Texas Instruments CBR 2 manual Hints for effective data collection, Getting better samples

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.

Track Normal

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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.

6 GETTING STARTED WITH THE CBR 2™ SONIC MOTION DETECTOR © 1997, 2004, 2006 TEXAS INSTRUMENTS INCORPORATED