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MX4428 MXP Engineering / Technical Manual Document: LT0273
MXP Loop Filter Board
Page 9-6 24 March 2006 Issue 1.5
9.4 PHOTO PROCESSING
9.4.1 SMARTSENSE PROCESSING
The smoke reading of the detector is returned as input AI0 from the device ASIC. Figure 9.2
shows a general view of the processing of the values received from the photoelectric sensor.
It is of interest that the value is multiplied by a factor (between 1 and 4) depending on the
factory calibration of the sensor, and again multiplied by a factor depending on the
temperature rate of rise (when “enhanced” operation is selected). In both cases the “tracked
value” is subtracted, the multiplication applied, and the “tracked value” added back in. This is
so that all scaling occurs relative to the clean air value.
When the MXP or detector powers up the clean air value or “tracked value” is initially set to
the average of the reading from the detector after about 30 seconds and the value stored in
the detector EEPROM, and then may continue to track up or down by 1 each poll for the
next 3 minutes.
When the “tracked value” is reset manually by command from the MX4428, it is set to a
value which will make CV equal to TV. Unless the calibration factor is exactly 1.0, the new
TV is not simply the same as the previous CV.
Subtractor
+
-
Calibration
Adder
Exponential
Filter
Subtractor
Enhancement Factor
derived from Heat RORCV
(1 if no enhancement)
Adder
Multiplier
Slope
Limiter
Very Slow Slope
Limiter
(1 step per Photo Tracking Interval)
TV = Tracked Value = Assumed Clean Air Value
Raw
Value
-
+
+
++
+
-
--
-
Alarm Threshold
Alarm
Subtractor
S
ubtractor
Comparator
C
omparator
CV
PreAlarm Threshold
PreAlarm
CVEnh
(CV to FIP)
SLV
Smoke Filter
Divisor
Smoke Step
Limit
Figure 9.2 Photo Processing Diagram - SmartSense
9.4.2 SMARTSENSE ENHANCEMENT
Figure 9.3 shows the amount by which smoke readings are increased depending on the
temperature rate of rise.
The default enhancement multiplier parameter is 12. The graph (
Figure 9.3) shows the effect
of this factor and also the effect of an alternative parameter of 6.
If enhancement is disabled, (or less efficiently if the enhancement multiplier parameter is set
to 0) the multiplier is exactly 1.