TWO-AXIS ACCELEROMETER

The Analog Devices ADXL202JE was chosen for its simple two-pin connection to the controller and its high accuracy when combined with two 16-bit timers on the ATmega128. We chose the external components to give accuracy to 0.4 mG (normal gravity is 1 G). The pins trigger an interrupt to read and rest the timer. A formula then converts the 16-bit value to a magnitude equal to the forces measured for that axis. Finally, the software on the pit crew’s end turns the two-axis information into a crosshair graph showing movement on a Cartesian grid along with a history trail to track movements.

The two-axis sensor was a big problem. During the initial testing phase with the original PCB, we found that the part was sending incorrect signals. We later discovered that the datasheet’s footprint was a mirror image. The wrong pads destroyed the first part and made the first PCB’s two-axis functions useless. We had to make another PCB immediately and make sure there were enough parts for a second sensor PCB. The time spent getting the new board was used to get the other parts on the incorrect board working and tested.