This application note describes how to use a Kionix MEMS tri-axis accelerometer as a

free-fall sensor for drop force modeling applications. Required theory, equations,

and sample event signatures are provided with this note as guidelines for

characterizing drop force models.

the X, Y, and Z outputs of the accelerometer using Equation 1 below. When a tri-

axis accelerometer is dropped in any orientation, it is in free-fall and the measured

acceleration on all three axes is 0g. Therefore, the total acceleration is zero as well.

Total acceleration can be monitored to sense that the accelerometer has been

dropped, and to measure its free-fall time. Note that this signature cannot

accurately be observed when using a dual axis accelerometer because, when

horizontally oriented, the X and Y-axis outputs are the same (0g), whether the

accelerometer is stationary or in free-fall.

The purpose of the drop test was to address a product warranty situation in which a

manufacturer guarantees that a product will survive a 4ft (1.219m) drop to concrete.

A Kionix KXM52 equipped development board was used to conduct this experiment.

The development board sends the analog accelerometer outputs to a Texas

Instruments MSP430F149 micro for ADC, and transmits via serial port to a PC for

data logging, processing, and plotting. Please refer to the Kionix application note

entitled “AN002 Interfacing the Kionix KXP94 or KXR94 Tri-Axis Accelerometer with

the Texas Instruments MSP430F149 Microprocessor to Measure Tilt and Other

experiment. With the PC logging data at a rate of 250 samples per second, the

enclosure was dropped from a height of approximately 4ft. The logged total

acceleration data for the entire event was plotted over time in order to observe the

signature in Figure 1 below. For this calculation, the total time in free-fall was

considered to be the interval of time between the point at which the object crossed

.5g on the way to free-fall to the point at which the object again crossed .5g upon

impact.

Figure 1: Drop Test Signature

Based on the data collected, the free-fall time was .498 seconds. Equation 2 can be

used to calculate the height at which the object was released to free-fall. In this

case, the object, in fact, did fall from a height of approximately 1.219m (4ft).