C
OPYING PERMITTED PROVIDED
TI
COPYRIGHT NOTICE IS INCLUDED
6
G
ETTING
S
TARTED WITH
CBR
© 1997 T
EXAS
I
NSTRUMENTS
I
NCORPORATED
Hints for effective data collection
Getting better samples
How does CBR 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.
CBR
, like any sonic motion detector, measures the time interval between transmitting the
ultrasonic pulse and the first returned echo, but
CBR
has a built-in microprocessor that does
much more. When the data is collected,
CBR
calculates the distance of the object from the
CBR
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
L1
,
L2
,
L3
, and
L4
.
Performing the same calculations as
CBR
is an interesting classroom activity.
➊
Collect sample data in
REALTIME=NO
mode. Exit the
RANGER
program.
➋
Use the sample times in
L1
in conjunction with the distance data in
L2
to calculate the
velocity of the object at each sample time. Then compare the results to the velocity data
in
L3
.
(
L2
n+1
+
L2
n
)
à
2
N
(
L2
n
+
L2
n-1
)
à
2
L3
n
=
L1
n+1
N
L1
n
➌
Use the velocity data in
L3
(or the student-calculated values) in conjunction with the
sample times in
L1
to calculate the acceleration of the object at each sample time. Then
compare the results to the acceleration data in
L4
.
Object size
Using a small object at a far distance from the
CBR
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
sends out a pulse, the pulse hits the object, bounces back, and is received by
the
CBR.
If an object is closer than 0.5 meters (1.5 feet), consecutive pulses may overlap and
be misidentified by
CBR
. The plot would be inaccurate, so position
CBR
at least 0.5 meters
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
), the return echo may be too
weak to be reliably detected by the
CBR
. This limits the typical reliably effective distance from
the
CBR
to the object to less than 6 meters (19 feet).