Problem
An important area of current meteorological research is in
automatic classification of weather system parameters such as current weather
i.e. rain, frost, fog etc. and the identification of remote object such as
cloud types. As part of this research, a method of measuring the changes in the
surface temperature of clouds was required. The cloud temperature profiles for
each of the individual low level cloud types was required, and is to be used in
conjunction with neural models to create a system which can automatically
identify prevailing weather conditions.
A radiometer design was optimised to operate within the
atmospheric windows of the Infra-red spectrum. Conventionally a radiometer
design, like most Infra-red instrumentation requires multi-stage low noise
pre-amplifiers to amplify the low level signals originating from the detector.
Features of the detector signal require multi-rate sampling in order to extract
details of importance. A range of different types of sensors are used within
the radiometer to monitor its operation, optimise the operating environment of
the cooler systems and to control the chopper mechanisms. |
Radiometer Design. Fig
1 |
Radiometer Schematic - NP4000 Technology.
Fig 2 |
Solution
The Keynes Controls 24 bit NP4000 instrumentation was chosen
for use in the radiometer experimentation. The design of the radiometer
instrumentation was based on the schematic shown in Fig 1. The electronic
interface used in the design is based on the circuit shown in Figure 2. The IR
signal originating from the detector, when the instrumentation is operating
contains a dark component (DC) and a chopped dynamic emissivity component. The
AC signal is the one from where the emissivity is made .
The multi-rate sampling capability of the NP4000
instrumentation is used to sample the dark component and AC signal at different
but suitable rates. Digital filters are used to condition the signals. The high
resolution of the 24 bit ADCs in collaboration with their integral low noise
pre-amps ensures that the digitisation process can be carried directly on the
output signal of the IR detector and that the highest possible S/N ratio is
maintained. The chopper tacho signal is detected using one of the NP4000
digital input cards. |
|
The NP4000 analogue input boards contain integral signal
condition for a wide range of sensors. The thermocouples mounted within the
radiometer are integrated directly to further analogue input channels and are
used to monitor the operating characteristics of critical components. The
chopper tacho signal is used within the system to determine the chopper
temperature when the detector is blocked. This operation provides reference
level for the incoming radiation. It is a simple matter to determine if the
incoming radiation is from a source which is hotter/colder than the chopper.
The simple expansion and wide range of control cards enabled
data to be obtained from a wide range of sensors. Data is automatically time
stamped ensuring correlation with other parameters being recorded. The NP4000
instrumentation also controlled the movement of TV camera and the switching on
off field lighting. Figure 3 shows a sample layout of our driver software used
to control the operations of this project.
Further details of the design of the radiometer can be
obtained from our sales office. |
Combined Instrumentation Results Screen.
Fig 3. Information from widely distributed instrumentation can be
easily combined and data processed operated by many user simultaneously. The
instrument driver supports multi-user, multi-machine operations as standard. A
user development kit ensures that software can be developed to fully utilise
the potential of the NP4000 Instrumentation. |
