Saturday, February 12, 2011

Vehicle Lateral Acceleration Meter

I have often been curious when driving through a sharp turn how much lateral acceleration I am able to reach. Being a Car and Driver subscriber for nearly 15 years now I am well aware of how many G's a vehicle can reach because of centrifugal force. The best vehicles with the best suspension and tires an reach just over 1G of lateral acceleration, so being a spirited driver I want to know what I am able to reach as my tires begin to chirp as I am accelerating out of the apex of a highway cloverleaf at nearly 65 mph.

You can buy inexpensive G meters that you can place on your dash that can show you and log this data along with other fun things like 0-60 times. But these devices are clunky and I hate having things stuck to my dash. In my car there is also no good location to place one. Thinking about this, it would be extremely easy to make one. Accurate accelerometers are dirt cheap, plus wouldn't it be nice to have the data overlaid on my vehicles in-dash navigation system for a nice clean look? I thought so :)

The system is very basic. I'm using a 3-axis accelerometer (only two of the three axis' are in use, the third can be used for acceleration at a later time. Now I am focused on lateral acceleration), a MAX7456 on-screen display generator, and a PIC18F2520 to make it all work.

The PIC utilizes it's 10 bit A/D converters to read in the accelerometer data from an ADXL330 which is good for plus or minus 3G. This data is then manipulated using some basic trig to remove any z-axis rotational data to eliminate false acceleration caused by body roll of the car in a turn. The PIC then sends data to the MAX7456 via i2c to generate a simple bar graph showing relative acceleration between 0G and 1G along with text of the actual current acceleration. The device was easy to calibrate as all you have to do is rotate the accelerometer exactly 90 degrees and get a reading of the earths gravity.

The video output of the MAX7456 will then be overlaid onto my in dash navigation system display. I am using a simple ntsc display for testing on the bench. Looking into the Lexus navigation system in my car I had assumed that the video would be simply composite ntsc, but this was not the case. The video from the nav computer to the in-dash display was RGB destroying my hopes of simply placing the MAX7456 inline with the nav system. Luckily there are inexpensive devices that can plug into the factory wiring harness that can accept composite video and display it on the display. Here is the video output on my bench at rest:

Here is output at 1G (device turned 90 degrees with z-axis compensation disabled):

Here is the device with me shaking it randomly.

Now I just need to get it on the pcb I am in the process of making and get it installed in my car. It has been so cold here in Michigan I haven't had the motivation to get out to the garage and get it going, but should be done soon for it's first test. Once tested the next obvious steps will be to add a datalogger and GPS to track both vehicle speed and acceleration.

1 comment:

  1. just .44 G of shake? C'mon man, I know you can shake it with more force ;)

    Pretty cool Brad, I like this project!