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Project UCI Stormcraft
Sponsor: Shorbagy Mohamed
Daniel Stoll, Abhijith Jose, Charles (Kolt) Stark
MAE 189: Capstone Design
University of California, Irvine
Professor Mark Walter and Professor David Copp
11/10/2023
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Table of Contents
Executive Summary.....................................................................................................................4
Chapter 1: Project Definition...................................................................................................... 6
Primary Research:................................................................................................................... 6
Problem Definition:.................................................................................................................. 7
Design Attributes table:......................................................................................................7
High-Level Work Breakdown Structure (WBS):................................................................. 8
Gantt Chart:....................................................................................................................... 9
Wiring Diagram:................................................................................................................. 9
Chapter 2: Conceptual/Preliminary Design.............................................................................10
Internal Subsystem................................................................................................................ 10
Communication Protocols:............................................................................................... 10
Microcontroller:.................................................................................................................11
Battery..............................................................................................................................11
Frame Subsystem..................................................................................................................12
Frame Design:................................................................................................................. 12
Anemometer:................................................................................................................... 13
Summary of Preliminary Design............................................................................................ 16
Description of Design:......................................................................................................16
SWOT Analysis:.....................................................................................................................21
Cost Estimates:......................................................................................................................21
Chapter 3: Critical Design/Analysis......................................................................................... 22
Detailed Engineering Analysis and Component Testing........................................................ 22
I2C Communication......................................................................................................... 27
Bluetooth Low-Energy......................................................................................................28
Pin Input Capture............................................................................................................. 29
Functional Architecture.................................................................................................... 30
Prototype Plan....................................................................................................................... 32
Validation Plan................................................................................................................. 32
Chapter 4: Prototype Performance and Final Design ....................................................... 34
Description of Final Design....................................................................................................34
Re-design.........................................................................................................................34
Electrical Component Assembly...................................................................................... 35
Prototype Verification.............................................................................................................37
Temperature and Humidity Requirements Verification.....................................................37
Temperature and Humidity Calibration.............................................................................37
Wind Speed Calibration................................................................................................... 38
Wind Direction Calibration............................................................................................... 39
Bluetooth Range Verification............................................................................................40
Chapter 5: Design Recommendations and Conclusions ...................................................42
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Accomplishments Summary............................................................................................ 42
Design Recommendations For the Future....................................................................... 42
Lessons Learned............................................................................................................. 43
Conclusion....................................................................................................................... 43
References................................................................................................................................. 44
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Executive Summary
In this new age of technology, information is superfluous and easy to access. Information
and data acquisition has only gotten faster and easier for the average person, though that is not to
say that it is idealized for each person and for each specific situation. Access to widely available
weather data surely may help people with their general needs such as knowing what temperature
it is outside to know what to wear, knowing the humidity outside for sensitive gardening needs,
or even knowing the wind speed to prepare for unsafe driving conditions. Even though severe
conditions for any of these cases will clearly be communicated to people on a need-to-know
basis from the government, this is still a market for very localized data from weather stations that
will give precise measurements for each person in their exact location. Something like this will
also prove valuable to those who want to have data that they can control instead of relying on an
over-the-air source that can have delays or consistent connections. A weather station would need
to integrate various sensors; temperature, humidity, and barometric pressure sensors operate by
altering the electrical current through the device based on changing atmospheric conditions. We
aim to design a weather station that should be able to be battery operated, have data wirelessly
transmitted, be compact, and have real-time updates. Requirements include an operational
humidity range between 20%-80%, a temperature range up to 110F, data updates at least every
30 seconds, and the ability to transmit information at least 150 feet.
Photo of Assembled Weather Station
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We made the following design decisions to achieve these goals. For the internal
subsystem, we chose to do a communication protocol of Bluetooth Low Energy due to
simplicity, and mobile phone compatibility; for a microcontroller, we chose the STM32 for its
documentation, and more importantly, the coder’s familiarity with the software. These solutions
to the design allowed us to be able to connect all the sensors that we need and be able to
wirelessly send the meteorological data over Bluetooth to a mobile phone. For the frame and
chassis subsystem, the two popular designs for current solutions for weather stations included a
stacked design, where the sensors are directly on top of one another, or a separated design, where
the sensors are separated from the middle of the chassis with arms. We ended up using a hybrid
design of both of these, due to ease of manufacturing, and compactness, both important
considerations for the weather station. Also, another design consideration was the method in
which a wind sensor would be implemented into the design, in which many different ways are
often used in other weather stations. We ended up using a cup anemometer design due to its
durability, ease of manufacturing, and cost. Finally, the mounting system was considered, which
was important as a weather station needs to be versatile in where it can be placed so that it can
get the specific meteorological data that the user needs. We ended up deciding on a universal
U-mounting pole due to its efficient use of space despite its versatility, cheap cost, and resistance
against obstacles, such as wind.
The results and verifications that we observed us to fulfill these constraints. The weather
station that was made was able to accomplish almost all of the goals that we set out to do. The
goal was to create a localized weather station that could measure the temperature, humidity, wind
speed, and wind direction. We set physical constraints on the capabilities of this weather station
and we were able to match all these goals. These requirements include the ability to sense
temperature up to 44°C; the weather station’s sensors were able to measure up to 50°C. Another
requirement was for it to detect humidity up to 80% in a given space, in which the weather
station was able to detect up to 85%. A requirement of information update at least every 30
seconds and a max wireless data transmission range of 150 feet from the microcontroller unit to
the data receiver was also important to the project. These requirements were fulfilled by the
ability of the microcontroller being able to transmit data once every second and having an RSSI
of -89.45 at 150 feet. Also, a size constraint that was set for the station was for it to fit within the
space of 1’ x 2’ x 3’; the dimensions of the weather station were 9.5” x 15.5” x 29.5”. Lastly, the
weather station should be able to withstand typical winds in the Orange County region, which
was 30 mph. Using the simulations that were shown above, the design was made to withstand
these wind speeds fairly easily. Though our three main needs of wireless data transmission,
compactness, and real-time updates were met, the wireless aspect could be improved as we were
not able to make the battery-powered circuits within the time limits given to us. This leads us to
some improvements we can make in the future.
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Chapter 1: Project Definition
Primary Research:
Weather stations offer the advantage of gathering real-time meteorological data, providing
valuable insights into local atmospheric conditions. Current solutions on the market were
typically equipped to measure temperature, humidity, barometric pressure, and wind conditions.
This information is a valuable asset for various applications from research and agriculture to an
end user simply planning their day’s activities.
Figure 1: Typical components of a Weather Station
(Sungeeta, n.d.)
To deliver this data, a weather station would need to integrate a variety of sensors. Temperature,
humidity, and barometric pressure sensors operate by altering the electrical current through the
device based on changing atmospheric conditions. Some technologies include principles of
capacitance, resistance, thermal conductivity and optics. Moreover, “cup-design” anemometers
are frequently used to measure wind speed while wind vanes determine wind direction. These
particular sensors will be discussed in more detail in Chapter 2.
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Problem Statement:
While there is weather data for general areas widely available for most people in urban areas,
weather stations such as this help provide very localized data to your specific area. It also cuts
out the intermediary step of separate off-site data sites that can be affected by area signal strength
and geological inconsistencies at finer scales. Personalized weather stations such as this allow for
accurate weather data within the closest possible range, and in real-time without relying on
external sources of bottlenecks or outages in the surrounding network or power. Weather stations
provide useful data for those that require very precise and quick updates of meteorological data
such as gardens that are extra sensitive to humidity changes, underwater fish tanks that are
sensitive to changes in temperature etc.
Problem Definition:
We need to read localized weather data including: temperature, humidity, barometric pressure,
wind speed and direction and transmit that information wirelessly. We will design and build a
weather station that is battery-operated and can operate within typical Orange County weather
conditions. The weather station needs to be battery-operated, have data wirelessly transmitted, be
compact and have real-time updates. Requirements include an operational humidity range
between 20%-80%, a temperature range up to 110F, data updates at least every 30 seconds, must
transmit information at least 150 feet, and it must last 7 days on a single charge.
Design Attributes table:
Table 1 : Design Attributes Table
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High-Level Work Breakdown Structure (WBS):
1. Problem definition (All)
a. Formalize Problem Statement
b. Determine Stakeholder Needs and Requirements
c. List design attributes and constraints
2. Solution (All team members)
a. Research solutions to specific subsystem
b. Compare and contrast available solutions
c. Generate prototype concept
3. Conceptual Design
a. Internal Subsystem (Daniel Stoll)
i. Research and compare communication protocols
ii.
Develop high-level wiring diagram
iii. Select appropriate MCU
b. Frame Subsystem (Abhijith Jose)
i. Research different methods of wind speed and direction recording
ii.
Modeling possible physical wire routing
iii. Ease of manufacturing and assembly-driven design
c. Mounting Subsystem (Charles (Kolt) Stark)
i. Research existing solutions of mounting methods for weather stations
ii.
Compare the advantages and disadvantages of different concepts
iii. Potential for placement on multiple angles or surfaces driven design
4. Prototype
a. Firmware and Data communication (Daniel Stoll)
i. Develop drivers for each sensor
ii.
Organize and process appropriate data
iii. Transmit information wirelessly to client
iv. Ensure appropriate voltage levels for each component
b. Manufacturing and Assembly (Abhijith Jose / Charles (Kolt) Stark)
i. Buying off-the-shelf parts
ii.
3D modeling CAD models and tolerancing
iii. 3D printing and manual manufacturing
iv. Solid model simulation testing for
c. Stress Testing (Abhijith Jose)
i. Wind vane directions testing
ii.
High-velocity wind speed testing
iii. Drop/durability testing
iv. Clamp strength testing
d. Sensor Validation (Daniel Stoll)
i. Ensure functional system integration
ii.
Test sensor accuracy
iii. Debug developed code
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Gantt Chart:
Figure 2 : Gantt Chart
Wiring Diagram:
Figure 3 : Wiring diagram
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Chapter 2: Conceptual/Preliminary Design
Internal Subsystem
Communication Protocols:
Among wireless communication protocols for Internet of Things (IoT) solutions, Long Range
(LoRa) , Zigbee, and Bluetooth Low-Energy (BLE) are the most common. LoRa stands out for
its long range capabilities, extending its functional range over several kilometers. It achieves this
in part by operating at much lower frequencies than the other two at the cost of requiring more
specialized hardware. Both Zigbee and BLE operate on the 2.4GHz ISM band like other
protocols such as wifi. This offers the advantage of cross compatibility, since a device only needs
to include a single antenna for a wide range of functionality. All three protocols are optimized for
low power consumption and as a result, suffer from limited bandwidth.
Table 2: Communication protocols compared
When comparing the protocols, all three provided adequate range for our purposes, effectively
eliminating LoRa from consideration. BLE and Zigbee are similar in many aspects but BLE is a
much more ubiquitous protocol and can be found in many consumer products, including many
smartphones. This native compatibility along with lower cost and complexity made it the ideal
candidate for our design.
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Microcontroller:
Table 3: Microcontrollers compared
There are countless microcontrollers from many different manufacturers that would serve our
needs. However, we narrowed our search to three candidates due to a combination of cost,
accessibility, and familiarity. The arduino platform provides a beginner friendly experience
which sacrifices hardware efficiency. Another candidate, the ESP-32 offers a wealth of features
at an affordable price, but its lack of power efficiency and personal familiarity made it a less than
ideal solution. Finally, STMicroelectronics’ STM32 Nucleo board provided many advanced
features, power efficiency, and excellent documentation. Based on this criteria, we decided to
proceed with this platform.
Battery
The stm32-Nucleo board provides two dedicated oscillators for use as the system clock source;
the HSI and HSE. The HSI is an oscillator embedded within the MCU itself and is the default
clock source upon reset. However, clock accuracy degrades with increasing temperature unlike
the HSE which is a dedicated oscillator independent of the MCU. According to the datasheet, the
board draws 72mA of current at maximum frequency and all peripherals enabled when using the
external oscillator (HSE) as the system clock source 1. With this information, we can calculate
the required battery capacity to ensure a 7 day operational period.
Capacity = Imax x 168 hr = 12,096 mAh
1
STMicroelectronics, “DS10693 Rev 10”, STM32F446xC/E, 2021
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Frame Subsystem
Frame Design:
Table 4: Frame Designs compared
Weather stations tend to come in many different shapes and sizes, but the following two shapes
were the ones that had the most benefits that were used by most weather stations. The frist design
is one where there sensors are stacked on top of each other, and the second design in which the
sensors are mounted off centered coming off a pole, held together by an arm for each sensor.
From the decision matrix, it is found the stacked design was not as easy to manufacture as it
requires some unconventional means of assembly to make sure wires do not get tangled. This,
though, is outweighed by the lower cost for the lower amount of material needed for the stacked
design, and that it fits the design constraints of being smaller, better.
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Anemometer:
Table 5: Anemometer Designs compared
An anemometer is a device that measures wind speed and direction. It is a common instrument
used in weather stations. There tends to be 3 different ways to make a sensor that senses wind
speed: a cup design, a vane design, and a hot-wire design. These 3 tend to be the most common
forms of wind speed sensing in the industry. Using the design matrix, it is found that the vane
design tends to be the most durable since it is protected by an outer plastic layer, as opposed to
the hot wire which has an extremely sensitive end that is not suited for outdoor use.
Manufacturing, though, is simpler on the cup design, since it will have to be made from scratch
for our design (explained later in the Preliminary Design Summary) Accuracy is be the best on
the Hot Wire design since its commercial/lab applications, whereas the vane design is completely
dependent on the direction of wind, thus making it practically useless. Though it may not be the
most compact, the previous considerations, including the very low cost, make the cup design the
best one to use for this weather station.
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Mounting Subsystem
According to a personal weather station siting guide from weather.gov, the placement and
method of mounting a weather station is crucial to the accuracy of its sensors and the readings it
displays (“Personal Weather Station -Siting.” Available:
https://www.weather.gov/media/epz/mesonet/CWOP-Siting.pdf).
Mounting Mechanism Design:
Table 6: Mounting Mechanism Existing Solutions Design Matrix
The method chosen for mounting our weather station was not only determined through the
design requirements to be able to withstand forces like wind speed and weight capacity, but it
was also impacted by our ability to redesign (or improve upon) already existing solutions and
manufacture them within the allotted time. While originally considering redesigning a tripod to
create a universal mounting mechanism, the time, materials and manufacturing tools needed to
create it within the scope of the project would prove too difficult to have a functioning prototype
before the scheduled deadline. After revisiting the problem definition and adjusting the design
constraints, a bracket mount was agreed upon to mount the weather station as it still allows us to
pursue the objective of creating a more portable and adjustable weather station.
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Bracket Mount:
Table 7: Bracket Mount Decision Matrix
The bracket mount concept for mounting our weather station was largely determined by our
design objective to have the station be capable of being placed on multiple surfaces or angles in
case of the need for relocation. When considering which bracket mount would be used in our
final design, we also wanted to account for the potential of screws failing by the threads
becoming stripped as a result of continual vibrations generated by wind forces. Our group leaned
toward, and decided on, using a universal mounting pole bracket as it most closely followed our
design objectives and met our design constraints.
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Figure 4 (left): sketches of bracket mount and dimensions; Figure 5 (Right): potential for
bracket mount at different angles and on different surfaces
(https://www.newegg.com/p/16R-06F4-00055?item=9SIBJPFK5H5714&source=region)
Summary of Preliminary Design
Description of Design:
The following concepts are the best choices for the preliminary design. For the internal
subsystem, the Bluetooth low Energy for the communication protocol, and the STM32 for the
microcontroller. For the Frame subsystem, the stacked frame design for the assembly, the cup
design for the anemometer design. Other things are likely a given since there are not many
variations on how they are designed for existing solutions, such as wind vane design or
temperature/pressure sensor holders. The following design was made using some estimated
dimensions of possible sensor choices for all the data objectives we had, with a large emphasis
on the anemometer design, and the interface connection to the wind vane.
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Figure 6: Preliminary design Overview
The Figure above shows an overview of the general structure for the weather station. Bear in
mind that since it is a preliminary design, not all parts are modeled one-to-one, neither is every
part shaped exactly right. This model shows where most of the sensors and subsystems will be in
place and how they will interact with each other. Starting from the bottom, shows a placeholder
yellow box for the clamp, (since it requires very little necessary modeling for an off the shelf
part) connected to the orange frame base that holds the microcontroller. Above that, is the 3D
printed housing for the humidity, temperature, and pressure sensors, which may possibly be
tweaked for better data transmission.
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Figure 7: Detailed look on anemometer design
The figure above shows a detailed view on how the anemometer will work. The cup design
choice, the large gray piece shown, will be a fully 3D printed part. The reason why this design
does ot allow for an off the shelf anemometer is because of the stacked frame design that was
selected. Since the sensors need to be placed on top of each other, the sensors must be on top of
each, but not interfere with each other as they both revolve. If the anemometer has one cable
going down to the microcontroller, there would be no way to have a cable from the wind vane on
top, down to the microcontroller, all the spinning cups will be tangled on the wires. Thus, the
system of a central column with wires running from inside the middle, which will be explained
in more detail in the next subsection. For the Anemometer itself, a hall sensor will be used,
mounted on the purple sensor housing, and a cable on the inside going to the microcontroller.
The cups will be printed with a hollow core that will be secured to a rotary ball bearing (light
blue), which is connected to a steel central column (dark blue). The wind speed will be
determined by the amount of the times, the pink magnet, mounted underneath the cups, which
will trigger the hall sensors every time a revolution has been made, being able to change rpm to a
tangential wind speed.
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Figure 8: A Detailed look on wind vane design
The figure above shows a detailed view of the design for the wind vane and the centralized
design discussed in the last section. In this case, a steel housing that interacts only slightly
smaller for the inner diameter of the bearing, so that attachments can be fairly simple. As the hall
sensor cables can be routed down towards the microcontroller, the wind vane sensor requires
cables that go through the anemometer through the central column. After it goes up the column,
it interacts with the printed connecting housing, which holds up the rotary connector. This is very
important as the sensors being used to find angle direction will be placed on the data collector,
which turns. To make sure that the cables do not twist on each other and break, a rotary
connector is used to interface to the magnetometer.
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Figure 9: Preliminary dimensions for design
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SWOT Analysis:
Figure 10: SWOT Analysis Chart
Cost Estimates:
Table 8: Preliminary Bill of Materials (Link)
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Chapter 3: Critical Design/Analysis
Detailed Engineering Analysis and Component Testing
Final Design Overview
Figure 10: Updated Final Chassis Design
Our team’s chassis design was originally designed based off of stacked weather station designs
where the individual sensors were stacked on top of each other. However, we revisited our design
constraints and decided to base our design off of a hybrid stacked and separated concept, where
the premise of holding all of the sensors on one rod remained the same, but the individual
components were kept separated on the same arm. We came to this decision after realizing that
we were unable to integrate store bought sensors into the stacked design. This allowed us to still
keep everything centralized to the same mounting mechanism and stacked everything together
except the wind vane and anemometer.
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Table 9: Updated Frame Design Matrix
As previously stated, the frame subsystem was drastically changed for the new current iteration.
This mishap of the previous iteration came from not considering the importance of the time to
manufacture. As shown at the bottom of Table 9, a key decision of time to manufacture was
weighted and added, which gave us the result that the hybrid design, where some parts are
stacked and other parts are put on arms, was the best design moving forward.
Table 10: Detailed BOM
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Table 10: Detailed BOM
The MCU housing (in green), the MCU cap (in red), and the pipe mounting bracket (in red) were
all modeled in SolidWorks. Once all of the design decisions were finalized for the stacked design
portion of the weather station, the MCU was printed at FABworks. The assembly lead put
together the microcontroller and sensor wiring, the MCU, radiation shield, screws, and the
anemometer and wind vane.
Sensors
The weather station that was being built had the requirements to be able to measure temperature,
humidity, wind speed, and wind direction. These all required seperate sensors to be mounted
onto the chassis, but each sensor requires its own chassis design to be able to have the most
accurate readings.
Wind speed: The wind speed sensor, or the anemometer, is a drastic change from the initial
design iteration of the midterm report. As described earlier, we decided to go with an
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off-the-shelf anemometer, to not only simplify the design and manufacturing, but also make sure
that it is properly calibrated from the factory and operational within the time limit given.
Wind direction: The wind direction sensor was also an off-the-shelf product that was bought to
make sure that we had a properly calibrated sensor, as opposed to manufacturing it from scratch
and saving time at the same time. The design to use off-the-shelf products also allowed for the
consistency in measurement data, if future iterations of the weather station were built. Both of
the wind speed and wind direction sensors with screw/nut mounted onto the wooden plank.
Temperature/Humidity: The sensors to sense temperature and humidity were very tiny sensors
that were wired up the microcontroller. Though there was no impact on the measurement validity
of the humidity sensor depending on where it was placed on the weather station, this did apply to
the temperature sensor. As direct sunlight can cause the temperature sensors to drastically
fluctuate based on the time of the day on the way that the weather station is oriented towards the
sun. To make sure that the weather station will read consistent temperature no matter the
orientation, we designed the radiation shield, as shown in Figure X, to protect the temperature
from direct sun rays. 2 bowls were used, held by nuts and washers, to be a small distance apart,
to allow for enough airflow into the temperature sensor so it can measure accurately.
Chassis
Figure 12: Photo of Assembled Weather Station
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MCU Housing: The generally micro controller housing unit, the green cylindrical object shown
in Figure X. The housing was made completely entirely through 3D printing. It is also the base in
which the entire weather station runs through. To create this design for the weather station, we
decided to design a way for it to be held up entirely through thread rods that run through the
entire chassis. We were able either print three holes or drill the holes accurately throughout all
the chassis parts, and run rods that hold everything together by nuts to stabilize the entire
weather station.
Cap & Bracket Mount: A concern that the team faced in finishing the assembled prototype was
finding a solution for how the weather station would be mounted to the metal pipe that came
with the universal mounting bracket. After some discussion, we agreed that a type of cap that
would cover and hold onto the MCU would be necessary. The design lead created a 3D model of
the cap, which would be friction-fitted to the MCU and would also have enough room for the
metal pipe to comfortably hold onto the chassis without potentially damaging the electrical
components. The design lead also created a set of brackets that would hold onto the metal pipe in
order to hold the chassis higher on the pipe as well as to prevent the pipe from pushing up on the
electrical components.
Figure 13: Cross-section view of cap to microcontroller housing unit
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Figure 14: Part of the set of pipe brackets for the metal pipe
Figure 15: MCU Cap and Pipe Bracket Subassembly
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After the MCU Cap and pipe brackets were 3D printed, we friction-fitted the cap to the MCU
and used some electrical tape, in addition to nuts and screws to help increase the friction of the
pipe bracket’s hold on the metal pipe. We also screwed the pipe bracket into the MCU Cap to
ensure that the pipe bracket would not rotate or be loosened. Some other design changes of note
include using a wood plank instead of a 3D printed piece to hold the anemometer and wind vane,
in addition to not being able to integrate a battery within the final prototype design. We used a
wood plank because there were fabrication errors with the 3D printing process, and so we ran out
of time to reprint it correctly. We were unable to integrate a battery also because of time, where
we incorrectly calculated the amount of space in the MCU holder and were at a stage in the
design process where a battery was not necessary for the functionality of the prototype testing.
I2C Communication
The Inter-Integrated Circuit Protocol (I2C) is a multi-master, multi-slave protocol that is
commonly used in inter-device communication. It exchanges data via half-duplex wiring,
utilizing one line for bidirectional data transmission while a dedicated clock line ensures timing
compliance. Compared to alternative serial transmission protocols such as UART, SPI, or USB,
the I2C master device can distinguish between connected devices using a unique address,
allowing multiple slave devices to be connected to the same bus. To interface with the humidity
and temperature sensor, an I2C connection was established when the slave address (0x40 in
hexadecimal) was sent to the slave device followed by a write bit. After the successful
transmission of each byte, the slave sends an “Ack” bit, acknowledging the request. The master
then sends the 0xE3 command to request a temperature measurement. A read request is initiated
to the slave address and a raw 2-byte temperature reading is returned. After the raw reading is
converted to an integer, the final temperature value is calculated using the formula given in
Figure 11.
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Figure 16: I2C Communication
Bluetooth Low-Energy
Bluetooth Low-Energy is a wireless communication technology that debuted with the Bluetooth
v4.0. At the core of the BLE protocol is the Generic Attribute Protocol or GATT, which defines a
hierarchy for organizing the data as it is shared across devices. GATT operates on a client-server
model where data is stored in attributes which can be read, written to by the client. The
hierarchical structure of GATT is organized into three main components: Services,
Characteristics and Descriptors. Each service contains characteristics which represent specific
pieces of data or functionalities. In our case, the GATT profile contains two services which hold
two characteristics each as shown in Figure 17. The weather service (a) contains the humidity
and temperature data while the wind service contains the wind speed and direction. When a
characteristic is selected, the user can choose to read or be notified of the given data at intervals
specified in the software.
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Figure 17: Bluetooth GATT profile
Despite its high energy efficiency, BLE is capable of a respectable range of around 300 feet.
However, in real world scenarios where obstacles are present, the effective range may be
considerably less. For our purposes, we require data to be successfully transmitted up to 150 feet
which would satisfy the vast majority of residential applications.
Pin Input Capture
Our chosen anemometer contains an optical sensor that outputs 20 5V pulses per shaft
revolution. Since the pins of STM-32 Nucleo board are only 3.3 V compliant, a 100Ω and 220Ω
resistor were used to divide the voltage to acceptable levels. Once the microcontroller receives
the signal from the anemometer, a timer in input capture mode measures the time period
between the rising edge of each signal as demonstrated in Figure 17. The final wind speed is
then calculated using the following formula:
𝑤𝑖𝑛𝑑 𝑠𝑝𝑒𝑒𝑑 =
1.7 * 𝐶𝑜𝑢𝑛𝑡𝑒𝑟_𝑣𝑎𝑙𝑢𝑒
20 * 𝑐𝑙𝑜𝑐𝑘_𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦
Figure 18: Pin Input Capture
Functional Architecture
Since our weather station needs to measure 4 distinct values, we need to process the captured
data and convert it into standard units in a process outlined in Figure 18. First, the raw
temperature and humidity readings are captured from the Adafruit si7021 temperature and
humidity sensor. They are converted to 16-bit integers to allow arithmetic to be performed. To
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convert the raw readings to Celsius and % Relative Humidity respectively, we use the following
formulas:
125×𝐻𝑢𝑚. 𝑅𝑒𝑎𝑑𝑖𝑛𝑔
65536
175.72×𝑇𝑒𝑚𝑝. 𝑅𝑒𝑎𝑑𝑖𝑛𝑔
−
65536
%𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝐻𝑢𝑚𝑖𝑑𝑖𝑡𝑦 =
𝐷𝑒𝑔𝑟𝑒𝑒𝑠 𝐶𝑒𝑙𝑠𝑖𝑢𝑠 =
− 6 (Silicon Labs, 2022)
46. 85 (Silicon Labs, 2022)
To measure the wind direction, the analog output of the wind vane was converted to digital
values from 0 to 4096, depending on the position of the needle. For example, 0 degrees relates to
a 0 reading whereas a 2048 reading relates to an angle of 180 degrees. The following formula
was used to convert captured values to angular degrees.
𝐴𝑛𝑔𝑢𝑙𝑎𝑟 𝐷𝑒𝑔𝑟𝑒𝑒𝑠 =
𝐴𝑛𝑔𝑒𝑙 𝑅𝑒𝑎𝑑𝑖𝑛𝑔
4096
× 360
Finally, the wind speed is measured using pin input captures and converted to linear speed in the
process outlined in the last section. These values are then transmitted to client devices via BLE
Figure 19: Functional Architecture
32
Prototype Plan
Validation Plan
We will validate the accuracy and functionality of our sensors to ensure compliance with the
design requirements. For the temperature and humidity sensor, we require an operational range
of up to 45° Celsius and 80% relative humidity. These extreme conditions will be recreated using
a hair dryer on heating mode to reach high temperatures and a sauna for high humidity
conditions. We will also test the received signal strength index (RSSI) at a distance of 50 and 150
feet.
Once the sensors are shown to operate in our desired conditions, their accuracy will be verified
using various reference measurements. The wind direction sensor will be validated by sweeping
the sensor over a certain reference angle, say 90°, and the measured value will be compared.
Once the prototype is assembled, the anemometer will be tested by placing it atop a car moving
at a predetermined speed to verify its accuracy. The temperature sensor will be plunged into an
ice-water bath to create a reference temperature of 0 degrees Celsius. Next, the humidity sensor
will be compared to an off-the-shelf humidity meter to ensure proper calibration. Finally, the
BLE effective range will be tested in an open area at 50 and 150 feet, while documenting the
change in received signal strength index (RSSI).
Design Verification
For the verification of our design, we decided to perform a stress test and perform a bending
moment calculation on the bracket mount under the wind pressure from wind speeds of 30 mph,
as it will be holding the weight of the weather station. The design lead created a mach replica of
the solid model assembly with each component of the chassis being substituted with a
33
rectangular prism that covers the maximum surface area possible for each one. The force of wind
exerted on each surface was done with the formula 𝐹𝑑 =
1
2
2
* 𝐶𝑑 * ρ * 𝐴 * 𝑉 . The surface area
was converted from mm2 to in2, the density of air in english units is 4.67 × 10^5 lb/in³, the wind
speed was converted from 30 mph to 528 in/sec, and we assumed that the rectangles were
considered flat plates so the drag coefficient for a flat plate is 1.05. Since our units were in
english, we had to convert the resulting value from the force of wind formula to newtons. 1
Newton is approximately 7.23 lb-ft/s2, so we converted that into lb-in/s2 by multiplying by 12,
which equates to 1 Newton being approximately 86.8 lb-in/s2. The value of the wind force
equation would be divided by 86.8 to get our force of wind in newtons to more easily understand
the SolidWorks simulation testing. An example calculation would be:
2
5
2
2
2
𝐴 (𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎) = 23. 2 𝑖𝑛 ; 𝐹𝑑 = 0. 5 * 1. 05 * 4. 67 𝑥 10 [(𝑙𝑏 * 𝑖𝑛)/𝑠 ] * 23. 2 [𝑖𝑛 ] * 528 [𝑖𝑛/𝑠]
2
𝐹𝑑 = 158. 4 [(𝑙𝑏 * 𝑖𝑛)/𝑠 ]; 158. 4 / 86. 8 = 𝐹𝑑 = 1. 83 𝑁
Area (in2)
Fd (lb-in/s2)
Fd (N)
23.2
11.4
12.8
11.4
2.12
0.324
28.4
19.8
14.3
22.7
15.0
158.0
77.9
87.4
77.9
14.5
2.21
194.0
135.0
97.4
155.0
102.0
1.83
0.898
1.01
0.898
0.167
0.0255
2.24
1.56
1.12
1.79
1.18
34
Figure 20: Chassis Assembly Block Layout For Simulation
35
Figure 21: Stress Test Performed With Applied Forces
Our stress test analysis of the chassis under 30 mph wind speeds resulted in a maximum stress of
4.295e+01 N/mm2 (MPa) of stress. According to SolidWorks the aluminum 3003 alloy applied to
the bracket mount has a yield strength of approximately 4.136e+01 N/mm2 (MPa), so under these
types of wind speeds our design may cause some deformation in the bracket mount.
Chapter 4: Prototype Performance and Final Design
Description of Final Design
Re-design
Nothing on the chassis, or sensor usage has changed from what was previously presented in
Chapter 3. Thus the re-design, drawings, BOM, and cost analysis of these parts stay the same.
The first redesign was explained earlier, which was made right after the midterm report
submission.
36
Electrical Component Assembly
First, the si7021 temperature and humidity sensor was soldered to a daughter board and the
power, ground, SDA, and SCL lines were connected to the microcontroller. The lines of the wind
speed meter were also soldered to this board along with the voltage divider. (Figure 22) After
ensuring secure connections, the daughter board was placed under the blue radiation shield with
cables running downward to the microcontroller. Once the daughterboard was in place, the
microcontroller was fitted into the green housing and any loose cables were secured with zip ties.
The wind direction and speed sensor were mounted to their respective sections and the cables
were fed through the provided holes in the radiation shield. (Figures 24)
Figure 22: Daughterboard
37
Figure 23: MCU in its housing
Figure 24: Wind Direction and Speed sensors
38
Prototype Verification
Temperature and Humidity Requirements Verification
Per our design requirements, we needed to verify that our sensor measured temperature up to 44
degrees Celsius and up to 80% Relative Humidity. To recreate extreme heat, a blow dryer with
heating functionality was directed at the sensor. Over a period of 80 seconds, the readings rose
from 27℃ to 49℃ (Figure 24) , demonstrating an appropriate temperature range. To verify the
appropriate humidity range, we placed the sensor in a bathroom and ran a hot shower. Over a
period of 120 seconds, the % relative humidity rose from 67% to 85%, easily exceeding our
requirement of 80%.
Figure 25: Temperature and Humidity Verification
Temperature and Humidity Calibration
Once we demonstrated the appropriate measuring range of the temperature and humidity sensor,
we needed to verify its accuracy. An ice bath was prepared to create a reference temperature of
0℃ and the sensor was carefully wrapped in tin foil. This kept the water from damaging the
sensor and provided a conductive medium for the sensor to interact with the ice bath. Once the
temperature readings stabilized, 20 measurements were taken, yielding an average of .05℃ and a
maximum error of +0.4 ℃ (Figure 26), matching the published accuracy of ± 0. 4℃. (Adafruit
Si7021 Temperature & Humidity Sensor Breakout Board, n.d.)
The humidity readings were compared to the reference values provided by an off the shelf
humidity meter. The reference sensor provided a constant reading of 56% relative humidity. The
adafruit Si7021 provided a higher resolution and yielded an average value of 56.04% and a
maximum error of +0.5%.
39
Figure 26: Temperature and Humidity Calibration
Wind Speed Calibration
We designed a wind speed testing methodology by placing the weather station atop a car and
moving at a constant speed of 14 m/s. Readings were taken every 0.25 seconds for a duration of
5 seconds. 3 trials were performed with means of 13.62 m/s, 15.33 m/s, and 13.19 m/s
respectively. However, there are potential drawbacks with this method since existing wind
patterns were not taken into account which could introduce significant variations in our result.
Factors such as turbulence generated by the vehicle's structure could affect the readings as well.
A more rigorous approach, such as a wind tunnel, would eliminate environmental variables to
provide a more accurate calibration process.
40
Figure 27: Wind Speed Calibration
Wind Direction Calibration
Wind direction sensor accuracy was measured by rotating the needle at 2 different reference
angles. 10 trials were performed at 90° and 180° and yielded average readings of 89.3° and
179.4° with a maximum error of ±5°, which exceeded the advertised accuracy of ±3°. For the
purposes of transmitting cardinal directions (north, south, etc), this level of accuracy is generally
acceptable.
Figure 28: Direction Sensor Calibration
41
Bluetooth Range Verification
Received Signal Strength Index (RSSI) is a metric used in wireless communication to quantify
the power level of received radio signals, typically measured in decibels (db). Higher RSSI
values generally correspond to stronger signals while lower values suggest weaker signals or a
greater distance from the transmitter as illustrated in Figure 28.
Figure 29: RSSI Guide
The RSSI values were taken at 50 feet and 150 feet. The latter value represents the maximum
distance that data would be expected to reach on a reasonably sized residential property. At 50
feet, the average RSSI was -65.3 decibels which represents an excellent signal strength. At 150
feet, the average RSSI was -89.5 which represents a fair signal, providing a more than acceptable
signal for our low-bandwidth purposes.
Figure 30: RSSI Verification
Safety and Risk Assessment (Failure Mode and Effects Analysis)
42
Potential
Failure Mode
Severity of
Failure (1-5)
Effects of
Failure
Failure Causes
Recommended
Solution(s) or
Action(s)
Water From
Rainfall
Damaging
Electrical
Components
5
Electrical
components
would need
replacing and
chassis would
need
reassembled
Water damage
from rainfall
placing electrical
tape around
wiring
Damage to
Temperature /
Humidity Sensor
From Heat
3
Weather station
would transmit
inaccurate
temperature
readings
Heating of
plastic bowls
from sunlight
could heat the
temperature
sensor to the
point of
damaging it
Replace bowls
with a more
heat-resistant
material
Metal Pipe
Sliding Up Into
MCU And
Potentially
Damaging
Electrical
Components
4
Damaged
electrical
components
would need
replaced and
chassis would
need
reassembled
Friction fit of
tape and bracket
is weakened and
can no longer
hold onto metal
pipe
Protect wiring
inside MCU
with protective
material
Bracket Mount
Failure
5
Potential
destruction of
chassis if mount
failure results in
weather station
falling
High wind
speeds cause
slight
deformities in
bracket mount
A a stronger
material instead
of aluminum for
the bracket
mount
Wood Plank
Rotting
2
Weakening of
wood could
cause screws to
become loose
Exposure to the
elements
Replace with a
different
material
Figure 31: Safety and Risk Assessment (Failure Mode and Effects Analysis)
43
Chapter 5: Design Recommendations and Conclusions
Accomplishments Summary
The weather station that was made was able to accomplish almost all of the goals that we set out
to do. The goal was to create a localized weather station that could measure the temperature,
humidity, wind speed, and wind direction. We set physical constraints on the capabilities of this
weather station and we were able to match all these goals. These requirements include the ability
to sense temperature up to 44°C; the weather station’s sensors were able to measure up to 50°C.
Another requirement was for it to detect humidity up to 80% in a given space, in which the
weather station was able to detect up to 85%. A requirement of information update at least every
30 seconds and a max wireless data transmission range of 150 feet from the microcontroller unit
to the data receiver was also important to the project. These requirements were fulfilled by the
ability of the microcontroller being able to transmit data once every second and having an RSSI
of -89.45 at 150 feet. Also, a size constraint that was set for the station was for it to fit within the
space of 1’ x 2’ x 3’; the dimensions of the weather station were 9.5” x 15.5” x 29.5”. Lastly, the
weather station should be able to withstand typical winds in the Orange County region, which
was able to withstand 30 mph. Using the simulations that were shown above, the design was
made to withstand these wind speeds fairly easily. Though our three main needs of wireless data
transmission, compactness, and real-time updates were met, the wireless aspect could be
improved as we were not able to make the battery-powered circuits within the time limits given
to us. This leads us to some improvements we can make in the future.
Design Recommendations For the Future
As previously mentioned, one important improvement that can be made is to make the weather
station completely wireless. In its current form, it does not need to be fully tethered for it to
work, but a “handshake” must be introduced by a computer on sensor measurement startup. In
the future, we will make it so that it can be connected to a battery-powered bank, allowing it to
start with only a power source, without an external user. Another future improvement to the
wireless connection includes a WiFi connection to allow for an Internet of Things (IOT)
integration for remote access of the weather station data, instead of relying on Bluetooth
transmission. Providing a more user-friendly graphics user interface (GUI) will also be helpful as
the current interaction can be confusing for some to clearly see the data that is transmitted in
real-time. Mechanically, an improvement we could have not made was to eliminate the use of
friction fitting joints from the assembly of the weather station. Friction fitting joints heavily rely
on the precision of the machining, inconsistency in repeated assembly, and heavy wear on
materials when disassembled and reassembled. Ideally, any use of friction fitting in the weather
station, such as holding the MCU holder unit to the bracket holder cap, should be replaced with
more secured housings. This can include using latches, screws and nuts etc. Lastly, an
improvement that can be made to have a positive environmental impact would be the
44
introduction of solar energy to power the weather station. Though the design was made to
accommodate any power source that we can feed through a USB connection, which is possible
from solar powered sources already, a future iteration that accommodates this within the design
to encourage more environmentally friendly power sources would be desirable.
Lessons Learned
This quarter has been very taxing for the entire team, in terms of workload and time. The project,
though it seemed simple at first, became quite a daunting task, especially while juggling all other
aspects of life at the same time. One important lesson that we learned was the importance of
communication and teamwork. Many times throughout the project, we had miscommunications
and disagreements that may have possibly slowed down the development of fast iterations of the
design process. As the weeks progressed, not only did we learn to design and iterate better, but
we learned to work as a team more. We found that communication over texts may not correctly
describe our intentions and desires at the moment, and that in-person or online meetings where
we can have conversations became the best way for us to progress as a team. Another important
lesson that we learned was the importance of the time limit within a given project and how things
should be placed accordingly. In the beginning of the project, we came up with a stacked design
that incorporated a fully custom-made wind speed sensor, with a complicated housing design.
We believed that this was the best design at the time as it had the best chassis shape that likely
kept the most data integrity and was the most compact. If these things may be true, the time limit
of the project would not have allowed us to create these things. Especially making an
anemometer from scratch as it would require extensive tolerancing to make it properly calibrated
to the standards that we are looking for. Realizing this earlier would have allowed us to move
forward quicker and implement some more of the future recommendations that were explained
above.
Conclusion
To conclude, this project was an incredible learning experience for us all. It allowed us to learn
boat teamwork, design constraints, time limit constraints, manufacturing, tolerancing,
calibration, to name a few. All these things have provided a great challenge to us as engineers
that hopefully mimic what we will experience in our real jobs in the upcoming future as we
move onto larger, more ambitious, large companies. This project allowed us to learn more about
engineering with the hands-on experience of designing something digitally using proper key
design justifications and realizing it's not physical form. The skills acquired this quarter will very
much be used to propel us all forwards as prepared engineers in our respective fields in the
future.
45
References
Adafruit Si7021 Temperature & Humidity Sensor Breakout Board. (n.d.). Adafruit. Retrieved
December 15, 2023, from https://www.adafruit.com/product/3251
Silicon Labs. (2022). Si7021-A20 [I2C HUMIDITY AND TEMPERATURE SENSOR].
Sungeeta, J. (n.d.). WEATHER STATION. Retrieved December 15, 2023, from
http://atmos.washington.edu/k12/grayskies/weatherstation/index.html