BioCell Build & Test Prep
Table of Contents
Team Vision for Build & Test Prep Phase
During this phase, it was the goal of the team to finalize the PCB and component selections. This was satisfactorily done, and each PCB was ordered through OSH Park. The bill of materials for the three boards, was generated through Digi-Key.
To prepare for software, and to validate Honeywell's HSCMAND060PA3A3 absolute pressure pressure sensor, a simple pressure test was constructed to measure pressure using the device.
It was also a goal to select and validate a Nichrome wire solution. The wire was selected to meet our 2 watt requirement, but was only done so through quantitative analysis.
Final PCB Designs
BioX Cell Processing PCB
This is a 4-layer PCB that supplies the processing power, control signals, and CO2 sensing solution for the BioX Cell system. The board receives power from the CBOB in the form of 3V and 5V. A SPI bus is also passed between these two systems. The BioX Cell Processing board was ordered from OSH Park for $20.00 for three boards.
BioX Cell Flash PCB
This is a 2-layer board that supplies the flash solution to the PCB. This receives 5V and 3V power for the LEDs and LED driver, respectively. The board also shares a connector with the ArduCam but does not use the SPI bus. The LED driver communicates through the I2C bus also delivered through this connector. Three boards were purchased through OSH Park for $10.00.
BioX Cell Sensor Sidewall PCB
This is a 2-layer board that lines the two sides of the chamber. On one side of the cell, the Arducam and every sensor will be populated to this board. The second board will only house the Nichrome heating solution. Both boards contain 16 LEDs (8 red, 8 blue) that are utilized for the growth function.
Enclosure DesignThe latest prototype moves to a more complicated design using a thin-wall aluminum tube as the starting point. An aluminum plate will be welded to the base to cap and seal the bottom of the tube. another aluminum plate will be welded near the top of the tube, this plate will incorporated threaded holes which will be used in conjunction with the top cap and a gasket to seal the top end.
Honeywell HSCMAND060PA3A3 Sensor ValidationUsing Honeywell's HSCMAND060PA3A3 and their sensor evaluation kit, a pressure validation test was conducted. A pressurized air tank was set to 40 P.S.I. (gauge pressure), and was released into the barbed fitting of the sensor. The results of the minute long test are included below in the CSV file. At steady state, the system measured 14.02 P.S.I, near atmospheric pressure, and with the external pressure applied, the sensor was able to measure ~35 P.S.I. (absolute pressure). There are some inconsistencies in this measurement because the pressure measured should have been one atmosphere above this level, but it was not. The fitting was not tight and it is possible pressure was lost to the environment but the difference in the expected and measure value seems abnormal.
The PDF document below is the measured pressure in a graph of pressure and time.
Nichrome Wire SolutionOur first attempt at using Nichrome wire proved there is work still yet to be done in order to have a working system. Without knowing the resistivity of the material, we purchased McMaster-Carr 8880K75 believing it would near sufficient. In order to dissipate 2W of power at 3.3 volts, the nichrome wire must provide 5.4 ohms resistance. With the cable we purchased, we found that 7.3 ft of material provided the resistance needed; this is far too long to fit in any one cell. With the length of wire we had, we measured the resistivity of the wire to be 1.6E-06 ohm*meters. Targeting a length of 16", we concluded that we a 26AWG equivalent area of Nichrome wire to provide the resistance, and power, we need. That will be delivered by 8880K79.
Bill of Materials
BioX Cell Processing BOMIn all, the components for the BioX Processing PCB, BIOX Processing BOM, costs $58.09 (without tax) for the three boards purchased from Osh Park. This Bill of Materials does not include the Texas Instrument's MSP432P401M or the Gas Sensing Solution's CozIR-LP CO2 Sensor because they were both sampled. It does, however, include the conformal coating needed for the three PCBs in the BioCell and every other system.
BioX Cell Flash and Sensor Sidewall BOMThe components for the BioX Sensor Flash PCB and BioX Sensor Sidewall PCB were purchased together: BIOX Flash & Sensor BOM. The total cost of the components needed to populate more than one board was $126.93. This is an addition to the $10.00 and $50.35 spent on the two boards, respectively. This does not include the cost of an ARDUcam.
Test Plan Summary
Here are few examples of the test plans that have been constructed to satisfy the engineering requirements associated with the BioCell:
6 Week Demo
- Have closed system test with temperature, pressure, and CO2 collection.
- Grow a plant.