Communication System Detailed Design
Table of Contents
Communication System Detailed Design
Since the end of the last phase, minor changes have been made to the communication system PCB schematic. One difference is that the SPI clock and data out will be buffered using signal drivers to ensure that they will reliably send data to all 5 targets. Another major change to the board is that there will be an option to run the 2m transceiver off of 7.4V battery power which will run into and out of the PCB. The option to use the board generated power or the battery power will depend on the state of a jumper near the connectoy
A 2-layer PCB layout was completed in this phase as well, with a board size of 10cm x 10cm.
Prototyping, Engineering Analysis, Simulation
Power Block Diagram
MathCAD Documents Calculations for the power consumption of the various parts under different use scenarios can be found here: Board Power Calculations
Calculations for the TPS54334 12V Buck converter can be found here: TPS54334 Calculations
Data Block Diagram
The below image shows how data and signals will be sent within the comm board and to the other boards in the entire system.
Raspberry Pi Pin out
The pinout for the Raspberry Pi has changed somewhat since last phase. Many GPIO pins were shuffled to aid in the simplicity of the PCB layout.
|Device||I2C Address||Final I2C Address|
|Temperature Sensor||1001xxx(A2A1A0)||1001000 (0x48)|
|Pressure Sensor||111011x(CSBn)||1110111 (0x77)|
If the watchdog timer does not provide a valid watchdog input before the timeout period expires(60s minimum), the WD asserts a watchdog (WDO) output that is low for ~200ms, which resets the raspberry pi.
Drawings, Schematics, Flow Charts, Simulations
Communication Software Flowcharts
The below flowcharts show the general states and tasks that the Raspberry Pi on the communication board will deal with.
Communication System Schematics
A link to a pdf of all the slides of the schematic can is here:
Communication System PCB Layout
It was desired that all PCBS being designed with the exception of the biocell be designed with dimensions under 10cm x 10cm to reduce size and cost of the PCB. As of now, the PCB is a two-layer board.
All layers of the PCB are shown in the picture below.
Bill of Material (BOM)
A high level BOM that contains most of the main parts of the system that will be purchased is listed below.
A link to this sheet, and all the other BOMs from in the project can be found here: (Communication BOM is on "Communication System BOM" sheet)
A link to the BOM for the PCB board being manufactured can be found here: Detailed Design Documents/Comms/Board_BOM.xlsx
Test Plans related to the communication subsystem that will verify that the engineering requirements are satisfied are listed below:
ER2: Commands decoded and executed at over 100m
Commands will be sent between the payload and the base station with a distance between them of at least 100m. This test can be conducted with the payload on the ground with the commands being one that can most easily be determined to have been executed. Commands that were received and executed and those that were not will be recorded. This will not be done in the 1 week lab test.
ER4: APRS Transmission Rate
The APRS will be set up to transmit commands at the desired rate. The APRS will be powered using 5V or 3.3V. The APRS will then start transmitting AX.25 APRS packets. The packets will be received by either the 2m receiving equipment in use or the general APRS network that will upload the results to the APRS network that is visible using the aprs.fi website on any computer that has internet access. This will be done during the lab test.
ER33: Telemetry Range
During a 3 hour flight the transmission range of the APRS will be tested so that at any point of its flight where it is within 50 miles of a APRS receiver, the APRS packet shall be received. This is to be tested separately on the ground by driving in a car up to 50 miles away. The APRS location shall be view able using the APRS.fi website, and there shall be no point where the APRS cannot be received.
During a flight, the data sent via the 2m transceiver that is successfully received at the base station will be compared against the relative altitude of the balloon using the pressure sensor on the communication board. During a ground test, a powered payload will be driven around so that the distance between the payload and 2m receiver at the base station will extend to 50 miles. The telemetered data shall be received at all times during the car ride(assuming line of sight).
ER34: Telemetry Data Rate
Data pertaining to the biocell such as photos, sensor data as well as platform sensor data and photos shall be telemeterd to a receiver to satisfy the data rate stated. The amount of data sent over a fixed period of time will be used to calculate the average amount of bits sent per second.
A link to the excel document that contains all of the test plans can be found here: Preliminary Detailed Design Documents/Test_Plan_w_ERs.xlsx