Lufft CHM 15k, User Manual

Page: 13 / 73

Operating Manual CHM 15k R13 / 07-2019
Technical Description 13
4.2.2 Function control and device status
Figure 3 Flow Chart - standard measuring cycle.
The functional control of the CHM 15k (measurement and evaluation) is performed by an FPGA and an
OMAP processor. The logging interval (not shown here) consist of multiple measuring cycles calculated
in the OMAP processor, while processes with higher time resolution of up to 1s time intervals are
processed in the FPGA.
Figure 3
shows the internal measurement cycle that takes place every second. The measured data and
the evaluation of the status parameters are checked after each measurement cycle. If the values are
outside the tolerances or there is a hardware error, the standard measurement cycle is reinitialized, and
an error message is generated and output.
However, there are parts that are read out and controlled with higher time resolution, such as the
temperature control or that run in the logging time interval, such as the evaluation of the window
contamination and the fan controller in case of precipitation.
The state of the receiver depends on the results of monitoring noise level, supply voltage and DC-current
with and without a test pulse. Due to its internal setup, the light source is essentially characterized by the
pulse repetition rate, which may decrease because of light source aging. The pulse rate is subject to
monitoring. For pulse rates below 4.5 kHz, an error message is generated. The light pulse reflection from
the window panel is also analyzed to monitor the window contamination. All obtained values are reported
in data telegrams and are part of the NetCDF files. In addition, there is a software watchdog to control
the firmware processes. Determined values and status values are output in the extended data telegram
and the NetCDF files. The standard message contains rough information about the status code (see
8.5 ).