Mobile Science

cceleration by Ray Wisman and Kyle Forinash, Indiana University SE.

 

Acknowledgements:

Tab bar icons by Joseph Wain @ glyphish.com.
Core-Plot for graphs, http://code.google.com/p/core-plot.
Clock image by http://commons.wikimedia.org/wiki/File:Crystal_Clear_app_clock.png
Colored LED Rectangles by http://www.clker.com
Sounds from http://developer.apple.com/iphone/library/samplecode/BubbleLevel/Introduction/Intro.html
 

Description

cceleration records, graphs and emails the iPod/iPhone/iPad accelerometer measurements.

 

The application is useful for measuring acceleration, velocity and distance resulting from motion along a single axis such as by an accelerating vehicle.

Up to 120K unfiltered measurements can be recorded at rates of 1 to 100 samples per second. The measurements are graphed as recorded and can also be emailed.

Controls

Start to record accelerometer measurements, marked by a distinct sound.

Stop to stop recording, marked by a distinct sound.

EMAIL the recorded measurements.

Samples/second rate from 1 to 100 set using the slider bar.

Calibrate toggles zeroing of X, Y and Z axis values to cancel effects due to gravity or other global acceleration of the device. With calibration ON, the initial one second of measurements are used to determine the average acceleration measure for each axis; for zeroing, the average is subtracted from each subsequent measure. A distinct sound marks the end of calibration and the start of data collection.

X, Y and Z tab buttons display the graph of the acceleration, velocity and distance measurements for that axis.

Time slider scrolls through the X, Y and Z axis values measured at the time selected. X, Y and Z axis values are displayed in real time by three progress bars and graphed.

 

Example

cceleration example is dropping an iPod from one meter.

 

The iPod was dropped with the +Z axis face-up.

The screen shot shows the Z axis acceleration, velocity and distance.

The accelerometer measured about one g during the free-fall, spiking as free-fall abruptly ended when the iPod stopped falling.

From the graph, free-fall began at about 0.4 seconds and ended at about 0.85 seconds, lasting about 0.45 seconds, accurate for a one meter fall.

As a visual check of accuracy, the distance graph (red) intersects the acceleration graph (blue) at the time of impact. That point is when measured acceleration is about one g and the distance is one meter, the expected result.

More precise times can be determined two ways: by moving the Time slider on the main screen to observe when the Z axis acceleration changed significantly or by emailing the raw acceleration data and examining in a spreadsheet.

Unfiltered accelerometer measurements are used to calculate Velocity and Distance as follows:

Velocityn = Velocityn-1 + Accelerometern * 9.8 * Sample rate

Distancen = Distancen-1 + Velocityn * Sample rate + 0.5 * Accelerometern * 9.8 * Sample rate2

Graphing

cceleration, velocity and distance along each axis of motion can be graphed individually or in any combination.

 

Pressing the Acceleration, Velocity or Distance buttons will hide/show the respective graphs.

Acceleration, Velocity and Distance can be graphed together or individually.

The vertical axis of the graph is scaled to fit the maximum and minimum value of any axis displayed.

Time is displayed horizontally.

Sliders set the Start and End of the time interval displayed.

A maximum of 500 data points between the Start and End points are graphed to maintain responsiveness.

Preferences

Email addressee and attachment file name preferences are in Settings.