Present weather sensor Optic Eye
by Per-Erik Persson and Patrik Jonsson
AerotechTelub AB
Box 360, SE-831 25 Östersund, Sweden
Tel. +46 63 15 60 00
Fax. +46 63 15 61 99
Per-Erik.Persson@aerotechtelub.se
Patrik.Jonsson@aerotechtelub.se
Abstract
By using a cheap and reliable present weather sensor in its road weather information system,
several road authorities have been able to improve monitoring, and effectively control the winter
road maintenance. The Optic Eye is designed for automatic weather stations and its design
provides for long time intervals between maintenance. The Optic Eye uses horizontal infrared
light beams to detect precipitation and the processor algorithm can distinguish between snow, rain
and sleet. The sensor reports type of precipitation and intensity as well as sensor status and need of
maintenance. No calibration procedures are needed and the maintenance procedure is very limited.
To eliminate that lenses are contaminated with condensed humidity the lenses and their holders are
heated. The Optic Eye is used on almost all of the 700 road weather stations in Sweden. The
Optic Eye is also used in several countries outside of Sweden.
Figure 1. Optic Eye mounted at the top of a mast
Background
A reliable and cheap sensor for detection of precipitation and for analyze of type of precipitation
was the demands set up by the Swedish National Road Administration when the Swedish Road
weather information system was undergoing improved functionality during the late 1980´s. The
previously used sensors were of the conductivity type and they are only able to deliver yes or no
data on precipitation. These sensors also didn’t function well for snow precipitation as the sensors
heating effect and winds made that snow did not stay on the sensor surface to melt.
AerotechTelub AB has been involved in design of the Swedish RWIS and started to design a new
optical present weather sensor in 1990. The idea was to analyze every precipitation particle as it
passes through a light beam. The first tests with a number of prototypes of the sensor showed that
there was fairly good conformity between the different prototypes and with a reference gauge
(tipping bucket). With this background the prototype sensor was further developed and put into
production.
A test batch was delivered for tests in the Swedish RWIS during one winter. Tests were also
performed by an independent consultant to find out the accuracy of the intensity measurement for
different types of precipitation, i.e. rain, snow and sleet. The tests showed that, with used
measurement technique, it is munch more difficult to have good accuracy for the snow intensity
than for rain intensity. For sleet (mixed rain and snow) it is impossible to have reliable intensity
data. Anyhow, the algorithms have been improved and adapted to these new test results from the
last decade so that the present version of sensor is able to perform good classification of detected
precipitation and to calculate the intensities for rain and snow. To perform the classification the
sensor needs information about air temperature and relative humidity as well as current wind
speed. This means that the sensor, to be able to classify precipitation, must be installed at a
weather station that measures these parameters.
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Figure 2. Optic Eye installed in a road weather monitoring and traffic control system
The intensity for snow is based on the geometrical build up of snow, not on the equivalent amount
of water from melted snow. In Sweden there are regulations on how much snow there could be on
the road before an entrepreneur must do snow removal. By acquiring and spreading information
about local precipitation amounts, from the present weather sensors to the entrepreneurs, they are
able to make decisions on when to do their job with removing snow. The RWIS in Sweden is in
this way used for automatic registration of need for snow removal and for paying the
entrepreneurs. The experience from the Swedish road administration is that the sensor rapidly
reports when precipitation starts and stops, and that the amount of rain detected is very accurate.
The sensing technique
The Optic Eye uses beams of infrared light, arranged as an horizontal cross, i.e. the sensor is
designed with two detector channels that can detect the passing precipitation particles. The beam
transmitters and receivers are light emitting diodes and photo diodes respectively. These optical
components are encapsulated in special plastic housings. The housings have two functions, both as
protective cover and for heating of the optical components. The later to avoid contamination of the
lenses with moist and snow. Heating effect is automatically controlled, by the heating resistors of
PTC type, to optimize function and to reduce power consumption. By using pipes of rather small
diameter to form holders for the transmitters and the receivers there is very little influence on the
airflow through the sensor crown and thus on the function during windy situations. This design
makes it possible to detect precipitation coming from any direction, e.g. drifting snow.
The transmitters are fed with pulses of current so that the infrared light is not transmitted
constantly. Detection of precipitation is done by analyzing the intensity of received light. With a
micro controller circuit and an advanced algorithm the sensor system can deliver information
about precipitation type and current intensity. The algorithm also includes functions that will
discriminate between precipitation and other particles passing the light beams. To have the sensors
behavior during rapid changes in weather situation user defined the algorithm includes pre-settable
integration time. With a short integration time the sensor gives rapid response to changed intensity
and type of precipitation, and vice versa.
Automatic Gain Control
With time and traffic passing the installation site the lenses will be contaminated with dust and
dirt. This will reduce the received signal level. The sensor includes amplification circuits that
automatically will compensate for this reduction, i.e. Automatic Gain Control. When too much dirt
is hindering the light beams and the received signal level is too low, the sensor output is
automatically deactivated. This alert is also transmitted to the control room for system
maintenance. As an option it is also possible to have circuits installed for warning on low signal
level. Warning will be sent before the sensor is deactivated. By using this option the planning of
maintenance of the present weather sensors will be easy – no sensors deactivated before cleaning.
Installations
The Optic Eye precipitation sensor has been installed in the Swedish RWIS at almost every
monitoring station (more than 650 sensors are delivered). Both Estonia and Lithuania have
installed Optic Eye at a number of stations. All of these installations have monitoring stations
delivered from AerotechTelub, and here the algorithm for classification of precipitation is included
in the GMS (General Monitoring Station).
There also is a stand alone version of the Optic Eye that has been delivered to customers who
have adapted their own monitoring system and stations to communicate with the stand alone
version of the sensor.
Demo software on diskette for test of the Optic Eye with a Windows presentation is available for
new customers.
Figure 3. Screen from demo software for presentation of data from Optic Eye
These present weather sensors are delivered to Norway, Italy, Germany, Estonia, Lithuania and to
USA as well as to Korea. In Norway the stand alone version of Optic Eye is installed in the
RWIS at a large number of sites. The stand alone version of Optic Eye communicates with a
host system via RS232. This means that sensor must be located within a few meters from the
monitoring station, if not short range modems are used.
Calibration and Maintenance
The sensor design and function is such that it needs no calibration, and the maintenance is limited
to cleaning of the optics lenses. The electronics are enclosed in an air tight box that holds a
desiccant cartridge which absorbs moisture. If there should bee build up of ice and snow in the
light beams the sensor alert will close down the sensor until snow and ice is removed or melted.
The power of the heating elements is enough to melt ice and snow on the detectors at temperatures
as low as –25C if there is no wind. Consequently the precipitation sensor might be closed down
temporarily during severe weather conditions but it will start functioning again a couple of hours
after the wind has stopped.
Due to the principle function of the sensor the detectors do not need any calibration to classify and
measure intensity correct. To be able to verify that sensors and acquisition system are performing
correctly there is an optical precipitation simulator available. This equipment is based on the latest
technology in Ferro Liquid Crystal screens (FLC). These FLC screens are arranged perpendicular
to the light beams and they are electrically controlled to function as a rapid shutter. By using this
simulator, which can be preset for type of precipitation and a number of intensity levels, the
system can easily be verified.