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Motorola Sensor Device Data
For More Information On This Product,
Go to: www.freescale.com
Prepared by: Carlos Miranda, Systems and Applications Engineer and
David Heeley, Systems and Applications Mechanical Engineer
INTRODUCTION
The Baseball Pitch Speedometer, in its simplest form, con-
sists of a target with acceleration sensors mounted on it, an
MCU to process the sensors’ outputs and calculate the ball
speed, and a display to show the result. The actual imple-
mentation, shown in Figure 1, resembles a miniature pitching
cage, that can be used for training and/or entertainment. The
cage is approximately 6 ft. tall by 3 ft. wide by 6 ft. deep. The
upper portion is wrapped in a nylon net to retain the baseballs
as they rebound off the target. A natural rubber mat, backed
by a shock resistant acrylic plate, serve as the target. Accel-
erometers, used to sense the ball impact, and buffers, used to
drive the signal down the transmission line, are mounted on
the back side of the target. The remainder of the electronics
is contained in a display box on the top front side of the cage.
Accelerometers are sensors that measure the accelera-
tion exerted on an object. They convert a physical quantity
into an electrical output signal. Because acceleration is a
vector quantity, defined by both magnitude and direction, an
accelerometer’s output signal typically has an offset voltage
and can swing positive and negative relative to the offset,
to account for both positive and negative acceleration. An
example acceleration profile is shown in Figure 2. Because
acceleration is defined as the rate of change of velocity with
respect to time, the integration of acceleration as a function
of time will yield a net change in velocity. By digitizing and
numerically integrating the output signal of an accelerometer
through the use of a microcontroller, the “area under the
curve” could be computed. The result corresponds to the net
change in velocity of the object under observation. This is the
basic principle behind the Baseball Pitch Speedometer.
Figure 1. David Heeley, mechanical designer of the Baseball Pitch Speedometer Demo,
tests his skills at Sensors Expo Boston ’97.
SEMICONDUCTOR APPLICATION NOTE
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