Flight Computer and Sensor Integration

Flight Computer and Sensor Integration

Brett Russell

Description of Interconnections

                  The flight computer must communicate with four sensors and the data logger during flight in order to record the data; below is how we connected all five of these devices to the Arduino.

Analog Pin	Peripherial Device	Digital Pin	Peripherial Device
0	Humidity Sensor	2	Serial RX
1	Temperature Sensor	3	Serial TX
2	Data Logger CTS
3	Accelerometer X
4	Pressure Sensor
5	Pressure Sensor
6	Accelerometer Y
7	Accelerometer Z

                                       Table 1: Device Interconnections

                  Each sensor must be grounded and provided a different supply voltage, 3.3V or 5V, listed in Table 2.  We opted to simplify the voltage regulation circuit to a 3.3V regulator placed off of the 5V Arduino rail, as the 3.3V rail does not work when it is externally powered, versus supplying a larger range of voltages using a voltage divider. (http://arduino.cc/en/Main/ArduinoBoardNano, Accessed: 3/4/2012)

Peripherial Device	Input Voltage	Voltage Ranges
Pressure Sensor	3.30 V	1.8 - 3.6 V
Humidity Sensor	3.30 V	4.0 - 5.8 V
Temperature Sensor	5.00 V	2.7 - 5.7 V
Accelerometer	3.30 V	1.8 - 3.6 V
USB Datalogger	5.00 V	4.75 - 5.25 V

Table 2: Device Input Voltages

Code

                  The Arduino communicates with the USB Data logger using serial communications sent from the built in FTDI chip.  The specific set of commands accepted by the data logger can be found in my previous post on the USB Data Logger. 

Handing Changing Sensor Data Length

Every sensor outputs data in different lengths and has different ranges; for instance, the temperature can range from -99 degrees Celsius to 99 degrees Celsius.  If the temperature happens to be 0.00 degrees Celsius  and changes to -0.02 degrees Celsius the data logger takes a different length command and the dtostrf(FLOAT, WIDTH, PRECISION, CHAR[]) function takes different arguments, seen in Figure 1.  Each situation per sensor needs to be handled in the code or the data logger will fail to write, never closing the file.  In any case if the file is not properly closed it becomes corrupted, rendering it unreadable, and can corrupt the rest of the data on the data logger.

Figure 1: Determining Data Length

Range (°C)	Output Chars
-999.99 to -100.00	7	Not Possible
-99.99 to -10.00	6	Possible
-9.99 to -0.01	5	Possible
0.00 to 9.99	4	Possible
10.00 to 99.99	5	Possible
100.00 to 999.99	6	Not Possible

                                                                           Table 3: Temperature Write Range

Handling Improper Data Reads

Once all possible cases are covered to send to the data logger, I placed a catch all for any erroneous readings that may be encountered due to unforeseen events.  This also preserves the row and column arrangement of the data in the excel file if a mishap were to occur.

  Figure 2: Erroneous Reading Catch    

Handling Write Speeds

The baud rate of the communication between the Arduino and the data logger is set when defining the SoftwareSerial variable; however, using 9600 the Arduino will occasionally send information too fast. 

                              Figure 3: Setting Serial Communication Rate

There are a few ways to fix this problem:

1.     Add delays between output commands

2.     Use the clear to send (CTS) signal from the data logger

3.     Change the baud rate

The first becomes an issue if an improper amount of time is allocated, but will work with a much slower write rate.  The third option would half the communication process, leading to inefficiencies like the first method.  We opted to use the second to maximize the amount of data we can output to the USB drive.  In order to use the CTS pin from the data logger, the request to send (RTS) pin must still be bridged to the CTS pin in order to successfully complete the devices initiation sequence.  The difference is also connecting the CTS pin to an analog pin on the Arduino, we used A2 seen in Table 1.

The easiest implementation of writeDelay() is shown in Figure 4, Serial outputs are embedded for testing purposes.  Although still in the early phases of testing, our experience has shown that the write delay function is seldom used in a one minute time frame and this function has doubled data output from using simple delays.

Figure 4: USB Data Logger Write Delay Function

Comments

                  The main loop program consists of a string of function calls that each read and print out different sensors data intertwined with write delay functions, this part of the loop can be seen in Figure 5.  

Figure 5: Write Loop if(File Open)    

***The total size of the program has reached 19994 bytes of a 30720 byte capacity***