BioCell Detailed Design
Table of Contents
Team Vision for Detailed Design Phase
After finalizing the selection of the carbon dioxide, temperature, relative humidity sensor focus was shifted to schematic design and board design of the sensor sidewall, LED sidewall, and LED flash board. Outstanding tasks of the Preliminary Detailed Design Review were the selection of an appropriate pressure sensor and resolving the issue of heating the cell.
During this phase schematic design was finalized and a first revision of each board was completed. For part reuse and to save design time, the LED sidewall board was deleted in favor of replicating the sensor sidewall board on each side of the cell. One board will be only populated with the necessary circuitry to support the growth functions while the other will be populated with every sensor. Additionally, nichrome wire which will be mounted to one of the sidewalls will be used to provide heat to the cell. This will be controlled by a GPIO on in the MSP432.
Drawings, Schematics, Flow Charts, Simulations
Schematic DesignAs discussed previously, the schematic design was finalized and updated with suggestions and corrections given at the last design review and the individual reviews following that presentation. This includes the addition of bulk capacitance to the power rail inputs of each board, bypass capacitance at each integrated circuit, and ensuring the proper LED forward current could be supported by the LED drivers. Capacitance was added in both respective areas. The LED driver on the Flash board was exchanged for a similar part that can support 120 milliamps on each channel. This ensures that that the infrared LEDs, a high current LED, will properly operate when demanded.
Sensor SidewallOne large change was the removal of the individual LED Sidewall. Instead, it is to be replaced with an unpopulated sensor sidewall. This reduce the design time and will be cheaper because only one more PCB must be manufactured.
Updates to the sensor sidewall include a temperature control solution, redefined board-to-board connectors and the selected pressure sensor.
To add heat to the system, a nichrome wire will be mounted to the board and controlled through GPIO. When it is determined that heat needs to be added, a NMOSFET will be turned on sinking current through the wire. Chosen to be 640 ohms, at 30 AWG, the heating solution will generate 2 watts of heat.
The selected pressure sensor is Honeywell's HSCMAND060PA3A3 I2C 12-bit 0-60PSI absolute pressure pressure sensor. It is an SMT package which fits well in the design, offers the pressure range needed, and does not require a large amount of volume within the cell. The new pressure sensor also simplifies the design because there is no longer an address collision on the I2C bus allowing just one bus to be used. Therefore, the only I2C bus in use is what had been previously defined as the A bus.
In the latest revision, the connectors needed to support the ARDUCAM and the LED Flash board have been refined to one connector. The ten pins needed to support both both boards functions (SPI (x4), I2C (x2), 5V, 3V, GND, LED Driver Reset) are included in one right angle connector. This connector will extend through the flash board and enter an 8-pin socket that will be attached to the ARDUCAM. The ARDUCAM does not require 5-volts or the LED Driver Reset. This changes saves board space on the Sidewall and allows the ARDUCAM to be as close as possible to the FLASH board of which the camera will poke through.
LED FLASHThe two major updates to the Flash board are the connector replacement and the swap of the 8-channel 50 milliamp LED driver with a sixteen channel 120 milliamp driver than can support all sixteen LEDs (red, blue, green, infrared) and their respective forward currents. Bulk and bypass capacitance has been added to this schematic as well.
Sensor SidewallThe sensor sidewall is planned to be a four-layer board and support camera, temperature, pressure, relative humidity, and growth functions. It is shown below.
LED FLASHThe LED Flash board is a two-layer board supporting only the growth functions. It is shown below.
CAD ModelThe below CAD Model builds on the previous design. the side wall and flash boards have been integrated based on the board layouts. connectors have been finalized and parts inserted into the assembly. Screw Terminals for nichrome wire have been added and a theoretical path for the nichrome wire has been modeled. The Arducam Mini model has been updated with more accurate dimensions. Two board supports have been added to support the cantilevered flash and camera boards as well as more rigidly connect the side boards. The Upper board mount will be primary mounting structure for the cell internals. The Pressure Sensor model has been added. An additional Board support may be added at the bottom.
The below section view better illustrates the connection methods of the boards. A major change is the combination of the arducam and flash board pins. previously each had their own right angle connector to the side board. there is now a sizable gap from this reduction in size. this will allow for more condensing of boards in a future revision. The boards and connectors are now layed out in such a way that they should be easily assembled.
The CAD is found at the link below:
Prototype 2 was constructed with inconclusive results. The prototype holds pressure for a very short time, ~1 hour. This is orders of magnitude off of the anticipated performance. a root cause of this poor performance has not yet been determined. In addition to the thread tape intended to seal the end caps to the tube caulk was added to the exposed joint between the end caps and the tube. This had no effect on the performance. The remainder of joints need to be investigated further.
If no cause can be determined a redesign will be required.
Camera LensThe ideal lense needs to have an acceptable level of focus for any height of plant in the tube. this ranges from 2-8 in from the camera lense. The camera lense must also follow the S-Type Lense (14mm OD) to fit within the constraints of the Cell. The lense also needs to allow IR light through to the sensor.
There are several lenses available that meet this requirement, a specific lense has not yet been chosen.