A novel automated enclosure for protecting solar-tracking atmospheric
instruments was designed, constructed, and successfully tested under various
weather conditions. A complete automated measurement system, consisting of
a compact solar-tracking Fourier transform infrared (FTIR) spectrometer (EM27/SUN)
and the enclosure, has been deployed in central Munich to monitor greenhouse
gases since 2016 and withstood all critical weather
conditions, including rain, storms, and snow. It provided ground-based
measurements of column-averaged concentrations of CO
The enclosure protects the instrument from harmful environmental influences while allowing open-path measurements in sunny weather. The newly developed and patented cover, a key component of the enclosure, permits unblocked solar measurements while reliably protecting the instrument. This enables dynamic decision regarding taking measurements, and thus increases the number of data samples. This enclosure leads to a fully automated measurement system, which collects data whenever possible without any human interaction. In the long term, the enclosure will provide the foundation for a permanent greenhouse gas monitoring sensor network.
Anthropogenic greenhouse gas (GHG) emissions into the atmosphere
have risen to a worrying level over the last few decades. There is
little doubt that this impacts the climate on Earth and eventually
the well-being of humanity. To understand greenhouse gas sources,
sinks, and transportation, reliable and precise atmospheric
concentration measurements are required. State-of-the-art
ground-based and spaceborne spectrometers are used to measure
column-averaged gas concentrations by analyzing the gas absorptions
for specific frequencies of sunlight. Ground-based solar-viewing
spectrometers use the sun as a light source. Gas molecules, such as
The Total Carbon Column Observing Network (TCCON;
A portable solar-tracking Fourier transform infrared (FTIR) spectrometer, the EM27/SUN
(
Much human effort is necessary to operate the instrument at a measurement site. A person needs to set up the system every time before measuring. This setup needs to be manually dismantled and stored safely, whenever the weather changes to rainy or stormy conditions. Besides wear and tear of the connectors, much costly labor is necessary. Even though the operators do not need to be highly trained, their permanent attention is required. Consequently, the very limited availability for measuring during short periods of good conditions reduces the amount of data collected.
3-D computer-aided design (CAD) model of the enclosure including the
base cabinet, the cover, the thermal electrical cooler, and the rain sensor.
These hardware components are explained in Sect.
We have developed an automated enclosure
(Fig.
To construct such an enclosure, several challenges must be faced. A safe and reliable protection against environmental influences, like dirt, thunderstorms, and even hail, is needed for the measurement system, though it is necessary for the solar-tracking mirrors of the instruments to be directly exposed to the sunlight during sunny weather conditions. Therefore, the prime directive for the enclosure design is reliable protection and maximized amount of measurement data. Other criteria include the best possible remote controllability via the internet, the thermal stability of the system, and a stable and uninterrupted power supply. The latter ensures error-free collection of the data and prevents the system from experiencing unpredictable situations. To conclude, the enclosure is designed for automatic operation without the need for human interaction.
An overview of the components of the automated enclosure is provided in Fig.
Enclosure overview: components and their arrangements. The laptop is placed on top of the EM27/SUN spectrometer to ensure good accessibility.
Dimensions and weight of the enclosure.
3-D model of the base cabinet.
In
As previously mentioned, the EM27/SUN spectrometer requires physical protection. Therefore, a stable and waterproof control cabinet is placed with its opening facing up, serving as the base cabinet for the enclosure. In this orientation, the door is located on top and allows easy access to all equipment inside. Rubber feet on the bottom ensure a slightly elevated stable position. The electrical components of the enclosure, such as power distribution and power supplies, are attached to a DIN rail. All wiring is stowed in cable ducts to keep the inside neatly organized, easy to access, and operating reliably.
The base cabinet depicted in
Fig.
Because the solar tracker extends through the top of the base
cabinet, it must be protected separately against harmful
environmental influences. Tests with a glass dome were performed at
the Karlsruhe Institute of Technology (KIT; see
A completely different concept is presented here. A newly
developed and patented cover is mounted on top of the opening,
covering the solar tracker. The cover is made of two rotationally
symmetric parts with cutouts, where one fits inside the other
(see Figs.
Drawings of the two parts of the cover. The inner cover with its
large opening is depicted in
The image shows the gaskets that were installed to seal the gap between the two covers, preventing water from being blown into the cover by strong winds.
The outer cover rotates about the vertical symmetry axis on top of the inner cover. Its weight is carried by eight ball bearings equally distributed around the lower end of the inner cover. While the inner cover is mounted in a stationary position on top of the base cabinet, the outer one rotates. The rotation is driven by a simple geared electric motor, which is mounted inside the common axis of the two covers.
The dimensions of the cover is given in
Table
The upper size limit for the opening is determined by the size of
the inner cover's remaining wall. During bad weather conditions or
darkness, the outer cover will rotate to a closed position where
the inner cover wall needs to be fully covered by the opening of
the outer one. Sealing concerns and a lower demand for positioning
precision led the design towards greater overlap. Finally, an
opening of 90
A rough estimation based on the 3-D CAD model shows that the permissible solar zenith angle range is
A gap between the two covers ensures a friction-free smooth
rotation of the outer cover. However, rain combined with strong
winds could blow water into the cover despite the overlap between
the inner and outer covers. Thus, a gasket as depicted in
Fig.
The outer cover needs to track the sun for undisturbed
measurements. Therefore, the actual position of the outer cover
needs to be determined. To this end, we have developed an encoder
using magnets and reed sensors that are placed inside of the
notches on the covers (Fig.
Three reed sensors are glued into the notches of the inner cover, and
each of the sensors has an electrical contact that closes if the
sensor is exposed to a magnetic field. Nineteen neodymium magnets
with a magnetic flux of 1.17 T each were distributed among the
notches of the outer cover. One reed sensor and a single magnet
positioned in the upper notches are used to detect the closed
position (absolute zero position). From then on, the motion
direction is determined and the number of magnets are counted. The
position can be determined with a precision of about
10
The image shows the cover with notches for magnets and reed sensors to track the outer cover's orientation.
To assess the distance and direction of any movement, two more reed
sensors are installed next to each other in the lower notch of the
inner cover. Whenever a magnet passes by the sensors, one sensor
will act a little earlier than the other. This delayed operation
can be used to determine the direction of movement. Multiple
magnets are distributed with alternating polarity around the outer
cover opposite the two sensors. Alternating the magnetic
polarity creates a very weak field between two magnets as shown in
Fig.
Comparison of the magnetic field strength
when placing the magnets of the same polarity
To make sure that the sun path to the tracking mirrors is never blocked, the solar-tracking mirrors' azimuthal orientation is read directly from the mirror control. The corresponding position for the outer cover is determined and set as the target position for the cover's control unit.
There are several thermal concerns regarding the
EM27/SUN. For instance, the solar tracker's stepping motors
may stall in freezing temperatures, possibly due to
greasing. Another point is the unknown impact of heat on the system.
Inside the EM27/SUN, an InGaAs detector senses the
intensity of the NIR light coming out of the interferometer.
Typical InGaAs detectors have a temperature-dependent transfer
function (
Thus, another much easier approach that even offers cooling below
the outside temperature was chosen. A thermoelectric cooler (TEC),
using a solid-state Peltier device, was installed at one of the
walls of the base cabinet. The TEC element is controlled by
electrical current and transfers heat energy into or out of the
enclosure. It does not require holes for ventilation and therefore
keeps the enclosure waterproof as well as closed to
animals. Further, by maintaining the temperature inside the
enclosure between 24 and 25
The table shows the expected maximum power dissipation of all components inside the enclosure.
To prevent an unintended heat exchange between the inside and
outside of the enclosure, thermal insulation is
installed. A 13
Because the enclosure houses many heat sources, cooling represents
a greater challenge than heating. The power necessary for cooling is
calculated by the given values of the components' specifications in
their data sheets. A maximum power of about 120
A Hydreon RG-11 optical rain sensor is installed on top of the enclosure next to the cover. When rain is detected, the integrated logic of the enclosure will automatically close the cover and send a message to the measurement system. This message may stop ongoing measurements or inform an administrator.
The sensor works as follows: an infrared LED emits light into a dome-shaped lens. The light is reflected by the lens surface and travels along its shape to the other side where it strikes a photodiode, which monitors the light intensity. When a water drop lands on the surface of the lens, less light will be detected by the photodiode because the drop allows the light to escape by refraction. RG-11 was chosen due to its dome-shaped surface. It is an improvement over a flat surface as the drops do not accumulate on the surface, which could produce a false signal after the rain stops.
As described in Sect.
Simplified model of the base cabinet and its door to calculate the joint positions and the strength of the gas spring.
A very simplified model, shown in Fig.
As previously mentioned, an electric motor rotates the outer cover. There is no mechanical fallback that can close the cover in case of a power outage. Hence, an uninterruptible power supply (UPS) is mandatory. Additionally, temporary power outages could erase measurements and place the system in an undefined state; therefore, the UPS is designed to supply sufficient energy to the measurement system for several minutes.
The flow chart visualizes the structure of the microcontroller's software.
While operating the EM27/SUN in the past, occasionally unexpected errors occurred, some of which could only be resolved by restarting the spectrometer or its camera. Thus, two relays are included so that a remote operator or controlling software can switch off the power to the spectrometer and disconnect its USB camera from the laptop.
The first relay is mounted on the DIN rail and connects the power
to the EM27/SUN. Besides resolving errors, this switch
allows powering the instrument off at night and thus helps to save
energy. The second relay is located on a USB intermediate plug.
It was developed to enable the solar tracker's USB camera to be
disconnected. In the past, the only effective solution to recover
from camera errors was to physically reconnect the camera.
Therefore, the relay on the intermediate plug simply disconnects
the 5
The control board of the enclosure handles low-level access to the enclosure's hardware. It provides protective safety features, like closing the cover whenever an error occurs. Furthermore, it receives commands from the laptop and operates the enclosure hardware accordingly. The control board works independently to guarantee a very high level of fail safety. Almost every signal on and off the board is designed for the highest possible fault tolerance to ensure reliable operation in every situation.
The control board implements the most critical safety features and
serves as a tool for the laptop to operate the enclosure. The
central brain of the board is an Atmel ATmega168
microcontroller. The system on chip (SOC), which includes
a small, 20
The flow chart shows the design of the cover control flow. The red arrows indicate values that can be changed or influenced by the user via the USB interface.
The image shows the graphical user interface called ECon. With the given controls and indicators, nearly every enclosure function can be controlled and observed.
Example of the bad weather the fully equipped enclosure was exposed to.
The enclosure control board is a central component of the enclosure. The software for the microcontroller on the board implements the enclosure's basic logic, controlling the electric motor of the cover and reading its position by evaluating the signals of the sensors in the cover. Moreover, it receives rain sensor and UPS signals and communicates with the measurement system via USB. Controlling some relays, the enclosure control board can even turn the spectrometer on or off inside the enclosure. Its design, with self-monitoring and a fail-safe circuit, guarantee high reliability and security, ensuring full control and the highest protection of the instrument inside.
To exploit the limited hardware resources as much as possible, the
software is split into a set of high- and low-priority tasks. The
most important high-priority task is the control of the cover as
explained in Sect.
Figure
Figure
An additional forced mode is included to allow full control in case of emergency. If an error occurs, the loop can be cut open via the interface, and a fixed value can be fed into the ramp generator. Hence, a remote operator can take full control of the cover and navigate it into a secure position if needed.
The microcontroller's hardware includes a watchdog feature that monitors the processor's operation. A watchdog is basically a timer that expects a reset signal in its configurable period. In normal operation the watchdog timer will be reset before a timeout occurs. However, if the processor gets hung at any position in the program, the watchdog timer will time out and consequently trigger a full reset, guaranteeing very high operational reliability.
Additionally, the software provides an optional UART watchdog.
When enabled, it will expect an “I am alive” message every 5
We also developed a computer program, written in python, to
offer a set of methods to control the enclosure's different functions
with a graphical user interface (GUI). The GUI is called
ECon (short for Enclosure Control) and appears
as a window on the desktop (Fig.
After constructing the enclosure, its functions were tested. First of all, the dryness inside the enclosure after extreme stormy and snowy conditions is confirmed. Secondly, the ability to regulate the temperature inside the enclosure is examined. Next, we investigate whether it blocks the sun during the course of the day. Finally, the remote controllability and operability are shown.
The images show screen shots of the laptop in the late evening
The diagram shows the internal and external temperature of the enclosure. The red bars illustrate time periods during which the measurement system was active and therefore produced heat that had to be pumped out of the enclosure by the TEC.
Visualization of the amount of daily sampled data with the EM27/SUN. The yellow region marks the time when the EM27/SUN was deployed inside of the enclosure. During the time periods indicated by the red regions, the EM27/SUN participated in measurement campaigns, first in Indianapolis and then in the San Francisco Bay Area, USA, respectively. Due to the shipping time to and back from the USA, measurements were not taken a month before and after the campaigns.
The enclosure encountered several extreme weather events. The
surveillance camera, installed to allow for remote
live viewing of the enclosure, captured the stormy conditions in
Fig.
In winter 2016, Munich experienced a large amount of snow, and the enclosure
proved to be robust against snowfall on the cover
(Fig.
During the 1.5-year test period, the enclosure showed no single
case of blocking the sun during measurements. The test azimuthal
angle ranges from 71
The enclosure's thermal regulation was tested by logging thermal
readings of the outside and inside temperatures during normal
operation (see Fig.
During construction, the TEC was designed to pump out the heat of
the measurement system. Cooling below the outside temperature were
considered a useful feature after the enclosure was assembled. The
built-in reserves of the TEC are not large enough to cool the
system more than just a few degrees below the outside temperature
(see Fig.
The measurement system can be fully remotely operated. Using
remote desktop software, any operator can log in from any
computer and take control of the system. Because of the enclosure
no physical attendance is required to start, stop, or monitor
measurements.
Figure
In future, rain prediction information can be integrated to assist in the decision making regarding cover control. One potential approach could be to assess real-time online rain radar data and meteorological forecasts to predict upcoming weather.
Further improvements of the enclosure could focus on a localized thermal regulation. One could specify the most sensitive components of the measurement system and pay particular attention to their thermal regulation, which could also meet the thermal demands while reducing the total energy consumption.
Another enclosure improvement could target a lightweight and portable version, which would be better suited for mobile measurements as needed in campaigns. The current enclosure was designed for stationary use and gives proof of concept. Nevertheless, there is high demand to use the enclosure for campaigns. This raises a whole new set of requirements such as reduced weight, size, and power consumption.
We designed, engineered, and assembled an
automated enclosure for an atmospheric solar-tracking instrument, the EM27/SUN, and successfully tested it under extreme weather conditions. The
automated solar-tracking system is located in central Munich
(48.15
The enclosure is suited for fully autonomous operation and enables easy handling of the measurement system. When potentially poor weather conditions arise, the system can be shut down and protected within seconds. In case of sudden rain, the enclosure reliably protects the measurement system by closing its cover. In this manner, the measurement system is sheltered from heavy storms, rainfall, and snowfall without the need for any physical human interaction.
Remote access can be obtained by any smartphone or computer. Thus, the system can be observed and controlled from anywhere in the world. This considerably reduces the inhibition level to start measurements, resulting in a significant increase in the amount of data collected. Furthermore, the automated enclosure reduces the need for costly human effort and optimizes the chance to sample good data, even during periods of unstable weather conditions. Accordingly, the measurement system is highly efficient, which should increase the significance of any scientific outcome derived from these data.
In sum, the enclosure allows for a fully automated measurement system, guaranteeing a maximum amount of data collection while minimizing the operational risks and costs, thus providing a foundation for long-term GHG monitoring sensor network.
All data are included in the paper. Inquiries about additional data can be directed to the corresponding author, Jia Chen (jia.chen@tum.de).
The authors declare that they have no conflict of interest.
We gratefully thank Gerold Wunsch for helping with the cover design and Florian Dietrich for further developing and operating the automatic system. We also acknowledge our Bruker colleagues Peter Maas and Gregor Surawicz for technical support. Further, we thank Frank Hase, Bruce Daube, Steven C. Wofsy, Andreas Meichelböck, Johannes C. Paetzold, Duc Hai Nguyen, and Patrick Aigner for fruitful discussions and Stephen Starck from the TUM language center for the helpful English editing. Jia Chen is supported by Technische Universität München – Institute for Advanced Study, funded by the German Excellence Initiative and the European Union Seventh Framework Programme under grant agreement no. 291763. This work was supported by the German Research Foundation (DFG) and the Technische Universität München within the funding programme Open Access Publishing. Edited by: Frank Hase Reviewed by: two anonymous referees