USER'S GUIDE ____________________________________________________________________
110 __________________________________________________________________ M211187EN-C
Software Description
The FS11 sensor has a fixed program tuned by the system parameters.
The program code includes the signal processing algorithms and the
system timing. After a hardware reset or a power-up, the program
initializes its working data structures and reads the system parameters
from the non-volatile FLASH memory into the data memory. The
correctness of the read parameters is verified with checksums.
The program is divided into tasks that are executed under the control of a
real time operating system kernel.
The program is organized around a data base that is used in storing
measured values. There is a server task that is responsible for updating
the stored values. Message generation and all requests are handled by a
separate task. There is also a task dedicated for the command interpreter.
Alarm generation is based on information collected during
measurements. The actual alarm logic is handled by a message generator
task.
Measuring Sequence
The FS11 sensor measures visibility at 15-second intervals. Fourteen
seconds of this period is devoted to the signal measurement by firing the
transmitter LED and sampling the receiver detector. After that there is a
1-second period for diagnostic measurements. In the following 15-second
interval, the 1-second diagnostic measurement period is devoted for
contamination measurement.
Visibility Measuring Principle
In the FSM102 receiver unit the signal generated by the PIN photodiode
is amplified and filtered with precision amplifiers. The filtered signal is
then detected by a phase sensitive lock-in amplifier and the result is
converted into a digital word by a 16-bit analog-to-digital converter. The
CPU in the controller board reads the digital words for further
processing.
The offset of the receiver is monitored regularly. Each data point consists
of a signal and background value. These values are subtracted to produce
a real-time, offset-corrected signal value.
