BioCell Selection process
Table of Contents
Bio Cell System Selection process
Functional DecompositionThe below diagram breaks down the high level goal of conducting a plant growth experiment into all of the required functions. These functions include measuring relative humidity, temperature, pressure, CO2 concentration, O2 concentration and pH levels. Other functions include storing and delivering water, CO2 and O2, controlling temperature and pressure in the cell, and providing specific wavelengths of light on command.
BenchmarkingDue to the novelty of a closed environment plant growth system of this complexity, it was difficult to find sufficient benchmarks. However, after researching fast plants, the target experiment's plant, information provided by Fast Plants, a deli-container watering system was selected. This system utilizes two cups, one placed inside the other, to provide a water storage and delivery system and an area to grow the fast plants. A link to the instructions can be found here: https://fastplants.org/wp-content/uploads/2016/12/DeliContainerGrowingSystem.pdf
Concept Development and Morphological Chart
Morphological ChartSystems Level Design Documents/BioX/BioXMorphChart.xlsx
The morphological chart is divided into the functions described by the functional decomposition diagram and the concept options that have been considered for each function. The choices will be defined and selected in the following Pugh Charts.
The RH sensor was considered to be the datum for an RH measurement. Other options considered are a pyschrometer, hygrometer, capacitive soil sensor, visual estimation and chemical changes. For evident reasons, the pyschrometer and hygrometer are inherently large and are not easily repeatable. The visual estimation and chemical change evaluation procedures are as feasible as the datum and offer an unacceptable accuracy measurement. Finally, the capacitive soil sensor is expected to be a larger system and consume a larger cell volume then the datum.
Final selection: RH Sensor.
The water distribution system considers both the water containment and delivery system. To contain the water, both centralized and decentralized systems were considered other than our datum, water gels. A centralized water containment system would offer a shared water source for all cells, whereas a decentralized system would offer a water source only to a particular cell. With the water gels considered, to be a datum, centralized options including an intravenous drip, diffusion and capillary action were considered. Alternatively, we considered a decentralized system that could be augmented with capillary action and a syringe action.
After consideration, the decentralized capillary option was chosen because it has been implemented in the deli-container watering system and has been proven to work. It will requiring a wicking material that will draw water from beneath the cell into the soil where it will diffuse and spread.
Final Selection: Decentralized Capillary Action
Atmospheric CO2 Measurement
The atmospheric CO2 measurement allows us to understand the concentration of gaseous CO2 molecules within the plant's environment. Ideally, we need to be able to measure this at a parts per million resolution. Therefore, we have considered an infrared sensor, colorimetrics, and spectrometry. Spectrometry is not ideal because of the size and weight it would add to the system despite the precise measurement it would deliver. Colorimetrics are not repeatable and would not offer the level of accuracy needed because of the image estimation needed. Therefore, the final selection is the concept's datum: an infrared sensor.Final Selection: Infrared Sensor
Ensure CO2 Concentration
To ensure that the carbon dioxide measure concentration does not deviate too far from the mean the following concepts were considered: actuated pressurized delivery, controlled reaction, closed system, 50% CO2 starting atmosphere and passive delivery were considered. The actuated pressurized delivery is considered to be the datum because canisters can be bought and release CO2 into the environment with few drawbacks. However, after consideration it was determined that a controlled reaction is the best to pursue because of the fine control that this system would offer over determining how much CO2 is introduced. Unlike the actuated pressurized delivery, the effect of the reaction will not deteriorate over time. A pressurized delivery will lose pressure and may be harder to control.
The other concepts have been proven to unfeasible. It is incorrect to say that the environment, at any given time, will be a closed system. Therefore one cannot rely on the starting concentration of CO2 to be constant. Additionally, a 50% CO2 starting environment is not feasible because of the impact it may have on the plant and the bacteria. It will ensure that there is enough CO2 to last the duration of the experiment but may compromise the experiment in doing so.
Final Selection: Controlled Reaction
Atmospheric O2 Measurement
The measurement of the oxygen concentration atmosphere is not so straightforward and will require future consideration. Every selection shown in this section, is not considered feasible due to the size, weight and cost. This applies to the datum as well. Therefore, a selection has not been made.
Final Selection: To be determined.
Ensure O2 Concentration
The control of the oxygen concentration follows the same arguments as carbon dioxide concentration control; all of the same options were considered. Therefore, our final selection is a controlled reaction that produces oxygen gas.
Final Selection: Controlled reaction.
To increase temperature the following were considered: a nichrome wire, resistive losses from the internal LEDS and an exothermic reaction. It was determined that a nichrome wire element will be the best selection because of the affinity it has for producing heat when power is supplied. An exothermic reaction may not be as controllable as the ones discussed in the atmospheric gas control. Also, LEDs are not a reliable source for heat because they efficiently convert electrical energy to light energy.
Final Selection: Nichrome Wire
Visible Light Delivery
The fast plant will require light and it is the environment's job to supply that. Therefore, colored LEDs, white light, filtered white light, sunlight, and ionized gases have all been considered. Due to the size and power consumption ionizing a gas it is not feasible. To provide sunlight would be power effective, but the size of the system needed to do so is not wanted. Having a white light inside the tube is an acceptable effort but the white light spreads power across frequencies that are not needed and is therefore an inefficient use of power. Furthermore, adding a filter to this light to provide the red, blue, and green wavelengths needed would only increase the size of the system. Therefore, it is best to provide red, green and blue LEDs to provide visible light to the plant.
Final Selection: Red, Green, Blue LEDs
Infrared Light Delivery
It has been decided to provide infrared light using an infrared LED because of the power efficiency and size of the LED. Filtering sunlight is not feasible for the reasons described above.
Final Selection: Infrared LED
All Pugh Charts pertaining to the experiment can be found here: Systems Level Design Documents/BioX/BioXExperimentPugh.xlsx
The structure's morphological chart can be found here: Systems Level Design Documents/BioX/BioXStructureMorphChart.xlsx
Several Concepts for different components of the structure are shown in this morphological chart. Concepts are selected from these for comparison.
The structure's Pugh chart can be found here: Systems Level Design Documents/BioX/BioXStructurePugh.xlsx
The first of the three concepts proved the most promising from the pugh chart. The cylindrical shape is feasible and works well with a proven method of sealing that is threaded joints. Limiting the sensors to entry from one end of the tube reduces the complexity and board mounting as many sensors as possible will ease manufacture. Clusters of four cells will reduce the amount of support and mounting needed for the cells overall.
The Concept Selection can be found here: Systems Level Design Documents/BioX/BioXConcept.xlsx
The Concept CAD file can be found here: Systems Level Design Documents/BioX/BioCell Concept.STEP