Phase 3: Preliminary Detailed Design

Team Vision for Preliminary Detailed Design Phase

Our team's goals for Phase Three of the BugTorch project are to produce a fully developed system architecture, a first iteration set of CAD models and drawings (including both parts and assemblies), and a first iteration electrical schematic. Feasibility analysis will be performed prior to development of initial prototypes to minimize losses in time due to mistakes in concepts. A bill of materials for long-lead items and budget analysis will also be created. Our team risk assessment and overall project plan will be updated and maintained throughout the phase. The deliverables for this phase will place the team in a favorable position for Phase 4, where the schematics and drawings will be iterated upon and improved.

At the time of the Phase 3 Review we have completed our system architecture and the first iterations of schematics. Our feasibility analysis determined our pump would be insufficient and that a more powerful one would be required. Our budget analysis has been completed and long-lead time items have been ordered.

Mechanical Analysis

With the mechanical engineering aspect of the BugTorch system, there are two main components: the process of pumping fuel from the main reservoir into the system, and the control of flow of fuel into each individual torch tank.

Reservoir Pump

To determine if the chosen fluid pump will meet the requirement of moving fluid through the torch network and into the torch fuel tanks this calculation will calculate the maximum pump head required including frictional losses in the tubing system. Since the tubing is flexible we will make the assumption that the tubing is 40 feet long in a straight line and has a diameter of one eighth inch, and ignore frictional losses in curves .After travelling through the tubing system to the torch, the fluid must be transported 6 feet vertically up the torch base at a minimum rate of approximately 0.013 gallons per minute to sustain the flame.

As a result the pump in the base of the torch must have a total head value of at least 6.004 feet (1.83 meters) to allow the last torch in the series to function properly. Our best candidate at this time has a head value of approximately 2 feet per the pump's specifications, this means the pump we had originally intended to use will not be sufficiently powerful to run the torch system under these conditions. Please note this model is very rough and the conditions are heavily subject to change in future analysis.

Fuel Control

In order to minimize power requirements, a purely mechanical valve is optimal. This first iteration is similar to a design given to the team by the customer, which utilizes a floating mechanic to open/close the fuel valve. Our design differs from the customer's other design in the fact that the fuel plunger and float are separated by a cord instead of being one complete piece. As the fuel level rises in the tank it pulls the cord up, which puts the plunger into the closed position.

This design ran the risk of being pushed closed prematurely by the flow of fuel, and since the float is inline with the central axis, it may be affected by the torch wick.

To combat this, the direction of "plugging" was reverse so that it pushes closed against the direction of flow. To do this, piece of fishing line/cord is run from the tip of the plunger, down through a hole in the fuel diffuser, and connects to a float that moves freely within the tank. As the fuel tank fills its lift the float, pulling on the connected line, which then pulls the plunger downward until it is in the closed position.

Electrical Analysis

To decide on each electrical component the team viewed our phase two bench-marking and made sure to design for compatibility and manufacturing.  There are currently four primary electrical components:

1. The base station micro-controller that communicates with all of the peripherals.
   1. To decide which controller to use was not very difficult as it had a few simple requirements.  All it needed was BT mesh capabilities and some peripheral control. The current choice is the esp32 for its reasonable price, large amount of support and extensive range of customization. 
      
2. A level sensor that can be used to tell torch fuel level and main tank fuel level.
   1. Currently the team is looking into capacitive sensing using the properties of impedance spectroscopy.  Taking an already designed sensor from Seed studio and modifying it to remain within size and accuracy constraints has provided a solid base for these sensors.
      
3. The torch micro-controller that will report the sensors current state, thus monitoring each torch.
   1. To fit inside the torch a very small controller has to be used.  To meet this requirement we have chosen to go with Nordic's NRF52840-DONGLE.  This controller can handle all the demands of the system.
      
4. Torch power system that will provide continuous power to each torch without need of ground wires.
   1. Without any physical pieces to prototype this phase it is difficult to choose a reliable power source that will also fit unnoticed within the torch.  Until we have been able to get hands on the above pieces the power remains up in the air.  The team believes the current best solution to be solar power by hiding flexible panels on the surface of the torch.
      
   2. When considering the power draw listed by the controllers datasheets and the expected draw of a capacitive sensor, the amount of power required should be relatively low.

Voltage, Power and Current Compatibility:

The table above shows the voltage requirements of the three primary electrical components of the system.  Because the components all function off of the same range of voltages the system will not require the use of a simple amplifier or a step-up/down converter.  This greatly simplifies the problem of making the pieces compatible as they already are.  Below are the power graphs for each component based around the expected voltages and clock speeds of controllers.

Figure 39-4 shows the current related to voltage for the sensor.  The maximum value shown is ~9.5uA giving the device a max power draw of 152uW.  From the datasheets once more the esp32 and the Nordic dongle have a max current around ~5mA.  At the max voltage this computes to a power draw per torch of 18.152mW.  With that being said two things can be concluded:  the power required by the base station will be low, max 18mW, to handle the data traffic from the torch, and the power at each torch is at max 18.152mW.

Therefore, if the torch were to run for 12 hours straight at the maximum voltage of 3.6 then it would require roughly 0.22Wh or 62mAh.  This calculation comes from using this formula: Q_(mAh) = 1000 × E_(Wh) / V_(V).  This means a 3.6V, 5.5mA (~100mAh) battery could power the system for a little over half a day.  If a 1000mAh batter is used it could run for five days straight etc...

Relevant Files

Developing a Feasible Design for the Overall System

In order to start developing a functional prototype, the team needed to decide on a system set-up that would be used throughout the duration of the project. The figure below showcases the decided set-up that will be used for analysis, prototyping, simulating, and testing. Please note the system is not drawn to scale.

System features are described and justified as follows:

• Electricity sourced from an outdoor home receptacle: Used to power the pump. It is assumed that the receptacle will provide 120V to the system for the pump to use.
• Central fuel tank: located close to the home and the pump. Occasional refilling of the fuel tank will need to be completed manually by the user to the main system.
• Distance between the first torch and the pump: The team decided using 15, 30, 60, and 90 ft of tubing would be appropriate for simulating various set ups. Generally, the distance between the pump and the first torch will be variable for each customized home set-up. In fuel analysis, these dimensions would allow for easy scaling. It was also noted in this discussion that the app should include in its setup menu a selection of various pump to first torch lengths, so that the microcontroller can communicate the appropriate flow rate of the fuel accordingly.
• Distances between torches: citronella oil is effective in approximately a three foot range. The ensure the spheres of coverage overlap, or team will be using a 5ft standard spacing between consecutive torches.

In order to proceed with system and subsystem development, the system was broken down into mechanical and electrical subsystems. Some focused analysis on fluid dynamics and power was conducted and described next.

Mechanical Considerations

Pump Flow Rate Feasibility

To determine if the chosen fluid pump will meet the requirement of moving fluid through the torch network and into the torch fuel tanks this calculation will calculate the maximum pump head required including frictional losses in the tubing system. Since the tubing is flexible we will make the assumption that the tubing is 40 feet long in a straight line and has a diameter of one eighth inch, and ignore frictional losses in curves .After travelling through the tubing system to the torch, the fluid must be transported 6 feet vertically up the torch base at a minimum rate of approximately 0.013 gallons per minute to sustain the flame.

As a result the pump in the base of the torch must have a total head value of at least 6.004 feet (1.83 meters) to allow the last torch in the series to function properly. Our best candidate at this time has a head value of approximately 2 feet per the pump's specifications, this means the pump we had originally intended to use will not be sufficiently powerful to run the torch system under these conditions. Please note this model is very rough and the conditions are heavily subject to change in future analysis.

Electrical Considerations

Sensor Feasibility

Concept 5 and 6 from the concept selection in the System Level design showcased using an ultrasonic sensor or a capacitive touch sensor would best meet our requirements for the project. Using the products benchmarked for the sensor selection, it was apparent that off-the-self ultrasonic sensor within our price range would not meet the range needed for accurate measurements and would be difficult to mount. As a result the team is looking into potentially redesigning the head of the torch using machinery on campus. This will need to be further discussed and decided upon in the next phase. In the meantime, Seeed Technology's Capacitive Water Level Sensor seems like a promising alternative that would be easy to integrate into our system. After some investigating, the team discovered it would be possible to redesign the product to fit our needs. Overall, it seems it would be better suited for our project with the current torch materials we have. More information on redesigning process of this sensor is described in the next section.

Power Feasibility

Since the System Level Design phase, the team has had to revisit some concepts selected surrounding the valves in order to proceed with power subsystem development for the individual torches. Upon review of the team's system level design, the client updated the team with his exploration in using a mechanical float to control the refilling of the individual torches with fuel. The team has taken the concept into consideration and focused on implementing it into their own system, thereby reducing reduce the amount of power required from the solar panels for each torch. At this time, the team has decided to move forward with prototyping a system without an electromechanical valve and understand the risks associated with doing so include potentially redesigning the power subsystem if a valve would need to be added down the line.

With the new design in mind, using the power information concluded from the Voltage Power and Current Compatibility table in Section 2: Prototyping, Engineering Analysis, and Simulation, a small power sub-system was developed as seem in the block diagram below.

The red lines represent the positive/Vcc leads and the black lines represent the negative/ground leads.

Five major components make up the solar powered subsystem: solar panel, charger, LiPo rechargeable battery, voltage regulator, and the load(s). The loads taken from the Voltage Power and Current Compatibility table in Section 2: Prototyping, Engineering Analysis, and Simulation were used to understand exactly how much power must be generated from the solar panels in order to operate which was approximately 220 mWh or 62 mAh. Looking at the specifications for the microcontroller, level sensor, and transceiver, it was noted that the operating voltage for the subsystem needs to be around 3.3V. After lots of searching only one rechargeable coin cell battery was found to fit the requirements. However, the battery was not ordered due to the fact it is an obsolete part and might jeopardize our system design later on. Therefore, a 3.7V LiPo rechargeable battery was implemented into the system design and ordered along with a voltage regulator. LiPo batteries are easy to come by and made into many shapes and sizes making it an ideal choice for the design. It should be known that the chosen voltage regulator also requires two different sized capacitors to operate, one 100nF and one 10uF capacitor. Next, the charger was chosen. Adafruit's Universal USB / DC / Solar Lithium Ion/Polymer charger was integrated into the design, since it is small and has a variety of ports and pinouts making prototyping easy.

The final component selected were the solar panel cells. The first couple parameters considered was the size and voltage a the maximum power point. This value should be relatively high (between 5-12V), so our team can ensure it will meet the demand of our circuit. The maximum power generated from this cell is 132.3mW in full day sun. According to the graph from the data sheet below, overcast skies can cause the power generation to decrease dramatically as it follows a logarithmic scale.

Assuming that part sun reduced the power generated logarithmically to 13.23mW and only 8 hours of daylight is received per day, a single IXOLARTM High Efficiency SolarMD cell (SM500K12L) could only produce approximately 106mW per day. This calculation was evaluated simply by multiplying the power generated in part sun by the hours of available sunlight. Based on the 0.22 Wh need from the loads, approximately three of the solar panels selected will be needed to power the torch.

The table below tabulates the components, quantities, output voltage/current, and the datasheet for each of the components selected for prototyping to further demonstrate feasibility.

Relevant Files

Mechanical Drawings

Torch Head Drawings

Below are three drawings of the torch head that will serve as the base of the BugTorch System. They are by no means an accurate, one-to-one representation; they only serve to give the team an ideal of the area we have to work within and outside the torch head itself.

Fluid Control Valve

The image below represents a more detailed prototype "inverted float plunger", which is designed to close when the fuel reaches a certain level.

Electrical Drawings

Sensor Schematics:

The figure below is the unmodified Seed Studio water level sensor.  This version of the sensor features an accuracy of +/- 5% which is too accurate for the requirements of the BugTorch.

 This modified version of the sensor, created in AutoCAD Eagle features one pad for measurement which will provide a binary value of the oil level in the torch.  This modification simplifies the design to meet the requirements of the project.  By removing an integrated circuit and several resistors, this version of the sensor will be a good fit for the size of the torch tank.  To combat erosion from the oil the PCB can be coated in nail polish or another varnish.

Micro-controller Flowcharts/Block Diagrams:

  

In order of appearance:

• Figure one: NRF-DONGLE block diagram.

1. 1.  The Nordic micro-controller has the two major pieces we need to monitor the torch. As pictured the device contains several general purpose input/output (GPIO) that can be configured to take in the I2C data from the sensor and send it over Bluetooth to a matching network.

• Figure two: esp32.

1. 1. As far as micro-controller kits go this one has a decent price and plenty of versatility.  It will be able to communicate with every torch via Bluetooth mesh communicating well over the required data rate necessary.  It also has a WiFi antenna which will be integral in sending data to be analyzed and displayed neatly by the BugTorch app.

• Figure three: ATiny automotive controller.

1. 1. Specified by Seed studio this controller is used to take in data from the environment using the capacitive sensor.  It then sends that data over I2C to the Nordic controller.  We have decided to stay with this controller because Seed studio already has C++ libraries designed to poll for the level data.

Relevant Files

Bill of Material (BOM)

The team ordered some parts for this phase. This page is about the stance of the budget and the BOM at the end of phase 3. The first figure shows the budget spreadsheet which explains in detail the current parts that the team has ordered and the amount of the budget spent on those parts. There are two similar orders from both Digi-Key and Mouser as there was an issue with the Digi-Key order with the parts being out of stock and needing to wait for the supplies to refill. We decided that it would be good to order some of the parts from Mouser since they had the parts in stock and wouldn't delay the progression with testing the components. The team at the end of phase 3 has spent $189.91 to test and prototype the system. The parts ordered are the main micro-controller for our system, jumper wires, prototype boards so the MCU can give access to the pinouts, the solar panels we will be testing with, a solar lithium charger to charge the batteries from the solar panels, 3.3V voltage regulator, and 2 different batteries. 

The figures below show the bill of materials (BOM) along with the budget chart of the BOM. The price is different from the budget as it doesn't include the prices of the shipping price and the BOM only includes the parts that will be on the final prototype therefore there will be a mismatch with the budget spreadsheet. The BOM includes the main MCU, prototype board, jumper wires, solar panels, lithium-ion charger, 1200mAh battery, and 350mAh battery. The total of the BOM currently is $171.63 which is equivalent to 4.3% of the budget. As we progress through this project the BOM and budget will be updated accordingly. 

Test Plans

In order to get sufficient testing done, the team plans to limit the initial prototype system to a single torch. This will allow the team to test major components of the whole system, such as: fuel level reading and reporting, pumping fuel to the torch from the reservoir, and making sure the tank in the torch does not overfill. Once these system operate under conditions satisfactory to our requirements, the team will expand the BugTorch System to include up to four torches in series, in order to evaluate how the system operates with multiple outputs.

To best define the system conditions under the average/recommended layout, testing and calculations will follow these standards:

• The distance from the reservoir and pump to the first torch in the system will be separated by 15/30/60/90 feet of tubing
• The distance from one torch to the next in the system will be separated by 5 feet of tubing

In order to comply with RIT Safety Guidelines, the team plans to do all current testing with water or s similar liquid in place of the citronella oil. We understand the risks of running tests with flammable liquids; water is a much safer and cost-effective alternate.

Updated System Flowcharts/ Block Diagram

Since the last review the team has updated some features of our system. The diagram above shows the new modifications made to the system and highlights the sub-systems focused on during this phase. One of the major modifications to this diagram was changing the individual torch "valve" to a "refilling mechanism" as a result of moving forward with a mechanism plug component. The other major modification was the addition of the solar power system highlighted in yellow. This boxed area shows the power and voltage generated from a solar panel transferred to the loads in the torch electronics subsystem indicated by the orange box. The large red box indicates the gateway electronics used to communicate with the individual torches as well as moderate the fuel level of the central fuel tank. After reviewing and updating this diagram, a few tasks surfaced and need to be worked on during the next phase. Moving into the next phase, it will be crucial for the team to think more about fuel sensing at the central fuel tank and how the valve near the pump and central fuel tank will be controlled. This may introduce additional subsystems and require more revisions to this diagram.

Risk Assessment

The team re-evaluated the risk management as Phase 3, preliminary detailed design phase, and the team has decided that the number of risks along with the total importance went up during this phase as more of the design was progressed. From the previous phase, 4 more risks were added and they are risk number 16-19. The team plans on creating a PCB and the risk with that is it needs to be able to withstand oil erosion. It's something that's really important to consider since the sensor PCB senses the oil level it's one of the most important components in the system and having it be defected by oil erosion can occur into troubles. To minimize this issue a protective coating needs to be looked into to minimize the erosion. The next new risk that was added was solar power not meeting power requirements. As the team is deciding to move towards solar power the risk is that the clouds can disrupt the solar power or there could be unexpected power draw that could fry the micro-controller. To minimize this risk the team would need to consider other power options that could be more reliable. Another risk that was brought up was regarding the parts that will be ordered from manufacturers. There could be a delay in the parts which would delay our progress with the prototyping and testing out components. The last risk that the team decided to add was the mechanical parts not being compatible. The effect that could cause is that there would need to be modifications with the parts selected. To minimize this the measurements of the torch need to be completed and after the compatibility needs to be confirmed too.

The risk burndown chart was also updated as the total importance and the number of risks went up this phase. The updated values are 19 risks with a total importance of 304.

Design Review Materials

Review material links:

• Pre-read: P21389_PDDR_Preread.pdf
• Presentation: Preliminary Detailed Design Review.pdf

Plans for next phase

At the time of the Phase 4 review, we expect to have a fully developed set of schematics that are ready for production and testing at the beginning of MSD 2. All relevant simulations and calculations should have been performed and necessary parts should be ordered and shipping. We will consolidate our project documentation and ensure our client has access in the event he wishes to make use of our prototype during the summer break.

Individual three week plans: