Chapter 5 _________________________________________________________ Theory of Operation
VAISALA _______________________________________________________________________ 121
Heaters Operation
There are two separate sets of heaters in the FSM102 Visibility
Measurement Unit: Hood heaters in the outside hoods of the optics
heads, and dew heaters inside the optics units for keeping the lenses
clear.
The hood heater operation is monitored with the temperature sensors
located on the glue sides of the hood heater foils. The temperature sensor
output is used for the stabilization of the hood temperatures by the
automatic heating control system. The system turns the heaters on when
the hood temperature drops below 15 °C and turns the heaters off when
the hood temperature rises above 20 °C.
If the sensor output is invalid, the heaters are turned off and a HOOD
HEATER FAULT indication is generated. If the measured heater
temperature has increased less than 2 °C within 5 minutes since the
automatic heater control system turned on the heater, an indication is also
generated.
The dew heaters prevent the formation of condensation on the optic
surfaces. They keep the receiver and transmitter temperature above the
ambient temperature. If the dew heater has been activated (with
commands SET DEW_HEATER VISIBILITY_SENSOR ON or SET
DEW_HEATER BL_SENSOR ON), the automatic heater control
system will turn on the heater when the outside temperature drops below
10 °C. The heater is automatically turned off when the outside
temperature rises above 12 °C.
If no current runs through the heater when it is in the ON state, a DEW
HEATER FAULT indication is generated.
Power Supplies
All internal power supply voltages are monitored, and if they are not
within ±1 V of their nominal values, a POWER SUPPLY alarm is
generated. The +5 V power supply voltage is monitored by a watchdog
circuitry and is not included in the monitored voltages. If the +5 V
voltage is below 4.5 V, the watchdog circuitry generates a reset and
keeps the CPU board in the reset state until the voltage rises above 4.6 V.
The presence of the AC (mains) voltage is monitored by measuring the
voltage input of FSC202. If the voltage input value (PVin) is above 15 V,
the AC (mains) voltage is connected. If the input voltage is below 15 V,
the FS11 sensor is working on the battery and a WORKING ON
BATTERY indication is generated. If the voltage input value drops
below 11 VDC, a BATTERY LOW warning is generated.
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