6
G
ETTING
S
TARTED WITH THE
CBR 2™
S
ONIC
M
OTION
D
ETECTOR
©
1997,
2004,
2006
T
EXAS
I
NSTRUMENTS
I
NCORPORATED
Hints for effective data collection
Getting better samples
How does the CBR 2™ sonic motion detector work?
Understanding how a sonic motion detector works can help you get better data plots. The
motion detector sends out an ultrasonic pulse and then measures how long it takes for that
pulse to return after bouncing off the closest object.
The
CBR 2™
motion detector, like any sonic motion detector, measures the time interval
between transmitting the ultrasonic pulse and the first returned echo, but the
CBR 2™
motion detector has a built-in microprocessor that does much more. When the data is
collected, the
CBR 2™
motion detector calculates the distance of the object from the
CBR 2™
motion detector using a speed-of-sound calculation. Then it computes the first and second
derivatives of the distance data with respect to time to obtain velocity and acceleration data.
It stores these measurements in lists.
Object size
Using a small object at a far distance from the
CBR 2™
motion detector decreases the
chances of an accurate reading. For example, at 5 meters, you are much more likely to
detect a soccer ball than a ping-pong ball.
Minimum range
When the
CBR 2™
motion detector sends out a pulse, the pulse hits the object, bounces
back, and is received by the
CBR 2™
motion detector
.
If an object is closer than 15
centimeters (about six inches), consecutive pulses may overlap and be misidentified by the
CBR 2™
motion detector. The plot would be inaccurate, so position the
CBR 2™
motion
detector at least 15 centimeters away from the object.
Maximum range
As the pulse travels through the air, it loses its strength. After about 12 meters (6 meters on
the trip to the object and 6 meters on the trip back to the
CBR 2™
motion detector), the
return echo may be too weak to be reliably detected by the
CBR 2™
motion detector. This
limits the typical reliably effective distance from the
CBR 2™
motion detector
to the object to
less than 6 meters (about 20 feet).
Sensitivity switch
The sensitivity switch has two modes—Track and Normal.
The Track mode is intended for activities using dynamics
tracks and carts; the Normal mode is intended for all other
activities, such as, walking, ball toss, bouncing ball,
pendulum, etc.
If you are getting lots of extra noise in your data, the sensitivity switch may be in the Normal
mode. Moving the sensitivity switch to the Track position, will reduce the sensitivity of the
sensor and may produce better data.
Track Normal
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