2. Prototyping, Engineering Analysis, Simulation

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