BioCell Preliminary Detailed Design
Table of Contents
Feasibility: Prototyping, Analysis, Simulation
Atmosphere ControlAn experiment was run by the biology department to determine the feasibility of growing a plant in a sealed chamber with no inputs.
Seeds were placed in test tubes, water was added and then a second tube was placed upside down on top of the first. These tubes were sealed together using tape. The plants grew normally under these conditions. This testing also determined that 2-3 ml was optimal.
This means water, CO2 and O2 will not need to be added at any point to maintain a livable atmosphere for the plant. As long as the test vessel remains sealed.
Pressure Duration TestingAn assembly of fittings and a pressure gauge was created to monitor the pressure inside of prototype cells. By pressurizing the prototype to 1 or more atmospheres gauge pressure the relative pressure experienced in flight can be simulated. Using an available prototype (prototype 1) a pressure duration test was conducted.
Link to the Pressure Duration Test Plan:Preliminary Detailed Design Documents/BioCell/Pressure Duration Test Plan.docx
Link to Pressure Duration Test 1 Data:Preliminary Detailed Design Documents/BioCell/Pressure Duration Test 1.xlsx
The acceptable pressure range was defined as 0.75atm to 1.5atm. The interval of the test in this range is bounded by the vertical green lines. This interval represents ~16 hours. The Target Duration is 6 weeks or 1008 hours. Prototype 1 survived 1/63rd of the required time. Contrary to appearance this is not a death sentence for maintaining pressure with no inputs. This prototype was 1/8th the volume of the final dimensions with similar sized sealing surfaces as the final dimensions.This means for the same leak rates as Prototype 1 a prototype in final dimensions would last up to 8x longer. This means only an 8x improvement in sealing is necessary to meet pressure requirements. Confidence in the seals on prototype 1 was initially low so this should be an easy improvement to make.
Bio Cell Thermal AnalysisA moderately simplified thermal analysis was conducted to calculate the power required to maintain an acceptable internal temperature in the Bio Cell. The model uses thermal resistances to account for an aluminum cell wall, an air chamber and an outer insulation wall. Convection, conduction and radiation were accounted for as shown below.
The inputs to the model are shown below.
For these inputs a steady state heat transfer rate of ~1.8W was calculated. This means that ~1.8W of heat will be required to maintain a suitable temperature for the plant. This is feasible as the cell will have 3.3W available.
The Matlab code for this analysis is found here: Preliminary Detailed Design Documents/BioCell/MSDBCThermalModel.m
A future Simulation will examine temperature gradients within the cell itself and contributions of other heat sources.
Selected Sensing Components
Temperature SensorTo measure temperature, the Analog Devices ADT7420 16-bit I2C digital temperature sensor was chosen. This is a low power device that can accurately measure temperatures between -20 and 105 degrees Celsius with 0.25 degree Celsius accuracy. The resolution of the 16-bit analog-to-digital converter guarantees that the resolution is as low as 0.0078 degrees Celsius. It will also operate in environments between -40 to 150 degrees Celsius.Operating Voltage: 3.3V
Power Consumption (Normal Mode): 700 microWatts
Serial Interface: I2C
Relative Humidity SensorThe Relative Humidity sensor chosen is Sensiron's SHT3x-DIS Humidity and Temperature Sensor. The sensor offers an accuracy of 1.5%RH and a resolution of 0.01%. The sensor is addressed with the I2C serial interface bus.
Operating Voltage: 3.3V
Power Consumption (measurement): 4.95 milliwatts Serial Interface: I2C
Datasheet: Preliminary Detailed Design Documents/BioCell/HT_DS_SHT3x_DIS.pdf
Pressure SensorCurrently, a pressure sensor has not been selected. The initially chosen part was found not to satisfy the pressure requirements set forth by the engineering requirements. As the design proceeds, a new sensor will be chosen to monitor this crucial function.
CO2 SensorGas Sensing Solutions' CozIR-LP-CO2 sensor has been chosen as the main design component to monitor the atmospheric concentration of the carbon dioxide. This sensor has a measurement of 0-2000 parts per million, well above and below the ideal concentration. At this operating range, the sensor offers a resolution of 20 parts per million.
The sensor utilizes the UART protocol to receive commands and transmit data. The sensor will communicate directly with the cell's microprocessor.
Additionally, this is a low-power device. The datasheet quotes a 9 milliwatt draw under a load of two conversions every second.
Operating Voltage: 3.3V
Power Consumption: 9mW
Serial Interface: UART
Datasheet: Preliminary Detailed Design Documents/BioCell/CozIR-LP-CO2-sensor-datasheet.pdf
Radiated Spectral Power
The sun radiates nearly 1300 watts per square meter across the visible spectrum at sealevel. However, of this spectrum about one-tenth of the received power is spread across the wavelengths that will be received by the plant (red and blue). This was calculated with the above figures. By restricting the wavelengths fo each red and blue wavelength, the individual power contributions can be calculated. Knowing the square area of the 2 inch round cell, a total of 0.343 watts is needed across the red and blue wavelengths to emulate the power received by the sun. Specifically, 0.132 watts is dedicated to red wavelengths and 0.211 watts is delivered in wavelengths associated with the color blue.
However, LEDS are not one hundred percent efficient and more power must be delivered tot he lighting solution than what is quoted above. Assuming a 30% efficiency for each LED type, 1.142 watts must be dedicated to LEDS. Individual input powers are 0.439 watts and 0.709 watts. Assuming 20 milliamp LEDs, and forward voltages of 2.2 (red) and 3.1 (blue) volts the light power produced by each LED is 0.044 and 0.062 watts, respectively. Therefore a minimum of 10 and 12 LEDs of each color to sufficiently provide the required light power.
These LEDs will be controlled by an eight-bit PWM driver. Therefore, the final design will include 16 of each type and the power will be controlled through use of individual duty cycles. This will allow the power to adjusted if the efficiency is larger or smaller than what was calculated.
LED DriverTo drive the LEDs mentioned above and curb the power to the desired output, Texas Instruments' TLC59208F 8-bit Fm+ I2C Bus LED Driver. This is an eight channel device that has the capability of sinking 50 milliamps through an LED. Each output, 0 through 7, can be individually controlled with internal registers creating various lighting solutions.
Each driver is individually addressable on the I2C bus and offers control over the internal register. Additionally, groups of drivers can be created and addressed with a group address allowing all LEDs within those groups to be controlled. With this feature the drivers will be grouped into the LEDs dedicated for growth solutions and for image solutions.
Operating Voltage: 3.3V
Output Current: 50 mA
Number of Outputs: 8
Serial Interface: I2C
Datasheet: Preliminary Detailed Design Documents/BioCell/8ch8bitLEDdriver.pdf
Drawings, Schematics, Flow Charts, Simulations
BioX Cell Schematics
The BioX Cell will contain five total boards: the main processing board, the Sensor Side Wall oard, the LED Side Wall board, the ArduCAM, and the Flash LED board. The main processing board will house the MSP432P401RIPZR and the CO2 sensor. Currently, the board will receive power from the cluster master and the the Cluster SPI interface. The MSP432 microcontroller will be a slave to this line. Power will be delivered at 3.3 volts. The processing board will then hold the system's sole UART communications. This board will then deliver power to the Sensor Side Wall and the LED Side Wall as well as communications interfaces: I2C and SPI. The ArduCAM and Flash LED boards are supported by the Sensor Side Wall board and communicate with the processor through this medium.
The Sensor Side Wall houses all environmental sensors, the ArduCAM and required growth LEDs. The LED Side Wall only houses the other half of the necessary growth LEDs. The FLASH LED board only houses the sixteen LEDs need to capture imagery.
Sensor Side Wall Board Schematic
The Sensor Wall Board accepts VDD, GND, the Cluster SPI interface, and the two I2C busses. These serial interfaces allow control over the ArduCAM camera, the LED drivers, and the environmental sensors. The ArduCAM will utilize the SPI interface bus and have a dedicated address on the I2C bus. The first I2C interface, I2C A will interface with the temperature, pressure, and relative humidity sensor. The second bus, I2C B bus will be dedicated for the LED drivers. In total, there are two LED drivers on this board alone. They control one-half of the growth LEDs.
The Sensor Wall board also supports the ArduCAM board and the supporting Flash board. The ArduCAM board will be passed the environmental SPI bus, I2C bus A and power. The Flash board will be passed power and I2C bus B.
LED Wall Board Schematic
The LED Wall Board, at this time, only supports growth LEDs with two LED drivers answering to the I2C interface bus B. In total there are eight LEDS of each color (red and blue).
Flash Board Schematic
The Flash Board supports the image sensing function. This board will contain two additional LED drivers that support four red, blue, green, and infrared LEDS.
BioX Serial InterfacesThe BioX cell system will make use of SPI, UART and I2C serial interfaces. Two SPI buses will be used to communicate with the cluster breakout and the necessary sensors within the environment. Two I2C busses will be used to communicate with the sensors and LED drivers, respectively. UART will be used to solely communicate with the on-board CO2 Sensor
Cluster SPIThe Cluster SPI will be the sole communication medium with the cluster and the payload. The MSP432 will be a slave on this line and will respond to commands sent by the master. All collected data that will be stored at the level of the master will be sent through this bus.
Environment SPIThe environment SPI offers the SPI bus for internal communications only. Specifically, this is included because it is needed for the ArduCAM. On this bus, the ArduCAM will be a slave to the MSP432, the master.
UARTThe UART is added for data transmission from the CO2 sensor to the microcontroller.
I2C Bus AI2C Bus A is the main data channel used to communicate with the BioX cell's included sensors. All sensors excluding the CO2 sensor, i.e. temperature, pressure, relative humidity and the ArduCAM will respond to commands using this line. Additionally, the temperature, pressure, and relative humidity sensor will use this line to transmit converted data back to the microcontroller.
I2C Bus BI2C Bus B will be used to interface with the LED drivers. These will be the only devices on this line. Commands will be sent to activate and deactivate the LEDs according to the current operating function, i.e growth or sensing.
BioCell CAD: Prototype 2STEP file found here: Preliminary Detailed Design Documents/BioCell/BIOCELL CAD PDD.STEP
BOM for Prototype 2: Preliminary Detailed Design Documents/BioCell/Cell Prototype 2 BOM.xlsx