If your Motion Detector data does not seem to be working correctly, consider the following suggestions:
When using the Motion Detector, it is important to realize that the ultrasound is emitted in a spreading cone about 30º wide. Anything within the cone of detection can cause a reflection. A common problem when using Motion Detectors is getting unintentional reflections from a desk, chair, or computer in the room.
For example, let's say you have a table two meters from the Motion Detector and a meter to the side. It may seem that the table is out of the way, but it may cause problems. 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 the table. 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 reflections from the surface of a table supporting the Motion Detector. They can be reduced by raising or tilting the Motion Detector above the table or covering the table with a soft cloth.
If you begin with a velocity or acceleration graph and obtain a confusing display, switch back to the position graph to see that it makes sense. Sometimes a stray object or one of the problems below will interfere, and it is much easier to detect the problem while looking at the position graph.
The Motion Detector does not properly detect objects closer than 0.2 meters (newer Motion Detectors and Go! Motion) or 0.4 meters (older Motion Detectors) 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.
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 velocity and acceleration graphs are noisy, try to increase the strength of the ultrasonic reflection from the target by increasing the target's area.
Newer Motion Detectors have a switch for changing the sensitivity. The track position, marked with a cart, is better for dynamics carts on a track. The normal position, marked with the ball and walker, is better for other applications. The normal position corresponds to the sensitivity of older Motion Detectors. The track position reduces the sensitivity to stray objects, but requires that the target yield a very strong reflection. If you have trouble with dropouts in the track position, try the other setting.