motion detector
Description D03517
Figure 1. The Motion Detector
Short description
The Ultrasonic Motion Detector is designed to measure continuously the
position of a body, without disturbing the movement of the body. The
measurements are based on the reflection of ultrasonic pulses, which are
emitted from the gold foil of the transducer. Distance is determined from the
travelling time of an ultrasonic pulse from the transducer to the object and
The motion detector is packaged with a universalmounting clam. Attach the clamp using the thumbscrew on
the clamp and the mounting threads on the back of the unit.
These threads are also compatible with typical tripodmounting hardware.
While motion detector is operating a slight clicking sound
from the motion detector will be heard and the green LED
will be lit. The minimum range of the motion detector is
about 0.5 meters. The maximum range is 6 m.
The motion detector can be connected to the following interfaces:
• UIB board (via a special cable 0775 that has to be ordered separately).
• CoachLab II (sonic inputs)
• Texas Instruments CBL™ data-logger (sonic input).
Suggestion for experiments
The motion detector can be used for studying a variety of motions including:
- students walking toward and away from the motion sensor;
- objects in simple harmonic motion, such as a weight hanging on a spring;
- pendulum motions;
- carts rolling on a table or track;
- air track experiments;
- bouncing objects;
- falling objects.
How the Motion Detector works
This Motion Detector emits short
bursts of ultrasonic sound waves
from the gold foil of the transducer.
These waves fill a cone shaped are
about 15° to 20° off the axis of the
centerline of the beam. The detector
then "listens" for the echo of these
ultrasonic waves returning to it.
By timing how long it takes for the ultrasonic waves to make the trip from the
detector to an object and back the distance to the object can be determined
(based on the speed of ultrasound in air).
Note that the motion detector will report the distance to the closest object that
produces a sufficiently strong echo. Object such as chairs and tables in the
cone of ultrasound can be picked up by the detector.
The sensitivity of the echo detection circuitry automatically increases, in steps,
every few milliseconds as the ultrasound travels outward. This is to allow for
echoes being weaker from distance objects.
For accurate measurements the object should have a flat front perpendicular to
the line between sensor and object.
Tips on getting good results with the Motion Detector
The most frequently reported problem with a motion detector is that it doesn't
work beyond a certain distance. Here are some things to check if you have
• Check for a stationary object (chair, table, etc.) in the cone of the
ultrasound. This object may be detected when you are trying to study an
object further away, It may not take a very large object to cause problems.
If you have trouble with a stationary object causing unwanted echoes, try
placing a cloth over it. This minimizes the sound reflection.
• Also note the cone of ultrasound extends downward from the center line.
This can cause problems if you are using the motion detector on a hard,
horizontal surface. In these cases, try pivoting the head of the Motion
Detector to aim it slightly upward.
Other troubleshooting tips
• If there is another source of ultrasonic waves in the same frequency range,
(like motors, and fans, air track blowers, the sound made by air exiting the
holes on an air track, and even students making loud noises) this will cause
erroneous readings.
• If the room in which the motion detector is being used has a lot of hard,
sound-reflecting surfaces, you can get weird effects caused by the
ultrasound bouncing around the room. Standing waves can be set up
between the detector and a sound reflector. Try placing a cloth horizontally
just in front of and below the detector. This sometimes helps eliminate
ultrasound that is "skipping" into the detector.
• Try changing the data collection rate (measurement frequency in Coach
program). Sometimes Motion Detectors work better at one data rate than
• If you are studying people moving, have them hold a large, flat object (e.g.
a large book) as a reflector. If you have an irregular reflecting surface,
sometimes the waves will be reflected back to the transducer, and
sometimes not. The result will seem erratic.
Using the Motion Detectors at the same time as other sensors
The Motion Detector can be used at the same time as a sensor connected to
one of the analog inputs. This requires Coach Junior or Coach 5 software.
Here are some examples:
- with a force sensor to study the relationship between force and motion;
- with a force sensor to study collisions and impulse;
- with a force sensor to study simple harmonic motion;
- with a light sensor to study the inverse square law;
- with a magnetic field sensor to study how magnetic field varies with
The name of the motion detector in the sensor library of Coach 5 program is
Ultrasonic Motion Detector (03517) (CMA).
Technical data
Frequency of ultrasound
40 kHz
Aperture = (top angle) / 2
Aprox. 15 - 20º with respect to the central axis
Minimum range
0.5 m
Maximum range
Typical accuracy
± 2 mm
1 mm
5 V external (supplied by the interface)
About 51 mA @ 5 VDC while running
Speed of ultrasound in air
used to calculate a position
343 m/s
Note: This product is to be used for educational purposes only. It is not
appropriate for industrial, medical, research, or commercial applications.
Kruislaan 404, 1098 SM Amsterdam, The Netherlands
Fax: +31 20 5255866, e-mail:,
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