Vernier Tech Info Library TIL #5

Question

My Motion Detector gives noisy readings or will not work at more than 2 m. What can I do about this?

Answer

Some General Tips for the Motion Detector:

1. The maximum sample rate that you can use for the motion detector is 50 Hz, which equates to a sample interval of 0.02 seconds. Since the motion detector emits ultrasound and detects an echo, the speed of sound is a limiting factor in the use of the sensor. So when you use the sensor, the equipment has to give the ultrasound enough time to go out to the object and back. Therefore we don't recommend sampling faster than 50 Hz, i.e. more often than 0.02 seconds. If the room has hard ceilings, floors, or walls, the Motion Detector may detect an ultrasonic pulse that has bounced around the room instead of the direct reflection from the target. This problem is worse at high data rates. Try slowing down the rate of data collection, i.e. less than or equal to 30/s.

2. In using the Motion Detector, it is important to realize that the ultrasound is emitted in a spreading cone about 20 degrees wide. Anything within the cone of ultrasound can cause a reflection and possibly a spurious measurement. These unintentional reflections may come from a desk, chair, or computer in the room. This object can be very small. Remember that the Motion Detector stops checking for echoes when it gets its first recognizable echo. It does not do the following (which might be better if it were possible): Check all echoes that come back and then select the strongest one.

3. As you start doing experiments, everything works fine, except that as the target object moves more than two meters away from the Motion Detector, the Motion Detector still reports two meters. It is reporting the distance to the first object in the cone of ultrasound that sends back a strong echo, which in this case is a table two meters away from the Motion Detector and some distance to the side. You may be able to reduce this problem by moving the table, covering it with a soft cloth, or angling the Motion Detector away from the table. You also can get reflection from the surface of a table supporting the Motion Detector. This can be reduced by raising or tilting the Motion Detector above the table or covering the table with a soft cloth.

4. Look at Distance first. If you begin with a velocity or acceleration graph and obtain a confusing display, switch back to the distance graph to see that it makes sense. Sometimes a stray object will interfere, and it is much easier to detect the problem while looking at the distance information.

5. With a CBL, just monitor the object's position in the raw sensor reading mode until you're sure the Motion Detector is properly positioned. Don't get too close. The Motion Detector does not properly detect objects closer than 0.45 meters due to the time it takes to switch from ultrasound emission to ultrasound detection mode. The maximum range is about six meters, but stray objects in the wide ultrasound cone can be problematic at this distance.

6. Soft targets. Sometimes a target may not supply a strong reflection of the ultrasound. For example, if the target is a person wearing a bulky sweater, the resulting graph may be noisy. It helps for the person to carry a book or other hard, flat object to create a stronger reflection. If the target is a ball, we have found that playground kick balls and glow in the dark "Gertie" balls (usually sound at science stores) work best.

7. If the velocity and acceleration graphs are noisy, and the motion you're studying isn't too quick, try increasing the averaging rate by increasing the number of points used to find the velocity. You can also try to increase the strength of the ultrasonic reflection from the target by increasing the target's area.

8. Multiple Motion Detectors in the room can 'talk' to each other when they are within each other's active cone.

9. Sometimes isolating the detector from ambient vibrations can help. We find putting a paper towel under the detector when we are using it on something like a ramp helps. Also, make sure that the Motion Detector is at least one meter from the computer and the monitor.

10. When using a Motion Detector on a dynamics track or air track, where the maximum distance to be measured is just a meter or two, consider putting a sock or a wrist band over the Motion Detector. This may sound strange, but it reduces the strength of minor echoes coming from small objects around the area. In many cases, this will improve the results significantly.

11. If using Motion Detectors with air tracks, try playing with the air pressure you deliver to the track. Sometimes the air currents created by an air track will cause spurious reflections of the ultrasound. Some air tracks can be used with motion detectors, others just never seem to work.

For detailed information about how our motion detector works, you can read the documentation from Polaroid that describes all of the technical specifications for the transducer. (To view this document, you need the Adobe Acrobat Reader).
http://www2.vernier.com/pdf/polaroid.pdf

Created by: jmcbride on August 30 1993
Last updated by: jhopkins on March 04 2008