Output correction system for analog sensor

This invention is directed to an output correction system for an analog sensor which outputs an analog signal corresponding to a quantity of state. The output correction system for the analog sensor comprises a control section which receives an output from the analog sensor obtained under condition where the quantity of state is zero and an output from the analog sensor obtained under pseudo-condition equivalent to a certain quantity of state; a first arithmetic section for calculating a gradient on the basis of the output under the zero condition and the output under the pseudo-condition; a storage section for storing output characteristics defined by said gradient; and a second arithmetic section for calculating a quantity of state corresponding to the output from the analog sensor on the basis of the output characteristics defined by the gradient.

BACKGROUND OF THE INVENTION AND RELATED ARTS 
This invention relates to a system for correcting an output of an analog 
sensor which outputs an analog signal corresponding to a quantity of state 
a physical quantity such as a smoke density or a temperature. 
As an output correction system for an analog sensor, there have been known 
a zero adjusting system and a span adjusting system. For example, in the 
case where a current of 4 to 20 mA is output for a change in a temperature 
or a smoke density, amplification characteristics of an output amplifier 
provided in the analog sensor are adjusted to adjust a zero point and a 
span (linear adjustment) of output characteristics. 
However, in such a conventional output correction system, it is necessary 
for each analog sensor to adjust its output characteristics and thus it 
takes much time to set completely all of the sensors. And also this makes 
the adjustment operation complicated and prevents accurate analog outputs 
from being obtained. 
SUMMARY OF THE INVENTION 
The present invention has been achieved to obviate the problems involved in 
the conventional techniques and it is an object of the present invention 
to provide an output correction system for an analog sensor which is 
capable of providing a true quantity of state or true value of a physical 
quantity from an analog output of an analog sensor, irrespective of the 
output characteristics of the analog sensor. 
To attain the object, the present invention features an output correction 
system for an analog sensor which outputs an analog signal corresponding 
to a given quantity of state, comprising a first arithmetic section which 
detects an output from the sensor when the quantity of state is zero and 
an output from the sensor when a pseudo-condition is produced equivalent 
to a predetermined quantity of state, and calculates a gradient on the 
basis of said output under the condition of the zero quantity of state and 
said output under said pseudo-condition. The system further comprises a 
second arithmetic section for computing a quantity of state corresponding 
to an output of the analog sensor on the basis of the sensor output 
characteristics defined by said gradient.

PREFERRED EMBODIMENTS OF THE INVENTION 
Preferred embodiments of the present invention will now be described, 
referring to the drawings. 
According to a first embodiment illustrated in FIGS. 1 to 7, a correcting 
system for an output of an analog sensor comprises a central signal 
station 1 and a plurality of analog fire detectors 3 which are connected 
in parallel with each other to a pair of power/signal lines 2a, 2b derived 
from the central signal station 1. The central signal station 1 includes a 
transmission unit 4 which controls transmission of analog data from the 
analog fire detectors 3 by polling and a central processing unit (CPU) 5 
which corrects the analog data obtained by polling and makes a fire 
determination on the basis of the corrected analog data. 
The analog fire detector 3 employed in the present invention may be a 
scattered-light type photoelectric smoke detector as illustrated in FIG. 3 
which detects a density of smoke caused by a fire in the form of an analog 
signal amount. 
As illustrated in FIG. 3, LED 7 of a light-emitting element and a 
photodiode 8 of a photo detector are mounted oppositely on a holder 6 
disposed within a smoke detecting chamber of the detector at such angles 
that light from LED 7 is not directly impinged upon the photodiode 8. The 
light from LED is irregularly reflected by particles of smoke entering a 
smoke detecting area 9 and the scattered light is incident upon the 
photodiode 8 to produce an analog signal corresponding to the density of 
smoke. The analog fire detector 3 further has a test LED 10 mounted on the 
holder at a position opposite to the photodiode 8 so that the photodiode 8 
may receive the light from the test LED 10 directly. 
This test LED is adapted to emit a light amount corresponding to the amount 
of scattered light obtained at a predetermined smoke density (for example, 
a smoke density of 5%/m which is a critical density for giving a fire 
detection signal). With this setting, the photodiode 8 outputs an analog 
signal corresponding to the smoke density of 5%/m. 
The amount of light may be adjusted by a variable resistor 12 to provide a 
pseudo-condition of entering smoke of the predetermined density by the 
test LED 10. The adjustment for producing the psuedo-smoke density by the 
test LED 10 is carried out as follows. When the assembling of an analog 
photoelectric smoke detector has been completed at a factory, smoke of the 
predetermined density (for example, a smoke density of 5%/m) is actually 
introduced to the smoke detector to measure an analog output (for example, 
an analog output current) obtained from the smoke detector at the 
predetermined smoke density. Subsequently, the test LED 10 is driven to 
emit light under the condition where no smoke enters the detector and then 
the amount of light emitted by the test LED 10 is adjusted by the variable 
resistor 12 until the analog output current of the detector is equivalent 
to that produced by smoke having the predetermined density. 
Once the adjustment of the light amount of the test LED has been completed, 
light of an amount corresponding to the scattered light obtainable upon 
entering of smoke having the predetermined density may be supplied to the 
photodiode 8 by driving the adjusted test LED 10 and without actually 
introducing smoke of the predetermined density into the detector. Thus, a 
pseudo-condition equivalent to that in which smoke of the predetermined 
density is in the detector can be produced. 
In this connection, it is to be noted that since the test LED 10 is 
disposed near the photodiode 8, the amount of light will hardly be changed 
even after a long use. This assures that a constant pseudo-condition of 
the predetermined smoke density is always produced by driving the test LED 
10. 
FIG. 4 is a block diagram of a circuit arrangement of an analog 
photoelectric smoke detector to which the correction system of the present 
invention having an arrangement for producing the pseudo-condition is 
applied. 
In FIG. 4, 13 is a light-emitting circuit for driving LED 7 to emit light 
intermittently with a predetermined period. 14 is a photodetecting circuit 
which receives, by the photodiode 8, light scattered by smoke entering the 
detector and outputs, to a transmission input/output circuit 15, an analog 
current having characteristics such that the current increases linearly in 
proportion to an increase of smoke density, for example, the output 
current is 4 mA at a smoke density of 0%/m and 25 mA at a smoke density of 
5%/m, i.e., a critical density for giving a fire detection signal. The 
transmission input/output circuit 15 discriminates its calling from the 
central signal station 1 through polling from the transmission unit 4 
provided in the central signal station 1 as illustrated in FIG. 1 and 
transmits an analog signal corresponding to a smoke density by allowing an 
analog current based on the output from the photodetecting circuit 14 to 
flow through the power/signal lines 2a, 2b derived from the central signal 
station 1 when the transmission input/output circuit 15 discriminates its 
calling. The transmission input/output circuit 15 drives the test LED 10 
to emit light through a test light-emitting circuit 16 upon receipt of a 
light emission drive signal for the test LED 10 from the central signal 
station 1 as will be described in detail later. The variable resistor 12 
and the test LED 10 are connected in series to an output of the test 
light-emitting circuit 16. More particularly, the test light-emitting 
circuit 16 is driven to emit light through test light emission control by 
the central signal station 1 or operation of a manual switch 17 to produce 
a pseudo-condition corresponding to smoke of a predetermined density, for 
example, a density of 5%/m, entering the detector. 
The details of CPU 5 provided within the central signal station 1 will be 
described. 
As illustrated in FIG. 2, CPU 5 comprises a control section 5a, a first 
arithmetic section 5b, a storage section 5c, a second arithmetic section 
5d and a fire determining section 5e. CPU 5 corrects analog data obtained 
through polling by the transmission unit 4 and makes a fire determination 
on the basis of the analog data obtained through the correction 
processing. 
The correction processing is carried out on the basis of the output 
characteristics of an analog sensor as shown in FIG. 5. In FIG. 5, the 
abscissa indicates a smoke density and the ordinate indicates an output 
current. Output characteristics expected for an analog sensor are linear 
characteristics as indicated by a broken line 18 which, for example, 
provide an output current of 4 mA at a smoke density of 0%/m and an output 
current of 25 mA at a smoke density of 5%/m, the critical density for 
giving a fire detection signal. 
However, an actual analog photoelectric smoke detector can not always have 
characteristics fully conformable to the desired characteristics 18. The 
actual output characteristics vary between individual detectors. 
Therefore, the following correction processing is carried out by CPU 5 so 
as to always obtain a true smoke density from the output current of the 
detectors even if the individual detectors have characteristics deviated 
from the expected characteristics 18. 
First, an analog output current Io (for example, Io=5 mA) is detected under 
a condition where the smoke density is zero. 
Then, the light amount of the test LED 10 is adjusted to a predetermined 
smoke density Ds (for example, Ds=5%/m) and the test LED 10 is driven to 
emit light to produce a pseudo-condition of smoke density of 5%/m. 
Thereafter, the detector output current Is obtained under this condition 
is measured. The adjustment and detection are carried out by the control 
section 5a. 
Subsequently, a gradient K of a straight line defining the actual output 
characteristic 20 as indicated by a solid line is computed by the first 
arithmetic section 5b on the basis of the zero output Io=5 mA and the 
pseudo-output Is=20 mA according to the following formula: 
EQU K=Ds/(Is-Io) 
Since Ds=5%/m, Is=20 mA and Io=5 mA, K will be 0.33. 
When the gradient K defining the actual output characteristics 20 has been 
obtained, the gradient constant K and the zero current Io data are stored 
in the storage section 5c and the data is transmitted to the second 
arithmetic section 5d. 
With respect to an output current Ix obtained thereafter, the second 
arithmetic section 5d carries out the following calculation 
EQU Dx=K(Ix-Io) 
to obtain a smoke density Dx corresponding to the actual output current Ix. 
The correction processing as described above assures that true smoke 
density can always be obtained on the basis of the actual analog output 
current and that accurate fire determination can be carried out on the 
basis of the thus obtained true smoke density. 
Now, the entire operation of the output correction system for an analog 
sensor will be described referring to FIGS. 6 and 7. 
FIG. 6 is a flowchart for the correction processing operation to be carried 
out by the present correction system. As shown in the figure, processing 
for obtaining the gradient of a line defining actual output 
characteristics of an analog fire detector 3 is carried out as an initial 
processing operation. 
The processing operation is initiated a predetermined period of time after 
a transient state has elapsed following the connection of a power source 
to the central signal station 1. At block 21, the sensor, i.e., analog 
fire detector 3 is called by polling and, at block 22, the zero data Io 
obtained under the condition where the smoke density is zero is read by 
the control section 5a. The reading of the zero data Io by this sensor 
polling is carried out several times for the same sensor or detector so 
that an average value of the zero data Io obtained by these polling 
operations repeated several times is regarded as final zero data Io. 
Further the average value of the zero data can be calculated by the 
running average or simple average. 
When the reading of the zero data Io has been completed, the step proceeds 
to block 23 to transmit a single for controlling the light emission of the 
test LED 10 provided in the detector 3 for driving the test LED 10. At 
block 24, test light-emission data Is obtained under the pseudo-condition 
produced by the test light emission is read by the control section 5a. The 
reading of the test light emission data Is is also repeated several times 
as many as the zero data Io, in response to instructions from the control 
section 5a, and an average value of the test light emission data obtained 
by the repeated test light emission is read as final test light-emission 
data Is. Further the average value of the zero data can be calculated by 
the running average or simple average. 
Subsequently, at block 25, the zero data Io, the test light-emission data 
Is and the present smoke density Ds for test light-emission are read out 
from ROM in the storage section 5c and the gradient constant K of the 
straight line defining the actual output characteristics is calculated by 
the first arithmetic section 5b. 
Thereafter, at block 26, the gradient constant K and the zero data Io are 
stored in RAM of the storage section 5c. After completion of these series 
of processing operations, the control section 5a checks at block 27 as to 
whether the polling of all the sensors has been finished or not. If 
finished, the initial processing operation is completed and if not 
finished, the step returns to block 21 to repeat similar processing 
operations for the following sensor. 
FIG. 7 is a flowchart showing a fire determination processing operation at 
the central signal station 1 after the gradient constant K and zero data 
Io of the straight line defining the actual output characteristics have 
been obtained as shown in FIG. 6. 
First, the analog photoelectric smoke detector as an analog sensor is 
called by polling at block 30. At block 31, the then analog data I is read 
by the control section 5a to transmit the same to the second arithmetic 
section 5d. Thereafter, a smoke density D is calculated, at block 32, on 
the basis of the gradient constant K and the zero data Io stored in the 
storage section 5c according to the following formula: 
EQU D=K(I-Io) 
Thus, a true smoke density D is always obtained irrespective of the output 
characteristics of the sensor. 
When the smoke density D has been obtained, it is checked by the fire 
determining section 5e, at block, 33 whether the smoke density D exceeds a 
critical smoke density for giving a fire detection signal, for example, 
10%/m or not. If the density D exceeds 10%/m, the step proceeds to block 
34 to carry out fire processing operation such as fire alarming or 
indication of fired area. If the density D is lower than 10%/m, the step 
proceeds to block 35 to compare the density D with a density for giving a 
pre-alarming, for example, a density of 5%/m. If the density D is higher 
than 5%/m, the step proceeds to block 36 to carry out a pre-alarming 
processing operation and if the density D is lower than 5%/m, the step 
returns to block 30 to carry out polling of the following sensor. 
A second embodiment of the present invention will be described referring to 
FIGS. 8 to 12. 
An output correction system for an analog sensor according to the present 
embodiment comprises, as illustrated in FIG. 8, a central signal station 
51 comprised of a main control section 52 for controlling the entire 
system and a transmission unit 4 and a plurality of analog fire detectors 
53 connected in parallel with each other to a pair of power/signal lines 
2a, 2b derived from the central signal station 51 so that each of the fire 
detectors can carry out the correction processing. 
The fire detector 53 comprises, as illustrated in FIG. 9, a light-emitting 
circuit 13 to which LED 7 is connected externally, a photodetecting 
circuit 14 to which a photodiode 8 is connected externally, and a test 
light-emitting circuit 16 to which a variable resistor 12, a test LED 10 
and a manual switch 17 are connected. These circuits are substantially the 
same, in arrangements and functions, as those employed in the first 
embodiment. LED 7, the photodiode 8 and the test LED 10 are also identical 
with those of the first embodiment as illustrated in FIG. 3. 
An output correction circuit 19 is connected to the photodetecting circuit 
14. This output correction circuit 19 corrects an output current obtained 
from the photodetecting circuit 14 to the predetermine output 
characteristics. For example, to output characteristics defined by a line 
in which the output current is 4 mA at a smoke density of 0%/m and 25 mA 
at a smoke density of 5%/m, that density being for giving a fire alarm 
signal, to generate a corrected analog output. 
More particularly, the actual output characteristics of the detector depend 
upon the photodetecting circuit 14 and do not always conform to the 
expected output characteristics for various reasons, and so they vary 
among the individual detectors. The output correction circuit 19 carries 
out output correction processing as will be described in detail later, 
with respect to such variances in actual output characteristics to 
generate a current output in conformity with the correct output 
characteristics for the transmission input/output circuit 15. 
This transmission input/output circuit 15 transmits analog data upon 
receipt of polling from the central signal station 1. More specifically, 
the transmission input/output circuit 15 discriminates its calling through 
polling from the central signal station 1 to transmit an output current 
obtained from the output correction circuit 19 at that time. The 
transmission input/output circuit 15 is further adapted to receive a 
control signal for actuating the test light-transmitting circuit 16 
according to instructions from the central signal station 1 to transmit 
the same to the test light-transmitting circuit 16. 
The arrangement of the output correction circuit 19 will now be described 
in detail. 
The output correction circuit 19 comprises, as illustrated in FIG. 10, a 
control section 19a, a first arithmetic section 19b, a storage section 
19c, a second arithmetic section 19d and a third arithmetic section 19e 
for correcting the output current from the photodetecting circuit 14 so as 
to output the corrected output current to the transmission input/output 
circuit 15. 
This correction processing is carried out on the basis of output 
characteristics of an analog sensor as shown in FIG. 11. In FIG. 11, the 
abscissa indicates a smoke density and the ordinate indicates an output 
current. The expected correct output characteristics are those indicated 
by a broken line 18. The correct characteristics 18 are in the form of 
straight line in which output current Io' is 4 mA at a smoke density of 
0%/m and 25 mA at a density of 5%/m for giving a fire detection signal. 
The gradient Ko of the straight line defining the output characteristics 
18 is preliminarily obtained. 
On the other hand, the output characteristics of an actual detector are 
deviated from the correct output characteristics 18 as actual output 
characteristics 20 designated by a solid line. In the actual output 
characteristics 20, the output current Io at a smoke density of 0%/m is 5 
mA and the output current Is is 20 mA at a pseudo-smoke density Ds of 5%/m 
produced by the light emission from the test LED 10. The output correction 
circuit 19, therefore, carries out the processing as will be described 
below to transmit an output current based on the correct output 
characteristics even if the actual characteristics are deviated from the 
correct output characteristics 18. 
First, an output current from the sensor is detected under the condition in 
which the smoke density is zero and, then, the test LED 10 is driven for 
emitting light to produce a sensor output current Is corresponding to the 
smoke density Ds. The detection is carried out by the control section 19a. 
Subsequently, the gradient Kr of the straight line 20 defining the actual 
characteristics is calculated by the first arithmetic section 19b on the 
basis of the sensor output Io at a smoke density of zero and the output 
current Is at the predetermined smoke density Ds as follows: 
EQU Kr=Ds/(Is-Io) (1) 
When the gradient Kr of the straight line defining the actual 
characteristics 20 is thus obtained, the gradient constant Kr and the zero 
data Io are stored at the storage section 19c to transmit the data to the 
second arithmetic section 19d. 
With respect to an output current Ir obtained thereafter, the following 
calculation is carried out by the second arithmetic section 19d to obtain 
a smoke density Dx when the output current Ir is obtained. 
EQU Dx=Kr(Ir-Io) (2) 
On the other hand, since the gradient Ko of the straight line defining the 
correct output characteristics 18 denoted by a broken line is 
preliminarily determined, there are the following relationships between 
the correct output current Ix and the smoke density Dx: 
EQU Dx=Ko(Ix-Io') (3) 
EQU Ix=(Dx/Ko)+Io' (4) 
Since the smoke density Dx with respect to the given output current Ir 
based on the actual characteristics have been obtained by the formula (2), 
Dx is substituted in the formula (4) to obtain the output current Ix based 
on the correct output characteristics 18 by the third arithmetic section 
19e. 
The corrected output current is received by the transmission unit 4 through 
the polling and the main control section 11 makes fire determination on 
the basis of the analog data obtained through the polling. The main 
control section 11 further has a function to transmit a control signal to 
the analog fire detector 53 as interrupt with a predetermined period or by 
a manual operation to drive the test LED 7 for emitting light so as to 
calculate the gradient of the line defining the actual output 
characteristics. 
The entire operation of the output correction system for an analog sensor 
will be described referring to FIG. 12. 
First, the control section 19a provided in the output correction circuit 19 
checks as to whether the system is in a test mode or not (block 40). When 
the control signal has been transmitted from the central signal station 1 
or the manual switch 17 has been operated, the system is in the test mode. 
At the time of connection of the fire alarm system to a power source, the 
system is thrown into the test mode as an initial processing. 
When the test mode is discriminated, the step proceeds to block 41 where 
the control section 19a reads the zero data Io at a smoke density of zero. 
Subsequently, the test LED 10 is driven for emitting light at block 42 and 
the test light-emission data Is is read at block 43. It is preferred that 
a plurality of zero data Io and test light-emission data Is be obtained 
and average values of the respective data be read as final zero data Io 
and test light-emission data Is at block 41 and block 43, respectively. 
Further the average value of the zero data can be calculated by the 
running average or simple average. 
When the zero data Io and the test light-emission data Is have been thus 
obtained, the step proceeds to block 44 to calculate the gradient Kr of 
the straight line defining the actual output characteristics by the first 
arithmetic section 19b according to the formula (1). The thus calculated 
gradient Kr and the zero data Io are stored in the storage section 19c at 
block 45. 
After the processing as described above has been completed, the system is 
thrown into an ordinary fire monitoring mode and, at block 46, the actual 
output Ir, namely, the output current Ir from the photodetecting circuit 
14 as shown in FIG. 9 is read and, at block 47, the smoke density Dx is 
calculated by the second arithmetic section 19d on the basis of the 
gradient Kr of the actual characteristics and the zero data Io according 
to the formula (2). Subsequently, at block 48, the smoke density Dx is 
substituted to the slope Ko which is constant and to the zero data Io' and 
the correct output current Ix is calculated by the third arithmetic 
section 19e on the basis of the correct output characteristics according 
to the formula (4). The control section 19a transmits the correct output 
current Ix to the transmission input/output circuit 15. The transmission 
input/output circuit 15 monitors polling from the central signal station 1 
at block 49. If there is polling from the central signal station 1, the 
correct output current Ix is transmitted to the central signal station 1 
at block 50. 
Although the scattered-light type photoelectric smoke detector is employed 
as an analog sensor in the foregoing embodiments, the analog sensor to 
which the present invention is applied is not limited to this type of 
smoke detector and extinction type smoke detector or an ionization type 
smoke detector may alternatively be employed. For example, in the case of 
the ionization type smoke detector, a pseudo-condition wherein smoke 
enters at a certain density is produced by electrically changing the 
potential of an intermediate electrode in an ionization smoke chamber 
which is provided with an external electrode, the intermediate electrode 
and an inner electrode including a radiation source. The output correction 
according to the present invention is realized by obtaining an output 
current for giving a fire detection signal under the pseudo-condition. The 
analog sensor to which the present invention is applied is not limited to 
the sensor for detecting a smoke density or a temperature due to a fire. 
The output correction system of the present invention is applicable to any 
sensor which outputs an analog signal corresponding to some suitable 
quantity of state to obtain a correct quantity of state irrespective of 
the output characteristics of the sensor. Further, although the 
calculation for correction is carried out at the sensor or at the central 
signal station in the foregoing embodiments, a repeater may be employed to 
carry out such correction calculation and transmit an analog amount or a 
fire signal to the central signal station. 
Further, instead of transmitting analog data to the central signal station, 
a threshold value of a predetermined level may be set in the sensor to 
allow only an alarming signal to be transmitted to the central signal 
station when the analog data exceeds the predetermined level. The 
threshold value may alternatively be set in the repeater.