How many G my car can pull?

In the previous article we study how to work with ADXL335 accelerometer. Today we will use it to measure car's acceleration. This is a sample program which get the values from sensor and send them through serial port to the computer. It measures every 20ms and do it 1000 times.

Now lets connect ADXL335 to the Arduino and then compile and upload the above code.

Sensor placed in such way that Z axis is directed to front of car.(You can see it in the picture, on top of red box).

I run the program, type something and press enter in the Serial Monitor and then drive as fast as possible and then brake the car. Copy the values to the Google Docs Spreadsheet and create a chart for the values of Z axis (in which direction the acceleration occurs). But before creating charts lets understand how those numbers map to the real acceleration values in g. The Z axis can be directed downward and measure value (+1g), then direct it upward and measure again(-1g) and taking account that the measured values dependance on acceleration is linear, calculate which value correspond to 0g and which interval correspond to 1g interval. I get following results (approximate results)
    0g --  355
    1g interval -- 70 interval
The measured values also contain a lot of noise, so we need to filter out them.

This is unfiltered chart: Z_i


This is filtered and converted to G chart: ((Z_i + Z_i+1) /2 - 355) / 70


This is filtered(another filter) and converted to G chart: ((Z_i + Z_i+1 + Z_i+2) / 3  - 355) / 705

In the above chart you can see the shift of gear from 1 to 2 and then brake. After car stops and acceleration is 0.

ADXL335 Accelerometer

Accelerometer is a sensor that measure linear acceleration. Today we will use ADXL335 accelerometer, we will write a program to display the measured values of acceleration of all 3 axes.

ADXL335 on a board(Manufactured by Sparkfun.com)

You can see axis scheme on the board, the x and y axises directions are clear, and the z axis direction is point out of the board. Remember this sensor has a sensing range of +/-3g, so you can't measure a higher accelerations :). But it is not a big disadvantage, because the calculation shows that more than 100 km/h we can get during 1 second of 3g acceleration. If you want to check it but you are lazy to do, you can use http://www.wolframalpha.com(3 * gravitational acceleration * 1 second in km/h).
The interface of this device is analog: The voltages on X(Y,Z) port depends on the linear acceleration throughout X(Y,Z) axis. The Arduino board has 6 analog-in ports, we can use 3 of them(0,1,2) to connect to X,Y,Z ports of ADXL335.

Analog ports

So lets open the Arduino IDE and write a simple program, which will periodically read the values on analog ports and send trough the Serial port.

In this program analogRead function read the voltage on the pin and return the integer value between 0 and 1023.
On the computer side we will read the information coming through Serial port an show the values on 3 horizontal progress bars.

This is C# code of program

Be careful, connect ADXL335 to the Arduino board, before connecting them to the PC, otherwise the board will hang and only solution is reconnecting.

This is a short video how it runs.