the driving circuit  21  shown in FIG.  6  and serves for electric power supply to the light emitting element  11  as well as pulse-like light emission control thereof. A light receiving circuit  42  is electrically connected to the light receiving element  12 , and an amplifier circuit  43  is connected to the output of the light receiving circuit  42  for amplifying the detection signal, the amplified detection signal being then inputted to the microcomputer  40 . 
     An oscillator circuit  44  is provided for supplying a clock pulse signal CK to the microcomputer  40 . Further provided is an EEPROM (Electrically Erasable Programmable Read-Only Memory)  45  which is connected to the microcomputer  40  for storing preset data such as addresses and others. 
     An alarm lamp  46  is provided as an alarming means for generating an alarm upon occurrence of abnormality such as a fire. The alarm lamp  46  is driven or electrically energized by a lighting circuit  47  under the control of the microcomputer  40 . 
     A receiving circuit  48  serves for receiving signals such as external signals sent from the receiver equipment  30  (see FIG.  6 ), which signal are then inputted to the microcomputer  40 . On the other hand, the output signals of the microcomputer  40  are sent to external apparatus via a transmitting circuit  49 . Incidentally, the receiving circuit  48  and the transmitting circuit  49  functionally correspond to the transmission circuit  24  shown in FIG.  6 . 
     A constant-voltage circuit  50  is provided for supplying electric power to the microcomputer  40  and others incorporated in the control unit  20  and other discrete circuits  41  to  49 . 
     A diode bridge circuit  51  serves for nullifying the poralities of terminals when the control unit  20  and the receiver equipment  30  of the center station (see FIG. 6) are interconnected by a signal line (not shown). 
     FIG. 8 is a signal waveform diagram for illustrating detection levels or pulses outputted from the light receiving element  12  in correspondence to the driving pulses P, respectively, in the state where the smoke density is zero when the driving pulses P are applied to the light emitting element  11 . 
     As can be seen in FIG. 8, a train of driving pulses P includes first pulses PI for fire detection and a second pulse P 2  for fault detection, wherein the second pulse P 2  is at a higher level than the first pulse P 1 . 
     At this juncture, it should be mentioned that the second pulse P 2  serves for the function for increasing or intensifying the light emission of the light emitting element  11  in addition to the function of the first pulse P 1 . As the alternative, the second pulse P 2  may be generated by increasing intermittently the amplification factor of the amplifier circuit  43  connected to the output of the light receiving circuit  42 . 
     The output period or cycle τ of the first pulses P 1  and the second pulses P 2  is set at an equi-interval (e.g. two seconds), wherein the second pulse P 2  for fault detection is generated once for four pulses (e.g. at the interval of eight seconds). 
     With the conventional photoelectric smoke detecting apparatus of the structure described above by reference to FIGS. 6 and 7, the smoke sensor  10  is driven in response to the driving pulse train P illustrated in FIG. 8, whereby emission of light rays L 1  and reception of the scattered light rays L 2  are carried out repetitively, as a result of which the detection value D is outputted from the light receiving element  12 . 
     On the other hand, the control unit  20  fetches the detection value D through the medium of the light receiving circuit  42 , the amplifier circuit  43  and the A/D converter  22  to thereby generate the analog data E indicative of the smoke density in accordance with the characteristic function table  23 T, the analog data signal E as generated being then sent to the receiver equipment  30  via the transmitting circuit  49  as shown in FIG. 7 (corresponding to the transmission circuit shown in FIG.  6 ). 
     Since the second pulse P 2  is contained in the driving pulse train P, the light emitting element  11  emits the light rays L 1  at a higher output level once for eight seconds. In response to the emitted light rays L 1  of the high intensity, the light receiving element  12  outputs the detection value D which can be used for detecting the noise level internally of the smoke sensor  10 . 
     At this juncture, it should be added that the characteristic function stored in the characteristic function table  23 T remains unchanged in the initial state without being corrected even when the characteristic function of the smoke sensor  10  has changed. 
     According to the International Standards FDK38U as well as the Japanese Standards FDK038-X, it is recommended that the fire detection or fault detection be performed at the output period τ of about two seconds and that the fault detection be performed once for four cycles (i.e., periodically at an interval of about eight seconds). 
     As is apparent from the foregoing description, in the photoelectric smoke detecting apparatus known heretofore, no compensating measures are taken or adopted against the change of the detection level. Consequently, when the characteristic function of the smoke sensor has changed, the analog data E indicating accurately the smoke density can no more be made available, giving rise to a problem that the fire state can not be determined with reasonable accuracy and reliability in the center station equipped with the receiver equipment  30 . 
     SUMMARY OF THE INVENTION 
     In the light of the state of the art described above, it is an object of the present invention to provide a photoelectric smoke detecting apparatus which is capable of making available the analog data signal indicating accurately the smoke density regardless of contamination of the smoke sensor by imparting to the photoelectric smoke detecting apparatus the function or capability for automatically or spontaneously compensating the time-dependent change of the detection value derived from the output of the light receiving element of the smoke sensor due to the contamination thereof. 
     In view of the above and other objects which will become apparent as the description proceeds, there is provided according to a general aspect of the present invention a photoelectric smoke detecting apparatus which includes a smoke sensor composed of a light emitting element and a light receiving element accommodated within a chamber enclosed by a labyrinth inner wall for outputting from the light receiving element a detection signal indicative of a detection value corresponding to a smoke density prevailing within the chamber enclosed by the labyrinth inner wall. The smoke detecting apparatus further includes a control unit for outputting analog data corresponding to the smoke density on the basis of the detection value. The control unit is comprised of a smoke arithmetic module having a characteristic function for converting the detection value to a smoke density value, a zero-density detection value storage device for storing a detection value at a time point when the smoke density is zero as a zero-density detection value, a change rate (i.e. ratio) arithmetic module designated for determining arithmetically a ratio of change (also referred to as the change ratio) of the zero-density detection value, and a compensation arithmetic module designed for compensating conversion characteristic for converting the detection value to the smoke density value by taking into account the above-mentioned ratio of change. Further, the compensation arithmetic module is designed as cause the smoke density arithmetic module to generate a smoke density value in such a manner that change of output characteristic of the detection value for the smoke density, which change bears dependency on ratio of the change, can be canceled out. 
     In a preferred mode for carrying out the present invention, the change ratio arithmetic module may be designed to arithmetically determine the change ratio as a value derived by dividing the zero-density detection value by an initial value thereof, wherein the compensation arithmetic module is so designed as to correctively increase the detection value as the change ratio of the zero-density detection value increases or alternatively decreases from a value “1 (one)”. 
     In another mode for carrying out the present invention, the change ratio arithmetic module should preferably be designed to arithmetically determine the change ratio in terms of an absolute value derived from division of a change quantity of the zero-density detection value from the initial zero-density detection value by the initial value, wherein the compensation arithmetic module is so designed as to correctively increase the detection value in dependence on increasing of the change ratio of the zero-density detection value. 
     In yet another mode for carrying out the present invention, the compensation arithmetic module should preferably be so designed as to correct the detection value in dependence on the change ratio and establish a detection value after compensation by adding or, alternatively, subtracting the change quantity of the zero-density detection value. 
     In still another mode for carrying out the present invention, the change ratio arithmetic module should preferable be designed to arithmetically determine the change ratio as a value derived by dividing the zero-density detection value by an initial value thereof, wherein the compensation arithmetic module is designed to correctively establish a slope of the currently valid characteristic function to be smaller than an initial slope thereof as the change ratio increases or alternatively decreases from a value “1 (one)”. 
     In a further mode for carrying out the present invention, the change ratio arithmetic module should preferably be designed to arithmetically determine the change ratio in terms of an absolute value derived from division of a change quantity of the zero-density detection value from the initial zero-density detection value by this initial value, wherein the compensation arithmetic module is designed to correctively establish a slope of the currently valid characteristic function to be smaller than an initial slope thereof in dependence on increasing of the change ratio. 
     In a yet further mode for carrying out the present invention, the compensation arithmetic module should preferably be designed to correct the slope of the characteristic function in dependence on the change ratio and establish a characteristic function after compensation by adding to or, alternatively, subtracting from the zero-density detection value the change quantity of the zero-density detection value. 
     In a still further preferred mode for carrying out the present invention, the control unit may include an analog-to-digital converter for converting the detection value to digital data, wherein the smoke density arithmetic module is designed to convert the digital data to the smoke density value. 
     In another preferred mode for carrying out the present invention, the compensation arithmetic module-may include a compensation range discriminating means for deciding whether the change ratio falls within a predetermined range for compensation and generating fault information when the change ratio departs from the predetermined range for compensation. 
     In yet another mode for carrying out the present invention, the compensation arithmetic module should preferably be designed that when a state in which the change ratio falls within the predetermined range for compensation has continued for a predetermined time duration, a value derived through average processing of the zero-density detection value over the predetermined time duration is employed as a final change ratio. 
     In still another preferred mode for carrying out the present invention, the compensation arithmetic module may include a compensating value setting module for fixedly placing therein a compensating value which corresponds to the change ratio. 
     In a further mode for carrying out the present invention, the compensation arithmetic module may preferably include a correcting value setting means for establishing a correcting value for correcting the compensating value in dependence on the zero-density detection value. 
     In a yet further mode for carrying out the present invention, the correcting value setting means may preferably include a correcting value storing means for storing the correcting value, wherein the correcting value can externally altered through input manipulation. 
     By virtue of the arrangements described above, there can be implemented a photoelectric smoke detecting apparatus which is capable of generating analog data which accurately indicates the smoke density regardless of contamination of the smoke sensor owing to the feature of self- or auto-compensation for aged deterioration or time-dependent change of the detection value outputted from the light receiving element of the smoke sensor due to contamination thereof 
     The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the course of the description which follows, reference is made to the drawings, in which: 
     FIG. 1 is a functional block diagram schematically showing a structure of a photoelectric smoke detecting apparatus according to a first embodiment of the present invention. 
     FIG. 2 is a view for illustrating changes of characteristic functions of detection values for smoke densities and a compensating arithmetic procedure according to the present invention. 
     FIG. 3 is a characteristic diagram for graphically illustrating a relation between change ratio of a slope of a characteristic function and change ratio of a zero-density detection value in the photoelectric smoke detecting apparatus. 
     FIG. 4 is a flow chart for illustrating an ordinary smoke density detection procedure in the photoelectric smoke detecting apparatus according to the first embodiment of the present invention. 
     FIG. 5 is a flow chart for illustrating a processing procedure executed upon detection of fault in the photoelectric smoke detecting apparatus according to the first embodiment of the invention. 
     FIG. 6 is a functional block diagram schematically showing a structure of a conventional photoelectric smoke detecting apparatus. 
     FIG. 7 is a circuit diagram schematically showing a circuit arrangement of the same. 
     FIG. 8 is a waveform diagram for illustrating detection levels or pulses outputted from a light receiving element of a smoke sensor in response to driving pulses. 
     FIG. 9 is a view for illustrating change of a characteristic function of a detection level for a smoke density in the state where a light emitting element and/or a light receiving element of the smoke sensor has been contaminated. 
     FIG. 10 is a view for illustrating change of a characteristic function of a detection level for the smoke density in the state where a labyrinth inner wall has been contaminated in white. 
     FIG. 11 is a view for illustrating change of a detection level for a smoke density in the state where a whole optical system of a smoke sensor has been contaminated in white. 
     FIG. 12 is a view for illustrating change of a detection level for a smoke density in the state where a labyrinth inner wall of the smoke sensor has been contaminated in black. 
     FIG. 13 is a view for illustrating change of a characteristic function of a detection level for a smoke density in the state where a whole optical system of a smoke sensor has been contaminated in black. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described in detail in conjunction with what is presently considered as preferred or typical embodiments thereof by reference to the drawings. In the following description, like reference characters designate like or corresponding parts throughout the several views. 
     Embodiment 1 
     FIG. 1 is a functional block diagram showing schematically a structure of the photoelectric smoke detecting apparatus according to a first embodiment of the present invention. In the figure, items similar to those described hereinbefore (see FIG. 6) are denoted by like reference characters with the equivalent being designated by like reference numerals affixed with “A”, and detailed description thereof is omitted. 
     It should first be mentioned that the arrangement of the photoelectric smoke detecting apparatus which is omitted from illustration in FIG. 1 is substantially same as that described hereinbefore, and that the circuit arrangement shown in FIG. 7 is also adopted in the photoelectric smoke detecting apparatus according to the instant embodiment of the invention. 
     Further, the timing at which the driving pulses P are applied to the light emitting element  11  is substantially same as the one illustrated in FIG.  8 . Changes of the characteristic function of the smoke sensor  10  due to contamination will be described later on by reference to FIGS. 9 to  13 . 
     Referring to FIG. 1, in the photoelectric smoke detecting apparatus according to the first embodiment of the invention, the control unit designed by  20 A in the instant case includes in addition to the driving circuit  21 , the A/D converter  22 , the smoke density arithmetic module  23 A and the transmission circuit  24  described previously in conjunction with the related art a zero-density detection value arithmetic module  25 , an initial zero-density detection value storage device  26 , a change ratio arithmetic module  27 , and a compensation arithmetic module  28 . 
     The zero-density detection value arithmetic module  25  is designed or programmed to arithmetically determine the detection value when the smoke density Ke is zero as a zero-density detection value VN on the basis of the digital data Dd of the detection value D outputted from the light receiving element  12  in response to the second pulse P 2  (see FIG.  8 ). On the other hand, the initial zero-density detection value storage device  26  is employed for storing the initial value of the zero-density detection value VN (i.e., the value before the smoke sensor  10  undergoes contamination) as the initial zero-density detection value VNO. 
     The ratio module  27  is designed or programmed to arithmetically determine, on the basis of the zero-density detection value VN and the initial zero-density detection value VNO, a ratio between the zero-density detection value VN and the initial zero-density detection value VNO (i.e., VN/VNO) or an absolute value acquired by dividing (or normnalizing) magnitude of the change (hereinafter also referred to as the change quantity) of the zero-density detection value VN from the initial zero-density detection value VNO by the initial value VNO (i.e., |(VN−VNO)/VNO|) as the ratio of change, (also referred to as the change ratio) ΔVN of the zero-density detection value. 
     The compensation arithmetic module  28  is designed or programmed to arithmetically determine a compensating value C for compensating the characteristic of conversion of the digital data Dd of the detection value D to the smoke density value VKe on the basis of the ratio or change ratio ΔVN. 
     The compensating value C determined arithmetically by the compensation arithmetic module  28  is inputted to the smoke density arithmetic module  23 A which responds thereto by generating the smoke density value VKe such that change in the output characteristic of the detection value D for the smoke density Ke, which change corresponds to the change ratio ΔVN, can be canceled out (see FIGS.  11  and  13 ). More specifically, the compensation arithmetic module  28  generates the compensating value C which correctively increases the digital data Dd of the detection value D correspondingly as the change ratio ΔVN of the zero-density detection value increases. To this end, the compensation arithmetic module  28  incorporates therein a compensating value setting module  28 T for storing fixedly the compensating values C corresponding to the change ratios ΔVN, respectively. 
     At this juncture, description will be made in detail of the change of the sensitivity characteristic of the smoke sensor. At first, tendency patterns of contamination will be discussed by referring to FIGS. 9 to  13  which illustrate tendencies of changes of the characteristic function in the level Vd of the detection value signal D (detection level) for the smoke density Ke [%/m] on a patten-by-pattern basis. 
     FIG. 9 is a view showing a tendency of the change of the detection level Vd 1  in the case where surfaces (lenses) of the light emitting element  11  and the light receiving element  12  are contaminated with a material or substance in white or black. 
     Further, FIG. 10 shows a tendency of the change of the detection level Vd 2  in the case where the labyrinth inner wall  14  is contaminated with a white material, while FIG. 11 is a view showing a tendency of the change of the detection level Vd 3  in the case where the whole smoke sensor  10  (the light emitting and light receiving elements  11  and  12  and the labyrinth inner wall  14 ) is contaminated with a white material. Parenthetically, the tendency of the change of the detection level Vd 3  illustrated in FIG. 11 can be approximated through synthesization of the characteristics illustrated in FIGS. 9 and 10, respectively. 
     Furthermore, FIG. 12 shows a tendency of the change of the detection level Vd 2  in the case where the labyrinth inner wall  14  is contaminated with a black material, while FIG. 13 is a view showing a tendency of the change of the detection level Vd 3  in the case where the smoke sensor  10  as a whole is contaminated with a black material. Parenthetically, the tendency of the change of the detection level Vd 3  illustrated in FIG. 13 can be approximated through synthesization of the characteristics illustrated in FIGS. 9 and 12, respectively. 
     In each of FIGS. 9 to  13 , a single-dotted broken line represents the initial characteristic function (i.e., characteristic function before being contaminated), and a solid line represents the characteristic function after contamination, wherein each of the characteristic functions is approximated by a linear function of a positive slope. Furthermore, in each of these figures, the characteristic function within a negative or minus range of the smoke density Ke which is not practically used for the data conversion is indicated by a broken line only for convenience of illustration for indicating the straight line representing the characteristic function as a whole. 
     Now, reference will first be made to FIG.  9 . As can easily be appreciated, the transmission quantity of light decreases at a predetermined rate as the contamination of the light emitting element  11  and the light receiving element  12  makes progress. Consequently, the slope (detection sensitivity of the sensor) of the straight line (solid line) representing the characteristic function of the detection level Vd 1  after contamination becomes more gentle when compared with that of the characteristic function before the contamination represented by the single-dotted broken line regardless of the color of the contaminant. 
     On the other hand, in the case illustrated in FIG. 10, the reflection quantity of light (i.e., quantity of light reflected by the labyrinth inner wall  14 ) which may also be referred to as the noise level increases by a predetermined value due to white contamination of the labyrinth inner wall  14 . As a result of this, the characteristic function of the detection level Vd 2  after contamination as represented by a solid line is shifted in the direction in which the detection level increases when compared with the characteristic function in the state not contaminated (represented by a single-dotted broken line). 
     Further in the case illustrated in Fig. 11, the characteristic function of the detection level Vd 3  after contamination represented by a solid line is shifted in the direction in which the detection level increases although the characteristic function after contamination exhibits a gentle slope as compared with the characteristic function in the state not contaminated as represented by a single-dotted broken line. Consequently, the level VN (zero-density detection value) for the smoke density Ke of zero increases beyond the initial zero-density detection value VNO. 
     By contrast, in the case illustrated in FIG. 12, the reflection quantity of light (noise level) decreases by a predetermined value due to black contamination of the labyrinth inner wall  14 , the characteristic function of the detection level Vd 2  after contamination as represented by a solid line is shifted in the direction in which the detection level decreases when compared with the characteristic function in the state not contaminated (represented by a single-dotted broken line). 
     Finally, in the case illustrated in FIG. 13, the characteristic function of the detection level Vd 3  after contamination (represented by a solid line) is shifted in the direction in which the detection level decreases after contamination and exhibits a gentle slope when compared with the characteristic function in the state not contaminated (represented by a single-dotted broken line). Consequently, the zero-density detection value VN decreases as compared with the initial zero-density detection value VNO. 
     FIG. 2 is a view for illustrating changes of the characteristic function of the level (detection level) Vd of the detection value D for the smoke density Ke [%/m] and a compensation arithmetic procedure on the presumption that the smoke sensor as a whole is contaminated with white material (corresponding to the case illustrated in FIG.  11 ). 
     Referring to FIG. 2, a single-dotted broken line YO represents a characteristic function before contamination (i.e., initial characteristic function) while a solid line Yd represents the characteristic function after the contamination (i.e., current characteristic function). Further, a double-dotted broken line Yc 1  represents a characteristic function obtained after a slope compensation arithmetic operation or procedure. The double-dotted broken line Yc 1  shows that the detection level Vd is correctively increased with a predetermined amplification factor which corresponds to the ratio of change (change ratio) ΔVN of the zero-density detection value VN. 
     As can be seen in FIG. 2, the slope of the characteristic function Yc 1  undergone the slope compensation arithmetic procedure as represented by the double-dotted broken line coincides with the slope of the initial characteristic function YO (represented by the single-dotted broken line). 
     In practice, the difference between the zero-density detection value VNc of the characteristic function after the slope compensation arithmetic and the initial zero-density detection value VNO is canceled out through translation or parallel displacement procedure. 
     FIG. 3 is a characteristic diagram for illustrating graphically a relation between the change ratio ΔA of the slope of the characteristic function and the change ratio ΔVN of the zero-density detection value VN. For simplification of the arithmetic operation, the change ratio ΔVN of the zero-density detection value VN is defined as VN/VNO with the change ratio ΔA of the slope of the characteristic function being defined by A/AO (where AO represents the slope of the initial characteristic function and A represents the slope of the characteristic function after contamination). 
     As can be seen in FIG. 3, as the deviation of the change ratio ΔVN of the zero-density detection value VN from the initial value “1−0” (VN=VNO) increases or decreases, the change ratio ΔA of the slope A of the characteristic function after contamination decreases. 
     In FIG. 3, the change ratio ΔVN of the zero-density detection value VN is taken along the abscissa (x-axis) while the change ratio ΔA of the slope A is taken along the ordinate (y-axis). Further, the function of the change ratio ΔA of the slope A within a range given by ΔVN≦1.0 is represented by a solid line Y 1 , while the function of the change ratio ΔA of the slope A within a range given by ΔVN≧1.0 is represented by a solid line Y 2 . In that case, the functions Y 1  and Y 2  can be approximated with the undermentioned expressions (1) and (2), respectively. 
     
       
           Y   1 = O. 1 X +0.9  (1) 
       
     
     
       
           Y   2 =− O. 1 X +1.1  (2) 
       
     
     Further, as can be seen in FIG. 3, a region extending around the change ratio ΔVN of “1.0” is defined as a sensitivity compensation range, while regions departed relatively far from the change ratio ΔVN of “1.0” are defined as fault ranges, respectively, in which a fault decision procedure is executed separately from the sensitivity compensation procedure which is carried out within the sensitivity compensation range. 
     In this conjunction, it should be mentioned that a temporal factor is involved in the fault range discrimination procedure validated within the fault range although detailed description thereof is omitted. Upon decision or detection of fault of the smoke sensor  10 , a relevant message is issued for prompting exchange of the smoke sensor  10  without carrying out the sensitivity compensation procedure. 
     It should further be added that the compensation arithmetic module  28  includes a fault range discriminating means for deciding whether the change ratio ΔVN falls within a predetermined range for compensation and generating fault information when the change ratio ΔVN departs from the predetermined range for sensitivity compensation (i.e., falls within the fault range), whereby a fault message is issued without carrying out the sensitivity compensation. 
     Next, referring to flow charts shown in FIGS. 4 and 5 together with FIGS. 2,  3 ,  7  to  13 , operation of the photoelectric smoke detecting apparatus according to the first embodiment of the invention will be described. 
     Referring to FIG. 4, it is first decided by the control unit  20 A in a step S 1  whether the fault detection procedure or routine is validated on the basis of the timing of the driving pulses P (see FIG.  8 ). 
     When it is determined in the step S 1  that the fault decision routine is validated at the output timing of the second pulse P 2  for the fault detection (i.e., when the decision step S 1  results in affirmation “YES”), then a compensating value determining routine or procedure (see FIG. 5) is validated (step S 2 ), whereon the routine illustrated in FIG. 4 comes to an end. 
     On the other hand, when it is determined in the step S 1  that the fault detection routine is not to be validated (i.e., when the decision step S 1  results in negation “NO”), this means that the first pulse P 1  for the fire detection (see FIG. 8) is generated. Consequently, the microcomputer  40  constituting a major part of the control unit  20 A (see FIG. 7) outputs the first pulse P 1  to the light emitting circuit  41 . 
     In response to the output of the light emitting circuit  41 , the light emitting element  11  is electrically energized to emit light rays while the control unit  20 A fetches the detection value D from the output of the light receiving element  12  via the A/D converter  22 . In succession, the control unit  20 A determines whether a compensation flag FC has been set (step S 3 ). 
     When it is determined in the step S 3  that the compensation flag has been set (i.e., when the decision step S 3  results in affirmation “YES”), then the compensation arithmetic module  28  executes the slope compensation arithmetic operation for the characteristic function Yd such that the characteristic function Yd represented by the solid line in FIG. 2 is angularly shifted to the characteristic function Yc 1  represented by the double-dotted broken line in the same figure (step S 4 ). 
     In succession, the compensation arithmetic module  28  arithmetically determines the translating (or parallel displacing) compensation value (step S 5 ) to thereby perform the translating compensation arithmetic operation so that the characteristic function Yc 1  represented by the double-dotted broken line in FIG. 2 is parallel-shifted or translated to the characteristic function YO represented by the single-dotted broken line in the same figure (step S 6 ). 
     In conjunction with the processing( step S 4 , it is supposed, by way of example, that the initial value VNO of the zero-density detection level of the photoelectric smoke detecting apparatus has already been set at the time point at which the sensitivity thereof was set in a manufacturing factor upon shipping therefrom. Then, the slope compensating value may be determined on the basis of the ratio of change ΔVN of the current zero-density detection value VN from the initial value VNO in the place where the photoelectric smoke detecting apparatus is installed, and then the slope or sensitivity compensation is performed for the current detection level Vd. 
     In this manner, the slope (sensitivity) of the characteristic function Yd (represented by the solid line) which has become more gentle due to contamination of the smoke sensor is so corrected that it coincides at least substantially with the slope of the initial characteristic function YO represented by the double-dotted broken line Yc 1 , as indicated by the double-dotted broken line Yc 1 . 
     In a step S 5 , the translating compensation value (parallel-displacement) is arithmetically determined on the basis of the initial zero-density detection value VNO and the slope compensating value (amplification factor) as determined. 
     In succession, in a step S 6 , the characteristic function Yc 1  of the detection level Vd resulting from the slope compensation (as represented by the double-dotted broken line Yc 1  in FIG. 2) is corrected by using the translating compensation value as determined. More specifically, the zero-density detection value VNc is shifted in the direction toward the origin (0) by the translating compensation value so that the current zero-density detection value VNc does actually coincide with the initial zero-density detection value VNO. 
     Through the procedure described above, the characteristic function of the digital data Dd based on the detection value D is corrected so that it coincides with the initial characteristic function (linear function). Thus, the conversion of the digital data Dd to the smoke density value VKe can be executed with very high accuracy on the basis of the initial characteristic function (linear function) by means of the smoke density arithmetic module  23 A. 
     In this conjunction, it is presumed that the smoke density Ke is taken along the abscissa (x-axis), while the detection level Vd is taken along the ordinate (y-axis) as shown in FIG.  2 . Then, the initial characteristic function YO represented by the single-dotted broken line in FIG. 2 as well as the characteristic function Yd after the contamination represented by the solid line in FIG. 2 can be approximated by the undermentioned expressions (3) and (4). 
     
       
           YO=AO·X+VNO   (3) 
       
     
     
       
           Yd=A·X+VN   (4) 
       
     
     where AO represents the slope of initial characteristic function, and A represents the slope of the post-contamination characteristic function. 
     On the other hand, the slope-compensated characteristic function Yc 1  (double-dotted broken line) can be approximated by the following expression (5). 
     
       
           Yc   1 = AO·X+VNc   (5) 
       
     
     Furthermore, the characteristic function Yc 2  can be approximated by the above-mentioned expression (3) after the translating compensation. It will be seen that the characteristic function Yc 2  coincides perfectly with the initial characteristic function YO after the translating or parallel-shifting compensation. 
     At this juncture, it should be mentioned that the initial zero-density detection value VNO (constant) represents the detection level (so-called noise level) in the state where no smoke exists and that the slope AO represents the sensitivity (ratio of change) of the detection level Vd in response to the change of the smoke density Ke. 
     In the processing routine illustrated in FIG. 4, the compensation processing steps S 4  to S 6  are executed when the zero-density detection value VN changes due to the so-called aged deterioration (i.e., deterioration as a function of time lapse) which may be regarded as being attributable to the contamination among others. In that case, the compensating value C is so selectively determined as to reduce the change ratio ΔVN. 
     The determined compensating value C is then used for determining a product with the value derived from subtraction or addition of the zero-density detection value VN from or to the detection value, whereon the conversion to the smoke density Ke is effectuated. Description which follows will be made on the assumption, by way of example only, that subtraction from the zero-density detection value VN is performed. In this case, a value which is obtained from a further correction is performed so that the initial characteristic function (straight line) passes through the origin. 
     More specifically, in the smoke density arithmetic module  23 A, a value (Vdc−VNO) obtained from subtraction of the initial zero-density detection value VNO from the detection level Vdc in succession to the compensation arithmetic operation performed on the basis of the compensating value C (steps S 4  to S 6  in FIG. 4) is converted to the smoke density value VKe, by referencing the characteristic function table  23 T (step S 7 ). 
     The smoke density value VKe is then supplied to the transmission circuit  24  to be converted to the analog data signal E which is then sent or transmitted to the receiver equipment  30 . Thus, the ordinary smoke density detection;,processing activated in response to the first pulse P 1  comes to an end. 
     As is apparent from the above, the ordinary smoke density Ke is determined by dividing by the slope AO the value obtained by subtracting the zero-density detection value VNO from the detection level Vdc after compensation thereof (digital data level). 
     Next referring to FIG. 5, description will be directed to the compensating value determining routine (step S 2  in the processing procedure illustrated in FIG. 4) which is executed when the driving pulse train P indicates the fault detection routine (i.e., the routine executed in response to the second pulse P 2 ). 
     First in a step S 11  shown in FIG. 5, decision is made whether the fault state is currently taking place. When it is decided that no fault occurs (i.e., when the decision step S 11  results in negation “NO”), then decision is made as to occurrence of a fire (step S 12 ). 
     When occurrence of the fault or the fire is decided in the step S 11  or step S 12  (i.e., when the decision step S 11  or S 12  results in “YES”), then the arithmetic operation for determining the compensating value C is skipped and the variables for arithmetically determining the compensating value such as accumulated zero-density detection value VNi and compensating counter value CNT are cleared to zero (step S 13 ), whereupon the processing routine illustrated in FIG. 5 is terminated. 
     On the other hand, in the case where it is decided in the step S 12  that no fire is taking place (i.e., when the decision step S 12  results in “NO”), then the compensating value C is arithmetically determined. To this end, the accumulated zero-density detection value VNi is updated to a value added with the currently obtained detection level Vd (step S 14 ) and the compensating counter value CNT is incremented (step S 15 ). 
     Subsequently, decision is made in a step S 16  whether the compensating counter value CNT has reached a value which corresponds to a standard update time period a (e.g. about 12 hours). When CNT&lt;α (i.e., when the decision step S 16  results in “NO”), then the processing routine illustrated in FIG. 5 is terminated. 
     By contrast, when it is decided in the step S 16  that CNT≧α (i.e., when the step S 16  results in “YES”), then a mean zero-density detection value VNm is determined on the basis of the accumulated zero-density detection value VNi and the compensating counter value CNT in accordance with the undermentioned expression (6) in a step S 17 . 
     
       
           VNm=VNi/CNT   (6) 
       
     
     In succession, the change ratio arithmetic module  27  determines the change ratio ΔVN on the basis of the mean zero-density detection value VNm and the initial zero-density detection value VNO in accordance with the undermentioned expression (7) (step S 18 ). 
     
       
         Δ VN=VNm/VNO   (7) 
       
     
     Subsequently, decision is made in a step S 19  whether the absolute value of deviation of the change ratio ΔVN from the initial value (=1) thereof is equal to or greater than a reference value β for performing the compensation. When this decision step results in “NO”, i.e., |1−ΔVN|&lt;β, the compensation flag FC is cleared or reset in a step S 20 , whereon the step S 13  is resumed. 
     By contrast, when the decision step S 19  results in affirmation “YES” (i.e., when |1−ΔVN|≧β), the compensation flag FC is set to “1” in a step S 21 , which is then followed by a step S 22  of determining a slope compensating value C 1  on the basis of the change ratio ΔVN by referencing the conversion table stored in the compensation arithmetic module  28 . 
     At this juncture, it should be mentioned that in the arithmetic processing step S 18 , the change ratio ΔVN of the zero-density detection value may be determined directly as the absolute value of the change ratio from the initial zero-density detection value VNO. In that case, the change ratio ΔVN can directly be compared with the reference value β. 
     In that case, by taking into account the relation between the change ratio ΔVN and the slope change ratio ΔA after contamination (i.e., linear proportional relation shown in FIG.  3 ), a corresponding table which allows the slope to be compensated straightforwardly may be prepared and stored in the ROM incorporated in the compensation arithmetic module  28  so that the slope compensating value C 1  can selectively be determined simply by referencing the table. 
     At this juncture, it should also be added that although the reference value β for effectuating the compensation can be set arbitrarily, it is preferred to set the reference value β to a value very close to zero so that the compensation can be validated even for the change of a small magnitude. 
     Finally, a correction processing of the slope compensating value C 1  (step S 23 ) is executed in succession to the step S 22  in consideration of the possibility that error is contained in the slope compensating value C 1  determined on the basis of the change ratio ΔVN. Thereafter, the step S 13  is resumed. 
     More specifically, in the step S 23 , a correcting value C 2  for correcting further the slope compensating value C 1  is determined for finely correcting the slope compensating value C 1  on the basis of the initial zero-density detection value VNO and the slope compensating value C 1 , and the corrected slope compensating value C 1  is established as the final sensitivity compensating value. 
     The correcting value C 2  employed for finely adjusting the slope compensating value C 1  may be set to an optimal value in advance through input operation with the aid of an external input device such as a keyboard and stored in the EEPROM incorporated in the compensation arithmetic module  28 . Incidentally, it is to be mentioned that the correcting value C 2  is a predetermined value which bears no relation to the change ratio ΔVN. 
     As mentioned above, the sensitivity compensating value determined from the change ratio ΔVN is stored in a memory incorporated in the compensation arithmetic module  28 . Accordingly, at the succeeding detection timing corresponding to the succeeding first pulse P 1 , the smoke density value VKe can be determined with high accuracy and reliability on the basis of the compensated detection level Vdc. 
     In that case, the characteristic function Yc 1  given by the expression (5) mentioned previously and undergone the slope compensation can be approximated in view of the expression (4) as follows: 
     
       
           Yc   1 =( A·X+VN )× C   1 × C   2   (8) 
       
     
     In the expression (8), the zero-density detection value VN is certainly known from the mean zero-density detection value VNm. However, since the slope A after contamination (i.e., post-contamination slope A) is unknown, the characteristic function Yc 1  is compensated for by making use of the slope compensating value C 1  and the correcting value C 2 . 
     At this juncture, it should also be added that the slope AO and the initial zero-density detection value VNO appearing in the expression (3) mentioned hereinbefore are known from the initial characteristic and that the zero-density detection value VN appearing in the expression (4) is also known from the mean zero-density detection value VNm of the detection level Vd. 
     The characteristic function Yc 1  given by the expression (8) is compensated through the translation processing described hereinbefore so that the condition that VN=VNO can be satisfied. However, in the characteristic function Yc 2  after the translating compensation, it is only required that the value of VN×C 1 ×C 2  appearing in the expression (8) coincides with that of the initial zero-density detection value VNO. Thus, the expression (8) can be approximated by the following expression (9): 
     
       
           Yc   2 = Yc   1 +( VNO−VN×C   1 × C   2 )  (9) 
       
     
     In the above expression (9), the term VNO−VN×C 1 ×C 2  can be rewritten as follows: 
     
       
           VNO−VN×C   1 × C   2 = VNO ×(1 −ΔVN×C   1 × C   2 )  (10) 
       
     
     As is obvious from the above expression (10), all the parameters assume known values. 
     Through the compensation arithmetic procedure described above, the final straight line Yc 2  after the sensitivity compensation can approximately be given by the undermentioned expression (11). 
     
       
           Yc   2 =( C   1 × C   2 × A ) X+VNO   (11) 
       
     
     When the slope (C 1 ×C 2 ×A) appearing in the expression (11) satisfies the relation given by the following expression (12), this means that the compensation has been carried out so that coincidence with the initial characteristic is realized. Namely, 
     
       
           C   1 × C   2 × A=AO   (12) 
       
     
     By the way, the update time period a for the slope compensating value C 1  may be determined by two parameters K 1  and K 2  stored in the EEPROM. Presuming, by way of example, that K 1 =100 and K 2 =54, the update time period α is then 12 hours=8 (sec)×100×54=43200 seconds. 
     In general, the update time period α may variably be set within a range of 8 seconds to 520200 seconds or 144.5 hours (=8 sec.×255×255). 
     Similarly, the reference value β of the change ratio ΔVN for effectuating the sensitivity compensation may variably be set in dependence on a parameter K 3  stored in the EEPROM. By way of example , when K 3 =95, the reference value β may be so set or selected that the sensitivity ation can be validated for the change ratio greater than 5%, i.e., when the change ratio ΔVN to or smaller than 95% (ΔVN≦95%). 
     The change ratio ΔVN can variably be set within a range of zero to 100%. The various parameter values mentioned above can be stored in the EEPROM. 
     Furthermore, as is apparent from the steps S 11  to S 13  shown in FIG. 5, the processing for updating the slope compensating value C 1  is not executed in the fault state where breakage, deviation from the upper/lower limit values or the like event occurs or in the case where the fire is taking place with the alarm lamp  46  (see FIG. 7) being lit. 
     In that case, the slope compensating value C 1  is held at the value validated immediately before occurrence of the fire or fault state. Upon restoration of the ordinary state, the compensation is performed with the value held at the time point immediately before the restoration. Subsequently, when the normal state has continued for the update time period a, the slope compensating value C 1  is updated. 
     The slope compensating value C 1  makes disappearance when the control unit  20 A is reset and thus the compensation is not carried out (the slope compensating value is not written in the EEPROM) until the update time period at has elapsed. 
     As will now be appreciated, when the light emitting element  11  and the light receiving element  12  of the smoke sensor  10  have been contaminated to such extent that the zero-density detection value VN changes from the initial zero-density detection value VNO, the compensation arithmetic module  28  sets the slope compensating value C 1  and the fine correcting value C 2  for correctively increasing the detection level Vd. 
     The smoke density arithmetic module  23 A converts the value obtained by subtracting the initial zero-density detection value VNO from the compensated detection level Vdc into the smoke density value VKe, which is then sent to the receiver equipment of the center station as the analog data signal E via the transmission circuit  24 . 
     Thus, with the receiver equipment  30  installed at the center station, the smoke density can constantly be detected discriminatively with high reliability on the basis of the analog data signal E representing the smoke density value VKe with enhanced accuracy even in the state where the smoke sensor  10  is contaminated. 
     Embodiment 2 
     In the case of the photoelectric smoke detecting apparatus according to the first embodiment of the invention, the compensation arithmetic module  28  is designed or programmed to arithmetically determine the compensating value for increasing the value of the detection level Vd on the basis of the change ratio ΔVN of the zero-density detection value VN so that the characteristic function Yd after contamination may coincide with the initial characteristic function YO. However, the compensation arithmetic module  28  may alternatively be designed or programmed to arithmetically determine a compensating value for decreasing the slope of the characteristic function for conversion of the detection value to the smoke density value VKe. 
     In that case, the compensation arithmetic module  28  is designed or programmed to arithmetically determine the compensating value C for compensatively correcting the slope of the characteristic function employed for converting the detection level Vd to the smoke density value VKe to be smaller than the initial slope AO in dependence on the increase of the change ratio ΔVN. 
     Further, in addition to the correction of the slope of the characteristic function in dependence on the change ratio ΔVN, the compensation arithmetic module  28  adds or subtracts through translation the change quantity of the zero-density detection value VN so that the characteristic function after compensation is compatible with the detection level Vd after contamination. 
     Furthermore, in conjunction with the photoelectric smoke detecting apparatus according to the first embodiment, description has been made exemplarily on the assumption that compensation for the change of the detection level Vd is performed after contamination of the smoke sensor with a white material (see FIG.  2 ). However, such compensation procedure can equally be applied to the compensation for the change oft he detection level Vd after contamination of the smoke sensor with a black material (see FIG.  13 ). In this case, the reliability of smoke and fire detection can equally be enhanced significantly. 
     Many features and advantages of the present invention are apparent from the detailed description and thus it is intended by the appended claims to cover all such features and advantages of the apparatus which fall within the true spirit and scope of the invention. Further, since numerous modifications and combinations will readily occur to those skilled in the art, it is not intended to limit the invention to the exact construction and operation illustrated and described. 
     By way of example, the foregoing description of the illustrated embodiments of the invention has been directed to the so-called analog type smoke/fire detecting apparatus or system in which the analog data signal E is generated to be sent to the center station through the medium of the transmission circuit  24 . However, such arrangement may equally be adopted in which the smoke density value VKe is directly made use of for discriminatively deciding the occurrence of fire event and the result of the decision is sent to the center or monitor station through the transmission circuit  24 . 
     Thus, many modifications, variations and equivalents of the present invention are possible in the light of the foregoing description. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.