1. RockOn! 2009 1 Accelerometer Calibration RockOn! 2009

2. 2 Accelerometer Calibration - Overview -The accelerometers are designed to produce a voltage proportional to the acceleration, with a 2.5V offset. -In other words, a sensor that is not accelerating should output 2.5V. Any acceleration will change that value based on a linear coefficient. -The offset and linear coefficient are often different between different sensors. -Usually the difference is small, but some units may vary by 1g or more. -To get the most accurate results, we want to individually calibrate each sensor.

3. RockOn! 2009 3 Accelerometer Calibration - Process - For best results, we want to measure the output over a wide range of accelerations and use that data to find the characteristic equation of the sensor. - To save on time, we will limit this to three data points per sensor. - Earth’s acceleration due to gravity is 1g. We can easily get measurements at 1g, 0g, and -1g simply by turning the sensor. - We will then use Excel to calculate the conversion.

4. RockOn! 2009 4 Accelerometer Calibration – Importing to Excel 1 - Open AccelCalibration.xls, which is on the workshop DVD. - All calculations will be done in this file.

5. RockOn! 2009 5 Accelerometer Calibration – Importing to Excel 2 - Open the accelerometer calibration data you collected during the software walkthrough. - Copy the first six columns into AccelCalibration.xls.

6. RockOn! 2009 6 Accelerometer Calibration – Importing to Excel 3 - Select the first cell in column B and press control – down. - This will scroll you to the end of the data. There may be some lines filled with 5s. Delete these. BeforeAfter

7. RockOn! 2009 7 Accelerometer Calibration – Importing to Excel 4 - Select the 1 and 2 in column A. -Double click the small square in the lower right corner of your selection. -This will fill column A with indexes down to the bottom of your data.

8. RockOn! 2009 8 Accelerometer Calibration – Creating Graphs 1 - Hold down Ctrl and click column labels A, B, C, and D to select those columns. - Column A will automatically become the X axis. - Click the “Chart Wizard” Button - Click “XY (Scatter)” and “Scatter with data points connected by lines without markers” - Click Finish. Or, if you want, click next twice and enter a chart title. Chart Wizard

9. RockOn! 2009 9 Accelerometer Calibration – Creating Graphs 2 - The graph you just made should be similar to this one. - Repeat the process for the high range accelerometers.

10. RockOn! 2009 10 Accelerometer Calibration – Interpreting Graphs 1 -This is the graph for the low range accelerometers. -Notice that areas of +1g, 0g, and -1g are clearly visible on the graphs as flat areas. -On this particular graph, the three sensors produce very similar outputs, but on careful examination slight shifts are visible. -1g +1g 0g

11. RockOn! 2009 11 Accelerometer Calibration – Interpreting Graphs 2 - This is the graph for the high range accelerometers. -Notice that the X and Y accelerometers seem to have produced similar outputs. -The Z accelerometer, on the other hand, reads about 70mV below the other two, the equivalent of more than 1g!

12. RockOn! 2009 12 Accelerometer Calibration – Manipulating Data 1 - Notice the list of values in column J. At the moment, they are just dummy values, so you need to fill them in. Read the actual values off the flattest parts of the graphs by moving your mouse over them. Read value from here (Y Low, -1g) And put it here

13. RockOn! 2009 13 Accelerometer Calibration – Manipulating Data 2 -The graphs and the corresponding equations will update automatically.

14. RockOn! 2009 14 Accelerometer Calibration – Converting Data 1 -Those equations can be used to convert from voltage to acceleration. -Copy column A into column AG to get the right number of indexes. -Then copy the equations from the top of each graph into the corresponding cell in row 2 of columns AH through AM.

15. RockOn! 2009 15 Accelerometer Calibration – Converting Data 2 -Select cell AH2. As before, double click the small square in the bottom right corner of the selected cell to fill the conversion down. -Repeat for the other accelerometers.

16. RockOn! 2009 16 Accelerometer Calibration – Converting Data 3 -Graph the converted data. You should now see all flat areas at -1g, 0g, or 1g. -Repeat for X, Y, and Z High accelerometers -You will use this with your flight data.