Re: CBL/Motion Detectors

["John Gastineau" <gastineau@badgerden.com>]

> Margaret J. Clarke
> all the rest of the week have TAs rushing frantically into my office
> saying "(One or more)  motion detector doesn't work......." to which
> I respond "PushinthecordstightlybesuretheCBLhasn'tturnedoff......." 
>  but more often than not,  this is not the problem.  They don't work
> well even for me..... 


Here are some ideas that I've gathered for using the motion detector. 
They're true for both the CBL and MBL environments. 

Some Tips for the Motion Detector


In using the Motion Detector, it's important to realize that the
ultrasound is emitted in a spreading cone about 30o wide. Anything
within the cone of ultrasound can cause a reflection and possibly a
spurious measurement. A common problem in 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. 

Look at Distance first

If you begin with a velocity or acceleration graph and obtain a
confusing display, switch back to distance 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 distance information. 

With CBL just monitor the object's position in the raw sensor reading 
mode until you're sure the MD is properly positioned. 

Don't get too close_ 

The Motion Detector does not properly detect objects closer than 0.4
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.  

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.

 Noisy graphs

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.


Detector differences

All Motion Detectors are not the same; there is some variation. You 
may find that one will work well in a given situation and another 
will be marginal. Such is life with these detectors. 
-=================
John E. Gastineau
304 296 1966
Morgantown, WV 
http://www.badgerden.com/~gastineau
email: gastineau@badgerden.com   <==== NEW as of 10/1/96