Smoke sensing apparatus of the light scattering type
FIELD OF THE INVENTION 
     This invention relates to the art of signal processing in a smoke sensing apparatus of the type sensing smoke on the basis of the quantity of light scattered by the smoke, and more particularly to an apparatus of the type described above which is suitable for use in, for example, an automobile for automatically starting operation of an air conditioner provided in the automobile. 
     BACKGROUND OF THE INVENTION 
     A prior art smoke sensing apparatus of the above type has such a construction that light emitted from a light emitting element and scattered by smoke is converted by a light receiving element into an electrical signal which is applied, after being amplified by an amplifier, to a comparator to be compared with a predetermined level. The comparator generates a control signal actuating an air conditioner when the level of the comparator input signal becomes higher than the predetermined level. However, the prior art smoke sensing apparatus has had the following problems: Firstly, the sensitivity of the smoke sensing apparatus tends to change depending on the temperature due to temperature-dependent variations of the characteristics of the elements including the light emitting and receiving elements and amplifier. Secondly, the level of the comparator input signal tends to vary due to degradations or secular variations of the characteristics of the light emitting and receiving elements due to a change in the quantity of reflected light not attributable to smoke, resulting in a reduced sensitivity or mal-operation of the apparatus due to the signal level variation. 
     SUMMARY OF THE INVENTION 
     With a view to solve the prior art problems points out above, it is a primary object of the present invention to provide a smoke sensing apparatus in which signal processing means are provided so that a high sensitivity can be stably maintained even when a signal level variation not attributable to smoke occurs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an electrical connection diagram showing the structure of a first embodiment of the smoke sensing apparatus according to the present invention. 
     FIG. 2 is a time chart illustrating the operation of the apparatus shown in FIG. 1. 
     FIG. 3 is an electrical connection diagram showing the structure of a second embodiment of the present invention. 
     FIG. 4 is a time chart illustrating the operation of the apparatus shown in FIG. 3. 
     FIGS. 5 and 6 are electrical connection diagrams showing the structure of a third embodiment and a fourth embodiment respectively of the present invention. 
     FIG. 7 is a time chart illustrating the operation of the apparatus shown in FIGS. 5 and 6. 
     FIG. 8 is an electrical connection diagram showing the structure of a fifth embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a first embodiment of the smoke sensing apparatus according to the present invention comprises a pulse oscillator 11, a switching element 12, a light emitting element 13, a light receiving element 14, a resistor 15 for generating a signal voltage, an amplifier 16, a sample and hold circuit 17, an integrating resistor 18, an integrating capacitor 19, a differential amplifier 20, and a comparator 21. 
     FIG. 2 is a time chart showing voltage waveforms appearing at various points of the circuit shown in FIG. 1. Curves 29, 30, 31 and 32 represent voltage waveforms appearing at points c, d, e and f in FIG. 1 respectively. A line 33 and a curve 34 represent a reference voltage applied to the comparator 21, and an output signal or control signal appearing at a point g (the output terminal) respectively. 
     With a view to obviate the prior art problems, that is, the temperature-dependent sensitivity change, th degradations or secular variations of the light emitting and receiving elements and the change in the quantity of light reflected inside the sensor, the present invention contemplates to effectively compensate such variations for ensuring that the smoke sensing apparatus can operate with a high sensitivity. Referring to FIG. 1, light emitted from the light emitting element 13 is reflected by dust particles such as those of smoke, and the reflected light incident upon the light reception part of the light receiving element 14 is converted into an electrical signal. The light emitting element 13 is actuated by a pulse signal applied from the pulse oscillator 11 through the switching element 12 so that it can generate a high optical output without being shortened in its useful service life. The electrical output signal appearing from the light receiving element 14 is converted by the resistor 15 into a voltage signal which is amplified by the amplifier 16. The resultant output signal of the amplifier 16 appearing at the point c has a waveform as shown by the dotted curve 29 in FIG. 2. 
     Such an output signal of the amplifier 16 is sampled in the sample and hold circuit 17 by the output of the oscillator 11 in synchronism with the optical output generation timing of the light emitting element 13. The resultant output signal of the sample and hold circuit 17 appearing at the point d has a waveform as shown by the solid curve 30 in FIG. 2. The voltage output signal of the sample and hold circuit 17 charges the capacitor 19 with a large time constant through the resistor 18, and the resultant voltage output signal appearing at the point e has a waveform as shown by the one-dot chain curve 31 in FIG. 2. 
     The section A in FIG. 2 corresponds to the absence of any smoke. It will be seen that the output voltage of the sample and hold circuit 17 appearing at the point d (the curve 30 in FIG. 2) does not change, and, since the capacitor 19 is full charged in such a case, the voltage appearing at the point e (the curve 31 in FIG. 2) is equal to that appearing at the point d. On the other hand, the section B in FIG. 2 indicates indicates the condition when smoke intervenes in the sensor part of the smoke sensing apparatus thereby increasing the quantity of reflected light. Consequently, the output voltage of the amplifier 16 appearing at the point c (the curve 29 in FIG. 2) increases, and the output voltage of the sample and hold circuit 17 appearing at the point d (the curve 30 in FIG. 2) increases correspondingly. At the same time, the capacitor 19 discharges through the resistor 18 to increase the voltage level appearing at the point e (the curve 31 in FIG. 2). However, this voltage increase is slow and gradual since the time constant is selected to be large. 
     The voltage appearing at the point e is applied to the differential amplifier 20 together with the voltage appearing at the point d. The differential amplifier 20 amplifies the voltage differential. Therefore, its output voltage appearing at the point f has a waveform as shown by the solid curve 32 in FIG. 2. When this output voltage exceeds the reference voltage V th  (represented by the line 33 in FIG. 2) of the comparator 21, the comparator output signal (the curve 34 in FIG. 2) indicative of sensed smoke appears at the point g as a control signal. 
     Although the voltage appearing at the point d may be subject to a gradual change owing to the sensitivity change due to the temperature-dependent variations of the characteristics of the light emitting and receiving elements and amplifier, the degradations or secular variations of the characteristics of the light emitting and receiving elements after a long time of use, and the change in the quantity of reflected light inside the sensor, the voltage appearing at the point d is equal to the voltage appearing at the point e in the absence of smoke. Therefore, no output appears from the differential amplifier 20 in the absence of smoke, and, only when the voltage at the point d increases for a short length of time due to intervention of smoke does the the differential amplifier 20 generate its output. Thus, the smoke sensing apparatus of the present invention can operate with the desired high sensitivity over a long period of time without the necessity for inspection and repair. 
     FIG. 3 shows a second embodiment of the present invention. In FIG. 3, the same reference numerals are used to designate the same parts appearing in FIG. 1. In the first embodiment, the pulse output of the amplifier 16 is converted into the d.c. voltage by the sample and hold circuit 17. In the second embodiment shown in FIG. 3, a peak hold circuit 40 including an operational amplifier 41, a diode 42 and a capacitor 43 together with a discharge resistor 44 is provided for converting a pulse waveform 48 into a saw-tooth waveform 49 as shown in FIG. 4. Then, the combination of a second operational amplifier 45, a second resistor 46 and a second capacitor 47 smoothes the saw-tooth waveform 49 into a d.c. voltage waveform 50 as shown in FIG. 4. Thus, the second embodiment achieves the effect similar to that exhibited by the first embodiment. 
     FIGS. 5 and 6 show a third embodiment and a fourth embodiment respectively of the present invention. In FIGS. 5 and 6, the same reference numerals are used to designate the same parts appearing in FIG. 1. In each of the third and fourth embodiments, a circuit for instantaneously charging the capacitor 19 in response to the turning-on of the power supply is added so that smoke can be sensed with a high sensitivity as soon as the power supply is turned on. In the case of the first embodiment shown in FIG. 1, the capacitor 19 is not charged at the time of turning-on of the power supply. As soon as the power supply is turned on, the voltage at the point e starts to gradually approach the voltage at the point d as shown by the solid curve 55 in FIG. 7. Until the voltage at the point e attains the level of the voltage at the point d, no output appears from the differential amplifier 20, since it does not quickly respond to a slight increase in the voltage at the point d when the quantity of smoke is slight. 
     In contrast, in the third embodiment in which a diode 51 is connected in parallel with the resistor 18, the capacitor 19 is charged as soon as the power supply is turned on, and the voltage at a point 1 becomes immediately equal to the voltage at a point k as shown by the one-dot chain curve 56 in FIG. 7, so that smoke present in a very slight quantity can be reliably sensed. In the fourth embodiment shown in FIG. 6, an operational amplifier 53 is connected in series with a diode 52 so that even the forward voltage of the diode can be ignored. 
     It is apparent that the function of part of the constituent elements of the aforementioned embodiments may be replaced by programmed processing by a digital computer. 
     FIG. 8 shows a fifth embodiment of the present invention. In FIG. 8, the same reference numerals are used to designated the same parts appearing in FIG. 1. Referring to FIG. 8, in response to the turning-on of an ignition key switch 23, the battery voltage +B is applied to a regulated voltage circuit (not shown) which applies a voltage regulated at a value required for operation of various circuits of the smoke sensing apparatus, thereby placing the smoke sensing apparatus in operation. 
     On the other hand, in response to the turning on of the ignition key switch 23, a monostable multivibrator 24 is triggered to generate a time signal including pulses having a pulse period of about 1 sec. In response to the application of this time signal, an analog switch 25 is closed at the time interval of about 1 sec. Therefore, in a short period of time after the turning-on of the key switch 23, the output voltage of the mean value circuit appearing at a point 1 equals the sensed voltage appearing at a point k, and, then, the apparatus starts its smoke sensing operation described above.