MPXM2102AS datasheet(132/670 Pages) MOTOROLA

132 / 670 page

AN1635

2–96

Motorola Sensor Device Data

www.motorola.com/semiconductors

IMPLEMENTATION — SOFTWARE

The operation of the Baseball Pitch Speedometer is very

simple. Upon power on reset, the output LEDs are initialized

to display “00” and “Best Speed.” The analog to digital con-

verter is turned on and the offset voltages of the accelerome-

ters are measured and stored. Finally, all the variables are

initialized and the MCU goes into a dormant state, where it will

wait for a negative edge input capture pulse to trigger it to

begin processing the crash signal.

Once the input capture flag is set, the MCU will immediately

begin the analog to digital conversion sequence. As it digitizes

the crash signature, it will calculate the absolute difference

between the current value and the stored offset voltage value.

It will integrate by summing up all the differences. Figure 2

shows a typical crash signature of the Baseball Pitch

Speedometer. As illustrated, starting at the point of impact (A),

the acceleration will initially ramp up, reaching a maximum,

then decrease as the target is displaced. Because the target

is constrained to the frame structure, the acceleration will

continue to decrease until it reaches a minimum (point B),

which correspond to the travel stop of the target. It is difficult

to determine exactly when point B will occur, because the

amplitude and duration of the initial acceleration pulse will vary

with ball speed. Therefore, the capture window duration is set

so that it will encompass most typical crash signatures, while

rejecting most of the secondary ripples that result as the

energy is dissipated by the system.

After integrating the four signals, the results are added

together to produce an overall sum. This procedure averages

out the individual responses and reduces measurement error

due to the variability of where the ball lands on the target. The

MCU then divides the grand sum by an empirically predeter-

mined constant of proportionality. The result will then go

through a binary to BCD conversion algorithm. A look–up table

is used to match the BCD numbers to their corresponding

7–segment display codes. The calculated speed is displayed

on the two digit 8–segment displays (one segment corre-

sponds to the decimal point), and the “Your Speed” LED is

turned on while the “Best Speed” LED is turned off. After a

duration of approximately five seconds, the LEDs are toggled

and stored best speed is redisplayed. The five second delay

is used to provide enough time for the user to check his/her

speed and also to allow the target to return to a rest state. The

system is now ready for another pitch. A complete listing of the

software is presented in the Appendix.

CONCLUSION

The Baseball Pitch Speedometer works fairly well, with an

accuracy of +/– 5 mph. The dynamic range of the system is

also worthy of note, measuring speeds from less than 10 mph

up to well above the 70 mph range. One key point to empha-

size, is that the system is empirically calibrated, and so to

maintain good accuracy the system should only be used with

balls of mass equal to those used during calibration.

Although intended mainly for training and recreational pur-

poses, the Baseball Pitch Speedometer demonstrates a very

important concept concerning the use of accelerometers.

Accelerometers can be used not only to detect that an event

such as impact or motion has occurred, but more importantly

they measure the intensity of such events. They can be used

to discern between different crash levels and durations. This

is very useful in applications where it is desired to have the

system respond in accord with the magnitude of the input

being monitored. An example application would be a smart

air bag system, where the speed at which the bag inflates is

proportional to the severity of the crash. The deployment rate

of the airbag would be controlled so that it does not throw the

occupant back against the seat, thus minimizing the possibil-

ity of injury to the occupant. Another application where this

concept may be utilized is in car alarms, where the response

may range from an increased state of readiness and monitor-

ing, to a full alarm sequence depending on the intensity of the

shock sensed by the accelerometer. This could be used to

prevent unnecessary firing of the alarm in the event that an

animal or person were to inadvertently bump or brush against

the automobile.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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